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Dr Chris Mills

Research Fellow
+44 (0)1483 682711
18 ATI 01



Research interests

Research collaborations

My publications


Engel E, Martínez E, Mills CA, Funes M, Planell JA, Samitier J (2009) Mesenchymal stem cell differentiation on microstructured poly (methyl methacrylate) substrates,Annals of Anatomy 191 (1) pp. 136-144
Recent studies on 2D substrates have revealed the importance of surface properties in affecting cell behaviour. In particular, surface topography appears to influence and direct cell migration. The development of new technologies of hot embossing and micro-imprinting has made it possible to study cell interactions with controlled micro features and to determine how these features can affect cell behaviour. Several studies have been carried out on the effect of microstructures on cell adhesion, cell guidance and cell proliferation. However, there is still a lack of knowledge on how these features affect mesenchymal stem cell differentiation. This study was designed to evaluate whether highly controlled microstructures on PMMA could induce rMSC differentiation into an osteogenic lineage. Structured PMMA was seeded with rMSC and cell number; cell morphology and cell differentiation were evaluated. Results confirm that microstructures not only affect cell proliferation and alignment but also have a synergistic effect with osteogenic medium on rMSC differentiation into mature osteoblasts. © 2008 Elsevier GmbH. All rights reserved.
Fernandez JG, Mills CA, Pla-Roca M, Samitier J (2007) Forced soft lithography (IFSL): Production of micro and nanostructures in thin freestanding sheets of chitosan biopolymer, ADVANCED MATERIALS 19 (21) pp. 3696-+ WILEY-V C H VERLAG GMBH
Martínez E, Lagunas A, Mills CA, Rodríguez-Seguí S, Estévez M, Oberhansl S, Comelles J, Samitier J (2009) Stem cell differentiation by functionalized micro- and nanostructured surfaces, Nanomedicine 4 (1) pp. 65-82 Future Medicine
Topographic micro and nanostructures can play an interesting role in cell behaviour when cells are cultured on these kinds of patterned substrates. It is especially relevant to investigate the influence of the nanometric dimensions topographic features on cell morphology, proliferation, migration and differentiation. To this end, some of the most recent fabrication technologies, developed for the microelectronics industry, can be used to produce well-defined micro and nanopatterns on biocompatible polymer substrates. In this work, osteoblast-like cells are grown on poly(methyl methacrylate) substrates patterned by nanoimprint lithography techniques. Examination of the cell-substrate interface can reveal important details about the cell morphology and the distribution of the focal contacts on the substrate surface. For this purpose, a combination of focused ion beam milling and scanning electron microscopy techniques has been used to image the cell-substrate interface. This technique, if applied to samples prepared by freeze-drying methods, allows high-resolution imaging of cross-sections through the cell and the substrate, where the interactions between the nanopatterned substrate, the cell and the extracellular matrix, which are normally hidden by the bulk of the cell, can be studied.
Mills CA, Chai KTC, Milgrew MJ, Glidle A, Cooper JM, Cumming DRS (2006) A multiplexed impedance analyzer for characterizing polymer-coated QCM sensor arrays,IEEE SENSORS JOURNAL 6 (4) pp. 996-1002 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
McMurray AA, Renault NJ, Kyselovik J, Mills CA, Santha H, Strohhöfer C, Ali Z (2007) A novel point of care diagnostic device: Impedimetric detection of a biomarker in whole blood, Annual International Conference of the IEEE Engineering in Medicine and Biology - Proceedings pp. 115-118
There is an unmet medical need for a more reliable and earlier assessment of patients suspected of having a deep vein thrombosis. We describe a novel approach which is developing a highly reliable, accurate, portable and handheld prototype medical diagnostic device to improve radically the speed, accuracy and reliability with which DVT and related blood clotting conditions can be assessed. The device will measure whole blood concentration of D-dimer, a recognized biomarker of increased blood clotting activity, and through innovation in the development of a novel detection, measurement and reporting system, will offer the opportunity to use the test in the point of care setting. The device combines innovation in antibody bio-engineering for high specificity immunoassay-based diagnostics and nano/micro engineered impedimetric analysis electrodes incorporating a biocompatible polymer substrate with development of a disposable microfluidic manifold specifically enabling diagnostics at the point-of-first-contact. © 2007 IEEE.
Intaniwet A, Mills CA, Shkunov M, Sellin PJ, Keddie JL (2012) Heavy metallic oxide nanoparticles for enhanced sensitivity in semiconducting polymer x-ray detectors,Nanotechnology 23 (23) 235502 IOP Publishing
Semiconducting polymers have previously been used as the transduction material in x-ray dosimeters, but these devices have a rather low detection sensitivity because of the low x-ray attenuation efficiency of the organic active layer. Here, we demonstrate a way to overcome this limitation through the introduction of high density nanoparticles having a high atomic number (Z) to increase the x-ray attenuation. Specifically, bismuth oxide (Bi
) nanoparticles (Z=83 for Bi) are added to a poly(triarylamine) (PTAA) semiconducting polymer in the active layer of an x-ray detector. Scanning electron microscopy (SEM) reveals that the Bi
nanoparticles are reasonably distributed in the PTAA active layer. The reverse bias dc currentvoltage characteristics for PTAABi
diodes (with indium tin oxide (ITO) and Al contacts) have similar leakage currents to ITO/PTAA/Al diodes. Upon irradiation with 17.5keV x-ray beams, a PTAA device containing 60wt% Bi
nanoparticles demonstrates a sensitivity increase of approximately 2.5 times compared to the plain PTAA sensor. These results indicate that the addition of high-Z nanoparticles improves the performance of the dosimeters by increasing the x-ray stopping power of the active volume of the diode. Because the Bi
has a high density, it can be used very efficiently, achieving a high weight fraction with a low volume fraction of nanoparticles. The mechanical flexibility of the polymer is not sacrificed when the inorganic nanoparticles are incorporated. © 2012 IOP Publishing Ltd.
Xu Y, Sellin PJ, Lohstroh A, Jie W, Wang T, Mills C, Veeramani P, Veale M (2009) Comparison of the x-ray spectroscopy response and charge transport properties of semi-insulating In/Al doped CdZnTe crystals,JOURNAL OF APPLIED PHYSICS 105 (8) ARTN 083101 AMER INST PHYSICS
Sam FLM, Dabera GDMR, Lai KT, Mills CA, Rozanski LJ, Silva SRP (2014) Hybrid metal grid-polymer-carbon nanotubes electrodes for high luminance organic light emitting diodes,Nanotechnology 25 (34) 345202 Institute of Physics
Organic light emitting diodes (OLEDs) incorporating grid transparent conducting electrodes (TCEs) with wide grid line spacing suffer from an inability to transfer charge carriers across the gaps in the grids to promote light emission in these areas. High luminance OLEDs fabricated using a hybrid transparent conducting electrode (TCE) composed of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS PH1000) or regioregular poly(3-hexylthiophene)-wrapped semiconducting single-walled carbon nanotubes (rrP3HT-SWCNT) in combination with a nanometre thin gold grid are reported here. OLEDs fabricated using the hybrid gold grid/PH1000 TCE have a luminance of 18,000 cd/m2 at 9 V; the same as the reference indium tin oxide (ITO) OLED. The gold grid/rrP3HT-SWCNT OLEDs have a lower luminance of 8,260 cd/m2 at 9 V, which is likely due to a rougher rrP3HT-SWCNT surface. These results demonstrate that the hybrid gold grid/PH1000 TCE is a promising replacement for ITO in future plastic electronics applications including OLEDs and organic photovoltaics (OPVs). For applications where surface roughness is not critical, e.g. electrochromic devices or discharge of static electricity, the gold grid/rrP3HT-SWCNT hybrid TCE can be employed.
Pumera M, Aldavert M, Merkoçi A, Alegret S, Mills C (2005) Direct voltammetric determination of gold nanoparticles using graphite-epoxy composite electrode, Electrochimica Acta 50 (18) pp. 3702-3707
A new voltammetric method for a direct determination of gold nanoparticles, based on adsorption and electrochemical detection of colloidal gold, is described. In this protocol, the absorption of gold nanoparticles onto the rough surface of graphite-epoxy composite electrode is followed by their electrochemical oxidation in 0.1 M HCl medium at a potential of +1.25 V. The resulting tetrachloroaurate ions generated near the electrode surface are detected by differential pulse voltammetry (DPV). The DPV response is linear in the range from 4.7 × 10 to 4.7 × 10 nanoparticles cm with a limit of detection of 1.8 × 10 gold nanoparticles cm. The surface characteristics of the composite electrode are investigated and the parameters that affect the complete analytical detection process of gold nanoparticles are optimized. © 2005 Elsevier Ltd. All rights reserved.
Mills CA, Pla-Roca M, Martin C, Lee M, Kuphal M, Sisquella X, Martinez E, Errachid A, Samitier J (2007) Structured thin organic active layers and their use in electrochemical biosensors, MEASUREMENT & CONTROL 40 (3) pp. 88-91 INST MEASUREMENT CONTROL
Mills CA, Sam FLM, Alshammari AS, Rozanski LJ, Emerson NG, Silva SRP (2013) Storage Lifetime of Polymer-Carbon Nanotube Inks for Use as Charge Transport Layers in Organic Light Emitting Diodes,IEEE/OSA Journal of display technology 10 (2) pp. 125-131 IEEE
The long-term stability of multi-wall carbon nanotubes (MWCNT) mixed with the hole-transport polymer Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been examined. These surfactant stabilised solutions, used as transport layers in organic light emitting diodes (OLEDs), are shown to be stable for periods of up to fifteen months, and show no signs of degrading soon after this time. In comparison, non-stabilised aqueous MWCNT solutions have been shown to aggregate within 30 minutes of production, and, although these aggregates can be re-dispersed, the solution displays an increase in smaller aggregates over time which cannot subsequently be re-dispersed by manual agitation. The stable MWCNT/PEDOT:PSS solutions have been used in ink-jet printing and as composite MWCNT/PEDOT:PSS films suitable as charge transport layers in spin coated organic light emitting diodes.
Mills CA, Engel E, Martinez E, Pla-Roca M, Funes M, Bessueille F, Errachid A, Samitier J (2006) Rapid Production of Transparent Micro/Nanostructured Polymer Substrates for Biomedical Surface Interaction Studies,In: Reece JP (eds.), New nanotechnology research 7: Rapid Production of Transparent Micro/Nanostructured Polymer Substrates for Biomedical Surface Interaction Studies pp. 199-234 Nova Science Pub Inc
This book presents the latest research in this frontier field.
Mills CA, Fernandez JG, Martinez E, Funes M, Engel E, Errachid A, Planell J, Samitier J (2007) Directional alignment of MG63 cells on polymer surfaces containing point microstructures., Small 3 (5) pp. 871-879
MG63 cells cultured on regular arrays of point microstructures (posts and holes) are shown to preferentially align at certain angles to the pattern of the structures, at 0 degrees, 30 degrees, and 45 degrees in particular. The effect is found to be more pronounced for post rather than hole structures (although no significant difference is found for the angles the cells make to the holes or posts) and is thought to be due to the fact that the cells use the posts as anchorage points to hold themselves to the surface. It is also shown that cells preferentially align with the structures depending on the dimensions of the structures and the distance between neighboring structures. This is important when designing structured surfaces for cell-surface interaction studies for materials to be used in, for example, drug delivery or tissue engineering.
Xu Y, Jie W, Sellin P, Wang T, Liu W, Zha G, Veeramani P, Mills C (2009) Study on temperature dependent resistivity of indium-doped cadmium zinc telluride, JOURNAL OF PHYSICS D-APPLIED PHYSICS 42 (3) ARTN 035105 IOP PUBLISHING LTD
Mills CA, Martinez E, Bessueille F, Villanueva G, Bausells J, Samitier J, Errachid A (2005) Production of structures for microfluidics using polymer imprint techniques,MICROELECTRONIC ENGINEERING 78-79 pp. 695-700 ELSEVIER SCIENCE BV
Silva SRP, Beliatis MJ, Jayawardena KDGI, Mills CA, Rhodes RW, Rozanski LJ (2014) Hybrid and nano-composite materials for flexible organic electronics applications, In: Logothetidis S (eds.), Handbook of flexible organic electronics: Materials, Manufacturing and Applications 3 Woodhead Publishing
Flexible organic electronics have recently progressed from ?organic-only? semiconductor devices, based on thin films of organic materials (small molecules and polymers), to hybrid and nano-composite materials - a family of truly advanced materials designed at the nanoscale which offer enhancements in device performance and a reduction in production costs over their traditional inorganic predecessors. These hybrid and nano-composite materials are attractive given the potentially wide range of available organic semiconductors (both small molecule and polymeric) and nanoparticle types (carbon allotropes, metal oxides, metal nanostructures etc.). Here, we emphasise the variety and potential of these materials and introduce some of the production methods, properties and limitations for their use in flexible electronics applications.
Marques de Oliveira IA, Vocanson F, Uttaro J-P, Asfari Z, Mills CA, Samitier J, Errachid A (2010) Characterization of a Self-Assembled Monolayer Based on a Calix[4]Crown-5 Derivate: Fabrication of a Chemical Sensor Sensitive to Calcium, JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY 10 (1) pp. 413-420 AMER SCIENTIFIC PUBLISHERS
Wyse CA, Preston T, Yam PS, Sutton DGM, Christley RM, Hotchkiss JW, Mills CA, Glidle A, Cumming DRS, Cooper JM, Love S (2004) Current and future uses of breath analysis as a diagnostic tool, VETERINARY RECORD 154 (12) pp. 353-360 BRITISH VETERINARY ASSOC
Mills CA, Beeley J, Wyse C, Cumming DRS, Glidle A, Cooper JM (2007) Polymer-based micro-sensor paired arrays for the determination of primary alcohol vapors,SENSORS AND ACTUATORS B-CHEMICAL 125 (1) pp. 85-91 ELSEVIER SCIENCE SA
Fernandez JG, Mills CA, Martinez E, Lopez-Bosque MJ, Sisquella X, Errachid A, Samitier J (2008) Micro- and nanostructuring of freestanding, biodegradable, thin sheets of chitosan via soft lithography., Journal of Biomedical Materials Research 85 (1) pp. 242-247
A technique for imparting micro- and nanostructured topography into the surface of freestanding thin sheets of chitosan is described. Both micro- and nanometric surface structures have been produced using soft lithography. The soft lithography method, based on solvent evaporation, has allowed structures approximately 60 nm tall and approximately 500 x 500 nm(2) to be produced on freestanding approximately 0.5 mm thick sheets of the polymer when cured at 293 K, and structures approximately 400 nm tall and 5 x 5 microm(2) to be produced when cured at 283 K. Nonstructured chitosan thin sheets (approximately 200 microm thick) show excellent optical transmission properties in the visible portion of the electromagnetic spectrum. The structured sheets can be used for applications where optical microscopic analysis is required, such as cell interaction experiments and tissue engineering.
Riul A, Taylor DM, Mills CA, Murphy PJ (2000) Langmuir and Langmuir-Blodgett (LB) films of 4-dicyanomethylene,4H-cyclopenta[2,1-b,3,4-b '] dithiophene, THIN SOLID FILMS 366 (1-2) pp. 249-254 ELSEVIER SCIENCE SA
Errachid A, Caballero D, Crespo E, Bessueille F, Pla-Roca M, Mills CA, Teixidor F, Samitier J (2007) Electropolymerization of nano-dimensioned polypyrrole micro-ring arrays on gold substrates prepared using submerged micro-contact printing,Nanotechnology 18 (48)
Cobaltabisdicarbollide-doped polypyrrole (PPy-[Co(C2B 9H11)2]) nanostructures have been produced by directed, potentiostatic electropolymerization using a patterned combination of conducting and insulating thiols (4-aminothiophenol and octadecylmercaptan, respectively). The different conducting characters of both self-assembled monolayers guides the PPy-[Co(C2B9H11) 2] deposition over the pattern. In this way we have produced doped, nanostructured annular polypyrrole rings,
Intaniwet A, Mills CA, Shkunov M, Thiem H, Keddie JL, Sellin PJ (2009) Characterization of thick film poly(triarylamine) semiconductor diodes for direct x-ray detection,JOURNAL OF APPLIED PHYSICS 106 (6) ARTN 064513 AMER INST PHYSICS
Jbari A, Bellarbi L, Samitier J, Errachid A, Zine N, Mills CA (2007) Multiplexed frequency spectrum analyzer instrumentation for the characterization of multiple QCM-based biosensors,2007 International Conference on Sensor Technologies and Applications, SENSORCOMM 2007, Proceedings pp. 436-440 IEEE
In this contribution, we present novel multiplexed frequency spectrum analyzer instrumentation to extract operational parameters and completely characterize the frequency response of an array of quartz_crystal microbalance sensors. The effectiveness of the proposed instrumentation is proven by experimental measurements over a range of frequencies. © 2007 IEEE.
Mills CA, Taylor DM, Riul A, Lee AP (2002) Effects of space charge at the conjugated polymer/electrode interface,JOURNAL OF APPLIED PHYSICS 91 (8) pp. 5182-5189 AMER INST PHYSICS
Mills CA, Intaniwet A, Shkunov M, Keddie JL, Sellin PJ (2009) Flexible radiation dosimeters incorporating semiconducting polymer thick films,Proceedings of SPIE - The International Society for Optical Engineering 7449 SPIE
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 (
Beeley JM, Mills C, Hammond PA, Glidle A, Cooper JM, Wang L, Cumming DRS (2004) All-digital interface ASIC for a QCM-based electronic nose,Sensors and Actuators, B: Chemical 103 (1-2) pp. 31-36 Elsevier
An all-digital interface, application specific integrated circuit (ASIC) has been developed for the control and data sampling of a quartz crystal microbalance (QCM)-based electronic nose. The ASIC is capable of measuring QCM resonant frequency between 0 and 11 MHz with a resolution of 1 Hz and ±1 Hz precision. The ASIC has been used to obtain measurements from polymer coated QCM sensors, in conjunction with polymer/carbon-black coated micro-resistance (¼R) sensors, in the detection of primary alcohols. A full system-on-a-chip (SoC) electronic nose, currently under test, which supports arrays of eight QCM and eight ¼R sensors along with on-chip processing capability, is also described. © 2004 Elsevier B.V. All rights reserved.
Pla-Roca M, Fernandez JG, Mills CA, Martínez E, Samitier J (2007) Micro/nanopatterning of proteins via contact printing using high aspect ratio PMMA stamps and nanoimprint apparatus.,Langmuir 23 (16) pp. 8614-8618 American Chemical Society
Micro- and nanoscale protein patterns have been produced via a new contact printing method using a nanoimprint lithography apparatus. The main novelty of the technique is the use of poly(methyl methacrylate) (PMMA) instead of the commonly used poly(dimethylsiloxane) (PDMS) stamps. This avoids printing problems due to roof collapse, which limits the usable aspect ratio in microcontact printing to 10:1. The rigidity of the PMMA allows protein patterning using stamps with very high aspect ratios, up to 300 in this case. Conformal contact between the stamp and the substrate is achieved because of the homogeneous pressure applied via the nanoimprint lithography instrument, and it has allowed us to print lines of protein approximately 150 nm wide, at a 400 nm period. This technique, therefore, provides an excellent method for the direct printing of high-density sub-micrometer scale patterns, or, alternatively, micro-/nanopatterns spaced at large distances. The controlled production of these protein patterns is a key factor in biomedical applications such as cell-surface interaction experiments and tissue engineering.
Ruiz A, Mills CA, Valsesia A, Martinez E, Ceccone G, Samitier J, Colpo P, Rossi F (2009) Large-Area, Nanoimprint-Assisted Microcontact Stripping for the Fabrication of Microarrays of Fouling/Nonfouling Nanostructures, SMALL 5 (10) pp. 1133-1137 WILEY-V C H VERLAG GMBH
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.
Rozanski LJ, Castaldelli E, Sam FLM, Mills CA, Demets GJ-F, Silva SRP (2013) Solution processed naphthalene diimide derivative as electron transport layers for enhanced brightness and efficient polymer light emitting diodes,Journal of Materials Chemistry C 1 (20) pp. 3347-3352 Royal Society of Chemistry
Increasing the efficiency and lifetime of polymer light emitting diodes (PLEDs) requires a balanced injection
and flow of charges through the device, driving demand for cheap and effective electron transport/hole
blocking layers. Some materials, such as conjugated polyelectrolytes, have been identified as potential
candidates but the production of these materials requires complex, and hence costly, synthesis routes.
We have utilized a soluble small molecule naphthalene diimide derivative (DC18) as a novel electron
transport/hole blocking layer in common PLED architectures, and compared its electronic properties to
those of the electron transport/hole blocking small molecule bathocuproine (BCP). PLEDs incorporating
DC18 as the electron transport layer reduce turn on voltage by 25%; increase brightness over three and
a half times; and provide a full five-fold enhancement in efficiencies compared to reference devices.
While DC18 has similar properties to the effective conjugated polyelectrolytes used as electron transport
layers, it is simpler to synthesise, reducing cost while retaining favourable electron transport properties,
and producing a greater degree of efficiency enhancement. The impact on device lifetime is
hypothesized to be significant as well, due to the air-stability seen in many naphthalene diimide derivatives.
Mills CA, Al-Otaibi H, Intaniwet A, Shkunov M, Pani S, Keddie JL, Sellin PJ (2013) Enhanced x-ray detection sensitivity in semiconducting polymer diodes containing metallic nanoparticles,Journal of Physics D: Applied Physics 46 (27) 275102 Institute of Physics
Semiconducting polymer X-radiation detectors are a completely new family of low-cost radiation detectors with potential application as beam monitors or dosimeters. These detectors are easy to process, mechanically flexible, relatively inexpensive, and able to cover large areas. However, their x-ray photocurrents are typically low as, being composed of elements of low atomic number (Z), they attenuate x-rays weakly. Here, the addition of high-Z nanoparticles is used to increase the x-ray attenuation without sacrificing the attractive properties of the host polymer. Two types of nanoparticles (NPs) are compared: metallic tantalum and electrically insulating bismuth oxide. The detection sensitivity of 5 µm thick semiconducting poly([9,9-dioctylfluorenyl-2,7-diyl]-co-bithiophene) diodes containing tantalum NPs is four times greater than that for the analogous NP-free devices; it is approximately double that of diodes containing an equal volume of bismuth oxide NPs. The x-ray induced photocurrent output of the diodes increases with an increased concentration of NPs. However, contrary to the results of theoretical x-ray attenuation calculations, the experimental current output is higher for the lower-Z tantalum diodes than the bismuth oxide diodes, at the same concentration of NP loading. This result is likely due to the higher tantalum NP electrical conductivity, which increases charge transport through the semiconducting polymer, leading to increased diode conductivity.
Mills CA, Escarre J, Engel E, Martinez E, Errachid A, Bertomeu J, Andreu J, Planell JA, Samitier J (2005) Micro- and nanostructuring of poly(ethylene-2,6-naphthalate) surfaces, for biomedical applications, using polymer replication techniques,NANOTECHNOLOGY 16 (4) pp. 369-375 IOP PUBLISHING LTD
Taylor DM, Mills CA (2001) Memory effect in the current-voltage characteristic of a low-band gap conjugated polymer,JOURNAL OF APPLIED PHYSICS 90 (1) pp. 306-309 AMER INST PHYSICS
Ruiz A, Zychowicz M, Buzanska L, Mehn D, Mills CA, Martinez E, Coecke S, Samitier J, Colpo P, Rossi F (2009) Single Stem Cell Positioning on Polylysine and Fibronectin Microarrays,MICRO AND NANOSYSTEMS 1 (1) pp. 50-56 Bentham Science Press
Arrays of human umbilical cord blood-neural stem cells have been patterned in high density at single cell resolution.
Pre-patterns of adhesive molecules, i.e. fibronectin and poly-L-lysine, have been produced on anti-adhesive poly
(ethylene) oxide films deposited by plasma-enhanced chemical vapour deposition, which prevents cell adsorption. The
structures consisted of adhesive squares and lines with 10¼m lateral dimensions, which correspond approximately to the
size of one cell nucleus, separated by 10¼m anti-adhesive gap. The stem cells cultured on these platforms redistribute their
cytoplasm on the permitted areas. Spherical cells were deposited on the square patterns in a single cell mode, while on the
lines they spread longitudinally; the extent of elongation being dependent on the specific (fibronectin) or non-specific
(poly-L-lysine) attachment biomolecule. The cell patterns were retained up to 12 days, which will be useful for recording
statistical data of individual chronic responses to chemical, physical or physiologically relevant stimuli.
Beliatis MJ, Rozanski LJ, Jayawardena KDGI, Rhodes RW, Anguita JV, Mills CA, Silva SRP (2014) Hybrid and Nano-composite Carbon
Sensing Platforms,
In: Demarchi D, Tagliaferro A (eds.), Carbon for Sensing Devices 5 pp. 105-132 Springer International Publishing Switzerland
Carbon nanomaterials offer a number of possibilities for sensing
platforms. The ability to chemically functionalize the surfaces of the
nano-carbon, using hybrid or nano-composite structures, can further
enhance the material properties. Complementary to the addition of
any requisite chemical or biochemical functionality, such
enhancements can take the form of improved electrical, optical or
morphological properties which improve the transduction capabilities
of the carbon nano-material, or facilitate detection of the transduced
signal, for example by improving charge transfer to detection
electronics. Here we review the methods of producing hybrid and
nano-composite carbon structures for sensing systems, highlighting
the advantages of the functionalization in each case and benchmark
their performance against existing carbon-only devices. Finally, we
detail some of the recent applications of hybrid and nano-composite
carbon technologies in a wide variety of sensor technologies.
Mills CA, Martinez E, Errachid A, Gomila G, Samso A, Samitier J (2005) Small scale structures: the fabrication of polymeric
nanostructures for biomedical applications using pattern
replication techniques
Contributions to Science 3 (1) pp. 47-56
Polymers are excellent candidates for the production of biomedical devices incorporating nanometric structures. Good optical transparency and sealing properties, low fabrication costs, fast design realization times, and, crucially, biocompatibility are all advantages that can be exploited by scientists for the production of such devices. Here, we review some of the methods and techniques used in the fabrication of polymeric nanostructures by pattern replication techniques that may be of relevance in the production of biomedical devices. Emphasis
is placed on imprint production of polymeric replicas, with master fabrication using focussed ion-beam technology, as a relatively simple method for reproducibly obtaining large numbers
of nanostructures. The use of these structures in polymercasting techniques is also described, together with some specific fabrication considerations. The maturity reached by
polymer-based nanotechnologies, together with the first polymer-based applications for single-cell analysis and for counting single DNA molecules, demonstrates that polymers constitute
a viable alternative to silicon-based nanotechnologies for biomedical applications.
Bessueille F, Pla-Roca M, Mills CA, Martinez E, Samitier J, Errachid A (2005) Submerged microcontact printing (S mu CP): An unconventional printing technique of thiols using high aspect ratio, elastomeric stamps,LANGMUIR 21 (26) pp. 12060-12063 AMER CHEMICAL SOC
Mills CA, Taylor DM, Murphy PJ, Dalton C, Jones GW, Hall LM, Hughes AV (1999) Investigations into a low band gap, semiconducting polymer, SYNTHETIC METALS 102 (1-3) pp. 1000-1001 ELSEVIER SCIENCE SA
Intaniwet A, Mills CA, Sellin PJ, Shkunov M, Keddie JL (2010) Achieving a Stable Time Response in Polymeric Radiation Sensors under Charge Injection by X-rays,ACS APPL MATER INTER 2 (6) pp. 1692-1699 AMER CHEMICAL SOC
Existing inorganic materials for radiation sensors suffer from several drawbacks, including their inability to cover large curved areas, lack of tissue equivalence toxicity, and mechanical inflexibility. As an alternative to inorganics, poly(triarylamine) (PTAA) diodes have been evaluated for their suitability for detecting radiation via the direct creation of X-ray induced photocurrents. A single layer of PTAA is deposited on indium tin oxide (ITO) substrates, with top electrodes selected from Al, Au, Ni, and Pd. The choice of metal electrode has a pronounced effect on the performance of the device; there is a direct correlation between the diode rectification factor and the metal-PTAA barrier height. A diode with an Al contact shows the highest quality of rectifying junction, and it produces a high X-ray photocurrent (several nA) that is stable during continuous exposure to 50 kV Mo K alpha X-radiation over long time scales, combined with a high signal-to-noise ratio with fast response times of less than 0.25 s. Diodes with a low band gap, 'Ohmic' contact, such as ITO/PTAA/Au, show a slow transient response. This result can be explained by the build-up of space charge at the metal-PTAA interface, caused by a high level of charge injection due to X-ray-induced carriers. These data provide new insights into the optimum selection of metals for Schottky contacts on organic materials, with wider applications in light sensors and photovoltaic devices.
Jayawardena KDGI, Rozanski LJ, Mills CA, Beliatis MJ, Nismy NA, Silva SRP (2013) ?Inorganics - in - Organics?: Recent Developments and Outlook for 4G Polymer Solar Cells, Nanoscale 5 (18) pp. 8411-8427 Royal Society of Chemistry
Recent developments in solution processable single junction polymer solar cells have led to a significant improvement in power conversion efficiencies from ~5% to beyond 9%. While much of the initial efficiency improvements were driven through judicious design of donor polymers, it is the engineering of device architectures through the incorporation of inorganic nanostructures and better processing that has continued the efficiency gains. Inorganic nano-components such as carbon nanotubes, graphene and its derivatives, metal nanoparticles and metal oxides that have been central role in improving device performance and longevity beyond those achieved by conventional 3G polymer solar cells. The present work aims to summarise the diverse roles played by the nanosystems and features in state of the art next generation (4G) polymer solar cells. The challenges associated with the engineering of such devices for future deployment are also discussed.
Piana C, Güll I, Wirth M, Gabor F, Gerbes S, Gerdes R, Mills C, Samitier J (2007) Influence of surface modification on vitality and differentiation of Caco-2 cells, Differentiation 75 (4) pp. 308-317
It is widely accepted that the functional and morphological differentiation of cells is initiated and determined by the interaction of molecules of the extracellular matrix and adhesion molecules of the cell membrane. To assess the influence of the underlying matrix on the characteristics of cells, enterocyte-like Caco-2 cells were cultivated on substrates commonly used for cell culture as well as on glass coated with hydrophobic layers. Providing the same starting conditions for growth, the parameters investigated on preconfluent Caco-2 cells were the number of adhering cells, the proliferative activity and the degree of differentiation indicated by the expression of three brush border enzymes. Whereas tissue culture treated polystyrene elicited highest rates of adhesion, proliferation, and differentiation, even glass altered the pattern of brush border enzyme expression. The hydrophobic surfaces strongly decreased the adhesion and the proliferation but the surviving cells exhibited a pronounced higher degree of differentiation. Interestingly, each sub-type of hydrophobic matrix triggered a different pattern of brush border enzyme expression. Thus, the development of a certain phenotype of a cell can not only be triggered by certain components of the extracellular matrix but also by artificially prepared surface coatings of the underlying matrix. In the future it seems to be feasible that cells can be programmed by tailoring the surface of the underlying substrate. © 2007, International Society of Differentiation.
Mills CA, Fernandez JG, Errachid A, Samitier J (2008) The use of high glass temperature polymers in the production of transparent, structured surfaces using nanoimprint lithography,MICROELECTRONIC ENGINEERING 85 (9) pp. 1897-1901 ELSEVIER SCIENCE BV
Martinez E, Rios-Mondragon I, Pla-Roca M, Rodriguez-Segui S, Engel E, Mills CA, Sisquella X, Planell JA, Samitier J (2007) Production of functionalised micro and nanostructured polymer surfaces to trigger mesenchymal stem cell differentiation, JOURNAL OF BIOTECHNOLOGY 131 (2) pp. S67-S67 ELSEVIER SCIENCE BV
Sam FLM, Razali MA, Jayawardena KDGI, Mills CA, Rozanski LJ, Beliatis MJ, Silva SRP (2014) Silver Grid Transparent Conducting Electrodes for Organic Light Emitting Diodes,Organic Electronics 15 (12) pp. 3492-3500 Elsevier
Polymer organic light emitting diodes (OLEDs) were fabricated using thin silver hexagonal grids replacing indium tin oxide (ITO) as the transparent conducting electrodes (TCE). Previous literature has assumed that thick metal grids (several hundred nanometres thick) with a lower sheet resistance ( 80 %) compared to thinner grids would lead to OLEDs with better performance than when thinner metal grid lines are used. This assumption is critically examined using OLEDs on various metal grids with different thicknesses and studying their performances. The experimental results show that a 20 nm thick silver grid TCE resulted in more efficient OLEDs with higher luminance (10 cd/A and 1460 cd/m2 at 6.5 V) than a 111 nm thick silver grid TCE (5 cd/A and 159 cd/m2 at 6.5 V). Furthermore, the 20 nm thick silver grid OLED has a higher luminous efficiency than the ITO OLED (6 cd/A and 1540 cd/m2 at 6.5 V) at low voltages. The data shows that thinner metal grid TCEs (about 20 nm) make the most efficient OLEDs, contrary to previous expectations.
Kuphal M, Mills CA, Korri-Youssoufi H, Samitier J (2012) Polymer-based technology platform for robust electrochemical sensing using gold microelectrodes,Sensors and Actuators, B: Chemical 161 (1) pp. 279-284 Elsevier
Li D, Mills CA, Cooper JM (2003) Microsystems for optical gas sensing incorporating the solvatochromic dye Nile Red,Sensors and Actuators, B: Chemical 92 (1-2) pp. 73-80
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
Smith C, Rhodes R, Beliatis M, Jayawardena K, Rozanski L, Mills C, Silva S (2014) Graphene oxide hole transport layers for large area, high efficiency organic solar cells,Applied Physics Letters 105 (7) 073304 American Institute of Physics
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.
Rodriguez-Trujillo R, Mills CA, Samitier J, Gomila G (2007) Low cost micro-Coulter counter with hydrodynamic focusing,MICROFLUIDICS AND NANOFLUIDICS 3 (2) pp. 171-176 SPRINGER HEIDELBERG
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.
Embrechts A, Feng CL, Mills CA, Lee M, Bredebusch I, Schnekenburger J, Domschke W, Vancso GJ, Schonherr H (2008) Inverted microcontact printing on polystyrene-block-poly(tert-butyl acrylate) films: A versatile approach to fabricate structured biointerfaces across the length scales,LANGMUIR 24 (16) pp. 8841-8849 AMER CHEMICAL SOC
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).
Renault NJ, Korri-Youssoufi H, Kyselovik J, Auger V, Errachid A, Mills CA, Strohhöfer C, Ali Z (2008) Impedimetric microanalysis system for deep vein thrombosis point-of-care testing,Proceedings of the 30th Annual International Conference of the IEEE Engineering in Medicine and Biology Society: Personalized Healthcare Through Technology pp. 1856-1856
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.
Caballero D, Pla-Roca M, Bessueille F, Mills CA, Samitier J, Errachid A (2006) Atomic force microscopy characterization of a microcontact printed, self-assembled thiol monolayer for use in biosensors,Analytical Letters 39 (8) pp. 1721-1734 Taylor and Francis
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
Dharmasena Randunu Devage Ishara Gihan, Jayawardena Imalka, Mills Christopher, Deane Jonathan, Anguita Jose, Dorey Robert, Silva S (2017) Triboelectric Nanogenerators: Providing a Fundamental Framework,Energy & Environmental Science 10 (8) pp. 1801-1811 Royal Society of Chemistry
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.
Liang H, Smith C, Mills C, Silva S (2015) The band structure of graphene oxide examined using photoluminescence spectroscopy,Journal of Materials Chemistry C 3 (48) pp. 12484-12491 The Royal Society of Chemistry
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.
Dharmasena R.D.I.G., Jayawardena K.D.G.I., Mills C.A., Dorey R.A., Silva Ravi (2018) A unified theoretical model for Triboelectric Nanogenerators,Nano Energy 48 pp. 391-400 Elsevier
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.
Dharmasena Randunu Devage Ishara Gihan (2019) Triboelectric self-powered energy systems.,
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.
Jayawardena K. D. G. Imalka, Thirimanne Hashini M., Tedde Sandro Francesco, Huerdler Judith E., Parnell Andrew J., Bandara R. M. Indrachapa, Mills Christopher A., Silva S. Ravi P. (2019) Millimeter-Scale Unipolar Transport in High Sensitivity Organic?Inorganic Semiconductor X-ray Detectors,ACS Nano American Chemical Society
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.
Legge Elizabeth J., Ahmad Muhammad, Smith Christopher T. G., Brennan Barry, Mills Christopher A., Stolojan Vlad, Pollard Andrew J., Silva S. Ravi P. (2018) Physicochemical characterisation of reduced graphene oxide for conductive thin films,RSC Advances 8 (65) pp. 37540-37549 Royal Society of Chemistry
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 Wiley
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.