Flexible radiation dosimeters have been produced incorporating thick films (>1 ¼m) of the semiconducting polymer
poly([9,9-dioctylfluorenyl-2,7-diyl]-co-bithiophene). Diode structures produced on aluminium-metallised poly(imide)
substrates, and with gold top contacts, have been examined with respect to their electrical properties. The results suggest
that a Schottky conduction mechanism occurs in the reverse biased diode, with a barrier to charge injection at the
aluminium electrode. Optical absorption/emission spectra reveal a band gap of 2.48 eV for the polymer. The diodes have
been used for direct charge detection of 17 keV X-rays, generated by a molybdenum source. Using operating voltages of
-10 and -50 V respectively, sensitivities of 54 and 158 nC/mGy/cm3 have been achieved. Increasing the operating
voltage shows that the diodes are stable up to approximately -200 V without significant increase in the dark current of
the device (
Mills CA, Martinez E, Errachid A, Engel E, Funes M, Moormann C, Wahlbrink T, Gomila G, Planell J, Samitier J (2007) Nanoembossed polymer substrates for biomedical surface interaction studies., Journal of Nanoscience and Nanotechnology 7 (12) pp. 4588-4594
Biomedical devices are moving towards the incorporation of nanostructures to investigate the interactions of biological species with such topological surfaces found in nature. Good optical transparency and sealing properties, low fabrication cost, fast design realization times, and biocompatibility make polymers excellent candidates for the production of surfaces containing such nanometric structures. In this work, a method for the production of nanostructures in free-standing sheets of different thermoplastic polymers is presented, with a view to using these substrates in biomedical cell-surface applications where optical microscopy techniques are required. The process conditions for the production of these structures in poly(methyl methacrylate), poly(ethylene naphthalate), poly(lactic acid), poly(styrene), and poly(ethyl ether ketone) are given. The fabrication method used is based on a modified nanoimprint lithography (NIL) technique using silicon based moulds, fabricated via reactive ion etching or focused ion beam lithography, to emboss nanostructures into the surface of the biologically compatible thermoplastic polymers. The method presented here is designed to faithfully replicate the nanostructures in the mould while maximising the mould lifetime. Examples of polymer replicas with nanostructures of different topographies are presented in poly(methyl methacrylate), including nanostructures for use in cell-surface interactions and nanostructure-containing microfluidic devices.
An optical gas sensor has been developed based on the fluorescence emission of the solvatochromic dye, Nile Red, immobilised within various polymers with different physical properties. Microsystems, made either in SU-8/glass or microstructure glass (MSG) substrates, were used to host the environmentally sensitive fluorescent dye within the polymer matrices. The MSG devices have been found to have superior sensitivity to analytes (up to seven times greater) and recovery times (up to 50% faster) than analogous structures made in SU-8. Measurement of the fluorescence at two separate wavelengths confirmed the ability of the MSG sensor array to produce a "fingerprint" response for separate analytes with a high degree of repeatability (the standard deviation of the average response to a given analyte was
Errachid A, Mills CA, Pla-Roca M, Lopez MJ, Villanueva G, Bausells J, Crespo E, Teixidor F, Samitier J (2008) Focused ion beam production of nanoelectrode arrays, MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS 28 (5-6) pp. 777-780 ELSEVIER SCIENCE BV
Rodríguez Seguí S, Pla M, Minic J, Pajot-Augy E, Salesse R, Hou Y, Jaffrezic-Renault N, Mills CA, Samitier J, Errachid A (2006) Detection of olfactory receptor I7 self-assembled multilayer formation and immobilization using a quartz crystal microbalance, Analytical Letters 39 (8) pp. 1735-1745
A self-assembled multilayer based on a mixed MHDA-Biotinyl PE self-assembled monolayer followed by the addition of a biotin-avidin system was built up on the gold electrode of a quartz crystal microbalance, which was used to monitor the deposition. With the view to producing an odorant sensing device, an olfactory receptor (OR), I7 OR, was immobilized on the self-assembled multilayer. The I7 OR originates from a large group of proteins belonging to the I subfamily of G protein-coupled receptors that binds odorant ligands. All formation steps were followed with the quartz crystal microbalance with dissipation measurements (QCM-D). Valuable information was obtained regarding the composition of each layer, providing evidence of the high dissipation effect of nanosomes adhesion. Also, based on the results, an explanation for multilayer formation and binding relations between components is proposed. Copyright © Taylor & Francis Group, LLC.
Fernandez JG, Mills CA, Rodriguez R, Gomila G, Samitier J (2006) All-polymer microfluidic particle size sorter for biomedical applications, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE 203 (6) pp. 1476-1480 WILEY-V C H VERLAG GMBH
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.
Martinez E, Rios-Mondragon I, Pla-Roca M, Rodriguez-Segui S, Engel E, Mills CA, Sisquella X, Planell JA, Sainitier J (2007) Cell-surface interactions studies to trigger stem cell differentiation, NANOMEDICINE-NANOTECHNOLOGY BIOLOGY AND MEDICINE 3 (4) pp. 349-349 ELSEVIER SCIENCE BV
Caballero D, Villanueva G, Plaza JA, Mills CA, Samitier J, Errachid A (2010) Sharp high-aspect-ratio AFM tips fabricated by a combination of deep reactive ion etching and focused ion beam techniques., Journal of Nanoscience and Nanotechnology 10 (1) pp. 497-501
The shape and dimensions of an atomic force microscope tip are crucial factors to obtain high resolution images at the nanoscale. When measuring samples with narrow trenches, inclined sidewalls near 90 degrees or nanoscaled structures, standard silicon atomic force microscopy (AFM) tips do not provide satisfactory results. We have combined deep reactive ion etching (DRIE) and focused ion beam (FIB) lithography techniques in order to produce probes with sharp rocket-shaped silicon AFM tips for high resolution imaging. The cantilevers were shaped and the bulk micromachining was performed using the same DRIE equipment. To improve the tip aspect ratio we used FIB nanolithography technique. The tips were tested on narrow silicon trenches and over biological samples showing a better resolution when compared with standard AFM tips, which enables nanocharacterization and nanometrology of high-aspect-ratio structures and nanoscaled biological elements to be completed, and provides an alternative to commercial high aspect ratio AFM tips.
Mills CA, Lacey D, Stevenson G, Taylor DM (2000) The preparation and characterisation of polymeric macrostructures (command surfaces) using electropolymerisation, JOURNAL OF MATERIALS CHEMISTRY 10 (7) pp. 1551-1554 ROYAL SOC CHEMISTRY
Beliatis MJ, Gandhi KK, Rozanski LJ, Rhodes R, McCafferty L, Alenezi MR, Alshammari AS, Mills CA, Jayawardena KD, Henley SJ, Silva SR (2014) Hybrid graphene-metal oxide solution processed electron transport layers for large area high-performance organic photovoltaics., Adv Mater 26 (13) pp. 2078-2083
Solution processed core-shell nano-structures of metal oxide-reduced graphene oxide (RGO) are used as improved electron transport layers (ETL), leading to an enhancement in photocurrent charge transport in PCDTBT:PC70 BM for both single cell and module photovoltaic devices. As a result, the power conversion efficiency for the devices with RGO-metal oxides for ETL increases 8% in single cells and 20% in module devices.
Gandhi KK, Nejim A, Beliatis MJ, Mills CA, Henley S, Silva SRP (2015) Simultaneous Optical and Electrical Modeling of Plasmonic Light Trapping in Thin-Film Amorphous Silicon Photovoltaic Devices, Journal of Photonics for Energy 5 (1) 057007 SPIE
Rapid prototyping of photovoltaic (PV) cells requires a method for the simultaneous simulation of the optical and electrical characteristics of the device. The development of nanomaterial enabled PV cells only increases the complexity of such simulations. Here, we use a commercial technology-computer-aided-design (TCAD) software, Silvaco Atlas, to design and model plasmonic gold nanoparticles integrated in optoelectronic device models of thin film amorphous silicon (a-Si:H) PV cells. Upon illumination with incident light, we simulate the optical and electrical properties of the cell simultaneously, and use the simulation to produce current-voltage (J-V) and external quantum efficiency (EQE) plots. Light trapping due to light scattering and localized surface plasmon resonance interactions by the nanoparticles has resulted in the enhancement of both the optical and electrical properties due to the reduction in the recombination rates in the photoactive layer. We show that the device performance of the modeled plasmonic a-Si:H PV cells depends significantly on the position and size of the gold nanoparticles, which leads to improvements either in optical properties only, or in both optical and electrical properties. The model provides a route to optimize the device architecture, by simultaneously optimizing the optical and electrical characteristics, which leads to a detailed understanding of plasmonic PV cells from a design perspective and offers an advanced tool for rapid device prototyping.
Riul A, Mills CA, Taylor DM (2000) The electrical characteristics of a heterojunction diode formed from an aniline oligomer LB-deposited onto poly(3-methylthiophene), JOURNAL OF MATERIALS CHEMISTRY 10 (1) pp. 91-97 ROYAL SOC CHEMISTRY
Fernandez JG, Mills CA, Samitier J (2009) Complex microstructured 3D surfaces using chitosan biopolymer., Small 5 (5) pp. 614-620
A technique for producing micrometer-scale structures over large, nonplanar chitosan surfaces is described. The technique makes use of the rheological characteristics (deformability) of the chitosan to create freestanding, three-dimensional scaffolds with controlled shapes, incorporating defined microtopography. The results of an investigation into the technical limits of molding different combinations of shapes and microtopographies are presented, highlighting the versatility of the technique when used irrespectively with inorganic or delicate organic moulds. The final, replicated scaffolds presented here are patterned with arrays of one-micrometer-tall microstructures over large areas. Structural integrity is characterized by the measurement of structural degradation. Human umbilical vein endothelial cells cultured on a tubular scaffold show that early cell growth is conditioned by the microtopography and indicate possible uses for the structures in biomedical applications. For those applications requiring improved chemical and mechanical resistance, the structures can be replicated in poly(dimethyl siloxane).
Deep Vein Thrombosis (DVT) and the associated condition of Pulmonary Embolism (PE) are the most common cause of unexpected death in developed nations. DVT is an internal clot formed in one of the body's deep veins, typically in the leg. If a part of the clot breaks free and moves into the lung, it can lead to pulmonary embolism (PE) which is often fatal. D-dimer is a recognised marker for the diagnosis of thrombus and is routinely used by skilled technical staff as part of an ELISA technique in hospital laboratories. Current D-dimer point-of-care tests are not sufficiently quantitative to allow them to be used to exclude DVT/PE. As a consequence, clinicians need to rely on the use of expensive Doppler ultrasound imaging (DUS), creating additional pressure on national health services. The DUS examination can take several days, during which time heparin is required to be administered to the patient. There is increasing in the development of low cost Lab-on-a-chip systems that will allow chemical and biological processing by non-specialist staff. A low cost, easy to use, portable and quantitative device for DVT/PE would be highly desirable since it would provide reliable diagnosis and aid faster treatment and recovery as well as lower healthcare provider costs.
Mills CA, Navarro M, Engel E, Martinez E, Ginebra MP, Planell J, Errachid A, Samitier J (2006) Transparent micro- and nanopatterned poly(lactic acid) for biomedical applications., Journal of Biomedical Materials Research Part A 76 (4) pp. 781-787 Wiley
The formation of structures in poly(lactic acid) has been investigated with respect to producing areas of regular, superficial features with dimensions comparable to those of cells or biological macromolecules. Nanoembossing, a novel method of pattern replication in polymers, has been used for the production of features ranging from tens of micrometers, covering areas up to 1 cm(2), down to hundreds of nanometers. Both micro- and nanostructures are faithfully replicated. Contact-angle measurements suggest that positive microstructuring of the polymer (where features protrude from the polymer surface) produces a more hydrophilic surface than negative microstructuring. The ability to structure the surface of the poly(lactic acid), allied to the polymer's postprocessing transparency and proven biocompatibility, means that thin films produced in this way will be useful for bioengineers studying the interaction of micro- and nanodimensioned features with biological specimen, with regard to tissue engineering, for example.
This paper describes friction experiments and pull-off force measurements using atomic force microscopy (AFM), between a nonfunctionalized silicon probe and a 2.5 ¼m diameter CH and COOH terminated thiol self-assembled monolayer pattern. The pattern is microcontact printed onto a gold-coated silicon wafer, in air, at room temperature, with a relative humidity around 30%, and used to examine probe-monolayer interactions. Atomic force microscopy imaging reveals that the patterns have been successfully reproduced on the substrate surface. We obtained force values of (8.67±2.60)·10 N, (2.68±1.09)·10 N, and (4.60±0.24)·10 N for CH terminated alkyl-thiol, COOH terminated thiol, and gold substrate respectively. Normalizing these values with the tip radius we obtained (0.87±0.27) N/m for CH terminated alkyl-thiol, (2.68±1.10) N/m for COOH terminated thiol, and (4.60±2.50) N/m for bare gold. These interactions are discussed in terms of the chemical affinity between the probe and the substrate. Copyright © Taylor & Francis Group, LLC.
Fernandez JG, Samitier J, Mills CA (2011) Simultaneous biochemical and topographical patterning on curved surfaces using biocompatible sacrificial molds, JOURNAL OF BIOMEDICAL MATERIALS RESEARCH PART A 98A (2) pp. 229-234 WILEY-BLACKWELL
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.
Carreno NLV, Escote MT, Valentini A, McCafferty L, Stolojan V, Beliatis M, Mills Chris, Rhodes R, Smith Christopher, Silva S (2015) Adsorbent 2D and 3D carbon matrices with protected magnetic iron nanoparticles, NANOSCALE 7 (41) pp. 17441-17449
ROYAL SOC CHEMISTRY
Imalka Jayawardena KDG, Li S, Sam LF, Smith Christopher, Beliatis MJ, Gandhi KK, Ranga Prabhath MR, Pozegic TR, Chen S, Xu X, Dabera DMR, Rozanski LJ, Sporea RA, Mills Chris, Guo X, Silva S (2015) High efficiency air stable organic photovoltaics with an aqueous inorganic contact, 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.
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 Ã?Ã* and À?À* transitions, a À band tail due to oxygen localized states, and a À 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 Ã?Ã* and À?À* transitions alone.
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.
Thirimanne Hashini, Jayawardena K, Parnell A, Bandara R, Karalasingam A, Pani Silvia, Huerdler J, Lidzey D, Tedde S, Nisbet Andrew, Mills Chris, Silva Ravi (2018) High sensitivity organic inorganic hybrid X ray detectors with direct transduction and broadband response, Nature Communications 9 2926
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.
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.
Hybrid inorganic-in-organic semiconductors are an attractive class of materials for optoelectronic applications. Traditionally, the thicknesses of organic semiconductors are kept below 1 ¼m 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 ?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.
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 11 pp. 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.
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
Mills Christopher A., Batyrev Erdni, Jansen Maurice J. R., Ahmad Muhammad, Pathan Tanveerkhan S., Legge Elizabeth J., Thakur Digvijay B., Patole Samson N., Brett Dan J. L., Shearing Paul R., van der Weijde Hans, Silva S. Ravi P. (2019) Improvement in the Electrical Properties of Nickel Plated Steel using Graphitic Carbon Coatings, Advanced Engineering Materials 21 (10) 1900408 pp. 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.