Margrit Hanbücken

Formation of noble-metal-Si(100) interfaces

Abstract From the very first stages of adsorption, in the course of the build up of the interfaces, the noble metals condensed on clean Si(111) surfaces behave quite distinctively. A large number of investigations has been devoted to the characterization of these systems, which appear in many aspects as prototypes for further studies in semiconductor interface physics. Although of increasing practical interest, the interfaces of Cu, Ag and Au with Si(100) have been comparatively much less studied in a fundamental way. In this paper we review the knowledge acquired on these systems, adding new experimental results (electron spectroscopy and -microscopy). A comparison between noble-metal/Si(100) and noble-metal/Si(111) interface properties will be included.

Surface spectroscopy studies of Pb monolayers on Si(111)

We have studied the adsorption process and the thermal desorption process of Pb on clean Si(111)7 × 7 substrates using LEED and AES. The growth kinetics follows a Stranski-Krastanov mode (2D adlayer + 3D islands). The adlayer is completed at one monolayer when deposited below the lead melting point. Upon heat treatment, three different surface phases resulting in a (3×3)R30 ° unit cell are identified. The last one, saturated at 1 ML, undergoes a reversible phase transition to a 1×1 structure. This phase transition is attributed to an order-order solid phase transition.

A novel self-ordered sub-10 nm nanopore template for nanotechnology.

The fabrication of cost-efficient wafer scale self-ordered arrays of vertical and insulating sub-10 nm nanopores with low porosity is demonstrated. These meet challenging applications like read heads with perpendicular to the plane giant magnetoresistance, calling for strongly localized currents. Purely electrical sequencing of DNA strands, requiring insulating membranes with reduced pore diameters can also be considered.

Combined AES, LEED, SEM and TEM characterizations of CuSi(100) interfaces

CuSi(100) interfaces prepared under UHV at different substrate temperatures (TS) have been characterized using in-situ Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) as well as ex-situ scanning and transmission electron microscopy (SEM, TEM). At room temperature (RT), the film grows in a layer by layer like mode. With increasing TS, the intensity of the Cu M2,3VV (61 eV) Auger transition decreases and at TS = 500°C no Cu Auger signal could be measured below θ ∼ 100. Yet SEM and TEM observations of these deposits show islands in epitaxial relation with the substrate. It can be determined from TEM images that these islands are covered with a Si skin (∼ 50 A; thick) and that they are deeply implanted in the Si substrate. This explains the AES measurements.

Photoemission and scanning tunneling microscopy on K/Si(100)

Abstract We have measured valence band photoemission, the Si 2p and K 3p transitions and the work function from K/Si(100)2 × 1 in the entire coverage range with K deposited at room temperature up to saturation coverage. In addition, we have studied this system by scanning tunneling microscopy (STM). In STM at about 1 6 − 1 4 of saturation coverage a surface structure with local order has been identified, which for higher coverage exhibits a transition to a 2 × 3 structure. At saturation coverage the tunneling current could be stabilized already at very low sample bias voltages in the mV range, which indicates the presence of a two-dimensional metallic K layer. The photoemission results do not show a shift of Si surface state emission towards higher binding energy with increasing K coverage. Metallic features in the vicinity of the Fermi level could not be identified in the entire coverage range. Absence of a visible metallic edge at saturation coverage may be explained by a small photoionization cross section of the K derived states for the used photon energy of 55 eV.

The first stages of the formation of the interface between gold and silicon (100) at room temperature

Abstract The formation of the AuSi(111) interface has been studied through many different approaches. Although of higher technical importance the AuSi(100) interface has received much less attention, especially in the early stages of its formation. We present a detailed study of this system at RT in the low coverage region, from zero to a few monolayers, using LEED, AES and transmission electron microscopy. The discussion of the experimental results is focussed on the onset of the intermixing reaction.

Atomic Layer Deposition of Pd Nanoparticles on TiO2 Nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties

Palladium nanoparticles are grown on TiO2 nanotubes by atomic layer deposition (ALD), and the resulting three-dimensional nanostructured catalysts are studied for ethanol electrooxidation in alkaline media. The morphology, the crystal structure, and the chemical composition of the Pd particles are fully characterized using scanning and transmission electron microscopies, X-ray diffraction, and X-ray photoelectron spectroscopy. The characterization revealed that the deposition proceeds onto the entire surface of the TiO2 nanotubes leading to the formation of well-defined and highly dispersed Pd nanoparticles. The electrooxidation of ethanol on Pd clusters deposited on TiO2 nanotubes shows not only a direct correlation between the catalytic activity and the particle size but also a steep increase of the response due to the enhancement of the metal-support interaction when the crystal structure of the TiO2 nanotubes is modified by annealing at 450 °C in air.

3D-nanoarchitectured Pd/Ni catalysts prepared by atomic layer deposition for the electrooxidation of formic acid

Summary Three-dimensionally (3D) nanoarchitectured palladium/nickel (Pd/Ni) catalysts, which were prepared by atomic layer deposition (ALD) on high-aspect-ratio nanoporous alumina templates are investigated with regard to the electrooxidation of formic acid in an acidic medium (0.5 M H2SO4). Both deposition processes, Ni and Pd, with various mass content ratios have been continuously monitored by using a quartz crystal microbalance. The morphology of the Pd/Ni systems has been studied by electron microscopy and shows a homogeneous deposition of granularly structured Pd onto the Ni substrate. X-ray diffraction analysis performed on Ni and NiO substrates revealed an amorphous structure, while the Pd coating crystallized into a fcc lattice with a preferential orientation along the [220]-direction. Surface chemistry analysis by X-ray photoelectron spectroscopy showed both metallic and oxide contributions for the Ni and Pd deposits. Cyclic voltammetry of the Pd/Ni nanocatalysts revealed that the electrooxidation of HCOOH proceeds through the direct dehydrogenation mechanism with the formation of active intermediates. High catalytic activities are measured for low masses of Pd coatings that were generated by a low number of ALD cycles, probably because of the cluster size effect, electronic interactions between Pd and Ni, or diffusion effects.

Large-scale ordered plastic nanopillars for quantitative live-cell imaging.

Surfaces exhibiting ordered nanopillars have a wide range of potential biomedical applications based on the altered adhesivity of living cells on nanopatterned surfaces compared to planar ones. Examples include scaffolding for tissue engineering, designer bandages for wound dressing, and antifouling surfaces for implants. Although numerous experiments performed over the last decade have confirmed that cells respond to the chemistry (biochemical 2D imprint) and geometry (topographical 3D relief) of their surroundings at the nanoscale, the fundamental processes by which cells recognize nanostructures is a subject of on-going research. In this context, there is a need to ensure that the nanostructured surfaces have large-scale coverage and are compatible with quantitative optical microscopy (QOM), an important tool for studying living cells, especially the dynamics thereof. While biochemical patterning is not expected to pose a special challenge for QOM, topographical patterning may do so. Well-known techniques for topographical patterning are nanoimprint lithography (NIL, including thermal embossing and UV curing) and self-assembly based on colloidal beads or phase separation of polymers, all of which achieve large coverage. NIL is relatively resource intensive and usually depends on conventional techniques like electron-beam lithography for the initial stamp. Self-assembly, although increasingly refined, has limited flexibility for the choice of motif. Transparent substrates made using these

An STM Study of Mechanochemically Prepared Si(111) Substrates: An Extended Set of Vicinal Surfaces

The morphology and surface structure of mechanochemically treated Si(111) samples, originally prepared for transmission electron microscopy studies, have been determined by scanning tunnelling microscopy (STM). During the preparation method, the Si(111) planes are cut under continuously changing polar angles in all azimuthal directions. The result is a dimple whose concave surface breaks up, under UHV heat treatment, into a large number of vicinal (111) surfaces. STM measurements, performed on different parts of the dimple, show the facetting of this surfaces into (111)-oriented, 7 × 7 reconstructed terraces, steps and step bunches. Characteristic features observed by STM on this type of vicinal substrate correspond to results reported on individually prepared vicinal surfaces.