Synthesis And Optical Characterization Of Novel Anisotropic Two-dimensional Materials

The large family of two-dimensional（2D）materials with various excellent and novel functions has provided the new way for the rapid development of information,energy,biology and related fields.Recently,anisotropic 2D materials,such as layered black phosphorus,have attracted intense attention.In comparison with highly isotropic 2D materials,like graphene and molybdenum disulfide（MoS₂）,these low-symmetry layered materials exhibit significant physical and chemical anisotropy behavior along different in-plane crystal directions,thus providing a new material platform for both of fundamental research and the development of novel micro-/nano-functional devices.As a new type of anisotropic 2D semiconductor,titanium trisulfide（TiS₃）exhibits high carrier mobility and excellent optoelectronic properties.However,the study on the preparation of anisotropic TiS₃ and its optical property has barely been explored to date.The aim of this thesis is to explore the optical anisotropy in 2D layered materials by mainly focusing on the preparation and optical property investigation of TiS₃ thin layers.On this basis,we have extended our study to the control of optical properties of isotropic 2D materials such as MoS₂.The main research work is shown as follows:（1）We for the first time successfully prepared high-quality thin layers of TiS₃ crystals on SiO₂/Si substrates using a vacuum solid phase sulfurization method.The effects of various reaction parameters were investigated.It was found that the as-grown TiS₃ samples have a smooth surface roughness with a film thickness ranging from 3 to 40 nm and a belt length up to 100μm.Based on atomic force microscopy and optical microscopy observations,we discussed the possible microscopic growth mechanism of layered TiS₃ and also proposed an in-plane/out-of-plane co-growth model.（2）We studied the anisotropic linear optical and nonlinear optical properties of the as-grown TiS₃ samples by angle-resolved polarization-dependent Raman spectroscopy and polarization-dependent four-wave mixing imaging technique,respectively.It was found that both optical methods can be used to accurately identify the crystalline orientation of TiS₃samples.We also for the first time observed that the layered TiS₃ has a strong four-wave mixing nonlinear optical response,which signal is highly sensitive to the changes in the number of layers.As a relatively new spectroscopy technique,four-wave mixing spectroscopy has the advantages of fast imaging,high resolution and no damage to the samples,which is expected to develop into a high-efficiency characterization means for 2D material analysis.（3）We have fabricated MoS₂/TiO₂ hybrid structures by the combination of mechanical exfoliation and dry transfer techniques,and observed a strong anisotropic enhancement of optical second-harmonic generation（SHG）in monolayer MoS₂.The experimental results indicated that the TiO₂ nanowires in the hybrid structure not only effectively increased the efficiency of the emitted SHG signal from monolayer MoS₂ but also led to highly anisotropic SHG enhancement.