P. Zeppenfeld

Where are the molecules adsorbed on single-walled nanotubes?

Volumetric measurements of Xe, CF4 and SF6 adsorption isotherms on single-walled carbon nanotubes grouped in bundles are reported. Adsorption in the grooves on the outer bundle surface, on the remaining part of the external surface, in the interstitial channels as well as on amorphous carbon impurities is discussed in the light of a comparison of these results with those recorded previously for methane on the same substrate.

Exciton-dominated optical response of ultra-narrow graphene nanoribbons

Narrow graphene nanoribbons exhibit substantial electronic bandgaps and optical properties fundamentally different from those of graphene. Unlike graphene--which shows a wavelength-independent absorbance for visible light--the electronic bandgap, and therefore the optical response, of graphene nanoribbons changes with ribbon width. Here we report on the optical properties of armchair graphene nanoribbons of width N=7 grown on metal surfaces. Reflectance difference spectroscopy in combination with ab initio calculations show that ultranarrow graphene nanoribbons have fully anisotropic optical properties dominated by excitonic effects that sensitively depend on the exact atomic structure. For N=7 armchair graphene nanoribbons, the optical response is dominated by absorption features at 2.1, 2.3 and 4.2 eV, in excellent agreement with ab initio calculations, which also reveal an absorbance of more than twice the one of graphene for linearly polarized light in the visible range of wavelengths.

Neutron diffraction and numerical modelling investigation of methane adsorption on bundles of carbon nanotubes

Abstract Neutron diffraction measurements on a powder sample of bundles of single-walled carbon nanotubes are used to gain insight into adsorption sites for methane molecules. In parallel, numerical methods, based on empirical force-fields, are employed to calculate the adsorption of methane in bundles of nanotubes and neutron diffraction patterns based on these structures. Adsorption sites in grooves on the outer surface of bundles and on the curved surfaces of the nanotubes are characterised by their binding energy and relative abundance. Comparison with published adsorption isotherms indicate that the more stable groove sites must also be complemented by interstitial sites. Such sites are found to be energetically unfavourable in homogeneous bundles composed of average diameter nanotubes, but can become favourable in bundles made of bigger diameter tubes and in more realistic, heterogeneous bundles. Nanotube deformation is an important factor, limiting the size of interstitial channels, but also allowing the stabilisation of adsorbed methane molecules. Diffraction patterns calculated for methane–nanotube structures with partially occupied interstitial sites are in best agreement with the experimental diffraction data.

Kinetic Monte Carlo simulation scheme for studying desorption processes

Abstract We introduce a discrete event kinetic Monte Carlo (KMC) simulation program that is capable of reproducing thermal desorption data. The KMC program is suitable for the analysis of both, desorption in 2D quasi-equilibrium and desorption influenced by kinetic effects. Whereas the KMC simulation exhibits a clear advantage over approaches using rate equations regarding kinetic influences, the well-known quasi-equilibrium desorption provides a crucial test for the KMC simulation. Under the appropriate circumstances, the KMC simulation is, indeed, in full agreement with the quasi-equilibrium analysis based on the Polanyi–Wigner equation.

Preparation of atomically flat Co(110) films on Cu(110)

It is shown that the growth mode of cobalt on the Cu(110) surface at 350 K can be changed from three-dimensional to extended layer-by-layer growth by pre- and co-adsorption of oxygen. The shape and the intensity of the growth oscillations observed in thermal energy helium atom scattering experiments depends sensitively on the total oxygen coverage. Helium diffraction reveals that different oxygen induced surface reconstructions of the top-most Co layer are responsible for the presence or absence of layer-by-layer growth in an extended coverage range.

Online measurement of the optical anisotropy during the growth of crystalline organic films

We report a reflectance difference spectroscopy (RDS) investigation of the growth of para-sexiphenyl (p-6P) on a TiO2(110) single crystal substrate at 100, 300, and 400K. The results demonstrate that RDS is a powerful technique to monitor organic thin film growth from the submonolayer regime to device relevant thicknesses. Based on the polarization dependence of the optical absorption at characteristic wavelengths, the orientation and the crystalline properties of the organic molecules can be directly determined from the RD spectrum with an extremely high sensitivity.

Adsorption and growth on nanostructured surfaces

Abstract The relaxation of surface stress or strain constitutes an important mechanism in the formation (self-assembly) and ordering of nanostructures at surfaces. These stable, pre-patterned surfaces can further be used as templates for the fabrication of regularly nanostructured metallic or molecular films. Under appropriate conditions preferential nucleation or selective growth can lead to the replication of the template pattern. Two examples are discussed: The deposition of cobalt on the reconstructed Au(111)-(∼22× 3 ) surface and the adsorption of molecular films on the Cu(110)-(2×1)O stripe phase.

A rotating-compensator based reflectance difference spectrometer for fast spectroscopic measurements

We present a new type of reflectance difference (RD) spectrometer for fast spectroscopic measurements based on a rotating-compensator (RC) design. The instrument uses a 1024 element Si photodiode linear array for simultaneous multiwavelength detection. High quality RD spectra covering a spectral range from 1.5 to 4.5 eV can be acquired within a few seconds. A detailed description of the working principle, the instrumentation, and the algorithms used for data collection and reduction is presented, followed by a discussion of errors introduced by lamp instability and optical imperfections of the compensator. Finally, to demonstrate the performance of the new RCRD spectrometer, we illustrate its application for the in situ, real-time monitoring of the initial stages of organic thin film growth of para-sexiphenyl (p-6P) on the Cu(110)-(2 x 1)O surface.

Growth and stability of cobalt nanostructures on gold (111)

Abstract The nucleation, growth and stability of cobalt nanostructures on the (∼22 × √3) reconstructed Au(111) surface have been studied using thermal energy helium-atom scattering (TEAS) and Auger electron spectroscopy (AES) in the temperature range 50–700 K. Helium atom diffraction provides information on the island distribution and step heights. The composition of the surface region was checked by AES. In addition, the presence of cobalt in the topmost layer was investigated by the adsorption of carbon monoxide, as monitored by the specularly reflected helium intensity. From the change in the CO adsorption characteristics on the Co-covered Au(111) surface and the AES data, we conclude that surface alloy formation or Au capping takes place between 300 and 350 K, whereas Co diffusion into the Au bulk only occurs above 450 K.

The influence of weak adsorbate–adsorbate interactions on desorption

We have studied the influence of weak lateral adsorbate–adsorbate interactions on submonolayer thermal desorption spectra using a comprehensively tested novel Kinetic Monte Carlo (KMC) simulation scheme. Both attractive and repulsive adsorbate–adsorbate interactions are investigated. It is shown, that the desorption order continuously changes from zero to first to second order like behavior when decreasing the adsorbate–adsorbate binding energy from positive (attractive) to negative (repulsive) values. The desorption of N2 on Cu(1 1 0)–(2×1)O and Xe on Pt(1 1 1) provide examples for systems crucially influenced by weak lateral interactions. Both systems are investigated by means of KMC simulations, which allow to quantitatively reproduce the experimental desorption spectra.