Charge Transport Methods And Mechanisms In Graphene Nanoribbons

Graphene nanoribbons(GNRs)are quasi-one-dimensional graphene-based materials that have broad application prospects in nanoelectronic and spintronic devices.Due to their remarkable electrical conductivity and their ability to be finely tuned by atomic structures,GNRs can improve the overall performance of devices and are promising candidates for next-generation semiconductor materials.Therefore,it is crucial to elucidate the nature of charge transport in GNRs.Charge transport is a typical nonadiabatic dynamics process,and a large-scale nonadiabatic dynamics approach is required to perform nonadiabatic dynamics in such large-scale GNRs of several hundred nanometers.The chemical and physical properties of GNRs are sensitive to their atomic structure.Therefore,all-atom nonadiabatic dynamics simulations are more useful to reveal the charge transport mechanisms of GNRs.However,the high scaling of electronic structure calculation methods limits the application of nonadiabatic dynamics to large-scale systems,making all-atom nonadiabatic dynamics mainly used for small systems.This thesis aims to develop an efficient and reliable all-atom large-scale nonadiabatic dynamics method to study the charge transport in GNRs.In Chapter 3,we propose a construction method for large-scale Hamiltonians based on the divide-and-conquer strategy.We use Wannier analysis of small building blocks to obtain the Hamiltonian on the Wannier basis,and artificial neural networks(ANN)are utilized to map the relationship between the local atomic structure environment and Hamiltonian elements.The well-trained ANN models can represent the Hamiltonian of large-scale systems at a lower computational cost while maintaining the first-principles level of accuracy.This method enables efficient large-scale electronic structure calculations and lays the foundation for all-atom large-scale nonadiabatic dynamics.Furthermore,we propose an effective Hamiltonian method inspired by the traditional diagonalization method,which selects a set of important adiabatic states from the previous time step to represent the Hamiltonian at the next time step.The effective Hamiltonian method can fine-tune the balance between accuracy and efficiency,and can also revert to the traditional method under extreme conditions.Finally,we integrate the molecular dynamics force field,the construction of the large-scale Hamiltonian method,and the effective Hamiltonian method to construct a novel framework of all-atom large-scale nonadiabatic dynamics which provides a comprehensive solution for all-atom large-scale nonadiabatic dynamics in realistic systems.In Chapter 4,we systematically investigate the charge transport properties and mechanism of zigzag GNRs(ZGNRs)and a corresponding cove-edge GNR using the developed method.The size-independent results in 3-CGNR are obtained with a mobility of～1700 cm~2/Vs.Taking 3-ZGNR as a reference,we elucidate the mechanism of the effect of edge modification on electron dynamics.Furthermore,we study the charge transport in a series of hybrid GNRs.The coexistence of localized and delocalized states leads to the observation of transient delocalization and transient localization mechanisms.The proposed all-atom nonadiabatic dynamics method has been successfully applied to study systems with>10,000 atoms,which is much larger than the system size that can be handled by traditional methods.In Chapter 5,we study the charge transport in five N-ZGNRs with different widths(N=3,4,5,6,and 7)and find that the width can dramatically change the electron dynamics properties and transport mechanisms.With the newly developed methods,we find that the mobilities of these ZGNRs range from 10~5～10~3cm~2/Vs,and the averaged electron delocalization sizes range from 10~3～10~1.The all-atom nonadiabatic dynamic simulations covering hopping to band transport mechanism have been achieved.In particular,the size-independent results have also been achieved even the charge transport in 3-ZGNR is close to ballistic conduction,i.e.,the ultrahigh mobility is 3×10~5cm~2/Vs.The effective Hamiltonian method successfully pushes the system size limit of all-atom nonadiabatic dynamics simulations to 50,000 atoms,showing a good universality for applications.In Chapter 6,we study the defect effect on the charge transport in ZGNR by introducing the Stone Wales(SW)defect.We systematically study four different concentrations of SW defects in 6-ZGNR and find that in most cases the defect can significantly suppress the charge transport behavior,such as reducing the mobility by an order of magnitude to 10~3 cm~2/Vs which is consistent with common understanding.However,for the system with the 100%concentration of SW defect,the mobility is3×10~4 cm~2/Vs which is higher than the pure 6-ZGNR.The averaged electron delocalization sizes increase from 45 to 240,indicating that the high concentration of SW defect can induce the charge delocalization for the transformation from hopping to band-like mechanism.