Properties of novel 2D materials by strain and thermal engineering

Abstract

During the past decade, two-dimensional (2D) materials become the hottest issue in the materials research community. Three important features, electron confinement, high mechanical flexibility and large specific surface area, make 2D materials unique and promising. Strain engineering is a facile and useful tool to modify the properties of 2D materials by utilizing the characteristic of high flexibility. Among various 2D materials, black phosphorus (BP) and molybdenum disulfide (MoS2) are the most notable members attracting lots of attention. It is valuable to explore the features of BP and MoS2 by strain engineering for future electronic and optoelectronic devices. In addition, a feasible and reliable chemical vapor deposition (CVD) method for the growth of few-layer black phosphorus (FLBP) is still absent. Hence, the study of thermal effect on FLBP is very conductive to facilitate the development of growth technique for FLBP. In this thesis, firstly, the anisotropic Raman response of the encapsulated FLBP by uniaxial strain was investigated comprehensively. The Raman shift rates of the A1g, B2g and A2g modes are dramatically distinct for the uniaxial strain applied along different directions. Especially, the B2g mode is extremely sensitive to the strain applied along the zigzag direction with an exceptional large Raman shift rate of ~ -11 cm-1/% strain and the Grüneisen parameter of ~2.5. Through density functional theory (DFT) calculations, we demonstrate that the effective Poisson's ratio applied to the encapsulated FLBP is estimated to be ~ 0 in the experiment. The bending technique can offer a quasi-pure uniaxial tensile strain to the encapsulated sample. The giant anisotropic strain response results from the co-effects of both the bond lengths and bond angles. Based on the anisotropic strain response in B2g mode, a novel method of identifying the crystallographic orientations is put forward by using the strained encapsulated FLBP and the conventional non-polarized Raman spectroscopy. Secondly, the shear mode and layer breathing mode in few-layer MoS2 were investigated by the low frequency Raman spectroscopy. The shear mode is demonstrated to be a more competent layer number indicator due to the obvious blue shifts about ~10 cm-1 from 3-layer sample to bulk material. It is found that the relationship between the layer number and peak position of the shear mode follows an enhanced linear-chain model very well. In addition, the shear mode is doubly degenerated to two sub-peaks and show opposite Raman shifts as the tensile strain increases. Thirdly, a series of atomic-scale morphological and spectroscopic measurements were utilized to reveal the thermal decomposition process of the solution-exfoliated FLBP. The sublimation temperature of FLBP was determined to be no less than 400 °C. Dynamic decomposition process of the BP flake demonstrates that bulk BP can be effectively thinned down to atomically thin layer through the thermal annealing. Finally, the properties of FLBP based field effect transistors are demonstrated by using the thermal thinning process.