Theoretical study of charge dynamics in two-dimensional materials

Abstract

Exfoliation of graphene in 2004 initiated intensive attention on layered two-dimensional (2D) materials. So far, except for graphene, a large family of 2D materials has been reported, such as transition metal dichalcogenides (TMDCs), hexagonal boron nitride, black phosphorene, metal nitrides/carbides and their van der Waals heterostructures, etc. The large number of species, unique electrical and optical properties and versatile functionalities render 2D materials as promising materials for electronic, optoelectronic and photovoltaic devices. This thesis investigates ultrafast charge dynamics in 2D material system based on real-time time-dependent density functional theory method within Ehrenfest framework. This work is divided into three major parts.

In first part, we investigate the ultrafast interlayer charge transfer process in the graphene/WS2 heterostructure. Our results demonstrate that photo-induced holes transfer from WS2 to graphene more efficient than electrons. The ultrafast charge dynamics arises from the coupling to nuclear vibrations and its amplitude and polarity show a strong dependence on the external electric fields. Further analysis reveals that carrier dynamics in the heterostructure is the result of competition between interlayer and intralayer relaxation process, which is governed by the couplings between carriers and their acceptor states. This work establishes a firm correlation between the charge dynamics and couplings between states in 2D heterostructures, and provide practical methodology to manipulate carrier dynamics at heterointerfaces.

In second part, we study the carrier multiplication (CM) phenomenon in six monolayer TMDCs MX2 (M = Mo, W; X = S, Se, Te). Our results present that CM is observed in all six TMDCs. The threshold energy of CM can be substantially reduced to 1.75 bandgap (Eg) via couplings to phonon modes. Since electron-phonon couplings can result in significant changes in electronic structures, even trigger semiconductor-metal transition, and eventually assist CM beyond threshold limit. Chalcogen vacancies can further decrease the threshold due to sub-gap defect states. In particular for WS2, CM occurs with excitation energy of only 1.51Eg. Our results identify TMDCs as attractive candidate materials for efficient photovoltaic devices with the advantages of high photo-conductivity and phonon-assisted CM characteristic.

In third part, we report the effect of doping levels on CM in graphene. Our calculation results indicate that doping level can introduce remarkable differences in CM conversion efficiency in graphene. Specifically, the CM quantum yield can be promoted from 1.41 to 1.89 when the Fermi level rising from 0.40 eV to 0.78 eV via n-doping. Consistently, time- and angle-resolved photoemission spectroscopy measurements on n-doped graphene present the same correlation between doping levels and CM conversion efficiency. Our results provide a practical strategy to promote the performance of graphene as a photovoltaic material by tuning doping levels.