Low-index Planes Research Articles

Structural Defects and Ferromagnetic Signature of V-Doped Sb2Te3 Thin Films Grown on SrTiO3(001) Produced by RF-Magnetron Sputtering.

Thin films of V-doped Sb2Te3 compounds were fabricated by co-deposition of Sb2Te3 and V using rf-magnetron sputtering onto SrTiO3(001) substrates kept at 570 K. The microstructures of the films were characterized using transmission electron microscopy (TEM) and electron diffraction. The crystal structure of the sputtered film (Sb38V2Te60) is the Bi2Te3-type structure with lattice parameters of a = 0.44 ± 0.03 nm and c = 3.02 ± 0.02 nm. A combination of cross-sectional and plan-view TEM observations revealed the preferential orientation of the c axis in the film's normal direction. A thin amorphous layer exists between the Sb2Te3 thin film and the SrTiO3 substrate. The interfacial amorphous layer relaxes the strain between the thin film and the substrate, and hence, it should promote the growth of a low-index atomic plane with a low surface free energy (i.e., (0001) of the Sb2Te3). The onset of ferromagnetic order was detected at temperatures below 70 K. A remarkable increase in magnetization was detected in the film's normal direction, which corresponds to the magnetic easy axis (i.e., c axis of the Sb2Te3). V3+ ions substituting Sb sites should contribute to ferromagnetism at low temperatures.

Early-Stage M23C6 Morphology at the Phase Boundary in Type 304L Austenitic Stainless Steel Containing δ Ferrite

The early-stage M23C6 morphology at the phase boundary between austenite and δ ferrite grain in Type 304L austenitic stainless steel was investigated with transmission electron microscopy (TEM). The M23C6 has coherency with austenite grains at phase boundary. The phase boundary between the M23C6 and austenite grains has curved appearance. The curved phase boundary might be a faceted interface composed of (111) plane and the other low-index planes on the atomic scale. The M23C6 morphology at the phase boundary was identified to be a complex pyramid-like shape that has {111}, {110} and {100} interfaces. The slight deviation in the angle measured between edges of carbide in the TEM images from that calculated between edges expected from the M23C6 morphology might be attributed to multilayer growth in the interior region of the M23C6 face.

Cavity Etching Behavior on the M‐Plane of Sapphire Crystal

AbstractWet etching has been a fundamental process for the fabrication of sapphire crystal substrates with critical position in the current semiconductor devices industry. The present work reveals the etching behaviors of cavities on the M‐plane {100} of sapphire crystal by the hot KOH etchant. It is shown that the boundary of cavities changes from smooth circle to octagon‐like and the edges evolve sharper as the etching temperature increases. Carefully analyzing the structure of the etched cavities suggests they are partially framed by low index planes (20) (20), (100), (010), and (110).

Distortion of SiC lattice induced by carbon-coating on (100) and (111) surfaces - ab-initio and molecular dynamics study

Lattice distortion in cubic SiC lattice induced by surface relaxation at C-terminated low-index atomic planes (100) and (111) was studied by means of molecular dynamics (MD) and ab-initio DFT simulations. DFT results served as a reference for choosing the best MD potential which was next used for examination of lattice distortion for models of SiC particles considerable larger than those accessible by quantum-mechanical calculations. The distortions underneath the surface were examined for stoichiometric lattice naturally terminated by a single monoatomic C layer and also for surfaces covered by one or two excessive carbon layers. It is found that interatomic distances underneath the surface change in lateral and normal directions down to the depth of about 2–3 nm. Surface relaxations at extended surfaces simulated under 2D periodic boundary conditions as well as at surfaces of free standing nanograins were analyzed and compared. It is shown that structural changes induced in SiC lattice by the strains appearing at the grain surfaces are functions of crystallographic orientation of the surface and in case of isolated nanocrystals also depend on their size and shape.

Interfacial Structure of Pt(110) Electrode during Hydrogen Evolution Reaction in Alkaline Solutions.

In alkaline solutions, interfacial cations affect the hydrogen evolution reaction (HER) activity of platinum electrodes. However, the effects of cations on the HER activity have not been previously investigated based on interfacial structures. In situ surface X-ray diffraction was performed on Pt(110), of which the HER activity is the highest in the low-index planes of Pt, at hydrogen evolution potentials in alkaline solutions, and revealed the interfacial structure of alkali metal cations (Li+ and Cs+). The interfacial structure of the Pt(110) electrode changed reversibly depending on the electrode potential. In the LiOH solution, where the HER activity was higher, the densely packed water layer in the electrical double layer acted as a hydrogen supplier. In the CsOH solution, where the HER activity was lower, the Cs+ cations were aligned in the missing rows of the 1 × 2 reconstructed Pt(110) surface, suggesting that the Cs+ hindered water from accessing the surface, resulting in a lower HER activity.

Porous gold nanomesh films electrodeposited in toluene-based dynamic soft template

Two-dimensional (2D) metallic nanomaterials have received much attention as next-generation materials. The creation of the characteristic pores into 2D metallic materials is expected to improve performance in sensing, surface-enhanced Raman spectroscopy, electrocatalytic activity, and so on. Here, we report that gold electrodeposition in a toluene-based bicontinuous microemulsion (BME), called a dynamic soft template, provides porous gold nanomesh (PGNM) film. The selective toluene adsorption onto Au{111} surfaces induces anisotropic 2D growth of gold in the aqueous phase of BME, and an intertwined network of water and toluene nanochannels in BME resulted in the formation of abundant in-plane nanopores. Toluene adsorption during Au electrodeposition makes the {111}-oriented Au surfaces more lipophilic, i.e., a structural change in BME is induced to expose the organic phase solution selectively to the toluene-adsorbed {111} Au surfaces. This dynamic change in the BME solution structure during electrodeposition might enhance the anisotropic growth of gold (dynamic soft templating). Electrochemical behaviors of low-index planes of Au electrode supported the growth mechanism of PGNM film.

Immobilization of gaseous elemental mercury by Ag nanoparticles: A combined DFT and experimental study

Silver nanoparticles (AgNPs) have attracted considerable attention as promising industrial adsorbents of gaseous mercury. The underlying mechanism of immobilization of Hg0 on different AgNP surfaces was investigated in theoretical and experimental approaches. The crystallographic properties of AgNPs obtained by X-ray diffraction and selected area electron diffraction were incorporated into comparative DFT calculations to evaluate the adsorption energy on AgNP surfaces such as low-index and edge planes. In addition, the energy pathway of oxidation of adsorbed Hg to HgO on AgNP surfaces was analysed by an ab-initio method, which suggested chemisorption rather than catalytic oxidation to be the main mechanism of mercury removal by AgNPs in the low-to-middle temperature range (≤326 °C). Mercury adsorption and desorption tests were conducted in a fixed-bed reactor with Ag nanoparticles and microparticles as adsorbent, where AgNPs were more efficient than microparticles in mercury removal, and the removal efficiency remained stable after four rounds of regeneration. Furthermore, temperature-programmed desorption analysis suggested a second-order desorption of Hg from AgNPs with an activation energy of 0.84 eV. X-ray photoelectron spectroscopy showed that the chemical characteristics of AgNPs remained stable while adsorbing Hg0 and that the adsorbed Hg was not in an oxidation state, validating the DFT results. By providing an integrated understanding of mercury adsorption on AgNPs, these computational and experimental results will help to guide the design and optimisation of Ag-based mercury adsorbents for industrial applications.

Crystallographic anisotropy of corrosion rate and surface faceting of polycrystalline 90Cu-10Ni in acidic NaCl solution

The orientation-dependent corrosion rate and surface faceting of polycrystalline 90Cu-10Ni in acidic NaCl solution was investigated statistically. A good linear relationship was found between corrosion rate and the deviation angle from the 〈111〉 direction of the grain. For the three principle low-index planes, the corrosion rates followed the sequence of (001) < (101) < (111). More interestingly, there is a clear correlation between crystallographic orientation and surface faceting of grains in 90Cu-10Ni. Grains orientated near the three principle low-index planes and the (112) plane exhibit four different types of submicron-scale post-corrosion morphologies, respectively. However, each type of the faceted structure is bounded by {001} planes which have the lowest corrosion rate, independent of the substrate grain orientation. This is supported by the crystallographic models and AFM topographic measurements. Current study provides a simple way to identify grain orientation by etching, and also promising solutions to optimize corrosion properties through tailoring surface texture and design micro- or nano-scale surface structure by controlled etching.

Facet-dependent interfacial segregation behavior of V-doped WC-Co cemented carbides

Understanding the interfacial segregation behavior in V-doped WC-Co hard metals is necessary to demystify the effect of dopant on microstructural evolution and mechanical properties. Herein, aberration-corrected scanning transmission electron microscopy was utilized to investigate the atomic level segregation mechanism at WC-Co phase boundaries and WC grain boundaries in V-doped WC-Co composites systematically. A key finding is that the segregation behavior largely depends on the orientation of low-index planes, particularly the basal and prismatic facets that prevail in WC-Co. V solute atoms show a relatively weak segregation at the WC prismatic plane-terminated interfaces, occurring largely within a monolayer with ∼10 at% V at the outermost surface. In contrast, segregation at the basal facets of WC grains distributes within a bilayer with a higher concentration of ∼25 at%. Additionally, this study shows that the facet-dependent interfacial segregation behavior is universal for both phase boundaries and grain boundaries, which often have at least one basal or prismatic WC facet. Our study presents a general solute segregation mechanism at an atomic level and demonstrates that the terminal planes of interfaces determine the atomic structure and segregation profile, while the interfacial orientation has little influence.

Crystal facet dependence of SiHCl3 reduction to Si mechanism on silicon rod

Clarifying the mechanism of SiHCl3 decomposition on silicon rod and in gas phase has a remarkable impact on intensifying the technology of polycrystalline silicon production from SiHCl3. Our work investigates the mechanism of SiHCl3 decomposition on low index plane of silicon crystal—(111), (110) and (100) by density functional theory (DFT), and further explores the mechanism in gas phase. SiHCl3 decomposition to Si is mainly formed on Si(111) facet due to the highest atomic density and the stronger Si-Si bond; the most favorable path is SiHCl3 → SiCl3 + H → SiCl2 + Cl → SiCl + Cl → Si + Cl. The three coordination Si atoms on Si(110) and Si(100) are not in the same plane; Si(110) and Si(100) are not beneficial to SiHCl3 decomposition to Si. In gas phase reaction, SiHCl3 decomposition to SiCl4 is more favorable than Si formation. In kinetics, when the temperature of silicon rod is higher than that in gas phase, the formation of Si has more advantages than SiCl4. The findings enhance our understandings about the crystal facet structure-dependent performances of silicon crystal in polycrystalline silicon production, and provide a useful guidance for promoting SiHCl3 decomposition to Si through exposing more (111) surface area of silicon rod and optimizing the flow field and temperature distribution in the reactor.

Intergranular precipitation-enhanced wetting and phase transformation in an Al0.4CoCrFeNi high-entropy alloy exposed to lead-bismuth eutectic

After exposure to oxygen-poor (10−13–10−14 wt%) liquid lead-bismuth eutectic (LBE) at 500 °C for 500 h, LBE penetrates more than one order of magnitude deeper in an FCC Al0.4CoCrFeNi high-entropy alloy (HEA) decorated with a network of BCC (Ni, Al)-rich intergranular (IG) precipitates than in a single-phase, FCC Al0.3CoCrFeNi HEA without the IG precipitate network. This deterioration of corrosion resistance is attributed to the energetic nature of the BCC/FCC interphase boundaries (IBs) and resultant IB wetting. The LBE ingress film selectively leaches nickel located at those low-indexed crystalline planes, resulting in phase transformation from FCC to BCC structure.

The role of fluorination during the physicochemical erosion of yttria in fluorine-based etching plasmas

A physicochemical mechanism acting between the reactive plasma and the material surface controls the erosion of polycrystalline ceramics in fluorine containing etching plasmas. In this study, a Y2O3/YOF composite was exposed to a fluorine etching plasma. Relocalization enables the direct correlation of crystalline orientation with material response. Our study reveals an orientation dependent surface fluorination of Y2O3, which controls the etching resistance and morphology formation. Orientations near the low index planes (001), (010) and (100) exhibit the lowest stability due to a homogeneous surface reaction. The presented results help to extend the mechanistic understanding of the plasma-material interaction of Y2O3.

Insights toward chlorine passivation effects on electronic structures of CdTe([formula omitted]) and CdTe([formula omitted]) surfaces via atomistic modeling

A systematic study of varied chlorine passivation concentrations (0, 25, 50, and 100%) along Te-terminated CdTe(1¯00) (2×1) and CdTe(1¯1¯1¯)(1×1) surfaces is presented using high-fidelity atomistic computational simulations to describe the CdTe surface electronic structure. The atomistic modeling approach incorporates quantum mechanical-based calculations via density functional theory coupled with a surface Green’s function formalism. Results show that fractional amounts of Cl manipulate the band alignment features of the CdTe(1¯00) (2×1) facet by introducing surface electronic states that bend the bands upward. The fully chlorine passivated (100%) CdTe(1¯00) (2×1) case reestablishes flat band conditions. As for the CdTe(1¯1¯1¯) (1×1) surface, a Cl concentration of 50% mitigates upward band bending effects as well as reduces the surface electronic states present along the highly polar facet. The study also indicates that Cl concentrations of at least 50% for the CdTe(1¯00) and CdTe(1¯1¯1¯) low-index plane orientations are necessary to decrease the CdTe electron affinity and could be used to improve band alignment found within CdTe solar cells. The investigation of Cl passivation effects on Te-terminated CdTe surfaces provides a unique atomic-scale perspective on the critical role Cl passivation has for electronic characteristics found in CdTe thin-film photovoltaic devices.

Facet dependent catalytic activity of Pd nanocrystals for the remedy of organic Pollutant: A mechanistic study

The facet effect and underlying molecular mechanisms of Palladium (Pd) nanocrystals have shown a potential candidate for various applications including catalysis and organic pollutant remediation. In this report, seed mediated synthesis was used to synthesize three distinct Pd nanostructures such as cuboctahedral (Coh), octahedral (Oh), and nanocubes (NCs) by optimizing the time and the temperature. The growth of nanocrystals were directed along different low index planes such as {100} for Pd NCs, {111} for Oh, and mixed planes of {100} and {111} for Coh. To understand the facet dependent catalysis, nanocrystals were employed for the reduction of 4-Nitroaniline (4-NA) to 4-Phenylenediamine (4-PDA), studied the effect of NO2 group and the degradation of cationic dye, Rhodamine B (RhB), and anionic dye, Methyl Orange (MO). By keeping the total surface area of particles unchanged, different volume of nanocrystal solutions were used to carry out facet-dependent analysis. The reduction and degradation processes were confirmed from the absorption study that the efficiency of Pd NCs was higher than Oh and Coh. The catalytic trends for the asymmetric growth of planes follows the order of NCs {100} > Oh {111} > Coh {100} {111}. The specific reactivity performance of the nanocrystals were confirmed using an analytical model.

Influence of Substrate Orientation and Oxygen Partial Pressure on the Morphology, Structure, and Electrical Property of Epitaxial Indium Tin Oxide Films

Indium tin oxide (ITO, tin-doped indium oxide) has been widely used in optoelectronic and other research fields because of its excellent electrical conductivity. However, there is a lack of systemic research on the influence of crystal orientation on the electrical properties of epitaxial ITO films. In this work, the epitaxial ITO films were deposited on low index planes of yttria-stabilized zirconia substrates by direct current magnetron sputtering at different oxygen partial pressures ranging from 0 to 3 Pa. The carrier concentration of the ITO film decreased with the increase of the oxygen partial pressure and further leading to the increase of overall resistivity. In addition, we also found that the Hall mobilities of ITO (100) films rapidly decreased with the oxygen partial pressure, while the Hall mobilities of ITO (111) films remained unchanged. The anisotropy of the carrier mobility was the main factor for the resistivity difference of ITO films with different orientations at high oxygen partial pressure.

Tailoring the hydrophilic and hydrophobic reaction fields of the electrode interface on single crystal Pt electrodes for hydrogen evolution/oxidation reactions

Interfacial hydrophobic/hydrophilic reaction fields significantly affect various reactions at the electrode surface. The hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR) have been investigated on single crystal Pt electrodes modified with hydrophobic/hydrophilic cations and anion-exchange copolymers in alkaline solutions. In alkali metal hydroxide solutions, Pt (110) exhibits the highest HER/HOR activity in the low-index planes of Pt. On the low-index planes of Pt, the hydrophilicity of the alkali metal cation in the supporting electrolyte activates the HER/HOR depending on its hydration energy. Hydrophilic cations at the interface facilitate the extraction of hydrogen from the hydrated water. The modification of anion-exchange copolymers with a hydrophobic skeleton on Pt (110) further enhanced the HER/HOR activity. The hydrogen bonding network formed around the hydrophobic species facilitated the mobility of water molecules and the OH− as the reactant/product of the HER/HOR. Appropriately forming hydrophilic and hydrophobic reaction fields at the interface improved the HER/HOR activity.

Lattice transformation in grain boundary migration via shear coupling and transition to sliding in face-centered-cubic copper

Migration of symmetric tilt grain boundaries (GBs) via shear coupling has been studied extensively in experiments and simulations. It was reported that shear coupling transitioned to GB sliding at high temperatures, but how such transition occurs at low temperatures has not been investigated. Lattice transformation on the atomic scale during shear coupling has not been fully understood. In this work, mode of motion of symmetric tilt GBs with [001] tilting axis in face centered cubic copper under a shear strain parallel to the boundary plane at 100 K was carefully characterized by tracking the positions of the corresponding planes in atomistic simulations and new features of GB motion were observed. The results show that the angles between the two low-index planes, (110) and (100), and the boundary plane can be used to define a nominal magnitude of shear s. Approximately, if one of these two planes has a value of s < 0.5, shear coupling occurs with this plane being the active invariant plane; if 0.5 < s < 0.6, GB moves by shear coupling + sliding, i.e. a hybrid mode by which shear coupling transitions to sliding; if both planes have a value of s> 0.6, only GB sliding occurs. Careful structural analyses show that, for all the GBs that undergo shear coupling, some GB atomic planes remain invariant, very similar to the first invariant plane in deformation twinning, whereas the other GB atomic planes swap their positions in the GB normal direction through highly coordinated and complex atomic shuffles. This behavior allows identification of transformation units that are reoriented toward the neighboring grain. Rate-limiting factors are identified for lattice transformation and can be used to infer a kinetics model for shear coupling.

A practical method to determine the five-parameter orientation of intragranular boundaries in polycrystals

Intragranular boundaries are important features of polycrystalline materials and impact on many physical and chemical properties. Knowledge of their physical orientation is often crucial to explain such properties. However, it has proved difficult to determine the complete orientation of intragranular boundaries, which involves the misorientation angle and axis about which the adjacent crystal lattices need to be rotated to bring them into coincidence and also the physical orientation of the boundary plane, expressed by the plunge and azimuth of its normal; five parameters in total. Here we present a simple and practical manual method to determine the complete intragranular boundary orientation in any crystal system. The method is developed on geometrical relationships exhibited between electron channelling patterns across a common boundary but then extended for use with electron backscattered diffraction patterns. The method recognises the channelling/diffraction band, equivalent to a crystal lattice plane, not displaced across a boundary; the boundary rotation axis must be the normal to this plane. Geometrical relationships between the boundary trace, the non-displaced band/plane and their respective plane normals constrain boundary orientation to two alternative symmetrically equivalent solutions and are evaluated via stereographic projection. The choice of solution is guided by comparison with the presence or absence of a similarly oriented band/plane observed in the original channelling/diffraction patterns. The method therefore conforms to the low-index crystallographic lattice plane and dislocation model for intragranular boundary formation and defines boundary orientation in terms of total angular misorientation due to tilt and twist components and the orientation of the boundary plane. Examples of intragranular boundary orientation determination using this method are provided in olivine. Results are compared to and differ from those obtained via conventional misorientation analysis, which only rotates adjacent crystal lattices into parallelism and does not consider boundary plane orientation. Potential extrapolation of the new method to intergranular boundaries is also considered

Gold-rutile interfaces with irrational crystallographic orientations

Gold-rutile interfaces have been extensively investigated particularly because of their importance to heterogeneous catalysts for many reactions such as CO oxidation and water-split reactions. Herein, we systemically study preferential orientation relationships (ORs) and atomic structures at gold-rutile interfaces using a model system of dewetted gold particles on a single-crystal rutile substrate. Four previously reported ORs are confirmed, and more importantly, a few irrational ORs (i.e., which cannot be described by low-indexed planes and directions) are newly discovered. These ORs agree with theoretical possibilities predicted according to the geometrical constraint of adjacent lattices. In addition, atomic rearrangement is observed at these interfaces, particularly in those with irrational ORs.

Properties of interfaces between copper and copper sulphide/oxide films

We compare different means of predicting and rationalizing properties of corrosion films aiming at gaining deeper understanding of the behaviour of copper in aqueous and sulphide-containing environments. Purely geometrical tools ranging from mean bulk information to anisotropic and facet-specific approaches are discussed, and their performances compared against DFT-based stability and property evaluations of a range of interfaces arising from combining low-index crystallographic planes of the compounds Cu, Cu2O, and Cu2S. We demonstrate the necessity to consider facet-specific interactions to understand nanolevel differences between Cu2S and Cu2O film behaviour, and that thin films cannot be directly described by bulk properties.