Calculation of the surface energy of bcc transition metals by using the second nearest-neighbor modified embedded atom method

The surface energies for 24 surfaces of all bcc transition metals Fe, Cr, Mo, W, V, Nb and Ta have been calculated by using the second nearest-neighbor modified embedded atom method. The results show that, for all bcc transition metals, the order among three low-index surface energies E_(1 1 0) < E_(1 0 0) < E_(1 1 1) is in agreement with experimental results and E_(1 1 0) is also the lowest surface energy for various surfaces. So that from surface energy minimization, the (1 1 0) texture should be favorable in the bcc films. This is also consistent with experimental results. The surface energy for the other surfaces increases linearly with increasing angle between the surfaces (h k l) and (1 1 0). Therefore, a deviation of a surface orientation from (1 1 0) can be used to estimate the relative values of the surface energy.     

Calculation of the surface energy of bcc-metals with the empirical electron theory

We have used the dangling bond analysis method (DBAM) based on the empirical electron theory (EET) to establish a database of surface energy for low index surfaces of the bcc-metals such as V, Cr, Fe, Nb, Mo, Ta, and W. And a brief introduction of the new surface energy models will be presented in this paper. Under the first-order approximation the calculated results are in agreement with experimental and other theoretical values. And the calculated surface energy shows a strong anisotropy. As we predicted, the surface energy of the close-packed (1 1 0) is the lowest one of all index surfaces. It is also found that the dangling bond electron density and the spatial distribution of covalent bonds have a great influence on surface energy of various index surfaces. The new calculation method for the research of surface energy provides a good basis for models of surface science phenomena, and the model may be extended to the surface energy estimation of more metals, alloys, ceramics, and so on, since abundant information about the valence electronic structure (VES) can be generated from EET.     

A density functional theory study of formaldehyde adsorption and oxidation on CeO2(1 1 1) surface

The effects of different oxygen species and vacancies on the adsorption and oxidation of formaldehyde over CeO₂(1 1 1) surface were systematically investigated by using density functional theory (DFT) method. On the stoichiometric CeO₂(1 1 1) surface, the C-H bond rupture barriers of chemisorbed formaldehyde are much higher than that of formaldehyde desorption. On the reduced CeO₂(1 1 1) surface, the energy barriers of C-H bond ruptures are less than those on the stoichiometric CeO₂(1 1 1) surface. If the C-H bond rupture occurs, CO and H₂ form quickly with low energy barriers. When O₂ adsorbs on the reduced (1 1 1) surface (O₂/O_v species), the C-H bond rupture barriers of formaldehyde are greatly reduced in comparison with those on the stoichiometric CeO₂(1 1 1) surface. If O₂ adsorbs on oxygen vacancy at sub-layer surface, its oxidative roles on formaldehyde are much similar to that of O₂/O_v species.     

Ga-induced superstructures on the Si(1 1 1) 7 × 7 surface

Monolayer Ga adsorption on Si surfaces has been studied with the aim of forming p-delta doped nanostructures. Ga surface phases on Si can be nitrided by N₂⁺ ion bombardment to form GaN nanostructures with exotic electron confinement properties for novel optoelectronic devices. In this study, we report the adsorption of Ga in the submonolayer regime on 7 × 7 reconstructed Si(1 1 1) surface at room temperature, under controlled ultrahigh vacuum conditions. We use in-situ Auger electron spectroscopy, electron energy loss spectroscopy and low energy electron diffraction to monitor the growth and determine the properties. We observe that Ga grows in the Stranski-Krastanov growth mode, where islands begin to form on two flat monolayers. The variation in the dangling bond density is observed during the interface evolution by monitoring the Si (LVV) line shape. The Ga adsorbed system is subjected to thermal annealing and the residual thermal desorption studied. The difference in the adsorption kinetics and desorption dynamics on the surface morphology is explained in terms of strain relaxation routes and bonding configurations. Due to the presence of an energetic hierarchy of residence sites of adatoms, site we also plot a 2D phase diagram consisting of several surface phases. Our EELS results show that the electronic properties of the surface phases are unique to their respective structural arrangement.     

The anomalous exhibition of GSF energy at surface of fcc metals

The generalized stacking fault (GSF) energy curves for (1 1 1) surface of fcc metals are calculated by the second nearest-neighbor modified embedded atom method (2NN-MEAM), in order to investigate the deformation mechanism of (1 1 1) surface. Except the energy reduce for all these metals, strange energy curves are found for Au, Pd and Pt, especially for Au. Combining the surface GSF energy data and the experimental results, we find that the deformation mechanism should be explained by not only the values of the stable stacking fault energy γ_sf and unstable stacking fault energy γ_usf, but the whole shape of a metal’s energy curve.     

An energetic criterion to compare the evolution of thermally-excited surface disturbances and nanoindentation-induced defects on thin polymer films

A criterion is developed to predict the resulting evolution process of the following surface defects on thin (17 nm) polystyrene (PS) films on silicon (Si): (i) nanoindentation-induced indents which grow after being heated above the glass transition temperature of PS, T_g, leading to dewetting; (ii) nanoindentation-induced indents which level at temperatures above the T_g, resulting in a flat polymer surface and (iii) indents which are formed and grow spontaneously by thermal treatment above the T_g (thermal film break up). The criterion is based on the concept of the excess surface energy, ΔF_γ, which was introduced in previous reports for cases (i) and (ii). Here, a similar energetic term is used which corresponds only to the effect of the depressions, ΔF_γ(D). The effect of the rims which surround the depressions in cases (i) and (ii) is not taken into account. Measurements of ΔF_γ(D), performed by atomic force microscopy, prior to any treatment above the T_g suggest that growing depressions (cases i and iii) correspond to ΔF_γ(D) > 1.5 × 10⁻¹⁶ J while for healing depressions (case ii) ΔF_γ(D) < 1.8 × 10⁻¹⁶ J. A critical region of ΔF_γ(D) exists from 1.5 × 10⁻¹⁶ J to 1.8 × 10⁻¹⁶ J. Depressions which correspond to this, rather short, region can either grow or heal.     

Vibrational entropy-driven dealloying of Mo(1 0 0) and W(1 0 0) surface alloys

The formation and stability of Cu, Ag and Au-induced c(2 × 2) alloys at the Mo(1 0 0) and W(1 0 0) surfaces have been investigated with low-energy electron microscopy and diffraction. The ordered alloys transform to disordered overlayer structures at elevated temperature. Comparison of the transformation temperatures with energetics obtained from first principles calculations reveals the vibrational entropic contribution to the system free energy that defines alloy thermal stability. Effective Debye temperatures for metal adatoms are determined that exhibit the expected mass and bond strength dependence.     

Bulk and surface properties of spinel Co3O4 by density functional calculations

DFT calculations are employed to bulk and surface properties of spinel oxide Co₃O₄. The bulk magnetic structure is calculated to be antiferromagnetic, with a Co²⁺ moment of 2.631 μ_B in the antiferromagnetic state. There are three predicted electron transitions O(2p) → Co²⁺(t_2g) of 2.2 eV, O(2p) → Co³⁺(e_g) of 2.9 eV and Co³⁺(t_2g) → Co²⁺(t_2g) of 3.3 eV, and the former two transitions are close to the corresponding experimental values 2.8 and 2.4 eV. The naturally occurring Co₃O₄ (1 1 0) and (1 1 1) surfaces were considered for surface calculations. For ideal Co₃O₄ (1 1 0) surfaces, the surface relaxations are not significant, while for ideal Co₃O₄ (1 1 1) surfaces the relaxation of Co²⁺ cations in the tetrahedral sites is drastic, which agrees with the experiment observation. The stability over different oxygen environments for possible ideal and defect surface terminations were explored.     

Interaction of CO with surface PdZn alloys

The adsorption and bonding configuration of CO on clean and Zn-covered Pd(1 1 1) surfaces was studied using low energy electron diffraction (LEED), temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). LEED and TPD results indicate that annealing at 550 K is sufficient to induce reaction between adsorbed Zn atoms and the Pd(1 1 1) surface resulting in the formation of an ordered surface PdZn alloy. Carbon monoxide was found to bond more weakly to the Zn/Pd(1 1 1) alloy surfaces compared to clean Pd(1 1 1). Zn addition was also found to alter the preferred adsorption sites for CO from threefold hollow to atop sites. Similar behavior was observed for supported Pd-Zn/Al₂O₃ catalysts. The results of this study show that both ensemble and electronic effects play a role in how Zn alters the interactions of CO with the surface.     

Phosphine and tertiarybutylphosphine adsorption on the indium-rich InP (0 0 1)-(2 × 4) surface

Phosphine and tertiarybutylphosphine adsorption on the indium-rich InP (0 0 1)-(2 × 4) surface at 25 °C have been studied by internal reflection infrared spectroscopy, X-ray photoelectron spectroscopy, and low energy electron diffraction. Both molecules form a dative bond to the empty dangling bonds on the In-P heterodimers and the second-layer In-In dimers and vibrate symmetrically at 2319 (2315) and 2285 (2281) cm⁻¹ and asymmetrically at 2339 (2339) and 2327 (2323) cm⁻¹. A fraction of these species dissociate into adsorbed PH₂ with the hydrogen and tertiarybutyl ligands transferring to nearby phosphorus sites. The calculated energy barriers for desorption (<11 kcal/mol) of these molecules is less than that for dissociation (>17 kcal/mol) and explains their low sticking probabilities at elevated temperatures under InP growth conditions.     

Theoretical and experimental studies of the structure and dynamics of the CaF2(1 1 1) surface

The structure and dynamics of the CaF₂(1 1 1) surface were investigated by means of low-energy electron diffraction (LEED) and molecular dynamics (MD) simulations at 300 K. LEED beam intensities were recorded as a function of electron energy and were analyzed with the tensor LEED approach. Positions as well as mean square amplitudes of the ions in the first layers were fitted to the experimental I(E) curves. According to both LEED and MD, the CaF₂(1 1 1) surface structure is similar to the bulk-terminated structure with only small relaxation of the outermost ions. Moreover, both methods show an enhancement of vibrational amplitudes in the outermost F-Ca-F triple layer.     

The structure of the V2O3(0 0 0 1) surface: A scanned-energy mode photoelectron diffraction study

Scanned-energy mode photoelectron diffraction (PhD), using the O 1s and V 2p photoemission signals, together with multiple-scattering simulations, have been used to investigate the structure of the V₂O₃(0 0 0 1) surface. The results support a strongly-relaxed half-metal termination of the bulk, similar to that found in earlier studies of Al₂O₃(0 0 0 1) and Cr₂O₃(0 0 0 1) surfaces based on low energy electron and surface X-ray diffraction methods. However, the PhD investigation fails to provide definitive evidence for the presence or absence of surface vanadyl (VO) species associated with atop O atoms on the surface layer of V atoms. Specifically, the best-fit structure does not include these vanadyl species, although an alternative model with similar relaxations but including vanadyl O atoms yields a reliability-factor within the variance of that of the best-fit structure.     

Ab initio studies of hydrogen desorption from low index magnesium hydride surface

The low index Magnesium hydride surfaces, MgH₂(0 0 1) and MgH₂(1 1 0), have been studied by ab intio Density Functional Theory (DFT) calculations. It was found that the MgH₂(1 1 0) surface is more stable than MgH₂(0 0 1) surface, which is in good agreement with the experimental observation. The H₂ desorption barriers vary depending on the crystalline surfaces that are exposed and also the specific H atom sites involved - they are found to be generally high, due to the thermodynamic stability of the MgH₂ system, and are larger for the MgH₂(0 0 1) surface. The pathway for recombinative desorption of one in-plane and one bridging H atom from the MgH₂(1 1 0) surface was found to be the lowest energy barrier amongst those computed (172 KJ/mol) and is in good agreement with the experimental estimates.     

The role of adsorption of dodecylethyldimethylammonium bromide and benzyldimethyldodecylammonium bromide surfactants in wetting of polytetrafluoroethylene and poly(methyl methacrylate) surfaces

The role of adsorption of dodecylethyldimethylammonium bromide (C₁₂(EDMAB)) and benzyldimethyldodecylammonium bromide (BDDAB) at water-air and polytetrafluoroethylene (PTFE)-water and poly(methyl methacrylate) (PMMA)-water interface, in wetting of PTFE and PMMA surface, was established from the measured values of the contact angle (θ) of aqueous C₁₂(EDMAB) and BDDAB solutions in PTFE (PMMA)-solution drop-air system, and from the measured values of the surface tension of aqueous C₁₂(EDMAB) and BDDAB solutions. Adsorption of C₁₂(EDMAB) and BDDAB at water-air interface was determined earlier from the Gibbs equation. Adsorption at solid-water interface was deduced from the Lucassen-Reynders equation based on the relationship between adhesion tension (γ_LV cos θ) and surface tension (γ_LV). The slope of the γ_LV cos θ-γ_LV curve was found to be constant and equal to −1, and about −0.3 for PTFE and PMMA surface, respectively (in the case of both surfactant studied: C₁₂(EDMAB) and BDDAB, and in the whole range of surfactants concentration in solution). It means that the amount of the surfactant adsorbed at the PTFE-water interface, Γ_SL, was essentially equal to its amount adsorbed at water-air interface, Γ_LV. However, Γ_SL at the PMMA-water interface was about three times smaller as compared to that at water-air interface. By extrapolating the linear dependence between γ_LV cos θ-γ_LV and dependence between cos θ-γ_LV and cos θ = 1 we determined the value of the critical surface tension of PTFE and PMMA surface wetting, γ_c. The obtained values of γ_c for PTFE surface were equal 23.4 and 23.8 mN/m, 23.1 and 23.2 mN/m for C₁₂(EDMAB) and BDDAB, respectively and they were higher than the surface tension of PTFE (20.24 mN/m). On the other hand, the obtained values of γ_c for PMMA surface were equal 31.4 and 30.9 mN/m, 31.7 and 31.3 mN/m for C₁₂(EDMAB) and BDDAB, respectively and they were smaller than the surface tension of PMMA (39.21 mN/m). Using the values of PTFE and PMMA surface tension and the measured values of the surface tension of aqueous C₁₂(EDMAB) and BDDAB solutions in the Young equation, the PTFE (PMMA)-solution interfacial tension, γ_SL, was also determined. Next, the work of adhesion (W_A) was deduced, and it occurred that the dependence between the W_A and the surface tension (γ_LV) for both studied solids was linear. However, the values of the W_A for PMMA change as a function of log C (C—surfactant concentration) changed from 91.7 to 68.5 mJ/m² and from 91.8 to 65.1 mJ/m² for C₁₂(EDMAB) and BDDAB, respectively. On the other hand, the work of adhesion of both studied surfactants solutions to the PTFE surface was practically constant (an average value was equal 45.8 and 45.4 mJ/m², respectively). These values were close to the value of the work of water adhesion to PTFE surface (45.5 mJ/m²).     

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.     

Ultra-thin Si overlayers on the TiO2 (1 1 0)-(1 × 2) surface: Growth mode and electronic properties

The growth of thin subnanometric silicon films on TiO₂ (1 1 0)-(1 × 2) reconstructed surfaces at room temperature (RT) has been studied in situ by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS), Auger electron and electron-energy-loss spectroscopies (AES and ELS), quantitative low energy electron diffraction (LEED-IV), and scanning tunneling microscopy (STM). For Si coverage up to one monolayer, a heterogeneous layer is formed. Its composition consists of a mixture of different suboxides SiO_x (1 < x ? 2) on top of a further reduced TiO₂ surface. Upon Si coverage, the characteristic (1 × 2) LEED pattern from the substrate is completely attenuated, indicating absence of long-range order. Annealing the SiO_x overlayer results in the formation of suboxides with different stoichiometry. The LEED pattern recovers the characteristic TiO₂ (1 1 0)-(1 × 2) diagram. LEED I-V curves from both, substrate and overlayer, indicate the formation of nanometric sized SiO_x clusters.     

Structure and stability of β-Mo2C bulk and surfaces: A density functional theory study

Density functional theory calculations have been performed on the structure and stability of β-Mo₂C bulk and the corresponding low-index surfaces. The eclipse configuration with a Mo-C-Mo-C stacking is the most stable, followed by the structure with a Mo-C-Mo-Mo-C stacking where there is an empty carbon layer every fourth layer. For (0 0 1) and (1 0 0) surfaces, the pure C terminations are more stable than the pure Mo terminations. For (0 1 0) and (1 1 1) surfaces, the Mo terminations are more stable than the C terminations. For the (0 1 1) surface, the mixed Mo/C termination is a little more stable than the Mo termination. Relaxation of these surfaces is moderate with no relaxation degree exceeding 12.8%. Among these surfaces, the mixed Mo/C termination of the (0 1 1) surface is the most stable with the lowest surface free energy, followed by the (1 0 1) surface and the T_Mo-2 termination of the (0 1 0) surface.     

Electron surface states in short-period superlattices: (GaAs)2/(AlAs)2(1 0 0)-c(4 × 4)

The electronic structure of (GaAs)₂/(AlAs)₂(1 0 0)-c(4 × 4) superlattice surfaces was studied by means of angular-resolved photoelectron spectroscopy (ARUPS) in the photon energy range 20-38 eV. Four samples with different surface termination layers were grown and As-capped by molecular beam epitaxy (MBE). ARUPS measurements were performed on decapped samples with perfect c(4 × 4) reconstructed surfaces. An intensive surface state was, for the first time, observed below the top of the valence band. This surface state was found to shift with superlattices’ different surface termination in agreement with theoretical predictions.     

Adsorption of ethene on Pt(1 1 1) and ordered PtxSn/Pt(1 1 1) surface alloys: A comparative HREELS and DFT investigation

The adsorption of ethene (C₂H₄) on Pt(1 1 1) and the Pt₃Sn/Pt(1 1 1) and Pt₂Sn/Pt(1 1 1) surface alloys has been investigated experimentally by high-resolution electron energy loss spectroscopy and temperature programmed desorption. The experimental results have been compared with density functional theory (DFT) calculations allowing us to perform a complete assignment of all vibration modes and loss features to the species present on the surfaces. On Pt(1 1 1) as well as on the Pt-Sn surface alloys an η² di-σ-bonded conformation of ethene has been found to be the most stable adsorbed form. In addition to this majority species a minor amount of π-bonded ethene has been identified, which is more abundant on the Pt₂Sn surface alloy than on the other surfaces. Additionally the HREELS spectra of ethene on Pt(1 1 1) and the Pt-Sn surface alloys differ only slightly in terms of the energetic positions of the loss peaks.