G. J. C. van Baarle

The ReactorSTM: Atomically resolved scanning tunneling microscopy under high-pressure, high-temperature catalytic reaction conditions

To enable atomic-scale observations of model catalysts under conditions approaching those used by the chemical industry, we have developed a second generation, high-pressure, high-temperature scanning tunneling microscope (STM): the ReactorSTM. It consists of a compact STM scanner, of which the tip extends into a 0.5 ml reactor flow-cell, that is housed in a ultra-high vacuum (UHV) system. The STM can be operated from UHV to 6 bars and from room temperature up to 600 K. A gas mixing and analysis system optimized for fast response times allows us to directly correlate the surface structure observed by STM with reactivity measurements from a mass spectrometer. The in situ STM experiments can be combined with ex situ UHV sample preparation and analysis techniques, including ion bombardment, thin film deposition, low-energy electron diffraction and x-ray photoelectron spectroscopy. The performance of the instrument is demonstrated by atomically resolved images of Au(111) and atom-row resolution on Pt(110), both under high-pressure and high-temperature conditions.

The ReactorAFM: Non-contact atomic force microscope operating under high-pressure and high-temperature catalytic conditions

An Atomic Force Microscope (AFM) has been integrated in a miniature high-pressure flow reactor for in-situ observations of heterogeneous catalytic reactions under conditions similar to those of industrial processes. The AFM can image model catalysts such as those consisting of metal nanoparticles on flat oxide supports in a gas atmosphere up to 6 bar and at a temperature up to 600 K, while the catalytic activity can be measured using mass spectrometry. The high-pressure reactor is placed inside an Ultrahigh Vacuum (UHV) system to supplement it with standard UHV sample preparation and characterization techniques. To demonstrate that this instrument successfully bridges both the pressure gap and the materials gap, images have been recorded of supported palladium nanoparticles catalyzing the oxidation of carbon monoxide under high-pressure, high-temperature conditions.

Imaging of vortex configurations in thin films by scanning-tunneling microscopy

We report on imaging of vortices in thin superconducting films using surface passivation with an ultrathin Au layer. This allows investigation of surfaces that oxidize easily, as well as the mounting of samples in air. We studied vortex configurations in a material with weak vortex pinning (a-Mo2.7Ge) and a strongly pinning material (NbN) at 4.2 K in magnetic fields up to 1.4 T. In a-Mo2.7Ge, we observe a well-ordered hexagonal lattice, with local defects beginning to appear around 1.0 T. In NbN, the vortex lattice is fully disordered.

STM imaging of vortex configurations in films of a-Mo3Ge through a Au layer

Abstract We report on visualization of the vortex configuration in thin films (50 nm) of amorphous (a-) Mo3Ge by means of scanning tunnelling spectroscopy (STS) at 4.2 K. We prepared the Mo3Ge films by rf-sputter deposition. To avoid oxidation during mounting of the film in the scanning tunnelling microscopy (STM), we deposited a thin (typically 3 nm) protecting Au layer immediately after deposition of the Mo3Ge layer. STM investigation showed the surface of this Au layer to be very smooth. STS in zero-field showed a proximity-induced gap everywhere on the surface and vortex lattices could be imaged in grid-mode in fields up to 0.6 T.

STM imaging of vortex structures in NbN thin films

We report on imaging of the vortex structure in NbN thin films by using low-temperature scanning tunnelling microscopy and spectroscopy at 4.2 K in magnetic fields up to 1.2 T. In order to avoid oxidation while retaining a smooth surface, a very thin film of Au (∼4 nm) was deposited immediately after sputtering of the NbN thin film (∼60 nm). The topography shows that the average grain size of the NbN thin film is 35 nm and the roughness of NbN/Au film surface is below 0.7 nm. From the spectroscopic measurements, vortices show a disordered structure in the whole field region measured, indicating the strong pinning effects in the NbN thin films.

Ground-state properties of thin films of the heavy-fermion system CeCu6 with varying thickness

Abstract Thin polycrystalline films of the heavy-fermion compound CeCu6 were prepared by sputter deposition. Films of thickness t=190 nm were previously found to be of good quality and to mimic the properties of bulk CeCu6 in resistivity measurements. We investigated the transport properties of films of varying thickness. As the thickness is decreased, the maximum in the resistivity is shifted to lower temperatures and the correlation coefficient A in the Fermi liquid regime (T2) decreases. This indicates that the coherence effects which cause the drop in the resistivity are weakened. The residual resistivity is independent of the thickness, proving that atomic disorder is not playing a major role.