FRG: Nanoscale Order in Amorphous Solids: Structure, Transformations, and Electronic Properties

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

This FRG project is a collaborative effort among five co-PIs, and additional collaborators with expertise in amorphous materials: preparation and analysis (Abelson, Bishop), FEM technique (Gibson, Voyles), TEM (Zuo), molecular dynamics and quantum mechanical simulations (Drabold), and statistical mechanics and topology of disordered networks (Goldbart). The project goals are to quantitatively determine the nanostructure in group IV and chalcogenide amorphous materials, and the relationship of this order with electronic properties and phase transformations. The approach is based on fluctuation electron microscopy (FEM)-a new analytical technique with the only purely structural view of MRO (medium range order) currently available. FEM in-volves a statistical analysis of the intensity scattered from nanometer-sized volumes of the sam-ple as a function of the volume, size, and scattering conditions. The project addresses nanoscale order in representative amorphous and glassy materials-amor-phous silicon and chalcogenides- which are without long range order. These materials are known to exhibit medium range order, but little is known about the nature of this ordering on the na-noscale. Medium-range order (MRO) is typically a structural correlation at length scales longer than the diameter of the third coordination shell but shorter than the scale at which ordering ap-pears as Bragg peaks in the structure factor. For amorphous silicon (a-Si), that translates to length scales of 1-4 nm. Structure at those length scales in the presence of disorder has historically been difficult to measure. Standard diffraction techniques yield little information at medium-range because of the isotropic nature of most disordered materials. The PIs have shown that strain affects small topologically crystalline regions in a-Si, rendering them not detectable in diffraction. %%% The project addresses fundamental research issues in areas of electronic materials science having technological relevance. An important feature of the project is the strong emphasis on education, and the integration of research and education. The combined resources, including experimental and theoretical methods, provide special opportunities for education and training of post doctoral associates, graduate and undergraduate students involved in highly interdisciplinary forefront research. ***