Adlayer Structure Research Articles

Sulfidation of a Cu submonolayer at the Au(1 1 1)/electrolyte interface – An in situ STM study

We describe the electrochemical preparation of an ultrathin copper sulfide film on Au(1 1 1) and its structural characterization by in situ STM. The first step, underpotential deposition of a Cu submonolayer from CuSO 4/H 2SO 4 solution, is followed by two electrolyte exchanges for (i) Cu-free (blank) H 2SO 4 solution and (ii) NaOH/Na 2S solution. The well-known (√3 × √3) R30° structure of the upd Cu layer is stable in the blank electrolyte for at least 2 h. After exposure to bisulfide, the Cu layer contracts and forms two-dimensional islands of two distinct ordered surface phases, i.e. a rectangular and, at higher potentials, a hexagonal phase, with Cu-free Au(1 1 1) regions between them, the latter exhibiting the characteristic (√3 × √3) R30°-S adlayer structure. Potential changes lead to a complex phase behaviour including HS − ⇆ S x oxidation/reduction and, at strongly anodic potentials, dissolution of the Cu adlayer.

In Situ Scanning Tunneling Microscopy of 1,6-Hexanedithiol, 1,9-Nonanedithiol, 1,2-Benzenedithiol, and 1,3-Benzenedithiol Adsorbed on Pt(111) Electrodes

Cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) were used to examine four dithiol molecules, including 1,6-hexanedithiol, 1,9-nonanedithiol, 1,2-benzenedithiol, and 1,3-benzenedithiol, adsorbed on well-ordered Pt(111) electrodes in 0.1 M HClO(4). The open-circuit potential (OCP) of Pt(111) electrodes decreased substantially from 0.95 to 0.3 V (versus reversible hydrogen electrode) upon the adsorption of dithol molecules, which indicates that these adsorbates injected electrons into the Pt electrode. For all dithiol molecules, ordered adlattices of p(2 x 2) and (square root 3 x square root 3)R30 degrees were formed when the dosing concentration was lower than 150 microM and the potential of Pt(111) was more negative than 0.5 V. Raising the potential of Pt(111) from 0.1 to 0.4 V or more positive values could transform p(2 x 2) to (square root 3 x square root 3)R30 degrees before it turned disarray. The insensitivity of the structure of dithiol adlayers with their chemical structures was explained by upright molecular orientation with the formation of one Pt-S bond per dithiol molecule. This molecular orientation was independent of the coverage of dithiol molecules, as nucleation seeds produced at the beginning of adsorption were also constructed with p(2 x 2). The triangular-shaped STM molecular resolution suggested 3-fold binding of sulfur headgroup on Pt(111). All dithiols were adsorbed so strongly on Pt(111) electrodes that switching the potential negatively to the onset of hydrogen evolution in 0.1 M HClO(4) or water reduction in 1 M KOH could not displace dithiol admolecules.

Morphology, interfacial electronic structure, and optical properties of oligothiophenes grown onZnSe(100)by molecular beam deposition

We report on the growth of two types of oligothiophenes, $\ensuremath{\alpha}$-sexithiophene and a substituted terthiophene, on $c(2\ifmmode\times\else\texttimes\fi{}2)$ reconstructed $\mathrm{ZnSe}(100)$ surfaces by molecular beam deposition. The study is aimed at a deeper understanding of the structural and electronic properties of organic/ inorganic semiconductor interfaces. The structure and morphology of the organic adlayer from submonolayer up to several monolayer coverage were characterized by atomic force microscopy. The interfacial electronic structure and the optical properties were investigated by photoemission and photoluminescence spectroscopy, respectively. The results reveal that the organic adlayer is bonded by van der Waals forces to the $\mathrm{ZnSe}(100)$ $c(2\ifmmode\times\else\texttimes\fi{}2)$ surface. The electronic structure and the optical properties of the two materials forming the interface remain unperturbed, indicating that surface passivation, which is found indispensable when using other covalent inorganic semiconductor surfaces such as $\mathrm{GaAs}$ as substrates for organic thin films, is not required to allow for an ordered growth of the organic adlayer on $\mathrm{ZnSe}(100)$.

Effect of sulfuric acid concentration on the structure of sulfate adlayer on Au(1 1 1) electrode

The adsorption of sulfate on Au(1 1 1) single-crystal electrode was examined in sulfuric acid solutions of various concentrations (0.5–7.5 M) by cyclic voltammetry and in situ scanning tunneling microscopy (STM). The well-known sulfate adlayer with (√3 × √7) unit cell was found in 0.5 M sulfuric acid by in situ STM. In 2.5 and 5.0 M sulfuric acid, a new order–disorder phase transition, from the (√3 × √7) adlayer to a disordered structure, was found to take place at ca. 1.1 V vs. Ag∣AgCl∣KCl(sat.). In 7.5 M sulfuric acid, no current spikes were seen in the CV profile, and in situ STM showed that the sulfate adlayer was disordered.

Scanning Tunnelling Microscopy on Ultrathin Organic Layers of Phthalocyanine and Naphthalocyanines on Highly Oriented Pyrolytic Graphite (0001)

The adsorption geometry of the monolayers of planar palladium phthalocyanine (PdPc), naphthalocyanine (Nc), and non-planar tin–naphthalocyanine (SnNc) on the basal plane (0001) of graphite have been studied. Monolayers were prepared using organic molecular beam epitaxy (OMBE) under ultrahigh vacuum (UHV) conditions at room temperature. Planar PdPc and Nc molecules form large areas of ordered monolayers with their molecular plane parallel to the substrate. PdPc forms quadratic and Nc forms distorted quadratic superstructures, however the superstructure is non-commensurate with the substrate lattice. Non-planar SnNc shows a commensurism with graphite and forms a hexagonal superstructure, which indicates a stronger molecule–substrate interaction compared to the planar derivatives revealing the influence of geometry on the adlayer structure.

Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy Studies of Planar and Nonplanar Naphthalocyanines on Graphite (0001). Part 1: Effect of Nonplanarity on the Adlayer Structure and Voltage-induced Flipping of Nonplanar Tin−Naphthalocyanine

The adsorption of base-free naphthalocyanine (Nc), a planar molecule, and tin-naphthalocyanine (SnNc), a nonplanar molecule, on a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface at low sample temperature (50 K) has been studied using a variable-temperature scanning tunneling microscope in ultra-high vacuum conditions. The planar molecules form large areas of defect-free ordered monolayer with high molecular packing density while the nonplanar molecules show different phases of adsorption with lower molecular packing density. The SnNc adlayers follow the same geometry as the graphite substrate and form pure phases of adsorption with either all molecules in a Sn(2+) up or Sn(2+) down geometry. Moreover, a one-dimensional selectivity is observed in still another phase of Sn(2+) down geometry. Multilayers show a completely different kind of adsorption in each case. Nc molecules show columnar pi-stacking whereas the SnNc molecules exhibit noncolumnar stacking. Distinctly, a voltage-induced flipping of nonplanar tin-naphthalocyanine in the monolayer has been observed which can possibly be applied to single-molecular information storage.

STM Investigation of the Photoisomerization of an Azobis-(benzo-15-crown-5) Molecule and Its Self-assembly on Au(111)

Scanning tunneling microscopy (STM) has been employed to investigate the photoisomerization of azobis-(benzo-15-crown-5) on Au(111). A self-assembled monolayer of azobis-(benzo-15-crown-5) with a (3 x 7) symmetry was observed on Au(111). After ex-situ UV-light irradiation, a new adlayer structure with a different molecular arrangement could be seen on Au(111), although the domain size of the ordered packing is small. On the basis of a high-resolution STM image and photochemical reaction result, schematic models have been proposed for the original and ex-situ irradiated adlayers. After in-situ UV-light irradiation, the ordered arrays disappeared and many bright clusters emerged at domain boundaries. The results presented here provide direct evidence at the molecular level for a photochemical reaction.

Time-Dependent Organization and Wettability of Decanethiol Self-Assembled Monolayer on Au(111) Investigated with STM

A detailed study on the time-dependent organization of a decanethiol self-assembled monolayer (SAM) at a designed solution concentration onto a Au(111) surface has been performed with scanning tunneling microscopy (STM). The SAMs were prepared by immersing Au(111) into an ethanol solution containing 1 microM decanethiol with different immersion times. STM images revealed the formation process and adlayer structure of the SAMs. It was found that the molecules self-organized into adlayers from random separation to a well-defined structure. From 10 s, small domains with ordered molecular organization appeared, although random molecules could be observed on Au(111) at the very initial stage. At 30 s, the SAM consisted of uniform short stripes. Each stripe consisted of sets of decanethiol mainly containing eight molecules. With the immersion time increasing, the length of the stripes increased. At 5 min, the alkyl chains overlapped each other between the adjacent stripes, indicating the start of a stacked process. After immersing Au(111) in decanethiol solution for 3 days, a densely packed adlayer with a (radical 3 x radical 3)R30 degrees structure was observed. The formation process and structure of decanethiol SAMs are well related to sample preparation conditions. The wettability of the decanethiolate SAM-modified Au(111) surface was also investigated.

Para-Sexiphenyl thin film growth on Cu(1 1 0) and Cu(1 1 0)–(2 × 1)O surfaces

We present a reflectance difference spectroscopy (RDS) study of para-sexiphenyl ( p-6P) thin film growth on Cu(1 1 0) and Cu(1 1 0)–(2 × 1)O substrates. The RDS spectra show pronounced anisotropies for p-6P films formed on both substrates at room temperature, demonstrating that the molecules are uniaxially aligned within the films. Based on the RD spectra and the evolution of the optical transitions with p-6P coverage the growth mode on both substrates could be identified. From the dominating RDS feature, assigned to the lowest energy HOMO–LUMO transition, the orientation of the molecular chain can be determined. On Cu(1 1 0), the p-6P molecular chains align in the [ 1 ¯ 1 0 ] direction, i.e., along the Cu atomic rows, whereas on the Cu(1 1 0)–(2 × 1)O surface, the molecules are oriented in the orthogonal [0 0 1] direction, i.e., along the “added” Cu–O rows of the Cu(1 1 0)–(2 × 1)O surface. The energetic position and line shape of the main RDS feature differs for the two substrates and varies with p-6P coverage. This fine structure is discussed in terms of different molecular conformations, adlayer structure and vibronic replicas.

Morphology Selected Molecular Architecture: Acridine Carboxylic Acid Monolayers on Ag (111)

The molecular architecture of acridine-9-carboxylic acid (ACA) grown on Ag (111) by physical vapor deposition was characterized by using UHV-STM and XPS. At lower coverage, ACA molecules exist in a 2-d gas phase on the surface at room temperature. With increased coverage (>0.4 ML), ACA molecules self-organize into distinctive adlayer structures that are correlated with underlying substrate morphology. On step-free Ag (111) regions, ACA molecules form large islands in coexistence with the 2-d ACA gas. These islands are commensurate with the Ag (111) substrate, indexed as (4 0, 2 4) in matrix notation, and can exceed 100 nm in size. There are two nonequivalent ACA molecules in each unit cell. XPS core level measurements reveal a hydrogen-bonding interaction between ACA molecules, with the ring nitrogen acting as the H-bond acceptor and the carboxyl proton acting as the H-bond donor. A structural model for this phase consists of chains of ACA molecules linked by head-to-tail hydrogen bonds along the substrate [10] direction. Alternating ACA tilting angles account for the two nonequivalent ACA molecules and the observed high packing density. Completely different molecular arrangements are observed on Ag (111) surface regions roughened by a higher density of crystallographic steps (terrace widths < or = 6 nm). Pairs of ACA molecules arrange in a zigzag pattern in a (12 2, 6 5) overlayer structure with a diluted packing density. The structural model for this lower density phase consists of carboxyl-carboxyl linked ACA dimers in a flat-lying molecular orientation.

Role of van der Waals interactions in adsorption of Xe on Cu(111) and Pt(111)

We consider some recently developed schemes for treating van der Waals interactions within the density functional theory (DFT) on the widely discussed example of adsorption of Xe on Cu(111) and Pt(111) surfaces. Consistent with the overall weakness of the Xe surface and Xe-Xe interactions we assess the performance of the schemes that are appropriate to systems consisting of nearly isolated fragments in which the coefficients of the van der Waals expansion are deduced from DFT calculations. Such generalized DFT calculations of potential energy surfaces yield the structure of Xe adlayers in good agreement with experiments and retrieve the dilation of commensurate monolayer phase in which the intralayer Xe-Xe radial force constants are strongly reduced. This provides a first principles interpretation of the observed vibrational properties of adlayers, in general, and the much debated dispersion of in-plane polarized vibrations, in particular.

Structural Evolution of Hexa-peri-hexabenzocoronene Adlayers in Heteroepitaxy on n-Pentacontane Template Monolayers

The growth and structure of self-assembled adlayers of hexakis(n-dodecyl)-peri-hexabenzocoronene (HBC-C12) adsorbed on highly ordered pyrolitic graphite (HOPG) decorated by an n-pentacontane (n-C50H102) monolayer have been investigated by scanning tunneling microscopy (STM). Whereas on HOPG the HBC-C12 molecules readily self-assemble into a unique stable 2D structure, on the [n-C50H102 monolayer/graphite] system we observe morphological phase transitions with formation of time dependent alpha, beta, and gamma phases (alpha-->beta-->gamma). The initial alpha-phase is similar to that obtained on bare graphite, while intermediate beta- and final gamma-structures present molecular dimers and rows, respectively. The observed two-dimensional polymorphism is due to weak interaction between HBC-C12 molecules and n-C50H102-modified graphite substrate. Our results constitute an important step toward the control of the growth and structure of highly ordered monolayers of functional conjugated molecules by modifying the graphite surface with an n-alkane monolayer of appropriate chain length.

Molecular Ordering and Adsorbate Induced Faceting in the Ag{110}−(S)-Glutamic Acid System

The adsorption of the amino acid, (S)-glutamic acid, was investigated on Ag{110} as a function of coverage and adsorption temperature using the techniques of scanning tunneling microscopy, low energy electron diffraction, and reflection absorption infrared spectroscopy. In the monolayer, (S)-glutamic acid was found to adsorb predominantly in the anionic glutamate form. Several discrete ordered adlayer structures were observed depending on preparation conditions. In addition, (S)-glutamic acid was found to induce both one- and two-dimensional faceting of the Ag{110} surface. In some cases, evidence was found that the 2-D faceting involved the creation of a chiral facet distribution. A comparison is made of the Ag/(S)-glutamic acid system with analogous studies of amino acids on Cu.

Structures of Glycine, Enantiopure Alanine, and Racemic Alanine Adlayers on Cu(110) and Cu(100) Surfaces

We have determined the structures of dense adlayers of glycine and alanine on the Cu(110) and Cu(100) surfaces using plane wave density functional theory. These calculations resolve several experimental controversies regarding these structures. Glycine exists on Cu(110) as a single adlayer structure, while on Cu(100) two distinct glycine adlayers coexist. The glycine structures serve as useful starting points for constructing alanine adlayer structures. We considered separately the adsorption of enantiopure alanine and racemic alanine on each surface. Adlayers of enantiopure alanine are found to be closely related to the adlayers observed for glycine. Racemic alanine adlayers on Cu(110) are structurally analogous to those observed for glycine on this surface and adopt a pseudo-racemate ordering. On Cu(100), in contrast to glycine, racemic alanine is found to adopt a single adlayer structure that is an ordered racemate. Spontaneous segregation of molecular enantiomers does not occur in racemic adsorbed mixtures on either surface. Consideration of the orientationally distinct domains that may exist for each adlayer on these surfaces provides important information for the interpretation of the adlayer domain boundaries that are commonly observed in scanning tunneling microscopy images of amino acid adlayers. Examining this set of amino acid adlayers provides useful insight into the range of subtle behaviors that can arise in these and related systems where chiral molecules form ordered adlayers on flat metal surfaces.

Molecular assemblies and redox reactions of zinc(II) tetraphenylporphyrin and zinc(II) phthalocyanine on Au(1 1 1) single crystal surface at electrochemical interface

Adlayers of zinc(II) 5,10,15,20-tetraphenyl-21 H,23 H-porphine (ZnTPP) and zinc(II) phthalocyanine (ZnPc) were formed on Au(1 1 1) substrate by immersion in benzene solutions containing these molecules, and they were investigated in 0.1 M HClO 4 by using cyclic voltammetry and in situ scanning tunneling microscopy. Highly ordered epitaxial layers of ZnTPP and ZnPc molecules were obtained by controlling the immersion time. In particular, the epitaxial growth of ZnPc was found to be strongly influenced by the underlying ZnPc adlayer structure. Ordered monolayers of ZnTPP and ZnPc molecules were observed on reconstructed Au(1 1 1) surfaces. At negative electrode potentials, an individual ZnTPP molecule was composed of a set of bright spots in the highly ordered ZnTPP adlayer, showing that a redox reaction proceeded in the framework of TPP. The highly ordered ZnPc adlayers also revealed a structural change induced by the redox reaction in the framework of Pc at negative potentials. Electrochemical potential played a significant role in the redox reactions of ZnTPP or ZnPc molecules on the Au(1 1 1) surface.

Dependence of Molecular Recognition of Fullerene Derivative on the Adlayer Structure of Zinc Octaethylporphyrin Formed on Au(100) Surface

Adlayers of ZnOEP were prepared on reconstructed Au(100)-(hex) and unreconstructed Au(100)-(1 x 1) surfaces by immersing into a benzene solution containing ZnOEP molecules, and the adlayer structures were characterized by scanning tunneling microscopy (STM). A hexagonally arranged ZnOEP array was formed on an Au(100)-(hex) surface, whereas a rectangularly arranged ZnOEP array was found on an Au(100)-(1 x 1) surface. The adlayer structure of ZnOEP was dependent upon the underlying Au atomic arrangements. Furthermore, an investigation of the spuramolecular assembly for these modified surfaces was carried out by using an open-cage C(60) derivative (opened C(60)). A supramolecular assembled adlayer with a 1:1 composition of opened C(60)/ZnOEP was formed on Au(100)-(hex), whereas aggregates of opened C(60) were found on the ZnOEP-modified Au(100)-(1 x 1) surface. Electrochemical responses of opened C(60) were significantly influenced by underlying ZnOEP arrays. This finding suggests that precise control of underlying ZnOEP adlayers with the Au atomic structure is important to recognize the opened C(60) on them.

Potential dependence of the saturation CO coverage of Pt electrodes: The origin of the pre-peak in CO-stripping voltammograms. Part 1: Pt(1 1 1)

Combined CO-stripping cyclic voltammetry and FT-IR spectroscopy measurements have allowed us to monitor changes in the coverage and structure of CO adlayers on Pt(1 1 1) electrodes in 0.1 M H 2SO 4 as a function of potential. Our results show that in CO-free solutions the maximum coverage is θ CO = 0.68 and that higher coverages can only be achieved in the presence of CO in the solution. Saturation coverages can only be reached if the potential at which the electrode is held during CO adsorption (dosing potential, E d) is more negative than 0.30 V vs. RHE. The lowest CO coverage at which hydrogen adsorption on the Pt(1 1 1) electrode is completely blocked is θ CO = 0.63, which corresponds to an E d = 0.50 V vs. RHE. Our results suggest that the process at the pre-peak (and, hence, the oxidation at low overpotentials of bulk CO in CO-saturated solutions), corresponds to the oxidation of adsorbed CO by reaction with oxygenated species nucleating at steps, the main peak appearing when nucleation of oxygenated species at the terraces also occurs.

Potential dependence of the saturation CO coverage of Pt electrodes: The origin of the pre-peak in CO-stripping voltammograms. Part 3: Pt(poly)

Abstract Combined CO-stripping cyclic voltammetry and FT-IR spectroscopy measurements have allowed us to monitor changes in the coverage and structure of CO adlayers on Pt(1 1 1) electrodes in 0.1 M H2SO4 as a function of potential. Our results show that in CO-free solutions the maximum coverage is θCO = 0.68 and that higher coverages can only be achieved in the presence of CO in the solution. Saturation coverages can only be reached if the potential at which the electrode is held during CO adsorption (dosing potential, Ed) is more negative than 0.30 V vs. RHE. The lowest CO coverage at which hydrogen adsorption on the Pt(1 1 1) electrode is completely blocked is θCO = 0.63, which corresponds to an Ed = 0.50 V vs. RHE. Our results suggest that the process at the pre-peak (and, hence, the oxidation at low overpotentials of bulk CO in CO-saturated solutions), corresponds to the oxidation of adsorbed CO by reaction with oxygenated species nucleating at steps, the main peak appearing when nucleation of oxygenated species at the terraces also occurs.

Adsorption of oxygen onPt3Sn(110)studied by STM and LEED

The adsorption of oxygen on the ${\mathrm{Pt}}_{3}\mathrm{Sn}(110)$ alloy surface was studied by means of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). After exposure to $2300\phantom{\rule{0.3em}{0ex}}\mathrm{L}$ ${\mathrm{O}}_{2}$ at $750\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ LEED shows additional $c(2\ifmmode\times\else\texttimes\fi{}2)$ spots with regard to the substrate $p(2\ifmmode\times\else\texttimes\fi{}1)$ pattern. This agrees straightforward with STM topographies revealing a thin layer of large protrusions, arranged in a pseudohexagonal lattice. This layer is split into domains separated by distinctive zigzagging boundaries. Post-annealing allowed to partially uncover substrate regions. That way it could be shown that the protrusions are located above Sn locations. Further post-annealing restored the original substrate completely. A model of the adlayer structure consistent with all STM and LEED findings is given. These features observed after high temperature oxygen exposure of the ${\mathrm{Pt}}_{3}\mathrm{Sn}(110)$ surface resemble to a large extent structures obtained on the ${\mathrm{Pt}}_{3}\mathrm{Sn}(111)$ surface under similar conditions. The observed structures can be understood in terms of $\mathrm{Sn}\penalty1000-\hskip0pt\mathrm{O}$ entities interacting with each other. However, results from the oxidation of ${\mathrm{Pt}}_{3}\mathrm{Sn}(111)$ indicate the formation of a Sn layer with chemisorbed oxygen on top, but not of separated $\mathrm{Sn}{\mathrm{O}}_{x}$ entities. The protrusion patterns observed with STM on both surfaces are a topographic signature of such $\mathrm{Sn}\penalty1000-\hskip0pt\mathrm{O}$ layers on ${\mathrm{Pt}}_{3}\mathrm{Sn}$.

The Effect of Polarity on Coadsorbed Molecular Nanostructures of Substituted Phthalocyanine and Thiol Molecules

Different adlayer structures were fabricated by coadsorption of differently substituted phthalocyanine/thiol molecules and investigated using scanning tunnelling microscopy (STM, see graphic). The molecular polarity was considered to be responsible for the formation of different adlayer structures.