Preparation And Physical Properties Of Indium Selenide And Bismuth Selenide Based Nanomaterials

Indium selenide(In₂Se₃)and bismuth selenide(BiSe),as typical narrow-band gap layered semiconductor compounds with unique crystal structure and excellent electrical and thermoelectric properties,have great potential in various important semiconductor devices including photodetectors,optical memory,and thermoelectric generators.It is of great importance in science and technology to make these materials into nanostructures and exploit their unique physical and chemical properties.In this thesis,the single-crystal?-In₂Se₃ nanobelts were synthesized via chemical vapor deposition without the assistant of catalysts,and the BiSe films were prepared by vacuum thermal evaporation method.The phases,microstructures,and physical properties of the prepared samples were investigated respectively,and the main results are as follows:(1)Preparation and properties of?-In₂Se₃ nanobelts.Single crystal?-In₂Se₃ nanobelts with high crystallinity were synthesized via chemical vapor deposition using Se powders as raw material and quartz glass loaded with In₂O₃ powders as substrate.Scanning electron microscopy(SEM)and atomic force microscopy(AFM)were used to characterize the size of nanobelts,and the results showed that these nanobelts were about tens of microns in length,hundreds of nanometers in width and tens of nanometers in thickness,showing typical belt-like morphlogy.Meanwhile,these products were also identified to be?-In₂Se₃structured nanobelts by transmission electron microscopy(TEM)and Raman spectroscopy(Raman).By analyzing the growth behavior of the nanobelts,it can be concluded that the?-In₂Se₃ nanobelts were obtained by direct selenization of In₂O₃ powders,and the growth was conctrolled by the typical vapor-solid(VS)growth mechanism.For the first time,the piezoelectric properties of single-crystalline?-In₂Se₃nanobelts were examined via piezoresponse force microscope(PFM)measurements,and the piezoelectric coefficient d₃₃of the 50 nm-thick nanobelt was approximately calculated to be 1.6 pm·V^-1.Moreover,the photodetector based on the?-In₂Se₃ nanobelts exhibited a high absorption for monochromatic light with a wavelength of 442 nm.The maximum photoresponsivity value and a response time could be obtained as 66 A·W^-1 and?1 s,respectively.Besides,the?-In₂Se₃nanobelt photodetector still maintained a clear photoelectric conversion behavior after 4 months of storage under an atmospheric environment,showing a good stability.(2)Preparation and properties of BiSe based nanocrystalline films.N-type BiSe nanocrystalline films with(00l)preferred orientation were synthesized on quartz glass substrates via vacuum thermal evaporation method.The Bi_1-xSb_xSe thermoelectric films with Sb concentrations of 0.15,0.24,and 0.35 were synthesized by co-evaporation of Sb and Bi₂Se₃.X-ray diffraction(XRD),SEM,Raman spectroscopy,and X-ray photoelectron spectroscopy(XPS)were used to characterize the phase,morphology,and element distribution of these films.It was found that Bi,Sb,and Se are evenly distributed in the samples.Sb entered the lattice of BiSe and randomly occupied the position of Bisite in Bi₂Se₃ quintuple layers and Bi₂ bilayer.With the increase of Sb doping concentration,the sizes of nanocrystals in the films decreased and the denser layered structure was formed,and the in-plane carrier mobility of samples effectively increased from 13.66 cm~2·V^-1·s^-1(BiSe)to 19.30 cm~2·V^-1·s^-1(Bi_0.65Sb_0.35Se),which resulted in the conductivity in a high range even although the carrier concentration decreased sharply.Under the optimization of both Seebeck coefficient and conductivity,Bi_0.76Sb_0.24Se exhibited the maximum power factor of 2.18?W·cm^-1·K^-2at 300 K,which is a little bit higher than that of undoped BiSe.In summary,?-In₂Se₃ nanobelts were grown by direct selenization of In₂O₃,and the piezoelectric properties of?-In₂Se₃ nanobelts were investigated for the first time,which further expands the research of In₂Se₃ nanostructures.Sb-doped BiSe films were prepared and the thermoelectric performance of Bi_0.76Sb_0.24Se was slightly improved with optimized carrier concentration and mobility,which would be helpful to explore other BiSe based thermoelectric materials.