The interfacial properties of MgCl(2) thin films grown on Si(111)7 x 7

Photoelectron spectroscopy with synchrotron radiation and low energy electron diffraction (LEED) were used in order to study the MgCl(2)Si(111) system. At submonolayer coverage of MgCl(2), a new LEED pattern was observed corresponding to a (sqr rt 3 x sqr rt 3)R30 degrees overlayer superimposed on the underlying reconstructed Si(111)7 x 7. The surface species at this stage are mainly molecular MgCl(2) and MgCl(x) (x<2) or MgO(x)Cl(y) attached to the Si substrate through Cl bridges coexisting with monodentate SiCl. The interfacial interaction becomes more pronounced when the submonolayer coverage is obtained by annealing thicker MgCl(2) layers, whereby desorption of molecular MgCl(2) is observed leaving on the nonreconstructed silicon surface an approximately 0.2 ML thick MgCl(x) layer which again forms the (sqr rt 3 x sqr rt 3 )R30 degrees superstructure.     

The reactive chemisorption of alkyl iodides at Cu(110) and Ag(111) surfaces: a combined STM and XPS study

The chemisorption of methyl and phenyl iodide has been studied at Cu(110) and Ag(111) surfaces at 290 K with STM and XPS. At both surfaces dissociative adsorption of both molecules leads to chemisorbed iodine, with the STM showing c(2 x 2) and (square root 3 x square root 3)R30 structures at the Cu(110) and Ag(111) surfaces, respectively. At the Cu(110) surface a comparison of coexisting c(2 x 2) I(a) and p(2 x 1) O(a) domains shows the iodine adatoms to be chemisorbed in hollow sites with evidence at low coverage for diffusion in the (110) direction. In the case of methyl iodide no carbon adsorption is observed at either the silver or the copper surfaces, but chemisorbed phenyl groups are imaged at the Cu(110) surface after exposure to phenyl iodide. The STM images show the phenyl groups as bright features approximately 0.7 nm in diameter and 0.11 nm above the iodine adlayer, reaching a maximum surface concentration after approximately 6 Langmuir exposure. However, the phenyl coverage decreases with subsequent exposures to PhI and is negligible by approximately 1000 L exposure, consistent with the formation and desorption of biphenyl. The adsorbed phenyls are located above hollow sites in the substrate, they are stabilized at the top and bottom of step edges and in paired chains (1.1 nm apart) on the terraces with a regular interphenyl spacing within the chains of 1.0 nm in the (110) direction. The interphenyl ring spacing and diffusion of individual phenyls from within the chains shows that the chains do not consist of biphenyl species but may be a precursor to their formation. Although the XPS data shows carbon present at the Ag(111) surface after exposure to PhI, no features attributable to phenyl groups were observed by STM.     

Specific ethene surface activation on silver oxide covered Ag[111] from the interplay of STM experiment and theory

High-resolution scanning tunneling microscopy (STM) images at 5 K, simultaneously resolving the molecular adsorbate and the honeycomb structure of the well-defined Ag[111]-p(4 x 4)+Ag(1.83)O substrate, assign the adsorption site for ethene on the silver oxide surface. Ethene molecules are exclusively adsorbed above a particular subset of Ag(delta)(+) sites in the hexagonal rings of the oxide. Extensive density functional theory (DFT) slab calculations confirm that this is the most stable site, with an adsorption energy of 0.4 eV (39 kJ mol(-1)). Adsorption is accompanied by a large deformation of the hexagonal oxide ring and a significant increase in the C-C bond length. STM image simulations provide qualitative agreement with the experimental images, and the molecular orientation is discussed with the help of simple molecular orbital arguments.     

Self-assembled monolayers of aromatic selenolates on noble metal substrates

Self-assembled monolayers (SAMs) formed from bis(biphenyl-4-yl) diselenide (BBPDSe) on Au(111) and Ag(111) substrates have been characterized by high-resolution X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, infrared reflection absorption spectroscopy, water contact angle measurements, and scanning tunneling microscopy (STM). BBPDSe was found to form contamination-free, densely packed, and well-ordered biphenyl selenolate (BPSe) SAMs on both Au and Ag. Spectroscopic data suggest very similar packing density, orientational order, and molecular inclination in BPSe/Au and BPSe/Ag. STM data give a similar intermolecular spacing of 5.3 +/- 0.4 A on both Au and Ag but exhibit differences in the exact arrangement of the BPSe molecules on these two substrates, with the (2 square root[3] x square root[3])R30 degrees and (square root[3] x square root[3])R30 degrees unit cells on Au and Ag, respectively. There is strong evidence for adsorbate-mediated substrate restructuring in the case of Au, whereas no clear statement on this issue can be made in the case of Ag. The film quality of the BPSe SAMs is superior to their thiol analogues, which is presumably related to a better ability of the selenolates to adjust the surface lattice of the substrate to the most favorable 2D arrangement of the adsorbate molecules. This suggests that aromatic selenolates represent an attractive alternative to the respective thiols.     

Studies of chemisorbed tetracene on si(111)-7x7

The chemisorption of tetracene on the Si(111)-7x7 surface was studied using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. On the basis of the STM results and dimension analysis, two types of binding configurations were proposed. One of the configurations involves the di-sigma reaction between two C atoms of an inner ring with an adatom-rest atom pair on the substrate to give rise to an unsymmetrical butterfly structure. Tetracene in another configuration possesses four C-Si bonds that are formed via di-sigma reactions between the C atoms at the terminal rings with two center adatom-rest atom pairs within one-half of the surface unit cell. Besides, two other binding modes were proposed based on the dimension compatibility between the tetracene C and the substrate Si dangling bonds even though their identifications through the STM images are nonexclusive. Structural modeling and adsorption energies calculations were carried out using the DFT method. Factors affecting the relative thermodynamic stabilities based on the calculation results and the relative populations of tetracene in the different binding configurations as observed experimentally were discussed.     

Scanning tunneling microscope imaging of (CH3S)2 on Cu(111)

Scanning tunneling microscope (STM) images of isolated molecules of dimethyl disulfide, (CH(3)S)(2), adsorbed on the Cu(111) surface were successfully obtained at a sample temperature of 4.7 K. A (CH(3)S)(2) molecule appears as an elliptic protrusion in the STM images. From density functional theory calculation, it was suggested that the bright part in the protrusion corresponds to the molecular orbital which is widely spread around H atoms in each CH(3) group in the (CH(3)S)(2) molecule. The STM images revealed that the molecules have a total of six equivalent adsorption orientations on Cu(111), which are given by the combination of three equivalent adsorption sites and two conformational isomers for each adsorption site.     

Adsorption mode of cinchonidine on Cu(111) surface

The adsorption mode of cinchonidine on Cu(111) was directly obtained by in situ STM. The molecules were found to adsorb on the substrate surface and form a long-range ordered adlayer with (4 x 4) symmetry. While the quinoline rings lie parallel to Cu(111), the chiral quinuclidine moiety extends out of the surface. The enantioselectivity of catalysts may relate to this special adsorption conformation of cinchonidine on the surface.     

A scanning tunneling microscopy study of self-assembled nickel(II) octaethylporphyrin deposited from solutions on HOPG

The adsorption of nickel(II) octaethylporphyrin (NiOEP) from benzene and chloroform solutions on highly ordered pyrolytic graphite (HOPG) was investigated with a scanning tunneling microscope (STM) operated in ambient conditions. STM images show that NiOEP self-assembles on the graphite surface and that the molecules lie flat and form 2D lattices with spacings of 1.58 +/- 0.03 nm by 1.46 +/- 0.06 nm with a lattice angle of 69 degrees +/- 4 degrees averaged over both solvents. We were unable to eliminate the possibility that one unit cell distance is twice the above-reported distance. The corresponding molecular packing density, 4.5 +/- 0.3 x 10(13) molecules/cm(2), was essentially the same for benzene and chloroform solution deposition. These results differ somewhat from the structure revealed by high-resolution STM images of NiOEP on Au (111). The lack of apparent height (image intensity) in the constant current STM image of the alkane region of alkane-substituted metal porphyrins is attributed to a combination of changes in alkane configuration relative to the ring and associated changes in electronic coupling with HOMO and LUMO.     

Study of adsorption and decomposition of H2O on Ge(100)

The adsorption and decomposition of water on Ge(100) have been investigated using real-time scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. The STM results revealed two distinct adsorption features of H2O on Ge(100) corresponding to molecular adsorption and H-OH dissociative adsorption. In the molecular adsorption geometry, H2O molecules are bound to the surface via Ge-O dative bonds between the O atom of H2O and the electrophilic down atom of the Ge dimer. In the dissociative adsorption geometry, the H2O molecule dissociates into H and OH, which bind covalently to a Ge-Ge dimer on Ge(100) in an H-Ge-Ge-OH configuration. The DFT calculations showed that the dissociative adsorption geometry is more stable than the molecular adsorption geometry. This finding is consistent with the STM results, which showed that the dissociative product becomes dominant as the H2O coverage is increased. The simulated STM images agreed very well with the experimental images. In the real-time STM experiments, we also observed a structural transformation of the H2O molecule from the molecular adsorption to the dissociative adsorption geometry.     

Ultrathin TiO(x) films on Pt(111): a LEED, XPS, and STM investigation

Ultrathin ordered titanium oxide films on Pt(111) surface are prepared by reactive evaporation of Ti in oxygen. By varying the Ti dose and the annealing conditions (i.e., temperature and oxygen pressure), six different long-range ordered phases are obtained. They are characterized by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). By careful optimization of the preparative parameters, we find conditions where predominantly single phases of TiO(x), revealing distinct LEED pattern and STM images, are produced. XPS binding energy and photoelectron diffraction (XPD) data indicate that all the phases, except one (the stoichiometric rect-TiO2), are one monolayer thick and composed of a Ti-O bilayer with interfacial Ti. Atomically resolved STM images confirm that these TiO(x) phases wet the Pt surface, in contrast to rect-TiO2. This indicates their interface stabilization. At a low Ti dose (0.4 monolayer equivalents, MLE), an incommensurate kagomé-like low-density phase (k-TiO(x) phase) is observed where hexagons are sharing their vertexes. At a higher Ti dose (0.8 MLE), two denser phases are found, both characterized by a zigzag motif (z- and z'-TiO(x) phases), but with distinct rectangular unit cells. Among them, z'-TiO(x), which is obtained by annealing in ultrahigh vacuum (UHV), shows a larger unit cell. When the postannealing of the 0.8 MLE deposit is carried out at high temperatures and high oxygen partial pressures, the incommensurate nonwetting, fully oxidized rect-TiO2 is found The symmetry and lattice dimensions are almost identical with rect-VO2, observed in the system VO(x)/Pd(111). At a higher coverage (1.2 MLE), two commensurate hexagonal phases are formed, namely the w- [(square root(43) x square root(43)) R 7.6 degrees] and w'-TiO(x) phase [(7 x 7) R 21.8 degrees]. They show wagon-wheel-like structures and have slightly different lattice dimensions. Larger Ti deposits produce TiO2 nanoclusters on top of the different monolayer films, as supported both by XPS and STM data. Besides the formation of TiO(x) surfaces phases, wormlike features are found on the bare parts of the substrate by STM. We suggest that these structures, probably multilayer disordered TiO2, represent growth precursors of the ordered phases. Our results on the different nanostructures are compared with literature data on similar systems, e.g., VO(x)/Pd(111), VO(x)/Rh(111), TiO(x)/Pd(111), TiO(x)/Pt(111), and TiO(x)/Ru(0001). Similar and distinct features are observed in the TiO(x)/Pt(111) case, which may be related to the different chemical natures of the overlayer and of the substrate.     

Deposition of thermally unstable molecules with the spray-jet technique on Au(111) surface

A porphyrin derivative (5,15-bis(4-ethynylphenyl)-10,20-bis(3,5-di-tert-butylphenyl)porphyrin: trans-BETBPP) possessing chemically reactive substituents was successfully deposited on an Au(111) surface with a new molecular beam deposition system with use of a spray-jet technique (Spray-jet-MBD) without denaturing the molecules. The deposited molecular overlayers were observed at 77 K under ultrahigh vacuum condition by scanning tunneling microscopy (STM). They form two different overlayer structures: a linear arrangement and a square lattice structure. In these overlayers, some molecules were accidentally moved by STM tip agitation, which indicates that the molecules were not polymerized during the deposition process.     

Comparative investigation of underpotential deposition of Ag from aqueous and ionic electrolytes: An electrochemical and in situ STM study

Underpotential deposition (UPD) of Ag on Au(111) has been studied with two different electrolytes: aqueous 0.1 M H2SO4 solution in comparison with the ionic liquid 1-butyl-3-methylimidazolium chloride BMICl + AlCl3. Of particular interest is the distinct behavior of 2D phase formation at both interfaces, which has been investigated by cyclic and linear sweep voltammetry in combination with in situ electrochemical scanning tunneling microscopy (STM). It is found that one monolayer (ML) of Ag is formed in the UPD region in both electrolytes. In aqueous solution, atomically resolved STM images at 500 mV versus Ag/Ag+ show a (3 x 3) adlayer of Ag, whereas after sweeping the potential just before the commencement of the bulk Ag deposition, a transition from expanded (3 x 3) to pseudomorphic ML of Ag on Au(111) occurs. In BMICl-AlCl3, the first UPD process of Ag exhibits two peaks at 410 and 230 mV indicating that two distinct processes on the surface take place. For the first time, STM images with atomic resolution reveal a transition from an inhomogeneous to an ordered phase with a (square root of 3 x square root of 3)R30 degrees structure and an adsorption of AlCl4- anions having a superlattice of (1.65 x square root of 3)R30 degrees preceding the deposition of Ag.     

Two-site adsorption model for the (square root 3 x square root 3)-R30 degrees dodecanethiolate lattice on Au(111) surfaces

The surface structure of dodecanethiolate self-assembled monolayers (SAMs) on Au(111) surfaces, formed from the liquid phase, have been studied by grazing incidence X-ray diffraction (GIXRD), scanning tunneling microscopy (STM), and electrochemical techniques. STM images show that the surface structure consists of (square root 3 x square root 3)-R30 degrees domains with only a few domains of the c(4 x 2) lattice. The best fitting of GIXRD data for the (square root 3 x square root 3)-R30 degrees lattice is obtained with alkanethiolate adsorption at the top sites, although good fittings are also obtained for the fcc and hcp hollow sites. On the basis of this observation, STM data, electrochemical measurements, and previously reported data, we propose a two-site model that implies the formation of incoherent domains of alkanethiolate molecules at top and fcc hollow sites. This model largely improves the fitting of the GIXRD data with respect to those observed for single adsorption sites and, also, for the other possible two-site combinations. The presence of alkanethiolate molecules adsorbed at the less favorable top sites could result from the adsorption pathway that involves an initial physisorption step which, for steric reasons, takes place at on top sites. Once the molecules are chemisorbed, the presence of energy barriers for alkanethiolate surface diffusion, arising mostly from chain-chain interactions, "freezes" some of them at the on top sites, hindering their movement toward fcc hollow sites. By considering the length of the hydrocarbon chain and the adsorption time, the two-site model could be a tool to explain most of the controversial results on this matter reported in the literature.     

Self-assembly and conformation of tetrapyridyl-porphyrin molecules on Ag(111)

We present a low-temperature scanning tunneling microscopy (STM) study on the supramolecular ordering of tetrapyridyl-porphyrin (TPyP) molecules on Ag(111). Vapor deposition in a wide substrate temperature range reveals that TPyP molecules easily diffuse and self-assemble into large, highly ordered chiral domains. We identify two mirror-symmetric unit cells, each containing two differently oriented molecules. From an analysis of the respective arrangement it is concluded that lateral intermolecular interactions control the packing of the layer, while its orientation is induced by the coupling to the substrate. This finding is corroborated by molecular mechanics calculations. High-resolution STM images recorded at 15 K allow a direct identification of intramolecular features. This makes it possible to determine the molecular conformation of TPyP on Ag(111). The pyridyl groups are alternately rotated out of the porphyrin plane by an angle of 60 degrees.     

Scanning tunneling microscopy investigation of the structure of methanethiolate on Ag(111)

Scanning tunneling microscopy (STM) has been used to investigate the structure of the ordered methanethiolate overlayer formed on Ag(111) by reaction at room temperature with dimethyl disulfide. High-resolution images show an ordered structure with three inequivalent atomic-scale protrusions within each ( radical7 x radical7)R19 degrees surface unit mesh which can be reconciled with methanethiolate species on a regular lateral submesh, similar to that proposed in the multilayer ( radical7 x radical7)R19 degrees -S sulfide phase previously reported. STM imaging during dynamic dosing also provides evidence for a significant change in the outermost layer Ag atom density, consistent with a reconstructed surface model. Possible models for this reconstruction are presented and discussed in the light of available information.     

Encapsulated Pd nanocrystals supported by nanoline-structured SrTiO3(001)

Palladium nanocrystals were grown on a nanostructured SrTiO(3)(001) surface and annealed in ultrahigh vacuum at 620 degrees C. This leads to the so-called strong metal-support interaction (SMSI) state, characterized by encapsulation of the metal clusters with an oxide layer. Scanning tunneling microscopy (STM) of the oxide adlayer on the Pd(111) cluster surface reveals two superstructures with different lattice parameters and crystallographic rotations. Interpretation of the STM images is most readily achieved via noncommensurate TiO(x)() surface layers which result in two distinct Moiré patterns.     

Self-assembly of alkanethiol monolayers on Ag-Au(111) alloy surfaces

The self-assembly of ethanethiol (C(2)) and 1-octanethiol (C(8)) on Ag-Au(111) alloy films was studied by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and scanning tunneling microscopy (STM), to illuminate how the monolayer structures and chemisorption-induced substrate defect structures depend on the alloy composition. The thiolate packing density at saturation increased approximately linearly with increasing Ag ratio. The CV data for reductive desorption of thiolates evidenced predominant or major contributions of Ag atoms to the substrate-sulfur interactions for the alloy surfaces. The STM study supported the lack of elemental periodicity on Ag-Au(111) and the consequent absence of periodicity in substrate-sulfur bonding. For C(8)-covered films, we observed systematic changes of substrate defect structures from elevated monatomic islands on Ag(111) to vacancy island structure on Au(111), in good correlation with the reductive desorption characteristics. The former type of defects can be explained best in terms of breakup of atomic terraces under excess thiolate packing density for Ag(111) and Ag-rich Ag-Au(111). As for the vacancy island formation, the present results are not agreeable with the chemical etching model but compatible with the lattice relaxation model.     

Submolecular imaging of chloronitrobenzene isomers on Cu(111)

We compare computer simulations to experimental scanning tunneling microscopy (STM) images of chloronitrobenzene molecules on a Cu(111) surface. The experiments show that adsorption induced isomerization of the molecules takes place on the surface. Furthermore, not only the submolecular features can be seen in the STM images, but different isomers can also be recognized. The Todorov-Pendry approach to tunneling produces simulated STM images which are in good accordance with the experiments. Alongside with STM simulations in a tight-binding basis, ab initio calculations are performed in order to analyze the symmetry of relevant molecular orbitals and to consider the nature of tunneling channels. Our calculations show that while the orbitals delocalized to the phenyl ring create a relatively transparent tunneling channel, they also almost isolate the orbitals of the substitute groups at energies which are relevant in STM experiments. These features of the electronic structure are the key ingredients of the accurate submolecular observations.     

The adsorption structure of NO on Pd(111) at high pressures studied by STM and DFT

Using a combination of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we study the adsorption structure of NO on Pd(111) at pressures of up to 720 Torr. From atomically resolved STM images, we identify, at high pressures, only the (2 x 2)-3NO structure, which is identical with the highest NO-coverage structure found at low pressure and low temperature. DFT calculations confirm that the (2 x 2)-3NO structure is indeed the most stable adsorption structure at high pressures. Contrary to recent suggestions in the literature, we therefore conclude that we find no evidence for a (3 x 3)-7NO structure on Pd(111) at high NO pressure.     

四氮杂杯芳烃三嗪衍生物在Au(111)表面的自组装结构的电化学STM研究

利用电化学扫描隧道显微镜和循环伏安法研究了一种新型的杂杯杂芳烃四氮杂杯芳烃三嗪衍生物在Au(111)表面的自组装结构. 高分辨的STM图像表明, 该杂杯杂芳烃可以在Au(111)表面形成长程有序的单层膜. 此外, 分子以1,3-交替构象吸附, 两个三嗪环平躺在表面, 而苯环倾斜吸附在基底上, 这是分子间与分子-基底间相互作用平衡的结果.