Non-adiabatic Molecular Dynamics Study Of Ultrafast Charge Separation In TMD Heterostructures

The dynamics of excited states involves the knowledges of potential energy surface (PES) of excited states and nonadiabatic couplings between them. The motions of the classical particles produce time variations of the hamiltonian that govern the quantum particles. This induces transitions among the quantum states of the fast coordinates. These changes of quantum state, in turn, alter the forces that govern the classical motion. In these circumstances, the tradi-tional Born-Oppenheimer (BO) approximation, in which the transition between quantum states is neglected, will not suffice. In chapter one, we start from Born-Oppenheimer molecular dynamics (MD) and the situation where it fails. Then two methods beyond BOMD were introduced - Ehrenfest Dynamics and Surface Hopping. In the last part, we introduced the non-adiabatic molecular dynamics (NAMD) proposed by Prezhdo that combines time-dependent density functional theory and fewest-switches surface hopping (FSSH).In chapter two, we briefly introduced the family of 2D layerd materials and the van der Waals (vdW) heterostructures formed by them. Among them, tran-sition metal dichalcogenides (TMD) gain a lot of interests recently due to their extraordinary optical and electronic properties. Moreover, ultrafast charge sep-aration was observed in heterostructures formed by monolayer TMDs in recent experiments. Efficient electron-hole spatial separation is of uttermost importance in solar-energy conversion, thus the understanding of ultrafast charge separation mechanism is valuable for novel 2D-devices design.In the third chapter, we applied NAMD proposed by Prezhdo to study the ultrafast charge separation process in MoS2/WS2 heterojunction, we show that instead of direct tunneling, the ultrafast interlayer hole transfer is strongly pro-moted by an adiabatic mechanism through phonon excitation.In chapter four, in order to further understand the ultrafast charge separation in TMD heterojunction, we applied NAMD to study the photoexcited carrier dynamics in two similar systems - MoS2/WS2 and MoSe2/WSe2.In the last chaper. a brief summary was given.