Coverage dependent supramolecular structures: C60:ACA monolayers on Ag(111)

The dependence of supramolecular structure on fractional molecular coverage has been investigated for acridine-9-carboxylic acid (ACA) and the C(60):ACA binary molecular system. The coverage-dependent phase diagram for ACA is first determined from room-temperature STM imaging. At low molecular coverages (theta < 0.4 ML, ML = monolayer), ACA forms a 2-D gas phase. Ordered ACA structures appear with increasing coverage: first a chain structure composed of ACA molecules linked by consecutive O-H...N hydrogen bonds (theta > 0.4 ML), then a dimer structure composed of ACA dimers linked by paired carboxyl-carboxyl hydrogen bonds (theta approximately equal to 1.0 ML). Structures of the C(60):ACA binary system depend on the coverage of predeposited ACA. At intermediate (0.4 ML approximately 0.8 ML) ACA coverages, C(60) deposition results in a hexagonal cooperative structure with the C(60) periodicity nearly 3 times that of the normal C(60) 2-D packing of 1 nm and exists in enantiopure domains. At higher ACA coverages, a C(60) quasi-chain structure is formed in which parallel C(60) chains are spaced by ACA dimer domains. The mechanistic role of the initial ACA phase in the formation of C(60):ACA supramolecular structures is described. Chemically intuitive molecular packing models are presented based on the observed STM images.     

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.     

Electronic structure of C60 on Au(887)

We present an analysis of the electronic structure of C60 adsorbed on a vicinal Au(111) surface at different fullerene coverages using photoemission, x-ray absorption, and scanning tunneling microscopy/spectroscopy (STS). STS provides a straightforward determination of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels with respect to the Fermi energy. At C60 coverages of 0.5 and 1 ML a 2.7 eV wide HOMO-LUMO gap is found. The near-edge x-ray absorption fine structure (NEXAFS) spectrum for the 0.5 ML C60 nanomesh structure displays a significant intensity at the low energy side of the LUMO exciton peak, which is explained as due to absorption into HOMO-LUMO gap states localized at individual C60 cluster edges. From 0.5 to 1 ML we observe a rigid shift of the HOMO-LUMO peaks in the STS spectra and an almost complete quenching of the gap states feature in NEXAFS.     

Preferential trapping of C60 in nanomesh voids

Surface nanotemplate-assisted molecular assembly offers great potential in the "bottom-up" construction of addressable molecular architectures for device miniaturization. Here, we report the fabrication of an extended 2D C60 nanomesh featuring a well-ordered nanocavity array by controlling the binary molecular phases of C60 and pentacene on Ag(111). Using low-temperature scanning tunneling microscopy, we demonstrate that the C60 nanomesh can serve as an effective template to selectively accommodate guest C60 molecules at the cavity sites, thereby leading to the formation of an ordered 2D C60 array.     

The first layer of water on Rh(111): microscopic structure and desorption kinetics

The adsorption states and growth process of the first water (D2O) layer on Rh(111) were investigated using infrared reflection absorption spectroscopy, temperature programed desorption, and spot-profile-analysis low energy electron diffraction. Water molecules wet the Rh(111) surface intact. At the early stage of first layer growth, a (square root 3 x square root 3)R30 degrees commensurate water layer grows where "up" and "down" species coexist; the up and down species represent water molecules which have free OD, pointing to a vacuum and the substrate, respectively. The up domain was a flatter structure than an icelike bilayer. Water desorption from Rh(111) was a half-order process. The activation energy and the preexponential factor of desorption are estimated to be 60 kJ/mol and 4.8 x 10(16) ML(1/2)/s at submonolayer coverage, respectively. With an increase in water coverage, the flat up domain becomes a zigzag layer, like an ice bilayer. At the saturation coverage, the amount of down species is 1.3 times larger than that of the up species. In addition, the activation energy and the preexponential factor of desorption decrease to 51 kJ/mol and 1.3 x 10(14) ML(1/2)/s, respectively.     

Effects and structures of the O/Cu surfactant layer in O-mediated film growth of Cu on Ru(0 0 0 1)

The O-mediated Cu-film growth on O-precovered Ru(0 0 0 1) is investigated by means of scanning tunneling microscopy for growth temperatures between 300 and 600 K. Cu-films on clean Ru(0 0 0 1) grow in a multilayer mode. For O precoverages (Θ) between 0.2 ML (monolayer) and the saturation coverage (Θ=0.5 ML), a layer-by-layer growth is observed at growth temperatures between 350 and 450 K. On Cu-islands, an O/Cu surfactant layer is formed, which floats on-top of the growing film and induces the layerwise Cu-film growth. The surface coverage of the O/Cu surfactant layer linearly rises with the O precoverage up to Θ≈0.4 ML, where it completely covers the surface. Two different types of the surfactant layer are identified, inducing different surfactant mechanisms. For Θ=0.1–0.4 ML, the O/Cu surfactant structure (A-type) displays some local order and induces inhomogeneous nucleation at the misfit-induced relaxation structure of the Cu-film. The layer-wise growth is explained by the concept of two mobilities, implying a large attempt frequency for adatom jumps over the interlayer diffusion barrier at the steps. For Θ = 0.4–0.5 ML, a disordered O/Cu surfactant layer is established (B-type), inducing homogeneous nucleation. The layer-wise Cu-film growth is attributed to a reduction of the effective interlayer diffusion barrier. Cu-film growth at 400 K on the ordered (3×2√3)O/Cu structure formed at temperatures around 520 K yields the conclusion that the O/Cu surfactant structures are composed of randomly arranged O–Cu–O strings and disrupted “Cu₂O(1 1 1)” fragments.     

Gold cluster formation on a fullerene surface

The growth of Au clusters on a fullerene thin film was investigated by in situ photoelectron spectroscopy in the ultraviolet (UPS) and x-ray (XPS) regime. Due to its highly corrugated surface fullerene films provide a wide range of bonding sites which could be exploited as molecular templates and serve to create a cluster superstructure. To gain insight into the fullerene-Au interaction two types of experiments were performed: (i) the deposition of Au on a fullerene surface, and (ii) the deposition of fullerenes on a Au surface. In both experiments an island growth mode is observed. The deposition of submonolayer amounts of C60 onto a gold film showed that the main interaction of the two species is due to chemisorption of the first C60 monolayer. In addition a constant band bending in the fullerene film is detected, but the UPS valence-band spectra show that there is no charge transfer from the Au to the C60 lowest unoccupied molecular orbital. In the reverse experiment, the cluster growth of Au on the corrugated C60 surface, the analysis of the Au core level does not reveal a specific bonding or nucleation site for Au atoms and clusters. This is in contrast to observations with Si clusters, which prefer to reside in the troughs between the fullerene molecules. The Au clusters grow continually from a size of about 55 atoms for the early stages of growth up to 150 atoms for the deposition of a nominal coverage of 1.5 nm. These data are derived from an analysis of the d-band splitting and the Au 4f core-level shift due to delayed photohole relaxation. The thermal stability of the Au-clusters-covered fullerene film was investigated by annealing in situ up to temperatures of 650 degrees C. For temperatures up to 450 degrees C a continuous growth of the clusters is detected, which is accompanied by a slight drop in Au concentration in the range of XPS for annealing temperatures higher than 350 degrees C. This may be due to a ripening of the clusters. The presence of Au apparently delays fullerene sublimation. The film shows a very good thermal stability and even after annealing at 650 degrees C there is still a fullerene film detectable in the photoelectron spectroscopy spectra.     

Atomic imaging of nucleation of trimethylaluminum on clean and H2O functionalized Ge(100) surfaces

The direct reaction of trimethylaluminum (TMA) on a Ge(100) surface and the effects of monolayer H(2)O pre-dosing were investigated using ultrahigh vacuum techniques, such as scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). At room temperature (RT), a saturation TMA dose produced 0.8 monolayers (ML) of semi-ordered species on a Ge(100) surface due to the dissociative chemisorption of TMA. STS confirmed the chemisorption of TMA passivated the bandgap states due to dangling bonds. By annealing the TMA-dosed Ge surface, the STM observed coverage of TMA sites decreased to 0.4 ML at 250?°C, and to 0.15 ML at 450?°C. XPS analysis showed that only carbon content was reduced during annealing, while the Al coverage was maintained at 0.15 ML, consistent with the desorption of methyl (-CH(3)) groups from the TMA adsorbates. Conversely, saturation TMA dosing at RT on the monolayer H(2)O pre-dosed Ge(100) surface followed by annealing at 200?°C formed a layer of Ge-O-Al bonds with an Al coverage a factor of two greater than the TMA only dosed Ge(100), consistent with Ge-OH activation of TMA chemisorption and Ge-H blocking of CH(3) chemisorption. The DFT shows that the reaction of TMA has lower activation energy and is more exothermic on Ge-OH than Ge-H sites. It is proposed that the H(2)O pre-dosing enhances the concentration of adsorbed Al and forms thermally stable Ge-O-Al bonds along the Ge dimer row which could serve as a nearly ideal atomic layer deposition nucleation layer on Ge(100) surface.     

Probing the buried C60/Au(111) interface with atoms

To characterize the C(60)/Au(111) interface, we send Au atoms "diving" through the C(60) layer and observe their behavior at the interface. Our observations show that the interfacial diffusion of gold atoms and the nucleation of small Au islands at the interface are strongly dependent on the local C(60)-Au(111) bonding which varies from one domain to another. The contrast-disordered domain consisting of a large fraction of molecules bonded to Au vacancies has a special structure at the interface allowing Au atoms to be inserted beneath the bright-looking molecules while the dim molecules present a much stronger resistance to the diffusing Au atoms. This leads to the formation of isolated Au islands with discrete sizes, with the smallest island just about 1 nm across.     

Co-adsorption of water and hydrogen on Ni(111)

We have studied the surface coverage dependence of the co-adsorption of D and D(2)O on the Ni(111) surface under UHV conditions. We use detailed temperature-programmed desorption studies and high resolution electron energy loss spectroscopy to show how pre-covering the surface with various amounts of D affects adsorption and desorption of D(2)O. Our results show that the effects of co-adsorption are strongly dependent on D-coverage. In the deuterium pre-coverage range of 0-0.3 ML, adsorption of deuterium leaves a fraction of the available surface area bare for D(2)O adsorption, which shows no significant changes compared to adsorption on the bare surface. Our data indicate phase segregation of hydrogen and water into islands. At low post-coverages, D(2)O forms a two-phase system on the remaining bare surface that shows zero-order desorption kinetics. This two phase system likely consists of a 2-D solid phase of extended islands of hexamer rings and a 2-D water gas phase. Increasing the water post-dose leads at first to 'freezing' of the 2-D gas and is followed by formation of ordered, multilayered water islands in-between the deuterium islands. For deuterium pre-coverages between 0.3 and 0.5 ML, our data may be interpreted that the water hexamer ring structure, (D(2)O)(6), required for the formation of an ordered multilayer, does not form anymore. Instead, more disordered linear and branched chains of water molecules grow in-between the extended, hydrophobic deuterium islands. These deuterium islands have a D-atom density in agreement with a (2x2)-2D structure. The disordered water structures adsorbed in-between form nucleation sites for growth of 3-D water structures. Loss of regular lateral hydrogen bonding and weakened interaction with the substrate reduces the binding energy of water significantly in this regime and results in lowering of the desorption temperature. At deuterium pre-coverages greater than 0.5 ML, the saturated (2x2)-2D structure mixes with (1x1)-1D patches. The mixed structures are also hydrophobic. On such surfaces, submonolayer doses of water lead to formation of 3-D water structures well before wetting the entire hydrogen-covered surface.     

Reversible Modification of the Structural and Electronic Properties of a Boron Nitride Monolayer by CO Intercalation

We demonstrate the reversible intercalation of CO between a hexagonal boron nitride (h‐BN) monolayer and a Rh(111) substrate above a threshold CO pressure of 0.01 mbar at room temperature. The intercalation of CO results in the flattening of the originally corrugated h‐BN nanomesh and an electronic decoupling of the BN layer from the Rh substrate. The intercalated CO molecules assume a coverage and adsorption site distribution comparable to that on the free Rh(111) surface at similar conditions. The pristine h‐BN nanomesh is reinstated upon heating to above 625 K. These observations may open up opportunities for a reversible tuning of the electronic and structural properties of monolayer BN films.     

Nucleation,morphology, and structure of sub-nm thin ceria islands on Rh(111)

The early stages of ceria growth on Rh(111) at high temperature have been investigated by low-energy electron microscopy and photoemission electron microscopy. Ceria was deposited by reactive Ce deposition at substrate temperatures between 700°C and 900°C in an oxygen ambient of 5 × 10⁻⁷ Torr. At 700°C, we observe a high nucleation density of 100-nm-sized islands. With elevated temperature, the average island size increases, and the nucleation density decreases. Triangularly shaped islands nucleate preferentially at step edges, with seemingly abrupt interfaces between Ce and Rh. At 900°C, the island edges are still straight, but during growth the islands lose their triangular form. Instead, growth along the substrate step edges becomes favorable, leading to a maze-like morphology. Atomic force microscopy reveals islands of 0.3 to 0.6-nm height, consistent with ceria islands formed by one or two trilayers (O―Ce―O) of ceria. Moreover, the second layer of the islands is also triangularly shaped, with lateral dimensions of 50 nm and similar step heights. IV-LEEM analysis leads to the conclusion that the rhodium surface is covered by a layer of reduced cerium oxide, which is partially overgrown by smaller islands of CeO₂.     

Structure analysis of SiCGe films grown on SiC

The island growth of SiCGe films on SiC at different temperatures has been investigated by SEM and transmission electron microscope (TEM). The island growth of SiCGe thin films depends on the processing parameter such as the growth temperature and follows the Stranski‐Krastanov (SK) mode. When the growth temperature is comparatively low, the thin film has two types of islands: one is a SiGe sphere‐like island of diamond‐cubic structure; another is a SiCGe cascading triangular island of zinc‐blende structure. As the growth temperature increases, the quantity of the sphere‐like islands reduces, while that of the cascading triangular islands increases. When the growth temperature is about 1060 °C, only cascading triangular islands are observed on the surface of the thin film, and the thickness of the 2D interfacial growth layer formed at the initial stage of the growth process is about 40 nm, which is twice of that grown at low temperature. Copyright © 2008 John Wiley & Sons, Ltd.     

水在金红石型TiO2（110）表面〈001〉阶梯边缘吸附的第一性原理研究

利用第一性原理计算方法,研究了水在金红石型TiO2（110）表面及〈001〉阶梯边缘处的吸附．关于水在（11O）表面上的吸附,研究表明,对不同的吸附率,水都是以分子模式吸附在表面．关于水在〈001〉9梯边缘处的吸附,研究表明,其吸附模式和吸附率有密切的联系．当水的吸附率为一个单层（1ML）时,分子吸附和解离吸附对应的吸附能分别为0．92和0．60eV,分子吸附模式更稳定．当吸附率降为1／2ML时,分子吸附和解离吸附所对应的吸附能分别为0．86和0．84eV,两种吸附模式都可能存在．在表面上,不同吸附模式的吸附能随吸附率变化趋势是一致的．而在〈001〉阶梯边缘处,对于不同的吸附模式,吸附能随吸附率的变化呈现出不同的变化趋势．这是由在〈001〉阶梯边缘处低吸附率时解离模式的独特结构引起的．     

Site-dependent electron-stimulated reactions in water films on TiO2(110)

Electron-stimulated reactions in thin [<3 ML (monolayer)] water films adsorbed on TiO(2)(110) are investigated. Irradiation with 100 eV electrons results in electron-stimulated dissociation and electron-stimulated desorption (ESD) of adsorbed water molecules. The molecular water ESD yield increases linearly with water coverage theta for 0< or =theta< or =1 ML and 11 ML, the water ESD yield per additional water molecule adsorbed (i.e., the slope of the ESD yield versus coverage) is 3.5 times larger than for theta<1 ML. In contrast, the number of water molecules dissociated per incident electron increases linearly for theta< or =2 ML without changing slope at theta=1 ML. The total electron-stimulated sputtering rate, as measured by postirradiation temperature programmed desorption of the remaining water, is larger for theta>1 ML due to the increased water ESD for those coverages. The water ESD yields versus electron energy (for 5-50 eV) are qualitatively similar for 1, 2, and 40 ML water films. In each case, the observed ESD threshold is at approximately 10 eV and the yield increases monotonically with increasing electron energy. The results indicate that excitations in the adsorbed water layer are primarily responsible for the ESD in thin water films on TiO(2)(110). Experiments on "isotopically layered" films with D(2)O adsorbed on the Ti(4+) sites (D(2)O(Ti)) and H(2)O adsorbed on the bridging oxygen atoms (H(2)O(BBO)) demonstrate that increasing the water coverage above 1 ML rapidly suppresses the electron-stimulated desorption of D(2)O(Ti) and D atoms, despite the fact that the total water ESD and atomic hydrogen ESD yields increase with increasing coverage. The coverage dependence of the electron-stimulated reactions is probably related to the different bonding geometries for H(2)O(Ti) and H(2)O(BBO) and its influence on the desorption probability of the reaction products.     

Direct and indirect causes of Fermi level pinning at the SiO/GaAs interface

The correlation between atomic bonding sites and the electronic structure of SiO on GaAs(001)-c(2x8)/(2x4) was investigated using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT). At low coverage, STM images reveal that SiO molecules bond Si end down; this is consistent with Si being undercoordinated and O being fully coordinated in molecular SiO. At approximately 5% ML (monolayer) coverage, multiple bonding geometries were observed. To confirm the site assignments from STM images, DFT calculations were used to estimate the total adsorption energies of the different bonding geometries as a function of SiO coverage. STS measurements indicated that SiO pins the Fermi level midgap at approximately 5% ML coverage. DFT calculations reveal that the direct causes of Fermi level pinning at the SiO GaAs(001)-(2x4) interface are a result of either local charge buildups or the generation of partially filled dangling bonds on Si atoms.     

Densely packed overlayer of iron phthalocyanine molecules grown on single-layer graphene

The FePc molecules form a series of order superstructures on single-layer graphene grown on Ru(0001) with increasing molecular coverage.     

第一性原理研究C在Fe(111)表面和次表面的积炭机理

使用密度泛函方法对C原子在Fe(111)表面吸附团聚和次表层的吸附扩散进行了研究。在炭覆盖度θ_C <1 ML时,C主要以孤立的原子态存在并导致表面重构;1 ML≤θ_C ≤2 ML,"mC₂+nC"为主要的吸附形式;θ_C≥2 ML时,复杂的吸附形态比如碳链和岛状碳团簇开始生成。这些复杂岛状碳团簇是Fe(111)表面石墨沉积或碳纳米管生长的成核中心。在次表层,C原子在八面体位稳定存在。C在表面的迁移能垒为0.45 eV,由表面迁移到次表面的的能垒为0.73 eV。虽然C₂团簇的生成是热力学有利的,但是C向次表层的迁移动力学上占优。     

C60 thin film growth on graphite: Coexistence of spherical and fractal-dendritic islands

The initial growth stage of C(60) thin film on graphite substrate has been investigated by scanning tunneling microscopy in ultrahigh vacuum at room temperature. The C(60) layer grows in a quasi-layer-by-layer mode and forms round, monolayer high islands on the graphite surface. The islands are confined by terraces on the graphite surface and the mobility of C(60) fullerenes across steps is low in all layers. The second and all subsequent layers adopt a fractal-dendritic shape, which was confirmed by calculating the fractal dimension (D=1.74 prior to island coalescence) and is in agreement with a diffusion limited aggregation. The profound differences between the growth of C(60) layers on graphite (first layer) and on C(60) surfaces (second and higher layers) are caused by the restriction of the C(60) mobility on the highly corrugated fullerene surfaces. The orientation of the fractal islands follows the hexagonal symmetry of the densely packed (111) surface of the fullerene lattice, which introduces a bias in the direction of molecule movement. The differences in surface topography on the nanoscale determine the mode of film growth in this van der Waals bonded system.     

Structural rearrangements during the initial growth stages of organic thin films of F16CuPc on SiO2

We present an experimental study on the first stages of the thin film growth of the organic molecule F(16)CuPc (hexadecafluoro-copper-phthalocyanines) on SiO(2). By means of in situ X-ray reflectivity, in situ grazing incidence X-ray diffraction (GIXD), and ex situ atomic force microscopy (AFM), we provide a detailed picture of the film growth mode and its structural evolution at the nanometer scale. We discovered the formation of a low-density layer of molecular aggregates with heights between 5 and 10 A at the interface with the SiO(2) and show that, on top of this interfacial layer, the nucleation and two-dimensional growth of elongated islands of upright standing molecules take place. Structural changes are observed, pointing to significant relaxations of the lattice parameters within the first layers of standing molecules.