Role of buffer layer in electronic structures of iron phthalocyanine molecules on Au(111)

We investigate the electronic structures of one and two monolayer iron phthalocyanine (FePc) molecules on Au(111) surfaces. The first monolayer FePc is lying flat on the Au(111) substrate, and the second monolayer FePc is tilted at～15° relative to the substrate plane along the nearest neighbour [101ˉ] direction with a lobe downward to the central hole of the unit cell in the first layer. The structural information obtained by first-principles calculations is in agreement with the experiment results. Furthermore, it is demonstrated that the electronic structures of FePc molecules in one-monolayer FePc/Au(111) system are perturbed significantly, while the electronic structures of FePc molecules in the second monolayer in two-monolayer FePc/Au(111) system remain almost unchanged due to the screening of the buffer layer on Au(111).     

Static investigation of the small clusters on the Cu(111) and Au(111) surfaces

The diffusion properties of small clusters Ag_n, Cu_n, and Au_n on the Cu(111) and Au(111) surfaces were studied using the molecular statics (MS) in order to understand the atomistic processes underlying the motion. In this work, the atomic interaction potential is modeled by a semi-empirical Embedded Atom Method (EAM), while the drag method is used to determine the static activation energy for each diffusion path. The presented results indicate that the dimer can diffuse on the (111) surface via the zig-zag and concerted motion mechanisms. The trimer diffuses according to the concerted motion mechanism. For the tetramer diffusion, the mechanism that consumes the least amount of energy is the zig-zag motion, in which only two atoms are needed for the diffusion process at one time. This allows finding a static activation energy smaller compared to the trimer diffusion.     

Relaxation and electronic states of Au(100), (110) and (111) surfaces

Using a first-principles method based on density functional theory, we investigate the surface relaxation and electronic states of Au(100), (110) and (111) surfaces. The calculated results show that the relaxations of the (100) and (110) surfaces of the metal are inward relaxations. However, the Au(111) surface shows an ‘anomalous’ outward relaxation, although several previous theoretical studies have predicted inward relaxations that are contrary to the experimental measurements. Electronic densities of states and the respective charge density distribution along the Z-axis of the relaxed surfaces are analyzed, and the origin of inward and outward relaxation is discussed in detail.     

Ar adsorptions on Al (111) and Ir (111) surfaces: a first-principles study

We investigate the adsorptions of Ar on Al (111) and Ir (111) surfaces at the four high symmetry sites, i.e., top, bridge, fcc- and hcp-hollow sites at the coverage of 0.25 monolayer (ML) using the density functional theory within the generalized gradient approximation of Perdew, Burke and Ernzerhof functions. The geometric structures, the binding energies, the electronic properties of argon atoms adsorbed on Al (111) and Ir (111) surfaces, the difference in electron density between on the Al (111) surface and on the Ir (111) surface and the total density of states are calculated. Our studies indicate that the most stable adsorption site of Ar on the Al (111) surface is found to be the fcc-hollow site for the (2 × 2) structure. The corresponding binding energy of an argon atom at this site is 0.538 eV/Ar atom at a coverage of 0.25 ML. For the Ar adsorption on Ir (111) surface at the same coverage, the most favourable site is the hcp-hollow site, with a corresponding binding energy of 0.493 eV. The total density of states (TDOS) is analysed for Ar adsorption on Al (111) surface and it is concluded that the adsorption behaviour is dominated by the interaction between 3s, 3p orbits of Ar atom and the 3p orbit of the base Al metal and the formation of sp hybrid orbital. For Ar adsorption on Ir (111) surface, the conclusion is that the main interaction in the process of Ar adsorption on Ir (111) surface comes from the 3s and 3p orbits of argon atom and 5d orbit of Ir atom.     

ESR and TPD Investigations of the Adsorption of Di-tert-butyl Nitroxide on Au(111) and NiO(111). Evidence for Long-Range Interactions

Electron spin resonance and temperature-programmed desorption spectra of thin layers of DTBN (di-tert-butyl nitroxide) adsorbed on Au(111) and NiO(111)/Au(111) surfaces have been measured. The temperature-programmed desorption data show a weak chemisorption of the DTBN molecules in the monolayer on both surfaces. On Au(111) as well as on NiO(111)/Au(111), the ESR signal from monolayer coverages is totally suppressed. This suppression continues into the multilayer regime on both substrates. Disturbances of the substrate/adsorbate interface have a strong influence on the range of the signal suppression. Possible reasons for this behavior are discussed.     

Adsorption of 5-halouracils on Au(111)

The immobilization of 5-halouracils on Au(111) has been studied by soft X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Multilayer and monolayer films of 5-chlorouracil were deposited on the Au(111) surface by evaporation in vacuum while films of 5-bromouracil were absorbed in two modes: from the gas phase under ultrahigh vacuum conditions and from solution. The photoemission spectra of C, N and O 1s, Br 3d and Cl 2p as well as the absorption spectra at the N and O K-edges were measured for monolayers of 5-halouracil films and the nature of the bonding with the Au(111) surface has been determined. From the angular dependence of the NEXAFS spectra at the O and K-edges, we conclude that these 5-halouracils are lying nearly parallel to the Au(111) surface. Distinct chemical states and surface adsorption geometry of the 5-BrU molecules for monolayer coverage prepared in two different ways have been found.     

Electric-dipole layer on Au(111) surfaces

We measured the barrier height (BH) a UHV scanning tunneling microscope on Au(111) surfaces with Au, Pt, and carbon nanotube tips. The 222?{^[¯] }3^[¯]\sqrt\Box3\Box]] reconstruction was observed with all the tips, and the current-voltage relation reflected the density of states of the tips. The BH measured on the reconstructed Au(111) surface using a modulation method showed a bias-polarity dependence as 30%, at low currents (<100 pA) irrespective of the tip material, while on unreconstructed Au surfaces, BH values were independent of the bias polarity, suggesting a dipole layer originating from the reconstructed Au(111) surface.     

Self-assembly of Mo<Subscript> 6</Subscript>S<Subscript> 8</Subscript> clusters on the Au(111) surface

The preferred adsorption sites and the propensity for a self-organised growth of the molybdenum sulfide cluster Mo₆S₈ on the Au(111) surface are investigated by density-functional band-structure calculations with pseudopotentials and a plane wave basis set. The quasi-cubic cluster preferentially adsorbs via a face and remains structurally intact. It experiences a strong, mostly non-ionic attraction to the surface at several quasi-isoenergetic adsorption positions. A scan of the potential energy surface exhibits only small barriers between adjacent strong adsorption sites. Hence, the cluster may move in a potential well with degenerate local energy minima at room temperature. The analysis of the electronic structure reveals a negligible electron transfer and S-Au hybridised states, which indicate that the cluster-surface interaction is dominated by S-Au bonds, with minor contributions from the Mo atom in the surface vicinity. All results indicate that Mo₆S₈ clusters on the Au(111) surface can undergo a template-mediated self-assembly to an ordered inorganic monolayer, which is still redox active and may be employed as surface-active agent in the integration of noble metal and ionic or biological components within nano-devices. Therefore, a classical potential model was developed on the basis of the DFT data, which allows to study larger cluster assemblies on the Au(111).     

Interaction of HNCO with Au(111) surfaces

The surface chemistry of isocyanic acid, HNCO, and its dissociation product, NCO, was studied on clean, O-dosed and Ar ion bombarded Au(111) surfaces. The techniques used are high resolution energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD). The structure of Ar ion etched surface is explored by scanning tunneling microscopy (STM). HNCO adsorbs molecularly on Au(111) surface at 100 K yielding strong losses at 1390, 2270 and 3230 cm^? 1. The weakly adsorbed HNCO desorbs in two peaks characterized by T_p = 130 and 145 K. The dissociation of the chemisorbed HNCO occurs at 150 K to give NCO species characterized by a vibration at 2185 cm^? 1. The dissociation process is facilitated by the presence of preadsorbed O and by defect sites on Au(111) produced by Ar ion bombardment. In the latter case the loss feature of NCO appeared at 2130 cm^? 1. Isocyanate on Au(111) surface was found to be more stable than on the single crystal surfaces of Pt-group metals. Results are compared with those obtained on supported Au catalysts.     

Electrocompression of the Au(111) surface layer during Au electrodeposition

In situ grazing-incidence x-ray diffraction studies of reconstructed Au(111) electrodes in aqueous electrolyte solutions are presented, which reveal a significantly increased compression of the Au surface layer during Au electrodeposition as compared to Au(111) surfaces under ultrahigh vacuum conditions or in the Au-free electrolyte. The compression increases towards more negative potentials, reaching 5.3% at the most negative potentials studied. It may be explained within a simple thermodynamic model by a release of potential-induced surface stress.     

Gold-adatom-mediated bonding in self-assembled short-chain alkanethiolate species on the Au(111) surface

Microscopic evidence for Au-adatom-induced self-assembly of alkanethiolate species on the Au(111) surface is presented. Based on STM measurements and density-functional theory calculations, a new model for the low-coverage self-assembled monolayer of alkanethiolate on the Au(111) surface is developed, which involves the adsorbate complexes incorporating Au adatoms. It is also concluded that the Au(111) herringbone reconstruction is lifted by the alkanethiolate self-assembly because the reconstructed surface layer provides reactive Au adatoms that drive self-assembly.     

Molecular simulation studies of self-assembled monolayers of alkanethiols on Au(111)

A review is presented of this group's recent molecular simulation studies of self-assembled monolayers (SAMs) of alkanethiols on Au(111) surfaces. SAMs are very useful for the systematic alteration of the chemical and structural properties of a surface by varying chain length, tail group and composition. The scientific and technological importance of SAMs cannot be overestimated. The present work has been centred on studies of atomic scale surface properties of SAMs. First, configurational-bias Monte Carlo simulations were performed in both semigrand canonical and canonical ensembles to investigate the preferential adsorption and phase behaviour of mixed SAMs on Au(111) surfaces. Second, a novel hybrid molecular simulation technique was developed to simulate atomic force microscopy (AFM) over experimental timescales. The method combines a dynamic element model for the tip-cantilever system in AFM and a molecular dynamics relaxation approach for the sample. The hybrid simulation technique was applied to investigate atomic scale friction and adhesion properties of SAMs as a function of chain length. Third, dual-control-volume grand canonical molecular dynamics (DCV-GCMD) simulations were performed of transport diffusion of liquid water and methanol through a slit pore with both inner walls consisting of Au(111) surfaces covered by SAMs under a chemical potential gradient. Surface hydrophobicity was adjusted by varying the terminal group of CH₃ (hydrophobic) or OH (hydrophilic) of the SAMs. Finally, ab initio quantum chemical calculations were performed on both clusters and periodic systems of methylthiols on Au(111) surfaces. Based on the ab initio results, an accurate force field capable of predicting c(4×2) superlattice structures over a wide range of temepratures for alkanethiols on Au(111) was developed. The extension of current work is discussed briefly.     

Au(111)表面甲基联二苯丙硫醇盐单层膜的原子结构

利用第一性原理研究了甲基联二苯丙硫醇盐(BP3S)单体、虚拟Au表面BP3S的分子链和单层膜及BP3S/Au(111)吸附系统的原子结构.计算表明BP3S单体呈对称结构,两苯环夹角为35°±10°.首先BP3S单体在虚拟Au(111)表面自组装成稳定的单一分子链.然后在虚拟Au(111)表面,分子链错位排列自组装成两种稳定的单层膜.在虚拟Au(111)-(3~(1/2)×7~(1/2))和Au(111)-(3~(1/2)×13~(1/2))表面,分子链与虚拟表面夹角分别为60°和30°.最后把两种稳定的单层膜吸附在Au(111)表面的四个吸附位,计算表明只有桥位和顶位稳定,且桥位的吸附能比顶位的吸附能低.比较吸附前后BP3S单层膜的结构变化,可知其变化不大,这说明吸附系统的结构参数主要取决于单层膜内的相互作用,衬底对其的影响不大.     

Density-functional study of Mo4S6 on Au(111)

The adsorption site and energetics of Mo₄S₆ on the unreconstructed (111) gold surface was determined by density-functional band-structure calculations. Mo₄S₆ adsorbs preferably via three S atoms at Au–Au bridging sites and three Mo atoms at adjacent threefold hollow sites of the Au(111) surface. The perfectly tetrahedral free cluster becomes trigonally distorted upon adsorption, which indicates the presence of attractive Au–S and Au–Mo interactions. The work of separation amounts to 4.2 eV including gradient corrections. Small peak shifts in the density of electronic states indicate electron transfer from Au to S states and from Mo to Au states, such that a Bader analysis yields a neutral cluster. Electron density difference maps, which visualize the electron redistribution between cluster and substrate in the interface region, show that the electron redistribution is confined to the cluster-surface contact region. PACS 61.46.+w; 73.22.-f; 72.20.-v     

Understanding formation of molecular rotor array on Au(111) surface

The motion of single molecules on surfaces plays an important role in nanoscale engineering and bottom-up construction of complex devices at single molecular scale. In this article, we review the recent progress on single molecular rotors self-assembled on Au(111) surfaces. We focus on the motion of single phthalocyanine molecules on the reconstructed Au(111) surface based on the most recent results obtained by scanning tunneling microscopy (STM). An ordered array of single molecular rotors with large scale is self-assembled on Au(111) surface. Combined with first principle calculations, the mechanism of the surface-supported molecular rotor is investigated. Based on these results, phthalocyanine molecules on Au (111) are a promising candidate system for the development of adaptive molecular device structures.     

Electronic transitions of Ar,Xe, N2, CO physisorbed on Ag(111) and Al(111)

High Resolution Electron Energy Loss Spectroscopy has been extended to study also the excitonic (low lying electronic) transitions of physisorbed rare gas atoms (Ar, Xe) and diatomic molecules (N₂, CO) on Ag(111) and Al(111) surfaces at ～20K. Electron Loss Spectra were performed using a pair of hemispherical analyzers mounted at a fixed scattering angle (90°). This spectrometer allowed high transmission in the range of 0–15eV loss energies and incident beam energies up to 2OeV. AES, LEED and UV Photoemission (HeI) were also used in situ to characterize these surfaces and to identify the adsorbed gases and delineate their absolute coverage regimes.In contrast to optical absorption experiments, we observe both, optical (dipole) forbidden and allowed electronic transitions which show vibrational line structure for condensed multilayers. By comparison to gas phase data we find only weak perturbations in the condensed state. The observed electronic excitations show changes in intensity and FWHM depending on the coverage of the adsorbed gases.The FWHM of the electronic excitations of CO and N₂ adsorbed in the monolayer regime is larger than in multilayers. Nitrogen, on both surfaces exhibits an increase from 60meV to 120meV (FWHM) whereas for CO the vibronic features are broadened out leaving peaks with FWHM of ～1eV.The intensities of the electronic losses for all gases are smaller in the first monolayer than in the second or in multilayers. At submonolayer coverage the loss intensifies due to electronic excitations are strongly reduced and no longer observable although vibrational bands and photoelectron spectra show the presence of physisorbed adsorbates.Our results will be compared to optical absorption experiments (ref.1) on similar systems and to atom-on-jellium calculations (ref.2).     

Local spectroscopy of two-dimensional states in Na films on Cu(111)

Alkali overlayers on noble metal surfaces exhibit two-dimensional electronic states. We investigated the electronic structure of Na on Cu(111) at T=5 K with scanning tunneling spectroscopy. In agreement with photoelectron spectroscopy we found two electronic bands close to the Fermi energy which correspond to quantum well states (QWS). In second monolayer islands, confinement of the QWS was observed.     

LEED study of the epitaxial growth of the thin film Au(111)/Ag(111) system

An analysis of LEED data from the Ag(111) surface at room temperature and 5° ? Θ ? 16°, φ = 12° has been carried out in order to test three different model potentials for the exchange and correlation part of the one-electron LEED potential. Clean Au(111) surfaces have been grown on Ag(111) at room temperature at a deposition rate of 0.15 Å s^?1. Similar method of calculation and potentials have been employed for the Au overlay er on Ag(111). After the deposition of ? 2.5 monolayers of Au/Ag(111) the growth of Au can proceed in two different ways. One of them matches satisfactorily with the theoretical calculation for the Au(111) overlayer on Ag(111) following the fcc sequence. The other seems to be concerned with the diffusion of Ag during the Au growth. Similar curves have been obtained during the diffusion of Ag through 350 Å of Au(111).     

Density functional theory calculation of platinum surface segregation energy in Pt3Ni (111) surface doped with a third transition metal

We have used density functional theory method to calculate the Pt surface segregation energy in the Pt₃Ni (111) surface doped with a third transition metal M and thus investigated the influence of component M on the extent of Pt segregation to the outermost layer of these Pt₃Ni/M (111) surface. As a third component in the Pt₃Ni/M (111) surface, V, Fe, Co, Mo, Tc, Ru, W, Re, Os, and Ir were predicted to lead to even more negative Pt surface segregation energies than that in the based Pt₃Ni (111) surface; Ti, Cr, Mn, Cu, Zr, Nb, Rh, Hf, and Ta would still retain the preference of Pt segregation to the surface but with less extent than the replaced Ni, while Pd, Ag, and Au would completely suppress the Pt segregation to the Pt₃Ni/M (111) surfaces. Furthermore, we examined the relation between the Pt surface segregation energy in the Pt₃Ni/M (111) surfaces and the material properties (lattice parameter, heat of solution, and Pt surface segregation energy) of binary alloys Pt₃M. It was found that the surface energy effect, strain effect, and heat of solution effect induced by the doped element M would collectively affect the Pt surface segregation energy in the Pt₃Ni/M (111) surfaces.     

Self-assembly and growth of manganese phthalocyanine on an Au（111） surface

Self-assembly and growth of manganese phthalocyanine (MnPc) molecules on an Au(111) surface is investigated by means of low-temperature scanning tunneling microscopy. At the initial stage, MnPc molecules preferentially occupy the step edges and elbow sites on the Au(111) surface, then they are separately adsorbed on the face-centered cubic and hexagonal closely packed regions due to a long-range repulsive molecule—molecule interaction. After the formation of a closely packed monolayer, molecular islands with second and third layers are observed.