Momentum resolved ground/excited states and the ultra-fast excited state dynamics of monolayer MoS2

TL;DR

This study uses advanced spectroscopy techniques to investigate the electronic structures and ultrafast excited state dynamics of wafer-scale monolayer MoS2, revealing detailed momentum-space information and rapid carrier scattering processes.

Contribution

It demonstrates the creation of large, high-quality monolayer MoS2 samples suitable for detailed momentum-resolved studies and provides new insights into its ultrafast excited state dynamics.

Findings

Mapped quasiparticle band structure in valence and conduction bands

Unveiled ultrafast inter- and intra-valley carrier scattering

Observed a rapid energy shift of ~0.2 eV in excited states

Abstract

The emergence of transition metal dichalcogenides (TMD) as crystalline atomically thin semiconductors has created a tremendous amount of scientific and technological interest. Many novel device concepts have been proposed and realized (1-3). Nonetheless, progress in k-space investigations of ground/excited state electronic structures has been slow due to the challenge to create large scale, laterally homogeneous samples. Taking advantage of recent advancements in chemical vapor deposition, here we create a wafer-size MoS2 monolayer with well-aligned lateral orientation for advanced electron spectroscopy studies (4-6). Low energy electron diffraction and scanning tunneling microscopy (STM) demonstrate atomically clean surfaces with in-plane crystalline orientation. The ground state and excited state electronic structures are probed using scanning tunneling spectroscopy (STS), angle-resolved photoemission (ARPES) and time-resolved (tr-)ARPES. In addition to mapping out the momentum-space quasiparticle band structure in the valence and conduction bands, we unveil ultrafast excited state dynamics, including inter- and intra-valley carrier scattering and a rapid downward energy shift by ~ 0.2eV lower than the initial free carrier state at Sigma point.