Wendy R. Flavell

Electronic and surface properties of PbS nanoparticles exhibiting efficient multiple exciton generation

Ultrafast transient absorption measurements have been used to study multiple exciton generation in solutions of PbS nanoparticles vigorously stirred to avoid the effects of photocharging. The threshold and slope efficiency of multiple exciton generation are found to be 2.5 ± 0.2 ×E(g) and 0.34 ± 0.08, respectively. Photoemission measurements as a function of nanoparticle size and ageing show that the position of the valence band maximum is pinned by surface effects, and that a thick layer of surface oxide is rapidly formed at the nanoparticle surfaces on exposure to air.

Controlled Synthesis of Tuned Bandgap Nanodimensional Alloys of PbSxSe1−x

Truly alloyed PbS(x)Se(1-x) (x = 0-1) nanocrystals (∼5 nm in size) have been prepared, and their resulting optical properties are red-shifted systematically as the sulfur content of the materials increases. Their optical properties are discussed using a modified Vegards approach and the bowing parameter for these nanoalloys is reported for the first time. The alloyed structure of the nanocrystals is supported by the energy-filtered transmission electron microscope images of the samples, which show a homogeneous distribution of sulfur and selenium within the nanocrystals. X-ray photoelectron spectroscopy studies on ligand-exchanged nanocrystals confirmed the expected stoichiometry and various oxidized species.

Novel quantum confined structures via atmospheric pressure MOCVD growth in asbestos and opals

Porous host materials of opal and asbestos have been infilled with InP by atmospheric pressure MOCVD. These were characterised by a variety of techniques including PL at 4 and 300 K. The PL gave clear evidence of quantum confinement with the InP bound exciton blue shifted by at least 0.2 eV. OpalInP gave the additional characteristic of controlling the stop-band of the host. These all represent a new method of producing easily contactable quantum device structures.

Lattice parameter changes in the mixed-oxide system Ce1–xLaxO2–x/2: a combined experimental and theoretical study

X-Ray diffraction measurements are reported of ceramic samples of the mixed-oxide system Ce1–xLaxO2–x/2 for 0<x<0.9, where x has been determined by EDXA. The solid-state reaction forms a highly crystalline product at an ageing temperature of 1200 °C. The solubility limit of La in the bulk CeO2 at this temperature has been determined as ca. 52% by monitoring the lattice parameter increase and the subsequent appearance of a second La2O3 phase. To model the experimental measurements, atomistic lattice calculations based on electron-gas shell-model potentials have been carried out to estimate the variation of the lattice parameter with La content.

A SPECTROSCOPIC STUDY OF ELECTRON AND ION-BEAM REDUCTION OF SNO2(110)

The reduction of SnO2(110) by 160 eV electron bombardment has been studied by XPS. Oxygen desorption is characterised by an initial cross section of 2.8 × 10−20 cm2. Growth of core peaks due to Sn(0) accompany oxygen loss; this contrasts with reduction under Ar ion bombardment, where only Sn(II) is formed. Plasmon loss satellites 14 eV below the new core peaks identify the Sn(0) as metallic tin. Careful analysis of peak intensities shows that electron beam reduction does produce some Sn(II) in addition to the Sn(0) species.

Near-unity quantum yields from chloride treated CdTe colloidal quantum dots.

Colloidal quantum dots (CQDs) are promising materials for novel light sources and solar energy conversion. However, trap states associated with the CQD surface can produce non-radiative charge recombination that significantly reduces device performance. Here a facile post-synthetic treatment of CdTe CQDs is demonstrated that uses chloride ions to achieve near-complete suppression of surface trapping, resulting in an increase of photoluminescence (PL) quantum yield (QY) from ca. 5% to up to 97.2 ± 2.5%. The effect of the treatment is characterised by absorption and PL spectroscopy, PL decay, scanning transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. This process also dramatically improves the air-stability of the CQDs: before treatment the PL is largely quenched after 1 hour of air-exposure, whilst the treated samples showed a PL QY of nearly 50% after more than 12 hours.

The high-resolution electron-energy-loss spectrum of TiO2(110)

Abstract High-resolution electron-energy-loss spectra of TiO 2 (110) are presented and compared with model calculations of dielectric loss functions. The influence of crystal anisotropy on loss spectra is rather small, producing only minor changes in the relative intensities of loss peaks. By contrast, electronic excitations associated with surface oxygen vacancy detects exert a major influence on the vibrational loss spectra and lead to pronounced changes in peak intensities and positions.

Chemical vapour deposition of ZrO2 thin films monitored by IR spectroscopy

Thin films of ZrO2 have been grown under kinetic control by decomposition of zirconium tetra-tert-butoxide onto quartz substrates. The resulting films have been characterised by optical and electron microscopy and X-ray diffraction. A phase transition from a poorly crystalline, metastable form of zirconia to the monoclinic phase showing a strong preferred orientation takes place as the substrate temperature is raised from 450 to 500 °C. The decomposition of the precursor has been followed by ex situ infrared spectroscopy, allowing monitoring of the gas-phase products as a function of substrate temperature. The possible mechanism for the decomposition reaction is discussed.

Surface Properties of Nanocrystalline PbS Films Deposited at the Water-Oil Interface: A Study of Atmospheric Aging

Nanocrystalline thin films of PbS are obtained in a straightforward reaction by precipitation at the interface between toluene (containing a Pb precursor) and water (containing Na2S). Lead thiobiuret [Pb(SON(CN(i)Pr2)2)2] and lead diethyldithiocarbamate [Pb(S2CNEt2)2] precursors are used. The films are characterized by X-ray diffraction and electron microscopy, revealing typical particle sizes of 10-40 nm and preferred (200) orientation. Synchrotron-excited depth-profiling X-ray photoelectron spectroscopy (XPS) is used to determine the depth-dependent chemical composition as a function of surface aging in air for periods of up to 9 months. The as-synthesized films show a 1:1 Pb/S composition. Initial degradation occurs to form lead hydroxide and small quantities of surface-adsorbed -SH species. A lead-deficient Pb1-xS phase is produced as the aging proceeds. Oxidation of the sulfur occurs later to form sulfite and sulfate products that are highly localized at the surface layers of the nanocrystals. These species show logarithmic growth kinetics, demonstrating that the sulfite/sulfate layer acts to passivate the nanocrystals. Our results demonstrate that the initial reaction of the PbS nanocrystals (forming lead hydroxide) is incongruent. The results are discussed in the context of the use of PbS nanocrystals as light-harvesting elements in next-generation solar technology.

Photoemission studies of oxide superconductors

A procedure has been developed for preparation of clean surfaces of oxide superconductors for study by electron spectroscopy. This involves annealing ceramic or thin film samples under an oxygen atmosphere in a preparation chamber that is an integral part of the spectrometer UHV system. The technique has allowed us to obtain high quality spectroscopic data from YBa,Cu,O,-, , Bi2Sr2Can-,Cun04+2n (n = 2, 3) and La2-,Sr,Cu04. Our results are considered in relation to other contemporary work. Current areas of controversy will be highlighted, including a discussion of Fermi edge and satellite structure in valence region spectra and evidence for mixed valency in Cu : 2p and 0 : 1s core regions.