Interpretation of high-resolution images of the best-bound wetting layers on Pt(111)

Two interpretations have been proposed of dark triangles observed in scanning tunneling microscopy (STM) images of the best bound, √37×√37-R25.3°, and √39×√39-R16.1° periodic water monolayers on Pt(111). In one, a "Y"-shaped tetramer of water molecules is removed, leaving a vacancy island behind; the other assumes the Y is replaced by a hexagon of H(2)O molecules, amounting to a di-interstitial. Consistent only with the di-interstitial model are thermal desorption and CO coadsorption data, STM line scans, and total energies obtained from density functional theory calculations.     

Structure of CO2 adsorbed on the KCl(100) surface

The structure and dynamics of the adsorbate CO(2)/KCl(100) from a diluted phase to a saturated monolayer have been investigated with He atom scattering (HAS), low-energy electron diffraction (LEED), and polarization dependent infrared spectroscopy (PIRS). Two adsorbate phases with different CO(2) coverage have been found. The low-coverage phase is disordered at temperatures near 80 K and becomes at least partially ordered at lower temperatures, characterized by a (2√2×√2)R45° diffraction pattern. The saturated 2D phase has a high long-range order and exhibits (6√2×√2)R45° symmetry. Its isosteric heat of adsorption is 26 ± 4 kJ mol(-1). According to PIRS, the molecules are oriented nearly parallel to the surface, the average tilt angle in the saturated monolayer phase is 10° with respect to the surface plane. For both phases, structure models are proposed by means of potential calculations. For the saturated monolayer phase, a striped herringbone structure with 12 inequivalent molecules is deduced. The simulation of infrared spectra based on the proposed structures and the vibrational exciton approach gives reasonable agreement between experimental and simulated infrared spectra.     

Adsorptive behavior of dimethylglyoxime on Au(111)

Dimethylglyoxime (DMG) adsorbed on Au(111) was investigated using electrochemical scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM experiments revealed three different structures of adsorbed DMG at open circuit potential (~0.07 V versus Ag/AgCl): (2√3×2√3)R30°-α, (2√3×4√3)R30°-β, and (2√3×4√3)R30°-γ. The coverage of adsorbed DMG obtained using XPS was 0.33. A combination of structural and quantitative information identified the adsorbed DMG as an anionic tetramer, held together by intermolecular hydrogen bonding and arrayed in three ordered patterns. Domains of adsorbed DMG underwent phase transitions between the observed structures, most likely due to the influence of the STM tip. However, a significant correlation between the observed structures and the imaging conditions was not found. The ordered layers existed only at open circuit potential as evidenced by their disappearance when the potential was shifted to 0.2 or -0.15 V. The ordered layers were also removed by immersion in a solution of Ni(2+), implying that the adsorbed DMG was converted to a soluble dimer complex with the Ni(2+) ion. This particular observation is discussed in terms of the rigidity of the organic network.     

Molecular orientations and interfacial structure of C60 on Pt(111)

Molecular orientations and assembled structures of C(60) molecules on Pt(111) have been characterized by low-temperature scanning tunneling microscopy for coverage between 0.1 ML and 1.5 ML. At room temperature, C(60) molecules preferentially decorate the steps and nucleate into single layer islands (SLIs) with hexagonal close-packed structures upon increasing coverage. C(60) islands comprise two differently oriented C(60)∕Pt(111)-(√13?×?√13) R13.9° phases, in which five types of molecular orientation of C(60) carbon cage configurations are clearly identified by the high-resolution scanning tunneling microscopy image. Further annealing treatment leads to more uniform molecular orientation without apparent aggregation of C(60) SLIs. As coverage increases above 1 ML, domains corresponding to (2√3?×?2√3) R30° superstructure appear. To explain the above transformation, an interfacial reconstruction model is proposed according to the detailed study of the molecular adsorption structures in different domains.     

Surface structure and phase transition of K adsorption on Au(111): by ab initio atomistic thermodynamics

We studied the interactions between atomic potassium (K) and Au(111) at a range of coverage (i.e., Θ(K) = 0.11-0.5 monolayer (ML)) by ab initio atomic thermodynamics. For K on-surface adsorption, we found that K energetically favors the three-fold hollow sites (fcc or hcp), while the most significant surface rumpling was obtained at the atop sites. The incorporation of gold atoms in the adsorbate layer gradually becomes energetically favorable with increasing K coverage. We proposed a possible model with a stoichiometry of K(2)Au for the (2 × 2)-0.5 ML phase observed in lower energy electron diffraction (LEED): one K at atop site and the other K as well as one Au adatom at the second-nearest fcc/hcp and hcp/fcc, respectively. Clear theoretical evidences were given for the ionic interaction of K on Au surface. Additionally, phase transitions were predicted based on chemical potential equilibrium of K, largely in line with the earlier reported LEED observations: the clean surface → (√3 × √3)R30° → (2 × 2), and (2 × 2) → (√3 × √3)R30° reversely at an elevated temperature.     

Dynamic double lattice of 1-adamantaneselenolate self-assembled monolayers on Au{111}

We report a complex, dynamic double lattice for 1-adamantaneselenolate monolayers on Au{111}. Two lattices coexist, revealing two different binding modes for selenols on gold: molecules at bridge sites have lower conductance than molecules at three-fold hollow sites. The monolayer is dynamic, with molecules switching reversibly between the two site-dependent conductance states. Monolayer dynamics enable adsorbed molecules to reorganize according to the underlying gold electronic structure over long distances, which facilitates emergence of the self-organized rows of dimers. The low-conductance molecules assume a (7 × 7) all-bridge configuration, similar to the analogous 1-adamantanethiolate monolayers on Au{111}. The high-conductance molecules self-organize upon mild annealing into distinctive rows of dimers with long-range order, described by a (6√5 × 6√5)R15° unit cell.     

First-principles study on the reconstruction induced by the adsorption of C60 on Pt(111)

The adsorption of C60 on a Pt(111) surface and the origins of the √13 × √13R13.9° or 2√3 × 2√3R30° reconstruction of the C60/Pt(111) system have been investigated by means of first-principles calculations. In agreement with the experimental observations, our calculations reveal that the C60 molecule binds covalently on the Pt(111) surface. The C60 molecule adsorbs on the Pt(111) surface with the center of a hexagonal ring located on top of a surface Pt atom. The surface Pt atom can be removed easily, forming a Pt vacancy upon the adsorption of C60 molecule. Our calculation results show that the strong covalent bonds between C60 and the Pt(111) surface and the formation of adatom-vacancy pairs in the C60/Pt(111) system may be the main driving forces promoting the substrate reconstructing pattern observed in experiments.     

Structural phases formed by NO(2)∕CO co-adsorption on Au{111} surfaces

Exposing a Au{111} surface to NO(2) and then to CO at temperatures around 120 K in ultra-high vacuum gives rise to molecular overlayers in which the two species are co-adsorbed, which we have investigated using low-temperature scanning tunnelling microscopy. Under NO(2)-rich conditions, a (√7 × √7)R19.1° phase with 3:1 NO(2):CO stoichiometry forms. Under CO-rich conditions, this phase co-exists with other phases having 2:1 and 1:1 NO(2):CO stoichiometries and different symmetries, and with bare Au surface. Structural models for these phases are discussed. Individual domains of the (√7 × √7)R19.1° phase are chiral, by virtue of the arrangement of their achiral components, an observation that may have more general implications.     

Ab initio studies of the electronic structure of L-cysteine adsorbed on Ag(111)

We have performed ab initio calculations for the adsorption of L-cysteine on Ag(111) using density functional theory. We have focused on two possible adsorbed species: the L-cysteine radical (?S-CH(2)-CH-NH(2)-COOH) adsorbed almost flat at a bridge site, slightly displaced toward an fcc location, and the zwitterionic radical Z-cysteine (?S-CH(2)-CH-NH(3)(+)-COO(-)) adsorbed at a bridge site, shifted to a hcp site forming a (4 × 4) unit cell (θ = 0.06) and a (√3 × √3) R 30° unit cell (θ = 0.33), respectively. Special attention has been paid to the electronic structure of the system. The adsorbate-silver bond formation has been exhaustively investigated by analyzing the density of states projected onto the different atoms of the molecule, and by charge density difference calculations. A complicated interplay between sp and d states of silver in the formation of bonds between the adsorbates and the surface has been found. The role of the carboxyl group in the interaction with the surface has been also analyzed.     

Formation of highly ordered and orientated gold islands: effect of immersion time on the molecular adlayer structure of pentafluorobenzenethiols (PFBT) SAMs on Au(111)

Self-assembled monolayers (SAMs) of pentafluorobenzenethiol (PFBT) on Au(111) substrates, prepared with different immersion times (ITs) at room temperature, were studied using scanning tunneling microscopy (STM) and infrared reflection-absorption spectroscopy (IRRAS). In the present study, the focus was on several important points of interest in the field of SAMs. First, the gold islands formed upon adsorption of PFBT molecules on the gold surface were monitored at different ITs in terms of their size, density, and shape. After short ITs (5 to 30 min), small gold islands with rounded shape were formed. These gold islands were arranged in a rather regular fashion and found to be quite mobile under the influence of the STM-tip during the scanning. When the IT was increased to 16 h, the results revealed the formation of highly ordered and orientated gold islands with very unusual shapes with straight edges meeting at 60° or 120° running preferentially along the [11(-)0] substrate directions. The density of the gold islands was found to decrease with increasing IT until they almost disappeared from the SAMs prepared after 190 h of IT. On top of the gold islands, the PFBT molecules were found to adopt the closely packed (10√3 × 2) structure. Second, a number of structural defects such as disordered regions at the domain boundaries and dark row(s) of molecules within the ordered domains of the PFBT SAMs were observed at different ITs. The SAMs prepared after 190 h of IT were found to be free of these defects. Third, at low and moderate ITs, a variation in the PFBT molecular contrast was observed. This contrast variation was found to depend mainly on the tunneling parameters. Finally, our results revealed that the organization process of PFBT SAMs is IT-dependent. Consequently, a series of structural phases, namely, α, β, γ, δ, and ε were found. The α-, β-, γ-, and δ-phases were typically accompanied by the ε-phase that appeared on top of gold islands. With increasing IT, the α→β→ γ→δ→ε phase transitions took place. The resulting ε-phase, which covered the entire gold surface after 190 h of IT, yielded well-ordered self-assembled monolayers with large domains having a (10√3 × 2) superlattice structure.     

4-Nitrophenyl sulfenyl chloride as a new precursor for the formation of aromatic SAMs on gold surfaces

4-Nitrophenyl sulfenyl chloride was used as a new precursor for the formation of densely packed aromatic SAMs on gold. The adsorption involves the reductive dissociation of the S-Cl bond. A well-ordered row structure corresponding to a √3 × 4 phase with a molecular area of 27.8 ?(2) is obtained.     

金属-有机配合物分子在Au(111)表面的吸附行为

用STM对含氧桥的金属-有机配合物[Cu₂(μ-O)(dptap)4(NO₃)₂]分子在Au(111)表面的吸附行为进行了研究. STM结果表明, 该分子同时存在非解离吸附和解离吸附, 大部分分子在Au(111)面形成有规则的排列, 少量分子发生解离吸附, 并形成(√3×√3)R30°Cu原子吸附结构. 探讨了两种吸附现象共存的起因.     

Relationship between the c(4×2) and the (√3×√3)R30° phases in alkanethiol self-assembled monolayers on Au(111)

Transformation between the two well-known phases of alkanethiol monolayers on Au(111), c(4×2) and (√3×√3)R30°, has been studied using scanning tunneling microscopy in ultra-high vacuum. Among the many versions of the c(4×2) phases observed, one particular structure where a lateral shift of adsorbate by as much as 0.17 nm within the unit cell is found. This lateral shift along the [112[combining macron]] direction corresponds to the movement of one adsorbed unit, towards its nearest neighbour from one hollow site to another (fcc to hcp, or hcp to fcc).     

Orientational ordering of the second layer of C60 molecules on Au(111)

We have studied the orientational ordering of the second layer of C(60) molecules on Au(111) using scanning tunnelling microscopy (STM) at 77 K. The orientation of individual molecules within the second layer follows a regular pattern, giving rise to a 2 × 2 superlattice. The long-range order of the 2 × 2 lattice depends on the structure of the first molecular layer with the best ordering found inside the R14° domain. The second layer formed on top of the contrast-disordered R30° domain consists of patches of bright and dim molecules. The contrast between bright and dim patches shows a clear dependence on the sample bias. This bias-dependent contrast is explained by considering the contributions to tunnel current from HOMO and LUMO mediated electron transfer processes. Scanning tunnelling spectroscopic measurement reveals the narrowing of the HOMO-LUMO gap for the layer of molecules in direct contact with the Au(111) substrate.     

Structure of Reconstructed Cu(100) Surface Induced by Dissociative Adsorption of Gaseous Oxygen

The reconstructed structures of Cu(100) surface induced by O₂ dissociative adsorption wereinvestigated by low energy electron diffraction and scanning tunneling microscopy. At lower oxygen coverage, it was found that two reconstructed structures, i.e. c(2×2)-O and (√2×2√2)R45°-O are coexistent. The domain size of the c(2×2)-O structure decreased with the increasing of O₂ exposure. The reconstructed structure at very small coverage was also investigated and a “zigzag” structure was observed at this stage. The “zigzag” structure was identified as boundaries of local c(2×2) domains. It was found that the strip region shows much stronger molecule-substrate interaction than that of oxygen covered regions, making it a proper template for patterned organic films. The sequence of the thermal stability was found as zigzag structure>c(2×2)>(√2×2√2)R45°-O.     

Probing the buried Pb/Si(111) interface with SPA LEED and STM on Si(111)-Pbα√3×√3

High resolution spot profile analysis low energy electron diffraction (SPA-LEED) and variable temperature scanning tunneling microscopy (STM) have been used to observe the growth of Pb on the Pb/Si(111)-α√3×√3 phase, which is driven by quantum size effects (QSE). A change in the rotation of the Pb grown islands with respect to the Si substrate has been observed with increasing coverage θ. At lower coverage, separated two-step islands are grown and are aligned with the [110] axis of the substrate. With increasing coverage above 1.5 ML, of the islands coalesce and form a bilayer, with additional islands grown on top. The preferred Pb island orientation changes to 5.6° with respect to the [110] direction. These changes at the metal/semiconductor buried interface are obtained both with SPA LEED and STM as changes to the period of the Moire pattern. The method of analysis of the corrugation period and rotation angle of the Moire pattern measured with diffraction and STM can be applied to obtain the structure of buried metal/substrate interfaces in other epitaxial systems.     

Electrodeposition of copper on a Pt(111) electrode in sulfuric acid containing poly(ethylene glycol) and chloride ions as probed by in situ STM

This study employed real-time in situ STM imaging to examine the adsorption of PEG molecules on Pt(111) modified by a monolayer of copper adatoms and the subsequent bulk Cu deposition in 1 M H(2)SO(4) + 1 mM CuSO(4)+ 1 mM KCl + 88 μM PEG. At the end of Cu underpotential deposition (~0.35 V vs Ag/AgCl), a highly ordered Pt(111)-(√3 × √7)-Cu + HSO(4)(-) structure was observed in 1 M H(2)SO(4) + 1 mM CuSO(4). This adlattice restructured upon the introduction of poly(ethylene glycol) (PEG, molecular weight 200) and chloride anions. At the onset potential for bulk Cu deposition (~0 V), a Pt(111)-(√3 × √3)R30°-Cu + Cl(-) structure was imaged with a tunneling current of 0.5 nA and a bias voltage of 100 mV. Lowering the tunneling current to 0.2 nA yielded a (4 × 4) structure, presumably because of adsorbed PEG200 molecules. The subsequent nucleation and deposition processes of Cu in solution containing PEG and Cl(-) were examined, revealing the nucleation of 2- to 3-nm-wide CuCl clusters on an atomically smooth Pt(111) surface at overpotentials of less than 50 mV. With larger overpotential (η > 150 mV), Cu deposition seemed to bypass the production of CuCl species, leading to layered Cu deposition, starting preferentially at step defects, followed by lateral growth to cover the entire Pt electrode surface. These processes were observed with both PEG200 and 4000, although the former tended to produce more CuCl nanoclusters. Raising [H(2)SO(4)] to 1 M substantiates the suppressing effect of PEG on Cu deposition. This STM study provided atomic- or molecular-level insight into the effect of PEG additives on the deposition of Cu.     

Low-temperature adsorption of H2S on Ag(111)

H(2)S forms a rich variety of structures on Ag(111) at low temperature and submonolayer coverage. The molecules decorate step edges, exist as isolated entities on terraces, and aggregate into clusters and islands, under various conditions. One type of island exhibits a (√37×√37)R25.3° unit cell. Typically, molecules in the clusters and islands are separated by about 0.4 nm, the same as the S-S separation in crystalline H(2)S. Density functional theory indicates that hydrogen-bonded clusters contain two types of molecules. One is very similar to an isolated adsorbed H(2)S molecule, with both S-H bonds nearly parallel to the surface. The other has a S-H bond pointed toward the surface. The potential energy surface for adsorption and diffusion is very smooth.     

Adsorption and thermal decomposition of 2-octylthieno[3,4-b]thiophene on Au(111)

The adsorption and thermal stability of 2-octylthieno[3,4-b]thiophene (OTTP) on the Au(111) surfaces have been studied using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). UHV-STM studies revealed that the vapor-deposited OTTP on Au(111) generated disordered adlayers with monolayer thickness even at saturation coverage. XPS and TPD studies indicated that OTTP molecules on Au(111) are stable up to 450K and further heating of the sample resulted in thermal decomposition to produce H(2) and H(2)S via C-S bond scission in the thieno-thiophene rings. Dehydrogenation continues to occur above 600K and the molecules were ultimately transformed to carbon clusters at 900K. Highly resolved air-STM images showed that OTTP adlayers on Au(111) prepared from solution are composed of a well-ordered and low-coverage phase where the molecules lie flat on the surface, which can be assigned as a (9×2√33)R5° structure. Finally, based on analysis of STM, TPD, and XPS results, we propose a thermal decomposition mechanism of OTTP on Au(111) as a function of annealing temperature.     

Titanium isopropoxide complexes of a series of sterically demanding aryloxo based [N2O2]2- ligands as precatalysts for ethylene polymerization

Several titanium isopropoxide complexes [N,N'-bis(2-oxo-3-R(1)-5-R(2)-phenylmethyl)-N,N'-bis(methylene-p-R(3)-C(6)H(4))-ethylenediamine]Ti(O(i)Pr)(2) [R(1) = t-Bu, R(2) = Me, R(3) = H (1b); R(1) = R(2) = t-Bu, R(3) = H, (2b); R(1) = R(2) = Cl, R(3) = H, (3b), R(1) = t-Bu, R(2) = Me, R(3) = Cl (4b); R(1) = R(2) = t-Bu, R(3) = Cl, (5b); R(1) = R(2) = R(3) = Cl, (6b)] supported over sterically demanding aryloxy based [N(2)O(2)]H(2) ligands have been designed as precatalysts for the ethylene polymerization. Specifically, the 1b-6b complexes, when treated with methylaluminoxane (MAO) under 88 ± 0.5 psi of ethylene at 30 °C for 3 h, produced polyethylene polymers of high molecular weight (M(w) = ca. 7.2-8.3 × 10(5) g mol(-1)) having broad molecular weight distribution (PDI = ca. 13.1-14.6). The 1b-6b complexes were conveniently synthesized from the direct reaction of the [N(2)O(2)]H(2) ligands, 1a-6a, with Ti(O(i)Pr)(4) in 69-86% yield.