2, University of Chinese Academy of Sciences, Beijing, , China
3, Vanderbilt University, Nashville, Tennessee, United States
Two-dimensional (2D) materials have fascinating properties due to their monolayer nature and are promising candidates for flexible nanoelectronic and optoelectronic applications. Defects are well known to have profound influence on the performance of these materials. In order to fully develop their potential, it is essential to understand the atomic structures and dynamical behaviors of the intrinsic defects, and their related electronic properties.
Excitation of dynamical evolution of defects and simultaneous atomic resolution imaging can be realized with an aberration corrected electron beam inside the scanning transmission electron microscope (STEM). This method offers time-resolved direct tracking of the atomic motion during the structural changes induced by the high energy electrons. By controlling the scanning pattern of the electron beam, we can even manipulate the evolution of the defects.
In this talk, I will first show the atomic scale characterizations of complex defect structures in common 2D materials, such as graphene and MoS2, and elaborate how they affect the physical properties of the materials by combing density functional theory (DFT) calculations. I will then demonstrate the atom-by-atom structural evolutions as monitored by sequential Z-contrast (STEM) imaging and its underlying physics, such as Se vacancy-induced inversion domain nucleation in MoSe2, the origin of novel 2D Pd2Se3 phase driven by interlayer fusion in layered PdSe2, and the electron beam induced synthesis of hexagonal MoSe2 from square FeSe [1-3]. At the end of the talk, I will discuss the in-situ fabrication of highly stable metallic nanowires with MX stoichiometry within the transition-metal dichalcogenide (TMD) monolayers by steering the electron beam with atomic precision . These nanowires can be made into different morphology and can be also alloyed either in cation or anion, effectively tuning their electronic structures . These tunable nanowires could, therefore, serve as ultrasmall interconnects in future flexible nanocircuits fabricated entirely from the same monolayer.
 Junhao Lin, et al., ACS Nano, 9, 5189 (2015)
 Junhao Lin, et al., Physical Review Letters, 119, 016101 (2017)
 John A. Brehm#, Junhao Lin#, et al., under review
 Junhao Lin, et al., Nature Nanotechnology, 9, 436 (2014)
 Junhao Lin, et al., ACS Nano, 10, 2782 (2016)