Colloidal quantum dots (QDs) are promising candidates for the next generation of solar cells due to their tunable band gaps, solution processability and the potential for multiple exciton generation. However their stability and the reduction of surface defects are big challenges and effective surface passivation is needed. Passivations via organic ligands have been shown to be imperfect and hinder the charge transfer in devices. Three different QD systems, chosen as exemplars of different approaches to surface passivation, have been investigated with synchrotron-radiation (SR) depth- profiling X-ray photoelectron spectroscopy (XPS). With this technique the chemical composition of the top few nanometres of a sample can be studied with depth. The study of CdTe QDs with and without a chloride treatment revealed the presence of stoichiometric particles prior to, and the likely coexistence of Cl atoms and organic ligands on the surfaces of the QDs after the treatment. The chloride treatment led to a better surface passivation of the QDs resulting in photoluminescence quantum yields of up to 97.2%. Shell thickness estimations using a core/shell/shell model were performed of the chloride treated sample and XPS highlighted the complexity of the structure of the sample. CdTe QDs passivated by a thick CdSe shell were investigated. Indications for an improvement of the stability of the QDs against oxidation were found. The Se:Te ratio was equivalent to a CdSe shell of 0.3-0.4 nm which was significantly smaller than intended, indicating that the butylamine ligand exchange and/or the washing of the sample reduced the thickness of the CdSe shell drastically. The third system studied was PbS QDs that were passivated with a thin CdS shell. XPS of the thoroughly washed QDs confirmed the presence of Cd in an amount equivalent to a 0.13-0.18 nm thick shell. This is thicker than the 0.05 nm shell expected from absorption spectroscopy. A study of ageing of the PbS/CdS QDs revealed that oxidation took place on the surface of the QDs. It was found that sulfur oxidised in stages leading to highly oxidised SO4^2- components. Upon long-term ageing Pb oxidised more rapidly than S, and either some Pb and/or Cd migration or some decomposition of the QDs occurred. The PbS/CdS nanoparticles were more stable than a comparable PbS colloidal quantum dot sample from the literature. The study of the PbS/CdS QDs prior to and after the second wash- ing cycle after a mercaptopropionic acid (MPA) ligand exchange revealed, amongst other things, the removal of MPA and a reduction of the Cd:Pb ratio indicating that (parts of) the QDs decomposed through the ligand exchange or the washing. In addition to the results of the nanoparticles studied some limitations of the study of colloidal QDs with SR depth-profiling XPS are discussed.

Thesis (Ph. D.)--University of Missouri-Columbia, 2002. / Typescript. Vita. Includes bibliographical references (leaves 98-103). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2002. / Typescript. Vita. Includes bibliographical references (leaves 98-103). Also available on the Internet.

Abstract In this thesis, the electronic structure of selected atoms was investigated by means of electron and uorescence spectroscopy. Synchrotron radiation was used to excitate atoms in gas phase. In particular, the photoionization and subsequent Auger decay processes in metal vapours were studied as well as the resonant Auger decay in rare gases. The experimental results were analyzed together with theoretical predictions obtained utilizing the multiconfiguration Dirac-Fock method.

This Licentiate thesis presents research accomplished at theSection of Atomic and Molecular Physics at the Royal Instituteof Technology in Stockholm using photon induced fluorescencespectroscopy (PIFS) during the last two years. The main results presented are summarized: - Neutral photodissociation in CO was observed aftersynchrotron photon excitation in the range 19-26 eV bycollecting dispersed fluorescence from excited neutral C atoms.Follow-up ab initio calculations point out CO Rydberg seriesconverging to the CO+ C and D states as precursors. - The branching ratio between N2 + (B-X)(v’=1,v’’=2) and (v’=0,v’’=1)transitions in the 20-46 eV energy range reveals strongnon-Franck-Condon effects. Ab initio calculations indicate thatthe autoionization of certain superexcited states areresponsible for some of the structures present in the branchingratio curve, confirming the important role of non-Rydbergdoubly excited resonant states (NRDERS) in de-excitationprocesses above the ionization potential. - Photon induced neutral dissociation processes in NO arereported. Neither Rydberg series nor other molecular states inNO known so far can account for the collected data. From abinitio calculations more information regarding the NO precursorstates and the mechanism behind the observed neutraldissociation were obtained. - The details of a new experimental set-up for gas phasefluorescence measurements using synchrotron radiation aredescribed. It is able to perform simultaneous measurements ofdispersed and total fluorescence in the visible range. Thefirst results obtained with this set-up are presented,concerning fluorescence after excitation of the N2 molecules inthe N 1s edge. These four studies conform the set of papers enclosed in theLicentiate thesis. Finally a pre-study to further apply PIFS to speciespreviously excited by microwave discharge is included as futureplans. / NR 20140805

The X-ray optics of synchrotron radiation beamlines are considered in this paper. The characteristics of synchrotron radiation which make it the premier source of light for studies in many regions of the electromagnetic spectrum are given. A chapter is devoted to the collimating, focusing and monochromating optics of two X-ray diffraction beamlines at the National Synchrotron Light Source. The beamlines are operated by the Naval Research Laboratory (NRL) and the Oak Ridge National Lab (ORNL). The major optical components of these beamlines are reviewed in regards to their function and their flexibility. A detailed analysis is performed on the NRL X-ray collimating mirror. The mirror is treated as an elastically bent beam. Deflection and slope error equations are developed which relate the shape of the bent mirror to its ideal surface. Visible light diffraction patterns collected from the mirror helped to establish operating conditions which provide good collimation. When the observed patterns are wed to the theoretical calculations, estimates of the average figure error are made. Finally, the effect of a highly collimated synchrotron beam on the reproducibility of the integrated intensities from polycrystalline materials is considered. The calculations show that except for the most fine grained materials, representative intensity measurements can only be made when the sample is permitted to move. / M.S.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

Precessional dynamics are exploited in the operation of high frequency magnetic devices such as magnetic disk drives, non reciprocal microwave devices and spin transfer oscillators. The trajectory of the precession and its damping are of crucial importance. This thesis presents the characterisation of a variety of magnetic thin film structures performed with a range of phase sensitive techniques. It is possible to obtain new insight by utilising the chemical and site specificity of X-ray Magnetic Circular Dichroism (XMCD) to isolate the precession in different chemical species or at distinct sites in the crystal structure of a chosen material. X-ray Ferromagnetic Resonance (XFMR) combines XMCD and Ferromagnetic Resonance (FMR) phenomena in a technique capable of measuring the FMR response of an alloy or multilayer with both chemical and site specificity. To complement the XFMR technique a low temperature Time-Resolved Magneto Optical Kerr Effect (TR-MOKE) setup has been developed. This allowed for the characterisation of samples at temperatures in the range 4 K to room temperature. A frequency swept Vector Network Analyser FMR (VNA-FMR) setup was developed to allow for a fast method for determining the resonance condition and damping of a range of ferromagnetic thin film samples. In addition a TR-X-ray Photoemission Electron Microscopy (TR-XPEEM) setup has been established which allows images to be obtained with magnetic contrast. The combination of the above techniques has lead to studies on rare earth capped spin valve free layers and the measurement of spin pumping in industrially relevant spin valves.

The ionization and thermal balances are considered for a gas that is ionized by a dilute radiation field, taking into account the diffuse ionizing radiation produced by the gas. A number of models are constructed in which the electron temperature and the ionization of the elements H, He, C, N, 0, Ne, and Mg are determined for optically thin and optically thick gases ionized by ultraviolet synchrotron radiation under different conditions. Conclusions are then drawn about the general characteristics of ionization by synchrotron radiation. It is shown that, in an optically thin gas, because of the insensitive frequency- dependence of synchrotron radiation each element occupies a number of different stages of ionization at any one point in the gas. It is also shown that in an optically thick gas the heavy elements remain ionized to much greater distances from the source than hydrogen and helium, and that the gas becomes thermally unstable when H and He have become almost completely neutral. In addition, observations of the emission -line intensities of the Crab Nebula are compared with a model of this object. Considerable disagreement exists between the observed and predicted intensities, and possible reasons for the discrepancy are discussed.