The structure of ordered Au films on TiOx

The growth of Au on an ultra-thin, ordered Mo(1 1 2)-(8 × 2)-TiO_x, was investigated using scanning tunneling microscopy (STM), low energy ion scattering spectroscopy (LEISS), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). Wetting of the TiO_x surface by Au was observed with STM and LEISS, and the ordering of the Au films was atomically resolved with STM. TPD showed that Au binds more strongly to the reduced TiO_x film than to bulk TiO₂, but more weakly than to the Mo substrate. The Au-TiO_x binding energy is greater than Au-Au in bulk Au. The oxidation state of Ti in the TiO_x film was deduced by XPS and from the Ti-O phonon shifts relative to bulk TiO₂. The TiO_x/Mo(1 1 2) film structure and those for the (1 × 1)- and (1 × 3)-Au/TiO_x/Mo(1 1 2) surfaces are discussed.     

Characterisation of alkyl-functionalised Si(1 1 1) using reflectometry and AC impedance spectroscopy

The past few years have seen a dramatic increase in the study of organic thin-film systems that are based on silicon-carbon covalent bonds for bio-passivation or bio-sensing applications. This approach to functionalizing Si wafers is in contrast to gold-thiol or siloxane chemistries and has been shown to lead to densely packed alkyl monolayers. In this study, a series of alkyl monolayers [CH₃(CH₂)_nCH=CH₂; n = 7, 9, 11, 13, 15] were directly covalent-linked to Si(1 1 1) wafers. The structures of these monolayers were studied using X-ray reflectometry (XRR) and AC impedance spectroscopy. Both techniques are sensitive to the variation in thickness with each addition of a CH₂ unit and thus provide a useful means for monitoring molecular-scale events. The combination of these techniques is able to probe not only the thickness, but also the interfacial roughness and capacitance of the layer at the immobilized surface with atomic resolution. Fundamental physical properties of these films such as chain canting angles were also determined.     

Self-assembled strontium ferrite dot array on Au underlayer

Strontium ferrite (SrM) thin films were deposited on thermally oxidized silicon wafer with Au underlayer. Gold underlayers were prepared at various substrate temperatures by using a magnetron sputtering system. C-axis oriented SrM perpendicular films and preferred (1 1 1) orientation of underlayer have confirmed by X-ray diffraction patterns. The intensity of (1 1 1) diffraction line for Au and that of (0 0 l) diffraction line for strontium ferrite decrease with increase in substrate temperature (T_u) The maximum coercivity and remanent squareness ratio in perpendicular direction, at T_u of 500 °C, are 5.4 kOe and 0.68, respectively. The strength of the intergranular interaction of SrM magnetic particles is described by the parameter Δm. The SrM/Au films prepared at T_u above 100 °C have smaller Δm peak values than that for SrM/Au films prepared at T_u of room temperature. This behavior is related to low magnetostatic coupling between the magnetic particles separated by the non-magnetic amorphous phase.     

Electrochemical stability of self-assembled monolayers of biphenyl based thiols studied by cyclic voltammetry and second harmonic generation

The reductive desorption of self-assembled monolayers (SAMs) of ω-(4′-methyl-biphenyl-4-yl)-alkanethiols (CH₃-C₆H₄-C₆H₄-(CH₂)_n-SH, BPn) on Au(1 1 1) on mica was studied in 0.5 M KOH solution as a function of the length of the aliphatic spacer chain (n = 1-6 and 12) and for two different preparations temperatures (295 K and 343 K). Second harmonic generation (SHG) was applied in situ parallel to cyclic voltammetry (CV). Odd-even differences in the structure of the BPn monolayers are clearly reflected in the electrochemical stability, as well as by the charge and shape of the desorption peak. For n = 1-5 a single desorption peak is detected whereas multiple peaks occur for BP6 similar to hexadecane thiol which was also studied for comparison. An increased preparation temperature affects the shape and width of the desorption peak but not the position. BP1 exhibits a temperature dependence different from the other homologues. The relationship between coverage monitored by SHG and desorption charge determined from the CVs is found to be linear and surprisingly independent from the details of the SAMs. The combined SHG and CV experiments suggest that capacitive and faradaic current are always closely coupled even for BP6 and hexadecane thiol which exhibit multiple desorption peaks.     

Observation of stable mixed alkanethiolate-chloride adlayer on Ag(1 1 1): structural correlation with pure alkanethiolate monolayers

The self-assembly of 1-alkanethiols, CH₃(CH₂)_n−1SH (n=2-16), on Ag(1 1 1) film initially covered with a native oxide monolayer and that on a chloride-covered Ag(1 1 1) were comparatively studied by X-ray photoelectron spectroscopy and scanning tunneling microscopy. The native oxide monolayer was readily substituted completely by thiolates irrespective of chain length, leading to a dense (√7×√7)R19.1° monolayer for n<3, or a distorted (√7×√7)R19.1° structure for n>3 accompanied by distinct island and fine domain structures previously reported by other groups. In contrast, the chloride-to-thiolate conversion was far from complete for long alkanethiols (n?8), and at sufficiently high conversion temperature (>50 °C), we found a highly stable mixed thiolate-chloride monolayer with a well-defined 2:1 S/Cl atomic ratio, suggesting the occurrence of a mixed (√7×√7)R19.1° adlayer ordering. The combined effects of substrate-molecule and intermolecular interactions behind these contrastive phenomena are discussed.     

Growth mode and electronic structure of Au nano-clusters on NiO(0 0 1) and TiO2(1 1 0)

The growth mode and electronic structure of Au nano-clusters grown on NiO and TiO₂ were analyzed by reflection high-energy electron diffraction, a field-emission type scanning electron microscope, medium energy ion scattering and photoelectron spectroscopy. Au was deposited on clean NiO(0 0 1)-1 × 1 and TiO₂(1 1 0)-1 × 1 surfaces at room temperature with a Knudsen cell at a rate of 0.25-0.35 ML/min (1 ML = 1.39 × 10¹⁵ atoms/cm²:Au(1 1 1)). Initially two-dimensional (2D) islands with thickness of one Au-atom layer grow epitaxially on NiO(0 0 1) and then neighboring 2D-islands link each other to form three-dimensional (3D)-islands with the c-axis oriented to the [1 1 1] direction. The critical size to form 3D-islands is estimated to be about 5 nm². The shape of the 3D-islands is well approximated by a partial sphere with a diameter d and height h ranging from 2.0 to 11.8 nm and from 0.95 to 4.2 nm, respectively for Au coverage from 0.13 to 4.6 ML. The valence band spectra show that the Au/NiO and Au/TiO₂ surfaces have metallic characters for Au coverage above 0.9 ML. We observed Au 4f spectra and found no binding energy shift for Au/NiO but significant higher binding energy shifts for Au/TiO₂ due to an electron charge transfer from Au to TiO₂. The work function of Au/NiO(0 0 1) gradually increases with increase in Au coverage from 4.4 eV (NiO(0 0 1)) to 5.36 eV (Au(1 1 1)). In contrast, a small Au deposition(0.15 to 1.5 ML) on TiO₂(1 1 0) leads to reduction of the work function, which is correlated with an electron charge transfer from Au to TiO₂ substrate.     

Chemisorption of alkyl thiols and S-alkyl thiosulfates on Pt(1 1 1) and polycrystalline platinum surfaces

The self-assembled monolayers prepared from 1-dodecanethiol (C₁₂SH) or S-dodecylthiosulfate (Bunte salt, C₁₂SSO₃Na) have been characterised on polycrystalline gold and platinum surfaces and on Pt(1 1 1). Contact angle and impedance measurements show that the film quality decreases in the order Au/C₁₂SH > Pt/C₁₂SH ∼ Au/C₁₂SSO₃Na > Pt/C₁₂S SO₃Na. XPS measurements show that the S-SO₃ bond of organic thiosulfates is broken on platinum surfaces and the state of the surface-bound sulfur is indistinguishable from that of thiolate. On platinum three sulfur species are formed upon SAM formation and we suggest that the catalytic activity of platinum is responsible for their existence in pristine monolayers.     

Defect assisted subwavelength resolution in III-V semiconductor photonic crystal flat lenses with n = −1

We propose a III-V semiconductor photonic crystal slab designed to operate as a n = −1 superlens at λ₀ = 1.55 μm. The structure consists of air holes arranged in a two-dimensional triangular lattice, of period a, nanopatterned in an InP/InGaAsP/InP slab. Exploiting the second pass-band regime (a/λ₀ ∼ 0.31), subwavelength resolutions as low as 0.38λ₀ for planar lenses have been obtained by the insertion of hexagonal nanocavities within the crystal.     

Self-assembled monolayers on magnesium alloy surfaces from carboxylate ions

Self-assembled monolayers (SAMs) of carboxylate salts (C_n−1H_2n−1COONa, n = 12, 16, 18) were successfully formed on Mg alloy substrate in ethanol solution and characterized by the contact angle measurement, ATR-FTIR, ellipsometry and XPS. The SAMs were regularly and densely anchored to the Mg alloy substrate via monodentate bonding with the tilting angles of about 40°, 33° and 27° for C12, C16 and C18 monolayers, respectively. Electrochemical impedance spectroscopy (EIS) proved the corrosion protection of SAMs for Mg alloy substrate with protective efficiency (PE) of even up to 98.5%.     

Thickness dependent stripe structure stability of Ag films on Si(1 1 1)-(4 × 1)-In substrate

The thickness dependent stripe structure stabilization of Ag films on Si(1 1 1)-(4 × 1)-In substrate is thermodynamically considered. It is found that for the stability of the structure, there is a competition between the sum of elastic energy and stacking fault energy in the film and the film-substrate interface energy. The presence of equilibrium of them leads to a critical film thickness. Beyond it, the stripe structure will transform into a flat one. Our prediction for n_c of Ag films shows reasonable agreement with experimental data. In addition, according to the established model, it is predicted that Au could also form the above stripe structure on this substrate with a similar n_c value of Ag.     

Formation of Ag2, Au2 and AgAu particles on MgO(1 0 0): DFT study on the role of support-induced charge transfer in metal-metal interactions

The formation of Ag₂, Au₂ and AgAu particles oriented perpendicularly to the MgO(1 0 0) surface was studied using the density functional theory. While the support induces a slight enhancement of the Ag-Ag bond (by 0.3-0.4 eV), the Au-Au bond is strongly enhanced (by 0.8-1.1 eV). Concerning the bimetallic particle, the Ag-Au bond stabilization depends on the relative position of each atom. Thus, in general terms, the strength of the metal-metal bond is determined by the nature of the terminal atom; the bond is stronger in Au-terminal particles. The partial electronic charge transfer to the terminal Au atom and its ability to polarize this charge are responsible for this energetic stabilization.     

Magneto-transport properties of [Co/Bi]n wire structures

The present study examines the artificial control of grain-boundary resistance and its contribution to the magneto-transport properties of [Co(1 nm)/Bi(2.5 nm)]_n (n=10 or 20) line structures on the Si(0 0 1)/SiN_x substrate. Conventional patterning and deposition processes are applied for the fabrication of a device that consists of five-line structures with a line width of 2 μm. A ΔR/R=80% ratio was observed in the five-line structure of [Co(1 nm)/Bi(2.5 nm)]₁₀ multilayers at 10 K. Our measurements indicate that grain-boundary effects can be associated with the large ΔR/R ratio of transverse magnetoresistance.     

Adsorption energy and geometry of physisorbed organic molecules on Au(1 1 1) probed by surface-state photoemission

The modification of the Au(1 1 1) Shockley-type surface state by an adsorbed monolayer of large π-conjugated molecules was investigated by high-resolution angle-resolved photoelectron spectroscopy (ARPES). We determined binding energy, band mass, and Rashba-splitting and discuss the results in the context of rare-gas adsorption on noble metals. This comparison allows the determination of the bonding strength of the adsorbates, found to be physisorptive with derived binding energies per molecule of 2.0 eV for perylene-tetracarboxylic-dianhydride (PTCDA) and 1.5 eV for naphtalene-tetracarboxylic-dianhydride (NTCDA). We will also present a superstructure model for the NTCDA/Au(1 1 1) system, deduced from low energy electron diffraction images (LEED) in combination with substrate band-backfolding.     

Au/TiO2/Ru(0 0 0 1) model catalysts and their interaction with CO

Au/TiO₂/Ru(0 0 0 1) model catalysts and their interaction with CO were investigated by scanning tunneling microscopy and different surface spectroscopies. Thin titanium oxide films were prepared by Ti deposition on Ru(0 0 0 1) in an O₂ atmosphere and subsequent annealing in O₂. By optimizing the conditions for deposition and post-treatment, smooth films were obtained either as fully oxidized TiO₂ or as partly reduced TiO_x, depending on the preparation conditions. CO adsorbed molecularly on both oxidized and reduced TiO₂, with slightly stronger bonding on the reduced films. Model catalyst surfaces were prepared by depositing submonolayer quantities of Au on the films and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. From X-ray photoelectron spectroscopy, a weak interaction between the Au and the TiO₂ substrate was found. At 100 K CO adsorption occurred on both the TiO₂ film and on the Au nanoparticles. CO desorbed from the Au particles with activation energies between 53 and 65 kJ/mol, depending on the Au coverage. If the Au deposit was annealed to 770 K prior to CO exposure, the CO adsorption energy decreased significantly. STM measurements revealed that the Au particles grow upon annealing, but are not encapsulated by TiO_x suboxides. The higher CO adsorption energy observed for smaller Au coverages and before annealing is attributed to a significantly stronger interaction of CO with mono- and bilayer Au islands, while for higher particles, the adsorption energy becomes more bulk-like. The implications of these effects on the known particle size effects in CO oxidation over supported Au/TiO₂ catalysts are discussed.     

Nonlinear optical properties of periodic gold nanoparticle arrays

Periodic Au nanoparticle arrays were fabricated on silica substrates using nanosphere lithography. The identical single-layer masks were prepared by self-assembly of polystyrene nanospheres with radius R = 350 nm. The structural characterization of nanosphere masks and periodic particle arrays was investigated by atomic force microscopy. The nonlinear optical properties of the Au nanoparticle arrays were determined using a single beam z-scan method at a wavelength of 532 nm with laser duration of 55 ps. The results show that periodic Au nanoparticle arrays exhibit a fast third-order nonlinear optical response with the nonlinear refractive index and nonlinear absorption coefficient being n₂ = 6.09 × 10⁻⁶ cm²/kW and β = −1.87 × 10⁻⁶ m/W, respectively.     

Coarsening of mass-selected Au clusters on amorphous carbon at room temperature

Low-energy cluster beam deposition was used to deposit mass-selected Au_n clusters (n = 4, 6, 13 and 20) on amorphous carbon (a-C) substrates. The resulting samples were stored at room temperature under ambient conditions for time periods up to 32 months to analyze the coarsening behaviour of the clusters. Cluster-size distributions were measured in regular time intervals by transmission electron microscopy (TEM). The TEM experiments show a significant increase of the average cluster size with time analogous to classical surface Ostwald ripening (OR). The coarsening of Au clusters can be well described by steady-state diffusion-limited kinetics. The derived surface mass-transport diffusion coefficients at room temperature range between 1.1 and 3.8·10⁻²⁵ m² s⁻¹ for our samples. A detailed analysis of values suggests that, the rate of the surface OR for mass-selected Au_n clusters increases with the cluster size in the sequence: Au₄ ≈ Au₆ < Au₁₃ < Au₂₀ for the investigated range of Au clusters. Given that the initial, on-surface cluster-size distributions are nominally monodisperse, classical OR with cluster coarsening based only on the Gibbs-Thomson effect cannot explain the observed coarsening. The activation of the coarsening process is rationalized by initial variations of the cluster sizes due to the deposition process itself and/or the interaction of the clusters with the substrate. Moreover, the presence of initial deposited Au clusters as different isomers with slightly different chemical potential on the substrate, may also initiate the coarsening by surface OR. Furthermore, we find that the coarsening is most pronounced for the paucidispersed sample with Au_m (10 ? m ? 20) clusters. A possible explanation of this behaviour is the presence of an initial distribution of different cluster sizes directly after deposition.     

Supramolecular assembly of biphenyl dicarboxylic acid on Au(1 1 1)

We investigate the structure of submonolayer film of 4,4′-biphenyl dicarboxylic acid (BDA) molecules on Au(1 1 1)-22 × √3 reconstructed surface with the use of scanning tunneling microscopy (STM). The BDA molecules form ordered structures on Au(1 1 1) surface which are commensurate with the substrate. We have concluded that the molecule-molecule interaction is mainly through hydrogen bonding formed by a straight dimer of BDA molecules. The straight dimer can be expressed as 4s + 2t or its six crystallographic equivalents using the unit vectors of the gold substrate of s and t. The length of hydrogen bonding (O-H-O) is estimated to be 0.31 nm assuming nearest neighbor distance of gold atoms of 0.275 nm. The ordering shows a clear contrast with the case of BDA on Cu(1 0 0) surface [S. Stepanow, N. Lin, F. Vidal, A. Landa, M. Ruben, J.V. Barth, K. Kern, Nanoletters 5 (2005) 901] in which a square type of ordering of molecules is observed by the formation of hydrogen bonding between a carboxylate (COO) and a benzene ring. The clear difference of the ordered structure on Cu(1 0 0) and Au(1 1 1) surface demonstrates that the absence (presence) of deprotonation of carboxyl group of BDA molecule on Au(1 1 1) (Cu(1 0 0)) switches the straight and square type ordering of BDA molecules.     

High-resolution photoemission studies of the interfacial reactivity and interfacial energetics of Au and Cu Schottky barriers on methyl-terminated Si(1 1 1) surfaces

The Schottky junction formation by the stepwise evaporation of gold and copper, respectively, onto methyl-terminated silicon, CH₃-Si(1 1 1), was investigated by synchrotron X-ray photoelectron spectroscopy. During the junction formation process, interface reactions occurred as revealed by the appearance of chemically shifted Si 2p components. Upon deposition of Au, the formation of about one monolayer of gold silicide, SiAu₃, with a Si 2p chemical shift of +0.75(2) eV, was observed. The SiAu₃ floated on top of the growing gold layer. Similarly, for the deposition of Cu, the methyl termination layer was partially disrupted, as indicated by the appearance of a −0.28(2) eV chemically shifted Si 2p component attributable to an interfacial copper silicide phase, SiCu₃. Hence, the termination of the Si(1 1 1) surface by methyl groups did not completely prevent interfacial reactions, but did reduce the amount interfacial reaction products as compared to bare Si(1 1 1)-(7 × 7) surfaces.Electron Schottky barrier heights of 0.78(8) eV (Au) and 0.61(8) eV (Cu) were measured. Within the experimental uncertainty the observed Schottky barriers were identical to those ones obtained on non-passivated, (7 × 7)-reconstructed Si(1 1 1) surfaces. Thus, the modification of the electronic properties of the silicon-metal contact requires the complete absence of interfacial reactions.     

The controlled excitation of forbidden transitions in the two-photon spectrum of strontium by using collisions and electric fields

We present experimental results involving controlled configuration mixing in two-photon spectroscopy of highly-excited states by exploiting a weak external electric field and collisions. The method has allowed new extensions to high members of the two-photon forbidden J = 3 odd-parity 5snf ¹F₃ and the J = 0, even-parity 5sns ¹S₀ Rydberg series of neutral strontium to be observed. We achieve resonant two-photon transverse excitation of a high density atomic jet by using a narrow bandwidth tunable dye laser in a heat pipe setup with sensitive ionization detection. Experimental term values are extended for the 5sns ¹S₀ series up to n = 46. By suitable exploitation of the composition and pressure of the buffer gases in conjunction with the electric field strength in the excitation region and the exciting laser beam intensity we have also extended observations up to n = 44 for the 5snf ¹F₃ series and up to n = 46 for the 5snp ¹P₁ series. Our results demonstrate a novel and remarkably simple experimental method to access high Rydberg states to which transitions are forbidden from the ground state by parity and other selection rules.     

Magnetic properties and microstructure study of high coercivity Au/FePt/Au trilayer thin films

High-coercivity Au(60 nm)/FePt(δ nm)/Au(60 nm) trilayer samples were prepared by sputtering at room temperature, followed by post annealing at different temperatures. For the sample with δ=60 nm, L1₀ ordering transformation occurs at 500 °C. Coercivity (H_c) is increased with the annealing temperature in the studied range 400–800 °C. The H_c value of the trilayer films is also varied with thickness of FePt intermediate layer (δ), from 27 kOe for δ=60 nm to a maximum value of 33.5 kOe for δ=20 nm. X-ray diffraction data indicate that the diffusion of Au atoms into the FePt L1₀ lattice is negligible even after a high-temperature (800 °C) annealing process. Furthermore, ordering parameter is almost unchanged as δ is reduced from 60 to 15 nm. Transmission electron microscope (TEM) photos indicate that small FePt Ll₀ particles are dispersed amid the large-grained Au. We believe that the high coercivity of the trilayer sample is attributed to the small and uniform grain sizes of the highly ordered FePt particles which have perfect phase separation with Au matrix.