Atomic Reconstruction Research Articles

Mechanical and electronic properties of 2D black phosphorene nanoribbons: A first-principles study

The mechanical and electronic properties of black phosphorene nanoribbons are studied using the first-principles calculations. The armchair black phosphorene nanoribbons (APNR) are obtained and the atomic reconstruction is found at the edge of the nanoribbons. In addition, the strain field is applied on the nanoribbons, and its stress-strain curve, Young’s modulus, and Poisson’s ratio are calculating by stretching APNR. It is found that the Young’s modulus enhanced with the increase of nanoribbons width. It is significantly lower than the Young’s modulus of monolayer black phosphorene. On the contrary, the Poisson’s ratio hardly changes with the increase of nanoribbons width. Finally, it is found that the band gap of APNR decreases with the increase of nanoribbons width and increases monotonically with the increase of strain.

Lattice distortion-enhanced superlubricity of (Mo, X)S2 (X = Al, Ti, Cr and V) with moiré superlattice.

Two-dimensional (2D) materials with the advantage of low interlayer shear strain are ultilized as lubricants in aerospace and precision manufacturing. Moiré superlattices (MSL), with attractive physical properties of electronic structures, interlayer hybridization and atomic forces, have been widely investigated in superlubricity, which is caused by elimination of interlayer lock-in by incommensurate atomic reconstruction. Although the foundations of superlubricity and the development of 2D lubricants via vanishing friction have been investigated, it is still important to comprehensively reveal the influence of MSL on the interlayer van der Waals (vdW) interactions of 2D lubricants. Here, the contributions of lattice distortions of solute-doped twisted bilayers (Mo, X)S2 (X = Al, Ti, V and Cr) to superlubricity are comprehensively investigated by high-throughput modelling and DFT-D2 calculations. It is revealed that the lattice distortion not only breaks the interlayer balance of repulsion and van der Waals interactions but also yields layer corrugation. These layer-corrugation-induced changes of the interlayer interactions and spacing distances are utilized to optimize lubricity, which matches with the experimental friction coefficients in the order of (Mo, Al)S2 > (Mo, Cr)S2 > MoS2 >(Mo, V)S2 >(Mo, Ti)S2. The evolutions of the band structures show an exponential relationship of the band edge width and layer deformations, paving a path to accelerate the development of advanced superlubricity materials via lattice distortions.

Anomalous fracture in two-dimensional rhenium disulfide.

Low-dimensional materials usually exhibit mechanical properties from those of their bulk counterparts. Here, we show in two-dimensional (2D) rhenium disulfide (ReS2) that the fracture processes are dominated by a variety of previously unidentified phenomena, which are not present in bulk materials. Through direct transmission electron microscopy observations at the atomic scale, the structures close to the brittle crack tip zones are clearly revealed. Notably, the lattice reconstructions initiated at the postcrack edges can impose additional strain on the crack tips, modifying the fracture toughness of this material. Moreover, the monatomic thickness allows the restacking of postcrack edges in the shear strain-dominated cracks, which is potentially useful for the rational design of 2D stacking contacts in atomic width. Our studies provide critical insights into the atomistic processes of fracture and unveil the origin of the brittleness in the 2D materials.

Dimensionality effects in crystal plasticity, from 3D silicon to 2D silicene

To characterize the effects of free surfaces on dislocation mobility, the edge dislocation glide process in thin silicon films is modeled using an interatomic potential and first principles calculations. The influence of film thickness is determined, starting with the simulation of a silicene monolayer and then increasing the number of layers. The energy barrier for dislocation glide in silicene was calculated to be 1.5 eV, indicating a relatively high mobility of dislocation defects. In thin Si films a glide mechanism via consecutive bond rotations was identified, with kink nucleation being observed at the free surfaces of the film with subsequent migration. The influence of the free edge in finite size films is shown to be negligible in relation to glide for dislocations at distances from the free edge larger than three Burgers vectors. Molecular dynamics simulations reveal the possibility of more complex but lower energy barrier atomic reconstructions triggered at the free surfaces of the film near the dislocation core that may increase dislocation mobility at higher temperatures.

Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSe2/MoSe2 bilayers.

Van der Waals heterostructures obtained via stacking and twisting have been used to create moiré superlattices1, enabling new optical and electronic properties in solid-state systems. Moiré lattices in twisted bilayers of transition metal dichalcogenides (TMDs) result in exciton trapping2-5, host Mott insulating and superconducting states6 and act as unique Hubbard systems7-9 whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures feature atomic reconstruction and domain formation10-14. However, due to the nanoscale size of moiré domains, the effects of atomic reconstruction on the electronic and excitonic properties have not been systematically investigated. Here we use near-0°-twist-angle MoSe2/MoSe2 bilayers with large rhombohedral AB/BA domains15 to directly probe the excitonic properties of individual domains with far-field optics. We show that this system features broken mirror/inversion symmetry, with the AB and BA domains supporting interlayer excitons with out-of-plane electric dipole moments in opposite directions. The dipole orientation of ground-state Γ-K interlayer excitons can be flipped with electric fields, while higher-energy K-K interlayer excitons undergo field-asymmetric hybridization with intralayer K-K excitons. Our study reveals the impact of crystal symmetry on TMD excitons and points to new avenues for realizing topologically non-trivial systems16,17, exotic metasurfaces18, collective excitonic phases19 and quantum emitter arrays20,21 via domain-pattern engineering.

Polarity-induced electronic and atomic reconstruction at NdNiO2/SrTiO3 interfaces

Superconductivity has recently been observed in Sr-doped NdNiO2 films grown on SrTiO3. Whether it is caused by or related to the interface remains an open question. To address this issue, we use density functional theory calculation and charge transfer self-consistent model to study the effects of polar discontinuity on the electronic and atomic reconstruction at the NdNiO2/SrTiO3 interface. We find that sharp interface with pure electronic reconstruction only is energetically unfavorable, and atomic reconstruction is unavoidable. We further propose a possible interface configuration that contain residual apical oxygen. These oxygen atoms lead to hybrids of dz2 and dx2-y2 states at the Fermi level, which weaken the single-band feature and may be detrimental to superconductivity.

Low-frequency Raman scattering in WSe2−MoSe2 heterobilayers: Evidence for atomic reconstruction

We investigate WSe2–MoSe2 heterobilayers with different twist angles θ±δ between the two layers by low-frequency Raman scattering. In sufficiently aligned samples with θ=0° or θ=60° and δ≲3°, we observe an interlayer shear mode (ISM), which is a clear sign of a commensurate bilayer structure, i.e., the layers must undergo an atomic reconstruction to form R-type or H-type stacking orders. We find slightly different ISM energies of about 18 cm–1 and 17 cm–1 for H-type and R-type reconstructions, respectively, independent of the exact value of θ±δ. Our findings are corroborated by the fact that the ISM is not observed in samples with twist angles, which deviate by δ>3° from 0° or 60°. This is expected since, in such incommensurate structures, with the possibility of Moiré-lattice formation, there is no restoring force for an ISM. Furthermore, we observe the ISM even in sufficiently aligned heterobilayers, which are encapsulated in hexagonal Boron nitride. This is particularly relevant for the characterization of high-quality heterostructure devices.

Oxygen-vacancy-induced atomic and electronic reconstructions in magnetic Sr(Ti0.875Fe0.125)O3-δ thin films

The atomic and electronic structures have been investigated for the multiferroic behavior in the perovskite oxides, which also can be tuned by oxygen vacancy for enhancing properties. Here epitaxial Sr(Ti0.875Fe0.125)O3-δ thin films were deposited on (001) SrTiO3 substrates by pulsed laser deposition and were post-annealed in an oxygen atmosphere. We found that the oxygen vacancies formed in high vacuum are the source of the macroscopic crystal distortion as the growth strain along out-of-plane. Moreover, it was determined that the full-filled Fe 3d states induced by oxygen vacancy effect are responsible for the decreased magnetization. This work demonstrates that the oxygen vacancy can both lead to atomic and electronic reconstructions in the perovskite films for manipulating ferroic properties.

Density functional calculations for structures and energetics of atomic steps and their implication for surface morphology on Si-face SiC polar surfaces

We perform large-scale density-functional calculations using the real-space finite-difference scheme endorsed by the Gordon Bell prize in 2011 that reveal detailed atomic and electronic structures of atomic steps on silicon carbide (SiC) polar surfaces for the first time. The accurate structural optimization elucidates characteristic atomic reconstruction among the upper and lower edge atoms, which is peculiar to compound semiconductors having both covalent and ionic nature. The calculated formation energies of all the possible atomic steps lead us to unequivocally identify the abundant atomic steps on the Si-face SiC polar surfaces. The energetics thus obtained for the atomic steps provides a natural and persuasive microscopic reason for the difference in the step morphology observed experimentally, i.e., the meandering and straight step edges depending on the inclined direction on the polar vicinal SiC surfaces. Electron states caused by those atomic steps are also calculated, which assists in the identification of the atomic steps by future experiments.

Self-Epitaxial Hetero-Nanolayers and Surface Atom Reconstruction in Electrocatalytic Nickel Phosphides.

Surface atomic, compositional, and electronic structures play decisive roles in governing the performance of catalysts during electrochemical reactions. Nevertheless, for efficient and cheap transition-metal phosphides used for water splitting, such atomic-scale structural information is largely missing. Despite much effort being made so far, there is still a long way to go for establishing a precise structure-activity relationship. Here, in combination with electron-beam bombardment and compositional analysis, our atomic-scale transmission electron microscopy study on Ni5P4 nanosheets, with a preferential (001) orientation, directly reveals the coverage of a self-epitaxial Ni2P nanolayer on the phosphide surface. Apart from the presence of nickel vacancies in the Ni5P4 phase, our quantum-mechanical image simulations also suggest the existence of an additional NiPx (0 < x < 0.5) nanolayer, characteristic of complex surface atom reconstruction, on the outermost surface of the phosphides. The surface chemical gradient and the core-shell scenario, probably responsible for the passivated catalytic activity, provide a novel insight to understand the catalytic performance of transition-metal catalysts used for electrochemical energy conversion.

Twist Angle-Dependent Atomic Reconstruction and Moiré Patterns in Transition Metal Dichalcogenide Heterostructures.

Van der Waals layered materials, such as transition metal dichalcogenides (TMDs), are an exciting class of materials with weak interlayer bonding, which enables one to create so-called van der Waals heterostructures (vdWH). One promising attribute of vdWH is the ability to rotate the layers at arbitrary azimuthal angles relative to one another. Recent work has shown that control of the twist angle between layers can have a dramatic effect on TMD vdWH properties, but the twist angle has been treated solely through the use of rigid-lattice moiré patterns. No atomic reconstruction, that is, any rearrangement of atoms within the individual layers, has been reported experimentally to date. Here, we demonstrate that vdWH of MoSe2/WSe2 and MoS2/WS2 at twist angles ≤1° undergo significant atomic level reconstruction leading to discrete commensurate domains divided by narrow domain walls, rather than a smoothly varying rigid-lattice moiré pattern as has been assumed in prior experimental work. Using conductive atomic force microscopy (CAFM), we show that TMD vdWH at small twist angles exhibit large domains of constant conductivity. The domains in samples with R-type stacking are triangular, whereas the domains in samples with H-type stacking are hexagonal. Transmission electron microscopy provides additional evidence of atomic reconstruction in MoSe2/WSe2 structures and demonstrates the transition between a rigid-lattice moiré pattern for large angles and atomic reconstruction for small angles. We use density functional theory to calculate the band structures of the commensurate reconstructed domains and find that the modulation of the relative electronic band edges is consistent with the CAFM results and photoluminescence spectra. The presence of atomic reconstruction in TMD heterostructures and the observed impact on nanometer-scale electronic properties provide fundamental insight into the behavior of this important class of heterostructures.

High‐Resolution Crystal Truncation Rod Scattering: Application to Ultrathin Layers and Buried Interfaces

AbstractIn crystalline materials, the presence of surfaces or interfaces gives rise to crystal truncation rods (CTRs) in their X‐ray diffraction patterns. While structural properties related to the bulk of a crystal are contained in the intensity and position of Bragg peaks in X‐ray diffraction, CTRs carry detailed information about the atomic structure at the interface. Developments in synchrotron X‐ray sources, instrumentation, and analysis procedures have made CTR measurements into extremely powerful tools to study atomic reconstructions and relaxations occurring in a wide variety of interfacial systems, with relevance to chemical and electronic functionalities. In this review, an overview of the use of CTRs in the study of atomic structure at interfaces is provided. The basic theory, measurement, and analysis of CTRs are covered and applications from the literature are highlighted. Illustrative examples include studies of complex oxide thin films and multilayers.

Atom probe analysis of Ni–Nb–Zr metallic glasses

Atomic short arrangements of two Ni–Nb–Zr glassy alloys (doped and undoped) were examined through the 3D atomic reconstruction technique, atom probe tomography (APT). The chemical short configurations predicted according to two distinct theoretical models were compared with APT reconstructions, and inconsistencies were found. Additionally, diffraction experiments confirmed that the alloys under investigation were amorphous in nature.

Unusual valence state in the antiperovskites Sr3SnO and Sr3PbO revealed by x-ray photoelectron spectroscopy

The class of antiperovskite compounds $A_3B$O ($A$ = Ca, Sr, Ba; $B$ = Sn, Pb) has attracted interest as a candidate 3D Dirac system with topological surface states protected by crystal symmetry. A key factor underlying the rich electronic structure of $A_3B$O is the unusual valence state of $B$, i.e., a formal oxidation state of $-4$. Practically, it is not obvious whether anionic $B$ can be stabilized in thin films, due to its unusual chemistry, as well as the polar surface of $A_3B$O, which may render the growth-front surface unstable. We report X-ray photoelectron spectroscopy (XPS) measurements of single-crystalline films of Sr$_3$SnO and Sr$_3$PbO grown by molecular beam epitaxy (MBE). We observe shifts in the core-level binding energies that originate from anionic Sn and Pb, consistent with density functional theory (DFT) calculations. Near the surface, we observe additional signatures of neutral or cationic Sn and Pb, which may point to an electronic or atomic reconstruction with possible impact on putative topological surface states.

Composite super-moiré lattices in double-aligned graphene heterostructures.

When two-dimensional (2D) atomic crystals are brought into close proximity to form a van der Waals heterostructure, neighbouring crystals may influence each other's properties. Of particular interest is when the two crystals closely match and a moiré pattern forms, resulting in modified electronic and excitonic spectra, crystal reconstruction, and more. Thus, moiré patterns are a viable tool for controlling the properties of 2D materials. However, the difference in periodicity of the two crystals limits the reconstruction and, thus, is a barrier to the low-energy regime. Here, we present a route to spectrum reconstruction at all energies. By using graphene which is aligned to two hexagonal boron nitride layers, one can make electrons scatter in the differential moiré pattern which results in spectral changes at arbitrarily low energies. Further, we demonstrate that the strength of this potential relies crucially on the atomic reconstruction of graphene within the differential moiré super cell.

Visualizing Progressive Atomic Change in the Metal Surface Structure Made by Ultrafast Electronic Interactions in an Ambient Environment.

Understanding the atomic and molecular phenomena occurring in working catalysts and nanodevices requires the elucidation of atomic migration originating from electronic excitations. The progressive atomic dynamics on metal surface under controlled electronic stimulus in real time, space, and gas environments are visualized for the first time. By in situ environmental transmission electron microscopy, the gas molecules introduced into the biased metal nanogap could be activated by electron tunneling and caused the unpredicted atomic dynamics. The typically inactive gold was oxidized locally on the positive tip and field-evaporated to the negative tip, resulting in the atomic reconstruction on the negative tip surface. This finding of a tunneling-electron-attached-gas process will bring new insights into the design of nanostructures such as nanoparticle catalysts and quantum nanodots and will stimulate syntheses of novel nanomaterials not seen in the ambient environment.

Visualizing Progressive Atomic Change in the Metal Surface Structure Made by Ultrafast Electronic Interactions in an Ambient Environment

AbstractUnderstanding the atomic and molecular phenomena occurring in working catalysts and nanodevices requires the elucidation of atomic migration originating from electronic excitations. The progressive atomic dynamics on metal surface under controlled electronic stimulus in real time, space, and gas environments are visualized for the first time. By in situ environmental transmission electron microscopy, the gas molecules introduced into the biased metal nanogap could be activated by electron tunneling and caused the unpredicted atomic dynamics. The typically inactive gold was oxidized locally on the positive tip and field‐evaporated to the negative tip, resulting in the atomic reconstruction on the negative tip surface. This finding of a tunneling‐electron‐attached‐gas process will bring new insights into the design of nanostructures such as nanoparticle catalysts and quantum nanodots and will stimulate syntheses of novel nanomaterials not seen in the ambient environment.

Distinctive Performance of Terahertz Photodetection Driven by Charge‐Density‐Wave Order in CVD‐Grown Tantalum Diselenide

AbstractThe quantum behavior of carriers in solid is the foundation of modern electronic and optoelectronic technology, but it is still facing huge challenges within inherited single‐particle quantum processes working at the millimeter wave/terahertz (THz) band. Here, a straightforward strategy for the direct detection of millimeter wave/THz photons in a sub‐wavelength metal‐TaSe2‐metal structure under strong interaction with a localized field of surface plasmon is proposed. By breaking the inversion symmetry under the perturbations of electric field and atomic reconstruction from van der Waals integration, the nonequilibrium electronic states under a radiant field can be manipulated in a collective fashion, leading to a large photocurrent responsivity over 40 A W−1 and noise equivalent power less than 1 pW Hz−1/2 even at room temperature. A more than 40‐fold enhancement in responsivity is achieved when transitioning from the normal phase to the CDW phase. The findings shed fresh light on the understanding of the delicate balance in the charge‐ordered phase, and facilitate the exploitation of a correlated electron system for optoelectronic applications in fields of security, remote sensing, and imaging.

Iterative Algorithm of Atomic Potential Reconstruction Based on DPC Signal from Thick Specimens

Iterative Algorithm of Atomic Potential Reconstruction Based on DPC Signal from Thick Specimens

Atomic and Electronic Reconstruction at the a-LAO/STO Interface by E-Beam Induced Crystallization

Atomic and Electronic Reconstruction at the a-LAO/STO Interface by E-Beam Induced Crystallization