2009. "Water as a Catalyst: Imaging Reactions of O-2 with Partially and Fully Hydroxylated TiO2(110) Surfaces." Journal of Physical Chemistry C 113(5):1908-1916. Abstract The reactions of molecular oxygen with bridging hydroxyl groups, OHb, formed by H2O dissociation on bridging oxygen vacancies of TiO2 (110) are studied at low and high OHb coverages as a function of the O2 exposure, using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and electron simulated desorption (ESD) techniques. On partially hydroxylated surfaces, the sudden simultaneous disappearance of oxygen vacancies and oxygen adatoms formed by O2 dissociation is observed at high O2 exposures. On fully hydroxylated TiO2 surfaces, which enable us to compare results of STM, TPD and ESD studies, most of OHb’s are removed via reacting with O2. Hence, fully hydroxylated TiO2 surfaces can be converted to nearly stoichiometric surfaces, albeit with some amount of adsorbed molecular water. Formation of mobile H2O molecules and water-assisted diffusion of the reactants plays an important role in the kinetics of the processes on both partially and fully hydroxylated surfaces.
2009. "Chemical Reactivity of Reduced TiO2(110): The dominant role of surface defects in oxygen chemisorption." Journal of Physical Chemistry C 113(28):12407-12411. doi:10.1021/jp901989x Abstract O2 chemisorption on reduced, rutile TiO2(110) with various concentrations of oxygen vacancies (Ov) and bridging hydroxyls (OHb) is investigated with scanning tunneling microscopy, temperature programmed desorption and electron-stimulated desorption. On the annealed surface, 2 oxygen molecules can be chemisorbed per Ov. The same amount of O2 chemisorbs on surfaces where each Ov is converted to two OHb’s by exposure to water (i.e. 1 O2 per OHb). Surfaces with few or no Ov’s or OHb’s can be created by exposing the hydroxylated surface to O2 at room temperature, and the amount of O2 that chemisorbs on these surfaces at low temperatures is only ~20% of the amount on the annealed (reduced) surface. In contrast, the amount of chemisorbed O2 increases by more than a factor of two when the OHb concentration is enhanced – without changing the concentration of sub-surface Ti interstitials. The results indicate that the reactivity of TiO2(110) is primarily controlled by the amount of electron-donating surface species such as Ov’s and/or OHb’s, and not Ti3+ interstitials.
2009. "Crystallographic Dependence of Visible-Light Photochemistry in Epitaxial TiO2-xNx Anatase and Rutile." Physical Review. B, Condensed Matter and Materials Physics 79(8):Art. No. 085401. Abstract All films were grown by plasma assisted molecular beam epitaxy (PAMBE) in a custom chamber described elsewhere (1). Epitaxial films of TiO2-xNx(001) (x ≤ ~0.02) anatase were grown by PAMBE on undoped or Nb-doped (0.02 at. %) SrTiO3(001) (STO) and undoped LaAlO3(001) (LAO). Similarly, TiO2-xNx(001) (x ≤ ~0.02) rutile epifilms were grown on rutile TiO2(110). The growth and physical properties of N-doped anatase on LAO(001) and N-doped rutile on TiO2(110) have been described in detail elsewhere (2-4). In what follows, we describe the growth details for N-doped anatase on STO(001). The PAMBE chamber is connected to an x-ray photoelectron spectrometer (XPS) chamber and a photodesorption chamber. The former is equipped with a Gamma Data/Scienta SES 200 analyzer and a monochromatic AlK x-ray source. The latter includes a molecular dosing apparatus for TMAA, a Hg arc lamp, and a quadrupole mass spectrometer. The STO substrates were etched in buffered HF and annealed in flowing O2 at 1 atm. at 950oC for 8 hours. The etch dissolved SrO terraces and the oxygen anneal resulted in mass transport of the discontinuous TiO2 microterraces, resulting in an atomically flat, TiO2 terminated surface with a minimum step height of 4 Å (5). This treatment left some residual fluorine on the surface which could not be removed by annealing. The measured F 1s binding energy was ~684.0 eV, which is close to that exhibited by SrF2 – 684.6 eV (6). Based on this binding energy and the high degree of thermal stability, we conclude that F substitutes for O in the lattice. Under this assumption and using atomic photoemission cross sections (7), the F mole fraction within the anion sublattice is estimated to be ~0.05 within the probe depth of XPS at normal emission (~45 Å).
2009. "Crystallographic Dependence of Visible-light Photoactivity in Epitaxial TiO2−xNx Anatase and Rutile." Physical Review. B, Condensed Matter and Materials Physics 79(8):Article number: 085401. doi:10.1103/PhysRevB.79.085401 Abstract Nitrogen-doped TiO2 materials have been shown to exhibit visible-light photoactivity, but the operative mechanism(s) are not well understood. Here we use structurally and compositionally well-defined epitaxial films of TiO2−xNx anatase (001) and rutile (110) (x~0.02) to show a qualitative difference between the visible-light activities for the two polymorphs. Holes generated by visible light at N sites in anatase (001) readily diffuse to the surface and oxidize adsorbed trimethyl acetate while the same in rutile (110) remain trapped in the bulk. In light of the low doping densities that can be achieved in phase-pure material, conventional wisdom suggests that holes should be trapped at N sites in both polymorphs. Although the detailed mechanism is not yet understood, these results suggest that the hole hopping probability is much higher along the [001] direction in N-doped anatase than along the [110] direction in N-doped rutile.
2009. "Two Pathways for Water Interaction with Oxygen Adatoms on TiO2(110)." Physical Review Letters 102(9):Art. No. 096102. doi:10.1103/PhysRevLett.102.096102 Abstract Scanning tunneling microscopy and density functional theory studies show that oxygen adatoms (Oa), produced during O2 exposure of reduced TiO2(110) surfaces, alter the water dissociation/recombination chemistry through two distinctive pathways. Depending on whether H2O and Oa are on the same or adjacent Ti4+ rows, Oa facilitates H2O dissociation and proton transfer to form a terminal hydroxyl pair, positioned along- or across-Ti row, respectively. The latter process has not been reported previously, and it starts from “pseudo-dissociated” state of water. In both pathways, the subsequent reverse proton transfer results in H2O recombination and statistical oxygen atom scrambling, as manifested by an apparent along- or across-row motion of Oa’s.
2009. "Imaging Consecutive Steps of O2 Reaction with Hydroxylated TiO₂(110): Identification of HO₂ and Terminal OH Intermediates." Journal of Physical Chemistry C 113(2):666-671. Abstract We report results of the combined experimental and theoretical investigation of the molecular oxygen reaction with a partially hydroxylated TiO₂(110) surface. The consecutive steps of both primary and secondary site-specific reactions have been tracked with high-resolution scanning tunneling microscopy (STM). For the first time, we have directly imaged stable, adsorbed hydroperoxyl (HO₂) species, which is believed to be a key intermediate in many heterogeneous photochemical processes but generally metastable and “elusive” until now. We also found terminal hydroxyl groups, another critical but never directly observed intermediates. A conclusive evidence that O₂ reacts spontaneously with a single bridging OH group as an initial reaction step is provided. The experimental results are supported by density functional theory (DFT) calculations that have determined species energies and configurations. Reported observations provide a basis for a consistent description of the elementary reaction steps and offer molecular-level insight into the underlying reaction mechanisms. In a broader perspective, the results are expected to have far reaching implications for various catalytic systems involving the interconversion of O₂ and H₂O.
2009. "Imaging Consecutive Steps of O2 Reaction with Hydroxylated TiO2(110): Identification of HO2 and Terminal OH Intermediates." Journal of Physical Chemistry C 113(2):666-671. doi: 10.1021/jp807030n Abstract The hydroperoxyl (HO2) species is believed to be a key intermediate in many heterogeneous photochemical processes, but generally metastable and thus hard to prove. We report here that for the first time, we directly imaged stable, adsorbed HO2 species during O2 reaction with a partially hydroxylated TiO2(110). We also found terminal hydroxyl groups, another critical but never directly observed intermediates. By imaging species and tracking site-specific reactions with high-resolution scanning tunneling microscopy, and determining the energies and configurations with density functional theory calculations, we provide molecular-level insight into the underlying reaction mechanisms. These results are expected to have far reaching implications for various catalytic systems involving the interconversion of O2 and H2O.
2009. "Band Offsets at the Epitaxial Anatase TiO2/n-SrTiO3(001) Interface." Surface Science 603(5):771-780. Abstract We have used high-energy-resolution x-ray photoelectron spectroscopy to measure valence band offsets at the epitaxial anatase TiO2(002)/n-SrTiO3(001) heterojunction prepared by molecular beam epitaxy, Within experimental error, the valance band offset is zero for anatase thicknesses between 1 and 7 monolayers. The conduction band offset is also zero by virtue of the fact that both anatase and SrTiO3 exhibit the same bandgap value (~3.2 eV). In one set of experiments, the interface included a partial monolayer of fluorine remaining from the HF etch that was used to prepare the substrate. The F could not be removed without Ar ion sputtering and annealing, which in turn resulted in ~0.15 eV of band bending, indicating the presence of interfacial defects. The band offsets were measured to be approximately 0 eV as well when the F was removed. Density functional theory predicts the valence band offset for the clean interface to be 0.5 eV. Inclusion of interfacial F reduces the theoretical band offset to 0.2 eV, much closer to experiment, and suggesting that the interface dipoles created by F and sputter defects have a major effect on the band offset.
2008. "Hole-mediated Photodecomposition of Trimehtyl Acetate on a TiO2(001) Anatase Epitaxial Thin Film Surface." Journal of Physical Chemistry C 112(50):20050-20056. doi:10.1021/jp8077997 Abstract Surfaces of titanium dioxide in both rutile and anatase polymorphs have attracted significant attention in catalysis and photochemistry. The (110) orientation of rutile, and to a lesser extent other rutile orientations, have been studied on an atomic scale, yielding information on surface structure and chemical reactivity. In contrast, the thermal and photochemistry of well-defined, single-crystal anatase surfaces had not been investigated, largely because of the metastable nature of anatase , as well as the lack of availability of high-quality surfaces. Here we describe a study of the adsorption and photoreactivity of an organic adlayer, trimethyl acetate (TMA), on structurally-excellent anatase (001) epitaxial thin films grown by oxygen plasma assisted molecular beam epitaxy (OPAMBE). High-resolution scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and photodesorption spectrometry have been used to study the chemisorptions and ultraviolet (UV) light-induced photodecomposition of TMA in ultrahigh vacuum. UV light promotes hole-mediated photodecomposition of TMA, resulting in decarboxylation to yield tert-butyl radical and CO2. The photochemical rate constant is equal to that measured for OPAMBE grown rutile TiO2(110) surfaces.
2008. "Intrinsic Diffusion of Hydrogen on Rutile TiO2(110)." Journal of the American Chemical Society 130(28):9080-9088. doi:10.1021/ja8012825 Abstract The combined experimental and theoretical study of intrinsic hydrogen diffusion on bridge-bonded oxygen (BBO) rows of TiO2(110) is presented. The sequences of isothermal scanning tunneling microscopy images demonstrate a complex behavior of hydrogen formed by water dissociation on BBO vacancies. Different diffusion rates are observed for the two hydrogens in the original geminate OH pair suggesting a presence of long lived polaronic state. For the case of separated hydroxyls, both theory and experiment yield comparable temperature-dependent diffusion rates. The density functional theory calculations show that the lowest energy diffusion path involves a simple hydrogen motion along the BBO from one BBO to its neighbour. The values of kinetic parameters (prefactors and diffusion barriers) determined experimentally and theoretically are significantly different and indicate the presence of a more complex diffusion mechanism. We speculate that the hydrogen diffusion proceeds via two step mechanism, the initial diffusion of localized charge followed by the diffusion of hydrogen. Both experiment and theory show the presence of repulsive OH-OH interactions.
2008. "Focused ion beam directed self-assembly (Cu2O on SrTiO3 ): FIB pit and Cu2O nanodot evolution." Superlattices and Microstructures 44(4-5):677-685. doi:10.1016/j.spmi.2008.01.016 Abstract A gallium focused ion beam has been used to create discrete pits on the surface of a SrTiO3 (100) surface with the idea that these pits will serve as the nucleation sites for subsequent Cu2O quantum dot growth. Immediately after pit formation and following wet chemical etching and thermal annealing of the surface, the concentration of gallium within these pits has been analyzed using a high-resolution Auger system,. Using atomic force microscopy, the geometry of the pits has also been determined following etching and annealing. Growth of Cu2O quantum dots on the patterned surfaces has been performed. Growth of Cu2O quantum dots within the pits is the primary mode of dot formation. In several samples, dot growth within pits appears to occur by a two-step process with pits filling prior to initiation of a second, distinct phase of quantum dot growth above the plane of the original SrTiO3 surface.
2008. "Transient Mobility of Oxygen Adatoms upon O2 Dissociation on Reduced TiO2 (110)." Journal of Physical Chemistry C 112(7):2649-2653. doi:10.1021/jp077677u Abstract Tracking the same region of the reduced TiO2 (110) surface by scanning tunneling microscopy before and after oxygen exposure at room temperature confirms that O2 molecules dissociate only at the bridging oxygen vacancies, with one O atom healing a vacancy and other O atom bonding at the neighboring Ti site as an adatom. Majority (~81%) of O adatoms are found separated from the original vacancy positions, by up to two lattice constants along [001] direction. Since at room temperature the thermal diffusion of O adatoms has been found to be rather small, with experimentally estimated activation energy of ~1.1 eV, we conclude that observed lateral distribution of the oxygen adatoms is attained through a nonthermal, transient mobility in the course of O2 dissociation. Unlike for other known cases of the dissociation of the diatomic molecules where both “hot” adatoms accommodate at the equivalent sites, in the studied system the oxygen atoms filling the vacancies are locked into the bridging oxygen rows and only the O adatoms are relatively free to move. The transient motion of the hyperthermal oxygen adatoms on the TiO2 (110) surface occurs exclusively along the Ti troughs.
2007. "Synthesis and Characterization of Compositionally Graded Si1-xGex Layers on Si substrate." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 261(1-2):723-726. Abstract Thin film of silicon germanium (Si1-xGex) with tailored composition was grown on Si (100) substrate at 650oC in an ultrahigh vacuum molecular beam epitaxy system. The nominal x-value is ranged from 0 to 0.14. The quality of the film was investigated by Rutherford backscattering spectrometry (RBS) in random and channeling geometries, glancing angle x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDX), and atomic force microscopy (AFM). RBS/Channeling measurements indicate that the strain associated with lattice mismatch is compressive in the film. Both RBS and EDX analyses indicate the compositional graded incorporation of Ge in the film with x ranging from 0 to 0.14. The film shows island growth with each island centering around an interface dislocation.
2007. "Epitaxial Growth and Microstructure of Cu2O Nanoparticle/thin Films on SrTiO3(100)." Nanotechnology 18:Art. No. 115601. doi:10.1088/0957-4484/18/11/115601 Abstract Cuprous oxide (Cu2O) was grown on SrTiO3 (STO)(100) by oxygen plasma assisted molecular beam epitaxy. Microstructure of the grown layer and Cu valence state were analyzed using x-ray diffraction (XRD), x-ray photo-electron spectroscopy (XPS), atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM) as well as electron diffractions. The grown layer was dominated by Cu2O phase, possessing an epitaxial orientation with the substrate such that: Cu2O[001]//STO[001] and Cu2O(100)//STO(100). Cu2O film morphologically shows dependence on the growth rate. Typically, a fast growth will lead to the formation of a thin film with a relatively smooth surface. A slow growth will lead to the development of nanoparticles, featuring the formation of Cu2O pyramid. The pyramids are invariantly defined by the Cu2O {111} planes. Given the fact that the {111} planes correspond to the lowest surface energy of Cu2O, a slow growth will lend the system enough time to allow it to adopt the pyramid configuration by which the overall energy of the system was minimized.
2007. "Direct Observation of Adsorption Evolution and Bonding Configuration of TMAA on TiO2(110)." Journal of Physical Chemistry C 111(11):4342-4346. doi:10.1021/jp067264d Abstract Trimethyl acetic acid (TMAA) adsorption evolution on the rutile TiO2(110) surface from submonolayer to saturation coverages was examined at the atomic level by scanning tunneling microscopy using the same area analysis approach. Upon TMAA deprotonation, no evidence of terminal OH group formation has been found. It has been suggested that uncommon geometry associated with detached hydrogen atom takes place instead, with proton bonding to pair bridging oxygen atoms. Such a configuration is likely to be stabilized by adjacent adsorbed TMA groups and, in turn, be a factor in the formation of TMA (2x1) reconstruction at saturation coverage. Our results indicate that TMAA adsorption on reduced TiO2 is virtually not affected by bridging oxygen vacancies or other surface defects.
2007. "Atomic-Scale Assembly of a Heterogeneous Catalytic Site." Journal of the American Chemical Society 129(46):14355-14361. doi:10.1021/ja074891n Abstract The (100) surface of a AuPd bulk alloy was imaged by scanning tunneling microscopy. Chemical-contrast revealed structures with short-range order involving surface Pd atoms at c(2x2) sites with respect to the surface lattice—a first time observation of a known active heterogeneous catalytic site with atomic resolution. Comparison of the Pd distribution in a AuPd(100) surface with that in a Au3Pd(100) surface shows the unique thermodynamic properties of the 50 : 50 alloy are decisive in controlling the formation of isolated Pd sites.
2007. "Spatial First-passage Statistics of Al/Si(111)-(root 3X root 3) Step Fluctuations." Physical Review. E, Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics 75(2 PT 1):443-448. doi:10.1103/PhysRevE.75.021603 Abstract Step edge fluctuations on a multi-component surface of Al/Si(111)-( ) were measured via scanning tunneling microscopy over a temperature range of 720K-1070K, for step lengths of 65-160 nm. Even though the time scale of fluctuations of steps on this surface varies by orders of magnitude over the indicated temperature ranges, measured first-passage spatial persistence and survival probabilities are shown to be temperature independent over the temperature and spatial range. The power law functional form for spatial persistence probabilities is confirmed and the spatial persistence exponent is measured to be 0.71 ± 0.13 in contrast to theoretical predictions of a value ≤ ½. The survival probability is found to scale directly with x/L, where x is the step edge displacement and L is the step length anallyzed, with minor corrections for the discrete measurement interval. The form of the survival agrees quantitatively with theoretical predictions, which reduce to an exponential decay at short distances (x/L <~ 0.3). The decay constant is found experimentally to be (0.076 ± 0.033)L Also, the functional form of the stationary single site height distribution is found to agree with predictions.
2007. "Preparation of TiO2(110)-(1x1) Surface via UHV Cleavage: An scanning tunneling microscopy study." Review of Scientific Instruments 78(11):Art. No. 113907. doi:10.1063/1.2814160 Abstract TiO2(110) surface was successfully prepared in-situ by UHV cleaving of a commercial TiO2 crystal.. STM imaging revealed atomically flat more than 1 m wide terraces with (110) orientation separated by steps running in [001] direction, with very low kink density. Atomically resolved STM images show periodicity in the [001] and [ ] directions with the unit cell parameters measured to ~3 Å and 6.5 Å respectively which are closed to the expected values of bulk terminated (1x1) surface.
2006. "Reproducible Tip Fabrication and Cleaning for UHV STM ." Ultramicroscopy 108(9):873-877. doi:10.1016/j.ultramic.2008.02.010 Abstract Several technical modifications related to the fabrication and ultra-high vacuum (UHV) treatment of the tips have been implemented to improve a reliability of the tip preparation for high-resolution scanning tunneling microscopy. The widely used drop-off technique for the tip electrochemical etching has been further refined to enable a reproducible fabrication of the tungsten tips with a radius 3 nm. Simple and flexible setup for the tip UHV annealing has been developed and employed. The Ar ion sputtering with subsequent annealing has been adopted for a final tip treatment. The proper tip preparation has been demonstrated by imaging an atomic structure of the rutile TiO2(110) surface.
2006. "Formation of Cu2O Quantum Dots on SrTiO3 (100): Self-Assembly and Directed Self-Assembly." Journal of Applied Physics 100(9):Art. No. 094315. doi:10.1063/1.2364038 Abstract Nanoscale islands of Cu2O have been synthesized on single-crystal SrTiO3 (100) substrates using oxygen plasma-assisted molecular-beam epitaxy (OPA-MBE). Island growth location has been controlled by using an ex-situ Ga+ focused ion beam (FIB) to modify the growth surface in discrete locations prior to island synthesis. Analysis of Cu2O dot growth on unmodified substrate regions revealed an evolution of dot size and array density. Atomic force microscopy studies show that certain FIB substrate modification and MBE growth condition combinations lead to directed self-assembly of islands. Islands initially formed in the FIB-generated surface topography and filled those features before nucleating on neighboring unmodified surface regions.
2005. "Formation of Epitaxial Oxide Nanodots on Oxide Substrate: Cu₂O on SrTiO₃(100)." Surface Science 589(1-3):120-128. Abstract X-ray photoelectron spectroscopy analysis during the oxygen plasma assisted molecular beam epitaxy, combined with atomic force microscopy, scanning Auger microscopy, and theoretical simulation studies have been used to evaluate the mechanism of single-phase Cu₂O nanodot formation on the SrTiO₃(100) surface. Formation of pure crystalline Cu₂O nanodots occurs rather in a narrow growth parameter window, outside which a coexistence of the multiple phases has been observed. Cuprous oxide nanodots on the SrTiO₃(100) substrate follow a growth mechanism which differs significantly from the growth modes observed for the majority of semiconductor quantum dots. Growth starts without wetting layer formation with appearance of well-ordered truncated square-based nanodots at submonolayer coverages. At the initial stages of growth, the nanodot size is only weakly changes with coverage and exponentially scales with temperature. After reaching a critical, temperature dependent dot density (~ 10¹³ cm-² for 760 K growth temperature), growth of mid-sized nanoclusters starts through coalescence, which is eventually followed by large dome-shaped cluster formation at higher coverages. The coexistence of the different types of the clusters at high coverages results in a multi-modal distribution of sizes and shapes.
2004. "Initial Stages of Oxide Nanodot Heteroepitaxial Growth: Cu2O on SrTiO3(100)." Applied Physics Letters 85(19):4481-4483. Abstract The growth mechanism in a heteroepitaxy of oxide nanodots is investigated by combination of the x-ray photoelectron spectroscopy (XPS), atomic force microscopy and theoretical modeling. In contrast to the majority of semiconductor systems, in the studied metal oxide system of Cu2O – SrTiO3(100) the growth process starts without wetting layer formation with appearance of small ( ~ 10 nm) square-based planar Cu2O nanodots. Continued deposition leads mainly to increase of the nanodot density, practically, without change of their size. Only after reaching some critical density (~ 1013 cm-2 for 760 K growth temperature), growth of scattered, significantly larger islands starts through the coalescence of small nanodots. XPS analysis suggests that the interface between small nanodots and substrate is abrupt with only weak Cu – O(SrTiO3) interaction.
2004. "Formation of Single-Phase Oxide Nanoclusters: Cu₂O on SrTiO₃(100)." Journal of Applied Physics 94:7926. Abstract Selective formation of the single phase nano-clusters of Cu₂O on SrTiO₃(100) substrates in the size range of 10-50 nm is found to occur only in a very narrow oxygen plasma assisted molecular beam epitaxy growth parameter window, in comparison with the bulk phase diagram (for oxygen pressure vs. temperature). X-ray photoelectron spectroscopy, Auger electron spectroscopy and x-rays diffraction analysis of nano-clusters, has detected the distinctive parameter regions, where multiple phase-like forms coexist (CuO/Cu₂O and Cu₂O/Cu), in agreement with theoretical prediction for small systems, and as opposite to the sharp phase boundaries for the bulk. Observed changes in the nano-cluster composition are found to correlate with differences in cluster morphologies.
2004. "Focused-Ion-Beam Directed Self-Assembly of Cu₂O Islands on SrTiO3(100)." Applied Physics Letters 84(25):5213-5215. Abstract Nanoscale islands of Cu₂O have been synthesized on single crystal SrTiO₃ (100) substrates using oxygen plasma assisted molecular beam epitaxy (MBE). Island growth location has been controlled by using an ex-situ Ga⁺ focused ion beam (FIB) to modify the growth surface in discrete locations prior to island sythesis. The FIB modifications have generated surface topography with lateral dimensions of 150-200 nm. Ex-situ AFM study after island growth reveals that certain FIB substrate modification and MBE growth condition combinations lead to directed self-assembly of metal oxide islands at the edges of the FIB modified zones.
2004. "Distinguishing Step Relaxation Mechanisms via Pair Correlation Functions." Physical Review. B, Condensed Matter and Materials Physics 70(23):article no. 235422. doi:10.1103/PhysRevB.70.235422 Abstract Theoretical predictions of coupled step motion are tested by direct STM measurement of the fluctuations of near-neighbor pairs of steps on Si(111)-R30° Al at 970K. The average magnitude of the pair-correlation function is within one standard deviation of zero, consistent with uncorrelated near-neighbor step fluctuations. The time dependence of the pair-correlation function shows no statistically significant agreement with the predicted t½ growth of pair correlations via rate-limiting atomic diffusion between adjacent steps. The physical considerations governing uncorrelated step fluctuations occurring via random attachment/detachment events at the step edge are discussed.
2002. "Dynamics of Step Fluctuations on a Chemically Heterogeneous Surface of Al/Si(111)-(√3x√3)." Physical Review. B, Condensed Matter and Materials Physics 66(8):085327-1 - 085327-5. Abstract The analysis of the dynamics of equilibrium step fluctuations has been extended to a chemically heterogeneous surface. The multicomponent reconstructed Al/Si~111!-~)3)! surface has been studied using variabletemperature scanning tunneling microscopy at elevated temperatures. The temporal correlation functions for both single steps and step arrays follow a t1/2 dependence over the entire temperature range ~770–1020 K!, consistent with a rate limiting mechanism of random attachment and detachment of atoms at step edges. The alternative mechanism, diffusion from step-to-step, is shown to be inconsistent with more detailed analytic approximations to the correlation function for the measured step-step separation. An activation energy of 1.9 eV and the major kinetic parameters that govern surface mass transport and step equilibration processes have been determined.
2002. "Experimental Persistence Probability for Fluctuating Steps." Physical Review Letters 89(13):136102. Abstract The persistence behavior for fluctuating steps on the Si(111)-( √ 3x √3) R30°-Al surface was determined by analyzing time-dependent STM images for temperatures between 770 and 970 K. Using the standard persistence definition, the measured persistence probability displays power-law decay with an exponent of ⊖ = 0.77 +- 0.03. This is consistent with the value of ⊖ = 3/4 predicted for attachment-detachment limited step kinetics. If the persistence analysis is carried out in terms of return to a fixed-reference position, the measured probability decays exponentially. Numerical studies of the Langevin equation used to model step motion corroborate the experimental observations.