Devices and systems based on two dimensional MoO3 and MoS2

Materials that are composed of atoms which are covalently bonded in two dimensional (2D) planar spaces are considered quasi 2D materials. Graphene is an excellent example of a 2D structure and of great interest due to its enhanced carrier mobilities. The enhancement in carrier mobility is due to the emergence of near massless fermions in 2D graphene. Such large carrier mobilities are of great value for the development of future transistors, as the current technologies for the fabrication of nanostructured silicon transistors are fast exceeding their limits in reducung size and enhancing their performance.The aim of this Ph.D. research is to synthesize and characterize the properties of 2D MoO 3 and MoS 2 , in order to realize 2D electronic devices with high carrier mobilities and understand the behaviour of these 2D materials upon small ion intercalation. The author of this thesis thoroughly reviewed the fundamental properties, as well as methods of synthesis and properties of layered MT&DCs. Based on the review, the author recognized some of the niche areas to be further investigated regarding 2D MoS2. The author identified the lack of a simple and efficient process for the synthesis of layered MoS 2 . Additionally, the author identified 2D molybdenum trioxide (MoO 3 ) as the core material suitable for the development of a new family of 2D electronic devices and systems. MoO 3 is an intrinsically high dielectric material with tuneable electronic properties. This tunability can be achieved via facile ion intercalation. In order to realize the aforementioned goals and to create new knowledge, the author implemented his research work in three distinct stages. The first stage involved realizing 2D MoS 2 flakes for the purpose of investigating the structural effects of ion intercalation in comparison to its bulk counterpart. The author developed a simple and effective method of synthesis involving the simultaneous co-evaporation of MoO 3 powder together with sulphur. The MoS 2 was characterized to be high purity layered nanostructures. Subsequently, the author realized 2D MoS 2 via mechanical exfoliation of the as obtained material. The author the conducted electrochemical lithium ion exposure studies and investigated the effects using Raman spectroscopy presenting a new insight regarding the vibrational properties of such systems. In the second stage, the author synthesized layered α-MoO 3 crystals by the thermal evaporation of MoO 3 powder. 2D MoO 3 layers were realized via micromechanical exfoliation of as obtained crystals. The author demonstrated various approaches of H + intercalation and partial reduction of MoO 3 , for the purpose of reducing the bandgap to be viable for FET applications. The author then developed FETs based on such 2D MoO 3 flakes with enhanced carrier mobilities exceeding 1100 cm 2 V –1 s –1 . Such mobility was significantly larger than the best of those reported for the 2D MoS 2 based FETs and en par with the best mobilities offered by silicon technology. Overall, the author strongly believes that the objectives achieved in this Ph.D. research work, have contributed significantly to the advancement of 2D electronic devices and systems as well as created exciting new knowledge

Atomically thin layers of MoS2 via a two step thermal evaporation-exfoliation method

Two dimensional molybdenum disulfide (MoS2) has recently become of interest to semiconductor and optic industries. However, the current methods for its synthesis require harsh environments that are not compatible with standard fabrication processes. We report on a facile synthesis method of layered MoS2 using a thermal evaporation technique, which requires modest conditions. In this process, a mixture of MoS2 and molybdenum dioxide (MoO2) is produced by evaporating sulfur powder and molybdenum trioxide (MoO3) nano-particles simultaneously. Further annealing in a sulfur-rich environment transforms majority of the excess MoO2 into layered MoS2. The deposited MoS2 is then mechanically exfoliated into minimum resolvable atomically thin layers, which are characterized using micro-Raman spectroscopy and atomic force microscopy. Furthermore Raman spectroscopy is employed to determine the effect of electrochemical lithium ion exposure on atomically thin layers of MoS2

Facile synthesis of layered hexagonal MoS2

In this work synthesis of layered molybdenum sulphide (MoS2) through a temperature-controlled thermal evaporation approach is reported. Simultaneous co-evaporation of molybdenum trioxide (MoO3) and sulphur in an argon environment is employed. The as-deposited thin films are characterized by diffraction and microscopy

ZnO based thermopower wave sources

Exothermic chemical reactions of nitrocellulose are coupled onto thermoelectric zinc oxide (ZnO) layers to generate self-propagating thermopower waves resulting in highly oscillatory voltage output of the order of 500 mV. The peak specific power obtained from ZnO based sources is approximately 0.5 kW kg-1

Transition metal oxides - Thermoelectric properties

Transition metal oxides (TMOs) are a fascinating class of materials due to their wide ranging electronic, chemical and mechanical properties. Additionally, they are gaining increasing attention for their thermoelectric (TE) properties due to their high temperature stability, tunable electronic and phonon transport properties and well established synthesis techniques. In this article, we review TE TMOs at cryogenic, ambient and high temperatures. An overview of strategies used for morphological, compositing and stoichiometric tuning of their key TE parameters is presented. This article also provides an outlook on the current and future prospects of implementing TMOs for a wide range of TE applications

CNT/PDMS composite membranes for H2 and CH4 gas separation

Polydimethylsiloxane (PDMS) composites with different weight amounts of multi-walled carbon nanotubes (MWCNT) were synthesised as membranes to evaluate their gas separation properties. The selectivity of the membranes was investigated for the separation of H2 from CH4 gas species. Membranes with MWCNT concentrations of 1% increased the selectivity to H2 gas by 94.8%. Furthermore, CH4 permeation was almost totally blocked through membranes with MWCNT concentrations greater than 5%. Vibrational spectroscopy and X-ray photoelectron spectroscopy techniques revealed that upon the incorporation of MWCNT a decrease in the number of available Si–CH3 and Si–O bonds as well as an increase in the formation of Si–C bonds occurred that initiated the reduction in CH4 permeation. As a result, the developed membranes can be an efficient and low cost solution for separating H2 from larger gas molecules such as CH4

Characterization of metal contacts for two-dimensional MoS2 nanoflakes

While layered materials are increasingly investigated for their potential in nanoelectronics, their functionality and efficiency depend on charge injection into the materials via metallic contacts.This work explores the characteristics of different metals (aluminium, tungsten, gold, and platinum) deposited on to nanostructured thin films made of two-dimensional (2D) MoS2 flakes. Metals are chosen based on their work functions relative to the electron affinity of MoS2. It is observed, and analytically verified that lower work functions of the contact metals lead to smaller Schottky barrier heights and consequently higher charge carrier injection through the contact

Black phosphorus: ambient degradation and strategies for protection

Elemental 2D black phosphorus (BP) is a highly anisotropic versatile material capable of exhibiting wide ranging electronic characteristics ranging from semi-metallic to semiconducting. Its thickness dependent tunable energy gap makes it an exciting prospect for deployment in a variety of applications. The main hurdle limiting diverse applications incorporating BP is its ambient instability. BP degrades rapidly under room conditions, affecting its structure and properties. In this report, we cover the recent progress that has occurred towards protecting BP from ambient degradation. We review the major developments in effectively countering the problem and compare their relative degrees of success. This is provided in the context of the mechanisms governing the atmospheric instability of this material. A targeted focus is kept on the various causes of degradation of BP in atmospheric conditions and the protection strategies that have been implemented so far

Large-area epitaxial growth of InAs nanowires and thin films on hexagonal boron nitride by metal organic chemical vapor deposition

Large-area epitaxial growth of III-V nanowires and thin films on van der Waals substrates is key to developing flexible optoelectronic devices. In our study, large-area InAs nanowires and planar structures are grown on hexagonal boron nitride templates using metal organic chemical vapor deposition method without any catalyst or pre-treatments. The effect of basic growth parameters on nanowire yield and thin film morphology is investigated. Under optimised growth conditions, a high nanowire density of 2.1×109cm-2is achieved. A novel growth strategy to achieve uniform InAs thin film on h-BN/SiO2/Si substrate is introduced. The approach involves controlling the growth process to suppress the nucleation and growth of InAs nanowires, while promoting the radial growth of nano-islands formed on the h-BN surface. A uniform polycrystalline InAs thin film is thus obtained over a large area with a dominant zinc-blende phase. The film exhibits near-band-edge emission at room temperature and a relatively high Hall mobility of 399 cm-2/(Vs). This work suggests a promising path for the direct growth of large-area, low-temperature III-V thin films on van der Waals substrates.Aswani Gopakumar Saraswathy Vilasam, Sonachand Adhikari, Bikesh Gupta, Sivacarendran Balendhran, Naoki Higashitarumizu, Julie Tournet, Lily Li, Ali Javey, Kenneth B Crozier, Siva Karuturi, Chennupati Jagadish, and Hark Hoe Ta

High-performance field effect transistors using electronic inks of 2D molybdenum oxide nanoflakes

Planar 2D materials are possibly the ideal channel candidates for future field effect transistors (FETs), due to their unique electronic properties. However, the performance of FETs based on 2D materials is yet to exceed those of conventional silicon based devices. Here, a 2D channel thin film made from liquid phase exfoliated molybdenum oxide nanoflake inks with highly controllable substoichiometric levels is presented. The ability to induce oxygen vacancies by solar light irradiation in an aqueous environment allows the tuning of electronic properties in 2D substoichiometric molybdenum oxides (MoO3-x). The highest mobility is found to be approximate to 600 cm(2) V-1 s(-1) with an estimated free electron concentration of approximate to 1.6 x 10(21) cm(-3) and an optimal I-On/I-Off ratio of >10(5) for the FETs made of 2D flakes irradiated for 30 min (x = 0.042). These values are significant and represent a real opportunity to realize the next generation of tunable electronic devices using electronic inks