Just a friendly reminder! This is TODAY!!
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Dear colleagues and friends,
You are cordially invited to attend my thesis defense. Details are below.
Title: Towards Valleytronics in Monolayer Transition Metal Dichalcogenides
Presenter: Brian Modtland, RLE
Supervisor: Marc Baldo
Date: Thursday, January 11, 2018
Time: 10:30am
Location: Marlar Lounge (37-252)
Abstract:
Monolayer transition metal dichalcogenides (TMDs) exhibit distinct electrical and optical
properties according to the relative occupation of each of two valleys in their dispersion
relation. The resulting valley degree of freedom is robust, linked to a large spin-orbit
splitting between valence bands, and shows promise in electro-optical devices or as an
information token for logic applications. In order to explore applications of these
properties, monolayer crystals are required that have reduced intervalley scattering. To
date, the majority of valley-related studies have focused on exfoliated samples for their
quality and ease of production. In this thesis, valley polarization is explored in
monolayer tungsten disulfide (WS2) synthesized by chemical vapor transport (CVT). This
novel method of bottom-up growth relies on halide-driven vapor transport commonly utilized
in bulk crystal growth. Using a small amount of sodium chloride salt as a source of
chlorine, non-volatile WS2 can react to form gaseous tungsten chloride and sulfur. With
an open tube system, a controlled reaction generates mono- and few- layer WS2 crystals.
These crystals have excellent optical properties and exhibit a degree of valley
polarization near 50% at 77 K and up to 30% at room temperature. This surpasses previous
values reported in WS2. By decoupling pump photon and thermal energy, valley
depolarization shows the characteristics of an electron-hole exchange interaction rather
than nonradiative scattering. These results offer the initial groundwork for future
devices that use the coupled valley-spin degree of freedom as a robust token of
information, promising reduced power consumption compared to conventional MOSFET-based
electronics.
Best regards,
Brian Modtland
PhD Candidate | EECS | MIT
Spin and Excitonics Engineering Group
Dept. of Energy - Center for Excitonics
(641) 780-8678
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