Victoria Ferguson, Bowei Li, Mehmet O Tas, Thomas Webb, Muhammad T Sajjad, Stuart A. J Thomson, Zhiheng Wu, Yonglong Shen, Guosheng Shao, José V Anguita, S. Ravi P Silva, Wei Zhang (2020)Direct Growth of Vertically Aligned Carbon Nanotubes onto Transparent Conductive Oxide Glass for Enhanced Charge Extraction in Perovskite Solar Cells, In: Advanced materials interfaces7(21)2001121
Vertically aligned carbon nanotubes (VACNTs) present an exciting avenue for nanoelectronics due to their predetermined orientation and exceptional transport capabilities along the tube length, with the potential to be employed in a variety of optoelectronic applications. However, growth of VACNTs using conventional chemical vapor deposition (CVD) methods requires elevated temperatures (>720 °C) and therefore, the suitability of commonly used transparent conductive oxide (TCO) glasses, such as fluorine‐doped tin oxide (FTO) and indium‐tin oxide (ITO), as the substrates for nanotube growth are limited by their temperature‐sensitive nature. Here, the successful growth of multi‐walled VACNTs directly onto commonly used TCO glasses, FTO and ITO, using the photo‐thermal chemical vapor deposition (PTCVD) growth method is reported. The benefit of reflection, within the infrared region, of the TCO substrate and the effect of surface roughness on the growth of VACNTs is investigated. The application of VACNTs on ITO in inverted planar perovskite solar cells is investigated, which shows superior charge transfer, larger grain sizes in the perovskite film, and a champion device efficiency approaching 16%.
Vertically aligned carbon nanotubes are grown directly onto temperature‐sensitive transparent conductive oxide glass; the morphology, quality and electrical properties are analyzed and used to fabricate optimized patterned carbon nanotube forest films which are used in perovskite solar cells to improve charge extraction resulting in a champion efficiency approaching 16%.
Electronic skins (e-skins), which can seamlessly adapt and adhere to the body to mimic the functionality of human skin, are a rapidly emerging research area. Such e-skins have the potential to revolutionize artificial prosthetics, robotics, human-machine interfacing, and health monitoring applications. Powering the e-skin is a critical challenge at present due to strict performance criteria, including flexibility, stretchability, mobility, and autonomous operation. One of the most promising approaches to overcome some of these challenges is to scavenge energy from the human body's movements and its surrounding environment. This paper outlines some of the key potential developments that enable energy harvesting through mechanical, thermal affects, and low light sources, as well as energy management and storage technologies, which could lead toward the construction of autonomous e-skin modules and self-powered sensing systems.
Triboelectric nanogenerators (TENGs) developed using eco-friendly natural materials instead of traditional electronic materials are more favorable for biocompatible applications, as well as from a sustainable life-cycle analysis perspective. Microarchitectured silkworm fibroin films with high surface roughness and an outstanding ability to lose electrons are used to design TENGs. An alcohol-annealing treatment is utilized to strengthen the resistance of the silk film (SF) against humidity and aqueous solubility. Herein, for the first time, the distance-dependent electric field theoretical model is employed to optimize the TENG parameters to achieve high output, which shows excellent agreement with the experimental outputs of SF-based TENG. The alcohol-treated microarchitectured SF (AT-MASF) with a polytetrafluoroethylene positive contact exhibits a stable and high electrical output even in harsh environments. These studies can lead us closer to the attractive future vision of realizing biodegradable TENG systems for harness/sensing various biomechanical activities even under real/humid environments. The potential and real-time application of the proposed AT-MASF-based TENG is demonstrated by directly employing its electric power to drive a number of low-power portable electronics and for sensing in human-body centric activities.
New materials and optimized fabrication techniques have led to steady evolution in large area electronics, yet significant advances come only with new approaches to fundamental device design. The multimodal thin‐film transistor introduced here offers broad functionality resulting from separate control of charge injection and transport, essentially using distinct regions of the active material layer for two complementary device functions, and is material agnostic. The initial implementation uses mature processes to focus on the device's fundamental benefits. A tenfold increase in switching speed, linear input–output dependence, and tolerance to process variations enable low‐distortion amplifiers and signal converters with reduced complexity. Floating gate designs eliminate deleterious drain voltage coupling for superior analog memory or computing. This versatile device introduces major new opportunities for thin‐film technologies, including compact circuits for integrated processing at the edge and energy‐efficient analog computation.
Next generation electronics are shaping the life of people by digitally connecting humans and everyday objects using smart technologies. A major challenge related to such technologies is powering the electronic devices while maintaining autonomy and mobility. Triboelectric Nanogenerators (TENGs) provide innovative solutions for powering next generation low-power electronics, by converting movement into electricity. However, these devices are still in their infancy with numerous drawbacks including high device impedance, low output power density and efficiency, mainly due to the lack of understanding of their working principles and optimization techniques. This thesis investigates the fundamental working principles of TENGs and some of their applications as energy harvesting devices. The electric field behaviour of different TENG architectures is studied using Maxwell’s equations, leading to the derivation of the distance-dependent electric field (DDEF) model. This new model is capable of fully explaining the electric field behaviour and working principle of TENGs, overcoming the drawbacks of previous models. The DDEF model is developed initially for the vertical contact-separation mode TENG and expanded to represent all working modes which utilise contact-separation movement, via the development of unified DDEF model. The models are then used to simulate the output trends of different experimental TENG devices. An experimental setup is developed and TENG devices fabricated to assess the DDEF model predictions, which verifies the higher accuracy of the new model over previous capacitor-based circuit models. Using the unified DDEF model as a framework, the effect of different structural and motion parameters of TENGs on their power output is studied. A number of new analysis techniques are introduced, including the TENG power transfer equation and TENG impedance plots, to identify the output trends and optimisation routes to design TENG devices, resulting an increase of power and reduction of TENG internal impedance by more than an order of magnitude. Finally, application of theoretical knowledge gained from the DDEF model is demonstrated by constructing a direct current output TENG device. This new design produces a constant power output subjected to continuous input motion, showing the potential to be used in self-powered electronic applications.
Inorganic-organic hybrid metal halide perovskites have shown great promise in realising high performing photovoltaic devices with low fabrication cost. In this regard, Pb-Sn mixed perovskites are considered as highly suitable candidate for all perovskite tandem solar cells and single junction solar cells with narrow bandgaps. However, the maximum achievable performances of Pb-Sn mixed perovskite solar cells (PSCs) are not yet obtained due to the lack of understanding of the material system as well prevailing issues due to unintentional doping as a result of Sn oxidation. In this thesis, the optimisation of a triple cation Pb-Sn mixed perovskite with a formula Cs0.05(FA0.83MA0.17)0.95Pb0.5Sn0.5I3 was investigated. Firstly, the effect of the “anti-solvent” on Pb-Sn mixed perovskites (fabricated through the commonly used anti-solvent dripping method) was investigated and compared to a Pb-only triple cation perovskite analogue, in terms of its effect on the film morphology, crystallisation dynamics and molecular interactions in the films. Secondly, the need for careful selection of anti-solvents for Sn-incorporated PSCs, compared to their Pb-based counterparts is elaborated. This is investigated using Pb-Sn mixed perovskite devices of an inverted architecture, with a device stack of ITO/PEDOT:PSS/Perovskite/PC60BM/ZnO/BCP/Ag. The above studies, in combination with the film processing conditions used is identified to enable the removal of unwanted Sn4+ dopants, through engineering the anti-solvent method for Pb-Sn mixed perovskites. Optimisation of the anti-solvent engineering process enables an 80% enhancement in efficiency when toluene is used as the antisolvent, in comparison to the other anti-solvents tested in this work. The champion power conversion efficiency of 11.6% achieved through this is further improved to 12.04% following optimised anti-solvent wash and thermal treatment. This is attributed to the complete removal of Sn4+ dopants following the optimisation process. Following the optimisation of the Pb-Sn mixed perovskite film preparation, a surface post-treatment method is developed based on guanidinium bromide which enables the efficiency to be improved even further to 14.4%. The surface post-treatment especially allows device fill factors of 83%, the highest reported for Pb-Sn mixed PSCs reported so far, approaching the Shockley-Queisser limit for fill factors in Pb-Sn perovskites. This work highlights the importance of careful selection of anti-solvents and post treatments on Pb-Sn mixed perovskite absorber layers, in managing defects and reducing non-radiative recombination for high efficiency low bandgap perovskite materials, and also enables further improvements in device efficiency for all-perovskite tandem solar cells.
X-ray detectors are an invaluable tool for healthcare diagnostics, cancer therapy, homeland security and non-destructive evaluation among many fields. However, potential uses for X-rays are limited by system cost and/or detector dimensions. Current X-ray detector sensitivities are limited by the bulk X-ray attenuation of the materials and consequently necessitates thick crystals with dimensions of ~ 1 mm – 1 cm, resulting in rigid structure, high operational voltages and high cost. Semiconducting polymer X-ray detectors are an emerging low cost technology. Their solution processable nature enables the fabrication of such detectors on flexible substrates over large areas using printing and roll-to-roll coating techniques. However, the low atomic number (Z) of organic materials results in low X-ray attenuation and hence, low X-ray sensitivities. This thesis focuses on direct detection of X-rays using organic semiconducting systems incorporating high Z Bi2O3 nanoparticles (NPs). For the work discussed, a thick organic bulk heterojunction (~10 – 20 μm) consisting of the p-type poly(3-hexylthiophene-2,5-diyl) (P3HT) and the n type [6,6]-Phenyl C71 butyric acid methyl ester (PC70BM) are utilised. The effectiveness on the utilisation of a dual carrier system is demonstrated through the fabrication of detectors in a diode architecture by varying the NP loading from 0 – ~50% (by wt.). These hybrid detectors demonstrate sensitivities of 1712 μC mGy-1 cm-3 when irradiated under 50 kV tungsten X-ray source and ~30 and 58 μC mGy-1 cm-3 under 6 and 15 MV X-rays generated from a medical linear accelerator. A flexible detector was fabricated which demonstrated a high sensitivity approaching 300 μC mGy-1 cm-3, highlighting the promise of the technology for dosimetry and imaging in non-planar architectures. These performances are discussed based on the structural and electrical characteristics of the hybrid thick film diodes. Possible mechanisms for the high sensitivity observed are proposed where photoconducting gain, impact ionisation and Mie scattering are identified as central contributors towards the detector sensitivity. Routes to increasing the detector thickness to 100 μm – 1 mm thickness range is demonstrated using a pellet based on powder sintering technique. These P3HT:PC70BM:Bi2O3 detectors operate as photoconductors showing X-ray sensitivities in the region of ~ 160 C mGy-1 cm-3 and a preliminary X-ray imager based on the pellets are fabricated. Finally, potential routes for further improvement of the detector characteristics are discussed.
As there are many different graphene production methods which vary in terms of quality, cost, production rate, yield, scalability, post-processing requirements, flake size distribution, and batch-to-batch repeatability, it has been challenging to develop commercial applications that exploit the extraordinary properties of graphene rather than purely using it as a marketing tool. This thesis reports on the study of ultrasonication, which is an established method to exfoliate graphene in the liquid phase, as well as a pre/post-processing technique in other graphene production methods. Due to a poor understanding of inertial cavitation, which is the fundamental mechanism driving graphene exfoliation during ultrasonication, this production method is commonly characterised by low exfoliation rates, reduced yields and limited scalability/controllability. By optimising the inertial cavitation dose, graphene yields of up to 19 ± 4 % can be exfoliated over just 3 hours of sonication at room temperature. Furthermore, inertial cavitation preferentially exfoliates larger flakes during ultrasonication and the size of the graphene flakes is correlated with, and therefore can be controlled by, inertial cavitation dose. Alongside small-scale exfoliation studies, a scalable graphene sonoreactor proof-of-concept (patent pending), has been designed and tested to generate high exfoliation rates and allow for in-situ size distribution control. More generally it is shown cavitation metrology is critical in developing efficient, controllable and repeatable ultrasonication strategies for the liquid phase exfoliation of 2D nanomaterials, and the many applications and industries in which ultrasonication is employed.
Thermoelectric (TE) materials are used within devices that can be used to convert heat energy directly into electrical energy. When a temperature gradient is applied across a TE device, it is observed that an electrical potential is established. An efficient TE device requires a high figure of merit (ZT) which means a high power factor and a low thermal conductivity are necessary. In this project, Carbon Nanotubes (CNTs) were selected for investigation as an alternative to commercial TE devices made from Bismuth Telluride, mainly due to their availability, low carbon foot print, high design capability, mechanical flexibility, low manufacturing cost and potential for better device performance. This work includes the fabrication process of CNT films which has been explored as well as doping them to n-type and p-type semiconductors. It also compares the effect of seven surfactants: Sodium dodecylbenzenesulfonates (SDBS), Sodium dodecyl sulfate (SDS), Pluronic F-127, Brij 58, Tween 80, Triton X-405 and Benzalkonium chloride (ADBAC). These surfactants are categorised depending on their hydrophilic group polarity (anionic, non-ionic and cationic). Samples exposed to ambient oxygen were found to exhibit p-type behaviour, while the inclusion of Polyethylenimine (PEI) results in n-type behaviour. The highest output power from the TE devices made of a single pair of p and n-type elements was measured to be as high as 1.5 nW/K (67 nW for a 45K temperature gradient), which is one of the highest obtained. This was achieved with Triton X-405. In addition, the electrical data obtained revealed that Triton X-405 has the highest Seebeck coefficient with 81 µV/K and a conductivity of 3.7E+03 S/m due to its short hydrophobic end and non-polar hydrophilic tail which constitutes one of the novelties of this PhD. On the other hand, the anionic surfactant SDBS with its positive end showed a 55 µV/K but a significantly higher electrical conductivity at around 2.6E+4 S/m which is believed to be due to the contribution of additional carriers (sodium ions) from the surfactant. Thermogravimetric analysis (TGA) conducted on the surfactants confirm the maximum operating temperature of each surfactant by showing their thermal degradation points. With this, it was observed that Triton X-405 and Tween 80 indicated a thermal degradation point around 364 ˚C and very low residue left of around 0.12% compared to 33% and 25% for SDBS and SDS respectively. In regard to the thermal behaviour of the CNT samples, it was revealed that CNT films with lengths above 1 cm showed heat losses due to emissivity, therefore, making longer films was deemed inefficient. Finally, a TE device is made from the best performing surfactant (Triton X-405) because of its optimum power factor, with 6 pairs of p and n type semiconducting CNT films. This device was used for a motorcycle exhaust in order to simulate heat waste harvesting which resulted in a ~ 42 mV output voltage at ~ 87 ⁰C temperature difference. This means that many alternating pairs of p-n devices are required to achieve a high output power.
The nanostructure of amorphous carbon thin films is described in terms of a disordered nanometer-sized conductive sp(2) phase embedded in an electrically insulating sp(3) matrix. It is shown that the degree of clustering and disorder within the sp(2) phase plays a determining role in the electronic properties of these films. Clustering of the sp(2) phase is shown to be important in explaining several experimental results including the reduction of the electron spin resonance linewidth with increasing spin density and the dispersion associated with the width of the Raman active G band. The influence of structural disorder, associated with sp(2) clusters of similar size, and topological disorder, due to undistorted clusters of different sizes, on both spin density and Raman measurements, is discussed. An extension of this description to intercluster interactions to explain some of the electrical transport and electron field emission behavior is also presented.
For carbon nanotubes (CNTs) to be exploited in electronic applications, the growth of high quality material on conductive substrates at low temperatures (<450°C) is required. CNT quality is known to be strongly degraded when growth is conducted on metallic surfaces, particularly at low temperatures using conventional chemical vapor deposition (CVD). Here, the production of high quality vertically-aligned CNTs at low substrate temperatures (350–440°C) on conductive TiN thin film using photo-thermal CVD is demonstrated by confining the energy required for growth to just the catalyst using an array of optical lamps and by optimizing the thickness of the TiN under-layer. The thickness of the TiN plays a crucial role in determining various properties including diameter, material quality, number of shells, and metallicity. The highest structural quality with a visible Raman D- to G-band intensity ratio as low as 0.13 is achieved for 100 nm TiN thickness grown at 420°C; a record low value for low temperature CVD grown CNTs. Electrical measurements of high density CNT arrays show the resistivity to be 1.25 × 10-2 Ω cm representing some of the lowest values reported. Finally, broader aspects of using this approach as a scalable technology for carbon nanomaterial production are also discussed.
Tungsten oxide nanowires are grown directly on tungsten wires and plates using thermal heating in an acetylene and nitrogen mixture. By heating the tungsten in nitrogen ambient, single crystal tungsten oxide nanowires can be synthesized via a self-assembly mechanism. It was found that the yield can be significantly increased with the addition of acetylene, which also results in thinner nanowires, as compared to nanowires synthesized in an oxidizing ambient. The tungsten oxide nanowires are 5 to 15 nm in diameter and hundreds of nanometers in length. In some cases, the use of acetylene and nitrogen process gas would result in tungsten oxide nanowires samples that appear visually,transparent. Comparison of the growth using the acetylene/nitrogen or then air/nitrogen mixtures is carried out. A possible synthesis mechanism, taking into account the effect of hydrocarbon addition is proposed.
The photoluminescence spectra of a series of 5-substituted pyridyl-1,2,3-triazolato PtII homoleptic complexes show weak emission tunability (ranging from λ=397-408 nm) in dilute (10-6 M) ethanolic solutions at the monomer level and strong tunability in concentrated solutions (10-4 M) and thin films (ranging from λ=487-625 nm) from dimeric excited states (excimers). The results of density functional calculations (PBE0) attribute this "turn-on" sensitivity and intensity in the excimer to strong Pt-Pt metallophilic interactions and a change in the excited-state character from singlet metal-to-ligand charge transfer (1MLCT) to singlet metal-metal-to-ligand charge transfer (1MMLCT) emissions in agreement with lifetime measurements. Turn-on tunability: A series of bis-4-(2-pyridyl)-1,2,3-triazolatoplatinum(II) complexes display variable emission tunability. At low concentration, the emission can be tuned only slightly by changing the nature of the substituent but at higher concentrations tunability is enhanced. This "turn-on" sensitivity in the excimeric emission is attributed to strong Pt-Pt metallophilic interactions and a change in the excited-state character.
A method to simultaneously synthesize carbon-encapsulated magnetic iron nanoparticles (Fe-NPs) and attach these particles to multi-walled carbon nanotubes (MWCNT) is presented. Thermal decomposition of cyclopentadienyliron dicarbonyl dimer [(C5H5)(2)Fe-2(CO)(4)], over a range of temperatures from 250 degrees C to 1200 degrees C, results in the formation of Fe-NPs attached to MWCNT. At the same time, a protective carbon shell is produced and surrounds the Fe-NPs, covalently attaching the particles to the MWCNT and leading to resistance to acid dissolution. The carbon coating varies in degree of graphitisation, with higher synthesis temperatures leading to a higher degree of graphitisation. The growth model of the nanoparticles and subsequent mechanism of MWCNT attachment is discussed. Adsorption potential of the hybrid material towards organic dyes (Rhodamine B) has been displayed, an indication of potential uses as a material for water treatment. The material has also been electrospun into aligned nanocomposite fibres to produce a soft magnetic composite (SMC) with future applications in sensors and fast switching solenoids.
We report prediction of selected physical properties (e.g. glass transition temperature, moduli and thermal degradation temperature) using molecular dynamics simulations for a difunctional epoxy monomer (the diglycidyl ether of bisphenol A) when cured with p-3,30 -dimethylcyclohexylamine to form a dielectric polymer suitable for microelectronic applications. Plots of density versus temperature show decreases in density within the same temperature range as experimental values for the thermal degradation and other thermal events determined using e.g. dynamic mechanical thermal analysis. Empirical characterisation data for a commercial example of the same polymer are presented to validate the network constructed. Extremely close agreement with empirical data is obtained: the simulated value for the glass transition temperature for the 60 C cured epoxy resin (simulated conversion a = 0.70; experimentally determined a = 0.67 using Raman spectroscopy) is ca. 70–85 C, in line with the experimental temperature range of 60–105 C (peak maximum 85 C). The simulation is also able to mimic the change in processing temperature: the simulated value for the glass transition temperature for the 130 C cured epoxy resin (simulated a = 0.81; experimentally determined a = 0.73 using Raman and a = 0.85 using DSC) is ca. 105–130 C, in line with the experimental temperature range of 110–155 C (peak maximum 128 C). This offers the possibility of optimising the processing parameters in silico to achieve the best final properties, reducing labour- and material-intensive empirical testing. 2013
Low-pressure oxygen and argon plasmas were used to pre-treat nylon fabrics, and the modified fabrics, together with the raw fabrics, were subsequently coated with single walled carbon nanotubes (SWCNTs) by a dip-drying process. Scanning electron microscopy (SEM) and Raman spectroscopy analyses indicated the attachment of SWCNTs onto nylon fabrics. After the coating with SWCNTs, the plasma modified fabrics exhibited sheet resistance of as low as 2.0 kΩ/sq. with respect to 4.9 kΩ/sq. of the raw fabrics, presumably owing to the increase of fibre surface roughness incurred by the plasma modification, which is evidenced by SEM analyses. Fourier transform infrared spectroscopy (FTIR) analysis indicates the incorporation of oxygen functionalities on fibre surfaces in the plasma modification. This is responsible for the variation of the electrical conductance of SWCNT-coated fabrics with the type of plasma and the duration of plasma ablation. © 2012 Elsevier B.V. All rights reserved.
R. M. I. Bandara, K. D. G. I. Jayawardena, S. O. Adeyemo, S. J. Hinder, J. A. Smith, H. M. Thirimanne, N. C. Wong, F. M. Amin, B. G. Freestone, A. J. Parnell, D. G. Lidzey, H. J. Joyce, R. A. Sporea, S. R. P. Silva (2019)Tin(iv) dopant removal through anti-solvent engineering enabling tin based perovskite solar cells with high charge carrier mobilities, In: Journal of Materials Chemistry C
Royal Society of Chemistry
We report the need for careful selection of anti-solvents for Sn-based perovskite solar cells fabricated through the commonly used anti-solvent method, compared to their Pb-based counterparts. This, in combination with the film processing conditions used, enables the complete removal of unwanted Sn4+ dopants, through engineering the anti-solvent method for Sn-based perovskites. Using a Cs0.05(FA0.83MA0.17)0.95Pb0.5Sn0.5I3 perovskite, charge carrier mobilities of 32 ± 3 cm2 V−1 s−1 (the highest reported for such systems through the optical-pump terahertz probe technique) together with ∼28 mA cm−2 short circuit current densities are achieved. A champion efficiency of 11.6% was obtained for solvent extraction using toluene (an 80% enhancement in efficiency compared to the other anti-solvents) which is further improved to 12.04% following optimised anti-solvent wash and thermal treatment. Our work highlights the importance of anti-solvents in managing defects for high efficiency low bandgap perovskite materials and develops the potential for all-perovskite tandem solar cells.
The silicon integrated electronics on glass or plastic substrates attracts wide interests. The design, however, depends critically on the switching performance of transistors, which is limited by the quality of silicon films due to the materials and substrate process constraints. Here, the ultrathin channel device structure is proposed to address this problem. In a previous work, the ultrathin channel transistor was demonstrated as an excellent candidate for ultralow power memory design. In this letter, theoretical analysis shows that, for an ultrathin channel transistor, as the channel becomes thinner, stronger quantum confinement can induce a marked reduction of OFF-state leakage current (I-OFF), and the subthreshold swing (S) is also decreased due to stronger control of channel from the gate. Experimental results based on the fabricated nanocrystalline silicon thin-film transistors prove the theoretical analysis. For the 2.0-nm-thick channel devices, I-ON/I-OFF ratio of more than 10(11) can be achieved, which can never be obtained for normal thick channel transistors in disordered silicon.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Single-walled carbon nanotube (SWNT) electrodes that are chemically and mechanically robust are fabricated using a simple drop cast method with thermal annealing and acid treatment. An electronic-type selective decrease in sheet resistance of SWNT electrodes with HNO3 treatment is shown. Semiconducting SWNTs show a significantly higher affinity toward hole doping in comparison to metallic SWNTs; a ≈12-fold and a ≈fivefold drop in sheet resistance, respectively. The results suggest the insignificance of the electronic type of the SWNTs for the film conductivity after hole doping. The SWNT films have been employed as transparent hole extracting electrodes in bulk heterojunction (BHJ) organic photovoltaics. Performances of the devices enlighten the fact that the electrode film morphology dominates over the electronic type of the doped SWNTs with similar sheet resistance and optical transmission. The power conversion efficiency (PCE) of 4.4% for the best performing device is the best carbon nanotube transparent electrode incorporated large area BHJ solar cell reported to date. This PCE is 90% in terms of PCEs achieved using indium tin oxide (ITO) based reference devices with identical film fabrication parameters indicating the potential of the SWNT electrodes as an ITO replacement toward realization of all carbon solar cells. Fabrication of electronic-type separated single-walled carbon nanotube (SWNT) electrodes for organic solar cells, using a simple drop cast method followed by thermal and acid treatment. The thermal and acid treatment processes significantly enhance the conductivity of the SWNT films, enabling the use of the conductivity-enhanced SWNT layers as hole extracting, transparent electrodes in organic bulk heterojunction solar cells.
Perovskite solar cells (PSCs) have emerged as a ‘rising star’ in recent years due to their high-power conversion efficiency (PCE), extremely low cost and facile fabrication techniques. To date, PSCs have achieved a certified PCE of 25.2% on rigid conductive substrates, and 19.5% on flexible substrates. The significant advancement of PSCs has been realized through various routes, including perovskite composition engineering, interface modification, surface passivation, fabrication process optimization, and exploitation of new charge transport materials. However, compared with rigid counterparts, the efficiency record of flexible perovskite solar cells (FPSCs) is advancing slowly, and therefore it is of great significance to scrutinize recent work and expedite the innovation in this field. In this article, we comprehensively review the recent progress of FPSCs. After a brief introduction, the major features of FPSCs are compared with other types of flexible solar cells in a broad context including silicon, CdTe, dye-sensitized, organic, quantum dot and hybrid solar cells. In particular, we highlight the major breakthroughs of FPSCs made in 2019/2020 for both laboratory and large-scale devices. The constituents of making a FPSC including flexible substrates, perovskite absorbers, charge transport materials, as well as device fabrication and encapsulation methods have been critically assessed. The existing challenges of making high performance and long-term stable FPSCs are discussed. Finally, we offer our perspectives on the future opportunities of FPSCs in the field of photovoltaics.
Carbon nanotubes (CNTs) have gained much interest from academia and industry due to their unique properties that include high electrical and thermal conductivity, high mechanical strength, high aspect ratio, high surface area and chemical resistance. Although composite structures containing CNTs are probably the most commercially advanced applications in the market, the area that holds most promise is in electronic applications. Low temperature CVD growth of high quality CNTs can be utilized in many applications particularly next generation IoTs, wearable electronic devices, TSVs, interconnects, and sensors. CNT growth temperature generally reported in literature ranges from 600 – 1000oC, which is not suitable for temperature sensitive substrates. However, there is ongoing research to achieve CNT growth at low temperatures, with a number reporting the growth below 550oC. In this review, we examine and discuss various techniques and approaches adopted to achieve growth of carbon nanotubes at low temperatures and its effect on various parameters of CNTs.
Metal halide perovskite solar cells are emerging candidates for next‐generation thin‐film photovoltaic devices with the potential for extremely low fabrication cost and high power conversion efficiency. Perovskite solar cells have demonstrated a rapid development in device performance over the last decade, from an initial 3.81% to a most recently certified 24.2%, though the challenges of long‐term stability and lead toxicity still remain. Carbon materials, ranging from zero‐dimensional carbon quantum dots to three‐dimensional carbon black materials, are promising candidates for the enhancement of both efficiency and stability of perovskite solar cells, offering unique advantages for incorporation into various device architectures. In this review article, we present a concise overview of important and exciting advancements of perovskite solar cells that incorporate different dimensions of carbon material in their device architectures in an effort to simultaneously improve device performance and long‐term stability. We also discuss the major advantages and potential challenges of each technique that has been developed in the most recent work. Finally, we outline the future opportunities toward more efficient and stable perovskite solar cells utilizing carbon materials.
A unified theoretical model applicable to different types of Triboelectric Nanogenerators (TENGs) is presented based on Maxwell’s equations, which fully explains the working principles of a majority of TENG types. This new model utilizes the distance-dependent electric field (DDEF) concept to derive a universal theoretical platform for all vertical charge polarization TENG types which overcomes the inaccuracies of the classical theoretical models as well as the limitations of the existing electric field-based model. The theoretical results show excellent agreement with experimental TENGs for all working modes, providing an improved capability of predicting the influence of different device parameters on the output behaviour. Finally, the output performances of different TENG types are compared. This work, for the first time, presents a unified framework of analytical equations for different TENG working modes, leading to an in-depth understanding of their working principles, which in turn enables more precise design and construction of efficient energy harvesters.
Steam treatment has been applied to our prefabricated highly aligned areas of electrospun carbon nanotube composite nano-fibres, leading to controlled and targeted removal of polymeric and amorphous carbon materials, resulting in areas of highly aligned, highly crystalline, pure nanotubes. Raman analysis shows how the ID to IG intensity ratio was reduced to 0.03, and the radial breathing mode peak intensity, used for nanotube diameter calculation, changes. Therefore, suggesting that some carbon nanotubes are more resistant to steam assisted oxidation, meaning that specific carbon nanotube diameters are preferentially oxidised. The remaining carbon nanotubes have displayed a significant improvement in both quality, with respect to defect density, and in crystallinity, resulting in an increased resistance to oxidation. These steam treated super resilient carbon nanotubes are shown to withstand temperatures of above 900 °C under ambient conditions. Applying this purification method to electrospun nano-fibres leads the way for the next generation of composite materials which can be used in high temperature extreme environments.
Abstract The predicted 50 billion devices connected to the Internet of Things by 2020 has renewed interest in polysilicon technology for high performance new sensing and control circuits, in addition to traditional display usage. Yet, the polycrystalline nature of the material presents significant challenges when used in transistors with strongly scaled channel lengths due to non-uniformity in device performance. For these new applications to materialize as viable products, uniform electrical characteristics on large areas will be essential. Here, we report on the effect of deliberately engineered potential barrier at the source of polysilicon thin-film transistors, yielding highly-uniform on-current (<8% device-to-device, accounting for material, as well as substantial geometrical, variations). The contact-controlled architecture of these transistors significantly reduces kink effect and produces high intrinsic gain over a wide range of drain voltage (2 – 20V). TCAD simulations associate critical grain boundary position and the two current injection mechanisms in this type of device, showing that, for the geometry considered, the most unfavorable location is ~150nm inside the source area. At this point, grain boundary contributes to increasing the resistance of the source pinch-off region, reducing the current injection from the bulk of the source area. Nevertheless, the effect is marginal, and the probability of a grain boundary existing at this position is low. This new understanding is instrumental in the design of new signal conversion and gain circuits for flexible and low-power sensors, without the need for complex compensation methods.
Silicon-based digital electronics have evolved over decades through an aggressive scaling process following Moore’s law with increasingly complex device structures. Simultaneously, large-area electronics have continued to rely on the same field-effect transistor structure with minimal evolution. This limitation has resulted in less than ideal circuit designs, with increased complexity to account for shortcomings in material properties and process control. At present, this situation is holding back the development of novel systems required for printed and flexible electronic applications beyond the Internet of Things. In this work we demonstrate the opportunity offered by the source-gated transistor’s unique properties for low-cost, highly functional large-area applications in two extremely compact circuit blocks. Polysilicon common-source amplifiers show 49 dB gain, the highest reported for a twotransistor unipolar circuit. Current mirrors fabricated in polysilicon and InGaZnO have, in addition to excellent current copying performance, the ability to control the temperature dependence (degrees of positive, neutral or negative) of output current solely by choice of relative transistor geometry, giving further flexibility to the design engineer. Application examples are proposed, including local amplification of sensor output for improved signal integrity, as well as temperature-regulated delay stages and timing circuits for homeostatic operation in future wearables. Numerous applications will benefit from these highly competitive compact circuit designs with robust performance, improved energy efficiency and tolerance to geometrical variations: sensor front-ends, temperature sensors, pixel drivers, bias analog blocks and high-gain amplifiers.
For hyperthermia to be used under clinical conditions for cancer therapeutics the temperature regulation needs to be precise and accurately controllable. In the case of the metal nanoparticles used for such activities, a high coercivity is a prerequisite in order to couple more energy in a single heating cycle for efficient and faster differential heating. The chemically stable Co–Zn ferrite nanoparticles have typically not been used in such self-regulating hyperthermia temperature applications to date due to their low Curie temperature usually accompanied by a poor coercivity. The latter physical property limitation under clinically applied magnetic field conditions (frequency: 100 kHz, intensity: 200 Oe) restricts the transfer of a reasonable heat energy, and thus limits the hyperthermia efficiency. Here, we report a novel Cr3+ substituted Co–Zn ferrite (Zn0.54Co0.46Cr0.6Fe1.4O4), whose Curie temperature and coercivity values are 45.7 °C and 174 Oe, respectively. Under clinically acceptable magnetic field conditions, the temperature of these nanoparticle suspensions can be self-regulated to 44.0 °C and, most importantly with a specific absorption rate (SAR) of 774 W kg−1, which is two-fold higher than the SAR standard for magnetic nanoparticles used in hyperthermia (300 W kg−1). The evaluation of the in vitro cytotoxicity of the nanoparticles reports a low toxicity, which points to a novel set of magnetic nanoparticles for use in self-regulating hyperthermia.
Graphene is a desirable material for next generation technology. However, producing high yields of single-layer flakes with industrially applicable methods is currently limited. We introduce a combined process for the reduction of graphene oxide (GO) via vitamin C (ascorbic acid) and thermal annealing at temperatures of <150 °C for times of <10 minutes, resulting in electrically conducting thin films with sheet resistances reducing by 8 orders of magnitude to as low as ∼1.3 kΩ □−1, suitable for microelectronics, display technology and optoelectronic applications. The in-depth physicochemical characterisation of the products at different stages of GO preparation and reduction allows for further understanding of the process and demonstrates the suitability for industrial production methodologies due to an environmentally-friendly reducing agent, solution processability and no requirement for high temperatures. The presence of the vitamin C lowers the temperature required to thermally reduce the GO into an electrically conducting thin film, making the technique suitable for thermally sensitive substrates, such as low melting point polymers. Simultaneous spray coating and reduction of GO allows for large area deposition of conductive coatings without sacrificing solution processability, often lost through particle agglomeration, making it compatible with industrial processes, and applicable to, for example, the production of sensors, energy devices and flexible conductive electrodes for touchscreens.
Large-scale incorporation of nanomaterials into manufactured materials can only take place if they are suitably dispersed and mobile within the constituent components, typically within a solution/ink formulation so that the additive process can commence. Natural hydrophobicity of many nanomaterials must be overcome for their successful incorporation into any solution-based manufacturing process. To date, this has been typically achieved using polymers or surfactants, rather than chemical functionalization, to preserve the remarkable properties of the nanomaterials. Quantifying surfactant or dispersion technique efficacy has been challenging. Here we introduce a new methodology to quantify dispersions applicable to high-weight fraction suspensions of most nanomaterials. It’s based on centrifuging and weighing residue of undispersed material. This enables the determination of the efficacy of surfactants to disperse nanomaterials (e.g. ultrasonication power and duration) and leads to increased nanomaterial solution loading. To demonstrate this technique, we assessed carbon nanotube dispersions using popular surfactants: Benzalkonium chloride (ADBAC), Brij®52, Brij®58, Pluronic®F127, sodium dodecyl sulfate (SDS), sodium dodecylbenzenesulfonate (SDBS), Triton™ X-100, Triton™X-405 and Tween®80, evaluating the dispersion outcome when varying sonicator power and horn depth, as well as imaging sono-intensity within the solution with luminol. The methodology is shown to be applicable for high-weight fraction nanomaterial suspensions, enabling greater deployment.
As a technique, electrospinning has been increasingly utilised for polymer nanofibre production, which has a growing list of advanced applications to which they are being applied. However, commercially scaling the process is challenging, especially when the uniformity of the nanofibres across the bulk of the material is important for the required application. At present, most commercially-scalable systems tend to rely on a drum or cylindrical-style electrode, where a multitude of electrospinning jets are formed with no specific controlled distribution or uniformity over its surface. These electrospinning systems also have the drawback of possessing a varying electrostatic field across the length of the electrode, resulting in a range of spinning conditions which result in an inconsistency in the produced nanofibres. Due to the high centrifugal stresses exerted on the polymer during electrospinning, controlling the electrostatic field is crucial for consistent nanofibre production, which forms the basis for applications such as cellular scaffolds and smart materials. In the work reported here, we utilise computational simulation to explore a range of electrode designs to achieve a large area electrospinning system with a balanced electrostatic field across its entire active surface. We demonstrate the output by producing a high-throughput of nanofibres with comparable properties to that of a traditional single spinneret system, but at a processing rate two orders of magnitude faster.
The ability to engineer a thin two-dimensional surface for light trapping across an ultra-broad spectral range is central for an increasing number of applications including energy, optoelectronics, and spectroscopy. Although broadband light trapping has been obtained in tall structures of carbon nanotubes with millimeter-tall dimensions, obtaining such broadband light–trapping behavior from nanometer-scale absorbers remains elusive. We report a method for trapping the optical field coincident with few-layer decoupled graphene using field localization within a disordered distribution of subwavelength-sized nanotexturing metal particles. We show that the combination of the broadband light–coupling effect from the disordered nanotexture combined with the natural thinness and remarkably high and wavelength-independent absorption of graphene results in an ultrathin (15 nm thin) yet ultra-broadband blackbody absorber, featuring 99% absorption spanning from the mid-infrared to the ultraviolet. We demonstrate the utility of our approach to produce the blackbody absorber on delicate opto-microelectromechanical infrared emitters, using a low-temperature, noncontact fabrication method, which is also large-area compatible. This development may pave a way to new fabrication methodologies for optical devices requiring light management at the nanoscale.
Bowei Li, Yuren Xiang, Imalka Jayawardena, Deying Luo, John Watts, Steven Hinder, Hui Li, Victoria Ferguson, Haitian Luo, Rui Zhu, Ravi Silva, Wei Zhang (2020)Tailoring Perovskite Adjacent Interfaces by Conjugated Polyelectrolyte for Stable and Efficient Solar Cells, In: Solar RRL
Interface engineering is an effective means to enhance the performance of thin‐film devices, such as perovskite solar cells (PSCs). Herein, a conjugated polyelectrolyte, poly[(9,9‐bis(3′‐((N,N‐dimethyl)‐N‐ethyl‐ammonium)‐propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)]di‐iodide (PFN‐I), is used at the interfaces between the hole transport layer (HTL)/perovskite and perovskite/electron transport layer simultaneously, to enhance the device power conversion efficiency (PCE) and stability. The fabricated PSCs with an inverted planar heterojunction structure show improved open‐circuit voltage (Voc), short‐circuit current density (Jsc), and fill factor, resulting in PCEs up to 20.56%. The devices maintain over 80% of their initial PCEs after 800 h of exposure to a relative humidity 35–55% at room temperature. All of these improvements are attributed to the functional PFN‐I layers as they provide favorable interface contact and defect reduction.
Photoluminescence (PL) spectra have been used to elucidate the band structure of graphene oxide (GO) reduced in aqueous solution. The GO reduction is measured in situ via the identification of four PL peaks produced from GO solutions with different concentrations. Using corresponding UV-visible and photoluminescence excitation (PLE) spectroscopy, and on progressing from high energy to low energy transitions, the four PL peaks are identified as σ–σ* and π–π* transitions, a π band tail due to oxygen localized states, and a π band tail due to trapped water, respectively. The labeling of the band structure has been used to challenge the prevailing assignation of the low energy transitions, reported in the literature, to molecular σ–σ* and π–π* transitions alone.
Carbon nanotubes (CNTs) are ideal candidates to be used as field emission sources. Electrodeposition could provide a viable method to deposit CNTs over large areas as part of an industrialized process. It has been shown that CNTs can be co-deposited with nickel onto various substrates, using a suitable CNT dispersant. In the present study, a multiwall carbon nanotube (MWNT): nickel (Ni) composite has been electrodeposited without the use of a dispersant. The field emission properties of the resulting electrodeposits were studied. Unpurified MWNTs grown by CVD were added to a Ni plating bath comprising of IM NiSO·6HO, 0.2M NiCl and 0.5M HBO. Due to their hydrophobic nature, MWNTs did not disperse naturally, so the plating solution was placed in a sonic bath for 15 minutes before electrodeposition. Electrochemical measurements were conducted using a μAutolab computer controlled potentiostat with a three-electrode configuration and typical cell volume of 10 cm. A spiral wound platinum wire served as the counter electrode with a Ag:AgCl wire reference electrode. Cu plates were used as cathodes, with an exposed surface area of 2 cm. After deposition, samples were thoroughly rinsed in deionised water to remove Ni salts. The resulting electrodeposits were imaged using a scanning electron microscope (FIG.1) Importantly, these deposits were observed after the samples were thoroughly rinsed in deionised water, suggesting that there is a strong adhesion between the nickel coated nanotubes and the substrate surface. FIG.1 (a) shows MWNTs (0.013 mg/ml) electrodeposited directly after sonication. Note that a thick Ni coating is not observed (see inset), and that uniform MWNT deposition is observed over a relatively large area. FIG.1(b) shows MWNTs deposited with the same solution after five minutes. A much thicker Ni coating indicates that a relatively higher concentration of Ni to MWNT was present. This was probably due to a rebundling of MWNTs over time, after the sonication process. FIG.1(c) and (d) show MWNTs deposited with a much lower concentration (0.005 mg/ml), and therefore relatively higher concentration of Ni, resulting in thicker Ni coating. Beads of Ni (visible in FIG.1(d)), approximately one micron in diameter completely encased the MWNTs, previously observed by Aria et al., using a poly(acrylic acid) dispersant. Subsequently, the substrates were subjected to field emission characterisation using a 5 mm spherical stainless steel anode. The emission current was recorded as a function of macroscopic electric field at a vacuum of around 10 mbar. The threshold field was taken to be the field at which an emission current of 1 nA was detected and the macroscopic field was calculated by dividing the applied voltage by the electrode gap, which was typically 25 μm. Threshold fields of the order 20 V/μm were observed (FIG.2), after conditioning. It is expected that by altering the deposition conditions, the much lower threshold fields would be observed. © 2005 IEEE.
Printing of highly conductive tracks at low cost is of primary importance for the emerging field of flexible, plastic, and large-area electronics. Commonly, this is achieved by printing of metallic conductive inks, often based on Ag or Cu nanoparticles dispersed in organic solvents. The solvents, which must be safely removed, have particular storage and handling requirements, thus increasing the process costs. By using water-based inks containing micron-sized silver flakes, both material and process costs can be reduced, making these inks attractive for industrial applications. However, the sintering of flake inks requires higher temperatures than nano-sized inks owing to the particles’ smaller surface area-to-volume ratio, meaning that when cured thermally the conductivity of many flake inks is lower than nanoparticle alternatives. This problem can be addressed by the application of visible light photonic curing; however, the substrate must be protected and so process parameters must be defined for each material/substrate combination. Here, we report results of a large-scale trial of photonic curing of aqueous flake silver inks on poly(ethylene terephthalate) substrates in an industrial setting. The resistivity of printed patterns after an optimized photocuring regime matched those reported for typical nanoparticle inks; on the order of 100 μΩ cm depending on substrate and geometry. Scanning electron microscopy revealed evidence for structural changes within the printed films consistent with localized melting and necking between adjacent particles, leading to an improved percolation network. Furthermore, in the large-scale industrial trial employing screen-printed silver lines, the manufacturing yield of conductive lines was increased from 44% untreated to 80% after photocuring and reached 100% when photocuring was combined with thermal curing. We believe this to be the first reported observation of an increase in the yield of printed electronic structures following photocuring. We propose a crack-healing mechanism to explain these increases in yield and conductivity. We further report on the effects of the photonic curing on the mechanical bending stability of the printed conductors and discuss their suitability for wearable applications.
NLV Carreno, MT Escote, A Valentini, L McCafferty, V Stolojan, M Beliatis, Chris Mills, R Rhodes, Christopher Smith, S Silva (2015)Adsorbent 2D and 3D carbon matrices with protected magnetic iron nanoparticles, In: NANOSCALE7(41)pp. 17441-17449
ROYAL SOC CHEMISTRY
We present a novel approach, which will potentially allow for low-temperature-substrate synthesis of carbon nanotubes using direct-current plasma-enhanced chemical vapour deposition. The approach utilizes top-down plasma heating rather than conventional heating from a conventional substrate heater under the electrode. In this work, a relatively thick titanium layer is used as a thermal barrier to create a temperature gradient between the Ni catalyst surface and the substrate. We describe the growth properties as a function of the bias voltage and the hydrocarbon concentrations. The heating during growth is provided solely by the plasma, which is dependent only on the process conditions, which dictate the power density and the cooling of the substrate, plus now the thermal properties of the "barrier layer". This novel approach of using plasma heating and thermal barrier allows for the synthesis of carbon nanotubes at low substrate temperature conditions to be attained with suitable cooling schemes.
Christopher A. Mills, Erdni Batyrev, Maurice J. R. Jansen, Muhammad Ahmad, Tanveerkhan S. Pathan, Elizabeth J. Legge, Digvijay B. Thakur, Samson N. Patole, Dan J. L. Brett, Paul R. Shearing, Hans van der Weijde, S. Ravi P. Silva (2019)Improvement in the Electrical Properties of Nickel Plated Steel using Graphitic Carbon Coatings, In: Advanced Engineering Materials21(10)1900408pp. 1-9
Thin layers of highly conductive graphitic carbon have been deposited onto nickel plated steel substrates using a direct photothermal chemical vapour deposition (PTCVD) technique. The coated nickel plated steel substrates have improved electrical properties (sheet resistance and interfacial contact resistance) compared to the pristine nickel plated steel, which makes it a cost effective alternative to stainless steel for steel producers to use in high-end electrical applications such as energy storage and microelectronics. The coated nickel plated steel has been found to have an approximately 10% reduction in sheet resistance, and a 200 times reduction in interfacial contact resistance (under compression at 140 N cm-2), compared to the pristine nickel plated steel. The interfacial contact resistance is also three times lower than that of a benchmark gold coated stainless steel equivalent at the same pressure.
Simulations of electric field enhancement factor, beta, for isolated metallic carbon nanotubes (CNT) are reported. We show that beta is dependent not only on the geometry of the tubes, but also on the location of the anode electrode. We also highlight the effect of field screening due to boundary conditions of the simulation package. Finally, we give an expression for beta as a function of CNT height, radius, and anode to cathode separation.
Pulse laser ablation and subsequent laser nanostructuring at room temperature has been employed to produce nanostructured Ni on SiO2/Si substrates for catalytic growth of carbon nanotubes. The resultant nanostructured surface is seen to consist of nanometer sized hemispherical droplets whose mean diameter is controlled by the initial metal thickness, which in turn is readily controlled by the number of laser pulses. Vertically aligned multiwall carbon nanotube mats were then grown using conventional plasma enhanced chemical vapor deposition. We show that within a single processing technique it is possible to produce the initial metal-on-oxide thin film to a chosen thickness but also to be able to alter the morphology of the film to desired specifications at low macroscopic temperatures using the laser parameters. The influence of the underlying oxide is also explored to explain the mechanism of nanostructuring of the Ni catalyst. (C) 2004 American Institute of Physics.
The observation of field induced electron emission from room temperature grown carbon nanofibers at low (5 V/mum) macroscopic electric fields is reported. The nanofibers were deposited using methane as a source gas in a conventional rf plasma enhanced chemical vapor deposition reactor using a Ni metal catalyst previously subjected to an Ar plasma treatment. Analysis of the scanning electron microscopy images of the nanofibers show them to possess an average diameter of 300 nm and that the nanofibers are observed to be radially dispersed over an area of 50 mum in diameter. No evidence of hysteresis in the current-voltage characteristic or conditioning of the emitters is observed. The mechanism for emission at low fields is attributed to field enhancement at the tips rather than from the surrounding amorphous carbon film which is shown to have a higher threshold field (20 V/mum) for emission.
Visible room-temperature photoluminescence (PL) was observed from hydrogen-free nanostructured amorphous carbon films deposited by pulsed laser ablation in different background pressures of argon (P-Ar). By varying P-Ar from 5 to 340 mTorr, the film morphology changed from smooth to rough and at the highest pressures, low-density filamentary growth was observed. Over the same pressure regime an increase in the ordering of sp(2) bonded C content was observed using visible Raman spectroscopy. The origin of the PL is discussed in terms of improved carrier localization within an increased sp(2) rich phase.
Self-organization of matter is essential for natural pattern formation, chemical synthesis, as well as modern material science. Here we show that isovolumetric reactions of a single organometallic precursor allow symmetry breaking events from iron nuclei to the creation of different symmetric carbon structures: microspheres, nanotubes, and mirrored spiraling microcones. A mathematical model, based on mass conservation and chemical composition, quantitatively explains the shape growth. The genesis of such could have significant implications for material design.
By electrospinning poly(ethylene oxide) (PEO)-blended sodium dodecyl sulfate (SDS) functionalized carbon nanotube (CNT) solutions, we engineered single- and double-walled nanotubes into highly aligned arrays. CNT alignment was measured using electron microscopy and polarised Raman spectroscopy. Mechanical tensile testing demonstrates that a CNT loading of 3.9wt% increases the ultimate tensile strength and ductility of our composites by over a factor of 3, and the Young's modulus by over a factor of 4, to ∼260MPa. Transmission electron microscopy (TEM) reveals how the aligned nanotubes provide a solid structure, preventing polymer chains from slipping, as well as polymer crystallisation structures such as ‘shish-kebabs’ forming, which are responsible for the improved mechanical properties of the composite. Differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) reveals micellar and hexagonal columnar structures along the axis of the fibers, some of which are associated with the presence of the CNT, where these hexagonal structures are associated with the SDS functionalization on the CNT surfaces. This work demonstrates the benefits of CNT alignment within composites, revealing the effectiveness of the electrospinning technique, which enables significantly improved functionality, increasing the utility of the composites for use in many different technological areas.
Carbon nanotubes (CNTs) can be used in many different applications. Field emission (FE) measurements were used together with Raman spectroscopy to show a correlation between the microstructure and field emission parameters. However, field emission characterization does not suffer from fluorescence noise present in Raman spectroscopy. In this study, Raman spectroscopy is used to characterize vertically aligned CNT forest samples based on their D/G band intensity ratio (ID/IG), and FE properties such as the threshold electric field, enhancement coefficient, and anode to CNT tip separation (ATS) at the outset of emission have been obtained. A relationship between ATS at first emission and the enhancement factor, and, subsequently, a relationship between ATS and the ID/IG are shown. Based on the findings, it is shown that a higher enhancement factor (3070) results when a lower ID/IG is present (0.45), with initial emissions at larger distances (47 lm). For the samples studied, the morphology of the CNT tips did not play an important role; therefore, the field enhancement factor (b) could be directly related to the carbon nanotube structural properties such as breaks in the lattice or amorphous carbon content. Thus, this work presents FE as a complementary tool to evaluate the quality of CNT samples, with the advantages of alarger probe size and an averaging over the whole nanotube length. Correspondingly, one can find the best field emitter CNT according to its ID/IG.
The field emission screening effect is one of great importance when aiming to design efficient and powerful cathodes. It has long been assumed that the degrading effect is at a minimum when neighboring emitters are at least twice their height from each other. In this work, we show that the screening effect is underestimated and diminishes at far greater separations of five times the height of the emitter. We further observe that to achieve maximum emission efficiency in an array, one requires a trade off between screening and emitter number, resulting in a separation of three times their height.
The concept of magnetic induction of hyperthermia was first proposed by Gilchrist et al. in 1957. The physics is based on the simple principle that when exposed to an alternating magnetic field, the magnetic media can transform the electromagnetic energy to thermal energy, causing the temperature increase of any surrounding media or tissue. In biological tissue, normal cells usually possess higher heat resistance and resilience to temperature than tumor cells. As such, cancerous cells can be selectively destroyed by increasing the local temperature of the tissue to a desired temperature range (42–46°C), while ensuring healthy cells are unharmed.
Metal-organic frameworks (MOFs) have emerged as an exciting class of porous materials that can be structurally designed by choosing particular components according to desired applications. De- spite the wide interest in and many potential applications of MOFs, such as in gas storage, catal- ysis, sensing and drug delivery, electrical semiconductivity and its control is still rare. The use and fabrication of electronic devices with MOF-based components has not been widely explored, despite the significant progress of these components made in recent years. Here we report the syn- thesis and properties of a new highly crystalline, electrochemically active, cobalt and naphthalene diimide-based MOF that is an efficient electrical semiconductor and has a broad absorption spec- trum, from 300 nm to 2500 nm. Its semiconductivity was determined by direct voltage bias using a four-point device, and it features a wavelength dependant photoconductive-photoresistive dual behaviour, with a very high responsivity of 2.5×105 A W−1.
Bowei Li, Yuren Xiang, K.D.G. Imalka Jayawardena, Deying Luo, Zhuo Wang, Xiaoyu Yang, John F. Watts, Steven Hinder, Muhammad T. Sajjad, Thomas Webb, Haitian Luo, Igor Marko, Hui Li, Stuart A.J. Thomson, Rui Zhu, Guosheng Shao, Stephen J. Sweeney, S. Ravi P. Silva, Wei Zhang (2020)Reduced bilateral recombination by functional molecular interface engineering for efficient inverted perovskite solar cells, In: Nano Energy105249
Interface-mediated recombination losses between perovskite and charge transport layers are one of the main reasons that limit the device performance, in particular for the open-circuit voltage (VOC) of perovskite solar cells (PSCs). Here, functional molecular interface engineering (FMIE) is employed to retard the interfacial recombination losses. The FMIE is a facile solution-processed means that introducing functional molecules, the fluorene-based conjugated polyelectrolyte (CPE) and organic halide salt (OHS) on both contacts of the perovskite absorber layer. Through the FMIE, the champion PSCs with an inverted planar heterojunction structure show a remarkable high VOC of 1.18 V whilst maintaining a fill factor (FF) of 0.83, both of which result in improved power conversion efficiencies (PCEs) of 21.33% (with stabilized PCEs of 21.01%). In addition to achieving one of the highest PCEs in the inverted PSCs, the results also highlight the synergistic effect of these two molecules in improving device performance. Therefore, the study provides a straightforward avenue to fabricate highly efficient inverted PSCs.
KDG Imalka Jayawardena, S Li, LF Sam, Christopher Smith, MJ Beliatis, KK Gandhi, MR Ranga Prabhath, TR Pozegic, S Chen, X Xu, DMR Dabera, LJ Rozanski, RA Sporea, Chris Mills, X Guo, S Silva (2015)High efficiency air stable organic photovoltaics with an aqueous inorganic contact, In: Nanoscale(34)pp. 14241-14247
The Royal Society of Chemistry
We report a ZnO interfacial layer based on an environmentally friendly aqueous precursor for organic photovoltaics. Inverted PCDTBT devices based on this precursor show power conversion efficiencies of 6.8–7%. Unencapsulated devices stored in air display prolonged lifetimes extending over 200 hours with less than 20% drop in efficiency compared to devices based on the standard architecture.
Electron field emission from an isolated carbon nanotube (CNT) was performed in situ in a modified scanning electron microscope, over a range of anode to CNT tip separations, D, of 1-60 mu m. The threshold field required for an emission current of 100 nA was seen to decrease from a value of 42 V mu m(-1) at an anode to CNT tip separation of 1 mu m, asymptotically, to approach 4 V mu m(-1) at a separation of 60 mu m. It is proposed that at low D, the electric field enhancement factor (beta) reduces as the anode electrode approaches the CNT mimicking a parallel plate configuration. Under "far field" conditions, where D > 3 h, where h is the CNT height, the CNT enhancement factor is no longer dependant on D, as shown by the asymptotic behavior of the threshold field, and is purely a factor of the CNT height and radius. For each CNT to tip separation, measured emission current data together with the threshold field and enhancement, are consistent with a Fowler-Nordheim analysis for the far field conditions, and dispels the need for a novel emission mechanism to explain the results as has been proposed recently. (c) 2005 American Institute of Physics.
Hashini Thirimanne, K Jayawardena, A Parnell, R Bandara, A Karalasingam, Silvia Pani, J Huerdler, D Lidzey, S Tedde, Andrew Nisbet, Chris Mills, Ravi Silva (2018)High sensitivity organic inorganic hybrid X ray detectors with direct transduction and broadband response, In: Nature Communications92926
Nature Publishing Group
X-ray detectors are critical to healthcare diagnostics, cancer therapy and homeland security, with many potential uses limited by system cost and/or detector dimensions. Current X-ray detector sensitivities are limited by the bulk X-ray attenuation of the materials and consequently necessitate thick crystals (~ 1 mm – 1 cm), resulting in rigid structure, high operational voltages and high cost. Here we present a disruptive, flexible, low cost, broad-band, and high sensitivity direct X-ray transduction technology produced by embedding high atomic number bismuth oxide nanoparticles in an organic bulk heterojunction. These hybrid detectors demonstrate sensitivities of 1712 µC mGy-1 cm-3 for “soft” X-rays and ~30 and 58 µC mGy-1 cm-3 under 6 and 15 MV “hard” X-rays generated from a medical linear accelerator; strongly competing with the current solid state detectors, all achieved at low bias voltages (-10 V) and low power, enabling detector operation powered by coin cell batteries.
A laser direct-writing method producing high-resolution patterns of gold, silver and alloy plasmonic nanoparticles implanted into the surface of glass substrates is demonstrated, by scanning a pulsed UV laser beam across selected areas of ultra-thin metal films. The nanoparticles are incorporated beneath the surface of the glass and hence the patterns are scratch-resistant. The physical mechanisms controlling the process are investigated and we demonstrate that this technique can be used to fabricate a wide range of plasmonic optical structures such as wavelength selected diffraction gratings and high-density substrates for lab-on-chip surface-enhanced Raman spectroscopy.
A low-cost lithographic technique to pattern poly (3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) films with 10 nm deep features of 700 nm periodicity is demonstrated. The use of these patterned films in poly (3-hexylthiophene) : [6,6]-phenylC(61)-butyric acid methyl ester organic photovoltaic devices leads to an increase in short circuit current (J(sc)), fill factor, and power conversion efficiency (PCE) with only a slight reduction in open circuit voltage. Patterning the PEDOT: PSS at 150 degrees C increases Jsc from 2.44 to 3.03 mA/cm(2) improving the PCE from 0.63% to 0.81% with similar increases due to patterning also being obtained at other temperatures.
S. R. P. Silva, JD Carey, G. Y. Chen, D. C. Cox, R. D. Forrest, C. H. Poa, R. C. Smith, Y. F. Tang, J. M. Shannon (2004)Nanoengineering of materials for field emission display technologies, In: IEE Proceedings in Circuits, Devices and Systems51pp. 489-496
The holy grail in terms of flat panel displays has been an inexpensive process for the production of large area 'hang on the wall' television that is based on an emissive technology. Electron field emission displays, in principle, should be able to give high quality pictures with good colour saturation, and, if suitable technologies for the production of cathodes over large areas were to be made available, at low cost. This requires a process technology where temperatures must be maintained below 450/spl deg/C throughout the entire production cycle to be consistent with the softening temperature of display glass. In this paper we propose three possible routes for nanoscale engineering of large area cathodes using low temperature processing that can be integrated into a display technology. The first process is based on carbon nanotube-polymer composites that can be screen printed over large areas and show electron field emission properties comparable with some of the best aligned nanotube arrays. The second process is based on the large area growth of carbon nanofibres directly onto substrates held at temperatures ranging from room temperature to 300/spl deg/C, thereby making it possible to use inexpensive substrates. The third process is based on the use of excimer laser processing of amorphous silicon for the production of lithography-free large area three terminal nanocrystalline silicon substrates. Each route has its own advantages and flexibility in terms of incorporation into an existing display technology. The harnessing of these synergies will be highlighted together with the properties of the cathodes developed for the differing technologies.
Electron paramagnetic resonance ~EPR! measurements have been made of amorphous hydrogenated carbon ~a-C:H! films grown by plasma enhanced chemical vapor deposition ~PECVD! with negative self-bias voltages Vb in the approximate range 10–540 V. For Vb,100 V, as the film changes from polymerlike to diamondlike, the changes in linewidth and shape are interpreted in terms of changes to two contributions—one due to dipolar interactions between the unpaired spins and one due to unresolved lines arising from hyperfine interactions with H1. The former yields a Lorentzian line, the latter a Gaussian, and the resultant spectrum has the Voigt shape. The empirical relation DBpp G ~in Gauss!5~0.1860.05!3~at.%H) between the peak-to-peak Gaussian contribution ~in Gauss! DBpp G and the hydrogen content in atomic percentage is obtained. For Vb.100 V the linewidth is shown to be dominated by the dipolar interactions and exchange and it decreases as Vb increases; the change is shown to arise primarily from a change in the exchange interaction. Evidence for this comes from measurements which show that the spin-lattice relaxation time appreciably shortens and the spin-spin relaxation time lengthens as the bias voltage is increased. The magnitude and variation with bias of the linewidth are consistent with the EPR signal originating from the p-type radicals.
The microstructure of filtered cathodic vacuum arc deposited tetrahedral amorphous carbon films is studied as a function of ion energy. An optimum energy window in the density and C–C sp3 content at an ion energy of ;90 eV observed in this study. It is shown that the density of the amorphous carbon films are closely related to the sp3 content. The observation of nanocrystals embedded in the amorphous carbon matrix is reported. Most of the crystals observed by transmission electron microscopy can be indexed to graphite, but some of the crystals can be indexed to cubic diamond. The chemical composition of the crystals is analyzed using electron energy loss spectroscopy ~EELS!. The only discernible EELS edge is that of C at an energy of 285 eV.
Hybrid inorganic-in-organic semiconductors are an attractive class of materials for optoelectronic applications. Traditionally, the thicknesses of organic semiconductors are kept below 1 μm due to poor charge transport in such systems. However, recent work suggests that charge carriers in such organic semiconductors can be transported over centimeter length scales opposing this view. In this work, a unipolar X-ray photoconductor based on a bulk heterojunction architecture, consisting of poly(3-hexylthiophene), a C70 derivative, and high atomic number bismuth oxide nanoparticles operating in the 0.1–1 mm thickness regime is demonstrated, having a high sensitivity of ∼160 μC mGy–1 cm–3. The high performance enabled by hole drift lengths approaching a millimeter facilitates a device architecture allowing a high fraction of the incident X-rays to be attenuated. An X-ray imager is demonstrated with sufficient resolution for security applications such as portable baggage screening at border crossings and public events and scalable medical applications.
We describe the development and first tests of ENOBIO, a dry electrode sensor concept for biopotential applications. In the proposed electrodes the tip of the electrode is covered with a forest of multi-walled Carbon Nanotubes (CNTs) that can be coated with Ag/AgCl to provide ionic–electronic transduction. The CNT brushlike structure is to penetrate the outer layers of the skin improving electrical contact as well as increase the contact surface area. In this paper we report the results of the first tests of this concept—immersion on saline solution and pig skin signal detection. These indicate performance on a par with state of the art researchoriented wet electrodes.
Compositional analysis of metal-containing carbon thin films and nanostructures produced by pulsed laser ablation of a carbon-nickel target revealed significantly higher fractions of nickel in the materials than in the target used to produce them. Ablation of mixed targets is used routinely in the synthesis of carbon nanotubes and to enhance the conductivity of amorphous carbon films by metal incorporation. In this extensive study we investigate the physical mechanisms underlying this metal-enrichment and relate changes in the dynamics of the ablation plumes with increasing background gas pressure to the composition of deposited materials. The failure to preserve the target atom ratios cannot, in this case, be attributed to conventional mechanisms for non-stoichiometric transfer. Instead, nickel-enrichment of the target surface by back-deposition, combined with significantly different propagation dynamics for C atoms, Ni atoms and alloy clusters through the background gas, appears to be the main cause of the high nickel fractions. © 2012 Elsevier Ltd. All rights reserved.
Metal halide perovskite solar cells are emerging candidates amongst the next-generation thin-film photovoltaic devices with extremely low fabrication cost and high power conversion efficiency. Defects (both in the bulk material and at the interfaces) are recognized as one of the most fundamental reasons for the compromised device performance and long-term stability of perovskite solar cells. In this review article, we analyse the possible origins of the defects formation in metal halide perovskites, followed by the rationalization of various approaches being utilized to reduce the density of defects. We demonstrate that defect engineering, including adding dopants in the precursor solutions, interface passivation, or other physical treatments (thermal or light stress) is an essential way to further boost the device performance and enhance their long-term stability. We note that although the exact mechanisms of defect elimination in some approaches are yet to be elucidated, the research on defect engineering is expected to have enormous impact on next wave of device performance optimisation of metal halide perovskite solar cells towards Shockley-Queisser limit.
Amorphous carbon films containing no hydrogen were irradiated with a pulsed UV laser in vacuum. Raman spectroscopy indicates an increase in the quantity of sp(2) clustering with the highest laser energy density and a commensurate reduction in resistivity. The reduction of resistivity is explained to be associated with thermally induced graphitization of amorphous carbon films. The high field transport is consistent with a Poole-Frenkel type transport mechanism via neutral trapping centers related to sp(2) sites which are activated under high fields. Decreasing the resistivity is an important feature for use of carbon as an electronic material. (C) 2008 American Institute of Physics.
We report prediction of selected physical properties (e.g. glass transition temperature, moduli and thermal degradation temperature) using molecular dynamics simulations for a difunctional epoxy monomer (the diglycidyl ether of bisphenol A) when cured with p-3,30 -dimethylcyclohexylamine to form a dielectric polymer suitable for microelectronic applications. Plots of density versus temperature show decreases in density within the same temperature range as experimental values for the thermal degradation and other thermal events determined using e.g. dynamic mechanical thermal analysis. Empirical characterisation data for a commercial example of the same polymer are presented to validate the network constructed. Extremely close agreement with empirical data is obtained: the simulated value for the glass transition temperature for the 60 C cured epoxy resin (simulated conversion a = 0.70; experimentally determined a = 0.67 using Raman spectroscopy) is ca. 70–85 C, in line with the experimental temperature range of 60–105 C (peak maximum 85 C). The simulation is also able to mimic the change in processing temperature: the simulated value for the glass transition temperature for the 130 C cured epoxy resin (simulated a = 0.81; experimentally determined a = 0.73 using Raman and a = 0.85 using DSC) is ca. 105–130 C, in line with the experimental temperature range of 110–155 C (peak maximum 128 C). This offers the possibility of optimising the processing parameters in silico to achieve the best final properties, reducing labour- and material-intensive empirical testing.
Christopher Smith, Christopher A. Mills, Silvia Pani, Rhys Rhodes, Josh J. Bailey, Samuel J. Cooper, Tanveerkhan S. Pathan, Vlad Stolojan, Daniel J. L. Brett, Paul R. Shearing, S. Ravi P. Silva (2019)X-ray micro-computed tomography as a non-destructive tool for imaging the uptake of metal nanoparticles by graphene-based 3D carbon structures, In: Nanoscale11pp. 14734-14741
Royal Society of Chemistry
Graphene-based carbon sponges can be used in different applications in a large number of fields including microelectronics, energy harvesting and storage, antimicrobial activity and environmental remediation. The functionality and scope of their applications can be broadened considerably by the introduction of metallic nanoparticles into the carbon matrix during preparation or post-synthesis. Here, we report on the use of X-ray micro-computed tomography (CT) as a method of imaging graphene sponges after the uptake of metal (silver and iron) nanoparticles. The technique can be used to visualize the inner structure of the graphene sponge in 3D in a non-destructive fashion by providing information on the nanoparticles deposited on the sponge surfaces, both internal and external. Other deposited materials can be imaged in a similar manner providing they return a high enough contrast to the carbon microstructure, which is facilitated by the low atomic mass of carbon.
Some of the key challenges in the applications of graphene and carbon nanotubes are associated with their poor attachment to the substrate and poisoning of the catalyst by environmental contamination prior to the growth phase. Here we report a ‘protected catalyst’ technique which not only overcomes these challenges but also provides a new material production route compatible with many applications of carbon nanomaterials. The breakthrough technique involves capping the catalyst with a protective layer of a suitable material (examples include TiN, Cr, Ta) which protects the catalyst from environmental contaminants such as oxidation, etchant attack, etc., whilst maintaining carbon supply to the catalyst for the CVD growth of desired nanomaterial. A thin Fe catalyst layer remained protected due to the capping layer in the CF4 based reactive-ion-etching of SiO2. We show that the carbon nanostructures grown using this technique exhibit significantly improved adhesion to the substrate in sonication bath tests. We demonstrate the fabrication of 3D structures and CNT based vias in a buried catalyst arrangement using the protected catalyst technique. The technique also allows better control over various growth parameters such as number of graphene layers, growth rate, morphology, and structural quality.
Visible room-temperature photoluminescence (PL) was observed from hydrogen-free nanostructured amorphous carbon films deposited by pulsed laser ablation in different background pressures of argon (PAr). By varying PAr from 5 to 340 mTorr, the film morphology changed from smooth to rough and at the highest pressures, low-density filamentary growth was observed. Over the same pressure regime an increase in the ordering of sp2 bonded C content was observed using visible Raman spectroscopy. The origin of the PL is discussed in terms of improved carrier localization within an increased sp2 rich phase.
This work reports the synthesis and characterisation of a core-shell n-octacosane@silica nano-encapsulated phase-change material obtained via interfacial hydrolysis and poly-condensation of tetraethyl orthosilicate in mini-emulsion. Silica has been used as the encapsulating material because of its thermal advantages relative to synthesised polymers. The material presents excellent heat storage potential, with a measured latent heat varying between 57.1 and 89 kJ∙kg-1 (melting point between 58 and 64°C) and a small particle size (between ~565 and ~227 nm). Degradation of the n-octacosane core starts between 150 and 180°C. Also, the use of silica as shell material gives way to a heat conductivity of 0.796 W∙m-1∙K-1 (greater than that of nano-encapsulated materials with polymeric shell). Charge/discharge cycles have been successfully simulated at low pressure to prove the suitability of the nano-powder as phase-change material. Further investigations will be carried out in the future regarding the use of the synthesised material in thermal applications involving nanofluids.
The microstructure of filtered cathodic vacuum arc deposited tetrahedral amorphous carbon films is studied as a function of ion energy. An optimum energy window in the density and C–C sp 3 content at an ion energy of ∼90 eV observed in this study. It is shown that the density of the amorphous carbon films are closely related to the sp 3 content. The observation of nanocrystals embedded in the amorphous carbon matrix is reported. Most of the crystals observed by transmission electron microscopy can be indexed to graphite, but some of the crystals can be indexed to cubic diamond. The chemical composition of the crystals is analyzed using electron energy loss spectroscopy (EELS). The only discernible EELS edge is that of C at an energy of 285 eV.
A UV pulsed laser writing technique to fabricate metal nanoparticle patterns on low-cost substrates is demonstrated. We use this process to directly write nanoparticle gas sensors, which operate via quantum tunnelling of electrons at room temperature across the device. The advantages of this method are no lithography requirements, high precision nanoparticle placement, and room temperature processing in atmospheric conditions. Palladium-based nanoparticle sensors are tested for the detection of water vapor and hydrogen within controlled environmental chambers. The electrical conduction mechanism responsible for the very high sensitivity of the devices is discussed with regard to the interparticle capacitance and the tunnelling resistance.
The formation of Ni nanostructures to act as catalysts in the growth of carbon nanotubes is reported. The changes in the surface morphology of Ni produced by three methods - thermal evaporation and annealing of thin films, pulsed laser ablation and annealing of Ni, and the use of metal containing macromolecules - have been investigated by atomic force microscopy and scanning electron microscopy. In the case of thermal annealing of thin metal films in the temperature range 300-500oC we observe an increase in the mean diameter of the islands formed, accompanied by a reduction in the mean island density with increasing temperature. We attribute this effect to mass transport of weakly bound individual Ni atoms and/or small island clusters across the surface to form larger isolated islands, in a process similar to Ostwald ripening. Using a pulsed KrF excimer laser for ablation of a Ni target we show that nanometre smooth Ni thin films can be produced provided a sufficient number of laser shots is used. The surface morphology of these smooth films can then be altered by laser annealing to form Ni droplets. It is found that the mean diameter of the Ni droplets depends not only on the initial Ni thickness but also the laser fluence. It is also found that the nanostructuring of the film depends on the presence of an oxide under layer, with a higher fluence required on thinner oxides and no nanostructuring observed on bare Si. Finally, we show that Ni nanostructuring can be formed by suitable annealing of a Ni containing aqueous dendrimer solutions.
Despite the "darker than black" association attributed to carbon nanotube forests, here is shown that it is also possible to grow these structures, over heat-sensitive substrates, featuring highly transmissive characteristics from the UV to infrared wavelengths, for forest heights as high as 20 μm. The optical transmission is interpreted in terms of light propagation along channels that are self-generated by localized bundling of tubes, acting as waveguides. A good correlation is shown between the distribution of diameter sizes of these sub-wavelength voids and the transmission spectrum of the forests. For the shorter visible and near-UV wavelengths, this model shows that light propagates by channeling along individual vertical voids in the forests, which elucidates the origin for the widely-reported near-zero reflectance values observed in forests. For the longer infrared wavelengths, the mode spreads over many nanotubes and voids, and propagates along a "homogeneous effective medium". The strong absorption of the forest at the shorter wavelengths is correlated in terms of the stronger attenuation inside a waveguide cavity, according to the λ attenuation dependency of standard waveguide theory. The realization of this material can lead to novel avenues in new optoelectronic device design, where the carbon nanotube forests can be used as highly conducting "scaffolds" for optically active materials, whilst also allowing light to penetrate to significant depths into the structure, in excess of 20 μm, enabling optical functionality. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Mixed halide Perovskite solar cells (PSCs) are commonly produced by depositing PbCl2 and CH3NH3I from a common solvent followed by thermal annealing, which in an up-scaled manufacturing process is likely to take place under ambient conditions. However, it has been reported that, similar to the effects of thermal annealing, ambient humidity also affects the crystallisation behaviour and subsequent growth of the Perovskite films. This implies that both of these factors must be accounted for in solar cell production. In this work, we report for the first time the correlation between the annealing time, relative humidity (RH) and device performance for inverted, mixed halide CH3NH3PbI(3−x)Cl x PSCs with active area ≈1 cm2. We find a trade-off between ambient humidity and the required annealing time to produce efficient solar cells, with low humidities needing longer annealing times and vice-versa. At around 20% RH, device performance weakly depends on annealing time, but at higher (30%–40% RH) or lower (0%–15% RH) humidities it is very sensitive. Processing in humid environments is shown to lead to the growth of both larger Perovskite grains and larger voids; similar to the effect of thermal annealing, which also leads to grain growth. Therefore, samples which are annealed for too long under high humidity show loss of performance due to low open circuit voltage caused by an increased number of shunt paths. Based on these results it is clear that humidity and annealing time are closely interrelated and both are important factors affecting the performance of PSCs. The findings of this work open a route for reduced annealing times to be employed by control of humidity; critical in roll-to-roll manufacture where low manufacturing time is preferred for cost reductions.
Graphene oxide (GO) is becoming increasingly popular for organic electronic applications. We present large active area (0.64 cm^2), solution processable, poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:[6,6]-Phenyl C71 butyric acid methyl ester (PCDTBT:PC70BM) organic photovoltaic (OPV) solar cells, incorporating GO hole transport layers (HTL). The power conversion efficiency (PCE) of ~5% is the highest reported for OPV using this architecture. A comparative study of solution-processable devices has been undertaken to benchmark GO OPV performance with poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) HTL devices, confirming the viability of GO devices, with comparable PCEs, suitable as high chemical and thermal stability replacements for PEDOT:PSS in OPV.
Post-synthesis separation of metallic (m-SWNTs) and semiconducting (s-SWNTs) single-wall carbon nanotubes (SWNTs) remains a challenging process. Gel agarose chromatography is emerging as an efficient and large scale separation technique. However, the full (100%) separation has not been achieved yet, mainly due to the lack of understanding of the underlying mechanism. Here, we study the temperature effect on the SWNTs separation via gel agarose chromatography, for four different SWNT sources. Exploiting a gel agarose micro-beads filtration technique we achieve up to 70% m-SWNTs and over 90% s-SWNTs, independent of the source material. The process is temperature dependent, with yields up to 95% for s-SWNT (HiPco) at 6 °C. Temperature affects the sodium dodecyl sulfate surfactant-micelle distribution along the SWNT sidewalls, thus determining the effectiveness of the SWNTs sorting by electronic type. The sorted SWNTs are then used to fabricate transistors with very low OFF-currents (∼10−13 A), high ON/OFF current ratio (>106) and charge carriers mobility ∼ 40 cm2 V−1 s−1.
Facile and low cost hydrothermal routes are developed to fabricate three-dimensional (3D) hierarchical ZnO structures with high surface-to-volume ratios and an increased fraction of (0001) polar surfaces. Hierarchical ZnO nanowires (ZNWs) and nanodisks (ZNDs) assembled from initial ZnO nanostructures are prepared from sequential nucleation and growth following a hydrothermal process. These hierarchical ZnO structures display an enhancement of gas sensing performance and exhibit significantly improved sensitivity and fast response to acetone in comparison to other mono-morphological ZnO, such as nanoparticles, NWs, or NDs. In addition to the high surface-to-volume ratio due to its small size, the nanowire building blocks show the enhanced gas sensing properties mainly ascribed to the increased proportion of exposed active (0001) planes, and the formation of many nanojunctions at the interface between the initial ZnO nanostructure and secondary NWs. This work provides the route for structure induced enhancement of gas sensing performance by designing a desirable nanostructure, which could also be extended to synthesize other metal oxide nanostructures with superior gas sensing performance.
The use of carbon nanotubes as a gene delivery system has been extensively studied in recent years owing to its potential advantages over viral vectors. To achieve this goal, carbon nanotubes have to be functionalized to become compatible with aqueous media and to bind the genetic material. To establish the best conditions for plasmid DNA binding, we compare the dispersion properties of single-, double- and multi-walled carbon nanotubes (SWCNTs, DWCNTs and MWCNTs, respectively) functionalized with a variety of surfactants by non-covalent attachment. The DNA binding properties of the functionalized carbon nanotubes were studied and compared by electrophoresis. Furthermore, a bilayer functionalization method for DNA binding on SWCNTs was developed that utilized RNA-wrapping to solubilize the nanotubes and cationic polymers as a bridge between nanotubes and DNA.
The nanostructure of amorphous carbon thin films is described in terms of a disordered nanometer-sized conductive sp2 phase embedded in an electrically insulating sp3 matrix. It is shown that the degree of clustering and disorder within the sp2 phase plays a determining role in the electronic properties of these films. Clustering of the sp2 phase is shown to be important in explaining several experimental results including the reduction of the electron spin resonance linewidth with increasing spin density and the dispersion associated with the width of the Raman active G band. The influence of structural disorder, associated with sp2 clusters of similar size, and topological disorder, due to undistorted clusters of different sizes, on both spin density and Raman measurements, is discussed. An extension of this description to intercluster interactions to explain some of the electrical transport and electron field emission behavior is also presented.
a-C:H films prepared by DC-magnetron sputtering in an H2:Ar mixture exhibit strong photoluminescence (PL) peaks superimposed upon the Raman scattering spectrum. PL becomes observable at a hydrogen content of ca. 34%, and increases exponentially thereafter, driven by the progressive saturation of carbon dangling bonds. In this %H range, hardness and elastic modulus decrease and CSS durability reaches an optimum. The Raman G peak position is very sensitive to deposition temperature (shift of 0.1 cm-1/°C) and was found to correlate with the sp3 /sp2 bonding ratio as measured by EELS, and therefore can also be used as a predictor of carbon tribological performance
Pulse laser ablation and subsequent laser nanostructuring at room temperature has been employed to produce nanostructured Ni on SiO2/Si substrates for catalytic growth of carbon nanotubes. The resultant nanostructured surface is seen to consist of nanometer sized hemispherical droplets whose mean diameter is controlled by the initial metal thickness, which in turn is readily controlled by the number of laser pulses. Vertically aligned multiwall carbon nanotube mats were then grown using conventional plasma enhanced chemical vapor deposition. We show that within a single processing technique it is possible to produce the initial metal-on-oxide thin film to a chosen thickness but also to be able to alter the morphology of the film to desired specifications at low macroscopic temperatures using the laser parameters. The influence of the underlying oxide is also explored to explain the mechanism of nanostructuring of the Ni catalyst.
The electrical properties of amorphous carbon are governed by the high localization of the sp2 π states, and conventional methods of altering the sp2 content result in macroscopic graphitization. By using ion beams we have achieved a delocalization of the π states by introducing nanoclustering and hence improving the connectivity between existing clusters, as demonstrated by the increase in the conductivity by two orders of magnitude without modification of the band gap. At higher doses, paramagnetic relaxation-time measurements indicate that exchange effects are present. This unveils the possibility of amorphous carbon-based electronics by tailoring the ion-beam conditions, which we demonstrate in the form of a rectifying device.
Double-walled carbon nanotubes (DWNTs) prepared by catalytic chemical vapour deposition were functionalized in such a way that they were optimally designed as a nano-vector for the delivery of small interfering RNA (siRNA), which is of great interest for biomedical research and drug development. DWNTs were initially oxidized and coated with a polypeptide (Poly(Lys:Phe)), which was then conjugated to thiol-modified siRNA using a heterobifunctional cross-linker. The obtained oxDWNT-siRNA was characterized by Raman spectroscopy inside and outside a biological environment (mammalian cells). Uptake of the custom-designed nanotubes was not associated with detectable biochemical perturbations in cultured cells, but transfection of cells with DWNTs loaded with siRNA targeting the green fluorescent protein (GFP) gene, serving as a model system, as well as with therapeutic siRNA targeting the survivin gene, led to a significant gene silencing effect, and in the latter case a resulting apoptotic effect in cancer cells.
Y Hayashi, T Fujita, T Tokunaga, K Kaneko, M Tanemura, T Butler, N Rupesinghe, JD Carey, SRP Silva, KBK Teo, GAJ Amaratunga (2008)Microstructure and local magnetic induction of segmented and alloyed Pd/Co nanocomposites encapsulated inside vertically aligned multiwalled carbon nanotubes, In: DIAMOND AND RELATED MATERIALS17(7-10)pp. 1525-1528
This paper reports the transparent field-emitters, produced by spin-coating acid-oxidised multiwall carbon nanotubes (o-MWNT) onto indium tin oxide (ITO)-coated glass slides via an o-MWNT ink. We report substantial changes in the morphology of the o-MWNT layer and an improvement in the FE properties after exposing the films to laser pulses of differing intensity. It is envisaged that this technique could be scaled up as an industrial process for producing truly large area, transparent and cheap FE substrates.
For the first time, thin insulating layers are used to modulate a depletion region at the source of a thin-film transistor. Bottom contact, staggered electrode transistors fabricated using RFsputtered IGZO as the channel layer, with a 3 nm ALD Al2O3 layer between the semiconductor and Ni source-drain contacts show behaviours typical of source-gated transistors (SGTs): low saturation voltage (VD_SAT ~ 3V), change in VD_SAT with gate voltage of only 0.12 V/V and flat saturated output characteristics (small dependence of drain current on drain voltage). The transistors show high tolerance to geometry variations: saturated current changes only 0.15x for channel lengths between 2 - 50 μm, and only 2x for sourcegate overlaps between 9 - 45 μm. A higher than expected (5x) increase in drain current for a 30K change in temperature, similar to Schottky-contact SGTs, underlines a more complex device operation than previously theorised. Optimizations for increasing intrinsic gain and reducing temperature effects are discussed. These devices complete the portfolio of contactcontrolled transistors, comprising devices with: Schottky contacts, bulk barrier or heterojunctions, and now, tunnelling insulating layers. The findings should also apply to nanowire transistors, leading to new low-power, robust design approaches as large-scale fabrication techniques with sub-nanometre control mature.
We report on high luminance organic light-emitting diodes using acid functionalized multi-walled carbon nanotube (o-MWCNTs) as efficient hole injector electrodes, using a simple, solution processable device structure. At only 10 V, the luminance approaches 50,000 cd/m with an external quantum efficiency over 2% and a current efficiency greater than 21 cd/A. The investigation of hole-only devices shows that the mechanism for hole injection changes from injection limited to bulk limited because of the higher effective work function of the anode modified by the o-MWCNTs. We expect the enhancement of the local electric field, brought about by both the dielectric inhomogeneities within the o-MWCNT containing anode and the high aspect ratio carbon nanotubes, improves hole injection from the anode to the organic active layer at much lower applied voltage. © 2012 Elsevier Ltd. All rights reserved.
The growth of carbon nanotubes from Ni catalysts is reversed and observed in real time in a transmission electron microscope, at room temperature. The Ni catalyst is found to be Ni3C and remains attached to the nanotube throughout the irradiation sequence, indicating that C most likely diffuses on the surface of the catalyst to form nanotubes. We calculate the energy barrier for saturating the Ni3C (2-13) surface with C to be 0.14 eV, thus providing a low-energy surface for the formation of graphene planes.
GD Dabera, KD Jayawardena, MR Prabhath, I Yahya, YY Tan, NA Nismy, H Shiozawa, M Sauer, G Ruiz-Soria, P Ayala, V Stolojan, AA Adikaari, PD Jarowski, T Pichler, SR Silva (2013)Hybrid carbon nanotube networks as efficient hole extraction layers for organic photovoltaics., In: ACS Nano7(1)pp. 556-565
Transparent, highly percolated networks of regioregular poly(3-hexylthiophene) (rr-P3HT)-wrapped semiconducting single-walled carbon nanotubes (s-SWNTs) are deposited, and the charge transfer processes of these nanohybrids are studied using spectroscopic and electrical measurements. The data disclose hole doping of s-SWNTs by the polymer, challenging the prevalent electron-doping hypothesis. Through controlled fabrication, high- to low-density nanohybrid networks are achieved, with low-density hybrid carbon nanotube networks tested as hole transport layers (HTLs) for bulk heterojunction (BHJ) organic photovoltaics (OPV). OPVs incorporating these rr-P3HT/s-SWNT networks as the HTL demonstrate the best large area (70 mm(2)) carbon nanotube incorporated organic solar cells to date with a power conversion efficiency of 7.6%. This signifies the strong capability of nanohybrids as an efficient hole extraction layer, and we believe that dense nanohybrid networks have the potential to replace expensive and material scarce inorganic transparent electrodes in large area electronics toward the realization of low-cost flexible electronics.
A comparison of the field emission properties of exposed nanotubes lying on a tipped carbon nanorope, with the emission properties from a sharpened iron tip of similar dimensions is performed. By varying the electrode separation it is observed that the threshold field for emission for both structures decreases as the electrode separation initially increases; however, for sufficiently large electrode separations, the threshold field is observed to reach an asymptotic value. Our results show that the field enhancement factor is fundamentally associated with the electrode separation, and depending on the experimental conditions in order to obtain a true value for electric field a set of alternative definitions for enhancement factors is required. We further confirm our experimental synopsis by simulation of the local electrostatic field which gives results similar to those obtained experimentally.
The observation of field induced electron emission from room temperature grown carbon nanofibers at low (5 V/µm) macroscopic electric fields is reported. The nanofibers were deposited using methane as a source gas in a conventional rf plasma enhanced chemical vapor deposition reactor using a Ni metal catalyst previously subjected to an Ar plasma treatment. Analysis of the scanning electron microscopy images of the nanofibers show them to possess an average diameter of 300 nm and that the nanofibers are observed to be radially dispersed over an area of 50 µm in diameter. No evidence of hysteresis in the current-voltage characteristic or conditioning of the emitters is observed. The mechanism for emission at low fields is attributed to field enhancement at the tips rather than from the surrounding amorphous carbon film which is shown to have a higher threshold field (20 V/µm) for emission.
The quest to develop materials that enables the manufacture of dimensionally ultra-stable structures for critical-dimension components in spacecraft, has led to much research and evolution of carbon-fibre reinforced polymer materials (CFRP) over many decades. This has resulted in structural designs that feature a near-zero coefficient of thermal expansion. However, the dimensional instabilities that result from moisture ingression and release remains the fundamental vulnerability of the matrix, which restricts many such applications. Here, we address this challenge by developing a space-qualifiable physical surface barrier that blends within the mechanical properties of the composite, thus becoming part of the composite itself. The resulting enhanced composite features mechanical integrity and strength that is superior to the underlying composite, whilst remaining impervious to moisture and outgassing. We demonstrate production capability on a model-sized component for Sentinel-5 mission and demonstrate such capability for future European Space Agency (ESA) and National Aeronautics and Space Administration (NASA) programs such as Copernicus Extension, Earth Explorer and Science Cosmic Visions.
A new model which comprehensively explains the working principles of contact-mode Triboelectric Nanogenerators (TENGs) based on Maxwell’s equations is presented. Unlike previous models which are restricted to known simple geometries and derived using the parallel plate capacitor model, this model is generic and can be modified to a wide range of geometries and surface topographies. We introduce the concept of a distance-dependent electric field, a factor not taken in to account in previous models, to calculate the current, voltage, charge, and power output under different experimental conditions. The versatality of the model is demonstrated for non-planar geometry consisting of a covex-conave surface. The theoretical results show excellent agreement with experimental TENGs. Our model provides a complete understanding of the working principles of TENGs, and accurately predicts the output trends, which enables the design of more efficient TENG structures.
The demand for high-density memory in tandem with limitations imposed by the minimum feature size of current storage devices has created a need for new materials that can store information in smaller volumes than currently possible. Successfully employed in commercial optical data storage products, phase-change materials, that can reversibly and rapidly change from an amorphous phase to a crystalline phase when subject to heating or cooling have been identified for the development of the next generation electronic memories. There are limitations to the miniaturization of these devices due to current synthesis and theoretical considerations that place a lower limit of 2 nm on the minimum bit size, below which the material does not transform in the structural phase. We show here that by using carbon nanotubes of less than 2 nm diameter as templates phase-change nanowires confined to their smallest conceivable scale are obtained. Contrary to previous experimental evidence and theoretical expectations, the nanowires are found to crystallize at this scale and display amorphous-to-crystalline phase changes, fulfilling an important prerequisite of a memory element. We show evidence for the smallest phase-change material, extending thus the size limit to explore phase-change memory devices at extreme scales.
Owing to superior energy efficiency, Light Emitting Diode (OLED) technology has become considerably commercialised over the last decade. Innovations in this field have been spurred along by the discovery of new molecules with good stability and high emission intensity, followed through by intense engineering efforts. Emissive transition metal complexes are potent molecular emitters as a result of their high quantum efficiencies related to facile intersystem crossing (ISC) between excited-state manifolds (efficient spin orbit coupling (SOC)) and resultant efficient emission from the triplet state (phosphorescence). These also allow rational tuning of the emission wavelengths. Tuning of the ground and excited state energies, and thus emission wavelength of these complexes can be achieved by subtle structural changes in the organic ligands. Pyridyl-triazole ligands have started receiving increasing attention in recent years as strong field ligands that are relatively straightforward to synthesise. In this study we explore the emission tunability of a newly synthesised series of 5-subsituted-Pyridyl-1,2,3-triazole-based ligands and their Pt(II) complexes. Studies have shown, substitution at the triazole moiety is less effective in achieving emission tunability. Alternatively we carried out the substitution at the 5th position of the pyridine ring with a wide range of electronically diverse, donor-acceptor groups (-N(CH3)2, -H, -CHO, -CHC(CN)2). The target ligands were approached through the serial application of the Sonogashira carbon–carbon coupling and the Sharpless copper-catalyzed Huisgen’s 1,3-dipolarcycloaddition procedures. As a result, coarse tunability of excimer emission was observed in thin-films, generating blue-(486 nm), green-(541 nm), orange-(601 nm) and red-(625 nm) luminescence respectively. This “turned-on” substituent effect was accounted for metallophilic Pt—Pt interaction-induced aggregates in the solid state. Excited state calculations reveal that the solid state emission is associated with 1MMLCT transitions. Lifetime measurements revealed the existence of two decay processes: one being fluorescence and the other process, either phosphorescence or delayed fluorescence. Further a linear-relationship between the Hammett parameters of the substituents and emission wavelengths was established. This allows a reliable emission predictability for any given substituent of 5-substituted pyridyl-1,2,3-triazole platinum complexes. In conclusion, we show a new approach in achieving coarse emission tunability in pyridyl-1,2,3-triazole based platinum complexes via subtle changes in the molecular structure and the importance of metallophilic interactions in the process. During the second phase of the study, the scope was broadened to examine the effects of heterocyclic nitrogens in the ligand skeleton. Fifteen different combinations of azole-azine linked ligand systems were synthesized, by systematically increasing the number of nitrogens and changing the ring position of the nitrogens in the skeleton. Later, the homoleptic platinum complexes of the respective ligands were synthesised, and the photo-physical characteristics were studied. The above mentioned changes in the ligand structure resulted in a 264 nm emission tunability, in the thin films of the complexes. Theoretical studies on the complexes revealed that based on the structure of the ligand, different metallophilic stacking behaviours and different origins of emission (fluorescence and phosphorescence) can result, which in turn give rise to tunable emission wavelengths.
The drive for miniaturisation of electronic circuits provides new materials challenges for the electronics industry. Indeed, the continued downscaling of transistor dimensions, described by Moore’s Law, has led to a race to find suitable replacements for current interconnect materials to replace copper. Carbon nanotubes have been studied as a suitable replacement for copper due to its superior electrical, thermal and mechanical properties. One of the advantages of using carbon nanotubes is their high current carrying capacity which has been demonstrated to be three orders of magnitude greater than that of copper. Most approaches in the implementation of carbon nanotubes have so far focused on the growth in vias which limits their application. In this work, a process is described for the transfer of carbon nanotubes to substrates allowing their use for more varied applications. Arrays of vertically aligned multiwalled carbon nanotubes were synthesised by photo-thermal chemical vapour deposition with high growth rates. Raman spectroscopy was used to show that the synthesised carbon nanotubes were of high quality. The carbon nanotubes were exposed to an oxygen plasma and the nature of the functional groups present was determined using X-ray photoelectron spectroscopy. Functional groups, such as carboxyl, carbonyl and hydroxyl groups, were found to be present on the surface of the multiwalled carbon nanotubes after the functionalisation process. The multiwalled carbon nanotubes were metallised after the functionalisation process using magnetron sputtering. Two materials, solder and sintered silver, were chosen to bind carbon nanotubes to substrates so as to enable their transfer and also to make electrical contact. The wettability of solder to carbon nanotubes was investigated and it was demonstrated that both functionalisation and metallisation were required in order for solder to bond with the carbon nanotubes. Similarly, functionalisation followed by metallisation was critical for bonding carbon nanotubes to sintered silver. A step by step process is described that allows the production of solder-carbon nanotubes and silver-carbon nanotubes interconnects. 4-point probe electrical characterisation of the interconnects was performed and the interconnects were shown to have a resistivity of 5.0 × 10-4 Ωcm for solder-carbon nanotubes and 5.2 × 10-4 Ωcm for silver-carbon nanotubes interconnects. Ramp to failure tests carried out on solder-carbon nanotubes interconnects showed current carrying capacity of 0.75 MA/cm2, only one order of magnitude lower than copper.
Manufacturing electronic devices by printing or coating is a key emerging technology, promising low cost and high throughput. Halide perovskites have emerged as high efficiency, solution processable photovoltaic materials, and within this thesis some of the issues relevant to their up-scaling are explored. Additionally, photo-curing – a post-processing technique with a wide range of applications in printed electronics – is investigated. Aqueous silver flake inks are a promising material for printed conductive applications, combining low cost and high conductivity. Within this thesis, photo-curing of these inks to further improve their conductivity is investigated. Photo-cured samples showed an 11x conductivity improvement compared with thermally cured samples. Furthermore, the manufacturing yield was doubled following photo-curing. These novel observations are explained, by recourse to percolation theory, by an increase in mean particle size. These results enable lower cost and increased yield in future manufacturing. Halide perovskite materials show great promise for solution processable photovoltaics. Within this thesis, the effects of ambient conditions during device processing are measured, in order to inform future up-scaled manufacturing. The chemical and morphological effects of ambient humidity in perovskite films are correlated with the annealing time used and final device performance. This work led to new insights into the combined effects of these two parameters, and a suggestion is made for reducing the annealing time. Finally, a barrier to commercialisation of perovskite solar cells is the use of toxic solvents in their fabrication. Within this thesis, a novel deposition technique is proposed, based on the synthesis of perovskite material in particulate form followed by re-dispersal in non-toxic solvents. This mitigates solvent toxicity, reduces sensitivity to ambient conditions, and in some cases enhances stability. Devices are fabricated based on this technique, and though performance remains low, a marked improvement is observed by the addition of conductive graphene flakes to the inks.
Graphene, a single layer of sp2-bonded carbon atoms, has received significant attention due to its exceptional opto-electronic properties and potentially scalable production processes. However, scalable graphene requires an underlying substrate, which is often a source of strain, doping and carrier scattering, limiting the mobility and quality of graphene. It was shown that by intercalating graphene on SiC by hydrogen, the interfacial layer, associated with n-doping and mobility degradation, is de-coupled from the substrate. The transformations of the H2-intercalation were demonstrated using Raman spectroscopy, while the SiC/interface changes were probed using surface enhanced Raman scattering. The H2-intercalation resulted in carrier type inversion, where the decoupled graphene change from n- to p-type, as well as showing mobility enhancement, up to more than four times, compared to as-grown graphene. Using calibrated Kelvin probe force microscopy, local work function maps were generated, demonstrating the changes in local electronic properties with nanoscale resolution. Furthermore, the layer structure, doping and strain induced by the underlying substrate are compared to CVD grown graphene transferred onto Si/SiO2. In addition to the substrate effects, the electronic properties of graphene are also significantly affected due to the direct exposure of π electrons to the environment. For the investigation of the environmental effects on graphene (i.e. H2O and NO2), a custom-built environmental transport properties measurement system was designed and developed, allowing magneto-transport measurements to be conducted in highly controlled environments. Using this system and calibrated local work function mapping, it is demonstrated that water withdraws electrons from graphene on SiC and SiO2 substrates, as well as acting as a source of impurity scattering. However, the sensitivity of graphene to water depends highly on the underlying substrate and substrate-induced doping. Moreover, it is shown that epitaxial graphene can successfully be used as the sensing material with detection down to 10 parts-per-billion molecules. Considering the environmental effects on the electronic properties of graphene, the importance of clearly reporting the measurement environmental conditions is high-lighted, whenever a routine characterisation for carrier concentration and mobility is reported.
Commercially available organic photovoltaics (OPV) are commonly fabricated using printing and coating techniques that allow for low-cost, high throughput processing of large-area OPV devices. However, the power conversion eﬃciency (PCE) of scaled-up OPVs is often lower than that of small-area ones. This is because the deposition techniques typically being used in industry are diﬀerent to those used in research laboratories (printing/coating vs. spin-coating). Thus, detailed studies of functional materials are required to tailor the characteristics of photoactive D/A blends of OPVs in order to preserve high PCE values for scaled-up device sizes. Therefore, the aims of this thesis were to enhance the PCEs of OPV cells made using a well-known donor material (P3HT), and to develop a structured approach to fabricating large-area OPVs thus easing the transfer of fabrication procedures from laboratory to industry. To achieve the ﬁrst goal, indene-C70-bis-adduct (IC70BA) was chosen as an acceptor material for a photoactive blend with P3HT. A review of P3HT:ICBA-based solar cells indicated a signiﬁcant variation of reported device PCE values (average of 4.66±1.45%). The majority of reported device eﬃciencies were measured for OPVs with photoactive areas rarely exceeding 0.1 cm2. Therefore, a detailed study of the intrinsic characteristics of the IC70BA molecule and the morphology of the P3HT:IC70BA blends was carried out in order to design the optimal fabrication conditions for achieving higher PCEs and up-scaled device areas. Record PCEs approaching 7% were accomplished in this thesis for OPVs with photoactive areas of 0.43 cm2. This was achieved by understanding the correlation between the isomeric properties of the IC70BA molecule and the resulting D/A blend morphology depending on the fabrication conditions used. The second goal of this thesis was accomplished by designing a slot-die coating equipment that allows for the deposition of functional materials over large-areas. Diﬀerent solubilised materials were deposited in ambient conditions on glass and plastic substrates in order to fabricate OPV devices. Two diﬀerent photoactive D/A systems were used: P3HT:IC70BA and PCDTBT:PC70BM. OPV cell and module PCEs approaching 4% were achieved for devices with photoactive areas of about 35 cm2. The quality of the slot-die coated layers was investigated using LBIC, PL, and Raman mapping. This will allow for future improvements in the coating process and, therefore, increased device PCEs and operational lifetimes. In conclusion, the results obtained in this thesis show a way of fabricating eﬃcient large-area OPVs without the use of the spin-coating deposition technique. The study of materials and the development of deposition procedures allows for an accessible transfer of research outcomes from the laboratory to industry.
Photovoltaic technologies are likely to become one of the world’s major renewable energy generators in the future provided they are able to meet the increasing world energy demands at a significantly lower generation cost compared to conventional non-renewable energy sources. Photovoltaic systems based on 1st generation mono or poly crystalline silicon wafers have already been commercially successful over the past two decades. As the technology further develops however, it faces fundamental limits to further reduce cost which are primarily due to processing of silicon wafers. Hence, a 2nd generation of “thin film” photovoltaic systems, such as amorphous and poly silicon, CdTe and CIGS, which use cheap materials and inexpensive manufacturing processes with relatively high power conversion efficiency, have been developed. In order to commercialise the 2nd generation technology successfully, the efficiency of the thin film photovoltaic panels needs to increase to compete with the 1st generation silicon photovoltaics. Plasmonic structures provide a route to increase the efficiency of 2nd generation thin film photovoltaic devices. With the unique properties of plasmonic structures, such as ability to guide and trap light at nanometre dimensions, light absorption in the photoactive layer of thin film photovoltaic device can be increased resulting in improved device performance. In this research, plasmonic nanoparticles are utilised as an anti-reflection coating on the front side of the PV, coupling light into the active PV layer, and as scattering centres at the back reflector, increasing the path length of the light through the photoactive layer. The optical and electrical effects of the plasmonic structures are modelled simultaneously using a commercial technology computer aided design (TCAD) simulation package to understand and optimise the plasmonic effects on the performance of the 2nd generation thin film amorphous silicon, and 3rd generation organic, photovoltaic devices. The thesis describes the first ever dedicated optoelectronic model to simultaneously simulate optical and electrical properties of plasmonic thin film photovoltaics devices in collaboration with the TCAD software developer Silvaco Inc. The model demonstrates a maximum 12% relative increase in the power conversion efficiency of plasmon enhanced n-i-p configured amorphous silicon thin film photovoltaic devices. This remarkable increase in the performance is due to the light trapping in the photoactive layer of the thin film amorphous silicon photovoltaic devices, which results in improvements in the both the optical and electrical properties. Experimental work was also carried out to observe the plasmonic effects of the metal nanoparticles on the performance of 3rd generation organic photovoltaic devices which were subsequently modelled using the simulation package. A 4% relative increase in the efficiency was achieved using gold nanoparticles. A plasmonic organic photovoltaic device model and material library for the commercial organic semiconductor P3HT:PCBM, has also been developed and benchmarked experimentally. The model has assisted in the understanding of the effect of the plasmonic gold nanoparticles on the increased performance, as well as degradation effects due to the incorporation of silver nanoparticles.
Carbon fibre reinforced plastics (CFRP) have revolutionised industries that demand high specific strength materials. With current advancements in nanotechnology there exists an opportunity to not only improve the mechanical performance of CFRP, but to also impart other functionalities, such as thermal and electrical conductivity, with the aim of reducing the reliance on metals, making CFRP attractive to many other industries. This thesis provides a comprehensive analysis of the nano-phase modification to CFRP by growing carbon nanotubes (CNTs) on carbon fibre (CF) and performing mechanical, electrical and thermal conductivity tests, with comparisons made against standard CFRP. Typical CFs are coated with a polymer sizing that plays a vital role in the mechanical performance of the composite, but as a consequence of CNT growth, it is removed. Therefore, in addition, an ‘intermediate’ composite was fabricated – based on CFs without a polymer sizing – which enabled a greater understanding of how the mechanical properties and processability of the material responds to the CNT modification. A water-cooled chemical vapour deposition system was employed for CNT growth and infused into a composite structure with an industrially relevant vacuum-assisted resin transfer moulding (VARTM) process. High quality CNTs were grown on the CF, resulting in properties not reported to date, such as strong intra-tow binding, leading to the possibility of a polymer sizing-free CFRP. A diverse set of spectroscopic, microscopic and thermal measurements were carried out to aid understanding for this CNT modification. Subsequent electrical conductivity tests performed in three directions showed 300%, 230% and 450% improvements in the ‘surface’, ‘through-thickness’ and ‘through-volume’ directions, for the CNT modified CFRP, respectively. In addition, thermal conductivity measurements performed in the through-thickness direction also gave improvements in excess of 98%, boding well for multifunctional applications of this hybrid material concept. A range of mechanical tests were performed to monitor the effect of the CNT modification, including: single fibre tensile tests, tow pull-out tests (from the polymer matrix), composite tensile tests, in-plane shear tests and interlaminar toughness tests. Single fibre tensile tests demonstrated a performance reduction of only 9.7% after subjecting the fibre to the low temperature CNT growth process, which is significantly smaller than previous reports. A reduction in tensile performance was observed in the composite tensile test however, with a reduction of 33% reduction in the ultimate tensile strength, but a 146% increase in the Young’s modulus suggests that the CNTs may have improved the interfacial interactions between the fibre and the polymer matrix. To support this, improvements of 20% in the in-plane shear stress and 74% and the shear chord modulus, were recorded. Negligible differences were observed using a pull-out test to directly measure the interfacial strength as a consequence of the inherently difficult mechanical test procedure. The fracture toughness was tested under mode-I loading of a double cantilever beam configuration and improvements of 83% for CNT modified composite alluded to CNT pull-out fracture mechanism and crack propagation amongst the microstructures. The changes in the physical properties are correlated to the microstructure modifications ensured by the low temperature CNT growth on the CF substrates used in the CFRP composites. This allows for a new generation of modified multifunctional CFRPs to be produced.
Organic Solar Cells can be made to be flexible, semi-transparent, and low-cost making them ideal for novel energy harvesting applications such as in greenhouses. However, the main disadvantage of this technology is its low energy conversion efficiency (<15%); mostly due to high recombination rates, compared with other higher performing technologies, such as thinfilm GaAs (>30% Efficiency), and Si-based (>20% Efficiency), solar cells, where recombination within these technologies is much less than Organic Solar Cells. There are still many challenges to overcome to improve the efficiency of Organic Solar Cells. Some of these challenges include: Maximising the absorption of the solar spectrum; improving the charge dynamics; and increasing the lifetime of the devices. One method to address some of these challenges is to include plasmonic nanoparticles into the devices, which has been shown to increase the absorption through scattering, and improve the charge dynamic through localised surface plasmon resonance effects. However, including nanoparticles into Organic Solar Cells has shown to adversely affect the performance of the devices in other ways, such as increasing the recombination of excitons. To address this, an additional (insulating) coating around the nanoparticles supresses this increase, and has shown to be able to increase the performance of the solar cells. In this work, we demonstrate the use of our all-inclusive optical model in the design and optimisation of bespoke colour-specific windows (i.e. Red, Green, and Blue), where the solar cells can be made to have a specific transparency and colour, whilst maximizing their efficiency. For example, we could specify that we wish the colour to be red, with 50% transmissivity; the model will then maximise the Power Conversion Efficiency. We also demonstrate how our extension to Mie theory can simulate nanoparticle systems and can be used to tune the plasmon resonance utilising different coatings, and configurations thereof.
Understanding how the brain works remains one of the key challenges for scientists. To further this understanding a wide variety of technologies and research methods have been developed. One such technology is conductive electrodes, used to measure the electrical signals elicited from neuronal cells and tissues. These electrodes can be fabricated as a singular electrode or as a multi-electrode array (MEA). This permits bio-electrical measurements from one particular area or simultaneous measurements from multiple areas, respectively. Studying electrical and chemical signals of individual cells in situ requires the use of electrodes with ≤20 µm diameter. However, electrodes of this size generally produce high impedance, perturbing recording of the small signals generated from individual cells. Nanomaterials, such as carbon nanotubes (CNTs), can be deposited to increase the real surface area of these electrodes, producing higher sensitivity measurements. This thesis investigates the potential for using photo-thermal chemical vapour deposition grown CNTs as the electrode material for a de novo fabricated MEA. This device aimed to measure electrochemical signals in the form of dopamine, an important mammalian neurotransmitter, as well as conventional bio-electrical signals that the device is designed for. Realising this aim began with improving CNT aqueous wetting behaviour via oxygen plasma functionalisation. This procedure demonstrated grafting of oxygen functional groups to the CNT structure, and dramatic improvements in aqueous wetting behaviour, with CNTs attached to the device. Subsequently, oxygen plasma functionalised CNT-based MEAs were fabricated and tested, allowing comparisons with a non-functionalised CNT MEA and a state-of-the-art commercial MEA. The functionalised CNT MEA demonstrated an order of magnitude improvement compared to commercial MEAs (2.75 kΩ vs. 25.6 kΩ), at the biologically relevant frequency of 1 kHz. This was followed by measurement of one of the best sensitivity density values, compared to the available literature, for the electrochemical detection of dopamine (9.48 µA µM-1 mm-2). The functionalised CNT MEA then illustrated some selectivity compared to common interferents, i.e. ascorbic acid, of a higher concentration. Nonetheless, imaging of the MEA revealed CNTs were being removed from the electrode areas due to extensive use. Therefore, the final results chapter aimed to develop a novel fabrication route for CNT-based MEAs that produced improved CNT retention on the electrodes. This next-generation functionalised CNT-based MEA displayed improved CNT retention, whilst also producing competitive electrochemical impedance values at 1 kHz (17.8 kΩ) and excellent electrochemical selectivity for dopamine vs. ascorbic acid. Overall, this thesis demonstrates the potential for using MEAs as electrochemical detectors of biological molecules, specifically when using functionalised CNTs as the electrode material.