In-situ XPS analysis of the atomic layer deposition of aluminium oxide on titanium dioxide

Ultra-thin aluminium oxide was grown on a rutile titanium dioxide surface by atomic layer deposition using trimethylaluminium and water precursors. This process, carried out using realistic temperatures and pressures (1 mbar, 450 K), was monitored in-situ using high-pressure X-ray photoelectron spectroscopy. This provides insight into the surface chemistry at the interface between the two oxide layers - specifically the reduction of titanium atoms from Ti4+ to Ti3+ upon dosing of trimethylaluminium. These defect states become locked into the heterojunction's interface, with implications to its electronic structure, and can act as an indicator as to when complete coverage of the rutile substrate is achieved

Resonant vibrations of microlitre liquid drops

The vibrational modes of water droplets supported on a surface (sessile drops) were excited by applying an impulse in the form of a short puff of nitrogen gas. An optical deflection technique was used to examine the modes of these oscillations in which laser light was scattered off the surface of the drops onto a photodiode. The time dependent intensity variations detected by the photodiode were Fourier transformed to give the vibrational spectra of the drops. The position and widths of the resonant peaks in these spectra were used along with a simple theory of droplet vibration to determine the surface tension and viscosity of the liquid. This theory models the resonant modes of the droplets as standing capillary wave states on the surface of a liquid bath of finite depth. Surfaces patterned with parallel, periodic grooves were made using a variety of techniques. When a droplet of water was placed on one of these surfaces, the droplet was shown to wet anisotropically to the surface elongating the shape of the drop. The vibrational response of these aspherical droplets were measured as described above and the resulting vibrational spectra were shown to contain two closely spaced resonant peaks. These two peaks are shown to correspond to standing wave states forming along the profile lengths of the drop in the directions of the major and minor axes. The vibrational response of droplets suspended from the end of a pipette tip (pendant drops) were also analysed using the same methods

Complex molecules on surfaces: in-situ electrospray deposition and photoelectron spectroscopy

Furthering our understanding of next generation hight harvesting devices has the potential to revolutionise energy storage and production. This thesis discusses the use of surface science techniques, principally soft X-ray photoelectron spectroscopies, to study two types of molecular solar cell: dye sensitised solar cells and organic photovoltaic devices. Much of this work relies on electrospray deposition, which is becoming a well established technique allowing the in-situ deposition of fragile, non-volatile molecules in high vacuum environments. This thesis explores the potential use of electrospray deposition to build model photovoltaic devices, including bi-layer and bulk heterojucntion structures, to extend the use of the technique beyond the simple "molecule on a surface" type of experiment that have previously dominated the high-vacuum electrospray deposition literature. Firstly this thesis presents the development of two instruments that aim to aid the characterisation of electrospray. A novel ultra-fast exposure microscopy system is presented that used a pulsed laser and purpose-designed cell containing a fluorescent dye to produce short (<20 ns) incoherent light pulses. Images of electrospray ionisation, highlighting the capabilities of the imaging system, are presented. Preliminary images showing the unusual behaviour when the liquid properties are severely changed by adding a high concentration of salt are also presented. We also include some proof-of-concept data, collected using a home-built image charge detection instrument, to measure velocity distributions of molecular ions that are landing on the surface. This instrument has led the way to a more refined image charge detection instrument that will be of great benefit to future studies. A series of three experiments are presented that highlight the capability and suitability of high vacuum deposition to produce the types of structures used in molecular photovoltaic devices. Time of flight secondary ion mass spectrometry was used to image a bi-layer structure, produced using electrospray from two molecules dissolved in the same solvent, showing there is little mixing between the layers. These structures are challenging to make using processes like spin casting where solvent compatibility results in mixing of the films. Two electrospray sources were used to co-deposit two molecules simultaneously from immiscible solvents, which is again not possible using standard techniques. Finally a complex polymer blend was deposited using electrospray, from a single solution, and analysed using X-ray photoelectron spectroscopy (XPS) finding evidence of preferential deposition of one species. Bi-isonicotionic acid (the ligand of many popular organometallic photo-sensitisers) has been deposited onto Ag(111) in-situ via sublimation. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) have been used to obtain chemical information and map out the density of states for the system. We find the LUMO (lowest unoccupied molecular orbital) lies below the Fermi level of the silver surface allowing ultra-fast bi-directional charge transfer between the molecule and surface. This was probed using resonant photoelectron spectroscopy (RPES) by observation of super-spectator and super-Auger decay mechanisms from the core excited state. In the final experimental chapter, films of phenyl-C61-butyric acid methyl ester (PCBM) and Poly(3-hexylthiophene-2,5-diyl) (P3HT), common acceptor and donor molecules used in organic photovoltaic devices, were successfully deposited in-situ in UHV using electrospray deposition on the Au(111) surface. Bi-layer heterojunction and bulk heterojunction structures of the two molecules were also produced using electrospray. Again, XPS, XAS and RPES were used to investigate chemical interactions, map out the density of states for the molecular systems and probe charge transfer between the molecules and surfaces. For PCBM, some interesting chemical behaviour regarding an oxygen group on the ligand is observed. Measurements also show the LUMO of PCBM lies below the surface Fermi level, but surprisingly no evidence of super-spectator or super-Auger decay is found. RPES was unable to show evidence of charge transfer between the two molecules in a bi-layer heterojunction structure. Finally, careful ionisation potential calibration of XPS spectra were used to try and infer the position of the valence band maximum of the two molecules when brought together in a heterojunction

Resonant vibrations of microlitre liquid drops

The vibrational modes of water droplets supported on a surface (sessile drops) were excited by applying an impulse in the form of a short puff of nitrogen gas. An optical deflection technique was used to examine the modes of these oscillations in which laser light was scattered off the surface of the drops onto a photodiode. The time dependent intensity variations detected by the photodiode were Fourier transformed to give the vibrational spectra of the drops. The position and widths of the resonant peaks in these spectra were used along with a simple theory of droplet vibration to determine the surface tension and viscosity of the liquid. This theory models the resonant modes of the droplets as standing capillary wave states on the surface of a liquid bath of finite depth. Surfaces patterned with parallel, periodic grooves were made using a variety of techniques. When a droplet of water was placed on one of these surfaces, the droplet was shown to wet anisotropically to the surface elongating the shape of the drop. The vibrational response of these aspherical droplets were measured as described above and the resulting vibrational spectra were shown to contain two closely spaced resonant peaks. These two peaks are shown to correspond to standing wave states forming along the profile lengths of the drop in the directions of the major and minor axes. The vibrational response of droplets suspended from the end of a pipette tip (pendant drops) were also analysed using the same methods

Adsorption and charge transfer interactions of bi-isonicotinic acid on Ag(111)

The adsorption and charge transfer dynamics of the organic molecule bi-isonicotinic acid (4,4′-dicarboxy-2,2′-bipyridine) on single crystal Ag(111) has been studied using synchrotron radiation-based photoemission, x-ray absorption and resonant core spectroscopies. Measurements for multilayer and monolayer coverage are used to determine the nature of the molecule-surface interactions and the molecular orientation. An experimental density of states for the monolayer with respect to the underlying metal surface is obtained by combining x-ray absorption spectroscopy at the N 1s edge and valence photoemission to measure the unoccupied and occupied valence states, respectively. This shows that the lowest unoccupied molecular orbital in the core-excited state lies energetically below the Fermi level of the surface allowing charge transfer from the metal into this orbital. Resonant photoelectron spectroscopy was used to probe this charge transfer in the context of super-spectator and super-Auger electron transitions. The results presented provide a novel interpretation of resonant core-level spectroscopy to explore ultra-fast charge transfer between an adsorbed organic molecule and a metal surface through the observation of electrons from the metal surface playing a direct role in the core-hole decay of the core-excited molecule

The remapped particle-mesh advection scheme

We describe the remapped particle-mesh method, a new mass-conserving method for solving the density equation which is suitable for combining with semi-Lagrangian methods for compressible flow applied to numerical weather prediction. In addition to the conservation property, the remapped particle-mesh method is computationally efficient and at least as accurate as current semi-Lagrangian methods based on cubic interpolation. We provide results of tests of the method in the plane, results from incorporating the advection method into a semi-Lagrangian method for the rotating shallow-water equations in planar geometry, and results from extending the method to the surface of a sphere

Exploring ultra-fast charge transfer and vibronic coupling with N 1s RIXS maps of an aromatic molecule coupled to a semiconductor

We present for the first time two-dimensional resonant inelastic x-ray scattering (RIXS) maps of multilayer and monolayer biisonicotinic acid adsorbed on the rutile TiO2(110) single crystal surface. This enables the elastic channel to be followed over the lowest unoccupied molecular orbitals resonantly excited at the N 1s absorption edge. The data also reveals ultra-fast intramolecular vibronic coupling, particularly during excitation into the LUMO-derived resonance. Both elastic scattering and the vibronic coupling loss features are expected to contain the channel in which the originally excited electron is directly involved in the core-hole decay process. This allows RIXS data for a molecule coupled to a wide bandgap semiconductor to be considered in the same way as the core-hole clock implementation of resonant photoemission spectroscopy (RPES). However, contrary to RPES measurements, we find no evidence for depletion of the participator channel under the conditions of ultra-fast charge transfer from the molecule to the substrate densities of states, on the timescale of the core-hole lifetime. These results suggest that the radiative core-hole decay processes in RIXS are not significantly modified by charge transfer on the femtosecond timescale in this system

Charge transfer from an adsorbed ruthenium-based photosensitizer through an ultra-thin aluminium oxide layer and into a metallic substrate

The interaction of the dye molecule N3 (cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4′-dicarbo-xylato)-ruthenium(II)) with the ultra-thin oxide layer on a AlNi(110) substrate, has been studied using synchrotron radiation based photoelectron spectroscopy, resonant photoemission spectroscopy, and near edge X-ray absorption fine structure spectroscopy. Calibrated X-ray absorption and valence band spectra of the monolayer and multilayer coverages reveal that charge transfer is possible from the molecule to the AlNi(110) substrate via tunnelling through the ultra-thin oxide layer and into the conduction band edge of the substrate. This charge transfer mechanism is possible from the LUMO+2 and 3 in the excited state but not from the LUMO, therefore enabling core-hole clock analysis, which gives an upper limit of 6.0 ± 2.5 fs for the transfer time. This indicates that ultra-thin oxide layers are a viable material for use in dye-sensitized solar cells, which may lead to reduced recombination effects and improved efficiencies of future devices

An in situ exploration of subsurface defect migration to a liquid water‐exposed rutile TiO2(110) surface by XPS

The ability of titanium dioixide to split water into OH− and H+ species is heavily dependent on the behaviour of defects in the crystal structure at or near the surface. We present an in situ study of defect migration in rutile TiO2(110) conducted using X‐ray photoelectron spectroscopy (XPS). First, surface and subsurface defects were created in the crystal by argon ion sputtering. Subsequent in situ exposure of the defective crystal to liquid water healed the surface defects, whereas the subsurface remained defective. The sample was then annealed while XPS was used to monitor the concentration of titanium defects. At low annealing temperatures, Ti3+ was observed to migrate from the subsurface to the surface. Further annealing gradually restored the surface and subsurface to the defect‐free Ti4+ form, during which the changes in abundance of Ti1+, Ti2+ and Ti3+ defects are discussed

A soft x-ray probe of a titania photoelectrode sensitized with a triphenylamine dye

We present a thorough soft x-ray photoelectron spectroscopy (XPS) study of a mesoporous titanium dioxide electrode sensitized with the dye 4-(diphenylamino)phenylcyanoacrylic acid, referred to as “L0.” Supported by calculations, the suite of XPS, x-ray absorption spectroscopy, and resonant photoelectron spectroscopy allows us to examine bonding interactions between the dye and the surface and the frontier electronic structure at the molecule-oxide interface. While placing these measurements in the context of existing literature, this paper is intended as a useful reference for further studies of more complex triphenylamine based sensitizers