Optical and electrical characterization of two-dimensional transition metal dichalcogenides

Abstract

Monolayer transition metal dichalcogenides (TMDs), an emerging family of two-dimensional (2D) semiconductors with a direct band gap at visible and near-infrared range, have been attracting great interests for their extraordinary optical and electronic properties, such as enhanced exciton effect, valley related physics, giant spin-obit coupling and spin-valley locking. In this thesis, the optical and electrical characterization of 2D TMDs is discussed, focusing on the study of exciton binding energy and the optical control of spin-polarized photocurrents of 2D TMDs. The exciton binding energy in monolayer WS2 was measured to be 0.71eV by two-photon photoluminescence excitation (TP-PLE) spectroscopy. This was directly extracted as the difference between the single particle band gap and the optical band gap. The former was measured from the onset of interband continuum in TP-PLE spectrum, while the latter was obtained from the ground-state exciton peak measured in linear absorption and photoluminescence spectra of the same sample. Spin-polarized photocurrents were generated by circularly polarized laser and electrically detected by our well-designed spin-valve-like device. Remarkably, spin polarization of the photocurrents was shown to be manipulated by tuning photon energy of the circularly polarized excitation light with the same helicity, utilizing the valley dependent optical selection rules, the giant spin-orbit coupling and the spin-valley locking in monolayer TMDs. (208 words)