Maxim Shuknov

Dr Maxim Shkunov


Senior Lecturer
+44 (0)1483 686082
15 ATI 02

About

University roles and responsibilities

  • Programme Director Nanotechnology and Renewable Energy MSc
  • Academic Integrity Officer
  • PGR Director, Advanced Technology Institute

    Research

    Research interests

    Teaching

    Publications

    M. Shkunov, A. N Solodukhin, P. Giannakou, L. Askew , Yu. N Luponosov, D. O Balakirev, N. K Kalinichenko, I. P. Marko, S. J. Sweeney, S. A Ponomarenko (2021)Pixelated full-colour small molecule semiconductor devices towards artificial retinas, In: Journal of Materials Chemistry C Materials for optical and electronic devices9pp. 5858-5867 Royal Society of Chemistry

    Opto-stimulation of semiconductor-biointerfaces provides efficient pathways towards eliciting neural activity through selective spectral excitation. In visual prosthesis, tri-colour stimulation capability is the key to restoring full-colour vision. Here we report on investigation of organic photoactive π-conjugated donor–acceptor small molecules based on triphenylamine whose absorption spectra are similar to those of the photoreceptors of the human eye. Photoactive device fabrication and characterisation towards full colour, pixelated retinal prosthesis based on inkjet printing of these molecules is demonstrated, with round pixels reaching 25 microns in diameter. Photo-response is studied via interfacing with biological electrolyte solution and using long-pulse, narrow-band excitation. Both photo-voltage and photo-current responses in the devices with a ZnO hole-blocking interlayer show clear signatures of capacitive charging at the electrolyte/device interface, also demonstrating spectral selectivity comparable to that of human eye’ cones and rods.

    JINXIN BI, JING ZHANG, PAVLOS GIANNAKOU, Toshan Wickramanayake, XUHUI YAO, MANMAN WANG, XUEPING LIU, MAXIM SHKUNOV, WEI ZHANG, YUNLONG ZHAO (2022)A Highly integrated flexible photo-rechargeable system based on stable ultrahigh-rate quasi-solid-state zinc-ion micro-batteries and perovskite solar cells, In: Energy Storage Materials51pp. 239-248 Elsevier B.V

    Miniaturized flexible photo-rechargeable systems show bright prospects for wide applications in internet of things, self-powered health monitoring and emergency electronics. However, conventional systems still suffer from complex manufacturing processes, slow photo-charging and discharging rate, and mismatch between photovoltaic and energy storage components in size, mechanics and voltage, etc. Here, we demonstrate a facile inkjet printing and electrodeposition approach for fabricating a highly integrated flexible photo-rechargeable system by combining stable and ultra-high-rate quasi-solid-state Zn-MnO2 micro-batteries (ZMBs) with flexible perovskite solar cells (FPSCs). In particular, Ni protective layer is first introduced into ZMBs to stabilize battery configuration and facilitate enhanced electrochemical performance. The optimized ZMB exhibits ultrahigh volumetric energy density of 148 mWh cm−3 (16.3 μWh cm−2) and power density of 55 W cm−3 (6.1 mW cm−2) at the current density of 400 C (5 mA cm−2), enabling them comparable with the state-of-the-art micro-batteries or supercapacitors fabricated by conventional methods. The embedded FPSCs show excellent photovoltaic performance, sufficient to charge ZMBs and create a self-charging system capable to offer energy autonomy in miniaturized wearable electronics. The integrated systems can achieve an ultrafast photo-charging within 30 s, with sufficient energy to power other functional electronics (e.g., LED bulb and pressure sensor) for tens of minutes. This prototype offers a promising scheme for next-generation miniaturized flexible photo-rechargeable systems.

    Pavlos Giannakou, Robert C.T. Slade, Maxim Shkunov (2020)Cyclic Voltammetry Studies of Inkjet-printed NiO supercapacitors: Effect of Substrates, Printing and Materials, In: Electrochimica Acta353136539 Elsevier

    In the past decade the family of digital printing technology has been developed beyond the visualisation of information into a generator of a whole new set of functionalities through printed electronics, which triggered an explosion of new ideas and alternative fabrication approaches towards lean and cost-efficient manufacturing processes. Among other electronic components, inkjet-printed electrochemical systems including supercapacitors, are growing exponentially with the aim to achieve advanced form factors (i.e. flexible, stretchable, conformal, etc.) and design versatility, to simplify their production process and to lower their manufacturing cost. To the best of our knowledge, despite the growing interest in inkjet-printed electrochemical capacitors, no studies have been found to investigate the effects of printing (including substrates and materials) as a production process on the electrochemical performance and response of the devices. In this work, cyclic voltammetry studies were performed to investigate the effect of substrate and printing resolution, different types of current collector silver inks, sintering temperatures and carbon residues in electrodes, different types of electrolytes, binders and other organic compounds in electrolytes, and the effect of electrode gaps on the electrochemical response and capacitance of inkjet-printed, coplanar NiO supercapacitors. The results of this study can be extended to other electrochemical systems realised through printing fabrication processes (e.g. batteries, electrolyte-gated transistors, etc.).

    L. Askew, M. Shkunov, A. Sweeney (2023)Development of polymeric blue prosthetic retina photoreceptors, In: MRS Advances8pp. 152-160

    Two conjugated polymers with absorption spectra like that of human short-wavelength photoreceptors, or blue cones, are characterised to act as essential components of a sub-retinal flexible prototype prothesis for people suffering from retinal disease affecting photoreceptor cells. Spectrophotometric and transient tests undertaken in unbiased photovoltaic mode in ambient conditions demonstrate that the addition of hole-blocking ZnO layer results in reliably forcing a favourable capacitive charging regime and acts to improve the photoresponse over ten times in one polymer and 45 times in another. We report that the addition of fullerene and non-fullerene acceptor molecules in bulk-heterojunction (BHJ)-active layers make an almost sevenfold measured improvement to extracellular photovoltage for devices operating in an electrolyte environment.

    Manuela Ciocca, Pavlos Giannakou, Paolo Mariani, Lucio Cinà, Aldo Di Carlo, Mehmet O. Tas, Hiroki Asari, Serena Marcozzi, Antonella Camaioni, Maxim Shkunov, Thomas M. Brown (2020)Colour-sensitive conjugated polymer inkjet-printed pixelated artificial retina model studied via a bio-hybrid photovoltaic device, In: Scientific Reports1021457 Nature Research

    In recent years, organic electronic materials have been shown to be a promising tool, even transplanted in vivo, for transducing light stimuli to non-functioning retinas. Here we developed a bio-hybrid optoelectronic device consisting of patterned organic polymer semiconductors interfaced with an electrolyte solution in a closed sandwich architecture in order to study the photo-response of photosensitive semiconducting layers or patterns in an environment imitating biological extracellular fluids. We demonstrate an artificial retina model composed of on an array of 42,100 pixels made of three different conjugated polymers via inkjet printing with 110 pixels/mm2 packing density. Photo-sensing through three-colour pixelation allows to resolve incoming light spectrally and spatially. The compact colour sensitive optoelectronic device represents an easy-to-handle photosensitive platform for the study of the photo response of artificial retina systems.

    S Georgakopoulos, RADU A SPOREA, MAXIM SHKUNOV (2022)Polymer source-gated transistors with low saturation voltage, In: Journal of materials chemistry. C, Materials for optical and electronic devices10(4)pp. 1282-1288

    A type of injection-limited transistor is demonstrated with a conjugated polymer semiconductor and fluoropolymer insulator. The source-gated transistor (SGT) is based on a source Schottky barrier, the effective height of which is controlled by the gate voltage, shifting the origin of current modulation from the channel to the source-semiconductor contact. SGTs fabricated in this work saturate at up to 30 times lower drain voltages than FETs at high gate voltages. Saturation in SGTs is retained for short channels without requiring downscaling of the insulator layer. As the transistor channel has reduced influence on current modulation, the SGT output current is only weakly dependent on the channel length contrary to traditional FETs. These features come at some current reduction due to the series resistance stemming from the source depletion region. The ability to function with thick insulators, low voltage operation and thus low power consumption, as well as the reduced sensitivity to channel length variations could be highly synergistic with printing techniques used to deposit materials in organic electronics.

    C Opoku, M Shkunov, L Chen, F Meyer (2011)Solution processable nanowire field-effect transistors, In: Materials Research Society Symposium Proceedings1287pp. 69-74

    Hybrid field-effect-transistors (FETs) with germanium nanowire (NW) arrays and organic gate dielectric are presented. The nanowire deposition steps are fully compatible with printed electronics route. NW FETs demonstrate good performance with On/Off ratios of ~10 and hole mobilities of ∼13 cm /Vs in both nitrogen and air atmosphere. These results suggest that the hybrid nanowire FETs could be used in large area inexpensive electronics. © 2011 Materials Research Society.

    RA Sporea, S Georgakopoulos, M Shkunov, JM Shannon, SRP Silva, X Xu, X Guo (2013)Leveraging contact effects for field-effect transistor technologies with reduced complexity and superior current uniformity, In: MRS Online Proceedings Library1553

    In order to achieve high performance, the design of devices for large-area electronics needs to be optimized despite material or fabrication shortcomings. In numerous emerging technologies thin-film transistor (TFT) performance is hindered by contact effects. Here, we show that contact effects can be used constructively to create devices with performance characteristics unachievable by conventional transistor designs. Source-gated transistors (SGTs) are not designed with increasing transistor speed, mobility or sub-threshold slope in mind, but rather with improving certain aspects critical for real-world large area electronics such as stability, uniformity, power efficiency and gain. SGTs can achieve considerably lower saturation voltage and power dissipation compared to conventional devices driven at the same current; higher output impedance for over two orders of magnitude higher intrinsic gain; improved bias stress stability in amorphous materials; higher resilience to processing variations; current virtually independent of source-drain gap, source-gate overlap and semiconductor thickness variations. Applications such as amplifiers and drivers for sensors and actuators, low cost large area analog or digital circuits could greatly benefit from incorporating the SGT architecture.

    Maxim Shkunov, Grigorios Rigas, Marios Constantinou (2017)Solution-Processable Nanowire Field-Effect Transistors, In: Khan Maaz (eds.), Nanowires - New Insightspp. 79-100 IntechOpen Limited

    Solution-processable single-crystalline inorganic semiconducting nanowires are excellent building blocks for printable electronics requiring high performance of semiconducting components. Excellent charge carrier mobilities of crystalline nanowires combined with solvent-based nanowire processing open up possibilities for low-cost nanowire electronics targeting a variety of applications ranging from flexible circuits to chemical and biological sensors. Nanowire field-effect transistors are key devices for most of such applications. Recent developments in controllable nanowire positioning and orientation on the substrates and electrical property selection provide the necessary technological breakthroughs enabling the fabrications of reproducible nanowire transistors. In this chapter, we discuss the nanowire assembly methods and high-spatial-resolution scanning probe microscopy techniques towards scalable fabrication of high-performance printable nanowire field-effect transistors.

    K Prabha Rajeev, C Opoku, V Stolojan, M Constantinou, M Shkunov (2017)Effect of Nanowire-dielectric Interface on the Hysteresis of Solution Processed Silicon Nanowire FETs, In: Nanoscience and Nanoengineering5(2)pp. 17-24 Horizon Research Publishing

    Silicon nanowires (Si NW) are ideal candidates for low-cost solution processed field effect transistors (FETs) due to the ability of nanowires to be dispersed in solvents, and demonstrated high charge carrier mobility. The interface between the nanowire and the dielectric plays a crucial role in the FET characteristics, and can be responsible for unwanted effects such as current hysteresis during device operation. Thus, optimal nanowire- dielectric interface is required for low-hysteresis FET performance. Here we show that NW FET hysteresis mostly depends on the nature of the dielectric material by directly comparing device characteristics of dual gate Si NW FETs with bottom SiO2 gate dielectric and top hydrophobic fluoropolymer gate dielectric. As the transistor semiconducting nanowire channel is identical in both tops and bottom operational regimes, the performance differences originate from the nature of the nanowire-dielectric interface. Thus, very high 30 volt hysteresis is observed for forward and reverse gate bias scans with SiO2 interface; however, hysteresis is significantly reduced to 6 volt for the fluoropolymer dielectric interface. The differences in hysteresis are ascribed to the polar OH- groups present at SiO2/Si nanowire interface, and mostly absent at fluoropolymer/Si nanowire interface. We further demonstrate that high density of charge traps for bottom gate SiO2 interface (1× 1013cm-2) is reduced by over an order of magnitude for top-fluoropolymer gate interface (7.5 × 1011 cm-2), therefore highlighting the advantage of hydrophobic polymer gate dielectrics for nanowire field-effect transistor applications.

    H. M. Cronin, K. D. G. Imalka Jayawardena, Z. Stoeva, M. Shkunov, S. R. P. Silva (2016)Effects of Ambient Humidity on the Optimum Annealing Time of Mixed-Halide Perovskite Solar Cells, In: Nanotechnology28(11) IOP Publishing Ltd

    Mixed halide Perovskite solar cells are commonly produced by depositing PbCl2 and CH3NH3I from a common solvent followed by thermal annealing, which in an up-scaled manufacturing process is likely to take place under ambient conditions. However, it has been reported that, similar to the effects of thermal annealing, ambient humidity also affects the crystallisation behaviour and subsequent growth of the Perovskite films. This implies that both of these factors must be accounted for in solar cell production. In this work, we report for the first time the correlation between the annealing time, relative humidity and device performance for inverted, mixed halide CH3NH3PbI(3-x)Clx Perovskite solar cells with active area ≈1 cm2. We find a trade-off between ambient humidity and the required annealing time to produce efficient solar cells, with low humidities needing longer annealing times and vice-versa. At around 20% RH, device performance depends relatively weakly on annealing time, but at higher (30 - 40% RH) or lower (0 - 15 % RH) humidities it is very sensitive. Processing in humid environments is shown to lead to the growth of both larger Perovskite grains and larger voids; similar to the effect of thermal annealing, which also leads to grain growth. Therefore, samples which are annealed for too long under high humidity show loss of performance due to low open circuit voltage caused by an increased number of shunt paths. Based on these results it is clear that humidity and annealing time are closely interrelated and both are important factors affecting the performance of Perovskite solar cells. The findings of this work opens a route for reduced annealing times to be employed by control of humidity; critical in roll-to-roll manufacture where low manufacturing time is preferred for cost reductions.

    M Shkunov, YG Rabobason, GP Rigas, S Swaisaenyakorn, B Mirkhaydarov, B Ravelo, PR Young, N Benjelloun (2016)Design of Flexible Passive Antenna Array on Kapton Substrate, In: Progress In Electromagnetics Research C63pp. 105-117 Progress In Electromagnetics Research

    Recently, the RF/microwave electronic technology evolved with the consideration of plastic and organic substrates. Such a technology offers two-folded benefits: in one side for lowering the fabrication cost and in another side for the possibility to bend electronic devices. Such a technology is particularly interesting for the implementation of antenna system. This paper is dealing with the design of flexible microstrip antenna 1:2 array. Theoretical approach on the typically symmetrical antenna 1:2 array is proposed. The design methodology of microstrip antenna combined with 1:2 T-power divider (T-PWD) is described. Based on the transmission line theory, the S-parameter model of the antenna system with non-standard reference load is established. Then, the microstrip antenna passive system is theoretical analysed in function of the physical dimensions of the designed structure. The feasibility of the flexible antenna passive system is investigated with the proof-of-concept (POC) designed on Kapton substrate. The POC prototype consisted of microstrip antenna 1:2 array is designed to operate at about 5.8 GHz. Comparisons between the full wave simulated and measured return losses were performed. Then, simulated radiation pattern highlights the efficiency of the fabricated prototype of passive antenna array.

    T Kreouzis, R Baldwin, M Shkunov, I McCulloch, M Heeney, W Zhang (2005)High Mobility Ambipolar Charge Transport in a Cross-Linked Reactive Mesogen at Room Temperature, In: Applied Physics Letters87(17)
    S Barard, M Heeney, L Chen, M Colle, M Shkunov, I McCulloch, N Stingelin, M Philips, T Kreouzis (2009)Separate charge transport pathways determined by the time of flight method in bimodal polytriarylamine, In: JOURNAL OF APPLIED PHYSICS105(1)ARTN 01370 AMER INST PHYSICS
    B Milian Medina, A Van Vooren, P Brocorens, J Gierschner, M Shkunov, M Heeney, I McCulloch, R Lazzaroni, J Cornil (2007)Electronic structure and charge-transport properties of polythiophene chains containing thienothiophene units: A joint experimental and theoretical study, In: CHEMISTRY OF MATERIALS19(20)pp. 4949-4956 AMER CHEMICAL SOC
    SIMON GEORGE KING, WESLEY GUY BUXTON, KASPAR MARTIN SNASHALL, B Mirkhaydarov, MAXIM SHKUNOV, S. Ravi P. Silva, VLAD STOLOJAN (2022)A route towards metal-free electrical cables via carbon nanotube wires, In: Carbon trends7100159 Elsevier Ltd

    Carbon nanotubes (CNTs) have unique properties with promise to outperform the electrical characteristics of bulk copper, giving rise to its primary driver for use in electronic devices. The challenge still hindering their full exploitation stems from an inability to manufacture them to long lengths, resulting in a requirement to align and entwine them into a yarn or wire. There have been several methods presented in achieving this, however, the common disadvantage has been that they are only applicable to specific types and morphologies of CNTs. In the work reported here, using electrospinning as a universally applicable route for any CNT type, we re-engineer and optimise the various formulation, fabrication and processing steps required to manufacture CNT wires. Through a series of investigations using a materials agnostic approach, we experimentally probe the choice of solvent, surfactant and thermal treatment temperature of the CNT inks, demonstrating the CNT-type optimum using a range of commercially available single- double- and multiwalled CNTs. Finally, this allowed us to develop and probe an electrical conditioning process to further enhance the electrical performance, achieving the highest reported un-doped electrical conductivity of 36,000 S⋅m− 1 for electrospun CNT wires, or a specific conductivity of 0.2×106S·m−1/g·cm−3. [Display omitted]

    Bobur Mirkhaydarov, Haris Votsi, Abhishek Sahu, Philippe Caroff, Paul R. Young, Vlad Stolojan, Simon King, Calvin C H Ng, Vijaya Devabhaktuni, Hoe H Tan, Chennupati Jagadish, Peter Aaen, Maxim Shkunov (2019)Solution‐Processed InAs Nanowire Transistors as Microwave Switches, In: Advanced Electronic Materials5(1)1800323 Wiley

    The feasibility of using self‐assembled InAs nanowire bottom‐gated field‐effect transistors as radio‐frequency and microwave switches by direct integration into a transmission line is demonstrated. This proof of concept is demonstrated as a coplanar waveguide (CPW) microwave transmission line, where the nanowires function as a tunable impedance in the CPW through gate biasing. The key to this switching capability is the high‐performance, low impedance InAs nanowire transistor behavior with field‐effect mobility of ≈300 cm2 V−1 s−1, on/off ratio of 103, and resistance modulation from only 50 Ω in the full accumulation mode, to ≈50 kΩ when the nanowires are depleted of charge carriers. The gate biasing of the nanowires within the CPW results in a switching behavior, exhibited by a ≈10 dB change in the transmission coefficient, S21, between the on/off switching states, over 5–33 GHz. This frequency range covers both the microwave and millimeter‐wave bands dedicated to Internet of things and 5G applications. Demonstration of these switches creates opportunities for a new class of devices for microwave applications based on solution‐processed semiconducting nanowires.

    Maxim Shkunov, Khue T. Lai, Christopher G. Poll, John W. Dear (2018)Solution-processable transparent conducting films by defunctionalization of amine functionalized carbon nanotubes, In: Journal of Photonics for Energy8(03)032221pp. 032221-1

    This paper presents a low temperature, solution-based processing method of highly transparent, sparse networks of carbon nanotubes via annealing process that dramatically improves the conductivity of thin films of octadecylamine functionalized highly soluble single-wall carbon nanotubes by up to five orders of magnitude. This increase in conductivity obtained at low temperatures allows for the creation of transparent conducting carbon nanotube (CNT) films via printed deposition of contacts for photovoltaic, light emitting, and display devices. An increase in films conductivity has been shown with process temperatures of 200°C at normal atmospheric pressure. The dependence between the sheet resistance of CNT layers and the annealing parameters is analyzed together with Raman and FTIR data, suggesting a relationship between the loss of octadecylamine functional groups along with the healing of CNT defects during the annealing process and the dramatic conductivity improvement of CNT layers.

    G Adamopoulos, A Bashir, S Thomas, WP Gillin, S Georgakopoulos, M Shkunov, MA Baklar, N Stingelin, RC Maher, LF Cohen, DDC Bradley, TD Anthopoulos (2010)Spray-Deposited Li-Doped ZnO Transistors with Electron Mobility Exceeding 50 cm(2)/Vs, In: ADVANCED MATERIALS22(42)pp. 4764-+ WILEY-V C H VERLAG GMBH
    S Georgakopoulos, D Sparrowe, Y Gu, MM Nielsen, F Meyer, M Shkunov (2012)Air-stable pi-conjugated amorphous copolymer field-effect transistors with high mobility of 0.3 cm(2)/Vs, In: APPLIED PHYSICS LETTERS101(21)ARTN 21330 AMER INST PHYSICS
    P Brocorens, A Van Vooren, ML Chabinyc, MF Toney, M Shkunov, M Heeney, I McCulloch, J Cornil, R Lazzoroni (2009)Solid-State Supramolecular Organization of Polythiophene Chains Containing Thienothiophene Units, In: ADVANCED MATERIALS21(10-11)pp. 1193-1198 WILEY-V C H VERLAG GMBH
    AS Alshammari, M Shkunov, SRP Silva (2013)Inkjet printed PEDOT: PSS/MWCNT nano-composites with aligned carbon nanotubes and enhanced conductivity, In: Physica Status Solidi - Rapid Research Letters

    Conductive patterns of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/multi-walled carbon nanotube (MWCNT) composites were deposited on glass substrates using a drop on demand (DOD) inkjet printer, with the concentration of CNT varied from 0.01 wt% to 0.05 wt%. We show that by increasing the concentration of the nanotubes in the ink, percolated networks of well distributed carbon nanotubes in the printed samples can be achieved. Moreover, the orientation of the nanotubes in the printed sample can be controlled using a novel simple approach. The impact of the nanotube alignment on the conduction properties of inkjet printed nano-hybrid materials is studied and shown in this Letter. Samples with aligned nanotubes show a 53% enhanced conductivity in comparison with the randomly oriented nanotubes. The results show that the electrical performance of the nano-composite can be improved further by controlling the dispersion and orientation of the nano-filler in the printed samples. Carbon nanotubes orientation control in the printed PEDOT:PSS/MWCNT nano-composite samples. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

    S Georgakopoulos, D Sparrowe, F Meyer, M Shkunov (2010)Stability of top- and bottom-gate amorphous polymer field-effect transistors, In: Applied Physics Letters97(24)
    DJ Crouch, PJ Skabara, JE Lohr, JJW McDouall, M Heeney, I McCulloch, D Sparrowe, M Shkunov, SJ Coles, PN Horton, MB Hursthouse (2005)Thiophene and selenophene copolymers incorporating fluorinated phenylene units in the main chain: Synthesis, characterization, and application in organic field-effect transistors, In: CHEMISTRY OF MATERIALS17(26)pp. 6567-6578 AMER CHEMICAL SOC
    Kiron Prabha Rajeev, Michael Beliatis, Stamatis Georgakopoulos, Vlad Stolojan, John Underwood, Maxim Shkunov (2019)Laser Patterned Polymer/Carbon Nanotubes Composite Electrodes for Flexible Silicon Nanowire Transistors, In: Journal of Nanoscience and Nanotechnology19(8)pp. 4765-4770

    Fabrication techniques such as laser patterning offer excellent potential for low cost and large area device fabrication. Conductive polymers can be used to replace expensive metallic inks such as silver and gold nanoparticles for printing technology. Electrical conductivity of the polymers can be improved by blending with carbon nanotubes. In this work, formulations of acid functionalized multiwalled carbon nanotubes (f-MWCNTs) and poly(ethylenedioxythiophene) [PEDOT]:polystyrene sulphonate [PSS] were processed, and thin films were prepared on plastic substrates. Conductivity of PEDOT:PSS increased almost four orders of magnitude after adding f-MWCNTs. Work function of PEDOT:PSS/f-MWCNTs films was ∼0.5 eV higher as compared to the work function of pure PEDOT:PSS films, determined by Kelvin probe method. Field-effect transistors source–drain electrodes were prepared on PET plastic substrates where PEDOT:PSS/f-MWCNTs were patterned using laser ablation at 44 mJ/pulse energy to define 36 μm electrode separation. Silicon nanowires were deposited using dielectrophoresis alignment technique to bridge laser patterned electrodes. Top-gated nanowire field effect transistors were completed by depositing parylene C as polymer gate dielectric and gold as the top-gate electrode. Transistor characteristics showed p-type conduction with excellent gate electrode coupling, with an ON/OFF ratio of ∼200. Thereby, we demonstrate the feasibility of using high workfunction, printable PEDOT:PSS/f-MWCNTs composite inks for laser patterned source/drain electrodes for nanowire transistors on flexible substrates.

    P Sonar, AC Grimsdale, M Heeney, M Shkunov, I McCulloch, K Muellen (2007)A study of the effects metal residues in poly(9,9-dioctylfluorene) have on field-effect transistor device characteristics, In: SYNTHETIC METALS157(21)pp. 872-875 ELSEVIER SCIENCE SA
    RJ Baldwin, T Kreouzis, M Shkunov, M Heeney, W Zhang, I McCulloch (2007)A comprehensive study of the effect of reactive end groups on the charge carrier transport within polymerized and nonpolymerized liquid crystals, In: JOURNAL OF APPLIED PHYSICS101(2)ARTN 0pp. ?-? AMER INST PHYSICS
    G Adamopoulos, A Bashir, WP Gillin, S Georgakopoulos, M Shkunov, MA Baklar, N Stingelin, DDC Bradley, TD Anthopoulos (2011)Structural and Electrical Characterization of ZnO Films Grown by Spray Pyrolysis and Their Application in Thin-Film Transistors, In: ADVANCED FUNCTIONAL MATERIALS21(3)pp. 525-531 WILEY-V C H VERLAG GMBH
    M Constantinou, GP Rigas, FA Castro, V Stolojan, KF Hoettges, M Hughes, E Adkins, BA Korgel, M Shkunov (2016)Simultaneous Tunable Selection and Self-Assembly of Si Nanowires from Heterogeneous Feedstock, In: ACS Nano10(4)pp. 4384-4394 American Chemical Society

    Semiconducting nanowires (NWs) are becoming essential nano-building blocks for advanced devices from sensors to energy harvesters, however their full technology penetration requires large scale materials synthesis together with efficient NW assembly methods. We demonstrate a scalable one-step solution process for the direct selection, collection and ordered assembly of silicon NWs with desired electrical properties from a poly-disperse collection of NWs obtained from a Supercritical Fluid-Liquid-Solid growth process. Dielectrophoresis (DEP) combined with impedance spectroscopy provides a selection mechanism at high signal frequencies (>500 kHz) to isolate NWs with the highest conductivity and lowest defect density. The technique allows simultaneous control of five key parameters in NW assembly: selection of electrical properties, control of NW length, placement in pre-defined electrode areas, highly preferential orientation along the device channel and control of NWs deposition density from few to hundreds per device. Direct correlation between DEP signal frequency and deposited NWs conductivity is directly confirmed by field-effect transistor and conducting-AFM data. Fabricated NW transistor devices demonstrate excellent performance with up to 1.6 mA current, 106-107 on/off ratio and hole-mobility of 50 cm2 V-1 s-1.

    M Shkunov, I McCulloch, W Zhang, M Heeney, C Bailey, M Giles, D Graham, M Shkunov, D Sparrowe, S Tierney (2003)Polymerisable liquid crystalline organic semiconductors and their fabrication in organic field effect transistors, In: Journal of Materials Chemistry
    JM Shannon, RA Sporea, S Georgakopoulos, M Shkunov, SRP Silva (2013)Low-Field Behavior of Source-Gated Transistors, In: IEEE Transactions on Electron Devices60(8)pp. 2444-2449 IEEE

    A physical description of low-field behavior of a Schottky source-gated transistor (SGT) is outlined where carriers crossing the source barrier by thermionic emission are restricted by JFET action in the pinch-off region at the drain end of the source. This mode of operation leads to transistor characteristics with low saturation voltage and high output impedance without the need for field relief at the edge of the Schottky source barrier and explains many characteristics of SGT observed experimentally. 2-D device simulations with and without barrier lowering due to the Schottky effect show that the transistors can be designed so that the current is independent of source length and thickness variations in the semiconductor. This feature together with the fact that the current in an SGT is independent of source-drain separation hypothesizes the fabrication of uniform current sources and other large-area analog circuit blocks with repeatable performance even in imprecise technologies such as high-speed printing.

    Kaspar Snashall, Marios Constantinou, Maxim Shkunov (2017)Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices, In: Journal of Visualized Experiments(130)e56408 Journal of Visualized Experiments

    Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.

    I Mcculloch, M Heeney, C Bailey, K Genevicius, I Macdonald, M Shkunov, D Sparrowe, S Tierney, R Wagner, WM Zhang, ML Chabinyc, RJ Kline, MD Mcgehee, MF Toney (2006)Liquid-crystalline semiconducting polymers with high charge-carrier mobility, In: NATURE MATERIALS5(4)pp. 328-333 NATURE PUBLISHING GROUP
    I McCulloch, C Bailey, K Genevicius, M Heeney, M Shkunov, D Sparrowe, S Tierney, W Zhang, R Baldwin, T Kreouzis, JW Andreasen, DW Breiby, MM Nielsen (2006)Designing solution-processable air-stable liquid crystalline crosslinkable semiconductors, In: PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES364(1847)pp. 2779-2787 ROYAL SOC
    Harry M. Cronin, Zlatka Stoeva, Martin Brown, Maxim Shkunov, S. Ravi P. Silva (2018)Photonic Curing of Low-Cost Aqueous Silver Flake Inks for Printed Conductors with Increased Yield, In: ACS Applied Materials & Interfaces10(25)pp. 21398-21410 American Chemical Society

    Printing of highly conductive tracks at low cost is of primary importance for the emerging field of flexible, plastic, and large-area electronics. Commonly, this is achieved by printing of metallic conductive inks, often based on Ag or Cu nanoparticles dispersed in organic solvents. The solvents, which must be safely removed, have particular storage and handling requirements, thus increasing the process costs. By using water-based inks containing micron-sized silver flakes, both material and process costs can be reduced, making these inks attractive for industrial applications. However, the sintering of flake inks requires higher temperatures than nano-sized inks owing to the particles’ smaller surface area-to-volume ratio, meaning that when cured thermally the conductivity of many flake inks is lower than nanoparticle alternatives. This problem can be addressed by the application of visible light photonic curing; however, the substrate must be protected and so process parameters must be defined for each material/substrate combination. Here, we report results of a large-scale trial of photonic curing of aqueous flake silver inks on poly(ethylene terephthalate) substrates in an industrial setting. The resistivity of printed patterns after an optimized photocuring regime matched those reported for typical nanoparticle inks; on the order of 100 μΩ cm depending on substrate and geometry. Scanning electron microscopy revealed evidence for structural changes within the printed films consistent with localized melting and necking between adjacent particles, leading to an improved percolation network. Furthermore, in the large-scale industrial trial employing screen-printed silver lines, the manufacturing yield of conductive lines was increased from 44% untreated to 80% after photocuring and reached 100% when photocuring was combined with thermal curing. We believe this to be the first reported observation of an increase in the yield of printed electronic structures following photocuring. We propose a crack-healing mechanism to explain these increases in yield and conductivity. We further report on the effects of the photonic curing on the mechanical bending stability of the printed conductors and discuss their suitability for wearable applications.

    A McGlashon, KS Whitehead, DDC Bradley, AJ Campbell, W Zhang, M Heeney, DM Smilgies, M Shkunov, K Genevicius, A Amassian, GG Malliaras, A Amassian (2017)Spectroscopic and morphological investigation of conjugated photopolymerisable quinquethiophene liquid crystals, In: Current Applied Physics

    3′-methyl-(5,5′′-bis[3-ethyl-3-(6-phenyl-hexyloxymethyl)-oxetane])-2,2′:5′,2′′-terthiophene (5T(Me)Ox) is a solution processable small molecule semiconductor displaying smectic-C and nematic liquid crystal phases. The pendant oxetane group can be polymerized in situ in the presence of a suitable photoacid at concentrations ≥1% by weight. Spin-coated films of pure 5T(Me)Ox and 5T(Me)Ox doped with the soluble photoacid were characterized by absorption and photoluminescent spectroscopy. Thick pristine films showed absorption and emission from a crystalline phase. Thin monolayer (

    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.

    GP Rigas, MM Payne, JE Anthony, PN Horton, FA Castro, M Shkunov (2016)Spray printing of organic semiconducting single crystals, In: Nature Communications7

    Single-crystal semiconductors have been at the forefront of scientific interest for more than 70 years, serving as the backbone of electronic devices. Inorganic single crystals are typically grown from a melt using time-consuming and energy-intensive processes. Organic semiconductor single crystals, however, can be grown using solution-based methods at room temperature in air, opening up the possibility of large-scale production of inexpensive electronics targeting applications ranging from field-effect transistors and light-emitting diodes to medical X-ray detectors. Here we demonstrate a low-cost, scalable spray-printing process to fabricate high-quality organic single crystals, based on various semiconducting small molecules on virtually any substrate by combining the advantages of antisolvent crystallization and solution shearing. The crystals’ size, shape and orientation are controlled by the sheer force generated by the spray droplets’ impact onto the antisolvent’s surface. This method demonstrates the feasibility of a spray-on single-crystal organic electronics.

    A Intaniwet, JL Keddie, M Shkunov, PJ Sellin (2011)High charge-carrier mobilities in blends of poly(triarylamine) and TIPS-pentacene leading to better performing X-ray sensors, In: Organic Electronics12(11)pp. 1903-1908 Elsevier

    A new class of X-ray sensor – in which there is a blend of poly(triarylamine) (PTAA) and 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene in the active layer of a diode structure – has been developed. The crystalline pentacene provides a fast route for charge carriers and leads to enhanced performance of the sensor. The first time-of-flight charge-carrier mobility measurement of this blend is reported. The mobility of PTAA and TIPS-pentacene in a 1:25 molar ratio was found to be 2.2 × 10−5 cm2 V−1 s−1 (averaged for field strengths between 3 × 104 and 4 × 105 V cm−1), which is about 17 times higher than that obtained in PTAA over the same range of field strengths. This higher mobility is correlated with a fourfold increase in the X-ray detection sensitivity in the PTAA:TIPS-pentacene devices.

    S Wood, G-P Rigas, A Zoladek-Lemanczyk, JC Blakesley, S Georgakopoulos, M Mas-Torrent, M Shkunov, FA Castro (2016)Precise Characterisation of Molecular Orientation in a Single Crystal Field-Effect Transistor Using Polarised Raman Spectroscopy, In: SCIENTIFIC REPORTS6ARTN 33057 NATURE PUBLISHING GROUP
    Abhishek Sahu, Peter Aaen, Arkadiusz Lewandowski, Maxim Shkunov, Greg P. Rigas, Paul T. Blanchard, Thomas M. Wallis, Vijay Devabhaktuni (2017)Robust Microwave Characterization of Inkjet Printed Coplanar Waveguides on Flexible Substrates, In: IEEE Transactions on Instrumentation & Measurement66(12)pp. 3271-3279 IEEE

    In this paper, we propose a robust microwave characterization of inkjet printed components on flexible substrates, which aim at measuring the material properties of silver nanoparticle inks and the supporting dielectric spacer employed during measurements. Starting with propagation constant extracted from multiline thru-reflect-line calibration with coplanar waveguide (CPW) standards and then proceeding with finite element modeling of CPWs, the proposed technique can dynamically produce an interpolated search space by automatic driving of simulation tools. In the final stage, the algorithm utilizes a least-square optimization routine to minimize the deviation between model and measurements. Our technique significantly reduces the computing resources and is able to extract the material parameters using even a nominal ink profile. Characteristic impedances for CPWs are extracted using series resistor measurements from 10 MHz to 20 GHz. It is also shown that the proposed characterization methodology is able to detect any changes in material properties induced by changes in fabrication parameters such as sintering temperature. Ink conductivities of approximately 2.973×10^7 S/m and spacer dielectric constant of 1.78 were obtained for the inkjet printed CPWs on PET. In addition, the inkjet printed CPWs sintered at 170°C and 220°C on Kapton had conductivities of 0.187×10^7 and 0.201×10^7 S/m respectively. We verified our technique by measuring the material parameters with conventional approach.

    Y Miyajima, M Shkunov, SRP Silva (2009)Amorphous carbon and carbon nitride bottom gate thin film transistors, In: APPLIED PHYSICS LETTERS95(10)ARTN 10210 AMER INST PHYSICS
    C Opoku, KF Hoettges, MP Hughes, V Stolojan, SRP Silva, M Shkunov (2013)Solution processable multi-channel ZnO nanowire field-effect transistors with organic gate dielectric, In: Nanotechnology24(40)pp. 405203-4(7pp)

    The present work focuses on nanowire (NW) applications as semiconducting elements in solution processable field-effect transistors (FETs) targeting large-area low-cost electronics. We address one of the main challenges related to NW deposition and alignment by using dielectrophoresis (DEP) to select multiple ZnO nanowires with the correct length, and to attract, orientate and position them in predefined substrate locations. High-performance top-gate ZnO NW FETs are demonstrated on glass substrates with organic gate dielectric layers and surround source-drain contacts. Such devices are hybrids, in which inorganic multiple single-crystal ZnO NWs and organic gate dielectric are synergic in a single system. Current-voltage (I-V) measurements of a representative hybrid device demonstrate excellent device performance with high on/off ratio of 10^7, steep subthreshold swing (s-s) of 400 mV/dec and high electron mobility of 35 cm2 V-1 s-1 in N2 ambient. Stable device operation is demonstrated after 3 months of air exposure, where similar device parameters are extracted including on/off ratio of 4x10^6, s-s 500 mV/dec and field-effect mobility of 28 cm2 V-1 s-1. These results demonstrate that DEP can be used to assemble multiples of NWs from solvent formulations to enable low-temperature hybrid transistor fabrication for large-area inexpensive electronics.

    Maxim Shkunov (2016)Source-Gated Transistors Based on Solution Processed Silicon Nanowires for Low Power Applications, In: Advanced Electronic Materials3(1)1600256 Wiley

    The solution-based assembly of field-effect transistors using nanowire inks, processed at low temperatures, offers an enormous potential for low power applications envisioned for the “Internet of Things,” including power management sensor circuits and electronics for in vivo bioimplants. Such low-temperature assembly, however, yields substantial contact potential barriers, with limited capacity for high current applications. In this study, the Schottky effect in a specific transistor configuration is utilized to achieve much reduced power consumption, with low saturation voltages (≈1 V), with relatively thick 230-nm SiO2 dielectrics. These source-gated transistors (SGTs) employ solution-deposited silicon nanowire arrays. A range of metal electrode work functions are investigated as device contacts and SGT operation is realized only in the structures with high source contact barriers. Such devices show very early drain current pinch-off, abruptly saturating at low drain voltages. The authors show that drain-current modulation is achieved via the gate field acting on the source barrier and lowering it through image force effects. Activation energy measurements reveal gate-induced source barrier lowering of ≈3 meV V−1. Numerical simulations show excellent correlation with the experimental data. These features, coupled with flat current saturation characteristics, are ideal for a range of low power applications, including wearable electronics and autonomous systems.

    R Rawcliffe, M Shkunov, M Heeney, S Tierney, I McCulloch, A Campbell (2008)Organic field-effect transistors of poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[2,3-b] thiophene) deposited on five different silane self-assembled monolayers, In: CHEMICAL COMMUNICATIONS(7)pp. 871-873 ROYAL SOC CHEMISTRY
    MN Shkunov, W Gellermann, ZV Vardeny (1998)Amplified resonant Raman scattering in conducting polymer thin films, In: Applied Physics Letters7320pp. 2878-2880
    MN Shkunov, MC DeLong, ME Raikh, ZV Vardeny, AA Zakhidov, RH Baughman (2001)Photonic versus random lasing in opal single crystals, In: Synthetic Metals1161-3pp. 485-491