HAXPES of protected ITO surfaces at the buried P3HT/ITO interface

The characterization of buried interfaces is difficult and often has to be performed by a post‐processing method where the interface is exposed. Hard energy X‐ray photoelectron spectroscopy offers the ability to tune the X‐ray energy and thereby change the information depth. In this work, an inorganic/organic interface was evaluated, namely the poly(3‐hexylthiophene) (P3HT) interface with indium tin oxide (ITO), with relevance to organic photovoltaic devices. P3HT/ITO buried interfaces were examined using three X‐ray energies where the ITO surface was prepared under different pretreatment conditions. The P3HT film protected the ITO surface from adventitious adsorbents and allowed for sensitivity to the buried ITO surface. Robust peak fitting parameters were obtained to model the O 1 s and In 3d lineshapes. The deconvolution of these lineshapes allowed for the clear identification of a surface layer on the ITO which is oxidized to a greater extent than the underlying bulk ITO. The surface oxide layer, composed of indium oxide and indium hydroxide, is deficient of oxygen vacancies and would therefore be expected to act as an insulating barrier on the ITO surface. Peak fitting conditions allowed for an estimation of the relative thicknesses of this insulating layer. Copyright © 2012 John Wiley & Sons, Ltd.     

Direct measurement of valence-charge asymmetry by x-ray standing waves

By monitoring valence-photoelectron emission under condition of strong x-ray Bragg reflection, we have determined that a majority of GaAs valence charge resides on the anion sites of this heteropolar crystal, in quantitative agreement with the GaAs bond polarity as calculated from the Hartree-Fock term values. In contrast, the valence-charge distribution in Ge is found to be symmetric. In both cases, the valence emission is found to be closely coupled to the atomic cores.     

Performance of a four‐element Si drift detector for X‐ray absorption fine‐structure spectroscopy: resolution, maximum count rate,and dead‐time correction with incorporation into the ATHENA data analysis software

The performance of a four‐element Si drift detector for energy‐dispersive fluorescence‐yield X‐ray absorption fine‐structure measurements is reported, operating at the National Institute of Standards and Technology beamline X23A2 at the National Synchrotron Light Source. The detector can acquire X‐ray absorption fine‐structure spectra with a throughput exceeding 4 × 10⁵ counts per second per detector element (>1.6 × 10⁶ total counts per second summed over all four channels). At this count rate the resolution at 6 keV is approximately 220 eV, which adequately resolves the Mn Kα and Kβ fluorescence lines. Accurate dead‐time correction is demonstrated, and it has been incorporated into the ATHENA data analysis program. To maintain counting efficiency and high signal to background, it is suggested that the incoming count rate should not exceed ～70% of the maximum throughput.     

Nondestructive compositional depth profiling using variable‐kinetic energy hard X‐ray photoelectron spectroscopy and maximum entropy regularization

We discuss the calculation of nondestructive compositional depth profiles from regularization of variable kinetic energy hard X‐ray photoelectron spectroscopy (VKE‐XPS) data, adapting techniques developed for angle‐resolved XPS. Simulated TiO₂/Si film structures are analyzed to demonstrate the applicability of regularization techniques to the VKE‐XPS data and to determine the optimum choice of regularization function and the number of data points. We find that using a maximum entropy‐like method, when the initial model/prior thickness is similar to the simulated film thickness, provides the best results for cases where prior knowledge of the sample exists. For the simple structures analyzed, we find that only five kinetic energy spectra are necessary to provide a good fit to the data, although in general, the number of spectra will depend on the sample structure and noisiness of the data. The maximum entropy‐like algorithm is then applied to two physical films of TiO₂ deposited on Si. Results suggest interfacial intermixing. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.     

Synthesis of large quantities of single-walled aluminogermanate nanotube

A simple aqueous synthesis yielded about 100 times more structurally well-organized single-walled aluminogermanate nanotubes than previously reported "standard" procedures. The structure analyses using XRD, IRTF, TEM, and XAS were greatly facilitated by the high concentrations available, and they ascertained the imogolite-like structure of the nanotubes. Simplicity and yield of the synthesis protocol are likely to favor commercial applications of theses materials as well as simplified syntheses of other nanophases.     

Local structure determination in strained-layer semiconductors

The theory of elasticity accurately describes the deformations of macroscopic bodies under the action of applied stress [1]. In this review, we examine the internal mechanisms of elasticity for strained-layer semiconductor heterostructures. In particular, we present extended x-ray-absorption fine structure (EXAFS) and x-ray diffraction (XRD) measurements to show how the bond lengths and bond angles in semiconductor thin-alloy films change with strain when they are grown coherently on substrates with different lattice constants. The structural distortions measured by experiment are compared to valence-force field (VFF) calculations and other theoretical models. Atomic switching and interfacial strain at buried interfaces are also discussed.