Atomic Reconstruction Research Articles

Influence of edge reconstruction on the electron transport in zigzag graphene nanoribbon

Edge reconstructions of graphene nanoribbons and their stable defective configurations were identified by experimental characterization. First principles calculations are performed to evaluate the effects of atomic edge arrangement on the electronic transport properties of zigzag graphene nanoribbons. It is found that these two defective edge structures affect effectively the high stable nanostructure configuration and give rise to pronounced modifications on electronic bands, leading to the shift of Fermi level as well as the occurrence of resonant energies. Both of these two atomic reconstructions would limit the electron transport around the Fermi level, and result in the complete resonant backscattering taking place at different locations. The suppression of conductance is not only related with increasing defect size, but more sensitive to the distribution of defect state, and the modifications on the electronic bands that are influenced by the edge reconstructions.

Geometric thermal phase diagrams for studying the thermal dynamic stability of hollow gold nanoballs at different temperatures

Thermal stability is one of the main concerns for the synthesis of hollow nanoparticles. In this work, molecular dynamics simulation gave an insight into the atomic reconstruction and energy evolution during the collapse of hollow gold nanoballs, based on which a mechanism was proposed. The stability was found to depend on temperature, its wall thickness and aspect ratio to a great extent. The relationship among these three factors was revealed in geometric thermal phase diagrams (GTPDs). The GTPDs were studied theoretically, and the boundary between different stability regions can be fitted and calculated. Therefore, the GTPDs at different temperatures can be deduced and used as a guide for hollow structure synthesis.

First-Principles Calculations of the Formation and Structures of Point Defects on GaN (0001) Surface

We studied the structures and energies associated with 8 types of point defects on the [0001] surface of hexagonal gallium nitride (GaN) by modeling: (1) Ga vacancies (VGa), (2) N vacancies (VN), (3) substitution of N by Ga (GaN), (4) substitution of Ga by N (NGa), (5) Ga octahedral interstitial defects (GaO), (6) Ga tetrahedral interstitial defects (GaT), (7) N octahedral interstitial defects (NO), and (8) N tetrahedral interstitial defects (NT). Using a plane-wave ultrasoft pseudopotential method, we calculate these defect structures, simulate the shift, bonding, and relaxation reconstruction of surface atoms in response to the formation of these defects and also calculate the formation energies of these defects. We find that the Ga-related defects only slightly affect the surface, whereas all N-related defects induce substantial surface reconstruction. In particular, the formation of NT not only induces distortion of the surface structure, but also significantly influences the structure of the deeper lattice space. Calculations of formation energies suggest that, in Ga-rich conditions, GaO forms most easily, followed by GaN, VN, and GaT. In comparison, in N-rich conditions, VGa forms most easily. In all environments, GaO, GaN, and VGa form more easily than VN, and the formation of octahedral interstitial defects requires less energy than tetrahedral interstitial defects, which suggests it is difficult to form tetrahedral interstitial defects in the GaN (0001) surface.

Magnetism at the edge: New phenomena at oxide interfaces

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Oxide interfaces with enhanced ion conductivity

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Refining carbon flux paths using atomic trace data

Pathway analysis tools are a powerful strategy to analyze 'omics' data in the field of systems biology. From a metabolic perspective, several pathway definitions can be found in the literature, each one appropriate for a particular study. Recently, a novel pathway concept termed carbon flux paths (CFPs) was introduced and benchmarked against existing approaches, showing a clear advantage for finding linear pathways from a given source to target metabolite. CFPs are simple paths in a metabolite-metabolite graph that satisfy typical constraints in stoichiometric models: mass balancing and thermodynamics (irreversibility). In addition, CFPs guarantee carbon exchange in each of their intermediate steps, but not between the source and the target metabolites and consequently false positive solutions may arise. These pathways often lack biological interest, particularly when studying biosynthetic or degradation routes of a metabolite. To overcome this issue, we amend the formulation in CFP, so as to account for atomic fate information. This approach is termed atomic CFP (aCFP). By means of a side-by-side comparison in a medium scale metabolic network in Escherichia Coli, we show that aCFP provides more biologically relevant pathways than CFP, because canonical pathways are more easily recovered, which reflects the benefits of removing false positives. In addition, we demonstrate that aCFP can be successfully applied to genome-scale metabolic networks. As the quality of genome-scale atomic reconstruction is improved, methods such as the one presented here will undoubtedly be of value to interpret 'omics' data.

Stabilities and Reconstructions of PbTe Crystal Surfaces from Density-Functional Theory

Lead telluride (PbTe) is a well-known functional material with current applications in fields such as nanostructured thermoelectrics. Here, we report comprehensive first-principles simulations for important crystal surfaces of rocksalt-type PbTe. Surface energies have been computed for pristine (001), (011), and (111) surfaces, and a number of relevant (111) surface reconstructions have been explored. Density-functional theory (DFT) was applied at various levels of sophistication and cost, namely, the local density approximation (LDA), the generalized gradient approximation (GGA), and the HSE06 hybrid functional. Overall, PbTe(001) is predicted to be noticeably stable, essentially corroborating previous computational studies that had been performed for this particular termination; here, however, we expand upon these findings and use the computed set of surface-energy data to predict the equilibrium crystallite shape (as reflected by the Wulff construction); the latter is strongly dominated by {001} facets throughout the entire chemical-potential space. For the PbTe(111) surface, atomic reconstructions are investigated, and they emerge as energetically more favorable than either of the two pristine terminations: this in contrast to the lighter homologues GeTe and SnTe. In particular, for PbTe, 2 × 1 and 2 × 2 reconstructions are predicted and assessed in comprehensive surface phase diagrams. The computational results compare favorably to previous experiments including evidence for a 2 × 2 reconstruction of the (111) surface; overall, they may contribute a useful piece of understanding of these important crystal surfaces.

Theoretical study on electronic and optical properties of In0.53Ga0.47As (1 0 0) β2 (2 × 4) surface

InxGa1−xAs is a perfect III–V compound semiconductor for the photoemissivelayer of the infrared-extension negative electron affinity photocathode. The InxGa1−xAs used as photoemissivelayer has a better lattice match with InP used as the substrate of the photocathode when the In component is 0.53. The electron structure, formation energy, work function and the optical properties of the In0.53Ga0.47As (100) β2 (2×4) surface, which are closely related with the photocathode emission properties according to the photocathode formation mechanism, are calculated and analyzed in this article. The changes including the decrease of the energy gap and the appearance of the new energy bands happen to the surface energy bands due to the reconstruction of the surface atoms. The surface work function of In0.53Ga0.47As (100) β2 (2×4) surface which is sensitive to the near-infrared light is 4.274eV. It is less than that of the GaAs (100) β2 (2×4) surface which is sensitive to the visible light. The absorption coefficient is studied a lot among the optical properties. The results show that the surface absorption coefficient is obviously bigger than that in the In0.53Ga0.47As bulk in the low energy region. Hence the In0.53Ga0.47As (100) β2 (2×4) surface is benefit for the formation of the negative electron affinity photocathode, which offers effective theoretical foundation for the photocathode structure design.

Structural and electronic inhomogeneity for graphene grown on the C-face of SiC: Insights from ab initio calculations

The graphitization of the SiC(0001¯) plane, commonly referred to as the C-face of SiC, takes place through the sublimation and reorganization of surface atoms upon high-temperature annealing. Often, such reorganization gives rise to ordered atomic reconstructions over the ideally flat (0001¯) plane. In this article, we use the density functional theory to model graphene/SiC(0001¯) interfaces with an (1×1), (2×2) and (3×3) SiC periodicity. Our results indicate that the interface geometry can be crucial for both the stability and the electronic characteristics of the first graphitic layer, revealing a complex scenario of binding, doping and electronic correlations. We argue that the presence of more than one interface geometry at different areas of the same sample could be a reason for structural inhomogeneity and n- to p-type transitions.

Single-cycle atomic force microscope force reconstruction: resolving time-dependent interactions

Here, we enhance the capabilities of the atomic force microscope (AFM) to show that force profiles can be reconstructed without restriction by monitoring the wave profile of the cantilever during a single oscillation cycle. Two approaches are provided to reconstruct the force profile in both the steady and transient states in what we call single-cycle measurements. The robustness of the formalism is tested numerically to recover complex but relevant interactions. With single-cycle measurements, we add high temporal resolution (possibly microsecond range) to the spatial resolution of AFM. The access to simultaneous high throughput and high sensitivity further opens the door to a variety of feedback options for imaging.

Atomic and Electronic Structure of V–Rh(110) Near-Surface Alloy

Vanadia-promoted Rh catalysts and Rh–V alloys have shown enhanced reactivity in several heterogeneous catalytic processes. However, little is known about the role of electronic and geometrical effects of the alloy phase on the chemical reactivity. In this article, we present experimental study on geometric and electronic structure of near-surface ordered alloys prepared by depositing V on Rh(110) surface and by subsequent annealing. STM, LEED, and XPD measurements were combined to determine the atomic structure. The electronic influence of the subsurface vanadium on the Rh(110) surface was elucidated by means of core-level and angle-resolved valence band photoemission spectroscopy. The influence of the alloying on the interaction with gas molecules was probed by CO adsorption. It was found that annealing at 973 K results in a (2 × 1) surface reconstruction of Rh surface atoms along [1̅10] rows induced by subsurface vanadium. Annealing at higher temperatures induces diffusion of vanadium into deeper layers and formation of a (1 × 2) missing-row reconstruction, which is metastable at the bare Rh surface. Photoemission spectroscopy revealed a rather small shift of the Rh valence band centroid upon alloying, which indicates only a small influence of the electronic effect on the reactivity. On the other hand, the (1 × 2) missing-row reconstruction brings about new adsorption sites and low-coordinated Rh atoms which are expected to significantly contribute to the enhanced reactivity of the Rh–V alloy.

Edge reconstruction limited electron transport of zigzag graphene nanoribbon

Recent experimental characterizations have clearly visualized edge reconstructions in graphene nanoribbon and stable defective configurations. We have performed first principles calculations to evaluate the effects of atomic edge arrangement on the electronic transport properties of zigzag graphene nanoribbons (ZGNR). It is found that different conductance behaviors and variation of resonant energies are influenced by atomic reconstruction among three defective edge configurations. It is predicted that the conductance in edge reconstructed ZGNR is not a monotonic function of the increasing concentration of defects in size, but the topology and the distribution of defects should be taken into account. Our findings suggest that the ability of tuning the electronic transport of ZGNR could be improved through edge reconstruction activated by energetic particle irradiation.

Interfacial and bulk electronic properties of complex oxides and buried interfaces probed by HAXPES

Designing, understanding and controlling the properties of engineered and functional materials, based on oxides and buried interfaces, is one of the most flourishing research fields and one of the major challenges faced by contemporary solid state science and technology. Often, a reliable spectroscopic analysis of such systems is hindered by surface effects, as structural distortion, stoichiometry changes, strong reactivity to external agent and major atomic and/or electronic reconstruction to name but a few. Hard X-Ray PhotoEmission Spectroscopy (HAXPES) is a powerful technique to overcome such limitations, allowing to monitor truly bulk sensitive properties. We report selected HAXPES results for manganese-based oxides, both in films and crystal forms, and for buried metal–organic interfaces, with the aim of highlighting some of the important features such technique brings in the analysis of electronic properties of the solids.

Defect analysis by statistical fitting to 3D atomicmaps

In this article we present a statistical fitting method for evaluation of atomic reconstructions which does not require a coarse-graining step. The fitting compares different models of chemical structure in their capability to explain the measured data set by a least square type merit function. Only preliminary qualitative assumptions about the possible chemical structure are required, while accurate quantitative parameters of the chosen model are delivered by fitting. The technique is particularly useful for singular defect structures with very high composition gradients, for which iso-concentration surfaces determined by coarse-graining become questionable or impossible. We demonstrate that particularly detailed information can be gained from triple junctions and grain boundaries.

Bonding and structure of a reconstructed (001) surface of SrTiO3 from TEM

The determination of the atomic structure and the retrieval of information about reconstruction and bonding of metal oxide surfaces is challenging owing to the highly defective structure and insulating properties of these surfaces. Transmission electron microscopy (TEM) offers extremely high spatial resolution (less than one ångström) and the ability to provide systematic information from both real and reciprocal space. However, very few TEM studies have been carried out on surfaces because the information from the bulk dominates the very weak signals originating from surfaces. Here we report an experimental approach to extract surface information effectively from a thickness series of electron energy-loss spectra containing different weights of surface signals, using a wedge-shaped sample. Using the (001) surface of the technologically important compound strontium titanate, SrTiO(3) (refs 4-6), as a model system for validation, our method shows that surface spectra are sensitive to the atomic reconstruction and indicate bonding and crystal-field changes surrounding the surface Ti cations. Very good agreement can be achieved between the experimental surface spectra and crystal-field multiplet calculations based on the proposed atomic surface structure optimized by density functional calculations. The distorted TiO(6-x) units indicated by the proposed model can be viewed directly in our high-resolution scanning TEM images. We suggest that this approach be used as a general method to extract valuable spectroscopic information from surface atoms in parallel with high-resolution images in TEM.

Periodic DFT Study of the Tetragonal ZrO2 Nanocrystals: Equilibrium Morphology Modeling and Atomistic Surface Hydration Thermodynamics

A thorough periodic DFT/PW91 study of water sorption (0.1 < Θ < 1) on tetragonal ZrO2 (P42/nmc) nanocrystals was performed by means of the plane-wave periodic DFT calculations complemented by atomistic thermodynamics. All (101), (001), (100), (111), and (110) planes exposed by faceted t-ZrO2 nanocrystals were taken into account, and their atomic structure, surface reconstruction, and stabilization upon water adsorption were systematically investigated and analyzed in detail. Using the calculated surface energies of the reconstructed planes, a doubly truncated tetragonal-bipyramidal shape of the tetragonal zirconia nanocrystallites in dry and wet conditions was predicted by means of the Wulff construction. The results remain in very good agreement with the experimental HR-TEM images. For each of the exposed planes, the computed changes in the free enthalpy of water adsorption under specified hydration conditions were used to construct two-dimensional surface coverage versus temperature and pressure diagram...

Effect of lattice mismatch on phonon transmission and interface thermal conductance across dissimilar material interfaces

When phonons transport across a material interface, they experience reflection, transmission, and mode conversion, which results in a local temperature jump at the interface and thus dramatically changes the thermal conductivity of nanostructured materials. Phonon transmission across lattice-matched interfaces has been studied extensively in recent years with the atomistic Green's function (AGF) approach, which usually uses one unit cell to represent the cross section along the interface. However, modeling phonon transmission across realistic material interfaces is much more challenging because realistic interfaces are usually lattice-mismatched ones with atomic reconstruction, defects, and species mixing, which demands a larger cross-sectional area for the AGF simulation. In this paper, an integrated molecular dynamics (MD) and AGF approach is developed to study the phonon transmission across lattice-mismatched interfaces. MD simulation is used to simulate atomic reconstruction close to the interface. The recursive AGF approach is then employed to calculate frequency-dependent phonon transmission across lattice-mismatched interfaces with defects and species mixing, which addresses the numerical challenge in calculating phonon transmission for a relatively large cross-sectional area with reduced computational cost. The study of the relaxed interface formed from two semi-infinite bulk materials shows that lattice mismatch increases the lattice disorder and decreases the adhesion energy, which in turn lowers phonon transmission and reduces the interface thermal conductance across lattice-mismatched interfaces. Low-frequency phonons can be significantly scattered by increasing the defect size across the interface, while high-frequency phonons can be scattered almost completely (phonon transmission 0.1) across an alloyed layer as thin as 2.27 nm. The effect of lattice mismatch on phonon transmission becomes smaller for interfaces with defects and species mixing. The effect of annealing temperature on the Si/Ge interface thermal conductance was studied. A significant reduction of the Si/Ge interface thermal conductance was observed for a lattice-mismatched interface when annealed at high temperature, which agrees well with the available experimental data in literature.

Enhanced carrier mobilities in two-dimensional electron gases at III-III/I-V oxide heterostructure interfaces

In this paper, density functional theory calculations are used to explore the electronic and atomic reconstruction at interfaces between III-III/I-V oxides. In particular, at these interfaces, two dimensional electron gases (2DEGs) with twice the interfacial charge densities of the prototypical LaTiO3/SrTiO3 heterostructure are observed. Furthermore, a significant decrease in the band effective masses of the conduction electrons is shown, suggesting that possible enhancements in electron mobilities may be achievable. These findings represent a framework for chemically modulating 2DEGs, thereby providing a platform through which the underlying physics of electron confinement can be explored with implications for modern microelectronic devices.

Nanoscale Surface Modification of a Prosthetic Material: Case of Ti6Al4V into Ringer's Solution

Nanoscale surface modification of Ti6Al4V prosthetic material was investigated at 37 degrees C into a physiological liquid named Ringer's solution. The root-mean-square surface roughness evolution of the material as a function of immersion time was evaluated by atomic force microscopy (AFM) and 3D reconstruction of scanning electron microscope images (SEM). The results obtained from both techniques clearly showed a decrease of the root-mean-square surface roughness during the first 6 hours of immersion in the physiological liquid that is followed by a stability of the roughness value at longer durations. Moreover, the study of the roughness parameters extracted from AFM measurements is used to explain the smoothing process occurring at the interface between the prosthetic material and the physiological liquid.

Prediction of thickness limits of ideal polar ultrathin films

Competition between electronic and atomic reconstruction is a constantly recurring theme in transition-metal oxides. We use density functional theory calculations to study this competition for a model system consisting of a thin film of the polar, infinite-layer structure ACuO2 (A=Ca, Sr, Ba) grown on a nonpolar, perovskite SrTiO3 substrate. A transition from the bulk planar structure to a chain-type thin film accompanied by substantial changes to the electronic structure is predicted for a SrCuO2 film fewer than five unit cells thick. An analytical model explains why atomic reconstruction becomes more favorable than electronic reconstruction as the film becomes thinner, and suggests that similar considerations should be valid for other polar films.