Can PAC Measurements be Used to Investigate Defects in Nano-Structures?

Abstract

The PAC-technique always claims to test the micro-surrounding of the probe atoms. Typically, the samples are macroscopic and more or less homogeneous and there is no debate about the usefulness of the method: substitutional sites, trapped vacancies or phase transitions are easily seen. Even the PAC-“fingerprint” of an amorphous material is known. In case of inhomogeneous samples, perhaps made out of different constituents, the question arises whether the PAC can contribute to the understanding of such materials or not. The article will show the different ways to introduce the probe atoms into the samples and discuss then the influence of these histories on the final site of the probe in the sample. In general, diffusion tends to place the probes into grain boundaries, whereas implantation reaches the bulk. This becomes important for nano-materials with their high fraction of internal surfaces. As a second, important difference for possible experiments the spatial distribution of the probes has to be considered. Implantation leads to a Gaussian shaped depth distribution of the probes. This corresponds – in a certain region – to a 3-dimensional distribution of probes in the sample, used e.g. when doping a semiconductor. In the production of special sensors (which apply e.g. the giant magnetoresistance (GMR) effect) one needs a different package, thin films (1-2 mono-layers). To apply PAC here, the probes have to be introduced during the fabrication of such a sensor. In these cases the probes have to be placed within a plane of nano-scale thickness.

Perturbed angular correlation studies of uniaxial compressive stressed zinc, titanium, rutile, Ti2AlN, and Nb2AlC

“…The influence of grain boundaries is believed to be small since the grain sizes D were on the order of several microns. Their contribution δ GB is proportional to 1/D and only relevant in nanocrystalline material [47]. Delaminations and cracks are not considered for the same reason but should contribute to δ as well.…”

Untersuchung der elektrischen Hyperfeinwechselwirkung in M_n+1AX_n-Phasen mittels der gestörten γ-γ-Winkelkorrelation

“…0.33 bis 0.35 gut mit dem hier ermittelten Wert von 0.37(2) überein (siehe Tabelle 7.4). Betrachtet man die große Dämpfung des PAC-Signals, dann ist wohl die einfachste Erklärung, dass sich die 181 Ta-Sonden in sehr kleinen Nanokristalliten von HfO 2 befinden, deren Verteilungsbreite aufgrund des großen Oberfläche-Volumen-Verhältnisses typischerweise deutlich zunimmt [155].…”

Elektrische Quadrupolwechselwirkung in defektreichen und deformierten MAX-Phasen

“…Unabhängig von diesem konzeptionellen Problem ist anzunehmen, dass die durch Korngrenzen verursachte Verbreiterung des Feldgradienten mit dem Verhältnis aus Oberfläche und Volumen der Kristallite verknüpft ist. Aus dieser Annahme ergibt sich ein zur inversen Korngröße proportionaler Beitrag, der erst für nanokristalline Materialien signifikant wird [191].…”