Interfacial Superstructures Research Articles

Deformation-induced structural multiplicity of tilt boundaries facilitating the formation of two-dimensional interfacial superstructures

Understanding the interfacial structures and the induced solute segregation behaviors at the atomic scale can facilitate the design of high-performance materials. However, the diversity and concealed nature of interfaces often make it challenging to reveal the interfacial microstructures and the accompanying solute segregation phenomena. Here, we report the discovery of 8 types of asymmetric tilt grain boundaries (GBs) with solute segregation in a deformed and annealed Mg-rare-earth binary model alloy, by means of Z-contrast high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) observations. Molecular dynamics (MD) simulations provide a visual depiction of the atomic-scale characteristics of the periodic extension and compression sites in these tilt GBs. The first-principles density functional theory (DFT) calculations unveil the energetics of the well-ordered segregation behavior of solutes at these interfaces. It is confirmed that the selective segregation of solutes at substitutional sites in the periodic misfit-dislocations separated tilt GBs leads to the formation of 8 types of unique two-dimensional interfacial superstructures. Additionally, the MD simulations have revealed that solute segregation plays a pivotal role in governing the shear strength of tilt GBs by influencing the nucleation of dislocations at interfaces. Our atomic-scale insights provide valuable guidance for understanding the formation of tilt GBs within the wrought alloys and their pivotal role in facilitating solute segregation, enabling the creation of specific two-dimensional superstructures at interfaces. Furthermore, these insights hold promise for advancing the design of high-performance alloys through the application of GB engineering.

Nonsymmetrical Segregation of Solutes in Periodic Misfit Dislocations Separated Tilt Grain Boundaries.

Interfacial segregation is ubiquitous in mulit-component polycrystalline materials and plays a decisive role in material properties. So far, the discovered solute segregation patterns at special high-symmetry interfaces are usually located at the boundary lines or are distributed symmetrically at the boundaries. Here, in a model Mg-Nd-Mn alloy, we confirm that elastic strain minimization facilitated nonsymmetrical segregation of solutes in four types of linear tilt grain boundaries (TGBs) to generate ordered interfacial superstructures. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy observations indicate that the solutes selectively segregate at substitutional sites at the linear TGBs separated by periodic misfit dislocations to form such two-dimensional planar structures. These findings are totally different from the classical McLean-type segregation which has assumed the monolayer or submonolayer coverage of a grain boundary and refresh understanding on strain-driven interface segregation behaviors.

Interfacial superstructures and chemical bonding transitions at metal-ceramic interfaces.

Metal-ceramic interfaces are scientifically interesting and technologically important. However, the transition of chemical bonding character from a metal to a nonoxide ceramic is not well understood. The effects of solute segregation and interfacial structural transitions are even more elusive. In this study, aberration-corrected electron microscopy is combined with atomic-resolution energy-dispersive x-ray and electron energy loss spectroscopy to investigate Ti-, V-, and Cr-segregated WC-Co interfaces as model systems. The experiments reveal the general anisotropic formation of reconstructed trilayer-like superstructures with segregant-specific compositional profiles that facilitate the transition from covalent to metallic electronic structures. Density functional theory calculations confirm the gradual increasing metallicity from WC to Co in the interfacial trilayers via increasing metallic solute concentration. This study uncovers unprecedented details of the sophisticated interfacial superstructures at metal-ceramic interfaces. It sheds light on how a metal transits to a ceramic at a "general" interface with strong segregation.

A highly asymmetric interfacial superstructure in WC: expanding the classic grain boundary segregation and new complexion theories

The discovery of a grain boundary superstructure with highly asymmetric and off-the-center segregation, along with interfacial disordering and symmetry change on the opposite sides, extends our knowledge of grain boundary segregation and complexions.

Ternary interfacial superstructure enabling extraordinary hydrogen evolution electrocatalysis

Realizing large-scale electrochemical hydrogen evolution in alkaline and neutral media by robust and non-noble-metal heterogeneous catalysts is highly ambitious due to the sluggish reaction kinetics at low H+ conditions. Herein, highly efficient hydrogen evolution reaction (HER) catalysts, comprising Ni, NiO clusters, and defective carbon, are successfully constructed via a facile and large-scale route. Multiple synchrotron radiation-based X-ray spectroscopic characterizations, combining high-resolution transmission electron microscopy measurements, indicate the formation of ternary interfacial superstructure with intimate interfacial coupling through abundant NiOC bonds. Impressively, the optimized catalyst loaded onto the usual glass carbon electrode exhibits exceptional catalytic activities with overpotentials of 64 and 76 mV to reach 10 mA cm−2 in 1 M KOH and 1 M phosphate buffer solution (PBS), respectively, representing one of the best non-noble-metal HER electrocatalysts to date. Insights into the metal/oxide interfacial effects through density functional theory calculations reveal that the interface sites could efficiently lower the energy barrier of the rate-determining step (RDS), contributing to the fast reaction kinetics. This work not only provides comprehensive insights into interfacial feature of highly active HER catalysts but also broadens the fundamental understanding of interfacial effects toward HER catalysis.

Influence of the electrode potential and in situ STM scanning conditions on the phase boundary structure of the single crystal Bi(1 1 1)|1-butyl-4-methylpyridinium tetrafluoroborate interface

Interfacial structure of the Bi(111)|1-butyl-4-methylpyridinium tetrafluoroborate (BMPyBF4) interface has been studied applying the in situ scanning tunnelling microscopy (STM), cyclic voltammetry, impedance spectroscopy (EIS), and also computational chemistry methods. Influence of electrode potential and in situ STM scanning conditions on the interfacial superstructure formation process has been demonstrated. Slow electrical double layer formation limited by the mass transfer step of BMPyBF4 adsorption onto the Bi(111) surface, established by using EIS and verified also by STM data, has been explained by the BMPyBF4 multilayered superstructure formation. Possible reasons for the low values of series capacitance for the Bi(111)|BMPyBF4 interface have been analyzed on the basis of modern theoretical models and explained by the semimetallic nature of bismuth, i.e., by the potential drop inside the Bi(111) electrode surface layer, and by the strong van der Waals type attractive forces between the adsorbed BMPy+ cations, generating compact adsorbed structures with the low effective dielectric constant.It was found that at the fixed Bi(111) potential, the observed multilayered structure depends on the STM image scanning rate, the tip potential (bias) and on the tunnelling current. Multilayered formations of BMPyBF4 at the Bi(111) electrode were observed within potential range from −1.2V to −0.6V (vs. Ag|AgCl in BMPyBF4), for the positively charged tungsten STM tip and at the high scanning rates. At more negative Bi(111) potentials (and the slower scanning rates), the atomic resolution was observed similarly to the Bi(111)|Na2SO4+H2SO4 aqueous solutions interface.

Directed self-assembly of Janus nanorods in binary polymer mixture: towards precise control of nanorod orientation relative to interface

The ability to control the organization of anisotropic nanoparticles, such as nanorods, with high precision would greatly facilitate the fabrication of functional materials. Using a hybrid computational model, we systematically investigate the directed self-assembly of Janus nanorods, with two chemically different surface compartments, in binary polymer mixtures. Our simulations demonstrate that the energetic contributions from the surface chemistry of the Janus nanorods, the rod–rod interaction, and the spatial confinement from the polymer phases can be tailored to tune the orientation angle of the nanorods with respect to phase interface, leading to the formation of “lying”, tilt, and “standing” interfacial superstructures. A detailed insight into the mechanism regarding this precise control of nanorod orientation at the interface is obtained by evaluating the rod–phase interaction energy and the entropic energy of the tethered ligands on the rods. Furthermore, since the Janus rods are localized at the interface between two polymer phases, the structural evolution of the polymer nanocomposites is dramatically curtailed. This kinetic arrest is found to depend on the surface chemistry and the aspect ratio of Janus rods. The results demonstrated in this paper offer a novel approach to achieve morphological and kinetic control in nanoscopic composites towards unique photovoltaic and mechanical properties.

Origin of Anomalous Electronic Structures of Epitaxial Graphene on Silicon Carbide

On the basis of first-principles calculations, we report that a novel interfacial atomic structure occurs between graphene and the surface of silicon carbide, destroying the Dirac point of graphene and opening a substantial energy gap there. In the calculated atomic structures, a quasiperiodic 6x6 domain pattern emerges out of a larger commensurate 6 sqrt [3] x 6 sqrt [3]R30 degrees periodic interfacial reconstruction, resolving a long standing experimental controversy on the periodicity of the interfacial superstructures. Our theoretical energy spectrum shows a gap and midgap states at the Dirac point of graphene, which are in excellent agreement with the recently observed anomalous angle-resolved photoemission spectra. Beyond solving unexplained issues in epitaxial graphene, our atomistic study may provide a way to engineer the energy gaps of graphene on substrates.

Relationship between interfacial superstructures and Schottky-barrier heights of Sb/GaAs contacts.

The interfacial superstructures of Sb/GaAs(001) contacts are observed by use of the grazing-incidence x-ray-diffraction method. The Schottky-barrier heights for these contacts are also determined by using the current-voltage or the capacitance-voltage methods. The Schottky-barrier height of the contact with the (1\ifmmode\times\else\texttimes\fi{}6) interfacial superstructure is found to be larger than that of the contact with the (1\ifmmode\times\else\texttimes\fi{}4) interfacial superstructure by as much as 0.1 eV. This result indicates the importance of the local electronic structure for Schottky-barrier formation at the metal-GaAs interfaces. The present results are discussed in connection with the current Schottky-barrier models.

Interfacial superstructures studied by grazing incidence x-ray diffraction

Surface superstructures (reconstructed structures) have been observed by many techniques. However, it has not been easy to confirm that a superstructure does exist at an interface between two solid layers. We have observed superstructures at the interface between two solid layers by a grazing incidence X-ray diffraction technique with use of synchrotron radiation. We have observed the interfacial superstructures between amorphous silicon (a-Si) and Si(111)-7×7, Ge0.2Si0.8(111)-5×5 or B(√3 ×√3)/Si(111) and have also shown the existence of the interface between an epitaxial Si(111) layer and B(√3×√3)/Si(111). Interfacial superstructures between Al or other metals and GaAs(001) have been found. Such superstructures may be relevant for the understanding of Schottky barrier formation. In addition, interfacial superstructures between AIN and GaAs(001) layers, both deposited on a GaAs(001) substrate by the metalorganic chemical vapor deposition (MOCVD) method, have been found to exist.

Theoretical contribution to the study of an a-Si/Ge0.2Si0.8(111) interfacial structure.

A recently discovered correlation between geometric and elastic-density factors to determine the interfacial superstructure of an a-Si/${\mathrm{Ge}}_{0.2}$${\mathrm{Si}}_{0.8}$(111) heterostructure subsequent to film growth is applied and the results are compared with experimental data obtained by synchrotron x-ray diffraction.

Interfacial Superstructures Studied by Grazing Incidence X-Ray Diffraction

ABSTRACTSurface superstructures (reconstructed structures) have been observed by many authors. However, it is not easy to confirm that a superstructure does exist at an interface between two solid layers. The present paper reports a direct observation, by a grazing incidence x-ray diffraction technique with use of synchrotron radiation, of superstructures at the interface. Firstly, the boron-induced R30° reconstruction at the Si interface has been investigated. At the a Si/Si(111) interface, boron atoms at 1/3 ML are substituted for silicon atoms, thus forming a R30° lattice. Even at the interface between a solid phase epitaxial Si(111) layer and a Si(111) substrate, the boron-induced R30° reconstruction has been also observed. Secondly, SiO2/Si(100)-2×l interfacial superstructures have been investigated. Interfacial superstructures have been only observed in the samples of which SiO2 layers have been deposited with a molecular beam deposition method. Finally, the interfaces of MOCVD-grown AIN/GaAs(100) have been shown to have 1×4 and 1×6 superstructures.

Interfacial Superstructure Studied by Grazing Incidence X-Ray Diffraction

Interfacial Superstructure Studied by Grazing Incidence X-Ray Diffraction

Correlation between geometric and elastic parameters in structural modification of interfacial superstructures.

Correlation between geometric and elastic parameters in structural modification of interfacial superstructures.

Grazing incidence X-ray diffraction study of interfacial superstructure: a-Si on Ge0.2Si0.8(111)-5×5

Grazing incidence X-ray diffraction study of interfacial superstructure: a-Si on Ge_0.2Si_0.8(111)-5×5