Construction Of Two-dimensional MoS₂-based Heterojunctions And Their Photodetection Properties

Two-dimensional（2D）transition metal chalcogenides（TMDCs）exhibit excellent electrical and optical properties etc.,due to their special crystalline structures,large specific surface area,and nano-scale effects,and thus they have great application prospect in optoelectronics.MoS₂ is the most typical TMDCs,and the direct bandgap semiconducting characters of the single-layer MoS₂ endow itself with excellent optical and electrical properties.Field-effect transistors and photodetectors based on MoS₂ have confirmed its excellent photoelectric performance,such as high current on-off ratio（up to 10⁸）and high responsivity(880 A W^-1),and so on.However,the drawbacks,for instance,UV-Vis spectral response range limited by the intrinsic band gap and long response time,etc.,greatly degrade its application potential at the same time.As it is well known,the properties of 2D materials are extremely sensitive to the environment,and thus one can modulate the properties of 2D materials via changing the local environment.We found that the optoelectronic properties of2D MoS₂ can be effectively enhanced and extended by building various MoS₂-based heterojunctions.Based on the above,this article mainly adopts chemical modification and epitaxial growth of heterojunction to optimize the optoelecetronic properties of 2D MoS₂.The main research contents and results are as follows:（1）Slow photoresponse is one of the key drawbacks for 2D MoS₂ due to the influence from the adsorption of impurities such as O₂/H₂O.In this work,p-GaSe/n-MoS₂ was synthesized by van der Waals epitaxial growth,then the heterojunction was studied by combining theoretical calculation and experiment.The presence of GaSe effectively facilitates the separation of electron-hole pairs and suppresses the absorption of impurities as well,which significantly decrease the lifetime of minor carriers in MoS₂,thus enhancing the reponse rate for three orders of magnitudes.The epitaxial growth of the vertical p-GaSe/n-MoS₂ junction by chemical vapor deposition is simple,but can effectively adjust the optoelectronic properties,which is one of the simplest ways to improve the response speed of MoS₂.This method of improving the response speed through the p-n junction grown by van der Waals expitaxial growth provides a new idea for the study on optimizing the optoelectronic properties of 2D materials.（2）In order to broaden the spectral response range of MoS₂,MoTe₂/MoS₂ bilayer heterojunction was prepared by using the method of“Stacking-mode confined growth”.This method effectively enhances the heterojunction interface contact and improves the interface cleanliness.PL revealed charge transfer between the MoTe₂ and MoS₂ layers,resulting in a significant PL quenching in the heterojunction region.The results of photodetection have confirmed that the formation of the heterojunction effectively extended the spectral response range of the MoS₂ to the near-infrared region（1100 nm）,and exhibited high responsivity of4.71 A/W（1100 nm）and excellent EQE of 1.935×10³%（300 nm）,which were better than the performances of MoS₂ and the related TMDCs heterojunction devices.（3）To achieve spectrally selective response based on MoS₂ in the near-infrared region,we used up-converting nanoparticles（UCNPs）to modify 2D MoS₂,and successfully realized photodetection for specific wavelength bands in the near-infrared region.The Ln-doped UCNPs is a unique luminescent material,which absorbs specific wavelength infrared light and emits visible light.In this composite system,2D MoS₂ is used as the supporter of the composite device because of its ultra-high specific surface area and superior optoelectronic properties,UCNPs are used as the absorbers of infrared light.Specially,the isolated absorption spectrum of the UCNPs grants the wavelength selection of the MoS₂ for the photoresponse in the near-infrared region.In addition,the position of the UCNPs absorption peak can be changed with the types of the doped rare earth elements;and MoS₂can also be replaced by other TMDCs,and thus this device design concept could be extended to other similar systems for modulation of response range.