Synthesis and Optoelectronic Properties of Two-Dimensional Rhenium Disulfide and Their Heterostructures

Author:Qin Jing Kai

Supervisor:shao wen zhu xu cheng yan


Degree Year:2019





Two-dimensional transition metal dichalcogenides(TMDs)have promising applications in electronics,biocatalysts and solar cell fields due to their unique electrical,optical,mechanical and physicochemical properties.Among of them,Rhenium disulfide(ReS2)has attracted great attention of scientists due to its special crystal structure.The coupling effect between individual ReS2 layers could be significantly weakened because of the Peirs distortion in its crystal lattice,which would lead to unchanged direct band gap as the ReS2 increasing from monolayer to bulk.The asymmetric structure could also result into a strong anisotropy in its electrical and optical properties.The unique properties make ReS2 a promising candidate for novel logic and optoelectronic devices.In this thesis,we successfully realized the controlled synthesis of high-quality 2D ReS2 films by chemical vapor deposition(CVD),and systematically investigated its electrical and optoelectrical properties.Using various approaches such as doping,epitaxy growth and QDs declaration,we could effectively modulate the intrinsic physical properties of ReS 2and make a series of important breakthrough in optical,electrical performance and sensor application.In order to obtain high-quality layer-area ReS2 films,various Re compound are taken as precursor,and influence of substrate,temperature and growth duration on the morphology of ReS2 were also investigated.The result shows mica substrate with atomic-scale surface roughness and high chemical inactivity plays the crucial role in the growth process.It could greatly decrease the surface diffusion energy barrier for atoms and facilitate crystallization of ReS2 along the in-plane direction.Besides,the electrical and optoelectrical performance of field effect transistors(FETs)based on ReS2 films were also demonstrated.Mo doped monolayer ReS2 was obtained using CVD with MoO3 as catalyst.On one hand,the MoO3 could react with Re to form low-meting-point ReO3 during growth process,which could easily volatilize under low temperature,ensuring the efficient production of ReS2.On the other hand,MoO3 could also be reduced by S,resulting into the Mo doping in ReS2.The Mo substitutinal doping in ReS2 would introduce a deep impurity level in the band gap,and the direct band gap of 1.4 eV could be changed to indirect 0.76 eV.The FETs devices based on Mo-doped ReS2exhibits a degenerate p-type transport behavior,which is totally different the ReS2device showing n-type characteristics.Trigonal Se nanoplates can be epitaxially grown on monolayer ReS2 by vacuum vapor deposition to form a novel 1D/2D vertical heterostructure.The lateral dimension of Se nanoplates ranges from 100–800 nm,with the thickness less than80 nm.DFT calculation shows the Se could covalently bonded with ReS2 to create a high-quality interface contact,which could greatly increase the charge transfer efficiency between Se and ReS2.The optoelectrical performance of ReS2 could be significantly improved after Se nanoplates growth,showing excellent response speed less than 10 ms and ultrahigh detectivity up to 8×1012.The difference of Fermi level in Se and ReS2 could result into the type-II band alignment at interface,which would result into a small off-state current under dark condition.Besides,with the applied light on and off,photo-exited carriers could be quickly transferred and combined at interface,leading to the shorten of response time and increasing of photocurrent.Thin Se nanobelts could also be obtained by vacuum vapor deposition using few-layer ReS2 films as template.That is because the unmourns edges in polycrystalline ReS2 could significantly decrease the surface diffusion energy barrier for atoms,which enables the Se nanobelts grow along in-plane direction by wire-by-wire assembly mode.Dual-channel FETs based on Se nanobelt/ReS2heterostructure exhibit great optoelectrical performance compared with bare ReS2devices.The enhancement properties could well explained by the decreased width of depletion areas in Se and ReS2,which are induced by build-in electric field at interface.We also developed a PMMA-free transfer process for monolayer ReS2based on liquid exfoliation technique.Few-layer ReS2 could also be obtained by controlling the ultrasonic parameters.This new approach could effectively avoid the residuals and impurity absorption introduced by PMMA-assisted transfer technique.Besides,it could also lighten the pollution of HF to the environment and ensure the personal safety of researchers.Hybrid CdSe-CdS-ZnS QDs/ReS2 films was fabricated by spin-coating QDs on the surface of monolayer ReS2.The result shows that QDs was used as light absorber to enhance the photovoltaic conversion efficiency of hybrid films.CH3S-in EDT could effectively cure the vacancies defects in ReS2 and decrease the electrons scattering during carriers transport.Besides,EDT could also act as a cross-linking agent to ensure the high-quality contact between QDs and ReS2.After QDs decoration,the carrier mobility of the FETs based on monolayer ReS2 could be increased from 3.5 to 9.8 cm2 V-11 s-1.In the same time,the photoresponse can be enhanced up to 604 A/W with shorten response time less than 3.2s.