Self-assisted Epitaxial Growth Of Two-Dimensional Wide Bandgap Semiconductor Sodium Tetratungstate Single Crystal

The superior physical properties of two-dimensional(2D)materials such as graphene(GN),black phosphorus(BP),boron nitride(BN),and molybdenum disulfide(Mo S₂)have significantly contributed to the innovation and development of electronic,photonic,and mechanical devices.However,from the perspective of bandgap,layered graphene is a kind of zero band gap material,is therefore still limited in terms of its possible electronics and optoelectronics applications.Black phosphorus features a layer-dependent direct bandgap,ranging from?0.3 e V in the bulk to?1.5 e V in the monolayer.In addition,the bandgap of molybdenum sulfide and tungsten sulfide are about 1.5 e V.A two-dimensional material with a wider band gap is boron nitride at 6.4e V.Therefore,there is a lack of two-dimensional materials in the 2?4 e V bandgap region.This part of the width of the bandgap corresponds to the traditional wide bandgap semiconductor materials,which are represented by silicon carbide and zinc oxide.They play extremely important role in the preparation of devices in high-power,high-frequency,high-speed,high-temperature and harsh environment.Nevertheless,owing to the limitation of three-dimensional structure of these materials,it is difficult to control their growth in two-dimensional scale.In order to find the material with suitable bandgap from the laboratory,We turned our attention to a material with a layered structure and appropriate bandgap,and this is a derivative of an oxide,which is sodium tetratungstate.Based on the ideas introduced above,two-dimensional sodium tetratungstate(Na₂W₄O₁₃)single crystal has been successfully fabricated on the fluorocrystalline mica(KMg₃(Al Si₃O₁₀)F₂)substrate by conventional chemical vapor deposition(CVD)combined with self-assisted epitaxy.Its bandgap is about 3.5 e V,similar to the band gap of third-generation semiconductors such as silicon carbide and zinc oxide.And this material can adapt to a variety of working environment such as in strong acid or high temperature conditions.The main contents and achievement s of this paper are as follows:(1)The reaction temperature of tungsten oxide was reduced by chemical vapor deposition with sodium chloride as self-service flux.In other hand,sodium ion as one of the raw materials can inhibit the introduction of impurities.(2)In this paper,we obtained the characterization information of two- dimensional sodium tetratungstate crystal and analyzed the influence factors of its growth.According to the data obtained,the program was adjusted to optimize the process,and the optimal ratio of sample growth was determined.(3)On the basis of analyzing the Raman scattering anisotropy and the calculation of crystal structure by the first principles,the epitaxial growth habit of the material on mica substrate is explored.