Single-dopant band bending fluctuations in MoSe2_2 measured with electrostatic force microscopy

In this work, we experimentally demonstrate two-state fluctuations in a metal-insulator-semiconductor (MIS) device formed out of a metallic atomic force microscopy tip, vacuum gap, and multilayer MoSe$_2$ sample. We show that noise in this device is intrinsically bias-dependent due to the bias-dependent surface potential, and does not require that the frequency or magnitude of individual dopant fluctuations are themselves bias-dependent. Finally, we measure spatial nonhomogeneities in band bending (charge reorganization) timescales.Comment: 6 main text pages, 8 supplemetary pages, 11 figure

Molecular quantum rings formed from a π-conjugated macrocycle

The electronic structure of a molecular quantum ring (stacks of 40-unit cyclic porphyrin polymers) is characterised via scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS). Our measurements access the energetic and spatial distribution of the electronic states and, utilising a combination of density functional theory and tight binding calculations, we interpret the experimentally obtained electronic structure in terms of coherent quantum states confined around the circumference of the π-conjugated macrocycle. These findings demonstrate that large (53 nm circumference) cyclic porphyrin polymers have the potential to act as molecular quantum rings

Single atoms, clusters and nanoparticles for catalysis:Synthesis, stabilisation and characterisation

The urgent need to shift towards more sustainable solutions in the industrial, energy, and transportation sectors on a global scale is clear. Producing fuels and chemicals from abundant molecules such as hydrogen and carbon dioxide may pose part of the solution, but making the relevant chemical reactions more energy efficient for this purpose relies on the design of active and stable catalysts, which are selective for the product of interest. In this thesis, experimental work is presented which has explored fundamental ways of altering the performance and stability of catalysts by changing their structure on the atomic scale. Model catalysts in the form of clusters and nanoparticles have been made in the laboratory and their catalytic performance has been tested using electrochemical or thermochemical methods. In addition, detailed studies of their properties have been carried out to establish structure-activity relationships. The characterisation of the catalysts was conducted using different surface science techniques including ion scattering spectroscopy, X-ray photoelectron spectroscopy, and scanning probe microscopy. In one project, the size and shape of gold nanoparticles were found to play a significant role in the selectivity of the electrochemical reduction of carbon dioxide. The work investigates how to obtain a higher yield of carbon monoxide, which is a useful feedstock for the production of higher-density hydrocarbon fuels. Nanoparticles around the size of 3 nm with a multiply twinned structure were found to exhibit optimal carbon monoxide selectivity, which can be attributed to the sites present in the nanoparticle surface.Another project investigated how the stability of gold nanoparticles for gas-phase carbon monoxide oxidation could be influenced by alloying the nanoparticles with titanium. The alloyed nanoparticles exhibited a core-shell structure that aided stability during activity tests through anchoring effects with a titanium dioxide support. The structure of the alloyed nanoparticles was observed to be stable even at prolonged exposure to elevated temperatures and reactive conditions.The last topic of focus is single atom catalysts, an area which has recently gained high interest in the literature. Preliminary results are presented from a project which aimed to create a two-dimensional model system with single atoms of platinum anchored within a carbon lattice via nitrogen doping. The activity of the samples for the hydrogen evolution reaction was tested, and extensive characterisation by spectroscopy and microscopy was carried out to study the surface structures.The combined results give new insights into how the structure of catalysts can be altered to optimise their performance and stability, and they help pave the way toward the development of more efficient and sustainable catalysts for the future of fuel and chemicals production. This work also summarises key experimental challenges in carrying out this type of surface science-based catalysis research, and demonstrates examples where carefully controlling factors such as the support properties and catalyst loading has been key for the outcome of the investigations