Stacking Of Atomic Layers Research Articles

Anharmonic Nuclear Motion and the Relative Stability of Hexagonal and Cubic ice

We use extensive first-principles quantum mechanical calculations to show that, although the static lattice and harmonic vibrational energies are almost identical, the anharmonic vibrational energy of hexagonal ice is significantly lower than that of cubic ice. This difference in anharmonicity is crucial, stabilising hexagonal ice compared with cubic ice by at least 1.4 meV/H2O, in agreement with experimental estimates. The difference in anharmonicity arises predominantly from molecular O-H bond stretching vibrational modes and is related to the different stacking of atomic layers.

Good reproductive preparation method of Li-intercalated hexagonal boron nitride and transmission electron microscopy – Electron energy loss spectroscopy analysis

A Li-intercalated hexagonal boron nitride (Li-h-BNIC) phase was synthesized using a highly reproducible method that involves annealing an Li3N and h-BN mixture at 1220 K. Powder X-ray diffraction, electrical conductivity measurements, transmission electron microscopy (TEM) and electron energy loss spectroscopy were performed. The stacking of BN atomic layers in the Li-h-BNIC phase is not the same as the two-layer stacking periodicity of h-BN. TEM observation suggests the existence of incommensurate periodicity along the intralayer direction. From the low-loss and core-loss spectra, the Li-h-BNIC phase is not metal as predicted by the first-principle calculations. Satellite peaks of 1 s to π* transition in the B K-edge core-loss spectrum indicate the presence of N atom vacancies modified by O atoms in the h-BN atomic layer.

Compositional modulated atomic layer stacking and uniaxial magnetocrystalline anisotropy of CoPt alloy sputtered films with close-packed plane orientation

An atomic layer stacking structure in hexagonal close packed (hcp) Co100−xPtx alloy films with c-plane sheet texture was directly observed by a high-angle annular dark-field imaging scanning transmission electron microscopy. The analysis of sequential and/or compositional atomic layer stacking structure and uniaxial magnetocrystalline anisotropy (Ku = Ku1 + Ku2) revealed that (1) integrated intensity of the superlattice diffraction takes the maximum at x = 20 at. % and shows broadening feature against x for the film fabricated under the substrate temperature (Tsub) of 400 °C. (2) Compositional separation structure in atomic layers is formed for the films fabricated under Tsub = 400 °C. A sequential alternative stacking of atomic layers with different compositions is hardly formed in the film with x = 50 at. %, whereas easily formed in the film with x = 20 at. %. This peculiar atomic layer stacking structure consists of in-plane-disordered Pt-rich and Pt-poor layers, which is completely different from the so-called atomic site ordered structure. (3) A face centered cubic atomic layer stacking as faults appeared in the host hcp atomic layer stacking exists in accompanies with irregularities for the periodicity of the compositional modulation atomic layers. (4) Ku1 takes the maximum of 1.4 × 107 erg/cm3 at around x = 20 at. %, whereas Ku2 takes the maximum of 0.7 × 107 erg/cm3 at around x = 40 at. %, which results in the maximum of 1.8 × 107 erg/cm3 of Ku at x = 30 at. % and a shoulder in compositional dependence of Ku in the range of x = 30–60 at. %. Not only compositional separation of atomic layers but also sequential alternative stacking of different compositional layers is quite important to improve essential uniaxial magnetocrystalline anisotropy.

Synthesis-atomic structure-properties relationships in metallic nanoparticles by total scattering experiments and 3D computer simulations: case of Pt-Ru nanoalloy catalysts.

An approach to determining the 3D atomic structure of metallic nanoparticles (NPs) in fine detail and using the unique knowledge obtained for rationalizing their synthesis and properties targeted for optimization is described and exemplified on Pt-Ru alloy NPs of importance to the development of devices for clean energy conversion such as fuel cells. In particular, PtxRu100-x alloy NPs, where x = 31, 49 and 75, are synthesized by wet chemistry and activated catalytically by a post-synthesis treatment involving heating under controlled N2-H2 atmosphere. So-activated NPs are evaluated as catalysts for gas-phase CO oxidation and ethanol electro-oxidation reactions taking place in fuel cells. Both as-synthesized and activated NPs are characterized structurally by total scattering experiments involving high-energy synchrotron X-ray diffraction coupled to atomic pair distribution functions (PDFs) analysis. 3D structure models both for as-synthesized and activated NPs are built by molecular dynamics simulations based on the archetypal for current theoretical modelling Sutton-Chen method. Models are refined against the experimental PDF data by reverse Monte Carlo simulations and analysed in terms of prime structural characteristics such as metal-to-metal bond lengths, bond angles and first coordination numbers for Pt and Ru atoms. Analysis indicates that, though of a similar type, the atomic structure of as-synthesized and respective activated NPs differ in several details of importance to NP catalytic properties. Structural characteristics of activated NPs and data for their catalytic activity are compared side by side and strong evidence found that electronic effects, indicated by significant changes in Pt-Pt and Ru-Ru metal bond lengths at NP surface, and practically unrecognized so far atomic ensemble effects, indicated by distinct stacking of atomic layers near NP surface and prevalence of particular configurations of Pt and Ru atoms in these layers, contribute to the observed enhancement of the catalytic activity of PtxRu100-x alloy NPs at x ∼ 50. Implications of so-established relationships between the atomic structure and catalytic activity of Pt-Ru alloy NPs on efforts aimed at improving further the latter by tuning-up the former are discussed and the usefulness of detailed NP structure studies to advancing science and technology of metallic NPs - exemplified.

Synthesis and crystal structure characterization of InGaZnO4 with a new defect structure

Single crystals of InGaZnO4 were synthesized in a sealed Pt-tube at elevated temperatures under normal pressure without flux. InGaZnO4 has a trigonal crystal system (R3¯m; No. 166) deduced from convergent beam electron diffraction (CBED). Single crystal structure refinement from XRD data at −150°C (a=3.275(1) Å; c=25.99(1) Å; Z=3) revealed an alternate stacking of InO6/3− and (Ga,Zn)O4/4+ layers, iso-structural to YbFe2O4. The cell parameters at room temperature were refined from powder X-ray diffraction data (a=3.284(3) Å and c=26.037(3) Å). The transparent crystals have a gray-bluish color and exhibit an excess of Ga expressed by the cation ratio of In:Ga:Zn=30.2(9):39(1):31(1) determined by energy dispersive X-ray spectroscopy (EDXS). While the isovalent substitution of In3+ by Ga3+ is known, an additional aliovalent substitution of Zn2+ by Ga3+ is observed. This results in the formula (In0.9Ga0.1)Ga1.06Zn0.91□0.03O4 where 3% of the cations in 4+1 coordination are vacant due to aliovalent substitution. Further evidence of that defect structure is found in the lattice parameter a which is substantially smaller than in stoichiometric InGaZnO4, as well as in the strong absorption in the near-IR from transitions correlated with the defect structure. The bluish color of the crystals is due to an enhanced absorption in the red of the visible spectrum. For the first time, we achieved to image InGaZnO4 at atomic resolution in the electron microscope proving a perfect periodic stacking of atomic layers.

Evaluation of Atomic Layer Stacking Structure and Curie Temperature of Magnetic Films for Thermally Assisted Recording Media (Invited)

Thermally assisted recording system is a promising candidate to overcome the trilemma of perpendicular magnetic recording hard disk drive development. In this paper, we introduce our current research about evaluation for the media material. In-plane X-ray diffraction technique is effective to evaluate atomic layer stacking structure of (111)-oriented face-centered cubic, c-plane-oriented hexagonal closed packed (hcp), and their intermediate structure with stacking faults of CoPt alloy thin film. Analytical results of Co50Pt50-based thin film shows that changing the valence electron number closer to 9 can effectively reduce the stacking fault. In practical, perfect hcp atomic layer stacking can be achieved by substituting Pt (group 10) with Rh (group 9). High-angle annular dark field of scanning transmission electron microscopy with probe diameter of 1 Å can effectively observe composition modulated atomic layer stacking with the super-lattice diffraction in Co-based alloy films. In practical, for Co80M20 (M: Ir, Pt) thin film sputtered under high substrate temperature, the irregular or alternately layered structure of M rich and M poor layer can be observed directly. To evaluate Curie temperature (TC), which is an important physical property of thermally assisted media, conduction electron spin-dependent scattering should be the focus. Fitting dielectric spectra for MnSb thin film with TC ~ 320°C measured with the ellipsometry and analyzing the Drude's term, temperature dependence of resistivity and scattering time at around TC was confirmed.

Giant negative uniaxial magnetocrystalline anisotropy of Co80Ir20 sputtered films with perfect hexagonal-close-packed and composition-modulated atomic layer stacking

Co80Ir20 films with negative uniaxial magnetocrystalline anisotropy (Ku) are investigated with respect to the regularity of the stacking sequence and atomic site arrangement. Substrate heating at 600 °C enhances the negative Ku of Co80Ir20 to −9.6 × 106 erg/cm3. X-ray diffraction analysis and scanning transmission electron microscopy of the Co80Ir20 film fabricated at 600 °C indicate (1) a near perfect hexagonal-close-packed (hcp) stacking structure and (2) an atomic layered structure that consists of randomly sequenced Ir-rich and Ir-poor layers. These hcp and composition-modulated atomic layer stacking structures are considered to be the reason for the enhancement of the negative Ku.

Ru下地層の傾斜結晶面上に成膜された<i>c</i>面配向Coスパッタ薄膜の原子積層構造および一軸結晶磁気異方性

Relation among structure of Ru underlayer and atomic layer stacking structure and uniaxial magnetocrystalline anisotropy (Ku) of sputtered Co films was discussed. Small torque with amplitude below theoretical limit |KuCobulk-2π(MsCobulk)2| = 6.8×106 erg/cm3 was obtained for Co film with thickness dCo ＜ 30 nm deposited on rough Ru underlayer. Structure analysis of the thin Co films with dCo ＜ 30 nm deposited on rough Ru underlayer demonstrated that: 1) Magnitude of stacking faults were less than 1%, which meant the Co films had almost perfect hcp stacking, 2) Lattice constant c was expanded about 0.6% with retaining lattice constant a, 3) Thin Co films had rough surface which reflected morphology of the Ru underlayer. According to these results, it was thought that initial Co growth reflected vertical stacking of Ru underlayer on inclined crystal plane of the rough Ru underlayer. Ku was derived by correcting torque amplitude with self-demagnetizing energy considering surface morphology of Co film. Ku took maximal value of 5.0×106 erg/cm3 at dCo = 30 nm, and rapidly decreased with decreasing dCo. Reduction of Ku in dCo ＜ 30 nm might be caused by decrement of crystallographic uniaxial anisotropy for Co film due to expansion of the c axis.

Artificially Stacked Atomic Layers: Toward New van der Waals Solids

Strong in-plane bonding and weak van der Waals interplanar interactions characterize a large number of layered materials, as epitomized by graphite. The advent of graphene (G), individual layers from graphite, and atomic layers isolated from a few other van der Waals bonded layered compounds has enabled the ability to pick, place, and stack atomic layers of arbitrary compositions and build unique layered materials, which would be otherwise impossible to synthesize via other known techniques. Here we demonstrate this concept for solids consisting of randomly stacked layers of graphene and hexagonal boron nitride (h-BN). Dispersions of exfoliated h-BN layers and graphene have been prepared by liquid phase exfoliation methods and mixed, in various concentrations, to create artificially stacked h-BN/G solids. These van der Waals stacked hybrid solid materials show interesting electrical, mechanical, and optical properties distinctly different from their starting parent layers. From extensive first principle calculations we identify (i) a novel approach to control the dipole at the h-BN/G interface by properly sandwiching or sliding layers of h-BN and graphene, and (ii) a way to inject carriers in graphene upon UV excitations of the Frenkell-like excitons of the h-BN layer(s). Our combined approach could be used to create artificial materials, made predominantly from inter planar van der Waals stacking of robust bond saturated atomic layers of different solids with vastly different properties.

Uniaxial magnetocrystalline anisotropy for c-plane oriented Co100−xMx (M: Cr, Mo, W) film with stacking faults

Stacking faults (SFs) in Co-based alloy grains in a Co100−xMx (M: Cr, Mo, and W) film are evaluated by means of in-plane x-ray diffraction. Moreover, the correlation between SFs and uniaxial magnetocrystalline anisotropy Ku is discussed in connection with the spin-orbit interaction. The ratio of the integrated intensities of the (10.0) to (11.0) diffractions corrected by Lorentz and atomic scattering factors has been proposed as an index for SFs in hcp films with a c-plane sheet texture. This ratio is equal to 0.25 for perfect hcp stacking, while it is 0 for perfect fcc specific stacking. It has a one-to-one correspondence with the probability of -A-B-C- atomic-layer stacking Pfcc. Using this index, pure sputtered Co films are found to have a Pfcc of 10%. The addition of only 5 at. % of Mo or W into the Co grains reduces Pfcc to 2%. Ku was found to increase with the addition of material (e.g., Ku was 4.0×106 ergs/cm3 for 5 at. % Mo), although the atomic magnetic moment of Co decreases monotonously. A Pfcc of 10% is found to lower Ku in a pure Co film by more than a factor of 2 when the spin-orbit interaction is taken into account.

Surface tension of liquid binary alloys – theory versus experiment

Abstract In the present study, we compare available experimental surface tension data with two theoretical concepts: the Butler equation and a multilayer model in which the surface is described as a stacking of atomic layers. The experimental data were measured systematically over a wide temperature and concentration range using electromagnetic levitation and the oscillating drop technique. Three classes of alloys are studied: alloys with rather weak or no segregation, alloys with strong segregation, and associate forming alloys.

Synthesis, crystal structures and ionic conductivities of Bi 14P 4O 31 and Bi 50V 4O 85. Two members of the series Bi 18−4mM 4mO 27+4m ( M=P, V) related to the fluorite-type structure

The two hitherto unknown compounds Bi 14P 4O 31 and Bi 50V 4O 85 were prepared by the direct solid-state reaction of Bi 2O 3 and (NH 4)H 2PO 4 or V 2O 5, respectively. Bi 14P 4O 31 crystallizes in a C-centred monoclinic symmetry ( C2/ c space group) with the unit-cell parameters: a = 19.2745 ( 2 ) Å , b = 11.3698 ( 1 ) Å , c = 52.4082 ( 2 ) Å and β = 93.63 ( 1 ) ° ( Z = 16 ). The symmetry of Bi 50V 4O 85 is also monoclinic ( I2/ m space group) with lattice parameters of a = 11.8123 ( 3 ) Å , b = 11.7425 ( 2 ) Å , c = 16.5396 ( 2 ) Å and β = 90.14 ( 1 ) ° ( Z = 2 ). Both structures correspond to a fluorite-type superstructure where the Bi and P or V atoms are ordered in the framework. An idealized structural model is proposed where the structures result of the stacking of mixed atomic layers of composition [Bi 14 M 4O 31] and [Bi 18O 27] respectively. This new family can be formulated Bi 18−4 mM 4 m O 27+4 m with M=P, V and where the parameter m ( 0 ⩽ m ⩽ 1 ) represents the ratio of the number of [Bi 14 M 4O 31] layers to the total number of layers in the sequence. Bi 14P 4O 31 corresponds to m = 1 when Bi 50V 8O 85 corresponds to m = 1 / 3 . In this last case, the structural sequence is simply one [Bi 14V 4O 31] layer to two [Bi 18O 27] layers. As predicted by the proposed structural building principle, Bi 14P 4O 31 is not a good ionic conductor. The conductivity at 650 °C is 4 orders of magnitude lower from those found in Bi 46 M 8O 89 ( M=P, V) ( m = 2 / 3 ) and Bi 50V 4O 85 ( m = 1 / 3 ).

Control of interlayer spacing in(Pb2Cu)Sr2(Dy,Ce)nCu2O6+2n−z/(Pb2Cu)Sr2Dy1−yCayCu2O8+wsuperconducting superlattices

We report on superconducting superlattices in which the distances between the superconducting layers are controlled at the shortest intervals so far achieved. This was accomplished by the artificial modification of an atomic layer stacking in the crystal structure of (Pb 2 Cu)Sr 2 Dy x Ce n - x Cu 2 O 2 n + 6 (Pb-32n2 phase: n=3-8), utilizing the tendency to self-organize a layered structure. Ah initio electronic structure calculations for the Pb-32n2 phase with n=1-3 suggest that the anisotropy of conduction increases with n. In [(Pb-32n2) 1 (Pb-3212) 3 ] 9 (n=3, 4, 6) superconducting superlattices, the activation energy of flux motion decreases with increasing n. The above observations and the change in the shape of resistive transitions suggest that the interlayer coupling decreases with increasing the distances between the superconducting layers, as expected. We discuss coupling mechanisms.

DOUBLE ATOMIC LAYERS OF GRAPHENE/MONOLAYER h-BN ON Ni(111) STUDIED BY SCANNING TUNNELING MICROSCOPY AND SCANNING TUNNELING SPECTROSCOPY

Using STM, we have directly confirmed the incommensurate stacking of double atomic layers of graphene and monolayer h-BN on Ni(111). The formation of a graphene layer weakens the interfacial interaction between monolayer h-BN and Ni(111), resulting in insulating h-BN layers, while a pristine monolayer h-BN on Ni(111) is metallic. The STS spectra of the double atomic layers showed a tunneling character with a band gap of 0.5 eV.

Artificial construction of SrO/(La, Sr)MnO3 layered perovskite superlattice by laser molecular-beam epitaxy

Epitaxial thin films of layered perovskite (La, Sr)3Mn2O7 have been artificially grown by atomic-layer stacking of SrO and (La, Sr)MnO3 [artificial SrO/(La, Sr)MnO3 superlattice] using laser molecular-beam epitaxy on an SrTiO3(001) substrate at low processing temperature. Reflection high-energy electron diffraction patterns indicate the flat surface of the films and the layer-by-layer growth mode. X-ray diffraction patterns confirm that c-axis-oriented Ruddlesden–Popper layered (La, Sr)3Mn2O7 thin films can be grown at a substrate temperature as low as 580 °C. The resulting film exhibited ferromagnetism with a Curie temperature of 75 K.

Growth of AlN and GaN thin films via OMVPE and gas source MBE and their characterization

Thin films of AlN and GaN are deposited primarily via the common forms of organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE). Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AlN and GaN primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces inversion domain boundaries in the AlN at the SiC steps; this sequence problem may discourage the nucleation of GaN. Films of AlN and GaN grown by MBE at 650°C are textured; monocrystalline films are achieved at 1050°C by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed.

The crystal structure of YbFe2Al10, a combined substitution and stacking variant of the ThMn12 and CeMn4Al8 type structures

Abstract YbFe2Al10 crystallizes with a new structure type, which was determined from single-crystal X-ray data: Cmcm, a = 896.6(1)pm, b = 1015.3(2)pm, c = 900.3(1) pm, Z = 4, R = 0.024 for 795 structure factors and 41 variable parameters. The structure is closely related to the tetragonal ThMn12 and CeMn4Al8 type structures, from which it can be derived by substitution of some manganese by aluminum atoms, and in addition it has a different stacking of atomic layers. The ytterbium atoms have 20 neighbors (4 Fe + 16 Al), the iron atoms have distorted icosahedral coordination (2 Yb + 10 Al), and all of the five different aluminum sites are coordinated by ytterbium, iron, and aluminum atoms with coordination numbers 12, 13, and 14.

Analysis of voltage periodic structures in the negative differential resistance region of double barrier resonant tunneling diodes

We present a comprehensive investigation of the Periodic Fine Structures (PFS's). These PFS's are observed in the Negative Differential Resistance (NDR) region of double-barrier resonant-tunneling diodes (DBRTD's). DBRTD's were fabricated on MBE material with different well widths and barrier material as well as different device areas and topologies. The voltage periodicity, frequency and the amplitude of the PFS's in the NDR region were studied as a function of these materials and the device parameters. The effect of self-oscillations of the DBRTD's on the PFS's was also investigated. The results indicate that the periodicity of the PFS's is associated with a reduction in the well width which gives rise to a periodic shift in the resonance conditions in the well. We propose that the narrowing of the well is due to a roughness of the well-barrier junction which is incremental in steps of a fraction of the lattice parameter in the [100] direction of crystal growth. We propose that this two-dimensional disorder is due to random stacking of atomic layers during growth of the diode active layers and it is specifically due to continued growth at the barrier and barrier walls when growth interruption takes place at the barrier and well junctions.

Issues and Examples Regarding Growth of AlN, GaN and AlxGa1−xN Thin Films via OMVPE and Gas Source MBE

ABSTRACTOrganometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE) are the most common methods for the growth of thin films of A1N and GaN. Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AIN, GaN and AlxGa1−xN thin films primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces double positioning boundaries in A1N and the alloys at the SiC steps; this sequence problem appears to discourage the two-dimensional nucleation of GaN. Films of these materials grown by MBE at 650°C are textured; monocrystalline films are achieved between 850°C (pure GaN) and 1050°C (pure A1N) by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed. Selected issues surrounding the growth of these materials with particular examples drawn from the authors' research are presented herein.

Calculated X‐ray Diffraction Data for Diamond Polytypes

Calculated X‐ray diffraction pattern data for diamond polytypes are presented, which provides the required parameters for the characterization of the proposed diamond polytypes. The literature on diamond polytypes contains some small but significant errors with respect to the details of the crystal structure, including space groups, the atom positions in the unit cell, and the atomic layer stacking. In this paper, the crystal structures of four different hexagonal diamond and two different rhombohedral diamond polytypes are theoretically calculated to provide correct crystallographic information with which the diamond polytypes in a given sample can be characterized.