Electron-induced isomerisation of dichlorobenzene on Cu(111) and Ag(111)

The constitutional isomerisation of single dichlorobenzene molecules adsorbed on the surfaces of Ag(111) and Cu(111) between their meta- and para-isomers is induced and investigated by means of a low temperature scanning tunneling microscope. On both substrates similar isomerisation thresholds are found indicating that the excitation mechanism of this reaction does not depend significantly on the underlying substrate. The isomerisation threshold voltage of (170 +/- 7) meV most likely corresponds to excitation of a C-C stretch mode whose gas-phase energies we calculated ab initio to lie at 174 and 172 meV for meta- and para-isomers respectively. Though the reaction is found to be localized on the submolecular scale, it depends heavily on the second substituent both in terms of excitation energy and reaction outcome.     

Photochemistry of cyclohexane on Cu(111)

The photochemistry of cyclohexane on Cu(111) and its excitation mechanism have been studied by temperature-programmed desorption, ultraviolet and X-ray photoelectron spectroscopy. Cyclohexane weakly adsorbed on Cu(111) has been known to show a broadened and redshifted CH stretching band, i.e., CH vibrational mode softening. Although no dehydrogenation takes place thermally on this surface and by the irradiation of photons at 5.0 eV, adsorbed cyclohexane is dissociated to cyclohexyl and hydrogen by the irradiation of photons at 6.4 eV. This is a marked contrast to cyclohexane in the gas phase where the onset of absorption is located at 7 eV. When the surface irradiated by 6.4-eV photons is further annealed, cyclohexyl is dehydrogenated to form cylcohexene that desorbs at 230 K. The systematic measurements of photochemical cross sections at 6.4 eV with linearly polarized light as a function of incident angle indicate that the electronic transition from the highest occupied band of cyclohexane to a partially occupied hybridized band near the Fermi level is responsible for the photochemistry. The hybridized band is formed by the interactions between the electronic states of cyclohexane and the metal substrate. The role of the hybridized band in the photochemistry and the CH vibrational mode softening is discussed.     

氧化锌有序结构在Au(111)和Cu(111)上的生长

利用NO₂或O₂作为氧化剂,研究了氧化锌在Au(111)和Cu(111)上的生长和结构。NO₂表现了更好的氧化性能,有利于有序氧化锌纳米结构或薄膜的生长。在Au(111)和Cu(111)这两个表面上,化学计量比氧化锌都形成非极性的平面化ZnO(0001)的表面结构。在Au(111)上,NO₂气氛下室温沉积锌倾向于形成双层氧化锌纳米结构;而在更高的沉积温度下,在NO₂气氛中沉积锌则可同时观测到单层和双层氧化锌纳米结构。O₂作为氧化剂时可导致形成亚化学计量比的ZnO_x结构。由于铜和锌之间的强相互作用会促进锌的体相扩散,并且铜表面可以被氧化形成表面氧化物,整层氧化锌在Cu(111)上的生长相当困难。我们通过使用NO₂作为氧化剂解决了这个问题,生长出了覆盖Cu(111)表面的满层有序氧化锌薄膜。这些有序氧化锌薄膜表面显示出莫尔条纹,表明存在一个ZnO和Cu(111)之间的莫尔超晶格。实验上观察到的超晶格结构与最近理论计算提出的Cu(111)上的氧化锌薄膜结构相符,具有最小应力。我们的研究表明,氧化锌薄膜的表界面结构可能会随氧化程度或氧化剂的不同而变化,而Cu(111)的表面氧化也可能影响氧化锌的生长。当Cu(111)表面被预氧化成铜表面氧化物时,ZnO_x的生长模式会发生变化,锌原子会受到铜氧化物晶格的限域形成单位点锌。我们的研究表明了氧化锌的生长需要抑制锌向金属基底的扩散,并阻止亚化学计量比ZnO_x的形成。因此,使用原子氧源有利于在Au(111)和Cu(111)表面上生长有序氧化锌薄膜。     

Plasma-assisted oxidation of Cu(100) and Cu(111)

Oxidized copper surfaces have attracted significant attention in recent years due to their unique catalytic properties, including their enhanced hydrocarbon selectivity during the electrochemical reduction of CO₂. Although oxygen plasma has been used to create highly active copper oxide electrodes for CO₂RR, how such treatment alters the copper surface is still poorly understood. Here, we study the oxidation of Cu(100) and Cu(111) surfaces by sequential exposure to a low-pressure oxygen plasma at room temperature. We used scanning tunnelling microscopy (STM), low energy electron microscopy (LEEM), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS) and low energy electron diffraction (LEED) for the comprehensive characterization of the resulting oxide films. O₂-plasma exposure initially induces the growth of 3-dimensional oxide islands surrounded by an O-covered Cu surface. With ongoing plasma exposure, the islands coalesce and form a closed oxide film. Utilizing spectroscopy, we traced the evolution of metallic Cu, Cu₂O and CuO species upon oxygen plasma exposure and found a dependence of the surface structure and chemical state on the substrate''s orientation. On Cu(100) the oxide islands grow with a lower rate than on the (111) surface. Furthermore, while on Cu(100) only Cu₂O is formed during the initial growth phase, both Cu₂O and CuO species are simultaneously generated on Cu(111). Finally, prolonged oxygen plasma exposure results in a sandwiched film structure with CuO at the surface and Cu₂O at the interface to the metallic support. A stable CuO(111) surface orientation is identified in both cases, aligned to the Cu(111) support, but with two coexisting rotational domains on Cu(100). These findings illustrate the possibility of tailoring the oxidation state, structure and morphology of metallic surfaces for a wide range of applications through oxygen plasma treatments.

A low-pressure oxygen plasma oxidized Cu(100) and Cu(111) surfaces at room temperature. The time-dependent evolution of surface structure and chemical composition is reported in detail for a range of exposure times up to 30 min.     

Structure and bonding of alkanethiols on Cu(111) and Cu(100)

The local structure of the sulfur atom of methanethiolate and ethanethiolate on the Cu(111) and Cu(100) surfaces was investigated from first principles employing the periodic supercell approach in the framework of density functional theory. On the 111 surface, we investigated the (square root 3 x square root 3)R30 degrees and (2 x 2) structures, whereas on the 100 surface, we investigated the p(2 x 2) and c(2 x 2) structures. The landscape of the potential energy surface on each metal surface presents distinctive features that explain the local adsorption structure of thiolates found experimentally. On the Cu(111) surface, the energy difference between the hollow and bridge sites is only 3 kcal/mol, and consequently, adsorption sites ranging from the hollow to the bridge site were observed for increasing surface coverages. On the Cu(100) surface, there is a large energy difference of 12 kcal/mol between the hollow and bridge sites, and therefore, only the 4-fold coordination was observed. The high stabilization of thiolates on the hollow site of Cu(100) may be the driving force for the pseudosquare reconstruction observed experimentally on Cu(111). Density of states analysis and density difference plots were employed to characterize the bonding on different surface sites. Upon interaction with the metal d bands, the pi* orbital of methanethiolate splits into several peaks. The two most prominent peaks are located on either edge of the metal d band. They correspond to bonding and antibonding S-Cu interactions. In the case of ethanethiolate, all the back-bonds are affected by the surface bonding, leading to alternating regions of depletion and accumulation of charge in the successive bonds.     

Chiral structures of 6,12-dibromochrysene on Au(111) and Cu(111) surfaces

Nanoscale low-dimensional chiral architectures are increasingly receiving scientific interest, because of their potential applications in many fields such as chiral recognition, separation and transformation. Using 6,12-dibromochrysene (DBCh), we successfully constructed and characterized the large-area two-dimensional chiral networks on Au(111) and one-dimensional metal-liganded chiral chains on Cu(111) respectively. The reasons and processes of chiral transformation of chiral networks on Au(111) were analyzed. We used scanning tunneling spectroscopy (STS) to analyze the electronic state information of this chiral structure. This work combines scanning tunneling microscopy (STM) with non-contact atomic force microscopy (nc-AFM) techniques to achieve ultra-high-resolution characterization of chiral structures on low-dimensional surfaces, which may be applied to the bond analysis of functional nanofilms. Density functional theory (DFT) was used to simulate the adsorption behavior of the molecular and energy analysis in order to verify the experimental results.     

Corrugation energy for octane on Cu(111)

The energy barrier for sliding of octane on Cu(111) is estimated from an experimental datum for the Brillouin-zone-center gap for translation of a monolayer solid of the octane.     

Hydrogen-bonded assembly of methanol on Cu(111)

Investigation of methanol's surface chemistry on metals is a crucial step towards understanding the reactivity of this important chemical feedstock. Cu is a relevant metal for methanol synthesis and reforming, but due to the weak interaction of methanol with Cu, an atomic scale view of methanol's coverage-dependent ordering and self-assembly on Cu(111), the most abundant facet of most nanoparticles, has not yet been possible. Low and variable temperature scanning tunneling microscopy coupled with density functional theory reveal a coverage-dependent range of highly ordered structures stabilized by two hydrogen bonds per molecule. While extended chains that resemble the hydrogen-bonded zigzag structures reported for solid methanol are an efficient way to pack methanol at higher coverages, lower surface coverages yield isolated hexamer units. These hexamers form the same number of hydrogen bonds as the chains but appear to repel one another on the surface. Annealing treatments lead to the desorption of methanol with almost no decomposition. This data serves as a useful guide to both the preferred adsorption geometries and energies of a variety of methanol structures on Cu(111) surfaces as a function of surface coverage.     

Imaging the Bonds of Dehalogenated Benzene Radicals on Cu(111) and Au(111)

Dissociative adsorption of doubly substituted benzene molecules leads to formation of benzyne radicals. In this study, co‐adsorbed hydrogen molecules are used in scanning tunneling hydrogen microscopy to enhance the contrast of the meta‐ and the para‐isomers of these radicals on Cu(111) and Au(111). Up to three hydrogen molecules are attached to one radical. One hydrogen molecule reveals the orientation of the carbon ring and its adsorption site, allowing discrimination between the two radicals. Two hydrogen molecules reflect the bond picture of the carbon skeleton and reveals that adsorption on Cu(111) distorts the meta‐ isomer differently from its gas‐phase distortion. Three hydrogen molecules allow us to determine the bond picture of a minor species.     

Two-dimensional Molecular Phase Transition of Alkylated-TDPB on Au(111) and Cu(111) Surfaces

The derivatives of aromatic cores bearing alkyl chains with different lengths are of potential interest in on-surface chemistry, and thus have been widely investigated both at liquid-solid interfaces and in vacuum. Here, we report on the structural evaluation of self-assembled 1,3,5-tri(4-dodecylphenyl)benzene(TDPB) molecules with increased molecular coverages on both Au(111) and Cu(111) surfaces. As observed on Au(111), rhombic and herringbone structures emerge successively depending on surface coverage. In the case of Cu(111), the same process of phase conversion is also observed, but with two distinct structures. In comparison, the self-assembled structures on Au(111) surface are packed more densely than that on Cu(111) surface under the same preparation conditions. This may fundamentally result from the higher adsorption energy of TDPB molecules on Cu(111), restricting their adjustment to optimize a thermodynamically favorable molecular packing.     

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.     

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.     

STM Images of Individual Porphyrin Molecules on Cu(100) and Cu(111) Surfaces

Simply sublime! Samples of monomeric and dimeric zinc porphyrins were sublimed onto a Cu surface under ultrahigh vacuum conditions. Images obtained by scanning tunneling microscopy at room and low temperature (98 K) show features attributed to individual porphyrin molecules with excellent resolution. In the case of the (relatively large) linear dimer shown, two distinct conformations were detected on a surface with low coverage area. R=CH₂CH₂COOCH₃.     

Probing enantioselectivity on chirally modified Cu(110), Cu(100), and Cu(111) surfaces

Temperature programmed desorption methods have been used to probe the enantioselectivity of achiral Cu(100), Cu(110), and Cu(111) single crystal surfaces modified by chiral organic molecules including amino acids, alcohols, alkoxides, and amino-alcohols. The following combinations of chiral probes and chiral modifiers on Cu surfaces were included in this study: propylene oxide (PO) on L-alanine modified Cu(110), PO on L-alaninol modified Cu(111), PO on 2-butanol modified Cu(111), PO on 2-butoxide modified Cu(100), PO on 2-butoxide modified Cu(111), R-3-methylcyclohexanone (R-3-MCHO) on 2-butoxide modified Cu(100), and R-3-MCHO on 2-butoxide modified Cu(111). In contrast with the fact that these and other chiral probe/modifier systems have exhibited enantioselectivity on Pd(111) and Pt(111) surfaces, none of these probe/modifier/Cu systems exhibit enantioselectivity at either low or high modifier coverages. The nature of the underlying substrate plays a significant role in the mechanism of hydrogen-bonding interactions and could be critical to observing enantioselectivity. While hydrogen-bonding interactions between modifier and probe molecule are believed to induce enantioselectivity on Pd surfaces (Gao, F.; Wang, Y.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2007, 129, 15240-15249), such critical interactions may be missing on Cu surfaces where hydrogen-bonding interactions are believed to occur between adjacent modifier molecules, enabling them to form clusters or islands.     

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.     

Conformation Controls Mobility: 2H-Tetranaphthylporphyrins on Cu(111)

The adsorption behavior and the mobility of 2H-Tetranaphthylporphyrin (2HTNP) on Cu(111) was investigated by scanning tunneling microscopy (STM) at room temperature (RT). The molecules adsorb, like the structurally related 2HTPP, in the “inverted” structure with the naphthyl plane restricted to an orientation parallel to the Cu surface. The orientation of the four naphthyl groups yields altogether 16 possible conformations. Due to the existence of rotamer pairs, 10 different appearances are expected on the surface, and all of them are identified by STM at RT. Most interestingly, the orientation of the naphthyl groups significantly influences the diffusion behavior of the molecules on Cu(111). We identify three different groups of conformers, which are either immobile, medium or fast diffusing at RT. The mobility seems to decrease with increasing size of the footprint of the conformers on the surface.     

Diffusion Dynamics of Cux Cluster on Cu(111) Surface

The diffusion dynamics of small two-dimensional atomic clusters Cux (1·x·8) on Cu(111) surface were studied using the molecular dynamics simulations and a modified analytic embedded-atom method in the temperature range from 200 K to 800 K. The cluster size and temperature dependence of the diffusion coefficients and migration energies are presented. Our simulations show that the diffusion migration energy of the Cu7 cluster is the highest and the prefactor for the Cu7 cluster is almost three orders of magnitude larger than that for single atom diffusion. This conclusion is consistent with the experimental results for similar metals. In addition, the dependence of cluster diffusion on film growth is also discussed.     

Surface chemistry of isopropoxy tetramethyl dioxaborolane on Cu(111)

The surface chemistry of isopropoxy tetramethyl dioxaborolane (ITDB), tetramethyl dioxaborolane (TDB), and 2-propanol is studied on a clean Cu(111) single crystal using temperature-programmed desorption (TPD). 2-Propanol is found to have two competing reactions on the copper surface. Dehydration results in water and propene formation, and dehydrogenation results in the formation of acetone and hydrogen. ITDB directly adsorbed on the surface reacts completely and does not molecularly desorb. TDB and 2-propanol decompose desorbing mainly 2,3-dimethyl 2-butene and acetone, respectively. Both of those products desorb above room temperature and are present in TPDs of ITDB. An additional acetone desorption peak was observed for ITDB at higher temperatures than acetone desorption from 2-propanol. This higher temperature peak at ～391 K was attributed to two acetone molecules forming from the tetramethyl end group resulting from a stronger bound surface species in ITDB compared to TDB despite their identical end groups. The copper surface seems to be reactive enough toward ITDB at room temperature that a potential boron-containing tribofilm could be produced for copper-copper sliding contacts. Despite their similarities, ITDB and TDB have different surface species present at room temperature, so their tribological properties will be investigated in the future.     

Pentagonal tiling with buckybowls: pentamethylcorannulene on Cu(111)

We present scanning tunnelling microscopy studies and first principles calculation on the 2D crystallization of pentagonal pentamethylcorannulene on a Cu(111) surface under ultrahigh vacuum in the temperature range of 50 K to 400 K. The observed 2D crystal phases and their packing densities are compared to tiling options of hard pentagons. Temperature change-induced reversible phase transitions reveal entropic effects in 2D crystallization. Only inclusion of dispersion interactions into density functional theory yields structures observed experimentally at low temperatures.