Anisotropy analysis of the surface energy of hcp (c/a < 1.633) metals

In this paper, anisotropy of the surface energy of 5 hcp metals Be, Hf, Ru, Ti and Y have been analyzed. The surface energies of three kinds of representative surfaces, (h 0 l), (h h l) and (h k 0) belong to [0 1 0], [] and [0 0 1] crystal band, respectively, have been calculated using the modified embedded atom method. For all 5 hcp metals, the (1 1 0) plane has the minimum surface energy in all 35 surfaces studied. Considering surface energy minimization solely, the (1 1 0) texture should be favorable in the hcp films. The fact that the short termination corresponds to much lower surface energy than long one implies the former is more stable for those surfaces having two possible terminations. Such as the prism plane (1 0 0), only the short termination was observed in experiment.     

Young's modulus surface and Poisson's ratio curve for cubic metals

The general expressions for the compliance , Young's modulus E(h k l) and Poisson's ratio υ(h k l, θ) along arbitrary loading direction [h k l] are given for cubic crystals. The representation surface for which the length of the radius vector in the [h k l] direction equals to E(h k l) and representation curve for which the length of the radius vector with angle θ deviated from the reference directions , and equals to υ(100, θ), υ(110, θ) and υ(111, θ) for example, are constructed for six FCC metals Ag, Al, Au, Cu, Ni, Pb and seven BCC metals Cr, Fe, Mo, Nb, Ta, V, W.     

Generalized-stacking-fault energy and surface properties for HCP metals: A first-principles study

We present first-principles calculations on the generalized-stacking-fault (GSF) energies and surface properties for several HCP metals on Mg, Be, Ti, Zn, and Zr, employing density functional theory (DFT) within generalized-gradient-approximation (GGA) and spin-polarized GGA (SGGA) using the Vienna ab initio simulation package (VASP). Using a supercell approach, stacking fault energies for the [1 1  0] and [1 0  0] slip systems, and surface properties on basal plane (0 0 0 1) have been determined. Our results show that GSF energy is sensitive to the primitive cell volumes and the ratio c/a for HCP metals. A spin-polarized calculations should be considered for transition-metal Ti, Zn, and Zr. The results for Mg from this work are good with ones from the previous ab initio and the experiments.     

Ab initio calculations of generalized-stacking-fault energy surfaces and surface energies for FCC metals

The ab initio calculations have been used to study the generalized-stacking-fault energy (GSFE) surfaces and surface energies for the closed-packed (1 1 1) plane in FCC metals Cu, Ag, Au, Ni, Al, Rh, Ir, Pd, Pt, and Pb. The GSFE curves along (1 1 1) direction and (1 1 1) direction, and surface energies have been calculated from first principles. Based on the translational symmetry of the GSFE surfaces, the fitted expressions have been obtained from the Fourier series. Our results of the GSFEs and surface energies agree better with experimental results. The metals Al, Pd, and Pt have low γ_us/γ_I value, so full dislocation will be observed easily; while Cu, Ag, Au, and Ni have large γ_us/γ_I value, so it is preferred to create partial dislocation. From the calculations of surface energies, it is confirmed that the VIII column elements Ni, Rh, Ir, Pd, and Pt have higher surface energies than other metals.     

GaN/LiNbO3 (0 0 0 1) interface formation calculated from first-principles

The stable adsorption sites for both Ga and N ions on the ideal and on the reconstructed LiNbO₃ (0 0 0 1) surface are determined by means of first-principle total energy calculations. A single N layer is found to be more strongly bound to the substrate than a single Ga layer. The adsorption of a GaN monolayer on the polar substrate within different orientations is then modeled. On the basis of our results, we propose a microscopic model for the GaN/LiNbO₃ interface. The GaN and LiNbO₃ (0 0 0 1) planes are parallel, but rotated by 30° each other, with in-plane epitaxial relationship [1 0 0]_GaN‖ [1 1  0]_LiNbO3. In this way the (0 0 0 1) plane lattice mismatch between GaN and LiNbO₃ is minimal and equal to 6.9% of the GaN lattice constant. The adsorbed GaN and the underlying LiNbO₃ substrate have parallel c-axes.     

Surface defect states of CdTexS1 − x quantum dots

Properties of surface defect states of CdTe_xS_1 − x quantum dots with an average diameter of 7 nm are investigated experimentally. The stoichiometric ratio is found to be for by use of the energy dispersive analysis of x-ray. The photoluminescence spectrum, the photoluminescence excitation spectrum, and the surface passivation are adopted to characterize the properties of surface defect states. The energy levels of surface defect states of CdTe_xS_1 − x quantum dots are also determined.     

Giant optical anisotropy in M-plane GaN/AlGaN quantum wells due to crystal-field effect

The optical polarization of GaN/AlGaN wurtzite quantum wells in various orientations is studied using an arbitrarily-oriented [hkil] Hamiltonian potential matrix. The optical matrix elements in the wurtzite quantum wells are calculated using the k⋅p finite difference scheme. The results reveal the presence of giant in-plane optical anisotropy (polarized normal to [0001]) in the M-plane (i.e., the -oriented layer plane) GaN/Al_0.2Ga_0.8N quantum well, due to the positive crystal-field split energy effect (ΔCR>0). The present theoretical results are consistent with the photoluminescence measurements presented in the literature [B. Rau, et al., Appl. Phys. Lett. 77 (2000) 3343].     

Surface segregation energies in low-index open surfaces of bimetallic transition metal alloys

We present a database of 24 × 24 segregation energies of single transition metal impurities in low-index surfaces of transition metal hosts, calculated using the localized self-consistent Green’s function (LSGF) method, in combination with the atomic sphere approximation including a multipole correction to the electrostatic potential and energy. The surface energies of facets for fcc and bcc transition metals, and the more stable of the two facets of hcp transition metals are also calculated and compared with available theoretical results. Insights derived should be useful for determining the nature of active sites in a variety of catalytic reactions employing bimetallic catalysts.     

The v6 = 1 and v6 = 2 vibrational states of DCF3

The high-resolution infrared spectra of DCF₃ were reinvestigated in the ν₆ fundamental band region near 500 cm⁻¹ and around 1000 cm⁻¹ with the aim to assign and analyze the overtone level of the asymmetric CF₃ bending vibration v₆ = 2.The present paper reports on the first study of both its sublevels (A₁ and E corresponding to l = 0 and ±2, respectively) through the high-resolution analysis of the overtone band and the hot and bands.The well-known “loop method”, applied to and , yielded ground state energy differences Δ(K, J) = E₀(K, J) − E₀(K − 3,J) for the range of K = 6 to 30.In the final fitting of molecular parameters, we used the strategy of fitting all upper state data together with the ground state rotational transitions.This is equivalent to that calculating separately the and coefficients of the K-dependent part of the ground state energy terms from the combination loops.All rotational constants of the ground state up to sextic order could be refined in the calculation.This led to a very accurate determination of C₀ = 0.18924413(25) cm⁻¹, , and also .In the course of analyzing simultaneously the overtone band together with the and ν₆ bands, the original assignment of the fundamental ν₆ band [Bürger et al., J. Mol. Spectrosc. 182 (1997) 34-49] was found to be incompatible with the present one. Assignments of the (k + 1, l₆ = +1)/(k − 1,l₆ = −1) levels had to be interchanged, which changed the value of Cζ₆ = −0.14198768(26) cm⁻¹ and the sign of the combination of constants C − B − Cζ in the v₆ = 1 level to a negative value.     

Surface reconstructions on InN and GaN polar and nonpolar surfaces

We report a systematic and comprehensive computational study of surface reconstructions on GaN and InN surfaces in various orientations, including the polar c plane as well as the nonpolar a and m planes. For GaN we have identified several new metallic reconstructions under highly Ga-rich conditions on the nonpolar planes. For InN we find several distinct differences from the GaN case: the absence of a nitrogen-adatom reconstruction on the (0 0 0 1) plane; the presence of a single, metallic reconstruction over the entire stability range on the plane; and In-adlayer reconstructions on the (m) plane. An interesting “inverted polarity” defect structure on the (m) plane is also revealed.     

Microstructure of epitaxial scandium nitride films grown on silicon

Epitaxial scandium nitride films (225 nm thick) were grown on (1 1 1)-oriented silicon substrates by molecular beam epitaxy (MBE), using ammonia as a reactive nitrogen source. Film microstructure was investigated using X-ray diffraction (XRD). The (1 1 1) ω-scan FWHM of 0.551° obtained for films grown at 850 °C is the lowest reported so far for ScN thin films. The principal orientation of ScN with respect to Si is (1 1 1)_ScN//(1 1 1)_Si and []_ScN//[]_Si, representing a 60° in-plane rotation of the ScN layer with respect to the Si substrate. However, some twinning is also present in the films; the orientation of the twinned component is (1 1 1)_ScN//(1 1 1)_Si and []_ScN//[]_Si, representing a ‘cube-on-cube’ orientation. The volume percentage of these twins in the films decreases with increasing film growth temperature.     

Anisotropy analysis of the surface stress and surface energy in Cu surfaces with the modified embedded atom method

Taking Cu as an example, the surface stress and surface energy in three low index surfaces and two families of representative surfaces and belong to [0 0 1]- and -rotating axis respectively, have been calculated using MEAM. For the three low index surfaces, the decrease in the surface energy is small after relaxation, while the surface stresses in the surface planes τ_xx and τ_yy show opposite changes (decreasing and increasing) for inward and outward relaxations. The resulting relaxation direction is related to the normal stress τ_zz before relaxation. For the surfaces of the and families, with the increasing angle α (between the and (1 0 0) planes, and between and (0 0 1) planes, respectively), the surface stress and surface energy go through an oscillatory change. The surface stress and surface energy are symmetric about the planes (1 0 0), (1 1 0) and (0 1 0) at α=0^°, 45^° and 90^°, and about the planes (0 0 1) and (1 1 0) at α=0^° and 90^° respectively, due to crystal symmetry.     

Theoretical study of CO and Pb adsorption on the Ni(1 1 1) and Ni3Al(1 1 1) surfaces

Adsorption of CO molecules and Pb atoms on the Ni(1 1 1) and Ni₃Al(1 1 1) substrates is studied theoretically within an ab initio density-functional-theory approach. Stable adsorption sites and the corresponding adsorption energies are first determined for stoichiometric surfaces. The three-fold hollow sites (fcc for Pb and hcp for CO) are found most favourable on both substrates. Next, the effect of surface alloying by a substitution of selected topmost substrate atoms by Pb or Ni atoms on the adsorption characteristics is investigated. When the surface Al atoms of the Ni₃Al(1 1 1) substrate are replaced by Ni atoms, the Pb and CO adsorption energies approach those for a pure Ni(1 1 1) substrate. The Pb alloying has a more substantial effect. On the Ni₃Al(1 1 1) substrate, it reduces considerably adsorption energy of CO. On the Ni(1 1 1) substrate, CO binding strengthens slightly upon the formation of the Ni(1 1 1)p(2×2)-Pb surface alloy, whereas it weakens drastically when the Ni(1 1 1)-Pb surface alloy is formed.     

Stable surface termination on vicinal 6H-SiC(0 0 0 1) surfaces

Ordered nanofacet structures on vicinal 6H-SiC(0 0 0 1) surfaces, consisting of pairs of a (0 0 0 1) basal plane and a facet, are investigated in terms of stable surface stacking of the (0 0 0 1) basal planes. The surface termination of S3 (or S3*), i.e., ABC (or A*C*B*), was suggested by a structural model based on quantized step-bunching, which typically gives a one-unit-cell bunched step configuration at the facet. Here, we evaluate the surface termination at basal planes covered with a layer of silicon oxynitride by means of quantitative low-energy electron diffraction (LEED) analysis combined with scanning tunneling microscopy (STM), and show the validity of the structural model proposed.     

Electro-optic behavior of lithium niobate at cryogenic temperatures

The unclamped relative permittivity, , and the Pockels coefficient, , of congruent lithium niobate at a frequency f = 5760 Hz have been determined at low temperatures (7 K < T < 300 K). A He cryostat setup mounted to one arm of an electronically phase-stabilized Michelson interferometer was utilized for the measurement of . A continuous decrease in both parameters was observed as T → 0 K with limiting values of and , respectively.     

Modelling of phase transitions and reaction at CO adsorption on oxygen precovered Pd(1 1 1)

Using the interaction parameters up to the third neighbors and activated form of O and CO diffusion and their reaction, the model has been proposed for Monte-Carlo simulations describing the catalytic O + CO → CO₂ reaction and occurring phase transitions on Pd(1 1 1) surface. Upon adsorption of CO the pre-adsorbed oxygen transforms from p(2 × 2)_O phase into and phases in the limit of room and moderate temperatures, respectively. We demonstrate that the kinetic effects determine both the occurrence of the p(2 × 1)_O and disappearance of the phases at moderate and low temperatures, respectively. Using reaction rate as a fit parameter, we show that at room temperature the start of the reaction can be synchronized with the occurrence of phase.