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Dr Richard Cross

Job: Senior Research Fellow

Faculty: Computing, Engineering and Media

School/department: School of Engineering and Sustainable Development

Research group(s): Emerging Technologies Research Centre (EMTERC)

Address: ذكذكتسئµ, The Gateway, Leicester, LE1 9BH, United Kingdom

T: +44 (0)116 250 6157

E: rcross@dmu.ac.uk

W: www.dmu.ac.uk/emterc

 

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Personal profile

Most of my research career has been focused on the growth of thin films for electronic devices such as thin film transistors (TFTs); mainly hydrogenated amorphous silicon and zinc oxide. My main interests now lie in the novel growth of nanoscale materials. Of particular interest going forward will be exploring the nano-bio interface, i.e. interfacing directly nanoscale materials/devices with biological systems. Examples would be neuronal interfaces and structures for intracellular recording/biomarker detection.

Research group affiliations

Emerging Technologies Research Centre (EMTERC)

Publications and outputs


  • dc.title: Porous surfaces: stability and recovery of coronaviruses dc.contributor.author: Owen, Lucy; Shivkumar, Maitreyi; Cross, R. B. M.; Laird, Katie dc.description.abstract: The role of indirect contact in the transmission of SARS-CoV-2 is not clear. SARS-CoV-2 persists on dry surfaces for hours to days; published studies have largely focused on hard surfaces with less research being conducted on different porous surfaces, such as textiles. Understanding the potential risks of indirect transmission of COVID-19 is useful for settings where there is close contact with textiles, including healthcare, manufacturing and retail environments. This article aims to review current research on porous surfaces in relation to their potential as fomites of coronaviruses compared to non-porous surfaces. Current methodologies for assessing the stability and recovery of coronaviruses from surfaces are also explored. Coronaviruses are often less stable on porous surfaces than non-porous surfaces, for example, SARS-CoV-2 persists for 0.5 h–5 days on paper and 3–21 days on plastic; however, stability is dependent on the type of surface. In particular, the surface properties of textiles differ widely depending on their construction, leading to variation in the stability of coronaviruses, with longer persistence on more hydrophobic materials such as polyester (1–3 days) compared to highly absorbent cotton (2 h–4 days). These findings should be considered where there is close contact with potentially contaminated textiles. dc.description: open access article

  • dc.title: Low temperature (<150 آ°C) hydrogenated amorphous silicon grown by PECVD with source gas heating dc.contributor.author: Cross, R. B. M. dc.description.abstract: Hydrogenated amorphous silicon (a-Si:H) is a semiconductor that is widely used in a variety of applications. A particularly important development has been the incorporation of this material into thin film transistor (TFT) arrays for the active matrix addressing of liquid crystal displays. Plasma Enhanced Chemical Vapour Deposition (PECVD) is one of the most successful techniques currently in use for the deposition of device quality a-Si:H. However, there is an increasing desire to improve process compatibility with low cost, plastic substrates. This entails trying to reduce the deposition temperature from approximately 250 - 300آ°C to below 150آ°C, whilst maintaining material quality. This thesis describes the design of a novel, low temperature PECVD system incorporating the facility to pre-heat the deposition source gases. The physical and electronic properties of a-Si:H deposited at <150آ°C are investigated, and the performance of TFT structures incorporating optimised material as the active layer is described. It is shown that the physical properties of a-Si:H produced at a substrate temperature of 125آ°C with the source gas line heated to 400 آ°C are commensurate with films deposited at 250-300 QC. The hydrogen content of the optimised film was found to be 10.5 %, with a Tauc bandgap of 1.66 e V. Pre-heating of the source gases also leads to an increase in the proportion of hydrogen bonded in the monohydride configuration. It is suggested that the diffusion of the film-forming gaseous species is enhanced by this technique, resulting in a reduction in the degree of disorder within the film and hydrogen elimination. Consequently, the concentration of hydrogen and the Tauc bandgap also decrease, leading to an increase in photoconductivity of one order of magnitude. TFTs exhibit a switching ratio of 1 Os, which is approximately an order of magnitude smaller than high temperature a-Si:H TFTs, but a comparable OFF current of approximately 10.12 A. However, the field effect mobility of these devices is very poor (10.3 cm2Vآ·l sآ·I). This is thought to be due to a high interface state density at the boundary between the low temperature, gas-heated a-Si:H layer and the high temperature silicon nitride gate insulator.

  • dc.title: The Influence of ZnO Layer Thickness on the Performance and Electrical Bias Stress Instality in ZnO Thin Film Transistors dc.contributor.author: Ngwashi, D. K.; Mih, T. A.; Cross, R. B. M. dc.description.abstract: Thin Film Transistors (TFTs) are the active elements for future large area electronic applications, in which low cost, low temperature processes and optical transparency are required. Zinc oxide (ZnO) thin film transistors (TFTs) on SiO2/n+-Si substrate are fabricated with the channel thicknesses ranging from 20 nm to 60 nm. It is found that both the performance and gate bias stress related instabilities of the ZnO TFTs fabricated were influenced by the thickness of ZnO active channel layer. The effective mobility was found to improve with increasing ZnO thickness by up to an order in magnitude within the thickness range investigated (20 – 60 nm). However, thinner films were found to exhibit greater stability in threshold voltage and turn-on voltage shifts with respect to both positive and negative gate bias stress. It was also observed that both the turn on voltage (Von) and the threshold voltage (VT) decrease with increasing channel thickness. Moreover, the variations in subthreshold slope (S) with ZnO thickness as well as variations in VT and Von suggest a possible dependence of trap states in the ZnO on the ZnO thickness. This is further correlated by the dependence of VT and Von instabilities with gate bias stress. dc.description: University of Buea supported the first author during the writing of this manuscript Open access article

  • dc.title: Understanding the effects of a polymer on the surface dissolution of pharmaceutical cocrystals using combined experimental and molecular dynamics simulation approaches dc.contributor.author: Li, M.; Kirubakaran, P.; Wang, Ke; Rosbottom, I.; Cross, R. B. M. dc.description.abstract: The molecular interactions between the surfaces of cocrystals [i.e., flufenamic acid and theophylline (FFA-TP), flufenamic acid and nicotinamide (FFA-NIC), and carbamazepine and nicotinamide (CBZ-NIC)] and the polymers [i.e., polyethyleneglycol (PEG), polyvinylpyrrolidone (PVP) and copolymer of vinylpyrrolidone (60%)/vinyl acetate (40%) (PVP-VA)] were investigated through combined experimental and molecular dynamics simulation approaches to resolve the mechanisms of cocrystal dissolution and precipitation. It was found that adsorption of the polymers on the surfaces of cocrystals might prevent the precipitation of the parent drug and alter the dissolution rate. The effect of polymers on precipitation could be determined by the cocrystal dissolution rate, the interactions of polymers with the surfaces of cocrystals, the characters of the noncovalent bonds formed between the polymers and the cocrystal surfaces, and the mobility and conformation of the polymers. The etching experiments of single cocrystals revealed that FFA-NIC and CBZ-NIC appeared as surface precipitation cocrystals while FFA-TP could lead to bulk precipitation. Both PVP and PVP-VA were good precipitation inhibitors for FFA-NIC and they could completely inhibit the recrystallization of FFA III on the surfaces of dissolving cocrystals. In addition, as the adsorption of the polymer was slower than dissolution rate of the cocrystals, PVP and PVP-VA could only partially inhibit the recrystallization of CBZ dihydrate on the surface of CBZ-NIC. While PEG had no inhibitory effect on the surface crystallization of FFA-NIC and CBZ-NIC, due to its weak interactions with the surfaces of the cocrystals, it enhanced the dissolution performance of FFA-TP. In contrast, PVP and PVP-VA reduced the dissolution rate of FFA-TP and subsequently undermined the performance of cocrystals. Taken together, the approach of combining experimental and molecular dynamics simulation provided insights into the mechanisms of cocrystal dissolution as well as the polymers acting as inhibitory excipients for precipitation/recrystallisation, making contribution to the development of novel formulations. dc.description: The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.

  • dc.title: Electrical Performance and Stability of ZnO Thin-Film Transistors Incorporating Gadolinium Oxide High-k Dielectrics dc.contributor.author: Ngwashi, Divine K.; Paul, Shashi; Devi, Anjana; Cross, R. B. M. dc.description.abstract: This work investigates the performance and gate bias stress instability of ZnO-based thin film transistors (ZnO-TFTs) incorporating amorphous gadolinium oxide, a high-k dielectric material. ZnO thin films produced via radio frequency (RF) reactive magnetron sputtering were used as channel layers. The source/drain electrodes were achieved by the thermal evaporation of aluminium on a bottom gate inverted staggered ZnO TFT structure. Gadolinium oxide (Gd2O3) deposited by metal-organic chemical vapour deposition (MOCVD) served as the gate dielectric. The electrical characterisation of the ZnO-TFTs produced showed improvement in performance and stability in comparison to thermally-grown SiO2-based ZnO TFTs fabricated under the same conditions. The effective channel mobility, on-off current ratio and subthreshold swing of the TFTs incorporating Gd2O3 dielectric were found to be 33.5 cm2 V-1s-1, 107, and 2.4 V/dec respectively when produced. The electrical characterisation of the same devices produced with SiO2 dielectrics exhibited effective mobility, on-off current ratio and subthreshold swing of 7.0 cm2 V-1s-1, 106 and 1.4 V/dec respectively. It is worth noting that, the ZnO active layer was sputtered under room temperature with no intentional heating and post-deposition annealing treatment. On application of gate bias stressing on these thin film transistors, it was observed that threshold voltage instability increased with stress period in all device types. Transistors incorporating Gd2O3 however, were found to exhibit lesser threshold voltage related instability with regards to gate bias stressing in comparison to similar devices incorporating SiO2 as gate dielectric. It was also observed that the effective mobility in both devices tend to stabilize with prolonged gate bias application. In this work, it is demonstrated that Gd2O3 dielectric is a potential alternative to SiO2 for the fabrication of ZnO TFTs with improved performance and electrical stability under prolonged use. dc.description: open access article

  • dc.title: Nitridation of optimised TiOâپ 2 nanorods through PECVD towards neural electrode application dc.contributor.author: Sait, R. A.; Govindarajan, S.; Cross, R. B. M. dc.description.abstract: A neural electrode interface material is a key component for effective stimulation and recording of neuralactivity. The fundamental requirement of a neural electrode is for it to be able to deliver adequate charge to targeted neuronal population. Coating electrode surfaces with nanostructured material not only provides an increase in surface area, providing relatively more active sites for charge delivery than planar systems, but also allows for the reduction of electrode dimension to reduce invasiveness and increase selectivity. In this work, titanium nitride nanowires (TiN-NWs) synthesised by novel nitridation process in Plasma Enhanced Chemical Vapour Deposition (PECVD) is suggested as an enhanced coating material for neural electrodes. The synthesis involved the solution growth of crystalline titanium oxide nanorods (TiOâپ 2-NRs) from a sputtered TiN nucleation layer followed by nitridation. TiOâپ 2-NRs exhibited high aspect ratio of 23.1 and were converted into TiN after one hour of nitridation at 600آ°C. Evidence of conversion was studied by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Transmission electron microscopy (TEM). The nitridation temperature and time reported here are the lowest and shortest as compared to the literature. The near-stoichiometric TiN-NWs (x=0.49) achieved in this work were used subsequently for electrochemical characterisation through Cyclic Voltammetry (CV). The capacitance of relatively high aspect TiN-NWs was 3.78mF/cmâپ 2âپ , which was a 5-fold enhancement compared to thin film of TiN layer (0.7 mF/cmâپ 2). A stability test of the nanowires were performed in which the capacitance remained relatively unchanged. dc.description: The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.

  • dc.title: A multi-faceted approach to determining the efficacy of metal and metal oxide nanoparticles against bacterial biofilms dc.contributor.author: Tejpal, Jyoti; Cross, R. B. M.; Owen, Lucy; Paul, Shashi; Jenkins, R. O.; Armitage, David; Laird, Katie dc.description.abstract: Antibacterial efficacy of nanoscale silver, copper (II) oxide and zinc oxide were assessed against Pseudomonas aeruginosa and Staphylococcus aureus biofilms in solution and on surfaces. Using a Center for Disease Control biofilm reactor, minimum biofilm reduction concentrations, the coefficient of determination (R2) and log(10) reductions were determined. Atomic absorption spectroscopy, scanning electron microscopy and confocal laser scanning microscopy were used to assess the disruption of the biofilms. The efficacy of thin films of zinc oxide and silver deposited via magnetron sputtering and thermal evaporation respectively was also assessed. Minimum biofilm reduction concentrations of zinc oxide or silver nanoparticles were 256 or 50 آµg/ml for P. aeruginosa and 16 or50 آµg/ml for S. aureus respectively. When tested in combination the nanoparticles concentrations were at least halved resulting in significant (p ≤0.05) biofilm reductions of 3.77 log(10) - 3.91 log(10). Biofilm growth on thin films resulted in reductions of up to 1.82 log(10). The results suggest that nanoparticle suspensions and thin films of zinc oxide and may have potential as antimicrobial treatments for hard to eliminate biofilms in a clinical environment.

  • dc.title: Design and characterization of a diamond-shaped monopole antenna dc.contributor.author: Maricar, Mohamed Ismaeel; Bahar, A.; Greedy, S.; Phang, S.; Gradoni, G.; Cross, R. B. M.; Creagh, S.; Tanner, G.; Thomas, D. W. P. dc.description.abstract: The resonant frequency of a diamond shaped monopole antennas is analysed by using a finite difference time domain method. The diamond shaped monopole antennas was fabricated and tested experimentally on RT Duroid material and then the results were directly compared with simple monopole antennas., Tthe work shows that the diamond shaped monopole antennas has smaller dimensions, higher bandwidth and gain at millimeteric wave frequency . Experimentally measured diamond shaped monopole antennas has good agreement with simulated results. dc.description: The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.

  • dc.title: Facile Preparation of Drug-Loaded Tristearin Encapsulated Superparamagnetic Iron Oxide Nanoparticles Using Coaxial Electrospray Processing dc.contributor.author: Rasekh, M.; Ahmad, Z.; Cross, R. B. M.; Hernandez-Gil, Javier; Wilton-Ely, James D. E. T.; Miller, Philip W. dc.description: The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.

  • dc.title: Synthesis and characterization of sputtered titanium nitride as a nucleation layer for novel neural electrode coatings dc.contributor.author: Sait, R. A.; Cross, R. B. M. dc.description.abstract: A growing demand for chronically implantable electrodes has led to a search for the most suitable neural electrode interface material. Nobel metals such as Platinum (Pt) are inadequate for electrode/neuron interfaces at small scales due to their poor electrochemical properties, low charge injection and high charge density per unit area. Titanium Nitride (TiN) has been implemented in neural electrodes application due to its outstanding properties. In this work, TiNx films were deposited by non-reactive Radio Frequency (RF) magnetron sputtering towards the development of a novel TiN nanowires (NWs) neural interface. Although, there is substantial work on this material, its growth using non- reactive RF magnetron sputtering has not been reported previously and optimised towards the growth of TiN NWs and their use in neural interface applications. The sputtering parameters of RF power and Argon (Ar) flow rate were varied in order to investigate their effects on the structural, electrical and electrochemical properties of the TiN films. A dense film morphology was observed in the Scanning Electron Microscopy (SEM) images of TiN thin films showing a columnar structure. The film preferential orientation was changed between (200) and (111) with Ar flow rate due to the variation of the kinetic energy (KE) of the sputtered atoms. The crystallites size obtained were in the range of 13 to 95 nm. Surface roughness was found to increase from 0.69 to 1.95 nm as Ar flow rate increased. TiNx films showed a good electrical resistivity of 216 آµâ„¦.cm. Stoichiometry was found to vary with sputtering conditions in which the nitrogen content was found to deplete from the film at low Ar flow rate. The electrochemical behaviour of TiN films was characterised and the highest capacitance value obtained was 0.146 mF/cm^2 dc.description: The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.

Research interests/expertise

Nanoscale materials (nanowires, nanotubes) for health applications

Exploring the nano-bio interface

Large area electronic materials

Thin films

TFTs

Areas of teaching

Analysis of micro and nanoscale materials

Growth/deposition of electronic materials

Fabrication of micro and nanoelectronic devices

Qualifications

BSc

MSc

PhD

Courses taught

Electrical and Physical Measurements (ENGT5129)

Fabrication Technologies (ENGT5127)

Membership of professional associations and societies

01/09-date: Material Research Society (full member)

Projects

RIF project: A Cleaner, Greener, Low Carbon Fabrication Process for Photovoltaic (PV) Solar Cells
Start Date: 01/04/10
End Date: 01/07/10
Role: Co-Worker
PI: Shashi Paul
Project Aims:
• To demonstrate, for the first time, a photovoltaic solar cell using a novel low temperature, high growth rate, and low-cost process (developed within EMTERC) based on thin films of silicon.
• Approach industries with a view to the commercialisation of the technology.
• To develop new and enhance existing collaborative links with major players in the field of PV both in the UK and Europe.

RIF project: Enhancing IR Micro Sensor Technology To More Accurately Measure the Temperature Profile of Electronic Devices
Start Date: 01/04/10
End Date: 01/07/10
Role: Co-Worker
PI: Chris Oxley
Project Aims:
• The objective of the work was to explore the feasibility of moving controllably a single characterised IR micro sensor in micron steps along the channel of a GaN transistor to measure more accurately the temperature profile of the channel region of the device.

Consultancy work

Area of expertise: Analysis of micro/nano materials

Previous consultancies: Analysis of optical coating for CVI Melles Grigot

Currently available: Yes

Current research students

Second supervisor for:

Keith McGrath
Sultan Alotaibi
Faleh Alotaibi

Professional esteem indicators

Reviewer for:

Applied Physics Letters
AIP Advances
Journal of Applied Physics
IEEE Transactions on Electron Devices
IEEE Electron Device Letters
IEEE Micro and Nano Letters
Journal of Physical Chemistry
Journal of the Electrochemical Society
Solid State Communications
Solid State Electronics
Materials Science and Engineering B
Electrochemical and Solid State Letters
Electronics and Telecommunications Research Institute Journal
Journal of Vacuum Science and Technology A

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