Structure Of Overlayers Research Articles

An XPD and LEED study of highly strained ultrathin Ni films on Pd(1 0 0)

The epitaxial growth of ultrathin Ni films on the Pd(100) surface was studied by means of X-ray photoelectron diffraction (XPD) and LEED experiments. In excellent numerical agreement with the predictions of elasticity theory, the data indicate the formation of tetragonally strained Ni epitaxial layers, which subsequently turns into a bulk-like Ni structure as the thickness of approximately 12MLE is exceeded. This study demonstrates that LEED and XPD methodologies are rather complementary in order to have a detailed picture of the evolution of the overlayer structure in different thickness regimes.

Self-assembling of potassium nanostructures on InAs(1 1 0) surface

Potassium induced one-dimensional nanostructures are formed by deposition on the InAs(1 1 0) surface held at about 420 K, as observed by scanning tunneling microscopy. The K adatoms self-aggregate in zigzag chains oriented along the [1 1 ̄ 0] direction even at very low coverage. By increasing the K density, the atomic chains pack together, though keeping a minimum distance of 18.2 Å. At the completion of the first layer, the nanostructures extend over the entire surface giving rise to a clear c(2 × 6) low energy electron diffraction pattern. At higher coverage, a second ordered overlayer is formed. The results are discussed in terms of recent theoretical calculations and experimental work on the model system of Cs chains self-assembled on the InAs(1 1 0) surface. The structural model proposed for a single chain is used to discuss the evolution of the overlayer structure as a function of coverage.

Structural and electronic properties of thallium overlayers on the Si(111)-7×7surface

We have investigated atomic arrangements and their electronic properties of the well-ordered thallium overlayer structures formed on the Si(111)-$7\ifmmode\times\else\texttimes\fi{}7$ surface. As for other trivalent atoms, Tl is found to form a well-defined $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ surface, indicating the absence of a so-called ``inert pair effect'' considered only for Tl. Another well ordered $1\ifmmode\times\else\texttimes\fi{}1$ surface at 1.0 monolayer appears to be semiconducting in our angle-resolved photoemission spectra dominated by a unique dispersive surface band near the Fermi level. Our theoretical calculations using density-functional theory show that Tl adatoms occupy the ${T}_{4}$ sites and saturate all the dangling bonds of surface Si atoms to make the surface semiconducting with a band gap of 0.34 eV. The filled surface band observed has been well reproduced in our band calculations.

Structure of phthalocyanine overlayers on the [formula omitted] surfaces of InSb and InAs

The structure of free-base phthalocyanine (H 2Pc) and copper phthalocyanine (CuPc) overlayers deposited on the indium-terminated InSb(1 0 0) and InAs(1 0 0) (4×2)/c(8×2) reconstructed surfaces have been studied by low-energy electron diffraction and van der Waals (vdW) intermolecular interaction energy calculations. The diffraction pattern for H 2Pc grown on InSb(1 0 0) is a superposition of a (3×3) and (√10×√10) R±18.4° structure, whilst no ordered structure is formed when H 2Pc is grown on InAs(1 0 0). In contrast, CuPc films exhibit a (3×3) structure on InSb(1 0 0) and a (√10×√10) R±18.4° structure on InAs(1 0 0). These differences can be rationalised by the vdW intermolecular interaction energy calculations of the quadratic unit cells of H 2Pc and CuPc. The results show that the lattice structure during the initial stages of thin film growth is influenced strongly by the intermolecular interaction energy of their unit cells.

A low temperature scanning tunneling microscope designed for imaging molecular adsorbates

We report on the development of a low temperature scanning tunneling microscope (STM) used for imaging molecular adsorbates. The microscope operates near 85 K and features in situ tip and sample exchange. Cooling is accomplished via a clamping mechanism that provides a direct mechanical link between a liquid nitrogen dewar and the STM stage. During imaging, the clamping mechanism is released, and the STM stage is isolated from vibrations using a dual spring suspension system that incorporates magnetic eddy current damping. Additional isolation is provided by uncoupling the liquid nitrogen dewar from the vacuum chamber. We have used this STM to image benzene and carbon monoxide coadsorbed on the Pd (1 1 1) surface. The resulting acquired images show both the ordered arrangement and internal structure of the benzene molecules. Depending on the dosing conditions, three different ordered overlayer structures were formed, only one of which has been previously reported. Future plans include using the low temperature imaging capability of the STM to observe molecular binding sites, orientation, and tilt, as well as dynamical processes on the surface, such as chemical reactions.

Scanning Tunneling Microscopy Investigation of Silver Deposition upon Au(111) in the Presence of Chloride

Using electrochemical measurements and in situ scanning tunneling microscopy (STM), we confirm the process of Ag growth and the overlayer structures of Ag underpotential deposition (upd) on Au(111) in the presence or absence of chloride anions. There are four distinct structures of the Ag adlayer on Au(111) in the presence of chloride. Adlayer structures (√3 × √3)R30°, (√19 × √19)R37°, (√13 × √13)R46°, and (6 × 6) are formed as a function of potential, and these structures are clearly matched to the features in a cyclic voltammogram. Compared with the (4 × 4) structure of Ag upd on Au(111) in a pure 0.1 M HClO4 solution, overlayer structures of Ag on Au(111) in a chloride-containing solution are more loosely packed. Therefore, we conclude that the structures of the Ag overlayer on Au(111) strongly interact with chloride anions, suggesting the bilayer structure is composed of both Ag and Cl.

Polarization Modulation Infrared Reflection Absorption Spectroscopy at Elevated Pressures: CO Adsorption on Pd(111) at Atmospheric Pressures

CO adsorption on a Pd(111) single-crystal surface was investigated using in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) within the pressure range 10-6−800 mbar. The coverage-dependent CO overlayer structures found on the Pd(111) surface are identical throughout this pressure regime, that is, no new surface species at elevated pressures or adsorbate-induced substrate reconstructions were observed. The transition from an adsorbate superstructure dominated by bridged-bound CO to an adsorbate overlayer having 3-fold/atop CO sites was followed by varying the adsorbate pressure over 9 orders of magnitude. The derived phase diagram indicates an apparent activation energy of 44.35 ± 1.63 kJ/mol for the bridged-to-3-fold-hollow/atop transition. A comparison between these data and recent sum frequency generation (SFG) results is made.

Ge growth on GaAs(001) surfaces studied by reflectance anisotropy spectroscopy

Surface modifications induced by germanium deposition onto clean GaAs(001) substrates have been monitored by reflectance anisotropy spectroscopy (RAS) and low energy electron diffraction (LEED). The clean GaAs(001) surfaces onto which Ge has been evaporated were c(4X4), (2X4), and (4 × 1) reconstructed. Regardless of the initial surface reconstruction, after deposition of 0.5 monolayers of Ge and further annealing at 850 K, we have always observed a (1 × 2) LEED pattern and the same characteristic RAS spectrum. On the contrary, overlayer structures obtained at intermediate stages between the clean surface and this (1 × 2) phase depend upon the initial surface reconstruction. Modifying the (1 × 2) reconstructed surface by deposition of additional monolayers of Ge or exposure to atmosphere, we have separated the surface, interface, and bulk contributions to the RAS spectra. Finally, monitoring the characteristic linear-electro-optical feature appearing at E 1 and E 1 + Δ 1 bulk critical points, we discuss how its change in amplitude and sign could be connected to a variation of the substrate doping induced by annealing.

New possibilities for tuning ultrathin cobalt film magnetic properties by a noble metal overlayer.

Complementary multiscale magneto-optical studies based on the polar Kerr effect are carried out on an ultrathin cobalt wedge covered with a silver wedge and subsequently with the Au thick layer. A few monolayers of Ag are found to have a substantial effect on magnetic anisotropy, the coercivity field, and Kerr rotation. The silver overlayer thickness-driven magnetic reorientation from easy axis to easy plane generates a new type of 90 degrees magnetic wall for cobalt thicknesses between 1.3 and 1.8 nm. The tuning of the wall width in a wide range is possible. Tailoring of the overlayer structure can be used for ultrathin film magnetic patterning.

Evidence of possible band symmetry effects in STM studies of Gd overlayers

The thinner Gd films form a rectangular structure on both W(1 1 2) (1–2 ML (monolayers)) and Mo(1 1 2) (3–10 ML), with crystallite domains exhibiting a large in-plane aspect ratio along the 〈 1 ̄ 1 ̄ 1〉 surface direction versus the 〈1 1 ̄ 0〉 direction. At slightly higher coverages on W(1 1 2) (and much higher coverages on Mo(1 1 2)), the Gd overlayer adopts an hexagonal packing consistent with that expected of the hcp crystal structure of Gd(0 0 0 1). The band symmetries are expected to differ substantially with structure, based on angle-resolved photoemission studies of Gd on Mo(1 1 2). With changes in the overlayer structure, the changes in the band symmetries, as well as occupied density of states, are believed to influence the tunneling current and hence the apparent corrugation observed in STM for Gd on W(1 1 2). Changes in the unoccupied states do not result in significant changes in the observed STM corrugation.

Phosphine adsorption and the production of phosphide phases on Cu(0 0 1)

The adsorption and thermal decomposition of PH 3 on a Cu(0 0 1) surface has been investigated by means of high-resolution helium atom scattering, Auger electron spectroscopy and temperature programmed desorption. Phosphine adsorption at T x <160 K produces a molecular overlayer structure with short-range order. Annealing of the low-temperature structure results in decomposition of the adsorbed PH 3, and desorption of hydrogen between T x =250 K and T x =320 K. Molecular phosphine was not seen to desorb. Phosphine deposition at room temperature or above produces well-ordered copper–phosphorus phases exhibiting either c(6×8) or c(6×6) diffraction patterns, depending upon the deposition conditions. Auger electron spectra indicate that these ordered phases contain approximately 1/3 ML or more of phosphorus. Possible atomic models for the overlayer structures are discussed.

Investigation of the surface phase diagram of Fe( [formula omitted])–S

Reconstructions and overlayer structures of the Fe(1 1 0) surface at different sulfur (S) coverage are presented by atomically resolved scanning tunneling microscopy combined with Auger electron spectroscopy and low energy electron diffraction investigations. S overlayers were obtained by annealing a Fe(1 1 0) single crystal to 700 °C in order to induce S segregation from the bulk. Fe(1 1 0)c(6×4), (3×1) and (1×1) reconstructions were found at sub or monolayer S coverage. At S coverage above one monolayer a (2×1) super structure was formed on top of the Fe(1 1 0)(1×1)–S structure. When increasing the coverage further, S grow in a zigzag formation from step edges across the terraces. These zigzag rows grow in the [1 1 ̄ 1] and [0 0 1] directions and form a quasi-ordered parallelogram structure.

Synthesis and structural study of nano/micro diamond overlayer films

The nano/micro diamond overlayer films have been successfully fabricated by means of bias growth technique using microwave plasma-enhanced chemical vapor deposition method. During the diamond growth, as a negative bias (−100 V) is applied midway on the substrate side, the nanocrystalline diamond layer can be controllably deposited on the as-grown microcrystalline diamond to form a nano/micro overlayer structure. Transmission electron microscopy images reveal that the nanocrystalline layer consists of ultra-fine diamond crystallites, of which the average grain size is only about 4–5 nm. The formation mechanism of diamond nanocrystals on diamond micrograins under the bias conditions is discussed. Moreover, by the repeat-biasing way, three or more layered diamond films can be produced as well.

Core-level photoemission study of the Pb overlayers on Si(001)

High-resolution core-level photoemission spectroscopy using synchrotron radiation and low-energy electron diffraction have been applied to study the intriguing structures of the Pb overlayers on Si(001) up to a coverage of 1 monolayer (ML). At a coverage of less than 0.6 ML, two distinct components are observed in Pb $5d$ core levels. This suggests that the Pb chains formed at this coverage range, as observed in the previous scanning tunneling microscopy studies, are composed of buckled Pb dimers consistent with the recent structure model. The Pb $5d$ core-level shift due to this buckling is measured to be 0.35 eV. The spectral line shape of Pb $5d$ at an intermediate coverage around 0.75 ML, exhibiting the $c(8\ifmmode\times\else\texttimes\fi{}4)$ phase in the low-energy electron diffraction, differs significantly from that of the lower coverages in contradiction to the available structure models with asymmetric buckled dimers. In contrast, the $2\ifmmode\times\else\texttimes\fi{}1\ensuremath{-}\mathrm{Pb}$ surface developed at a full monolayer coverage exhibits only single component in Pb $5d.$ This result is compatible only with the close-packed symmetric dimer model. The spectral line shape of Pb $5d$ suggests a gradual change from a semiconducting to a metallic surface at 0.75--1.0 ML, which agrees with the previous valence-band photoemission study indicating the metallic nature of the $2\ifmmode\times\else\texttimes\fi{}1$ phase. Detailed analyses of the Si $2p$ core levels for the well-ordered $2\ifmmode\times\else\texttimes\fi{}2,$ $c(8\ifmmode\times\else\texttimes\fi{}4),$ and $2\ifmmode\times\else\texttimes\fi{}1$ phases at 0.5, 0.75, and 1.0 ML, respectively, are given for the discussion of the Pb-induced Si surface reconstructions.

Determination of the coverage dependent work function for Li adsorbed on Ru( [formula omitted

The coverage dependent work function change ΔΦ was measured for Lithium adsorbed on Ru(0 0 1) at T=100 K. A highly flux stable source of atomic Lithium was employed to ensure linear dependence of coverage with time. An absolute coverage calibration with an uncertainty of about 3% was achieved through observation of the ( 3 × 3 )R30° overlayer structure by LEED. The temperature programmed desorption monolayer was thus found to be n/ n Ru=0.76±0.02. Continuous work function measurements were performed using the diode method, while ultraviolet photoelectron spectroscopy has been used to check the work function change at selected points. Our new data are intended as a benchmark for theoretical calculations of work function changes in these systems. Considerable deviations between experiment and theory were the reason to remeasure ΔΦ versus coverage in a precise way. The discrepancy towards theory remained. This instigated a new effort in the calculation of work function changes with coverage. The agreement between the ab initio theory and experiment is now close to perfect.

An experimental study on adsorption of benzene on Co(0 0 0 1)

The adsorption of benzene on Co(0 0 0 1) was studied by X-ray photoelectron spectroscopy, temperature programmed desorption, low energy electron diffraction (LEED) and work function measurements. The adsorption was found to be molecular at room temperature and to saturate at a fractional coverage of 0.125 ML. With LEED a c(2 3 ×4 ) overlayer structure was seen. Below 220 K at high exposures a p( 7 × 7 ) R19° LEED pattern was observed corresponding to a coverage of 0.143 ML. Temperature programmed desorption measurements stated that benzene starts to decompose around 340 K to hydrogen and a hydrocarbon fragment, most likely C 6H 5. While the hydrogen desorbed, the hydrocarbon stayed at the surface. The desorption of molecular benzene was negligible. The activation energy for the dehydrogenation was calculated to be about 102 kJ/mol. The work function of Co(0 0 0 1) decreased by 1.3 eV upon saturation with benzene. The induced dipole moment was calculated to be 1.9 Debye/molecule.

The structure and reactivity of quinoline overlayers and the adsorption geometry of lepidine on Pt{1 1 1}: model molecules for chiral modifiers in enantioselective hydrogenation

NEXAFS, variable temperature STM and TPR have been used to study model adsorbates of relevance to heterogeneous chiral hydrogenation on platinum surfaces. In both the presence and absence of co-adsorbed hydrogen, quinoline molecules are uniformly though randomly distributed on Pt {1 1 1} at 300 K. At 350 K, irreversible surface reactions denude the Pt terraces of modified adsorption sites. Under conditions of hydrogen starvation, extensive polymerisation of quinoline occurs. In the presence of hydrogen, polymerisation is accompanied by hydrogenolysis. In both cases, the {1 1 1} terraces are depleted of quinoid-modified adsorption sites. Quinoline lies approximately flat at 300 K ( α∼15°), bonded to the surface predominantly via the aromatic π system. This adsorption geometry remains largely unaffected by heating to 360 K, although the data suggest that molecular tilt increases somewhat with surface coverage ( α∼20°). Lepidine exhibits a greater tilt angle than quinoline on Pt {1 1 1} ( α∼32°), the likely result of steric hindrance due to the substituent at the C4 ′ position––a characteristic that is present in all known efficient chiral modifiers for the reaction of interest. These findings are discussed with respect to the mechanism of chiral induction and the origin of enantioselectivity collapse in the asymmetric hydrogenation of α-ketoesters on Pt catalysts.

Epitaxial growth of Sb thin film and chemical reaction of Sb-induced surface reconstruction on Si(1 1 3)3×2

Antimony layers deposited on a Si(1 1 3)3×2 surface are investigated with low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). No long-range order is obtained in the room temperature grown Sb layer. Post-growth annealing at about 100°C restores long-range order and produces two-domain sixfold symmetric hexagonal structure. The tilted angle between two domains is 33.6°, which corresponds to the angle between [ 7 ̄ 4 1] and [4 7 ̄ 1] direction on bulk-terminated Si(1 1 3) plane. Sb (MNN) and Si (LMM) AES peak-to-peak intensities as a function of Sb deposition time at 100°C indicate that the overlayer structure grows a layer-by-layer fashion on the Si(1 1 3) surface at least up to two full molecule layer. Simple overlayer structure model, which consists of Sb n ( n=1–4) molecules, is suggested. Depending on the substrate temperatures above 300°C and Sb coverages, the 1×1, 2×2, and 2×5 reconstructions are formed successively. Oxygen exposure shows quite different surface chemical reactivities depending on the surface structures. The 1×1 and 2×2 surfaces are strongly passivated, on the other hand, the 2×5 surface reacts with oxygen relatively well. The different chemical reactivity is explained by the existence of the Sb lone pairs and Si dangling bonds in the unit cells.

Elastic electron backscattering from overlayer/substrate systems

AbstractElastic electron backscattering probability for Au overlayers deposited on Ni were determined experimentally in the energy range 200–1000 eV. These measurements were accompanied by characterization of the overlayer structure and the mean overlayer thickness. The elastic backscattering probabilities also were derived from extensive Monte Carlo simulations of the electron transport in overlayer/substrate systems. Two overlayer structures were considered in these calculations: a uniform Au layer, and isolated Au islands located on the uncovered substrate. Analysis of the shape of Au 4d spectra with QUASES‐Tougaard software indicated that the latter structure is more probable. It has been found that the experimental elastic backscattering probabilities compare better with the theoretical probabilities calculated for the island structure. The possibility of characterization of the overlayer thickness and structure from the elastic backscattering probabilities has been discussed. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Characterization of molecular overlayers on metal surface in dynamic equilibrium by scanning tunneling microscope

The overlayer structure of [tetra-(3,5-di-tertiary-butylphenyl)porphyrinato] copper (Cu-TBPP) molecules on Ag (001) and chloro(subphthalocyaninato)boron (III) (SubPc) molecules on Ag (111) were analyzed by a UHV scanning tunneling microscope. In the case of approximately 20% coverage, both molecules showed high mobility on the substrates, which caused them to form an overlayer island on terraces. Cu-TBPP molecules formed an oblique lattice on the Ag (001) surface and SubPc molecules formed a hexagonal lattice on Ag (111). From a high-resolution STM image of the overlayer, characteristic protrusion corresponding to the molecular shapes, such as four-lobes of Cu-TBPP and three-lobes of SubPc, were clearly identified.