Controlling Mechanism Of Thickness-dependent Ultrafast Charge Transfer In BP/MoS₂ Heterostructures

Two-dimensional（2D） van der Waals（vdW） heterostructures emerge as promising platform for optoelectronic and photovoltaic application based on feasible fabrication and without constraint of lattice mismatch.The ultrafast separation（transfer）and recombination of photoexcited carriers are the most basic steps in photo-conversion processes such as optoelectronics,photovoltaics and photocatalysis.However,compared with the recombination process which is widely concerned,there are few studies on ultrafast separation process and its regulation method after carrier excitation.Here,taking the vdW black phosphorus（BP）/MoS₂ heterostructure as an archetype,our ab initio nonadiabatic molecular dynamics（NAMD）simulations demonstrate that the interlayer carrier dynamics are thickness dependent.Specifically,type-II band alignment is formed in the present BP/MoS₂ system,and the electron transfer from the monolayer（1L）-BP to MoS₂ occurs quickly within 55 fs due to the large nonadiabatic coupling（NAC）induced by orbital hybridization.Contrastingly,hole transfer can only be observed within 1 ps with BP’s layer number N≥2,based on the excitation of low-frequency acoustic phonon and interlayer shear mode within 100cm^-1 which reduces the band alignment and enhance the interlayer coupling.Our findings may direct a new avenue towards the improvements of ultrafast carrier dynamics of 2D heterostructures for photoconversion.The specific content of this thesis is organized as follows:Chapter one is the introduction,which mainly focuses on two-dimensional materials and two-dimensional van der Waals heterostructures.Firstly,we introduce the development of 2D materials and mainly introduce the structure and properties of2D materials BP and MoS₂.Then,the characteristics and applications of vdW heterostructures formed by stacking two-dimensional materials is described.Finally,the BP/MoS₂ heterostructure and its research status are introduced.The second chapter explains the theoretical methods involved in this study,including density functional theory,non-adiabatic molecular dynamics,etc.And some computing programs used in this study are also introduced briefly.In chapter three,the dynamic process of electron and hole transfer in 1L-BP/MoS₂heterostructure is studied.It is found that the electron transfer process can complete the transfer from 1L-BP to MoS₂ in 55 fs.In contrast,in the calculation time of 2 ps,less than 3%of the hole transfer were completed.The analysis found that the ultrafast electron transfer benefits from the large nonadiabatic coupling（NAC）elements.The large NAC is caused by strong orbital hybridization and small mean band gap between energy levels.Similarly,the process of hole transfer is inhibited due to the large separation of energy levels.The results of single phonon mode NAMD calculation show that the fast electron transfer process involves the participation of multiple phonon modes.In chapter four,the electron and hole transfer in NL-BP/MoS₂（N=2,3）heterostructures are studied.The results show that when the BP layer thickness increases,the electron transfer is further accelerated,and the hole transfer is successfully transformed from inhibited to occurred within 1 ps.The analysis shows that the acceleration of electron transfer is mainly due to the decrease of band offset under the condition of strong orbital hybridization between BP and MoS₂.The success of hole transfer process is due to the excitation of low frequency ZA phonon mode,which causes large and long period energy level oscillation and enhances orbital coupling.Subsequently,we extended this conclusion to bulk-BP/MoS₂ heterostructure.Through data fitting,it is found that the relationship between the time scale of electron（hole）transfer and BP layer number can be well described by linear（exponential）function.In addition,the study of BP/2L-MoS₂ heterostructure found that due to different mechanisms,the increase of MoS₂ layer thickness accelerated the hole transfer to a lesser extent.In chapter five,we summarize and look forward to the results and significance of this thesis.In here,the qualitative law of charge transfer in heterostructures with layer thickness is given,and it is proved that layer thickness is an effective way to regulate charge transfer.This conclusion is expected to be extended to other 2D vdW heterostructures to guide their applications in light conversion and other fields.