Surface Energy Relaxation Research Articles

Epitaxial growth of NiO layers on MgO(0 0 1) and MgO(1 1 0)

About 33 nm thick NiO layers have been epitaxially grown on MgO substrate at 900°C. Investigations by X-ray diffraction, atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM) give evidence for a close relationship between surface morphology and substrate orientation. Deposited on MgO(0 0 1) surface, the NiO layer grows layer by layer leading to a flat NiO(0 0 1) surface. On the contrary, the surface of the NiO layers grown on MgO(1 1 0) has a roof-like morphology consisting of (1 0 0) and (0 1 0) facets elongated along the [0 0 1] direction. This particular surface configuration comes from a mechanism of surface energy relaxation during the growth process.

A density functional study of plutonium dioxide

The electronic and geometric structures of bulk PuO2 and its (110) surface have been studied using a periodic model within the generalized gradient approximation (GGA) of density functional theory (DFT). The sixty core electrons of the Pu atom have been represented by a relativistic effective core potential and scalar relativistic effects have been incorporated on the valence orbitals. For bulk PuO2, we predict an equilibrium lattice constant of 10.10 a.u. and a cohesive energy of 17.28 eV, in good agreement with experimental data. For the (110) surface, upon relaxation, the distance between the top layer and the next layer is found to decrease by 0.12 A, i.e. 5.3% of the corresponding interlayer distance in the bulk. The distance between the two oxygen atoms on the top layer is found to increase by 0.15 A, i.e. 5.6% of the corresponding bulk value. The small surface relaxation energy of 0.268 eV per unit cell indicates the fair stability of this surface. The effective charges on Pu and O atoms show that the chemical bonding in this system is not purely ionic. Together with the metallic feature of the density of states (DOS) on the surface, the effective charge distribution provides some basis for understanding surface reactivity and corresponding support for catalysis.

Relaxation model of coherent island formation in heteroepitaxial thin films

A model for coherent island formation in heteroepitaxial thin films is presented by focusing on the interplay between surface and strain relaxation energies. The resulting free energy is mapped onto the Landau free energy, and yields the local surface slope as the order parameter for island formation. Isotropic islands are found to exhibit second-order transitions. We argue that our model is appropriate for describing the nucleation and stability of island formation. The spinodal curve indicates that there exists an unstable regime where spinodal-like islands emerge as low-slope islands, even in a highly mismatched heteroepitaxial system.

Theory of PbTiO3, BaTiO3, and SrTiO3 surfaces

First-principles total-energy calculations are carried out for (001) surfaces of the cubic perovskite ATiO3 compounds PbTiO3, BaTiO3, and SrTiO3. Both AO-terminated and TiO2-terminated surfaces are considered, and fully-relaxed atomic configurations are determined. In general, BaTiO3 and SrTiO3 are found to have a rather similar behavior, while PbTiO3 is different in many respects because of the partially covalent character of the Pb-O bonds. PbTiO3 and BaTiO3 are ferroelectrics, and the influence of the surface upon the ferroelectric distortions is studied for the case of a tetragonal ferroelectric distortion parallel to the surface. The surface relaxation energies are found to be substantial, i.e., many times larger than the bulk ferroelectric well depth. Nevertheless, the influence of the surface upon the ferroelectric order parameter is modest, and is qualitatively as well as quantitatively different for the two materials. Surface energies and electronic properties are also computed. It is found that for BaTiO3 and SrTiO3 surfaces, both AO-terminated and TiO2-terminated surfaces can be thermodynamically stable, whereas for PbTiO3 only the PbO surface termination is stable.

Ab initio study of SrTiO 3 surfaces

We present first-principles total-energy calculations of (001) surfaces of SrTiO 3. Both SrO-terminated and TiO 2-terminated surfaces are considered, and the results are compared with previous calculations for BaTiO 3 surfaces. The major differences are in the details of the relaxed surface structures. Our calculations argue against the existence of a large ferroelectric relaxation in the surface layer, as had been previously proposed. We do find some indications of a weak surface ferroelectric instability, but so weak as to be easily destroyed by thermal fluctuations, except perhaps at quite low temperatures. We also compute surface relaxation energies and surface electronic band structures, obtaining results that are generally similar to those for BaTiO 3.

Ab initiostudy of BaTiO3surfaces

We have carried out first-principles total-energy calculations of (001) surfaces of the tetragonal and cubic phases of BaTiO${}_{3}$. Both BaO-terminated (type I) and TiO${}_{2}$-terminated (type II) surfaces are considered, and the atomic configurations have been fully relaxed. We found no deep-gap surface states for any of the surfaces, in agreement with previous theoretical studies. However, the gap is reduced for the type-II surface, especially in the cubic phase. The surface relaxation energies are found to be substantial, i.e., many times larger than the bulk ferroelectric well depth. Nevertheless, the influence of the surface upon the ferroelectric order parameter is modest; we find only a small enhancement of the ferroelectricity near the surface.

Surface energy relaxation of conduction electrons in bismuth using transverse electron focusing technique

The position of the first and second lines of transverse electron focusing on the scale of magnetic fields is analyzed to determine the loss of excess energy of conduction electrons in bismuth as a result of collisions with the crystal boundary upon normal incidence as well as the shape of the functional dependence of this quantity on the initial excess energy. It is found that beginning from relatively low values of excess energy (∼3 meV), a sharp drop in energy occurs upon reflection of electrons at the boundary.

Adatom-adatom and adatom-surface interactions: Islands and chains of Cl on GaAs(110).

Calculations are presented that quantify the adatom-surface and adatom-adatom interactions for Cl adsorption on GaAs(110). Expanding on a previous study of uniform Cl adatom islands, the present work investigates the surface relaxation energy in response to uniform Cl coverage. Compared to the bare (110) surface relaxation, a Cl adatom adsorbed near the surface Ga atom (the A site) causes relaxation back toward the zinc-blende atomic positions, which lowers the cell energy by about 1 eV, while a Cl adatom adsorbed on top of a surface As (the B site) has very little effect on the surface geometry. Additionally, the stability of linear chains of adatoms, similar to chains seen experimentally, and their effect on the surface GaAs geometry is investigated. We find an attractive Cl-Cl pair interaction, which is approximately 1.25 eV per Cl(A)-Cl(B) pair in the linear chain geometry. \textcopyright{} 1996 The American Physical Society.

Processes of surface energy relaxation in topochemical reactions : I. Uvarova. (Inst. for Problems in Materials Science, Kiev, Ukraine,) Science of Sintering, vol 26, no 3, 1994, 245–250

Processes of surface energy relaxation in topochemical reactions : I. Uvarova. (Inst. for Problems in Materials Science, Kiev, Ukraine,) Science of Sintering, vol 26, no 3, 1994, 245–250

Nano-fabrication on silicon at high temperature in a UHV-STM

The microscopic manipulation of atoms at high temperature has been attempted. A UHV-STM is designed to facilitate sample heating up to 1300 degrees C after sample introduction. The UHV-STM employs stacked piezoelectric elements to control the three-dimensional drive of the tip. Nano-fabrication utilizes this fine positioning mechanism and the imaging capability of the STM for ultrafine fabrication of the surface. The technique for nano-fabrication usually changes the bias voltage between the tip and sample during pattern writing while maintaining the tip-sample distance. This method is applied to a high-temperature silicon sample in which atoms on the sample surface are migrated towards the tip. As a result, a pyramid or a crater is formed on the sample. The authors succeeded in creating a hexagonal pyramid and crater on an Si(111) surface and a quadrangular pyramid on an Si(100) surface at 600 degrees C. (5*5) domains on the Si(111) surface can be observed on narrow terraces due to the relaxation of surface energy.

Observation of surface reconstruction and nano-fabrication on silicon under high temperature using a UHV-STM

In this paper we discuss three experiments on silicon (100) and (111) surface at high temperature obtained with an ultra-high-vacuum scanning tunneling microscope (UHV-STM). First, the initial stage of the crystalization process has directly been investigated on the Si(111) surface at a phase transition temperature of about 860°C. (7 × 7) domains nucleated from the step edges and expanded towards inner regions of the terraces, and the steps become straight [ 1 ¯ 1 ¯ 2 ] steps. Second, the high-temperature nano-fabrication method has been applied to Si surfaces. We succeeded in creating a hexagonal pyramid and a crater on the Si(111) surface, and a quadrangular pyramid on the Si(100) surface at 600°C. (5 × 5) domains can be observed on narrow terraces due to the relaxation of surface energy. Finally, we attempted to deposit gold (Au) atoms on silicon surfaces. Au atoms deposited on a high-temperature silicon surface migrated to the observation area while forming 5 × 1 structures. Then the Au atoms diffused into the bulk structure of silicon, and silicon (7 × 7) domains covered the surface again.

Nonlinear voltage-temperature characteristics of submicron semiconductor layers

Voltage-temperature characteristics (VTC) of current carriers in semiconductor layers that are substantially narrower than the bulk cooling length are constructed in the temperature approximation. The nature of the VTC nonlinearity is shown to crucially depend on the ratio of the bulk to the surface energy relaxation lengths of electrons. Because these lengths depend on the electron temperature, with changing external electric fields, the contributions of these surface and bulk scattering mechanisms to energy relaxation processes may vary, leading to visible changes of VTC.

7324. Interfacial reactions in AgZn thin film couples: A K Bandyopadhyay et al, Thin Solid Films, 186, 1990, 87–98

The surface relaxation and surface energy of both the mixed AuCu and pure Cu terminated Cu3Au (0 0 1) surfaces are simulated and calculated by using the modified analytical embedded-atom method. We find that the mixed AuCu termination is energetically preferred over the pure Cu termination thereby the mono-vacancy diffusion is also investigated in the topmost few layers of the mixed AuCu terminated Cu3Au (0 0 1) surface. In the mixed AuCu terminated surface the relaxed Au atoms are raised above Cu atoms for 0.13 Å in the topmost layer. All the surface atoms displace outwards, this effect occurs in the first three layers and changes the first two inter-layer spacing. For mono-vacancy migration in the first layer, the migration energies of Au and Cu mono-vacancy via two-type in-plane displace: the nearest neighbor jump (NNJ) and the second nearest neighbor jump (2NNJ), are calculated and the results show that the NNJ requires a much lower energy than 2NNJ. For the evolution of the energy requirements for successive nearest neighbor jumps (SNNJ) along three different paths: circularity, zigzag and beeline, we find that the circularity path is preferred over the other two paths due to its minimum energy barriers and final energies. In the second layer, the NN jumps in intra- and inter-layer of the Cu mono-vacancy are investigated. The calculated energy barriers and final energies show that the vacancy prefer jump up to a proximate Cu site. This replacement between the Cu vacancy in the second layer and Cu atom in the first layer is remunerative for the Au atoms enrichment in the topmost layer.

Dimensionality and size effects in simple metals.

A detailed discussion of quantum-size effects and the dimensionality in simple (s-p bonded) metals is presented by using aluminum as a prototype. The density of states, work function, surface energy, surface relaxation, and subband energies are calculated for films of varying thicknesses. Oscillatory variations of various physical properties correlate well with the surface charge density which itself varies with the film thickness. All calculations are performed using the self-consistent pseudopotential method and the planarly averaged one-dimensional potential generated from it. Total-energy calculations and forces on various atomic planes lead to important conclusions about surface relaxation.

All-electron local-density determination of the surface energy of transition metals: W(001) and V(001).

The surface energies of W(001) and V(001) are determined theoretically within local-density-functional theory employing the all-electron full-potential linearized augmented-plane-wave method. The calculated values are 5.1 J/${\mathrm{m}}^{2}$ for W(001)---in good agreement with experiment---and 3.4 J/${\mathrm{m}}^{2}$ for V(001). Multilayer surface-relaxation calculations show that the surface-relaxation energy is small and amounts to only a few percent of the surface energy. The surface entropy at sufficient high temperatures for the W(001) surface, as deduced from a comparison of our calculated result and high-temperature experimental surface-energy values, is large (1.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ J/${\mathrm{m}}^{2}$ deg).

Relaxation of the surface energy of solid polymers

The surface energy of a polymer can be increased by compression moulding against a metal substrate. After removal of the substrate relaxation to the equilibrium value sets in. We determined the rate of polymer surface energy relaxation as a function of temperature. For a vinyl chloride-vinyl acetate copolymer we determined an activation energy of the relaxation process that could be correlated to segmental motions in this polymer. For a plasticized polyvinylchloride we found a lower activation energy and a larger rate of relaxation, which is the result of the action of plasticizers on segmental motions. In the case of polyethylene the results indicate segmental motions in amorphous regions in the polymer. With polyethylene the activation energy drops when nearing the melt temperature. The movements of molecular segments correspond to a desorption process at the polymer surface, after removal of the substrate. This agrees with the adsorption during compression moulding, as repotted in earlier work.

Surface relaxation energies in core level spectroscopies of adsorbed atoms and molecules

Core level holes which are created in electron emission spectroscopies of atoms and molecules adsorbed or condensed onto metal surfaces induce a screening charge at the surface. The Coulomb interaction between the induced and the hole charge, called the extra-atomic relaxation energy, shifts the apparent binding energy of the ejected electron from the value inferred from an orbital energy. In this paper, linear response screening shifts are calculated for a number of different response functions which also allow for the polarizability of the adsorbed layers. The relation between classical image potential shifts and those obtained here is quantitatively compared. As an example, the X-ray photoelectron spectrum of SF 6 physisorbed on Ru surfaces is interpreted in terms of the present theory.