PHYS/ASTR Colloquium: "Tailoring and Manipulating Symmetry in Functional Nanomaterials at the Atomic and Nano-scale" - Dr. Aiming Yan (Asst. Prof., U.C. Santa Cruz)

Abstract: Emerging nanomaterials that are zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) have attracted tremendous interest because of their unique physical and chemical properties that enable novel functionalities. Importantly, such properties and functionalities in emerging nanomaterials are dictated by the microscopic structure, such as local crystal symmetry, which can be broken via introduction of structural disorders (such as defects, stacking faults and grain boundaries). Such control of symmetry in low-dimensional materials have enabled many new opportunities to realize new functionalities, such as control of quantum degrees of freedom in 2D materials, nanoscale actuators based on 1D nanostructures and magnetism at the 2D limit. In my talk, I will first discuss the manipulation of inversion symmetry that is manifested as different stacking sequences in few-layer molybdenum disulfide (MoS₂, a representative 2D material). Additionally, I will show that such control of symmetry can be achieved at different length scales, ranging from the wafer-scale to the atomic-scale, by using chemical synthesis method and in-situ high-resolution transmission electron microscopy (TEM), respectively. This control of symmetry at different length scales enables the realization of emergent valley control in 2D materials such as MoS₂ and could usher in a new type of device functionality– valleytronics. Next, I will focus on another example in which the nanoscale geometric symmetry is tailored in an individual carbon nanotube (CNT, a representative 1D nanostructure) by using a nanomanipulator inside a TEM. In this study, the transition between two metastable states– the symmetric inflated (tubular) state and the asymmetric collapsed state of a CNT can be realized. Such transition is potentially useful for the development of new nano-electromechanical actuators. At the end, I will talk about one of our recent efforts on a new type of 2D materials- 2D magnets, and how we can manipulate their long-range magnetic ordering via their stacking sequences.