A.J. Scott

Probing the bonding and electronic structure of single atom dopants in graphene with electron energy loss spectroscopy

A combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations reveal striking electronic structure differences between two distinct single substitutional Si defect geometries in graphene. Optimised acquisition conditions allow for exceptional signal-to-noise levels in the spectroscopic data. The near-edge fine structure can be compared with great accuracy to simulations and reveal either an sp(3)-like configuration for a trivalent Si or a more complicated hybridized structure for a tetravalent Si impurity.

Evidence for the solubility of boron in graphite by electron energy loss spectroscopy

We present the results of transmission electron microscopy (TEM) and electron energy loss spectrometry (EELS) studies on two carbon–boron alloys both prepared by chemical vapour deposition at ca. 1000°C and differing in their [Boron]/[Carbon] atomic ratio as well as in their morphology. In both samples, impurity concentrations, principally oxygen and nitrogen, were found to be low relative to boron dopant levels. For low boron contents, typically around 5–10 at.%, the sample consisted of onion-like spherical particles approximately 10 nm in diameter which exhibited a non-homogeneous distribution of boron, concentrated at a level of 5–6 at.% in the centre. For this sample, studies of the B–K- and C–K-ELNES (electron energy loss near-edge structure) together with associated modelling of the unoccupied density of electronic states, indicate a substitution of boron atoms on threefold coordinated sp2-sites within the graphite network. For higher boron doping levels, typically 25 at.%, the sample consisted of homogeneous thin films. In this case, the change in shape of the B–K-ELNES indicates that boron has higher coordinations than planar trigonal together with possibly some residual sp2 sites. This study unambiguously demonstrates the presence of boron substitution solely within an sp2-bonded graphite network in the case of low boron contents and, when combined with other studies, gives an indication of the solubility limit for boron in graphite for the chemical vapour deposition (CVD) process.

An investigation of commercial gamma-Al2O3 nanoparticles

We present an analysis of the morphology and crystal structure of commercial gamma-alumina nanoparticles. Characterisation was carried out with a combination of several techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), selected area electron diffraction (SAED), in parallel with X-ray powder diffraction (XRD). TEM analysis of crystallite size and structure is in good agreement with XRD. HRTEM showed evidence for surface reconstruction on facetted cubeoctahedral nanoparticles. This is similar to results observed for cubeoctahedral nanoparticles of magnetite Fe3O4.

Electronic Structure Modification of Ion Implanted Graphene: The Spectroscopic Signatures of p- and n-Type Doping.

A combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations is used to describe the electronic structure modifications incurred by free-standing graphene through two types of single-atom doping. The N K and C K electron energy loss transitions show the presence of π* bonding states, which are highly localized around the N dopant. In contrast, the B K transition of a single B dopant atom shows an unusual broad asymmetric peak which is the result of delocalized π* states away from the B dopant. The asymmetry of the B K toward higher energies is attributed to highly localized σ* antibonding states. These experimental observations are then interpreted as direct fingerprints of the expected p- and n-type behavior of graphene doped in this fashion, through careful comparison with density functional theory calculations.

A Study of Commercial Nanoparticulate γ‐Al2O3 Catalyst Supports

This study investigates a range of commercially available γ‐Al2O3 powders by using a combination of integrated experimental techniques. These included general measurements of powder properties by using helium density, BET surface area, and scanning electron microscopy (SEM) analyses. In addition, dynamic light scattering and zeta potential measurements were used to investigate nanoparticle dispersions. Bulk crystal structures were analysed by using comparative X‐ray and neutron powder diffraction (XRD and NPD) analyses. Conventional transmission electron microscopy (TEM) was used to determine particle morphology, particle size, composition, and structure. Aberration‐corrected TEM was used to investigate the crystallinity of nanoparticles including the existence of any surface reconstruction on commonly observed facetted, cubeoctahedral γ‐Al2O3 nanoparticles. From the observation of peak splittings in diffraction data, we favour a description of the γ‐Al2O3 structure based on a distortion of the conventionally accepted face‐centred cubic (Fd

A systematic approach to choosing parameters for modelling fine structure in electron energy-loss spectroscopy.

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Universal synthesis method for mixed phase TiO2(B)/anatase TiO2 thin films on substrates via a modified low pressure chemical vapour deposition (LPCVD) route

m) structure into a tetragonal I41/amd structure. Distinct differences between TEM, XRD, and NPD data indicate the presence of some cation disorder within a rigid close‐packed oxygen framework. The Rietveld refinement of the NPD data suggests a high level of microstrain of 1.2 %. An improvement to the model is achieved by reducing the aluminium content in the unit cell, which is commensurate with the migration of aluminium ions to the surface and some degree of nonstoichiometry in the particle core. Aberration‐corrected TEM imaging and exit wave reconstruction confirm previous evidence for the presence of enhanced surface contrast at {1 1 1} surface facets, which we associate with the presence of excess cation termination. In addition, these {1 1 1} facets are observed to be heavily stepped. These results may have important implications for the thermal stability of metal catalyst nanoparticles on these high‐surface area supports; the migration of aluminium ions to the surface provides clear evidence of why these materials perform so well as catalyst supports.

Incisive probing of intermolecular interactions in molecular crystals: core level spectroscopy combined with density functional theory.

A potential methodology is presented for the systematic prediction of EELS edges using DFT, suitable for codes that calculate ELNES for a specific atom in a unit cell. The method begins with the selection of a unit cell, chosen as the smallest cell that still provides a physically valid representation of the bulk material. Within this small cell, a single electron core-hole is included in the atom for which the EELS ionisation edge is to be calculated. The basis-set size and k-point mesh of the DFT calculation are converged specifically against the predicted EELS result. Subsequently, the cell size is increased until the theoretical core-holes no longer interfere. At this point one can then modify the exact core-hole approximation. This methodology was applied to the new EELS module of the CASTEP pseudopotential DFT code, as well as the all-electron code Wien2k. Aluminium K edges were investigated for various aluminium metal systems. It was observed that as the cell size was increased the predicted EELS result became less sensitive to the exact core-hole approximation used. It was noted however that due to high screening in metals a ground state single cell calculation is often acceptable. The semiconductor aluminium nitride (wurtzite form) was also investigated. It was observed that for both Wien2k and CASTEP, with careful convergence of the key DFT code parameters, single cell ground state calculations gave a reasonable agreement with experiment, contrary to what might be expected for a semiconductor with a large band gap. This was particularly true of the Wien2k result. Given the greater computational effort required for supercell calculations, these results are likely to form the beginnings of a detailed investigation into accepted methods of ELNES predictions.

Atomic-Scale Surface Roughness of Rutile and Implications for Organic Molecule Adsorption

A universal method for the synthesis of mixed phase TiO2 bronze (B)/anatase titania thin films by Low Pressure Chemical Vapour Deposition (LPCVD) onto any substrate is presented. General LPCVD conditions were titanium isopropoxide (TTIP) and N2 gas as the precursor and carrier gas respectively, 600 °C nominal reaction temperature, and 15 min reaction time; a range of different substrates were investigated including: a silicon wafer, fused quartz, highly ordered pyrolytic graphite (HOPG) and pressed graphite flake (grafoil). X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy were used to characterise the thin films which exhibited a columnar morphology together with smaller equi-axed particles. Pre-treatment of substrates by spraying with a Na-containing solution was found to encourage the crystallization of TiO2(B) during the LPCVD process. Increasing the concentration of Na in the pre-treatment process resulted in a higher proportion of TiO2(B) in the thin films up to an optimum condition of 0.75% w/v of Na. Na diffusion from the substrate surface into the adjacent TiO2 is the proposed mechanism for promoting TiO2(B) formation as opposed to the anatase phase with Density Functional Theory (DFT) modelling suggesting the presence of Na stabilises the TiO2(B) phase. Dye degradation tests indicate an increased photocatalytic activity for mixed phase anatase/TiO2(B) thin films.

Electron energy-loss spectroscopy (EELS) studies of an yttria stabilized TZP ceramic

The α-form of crystalline para-aminobenzoic acid (PABA) has been examined as a model system for demonstrating how the core level spectroscopies X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) can be combined with CASTEP density functional theory (DFT) to provide reliable modeling of intermolecular bonding in organic molecular crystals. Through its dependence on unoccupied valence states NEXAFS is an extremely sensitive probe of variations in intermolecular bonding. Prediction of NEXAFS spectra by CASTEP, in combination with core level shifts predicted by WIEN2K, reproduced experimentally observed data very well when all significant intermolecular interactions were correctly taken into account. CASTEP-predicted NEXAFS spectra for the crystalline state were compared with those for an isolated PABA monomer to examine the impact of intermolecular interactions and local environment in the solid state. The effects of the loss of hydrogen-bonding in carboxylic acid dimers and intermolecular hydrogen bonding between amino and carboxylic acid moieties are evident, with energy shifts and intensity variations of NEXAFS features arising from the associated differences in electronic structure and bonding.