Electronic states of a C70 monolayer on the surface of Ag(111)

We have investigated the electronic states of a C(70) monolayer on the surface of Ag(111) (1 ML C(70)/Ag(111)) using synchrotron radiation photoelectron spectroscopy and soft x-ray absorption spectroscopy techniques. The experimental data exhibit metallic properties and at least 2.6 e(-) charge transfer per C(70) molecule. The screening effect of Ag(111) on the electronic structure of C(70) is remarkable; it greatly reduces or even eliminates the on-site Hubbard energy. The work functions of the C(70) multilayer and monolayer are determined as 4.53 eV and 4.52 eV respectively. The energy levels of C(70) align with the Fermi level of the Ag(111) substrate, and the shift of the vacuum level caused by C(70) adsorption is negligible. Potassium doping indicates that 1 ML C(70)/Ag(111) can still accommodate about nine electrons and that the sample remains metallic at any doping level.     

Xe monolayer adsorption on Ag(111) I. Structural properties

The monolayer of adsorbed xenon on the (

1) surface of silver was studied using low- energy electron diffraction. The structure is an hexagonal lattice aligned with but out of registry with the substrate. The Xe-Xe spacing at 25 K is 4.44 ± 0.01 Å in the full monolayer and 4.47 ± at less than full coverage. The Xe-Ag spacing is 3.5 ± 0.1 Å. Here $\text{〈u}{}_{\mathrm{\perp }}\text{〉}{}^2\text{T}$, the meansquare vibrational amplitude normal to the surface divided by the temperature is (1.83 ± 0.4 × 10^?4 A²/K. The heat of adsorption is 0.28 ± 0.03 eV/atom.     

Adsorption behavior of Fe atoms on a naphthalocyanine monolayer on Ag(111) surface

Adsorption behavior of Fe atoms on a metal-free naphthalocyanine(H2Nc) monolayer on Ag(111) surface at room temperature has been investigated using scanning tunneling microscopy combined with density functional theory(DFT)based calculations. We found that the Fe atoms were adsorbed on the centers of H2 Nc molecules and formed Fe–H2Nc complexes at low coverage. DFT calculations show that Fe sited in the center of the molecule is the most stable configuration, in good agreement with the experimental observations. After an Fe–H2Nc complex monolayer was formed, the extra Fe atoms self-assembled to Fe clusters of uniform size and adsorbed dispersively at the interstitial positions of Fe–H2Nc complex monolayer. Therefore, the H2 Nc monolayer grown on Ag(111) could be a good template to grow dispersed magnetic metal atoms and clusters at room temperature for further investigation of their magnetism-related properties.     

Umklapp-mediated quantization of electronic states in Ag films on Ge(111)

We employ angle-resolved photoemission to study the electronic structure of atomically uniform films of Ag grown on Ge(111). A new kind of quantum well state is observed near a specific emission direction away from the surface normal. In contrast with the usual quantum well state arising from electron confinement by specular reflections at the surface and interface of the film, the new kind involves retroreflections, or umklapp reflections, at the interface. It requires four reflections, instead of the usual two reflections, to complete a coherent interference path.     

Impurity effect on surface states of Bi(111) ultrathin films

The surface impurity effect on the surface-state conductivity and weak antilocalization(WAL) effect has been investigated in epitaxial Bi(111) films by magnetotransport measurements at low temperatures. The surface-state conductivity is significantly reduced by the surface impurities of Cu, Fe, and Co. The magnetotransport data demonstrate that the observed WAL is robust against deposition of nonmagnetic impurities, but it is quenched by the deposition of magnetic impurities which break the time reversal symmetry. Our results help to shed light on the effect of surface impurities on the electron and spin transport properties of a 2D surface electron systems.     

Adsorption of thin films of titanium on tungsten (111) surface

The structural and electronic properties of titanium-covered (111) surface of tungsten are studied via first-principles density-functional calculations. The dominant role played by substrate–adlayer interactions in the formation of thin films of Ti on W(111) explains the observed isomorphous growth for Ti depositions up to a full physical overlayer, with adatoms most favorably arranged in substrate-lattice-continuation sites. The optimized atomic configurations indicate substantial interplane relaxations following the contraction–contraction–expansion pattern, characteristic also of the clean W(111) substrate. Modification of the surface electronic properties of the Ti/W(111) system is examined for increasing adsorbate coverage with emphasis placed on the formation and localization properties of adsorbate-induced resonances.     

Absence of filled surface states in the s-p gap of clean(111) surface of Ag

The nature and location of the surface states, present at the (111) face of Ag, are investigated through a theoretical calculation of the band structure and local density of states based on a LCAO method. Previous assignments of surface structures in the photoemission spectra are critically discussed in the light of our results.     

Passivation of Si(111) surfaces with electrochemically grafted thin organic films

Ultra thin organic films (about 5 nm thick) of nitrobenzene and 4-methoxydiphenylamine were deposited electrochemically on p-Si(111) surfaces from benzene diazonium compounds. Studies based on atomic force microscopy, infrared spectroscopic ellipsometry and x-ray photoelectron spectroscopy showed that upon exposure to atmospheric conditions the oxidation of the silicon interface proceed slower on organically modified surfaces than on unmodified hydrogen passivated p-Si(111) surfaces. Effects of HF treatment on the oxidized organic/Si interface and on the organic layer itself are discussed.     

Scanning tunneling microscopy study of a Ag monolayer on Cu(111)

The structure of submonolayer Ag deposited on Cu(111) has been examined with scanning tunneling microscopy. The long diffusion length of Ag leads to the formation of large (111)-like islands nucleating from the “downhill” side of Cu step edges. The growth fronts of the Ag islands are mostly straight and lie along the 〈1̄10〉 directions. The Ag overlayer is apparently slightly compressed relative to bulk Ag, and forms a mixture of superstructures rotated slightly relative to the substrate.     

Oxygen adsorption on a Pd(111) surface

The interaction of oxygen with Pd(111) has been studied by means of AES, ELS, thermal desorption (TD), electron stimulated desorption (ESD) and work function measurements. It was found that a very small part ( ～ 2–3%) of the available adsorption sites are contributing to the O⁺ electron stimulated yield, the population of the latter being accompanied by enormously large work function changes (up to ～ 0.9 eV). A mechanism of adsorption and depopulation of these sites involving oxygen bulk and surface diffusion has been proposed.     

Ar and Kr adsorption on Ag(111)

Precise structural and thermodynamic studies of Kr and of Ar adsorbed on Ag(111) are made using low energy electron diffraction. The phase diagram, lattice constants of the unconstrained monolayer and of the monolayer in equilibrium with the bilayer, latent heats of adsorption and isosteric heats are measured. The results are similar to those of an earlier study of Xe adsorbed on Ag(111). The results are compared to model calculations using effective lateral interactions which are similar to those for Xe/Ag(111). Comparison of the results for Xe, Kr, and Ar on Ag(111) is made using corresponding states scalings. A comparison is also made with properties of the non-registry phases of Xe, Kr, and Ar on basal plane graphite.     

Xe adsorption on Ag(111): Experiment

Precise structural and thermodynamic studies of Xe adsorbed on Ag(111) are made using low-energy diffraction. Spacings are measured relative to those of bulk adsorbed film. Effective equilibrium experiments are done using a directed gas beam and cryopumping. The density of only that gas in the coherently diffracting area is measured by the attenuation of the Ag(01) beam. With decreasing temperature at fixed gas flux one observes the sequential of 2D gas, monolayer, bilayer and bulk films. The effective pressure of the impinging gas is determined from the known bulk vapor pressure so bypassing absolute pressure measurements. The phase diagram, lattice spacings in the film at equilibrium and at zero pressure, latent heats of adsorption and isosteric heats are determined.     

Time-resolved measurements of electron transfer processes at the PTCDA/Ag(111) interface

The electron transfer processes at the interface between 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and Ag(111) have been studied using time- and angle-resolved two-photon photo-emission (2PPE). For this system a dispersing unoccupied interface state can be identified that is located 0.6 eV above the Fermi level with an effective electron mass of 0.39 m_e at the [`(G)]\overline{\Gamma}-point. The lifetime of 54 fs for the interface state is relatively short indicating a large penetration of the wavefunction into the metal. Supported by model calculations this interface state is interpreted as predominantly arising from an upshift of the occupied Shockley surface state of the clean metal substrate due to the interaction with the PTCDA overlayer. Coverage dependent measurements show a second long-lived component in the time-resolved measurements for higher PTCDA coverages that can be associated to charge transfer processes from the PTCDA multilayers into the metal substrate. Additionally the influence of the PTCDA adlayers on the image-potential states is studied indicating that the n = 1 image-potential state is localized in the first two monolayers of the PTCDA film.     

Comparison of electronic structure between monolayer silicenes on Ag(111)

√√The electronic structures of monolayer silicenes(4 × 4 and■×■R13.9o) grown on Ag(111) surface are studied by scanning tunneling spectroscopy(STS) and density functional theory(DFT) calculations. While both phases have similar electronic structures around the Fermi level, significant differences are observed in the higher energy unoccupied states.The DFT calculations show that the contributions of Si 3pz orbitals to the unoccupied states are different because of their different buckled configurations.     

Two commensurate hydrogen-bonded monolayer structures of melamine on Ag(111)

Melamine (1,3,5-triazine-2,4,6-triamine) was deposited on the Ag(111) surface under ultrahigh vacuum conditions. It forms two different monolayer structures, which were investigated by low energy electron diffraction and scanning tunneling microscopy. The α-phase is a honeycomb structure containing two molecules per unit-cell. The molecular orientation within the unit-cell is determined by six hydrogen bonds. The α-phase is kinetically preferred upon deposition at room-temperature and can be transferred to the thermodynamically more stable β-phase by annealing at 333 K. The β-phase has an oblique unit-cell containing four molecules and shows a higher surface density with additional hydrogen bonds between adjacent amino groups. Both structures are commensurate. While the structural motif of the α-phase has been observed before on Au(111) and Ag–Si(111) surfaces, the structure of the β-phase has been so far only theoretically predicted.