Mechanically Induced Metal–Insulator Transition in Carbyne

First-principles calculations for carbyne under strain predict that the Peierls transition from symmetric cumulene to broken-symmetry polyyne structure is enhanced as the material is stretched. Interpretation within a simple and instructive analytical model suggests that this behavior is valid for arbitrary 1D metals. Further, numerical calculations of the anharmonic quantum vibrational structure of carbyne show that zero-point atomic vibrations eliminate the Peierls distortion in the mechanically free chain, preserving the cumulene symmetry. The emergence and increase of Peierls dimerization under tension then implies a qualitative transition between the two forms, which our computations place around 3% strain. Thus, the competition between the zero-point vibrations and mechanical strain determines a switch in symmetry resulting in the transition from metallic state to a dielectric, with a small effective mass and a high carrier mobility. In any practical realization, it is important that the effect is also chemically modulated by the choice of terminating groups. These findings are promising for applications such as electromechanical switching and band gap tuning via strain, and besides carbyne itself, they directly extend to numerous other systems that show Peierls distortion

Mechanochemistry of One-Dimensional Boron: Structural and Electronic Transitions

Recent production of long carbyne chains, concurrent with advances in the synthesis of pure boron fullerenes and atom-thin layers, motivates an exploration of possible one-dimensional boron. By means of first-principles calculations, we find two isomers, two-atom wide ribbon and single-atom chain, linked by a tension-driven (negative-pressure) transformation. We explore the stability and unusual properties of both phases, demonstrating mechanical stiffness on par with the highest-performing known nanomaterials, and a phase transition between stable 1D metal and an antiferromagnetic semiconductor, with the phase boundary effectively forming a stretchable 1D Schottky junction. In addition, the two-phase system can serve as a constant-tension nanospring with a well-calibrated tension defined by enthalpic balance of the phases. Progress in the synthesis of boron nanostructures suggests that the predicted unusual behaviors of 1D boron may find powerful applications in nanoscale electronics and/or mechanical devices

Dirac Cones and Nodal Line in Borophene

Two-dimensional single-layer boron (borophene) has emerged as a new material with several intriguing properties. Recently, the β₁₂ polymorph of borophene was grown on Ag(111), and observed to host Dirac fermions. Similar to graphene, β₁₂ borophene can be described as atom-vacancy pseudoalloy on a closed-packed triangular lattice; however, unlike graphene, the origin of its Dirac fermions  is yet unclear. Here, using first-principles calculations, we probe the origin of Dirac fermions in freestanding and Ag(111)-supported β₁₂ borophene. The freestanding β₁₂ sheet hosts two Dirac cones and a topologically nontrivial Dirac nodal line with interesting Dirac-like edge states. On Ag(111), the Dirac cones develop a gap, whereas the topologically protected nodal line remains intact, and its position in the Brillouin zone matches that of the Dirac-like electronic states seen in the experiment. The presence of nontrivial topological states near the Fermi level in borophene makes its electronic properties important for both fundamental and applied research

Chemical Trends of Electronic Properties of Two-Dimensional Halide Perovskites and Their Potential Applications for Electronics and Optoelectronics

Two-dimensional (2D) halide perovskites with the formula of A₂M^IVX₄^VII are now emerging as a new family of 2D materials and promising candidates for nanoelectronics and optoelectronics. Potentially, there could be abundance of 2D halide perovskites by varying the compositions of A, M and X and their properties can be widely tuned to satisfy the requirements of the practical applications. While several samples have been experimentally realized, most of them are currently unexplored and their chemical trends in relation to the chemical compositions are yet not well understood, which thus drags down the exploration of their potential applications. In this work, using first-principles calculation methods, we systematically investigate the properties of 2D halide perovskites, including their structural stabilities, electronic, optical, and transport properties. The chemical trends in this novel family of 2D materials are established and we find that the bandgaps increase with increased lattice distortions by changing A ion from Cs⁺ to CH₃NH₃⁺, increase with M^IV ion changing from Sb to Pb, and decrease with X changing from Cl to Br to I. Some of the studied systems like Cs₂SnI₄ are identified with good optical properties for photovoltaics and most of the systems have good motilities suitable for electric devices like transistors. The abundance of potential 2D halide perovskites not only enriches current 2D families but also offers more possibility for electrical and optoelectrical applications. Our work is expected to provide theoretical understanding and guidance for the further study of these 2D halide perovskites

Predicting Dislocations and Grain Boundaries in Two-Dimensional Metal-Disulfides from the First Principles

Guided by the principles of dislocation theory, we use the first-principles calculations to determine the structure and properties of dislocations and grain boundaries (GB) in single-layer transition metal disulfides MS₂ (M = Mo or W). In sharp contrast to other two-dimensional materials (truly planar graphene and <i>h</i>-BN), here the edge dislocations extend in third dimension, forming concave dreidel-shaped polyhedra. They include different number of homoelemental bonds and, by reacting with vacancies, interstitials, and atom substitutions, yield families of the derivative cores for each Burgers vector. The overall structures of GB are controlled by both local-chemical and far-field mechanical energies and display different combinations of dislocation cores. Further, we find two distinct electronic behaviors of GB. Typically, their localized deep-level states act as sinks for carriers but at large 60°-tilt the GB become metallic. The analysis shows how the versatile GB in MS₂ (if carefully engineered) should enable new developments for electronic and opto-electronic applications

Can Two-Dimensional Boron Superconduct?

Two-dimensional boron is expected to exhibit various structural polymorphs, all being metallic. Additionally, its small atomic mass suggests strong electron–phonon coupling, which in turn can enable superconducting behavior. Here we perform first-principles analysis of electronic structure, phonon spectra, and electron–phonon coupling of selected 2D boron polymorphs and show that the most stable structures predicted to feasibly form on a metal substrate should also exhibit intrinsic phonon-mediated superconductivity, with estimated critical temperature in the range of <i>T</i>_c ≈ 10–20 K

Strain-Robust and Electric Field Tunable Band Alignments in van der Waals WSe₂–Graphene Heterojunctions

We study the band alignments and band structures of van der Waals WSe₂–graphene heterojunctions by varying out-of-plane external electric field and in-plane mechanical strain using density-functional calculations. We find that the electronic properties of WSe₂–graphene heterojunctions are insensitive to the change of the mechanical strain, showing strong robustness. However, the external electrical field intensity is able to significantly change the band alignments of WSe₂–graphene heterojunctions, while a constant band gap value of WSe₂ in the heterojunctions is nearly maintained. We further show that the highest hole concentration injected by the external electric field is estimated as high as 6.40 × 10¹² cm^–2, while the highest electron density is about 3.00 × 10¹² cm^–2. These findings suggest that the WSe₂–graphene heterojunctions are a promising structure instrumental for electronic device applications

Two-Dimensional Boron Polymorphs for Visible Range Plasmonics: A First-Principles Exploration

Recently discovered two-dimensional (2D) boron polymorphs, collectively tagged borophene, are all metallic with high free charge carrier concentration, pointing toward the possibility of supporting plasmons. Ab initio linear response computations of the dielectric function allow one to calculate the plasmon frequencies (ω) in the selected example structures of boron layers. The results show that the electrons in these sheets indeed mimic a 2D electron gas, and their plasmon dispersion in the small wavevector (<b>q</b>) limit accurately follows the signature dependence ω ∝ √<i>q</i>. The plasmon frequencies that are not damped by single-particle excitations do reach the near-infrared and even visible regions, making borophene the first material with 2D plasmons at such high frequencies, notably with no necessity for doping. The existence of several phases (polymorphs), with varying degree of metallicity and anisotropy, can further permit the fine-tuning of plasmon behaviors in borophene, potentially a tantalizing material with utility in nanoplasmonics

Realizing Indirect-to-Direct Band Gap Transition in Few-Layer Two-Dimensional MX₂ (M = Mo, W; X = S, Se)

In the applications of two-dimensional (2D) transition metal dichalcogenides (TMDs) for solar cell and optoelectronic devices, two challenging issues remain: (1) the direct-to-indirect band gap transition from single layer to a few layers and (2) the absence of an effective and robust doping procedure. In this study, we explore the feasibility to realize indirect-to-direct band gap transition and control the Fermi level by intercalating few-layer TMDs with embedded metals. Specifically, utilizing density functional theory calculations, we examine the electronic properties of few-layer MX₂ (M = Mo, W; X = S, Se) intercalated with metals (Zn, Sn, Mg and Ga). Our calculation results reveal that (1) Ga intercalation can realize an indirect-to-direct band gap transition in few-layer TMDs, and as a result, the absorption efficiency is increased by two orders compared with that of pristine MX₂; and (2) intercalated Ga acts as an n-type shallow donor, which markedly increases the charge density and electrical conductivity. Therefore, Ga intercalation may provide a potential practical route for manipulating few-layer TMDs for high performance solar and optoelectronic devices

Environment-Controlled Dislocation Migration and Superplasticity in Monolayer MoS₂

The two-dimensional (2D) transition metal dichalcogenides (TMDC, of generic formula MX₂) monolayer displays the “triple-decker” structure with the chemical bond organization much more complex than in well-studied monatomic layers of graphene or boron nitride. Accordingly, the makeup of the dislocations in TMDC permits chemical variability, depending sensitively on the equilibrium with the environment. In particular, first-principles calculations show that dislocations state can be switched to highly mobile, profoundly changing the lattice relaxation and leading to superplastic behavior. With 2D MoS₂ as an example, we construct full map for dislocation dynamics, at different chemical potentials, for both the M- and X-oriented dislocations. Depending on the structure of the migrating dislocation, two different dynamic mechanisms are revealed: either the direct rebonding (RB) mechanism where only a single metal atom shifts slightly, or generalized Stone–Wales (SW^g) rotation in which several atoms undergo significant displacements. The migration barriers for RB mechanism can be 2–4 times lower than for the SW^g. Our analyses show that within a range of chemical potentials, highly mobile dislocations could at the same time be thermodynamically favored, that is statistically dominating the overall material property. This demonstrates remarkable possibility of changing material basic property such as plasticity by changing elemental chemical potentials of the environment