Optoelectronic Properties of Two-Dimensional Molybdenum Ditelluride

In this thesis the layered, two-dimensional material MoTe2 is examined experimentally for its optoelectronic properties, using a field effect transistor device configuration. MoTe2 experiences a strong light matter interaction, which is highly dependent on the conditions of the measurement, and the wavelength of light used. Light is able to: produce a photocurrent in MoTe2, desorb adsorbates from the surface, and even controllably thin by a single layer at a time. A theoretical study on MoTe2 also provides insights on the source of some of these interesting light matter interactions. MoTe2 is found to be a fast and responsive photodetector when illuminated with red laser light in ambient conditions, with increases in current stemming from the photovoltaic effect. Due to the generated charge carriers from the photovoltaic effect, conductivity can increase by increasing the Fermi energy of the material, or by a photogating effect where excited charges are trapped and behave as an artificial gate for the field effect transistor. The mechanisms of charge trapping are experimentally investigated due to their prevalence in the photodetection mechanisms. A theoretical study points towards the existence of two types of trap states, in not just MoTe2 but all transition metal dichalcogenides, with shallow traps closer to the valence band edge (τ ~ 500 s) and deeper traps (τ ~ 1000 s), further away from the valence band edge. MoTe2, under the effects of higher energy photons from blue and green lasers, showed different photocurrent mechanisms to red light. From the increased energy of the photons, photo-desorption of adsorbates on the surface of MoTe2 occurred causing a decrease in the overall current, in a rarely seen photocurrent mechanism. Again, both shallow and deep traps are evident from the experimental measurements, with the shallow traps being removed when illuminated by higher energy photons. Finally, a humidity assisted photochemical layer-by-layer etching process was developed with an in-situ Raman spectroscopy system, able to thin MoTe2 by a single layer at a time with 200 nm spatial resolution. MoTe2 FETs were created with thinned channels to examine the effect of the thinning technique on optoelectronic properties. Some improvement in optoelectronic performance (higher responsivity, higher mobility) was seen for the thinned channel devices, with great improvement observed for monolayer MoTe2.Engineering and Physical Sciences Research Council (EPSRC

Humidity‐Controlled Ultralow Power Layer‐by‐Layer Thinning, Nanopatterning and Bandgap Engineering of MoTe2

This is the final version. Available on open access from Wiley via the DOI in this recordA highly effective laser thinning method is demonstrated to accurately control the thickness of MoTe2 layers. By utilizing the humidity present in the ambient atmosphere, multilayered MoTe2 films can be uniformly thinned all the way down to monolayer with layer-by-layer precision using an ultralow laser power density of 0.2 mW µm−2. Localized bandgap engineering is also performed in MoTe2, by creating regions with different bandgaps on the same film, enabling the formation of lateral homojunctions with sub-200 nm spatial resolution. Field-effect transistors fabricated from these thinned layers exhibit significantly improved electrical properties with an order of magnitude increase in on/off current ratios, along with enhancements in on-current and field-effect mobility values. Thinned devices also exhibit the fastest photoresponse (45 µs) for an MoTe2-based visible photodetector reported to date, along with a high photoresponsivity. A highly sensitive monolayer MoTe2 photodetector is also reported. These results demonstrate the efficiency of the presented thinning approach in producing high-quality MoTe2 films for electronic and optoelectronic applications.Office of Naval Research GlobalEngineering and Physical Sciences Research Council (EPSRC)Defence Science and Technology Laborator

Fast High-Responsivity Few-Layer MoTe2 Photodetectors

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.The Transition Metal Dichalcogenide MoTe2 is fabricated via mechanical exfoliation into few-layer Field Effect Transistors (FETs) having a hole mobility of 2.04 V/cm2/s. Four-layer MoTe2 FETs show a high photoresponsivity of 6 A/W and a response time, at around 160 μs, over 100 times faster than previously reported for MoTe2. Few-layer MoTe2 thus appears as a strong candidate for high speed and high sensitivity photodetection applications.CDW would like to acknowledge funding via EPSRC grants EP/M015173/1 and EP/M015130/1. TJO acknowledges funding from the EPSRC Centre for Doctoral Training in Metamaterials, grant number EP/L015331/

Role of charge traps in the performance of atomically-thin transistors

Transient currents in atomically thin MoTe2 field-effect transistors (FETs) are measured during cycles of pulses through the gate electrode. The curves of the transient currents are analyzed in light of a newly proposed model for charge-trapping dynamics that renders a time-dependent change in the threshold voltage as the dominant effect on the channel hysteretic behavior over emission currents from the charge traps. The proposed model is expected to be instrumental in understanding the fundamental physics that governs the performance of atomically thin FETs and is applicable to the entire class of atomically thin-based devices. Hence, the model is vital to the intelligent design of fast and highly efficient optoelectronic devices

Photoconductivity of Few-Layer MoTe2

This is the final version of the paper. Available from metaconferences.org via the URL in this record.A photoconductivity study of few-layer MoTe2 in a field effect transistor (FET) configuration was performed to find the photoresponsivity and photocurrent response of the material. The mechanisms for MoTe2 with no applied gate voltage were found to be dominated by the photovoltaic effect, showing its potential for use in solar cells. Due to the band gap of MoTe2 being 1.1 eV, MoTe2 is a suitable photodetector for optical wavelengths and potentially the near infrared

2D MoTe2 nanosheets by atomic layer deposition: Excellent photo-electrocatalytic properties

Herein, the synthesis of MoTe2 nanosheets by means of Atomic Layer Deposition (ALD) is demonstrated for the first time. ALD enables tight control over the thickness and the composition of the deposited material, which are highly appealing features for the nanostructure fabrication. The growth of ALD MoTe2 was studied on substrates of different nature, including TiO2 nanotube (TNT) layers used as active supporting material for fabricating hierarchical nanotubular MoTe2/TNT heterostructure. The combination of newly synthesized Te precursor with commercial Mo precursor rendered the growth of 2D flaky MoTe2 nanosheets mostly out-of-plane oriented. The as-deposited MoTe2 was extensively characterized by different techniques which confirmed its chemical composition and revealed 2D flaky nano-crystalline structures. In parallel, MoTe2/TNT layers were employed to explore and exploit both photoand electrocatalytic properties. The synergy stemming from the out-of-plane MoTe2 nanosheet orientation, with an optimized amount of catalytic active edges, and the fast electron transfer through 1D TiO2 nanotubes triggered the catalytic properties for both, organic pollutant degradation and hydrogen evolution reaction (HER) applications. Remarkably, the application of a cathodic potential originated a gradual HER electrochemical activation over time driving to a higher current density and an overpotential drop. (c) 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/

Marketing leisure services

This thesis discusses the extension of the marketing concept, marketing and marketing management to the service sector of the economy. The study begins with an explanation of the current literature about marketing in the service sector. As the sector includes a large number of different industries, however, it was not considered practicable to undertake empirical research throughout the area but to select one service industry for further examination. Leisure services was chosen and the relevant literature in this area has been discussed in the thesis. [Continues.

A thin-reflector microfluidic resonator for continuous-flow concentration of microorganisms: a new approach to water quality analysis using acoustofluidics

An acoustofluidic device has been developed for concentrating vegetative bacteria in a continuous-flow format. We show that it is possible to overcome the disruptive effects of acoustic streaming which typically dominate for small target particles{,} and demonstrate flow rates compatible with the testing of drinking water. The device consists of a thin-reflector multi-layered resonator{,} in which bacteria in suspension are levitated towards a glass surface under the action of acoustic radiation forces. In order to achieve robust device performance over long-term operation{,} functional tests have been carried out to (i) maintain device integrity over time and stabilise its resonance frequency{,} (ii) optimise the operational acoustic parameters{,} and (iii) minimise bacterial adhesion on the inner surfaces. Using the developed device{,} a significant increase in bacterial concentration has been achieved{,} up to a maximum of ~60-fold. The concentration performance of thin-reflector resonators was found to be superior to comparable half-wave resonators

Microtopographies control the development of basal protrusions in epithelial sheets

Cells are able to develop various types of membrane protrusions that modulate their adhesive, migratory, or functional properties. However, their ability to form basal protrusions, particularly in the context of epithelial sheets, is not widely characterized. The authors built hexagonal lattices to probe systematically the microtopography-induced formation of epithelial cell protrusions. Lattices of hexagons of various sizes (from 1.5 to 19 mu m) and 5-10 mu m height were generated by two-photon photopolymerization in NOA61 or poly(ethylene glycol) diacrylate derivatives. The authors found that cells generated numerous, extensive, and deep basal protrusions for hexagons inferior to cell size (3-10 mu m) while maintaining a continuous epithelial layer above structures. They characterized the kinetics of protrusion formation depending on scaffold geometry and size. The reported formation of extensive protrusions in 3D microtopography could be beneficial to develop new bio-materials with increased adhesive properties or to improve tissue engineering