Buckling Of Atoms Research Articles

Achieving a Large Energy Gap in Bi(110) Atomically Thin Films

Metal–insulator transition has long been one of the key subjects in condensed matter systems. Herein, the emergence of a large energy gap (Eg, 0.8–1.0 eV) in Bi(110) two‐atomic‐layer nanoribbons grown on a SnSe(001) substrate is reported, which normally has an intrinsic semimetal‐like characteristic. The existence of this abnormally large Eg in Bi(110) is, however, determined by Bi coverage. When coverage is above ≈64 ± 2%, Eg vanishes, and instead, a Bi(110) semimetal‐like phase appears through a singular insulator–metal transition. Measurements using qPlus atomic force microscopy demonstrate that either insulating or semimetal‐like Bi(110) possesses a distorted black phosphorous structure with noticeable atomic buckling. Density function theory fully reproduces the semimetal‐like Bi(110) on SnSe(001). However, none of the insulating phases with this large Eg could be traced. Although the underlying mechanism of the large Eg and the insulator‐metal transition requires further exploration, experiments demonstrate that similar results can be achieved for Bi grown on SnS, the structural analog of SnSe, exhibiting an even larger Eg of ≈2.3 eV. The experimental strategy may be generalized to utilization of group‐IV monochalcogenides to create Bi(110) nanostructures with properties unachievable on other surfaces, providing an intriguing platform for exploring the interesting quantum electronic phases.

One-Dimensional Periodic Buckling at a Symmetry-Incompatible Heterointerface of the NaCl(001) Monolayer on Ir(111)

Monolayer and multilayer NaCl(001) films are grown on an Ir(111) substrate, and strain relaxation modes are investigated by scanning tunneling microscopy (STM) and density functional theory calculations. We report that monolayer NaCl(001) exhibits an apparent one-dimensional periodic buckling along the [010] direction, which separates one-dimensional domains of pristine lattices. An atomic model of the monolayer NaCl(001)-(2 × 6) lattice on an Ir(111)-(4 × 73) substrate is proposed, and related simulation reproduces all the STM features. The calculations suggest that the 4–6 symmetry incompatibility of the NaCl(001)/Ir(111) interface leads to the formation of domain boundaries with alternately arranged top-sited and bridge-sited adatom lines along [100], resulting in one-dimensional atomic buckling. We also observe that the atomic buckling is veiled as the NaCl(001) increases to bilayer and above, suggesting that this relaxation mode is finely tuned by the crystal thickness.

Electronic and topological properties of Bi(110) ultrathin films grown on a Cu(111) substrate

The electronic and topological properties of Bi(110) ultrathin films epitaxially grown on a Cu(111) substrate are investigated using scanning tunneling microscopy/spectroscopy, combined with density functional theory calculations. Bilayer-by-bilayer growth of the ultrathin Bi(110) films is observed, which manifests a structure of black-phosphorus-like Bi bilayers (BLs), with thickness up to 4 BLs. The surface atomic buckling heights in the 1-BL and 2-BL films are clearly identified to depend on the stacking modes with respect to the well-ordered Bi atoms in the adlayer covered on the Cu(111) surface. Our results demonstrate that while the electronic and topological properties of 1-BL films greatly depend on the stacking modes between the Bi(110) bilayer and the adlayer, the 2-BL films show well-decoupled electronic properties from the Cu substrate and nontrivial topologies robust against surface atomic buckling height benefitting from the interbilayer coupling. Our calculations further show that besides the parameter of buckling heights, the topological nontrivial-to-trivial transition can also be induced by changing the interbilayer distance and the vertical intralayer bond length.

Flexible atomic buckling and homogeneous edge states in few-layer Bi(110) films

Flexible atomic buckling and homogeneous edge states in few-layer Bi(110) films

High-buckled stanene with a topologically nontrivial energy gap

We demonstrate an efficient way of tuning the atomic buckling in stanene to open a topologically nontrivial energy gap. By tuning the growth kinetics, we obtain an unexpected high-buckled stanene on the Bi(111) substrate. Scanning tunneling microscopy study combined with density functional theory calculation confirms that the stanene is a distorted 1 × 1 structure with a high-buckled Sn in every three 1 × 1 unit cells. The high-buckled stanene favors a large band inversion at the Γ point, and the spin–orbit coupling opens a topologically nontrivial energy gap. This study provides an alternate way to tune the topology of monolayer topological materials.

Quantitative determination of atomic buckling of silicene by atomic force microscopy

The atomic buckling in 2D "Xenes" (such as silicene) fosters a plethora of exotic electronic properties such as a quantum spin Hall effect and could be engineered by external strain. Quantifying the buckling magnitude with subangstrom precision is, however, challenging, since epitaxially grown 2D layers exhibit complex restructurings coexisting on the surface. Here, we characterize using low-temperature (5 K) atomic force microscopy (AFM) with CO-terminated tips assisted by density functional theory (DFT) the structure and local symmetry of each prototypical silicene phase on Ag(111) as well as extended defects. Using force spectroscopy, we directly quantify the atomic buckling of these phases within 0.1-Å precision, obtaining corrugations in the 0.8- to 1.1-Å range. The derived band structures further confirm the absence of Dirac cones in any of the silicene phases due to the strong Ag-Si hybridization. Our method paves the way for future atomic-scale analysis of the interplay between structural and electronic properties in other emerging 2D Xenes.

Electronic States of Silicene Allotropes on Ag(111).

Silicene, a honeycomb lattice of silicon, presents a particular case of allotropism on Ag(111). Silicene forms multiple structures with alike in-plane geometry but different out-of-plane atomic buckling and registry to the substrate. Angle-resolved photoemission and first-principles calculations show that these silicene structures, with (4×4), (√13×√13)R13.9°, and (2√3×2√3)R30° lattice periodicity, display similar electronic bands despite the structural differences. In all cases the interaction with the substrate modifies the electronic states, which significantly differ from those of free-standing silicene. Complex photoemission patterns arise from surface umklapp processes, varying according to the periodicity of the silicene allotropes.

Topological properties determined by atomic buckling in self-assembled ultrathin Bi(110).

Topological insulators (TIs) are a new type of electronic materials in which the nontrivial insulating bulk band topology governs conducting boundary states with embedded spin-momentum locking. Such edge states are more robust in a two-dimensional (2D) TI against scattering by nonmagnetic impurities than in its three-dimensional (3D) variant, because in 2D the two helical edge states are protected from the only possible backscattering. This makes the 2D TI family a better candidate for coherent spin transport and related applications. While several 3D TIs are already synthesized experimentally, physical realization of 2D TI is so far limited to hybrid quantum wells with a tiny bandgap that does not survive temperatures above 10 K. Here, combining first-principles calculations and scanning tunneling microscopy/spectroscopy (STM/STS) experimental studies, we report nontrivial 2D TI phases in 2-monolayer (2-ML) and 4-ML Bi(110) films with large and tunable bandgaps determined by atomic buckling of Bi(110) films. The gapless edge states are experimentally detected within the insulating bulk gap at 77 K. The band topology of ultrathin Bi(110) films is sensitive to atomic buckling. Such buckling is sensitive to charge doping and could be controlled by choosing different substrates on which Bi(110) films are grown.

Molecular Dynamics Simulations of Surface Oxidation on Pt(111) and Pt/PtCo/Pt3Co(111)

Classical molecular dynamics simulations of surface oxidation in the presence of atomic oxygen and water are performed on Pt(111) and Pt/PtCo/Pt3Co(111) to determine the surface evolution during the oxidation process. On the basis of density functional theory calculations, the model considers electrostatic interactions between the adsorbates and the two topmost layers of the surface as a function of oxygen coverage and it is able to reproduce the main features of the oxidation phenomena observed experimentally. The results indicate that oxygen and water induce changes in the structure and local composition of the near-surface layers which may affect the activity and stability of the catalyst. Such changes are strongly dependent on the amount of adsorbed oxygen, and include oxygen absorption in the subsurface layers at high coverages, and significant atomic buckling and surface reconstruction. In the Pt-skin alloy surface, migration of cobalt atoms to the surface is accompanied by atomic buckling and detachment. The reverse “reduction” process is also simulated by gradually removing the electrostatic interactions between the top layers and adsorbates.

The symmetry of the boron buckyball and a related boron nanotube

We investigate the symmetry of the boron buckyball and a related boron nanotube. Using large-scale ab initio calculations up to second-order Møller–Plesset perturbation theory, we have determined unambiguously the equilibrium geometry/symmetry of two structurally related boron clusters: the B 80 fullerene and the finite-length (5 0) boron nanotube. The B 80 cluster was found to have the same symmetry, I h, as the C 60 molecule since its 20 additional boron atoms are located exactly at the centers of the 20 hexagons. Additionally, we also show that the (5 0) boron nanotube does not suffer from atomic buckling and its symmetry is D 5d instead of C 5v as has been described by previous calculations. Therefore, we predict that all the boron nanotubes rolled from the α-sheet will be free from structural distortions, which has a significant impact on their electronic properties.

Ab initiostudy of superconducting hexagonalBe2Liunder pressure

Following the recent interest on lithium alloys under pressure, we present first-principles calculations of the electronic band structure and lattice dynamics for the ${\text{MgB}}_{2}$-like layered ${\text{Be}}_{2}\text{Li}$ at 80 GPa. Under pressure, the increasing electronic localization reduces the effective dimensionality of ${\text{Be}}_{2}\text{Li}$, showing both two-dimensional and one-dimensional conducting electronic channels. The numerical results reveal the presence of an ultrasoft strongly anharmonic ${B}_{2g}$ phonon mode associated to the atomic buckling in the layers, which shows a high electron-phonon interaction near the zone center and (besides the moderately coupled ${E}_{2g}$ in-plane and ${A}_{1g}$ interlayer Be modes) is responsible for a predicted superconducting transition around 2.5 K.