ML Au Research Articles

Formation of Graphene on Gold–Nickel Surface Alloys

Nickel (Ni)-catalyzed growth of a single- or rotated-graphene layer is a well-established process above 800 K. In this report, a Au-catalyzed, low-temperature, and facile route at 500 K for graphene formation is described. The substantially lower temperature is enabled by the presence of a surface alloy of Au atoms embedded within Ni(111), which catalyzes the outward segregation of carbon atoms buried in the Ni bulk at temperatures as low as 400-450 K. The resulting surface-bound carbon in turn coalesces into graphene above 450-500 K. Control experiments on a Ni(111) surface show no evidence of carbon segregation or graphene formation at these temperatures. Graphene is identified by its out-of-plane optical phonon mode at 750 cm-1 and its longitudinal/transverse optical phonon modes at 1470 cm-1 while surface carbon is identified by its C-Ni stretch mode at 540 cm-1, as probed by high-resolution electron energy-loss spectroscopy. Dispersion measurements of the phonon modes confirm the presence of graphene. Graphene formation is observed to be maximum at 0.4 ML Au coverage. The results of these systematic molecular-level investigations open the door to graphene synthesis at the low temperatures required for integration with complementary metal-oxide-semiconductor processes.

Properties of Au film on Ru(0001) revealed by AES, LEED, and DEPES

In this work the application of the surface sensitive experimental methods such as Auger electron spectroscopy, low energy electron diffraction, and directional elastic peak electron spectroscopy is shown to characterize the growth mode, the long and short range order of Au layers on the Ru(0001) surface. The adsorption was carried out at different substrate temperatures from room temperature till 460 °C. Auger signal from the substrate and the adsorbate recorded during the continuous Au adsorption indicates the formation of the first and then the second gold wetting layer at elevated substrate temperatures. At room temperature the formation of the second layer just before the completion of the first monolayer occurs. Further adsorption leads to the growth of three dimensional gold islands on 2 ML of Au on Ru(0001). At elevated temperatures the reduction of the Auger signal drop rate from the substrate and the signal rise rate from the adsorbate after the formation of 2 ML of Au can be rationalized by the coalescence of three dimensional Au islands and/or the decrease of the sticking coefficient of gold. Above 460 °C the saturation of Auger signals corresponding to 2 ML of Au is observed. The LEED pattern observations reveal the long range order within the clean and covered by about 5 ML of gold Ru(0001). The diffraction spots of the adsorbate reflect the Au(111) arrangement. The application of the DEPES method to the clean and covered by gold Ru(0001) surface reveals the presence of A and B type terraces on the Ru(0001) surface, as well as mutually rotated by 180° adsorbate domains, which exhibit the 12¯1111Au//01¯100001Ru and 1¯21¯111Au//01¯100001Ru orientations with respect to the crystalline substrate. The detailed R-factor analysis of experimental and theoretical DEPES distributions, the latter obtained with the use of the multiple scattering calculations, reveals equal population of A and B type Ru(0001) terraces, as well as unrotated and rotated Au(111) domains, which results in the six-fold symmetry DEPES intensity distributions for clean and covered substrate. The discussion of DEPES results concerns the adsorbate and substrate structures, inelastic mean free paths, and scattering properties of individual Au and Ru atoms described by the scattering factors and differential scattering cross sections.

Nanostructure phase and interface engineering via controlled Au self-assembly on GaAs(001) surface

We have investigated the temperature-dependent morphology and composition changes occurring during a controlled self-assembling of thin Au film on the Gallium arsenide (001) surface utilizing electron microscopy at nano and atomic levels. It has been found that the deposition of 2 ML of Au at a substrate temperature lower than 798 K leads to the formation of pure Au nanoislands. For the deposition at a substrate temperature of about 798 K the nanostructures of the stoichiometric AuGa phase were/had been grown. Gold deposition at higher substrate temperatures results in the formation of octagonal nanostructures composed of an AuGa2 alloy. We have proved that the temperature-controlled efficiency of Au-induced etching-like of the GaAs substrate follows in a layer-by-layer manner leading to the enrichment of the substrate surface in gallium. The excess Ga together with Au forms liquid droplets which, while cooling the sample to room temperature, crystallize therein developing crystalline nanostructures of atomically-sharp interfaces with the substrate. The minimal stable cluster of 3 atoms and the activation energy for the surface diffusion Ed=0.816+-0.038eV was determined. We show that by changing the temperature of the self-assembling process one can control the phase, interface and the size of the nanostructures formed.

(Tl, Au)/Si(1 1 1) 2D compound: an ordered array of identical Au clusters embedded in Tl matrix

Formation of the highly-ordered -periodicity 2D compound has been detected in the (Tl, Au)/Si(1 1 1) system as a result of Au deposition onto the Tl/Si(1 1 1) surface, its composition, structure and electronic properties have been characterized using scanning tunneling microscopy, angle-resolved photoelectron spectroscopy and density-functional-theory calculations. On the basis of these data, the structural model of the Tl–Au compound has been proposed, which adopts 12 Tl atoms and 10 Au atoms (in total, 22 atoms) per unit cell, i.e. ∼1.71 ML of Tl and ∼1.43 ML of Au (in total, ∼3.14 ML). Qualitatively, the model can be visualized as consisting of truncated-pyramid-like Au clusters with a Tl atom on top, while the other Tl atoms form a double layer around the Au clusters. The (Tl, Au)/Si(1 1 1) compound has been found to exhibit pronounced metallic properties at least down to temperatures as low as ∼25 K, which makes it a promising object for studying electrical transport phenomena in the 2D metallic systems.

Growth and motion of liquid alloy droplets of Au on Ge(1 1 0)

The growth of low-dimensional nanostructures of Au on Ge(110) and their temperature-induced motion were observed with Low Energy Electron Microscopy (LEEM). Ge(110) was dosed with 0.5–4 ML of Au and heated to 850 °C. Above 500 °C, liquid AuGe eutectic alloy islands grew on the surface. Islands were 0.3–3.0µm in width, 1–10µm in length, and elongated in the [11¯0] direction. Above 600 °C, islands began moving with speeds of 0.1–1.0µm/s, absorbing smaller stationary islands upon collision and increasing in size to more than 100µm in width. A temperature gradient of ∼0.017 °C/µm across the sample results in a gradient in the Ge concentration across the islands, inducing their movement in the direction of increasing temperature. Optical microscopy confirmed that the large islands moved from the cooler edges of the sample towards its hotter center. The mechanism for motion of the droplets is discussed, and the island velocities fit well to a model for diffusion-driven motion of the liquid droplet. When the temperature was subsequently lowered, islands became supersaturated with Ge, which crystallized on the island edges.

Reflectance anisotropy spectroscopy of the Si(111)−(5×2)Au surface

Hybrid DFT calculations of atomic structure, electronic band structure, and reflectance anisotropy (RA) are used to correlate atomic and electronic structures and optical transitions of eight structures for the $\mathrm{Si}(111)\text{\ensuremath{-}}(5\ifmmode\times\else\texttimes\fi{}2)\mathrm{Au}$ phase with Au coverage and Si adatom site occupancy. The structure recently proposed by Kwon and Kang (KK) [S. G. Kwon and M. H. Kang, Phys. Rev. Lett. 113, 086101 (2014)] and strongly supported by surface x-ray diffraction [T. Shirasawa et al., Phys. Rev. Lett. 113, 165501 (2014)] is found to be the most stable in the Au coverage range from 0.6 to 0.8 ML. The band structure of the Si adatom covered $(5\ifmmode\times\else\texttimes\fi{}4)$ surface has a surface state whose dispersion and position agree very well with the prominent ${S}_{1}$ surface state observed in angle-resolved photoemission experiment. Reflectance anisotropy spectra in the energy range 0.5 to 5 eV for the eight surface structures show variations with Au and Si adatom coverage which can be explained in terms of filling of this surface state. The best agreement between predicted and measured RA spectra, which are sensitive to both Au atom and Si adatom coverage, is found for coverages from 0.7 to 0.75 ML Au, when the surface has regions where Si adatoms are absent and where there are sufficient adatoms to cause surface-state band filling, in equal parts. Surface formation energy calculations favor a coverage of 0.7-ML Au. The structures of the $(5\ifmmode\times\else\texttimes\fi{}2)$ KK and $(6\ifmmode\times\else\texttimes\fi{}6)\mathrm{Au}$ phases on the Si(111) surface are compared and it is shown that the repeat unit of the KK phase also occurs in the $(6\ifmmode\times\else\texttimes\fi{}6)$ structure.

Adsorption and Adhesion of Au on Reduced CeO2(111) Surfaces at 300 and 100 K

The adsorption of vapor-deposited Au onto CeO2-x(111) thin films (x = 0.05 and 0.2) at 300 and 100 K was studied using single crystal adsorption calorimetry (SCAC). The morphology of Au on these films was investigated using He+ low-energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS) by monitoring the changes in substrate and adsorbate signals with Au coverage. Both techniques indicate that Au grows on CeO1.95(111) as three-dimensional particles in the approximate shape of hemispherical caps with a density of 2.8 × 1012 particles/cm2 at 300 K and 7.8 × 1012 particles/cm2 at 100 K. At 300 K, Au initially grows on CeO1.80(111) with a shape similar to hemispherical caps with a density of 5.4 × 1012 particles/cm2 until ∼1.6 ML Au coverage, above which the Au particles become thicker than hemispherical caps. At 300 K, the initial heat of adsorption of Au onto CeO1.95(111) is 259 kJ/mol, which is 37 kJ/mol lower than that on CeO1.80(111). This indicates stronger binding of Au to ...

Au-induced deep groove nanowire structure on the Ge(001) surface: DFT calculations

The atomic geometry, stability, and electronic properties of self-organized Au induced nanowires on the Ge(001) surface are investigated based on the density-functional theory in GGA and the stoichiometry of Au. A giant Ge zigzag chain structure is suggested for 0.75 ML Au coverage, which displays c(8×2) deep groove zigzag nanowire structure simulated STM images. The top layer Ge and Au atomic disorder introduces the chevron units into the zigzag nanowire structure STM image as per the experimental observations. The zigzag Ge nanowire exhibits a semi-metallic characteristic, and the electric transport occurs in between the Ge zigzag nanowire and the subsurface. The system exhibits obvious electronic correlations among the Ge nanowire, the nano-facet Au trimers and the deeper layer Ge atoms, that play an important role in the electronic structure. At surface Brillouin zone boundaries, an anisotropic two-dimensional upward parabolic surface-state band is consistent with the ARPES spectra reported by Nakatsuji et al. [Phys. Rev. B 80, 081406(R) (2009); Phys. Rev. B 84, 115411 (2011)]; this electronic structure is different from the quasi-one-dimensional energy trough reported by Schäfer et al. [Phys. Rev. Lett. 101, 236802 (2008); Phys. Rev. B 83, 121411(R) (2011)].

Dynamics of thermally induced assembly of Au nanoislands from a thin Au layer on Ge(001)

The dynamics of morphology changes upon post-deposition annealing of a thin film of 6 ML Au deposited on a (2 × 1) reconstructed Ge(001) surface was studied in a systematic way by scanning electron and atomic force microscopy. It has been found that annealing of the initial, continuous Au layer leads to the development of nanoislands which are crystalline and epitaxial to the Ge(001) substrate surface. The nanoislands formed at annealing temperatures lower than the bulk eutectic temperature for the Au–Ge system (634 K) are rectangular, with a flat top, while the nanoislands developed by annealing at temperatures higher than the bulk eutectic temperature have more complex, octagonal shapes. Moreover, the octagonal Au nanoislands are nearly half-buried in the Ge substrate and surrounded by a crystalline Ge rim.

Growth and Structure of Ni–Au Bimetallic Particles on Reducible CeO2(111)

The growth and sintering behavior of Ni–Au have been investigated on CeOx(111) (1.5 < x < 2) thin films with controlled oxidation states under ultrahigh vacuum conditions. Scanning tunneling microscopy studies reveal that pure Au grows three-dimensional particles with a submonolayer coverage at room temperature, while Ni prefers two-dimensional islands on ceria as a result of a stronger metal–ceria interaction. With heating, Au experiences extensive particle growth compared to Ni. In the study, bimetallic Ni–Au particles were prepared by deposition of 0.3 ML Ni followed by 0.3 ML Au on ceria. A larger fraction of deposited Au on the Ni particles dispersed on ceria. A small percent of Au was observed to form new pure Au particles on both ceria surfaces. With heating, both Au and Ni–Au particles sinter on CeO1.8 resulting in a bimodal particle size distribution. However, mostly Ni–Au bimetallic particles are present on CeO2. The different sintering behavior is attributed to the surface defects present on the reduced ceria. Such behavior was also observed in the study of ceria-supported Pt–Au bimetallic particles. Our study demonstrates that bimetallic Ni–Au particles can be prepared by deposition of Au on the existing Ni particles as “seeds” on both oxidized and partially reduced ceria. The growth and sintering behavior of Ni–Au bimetallic surfaces are dependent on the nature of ceria supports. Our study can provide morphological and size information for the understanding of the activity of ceria-supported Ni–Au catalysts.

Effect of C60 layer on the growth mode and conductance of Au and Ag films on Si(111)3-Au and Si(111)3-Ag surfaces

The surface morphology and electrical conductance of C60-precoated Si(111)3-Au and Si(111)3-Ag surfaces have been monitored in the course of Au and Ag depositions. In both cases, the deposited metal atoms penetrate through the fullerene layers. However, the similarity in the growth mode does not result in similar dependencies of conductance versus deposited metal dose. Deposition of Au onto the C60-precoated Si(111)3-Au surface leads to a monotonic gradual increase in conductance starting from ∼0.4 ML Au coverage. Deposition of Ag onto the C60-precoated Si(111)3-Ag surface results in a nonmonotone peak-like dependence with a maximum at ∼0.5 ML of Ag. Both dependencies can be explained in terms of the acceptor-type behavior of the fullerenes, which trap the electrons donated by Au or Ag atoms. The difference between the two dependencies is a consequence of the difference in conduction mechanisms at the original surfaces, namely, the main conductance channel in Si(111)3-Au is the space-charge layer, while in Si(111)3-Ag it is the surface state band.

The Study of Chiral Adsorption Systems Using Synchrotron-Based Structural and Spectroscopic Techniques: Stereospecific Adsorption of Serine on Au-Modified Chiral Cu{531} Surfaces

We apply modern synchrotron-based structural techniques to the study of serine adsorbed on the pure and Au-modified intrinsically chiral Cu{531} surface. XPS and NEXAFS data in combination with DFT show that on the pure surface both enantiomers adsorb in μ4 geometries (with de-protonated β-OH groups) at low coverage and in μ3 geometries at saturation coverage. Significantly larger enantiomeric differences are seen for the μ4 geometries, which involve substrate bonds of three side groups of the chiral center, i.e. a three-point interaction. The μ3 adsorption geometry, where only the carboxylate and amino groups form substrate bonds, leads to smaller but still significant enantiomeric differences, both in geometry and the decomposition behavior. When Cu{531} is modified by the deposition of 1 and 2 ML Au the orientations of serine at saturation coverage are significantly different from those on the clean surface. In all cases, however, a μ3 bond coordination is found at saturation involving different numbers of Au atoms, which leads to relatively small enantiomeric differences.

Growth of the Au-induced c(8 × 2) structure on vicinal Ge(001)

We have studied the formation of Ge(001) c(8 × 2)–Au surfaces on vicinal samples by scanning tunneling microscopy. The vicinal samples are tilted 1° toward [110]. The c(8 × 2)–Au surface is prepared by depositing 0.75 ± 0.05 ML of Au onto a germanium surface held at 800 K. The anisotropy introduced by the atomic steps of the vicinal surface and the preferential etching of S B steps during Au deposition is sufficient to introduce a preferred growth direction for the c(8 × 2)–Au phase. The result is a sample on which 78% of the surface is populated by Au-induced chains oriented parallel to the step direction. These parallel Ge(001) c(8 × 2)–Au domains are separated by single or multiple height D A steps (0.28 nm high).

Characterization of Pt–Au and Ni–Au Clusters on TiO2(110)

The surface composition and properties of Pt–Au and Ni–Au clusters on TiO2(110) have been studied by scanning tunneling microscopy (STM), low energy ion scattering (LEIS) and soft X-ray photoelectron spectroscopy (sXPS). STM studies show that bimetallic clusters are formed during sequential deposition of the two metals, regardless of the order of deposition. At the 2 ML of Au/2 ML of Pt or Ni coverages studied here, the second metal contributes to the growth of existing clusters rather than forming new pure metal clusters. LEIS experiments demonstrate that the surfaces of the bimetallic clusters are almost 100% Au when 2 ML of Au is deposited on top of 2 ML of Pt or Ni. However, a much larger fraction of Pt or Ni (50 and 20%, respectively) remains at the surface when 2 ML of Pt or Ni is deposited on 2 ML of Au, most likely due to limited diffusion of atoms within the clusters at room temperature. According to sXPS investigations, the binding energies of the metals in the bimetallic clusters are shifted from those observed for pure metal clusters; the Pt(4f7/2) and Ni(3p3/2) peaks are shifted to lower binding energies while the position of the Au(4f7/2) peak is dominated by surface core level shifts. Pure Pt clusters as well as 0.4 ML of Au on 2 ML of Pt clusters reduce the titania support upon encapsulation after annealing to 800 K, whereas 2 ML of Au on 2 ML of Pt clusters do not reduce titania, presumably because there is no Pt at the surface of the clusters. Pure Ni clusters are also known to become encapsulated upon heating, but the reduction of titania is much less extensive compared to that of pure Pt clusters.

Characterization of Ultra-Thin Films of Pd Deposited on Au(111)

Ultra-thin films (1 and 3 monolayers) of Pd were deposited on the Au(111) surface and then characterized by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger spectroscopy (XAES), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). For the 1 ML Pd film annealed at 450 °C, XPS and XAES results indicated that Pd had diffused into the Au substrate. For the 3 ML Pd film deposited at room temperature, the comparison between experimental and theoretical XPD results indicated approximately 30% of the surface was formed by 2 ML Au layers, and 70% of the surface, by 1 ML Au layers.

Low-temperature decomposition of methanol on Au nanoclusters supported on a thin film of Al2O3/NiAl(100)

With a variety of surface probe techniques, we investigated low-temperature decomposition of methanol on Au nanoclusters formed by vapor deposition onto an ordered Al(2)O(3)/NiAl(100) thin film. Upon adsorption of methanol on the Au clusters (with mean diameter 1.5-3.8 nm and height 0.45-0.85 nm) at 110 K, some of the adsorbed methanol dehydrogenates directly into carbon monoxide (CO); the produced hydrogen atoms (H) begin to desorb near 125 K whereas most of the CO desorbs above 240 K. The reaction exhibits a significant dependence on the Au coverage: the produced CO increases in quantity with the Au coverage, reaches a maximum at about 1.0-1.5 ML Au, whereas decreases with further increase of the Au coverage. The coverage-dependence is rationalized partly by an altered number of reactive sites associated with low-coordinated Au in the clusters. At least two kinds of reactive sites for the low-temperature decomposition are distinguished through distinct C-O stretching frequencies (2050 cm(-1) and 2092 cm(-1)) while the produced CO co-adsorbs with H and methanol.

Medium energy ion scattering investigation of methylthiolate-induced modification of the Au(111) surface

100 keV H + scattering has been used to investigate the structure of the methylthiolate/Au(111) interface in the Au(111)(√3 × √3)R30° phase. Adsorption of the thiolate onto the clean Au(111) surface leads to a large drop in the scattered ion yield due to the lifting of the clean surface ‘herring-bone’ reconstruction, but the thiolate-covered surface shows an ion yield higher than that of an unreconstructed Au(111) surface, providing direct evidence of a significant number of Au atoms that are displaced from their bulk-terminated positions at the buried interface. Simulations for two different Au adatoms models at the interface, namely, the Au-adatom-monothiolate (AAM) and Au-adatom-dithiolate (AAD) models, show significant sensitivity to the exact values of interlayer spacings and atomic vibrational amplitudes, but the comparison with experimental results appears to favour the AAD model with 0.17 ML Au adatoms in bridging sites at the interface.

Decomposition of Dimethyl Methylphosphonate on Pt, Au, and Au−Pt Clusters Supported on TiO2(110)

The decomposition of dimethyl methylphosphonate (DMMP) was studied by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) on TiO(2)-supported Pt, Au, and Au-Pt clusters as well as on TiO(2)(110) itself. In agreement with previous work, TPD experiments for DMMP on TiO(2)(110) showed that methyl and methane were the main gaseous products. Multiple DMMP adsorption-reaction cycles on TiO(2)(110) demonstrated that active sites for DMMP decomposition were blocked after a single cycle, but some activity for methyl production was sustained even after five cycles. Furthermore, the activity of the TiO(2) surface could be regenerated by heating in O(2) at 800 K or heating in vacuum to 965 K to remove surface carbon and phosphorus, which are byproducts of DMMP decomposition. On 0.5 ML Pt clusters deposited on TiO(2)(110), TPD studies of DMMP reaction showed that CO and H(2) were the main gas products, with methyl and methane as minor products. The Pt clusters were more active than TiO(2) both in terms of the total amount of DMMP reaction and the ability to break C-H, P-O, and P-OCH(3) bonds in DMMP. However, the Pt clusters had no sustained activity for DMMP decomposition, since the product yields dropped to zero after a single adsorption-reaction cycle. This loss of activity is attributed to a combination of poisoning of active sites by surface phosphorus species and encapsulation of the Pt clusters by reduced titania after heating above 600 K due to strong metal support interactions (SMSI). On 0.5 ML Au clusters, CO and H(2) were also the main products detected in TPD experiments, in addition to methane and methyl produced from reaction on the support. The Au clusters were less active for DMMP decomposition to CO and H(2) as well as P-O bond scission, but surface phosphorus was removed from the Au clusters by desorption at approximately 900 K. Au-Pt bimetallic clusters on TiO(2)(110) were prepared by depositing 0.25 ML of Pt followed by 0.25 ML of Au, and the bimetallic surfaces exhibited activity intermediate between that of pure Pt and pure Au in terms of CO and H(2) desorption yields. However, there is evidence that the production of methane from DMMP decomposition occurs at Au-Pt sites.

Manipulation of spin reorientation transition by Au capping in body-centered cubic Ni(001)film

The thickness dependent magnetic properties of artificially prepared ultrathin body-centeredcubic Ni films have been explored using the all electron full potential linearized augmentedplane wave (FLAPW) method. We have considered two types of BCC Ni(001) films: (i) pureBCC Ni(001) and (ii) Au capped BCC Ni(001) in the range from 1 monolayer (ML) to 4 MLof Au capping coverage. The average magnetic moment of pure BCC Ni(001) is about0.63 μB and a typical surface enhancement is found with a magnetic moment of0.78 μB. In thepresence of an Au capping layer, the magnetic moment of interface Ni is strongly suppressed to approximately0.5 μB and this causes a reduction of average magnetic moment. Nevertheless, the Auadlayer has no meaningful induced magnetic moment. The BCC pure Ni(001)films always have in-plane magnetization up to 11 ML, but very interestingly theAu/Ni(001) shows a thickness dependent spin reorientation transition (SRT) fromin-plane to perpendicular to the film surface. However, the thickness dependentSRT shows very irregular behavior. In addition, the calculated x-ray absorptionspectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) have been presented.

Coexistence of Ferromagnetism and Paramagnetism in a Ferromagnetic Monolayer

The ferromagnetic-paramagnetic phase transition in an Fe monolayer grown on a 2 ML Au/W(110) substrate and capped with a 1 ML Au layer was studied with spin polarized low electron energy microscopy. When the data are analyzed as in previous studies, not only the Curie temperature T(C) but also the effective critical exponent beta depends upon the terrace width. Taking finite size effects into account gives the terrace width-independent two-dimensional Ising model value beta=1/8.