Chiral recognition of organic molecules by atomic kinks on surfaces

Two distinct non-mirror-symmetric conformations of D- and L-cysteine were found after adsorption on Au(17 11 9)S. This demonstrates chiral heterorecognition, i.e., enantioselectivity of S kinks on vicinal Au(111). The structures as determined by angle scanned x-ray photoelectron diffraction agree well with those from density functional theory calculations. The calculations predict adsorption energies of approximately 2 eV where D-cysteine binds 140 meV stronger than L-cysteine. The classical three point contact model for molecular recognition fails to explain these findings.     

Adatom diffusion on vicinal surfaces with permeable steps

We study the behavior of single atoms on an infinite vicinal surface assuming certain degree of step permeability. Assuming complete lack of re-evaporation and ruling out nucleation the atoms will inevitably join kink sites at the steps but will do many attempts before that. Increasing the probability for step permeability or the kink spacing lead to increase of the number of steps crossed before incorporation of the atoms into kink sites. The asymmetry of the attachment-detachment kinetics (Ehrlich-Schwoebel effect) suppresses the step permeability and completely eliminates it in the extreme case of the infinite Ehrlich-Schwoebel barrier. A negligibly small drift of the adatoms in a direction perpendicular to the steps leads to a significant asymmetry of the distribution of the permeability events, the atoms thus visiting more distant steps in the direction of the drift. The curves are fitted with an exponential function containing a constant which can be considered as a length scale of the effect of the drift. Some conclusions concerning the stability of the vicinals are drawn.      

Misoriented surfaces with randomly distributed steps

Models for surfaces with randomly distributed steps having a peferred direction, i.e., an unequal probability of encountering an up or down step when going from one lattice site to the adjacent one, are constructed. The terrace widths obey a geometric distribution function. A closed-form solution is obtained for the angular distribution of the electron diffraction beam intensity. We show that both the angular width of the diffracted beam and the Bragg peak position oscillate as a function of the incident electron energy. Practical implications of these results are discussed.     

Diffraction from surfaces with interacting steps

Diffraction techniques are potentially powerful probes of atomic steps on crystalline surfaces. Correlation among the surface steps yields information on the interactions among steps. We consider surfaces with a cut-off geometric distribution of terrace widths which result from strong, short-range step-step interactions. We calculate the diffraction angular profile analytically and compare this profile with the two extreme cases of negligible and long-range interactions. For a particular case of mean terrace width of seven atoms, we show that a strong interaction with range greater than only two atoms produces measurable splitting in the diffracted intensity. The reciprocal space characteristics of this model surface are expounded.     

Adsorption characteristics of atomic nitrogen on ruthenium surfaces

We have studied the adsorption, vibration, and diffusion of N atoms on Ru(0 0 0 1), , and surfaces by means of the 5-parameter Morse potential (5-MP) of interaction between atomic nitrogen and a metal surface. The adsorption sites, adsorption geometry, binding energy and eigenvibration of atomic nitrogen on the different ruthenium surfaces are calculated. It is shown that atomic nitrogen always preferably occupies the high coordination sites on Ru surfaces. The 4-fold site is the preferable adsorption site for atomic nitrogen on both open and surfaces while 3-fold site is the most stable adsorption site for atomic nitrogen on both Ru(0 0 0 1) and surfaces. Moreover, we find the lowest energy pathway of diffusion and diffusion barriers of atomic nitrogen on the surfaces.     

Free energies of steps on (111) fcc surfaces

For a hexagonal two-dimensional lattice we derive, using a surprisingly simple route, exact expressions for the step free energies along the high symmetry directions, 〈1-10〉 and 〈11-2〉. If we consider only nearest-neighbor interactions, ε, and ignore step overhangs the step free energy vanishes at a temperature . In a more sophisticated model that incorporates step overhangs we find a reduction of T_R to about 0.87ε/k_b. The obtained step free energy expressions are also valid for the free energy of walls between two regions of opposite spins of the triangular 2D Ising system.     

Electronic and structural properties of steps on cleaved semiconductor surfaces

The work function of clean, cleaved p-Si(111) surfaces was measured in dependence of the density of cleavage steps. The contact potential difference CPD was observed first to increase then to decrease linearly with increasing step density. This dependence is caused by an increase in band bending, which saturates at a density of approximately 1 × 10⁶ steps per cm, and a decrease of the surface dipole which is proportional to the density of steps. Since the 2 × 1 reconstruction of the cleaved Si surface is ionic and buckled it increases the surface dipole moment compared with the 1 × 1 structure. Thus, a decrease of the surface dipole may be caused by removal of the 2 × 1 reconstruction on part of the step terraces. The experimental data give a width of about 28 Å for the unreconstructed stripe along the step edge.     

Diffraction from surfaces with randomly distributed steps

A closed-form solution is obtained for the angular distribution of intensity in diffraction from a surface on which the terrace size distribution is given by the geometric distribution, i.e. a surface in which the occurrence of steps is random. Several distributions of step heights that are integral multiples of the monatomic step height are considered. It is shown that a random occurrence of monatomic steps will cause some multiatomic steps. If a very broad distribution of step heights is assumed, the beam width no longer oscillates with energy but approaches a constant value except at the characteristic energies of zero width. Comparisons are made with a previous model and with measurements on GaAs(110).     

Electron states at steps in semiconductor surfaces

Based on the Bethe-Peierls approximation we have developed a method to study surfaces where local deviations from periodicity are present. Since the method leaves out everything but short-range order, the effect of particular local configurations are easily studied. In particular, we apply the method to study the electronic structure of steps in Si surfaces. The results are in fair agreement with photoemission experimental data.     

Evidence for atomic recondensation on ion-bombarded solid surfaces

Direct evidence is presented that vapor-phase crystal growth is induced on solid surfaces bombarded with inert-gas ions. Surface cones formed on AlGaAs/GaAs superlattice targets exposed to a few keV Ar⁺ ions were characterized as polycrystalline cone tips protruding beyond the original target surface. Fine particles were also found to grow in close vicinity to the cone evolution sites, thus demonstrating that target atoms ejected from the impact area condensed into polycrystalline cone tips and particles, as a result of an enhanced particle supply to the projected areas. Since this finding is not within the current knowledge of ion-solid interactions, a new theoretical approach is required for its interpretation.     

Spheroidal cap configurations of atomic clusters on planar surfaces

Deposited atomic cluster configurations are investigated under the hypothesis of spheroidal cap shapes being a stable geometry. The macroscopic-microscopic method is employed to calculate the deformation energy. A new specialized single-particle model is developed in order to account for the quantum effects, and the liquid-drop approach is used to calculate the macroscopic part of the energy. The minima within the total deformation energy are interpreted as equilibrium states of atomic clusters on surfaces. Calculations have been performed for the metallic clusters of Na with atom numbers N = 20, 70 and 200.     

Non-conserved dynamics of steps on vicinal surfaces during electromigration-induced step bunching

We report new results on the non-conserved dynamics of parallel steps on vicinal surfaces in the case of sublimation with electromigration and step-step interactions. The derived equations are valid in the quasistatic approximation and in the limit f ^-1 ≫ l _D  ≫ l _± ≫ l _i , where f is the inverse electromigration length, l _D the diffusion length, l _± the kinetic lengths and l _i the terrace widths. The coupling between crystal sublimation and step-step interactions induces non-linear, non-conservative terms in the equations of motion. Depending on the initial conditions, this leads to interrupted coarsening, anticoarsening of step bunches or periodic switching between step trains of different numbers of bunches.     

Optical bistability via atomic coherence

Optical bistability in a unidirectional cavity has been analyzed based on a ground-state doublet three-level atomic model in which atoms are initially prepared in a coherent superposition state. It is shown that bistability can be realized due to the initial coherence. Furthermore, the bistable hysteresis cycle becomes wider as the initial polarization increases, and the bistability can be controlled by the initial coherence.     

Multiphonon atomic scattering by solid surfaces

A multiphonon solution for the problem of atom-surface scattering at thermal energies is proposed. The scattering equations are solved using the assumption of low inelastic scattering intensities, and the theoretical formalism is basically an improvement of a previous work on one-phonon scattering. Present results reduce to the one-phonon expression of the previous work when the appropriate limit is taken.     

Optical properties of thin films on rough surfaces

A general theory, of second order in the film thickness and surface roughness over the wavelength, is developed of the optical properties of a thin film. These properties are described by a small number of electromagnetic constitutive coefficients. Formulae for these coefficients are derived in terms of the height-height correlation functions of the upper and lower surfaces of the film, and its average thickness. The reflectance, transmittance and ellipsometric coefficient are expressed in terms of the constitutive coefficients, for arbituary angles of incidence.