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We present a real-time investigation of ultra-fast carrier dynamics in single-wall carbon nanotube bundles using femtosecond time-resolved photoelectron spectroscopy. The experiments allow us to study the processes governing the sub-picosecond and the picosecond dynamics of non-equilibrium charge carriers. On the sub-picosecond time scale the dynamics are dominated by ultra-fast electron–electron scattering processes, which lead to internal thermalization of the laser-excited electron gas. We find that quasiparticle lifetimes decrease strongly as a function of their energy up to 2.38 eV above the Fermi level – the highest energy studied experimentally. The subsequent cooling of the laser-heated electron gas to the lattice temperature by electron–phonon interaction occurs on the picosecond time scale and allows us to determine the electron–phonon mass-enhancement parameter λ. The latter is found to be over an order of magnitude smaller if compared, for example, with that of a good conductor such as copper. [ABSTRACT FROM AUTHOR]

The orientation and the intramolecular relaxation due to adsorption of the chiral phenanthrene-derivative heptahelicene, C[sub 30]H[sub 18], on Cu(111) and Cu(332) surfaces have been investigated by means of angle-scanned full-hemispherical x-ray photoelectron diffraction. Although the C 1s diffraction patterns of the adsorbed submonolayer coverage helicene films exhibit scattering anisotropies of less than two percent, a detailed analysis involving simple molecular mechanics calculations of the atomic coordinates, photoelectron diffraction single-scattering cluster calculations and an R-factor analysis permits the determination of the helicene molecular orientation. On Cu(111), the molecules are found to bind to the substrate surface via their terminal phenanthrene group oriented parallel to the surface plane, while on Cu(332) the three terminal C-6 rings are oriented parallel to the (111) terrace plane. Six azimuthal molecular orientations are found to coexist on Cu(111), on Cu(332), however, the step–molecule interaction leads to a unique azimuthal alignment of the heptahelicene molecules. The heptahelicene on Cu(332) system thus represents a chiral surface with single-phase orientational order. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a comprehensive study on the binding and ordering of C[sub 60] on a Pd(110) surface employing scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD). Three well-ordered structures evolving at elevated temperatures were identified, with (&sup7; [sub -/+] &sup-1; [sub ±5), (4 × 5), (4 × 8) unit cells and a coverage of 1, 0.82 and 0.77 physical monolayers, respectively. The rearrangement of Pd substrate atoms turns out to play a crucial role in the evolution of thin C[sub 60] films. In STM images alternating bright and dark molecular rows are found for the regular structures. This height difference is attributed to a restructuring of the Pd substrate. Whereas the C[sub 60] molecules of the bright molecular rows are embedded in one layer deep microscopic pits, C[sub 60] accommodated in two layer deep pits account for the dark molecular rows. XPD results complete the understanding of the film structure. They reveal that the orientation of the C[sub 60] cages is unique in the regular structures; the molecules are facing with a 5–6 bond towards the substrate. [ABSTRACT FROM AUTHOR]

Abstract: We compare photoelectron spectra (PES) and theoretical densities-of-states of C60/Al(111), C60/Al(110) and C60/Al(100). The splitting observed on all three surfaces is attributed to final state charging. This splitting can also be used to improve estimates of the charging energy U of adsorbed molecules and as a criterion for the existence of strong covalent bonding to the electrode. Au exhibits weak bonding in available data, whereas Ag(100), with measurable charge transfer, appears qualitatively similar to Al. [Copyright &y& Elsevier]

The adsorption of the helically shaped polyaromatic hydrocarbon heptahelicene (C30H18) on Ni(1 1 1) was studied by means of STM, TPD, LEED, ToF–SIMS, XPS, and AES at temperatures between 130 and 1000 K. The molecule in the monolayer remains intact up to 500 K. Above that temperature a stepwise decomposition into carbon and hydrogen occurs; the latter desorbing subsequently as H2. At submonolayer coverages the molecules are adsorbed randomly. The saturated monolayer shows long-range order, but no differences between the structures generated by a pure enantiomer or the racemic mixture have been observed. [Copyright &y& Elsevier]

The adsorption of terephthalic acid [C6H4(COOH)2, TPA] on a Pd(111) surface has been investigated by means of scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy, and near-edge x-ray absorption fine structure spectroscopy under ultrahigh vacuum conditions at room temperature. We find the coexistence of one- (1D) and two-dimensional (2D) molecular ordering. Our analysis indicates that the 1D phase consists of intact TPA chains stabilized by a dimerization of the self-complementary carboxyl groups, whereas in the 2D phase, consisting of deprotonated entities, the molecules form lateral ionic hydrogen bonds. The supramolecular growth dynamics and the resulting structures are explained by a self-limiting deprotonation process mediated by the catalytic activity of the Pd surface. Our models for the molecular ordering are supported by molecular mechanics calculations and a simulation of high resolution STM images. [ABSTRACT FROM AUTHOR]

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°. [ABSTRACT FROM AUTHOR]

Presents a study that investigated the enhancement of the structural sensitivity of x-ray photoelectron diffraction through the acquisition of a full hemispherical diffraction pattern of chemically shifted core levels. Background on diamondlike carbon films; Discussion on complex systems to resolve the local order per element and per chemical environment; Effects of the sample temperature and ion dose on the structure of deposited layers.

Electronic and thermal properties of chevron-type graphene nanoribbons can be widely tuned, making them interesting candidates for electronic and thermoelectric applications. Here, we use post-growth silicon intercalation to unambiguously access nanoribbons’ energy position of their electronic frontier states. These are otherwise obscured by substrate effects when investigated directly on the growth substrate. In agreement with first-principles calculations we find a band gap of 2.4 eV. [ABSTRACT FROM AUTHOR]

Despite the development of crystal engineering, it remains a great challenge to predict the crystal structure even for the simplest molecules, and a clear link between molecular and crystal symmetry is missing in general. Here we demonstrate that the two- dimensional (2D) crystallization of heterocirculenes on a Au(111) surface is greatly affected by the molecular symmetry. By means of ultrahigh vacuum scanning tunneling microscopy, we observe a variety of 2D crystalline structures in the coverage range from submonolayer to monolayer for D8h-symmetric sulflower (C16S8), whereas D4h-symmetric selenosulflower (C16S4Se4) forms square and rectangular lattices at submonolayer and monolayer coverages, respectively. No long-range ordered structure is observed for C1h-symmetric selenosulflower (C16S5Se3) self-assembling at submonolayer coverage. Such different self-assembly behaviors for the heterocirculenes with reduced molecular symmetries derive from the tendency toward close packing and the molecular symmetry retention in 2D crystallization due to van der Waals interactions. [ABSTRACT FROM AUTHOR]

The mechanisms for the dehydrogenation reaction of cyclohexaphenylene at a copper surface are investigated with the help of density functional theory and metadynamics. Our results represent a showcase for an approach that describes the surface using many-body classical potentials and molecule-surface interactions with a van der Waals model. Starting from the experimental observation that dispersion-assisted mechanisms are at least as important as catalytic processes for the description of the reaction, we fully describe the former, we identify intermediate states and estimate the free energy barriers that characterize the reaction. [ABSTRACT FROM AUTHOR]

Abstract: The structure of ultrathin NaCl films on Au(111) and on Au(111212), as well as the one of bimolecular 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) and 1,4-bis-(2,4-diamino-1,3,5,-triazine)-benzene (BDATB) islands on NaCl films on both surfaces have been studied with a low-temperature scanning tunnelling microscope. We show that intermixed bimolecular assemblies based on selective three-fold hydrogen-bonding (H-bonding), that have previously been observed on Au(111) and on Au(111212), can also be stabilized on insulating NaCl films on Au, however, only if these films are grown on Au(111212) and not on Au(111). The behaviour of the heterocomplex structures is found to be largely influenced by the structural properties of the underlying substrate and by the number of NaCl layers. On a partly NaCl-covered Au(111) surface, the excess of molecules after completion of the first layer on Au prefers to form a second molecular layer based on ordered heterocomplex structures rather than to adsorb on the NaCl islands. The use of a vicinal surface together with the strong cohesion characteristic of the NaCl film introduces smooth elastic deformations on the NaCl(001) plane. As a consequence, the periodically modified structure of the overlayer provides preferential binding sites and allows adsorption of two-dimensional molecular structures. In contrast to what is observed on Au(111212), the molecular domains on the NaCl film do not follow the Au step directions, but the NaCl(001) high symmetry directions. Our results provide a strategy to increase the adsorption energy of flat molecules on insulating layers by choosing a vicinal metal substrate. [Copyright &y& Elsevier]

Abstract: This study first reports the initial growth stages of sodium chloride (NaCl) on Ag(110) at room temperature. NaCl grows in bi-layer mode along its [100] axis and gives rise to (4×1) and (1×2) reconstructed domains for coverages lower than two monolayers (ML), a minimal thickness inducing a bi-dimensional closed film. In addition, a 10ML NaCl film has been examined by low energy electron diffraction (LEED). LEED analysis leads to the dissociation of the NaCl deposit in a few minutes. The NaCl dissociation implies Cl desorption from the surface and Na remaining on it. The residual Na is arranged in the form of a (2×1) surface reconstruction and is found to be strongly bounded to the Ag substrate. These findings have been established by using the X-ray photoelectron spectroscopy technique. [Copyright &y& Elsevier]

We demonstrate the suitability of the Ag/Pt(111) strain-relief pattern as efficient template for controlling the formation of well defined heteromolecular nanostructures. Two different species of molecular building blocks with complementary end-group functionalities are combined on this surface, which results in the formation of robust bimolecular nanoribbons driven by the interplay of the site specific adsorption on the strain-relief pattern with the highly directional intermolecular hydrogen-bonding intrinsic to the free bimolecular system. [ABSTRACT FROM AUTHOR]