Edwin Mayes

Stable n-channel metal-semiconductor field effect transistors on ZnO films deposited using a filtered cathodic vacuum arc

We report on the properties of metal-semiconductor-field-effect-transistors (MESFETs) on ZnO films grown using the filtered cathodic vacuum arc (FCVA) technique. FCVA ZnO films deposited on a-plane sapphire at 200 °C showed good structural and electrical properties that improved further on annealing at 800 °C in oxygen, due to the formation of larger grains with lower inter-grain transport barriers. MESFETs with silver oxide and iridium oxide Schottky gates on these annealed films showed excellent long-term stability with low ideality factors (<1.3), low gate leakage, and channel mobilities up to 50 cm2/Vs that were unchanged with both age and stress testing.

Oxygen-deficient photostable Cu2O for enhanced visible light photocatalytic activity

Oxygen vacancies in inorganic semiconductors play an important role in reducing electron-hole recombination, which may have important implications in photocatalysis. Cuprous oxide (Cu2O), a visible light active p-type semiconductor, is a promising photocatalyst. However, the synthesis of photostable Cu2O enriched with oxygen defects remains a challenge. We report a simple method for the gram-scale synthesis of highly photostable Cu2O nanoparticles by the hydrolysis of a Cu(i)-triethylamine [Cu(i)-TEA] complex at low temperature. The oxygen vacancies in these Cu2O nanoparticles led to a significant increase in the lifetimes of photogenerated charge carriers upon excitation with visible light. This, in combination with a suitable energy band structure, allowed Cu2O nanoparticles to exhibit outstanding photoactivity in visible light through the generation of electron-mediated hydroxyl (OH˙) radicals. This study highlights the significance of oxygen defects in enhancing the photocatalytic performance of promising semiconductor photocatalysts.

Competitive Inhibition of the Enzyme-Mimic Activity of Gd-Based Nanorods toward Highly Specific Colorimetric Sensing of l-Cysteine

Gd-based nanomaterials offer interesting magnetic properties and have been heavily investigated for magnetic resonance imaging. The applicability of these materials beyond biomedical imaging remains limited. The current study explores the applicability of these rare-earth nanomaterials as nanozyme-mediated catalysts for colorimetric sensing of l-cysteine, an amino acid of high biomedical relevance. We show a facile solution-based strategy to synthesize two Gd-based nanomaterials viz. Gd(OH)3 and Gd2O3 nanorods. We further establish the catalytic peroxidase-mimic nanozyme activity of these Gd(OH)3 and Gd2O3 nanorods. This catalytic activity was suppressed specifically in the presence of l-cysteine that allowed us to develop a colorimetric sensor to detect this biologically relevant molecule among various other contaminants. This suppression, which could either be caused due to catalyst poisoning or enzyme inhibition, prompted extensive investigation of the kinetics of this catalytic inhibition in the presence of cysteine. This revealed a competitive inhibition process, a mechanism akin to those observed in natural enzymes, bringing nanozymes a step closer to the biological systems.

Degradation of black phosphorus is contingent on UV–blue light exposure

Layered black phosphorous has recently emerged as a promising candidate for next generation nanoelectronic devices. However, the rapid ambient degradation of mechanically exfoliated black phosphorous poses challenges in its practical implementation in scalable devices. As photo-oxidation has been identified as the main cause of degradation, to-date, the strategies employed to protect black phosphorous have relied upon preventing its exposure to atmospheric oxygen. These strategies inhibit access to the material limiting its use. An understanding of the effect of individual wavelengths of the light spectrum can lead to alternatives that do not require the complete isolation of black phosphorous from ambient environment. Here, we determine the influence of discrete wavelengths ranging from ultraviolet to infrared on the degradation of black phosphorous. It is shown that the ultraviolet component of the spectrum is primarily responsible for the deterioration of black phosphorous in ambient conditions. Based on these results, new insights into the degradation mechanism have been generated which will enable the handling and operating of black phosphorous in standard fabrication laboratory environments.Black phosphorous degradation: UV light contributes the most to photo-oxidationThe ultraviolet component of the light spectrum contributes significantly to the ambient degradation of ultra-thin black phosphorous. A team led by Sumeet Walia at RMIT University in Melbourne investigated the deterioration of layered black phosphorous under environmental conditions, upon exposure to individual wavelengths of light at progressive time durations. Morphological variations, indicative of material degradation, were found to be most prominent under exposure to 280 nm light, followed by 455 nm light. Conversely, longer wavelengths did not induce any discernible photo-oxidation. These results indicate that ultraviolet light is readily absorbed by black phosphorous resulting in a substantial decline of its electronic properties, whereas blue light causes less severe surface deterioration. An ultraviolet-deficient environment could therefore be instrumental to preventing black phosphorous photo-oxidation, and could be as effective as surface passivation by means of encapsulating layers.

Ultraviolet detection from graphitic-C/Zn1−xMgxO Schottky devices fabricated at moderate temperatures

Ultraviolet (UV) Schottky detector devices were fabricated on polycrystalline wurtzite Zn1−xMgxO films energetically deposited onto a-plane sapphire at room-temperature (RT) and 200 °C. The unintentionally doped, transparent, n-Zn1−xMgxO films exhibit low surface roughness (<5% of film thickness), moderate carrier concentration, and Hall mobility up to 15 cm2 V−1 s−1. The direct bandgaps of the RT and 200 °C films (x = 0.24 and x = 0.20) were 3.57 eV and 3.40 eV. Schottky diodes with graphitic anodes formed on these films exhibited barrier heights up to 0.88 eV and ideality factors as low as 1.97. Spectral response measurements demonstrated UV/visible photo-current ratios up to ∼104.

Co-deposition of band-gap tuned Zn1-xMgxO using high impulse power- and dc-magnetron sputtering

High impulse power- and direct current- magnetron sputtering have been used to reactively co-deposit Zn1-xMgxO onto a 100 mm diameter a-plane sapphire wafer at 200 C. The Zn1-xMgxO film exhibited low surface roughness, high transparency, high electrical resistivity and a Mg fraction (x) depending on substrate location. The optical bandgap of the film varied monotonically with x up to the miscibility limit of ∼0.32, beyond which a mixed cubic/wurtzite structure formed. Annealing at 550 C in forming gas (95% N2, 5% H2), caused reduced compressive stress and dramatically reduced electrical resistivity. The latter was attributed to shallow doping by hydrogen bound to oxygen vacancies and these changes occurred in the wurtzite Zn1-xMgxO without detectable phase transformation. A filtered UV detector, with active and filter layers fabricated from the co-deposited film, exhibited sensitivity to UV in a 330-355 nm pass-band and approximately three orders of magnitude UV-to-visible rejection.

The relationship between microstructure and electrical breakdown in cathodic arc deposited hafnium oxide films

Hafnium oxide films were deposited with a range of substrate temperatures using a filtered cathodic vacuum arc deposition system. The microstructure, electronic structure, and electrical breakdown of the films were characterized. In films deposited at temperatures above 200 °C, the microstructure became more ordered and x-ray diffraction indicated that the dominant phase was monoclinic hafnium oxide. Evidence for the presence of the tetragonal phase was also found in the films deposited at temperatures above 400 °C. The near edge structure of the oxygen K-edge measured using x-ray absorption spectroscopy, provided further evidence that films prepared at high temperatures contained a combination of the monoclinic and tetragonal phases. Films deposited at room temperature were disordered and exhibited the best electrical breakdown characteristics. The electrical breakdown of the films deteriorated as the crystallinity increased with increasing deposition temperature. These results support the proposition th...

A comparative study of the effect of submicron porous and smooth ultrafine‐grained Ti‐20Mo surfaces on osteoblast responses

The surface of an orthopaedic implant plays a crucial role in determining the adsorption of proteins and cell functions. A detailed comparative study has been made of the in vitro osteoblast responses to coarse-grained (grain size: 500 μm), ultrafine-grained (grain size: 100 nm), coarse-porous (pore size: 350 nm), and fine-porous (pore size: 155 nm) surfaces of Ti-20Mo alloy. The purpose was to provide essential experimental data for future design of orthopaedic titanium implants for rapid osseointegration. Systematic original experimental data was produced for each type of surfaces in terms of surface wettability, cell morphology, adhesion, growth, and differentiation. Microscopic evidence was collected to reveal the detailed interplay between each characteristic surface with proteins or cells. Various new observations were discussed and compared with literature data. It was concluded that the coarse-porous surfaces offered the optimum topographical environment for osteoblasts and that the combination of ultrafine grains and considerable grain boundary areas is not an effective way to enhance cell growth and osteogenic capacity. Moreover, pore features (size and depth) have a greater effect than smooth surfaces on cell growth and osteogenic capacity. It proves that cells can discern the difference in pore size in the range of 100-350 nm.

Data related to the nanoscale structural and compositional evolution in resistance change memories

The data included in this article provides additional supplementary information on our recent publication describing “Inducing tunable switching behavior in a single memristor” [1]. Analyses of micro/nano-structural and compositional changes induced in a resistive oxide memory during resistive switching are carried out. Chromium doped strontium titanate based resistance change memories are fabricated in a capacitor-like metal-insulator-metal structure and subjected to different biasing conditions to set memory states. Transmission electron microscope based cross-sectional analyses of the memory devices in different memory states are collected and presented.

Energetic deposition, measurement and simulation of graphitic contacts to 6H-SiC

Junctions between energetically deposited graphitic carbon and n-type 6H-SiC have been fabricated. Their current-voltage characteristics have been measured and compared with simulations using Sentaurus TCAD finite element software. Agreement between the experimental and simulated current-voltage characteristics was achieved using parameters derived from electrical measurements and electron microscopy. Using the best-fit models, the effects of interfacial layers and contact work function variations were elucidated to provide guidance for improved device performance.