Negative to Positive Crossover of Magnetoresistance in Layered WS2 with Ohmic Contact

The discovery of graphene has ignited intensive investigation on two dimensional (2D) materials. Among them, transition metal dichalcogenide (TMDC), a typical representative, attracts much attention due to the excellent performance in field effect transistor (FET) related measurements and applications. Particularly, when TMDC eventually reaches few-layer dimension, a wide range of electronic and optical properties, in striking contrast to bulk samples, are detected. In this Letter, we synthesized single crystalline WS2 nanoflakes by physical vapor deposition (PVD) method and carried out a series of transport measurements of contact resistance and magnetoresistance. Focused ion beam (FIB) technology was applied to deposit Pt electrodes on WS2 flakes. Different from the electron beam lithography (EBL) fabricated electrodes, FIB-deposited leads exhibited ohmic contact, resolving the dilemma of Schottky barrier. Furthermore, a temperature-modulated negative-to-positive transition of magnetoresistance (MR) associated with a crossover of carrier type at similar temperature was demonstrated. Our work offers a pathway to optimize the contact for TMDC and reveals the magnetoresistance characteristics of WS2 flakes, which may stimulate further studies on TMDC and corresponding potential electronic and optoelectronic applications

Atomic Level Insights for Photocatalytic CO2 Reduction

As the concentration of greenhouse gases in the atmosphere continues to rise, so does average global temperature. This process is irreversible over a short time. The development of renewable clean energy is important therefore to survival and sustainability. As a potential practical method of solar energy utilization, photocatalysis can convert CO2 into high-value-added fossil fuels and assist to obviate global temperature and solve energy shortage. Photocatalytic CO2 reduction (PCRR) is an artificial photosynthesis, that includes three steps: 1) Light absorption, 2) Charge separation and transfer, and 3) Surface redox reaction. However, high C=O double bond energy and the symmetrical molecular structure of CO2 molecules make it necessary to overcome a significantly high energy barrier for adsorption and activation. Therefore, a simple, environmentally friendly and universal method is urgent to be developed to activate the catalyst surface and significantly boost affinity for CO2 molecules. Additionally, photocatalytic CO2 reduction reaction involves complex, multi-electron transfer paths, leading to complex catalytic products and reaction kinetics. However, the construction of photocatalytic reaction systems that exhibit excellent performance, stability and low cost and are environmentally benign is a significant practical challenge. This Thesis aim is to determine the fundamentals of the atomic level and to design novel nanostructured photocatalysts for CO2 reduction reactions. Importantly, this involves detailed investigations on structure and performance, especially for charge transfer and surface reactions that can be reliably used to guide design of catalysts. The significance, context and scope of this Thesis is presented in Chapter 1. A critical review of research progress for CO2 photoreduction from atomic level understanding is provided in Chapter 2. This chapter critically evaluates the challenges and discusses atomic level active sites for PCRR, including defects, single atoms, functional groups and frustrated Lewis pairs and relationships between reactive sites and PCRR performance and impacts on selectivity, stability, efficiency and reactivity. Chapter 3 focuses on synthesizing reactive sites on the surface of transition metal dichalcogenides (TMDs) to alter the inert surface. A heterojunction between CdS nanoparticles and ultra-thin ReS2 nanosheets (6 nm thickness) is synthesized and the mechanism for CO2 photoreduction is determined for this catalytic system. Photoelectrons separation and transfer and in situ formed defects on the surface of ReS2 are confirmed to act as reactive sites to boost photocatalytic performance. Carbon contaminations have a significant influence on photocatalytic performance because carbon-contained products generated via CO2 conversion remain at a low level. In Chapter 4, a targeted series of experiments are described to identify carbon impurities and to establish the influence of carbon impurities on subsequent performance tests. Based on findings from these, reliable experimental protocols to obviate potential carbon contamination effect are proposed in detail. In Chapter 5, a straightforward strategy is developed for synthesis of surface defective bismuth oxide nanosheets. The (010) surface of nanosheets are actively regulated via oxygen vacancies. The surface-regulated Bi2MoO6 nanosheets exhibit practically promising PCRR performance. Findings from judiciously combined theoretical computations, kinetics analyses and in situ infrared spectroscopy are used to confirm that surface unsaturated metal atoms serve as the active sites, and that the hydrogenation of *OCH3 for CH4 formation is the rate-limiting step. Findings are shown to increase fundamemtal understanding of the regulated surface, and to form a basis for large-scale application. In the concluding experimental chapter, Chapter 6, dual single atoms for efficient CO2 conversion to value-added chemicals are explored. In situ characterizations and simulations evidence complex surface reaction and free energy diagram. Charge transfer is determined via in situ X-ray Photon Spectra (XPS) to demonstrate that the photogenerated electrons transfer direction is reversed in dual single atoms system compared with single-atom system. Findings are shown to develop understanding of the charge transfer between dual single atoms and single-atom research and mechanism. The conclusions available from findings from this research on CO2 photoreduction, togther with a perspective are presented in Chapter 7.Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering and Advanced Materials, 202

Fabrication and characterisation of planar and tubular solid oxide fuel cell anodes

Solid Oxide Fuel Cells (SOFCs) are energy conversion devices that convert the chemical energy of a fuel directly into electricity and useful heat, where the latter is recovered at the device’s high working temperature. SOFCs have become important in the fuel cell field due to their high energy conversion efficiency, wide range of fuels and environmental friendliness. However, one of the main obstacles to put SOFCs into mass production is their high fabrication costs including the cost of components such as the electrodes – anodes and cathodes. This project investigates the possibility of manufacturing anodes for tubular SOFCs by a novel co-deposition process which can reduce costs compared with conventional fabrication techniques. Anode requires a ceramic phase to help match the coefficient of thermal expansion of the electrolyte and a metallic phase to conduct electrons to the outside circuit. Both of these can be achieved via a novel nickel-ceramic electroless co-deposition technique. Both ceramic and metallic parts of the anode are deposited together in one single process and avoid the expensive sintering process which is involved in traditional techniques. The elimination of multi-stage processing and high thermal consumption reduces the time and cost of the anode fabrication process. The main challenge in this project is to increase the content of ceramic particles embedded in the nickel. The variables investigated are (i) ceramic particle size, (ii) plating time and (iii) the plating performance both on the inside and outside of tubular surfaces. Initial experiments were carried out on planar surface

Research on Method of Health Assessment about the Destruction Equipment for High-risk Hazardous Chemical Waste

AbstractThe destroying tasks of high-risk hazardous chemical waste have a strict request to the health status of destruction equipment.The paper proposes the health status classification method based on time between failures for the destruction of equipment, set up health status assessment model based on Time-varying Bayesian Networks and the time slice, which can take advantage of history fault information and health status monitoring indicator information to health status assessment for the destruction equipment, and which provides a reliable and safe evaluation method

Numerical analysis of a time discretized method for nonlinear filtering problem with L\'evy process observations

In this paper, we consider a nonlinear filtering model with observations driven by correlated Wiener processes and point processes. We first derive a Zakai equation whose solution is a unnormalized probability density function of the filter solution. Then we apply a splitting-up technique to decompose the Zakai equation into three stochastic differential equations, based on which we construct a splitting-up approximate solution and prove its half-order convergence. Furthermore, we apply a finite difference method to construct a time semi-discrete approximate solution to the splitting-up system and prove its half-order convergence to the exact solution of the Zakai equation. Finally, we present some numerical experiments to demonstrate the theoretical analysis

Retrieval Oriented Masking Pre-training Language Model for Dense Passage Retrieval

Pre-trained language model (PTM) has been shown to yield powerful text representations for dense passage retrieval task. The Masked Language Modeling (MLM) is a major sub-task of the pre-training process. However, we found that the conventional random masking strategy tend to select a large number of tokens that have limited effect on the passage retrieval task (e,g. stop-words and punctuation). By noticing the term importance weight can provide valuable information for passage retrieval, we hereby propose alternative retrieval oriented masking (dubbed as ROM) strategy where more important tokens will have a higher probability of being masked out, to capture this straightforward yet essential information to facilitate the language model pre-training process. Notably, the proposed new token masking method will not change the architecture and learning objective of original PTM. Our experiments verify that the proposed ROM enables term importance information to help language model pre-training thus achieving better performance on multiple passage retrieval benchmarks.Comment: Search LM part of the "AliceMind SLM + HLAR" method in MS MARCO Passage Ranking Leaderboard Submissio

8,10-Diiodo-2,6-dioxo-4λ3-ioda-3,5-dioxatricyclo­[5.3.1.04,11]undeca-1(11),7,9-triene-9-carb­oxy­lic acid

In the title compound, C9HI3O6·2H2O, the mol­ecule is located on a twofold axis that gives rise to disorder of the carboxyl group. This disorder is correlated with the disorder of one of the H atoms of the water mol­ecule. The carboxyl group is twisted relative to the attached benzene ring by 75.1 (4)°. The intra­molecular I⋯O distance is 2.112 (6) Å. Mol­ecules are linked via O—H⋯O hydrogen bonding, C—I⋯O halogen bonding, with I⋯O distances in the range 3.156 (5)–3.274 (6) Å, and dipolar C=O⋯C=O inter­actions between the carboxyl and carboxyl­ate groups, with an O⋯C distance of 2.944 (10) Å