Thierry G.G. Maffeis

• Doctor of Engineering
• Professor (Associate) at Swansea University

Optimised ZnO nanorod characteristics are essential for novel devices to operate efficiently, especially shifting the n-type nature towards intrinsic or p-type. The effects of argon bombardment for varying amounts of time on the surface chemistry and Fermi level of ZnO nanorods have been studied using XPS. Bombardment at 5 keV removed surface conta...

Metal oxide semiconductors such as ZnO have attracted much scientific attention due their material and electrical properties and their ability to form nanostructures that can be used in numerous devices. However, ZnO is naturally n-type and tailoring its electrical properties towards intrinsic or p-type in order to optimise device operation have pr...

Optimised ZnO nanorods characteristics are essential for novel devices to operate efficiently. The effects of vacuum annealing on the electrical transport properties and defect chemistry of ZnO nanorods have been studied. Annealing to 500 °C removed surface contamination causing reduced resistance while annealing to 600 °C created acceptor defects,...

ZnO nanosheets are polycrystalline nanostructures that are used in devices including solar cells and gas sensors. However, for efficient and reproducible device operation and contact behaviour the conductivity characteristics must be controlled and surface contaminants removed. Here we use low doses of argon bombardment to remove surface contaminat...

Zinc oxide nano-wires (ZnO NWs) are synthesized reproducibly with high yield via a low temperature hydrothermal technique. The influence of the growth duration time, growth temperature, zinc precursor and base concentration of Na2CO3 on the morphology of NWs is investigated. The growth products are characterised using scanning electron microscopy (...

Highly sensitive gold nanoparticle decorated zinc oxide nanosheet gas sensors have been fabricated using simple and rapid chemical methods capable of producing a normalised current gain of 2.54 (at 10V) in dry air containing 2.5ppm of hydrogen gas at 200C and a current gain of 382.53 under 125ppm. Compared to undecorated sheet based sensors where a...

Background: The rapid production and incorporation of engineered nanomaterials into consumer products alongside research suggesting nanomaterials can cause cell death and DNA damage (genotoxicity) makes in vitro assays desirable for nanosafety screening. However, conflicting outcomes are often observed when in vitro and in vivo study results are c...

Herein, we experimentally test a mathematical model of the reactions on the surface of a zinc oxide nanosheet-based carbon monoxide sensor. The carbon monoxide is assumed to react with surface oxygen via an Eley–Rideal mechanism, considering only the direct reaction between the two species. We demonstrate that the measured resistance responses of t...

ZnO nanorods are used in devices including field effects transistors, piezoelectric transducers, optoelectronics and gas sensors. However, for efficient and reproducible device operation and contact behaviour, surface contaminants must be removed or controlled. Here we use low doses of argon bombardment to remove surface contamination and make repr...

The ability to control the properties of electrical contacts to nanostructures is essential to realize operational nanodevices. Here, we show that the electrical behavior of the nanocontacts between free-standing ZnO nanowires and the catalytic Au particle used for their growth can switch from Schottky to Ohmic depending on the size of the Au parti...

Endometrial stromal and epithelial cell function is typically studied in vitro using standard two-dimensional monocultures, but these cultures fail to reflect the complex three-dimensional (3D) architecture of tissue. A 3D model of bovine endometrium that reflects the architectural arrangement of in vivo tissue would beneficially assist the study o...

ZnO is a wide bandgap semiconductor that has many potential applications including solar cell electrodes, transparent thin film transistors and gas/biological sensors. Since the surfaces of ZnO materials have no amorphous or oxidised layers, they are very environmentally sensitive, making control of their semiconductor properties challenging. In pa...

We demonstrate here a method using a multi-probe UHV instrument to isolate and measure individual metal contacts controllably fabricated on the tips of free standing ZnO nanowires (NWs). The measurements show Au can form reliable Ohmic and rectifying contacts by exercising control over the surface properties. In the as-grown state the Au contacts d...

The ectoparasitic copepod, Nicothoë astaci (the 'lobster louse'), infests the gills of the European lobster, Homarus gammarus. There have been limited studies on this haematophagous species; therefore knowledge of this parasite is rudimentary. The current study examines the surface morphology of this parasitic copepod, detached from the host, conce...

Vertical arrays of nanostructures have been widely used as major components in some of the most ground-breaking modern research-based devices, and ZnO nanowires have received particular attention because of their favorable electronic properties. Using a local multiprobe technique to measure the properties of individual ZnO nanowires in vertical arr...

We have developed a low-cost technique using a conventional microwave oven to grow layered basic zinc acetate (LBZA) nanosheets (NSs) from a zinc acetate, zinc nitrate and HMTA solution in only 2 min. The as-grown crystals and their pyrolytic decomposition into ZnO nanocrystalline NSs are characterized by scanning electron microscopy (SEM), energy-...

Metarhizium anisopliae, a fungal pathogen of terrestrial arthropods, kills the aquatic larvae of Aedes aegypti, the vector of dengue and yellow fever. The fungus kills without adhering to the host cuticle. Ingested conidia also fail to germinate and are expelled in fecal pellets. This study investigates the mechanism by which this fungus adapted to...

ZnO nanowire (NW) arrays were examined with Transmission Electron Microscopy (TEM) in cross-section after preparation by Focused Ion Beam (FIB) milling. This technique revealed that ZnO nanowires grown using a Au catalyzed vapor technique typically have Au particles at the NW tips, and also randomly dispersed across the base crystal growth that joi...

If the silicon industry is to successfully integrate ZnO nanowires (NWs) into existing devices to fully utilise the piezoelectric or optical properties of ZnO NWs, then a detailed understanding of the effect of metal interconnects on the morphology of the NWs during growth needs to be obtained. In this study, ZnO NWs were hydrothermally grown at 90...

The cuticle morphology of the American lobster Homarus americanus is a highly studied area, however its European counterpart, Homarus gammarus is not so well examined. Recent studies show a difference in diseases found between the species, as well as reports of H. americanus as an invasive species in European waters. This study looked into the diff...

In this study, the use of flexographic printing was investigated for low cost, high volume production of devices incorporating nanowires through the printing of zinc acetate precursors on a substrate used to form zinc oxide (ZnO) seeds for the growth of nanowires using a hydrothermal growth technique. The printing of precursors allows the selective...

A quantitative method to measure the reduction in oxide species on the surface of electrochemically etched tungsten tips during direct current annealing is developed using energy dispersive x-ray spectroscopy. Oxide species are found to decrease with annealing current, with the trend repeatable over many tips and along the length of the tip apex. A...

Zinc oxide nanowires have been synthesized hydrothermally using varying initial growth parameters, namely concentrations of precursors and temperature. SEM was then used to characterize the shape and size of the nanowires. PL was also implemented to examine the effects of altering the growth conditions on the defect band of the zinc oxide nanowires...

The rapid growth of the nanotechnology industry increas- ingly demands the use of in-vitro toxicology assays for cost effective safety assessment. The development of a 3D micro- nucleus assay utilising EpiDermTM tissue is therefore of key interest as human skin represents a key barrier/exposure route to environmental and occupational nanomaterial e...

The hydrothermal growth of ZnO nanowires on different surfaces including Gold (Au), Nickel (Ni), Silicon (Si) and Silicon Nitride (Si3N4) on a same silicon substrate is presented. It is shown that the growth rate of the nanowires is reduced on the metal surfaces compared to the Si and Si3N4 surfaces. The contamination of the growth solution by meta...

The pyrolytic decomposition of layered basic zinc acetate (LBZA) nanobelts (NBs) into nanocrystalline ZnO NBs is investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). We also report on the gas sensing response of the resulting...

The surface properties of vertically aligned ZnO nanowires grown by chemical vapour deposition on GaN using a gold layer as a catalyst are investigated by X-ray Photoelectron Spectroscopy as a function of annealing temperature in ultra high vacuum (UHV). The nanowires are 8.5 mum long and 60 nm wide. 87% of the surface carbon content was removed af...

We investigate the use of energy delivery using microwave radiation for both synthesis of nanoparticles as well as a hybrid imaging technique known as thermoacoustic tomography (TAT). In each instance, the absorption of microwave radiation is converted into heat. In the case of nanoparticle synthesis, water is used as the solvent and heated to indu...

With the rapid expansion in the nanotechnology industry, it is essential that the safety of engineered nanomaterials and the factors that influence their associated hazards are understood. A vital area governing regulatory health risk assessment is genotoxicology (the study of genetic aberrations following exposure to test agents), as DNA damage ma...

We present an AFM and STM-STS investigation of the surface of ZnO nanobelts grown by chemical vapour deposition.AFM images showed a type 1 (high aspect ratio) nanobelt lying across a type 2 (low aspect ratio) nanobelt, bending at an angle of 20.9° without breaking.Terraces 10 atomic layer thick were also observed, with step edges running along the...

The formation of the Ni/Al0.2Ga0.8N Schottky contacts has been investigated by x-ray photoelectron spectroscopy. In situ scanning tunneling microscopy was used in parallel to investigate the morphology of the Ni covered surface after the last deposition. In the same way, results are presented through two perspectives: the intensity of core-level si...

Au/Cr Ohmic contacts on p-type, heavily boron-doped, single-crystal CVD diamond have been studied using X-ray photoelectron spectroscopy (XPS) and electrical measurements. The interaction of chromium, a carbide-forming metal, with the diamond surface is discussed. The Cr/diamond contact formation process has been studied as a function of chromium t...

The investigation of the silicon carbide surface after a sacrificial silicon oxidation technique is reported. Oxidation of SiC is a necessary step in the fabrication of MOS devices and device termination features such as field plates. Device processing requires the etching of windows through the oxide layer to form features such as metal / SiC cont...

Cross-sectional scanning tunneling microscopy and spectroscopy have been used to probe the unreconstructed (1 1 0) surface of a commercially available buried heterostructure laser in ultra high vacuum. Complex re-growth above the non-linear blocking layers is shown to induce tensile strain in the device. Spectroscopic measurements show an increase...

Atomic force and high resolution scanning tunneling analyses were carried out onnanostructured WO3 films. It turned out that the band gap measured by scanning tunnelingspectroscopy at surface is lower than the band gap reported in the literature. This effect isattributed to the high density of surface states in this material, which allows tunneling...

We developed a model for n-type metal-oxide semiconductors, which allows one to calculate the density of charged surface states on nanostructured grains, once the Schottky barrier height is known. We characterised structurally and electrically two sets of polycrystalline SnO2 films with average grain radius of 30 and 10 nm. The purpose of this expe...

We have investigated the effect of O2 and reducing gases (CH4 and CO) exposure on nanocrystalline SnO2 in vacuum and at elevated temperatures (120°C) using three different techniques: X-ray photoelectron spectroscopy, in vacuum resistance measurements and scanning tunnelling microscopy and spectroscopy. XPS and resistance measurements showed that O...

In this work, we report on the ability to write features less than 15nm in size on nanocrystalline SnO2 by applying negative voltage pulses onto an STM tip. The fact that these features can be erased by scanning with a positive tip bias, and the strong dependence of the apparent height of the features with scanning bias after writing seems to indic...

An initial report on an investigation of the interface between sacrificial silicon dioxide on SiC and a conventional thermally grown silicon dioxide layer on SiC is presented. The ellipsometry technique is used as a key tool in evaluating the quality and structure of the interface formed. Three samples from the same n-type 4H-SiC wafer were prepare...

In this paper we present two examples of how scanning probe microscopy (SPM) techniques can be used to correlate the electronic properties with the surface topography of electronic devices down to the nanometre range. There is an increasing need for such techniques as the size of devices shrinks continually and therefore nanometre-sized features be...

We collected high resolution scanning tunneling (STM) images on thin films of WO3. This provided information about the morphology and the nanostructure of the samples. The surface gap was then determined through scanning tunneling spectroscopy (STS) and was found to be much narrower than the bulk gap determined via photoluminescence. This effect is...

The influence of premetallization surface preparation on the structural, chemical, and electrical properties of Au–nGaN interfaces has been investigated by x-ray photoemission spectroscopy (XPS), current-voltage measurement (I-V) and cross-section transmission electron microscopy (TEM). XPS analysis showed that the three GaN substrate treatments in...

Gold Schottky contacts formed in situ on n-type GaN after a 600 °C anneal have been characterized by current-voltage (I-V) measurements and x-ray photoelectron spectroscopy (XPS). The mean Schottky barrier height and lowest ideality factor were found to be 1.24 eV and 1.03, respectively, as measured by I-V. The highest barrier measured was 1.35 eV...

The Ga deposition, reduction, and re-evaporation technique commonly used to produce clean n-GaN surfaces and Ag–GaN interface formation on the resultant surface, have been investigated by Soft X-ray Photoelectron Spectroscopy (SXPS) and current–voltage measurements. SXPS studies have indicated that Ga deposition produces a band-bending of ΔEk = + 1...

Thesis (Ph. D.)--University of Sheffield, 2001.