Effective chemical potential for non-equilibrium systems and its application to molecular beam epitaxy of Bi2Se3

Understanding High-Temperature Chemical Reactions on Metal Surfaces: A Case Study on Equilibrium Concentration and Diffusivity of C x H y on a Cu(111) Surface

Chemical reactions on metal surfaces are important in various processes such as heterogeneous catalysis and nanostructure growth. At moderate or lower temperatures, these reactions generally follow the minimum energy path, and temperature effects can be reasonably described by a harmonic oscillator model. At a high temperature approaching the melting point of the substrate, general behaviors of surface reactions remain elusive. In this study, by taking hydrocarbon species adsorbed on Cu(111) as a model system and performing extensive molecular dynamics simulations powered by machine learning potentials, we identify several important high-temperature effects, including local chemical environment, surface atom mobility, and substrate thermal expansion. They affect different aspects of a high-temperature surface reaction in different ways. These results deepen our understanding of high-temperature reactions.

Atomic Properties of Monoclinic Ag2Se Thin Film Grown on SrTiO3 Substrate by Molecular Beam Epitaxy

Silver chalcogenides have attracted a great deal of interest due to their promise for exhibiting novel topological properties. Using scanning tunneling microscopy/spectroscopy (STM/S), we have characterized the atomic structure and electronic properties of a monoclinic Ag₂Se thin film, similar to β-Ag₂Te, grown on a SrTiO₃ (STO)(001) substrate by molecular beam epitaxy (MBE). Three different types of Ag₂Se atomic terminations are observed on the surface: (i) homogeneous hexagonal-like, (ii) rough mixed, and (iii) flat zigzag-striped structures. Structural analysis indicates that the different atomic terminations stem from different growth directions, which can be attributed to the lattice mismatch between the substrate and the Ag₂Se film. STS analysis of these atomic terminations uncovers different features near the Fermi level, indicating constituent- and direction-dependent electronic properties. This Letter presents a practical method to grow monoclinic thin film Ag₂Se and provides insight into its physical properties.

Role of Buffer Layer and Building Unit in the Monolayer CrI3 Growth: A First-Principles Perspective

CrI₃, a two-dimensional layered material, has recently attracted a lot of research interest due to its exotic magnetic property. However, the synthesis of a CrI₃ monolayer (ML) by vapor-phase deposition has not been achieved by current experimental endeavors, which require a better understanding of the vapor-phase growth mechanism involved. In this study, we theoretically simulate the growth of the CrI₃ ML on the Si(111) surface by using a first-principles calculation. Our key finding is that an iodine buffer layer on the Si surface is crucial to the formation of the CrI₃ ML by stabilizing the precursor and also reacting with the precursor. Moreover, our simulation reveals that the CrI₂ cluster as the growth building unit, which can be formed by controlling the chemical potential of the I source, is preferred for the formation of CrI₃ ML. We think that our work will provide insightful guidance for the experimental synthesis of CrI₃ ML in the future.