Detecting stable adsorbates of (1S)-camphor on Cu(111) with Bayesian optimization.

Identifying the atomic structure of organic-inorganic interfaces is challenging with current research tools. Interpreting the structure of complex molecular adsorbates from microscopy images can be difficult, and using atomistic simulations to find the most stable structures is limited to partial exploration of the potential energy surface due to the high-dimensional phase space. In this study, we present the recently developed Bayesian Optimization Structure Search (BOSS) method as an efficient solution for identifying the structure of non-planar adsorbates. We apply BOSS with density-functional theory simulations to detect the stable adsorbate structures of (1S)-camphor on the Cu(111) surface. We identify the optimal structure among eight unique types of stable adsorbates, in which camphor chemisorbs via oxygen (global minimum) or physisorbs via hydrocarbons to the Cu(111) surface. This study demonstrates that new cross-disciplinary tools, such as BOSS, facilitate the description of complex surface structures and their properties, and ultimately allow us to tune the functionality of advanced materials.

Anomalous Shape Evolution of Ag2O2 Nanocrystals Modulated by Surface Adsorbates during Electron Beam Etching.

An understanding of nanocrystal shape evolution is significant for the design, synthesis, and applications of nanocrystals with surface-enhanced properties such as catalysis or plasmonics. Surface adsorbates that are selectively attached to certain facets may strongly affect the atomic pathways of nanocrystal shape development. However, it is a great challenge to directly observe such dynamic processes in situ with a high spatial resolution. Here, we report the anomalous shape evolution of Ag2O2 nanocrystals modulated by the surface adsorbates of Ag clusters during electron beam etching, which is revealed through in situ transmission electron microscopy (TEM). In contrast to the Ag2O2 nanocrystals without adsorbates, which display the near-equilibrium shape throughout the etching process, Ag2O2 nanocrystals with Ag surface adsorbates show distinct facet development during etching by electron beam irradiation. Three stages of shape changes are observed: a sphere-to-a cube transformation, side etching of a cuboid, and bottom etching underneath the surface adsorbates. We find that the Ag adsorbates modify the Ag2O2 nanocrystal surface configuration by selectively capping the junction between two neighboring facets. They prevent the edge atoms from being etched away and block the diffusion path of surface atoms. Our findings provide critical insights into the modulatory function of surface adsorbates on the shape control of nanocrystals.

Electrothermal Local Annealing via Graphite Joule Heating on Two-Dimensional Layered Transistors.

A simple but powerful device platform for electrothermal local annealing (ELA) via graphite Joule heating on the surface of transition-metal dichalcogenide, is suggested here to sustainably restore intrinsic electrical properties of atomically thin layered materials. Such two-dimensional materials are easily deteriorated by undesirable surface/interface adsorbates and are screened by a high metal-to-semiconductor contact resistance. The proposed ELA allows one to expect a better electrical performance such as an excess electron doping, an enhanced carrier mobility, and a reduced surface traps in a monolayer molybdenum disulfide (MoS2)/graphite heterostructure. The thermal distribution of local heating measured by an infrared thermal microscope and estimated by a finite element calculation shows that the annealing temperature reaches up to >400 K at ambient condition and the high efficiency of site-specific annealing is demonstrated unlike the case of conventional global thermal annealing. This ELA platform can be further promoted as a practical gas sensor application. From an O2 cycling test and a low-frequency noise spectroscopy, the graphite on top of the MoS2 continuously recovers its initial condition from surface adsorbates. This ELA technique significantly improves the stability and reliability of its gas sensing capability, which can be expanded in various nanoscale device applications.