Monday, 6 March 2017

Phase Transitions and Spectroscopy in Flatland

Phase Transitions and Spectroscopy in FlatlandThe investigation of two-dimensional materials beyond graphene is a fascinating and highly active area of research. Transition metal dichalcogenides (TMDCs) are layered materials similar to graphite, with a chemical structure of MX2 where M is a transition metal atom and X is a chalcogen atom. Single monolayers of TMDCs in many cases are semiconducting and can be addressed via optical techniques. We will use optical techniques to study interactions between TMDCs and other nanomaterials such as nanocrystals, nanotubes, and other TMDCs in the hopes of advancing photovoltaic technologies

In this talk, I present the vibrational properties and structural phases of 1T’-MoTe2 and Mo1-xWxTe2 alloys. Temperature-dependent and polarization-resolved Raman spectroscopy are the primary tools in these studies, and are complimented by aberration-corrected scanning transmission electron microscopy (STEM) as well as x-ray diffraction (XRD) measurements. Temperature-dependent Raman measurements provide evidence for a transition from 1T’-MoTe2 to a distorted orthorhombic phase (Td) below 250 K and facilitate identification of the anharmonic contributions to the optical phonon modes in bulk MoTe2 and Mo1-xWxTe2 alloys.4,5 At temperatures ranging from 100 K to 200 K, we find that all modes redshift linearly with temperature; however, below 100 K we observe nonlinear frequency shifts in some modes. We demonstrate that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. We also explore the composition-dependent optical properties of MoxW1-xTe2 alloys and identify clear signatures of the 2H, 1T’, and Td crystal structures.

Speaker: Dr. Patrick Vora, Department of Physics and Astronomy, George Mason University @HitItWithLight

Location: Room 203, Arts/Science Seminar Room