Dr Chris Mills

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



Research interests

Research collaborations


FLM Sam, MA Razali, KDGI Jayawardena, CA Mills, LJ Rozanski, MJ Beliatis, SRP Silva (2014)Silver Grid Transparent Conducting Electrodes for Organic Light Emitting Diodes, In: Organic Electronics15(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 (< 10 Ω/□) and a similar light transmission (> 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.

Christopher Smith, R Rhodes, M Beliatis, K Jayawardena, L Rozanski, Chris Mills, S Silva (2014)Graphene oxide hole transport layers for large area, high efficiency organic solar cells, In: Applied Physics Letters105(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.

Elizabeth J. Legge, Muhammad Ahmad, Christopher T. G. Smith, Barry Brennan, Christopher A. Mills, Vlad Stolojan, Andrew J. Pollard, S. Ravi P. Silva (2018)Physicochemical characterisation of reduced graphene oxide for conductive thin films, In: RSC Advances8(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

MJ Beliatis, LJ Rozanski, KDGI Jayawardena, RW Rhodes, JV Anguita, CA Mills, SRP Silva (2014)Hybrid and Nano-composite Carbon Sensing Platforms, In: D Demarchi, A Tagliaferro (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.

HM Thirimanne, KDGI Jayawardena, JV Anguita, V Stolojan, CA Mills, SRP Silva (2016)Charge Funneling through Metal Electrode Structuring for High-Efficiency Gains in Polymer Solar Cells, In: ADVANCED ELECTRONIC MATERIALS2(9)ARTN 16000 WILEY-BLACKWELL
A Errachid, D Caballero, E Crespo, F Bessueille, M Pla-Roca, CA Mills, F Teixidor, J Samitier (2007)Electropolymerization of nano-dimensioned polypyrrole micro-ring arrays on gold substrates prepared using submerged micro-contact printing, In: NANOTECHNOLOGY18(48)ARTN 4pp. ?-? IOP PUBLISHING LTD
CA Mills, M Navarro, E Engel, E Martinez, MP Ginebra, J Planell, A Errachid, J Samitier (2006)Transparent micro- and nanopatterned poly(lactic acid) for biomedical applications., In: Journal of Biomedical Materials Research Part A76(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.

Y Xu, PJ Sellin, A Lohstroh, W Jie, T Wang, C Mills, P Veeramani, M Veale (2009)Comparison of the x-ray spectroscopy response and charge transport properties of semi-insulating In/Al doped CdZnTe crystals, In: JOURNAL OF APPLIED PHYSICS105(8)ARTN 0pp. ?-? AMER INST PHYSICS
M Pumera, M Aldavert, A Merkoçi, S Alegret, C Mills (2005)Direct voltammetric determination of gold nanoparticles using graphite-epoxy composite electrode, In: Electrochimica Acta50(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.

CA Mills, J Beeley, C Wyse, DRS Cumming, A Glidle, JM Cooper (2007)Polymer-based micro-sensor paired arrays for the determination of primary alcohol vapors, In: SENSORS AND ACTUATORS B-CHEMICAL125(1)pp. 85-91 ELSEVIER SCIENCE SA
H Liang, Christopher Smith, Chris Mills, S Silva (2015)The band structure of graphene oxide examined using photoluminescence spectroscopy, In: Journal of Materials Chemistry C3(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 σ–σ* and π–π* transitions, a π band tail due to oxygen localized states, and a π band tail due to trapped water, respectively. The labeling of the band structure has been used to challenge the prevailing assignation of the low energy transitions, reported in the literature, to molecular σ–σ* and π–π* transitions alone.

JG Fernandez, CA Mills, M Pla-Roca, J Samitier (2007)Forced soft lithography (IFSL): Production of micro and nanostructures in thin freestanding sheets of chitosan biopolymer, In: ADVANCED MATERIALS19(21)pp. 3696-+ WILEY-V C H VERLAG GMBH
E Martinez, E Engel, C Lopez-Iglesias, CA Mills, JA Planell, J Samitier (2008)Focused ion beam/scanning electron microscopy characterization of cell behavior on polymer micro-/nanopatterned substrates: A study of cell-substrate interactions, In: MICRON39(2)pp. 111-116 PERGAMON-ELSEVIER SCIENCE LTD
A Jbari, L Bellarbi, J Samitier, A Errachid, N Zine, CA Mills (2007)Multiplexed frequency spectrum analyzer instrumentation for the characterization of multiple QCM-based biosensors, In: 2007 International Conference on Sensor Technologies and Applications, SENSORCOMM 2007, Proceedingspp. 436-440

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.

NLV Carreno, MT Escote, A Valentini, L McCafferty, V Stolojan, M Beliatis, Chris Mills, R Rhodes, Christopher Smith, S Silva (2015)Adsorbent 2D and 3D carbon matrices with protected magnetic iron nanoparticles, In: NANOSCALE7(41)pp. 17441-17449 ROYAL SOC CHEMISTRY
MJ Beliatis, KK Gandhi, LJ Rozanski, R Rhodes, L McCafferty, MR Alenezi, AS Alshammari, CA Mills, KDGI Jayawardena, SJ Henley, SRP Silva (2014)Hybrid Graphene-Metal Oxide Solution Processed Electron Transport Layers for Large Area High-Performance Organic Photovoltaics, In: ADVANCED MATERIALS26(13)pp. 2078-2083 WILEY-V C H VERLAG GMBH
Randunu Devage Ishara Gihan Dharmasena, Imalka Jayawardena, Christopher Mills, Jonathan Deane, Jose Anguita, Robert Dorey, S Silva (2017)Triboelectric Nanogenerators: Providing a Fundamental Framework, In: Energy & Environmental Science10(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.

CA Mills, J Escarre, E Engel, E Martinez, A Errachid, J Bertomeu, J Andreu, JA Planell, J Samitier (2005)Micro- and nanostructuring of poly(ethylene-2,6-naphthalate) surfaces, for biomedical applications, using polymer replication techniques, In: NANOTECHNOLOGY16(4)pp. 369-375 IOP PUBLISHING LTD
E Engel, E Martinez, CA Mills, M Funes, JA Planell, J Samitier (2009)Mesenchymal stem cell differentiation on microstructured poly (methyl methacrylate) substrates, In: ANNALS OF ANATOMY-ANATOMISCHER ANZEIGER191(1)pp. 136-144 ELSEVIER GMBH, URBAN & FISCHER VERLAG
M Kuphal, CA Mills, H Korri-Youssoufi, J Samitier (2012)Polymer-based technology platform for robust electrochemical sensing using gold microelectrodes, In: Sensors and Actuators, B: Chemical161(1)pp. 279-284 Elsevier
M Kurzyp, CA Mills, R Rhodes, TR Pozegic, Christopher Smith, MJ Beliatis, LJ Rozanski, A Werbowy, SRP Silva (2015)Filtration properties of hierarchical carbon nanostructures deposited on carbon fibre fabrics, In: JOURNAL OF PHYSICS D-APPLIED PHYSICS48(11)ARTN 11530 IOP PUBLISHING LTD
A Embrechts, CL Feng, CA Mills, M Lee, I Bredebusch, J Schnekenburger, W Domschke, GJ Vancso, H Schonherr (2008)Inverted microcontact printing on polystyrene-block-poly(tert-butyl acrylate) films: A versatile approach to fabricate structured biointerfaces across the length scales, In: LANGMUIR24(16)pp. 8841-8849 AMER CHEMICAL SOC
SRP Silva, MJ Beliatis, KDGI Jayawardena, CA Mills, RW Rhodes, LJ Rozanski (2014)Hybrid and nano-composite materials for flexible organic electronics applications, In: S Logothetidis (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.

A Intaniwet, CA Mills, PJ Sellin, M Shkunov, JL Keddie (2010)Achieving a Stable Time Response in Polymeric Radiation Sensors under Charge Injection by X-rays, In: ACS APPL MATER INTER2(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.

CA Mills, DM Taylor, A Riul, AP Lee (2002)Effects of space charge at the conjugated polymer/electrode interface, In: JOURNAL OF APPLIED PHYSICS91(8)pp. 5182-5189 AMER INST PHYSICS
Christopher Smith, Christopher A. Mills, Silvia Pani, Rhys Rhodes, Josh J. Bailey, Samuel J. Cooper, Tanveerkhan S. Pathan, Vlad Stolojan, Daniel J. L. Brett, Paul R. Shearing, S. Ravi P. Silva (2019)X-ray micro-computed tomography as a non-destructive tool for imaging the uptake of metal nanoparticles by graphene-based 3D carbon structures, In: Nanoscale11pp. 14734-14741 Royal Society of Chemistry

Graphene-based carbon sponges can be used in different applications in a large number of fields including microelectronics, energy harvesting and storage, antimicrobial activity and environmental remediation. The functionality and scope of their applications can be broadened considerably by the introduction of metallic nanoparticles into the carbon matrix during preparation or post-synthesis. Here, we report on the use of X-ray micro-computed tomography (CT) as a method of imaging graphene sponges after the uptake of metal (silver and iron) nanoparticles. The technique can be used to visualize the inner structure of the graphene sponge in 3D in a non-destructive fashion by providing information on the nanoparticles deposited on the sponge surfaces, both internal and external. Other deposited materials can be imaged in a similar manner providing they return a high enough contrast to the carbon microstructure, which is facilitated by the low atomic mass of carbon.

KK Gandhi, A Nejim, MJ Beliatis, CA Mills, S Henley, SRP Silva (2015)Simultaneous Optical and Electrical Modeling of Plasmonic Light Trapping in Thin-Film Amorphous Silicon Photovoltaic Devices, In: Journal of Photonics for Energy5(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.

JG Fernandez, CA Mills, J Samitier (2009)Complex microstructured 3D surfaces using chitosan biopolymer., In: Small5(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).

CA Mills, KTC Chai, MJ Milgrew, A Glidle, JM Cooper, DRS Cumming (2006)A multiplexed impedance analyzer for characterizing polymer-coated QCM sensor arrays, In: IEEE SENSORS JOURNAL6(4)pp. 996-1002 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
CA Mills, E Martinez, F Bessueille, G Villanueva, J Bausells, J Samitier, A Errachid (2005)Production of structures for microfluidics using polymer imprint techniques, In: MICROELECTRONIC ENGINEERING78-79pp. 695-700
M Pla-Roca, JG Fernandez, CA Mills, E Martínez, J Samitier (2007)Micro/nanopatterning of proteins via contact printing using high aspect ratio PMMA stamps and nanoimprint apparatus., In: Langmuir23(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.

SR Segui, M Pla, J Minic, E Pajot-Augy, R Salesse, Y Hou, N Jaffrezic-Renault, CA Mills, J Samitier, A Errachid (2006)Detection of olfactory receptor I7 self-assembled multilayer formation and immobilization using a quartz crystal microbalance, In: ANALYTICAL LETTERS39(8)pp. 1735-1745
A Ruiz, M Zychowicz, L Buzanska, D Mehn, CA Mills, E Martinez, S Coecke, J Samitier, P Colpo, F Rossi (2009)Single Stem Cell Positioning on Polylysine and Fibronectin Microarrays, In: Z Ali (eds.), MICRO AND NANOSYSTEMS1(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 10μm lateral dimensions, which correspond approximately to the size of one cell nucleus, separated by 10μm 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.

IA Marques de Oliveira, F Vocanson, J-P Uttaro, Z Asfari, CA Mills, J Samitier, A Errachid (2010)Characterization of a Self-Assembled Monolayer Based on a Calix[4]Crown-5 Derivate: Fabrication of a Chemical Sensor Sensitive to Calcium, In: JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY10(1)pp. 413-420 AMER SCIENTIFIC PUBLISHERS
CA Mills, E Martinez, A Errachid, G Gomila, A Samso, J Samitier (2005)Small scale structures: the fabrication of polymeric nanostructures for biomedical applications using pattern replication techniques, In: Contributions to Science3(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.

FLM Sam, GDMR Dabera, KT Lai, CA Mills, LJ Rozanski, SRP Silva (2014)Hybrid metal grid-polymer-carbon nanotubes electrodes for high luminance organic light emitting diodes, In: Nanotechnology25(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.

CA Mills, E Engel, E Martinez, M Pla-Roca, M Funes, F Bessueille, A Errachid, J Samitier (2006)Rapid Production of Transparent Micro/Nanostructured Polymer Substrates for Biomedical Surface Interaction Studies, In: JP Reece (eds.), New nanotechnology research(7: Rap)pp. 199-234 Nova Science Pub Inc

This book presents the latest research in this frontier field.

Y Xu, W Jie, P Sellin, T Wang, W Liu, G Zha, P Veeramani, C Mills (2009)Study on temperature dependent resistivity of indium-doped cadmium zinc telluride, In: JOURNAL OF PHYSICS D-APPLIED PHYSICS42(3)ARTN 0pp. ?-? IOP PUBLISHING LTD
R Rodriguez-Trujillo, CA Mills, J Samitier, G Gomila (2007)Low cost micro-Coulter counter with hydrodynamic focusing, In: MICROFLUIDICS AND NANOFLUIDICS3(2)pp. 171-176 SPRINGER HEIDELBERG
A Riul, CA Mills, DM Taylor (2000)The electrical characteristics of a heterojunction diode formed from an aniline oligomer LB-deposited onto poly(3-methylthiophene), In: JOURNAL OF MATERIALS CHEMISTRY10(1)pp. 91-97 ROYAL SOC CHEMISTRY
C Nagavolu, K Susmitha, M Raghavender, L Giribabu, KBS Rao, Christopher Smith, CA Mills, SRP Silva, VVSS Srikanth (2016)Pt-free spray coated reduced graphene oxide counter electrodes for dye sensitized solar cells, In: SOLAR ENERGY137pp. 143-147 PERGAMON-ELSEVIER SCIENCE LTD
FLM Sam, CA Mills, LJ Rozanski, SRP Silva (2013)Thin film hexagonal gold grids as transparent conducting electrodes in organic light emitting diodes, In: Laser and Photonics Reviews8(1)pp. 172-179 Wiley

Indium Tin Oxide (ITO) coated glass is currently the preferred transparent conducting electrode (TCE) for organic light emitting diodes (OLEDs). However, ITO has its drawbacks, not least the scarcity of Indium, high processing temperatures, and inflexibility. A number of technologies have been put forward as replacements for ITO. In this paper, an OLED based on a gold grid TCE is demonstrated, the light emission through the grid is examined, and luminance and current measurements are reported. The gold grid has a sheet resistance of 15 Ω□-1 and a light transmission of 63 % at 550 nm, comparable to ITO, but with advantages in terms of processing conditions and cost. The gold grid OLED has a lower turn-on voltage (7.7 V versus 9.8 V) and achieves a luminance of 100 cdm-2 at a lower voltage (10.9 V versus 12.4 V) than the reference ITO OLED. The lower turn-on voltage and the uniformity of the light output through the gold grid TCE are discussed, and the conduction mechanisms in the ITO and gold grid TCE OLEDs are examined.

KDG Imalka Jayawardena, S Li, LF Sam, Christopher Smith, MJ Beliatis, KK Gandhi, MR Ranga Prabhath, TR Pozegic, S Chen, X Xu, DMR Dabera, LJ Rozanski, RA Sporea, Chris Mills, X Guo, S Silva (2015)High efficiency air stable organic photovoltaics with an aqueous inorganic contact, In: Nanoscale(34)pp. 14241-14247 The Royal Society of Chemistry

We report a ZnO interfacial layer based on an environmentally friendly aqueous precursor for organic photovoltaics. Inverted PCDTBT devices based on this precursor show power conversion efficiencies of 6.8–7%. Unencapsulated devices stored in air display prolonged lifetimes extending over 200 hours with less than 20% drop in efficiency compared to devices based on the standard architecture.

JM Beeley, C Mills, PA Hammond, A Glidle, JM Cooper, L Wang, DRS Cumming (2004)All-digital interface ASIC for a QCM-based electronic nose, In: Sensors and Actuators, B: Chemical103(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 (μR) 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 μR sensors along with on-chip processing capability, is also described. © 2004 Elsevier B.V. All rights reserved.

CA Mills, Y-F Chan, A Intaniwet, A Nisbet, JL Keddie, PJ Sellin (2013)Direct Detection of 6 MV X-rays from a Medical Linear Accelerator using a Semiconducting Polymer Diode, In: Physics in Medicine and Biology58(13)pp. 4471-4482 Institute of Physics

Recently, a new family of low-cost x-radiation detectors have been developed, based on semiconducting polymer diodes, which are easy to process, mechanically flexible, relatively inexpensive, and able to cover large areas. To test their potential for radiotherapy applications such as beam monitors or dosimeters, as an alternative to the use of solid-state inorganic detectors, we present the direct detection of 6 MV x-rays from a medical linear accelerator using a thick film, semiconducting polymer detector. The diode was subjected to 4 ms pulses of 6 MV x-rays at a rate of 60 Hz, and produces a linear increase in photocurrent with increasing dose rate (from 16.7 to 66.7 mGy s(−1)). The sensitivity of the diode was found to range from 13 to 20 nC mGy(−1) cm(−3), for operating voltages from −50 to −150 V, respectively. The diode response was found to be stable after exposure to doses up to 15 Gy. Testing beyond this dose range was not carried out. Theoretical calculations show that the addition of heavy metallic nanoparticles to polymer films, even at low volume fractions, increases the x-ray sensitivity of the polymer film/nanoparticle composite so that it exceeds that for silicon over a wide range of x-ray energies. The possibility of detecting x-rays with energies relevant to medical oncology applications opens up the potential for these polymer detectors to be used in detection and imaging applications using medical x-ray beams.

D Li, CA Mills, JM Cooper (2003)Microsystems for optical gas sensing incorporating the solvatochromic dye Nile Red, In: SENSORS AND ACTUATORS B-CHEMICAL92(1-2)pp. 73-80 ELSEVIER SCIENCE SA
CA Mills, M Pla-Roca, C Martin, M Lee, M Kuphal, X Sisquella, E Martinez, A Errachid, J Samitier (2007)Structured thin organic active layers and their use in electrochemical biosensors, In: MEASUREMENT & CONTROL40(3)pp. 88-91 INST MEASUREMENT CONTROL
CA Mills, FLM Sam, AS Alshammari, LJ Rozanski, NG Emerson, SRP Silva (2013)Storage Lifetime of Polymer-Carbon Nanotube Inks for Use as Charge Transport Layers in Organic Light Emitting Diodes, In: IEEE/OSA Journal of display technology10(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.

DM Taylor, CA Mills (2001)Memory effect in the current-voltage characteristic of a low-band gap conjugated polymer, In: JOURNAL OF APPLIED PHYSICS90(1)pp. 306-309 AMER INST PHYSICS
CA Mills, H Al-Otaibi, A Intaniwet, M Shkunov, S Pani, JL Keddie, PJ Sellin (2013)Enhanced x-ray detection sensitivity in semiconducting polymer diodes containing metallic nanoparticles, In: Journal of Physics D: Applied Physics46(27)275102pp. ?-? 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.

JG Fernandez, J Samitier, CA Mills (2011)Simultaneous biochemical and topographical patterning on curved surfaces using biocompatible sacrificial molds, In: JOURNAL OF BIOMEDICAL MATERIALS RESEARCH PART A98A(2)pp. 229-234 WILEY-BLACKWELL
R.D.I.G. Dharmasena, K.D.G.I. Jayawardena, C.A. Mills, R.A. Dorey, Ravi Silva (2018)A unified theoretical model for Triboelectric Nanogenerators, In: Nano Energy48pp. 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.

JG Fernandez, CA Mills, R Rodriguez, G Gomila, J Samitier (2006)All-polymer microfluidic particle size sorter for biomedical applications, In: PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE203(6)pp. 1476-1480
D Caballero, M Pla-Roca, F Bessueille, CA Mills, J Samitier, A Errachid (2006)Atomic force microscopy characterization of a microcontact printed, self-assembled thiol monolayer for use in biosensors, In: Analytical Letters39(8)pp. 1721-1734

This paper describes friction experiments and pull-off force measurements using atomic force microscopy (AFM), between a nonfunctionalized silicon probe and a 2.5 μm 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.

KDGI Jayawardena, LJ Rozanski, CA Mills, SRP Silva (2015)The true status of solar cell technology, In: NATURE PHOTONICS9(4)pp. 207-208 NATURE PUBLISHING GROUP
A Intaniwet, CA Mills, M Shkunov, H Thiem, JL Keddie, PJ Sellin (2009)Characterization of thick film poly(triarylamine) semiconductor diodes for direct x-ray detection, In: JOURNAL OF APPLIED PHYSICS106(6)ARTN 0pp. ?-? AMER INST PHYSICS
Hashini Thirimanne, K Jayawardena, A Parnell, R Bandara, A Karalasingam, Silvia Pani, J Huerdler, D Lidzey, S Tedde, Andrew Nisbet, Chris Mills, Ravi Silva (2018)High sensitivity organic inorganic hybrid X ray detectors with direct transduction and broadband response, In: Nature Communications92926 Nature Publishing Group

X-ray detectors are critical to healthcare diagnostics, cancer therapy and homeland security, with many potential uses limited by system cost and/or detector dimensions. Current X-ray detector sensitivities are limited by the bulk X-ray attenuation of the materials and consequently necessitate thick crystals (~ 1 mm – 1 cm), resulting in rigid structure, high operational voltages and high cost. Here we present a disruptive, flexible, low cost, broad-band, and high sensitivity direct X-ray transduction technology produced by embedding high atomic number bismuth oxide nanoparticles in an organic bulk heterojunction. These hybrid detectors demonstrate sensitivities of 1712 µC mGy-1 cm-3 for “soft” X-rays and ~30 and 58 µC mGy-1 cm-3 under 6 and 15 MV “hard” X-rays generated from a medical linear accelerator; strongly competing with the current solid state detectors, all achieved at low bias voltages (-10 V) and low power, enabling detector operation powered by coin cell batteries.

K. D. G. Imalka Jayawardena, Hashini M. Thirimanne, Sandro Francesco Tedde, Judith E. Huerdler, Andrew J. Parnell, R. M. Indrachapa Bandara, Christopher A. Mills, S. Ravi P. Silva (2019)Millimeter-Scale Unipolar Transport in High Sensitivity Organic–Inorganic Semiconductor X-ray Detectors, In: 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 μm due to poor charge transport in such systems. However, recent work suggests that charge carriers in such organic semiconductors can be transported over centimeter length scales opposing this view. In this work, a unipolar X-ray photoconductor based on a bulk heterojunction architecture, consisting of poly(3-hexylthiophene), a C70 derivative, and high atomic number bismuth oxide nanoparticles operating in the 0.1–1 mm thickness regime is demonstrated, having a high sensitivity of ∼160 μC mGy–1 cm–3. The high performance enabled by hole drift lengths approaching a millimeter facilitates a device architecture allowing a high fraction of the incident X-rays to be attenuated. An X-ray imager is demonstrated with sufficient resolution for security applications such as portable baggage screening at border crossings and public events and scalable medical applications.

CA Mills, E Martinez, A Errachid, E Engel, M Funes, C Moormann, T Wahlbrink, G Gomila, J Planell, J Samitier (2007)Nanoembossed polymer substrates for biomedical surface interaction studies., In: Journal of Nanoscience and Nanotechnology7(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.

D Caballero, G Villanueva, JA Plaza, CA Mills, J Samitier, A Errachid (2010)Sharp high-aspect-ratio AFM tips fabricated by a combination of deep reactive ion etching and focused ion beam techniques., In: Journal of Nanoscience and Nanotechnology10(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.

Christopher A. Mills, Erdni Batyrev, Maurice J. R. Jansen, Muhammad Ahmad, Tanveerkhan S. Pathan, Elizabeth J. Legge, Digvijay B. Thakur, Samson N. Patole, Dan J. L. Brett, Paul R. Shearing, Hans van der Weijde, S. Ravi P. Silva (2019)Improvement in the Electrical Properties of Nickel Plated Steel using Graphitic Carbon Coatings, In: Advanced Engineering Materials21(10)1900408pp. 1-9 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.

F Bessueille, M Pla-Roca, CA Mills, E Martinez, J Samitier, A Errachid (2005)Submerged microcontact printing (S mu CP): An unconventional printing technique of thiols using high aspect ratio, elastomeric stamps, In: LANGMUIR21(26)pp. 12060-12063 AMER CHEMICAL SOC
A Riul, DM Taylor, CA Mills, PJ Murphy (2000)Langmuir and Langmuir-Blodgett (LB) films of 4-dicyanomethylene,4H-cyclopenta[2,1-b,3,4-b '] dithiophene, In: THIN SOLID FILMS366(1-2)pp. 249-254 ELSEVIER SCIENCE SA
A Errachid, CA Mills, M Pla-Roca, MJ Lopez, G Villanueva, J Bausells, E Crespo, F Teixidor, J Samitier (2008)Focused ion beam production of nanoelectrode arrays, In: MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS28(5-6)pp. 777-780
NJ Renault, H Korri-Youssoufi, J Kyselovik, V Auger, A Errachid, CA Mills, C Strohhöfer, Z Ali (2008)Impedimetric microanalysis system for deep vein thrombosis point-of-care testing, In: Proceedings of the 30th Annual International Conference of the IEEE Engineering in Medicine and Biology Society: Personalized Healthcare Through Technologypp. 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.

LJ Rozanski, E Castaldelli, FLM Sam, CA Mills, GJ-F Demets, SRP Silva (2013)Solution processed naphthalene diimide derivative as electron transport layers for enhanced brightness and efficient polymer light emitting diodes, In: Journal of Materials Chemistry C1(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.