Growth of Sn on Mo(110) studied by AES and STM

Scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES) have been used to investigate the growth behavior of ultra-thin Sn films on a Mo(110) surface at room temperature. An analysis of STM and AES measurements indicates that layer-by-layer growth (Frank-van der Merwe mode) for the first two layers of Sn is observed. For submonolayer coverage, tin prefers to nucleate randomly and creates one atom high islands on Mo terraces. In the completed first and second layer, no ordered regions were observed. As the sample is post-annealed to 800 K, the rearrangement of an existing film suggests a Sn–Mo surface alloy formation.     

Chlorine superstructures on Mo(110) investigated by LEED and AES

C1₂ was adsorbed onto a clean Mo(110) surface at room temperature. Four well-defined structures occurred as coverage increased. Below half a monolayer (θ <0.5) a p'(2 × 1) pattern (with respect to the rectangular surface mesh) with p2mg glide-plane symmetry formed. Near θ = 0.5, a p'(1 × 1) structure formed, next p'(1 × 2) and at saturation (θ?0.8) p'(1 × 3). Physically plausible models based on compression structures (relatively even distribution influenced by adatom-adatom repulsion) with bonding sites restricted to the vicinity of quasi-four-fold hollows and three-fold wells are devised to explain these patterns. These models are suitable for the reinterpretation of other investigations of the related systems S on Mo(110) and Cl on Ta(110).     

Oxidation of W(110) studied by LEED and STM

The oxidation of W(110) was studied over a temperature a range of 1000 K to 1600 K at 1 × 10^? 6 Torr oxygen. The subsequent oxide structure was then characterized using Low Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM). It was found that the resulting structure was remarkably similar to that of Mo(110) oxidized under similar conditions. Using the Mo(110) oxide structure as our basis, along with atomic resolution STM images, we have constructed a model for the surface oxide of W(110).     

Growth of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on Cu(110) studied by STM

. The structure of thin 3,4,9,10-perylenetetracarb-oxylic-dianhydride (PTCDA) films on Cu(110) was studied by scanning tunnelling microscopy (STM) from submonolayer to monolayer coverage. While no long-range ordering was found after deposition at room temperature, the formation of a well-defined superstructure is observed after thermal annealing. It appears that the formation of the superstructure is driven by the interaction between the oxygen atoms of the PTCDA and the copper atoms of the substrate. While the distance between the molecules fits well to the atomic lattice of the Cu(110) surface along the [1[`(^[¯])]1^[¯]\bar\Box1\Box]]0] direction, the mismatch along the [001] direction leads to a periodic buckling normal to the surface accompanied by a restructuring of the substrate.     

Adsorption of oxygen on Ag(110) studied by high resolution ELS and TPD

Adsorption of oxygen on Ag(110) has been studied by high resolution electron energy loss spectroscopy (ELS) and temperature programmed desorption (TPD) in the temperature range from ? 160°C to 310°C. At ? 160°C oxygen is absorbed as a diatomic species. The low vibrational frequency of the O-O stretch vibration is explained in terms of charge transfer from the metal into the π¹ antibonding orbital and donation from the π bonding orbital to the metal. A tentative model is presented, according to which the molecule is adsorbed in the grooves of the (110) surface with its axis parallel to the surface. It is explicitly shown that this diatomic species is the precursor for dissociative adsorption of oxygen at temperatures above ? 100°C. Upon dissociation part of the diatomic species is desorbed. Between ? 100°C and + 310°C a single type of adsorbed atomic oxygen is observed which is desorbed at 310°C. Above 150°C adsorbed atomic oxygen also diffuses to subsurface sites. Below 450°C subsurface oxygen neither desorbs nor diffuses into the bulk, although it does exchange with adsorbed atomic oxygen at a temperature below 310°C. Therefore, both forms of atomic oxygen coexist at temperatures at which ethylene epoxidation occurs.     

Growth and ordering of Ni(II) diphenylporphyrin monolayers on Ag(111) and Ag/Si(111) studied by STM and LEED

The room temperature self-assembly and ordering of (5,15-diphenylporphyrinato)nickel(II) (NiDPP) on the Ag(111) and Ag/Si(111)-(√3 × √3)R30° surfaces have been investigated using scanning tunnelling microscopy and low-energy electron diffraction. The self-assembled structures and lattice parameters of the NiDPP monolayer are shown to be extremely dependent on the reactivity of the substrate, and probable molecular binding sites are proposed. The NiDPP overlayer on Ag(111) grows from the substrate step edges, which results in a single-domain structure. This close-packed structure has an oblique unit cell and consists of molecular rows. The molecules in adjacent rows are rotated by approximately 17° with respect to each other. In turn, the NiDPP molecules form three equivalent domains on the Ag/Si(111)-(√3 × √3)R30° surface, which follow the three-fold symmetry of the substrate. The molecules adopt one of three equivalent orientations on the surface, acting as nucleation sites for these domains, due to the stronger molecule-substrate interaction compared to the case of the Ag(111). The results are explained in terms of the substrate reactivity and the lattice mismatch between the substrate and the molecular overlayer.     

Atomic chemisorption of chlorine on Ag(110) studied by high-resolution electron energy loss spectroscopy

We have studied atomic chemisorption at room temperature of chlorine on Ag(110) using high-resolution electron energy loss spectroscopy (HREELS), supplemented by XPS and LEED. The ClAg vibration energy (around 25 meV) and the line-width of this loss peak show well resolved variations with both chlorine coverage and substrate temperature T. The observed shift with T is related to the anharmonicity of the potential. Based on the Morse potential we derive an anharmonicity parameter x_a = 6.2 × 10⁻² for the (2 × 1)Cl-overlayer. This indicates that the anharmonicity is enhanced by about a factor of two as compared to the bulk. In contrast, we find x_a < 0.2 × 10⁻² for c(4 × 2)Cl. By comparison to other data we conclude that the (2 × 1)Cl-phase is a simple overlayer, with no significant reconstruction of the topmost substrate layer.     

A reversible reaction forming (---Cu---O---) strings and (Cu)6-clusters on Ag(110) shown by STM

An artificial new surface of (---Cu---O---) chains grown on Ag(110) surface was prepared by reacting a surface with Cu atoms, where the (---Cu---O---) chains grow in the [1 0] direction and are self-assembled on the Ag(110) surface in a (2 x 2)-p2mg structure. When the Cu---O/Ag(110) surface was heated in vacuum, the (---Cu---O---) chain decomposed to uniform cluster dots arranged along the [1 0] direction, where the cluster dots were composed of six Cu atoms. When the Ag(110) surface with the Cu---cluster dots was exposed to O₂, the (---Cu---O---) lines were redrawn along the [1 0] direction by reacting a s in the [1 0] direction with O₂. This is a reversible chemical reaction in one dimensional regime proved in atomic resolution.     

CO dynamics induced by tunneling electrons: differences on Cu(110) and Ag(110)

The electronic current originating in a scanning tunneling microscope (STM) can be used to induce motion and desorption of adsorbates on surfaces. The manipulation of CO molecules on noble metal surfaces is an academic case that has received little theoretical attention. Here, we do thorough density functional theory calculations that explore the chemisorption of CO on Cu(110) and Ag(110) surface and its vibrational properties. The STM induced dynamics are explored after excitation of the highest lying mode, the C–O stretch. In order to give a complete account of this dynamics, the lifetime of the different CO modes is evaluated (by only including the mode decay into electronic excitations of the host surface) as well as the intermode coupling. Hence, after excitation of the stretch mode, the lower-energy modes are populated via intermode coupling and depopulated by electron-hole excitations. This study reveals the intrinsic features of the STM induced motion of CO on Cu(110) and Ag(110).     

Chemisorption of CO on Cu(100), Ag(110) and Au(110)

The adsorption of CO on Cu, Ag and Au is studied using core and valence photoemission, X-ray absorption and autoionization of core excited states. The purpose is to investigate the nature of the adsorption bond starting out from the well-established chemisorption system CO/Cu(100)-c(2 × 2), and from the results we suggest that CO forms chemisorbed phases also on Ag(110) and Au(110). The photoemission spectra show strong shake-up satellites both for the valence levels and the core levels. The separation to the satellite appearing closest to the main line is observed to follow the position of the substrate d-band relative to the Fermi level. The CO adsorption strength for the noble metals is deduced to decrease in the order Cu-Au-Ag. This is based on the widths of the XA resonances, which are related to the adsorbate-substrate interaction strength of the core excited states, and the relative shake-up intensities, which are expected to increase with a decreasing adsorption strength in the ground state. The same trends regarding the shake-up intensities are observed both for the valence and core levels.     

Ti/CeOx(111) interfaces studied by XPS and STM

Low coverage of Ti was deposited on the well-ordered CeO_x(111) (1.5 < x < 2) thin films grown on Ru(0001) by physical vapor deposition at room temperature. The structure and interaction of Ti/ceria interfaces were investigated with X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) techniques under ultrahigh vacuum conditions. XPS data indicate that the deposition of Ti on both oxidized and reduced ceria surfaces causes the partial reduction of Ce from + 4 to + 3 state. Ti is formally in the + 4 state. STM data show the formation of small atomic-like titania features at 300 K, which coalesce to form chain structures upon heating. It is demonstrated in the study that the deposition of Ti can form mixed metal oxides at the interface and modify both electronic and structural properties of the ceria support. The structural study of Ti/ceria interfaces can be a key for understanding the higher catalytic activity of the Ti–CeO_x mixed oxide catalysts as compared with the individual pure oxides.     

High-resolution photoemission from Ag(100), Ag(110), and Ag(111)

Energy distribution curves of photoelectrons emitted normal to (100), (110), and (111) faces of silver have been obtained at photon energies of 21.22, 16.85, and 11.83 eV. The results are compared with Christensen's relativistic band structure calculation of bulk silver yielding a close correspondence between experiment and theory. A surface state in the L gap immediately below the Fermi level is identified.     

C60 adsorption on Cu(110) studied by optical spectroscopy

The formation of the interface between C₆₀ thin films and the Cu(110) surface has been investigated in situ using reflectance difference spectroscopy (RDS). The electronic interaction between C₆₀ molecules in the first monolayer and the substrate inhibits low‐energy intramolecular transitions, whereas the C₆₀ molecules above the first monolayer are effectively decoupled from the substrate. The morphology of C₆₀ thin films prepared at room temperature is thermally stable up to 500 K. Above this threshold, optical spectroscopy and low energy electron diffraction (LEED) indicate the formation of rather large three dimensional C₆₀ islands on a one monolayer thick wetting layer. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The epitaxy of gold on (110) tungsten studied by leed

Growth of gold condensed on the (110) plane of tungsten has been studied using LEED and AES. Three ordered surface structures were observed when condensation takes place at or above 700 K, and no detectable order is seen below this temperature. Structure 1 is developed as the coverage approaches one monolayer and has gold atoms held in the W(110) array with a resultant 2% reduction in gold atom diameter. The second gold layer adopts the Au(111) structure with $\text{Au}\text{[1}\text{2}\text{1]}$ rotated by 2.5° from $\text{W}\text{[1}\text{1}\text{0]}$ and the first gold layer may also be constrained to adopt this structure. Deposition of more gold produces three dimensional crystallites with Au(111)∥W(110) which are double-positioned with their 121 directions parallel to the 121 directions of tungsten. Addition of half a monolayer of oxygen before condensation, completely prevents formation of structures 1 and 2. Instead, at coverages of 3 or more monolayers, three dimensional crystallites develop with Au(111) ∥ W(110) and $\text{Au}\text{[1}\text{2}\text{1] ∥}\text{W}\text{[1}\text{1}\text{0]}$. This behaviour is compared with the reported behaviour of copper and silver on W(110).     

Ferromagnetism in the thermodynamically stable monolayer Fe(110) on W(110), coated by Ag

Ferromagnetic order in the thermodynamically stable, pseudomorphic monolayer Fe(110) on W(110), coated by Ag, was studied in situ in UHV using Conversion Electron Mössbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). Films near the monolayer coverage, prepared at 475 K, consist of nearly independent monolayer and double-layer patches. The properties of monolayer patches are nearly independent of the mean film thickness resulting in excellent conditions to determine the true monolayer properties. The Curie temperature is reduced toT _{c, mono}= 282 K = 0.27 T_c,bulk, the ground state hyperfine field is reduced toB _hf(0)=11.9 T = 0.35B _hf,bulk(0) and the magnetic moment per atom is enhanced to(0) = 2.53 _B=1.14(0)_bulk. Remanent magnetization is detected forT 260 K=0.92T _{c, mono}, square loop magnetization forT 230 K=0.82T _{c, mono}. Unusual properties of the phase transition are detected by the combination of both experimental techniques.