Scattering Tools for Nanostructure Phonon Engineering

Abstract

Abstract : The vibrational properties of solids have crucial roles underpinning functional properties ranging from thermal conductivity to electron mobility. It has long been known that the modification of the mechanical boundary conditions imposed by the minimal spatial extent of nanostructures results in changes in the phonon dispersion. These changes include the creation of vibrational modes with symmetries not permitted by the bulk crystal, but allowed in nanostructures. The new modes are predicted to have wavevectors with magnitudes that range across the entire Brillouin zone of reciprocal space. These new modes present a serious characterization challenge because existing experimental techniques for the characterization of phonons in nanomaterials, such as Raman scattering, are sensitive only to phonon modes with wavevectors of extremely small magnitude. Fundamentally the wavevectors that can be probed by Raman scattering are limited by the small momentum of photons in the visible spectrum. Our work supported by this grant from the AFOSR has addressed the phonon characterization problem using x-ray scattering. Our approach is based on an adapting x-ray thermal diffuse scattering (TDS) techniques to nanoscale systems. With this approach we can probe phonons across the full range of wavevectors, the entire Brillouin zone, from sample volumes as small as 4 cubic microns. We have demonstrated the TDS approach using silicon nanomembranes with a series of thicknesses as small as 4 nm. TDS techniques required membranes far flatter than what had been possible previous fabrication techniques and we have thus developed an edge-induced flattening technique that provides unprecedented flatness. The TDS experiments were conducted using synchrotron x-rays at the experimental facilities of the Advanced Photon Source. A series of new data analysis techniques were developed to allow quantitative analysis of the x-ray scattering results.