For carbon nanotubes (CNTs) to be exploited in electronic applications, the growth of high quality material on conductive substrates at low temperatures (
Castellino M, Stolojan V, Virga A, Rovere M, Cabiale K, Galloni MR, Tagliaferro A (2013) Chemico-physical characterisation and in vivo biocompatibility assessment of DLC-coated coronary stents, Analytical and Bioanalytical Chemistry 405 (1) pp. 321-329
The vast majority of stent thrombosis occurs in the acute and sub-acute phases and is more common in patients with acute coronary syndromes, due to the thrombotic milieu where stent struts are positioned. Stent thrombosis is likely due to incomplete tissue coverage of metallic stents as the contact between metallic stents and blood elements may lead to platelet adhesion and trigger vessel thrombosis. If a stent is covered after 7 days, the risk that it will be found uncovered at later stages is very low (
In this work, Co ions were implanted into thermally oxidised SiO2 layers on silicon substrates. The implantation energy was 50 keV and the doses were 1, 3, 5 and 7 x 10(16) Co+/cm2. The field emission (FE) properties of these layers were studied and correlated with results from atomic force microscopy and transmission electron microscopy measurements. Other than that for the lowest dose sample, crystallised Co nanoclusters, with sizes ranging from 1.8 to 5.7 nm, are observed in these Co-implanted layers. The higher dose samples exhibit excellent FE properties and give an emission current of 1 nA at electric fields as low as 5 V/microm, for a dose of 5 x 10(16) Co+/cm2, compared with 120 V/microm for the lowest dose samples. We attribute the excellent FE properties of these layers to the formation of Co nanoclusters, with the electrical inhomogeneity giving rise to local field enhancement. Finally, repeatable staircase-like current-field (I-F) characteristics are observed in FE measurements of these higher dose samples as compared to conventional Fowler-Nordheim-type I-F characteristics in the lower dose sample. We believe this data may be a result of Coulomb blockade effects arising from the isolated low-capacitance metal quantum dots formed by controlled ion implantation.
Boyd RD, Young TJ, Stolojan V (2012) Characterisation of gold nanoparticles and rods using high angle annular dark field imaging, Journal of Nanoparticle Research 14 (4)
This study demonstrates use of high angle annular dark field (HAADF) imaging for the characterisation of gold nano-objects. HAADF, which is based on scanning transmission electron microscopy (STEM), has been used to measure gold spherical colloids and 1:6 aspect ratio gold nanorods. It has been demonstrated that this approach offers a realistic way of representing the dimensions (length, width and height) of a statistically relevant number of nanoobjects (at least 200) in a realistic period of time. This new approach does not involve tilting the sample and as such, is more time efficient than traditional electron tomography and does not require sophisticated software analysis. © 2012 Springer Science+Business Media B.V.
Pd/Co-based metal-filled carbon nanotubes (MF-CNTs) were synthesized by a microwave plasma-enhanced chemical vapor deposition method using a bias-enhanced growth technique. Pd/Co-based MF-CNTs were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) electron energy loss spectroscopy (EELS), and Raman spectroscopy. MF-CNTs were well-aligned and uniform in size on a Si substrate. Both multiwall nanotube carbon nanotubes (CNTs) and herringbone (or stacked cups structure) structures were observed. High-resolution TEM revealed that MF-CNTs were composed of highly ordered graphite layers, and the elemental maps of EELS indicate that both Co and Pd metals are present inside the nanotubes. TEM results clearly showed that both Pd and Co metals were successfully encapsulated into the CNTs. We observed a low value for the Raman intensity ratio between D (1355 cm(-1)) and G (1590 cm(-1)) bands with no shift of the G-peak position and no broadening of the G-peak, indicative of high-quality Pd/Co-based MF-CNTs. Based on TEM characterization, we propose a description for the encapsulating mechanisms.
Palumbo M, Henley SJ, Lutz T, Stolojan V, Cox D, Silva SRP (2008) Engineering the shape of zinc oxide crystals via sonochemical or hydrothermal solution-based methods, Materials Research Society Proceedings 1087 pp. 59-65
Recent results in the use of Zinc Oxide (ZnO) nano/submicron crystals in fields as diverse as sensors, UV lasers, solar cells, piezoelectric nanogenerators and light emitting devices have reinvigorated the interest of the scientific community in this material. To fully exploit the wide range of properties offered by ZnO, a good understanding of the crystal growth mechanism and related defects chemistry is necessary. However, a full picture of the interrelation between defects, processing and properties has not yet been completed, especially for the ZnO nanostructures that are now being synthesized. Furthermore, achieving good control in the shape of the crystal is also a very desirable feature based on the strong correlation there is between shape and properties in nanoscale materials. In this paper, the synthesis of ZnO nanostructures via two alternative aqueous solution methods - sonochemical and hydrothermal - will be presented, together with the influence that the addition of citric anions or variations in the concentration of the initial reactants have on the ZnO crystals shape. Foreseen applications might be in the field of sensors, transparent conductors and large area electronics possibly via ink-jet printing techniques or self-assembly methods.
Dabera GDMR, Jayawardena KDGI, Prabhath MRR, Yahya I, Tan YY, Nismy NA, Shiozawa H, Sauer M, Ruiz-Soria G, Ayala P, Stolojan V, Adikaari AADT, Jarowski PD, Pichler T, Silva SRP (2013) Hybrid carbon nanotube networks as efficient hole extraction layers for organic photovoltaics, ACS Nano 7 (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 mm2) 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. © 2012 American Chemical Society.
MiyajiM A Y, Tison Y, Giusca CE, Stolojan V, Watanabe H, Habuchi H, Henley SJ, Shannon JM, Silva SRP (2011) Probing the band structure of hydrogen-free amorphous carbon and the effect of nitrogen incorporation, Carbon 49 (15) pp. 5229-5238
Amorphous carbon and carbon nitride bottom gate thin film transistors have been fabricated, which show bulk carrier field effect mobilities around 10 -3 (cm 2 V -1 s -1) which is orders of magnitude higher than the previously reported values with p-channel devices at high electric fields between source and drain. The incorporation of nitrogen atoms into the amorphous carbon films deposited by pulsed laser deposition was studied using a wide range of techniques in order to understand the role of nitrogen in the conduction mechanism at high fields. The density of the states (DOS) was measured with several techniques such as electron energy loss spectroscopy, scanning tunnelling spectroscopy, and ultraviolet photoelectron spectroscopy, whereas the joint density of states (JDOS), corresponding to the transitions of electrons from the valence to the conduction bands, were obtained by optical transmittance and photothermal deflection spectroscopy. These measurements when combined can provide unparallel data on the shape and magnitude of the energy band states which are crucial to the understanding of the materials properties and thus opto-electronic applications for these thin films. In this report, the conduction mechanism will be discussed with a band diagram drawn based on the experimentally obtained DOS and JDOS measurements. © 2011 Elsevier Ltd. All rights reserved.
Octopus-like carbon nanofibres with leg diameters as small as 9 nm are reported, with a high yield over large areas, using a unique photo-thermal chemical vapour deposition system. The branched nature of these nanostructures leads to geometries ideal for increasing the surface area of contacts for many electronic and electrochemical devices. The manufacture of these structures involves a combination of a polyacrylonitrile/polysiloxane film covering the surface of cupronickel catalysts, supported on silicon. Acetylene is used as the carbon feedstock. High-resolution electron microscopy revealed a relationship between the geometry of the nanoparticles and the catalytic growth process, which can be tuned to maximise geometries (and therefore the surface area) and was obtained with a catalyst size of 125 nm. The technique proposed for growing these carbon octopi nanostructures is ideal to facilitate a new in situ transfer film process to place high-density carbon structures on secondary surfaces to produce high capacitance all-carbon contacts.
Stolojan V (2015) Nanometrology Using the Transmission Electron Microscope, Morgan & Claypool Publishers
The Transmission Electron Microscope (TEM) is the ultimate tool to see and measure structures on the nanoscale and to probe their elemental composition and electronic structure with sub-nanometer spatial resolution. Recent technological breakthroughs have revolutionized our understanding of materials via use of the TEM, and it promises to become a significant tool in understanding biological and bio-molecular systems such as viruses and DNA molecules. This book is a practical guide for scientists who need to use the TEM as a tool to answer questions about physical and chemical phenomena on the nanoscale.
Duncan R, Stolojan V, Lekakou C (2007) Manufacture of carbon multi-walled nanotubes by the arc discharge technique, World Congress on Engineering 2007, Vols 1 and 2 pp. 1391-1395 INT ASSOC ENGINEERS-IAENG
Crystallised Co nanoparticles were synthesized by Co+ implantation onto thermally oxidised SiO2 layers on silicon substrate. The implantation energy was 50 keV and the doses ranged from 1 to 7times1016 Co+/cm2. The possibility of controlling the size and distribution of the nanoclusters by changing implantation conditions (e.g. dose and energy) is the main advantage of this technique. Atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (X-TEM) were used to characterize the nanoclusters. The staircase I-V curve also shows that the metallic quantum dots embedded in a thin SiO2 layer on silicon substrate has effective Coulomb blockade at room temperature
Chen GY, Stolojan V, Silva SRP (2010) Top-Down Heating for Low Substrate Temperature Synthesis of Carbon Nanotubes, JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY 10 (6) pp. 3952-3958 AMER SCIENTIFIC PUBLISHERS
Monclus MA, Baker MA, Rebhoz C, Stolojan V, Gibson PN, Leyland A, Matthews A (2006) Nanostructural studies of PVD TiAlB coatings, SURFACE AND INTERFACE ANALYSIS 38 (4) pp. 731-735 WILEY-BLACKWELL
Miyajima Y, Shannon JM, Henley SJ, Stolejan V, Cox DC, Silva SRP (2007) Electrical conduction mechanism in laser deposited amorphous carbon, THIN SOLID FILMS 516 (2-4) pp. 257-261 ELSEVIER SCIENCE SA
Chemical vapor-synthesized carbon nanotubes are typically grown at temperatures around 600 °C. We report on the deployment of a titanium layer to help elevate the constraints on the substrate temperature during plasma-assisted growth. The growth is possible through the lowering of the hydrocarbon content used in the deposition, with the only source of heat provided by the plasma. The nanotubes synthesized have a small diameter distribution, which deviates from the usual trend that the diameter is determined by the thickness of the catalyst film. Simple thermodynamic simulations also show that the quantity of heat, that can be distributed, is determined by the thickness of the titanium layer. Despite the lower synthesis temperature, it is shown that this technique allows for high growth rates as well as better quality nanotubes. © 2005 American Institute of Physics.
Peng N, Jeynes C, Bailey MJ, Adikaari D, Stolojan V, Webb RP (2009) High concentration Mn ion implantation in Si, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS 267 (8-9) pp. 1623-1625 ELSEVIER SCIENCE BV
Chen GY, Jensen B, Stolojan V, Silva SRP (2010) Growth of carbon nanotubes at CMOS compatible temperatures, Carbon
Ohashi F, Chen GY, Stolojan V, Silva SRP (2008) The role of the gas species on the formation of carbon nanotubes during thermal chemical vapour deposition, NANOTECHNOLOGY 19 (44) ARTN 445605 IOP PUBLISHING LTD
By virtue of their unique electronic properties, nanometer-diameter sized single-walled carbon nanotubes represent ideal candidates to function as active parts of nanoelectronic memory storage devices. We show for the first time that GeTe, a phase change material, currently considered to be one of the most promising materials for data-storage applications, can efficiently be encapsulated within single-walled carbon nanontubes of 1.4 nm diameter. Structural investigations on the encapsulated GeTe nanowires have been carried out by high resolution transmission electron microscopy. The electronic interactions between the filling material and the host nanotube have been examined using ultraviolet photoelectron spectroscopy experiments and show that the electronic structure of the encapsulating nanotube and that of the encased filling are not perturbed by the presence of each of the other component.
The newly formed hybrids offer potential to operate as active elements in non-volatile electronic memory storage devices.
Semiconducting nanowires (NWs) are becoming essential nano-building blocks for advanced devices from sensors to energy harvesters, however their full technology penetration requires large scale materials synthesis together with efficient NW assembly methods. We demonstrate a scalable one-step solution process for the direct selection, collection and ordered assembly of silicon NWs with desired electrical properties from a poly-disperse collection of NWs obtained from a Supercritical Fluid-Liquid-Solid growth process. Dielectrophoresis (DEP) combined with impedance spectroscopy provides a selection mechanism at high signal frequencies (>500 kHz) to isolate NWs with the highest conductivity and lowest defect density. The technique allows simultaneous control of five key parameters in NW assembly: selection of electrical properties, control of NW length, placement in pre-defined electrode areas, highly preferential orientation along the device channel and control of NWs deposition density from few to hundreds per device. Direct correlation between DEP signal frequency and deposited NWs conductivity is directly confirmed by field-effect transistor and conducting-AFM data. Fabricated NW transistor devices demonstrate excellent performance with up to 1.6 mA current, 106-107 on/off ratio and hole-mobility of 50 cm2 V-1 s-1.
Zeze DA, Carey JD, Stolojan V, Weiss BL, Silva SRP (2006) Damage effects in Pyrex by CF4 reactive ion etching in dual RF-microwave plasmas, MICRO & NANO LETTERS 1 (2) pp. 103-107 INST ENGINEERING TECHNOLOGY-IET
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.
Excimer laser-crystallized silicon solar cells fabricated show a steady increment of the current densities with exposure to simulated sunlight, over a 30 min period. The current density of the amorphous silicon cell under identical conditions remains steady, with no significant change. The process was observed to be reversible upon cooling, and the performance increase is attributed to the energy barrier introduced by the enhanced bandgap of a nanocrystalline silicon middle layer, created as a result of the crystallization. It is suggested that the thermal energy due to prolonged illumination allows carriers to cross the barrier increasing output currents. © 2008 Elsevier B.V. All rights reserved.
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.
Carbon nanotubes (CNTs) in the form of interconnects have many potential applications, and their ability to perform at high temperatures gives them a unique capability. We show the development of a novel transfer process using CNTs and sintered silver that offers a unique high-temperature, high-conductivity, and potentially flexible interconnect solution. Arrays of vertically aligned multiwalled carbon nanotubes of approximately 200 ¼m in length were grown on silicon substrates, using low-temperature photothermal chemical vapor deposition. Oxygen plasma treatment was used to introduce defects, in the form of hydroxyl, carbonyl, and carboxyl groups, on the walls of the carbon nanotubes so that they could bond to palladium (Pd). Nanoparticle silver was then used to bind the Pd-coated multiwalled CNTs to a copper substrate. The silver?CNT?silver interconnects were found to be ohmic conductors, with resistivity of 6.2 × 10?4 ©m; the interconnects were heated to temperatures exceeding 300 °C (where common solders fail) and were found to maintain their electrical performance.
Jin Y, Curry RJ, Sloan J, Hatton RA, Chong LC, Blanchard N, Stolojan V, Kroto HW, Silva SRP (2006) Structural and optoelectronic properties of C-60 rods obtained via a rapid synthesis route, JOURNAL OF MATERIALS CHEMISTRY 16 (37) pp. 3715-3720 ROYAL SOC CHEMISTRY
Carreno N, Escote M, Valentini A, McCafferty L, Stolojan V, Beliatis M, Mills C, Rhodes R, Smith C, Silva S (2015) Adsorbent 2D and 3D carbon matrices with protected magnetic iron nanoparticles, NANOSCALE 7 (41) pp. 17441-17449
ROYAL SOC CHEMISTRY
Yahya I, Stolojan V, Clowes S, Mustaza SM, Silva SRP (2010) Carbon nanotube field effect transistor measurements in vacuum, IEEE Proceedings of International Conference on Semiconductor Electronics pp. 224-228 IEEE
Three terminal measurements on a carbon nanotube field effect transistor (CNTFET) were carried out in high vacuum and the ambient, and its performance compared. The on-off current ratio, ION/IOFF, were 102 and 105 for devices operated in high vacuum and in ambient air, respectively. Here, we show that the conversion of p-type to ambipolar behavior may largely be attributed to the O2 in ambient doping the single walled carbon nanotubes (SWCNTs) in the active channel which consists of bundles of SWCNTs. Switching behaviour of these devices, with respect to constituent types of SWCNTs in the bundles will be discussed.
Ohashi F, Chen GY, Stolojan V, Silva SRP (2008) Influences of hydrogen gas on carbon nanotube growth, Materials Research Society Symposium Proceedings 1081 pp. 87-93
For practical deployment of carbon nanotubes, an understanding of their growth mechanism is required in order to obtain better control over their crystallinity, chirality and other structural properties. In this study, we focus on the influences of gas species on carbon nanotube synthesis using thermal chemical vapour deposition. The influence of methane, hydrogen, and helium gases was investigated from the viewpoint of gas chemistry in relation to the nanotube structural change, by varying the growth pressure, the gas-flow ratio and the growth temperature. Simple changes in the hydrogen gas concentration during different growth stages have been found to induce surprising changes to the nanotube formation. The structure of the tubular carbon growth changed from amorphous to graphitic as the growth temperature and the concentration of hydrogen in the initial periods of growth decreases. The excess hydrogen tends to give rise to poor crystalline carbon nanofibres but has the effect of increasing the yields. Hydrogen gas is typically used in reducing metal catalyst particles during the pre-treatment and the carbon nanotube growth periods. We show that while hydrogen species can improve yield, it can also result in the degradation of the nanotube's crystallinity. The use of hydrogen in the growth process is one of the key parameters for enhanced control of carbon nanotube/nanofibre growth and their resulting crystallinity.
Energy loss spectroscopic profiling is a way to acquire, in parallel, spectroscopic information across a linear feature of interest, using a Gatan imaging filter (GIF) fitted to a transmission electron microscope (TEM). This technique is capable of translating the high spatial resolution of a bright field image into a sampling of the spectral information with similar resolution. Here we evaluate the contributions of chromatic aberration and the various acquisition parameters to the spatial sampling resolution of the spectral information, and show that this can reach 0.5 nm, in a system not ordinarily capable of forming electron probes smaller than 2 nm. We use this high spatial sampling resolution to study the plasmon energy variation across amorphous carbon superlattices, in order to extract information about their structure and electronic properties. By modelling the interaction of the relativistic incident electrons with a dielectric layer sandwiched between outer layers, we show that, due to the screening of the interfaces and at increased collection angles, the plasmon energy in the sandwiched layer can still be identified for layer thicknesses down to 5 A. This allows us to measure the change in the well bandgap as a function of well width and to interpret it in terms of the changes in the sp2 -fractions due to the deposition method, as measured from the carbon K-edges, and in terms of quantum confinement of the well wavefunction by the adjacent barriers.
Adikaari AADT, Carey JD, Stolojan V, Keddie JL, Silva SRP (2006) Bandgap enhancement of layered nanocrystalline silicon from excimer laser crystallization, NANOTECHNOLOGY 17 (21) pp. 5412-5416 IOP PUBLISHING LTD
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.
Sharma P, Anguita JV, Stolojan V, Henley SJ, Silva SRP (2010) The growth of silica and silica-clad nanowires using a solid-state reaction mechanism on Ti, Ni and SiO2 layers, NANOTECHNOLOGY 21 (29) ARTN 295603 IOP PUBLISHING LTD
Rajeev KP, Opoku C, Stolojan V, Constantinou M, Shkunov M (2015) Electrical analysis of hysteresis in solution processed silicon nanowire
field effect transistors,
Silicon nanowires (Si NW) are ideal candidates for solution processable field
effect transistors (FETs). The interface between the nanowire channel and the
gate dielectric plays a crucial role in the FET performance, and it can be
responsible for unwanted effects such as hysteresis of the I-V characteristics
due to threshold voltage shift when the gate voltage is applied. Using
gate-voltage bias stress measurements we show that a large hysteresis of up to
40V in Si NW FETs with SiO2 dielectric is mainly due to the holes traps at the
nanowire/SiO2 interface. An approach for reducing this hysteresis to just 2.5V
using solution processable fluoropolymer dielectric Cytop in the top-gate
configuration is demonstrated. Experimental results suggest that the density of
surface traps in Si nanowire transistors is dictated mainly by the nature of
the dielectric layer. The influence of the gate dielectric was studied by
assessing the field effect transport behaviour of a representative double gate
FETs based on SiO2 bottom dielectric and top Cytop dielectric layer. Such
devices were characterised, revealing an order of magnitude higher hole traps
density at the nanowire/SiO2 interface (1x10^13cm-2) compared to that of
nanowire/fluoropolymer interface (7.5x10^11cm-2).
Smith AJ, Gwilliam RM, Stolojan V, Knights AP, Coleman PG, Kallis A, Yeong SH (2009) Enhancement of phosphorus activation in vacancy engineered thin silicon-on-insulator substrates, Journal of Applied Physics 106 (10)
The concentration of vacancy-type defects in a silicon-on-insulator substrate consisting of a 110 nm silicon overlayer and a 200 nm buried oxide has been quantified using variable energy positron annihilation spectroscopy following 300 keV Si+ ion implantation to a dose of 1.5× 1015 cm-2 and subsequent annealing at temperatures ranging from 300 to 700 °C. The preferential creation of vacancies (relative to interstitials) in the silicon overlayer leads to a net vacancy-type defect concentration after annealing. Assuming that the defects have a structure close to that of the divacancy we determine the concentration to range from 1.7× 1019 to 5× 1018 cm-3 for annealing temperatures ranging from 300 to 700 °C. The measured defect concentration is in excellent agreement with that predicted via Monte Carlo simulation. The impact of this net vacancy population on the diffusion and activation of phosphorus introduced by a 2 keV implantation to a dose of 1× 10 15 cm-2 has been observed. For samples that combine both Si+ and P+ implantations, postimplantation phosphorus diffusion is markedly decreased relative to that for P+ implantation only. Further, a fourfold increase in the electrical activation of phosphorus after postimplantation annealing at 750 °C is observed when both implantations of Si+ and P+ are performed. We ascribe this affect to the reduction in phosphorus-interstitial clusters by the excess vacancy concentration beyond the amorphous/crystalline interface created by the P+ implantation. © 2009 American Institute of Physics.
Howard KT, Bailey MJ, Berhanu D, Bland PA, Cressey G, Howard LE, Jeynes C, Matthewman R, Martins Z, Sephton MA, Stolojan V, Verchovsky S (2013) Biomass preservation in impact melt ejecta, Nature Geoscience 6 (12) pp. 1018-1022
Meteorites can have played a role in the delivery of the building blocks of life to Earth only if organic compounds are able to survive the high pressures and temperatures of an impact event. Although experimental impact studies have reported the survival of organic compounds, there are uncertainties in scaling experimental conditions to those of a meteorite impact on Earth and organic matter has not been found in highly shocked impact materials in a natural setting. Impact glass linked to the 1.2-km-diameter Darwin crater in western Tasmania is strewn over an area exceeding 400 km 2 and is thought to have been ejected by a meteorite impact about 800 kyr ago into terrain consisting of rainforest and swamp. Here we use pyrolysis-gas chromatography-mass spectrometry to show that biomarkers representative of plant species in the local ecosystem - including cellulose, lignin, aliphatic biopolymer and protein remnants - survived the Darwin impact. We find that inside the impact glass the organic components are trapped in porous carbon spheres. We propose that the organic material was captured within impact melt and preserved when the melt quenched to glass, preventing organic decomposition since the impact. We suggest that organic material can survive capture and transport in products of extreme impact processing, at least for a Darwin-sized impact event. © 2013 Macmillan Publishers Limited.
Shang N, Chen GY, Tan YY, Stolojan V, Papakonstantinou P, Silva SRP (2010) Direct catalytic growth of high-density carbon nanotubes on nanoclusters at low temperatures, Proceedings of 8th International Vacuum Electron Sources Conference and Nanocarbon pp. 389-389
Carbon nanotubes (CNTs) have received extensive attention due to their one-dimensional structure and ability to demonstrate many novel physical and chemical phenomena in the quantum scale. However, the application of CNTs in electronics is hindered due to their higher growth temperatures which are usually in excess of 500 °C, which is not compatible with current semiconductor technology in industry. Low temperature growth is necessary for integrating CNTs into standard semiconductor devices such as CMOS and large-scale integrated circuits. To date, various techniques have been utilised to lower the CNT growth temperature by: 1. using various carbon sources with lower dissociation temperature; 2. exploring metal catalyst films of the low melting point or metal nanoparticles as catalysts; and, 3. introducing a plasma during deposition to increase the dissociation and ionization of feed gases. In this study, we report the low temperature growth of vertically aligned high-density CNTs by a DC plasma chemical vapour deposition method, using Ni nanoclusters as catalysts. The Ni nanoclusters are free from a high-temperature formation process compared to the film based catalysts and directly demonstrate catalytic growth of CNTs at substrate temperatures as low as 390 °C. The density of as-grown CNTs is up to 10 /cm , as shown in Figure 1. Transmission electron microscopy studies show the CNTs are made of crystalline graphene shells and have a uniform diameter distribution. The field electron emission properties of the samples are investigated.
Sharma P, Stolojan V, Silva SRP (2011) Raman analysis of oxide cladded silicon core nanowires grown with solid silicon feed stock, Journal of Nanoparticle Research pp. 1-7
Munz M, Langridge MT, Devarepally KK, Cox DC, Patel P, Martin NA, Vargha G, Stolojan V, White S, Curry RJ (2013) Facile synthesis of titania nanowires via a hot filament method and conductometric measurement of their response to hydrogen sulfide gas, ACS Applied Materials and Interfaces 5 (4) pp. 1197-1205
Titania nanostructures are of increasing interest for a variety of applications, including photovoltaics, water splitting, and chemical sensing. Because of the photocatalytical properties of TiO2, chemical processes that occur at its surface can be exploited for highly efficient nanodevices. A facile and fast synthesis route has been explored that is free of catalysts or templates. An environmental scanning electron microscopy (ESEM) system was employed to grow titania nanowires (NWs) in a water vapor atmosphere (
The growth of graphene on Ni using a photo-thermal chemical vapor deposition (PT-CVD) technique is reported. The non-thermal equilibrium nature of PT-CVD process resulted in a much shorter duration in both heating up and cooling down stages, thus allowing for a reduction in the overall growth time. Despite the reduced time for synthesis compared to standard thermal chemical vapor deposition (T-CVD), there was no decrease in the quality of the graphene film produced. Furthermore, the graphene formation under PT-CVD is much less sensitive to cooling rate than that observed for T-CVD process. Growth on Ni also allows for the alleviation of hydrogen blister damage that is commonly encountered during growth on Cu substrates and a lower processing temperature. To characterize the film?s electrical and optical properties, we further report the use of pristine PT-CVD grown graphene as the transparent electrode material in an organic photovoltaic device (OPV) with poly(3-hexyl)thiophene (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) as the active layer where the power conversion efficiency of the OPV cell is found to be comparable to that reported using pristine graphene prepared by conventional CVD.
Tison V, Stolojan V, Watts PCP, Cox DC, Chen GY, Silva SRP (2008) Gas Sensing Properties of Vapour-Deposited Tungsten Oxide Nanostructures, MICROSCOPY OF SEMICONDUCTING MATERIALS 2007 120 pp. 281-284 SPRINGER-VERLAG BERLIN
Zhang W, Stolojan V, Silva SR, Wu CW (2013) Raman, EELS and XPS studies of maghemite decorated multi-walled carbon nanotubes., Spectrochim Acta A Mol Biomol Spectrosc 121 pp. 715-718
Iron oxide particles with the diameter being 5-10 nm were attached onto the sidewalls of multi-walled carbon nanotubes (MWCNTs) by the thermal decomposition of cyclopentadieny iron (II) dicarbonyl dimmer. The red shift of G-mode from 1579 cm(-1) to 1571 cm(-1) in the Raman profile of the decorated MWCNTs is indicative of the attachment of nanoparticles. Electron energy loss spectroscopy and X-ray photoelectron spectroscopy analyses reveals that the attached nanoparticles are composed of a maghemite phase. Transmission electron microscopy suggests the maghemite particles are covered with amorphous carbon materials and form a core-shell structure.
Chen JS, Stolojan V, Silva SRP (2015) Towards type-selective carbon nanotube growth at low substrate temperature via photo-thermal chemical vapour deposition, Carbon 84 (1) pp. 409-418
© 2014 Elsevier Ltd. All rights reserved.Carbon nanotubes have been intensively researched for electronic applications, driven by their excellent electronic properties, where the goals are control and reproducibility of growth, semiconducting/metallic type selectivity and maintaining high quality of carbon nanotubes, in a process that is temperature-compatible with the electronics. Photo-thermal chemical vapour deposition can achieve these goals and, through a thorough investigation of the parameter space, we achieve very high nanotube-quality and growth rates, and produce a phase-diagram that reveals distinct regions for growing semiconducting and metallic single-walled nanotubes, as well as multi-walled. Correlation with the carbon-catalyst phase diagram allows for the development of a novel growth model. We propose that the temperature-gradient induces carbon diffusivity-gradient across the catalyst to yield the high growth rate. This is attributed to the increase of a-iron of catalyst. The growth control demonstrated here allows for integration of the nanotube growth process by photo-thermal deposition into mainstream electronics manufacture.
Stolojan V (2006) Nanomaterials: Nanotube growth surface-driven, Advanced Composites Bulletin (NOV.)
Using electron beam irradiation in an electron microscope, researchers at the Advanced Technology Institute, University of Surrey, UK, obtained evidence for the relationship between catalyst and carbon in the growth of carbon nanotubes. By considering the effects of heating and irradiation, the group observed that the carbon atoms at the catalyst surface are easily removed followed by a rapid rearrangement of the nanotube's atoms around the catalyst. Furthermore, they discovered that changes in the nanotube's growth direction are linked to a sudden rotation of the catalyst.
Stolojan V (2008) Scanning Transmission Electron Microscopy: A Tool for Biology and Materials Science, Microscopy and Analysis 22 (4) pp. 15-18 John Wiley & sons
A dedicated scanning transmission electron microscope is ideally coupled with energy dispersive x-ray and electron energy loss spectroscopies to obtain information about the chemical composition, morphology and electronic structure on the nanoscale. With several signals being available simultaneously with the pass of a sub-nanometresized beam, this instrument can answer questions from a broad range of research areas, in a timely fashion. The user-friendliness of the instrument comes at almost no cost in performance, making it an ideal multi-tool in a teaching environment.
Giusca CE, Tison Y, Stolojan V, Borowiak-Palen E, Silva SRP (2007) Inner-tube chirality determination for double-walled carbon nanotubes by scanning tunneling microscopy, NANO LETTERS 7 (5) pp. 1232-1239 AMER CHEMICAL SOC
Nicolosi V, Nellist PD, Sanvito S, Cosgriff EC, Krishnamurthy S, Blau WJ, Green MLH, Vengust D, Dvorsek D, Mihailovic D, Compagnini G, Sloan J, Stolojan V, Carey JD, Pennycook SJ, Coleman JN (2007) Observation of van der Waals driven self-assembly of MoSI nanowires into a low-symmetry structure using aberration-corrected electron microscopy, ADVANCED MATERIALS 19 (4) pp. 543-+ WILEY-V C H VERLAG GMBH
Tison Y, Giusca CE, Stolojan V, Hayashi Y, Silva SRP (2008) The inner shell influence on the electronic structure of double-walled carbon nanotubes, ADVANCED MATERIALS 20 (1) pp. 189-+ WILEY-V C H VERLAG GMBH
Henley SJ, Beliatis MJ, Stolojan V, Silva SR (2012) Laser implantation of plasmonic nanostructures into glass., Nanoscale
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.
Tan YY, Jayawardena KDGI, Adikaari AADT, Tan LW, Anguita JV, Henley SJ, Stolojan V, Carey JD, Silva SRP (2012) Photo-thermal chemical vapor deposition growth of graphene, Carbon 50 (2) pp. 668-673
The growth of graphene on Ni using a photo-thermal chemical vapor deposition (PT-CVD) technique is reported. The non-thermal equilibrium nature of PT-CVD process resulted in a much shorter duration in both heating up and cooling down stages, thus allowing for a reduction in the overall growth time. Despite the reduced time for synthesis compared to standard thermal chemical vapor deposition (T-CVD), there was no decrease in the quality of the graphene film produced. Furthermore, the graphene formation under PT-CVD is much less sensitive to cooling rate than that observed for T-CVD process. Growth on Ni also allows for the alleviation of hydrogen blister damage that is commonly encountered during growth on Cu substrates and a lower processing temperature. To characterize the film's electrical and optical properties, we further report the use of pristine PT-CVD grown graphene as the transparent electrode material in an organic photovoltaic device (OPV) with poly(3-hexyl)thiophene (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) as the active layer where the power conversion efficiency of the OPV cell is found to be comparable to that reported using pristine graphene prepared by conventional CVD. © 2011 Elsevier Ltd. All rights reserved.
The synthesis of high-quality nanomaterials depends on the efficiency of the catalyst and the growth temperature. To produce high-quality material, high-growth temperatures (often up to 1000 C) are regularly required and this can limit possible applications, especially where temperature sensitive substrates or tight thermal budgets are present. In this study, we show that high-quality catalyzed nanomaterial growth at low substrate temperatures is possible by efficient coupling of energy directly into the catalyst particles by an optical method. We demonstrate that using this photothermal-based chemical vapor deposition method that rapid growth (under 4 min, which includes catalyst pretreatment time) of high-density carbon nanotubes can be grown at substrate temperatures as low as 415 C with proper catalyst heat treatment. The growth process results in nanotubes that are high quality, as judged by a range of structural, Raman, and electrical characterization techniques, and are compatible with the requirements for interconnect technology. © 2013 American Chemical Society.
In this work, Ag-Si O2 nanocomposite layers were synthesized by introducing Ag nanoclusters into thermally oxidized Si O2 layers, using ion implantation. The field-emission (FE) properties of these layers were studied and correlated with the results from atomic force microscopy and transmission electron microscopy measurements. These nanocomposites exhibit good FE properties and give an emission current of 1 nA at electric fields as low as 13 V¼m, for a dose of 5× 1016 Ag+ cm2, compared with 204 V¼m for "bare" Si O2 layers. It is clearly demonstrated that the good FE properties of these nanocomposites are attributed to two types of local-field enhancement: one due to the surface morphology and the other due to electrical inhomogeneity. The isolated conductive Ag nanoclusters embedded in the electrically insulating Si O2 matrix provide a field enhancement due to the electrical inhomogeneity effect. Moreover, the implanted Ag ions diffuse to the surface, during the implantation process, and create dense surface-protrusion structure which provides a geometric local-field enhancement. The local-field-enhancement mechanisms in these samples are critically dependent on the implantation dose of Ag. © 2006 American Vacuum Society.
The present work focuses on nanowire (NW) applications as semiconducting elements in solution processable field-effect transistors (FETs) targeting large-area low-cost electronics. We address one of the main challenges related to NW deposition and alignment by using dielectrophoresis (DEP) to select multiple ZnO nanowires with the correct length, and to attract, orientate and position them in predefined substrate locations. High-performance top-gate ZnO NW FETs are demonstrated on glass substrates with organic gate dielectric layers and surround source-drain contacts. Such devices are hybrids, in which inorganic multiple single-crystal ZnO NWs and organic gate dielectric are synergic in a single system. Current-voltage (I-V) measurements of a representative hybrid device demonstrate excellent device performance with high on/off ratio of 10^7, steep subthreshold swing (s-s) of 400 mV/dec and high electron mobility of 35 cm2 V-1 s-1 in N2 ambient. Stable device operation is demonstrated after 3 months of air exposure, where similar device parameters are extracted including on/off ratio of 4x10^6, s-s 500 mV/dec and field-effect mobility of 28 cm2 V-1 s-1. These results demonstrate that DEP can be used to assemble multiples of NWs from solvent formulations to enable low-temperature hybrid transistor fabrication for large-area inexpensive electronics.
Superlattices are periodic structures where the constituents alternate between low- and high-bandgap materials; the resulting quantum confinement tailors the resulting device properties and increases their operating speed. Amorphous carbon is an excellent candidate for both the well and barrier layers of the superlattices, leading to a fast and reliable device manufacturing process. We show theoretically and experimentally that, using low energy-loss spatially resolved spectroscopy, we can characterize the component layers of a superlattice. We measure quantum confinement of the electron wave function in the superlattice's wells and calculate the effective tunneling mass for amorphous carbon superlattices as m* =0.067 me. This effective mass makes diamondlike carbon films as feasible candidate for electronic devices. © 2006 American Institute of Physics.
Shiozawa H, Skeldon AC, Lloyd DJ, Stolojan V, Cox DC, Silva SR (2011) Spontaneous emergence of long-range shape symmetry, Nano Letters 11 (1) pp. 160-163
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.
Tsang WM, Henley SJ, Stolojan V, Silva SRP (2006) Negative differential conductance observed in electron field emission from band gap modulated A-C nanolayers, IVNC and IFES 2006 - Technical Digest - l9th International Vacuum Nanoelectronics Conference and 50th International Field Emission Symposium pp. 199-200
Aspheric lenses are the most common method for correcting for spherical aberrations but, in microlens production, highly-controlled lens profiles are hard to achieve. We demonstrate a technique for creating bespoke, highly-accurate aspheric or spherical profile silicon microlens moulds, of almost any footprint, using focused ion-beam milling. Along with this, we present a method of removing induced ion-beam damage in silicon, via a hydrofluoric acid etch, helping to recover the surface's optical and chemical properties. In this paper, we demonstrate that our milled and etched moulds have a roughness of 4.0-4.1 nm, meaning they scatter less than 1% of light, down to wavelengths of 51 nm, showing that the moulds are suitable to make lenses that are able to handle light from UV up to infra-red. Using empirical experiments and computer simulations, we show that increasing the ion-dose when milling increases the amount of gallium a hydrofluoric acid etch can remove, by increasing the degree of amorphisation within the surface. For doses above 3000 ¼C/cm this restores previous surface properties, reducing adhesion to the mould, allowing for a cleaner release and enabling higher quality lenses to be made. Our technique is used to make aspheric microlenses of down to 3 ¼m in size, but with a potential to make lenses smaller than 1 ¼m. © 2013 Elsevier Ltd. All rights reserved.
One dimensional single-crystal nanorods of C60 possess unique optoelectronic properties including high electron mobility, high photosensitivity and an excellent electron accepting nature. In addition, their rapid large scale synthesis at room temperature makes these organic semiconducting nanorods highly attractive for advanced optoelectronic device applications. Here, we report low-cost large-area flexible photoconductor devices fabricated using C60 nanorods. We demonstrate that the photosensitivity of the C60 nanorods can be enhanced ~400-fold via an ultralow photodoping mechanism. The photodoped devices offer broadband UV-vis-NIR spectral tuneability, exhibit a detectivitiy>10(9) Jones, an external quantum efficiency of ~100%, a linear dynamic range of 80?dB, a rise time 60?µs and the ability to measure ac signals up to ~250?kHz. These figures of merit combined are among the highest reported for one dimensional organic and inorganic large-area planar photoconductors and are competitive with commercially available inorganic photoconductors and photoconductive cells. With the additional processing benefits providing compatibility with large-area flexible platforms, these devices represent significant advances and make C60 nanorods a promising candidate for advanced photodetector technologies.
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.
Saavedra MS, Sims GD, McCartney LN, Stolojan V, Anguita JV, Tan YY, Ogin SL, Smith PA, Silva SRP (2012) Catalysing the production of multiple arm carbon octopi nanostructures, Carbon
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 use of high quality semiconducting nanomaterials for advanced device applications has been hampered by the unavoidable growth variability of electrical properties of one-dimensional nanomaterials, such as nanowires and nanotubes, thus highlighting the need for the characterization of efficient semiconducting nanomaterials. In this study, we demonstrate a low-cost, industrially scalable dielectrophoretic (DEP) nanowire assembly method for the rapid analysis of the electrical properties of inorganic single crystalline nanowires, by identifying key features in the DEP frequency response spectrum from 1 kHz to 20 MHz in just 60 s. Nanowires dispersed in anisole were characterized using a three-dimensional DEP chip (3DEP), and the resultant spectrum demonstrated a sharp change in nanowire response to DEP signal in 1?20 MHz frequency range. The 3DEP analysis, directly confirmed by field-effect transistor data, indicates that nanowires of higher quality are collected at high DEP signal frequency range above 10 MHz, whereas lower quality nanowires, with two orders of magnitude lower current per nanowire, are collected at lower DEP signal frequencies. These results show that the 3DEP platform can be used as a very efficient characterization tool of the electrical properties of rod-shaped nanoparticles to enable dielectrophoretic selective deposition of nanomaterials with superior conductivity properties.
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.
In this thesis we discuss the manufacture and characterisation of micro-optical elements, for guiding light into sub-wavelength beams & spots, and for use in super-resolution imaging.
A physical limit exists in microscopy where it is impossible to view object smaller than half the illuminating wavelength, via conventional means. In white light microscopy this creates an resolution limit of 321nm (at a wavelength of 500nm, in air). This places a limit on the smallest objects a researcher can study using optical microscopy.
We present a method for fabricating plano-convex lenses which, when placed in near proximity to the samples, boost magnification of conventional microscopes by up-to 2.5x and resolve features below 200nm, with white light illumination.
We also demonstrate a curved axicon Bessel-beam former, that produces long (17 micrometer) non-diffracting beams of light, that can be sub-wavelength in width, down to 2/3rds the wavelength.
In this thesis we contribute the following to current knowledge:
We describe a focused ion-beam milling technique to form bespoke geometry of parabolic & spherical curvature, including reflective dishes, of diameter 1-10 microns, with a surface roughness of 4.0-4.1nm.
As part of this work, we calculate the efficiency of a new technique for removing ion-beam induced damage, using wet-chemical etching. Here we show that increasing the ion-dose above 3000 µC/cm^2 allows a higher percentage of the implantation and amorphisation damage to be removed, and leaves less than 0.5% of the gallium remaining in the surface.
We use the ion-milled dishes to form lens moulds; we double-replicate the brittle silicon mould, to create a hard wearing rubber mould. As multiple rubber moulds can be created per silicon mould the process becomes industrially scalable. A thin-film of polymer lenses is then formed from the mould.
We characterise these lenses, demonstrating 1.2-2.5x magnification and resolution of 200nm. We demonstrate their use by imaging two biological samples, one fixed & stained, and one unlabelled in water.
Additionally, using computer simulations alongside the focused ion-beam manufacturing technique, we demonstrate a curved axicon lens structure, that forms long, non-diffracting beams of intense light. We model and experimentally analyse how the lens profile and high-to-low refractive index change forms the beam, and show that increasing the refractive index change decreases the beam width but at a loss of light transmission.
The semiconductor zinc oxide (ZnO) is a promising material for applications in optoelectronics, photochemistry and chemical sensing. Furthermore, ZnO structures can be grown with a large variety of sizes and shapes. Devices with ZnO rods or wires as their core elements can be used in solar cells, gas sensors or biosensors. In this article, an easy approach for the non-aqueous wet chemical synthesis of ZnO structures is presented that employs the solvent trioctylamine (TOA) and the surfactant hexamethylenetetramine (HMTA). Using the thermal decomposition method, rod-shaped structures were grown that are suitable for the fabrication of electrical devices. A detailed study was carried out to investigate the effects of various reaction parameters on the growth process. Both the concentration of the surfactant HMTA and the zinc precursor zincacetylacetonate (Zn(acac)2) were found to show strong effects on the resulting morphology. In addition to structural characterisation using XRD, SEM and TEM, also optical properties of rod-shaped ZnO structures were measured. Rod-shaped structures were obtained for the following conditions: reaction time 4 h, reaction temperature 70 °C, 1 mmol of Zn(acac)2, 4 mmol of HMTA and 25 mL of the solvent TOA. Photoluminescence and photoluminescence excitation spectroscopy of samples grown under these conditions provided information on levels of defect states that could be critical for chemical sensing applications. Two narrow peaks around 254 and 264 nm were found that are well above the band gap of ZnO.
This paper proposes and demonstrates a new multiquantum well (MQW) laser structure with a temperature-insensitive threshold current and output power. Normally, the mechanisms that cause the threshold current (Ith) of semiconductor lasers to increase with increasing temperature T (thermal broadening of the gain spectrum, thermally activated carrier escape, Auger recombination, and intervalence band absorption) act together to cause Ith to increase as T increases. However, in the design presented here, carriers thermally released from some of the QWs are fed to the other QWs so that these mechanisms compensate rather than augment one another. The idea is in principle applicable to a range of materials systems, structures, and operating wavelengths. We have demonstrated the effect for the first time in 1.5 ¼m GaInAsP/InP Fabry-Perot cavity edge-emitting lasers. The results showed that it is possible to keep the threshold current constant over a temperature range of about 100 K and that the absolute temperature over which the plateau occurred could be adjusted easily by redesigning the quantum wells and the barriers between them. TEM studies of the structures combined with measurements of the electroluminescent intensities from the wells are presented and explain well the observed effects.
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 (
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.
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 use of 2,3,4,5,6-pentafluorobenzyl methacrylate (PFBMA) as a core-forming monomer in ethanolic RAFT dispersion polymerization formulations is presented. Poly[poly(ethylene glycol) methyl ether methacrylate] (pPEGMA) macromolecular chain transfer agents were chain extended with PFBMA leading to nanoparticle formation via polymerization-induced self-assembly (PISA). pPEGMA-pPFBMA particles exhibited the full range of morphologies (spheres, worms, and vesicles) including pure and mixed phases. Worm phases formed gels that underwent a thermo-reversible degelation and morphological transition to spheres (or spheres and vesicles) upon heating. Post-synthesis, the pPFBMA cores were modified through thiol?para-fluoro substitution reactions in ethanol using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the base. For monothiols, conversions were 64% (1-octanethiol) and 94% (benzyl mercaptan). Spherical and worm-shaped nano-objects were core cross-linked using 1,8-octanedithiol, which prevented their dissociation in non-selective solvents. For a temperature-responsive worm sample, cross-linking additionally resulted in the loss of the temperature-triggered morphological transition. The use of the reactive monomer PFBMA in PISA formulations presents a simple method to prepare well-defined nano-objects similar to those produced with non-reactive monomers (e.g. benzyl methacrylate) and to retain morphologies independent of solvent and temperature.
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
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
Silicon nanowires (Si NW) are ideal candidates for low-cost solution processed field effect transistors (FETs) due to the ability of nanowires to be dispersed in solvents, and demonstrated high charge carrier mobility. The interface between the nanowire and the dielectric plays a crucial role in the FET characteristics, and can be responsible for unwanted effects such as current hysteresis during device operation. Thus, optimal nanowire- dielectric interface is required for low-hysteresis FET performance. Here we show that NW FET hysteresis mostly depends on the nature of the dielectric material by directly comparing device characteristics of dual gate Si NW FETs with bottom SiO2 gate dielectric and top hydrophobic fluoropolymer gate dielectric. As the transistor semiconducting nanowire channel is identical in both tops and bottom operational regimes, the performance differences originate from the nature of the nanowire-dielectric interface. Thus, very high 30 volt hysteresis is observed for forward and reverse gate bias scans with SiO2 interface; however, hysteresis is significantly reduced to 6 volt for the fluoropolymer dielectric interface. The differences in hysteresis are ascribed to the polar OH- groups present at SiO2/Si nanowire interface, and mostly absent at fluoropolymer/Si nanowire interface. We further demonstrate that high density of charge traps for bottom gate SiO2 interface (1× 1013cm-2) is reduced by over an order of magnitude for top-fluoropolymer gate interface (7.5 × 1011 cm-2), therefore highlighting the advantage of hydrophobic polymer gate dielectrics for nanowire field-effect transistor applications.
A method of collecting composition data and
examining structural features of pearlite lamellae and the parent
austenite at the growth interface in a 13wt. % manganese steel has
been demonstrated with the use of Scanning Transmission Electron
Microscopy (STEM). The combination of composition data and the
structural features observed at the growth interface show that
available theories of pearlite growth cannot explain all the
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.
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 (
Carbon nanotubes have shown their abilities in a wide range of electronic applications due to their unique electronic properties. In order to match the different needs of applications, the issue of selectively growing specific types of single-walled carbon nanotubes has received considerable attention. In this study, a parametric study is implemented to solve this issue.
Firstly, the growth windows for selectively synthesising high quality single-walled carbon nanotubes via photo-thermal chemical vapour deposition (PTCVD) are determined. The growth process of the PTCVD is free of oxygen-containing precursors and corrosive catalysts, and is fully compatible with the integrated circuit process. Only acetylene and hydrogen are used and the catalyst is a layer of sputtered iron. The multi-variables, which include the process temperature, reactant gas ratio and total flow rate, are studied in terms of their influence on the growth rate, the quality and the preferential growth of carbon nanotubes. The highest growth rate obtained in this study is 442 nm/s, which is the highest growth rate reported so far, without using water and/or a corrosive catalyst to assist the growth. By studying the growth rate, we find that it can be correlated to the bulk iron and carbon phase diagram. Generally, above the eutectoid temperature of a and g iron, the growth rate decreases with increasing temperature and inversely, the growth rate is enhanced with increasing temperature below the eutectoid temperature of the a iron and carbide. Moreover, a novel growth model is also proposed to interpret the high growth rate. Owing to the topdown heating of the PTCVD, three factors are concluded to enhance the growth rate that are the gradients of the temperature and the carbon concentration and the chemical potential along the axis of the catalyst.
The selective growth of high-quality single-walled carbon nanotubes is achieved by optimising the reactant gas ratio and the process temperature, and is confirmed by the radial breathing modes in Raman spectroscopy. The growth window for semiconducting single walled carbon nanotubes is relatively larger than that for growing metallic single-walled carbon nanotubes. The semiconducting single-walled-carbon nanotubes prefer to grow above 800°C with the acetylene ratio being below 10%. The metallic single-walled carbon nanotubes tend to grow between 750 and 800°C,which correspond 420 and 450°C at the substrate temperature and the acetylene ratio of 16-18% are suggested.
The preferential growth of the semiconducting and metallic single-walled carbon nanotubes are confirmed again by analysising the Breit-Wigner-Fano lineshape of the G^(-)-band and the 2D-band. The results are highly consistent with those deduced from analysing the radial breathing modes
Finally, the field emission properties of different types of carbon nanotubes are investigated. We find that multi-walled carbon nanotubes have the better performance compared to semiconducting and metallic single-walled carbon nanotubes. Moreover, different morphologies of the carbon nanotubes and different substrates are also studied with respect to the field emission properties. Consequently, honeycomb-patterned multi-walled carbon nanotubes are grown on the a layer of indium tin oxide on a glass slide that can be used for the flat panel display and lightings.
Modern society is ever in demand for higher performing materials, with increased efficiency. Recognising this need, the work discussed here details the steps taken to develop and engineer a cost-effective manufacturing process, which could be easily commercially scalable for the production of large-areas of aligned carbon nanotubes. These aligned carbon nanotubes can then be directly applied in areas such as advanced ?multi-functional? composites. Of the available routes, the electrospinning technique demonstrated to be one of extreme promise towards achieving this goal. This thesis guides and justifies the investigative steps taken in scientifically engineering a suitable electrospinning method to achieve high-aligned arrays of carbon nanotubes. This includes the design and development of a novel, large-area high-throughput needleless electrospinning system, which is capable of not only producing nano-fibres in excess of 160 g per hour (700 times faster than conventional single needle electrospinning), but also in an aligned orientation, using purely aqueous based polymeric solutions. This success has led to the successful production of the World?s first large area sheets of highly aligned arrays of single walled carbon nanotubes by electrospinning. The analysis of these sheets found substantial increases in both mechanical and electrical performance. For the aligned nanotube-loaded nano-fibres, the tensile strength increased up to 320%, ductility increased up to 315% and Young?s modulus increased up to 430% (compared to the original polymer performances). The realisation of the significant enhancements CNTs pose on a composite material, led to an investigation into the chemical interactions that lead to these results. This resulted in the discovery of a new small angle X-ray scattering peak, which we attributed to a crystalline interface between the polymer and carbon nanotubes, giving rise to the enhancements seen during mechanical testing. In addition to mechanical performance, there was also a significant increase in electrical conductivity of 108 S/m, an improvement of 8 orders of magnitude compared to the original polymer. These results, combined with the realisation of industrially viable throughput, provide promise for impressive application into advanced multi-functional composites.
While the primary objectives of this research focused on large area electrospinning, the work outlined in this thesis also discusses investigations into other important aspects, and significant scientific discoveries. These scientific achievements include the introduction of a novel, micro-centrifugal dispersion assessment method, for the efficient surfactant functionalisation of nano-materials. This method allows for a fast and effective assessment of a material suspension, without the need for any equipment other than a simple centrifuge and a balance. This process leads to fast and efficient use of surfactants, producing greater loadings of nano-materials which can be suspended within a solvent for further processing.
As a method to recover the nanotubes once they have been processed and aligned, this thesis also explores post processing of the aligned nanotube-loaded sheets using steam purification. This led to the complete recovery, and purification, of the high quality aligned CNTs, which were found to significantly increase the resulting nanotubes resistance to oxidation, increasing their oxidation temperature in excess of over 900° C, a previously unreported achievement. The mechanisms behind the underlying chemistry were further probed using Raman spectroscopic analysis, this revealed how selective oxidation of CNTs was limited to that of metallic CNTs, leaving the remaining material as only semi-conducting species. This selective oxidation process could lead to selective manufacture of specific CNT species, allowing for better suited application in electrical devices.
Mirkhaydarov Bobur, Votsi Haris, Sahu Abhishek, Caroff Philippe, Young Paul R., Stolojan Vlad, King Simon, Ng Calvin C H, Devabhaktuni Vijaya, Tan Hoe H, Jagadish Chennupati, Aaen Peter, Shkunov Maxim (2019) Solution?Processed InAs Nanowire Transistors as Microwave Switches, Advanced Electronic Materials 5 (1) 1800323
The feasibility of using self?assembled InAs nanowire bottom?gated field?effect transistors as radio?frequency and microwave switches by direct integration into a transmission line is demonstrated. This proof of concept is demonstrated as a coplanar waveguide (CPW) microwave transmission line, where the nanowires function as a tunable impedance in the CPW through gate biasing. The key to this switching capability is the high?performance, low impedance InAs nanowire transistor behavior with field?effect mobility of H300 cm2 V?1 s?1, on/off ratio of 103, and resistance modulation from only 50 © in the full accumulation mode, to H50 k© when the nanowires are depleted of charge carriers. The gate biasing of the nanowires within the CPW results in a switching behavior, exhibited by a H10 dB change in the transmission coefficient, S21, between the on/off switching states, over 5?33 GHz. This frequency range covers both the microwave and millimeter?wave bands dedicated to Internet of things and 5G applications. Demonstration of these switches creates opportunities for a new class of devices for microwave applications based on solution?processed semiconducting nanowires.
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 ÚC 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 pC temperature difference. This means that many alternating pairs of p-n devices are required to achieve a high output power.
CNTs can have the ability to act as compliant small-scale springs or as shock resistance micro-contactors. This work investigates the performance of vertically-aligned CNTs (VA-CNTs) as micro-contactors in electromechanical testing applications for testing at wafer-level chip-scale-packaging (WLCSP) and wafer-level-packaging (WLP). Fabricated on ohmic substrates, 500-¼m-tall CNT-metal composite contact structures are electromechanically characterized. The probe design and architecture are scalable, allowing for the assembly of thousands of probes in short manufacturing times, with easy pitch control. We discuss the effects of the metallization morphology and thickness on the compliance and electromechanical response of the metal-CNT composite contacts. Pd-metallized CNT contactors show up to 25/¼m of compliance, with contact resistance as low as 460/m© (3.6/k©/¼m) and network resistivity of 1.8/×/10?5/©/cm, after 2500 touchdowns, with 50/¼m of over-travel; they form reproducible and repeatable contacts, with less than 5% contact resistance degradation. Failure mechanisms are studied in-situ and after cyclic testing and show that, for top-cap-and-side metallized contacts, the CNT-metal shell provides stiffness to the probe structure in the elastic region, whilst reducing the contact resistance. The stable low resistance achieved, the high repeatability and endurance of the manufactured probes make CNT micro-contacts a viable candidate for WLP and WLCSP testing.
Smith Christopher, Mills Christopher A., Pani Silvia, Rhodes Rhys, Bailey Josh J., Cooper Samuel J., Pathan Tanveerkhan S., Stolojan Vlad, Brett Daniel J. L., Shearing Paul R., Silva S. Ravi P. (2019) X-ray micro-computed tomography as a non-destructive tool for imaging the uptake of metal nanoparticles by graphene-based 3D carbon structures, Nanoscale
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.
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