Colloquium

Monday 6 September 2021

Speaker

George Kioseoglou

Affiliation

Dept. of Materials Science and Technology, University of Crete and Institute of Electronic Structure and Laser, FORTH

Title

2D Materials beyond graphene- Spin Valley Physics in TMDs and their heterostructures

Room

Online presentation. Please visit the link.

Time

16:00

Language

English

Abstract

Motivated by the success of graphene, 2D materials have become the focus of intense research due to their unique physical and chemical properties. Specifically, monolayers of transition metal dichalcogenides (TMDs) of the form MX₂ (M = Mo, W and X = S, Se), represent a large family of 2D direct-gap semiconductors where Coulomb-bound electron-hole quasiparticles (excitons,trions and biexcitons) of high binding energy dominate the emission spectra. In addition, the low-dimensional hexagonal structure leads to two inequivalent K-points in the Brillouin zone. This valley index and spin are intrinsically coupled, and spin-dependent selection rules enable one to independently populate and interrogate a unique K-valley with circularly polarized light [1-4]. There is an intense experimental and theoretical effort to understand the parameters affecting the polarization in TMDs such as photoexcitation energy [3,4], temperature [4], carrier density, e-ph interaction and dielectric environment. In the first part, we 'll review the drastic effect the dielectric environment has on the optical properties as well as on the spin-valley polarization of TMDs and their heterostructures. Specifically, we 'll discuss the extraordinary spin-valley polarization phenomena in WS₂/Graphene, WS₂/h-BN and suspended WS₂ monolayers compared to the common WS₂/SiO₂/Si system in a range of temperatures from 4K up to room temperature [6]. Spin-valley relaxation phenomena and the related scattering mechanisms of both neutral (X⁰) and charged excitons (X^-) are examined in resonant and off-resonant excitation conditions. In the second part, we 'll present our efforts to systematically control the e-density in WS₂ monolayers through photochemical doping (in a precursor chlorine atmosphere) and therefore control the emitted circular polarization [7,8]. The increase of the photochlorination time gives rise to a systematic red-shift in the PL energy of the X⁰ with subsequent increase of its intensity indicating a reduction of the e-density. A spin-valley polarization tunability by more than 40% in 1L-WS₂ on hBN has been demonstrated. The decrease in circular polarization after photochlorination is attributed to the significant reduction of the active defect sites and consequently to the increase in the non-radiative exciton lifetime.

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4. A.T. Hanbicki et al., Scientific Reports 6, 18885 (2016),
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7. I. Demeridou et al, 2D Mater. 6, 015003 (2019),
8. I. Demeridou, et al., Appl. Phys. Lett. 118, 123103 (2021)