Adatom Capture Research Articles

Quinone-Facilitated Coordinated Bipyrene and Polypyrene on Au(111) by Capture of Gold Adatoms

Precise “bottom-up” synthesis by surface-assisted chemical reaction provides advanced approaches for the fabrication of two-dimensional (2D) robust covalent nanostructures, which are bound to conta...

Tailoring the morphology and luminescence of GaN/InGaN core–shell nanowires using bottom-up selective-area epitaxy

Controlled bottom-up selective-area epitaxy (SAE) is used to tailor the morphology and photoluminescence properties of GaN/InGaN core–shell nanowire arrays. The nanowires are grown on c-plane sapphire substrates using pulsed-mode metal organic chemical vapor deposition. By varying the dielectric mask configuration and growth conditions, we achieve GaN nanowire cores with diameters ranging from 80 to 700 nm that exhibit various degrees of polar, semipolar, and nonpolar faceting. A single InGaN quantum well (QW) and GaN barrier shell is also grown on the GaN nanowire cores and micro-photoluminescence is obtained and analyzed for a variety of nanowire dimensions, array pitch spacings, and aperture diameters. By increasing the nanowire pitch spacing on the same growth wafer, the emission wavelength redshifts from 440 to 520 nm, while increasing the aperture diameter results in a ∼35 nm blueshift. The thickness of one QW/barrier period as a function of pitch and aperture diameter is inferred using scanning electron microscopy, with larger pitches showing significantly thicker QWs. Significant increases in indium composition were predicted for larger pitches and smaller aperture diameters. The results are interpreted in terms of local growth conditions and adatom capture radius around the nanowires. This work provides significant insight into the effects of mask configuration and growth conditions on the nanowire properties and is applicable to the engineering of monolithic multi-color nanowire LEDs on a single chip.

Massive Surface Reshaping Mediated by Metal–Organic Complexes

We present a unique remodeling of flat copper terraces induced by organic molecules. In particular, we show how metal–phthalocyanines (MPc, M = Zn and Cu) tailor the formation of regular arrays of Cu nanostripes on its (110) surface. The MPc mediated reshaping of Cu(110) terraces is found to involve a massive reorganization of Cu adatoms, at variance with the conventional changes of metal reconstructions upon molecular adsorption observed so far. By combining experimental and theoretical surface science techniques, we reveal the key role played by the metal atom at the molecular center in reshaping the terrace structure. Moreover, we demonstrate that extra Cu adatoms surrounding the molecules stabilize the molecular decoration of the Cu nanostripes. The massive surface reshaping is thus due to molecular mediated unidirectional blocking of adatom surface diffusion followed by adatom capture and accumulation.

DFT-D Studies of Single Porphyrin Molecule on Doped Boron Silicon Surfaces

We present a theoretical study in the framework of density functional calculations, taking into account the van der Waals interactions (DFT-D) of isolated Cu-5,10,15,20-tetrakis(3,5-di-tert-butyl-phenyl) porphyrin (Cu-TBPP) molecules in a C2v conformation adsorbed on a Si(111)√3x√3R30°-boron surface [denoted Si(111)-B]. With this approach, we investigate interactions between perfect or boron-defect Si(111)-B substrates and the Cu-TBPP molecule as well as the consequences of demetallation of Cu-TBPP. For each model, we determine the structural equilibrium, the spatial charge-density distribution and the electronic properties of the ground state. We conclude that there is potential for Si adatom capture by a porphyrin without strong modification of the porphyrin response, as seen from simulated scanning tunneling microscopy (STM) images.

Understanding the Interaction of the Porphyrin Macrocycle to Reactive Metal Substrates: Structure, Bonding, and Adatom Capture

We investigate the adsorption and conformation of free-base porphines on Cu(110) using STM, reflection absorption infrared spectroscopy, and periodic DFT calculations in order to understand how the central polypyrrole macrocycle, common to all porphyrins, interacts with a reactive metal surface. We find that the macrocycle forms a chemisorption bond with the surface, arising from electron donation into down-shifted and nearly degenerate unoccupied porphine π-orbitals accompanied with electron back-donation from molecular π-orbitals. Our calculations show that van der Waals interactions give rise to an overall increase in the adsorption energy but only minor changes in the adsorption geometry and electronic structure. In addition, we observe copper adatoms being weakly attracted to adsorbed porphines at specific molecular sites. These results provide important insights into porphyrin-surface interactions that, ultimately, will govern the design of robust surface-mounted molecular devices based on this important class of molecules.

Diffusion-Assisted Formation Mechanism of Molecular Break Junctions: A First-Principles Study of Benzenethiolate on Au(111)

Density-functional theory (DFT) calculations have been used to obtain the binding energies of benzenethiolate on Au(111) under applied stress. The binding energetics to ideal Au(111), as well as to Au adatoms and small 2−3 adatom islands, have been calculated as functions of the normal displacement applied to the free end of the molecule. We find that binding to adatoms and adatom islands is energetically preferred over breaking the Au−S bond for sufficiently large displacements. On the basis of these results, we propose a diffusion-assisted mechanism of pyramidal break junction formation on Au(111) that involves creation and capture of surface adatoms, as well as surface or bulk vacancies. Quantitative estimates of stretching rates that are compatible with the required diffusion times are given on the basis of random walk theory.

Self-organized quantum dot arrays: Kinetic mapping of adatom capture

Deterministic synthesis of self-organized quantum dot arrays for renewable energy, biomedical, and optoelectronic applications requires control over adatom capture zones, which are presently mapped using unphysical geometric tessellation. In contrast, the proposed kinetic mapping is based on simulated two-dimensional adatom fluxes in the array and includes the effects of nucleation, dissolution, coalescence, and process parameters such as surface temperature and deposition rate. This approach is generic and can be used to control the nanoarray development in various practical applications.

Potential transients on a renewed electrode as a source of information on the surface concentration of catalytically active adatoms

Potential transients on renewable gold and silver electrodes in cyanide solutions containing catalytically active ions of lead and thallium are measured. The transients are recorded both on freshly renewed electrodes and on renewed electrodes kept in solution for specified time periods Δt. The behavior of transients following a change in the concentration of lead and thallium ions, current density, and Δt qualitatively conforms to regularities that could be expected if the catalytically active adatoms were inserted into the deposit similarly to the admixtures. Methods for determining coefficients of capture of such adatoms by the growing metal deposit are suggested. Factors that hamper determination of these coefficients and make it worthwhile to select a method for their determination that would use potential transients are considered. The capture coefficient for the incorporation of lead adatoms in the growing silver deposit is nearly 20 times that for gold.

Partial self-ordering observed in silicon nanoclusters deposited on silicon oxide substrates by chemical vapor deposition

The formation of Si dots by chemical vapor deposition is studied from the very early stages of the dot formation up to about 25% of substrate coverage. Structural characterization is mainly performed by means of energy filtered transmission electron microscopy, which couples chemical information to very high spatial resolution. The dots are shown to be surrounded by Si-free regions and this is attributed to the Si adatom capture mechanism from each nucleus. The data are discussed in the framework of a self-similar model, which takes into account the dot local environment, the adatom diffusion and the continuous nucleation of new islands. From the fit to the data the correlation between the dot size and the capture area is obtained and the number of deactivated nucleation sites is quantified.

Modeling of Island Formation During Submonolayer Deposition: A Stochastic Geometry-Based Simulation Approach

Homogeneous nucleation and growth of two-dimensional islands during submonolayer deposition has been analyzed extensively by kinetic Monte Carlo (KMC) simulation of atomistic models and more recently by Burton--Cabrera--Frank-type continuum formulations for diffusion and aggregation of the deposited adatoms. Here, we develop an alternative geometry-based simulation (GBS) approach. This approach replaces an explicit treatment of adatom diffusion (either within an atomistic or continuum framework) with a formulation based on the stochastic geometry of depletion zones or zones surrounding islands. We consider models with a prescribed critical size, i, above which islands are stable. For canonical models with small i, we show that prediction of the nucleation rate and island density by GBS (or any other multiscale approach) requires precise description of adatom capture by critical clusters, for which we extend existing theory. We also present results for the island size distribution including the...

Nucleation and growth ofPd−Aualloy particles on crystalline graphite at elevated temperatures

In situ transmission electron microscopy (TEM) and locally resolved, energy dispersive x-ray spectroscopy (EDX) were employed to investigate the nucleation, growth, and composition of individual $\mathrm{Pd}\ensuremath{-}\mathrm{Au}$ alloy particles on highly oriented, pyrolithic graphite (HOPG). At substrate temperatures up to $480\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, the condensation coefficient of Au is very low on defect free substrate areas, while that of Pd is much greater. During simultaneous deposition of the components, condensation of Au was enhanced by the presence of Pd, and the composition of individual alloy particles was found to vary with size, as well as with the size of the respective capture area. Simulations of the kinetics of adatom diffusion and capture allowed us to determine the ratio of the adatom mean walk distances before desorption ${\ensuremath{\lambda}}_{\mathrm{Pd}}∕{\ensuremath{\lambda}}_{\mathrm{Au}}$ to about 3.4. Based on an extrapolated estimate from earlier work of ${\ensuremath{\lambda}}_{\mathrm{Au}}=2.9\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, this would result in ${\ensuremath{\lambda}}_{\mathrm{Pd}}=9.9\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at $480\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$.

Si epitaxial growth on Br-Si(100): How steric repulsive interactions influence overlayer development

Scanning tunneling microscopy results show the consequences of Si adatom deposition onto Br-saturated $\mathrm{Si}(100)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$. Those adatoms undergo an exchange reaction with Br and they are immobile at room temperature. In the low coverage regime, annealing to $650\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ leads to dimerization, limited ordering, and the formation of short Si chains. Adatom capture by those chains produces features of even and odd numbers of atoms. Annealing at $700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ eliminates the odd chains, but diffusion is highly constrained by Br site blocking. With subsequent Si depositions, there is further nucleation of chains and chain growth. The local patterning of the Si chains reveals the influence of the Br steric repulsive interactions as out-of-phase structures were favored over in-phase structures around any given chain. Eventually, $(3\ifmmode\times\else\texttimes\fi{}2)$ adlayer patches develop. Second layer chains appear after the deposition of $\ensuremath{\sim}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$, with layer-2 nucleation at antiphase domain boundaries. Bromine loss was observed, even at $650\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and it is probably tied to the weaker bonding to single isolated Si adatoms.

Nucleation and growth of Ag films on a quasicrystalline AlPdMn surface

Nucleation and growth of thin films of Ag on the fivefold surface of an ${\mathrm{Al}}_{72}{\mathrm{Pd}}_{19.5}{\mathrm{Mn}}_{8.5}$ icosahedral quasicrystal is studied with scanning-tunneling microscopy. For low coverages, flux-independent island nucleation is observed involving adatom capture at ``traps.'' With increasing coverage, islands start growing vertically, but then spread, and ultimately form hexagonal nanocrystals. These have fcc symmetry and pyramidlike multilayer stacking along the 〈111〉 direction. The constituent hexagonal islands have five different orientations, rotated by 2\ensuremath{\pi}/5, thus reflecting the symmetry of the substrate.

Reconstruction determined submonolayer growth of Ag on Si( [formula omitted])-(7×7) surface

The submonolayer growth of Ag on Si(1 1 1)-(7×7) surface at temperatures from 420 to 540 K was studied. Island densities, size distributions and average number of Ag atoms per occupied half-unit cell (HUC) of 7×7 reconstruction were investigated. At higher coverage large 2D islands with jagged shapes were observed. A scenario of the growth was outlined, based on the assumptions of existence of saturated Ag islands on the surface which cannot overgrow HUC boundaries by adatom capture. Such a model explains morphology of the large islands as well as the presence of the large amount of small islands formed inside HUCs. The capacity of a single HUC was found to be ≈18 Ag atoms and the capacity of HUCs covered by the large islands was found to be ≈31 Ag atoms on average.

Nucleation, adatom capture, and island size distributions: Unified scaling analysis for submonolayer deposition

We consider the irreversible nucleation and growth of two-dimensional islands during submonolayer deposition in the regime of large island sizes. A quasihydrodynamic analysis of rate equations for island densities yields an ordinary differential equation (ODE) for the scaling function describing the island size distribution. This ODE involves the scaling function for the dependence on island size of {open_quotes}capture numbers{close_quotes} describing the aggregation of diffusing adatoms. The latter is determined via a quasihydrodynamic analysis of rate equations for the areas of {open_quotes}capture zones{close_quotes} surrounding islands. Alternatively, a more complicated analysis yields a partial differential equation (PDE) for the scaling function describing the joint probability distribution for island sizes and capture zone areas. Then, applying a moment analysis to this PDE, we obtain refined versions of the above ODE{close_quote}s, together with a third equation for the variance of the cell area distribution (for islands of a given size). The key nontrivial input to the above equations is a detailed characterization of nucleation. We analyze these equations for a general formulation of nucleation, as well as for an idealized picture considered previously, wherein nucleated islands have capture zones lying completely within those of existing islands.

Rate equation approach to film growth

Recent progress of the rate equation method in the field of film growth, covers the modeling of island density behavior in the whole range of surface coverages and the stress-driven 2D–3D transition in island morphology, as it occurs in the Stranski–Krastanov growth mechanism. In the former case, rate coefficients are evaluated both on the basis of stochastic approaches which also allow one to deal with spatial correlation effects among nuclei and exploiting computer simulations. In the latter case, rate coefficients for adatom capture and for 2D–3D island transition are computed, respectively, by solving the diffusion equation and exploiting the quasi equilibrium approximation.

STM study of nucleation of Ag on Si(111)-(7×7) at submonolayer coverage

The initial stages of nucleation of Ag on Si(111)-(7×7) surface – half unit cell (HUC) filling – were studied in situ by UHV STM. The dependence of the density of Ag-occupied HUCs and of the preference in occupying HUCs on substrate temperature, deposition rate and amount of Ag deposited were measured. The density of filled HUCs was found to be independent of the deposition rate at room temperature. The density is independent of temperature up to 380 K (for a deposition rate of 0.0075 ML s −1) when thermally activated diffusion of Ag adatoms between single HUCs starts to influence growth. At lower temperatures, thermally activated hopping between HUCs is negligible. However, short-range correlations found in the structure of occupied HUCs together with the low density of occupied HUCs at coverage 0.1 ML at room temperature show that non-activated adatom capture by adjacent occupied HUCs plays an important role.

Island formation during deposition or etching

The behavior of a non-equilibrium lattice-gas model for irreversible formation of two-dimensional islands during submonolayer deposition or etching is examined in detail. In particular, recent developments in describing capture of diffusing species by islands of various sizes are reviewed. Both exact and geometric descriptions of capture are discussed, elucidating the role of the local environment of the islands, and its dependence on island size, on adatom capture and individual island growth rates. Limitations in mean-field treatments of capture are noted. These results are applied to characterize the growth of Ag islands on Ag(100), Cu/Co islands on Ru(0001), and pits on Si(001) etched with molecular oxygen.

Adatom capture by arrays of two-dimensional Ag islands on Ag(100)

We examine the capture of diffusing Ag adatoms by arrays of two-dimensional Ag islands subsequent to deposition on Ag(100) at room temperature. This is achieved by a combination of scanning tunneling microscopy experiments, kinetic Monte Carlo simulations, and diffusion equation analyses. The dependence of the capture rates on Ag-island size is shown to reflect larger island-free regions surrounding the larger islands, i.e., a strong correlation between island sizes and separations. This feature, and the influence of the local environment of the islands on capture, are elucidated by introducing suitable tessellations of the surface into ``capture zones'' for each island. We show that a Voronoi-type tessellation based on the distance from the island edges accurately reflects adatom capture. However, a tessellation exactly describing adatom capture is only obtained from a solution of the steady-state equation describing adatom deposition, diffusion, and capture by an array of islands distributed as in experiment. The stochastic nature of adatom capture is also quantified by analysis of the dependence on the deposition location of the probability for diffusing adatoms to be captured by a specific island. The experimental island size dependence of adatom capture is found to be entirely consistent with that obtained from a ``canonical'' model for the irreversible nucleation and growth of square islands.

Island Size and Environment Dependence of Adatom Capture: Cu/Co Islands on Ru(0001)

We quantify the rate of capture by Co islands on Ru(0001) of additionally deposited Cu atoms, using scanning tunneling microscopy. The dependence of the capture rates on Co-island size is shown to reflect larger island-free areas surrounding bigger islands, a feature neglected in mean-field treatments. We also find a strong direction dependence in Cu adatom capture, reflecting the local environment of individual islands. These features are elucidated by simulations and diffusion equation analyses. {copyright} {ital 1998} {ital The American Physical Society}