Study Of Two-dimensional Layer MoS₂ Thin Film Heterojunction Devices And Their Applications

In recent years,two-dimensional（2D）layered materials represented beyondgraphene have attracted wide attention due to their unique properties.The monolayer of these materials usually has a thickness of less than 1 nm,exhibiting excellent physical properties and application prospects for devices.Hence,they are very attractive materials for electronic and optoelectronic devices.Compared with other materials,they have high carrier mobility,high conductivity,transparency and excellent mechanical properties.However,due to the zero bandgap of graphene,the application of electronic and optoelectronic devices has been greatly limited.Graphene-like 2D materials,especially transition metal dichalcogenides（TMDCs）,have become hotspots for researchers.Among these 2D nano-materials,molybdenum disulfide（MoS₂）is one of the most studied layered materials due to its inherent and thickness-dependent band-gaps,large surface-to-volume ratio,and excellent optical transparency.This makes it a promising material for optoelectronic applications and is considered to be a 2D semiconductor material that promises to continue Moore’s Law.Crystals of MoS₂ are composed of vertically stacked,weakly interacting layers through van der Waals interactions with the indirect-gap with 1.2 eV.When the layers of MoS₂ decreased to monolayer,it will transform to a direct-gap with 1.9 eV.This unique property grants the great advantages for high-performance devices with high on/off ratio and low power consumption Therefore,by controlling the experimental conditions to prepare thickness controllable MoS₂ thin films,and explore the applications of MoS₂ thin film heterojunction device has great significance.In this paper,a large-area,high-quality,and controllable thickness of MoS₂ thin film was prepared by thermal decomposition method to construct a MoS₂heterojunction structure and explore its application in electronic and optoelectronic devices.The main performances are as follows:The prepared precursor solution by dipping coating and spinning coating onto a Si/SiO2 substrate,and a controllable preparation of a large-area MoS2 thin film was achieved by a two-step annealing thermal decomposition with CVD tube furnace.By analyzing the results of XRD,XPS,Raman,AFM,TEM and HRTEM show that we have successfully prepared a large area of MoS₂ thin film.The MoS₂/Si heterostructures were constructed in situ by synthesis MoS₂ on planeSi substrates,and the photoelectric characteristics of the prepared device were studied.Such MoS₂/Si heterostructures exhibited high sensitivity to light illumination with wavelengths ranging from the deep ultraviolet to the near infrared.Photoresponse analysis reveals that a high responsivity of 23.1 A/W,a specific detectivity of 1.63×10¹² Jones,and a fast response speed of 21.6/65.5μs were achieved.Notably,the MoS₂/Si heterojunction photodetector could operate with excellent stability and repeatability over a wide frequency range up to 150 kHz.The high performance could be attributed to the high-quality heterojunction between MoS₂ and Si obtained by the in situ fabrication process.Such high performance with broadband response suggests that MoS₂/Si heterostructures could have great potential in optoelectronic applications.In order to improve the performance of the MoS₂ heterojunction photodetector,photovoltaic MoS₂/Si nanowire array（SiNWA）heterojunction photodetectors（PDs）are constructed and investigated,which exhibit excellent photoresponse properties to light illumination at wavelengths from the deep ultraviolet to the near-infrared.Further analysis reveals that a high responsivity of 53.5?A/W and a specific detectivity of 2.8?×?10¹³ Jones,as well as fast response speeds of 2.9/7.3?μs at 50 kHz are achieved in a MoS₂/SiNWA heterojunction device.The high performances could be attributed to the high-quality heterojunction between MoS₂ and the SiNWA.Such high performances of MoS₂/SiNWA PDs are much better than those of previously reported MoS2-based PDs,suggesting that MoS2/SiNWA heterojunction devices have great potential in optoelectronic applications.The MoS₂/Si nanowires array（SiNWA）heterojunctions were constructed andinvestigated,which exhibited highly humidity-dependent electrical properties in both forward and reverse voltages.Further analysis reveals that such MoS₂/SiNWA heterojunction devices are highly sensitive,excellent stability,reproducibility and fast response speeds The sensitivities and response/recovery speeds were obtained to be392%and 26.4/15.1s at 95%RH under voltage bias of+5V,and 2967%and22.2/11.5s at 95%RH under voltage bias of-5V,respectively.Such high-performances of dual-mode Mo S₂/SiNWA heterojunction humidity sensor are superior to the previous reported results,revealing that they will have great potential applications in highly sensitive humidity sensors.In addition,MoS₂/Si nanowire array（SiNWA）heterojunctions exhibiting excellentgas sensing properties were constructed and investigated.Further analysis reveals that such MoS₂/SiNWA heterojunction devices are highly sensitive to nitric oxide（NO）gas under reverse voltages at room temperature（RT）.The gas sensor demonstrated a minimum detection limit of 10 ppb,which represents the lowest value obtained for MoS₂-based sensors,as well as an ultrahigh response of 3518%（50 ppm NO,～50%RH）,with good repeatability and selectivity of the MoS₂/SiNWA heterojunction.The sensing mechanisms were also discussed.The performance of the MoS₂/SiNWA heterojunction gas sensors is superior to previous results,revealing that they have great potential in applications relating to highly sensitive gas sensors.