Atomic-scale Features Research Articles

TiAl3 nucleation mechanism and atomic-scale interface features in the Al/Ti composite structures

In this work, Al/Ti composite structures are obtained by isothermal compression at 550 °C and then annealed at 500–650 °C, respectively. The element diffusion behavior , TiAl 3 nucleation mechanism and atomic-scale structure at the Al/Ti interface are systematically investigated by different material characterization methods. It is found that with the increasing annealing temperature, the average grain size of TiAl 3 increases from 0.47 to 2.27 μm and the diffusion rates of Al and Ti significantly accelerate, accompanied by the change of interlayer thickness from 0.4 to 74.6 μm. The diffusion of Al and Ti along the grain boundary of TiAl 3 is observed directly, but Al is dominant diffusion element. The nucleation density of TiAl 3 at the Ti/TiAl 3 interface is higher than that at Al/TiAl 3 interface. Moreover, the growth of the TiAl 3 layer is hindered by the enrichment of Si, V, and Mg. The atomic arrangement at the three-dimensional Ti/TiAl 3 interface is disturbed by the aggregation of Si, Al, and V, thus forming the short-range ordered Ti–Al(Si, V) structures with serious lattice mismatch . With an increase in the annealing temperature, the incoherent Al/TiAl 3 interface with obvious misfit dislocations transforms into the semi-coherent one with low lattice mismatch.

DFT Examination of Electronic and Structural Features of Favipiravir for Iron Chelation

Density functional theory calculations were performed to examine electronic and structural features of favipiravir (Fav) for iron (Fe) chelation. Fav was well known for the possible medication of COVID-19; however, its mechanism of action has still been a challenging issue. Therefore, this work was done to provide information regarding the possible action of Fav for participating in the Fe chelation process. To this aim, various types of molecular and atomic descriptors were obtained to discuss the topic of this work. Obtained values of energies indicated different levels of stability for pure Fav compounds, in which such variations were also found for FavFe complexes. Molecular orbital-related features showed a different tendency to contribute to reactions for both pure and complex Fav models, in which changes of the energy levels of molecular orbitals raise the detection function of Fe for Fav compounds. Atomic-scale features also indicated direct and indirect roles of atomic sites for formations of FavFe complex models. As a consequence, the idea of Fe chelation by Fav compound was affirmed regarding the obtained results with providing detailed information for investigating the mechanism of action of Fav in treatment of COVID-19.

Locating Single-Atom Optical Picocavities Using Wavelength-Multiplexed Raman Scattering.

Transient atomic protrusions in plasmonic nanocavities confine optical fields to sub-1-nm3 picocavities, allowing the optical interrogation of single molecules at room temperature. While picocavity formation is linked to both the local chemical environment and optical irradiation, the role of light in localizing the picocavity formation is unclear. Here, we combine information from thousands of picocavity events and simultaneously compare the transient Raman scattering arising from two incident pump wavelengths. Full analysis of the data set suggests that light suppresses the local effective barrier height for adatom formation and that the initial barrier height is decreased by reduced atomic coordination numbers near facet edges. Modeling the system also resolves the frequency-dependent picocavity field enhancements supported by these atomic scale features.

A moiré theory for probing grain boundary structure in graphene

Multiscale microscopy spanning the atomistic, moiré, and meso scales has enabled engineering the equilibrium structure of graphene. However, temporal restrictions on in-operando imaging techniques make the moiré scale the finest accessible spatial resolution, thereby limiting our understanding of atomistic mechanisms of non-equilibrium processes in graphene. In order to include atomic scale features with in-operando microscopy, we develop a moiré metrology theory that infers the atomic scale structure from the moiré scale, creating a bridge to in-operando microscopy. The theory is based on atomic scale models that govern the atomistic structure and are promoted to the moiré scale by simulation. We introduce this through a relevant application: nuclei coalescence of graphene during chemical vapor deposition. We develop two mechanistic atomic scale models that govern the propagation and structure of grain boundaries, illuminating how edge dislocations, disconnections, and grain boundaries form from the attachment of individual dimers. The atomistic models are brought to the moiré scale through bond convolution simulations and the resultant moiré metrology theory is tested on results from in-operando scanning tunneling microscopy. By showing that we can identify atomic scale defects from moiré patterns, we highlight how moiré metrology can enable decision making during growth from in-operando observation of graphene structure, paving the way for the design of graphene atomistic structure under scalable synthesis conditions.

Negative differential resistance in Si/GaAs tunnel junction formed by single crystalline nanomembrane transfer method

Negative differential resistance (NDR) region was established in Si/GaAs tunnel junction (TJ) formed by single crystalline nanomembrane (NM) transfer method. P + Si NM was transfer-printed onto n + GaAs epi-wafer, leading to the formation of Si/GaAs pn TJ diode comprised of crystalline semiconductors with no biaxial strain. Atomic-scale features at the Si/GaAs junction interface were analyzed by high-resolution transmission-electron-microscopy. The mechanism for NDR phenomenon in the electrical characteristics of Si/GaAs TJ diode was explained by the energy band diagram with a quantum mechanical band-to-band tunneling of carriers. The peak-to-valley-ratio value of TJ diode was 2.32. The results can be applicable to the fabrication of low-power circuits with a combination of lattice-mismatched crystalline semiconductors.

Barbituric Acid Tautomers: DFT Computations of Keto-Enol Conversions, Frontier Molecular Orbitals and Quadrupole Coupling Constants

In this work, density functional theory (DFT) computations were performed to investigate tautomeric formation processes of barbituric acid (BA). Ten tautomers were totally investigated for the purpose based on the movement of hydrogen atoms among nitrogen and oxygen atoms providing one pure keto form (BA1) and nine other keto-enol forms. The structures were optimized, and BA1 was found to be the most stable one, and both BA3 and BA7 were found to be the most unstable ones. The point was that the ring structure was broken for both BA3 and BA7, but the structure's stability was still approved. Indeed, such serious tautomeric conversion with breaking the structure warns for using such BA bio-organic molecules for further applications, especially in pharmacy-related ones, in which side effects or byproduct synthesis might appear. Further analyses of frontier molecular orbitals features indicated the effects of such tautomerism processes on all model systems, in which more details were obtained by atomic-scale quadrupole coupling constant (Qcc). All obtained results approved significant changes of tautomers regarding molecular and atomic scale features with more or less significant effects regarding the original BA1 reference model.

A DFT approach on tioguanine: Exploring tio-tiol tautomers, frontier molecular orbitals, IR and UV spectra, and quadrupole coupling constants

Tioguanine (TG) is a drug for medication of several types of cancers with favorable efficacy and unfavorable side effects. TG is a modification of guanine nucleobase including two fused heterocyclic rings easy for contributing to tautomerism mechanism, which was investigated in this work based on density functional theory (DFT) calculations. The tautomerism processes of tio-tiol conversion could show serious impacts on the expected function of TG, in which the results of this work indicated that such tautomers could be easily formed with small magnitude of switching energy between the tautomers. Frontier molecular orbitals analyses showed serious variations in the electronic features of TG molecules in tautomeric forms, in which the results were further described by the atomic scale quadrupole coupling constant (Qcc) evaluations. Although the switching energy was small enough to see all the tautomers in the same mode, but molecular and atomic scale features revealed different achievements for such systems as possible reasons of side effects arising during medications. As a final remark of this work, it could be mentioned that TG drug might be modified to show improved efficacies for the patients with lower side effect impacts; otherwise, the tautomeric processes could lead to existence of different side effects.

Atomistic modeling of energy band alignment in CdSeTe surfaces

Atomistic modeling based on Desnity Functional Theory (DFT)- 1/2 method coupled with surface Green’s function has been employed to investigate the energy band alignment results in cadmium selenium telluride (CdSeTe) surfaces. The structural and electronic properties of the bulk ternary alloy were established before cleaving the CdSeTe low index surface facets. The dependency of atomic-scale electronic features on different plane orientations was explored by studying the energy band alignment diagrams in the unreconstructed and reconstructed surfaces. While the low index CdSeTe surface geometry reconstructions replicate the CdTe surface geometries, the energy band alignment features of the unreconstructed and reconstructed CdSeTe surfaces differ from those observed in CdTe low index surfaces. The structurally relaxed reconstructed CdSeTe surface results in purely flat bands as opposed to an internal cusp feature observed in CdTe surfaces. The presence of an internal energy cusp in CdTe(111) surfaces is expected to play a key role in enhancing the hole charge transport towards the back of CdTe-only solar cell device and the absence of such feature in CdSeTe surface may explain one of the reason for the lower performance of CdSeTe-only solar cell seen experimentally.

Thermodynamics of Spinel-like Cation Partial Ordering in Ultrahigh Power and Energy Density Li-Ion Batteries for Fast-Charging Electric Vehicles

Recently, a class of new materials exhibiting partial spinel-like cation order, but with excess Li and excess cation content was found to combine very high rate performance with high energy density [1]. Such spinels with a cation to anion ratio higher than 3 are unusual as it requires the face sharing of some tetrahedral and octahedral cations. In this presentation we will report on the ab-initio modeling of the metastable cation arrangements in these lithium manganese oxyfluorides that are responsible for the high-rate performance. The high dimensional configurational space of the materials is described by the cluster-expansion technique and parameterized with multiple hundreds of DFT calculations.Local, atomic-scale features, such as Li-Mn dumbbell formation, antisite defects, and interstitial defects giving rise to face-sharing cations which are challenging to resolve with state-of-the-art diffraction techniques are possible to directly evaluate in our computational approach. We present a computational analysis on how cation excess in spinel-like materials is accommodated and how local cation perturbations ultimately influence short-range order and bulk Li transport. Our understanding of how features in cation-excess spinel-like Li-Mn-O-F materials influence their high rate capabilities may motivate and accelerate design of other high-performance, fast-charging battery materials.[1] Ji, H., Wu, J., Cai, Z. et al. Ultrahigh power and energy density in partially ordered lithium-ion cathode materials. Nat Energy 5, 213–221 (2020).

Atomic-Scale Imprinting by Sputter Deposition of Amorphous Metallic Films.

With its ease of implementation, low cost, high throughput, and excellent feature replication accuracy, nanoimprinting is used to fabricate structures for electrical, optical, and biological applications or to modify surface properties. If ultraprecise and/or subnanometer-sized patterns are desired, nanoimprinting has shown only limited success with polymers, silica glasses, or crystalline materials. In contrast, the absence of an intrinsic length scale that would interfere with imprinting resolution enables bulk metallic glasses (BMGs) to replicate structures down to the atomic scale through thermoplastic forming (TPF). However, only a small number of BMG-forming alloys can be used for TPF-based atomic-scale imprinting. Here, we demonstrate an alternative sputter deposition-based approach for the replication of atomic-scale features that is suited for a very broad range of amorphous alloys, thereby dramatically extending the available chemistries. Additional advantages are the method's scalability, its ability to replicate a wide range of molds, its low material consumption, and the fact that the films can readily be applied onto almost any workpiece, which together open up new avenues to atomically defined surface structuring and functionalization. Our method constitutes the advancement from proof of concept to a practical and highly versatile toolbox of atomic-scale imprinting to be explored for the science and technology of atomic-scale imprinting.

Atomic-resolution analytical scanning transmission electron microscopy of topological insulators with a layered tetradymite structure

The recent discovery of topological insulators has uncovered exciting new quantum materials with potential applications in the emergent fields of topological spintronics and topological quantum computation. At the heart of uncovering the new physical properties of these materials is the characterization of their atomic structures, composition, defects, and interfaces. The technique of atomic-resolution analytical scanning transmission electron microscopy has already provided many insights and holds great promise for future discoveries. This perspective discusses advances that have been achieved in the atomic-scale characterization of topological insulators with a layered tetradymite structure, and it proposes future directions to link atomic-scale features to exciting new physical phenomena.

Graph theory approach to determine configurations of multidentate and high coverage adsorbates for heterogeneous catalysis

Heterogeneous catalysts constitute a crucial component of many industrial processes, and to gain an understanding of the atomic-scale features of such catalysts, ab initio density functional theory is widely employed. Recently, growing computational power has permitted the extension of such studies to complex reaction networks involving either high adsorbate coverages or multidentate adsorbates, which bind to the surface through multiple atoms. Describing all possible adsorbate configurations for such systems, however, is often not possible based on chemical intuition alone. To systematically treat such complexities, we present a generalized Python-based graph theory approach to convert atomic scale models into undirected graph representations. These representations, when combined with workflows such as evolutionary algorithms, can systematically generate high coverage adsorbate models and classify unique minimum energy multidentate adsorbate configurations for surfaces of low symmetry, including multi-elemental alloy surfaces, steps, and kinks. Two case studies are presented which demonstrate these capabilities; first, an analysis of a coverage-dependent phase diagram of absorbate NO on the Pt3Sn(111) terrace surface, and second, an investigation of adsorption energies, together with identifying unique minimum energy configurations, for the reaction intermediate propyne (CHCCH3*) adsorbed on a PdIn(021) step surface. The evolutionary algorithm approach reproduces high coverage configurations of NO on Pt3Sn(111) using only 15% of the number of simulations required for a brute force approach. Furthermore, the screening of potentially hundreds of multidentate adsorbates is shown to be possible without human intervention. The strategy presented is quite general and can be applied to a spectrum of complex atomic systems.

Random telegraph signals caused by a single dopant in a metal–oxide–semiconductor field effect transistor at low temperature

While the importance of atomic-scale features in silicon-based device for quantum application has been recognized and even the placement of a single atom is now feasible, the role of a dopant in the substrate has not attracted much attention in the context of quantum technology. In this paper, we report random telegraph signals (RTSs) originated from trapping and detrapping of an electron by a donor in the substrate of a p-type metal–oxide–semiconductor field-effect-transistor. RTSs, not seen when the substrate was grounded, were observed when a positive bias was applied to the substrate. The comprehensive study on the signals observed reveals that the nature of the RTSs is discrete threshold voltage variations due to the change in the depletion layer width depending on the charge state of a single dopant, neutral or positively charged.

Next generation SERS- atomic scale platform for molecular level detection

The research with ultra-sensitive detection by Raman enhancement has traditionally focused on Surface Enhanced Raman Scattering (SERS) or Tip Enhanced Raman Scattering (TERS) from predominantly Raman active materials (monolayers of metallic nanoparticles of gold and silver). However, these traditional SERS & TERS methods have demonstrated limited applicability due to many inherent disadvantages. In this study, we have demonstrated the next generation of Enhanced Raman Sensing by adding a third dimension, by introducing a concept of employing atomic scale features (voids in nanostructures) for molecular level detection. Unlike traditional SERS/TERS, the atomic scale platform also exhibited immunity to the excitation wavelength showing potential for universal sensor. Additionally, this portable platform demonstrated a wide spectrum of applications from trace level detection of environmental pollutants, Raman reporter molecule sensing, detection of biomarkers associated with cancer and detection of cancer from the DNA, opening new doors for many non-invasive, ultrasensitive and remote sensing applications. The atomic scale platform was synthesized with two features- multi-tip probes encapsulated with multiple layers of atomic scale voids. Multi-photon ionization mechanism was used for the synthesis. We have witnessed exponentially increased and highly reproducible Raman enhancement from a traditionally Raman inactive material (semiconductor based ZnO) demonstrating molecular level detection. Utilization of atomic-scale features (voids in nanostructures) for activation of a typically Raman inactive material for molecular level detection will create a paradigm for future studies in the field of Raman enhancement for Raman active as well as Raman inactive materials. This mechanism will initiate avenues for many new materials to be candidates for Raman sensing.

Oxygen transport in Pr nickelates: Elucidation of atomic-scale features

Pr2NiO4+δ oxide with a layered Ruddlesden–Popper structure is a promising material for SOFC cathodes and oxygen separation membranes due to a high oxygen mobility provided by the cooperative mechanism of oxygen migration involving both interstitial oxygen species and apical oxygen of the NiO6 octahedra. Doping by Ca improves thermodynamic stability and increases electronic conductivity of Pr2−xCaxNiO4+δ, but decreases oxygen mobility due to decreasing the oxygen excess and appearing of 1–2 additional slow diffusion channels at x ≥ 0.4, probably, due to hampering of cooperative mechanism of migration. However, atomic-scale features of these materials determining oxygen migration require further studies. In this work characteristics of oxygen diffusion in Pr2−xCaxNiO4+δ (x = 0–0.6) are compared with results of the surface analysis by X-ray photoelectron spectroscopy and modeling of the interstitial oxygen migration by the plane-wave density functional theory calculations. According to the X-ray photoelectron spectroscopy data, the surface is enriched by Pr for undoped sample and by Ca for doped ones. The O1s peak at ~531 eV corresponding to a weakly bound form of surface oxygen located at Pr cations disappears at ~500 °C. Migration of interstitial oxygen was modeled for a I4/mmm phase of Pr2NiO4+δ. The interstitial oxygen anion repulses the apical one in the NiO6 octahedra pushing it into the tetrahedral site between Pr cations. The calculated activation barrier of this migration is equal to 0.585 eV, which reasonably agrees with the experimental value of 0.83 eV obtained by the oxygen isotope exchange method. At the same time, for the model compound Ca2NiO4+δ, obtained by isomorphic substitution of Pr by Ca in Pr2NiO4+δ, calculations implied formation of the peroxide ion comprised of interstitial and lattice oxygen species not revealed in the case of incomplete substitution (up to PrCaNiO4+δ composition).Hence, calculations in the framework of the plane-wave density functional theory provide a realistic estimation of specificity of oxygen migration features in Pr2NiO4+δ doped by alkaline-earth metals.

Effect of Atomic Corrugation on Adhesion and Friction: A Model Study with Graphene Step Edges.

This Letter reports that the atomic corrugation of the surface can affect nanoscale interfacial adhesion and friction differently. Both atomic force microscopy (AFM) and molecular dynamics (MD) simulations showed that the adhesion force needed to separate a silica tip from a graphene step edge increases as the side wall of the tip approaches the step edge when the tip is on the lower terrace and decreases as the tip ascends or descends the step edge. However, the friction force measured with the same AFM tip moving across the step edge does not positively correlate with the measured adhesion, which implies that the conventional contact mechanics approach of correlating interfacial adhesion and friction could be invalid for surfaces with atomic-scale features. The chemical and physical origins for the observed discrepancy between adhesion and friction at the atomic step edge are discussed.

Investigation of the interaction between carbon nanotube tip and silicon sample through molecular dynamic simulation

Atomic Force Microscopy (AFM) has become a standard tool in experimental surface metrology, revealing atomic scale features of the surfaces. Carbon nanotube represents ideal structure for AFM tips because of their tiny diameter, high aspect ratio and high strength. This article employs molecular dynamics (MD) simulation approach to investigate the atomic-scale interaction between the AFM tips represented by carbon nanotube and sample. The interaction force between two different types of single walled carbon nanotube (SWCNT) tips including open ended carbon nanotube and capped carbon nanotube while the deformation mechanism during the measurement was discussed in detail. Firstly, the interaction force between SWCNT tips and sample in contact mode was simulated by using hybrid potential. The stiffness of the two different types of single walled carbon nanotubes were verified. The simulation results indicate that capped SWCNT tip is more prone to tilt than open ended SWCNT tip, but the ability to resist deformation is stronger. Then the dynamic model of tip-sample system in tapping mode was established. The relationship between the tip of load and the distance in the process of approaching to the sample was obtained by curve fitting. The numerical solution was achieved by solving the dynamic equation and frequency spectrum was obtained with the method of Fourier transforms.

Acoustic processes in materials

Abstract

Evidence of Spin Frustration in a Vanadium Diselenide Monolayer Magnet.

Monolayer VSe2 , featuring both charge density wave and magnetism phenomena, represents a unique van der Waals magnet in the family of metallic 2D transition-metal dichalcogenides (2D-TMDs). Herein, by means of in situ microscopy and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X-ray and angle-resolved photoemission, and X-ray absorption, direct spectroscopic signatures are established, that identify the metallic 1T-phase and vanadium 3d1 electronic configuration in monolayer VSe2 grown on graphite by molecular-beam epitaxy. Element-specific X-ray magnetic circular dichroism, complemented with magnetic susceptibility measurements, further reveals monolayer VSe2 as a frustrated magnet, with its spins exhibiting subtle correlations, albeit in the absence of a long-range magnetic order down to 2 K and up to a 7 T magnetic field. This observation is attributed to the relative stability of the ferromagnetic and antiferromagnetic ground states, arising from its atomic-scale structural features, such as rotational disorders and edges. The results of this study extend the current understanding of metallic 2D-TMDs in the search for exotic low-dimensional quantum phenomena, and stimulate further theoretical and experimental studies on van der Waals monolayer magnets.

Fewer Defects in the Surface Slows the Hydrolysis Rate, Decreases the ROS Generation Potential, and Improves the Non‐ROS Antimicrobial Activity of MgO

Magnesium oxide (MgO) is recognised as exhibiting a contact-based antibacterial activity. However, a comprehensive study of the impact of atomic-scale surface features on MgO's antibacterial activity is lacking. In this study, the nature and abundance of the native surface defects on different MgO powders are thoroughly investigated. Their impacts on the hydrolysis kinetics, antibacterial activity against Escherichia coli (ATCC 47076), Staphylococcus epidermidis and Pseudomonas aeruginosa and the reactive oxygen species (ROS) generation potential are determined and explained. It is shown that a reduction in the abundance of low-coordinated oxygen atoms on the surface of the MgO improves its resistance to both hydrolysis and antibacterial activity. The ROS generation potential, determined in-situ using a fluorescence microplate assay and electron paramagnetic resonance spectroscopy, is not an inherent property of the studied MgO, rather it is a side product of hydrolysis (only for the most highly defected MgO particles) and/or a consequence of the MgO/bacteria interaction. The evaluation of the mutual correlations of the hydrolysis, the antibacterial activity and the ROS generation, with their origin in the surface defects' peculiarities, led to the conclusion that the acid/base reaction between the MgO surface and the bacterial wall contributes considerably to the MgO's antibacterial activity.