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Dive into the research topics where S J Gurman is active.

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Featured researches published by S J Gurman.


Journal of Physics: Condensed Matter | 1990

Reverse Monte Carlo simulation for the analysis of EXAFS data

S J Gurman; R L McGreevy

The authors describe the application of the reverse Monte Carlo simulation technique to the analysis of EXAFS data. This technique generates a series of three dimensional particle configurations which are consistent with the experimental EXAFS spectrum. No input information other than the density and the chemical composition of the sample is required. They present initial results for crystalline and amorphous silicon and preliminary results for silver bromide. Since the method works directly from an experimental spectrum it promises to be extremely powerful in modelling the structures of amorphous materials.


Journal of Physics: Condensed Matter | 1993

The effect of varying substrate temperature on the structural and optical properties of sputtered GaAs films

S.H. Baker; S C Bayliss; S J Gurman; N Elgun; J S Bates; E.A. Davis

Stoichiometric GaAs films have been prepared by RF sputtering over an appreciable range of substrate temperature Ts. The structural and optical properties of these films have been investigated by means of a variety of experimental techniques. Transmission electron microscope (TEM) experiments reveal that for Ts>or= approximately 70 degrees C the structure consists of nanocrystals embedded in an amorphous network. EXAFS, XPS and infrared experiments are consistent with each other, indicating that configurational disorder in the amorphous network of the GaAs films decreases as Ts is raised. Most of the disorder relates to bond-angle disorder; the spread in bond angle around Ga atoms is found to be less than that around As atoms for a given substrate temperature. The changes observed in the fundamental absorption edge with increasing Ts are discussed in relation to various possible defects in amorphous GaAs.


web science | 1992

Structural investigation of the a-Ga1−xAsx system

S.H. Baker; M.I. Manssor; S J Gurman; S.C. Bayliss; E.A. Davis

a-Ga 1− x As x ( x >0.5) films have been prepared by rf sputtering. The local structure and bonding configurations in these films have been investigated by EXAFS measurements. The As-coordination is shown to change from fourfold at stoichiometry towards threefold for As-rich films, while the Ga-coordination remains at 4 throughout the composition range studied. The EXAFS data are correlated with optical (visible and infrared) absorption experiments undertaken on the same films to provide a consistent picture of the local structure.


Journal of Synchrotron Radiation | 1999

Amplitude reduction in EXAFS

Mervyn Roy; S J Gurman

In real systems, inelastic processes remove photoelectrons from the elastic scattering channel. This reduces the amplitude of the EXAFS causing disagreement between the experimental and theoretically predicted amplitudes. Traditionally these discrepancies were treated by including two semi empirical reduction factors in the data analysis: a mean tree path term, which models the so called extrinsic loss processes, and a constant amplitude reduction factor which accounts for many electron excitations at the absorbing atom. The extrinsic inelastic effects may, however, be modelled more rigorously using a complex exchange and correlation potential. For example the Hedin-Lundqvist (H-L) potential used in most EXAFS data analysis programs. We present a method by which the losses caused by such a potential may be evaluated quickly and easily in the first Born approximation. The losses produced by the H-L potential significantly overestimate those produced by the mean free path alone. Instead the losses appear to agree well with the total reduction given by the semi-empirical reduction factors. These losses do not exhibit the correct low or high energy behaviour but do show excellent agreement with experiment over the range of a typical EXAFS spectrum. We therefore conclude, that the semi-empirical reduction parameters should not be included when data fitting using the H-L potential.


Electrochimica Acta | 2001

Dynamic EXAFS study of discharging nickel hydroxide electrode with non-integer Ni valency

N.R.S Farley; S J Gurman; A.R. Hillman

Abstract A dynamic in situ X-ray absorption study, in the energy dispersive mode, has revealed short-range structural changes taking place during the discharge of a nickel hydroxide film electrode. A Jahn–Teller distortion has been identified in the first Ni–O shell at overcharge, which diminishes during the discharge process. Throughout discharge, the first Ni–Ni shell moves antagonistically to Ni–O coordination. These structural changes and Ni K-edge shifts have been used as measures of Ni valency during film discharge. Correlation of the results with electrode charge leads to a simple mechanism, in terms of electron population at Ni sites, for the discharge process.


Journal of Non-crystalline Solids | 2002

Structure of amorphous GexSe1-x and GexSeyZnz thin films : an EXAFS study

J. Choi; S J Gurman; E.A. Davis

Abstract Extended X-ray absorption fine structure (EXAFS) measurements have been made on thin films of a-Ge x Se 1− x with 0.2 x x x Se y Zn z films show that the lengths of Ge–Zn and Se–Zn bonds are 2.57±0.05 and 2.44±0.03 A, independent of the composition. Results on the atomic coordinations reveal that Zn is fourfold coordinated and preferentially bonds to Se atoms.


web science | 1999

Simple cell for in situ X-ray absorption spectroelectrochemistry

N.R.S Farley; S J Gurman; A.R. Hillman

Abstract A simple, easily constructed in situ X-ray absorption spectroelectrochemical cell is described. Its capabilities for acquiring good quality, reproducible X-ray absorption spectra in the transmission, fluorescence and energy dispersive modes, in tandem with standard electrochemical measurements, are demonstrated using electrodeposited nickel hydroxide films.


Journal of Non-crystalline Solids | 1996

Optical properties and structure of unhydrogenated, hydrogenated, and zinc-alloyed a-GexSe1-x films prepared by radio-frequency sputtering

J. Choi; Anirudh Singh; E.A. Davis; S J Gurman

The optical absorption edges of films in the a-Ge x Se 1-x system, prepared by radio-frequency sputtering, have been determined over the composition range 0.2 < x < 1. The largest optical gap occurs at a composition corresponding to GeSe 2 , as for glasses and evaporated films. Hydrogenation and alloying with up to 27% zinc both lead to only small changes in the absorption edges. The electrical conductivity activation energies are lower in films alloyed with zinc. Extended X-ray absorption fine structure (EXAFS) measurements confirm a chemically ordered random network model with Ge and Zn both being four-fold coordinated and Se two-fold coordinated. The optical data are interpreted in terms of changes in the local structure with composition.


Journal of Physics: Condensed Matter | 2004

Atomic structure of embedded Fe nanoclusters as a function of host matrix material: a synchrotron radiation study

S.H. Baker; Mervyn Roy; S J Gurman; S. Louch; A. Bleloch; C. Binns

The atomic structure of Fe nanoclusters embedded in a range of matrix materials has been studied using synchrotron radiation. In particular, the effect of embedding the clusters in Ag, amorphous carbon (a-C) and a porous C60 matrix is investigated. The embedded cluster samples were prepared by co-deposition using a gas aggregation cluster source. Samples with both dilute and high-volume-filling fraction of clusters, at 4 and 40% respectively, were prepared. Fe K edge EXAFS measurements were used to probe the structure within the clusters. In a Ag matrix, the Fe clusters retain the b.c.c. structure of bulk Fe while in a-C there is evidence for both b.c.c. and f.c.c. structures in the clusters. These results are independent of cluster volume-filling fraction over the range investigated. When embedded in a porous C60 matrix, the Fe clusters oxidize to Fe2O3.


Journal of Synchrotron Radiation | 2001

An investigation of the use of the Hedin-Lundqvist exchange and correlation potential in EXAFS data analysis

Mervyn Roy; S J Gurman

In real systems, inelastic processes remove photoelectrons from the elastic scattering channel. This reduces the amplitude of the EXAFS. Traditionally the discrepancies between experimental and theoretical amplitudes were treated by including two semi-empirical reduction factors in the data analysis. Some inelastic effects may, however, be modelled more rigorously using a complex exchange and correlation potential, for example the Hedin-Lundqvist (HL) potential used in most EXAFS data-analysis programs. In this paper a systematic study of the effects of the HL potential on the calculated EXAFS amplitudes is presented. Expressions are derived whereby the EXAFS amplitudes may be examined in the presence of an arbitrary complex potential independently to the rest of the EXAFS signal. These results are used to study the effects of the HL potential on EXAFS data analysis in detail.

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Dive into the S J Gurman's collaboration.

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E.A. Davis

University of Leicester

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S.H. Baker

University of Leicester

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Mervyn Roy

University of Leicester

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C. Binns

University of Leicester

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N Elgun

University of Leicester

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S C Bayliss

University of Leicester

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S.C. Bayliss

Loughborough University

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J C Amiss

Sheffield Hallam University

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J. Choi

University of Leicester

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S. Louch

University of Leicester

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