Measuring the Au(1 1 1) surface state at the solid–liquid interface

We present the first direct observation of an occupied noble metal surface state at the metal/liquid interface. The Au(1 1 1) Shockley-like surface state was measured by scanning tunneling spectroscopy (STS) at the Au(1 1 1)/n-tetradecane interface. These results show that the surface state of gold survives in a liquid environment, and can be probed by STS. More generally it indicates that STS can be used to study electronic properties of surfaces at the solid/liquid interface, and that spectra can be directly compared to measurements and calculations of a surface’s electronic structure in ultra-high vacuum.     

Potential-induced structure transitions in self-assembled monolayers: ethanethiol on Au(1 0 0)

We have characterized the structural behaviour of ethanethiol self-assembled monolayers (SAMs) on Au(1 0 0) in 0.1 M H₂SO₄ as a function of electrode potential, using in-situ scanning tunneling microscopy (STM). After modification of the Au(1 0 0) electrode in an ethanolic solution of ethanethiol, STM images in air reveal a disordered thiol adlayer and a surface that is covered by 25% of monoatomic high gold islands, which originate from lifting of the (hex) reconstruction during thiol adsorption. In contrast to alkanethiol SAMs on Au(1 1 1), no vacancy islands are seen on the Au(1 0 0) surface. After contact of the SAM-covered Au(1 0 0) electrode with 0.1 M H₂SO₄ under potential control, two different structures are observed, depending on the potential range positive or negative of +0.3 V vs. SCE. In both cases the emerging ordered structures are quadratic, their unit cells being rotated by 45° with respect to the main crystallographic axes of the substrate. However, the ordered structure at negative potentials is more densely packed than the one at positive potentials, and in addition the surface reveals an almost 50% coverage of monoatomic high gold islands. The structure of the SAM changes reversibly with the electrode potential, the long range order gradually decreasing with each transition. Concomittant with this structure transition monoatomic deep holes are created when the potential is stepped from the cathodic to the anodic region. The experimental observations are rationalized by a high mobility of the gold thiolate moiety, causing the surface density of the SAM-covered gold to change drastically with potential.     

Enhanced dispersion and stability of gold nanoparticles on stoichiometric and reduced TiO2(1 1 0) surface in the presence of molybdenum

Mo, Au and their coadsorbed layers were produced on nearly stoichiometric and oxygen-deficient titania surfaces by physical vapor deposition (PVD) and characterized by low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and scanning tunnelling microscopy (STM). The behavior of Au/Mo bimetallic layers was studied at different relative metal coverages and sample temperatures.

STM data indicated clearly that the deposition of Au on the Mo-covered stoichiometric TiO₂(1 1 0) surface results in an enhanced dispersion of gold at 300 K. The mean size of the Au nanoparticles formed at 300 K on the Mo-covered TiO₂(1 1 0) was significantly less than on the Mo-free titania surface (2 ± 0.5 nm and 4 ± 1 nm, respectively). Interestingly, the deposition of Mo at 300 K onto the stoichiometric TiO₂(1 1 0) surface covered by Au nanoparticles of 3–4 nm (0.5 ML) also resulted in an increased dispersity of gold. The driving force for the enhanced wetting at 300 K is that the Au–Mo bond energy is larger than the Au–Au bond energy in 3D gold particles formed on stoichiometric titania. In contrast, 2D gold nanoparticles produced on ion-sputtered titania were not disrupted in the presence of Mo at 300 K, indicating a considerable kinetic hindrance for breaking of the strong Au-TiO_x bond.

The annealing of the coadsorbed layer formed on a strongly reduced surface to 740 K did not cause a decrease in the wetting of titania surface by gold. The preserved dispersion of Au at higher temperatures is attributed to the presence of the oxygen-deficient sites of titania, which were retained through the reaction of molybdenum with the substrate. Our results suggest that using a Mo-load to titania, Au nanoparticles can be produced with high dispersion and high thermal stability, which offers the fabrication of an effective Au catalyst.     

Photoemission, NEXAFS and STM studies of pentacene thin films on Au(1 0 0)

We have investigated the initial stages of the growth of pentacene thin films on the Au(1 0 0) substrate using synchrotron radiation photoelectron spectroscopy (PES), near edge X-ray absorption fine structure (NEXAFS) and scanning tunnelling microscopy (STM). Results indicate a well-ordered structure with the pentacene molecules adopting a predominantly flat orientation with respect to the substrate for coverages of less than three monolayers. NEXAFS and photoemission data indicates the presence of a second molecular orientation for thicker films, with the introduction of a slight tilting away from planar bonding geometry at higher pentacene coverages. STM images of coverages less than three monolayers indicate a well-ordered pentacene structure allowing for the calculation of pentacene unit cell parameters. The pentacene molecular rows adopt a side-by-side bonding arrangement on the surface. For pentacene deposited at room temperature, step edges were observed to act as nucleation centres for film growth. Annealing of the substrate to 373 K was found to remove excess molecules and improve film quality, but did not otherwise change the bonding geometry of the pentacene with respect to the surface.     

Enhanced transient reactivity of an O-sputtered Au(1 1 1) surface

The interaction of SO₂ with oxygen-sputtered Au(1 1 1) (θ_oxygen  0.35 ML) was studied by monitoring the oxygen and sulfur coverages as a function of SO₂ exposure. The morphology of the sputtered Au is relatively smooth on a long length scale, but rough on a finer scale with islands averaging 15 nm. The rough surface is not stable to scanning with the STM. Two reaction regimes were observed: oxygen depletion followed by sulfur deposition. An enhanced, transient sulfur deposition rate is observed at the oxygen depletion point. This effect is specifically pronounced if the Au surface is continuously exposed to SO₂. The enhanced reactivity towards S deposition seems to be linked to the presence of highly reactive, under-coordinated Au atoms. Adsorbed oxygen appears to stabilize, but also to block these sites. In absence of the stabilization effect of adsorbed oxygen, i.e. at the oxygen depletion point, the enhanced reactivity decays on a timescale of a few minutes. These observations shed a new light on the catalytic reactivity of highly dispersed gold nanoparticles.     

Local Au coverage as driving force for Au induced faceting of vicinal Si(0 0 1): a LEEM and XPEEM study

Adsorption of Au at 850°C on a regular stepped 4° vicinal Si(0 0 1) surface results in a dramatic change of the step morphology: the surface decomposes into areas which are perfectly flat with a (0 0 1) orientation and (1 1 9) facets. Low energy electron microscopy shows the dynamics of the faceting process in real space while X-ray photoemission electron microscopy (XPEEM) allows a spatially resolved determination of the Au coverage at different stages of the faceting process. At a critical Au coverage of ≈1/3 ML (0 0 1) terraces are formed which extend anisotropically along the step edges of the surface. The steps in between the terraces bunch and form step bands in order to conserve the macroscopic miscut of the sample. Driving force for this morphological transformation is a complex (5×3.2) reconstruction formed on the (0 0 1) terraces. XPEEM shows this phase separation also for the Au coverage: on the (0 0 1) terraces the Au coverage is up to 40% higher compared to the step bands. With further increasing Au coverage the width of the Au rich terraces increases while the step bands become steeper. In a second step Au adsorbs on the step bands transforming them into well defined and smooth (1 1 9) facets.     

Surface and subsurface distortions of the Au{110}-(1×2) structure

The reconstruction of the Au{110}-(1×2) missing-row surface has been studied by means of the new scattering and recoiling imaging spectrometry (SARIS) technique. The three-dimensional focusing patterns observed for scattering of 4 keV He⁺, Ne⁺ and Ar⁺ ions are highly sensitive to the structure of both the surface and subsurface layers. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) were used to simulate the scattering patterns. Using an R-factor comparison of the experimental and simulated images, it is demonstrated that SARIS is sensitive to changes of the order of 0.02 Å in the structural parameters of this Au surface. These parameters involve interlayer spacings, row pairing and row buckling in the first-through fifth atomic layers. Results for the shallow surface layers are in general agreement with the those of previous studies. The new results include structural parameters for the deeper subsurface layers and the observation of an oscillatory behavior of the layer spacings which is damped towards deeper layers.     

One-dimensional structural and electronic properties of magnetite Fe3O4(1 1 0)

We study the structure and the electronic properties of the (1 1 0) surfaces of magnetite Fe₃O₄ thin films by scanning tunneling microscopy (STM) and spectroscopy (STS). The STM images show a surface reconstruction consisting of ridges along the direction. Based on atomically resolved STM images we present a model for the observed ridge reconstruction of the surface, in agreement with a bulk-truncated layer containing both octahedral and tetrahedral iron ions. The metallic and semiconductor-like shapes of the measured current-voltage (I-V) curves indicate a non-uniform segregation of magnesium through the film. The weak contrast between the tops and valleys of ridges measured in the STS current maps is attributed to tetrahedral and octahedral coordination at the tops and the valleys, respectively. This attribution is in agreement with the proposed structure model. We observe a contrast enhancement at a tip change accompanied by a corrugation enhancement. This tip change is induced by picking up material from the sample, resulting in a magnetic tip. Thus, the contrast enhancement is attributed to detection of spin polarized current.     

Structure and electronic properties of ultrathin Co films on W(1 1 0)

The structure and electronic properties of ultrathin Co films on W(1 1 0) grown by molecular beam epitaxy in UHV were investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy and spectroscopy (STM and STS). For coverages above 0.7 ML the pseudomorphic (ps) monolayer is transformed gradually into close-packed (cp-) monolayer areas, showing up as separated islands that increase in size with coverage until the cp-monolayer is complete. Two different structures of the cp-monolayer were observed by atomically resolved STM, both leading to a 8 × 1 superstructure in the LEED pattern. Higher coverages continue to grow in the Stransky–Krastanov growth mode forming simultaneously double layer islands and triple layer islands in fcc(1 1 1) and hcp(0 0 0 1) stacking. STS reveals tunneling spectra that differ considerably depending on the thickness and on the structure. Two different classes of triple layer islands can be distinguished by a resonant peak at +0.3 eV appearing in only one of the two classes. We attributed this behavior to a different stacking according to a fcc or hcp structure.     

Overpotential deposition of Ag monolayer and bilayer on Au(1 1 1) mediated by Pb adlayer underpotential deposition/stripping cycles

Ultra-thin Ag films on the Au(1 1 1) surface were prepared via overpotential deposition (OPD) in the presence of Pb²⁺ ions. By carrying out repetitive Pb adlayer underpotential deposition (UPD) and stripping cycles during Ag bulk deposition, the two-dimensional growth of Ag films was significantly enhanced in high OPD. The Ag monolayer sample was made by comparing the voltammetry curves, in which the signatures for Pb adlayer UPD on Au(1 1 1) changed to that on Ag(1 1 1). As demonstrated by the X-ray specular reflectivity measurements, nearly complete monolayer and bilayer films can be made with optimized deposition procedures. On subatomic scale, however, we found that these films have significant higher root-mean-square displacement amplitudes than those underpotentially deposited Ag monolayer and bilayer on either Au(1 1 1) or Pt(1 1 1).     

Direct evidence for charge ordering and electronic phase separation in BixSr1−xMnO3 at room temperature

We present a study of charge ordering and electronic phase separation (EPS) phenomenon in Bi_xSr_1−xMnO₃, for an exhaustive range of x (0.25x0.75), by STM/STS at room temperature (RT) and specific heat measurements at high temperatures (350–650 K). Atomically resolved STM images of the samples, in real space, show the presence of stripe-like charge-ordered (CO) phase coexisting with charge-disordered (CD) phase. The STM images further reveal that the fraction of CO phase increases with an increase in x. The conductance spectra of these phases measured at nano level by STS are discussed. The transition to CO phase above RT is corroborated by specific heat measurements in all samples, giving a T_CO(x) phase diagram for this system.     

Investigation of the Shockley surface state on clean and air-exposed Au (1 1 1)

Scanning tunneling microscopy (STM) and spectroscopy (STS) carried out in vacuum and air were used to study the electronic structure of the Au (1 1 1) surface in the range of 0.0-0.7 eV below the Fermi level. The STS experiment carried out in UHV showed the existence of the Shockley surface state (SS) located 0.48 eV below the Fermi level. STS carried out in air showed strong local maximum located 0.35 eV below the Fermi level. This maximum was ascribed to the SS shifted toward lower energy due to carbon and oxygen overlayer. To confirm that the SS could exist on the sample exposed to air we did ultraviolet photoemission spectroscopy (UPS) experiment on air-treated and clean Au (1 1 1). Our results suggest that the SS position initially measured at 0.38 eV below the Fermi level was shifted to 0.27 eV after air treatment. Additionally, the level of contamination was measured using X-ray photoelectron spectroscopy (XPS).     

High temperature STM/STS investigations of resonant image states on Au(1 1 1)

Scanning tunneling microscopy (STM) and spectroscopy (STS) were used to study the electronic structure of Au(1 1 1) surface in the range of 2-5.5 eV above the Fermi level. In this paper, we concentrate firstly on the position of the upper band gap edge (BE) existing in [1 1 1] direction in Au(1 1 1) and secondly on the position of the resonant image potential surface state (RIS) located in the bulk states approximately 1.1 eV above BE. The experiment was carried out in UHV at two temperatures 294 K and 580 K. Our high temperature STS (HT-STS) results clearly show the presence of RIS and BE local maxima at both temperatures. What is more, a slight shift towards the Fermi level of BE and RIS was observed. Those shifts were the consequence of the change of [1 1 1] band gap and lowering gold work function due to the thermal extension of interatomic distances. Finally, estimation of the work function was given at 294 K and 580 K.     

Growth of metallic nanowires on anisotropic Si substrates: Pb on vicinal Si(0 0 1), Si(7 5 5), Si(5 3 3), and Si(1 1 0)

The spontaneous formation of mesoscopic Pb-wires, on 4° off-cut Si(0 0 1) vicinal surface, Si(7 5 5), Si(5 3 3), and Si(1 1 0) substrates was studied by low-energy electron microscopy. Before the deposition of Pb the substrates were modified by predeposition of a submonolayer amount of Au followed by annealing. The Au-induced reconstruction creates quasi-one-dimensional facets and superstructures. Their width ranged from several hundred nm in the case of the vicinal Si(0 0 1) down to atomic scale size, for the Si(1 1 0) surface. The best-developed arrays of parallel aligned mesoscopic wires were obtained during the deposition of Pb on substrates cooled slightly below room temperature. Wires with length to width ratio reaching 130 were produced on the Si(7 5 5) and the Si(5 3 3) substrates. The width of these nanowires was uniform over the whole substrate and was about 60 nm. The driving forces for the formation of the mesoscopic wires are the anisotropic strain due to the large misfit between the Pb and the Si lattice and one-dimensional diffusion of Pb.     

In-situ formation and detailed analysis of imine bonds for the construction of conjugated aromatic monolayers on Au(111)

We present the synthesis of 4′-amino-4-mercaptobiphenyl (AMB) and its deposition from solution onto Au(111) substrates. The resulting organic thin films were characterized by contact angle, infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) measurements. It is demonstrated that the majority of AMB molecules are coupled to the gold surface via S–Au covalent bonds, although only little orientational order of the AMB layer could be detected by NEXAFS. Furthermore, aromatic imine bonds between AMB and 4-hydroxybenzaldehyde (HB), 4-carboxybenzaldehyde (CB), 4-methylbenzaldehyde (MB), or 4-(trifluoromethyl) benzaldehyde (TMB) have been successfully formed. As a result of the limited order, this coupling reaction was incomplete. Nevertheless, the experimental results confirmed the formation of conjugated aromatic imine bonds.     

Giant alkali-metal-induced lattice relaxation as the driving force of the insulating phase of alkali-metal/Si(111):B

Ab initio density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the 2sqrt[3]×2sqrt[3]R30 surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. A large K-induced vertical lattice relaxation occurring only for 3/4 of Si adatoms is shown to quantitatively explain both the chemical shift of 1.14 eV and the ratio 1/3 measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character. Finally, the calculated STM images agree with our experimental results. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effects.     

STM studies of the reconstructed Au(1 1 1) thin-film at elevated temperatures

High temperature scanning tunneling microscope (HT-STM) was used to investigate a reconstructed Au(1 1 1) film evaporated on mica. The experiment was carried out at elevated temperatures in the range of 300-500 K. A herringbone reconstruction was observed at a wide range of temperatures. However, at the highest temperatures studied a break down of the reconstruction long range order was noticed. Finally, the presence of a triangular-like reconstruction was reported. Changes in the reconstruction were explained in terms of the change in surface stress arising as a result of the tension at the gold-mica interface.     

Scanned-probe topological and spectroscopic study of surface states on clean and Si-deposited GaAs (0 0 1)-c(4×4) surfaces

Microscopic topological and spectroscopic properties of MBE-grown GaAs c(4×4) surfaces without and with monolayer Si deposition were investigated by the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Empty state STM images on as-grown surface showed bright and dark cells, and they exhibited strong correlation with the spatial distribution of normal and anomalous conductance gaps of the STS spectra. Bias dependent STM images indicated presence of pinning areas with continuous space and energy distribution of surface gap states. By deposition of monolayer Si, dark areas reduced a great deal and the rate of finding normal STS spectra increased, indicating large reduction of surface states.     

Ordered arrangement of 9-aminoanthracene on Au(1 1 1) surfaces: A scanning tunneling microscopy study

We present a scanning tunneling microscopy (STM) investigation of 9-aminoanthracene (AA) on the reconstructed Au(1 1 1) surface. The bare Au(1 1 1) surface shows the herringbone reconstruction which is conserved upon deposition of the organic molecules. Most of the AA molecules are found to decorate the regions of fcc-stacking of the gold surface where a periodic linear arrangement is observed. The orientation of the long molecule axis of individual molecules is along the -directions of the Au substrate. In addition, for individual domains of the surface reconstruction, one of the three possible orientations is preferred. On substrate areas which exhibit a high step density, the steps are completely decorated by AA molecules. A detailed analysis of the STM images reveals that the molecules are located on top terrace levels. The fine structure of individual molecules on the terrace shows a clear dependence on the tunneling voltage and resembles the molecular orbitals of the free AA molecule.     

Surface structure evolution during Sb adsorption on Si(1 1 1)–In(4×1) reconstruction

The Sb adsorption process on the Si(1 1 1)–In(4×1) surface phase was studied in the temperature range 200–400 °C. The formation of a Si(1 1 1)–InSb (2×2) structure was observed between 0.5 and 0.7 ML of Sb. This reconstruction decomposes when the Sb coverage approaches 1 ML and Sb atoms rearrange to and (2×1) reconstructions; released In atoms agglomerate into islands of irregular shapes. During the phase transition process from InSb(2×2) to Sb (θ_Sb>0.7 ML), we observed the formation of a metastable (4×2) structure. Possible atomic arrangements of the InSb(2×2) and metastable (4×2) phases were discussed.