Atomic Structure Of Interface Research Articles

In Situ Phosphatizing of Triphenylphosphine Encapsulated within Metal-Organic Frameworks to Design Atomic Co1-P1N3 Interfacial Structure for Promoting Catalytic Performance.

The engineering coordination environment offers great opportunity in performance tunability of isolated metal single-atom catalysts. For the most popular metal-Nx (MNx) structure, the replacement of N atoms by some other atoms with relatively weak electronegativity has been regarded as a promising strategy for optimizing the coordination environment of an active metal center and promoting its catalytic performance, which is still a challenge. Herein, we proposed a new synthetic strategy of an in situ phosphatizing of triphenylphosphine encapsulated within metal-organic frameworks for designing atomic Co1-P1N3 interfacial structure, where a cobalt single atom is costabilized by one P atom and three N atoms (denoted as Co-SA/P-in situ). In the acidic media, the Co-SA/P-in situ catalyst with Co1-P1N3 interfacial structure exhibits excellent activity and durability for the hydrogen evolution reaction (HER) with a low overpotential of 98 mV at 10 mA cm-2 and a small Tafel slope of 47 mV dec-1, which are greatly superior to those of catalyst with Co1-N4 interfacial structure. We discover that the bond-length-extended high-valence Co1-P1N3 atomic interface structure plays a crucial role in boosting the HER performance, which is supported by in situ X-ray absorption fine structure (XAFS) measurements and density functional theory (DFT) calculation. We hope this work will promote the development of high performance metal single-atom catalysts.

An atomic-scale investigation on the initial precipitation behavior of nitrides in TiAl alloys

In the present work, the initial precipitation behavior of nitrides in TiAl alloys was investigated by monitoring the morphology, distribution, crystallographic orientation relationship and interface atomic structure of the nitride precipitates using TEM and HRTEM. It is found that there are two kinds of nitride precipitates (Ti2AlN and Ti3AlN) forming in the initial precipitation stage of Ti-49Al-1N alloys. The rod-like Ti2AlN precipitates tend to distribute at γ(TiAl)/α2(Ti3Al) interface with the crystallographic orientation relationship of [112‾0]Ti3Al//[112‾0]Ti2AlN//[1‾01]TiAl, (0001)Ti3Al//(0001)Ti2AlN//(111)TiAl. Both the Ti3Al(0001)/Ti2AlN(0001) interface and Ti2AlN(0001)/TiAl(111) interface display a plane-to-plane matching with coherent atomic correspondence. The plate-like Ti3AlN precipitates tend to distribute in the γ(TiAl) matrix with the crystallographic orientation relationship of [1‾10]Ti3AlN//[1‾10]TiAl, (111)Ti3AlN//(111)TiAl. The Ti3AlN(111)/TiAl(111) interface displays an edge-to-edge matching with coherent atomic correspondence. The morphology, distribution, crystallographic orientation relationship and interface atomic structure of nitride precipitates are determined by the crystal structure and lattice misfit of nitride precipitates and matrix. The understanding here will be useful in guiding morphology design and dispersion controlling of the nitride precipitates in TiAl alloys, and will further be benefit for improving the mechanical properties of TiAl alloys.

Interfacial atomic structure and electrical activity of nano-facetted CSL grain boundaries in high-performance multi-crystalline silicon

The interfacial structure of coincidence site lattice grain boundaries in multi-crystalline silicon plays a decisive role in their electrical behavior as revealed by high-resolution (scanning) transmission electron microscopy investigations. Considering only the global misorientation of the adjacent grains can lead to a false correlation between the structural and electrical properties of a grain boundary. The grain boundary habit plane as well as local deviations in the orientation and misorientation that induce additional structural defects need to be analyzed. Indeed, a Σ9 {221} grain boundary, presenting a perfect coincidence and periodicity at the atomic scale, appears electrically non-active. However, a grain boundary also identified as Σ9 {221} at the mesoscale is found to be composed of nano-sized triangular structures involving Σ3 {111} and Σ3 {112} facets at the nano-scale. This leads to the formation of grain boundary kinks and triple junctions that induce additional structural defects and turn the overall grain boundary interface electrically active. The possible origin of such a grain boundary dissociation, as well as its impact on the electrical activity, is discussed.

Thermal studies of individual Si/Ge heterojunctions — The influence of the alloy layer on the heterojunction

Phonon transport across an interface is of fundamental importance to applications ranging from electronic and optical devices to thermoelectric materials. The phonon scattering by an interface can dramatically suppress the thermal transport, which can benefit thermoelectric applications but create problems for the thermal management of electronic/optical devices. In this aspect, existing molecular dynamics simulations on phonon transport across various interfaces are often based on estimates of atomic structures and are seldom compared with measurements on real interfaces. In this work, planar Si/Ge heterojunctions formed by film-wafer bonding are measured for the interfacial thermal resistance (RK) that is further compared with predictions from existing simulations and analytical models. The twist angle between a 70-nm-thick Si film and a Ge wafer is varied to check the influence of the crystal misorientation. Detailed transmission electron microscopy studies are carried out to better understand the interfacial atomic structure. It is found that the alloyed interfacial layer with mixed Si and Ge atoms dominates the measured thermal resistance (RK). Some oxygen impurities may also help to increase RK due to the formation of glassy structures. Following this, RK reduction should be focused on how to minimize the interdiffusion of Si and Ge atoms during the formation of a Si/Ge heterojunction.

Complex Geometric Structure of a Simple Solid-Liquid Interface: GaN(0001)-Ga.

The equilibrium atomic interface structure between Ga and GaN(0001) is shown to contain substrate surface vacancies followed by substrate-induced layering and preferential lateral ordering in the liquid. The uncovered presence of point defects, in the form of vacancies at both sides of the solid-liquid interface, is an important structural feature which governs the local physical properties. Our x-ray diffraction study reveals that the layering is very stable and persists up to a temperature of 1123K and a nitrogen pressure of 32bar. The Ga layer spacing agrees remarkably well with the Friedel oscillation period for this system.

Characterization of InAs/GaSb superlattices grown by MOCVD with atomic resolution

High-angle annular dark-field (HAADF) imaging and electron energy loss spectroscopy (EELS) in a Cs-corrected scanning transmission electron microscope were utilized to analyze the interfacial atomic structure of InAs/GaSb superlattices (SLs) grown by metalorganic chemical vapor deposition (MOCVD) on InAs substrates. Despite high growth temperature, narrow interface (IF) widths of less than 2.5 monolayers (MLs) and 3.8 MLs were extracted from HAADF and EELS, respectively, indicating that the IF quality of MOCVD-grown InAs/GaSb SLs is comparable to those grown by molecular beam epitaxy. GaAs-type IFs are considered to account for the narrow IF width. In addition, GaSb-on-InAs IFs were found to be sharper and more strained than InAs-on-GaSb IFs, which is correlated with the special gas supply and switching sequence during MOCVD growth. The strain profile deduced from the HAADF image suggests that little Sb is incorporated into InAs sublayers and 7% In is incorporated into GaSb sublayers.

Improvement of electrical performance by surface structure of Ni-material as a high-performance asymmetric supercapacitor electrode

Binder-free Ni-material array electrodes on Ni-foam are fabricated using a facile hydrothermal method for high-performance supercapacitors. Instead of a binder, urea is added to increase the adhesion of Ni-foam as both the substrate and electrode material. In this paper, we present the contribution of urea to the formation of the material. The surface characteristics and the electrical properties of the Ni-material array change with the amount of urea. When 2.4 g of urea is added to the sample for the first time, a Ni(HCO3)2 microrod electrode is obtained; it shows large wire growth on the Ni-foam, which can be explained about the abnormal grain growth behavior because there is little energy barrier for atomic attachments with a disordered or rough interface atomic structure. Its specific capacitance is 346 F g−1 at a current density of 3 A g−1. In contrast, when the amount of urea is reduced to 0.8 g, a Ni2(CO3) (OH)2 array electrode is produced; it exhibits a nano-size wire array and a specific capacitance of 833.2 F g−1 at a current density of 3 A g−1. Thus, it is confirmed that the large surface area of Ni2(CO3) (OH)2 exhibits outstanding electrical properties. And, the fabricated asymmetric supercapacitor of Ni2(CO3) (OH)2 composite as the positive electrode and graphene as the negative electrode, which has a high energy density of 22.1 W h kg−1 at a power density of 673.1 W kg−1. In addition, this obtained asymmetric supercapacitor can turn on the LED light.

Arrayed silk fibroin for high-performance Li metal batteries and atomic interface structure revealed by cryo-TEM

An upright silk fibroin/lithium foil alternating array is fabricated to facilitate an unexpected SEI with a robust layered structure of lithium deposited on the anode (revealed by cryogenic electron microscopy), thus significantly suppressing dendrites.

Relationship between electrical properties and interface structures of SiO2/4H-SiC prepared by dry and wet oxidation

We have investigated the relationship between the electrical properties and interfacial atomic structure of SiO2/4H-SiC interfaces, prepared by dry and wet thermal oxidation procedures with 4H-SiC (0001) and 4H-SiC (000-1) substrates, using extended x-ray absorption fine structure (EXAFS) spectroscopy and electrical methods. From the current–voltage (I–V) and capacitance–voltage (C–V) measurements, the gate leakage current onset and density of interface states were shown to depend on the thermal oxidation procedure used for creating SiO2/4H-SiC (000-1) interfaces. This dependence was not observed for SiO2/4H-SiC (0001) interfaces. From EXAFS oscillations, we found that C and Si vacancies formed at the SiC side of SiO2/4H-SiC (0001) and SiO2/4H-SiC (000-1) interfaces, respectively. Compressive stress at the SiC sides of SiO2/4H-SiC (0001) and SiO2/4H-SiC (000-1) interfaces prepared with the dry thermal oxidation procedure caused decreases in bond lengths. Stress at the interface was smaller in the case of wet oxidation. Wet oxidation released the interface stress more effectively for 4H-SiC (000-1) substrates than for 4H-SiC (0001) substrates. A high gate leakage current onset related to a high interface charge, while a high density of interface states related to a high interface stress and a high interface roughness.

Regulation of the Interface Binding and Elastic Properties of SiC/Ti via Doping‐Induced Electronic Localization

The interface between metal matrix and reinforcement significantly affects mechanical properties of metal matrix composites (MMCs), but the interplay mechanism among the interface microstructure, the interface binding, and the mechanical properties is still unclear. Herein, the interface microstructure and interface binding of β‐SiC (111)/α‐Ti (0001) composite are regulated through the introduction of Sc, Y, Mg, Al, Si, Ni, Cu, and Mn dopants. Adhesive work and elastic properties of different doped interfaces are studied by first‐principles calculations. It is found that the elastic modulus does not directly relate to the adhesive work. The analysis of the electronic structure reveals that dopants change interface bonding states, which results in the localization of the interface electron distribution and enhances the elastic modulus. However, the lattice distortion induced by the dopants causes a significant decrease in the elastic modulus. Combined with the analysis of the adhesive work and interface electronic and atomic structure, the microscopic mechanism of the dopant‐induced electronic localization to regulate elastic properties of SiC/Ti composite is established. The new mechanism is helpful to understand the relationship among the interface microstructure, the interface binding, and the elastic properties, and to design MMCs with improved elastic properties.

Single Halide Perovskite/Semiconductor Core/Shell Quantum Dots with Ultrastability and Nonblinking Properties.

The further practical applications of halide perovskite quantum dots (QDs) are blocked by problems of instability and nonradiative Auger recombination manifested as photoluminescence blinking. Here, single core/shell structured perovskite semiconductor QDs are successfully fabricated by capping CsPbBr3 QD core with CdS shell. It is demonstrated that CsPbBr3/CdS core/shell QDs exhibit ultrahigh chemical stability and nonblinking photoluminescence with high quantum yield due to the reduced electronic traps within the core/shell structure. Efficiency of amplified spontaneous emission exhibits obvious enhancement compared to that of pure CsPbBr3 QDs, originating from the mitigated competition between stimulated emission and suppressed nonradiative biexciton Auger recombination. Furthermore, low‐threshold whispering‐gallery‐mode lasing with a high‐quality factor is achieved by incorporating CsPbBr3/CdS QDs into microtubule resonators. Density functional theory (DFT)‐based first‐principles calculations are also performed to reveal the atomic interface structure, which supports the existence of CsPbBr3/CdS structure. An interesting feature of spatially separated charge density at CsPbBr3/CdS interface is found, which may greatly contribute to the suppressed Auger recombination. The results provide a practical approach to improve the stability and suppress the blinking of halide perovskite QDs, which may pave the way for future applications for various optoelectronic devices.

On the compositional partitioning during phase transformation in a binary ferromagnetic MnAl alloy

We introduce a new perspective on the classical massive mode of solid-state phase transformation enabled by the correlative use of atomic-scale electron microscopy and atom probe tomography. This is demonstrated in a binary MnAl alloy which has Heusler-like characteristics. In this system, the τ phase formed by a massive transformation from the high-temperature ε phase is metastable and ferromagnetic. The transformation results in a high density of micro-twins inside the newly grown τ phase. Atomic-scale compositional analysis across the interface boundaries and atomic structure of the micro-twins reveals the involvement of both structural modification and also the compositional partitioning during the growth of the τ phase. This is assisted by the migrating τ/ε interface boundary during transformation. Finally, the role of micro-twins on nucleating the equilibrium phases and the influence of the defects and phase formation on the magnetic properties are discussed.

Spatial Mapping of Hot-Spots at Lateral Heterogeneities in Monolayer Transition Metal Dichalcogenides.

Lateral heterogeneities in atomically thin 2D materials such as in-plane heterojunctions and grain boundaries (GBs) provide an extrinsic knob for manipulating the properties of nano- and optoelectronic devices and harvesting novel functionalities. However, these heterogeneities have the potential to adversely affect the performance and reliability of the 2D devices through the formation of nanoscopic hot-spots. In this report, scanning thermal microscopy (SThM) is utilized to map the spatial distribution of the temperature rise within monolayer transition metal dichalcogenide (TMD) devices upon dissipating a high electrical power through a lateral interface. The results directly demonstrate that lateral heterojunctions between MoS2 and WS2 do not largely impact the distribution of heat dissipation, while GBs of MoS2 appreciably localize heating in the device. High-resolution scanning transmission electron microscopy reveals that the atomic structure is nearly flawless around heterojunctions but can be quite defective near GBs. The results suggest that the interfacial atomic structure plays a crucial role in enabling uniform charge transport without inducing localized heating. Establishing such structure-property-processing correlation provides a better understanding of lateral heterogeneities in 2D TMD systems which is crucial in the design of future all-2D electronic circuitry with enhanced functionalities, lifetime, and performance.

Atomic structure and crystallography of joints in SnO2 nanowire networks

Joints of three-dimensional (3D) rutile-type (r) tin dioxide (SnO2) nanowire networks, produced by the flame transport synthesis (FTS), are formed by coherent twin boundaries at (101)r serving for the interpenetration of the nanowires. Transmission electron microscopy (TEM) methods, i.e. high resolution and (precession) electron diffraction (PED), were utilized to collect information of the atomic interface structure along the edge-on zone axes [010]r, [111]r and superposition directions [001]r, [101]r. A model of the twin boundary is generated by a supercell approach, serving as base for simulations of all given real and reciprocal space data as for the elaboration of three-dimensional, i.e. relrod and higher order Laue zones (HOLZ), contributions to the intensity distribution of PED patterns. Confirmed by the comparison of simulated and experimental findings, details of the structural distortion at the twin boundary can be demonstrated.

Ab Initio Investigation of Charge Trapping Across the Crystalline- Si –Amorphous- SiO2 Interface

Author(s): Liu, YY; Zheng, F; Jiang, X; Luo, JW; Li, SS; Wang, LW | Abstract: Accurate microscopic description of the charge-trapping process from semiconductor to defects in the dielectric-oxide layer is of paramount importance for understanding many microelectronic devices such as complementary metal-oxide-semiconductor (CMOS) transistors, as well as electrochemical reactions. Unfortunately, most current microscopic descriptions of such processes are based on empirical models with parameters fitted to experimental device performance results or simplified approximations like the Wentzel-Kramers-Brillouin (WKB) method. Some critical questions are still unanswered, including: What controls the charge-hopping rate, the coupling strength between the defect level to semiconductor level, or the energy difference? How does the hopping rate decay with defect-semiconductor distance? What is the fluctuation of the defect level, especially in amorphous dielectrics? Many of these questions can be answered by ab initio calculations. However, to date, there are few ab initio studies for this problem mainly due to technical challenges from atomic-structure construction to large-system calculations. Here, using the latest advances in calculation methods and codes, we study the carrier-trapping problem using density-functional theory (DFT) based on the Heyd-Scuseria-Ernzerhof (HSE) exchange correlation functional. The valence bond random-switching method is used to construct the crystalline-Si-amorphous-SiO2 (c-Si/a-SiO2) interfacial atomic structure, and the HSE yields a band offset that agrees well with experiments. The hopping rate is calculated with the Marcus theory, and the hopping-rate dependences on the gate potential and defect distances are revealed, as well as the range of fluctuation results from amorphous structural variation. We also analyze the result with the simple WKB model and find a major difference in the description of the coupling constant decay with the defect-semiconductor distance. Our results provide the ab initio simulation insights for this important carrier-trapping process for device operation.

Role of the Short‐Range Order in Amorphous Oxide on MoS2/a‐SiO2 and MoS2/a‐HfO2 Interfaces

The interface structures between MoS2 and amorphous oxides of SiO2 and HfO2 (MoS2/a‐SiO2 and MoS2/a‐HfO2) are investigated through ab‐initio molecular dynamics simulations. The intrinsic short‐range order (SRO) of the amorphous oxide, such as the bond length, coordination number, and connectivity of the oxide polyhedron, is crucial in the interfacial properties. The amorphous SiO2 showing the strict SRO induces a strong interaction with MoS2 and forms interfacial bonds of Si–S, resulting in a significantly increased effective mass of MoS2. By contrast, the amorphous HfO2 has more flexible SRO and weakly interacts with MoS2, which explains the maintenance of the band dispersion and effective mass of MoS2 in contact with amorphous HfO2. These calculation results indicate a possible mechanism of the higher mobility of MoS2/a‐HfO2 over MoS2/a‐SiO2 by accounting for the interfacial atomic structure.

Depth-resolved local atomic structure of Fe/Cr multilayer film with GMR effect: Experimental results

A subnanometric resolution method for studying the local atomic structure of interface and surface of low contrast multilayered nanoheterostructure thin films is applied to the Fe/Cr multilayer sample with GMR (Giant Magnetoresistance) effect. We consider combination of the X-ray reflectivity (XRR) and the Extended X-ray absorption Fine Structure (EXAFS) spectroscopy with angular resolution. The XRR experiment has been carried out according to standard procedure using a specialized X-ray diffractometer «Empyrean» at the Institute of Metal Physics (Ekaterinburg). We have applied a measurement method proposed earlier by V. P. Romanov et al. This approach allowed us to separate the diffuse and pure specular contributions. To determine the concentration depth-profile of element from XRR data, we have implemented the Levenberg-Marquardt (L-M) algorithm for nonlinear inverse problem. Thus, the phase problem for X-ray specular reflectivity has been solving. The EXAFS measurements has been performed using synchrotron facilities (National Research Centre “Kurchatov Institute”). {The Fe and Cr K absorption spectra} were recorded in fluorescence mode with angular resolution. The experimental results for depth-resolved local atomic structure of Fe/Cr multilayer with GMR effect has been obtained. A depth resolution of 2-3 Å was reached. Atomic structure of Fe and Cr atoms located at seven depth points including interface for the Al2O3/Cr(100 Å)/[Fe(8 Å)/Cr(10,5 Å)]2/Cr(20 Å) multilayered sample was obtained. It was shown what chromium oxides were present on the surface of the sample..

Molecular dynamics simulations of the tensile responses and fracture mechanisms of Ti2AlN/TiAl composite

In the present work, molecular dynamics simulations are carried out to investigate the tensile responses and fracture mechanisms of Ti2AlN/TiAl composite. It is found that the Ti2AlN/TiAl composite with different interface atomic structure shows significantly different tensile responses and fracture mechanisms. When the tensile loading is applied normal to the Ti2AlN(0 0 0 1)/TiAl(1 1 1) coherent interface, the micro-void would nucleate from the interface, rapidly propagate along the interface and evolve into the complete-crack, leading to the brittle fracture behavior of Ti2AlN/TiAl composite. The formation of stress concentration and Lomer-Cottrell lock in the misfit dislocations at the coherent interface is the major mechanism for the nucleation of micro-void. When the tensile loading is applied normal to the Ti2AlN(1 0 1¯ 3)/TiAl(1 1 1) incoherent interface, micro-voids can nucleate from the weak interactive regions at the incoherent interface, but they cannot propagate rapidly along the incoherent interface, which indicates that the Ti2AlN(1 0 1¯ 3)/TiAl(1 1 1) incoherent interface system shows ductile fracture behavior. The ductile fracture mechanism is twofold: firstly, the dislocations in Ti2AlN and TiAl slab can nucleate at the tip of micro-crack and effectively blunt the propagation of micro-crack along the incoherent interface; secondly, the strong NTi and NAl bonds at the incoherent interface can give a contribution to the overall stability of Ti2AlN(1 0 1¯ 3)/TiAl(1 1 1) incoherent interface.

Bulklike band-offset mystery solved through energy minimization: Lessons from perovskite oxide heterojunctions

The formation of band offset (BO) at isovalent semiconductor heterojunctions has been branded ``bulklike'' because of an insensitivity of the BO to the interface orientation and atomic structure and a transitivity in BOs. Even though tunability and nontransitivity of BO are frequently found for heterovalent interfaces, empirical theories with built-in bulklike characteristics have thus far dominated the explanation of experimental BOs. Presently, the distribution of charge density and the formation of BO at a large number of interfaces between lattice-matched perovskite oxides are studied in detail using density functional theory. Ionic screening is found to dominate the formation of the BO, as a sharp dependence of the (apparently tunable) BO on atomic structure for unrelaxed interfaces is essentially washed out upon lattice relaxation. Numerical experimentation with deliberate embedding of dipolar layers in perovskite oxides and their interfaces corroborates the effectiveness of ionic screening. The relaxed, converged (bulklike) BOs are found to be in good agreement with the prediction of the neutral polyhedra theory (NPT). The success of the NPT, presently for ionic interfaces and previously for covalent zinc-blende interfaces, unmasks a possible connection between the partition into neutral symmetric cells and the energy-minimization requirement on the interface charge distribution. The independence of the BO on interface specifics, i.e., bulklike behavior, is shown to stem directly from such a property of charge distribution with minimized electrostatic energy. As energy minimization governs the formation of charge distribution in general, NPT is expected to describe the band offset of a wide variety of material interfaces.

The effect of interface atomic structure on the deformation mechanisms of Ti2AlN/TiAl composite under nanoindentation using MD simulations

In this study, we conduct molecular dynamics simulations to investigate the effect of interface atomic structure on the deformation mechanisms of Ti2AlN/TiAl composite under nanoindentation. It is found that the interface atomic structure has a remarkable effect on the deformation behavior of Ti2AlN/TiAl composite due to significantly different interface-dislocation interactive mechanisms. For the Ti2AlN(0 0 0 1)/TiAl(1 1 1) coherent interface system, although plenty of dislocations nucleate and propagate through the TiAl layer during the indentation process, there are no dislocations transmitting across the coherent interface. The formation of stair-rod dislocation and dislocation tangle is the major mechanism for blocking the slip motion of dislocations by the coherent interface. Thus, the Ti2AlN(0 0 0 1)/TiAl(1 1 1) coherent interface is beneficial for the strengthening effect of Ti2AlN/TiAl composite. For the Ti2AlN(1 0 3)/TiAl(1 1 1) incoherent interface system, the incoherent interface has some ability to annihilate the dislocations nucleated from the TiAl during the indentation process, but it cannot simultaneously annihilate plenty of dislocations. These dislocations accumulate in the incoherent interface and cause the stress concentration, providing the driving force for dislocation nucleation in Ti2AlN from the poor matching regions in the Al atomic arrays at the incoherent interface. Therefore, the incoherent interface can provide the access of dislocation transmission from TiAl to Ti2AlN, which benefits the ductility of Ti2AlN/TiAl composite.