Symmetry-driven optoelectronics in two-dimensional layered materials

Crystal symmetry and its breaking are the core of modern condensed matter physics and materials science research. They fundamentally determine the crystal topology, pairing mechanism and optoelectronic properties. They play a very important role in many novel phenomena such as quantum spin Hall effect in topological insulators, chiral fermion in Weyl semimetals and high temperature superconductivity in cuprates. Symmetry and related optoelectronic properties become more prominent in reduced dimensional systems, which have rich interfacial physics. For instance, in recent emerging 2D layered materials, inversion symmetry breaking and three-fold rotational crystal symmetry bring a unique valley degree of freedom. Meanwhile, large tunability of 2D materials are subject to external mechanical, electrical stimuli, or interfacial effects, which make symmetry engineering more feasible for potential applications. This dissertation covers experimental investigation in the emerging fields of two-dimensional valleytronics and polar structural phases in 2D layered materials. Both are closely linked with fundamental crystal symmetry and specific symmetry breaking. The dissertation will first present the discovery of optical selection for nonlinear optical process in monolayer WS2, while also taking into account both valley and excitonic degrees of freedom (DOF). The three-fold rotational crystal symmetry together with inversion symmetry breaking enables access to excitonic fine structures in specific valley through nonlinear optical processes with larger than 95% valley excitation efficiency. Such a discovery establishes a foundation for the control of optical transitions. This is crucial for valley optoelectronic device applications, such as 2D valley-polarized THz sources with 2p–1s transitions, and coherent control for quantum computing. Furthermore, the dissertation includes the first demonstration of electrical valley generation through ferromagnetic spin injection. The inversion symmetry breaking in monolayer TMDC leads to a unique spin-valley locking relationship, which is the key to achieve electrical valley generation. The electrical valley generation efficiency is up to 45%. Such high-fidelity achieved by electrical control opens the door towards a new paradigm of electronics that manifests all three DOFs—charge, spin, and valley—for information processing. On the other hand, the mirror symmetry breaking in 2D polar materials allows the exploration of novel structural ordering and associated optoelectronic properties in monolayer Janus MoSSe and ferroelectric ultrathin In2Se3 crystal. Significant vertical dipoles were observed in both materials by second harmonic generation (SHG) and piezoforce microscopy (PFM). In addition, the measured piezoelectric coefficient for a 3-nm-thick In2Se3 is about d33 = 0.5 pm/V and shows very high ferroelectric transition temperature Tc up to 700 K. These discoveries are applicable to electromechanical sensors and memory devices at molecular level.Finally, the demonstration of electrostatic doping induced structural phase transition in monolayer MoTe2 was discussed as an example for crystal symmetry manipulation. Such transition involves substantial crystal symmetry changes: from hexagonal to monoclinic and from inversion symmetry breaking to inversion symmetry preserved. SHG intensity modulation was observed more than one order during phase transition. This crystal symmetry engineering not only shows the capability for dynamic structural engineering at 2D limits, but also highlights the important role of electrostatic doping in controlling different phases, which benefits from weakly electrostatic screening in low-dimensional systems