Studies On Synthesis And Humidity-sensing Properties Of Two-dimensional SnSe Nanosheets

Humidity sensor plays an increasingly important role in industrial process control,agricultural environmental monitoring,medical treatment,environmental protection,nuclear energy,and daily life.In order to accommodate the increasing technical requirements such as higher sensitivity,shorter response/recovery time,excellent long-term stability,etc,people are working on the development of new humidity-sensing materials.In recent years,two-dimensional semiconductor materials exhibit excellent humidity sensitivity including high sensitivity,good stability,etc,due to their large specific surface area.They are regarded as powerful candidates for next-generation humidity sensor,but still have some drawbacks,such as slow response,and low water molecular desorption rate,which have caused some obstacles to its further application.In this dissertation,nanotechnology and material composite methods are comprehensively used.Firstly,a single large-size SnSe nanosheet is used as humidity-sensing material.Then,the hydrophilic inorganic salt LiCl is used to form a composite humidity-sensing material to accelerates the adsorption and desorption of H₂O molecules.Further,a humidity sensor based on LiCl:SnSe is constructed.The main research contents and results of this article are as follows:（1）Preparation of SnSe nanosheets.SnSe nanosheets are prepared on Mica substrate by"face-down"process of physical vapor deposition（PVD）.The effects of deposition temperature,deposition time,argon flow rate and pressure on the morphology of SnSe nanosheets are investigated.Through the process optimization,SnSe nanosheets with lateral dimensions ranging from a few micrometers to hundreds of micrometers and thicknesses of more than ten nanometers to hundreds of nanometers are prepared.At the same time,based on some of the above-optimized process parameters,a few layers（several nanometers thick）of SnSe nanosheets with a lateral dimension of several micrometers are prepared by the process of"face-up".（2）Preparation of SnSe nanosheet resistive humidity sensor and LiCl composite modification.Firstly,SnSe nanosheet resistance type humidity sensor is prepared and its humidity sensitivity is tested.The results show that the SnSe nanosheet sensor exhibits humidity sensitivity,its current value increases with relative humidity,and it has wide detection range（N₂⁹5%RH）,good sensitivity,repeatability,and long-term stability.The maximum sensitivity,response time,and recovery time are 1.4×10⁴%,22 s and 7 s,respectively.Then,LiCl is composited onto the surface of the SnSe nanosheet by impregnation method to obtain composite humidity-sensing material.Humidity sensor based on LiCl:SnSe is constructed for humidity sensitivity test.The results show that the composite LiCl has a significant improvement on the humidity-sensing performances of SnSe nanosheet.At relative humidity levels of 11%,34%,43%,51%,64%,76%,85%,95%,the current value is increased to 111%,170%,110%,124%,288%,279%,283%,and 318%of SnSe nanosheet humidity sensor,respectively.The maximum sensitivity（3.21×10⁵%）is increased to 23 times higher than that of SnSe nanosheets.The response time（12 s）and the recovery time（4 s）are reduced by 45.5%and 42.9%,respectively.（3）Theoretical calculation of the microscopic mechanism of humidity sensitivity of SnSe nanosheet.Using the first-principles calculation method and according to the principle of minimum energy,the most stable position of water molecules adsorbed on the surface of single-layer SnSe is determined.Then,the intrinsic model and the H₂O molecule adsorption system at most stable adsorption position are calculated,respectively.Finally,the microscopic mechanism of adsorption of SnSe by H₂O molecules is analyzed based on charge transfer and bonding.The results show that C（on top of quadrilateral center）is the most stable adsorption site.The bader charge analysis shows that there is a charge transfer of 0.03 e from SnSe to H2O molecule.The H2O molecule plays the role of acceptor,which is equivalent to p-type doping.The adsorption method is chemical adsorption by forming the ionic bond.