Atomic-scale dynamics of triangular hole growth in monolayer hexagonal boron nitride under electron irradiation

The production of holes by electron beam irradiation in hexagonal boron nitride (hBN), which has a lattice similar to that of graphene, is monitored over time using atomic resolution transmission electron microscopy. The holes appear to be initiated by the formation of a vacancy of boron and grow in a manner that retains an overall triangular shape. The hole growth process involves the formation of single chains of B and N atoms and is accompanied by the ejection of atoms and bundles of atoms along the hole edges, as well as atom migration. These observations are compared to density functional theory calculations and molecular dynamics simulations.open1

Magic Clusters of MoS₂ by Edge S₂ Interdimer Spacing Modulation

Edge atomic and electronic structures of S-saturated Mo-edge triangular MoS₂ nanoclusters are investigated using density functional theory calculations. The edge electrons described by the S₂-p_<i>x</i>p_<i>x</i>π* (S₂-Π_<i>x</i>) and Mo-d_<i>xy</i> orbitals are found to interplay to pin the S₂-Π_<i>x</i> Fermi wavenumber at <i>k</i>_F = 2/5 as the nanocluster size increases, and correspondingly, the ×5 Peierls edge S₂ interdimer spacing modulation is induced. For the particular sizes of <i>N</i> = 5<i>n</i> – 2 and 5<i>n</i>, where <i>N</i> is the number of Mo atoms at one edge representing the nanocluster size and <i>n</i> is a positive integer, the effective ×5 interdimer spacing modulation stabilizes the nanoclusters, which are identified here to be the magic S-saturated Mo-edge triangular MoS₂ nanoclusters. With the S₂-Π_<i>x</i> Peierls gap, the MoS₂ nanoclusters become far-edge S₂-Π_<i>x</i> semiconducting and subedge Mo-d_<i>xy</i> metallic as <i>N</i> → ∞

Monolayer MoS2 Bandgap Modulation by Dielectric Environments and Tunable Bandgap Transistors

Semiconductors with a moderate bandgap have enabled modern electronic device technology, and the current scaling trends down to nanometer scale have introduced two-dimensional (2D) semiconductors. The bandgap of a semiconductor has been an intrinsic property independent of the environments and determined fundamental semiconductor device characteristics. In contrast to bulk semiconductors, we demonstrate that an atomically thin two-dimensional semiconductor has a bandgap with strong dependence on dielectric environments. Specifically, monolayer MoS2 bandgap is shown to change from 2.8 eV to 1.9 eV by dielectric environment. Utilizing the bandgap modulation property, a tunable bandgap transistor, which can be in general made of a two-dimensional semiconductor, is proposed