Author : Keliang He
Publisher :
ISBN 13 :
Total Pages : 156 pages
Book Rating : 4.:/5 (116 download)
Book Synopsis Optical Spectroscopy of Two-Dimensional Transition Metal Dichalcogenides (TMDCs) by : Keliang He
Download or read book Optical Spectroscopy of Two-Dimensional Transition Metal Dichalcogenides (TMDCs) written by Keliang He and published by . This book was released on 2014 with total page 156 pages. Available in PDF, EPUB and Kindle. Book excerpt: Atomically thin transition metal dichalcogenides (TMDCs), with the chemical stoichiometry MX2, where M is group VI transition metal atoms including Mo, W; and X is chalcogen atoms of S, Se and Te, have been emerging as a fast growing research field. Similar to graphene, a layered structure with weak van der Waals coupling between neighboring layers allows for them to be easily separated into single to few-layer thick samples. Different from graphene of zero band gap, they exhibit a band gap with magnitude between 1.5 and 2.0 eV and show current on-off ratios exceeding 108 in field-effect transistors. It has been discovered that an indirect to direct band-gap transition occurs in the monolayer limit at the K point of the Brillouin zone, which significantly enhances the photoluminescence quantum yield. Moreover, monolayer TMDCs are non-centrosymmetric arising from A and B sublattices being occupied by M and X atoms, respectively, which makes them an ideal laboratory to study valleytronics. The electronic structure of atomically thin TMDCs, mainly focusing on MoS2 and MoSe2, is investigated with optical absorption, photoluminescence and ultrafast pump-probe spectroscopic techniques in this thesis. First, we demonstrate continuous tuning of the electronic structure of atomically thin MoS2 on flexible substrates by applying a uniaxial tensile strain. A redshift at a rate of ~70 meV per percent applied strain for both mono- and bilayer MoS2 direct gap transitions, and at a rate 1.6 times larger for bilayer MoS2 indirect gap transitions, have been determined.Second, we investigate the optical response of monolayer MoS2 as a function of carrier density. Tightly bound negative trions, a quasi-particle composed of two electrons and a hole, are identified in doped monolayer MoS2. They possess a large binding energy (~ 20 meV), rendering trion effects significant even at room temperature. Third, we demonstrate that optical pumping with circularly polarized light can achieve complete dynamic valley polarization in monolayer MoS2. Moreover, we found that this polarization is retained for longer than 1 ns. Finally, preliminary results on the ultrafast dynamics in mono- and bilayer MoSe2 are presented. The photo-excited carrier dynamics is investigated via optical pump-probe technique.
