Synthesis And Optical Characterization Of Two Dimensional Transition Metal Dichalcogenides

The rich and diverse physics of transition metal dichalcogenides(TMDs)together with their novel optical and electrical properties offers unprecedented opportunities for electronic and optoelectronic applications.Atomically thin TMDs show attractive band gaps and exhibit extraordinary light absorption and emission properties covering the visible to near-infrared range,making them promising candidates for optoelectronic devices such as photodetectors,photovoltaics,and light emitters.Chemical vapor deposition(CVD)is the most promising method to produce TMD large-scale films or high-quality single crystals.In this study,we studied the layer-controlled growth,shape evolution and size regulation of CVD TMDs.We also investigated the fundamental optical and electrical properties of CVD grown MoS2,WS2 and ReS2,which are strongly correlated to the sample quality and growth conditions.Firstly,we produced single-,bi-and few-layered WS2 single crystals by varying the time of introducing sulfur(S)vapor into the CVD system,S-precursor amount and S-precursor exposure time.We present the most optimum combination of these parameters for producing as-grown WS2 grains with the most appropriate shape and size.Single-step growth providing better controllability of the chalcogen precursor allows the fine tuning of layer number and crystal quality.The production of different shapes of single-crystal WS2 with a viable growth mechanism,and a thickness and size that can be easily varied,suggests the versatility of our growth strategy.Furthermore,the calculated field effect mobilities based on layer number and the demonstration of a high performance phototransistor based on trilayer WS2 nanosheet with prominent photoresponse facilitate the practical application of our growth recipe to other TMDs.Secondly,a systematic spectroscopic approach for the evaluation of intrinsic optical quality of CVD grown MoS2 is presented.The amount of structural defects in CVD grown MoS2 single crystals with different crystal sizes and growth temperatures was established by low-temperature defect-induced bound-exciton emission,which correlated well with the difference in PL intensities measured in air and vacuum.The structural defects in CVD MoS2 could affect not only its optical quality but also its electrical performance,as demonstrated by electrical transport measurements.This approach is applicable to assess the intrinsic optical quality of MoS2 and other TMDs materials.Lastly,a scalable,two-step vapor phase growth process of in-plane heterostructures of atomic layers with the same lattice symmetry(i.e.WS2-MoS2)and different lattice symmetry(ReS2-MoS2)is reported.Raman and photoluminescence spectroscopies have been employed to verify the lateral heterojunctions.The as-grown heterostructures are further demonstrated to be p-n junctions,indicating their potential for novel devices.The present work on these TMD layers not only provides a foundation to further develop processes for the synthesis of other TMDs and heterostructures along with a critical understanding on the optical and electrical properties of 2D semiconductors,but also opens up many opportunities for their potential applications in atomically thin electronic and optoelectronic devices.