J. Pant

Combining x-ray and optical spectroscopies in the study of dilute semiconductor nanoparticle composites

We discuss a methodology for the cooperative analysis of optical and x-ray spectroscopies to deduce the thermophysical properties of dilute suspensions of semiconductor nanoparticles in a wide band gap host. X-ray spectroscopy is used to determine concentration and bonding of selected elements. Resonant Raman spectroscopy establishes limits on composition and strain in particles. Analysis of optical absorption, with constraints provided by x-ray and Raman measurements, yields the particle size distribution and concentration. As an example of this approach, we study borosilicate glasses doped with ∼0.1 wt% CdS and heat treated to produce nanoparticles.

Order in CdS nanoparticles in glass

We have employed X-ray absorption spectroscopy to study the local bonding of Cd atoms in borosilicate glasses doped with Cd and S after heat treatment at temperatures from 550 to 750°C. Varying temperature and time can form CdS particles with average size from a few to several nanometers. In crystals grown below ∼700°C, the apparent number of second neighbors found from analysis of X-ray absorption fine structure is only a small fraction of that expected for well-ordered crystallites of these sizes. We attribute this apparent coordination deficit to substantial short-range structural disorder.

Homogeneous nucleation of CdS nanocrystals in glass

Abstract The precipitation of CdS and Cd 1− x Zn x S crystallites in borosilicate glass was studied using optical absorption, resonant Raman scattering and X-ray absorption spectroscopy. Evolution of the optical spectrum indicates that precipitates form by homogeneous nucleation of CdS from the glass, followed by Lifshitz-Slyozov coarsening.

XAS study of CdS nanocrystals formed in glass

Abstract We have measured and analyzed optical absorption and X-ray absorption spectra of CdS nanocrystals grown by solid-state reaction in a borosilicate glass. We observe and quantify the progressive replacement of O nearest neighbors of Cd by S with heat treatment and the appearance and growth of a well-defined second-nearest-neighbor peak, and correlate these changes with the evolving optical properties.

Evolution of Sn Environment in AlGaAs Alloys

We have performed fluorescence-detection XAFS measurements on Sn doped AlxGa1-xAs for x = 0.11 and 0.23. The Sn environment is found to change slowly with time.

XAS study of the liquid/UPD Cu/Pt interfacial region

Abstract The Cu atomic-scale environment in a layer of Cu deposited electrochemically on a polycrystalline Pt electrode has been studied in situ using X-ray absorption spectroscopy. Measurements were made on the layer with the X-ray electric field polarized both parallel and perpendicular to the surface. Spectra are interpreted using a model involving O, Cu and Pt near neighbors.

XAS study of Sn-related DX centers in (Al, Ga)As

Abstract We have used fluorescence-detection X-ray absorption spectroscopy to measure the Sn atomic-scale environment in single-crystal Sn doped Al 0.23 Ga 0.77 As in two states for direct comparison: in the dark at 80 K, where Sn atoms are in DX states; after photoexcitation at 20 K, where they are in shallow donor states. The observed differences in Sn near neighbors are not consistent with the widely accepted model for DX centers put forth by Chadi and Chang (PRL 1988).

Oscillating sample holder for XAS studies of single-crystal samples☆

Abstract We have designed and built a unique magnetically driven sample oscillator. Its effectiveness in enabling X-ray absorption spectroscopy studies of single-crystal samples is demonstrated.

Amplitude correction for grazing incidence surface XAS experiments

Abstract A technique of amplitude normalization for fluorescence detection surface XAS experiments is described. Fluorescence yield as a function of energy is normalized using a model for the electric field standing wave. Model parameters are fixed by variational optimization of the measured specular reflectivity. This method is applied to an in situ measurement of a nominal monolayer of Cu atoms underpotentially deposited onto polycrystalline Pt.

In-situ Study of an Underpotential-Deposited Cu Layer on Pt

We have performed in-situ fluorescence-detection x-ray absorption spectroscopy studies of the Cu environment in a layer of Cu deposited electrochemically on a large polycrystalline Pt electrode. We report spectra acquired with x-ray electric field vector parallel and perpendicular to the surface. This is the first time that these two measurements have been done on the same layer.