The Optical Properties Of Two-dimensional Materials Of Tungsten Sulfide And Molybdenum Sulfide Were Prepared By CVD Method And The Characteristics Of Field Effect Transistor Devices

Since the discovery of graphene in 2004,atomically thin two dimensional materials have attracted broad interest,due to their unique electrical and optical properties.Transition metal dichalcogenides?TMDs?have shown great promise for microelectronics and photoelectronis.Atomically WS₂ have been investigated,which exhibited many appealing properties for a wide range of applications,such as transistors,photodetectors,solar cells and so on.In my study,we systematically investigated the preparation and application of WS₂ and Mo?1-x?W_xS₂ alloy.The results obtained are as follow.?1?The Monolayer of WS₂ has been grown by Chemical Vapor Depositin?CVD?at optimized conditions.The obtained WS₂ monolayer of?20 ?m in size and thickness is around?0.95 nm.There is a significant difference has been found in between monolayer and multilayer WS₂ by Raman spectroscopy study.In both mono and multilayers show two prominent peak,which are associated with well known E_2g¹ and A1g modes of WS₂.Compared to E_2g¹ mode and A1g mode for monolayer WS₂,the E_2g¹ mode is practically stronger than A1g mode,with the WS₂ layer thickness.The E_2g¹ mode shows a red-shift and the A1g mode shows a blue-shift.From the photoluminescence?PL?study,it is also found that the peak intensity of WS₂ monolayer was noticeably stronger than that of multilayer.We also study on the thermal conductivities of monolayer WS₂,which was determined by temperature dependences of E_2g¹ and A1g Raman modes.The frequency shift of the A1g mode with temperature is larger than that of the mode E_2g¹ for WS₂ monolayer,which is attributed to stronger electron-phonon coupling in the A1g1 mode.By analyzing PL data on the monolayer WS₂ with variation in temperature,it has been found that the bandgap gap changes with the lattice expansion effect due to the temperature variation.?2?The high-quality ternary alloy of monolayer Mo?1-X?W_xS₂ with Mo dopping in WS₂ was synthesized via CVD.It is proved that Raman spectroscopy can accurtately distinguish ternary alloys and heterojunctions.The compostion can be estimated inversely by PL spectra.The PL spectrum indicated that the band gap of alloy of Mo?1-x?W_xS₂ could be tuned precisely between 1.82 eV?MoS₂?and 1.95 eV?WS₂?by changing the ratio of Mo/W compositions.We calculated that monolayer as-grown Mo?1-x?W_xS₂ component x was 0.68.Temperature-dependent Raman results of the as-grown Mo?1-X?W_xS₂ flake explore the ternary Mo?1-x?W_xS₂ alloy,which is result from low free energy and internal energy of ternary Mo?1-x?W_xS₂ alloy.?3?For the first time,we have fabricated back-gated Mo?1-x?W_xS₂ field effect transistors with high on/off ratio??105?by electron beam lithography followed by lift-off process.Over the different electrical measurement temperature?20 K-300 K?,the metal-insulator transition?MIT?is not very clerly observed,it may be due to the low electron charge concentration?less than 1×1013 cm-2?and limited gate sweep voltage 70 V?devices breakdown with further increasing measurement voltage?.Furthermore,we have measured the transport properties of monolayer Mo?1-x?W_xS₂ transistors from 300 K to 20 K and simulated the data by activation transport model.At high temperatures?>80 K?,the carrier transport is thermally activated whereas atlow temperature range?<80 K?,the carrier hopping transport through localized states.To gain further insight into the charge scattering mechanism,a monotonous increase of the mobility as the temperature is decreased and become saturated at low temperature?below?80 K?.Further it reaches to?30 cm2V-1s-1 at 300 K.Below 80 K,the field effect mobility saturates to an intrinsic value limited by Coulomb scattering.