H. Hövel

Controlled cluster condensation into preformed nanometer-sized pits

We have performed scanning tunneling microscopy (STM) in ultrahigh vacuum and transmission electron microscopy (TEM) on silver and gold clusters grown in preformed nanometer-sized pits on the surface of highly oriented pyrolytic graphite. We describe the preparation method and evaluate the three-dimensional shape of the clusters using a combination of STM and TEM applied to the same cluster sample. The nanometer-sized pits were essential to fix the clusters in position when using STM. The influence of the tip shape on the STM imaging of nanometer-sized clusters is discussed.

Penetration of thin C60 films by metal nanoparticles.

Metal nanoparticles supported by thin films are important in the fields of molecular electronics, biotechnology and catalysis, among others. Penetration of these nanoparticles through their supporting films can be undesirable in some circumstances but desirable in others, and is often considered to be a diffusive process. Here, we demonstrate a mechanism for the penetration of thin films and other nanoscopic barriers that is different from simple diffusion. Silver clusters that are soft-landed onto a monolayer of C(60) supported by gold sink through the monolayer in a matter of hours. However, the clusters are stable when landed onto two monolayers of C(60) supported on gold, or on one monolayer of C(60) supported on graphite. With backing from atomistic calculations, these results demonstrate that a metallic substrate exerts attractive forces on metallic nanoparticles that are separated from the substrate by a single monolayer.

Growth mechanisms of carbon nanotubes using controlled production in ultrahigh vacuum

We present a method for the preparation of single walled carbon nanotubes (SWNTs) on a highly oriented pyrolytic graphite (HOPG) surface in ultrahigh vacuum (UHV), for which the preparation parameters for the production of metal clusters, fixed to nanometer sized pits on the surface, and the subsequent deposition of carbon can be controlled separately. Using cobalt as the cluster metal we carried out a comprehensive study concerning the influence of the substrate temperature (up to 900 °C) and the effective film thickness for the carbon evaporation. With scanning tunneling microscopy in UHV at room temperature and at T=77 K we observed single, separated SWNTs of about 50 nm length, which frequently were angled or branched and included junctions between sections of different tube diameters. With a statistical evaluation of tube diameters, tube lengths, and cluster heights, we obtained new insights into the growth mechanisms. An increase of tube diameters with increasing substrate temperature and a strong c...

Crystalline structure and orientation of gold clusters grown in preformed nanometer-sized pits

Abstract Gold clusters were produced by condensing evaporated gold in nanometer-sized preformed pits on the surface of highly oriented pyrolytic graphite (HOPG). The height of the clusters was 6.7 ± 0.7 nm as measured with scanning tunneling microscopy in ultrahigh vacuum, the lateral width was 10.1 ± 1.9 nm as determined with transmission electron microscopy (TEM). Using TEM for electron diffraction, we obtained information on the crystalline structure of the clusters. The intensity of the observed diffraction rings shows the preferential orientation of the clusters with the (111) plane of the gold lattice parallel to the (0001) surface of HOPG. This was compared to the diffraction pattern of gold clusters produced in the gas phase by inert-gas evaporation and deposited on a flat HOPG surface at room temperature as complete units which showed no preferential orientation. The directional alignment in the surface plane as it is described in the literature for larger gold crystallites grown on a flat HOPG surface is not observed for the nanometer-sized clusters grown in pits.

Controlled fabrication of nanopit patterns on a graphite surface using focused ion beams and oxidation

We produced nanopits on a highly oriented pyrolytic graphite substrate arranged in a given pattern with a combination of focused ion beam (FIB) irradiation and an oxidation process. The FIB irradiation was carried out using a dedicated FIB nanofabrication tool [J. Gierak et al., Appl. Phys. A: Mater. Sci. Process. A80, 187 (2005)]. After oxidation of the sample surface, defects produced by single ions were imaged as one monolayer deep nanopits with scanning tunneling microscopy. The penetration depth of the ions could be measured by oxidation of the defective volume produced on points irradiated with high ion doses. An array of well separated nanopits with a periodicity of 50nm could be produced.

Carbon-nanotubes on graphite: alignment of lattice structure

We study the alignment of the lattice structure of single walled carbon-nanotubes (SWNT) produced on a highly oriented pyrolytic graphite (HOPG) surface using controlled growth in ultrahigh vacuum (UHV). With scanning tunnelling microscopy (STM) at low temperature (T = 77 K) the HOPG and the SWNT were imaged in UHV simultaneously with atomic resolution. The lattice structure of the SWNT was compared to the HOPG. For this purpose we present a technique for evaluating the mutual orientation which uses the measured lattice vectors of the HOPG surface as a reference for distortion and drift effects in the STM image. The quantitative evaluation showed an alignment of the SWNT lattice structure to the structure of the substrate.

MIE-PLASMON SPECTROSCOPY: A TOOL OF SURFACE SCIENCE

Clusters are regarded as tiny entieties consisting of few atoms or molecules only (of the order of 101 to 105 atoms/cluster); alternatively, they can be regarded as atomic systems with extremely high specific surface (i.e. surface to volume ratio). These are, of course, two sides of the same medal.

An internet-based synchrotron experiment for students measuring the X-ray magnetic circular dichroism of a PtFe alloy

A new internet-based synchrotron experiment for students is presented. A polarimeter and computer software have been developed for measuring via the internet the X-ray magnetic circular dichroism of PtFe around its Pt L(II) and L(III) absorption edges. From the experiment, students can examine the X-ray magnetic circular dichroism of a thin PtFe foil utilizing circular-polarized synchrotron radiation emitted by the superconducting asymmetric wiggler at the synchrotron radiation source DELTA of the University of Dortmund.

Optical Properties of Free and Embedded Metal Clusters: Recent Results

The electrodynamic description of optical properties of larger metal clusters is – in a phenomenological sense – fully understood and accepted. Based upon Maxwell’s equations, the Mie equations are as valid as the Fresnel formulae are for plane sample geometries. They describe fully the optical response of a cluster, including the unique Mie resonances, i.e. plasmon (or phonon) polaritons. Like Fresnel formulae, Mie’s equations do not contain informations about particular properties of the cluster material. These enter by the dielectric functions e(ω) which have to be taken from other experiments (on bulk material in the classical Mie theory), or from solid state theory.

Analysis of photoacoustic IR spectra of aerogel and silica powder

Especially for solid-gas systems with a small volume fraction of the solid material, photoacoustic spectroscopy (PAS) is a very useful tool for the determination of the optical properties. For such materials, the photoacoustic signal is extremely large because of their low thermal conductivity. With other methods, the measurement of reflectance and transmittance is very difficult for such samples. The expansion of the interstitial gas in porous samples is responsible for a significant part of the photoacoustic signal. It is important to include this contribution in the quantitative interpretation of the PAS experiments. In addition to the optical properties of the sample, the interstitial gas expansion yields information about the structure of the pores. A model calculation is presented for the analysis of photoacoustic spectra in the infrared regime (ṽ = 400…2000 cm−1; λ = 5…25 μm) of silica powder, monolithic aerogel and opacified aerogel powder.