Effect of electron-phonon interaction on surface states in a ternary mixed crystal

A variational theory is proposed to study the surface states of electrons in a semi-infinite ternary mixed crystal, by taking the effect of electron-surface optical (SO) phonon interaction into account. The energy and the wave function of the electronic surface-states are calculated. The numerical results of the energies of the surface states of the polarons and the self-trapping energies are obtained as functions of the composition x and surface potential V₀ for several ternary mixed crystal materials. The results show that the electron-phonon interaction lowers the surface-state levels with the energies from several to scores of meV. It is also found that the self-trapping energy of the surface polaron has a minimum at some middle value of the composition x. It is indicated that the electron-phonon coupling effect can not be neglected. Received 4 January 1999 and Received in final form 7 January 2000     

The three-dimensional equilibrium crystal shape of Pb: Recent results of theory and experiment

The three-dimensional equilibrium crystal shape (ECS) is constructed from a set of 35 orientation-dependent surface energies of fcc Pb which are calculated by density functional theory in the local-density approximation and distributed over the [110] and [001] zones of the stereographic triangle. Surface relaxation has a pronounced influence on the equilibrium shape. The (111), (100), (110), (211), (221), (411), (665), (15,1,1), (410) and (320) facets are present after relaxation of all considered surfaces, while only the low-index facets (111), (100) and (110) exist for the unrelaxed ECS. The result for the relaxed Pb crystal state is in support of the experimental ECS of Pb at 320–350 K. On the other hand, approximating the surface energies of vicinal surfaces by assuming a linear relationship between the Pb(111) first-principles surface energy and the number of broken bonds of surface atoms leads to a trivial ECS that shows only (111) and (100) facets, with a sixfold symmetric (111) facet instead of the correct threefold symmetry. It is concluded that the broken bond rule in this simple linear form is not a suitable approximation for obtaining the proper three-dimensional ECS and correct step formation energies. PACS 05.70.Np; 61.50.Jr; 68.35.Md; 71.15.Mb     

Chain conformations on the surface of polyethylene single crystals

New conformations for the shortest folds of polyethylene single crystals were determined from the principles: (1) the conformations of polymethylene chains are basically determined by the rotational isomeric approximation, (2) the crystal part is deformed where it is connected with the fold part, and (3) the conformation of the fold must be determined as the minimum of surface energy, not of conformation energy. It was shown that the surface energy, assuming the crystal state as the reference state, was composed of four terms: (i) the deformation energy of the crystal part, (ii) the sublimation energy of the fold part, (iii) the conformation energy of the fold part, and (iv) the interaction energy between folds. The conformations of the shortest fold were basically (GTGGTGGG) for (110)-folds and (G'G'TTGG) for (200)-folds. The setting angles of all the chains in the crystal part are rotated from their normal angle (41°) to 41 + a° at the boundary between the two parts. The fold part is deformed so as to fit in with the deformed boundary of the crystal part. The conformations with minimum surface energy were obtained at a = 30° for both (110)-and (200)-folds. Their surface energies were about 16 kcal/mole of fold (300 erg/cm2). The surface energies, assuming the liquid state as the reference state, were about 7 kcal/mole of fold (130 erg/cm²).     

XPS studies for NaCl deposited on the Ni(111) surface

Vapor deposited NaCl on the Ni(111) surface was characterized by XPS. Three types of NaCl were found on the surface. First, the NaCl interacted strongly with Ni giving the Nals peak at 1072.2 eV, the C12p peak at 198.8 eV and the NaKLL peak at 494.3 eV. The 494.3 eV peak shifted to 493.4 eV when measured at ∼ 500 K. The energy of the NaKLL peak and the modified Auger parameter for sodium were nearer to those for Na metal than to those for bulk NaCl. The species was assigned to NaCl which was freed from the potential of the NaCl crystal and which had a weakened NaCl bond. The other types were characterized by using H₂O as a probe. One set of the peaks, the Nals peak at 1072.8 eV, the C12p peak at 199.6 eV and the NaKLL peak at 497.7 eV, was assigned to NaCl which was in the form of a very thin crystal. The other set, the Nals peak at 1073.5 eV, the C12p peak at 200.1 eV and the NaKLL peak at 498.5 eV, was obtained during crystal growth of NaCl as large islands upon annealing. The energies suffered from the charging effect. The effect of coadsorbing oxygen was also studied.     

Distribution of atoms in a ternary alloy with a free surface in the self-consistent field approximation

Analytical form for the functions describing atomic distribution of the components in the subsurface layer of a ternary alloy has been found in a closed form in the self-consistent field approximation, without taking correlation into account. The functions obtained allow one to construct concentration profiles of the components near the surface, to calculate the component surface concentration, and to investigate the dependence of the composition on the surface of the alloy crystal on temperature, and surface relaxation of coordination numbers and energies of the nearest neighbor interatomic interaction in an alloy. An analysis of the character of temperature dependencies of the composition on the surface has been performed for various alloys.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 47–52, August, 1982.     

Study of surface cell Madelung constant and surface free energy of nanosized crystal grain

Surface cell Madelung constant is firstly defined for calculating the surface free energy of nanosized crystal grains, which explains the physical performance of small crystals and may be greatly beneficial to the analysis of surface states and the study of the dynamics of crystal nucleation and growth. A new approximative expression of the surface energy and relevant thermodynamic data are used in this calculation. New formula and computing method for calculating the Madelung constant α of any complex crystals are proposed, and the surface free energies and surface electrostatic energies of nanosized crystal grains and the Madelung constant of some complex crystals are theoretically calculated in this paper. The surface free energy of nanosized-crystal-grain TiO₂ and the surface electrostatic energy(absolute value) of nanosized-crystal-grain α -Al₂O₃ are found to be the biggest among all the crystal grains including those of other species.     

Determining surface free energies of crystals with highly disordered surfaces from simulation

A method for calculating surface free energies by gradually creating slabs from a simulation of bulk crystals is tested for three low index faces of a Lennard-Jones crystal at temperatures up to the melting point. The path by which the interactions between atoms in different slabs are turned off must be chosen with care; here it is done in three stages, first the interaction energy is reduced until the well in the pair potential is considerably less than kT, then the effective particle sizes are reduced and finally the interaction energy is reduced to zero. The results show a slow and steady decrease in the surface free energy with temperature, while both the surface energy and the surface entropy increase rapidly when the upper layers of the crystal disorder. There is no evidence for a first order surface roughening transition although the (111) surface shows a sharper onset of disorder than do the (110) and (100) surfaces.     

Effect of partial recovery of zinc surface at room temperature after basal plane indentation

The effect of partial recovery of zinc surface at room temperature after basal plane indentation was detected and studied with a scanning tunneling microscope. It was found that indentation depth recovery depending on time occurs in one stage and recovery of lateral sizes in two stages with different activation energies, which are less than that of bulk zinc self-diffusion. Possible mechanisms of surface self-diffusion of zinc atoms leading to surface crystal growth were discussed. It was shown that the dynamics of effects observed on the sample surface in the indentation area after 15-min annealing at 100°C is associated with emergence of thermally generated vacancies from the sample bulk.     

One and two mode behaviors of surface phonon-polaritons of ternary mixed crystal films

One and two mode behaviors of surface phonon-polaritons of ternary mixed crystal (TMC) films are studied in the framework of the modified random-element-isodisplacement model and the Born-Huang approximation, based on the Maxwell's equations with the usual boundary conditions. The numerical results for the frequencies and splitting energies of the surface phonon-polaritons as functions of the composition in several II-VI and III-V compound semiconductor ternary mixed crystal films are obtained. The “two-mode” and “one-mode” behaviors for different types of systems are clearly shown in the curves of the splitting energies of surface phonon-polaritons. The theoretical conclusion obtained is agreement with the reported experiment results for bulk TMC systems.     

The inelastic scattering of He from Cu(001) at 20 K

The inelastic scattering of He atoms from a (001) surface of copper at 20 K has been studied. The energy analyzer consists of a LiF crystal situated close to the normal of the copper sample crystal. Incident energies of 23 and 64 meV have been used and evidence has been obtained of energy gain and energy loss events. The energy analyzer can be used in two modes: (1) to fix the analyzer crystal to receive atoms of a certain energy range while scanning the sample crystal or (2) to scan the analyzer crystal for a particular setting of the sample crystal. No evidence of phonons from a surface wave branch was found and most of the inelastic events occur very close to the forward direction.     

单晶金刚石边缘表面倾斜角度对同质外延生长的影响

利用微波等离子体化学气相沉积法,对单晶金刚石(100)晶面边缘进行精细切割抛光处理,形成偏离(100)晶面不同角度的倾斜面,在CH_4/H_2反应气体中进行同质外延生长,研究单晶金刚石边缘不同角度倾斜面对边缘金刚石外延生长的影响.实验结果表明,边缘倾斜面角度对边缘的单晶外延生长质量有影响,随着单晶金刚石边缘倾斜面角度的增大,边缘多晶金刚石数量先减少后增多,在倾斜角3.8°时边缘呈现完整的单晶外延生长特性.分析认为,边缘不同角度的倾斜面会改变周围电场强度和等离子体密度,导致到达衬底表面的含碳前驱物发生改变,倾斜面台阶表面的含碳前驱物浓度低于能形成层状台阶生长的临界浓度是减弱单晶金刚石生长过程中边缘效应的主要原因.     

Hybrid HF–DFT modeling of monolayer water adsorption on (001) surface of cubic BaHfO3 and BaZrO3 crystals

First-principles calculations have been used to study the atomic structure, preferred sites and adsorption energies for water adsorption at different terminations of the cubic phase of perovskite-structured BaHfO₃ and BaZrO₃. By considering different initial positions of water molecules, the possibility of water dissociation has been investigated. It is demonstrated that the site selectivity and the form of adsorbed molecule can be affected by the choice of surface unit cell. Dissociative adsorption was found to be favorable for all surfaces in consideration. Hydroxylation of ZrO₂- and HfO₂-terminated surfaces is accomplished by a noticeable reconstruction of the surface structure of cubic phase towards the orthorhombic phase. Calculated atomic charges in bare and hydroxylated surfaces show that BaHfO₃ crystal is slightly more ionic than BaZrO₃.     

Thermally enhanced neutralization in hyperthermal energy ion scattering

Neutralization probabilities are presented for hyperthermal energy Na+ ions scattered from a Cu(001) crystal as a function of surface temperature and scattered velocity. A large enhancement in neutralization is observed as the temperature is increased. Velocity-dependent charge transfer regimes are probed by varying the incident energy, with the most prominent surface temperature effects occurring at the lowest energies. The data agree well with results obtained from a model based on the Newns-Anderson Hamiltonian, where the effects of both temperature and velocity are incorporated.     

Kinetics of the decomposition of disilane molecules on a silicon growth surface in vacuum chemical epitaxy

In the framework of the kinetic approach based on data of technological experiments, the range of characteristic rates of decomposition of disilane radical molecules adsorbed on the surface during the growth of a silicon layer is determined. The relationship between the rate of incorporation of silicon atoms into a growing crystal and the characteristic rate of pyrolysis of hydride molecules on the growing surface is established. The temperature dependences of the decomposition rate of disilane molecules exhibit an unusual activationless behavior in the growth temperature range. The form of the observed dependences is determined by the pyrolysis model, conditions of transferred of hydrogen from an adsorbed molecule onto the surface of the growing layer, being a function of the gas pressure and temperature in the reactor. It is demonstrated that the basic features of the behavior of the decomposition rate of disilane molecules are controlled by the specifics of the interaction of the silicon dihydride molecular beam with the growth surface under conditions of low and high degrees of bonding of hydrogen to free surface bonds. The temperature dependences are qualitatively described by a relation composed of two activation curves with different activation energies at low and high temperatures and preexponential factors depending on the surface coverage by hydrogen atoms.     

Surface energy anisotropy of iron surfaces by carbon adsorption

To investigate the surface energy anisotropy of carbon-adsorbed iron surfaces related to carbon nanotube growth we have performed self-consistent pseudopotential density-functional calculations. The iron particle’s equilibrium shape is obtained from the Wulff construction using the calculated surface energies. We investigate the adsorption and diffusion of carbon atoms on the iron surfaces. It is found that the desorption energy of the carbon atoms and the activation energy for carbon diffusion are very different on different facets. Using the energetics of carbon-adsorbed iron surfaces, we evaluate the formation energies of the surfaces as a function of carbon chemical potential. Since the surface energies of the low-index iron facets are affected differently by the presence of carbon, the crystal shape is changed correspondingly.     

Experimental observation of quantum reflection far from threshold

We present first experimental evidence for quantum reflection, originating exclusively from an attractive potential between an atom and a solid surface, at energies far from the threshold E(i)-->0. The system of light and stable 3He atoms scattering from an alpha-quartz crystal allows confirmation of recent theory on quantum reflection up to its asymptotic behavior, determined by the nonretarded van der Waals potential -C(3)/r(3). From the data, the gas-solid interaction potential is deduced quantitatively, covering the energy region, in which retardation plays a role.     

Experimental measurements of the energetics of surface reactions

Microcalorimetric measurements of the adsorption energies of well-defined surface species are reviewed, using selected examples mainly from our own group to demonstrate the types of information that can be achieved with this technique. The adsorption energies of molecules on single crystal transition metal surfaces to produce well-characterized molecular or dissociated adsorbates allow determination of the standard enthalpies of formation of key catalytic reaction intermediates. The adsorption energies for metal atoms during metal thin-film growth allow quantitative estimates of metal-substrate bond energies, metal film/substrate adhesion energies and the energetic costs associated with lattice mismatch during thin film growth. Results for several metals on MgO(1 0 0) reveal that they bind weakly to terrace sites. Metals from the right side of the periodic table also bind weakly to step and kink sites (although more strongly than on terraces), whereas alkali and alkaline earth metals bind very strongly to these defects. At 300 K, metals tend to form 2D or 3D clusters nucleated on MgO(1 0 0) defects, via a transiently adsorbed precursor (mobile adatoms on terraces). Calorimetric measurement of the energy of metal atoms in supported 3D metal nanoparticles as a function of particle size reveals a very strong size dependence below 6 nm diameter. Metal atoms also adsorb weakly on polymer surfaces and nucleate 3D metal particles, sometimes in kinetic competition with migration to and strong reaction with the more reactive, subsurface organic functional groups. Measurements of the energies for adsorbed proteins on calcium phosphate crystals, which have been structurally characterized by NMR, reveal extremely weak binding dominated by the entropy gained from release of organized water. These experimental measurements of the energies of well-defined adsorbates serve as benchmarks against which to compare theoretical computations for accuracy, thus enabling improvement upon quantum mechanical methods. Comparisons of calorimetric adsorption energies on single crystal surfaces with state-of-the-art DFT calculations show that the latter can often be in substantial (?20%) error.     

Combined channeling study of α-LiIO3 crystal in an electrostatic field

The combined channeling backscattering and channeling nuclear reaction analysis of the α-LiIO₃ monocrystal in an electrostatic field has been performed by proton beam at different energies to identify the behaviour of Li and I ions in the crystal. The thickness of the crystal was about 2 mm and has been cut perpendicular to C-direction. The channeling parameters like half angular width and minimum yield of 〈001〉 axial channeling have been measured precisely and the channeling behaviour of different ions in the crystal has been observed and measured quantitatively first time. The ionic d.c. conductivity can be calculated from the surface peak of the aligned channeling spectrum directly.     

Emission of single gamma rays by electrons with energies of hundreds of GeV in oriented crystals

Cross sections are calculated for the emission of single hard photons by electrons with energies of 150–1000 GeV as they pass through oriented crystals at small angles to the crystallographic axes. The contribution of incoherent emission at isolated atoms of the crystal is taken into account in the calculations, along with the emission in the continuum potential. The calculations are compared with the customary Bethe-Heitler spectrum for a thick amorphous target with allowance for photon absorption due to electron-positron pair production. It is shown that, in this range of energies, an oriented crystal can be more efficient than a thick amorphous target for creating a larger number of hard gamma rays with energies comparable to the energies of the emitting electrons. Zh. Tekh. Fiz. 68, 37–41 (September 1998)