BEGIN:VCALENDAR VERSION:2.0 PRODID:Data::ICal 0.22 BEGIN:VEVENT DESCRIPTION:Patrick Vora\, George Mason University\n\n
The progress of mo dern technology is increasingly driven by the development and evaluation o f novel materials. Layered two-dimensional (2D) materials\, collectively r eferred to as van der Waals solids\, are receiving intense interest in the community due to their unconventional and diverse electronic behaviors. T ransition metal dichalcogenides (TMDs) are a class of 2D material followin g the basic chemical formula MX2\, where M = (Mo\, W\, Nb\, Re\, &hellip\; ) and X = (S\, Se\, Te\, &hellip\;). Varying the chemical composition allo ws access to semiconducting\, semi-metallic\, superconducting\, or magneti c behaviors in the 2D limit. Of recent interest are telluride based TMDs s uch as MoTe2 and WTe2 which are predicted exhibit controllable structural phase transitions appropriate for phase change memory applications as well as topologically protected and spin polarized electronic states.
\n\n< p>In this colloquium\, I will present our efforts to understand the temper ature-dependent optical properties of MoTe2 and MoxW1-xTe2 using temperatu re-dependent and polarization-resolved Raman spectroscopy. We have used th is technique to identify the anharmonic contributions to the optical phono n modes in bulk MoTe2 occupying the distorted orthorhombic (T­\;d) latt ice structure. At temperatures ranging from 100 K to 200 K\, we find that all modes redshift linearly with temperature however\, below 100 K we obse rve nonlinear frequency shifts in some modes. We show that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. Furthermore\, the highest frequency Raman modes show lar ge changes in intensity and linewidth near 250 K that correlate well with a structural phase transition.\n\nWe also explore the composition-d ependent optical properties of MoxW1-xTe2 alloys. Our observations identif y signatures of the hexagonal (H)\, monoclinc (1T&rsquo\;) and Td structur al phases. Polarization-resolved Raman measurements allow for the assignme nt of all vibrational modes as well as the evolution of mode symmetry and frequency with composition. We discover a previously unobserved WTe2 mode as well as a Raman-forbidden MoTe2 mode that is activated by compositional disorder. The primary WTe2 Raman peak is asymmetric for x <\;= 0.1\, an d is well fit by the spatial correlation model. From these fits\, we extra ct the spatial phonon correlation length which serves as an indirect measu re of the WTe2 domain size. Our study is foundational for future studies o f MoxW1-xTe2 and provides new insights into the impact of disorder in tran sition metal dichalcogenides.
\n DTSTART:20161117T160000Z LOCATION:Physics Building\, Room 313 SUMMARY:OPTICS IN FLATLAND: RAMAN SPECTROSCOPIC PROBES OF STRUCTURAL SYMMET RY END:VEVENT END:VCALENDAR