First-Principles Simulations Of Excited States Dynamics In Low-Dimensional Materials

Interplay among different degrees of freedom（e.g.,electrons,phonons and spins,plasmons）is of paramount importance in understanding and optimizing the properties of quantum materials.The isolation or synthesis of two-dimensional（2D）materials such as graphene and transition metal dichalcogenides has opened up new venues for designing novel optoelectrical and photovoltaic devices.Photoexcitation induced dynamics in two-dimensional materials（e.g.,interfacial charger transfer and photoinduced phase transition）has been a hot topic in condensed physics and material science.In this thesis,we focus on the photoexcitation induced processes in low-dimensional materials.The systematic studies on two-dimensional heterostructures,borophene and charge density waves（CDW）materials aim at clarifying the interesting issues of the structural and electronic properties and dynamics upon excitation using density functional theory and time-dependent density functional theory.The main contents of this thesis are listed as below.1.We demonstrated photoexcitation induced carrier dynamics in low-dimensional materials.Systematical study on the interlayer charge transfer,carrier relaxation and energy transfer in MoS₂/WS₂,MoS₂/WeS₂ and Au₅₅/MoS₂ heterostructures with time-dependent density functional theory methods.Our results established the interlayer stacking as an effective freedom to control ultrafast charge transfer dynamics in 2D heterostructures.In addition,we discovered an interfacial pathway at the MoS₂/WSe₂interface for the relaxation of photoexcited hot electrons through interlayer hopping,which is significantly faster than intralayer relaxation.This established a microscopic picture between interlayer interactions and charge dynamics.We also studied hot-carrier dynamics at interfaces of semiconductors and plasmonic nanoclusters.It is demonstrated that two different mechanisms can coexist in a nanoparticle-semiconductor hybrid nanomaterial,both leading to faster transfer than carrier relaxation.These findings are of importance for understanding many experimentally observed photoinduced processes.2.We studied photoexcitation induced quantum dynamics of charge density wave and the emergence of a collective mode in 1T-TaS₂.As a well-known CDW material,1T-TaS₂ has been widely studied experimentally thanks to its intriguing photoexcited responses.The photoexcitation induced ultrafast dynamics in 1T-TaS₂ was investigated using time-dependent density functional theory molecular dynamics.We discovered the crucial role of electron-electron correlation and electron-phonon coupling and revealed a novel collective mode induced by photodoping.In addition,our finding validates conventional hot electron model is inadequate to explain photoinduced dynamics.Our results provide a deep insight on coherent electron and lattice quantum dynamics during the formation and excitation of CDW in 1T-TaS₂.3.Borophenes have emerged as the new platform to explore the properties of 2D metallic materials.Recent experiments show two-dimensional boron sheets grown on Ag（111）substrates.Furthermore,we explored the intrinsic resistivity of borophenes,which is highly dependent on the polymorphs,temperatures and the carrier densities.The temperature-dependent resistivity in borophene is well-described by Bloch–Grüneisen model,and it exhibits a universal scaling behavior.The work opened a new avenue for the electric devices based on borophene.We explored the excited-state dynamics in the low-dimensional materials.These studies would make a contribution to the future application of low-dimensional materials and van der Waals heterostructures in new optoelectrical,photovoltaic and photocatalysis etc.