Strain effects on semiconducting two-dimensional crystals
Date of Issue2016
School of Physical and Mathematical Sciences
Two-dimensional (2D) semiconducting layered materials have attracted widespread research interest recently not only from the fundamental point of view, but also because of their great possibilities in next-generation electronic devices, valleytronics, photodetectors, and flexible optoelectronics applications due to their as-born bandgaps and other unique properties. Strain, which can be used to alter and control the properties of these 2D semiconducting materials, thus enables the exploration of novel fundamental physics and applications of these materials. In this thesis, we present the band structure evolutions and lattice vibrational responses in 2D MS2 (M=Mo, W) and BP crystals under uniaxial tensile strain through a combination of in situ photoluminescence and Raman spectroscopy studies, as well as density functional theory (DFT) calculations. As uniaxially strained monolayer WS2 is theoretically predicted to undergo a direct-indirect bandgap transition which may limit its optoelectronic applications, it is important to accurately determine the critical strain to induce such transition. We have experimentally demonstrated the possibility of tuning different optical transition energies and their relative spectral weight in monolayer WS2 by applying uniaxial strain. This tuneable optical property is attributed to the strain-induced direct-indirect bandgap transition and confirmed by DFT calculations. In addition, slight lowering of the trion dissociation energy with increasing uniaxial strain is observed. Not only the band structure engineering, but also the crystallographic orientation determination could be achieved by the strain in TMDs. Crystal orientation can significantly determine the properties of TMD materials, which necessitates a convenient and reliable way to identify it. Our in situ Raman spectroscopy study reveals uniaxial tensile strain can soften the in-plane E' phonon mode and even lift its two-fold degeneracy as reflected by a doublet splitting. We further show that the polarizations of the scattered light from these two splitted modes are linear and orthogonal. Moreover, the polarization dependence of the two sub-bands could be adopted to identify the crystal orientation and confirms the observed zigzag-oriented edge of monolayer WS2 grown by chemical vapour deposition method in previous studies. BP possesses the remarkably anisotropic mechanical properties as a result of its unique puckered structure, so it is critical to understand how the anisotropic behaviors can be modulated by strain for the better integration of BP into various technologies, such as flexible electronics. It is found that the out-of-plane A_g^1 mode is sensitive to uniaxial tensile strain along the armchair direction while the in-plane B2g and A_g^2 modes are susceptible to the zigzag direction strain. Our DFT calculation results clearly illustrate the anisotropic influence of uniaxial strain on structural properties of few-layer BP owing to its unique puckered crystal structure and could be used to elucidate the striking dependence of strained phonon frequencies on crystal orientations. Our work suggests that strain engineering holds a promising future for extensive modulation of optical and mechanical properties in semiconducting layered materials. This study indeed enriches our understanding of strained states of 2D crystals and further lays a foundation for developing various applications of such emerging semiconducting layered materials based on their strain dependent properties, such as flexible optoelectronic devices.