The Study On Controllable Growth, Doping And Optoelectronic Devices Of Two-dimensional WS₂ Atomic Crystals

Two dimensional（2D）WS₂ offers great prospects for use in next-generation electronic and optoelectronic devices due to its layered structure and thickness-dependent band gap.However,the layer controllable growth of WS₂ single crystals remains a challenge.Besides,the charge neutrality level（CNL）in both CVD-grown and exfoliated WS₂are close to the middle of the band gap,leading to large Schottky barrier with metal contacts.As a consequence,WS₂-based electronic and optoelectronic devices generally suffer low on-current and low photosensitivity.The present thesis is devoted to addressing these issues.First,carbon nanoparticles on sapphire substrate by burning of candle or decomposition of C₂H₄ were employed for growth of bilayer WS₂ flakes via chemical vapor deposition（CVD）.The results show that the introduction of carbon nanoparticles promotes the formation of bilayer WS₂ triangles with high density,uniformity and high quality,while carbon-free leads to growth of almost all monolayers.In addition,WS₂ devices were fabricated and their electronic transport properties were investigated.The results show that the CVD-grown bilayer WS₂device has much higher field effect mobility（μ）and comparable on/off current ratio(I_on/I_off)compared with CVD-grown monolayer WS₂ device(bilayer WS₂ device:μ=34 cm²V^-1s^-1,I_on/I_off=6×10⁸;monolayer WS₂device:μ=0.8 cm²V^-1s^-1,I_on/I_off=1×10⁸).Second,we achieved reversible electron doping in monolayer WS₂via a chemical strategy.A surface charge transfer n-doping in monolayer WS₂ by tirethylamine（TEA）modification was developed.The mobility of monolayer WS₂ is improved from 9.8（pristine）to²8.6 cm²V^-1s^-1（after treatment）,and the photoresponsivity is also significantly enhanced from 6.4×10^-3 to 21 AW^-1after surface doping.It is obviously observed that the optical properties are also modified due to the doping.For instance,the monolayer WS₂ shows red-shift of 2LA（M）Raman mode and weakened PL emmission after TEA modification.The characteristic peaks of X-ray photoelecteon spectroscopy shift as well after doping.It is noteworthy that the n-doped WS₂ can be reversed to be close to the pristine state after washing with acetone.Third,Nb-doped WS₂（Nb:WS₂）triangles were synthesized using the mix powder of WCl₆ and NbCl₅ as precursor via one-step CVD method.The structure and optical properties were characterized by Raman and PL spectra.The results demonstrate the appearance of A_1gg mode of NbS₂ and quenching of PL emission.The devices based on Nb:WS₂ were prepared and their electron transport properties were studied.Bipolar conductive behavior is found,indicating that the doping of Nb can modulate the electron transport properties of WS₂.