SPIN- AND VALLEY- DEPENDENT EXCITONS IN ATOMICALLY THIN TRANSITION METAL DICHALCOGENIDES
Open Access
- Author:
- Wang, Zefang
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 10, 2018
- Committee Members:
- Mauricio Terrones, Dissertation Advisor/Co-Advisor
Mauricio Terrones, Committee Chair/Co-Chair
Jie Shan, Committee Member
Moses H.W. Chan, Committee Member
Venkatraman Gopalan, Outside Member - Keywords:
- 2D
exciton
transition metal dichalcogenides
valley - Abstract:
- Transition metal dichalcogenides (TMDCs) exhibit remarkable electronic properties when thinned down to the monolayer limit. Among them, atomically thin semiconducting TMDCs, such as MoS2, MoSe2, WSe2, etc., attract board interest due to their unique electronic and optoelectronic properties. Electrons in 2D TMDCs acquire not only spin but also valley degree of freedom, and the spin and valley are coupled due to broken inversion symmetry and strong spin-orbit coupling. Opposite valleys are associated with opposite Berry curvature, giving rise to interesting valley physics and valleytronic applications. Another unique aspect of atomically thin TMDCs is strong excitonic effect. Owing to quantum confinement, monolayer semiconducting TMDCs become direct bandgap semiconductor in contrast to indirect bandgap in their bulk counterparts, and the excitonic effect gets greatly boosted due to the reduced dimensionality. Strong excitonic effect gives rise to strong light-matter interaction, making optical spectroscopy a powerful tool to access intriguing spin and valley properties of 2D TMDCs. In this dissertation, we explore the spin and valley dependent properties of monolayer TMDCs with optical and electrical transport techniques with high quality devices. In the first part, we study the electronic band structure in K/K’ valleys of Brillouin zone of monolayer WSe2 and MoSe2 by optical reflection and photoluminescence spectroscopy on dual gate field-effect transistors. Our experiment reveals the distinct spin polarization in the conduction band of these compounds by a systematic study of the doping dependence of A and B excitonic resonances. We obtained conduction band spin splitting delta_c is approximately 40meV for WSe2 and delta_c is approximately -30meV for MoSe2, which are in good agreements with first principle calculations. In the second part, we examined Landau level structure in monolayer WSe2 at the presence of out-of-plane magnetic field. It is proposed by theory that the Berry curvature in valley degree of freedom together with strong spin-orbit interaction can generate unconventional Landau levels under a perpendicular magnetic field. We applied handedness-resolved optical reflection spectroscopy and observed fully valley- and spin-polarized LLs in high quality WSe2 monolayer field-effect transistor and therefore derived LL structure. We also measured a sizeable doping-induced mass renormalization driven by strong Coulomb interactions. In the third part, we continued to explore the strong Coulomb interactions by studying the valley magnetic response in 2D TMDCs. We measured doping dependency of the valley Zeeman splitting of the charged exciton emission in monolayer WSe2 under an out-of-plane magnetic field. A nonlinear valley Zeeman effect correlated with an over fourfold enhancement in the g-factor, is observed. This enhancement occurs when Fermi level crosses the spin-split upper conduction band, corresponding to a change of spin-valley degeneracy from two to four, and can be understood as a consequence of a sharp increase in the exchange interaction when the number of electron species doubles. This interaction-enhanced valley magnetic response suggests 2D TMDCs as a new platform for exploring strongly interacting electron system with multiple internal degrees of freedom. In the final part, a study on interlayer exciton in bilayer WSe2 is presented. Interlayer excitons are sought for creating high exciton density and optoelectronic applications due to their long lifetime. Here we demonstrate highly tunable interlayer excitons by an out-of-plane electric field in bilayer WSe2. Continuous tuning of the exciton dipole from negative to positive orientation has been achieved and a large linear field-induced redshift up to ~100meV has been observed in exciton resonance energy. The Stark effect is accompanied by an enhancement of exciton lifetime by more than two orders of magnitude to >20ns. The exciton density as high as 1.2×10^11 cm^(−2) can be created by moderate continuous-wave optical pumping. Our result has paved the way for realization of degenerate exciton gases in 2D TMDCs.