B.G. Frederick

Face specificity and the role of metal adatoms in molecular reorientation at surfaces

Abstract Using reflection absorption infrared spectroscopy (RAIRS), the coverage-dependent reorientation of the benzoate species on the (110) and (111) faces of copper is compared and contrasted. Whereas on Cu(110) benzoate reorients from a flat-lying to an upright orientation with increasing coverage, on Cu(111), at all coverages, benzoate is aligned normal to the surface. The formation of periodic, flat-lying copper–benzoate structures has been attributed to the availability of metal adatoms, which differs dramatically between the (111) and (110) faces. We discuss the face specificity of molecular orientation by comparing calculated formation energies of adatom vacancies from ledges and kink sites on (100), (110) and (111) faces. Further support for this model is given by the evaporation of sodium, either by pre- or post-dosing, onto low-coverage benzoate/Cu(111), which induces benzoate to convert from a perpendicular to a parallel orientation. Likewise, coevaporation of Cu while dosing benzoic acid onto the Cu(111) surface also results in a majority of flat-lying benzoate species. Finally, for adsorption on the p(2×1)O/Cu(110) reconstruction, benzoate occurs only as the upright species, which is consistent with reducing the copper mobility and availability on the (110) face. We therefore suggest the possible role of metal adatoms as a new mechanism in controlling adsorbate orientation and therefore face specificity in surface reactions.

Orientation and bonding of benzoic acid, phthalic anhydride and pyromellitic dianhydride on Cu(110)

Abstract The interaction of the polyimide precursor pyromellitic dianhydride (PMDA), and the related compounds benzoic acid and phthalic anhydride, with Cu(110) has been studied by high resolution electron energy loss spectroscopy (HREELS). For benzoic acid, deprotonation of the carboxylic acid group occurs on adsorption leading to the formation of a surface benzoate species (C6H5COO-). Bonding to the surface occurs through a carboxylate linkage via two equivalent oxygen atoms. The HREEL spectrum is characterised by an intense dipole active band, the symmetric OCO stretching vibration, at ∼ 1420 cm −1. The plane of the carboxylate group is aligned perpendicular to the surface as is the plane of the benzene ring. A similar species is found following exposure of Cu(110) to phthalic anhydride. The carboxylate linkage results from disruption of the anhydride ring with loss of the CO character (C6H4COO-). In the case of the dianhydride species PMDA, only one of the anhydride units is used in bonding to the surface; the second unit points away from the surface and is characterised by the symmetric anhydride stretch at 1255 cm−1 and weak OO stretching vibrations at ∼ 1850 cm−1. In both cases, changes in the intensity of some of the bands compared with benzoic acid suggest that the carboxylate group is tilted away from the surface normal due to an interaction between one of the carbons of the aromatic ring and the copper surface. This implies that the plane of the aromatic ring is now twisted out of the plane of the carboxylate group and, although still perpendicular to the surface, the axis is tilted to allow one of the β-carbon atoms to interact with the surface. In all cases, off-specular measurements at a primary electron energy of ∼ 8 eV are dominated by the intense CH stretching vibration. Measurements of the intensity of this mode, in the surface benzoate species, as a function of incident electron energy suggest that excitation of this mode occurs via a resonance scattering mechanism.

Long-range periodicity in c(8 × 2) benzoate/Cu(110): a combined STM, LEED and HREELS study

Abstract The adsorption of benzoic acid on Cu(110) between 300 and 350K results in benzoate species oriented perpendicular to the surface with a c(8 × 2) periodicity at saturation coverage. STM images of the ordered structure and non-dipolar scattering in HREELS demonstrate alignment in the [110] azimuth. We interpret the tunnelling mechanism in STM, negative ion resonance scattering in HREELS, and assign low-frequency vibrational modes utilising ab initio molecular orbital calculations. We propose a model with four molecules per c(8 × 2) unit cell (ϑ = 0.25ML) arranged in two out-of-phase, zig-zag rows along the [001] direction with the car☐ylates bound in short bridge sites alternately three and five lattice constants apart along the [110] direction. Adsorbate-induced dipole-active phonons in the frequency range 65–185 cm −1 in HREELS support the adsorption model.

Molecularly induced step faceting on Cu(110) surfaces

Abstract Scanning tunneling microscopy (STM) has been used to study structures formed by adsorbed formate and benzoate species on Cu(110) surfaces. Partially oxygen-precovered and clean surfaces were exposed to formic and benzoic acids, respectively. STM images show that both adsorbed formate and benzoate result in a tendency for bunched steps to align along 〈1 1 − 2〉 ; directions and to become equidistant: effectively creating small facet planes on the surface. In the case of benzoate, these planes can be described as (11 13 1) and (11 13 1 − ) planes. In the case of formate, ordered arrays of facet planes can form on step edges giving the steps a “sawtooth” appearance. Under some conditions adsorbed formate also appears to stabilize finger-like islands and pits.

Spectral restoration in HREELS

Abstract The spectral resolution of HREELS is determined by both instrumental and sample-related factors. Despite substantial progress in electron optic design, these factors are still a limitation in assigning the vibrational modes of complex molecules and in determining linewidths. To the extent that all spectral features are broadened in the same way and within signal to noise limitations, currently available spectral restoration methods may provide a reliable solution. In principle, the measured elastic peak provides a template for deconvolution of these factors from the observed lineshapes. We compare results of a direct deconvolution method, in which division in the Fourier domain is followed by a linearised maximum entropy filter, with iterative methods based on maximum likelihood, maximum entropy and Bayesian principles involving only multiplication (i.e. convolution) in the Fourier domain. These methods are applicable both to spectra of modest resolution (intrinsic linewidths > > instrumental linewidth) and to high resolution spectra (linewidths ≈ instrumental resolution) in which spectral lineshapes are measurably different. The asymmetry and tailing of the elastic peak, which are critical to restoration of low frequency features, are determined by a number of processes such as multiple scattering, the thermal population of vibrational states, and low energy continuum excitations (e–h pairs, phonons) of the substrate. These factors, as well as practical considerations, are discussed in relation to measuring data for optimal spectral restoration.

HREELS and RAIRS? A complete vibrational study of the surface benzoate species adsorbed on copper

Abstract Despite major instrumental advances in both high resolution electron energy loss spectroscopy (HREELS) and reflection absorption infrared spectroscopy (RAIRS), it is still advantageous to use both techniques to gain a more complete understanding of the vibrational properties of molecules adsorbed at surfaces. The strengths and limitations, even with state of the art techniques, are illustrated with an analysis of results for the surface benzoate species (C 6 H 5 CO 2 − ) chemisorbed on Cu(110) and Cu(100) surfaces. In particular, these are demonstrated with regard to understanding the bonding and orientation of the molecular species at the surface and the importance of spectral resolution, spectral range, excitation mechanism and signal-to-background ratio.

Multiple scattering contributions and defining the background for resolution enhancement in HREELS

We examine the consequences of multiple scattering in HREELS and propose a new approach to defining the background for the purpose of resolution enhancement when the contribution of broad, continuum excitations interferes with discrete features of interest, e.g. adsorbate features. The method, which should be generally applicable to metals, oxides and semiconductors, is based on the Fourier-log deconvolution procedure for removing the Poisson distribution of multiple losses. The effects of noise and variation in instrumental sensitivity and transmission functions are considered. Statistical testing of the performance of currently available Bayesian resolution enhancement codes has been evaluated with synthetic spectra.

Electron-stimulated disordering in c(8 × 2) benzoate/Cu(110): a combined STM, LEED and HREELS study

Abstract We demonstrate a novel, negative-ion-mediated, electron-stimulated disordering process in LEED and, at nanoscopic scales, in the STM. Electron-stimulated disordering of the c(8 × 2) benzoate/Cu(110) structure in LEED is reversible, and HREELS shows that it is non-destructive to the adsorbed species. The disordering kinetics in LEED are pseudo first-order above a threshold between 5 and 7 eV with a large cross-section (∼ 10 −16 cm 2 ). The threshold correlates with strong negative ion resonance scattering in HREELS centred at 7–8 eV. Defect-controlled kinetics in LEED are observed at 5 eV and correlate with the spatial dependence in the STM disordering, which initiates preferentially at domain boundaries and is inhibited at step edges. Both the STM- and electron-beam-induced disordering are consistent with a negative ion resonance mechanism for efficient energy transfer from the tunnelling/incident electron to adsorbate internal vibrational modes which then relax by excitation of the frustrated translation. The influence of field effects on the potential energy surface is also considered.

Orientation and periodicity in the c(4 × 8) and p(2 × 1) structures of 3-thiophene carboxylic acid on Cu(110)

Abstract The chemisorption of 3-thiophene carboxylic acid on Cu(110) between 300 and 350 K has been investigated by high resolution electron energy loss spectroscopy (HREELS), scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). Ab initio molecular orbital calculations of the molecular ion aided in vibrational frequency assignments, interpretation of STM images and estimation of intra- and inter-molecular interactions influencing formation of the c(4 × 8) and p(2 × 1) structures. HREELS shows that at low coverage, the molecule lays flat with its π orbitals interacting with the surface. Increasing the coverage induces the molecules to reorient perpendicular to the surface and form a c(4 × 8) intermediate structure. Impact scattering in HREELS demonstrates that the molecules are preferentially aligned with the thiophene ring in the [110] azimuth. STM images suggest that the upright carboxylate species form rows of four adjacent molecules face-to-face along the [001] direction separated by four lattice constants in [110]. Subsequent rows are shifted by two lattice constants along [110], resulting in an overall c(4 × 8) periodicity and a coverage of 0.25 ML. With increasing coverage, the c(4 × 8) structure changes to a p(2 × 1) structure. A model with the carboxylates bound in short bridge sites two lattice constants apart along [110] with a local coverage of 0.5 ML is proposed. Steric repulsion in the p(2 × 1) structure results in rotation of the thiophene ring by an estimated 30° away from the [110] direction, consistent with impact scattering HREELS measurements. Calculated dipole-dipole repulsion between the carboxylate groups is large compared to any dipole-dipole attraction which could result from anti-parallel alignment of the static dipole moments of the thiophene rings.

Chemisorption studies related to reactive organic film growth

Abstract Metal surfaces can be functionalised by the chemisorption of a bifunctional molecule. One functionality is used to form the chemisorption bond, while the second is directed away from the substrate and available for bonding to a second, potentially different, molecule. The process can be continued to grow an organic film in a layer-by-layer manner using controlled deposition and reaction sequences. These possibilities are demonstrated by the interaction of pyromellitic dianhydride with copper and nickel surfaces and its subsequent reaction with aniline and p-phenylene diamine to form imide links via the intermediate amic acid. The chemisorption behaviour of pyromellitic dianhydride which involves opening one of the anhydride rings, loss of CO and formation of a surface carboxylate link has been characterised by X-ray PES and EELS, and supported by similar measurements on the chemisorption of phthalic anhydride and benzoic acid on a copper surface. The reactions are also followed by these techniques.