arXiv subjects

Klaus Kern

Publications and source records attributed to Klaus Kern.

80 records · Page 5Linked to original sources

arXiv2008

Iron nanoparticle formation in a metal-organic matrix: from ripening to gluttony

A simple method for the fabrication of metal nanoparticles is introduced. Heating metal-organic crystals in vacuum results in the formation of well defined metal particles embedded in a carbon matrix. The method is demonstrated for iron-phthalocyanine. At 500C homogeneously distributed iron nanoparticles with a reasonably narrow size distribution form by nucleation and ripening. After this initial phase the formation kinetics changes drastically. The particles move in the matrix to incorporate material. The "gluttony" phase shows astonishing similarities with the search for nutrition of living micro-organisms. Particle formation, ripening and gluttony are followed in-situ by transmission electron microscopy.

cond-mat.mtrl-sci↗

arXiv2006

Enhanced field emission from multiwall carbon nanotube films by secondary growth

We have studied nickel, gold, and ferritin coatings on catalytically grown multiwall carbon nanotubes, as well as the generation of secondary nanotubes by resubmitting the decorated nanotubes to the chemical vapor deposition process. Nickel layers sputtered on nanotubes show a stronger interaction with the nanotube walls than gold coatings. At ambient temperature this results in a metal film that is more homogeneous for Ni than for Au. Surface mass transport at elevated temperatures leads to a transformation of the coating to nanoscale clusters on the nanotube surface. The resulting Au clusters are sphere-like with a very small contact area with the nanotube whereas the Ni clusters are stretched along the tube axis and have a large contact area. Secondary nanotubes were established by growing nanotubes directly on the walls of primary nanotubes. Thin Ni layers or ferritin served as catalyst. We compared the field emission properties of samples with and without secondary nanotubes. The presence of secondary nanotubes enhances the field emission substantially.

cond-mat.mtrl-sci↗

arXiv2005

Formation of metallic nano-crystals from gel-like precursor films for CVD nanotube growth: An in-situ TEM characterization

The evolution of a continuous Fe(NO3)3 catalyst film was observed by in-situ annealing in a Transmission Electron Microscope (TEM). Such catalysts are routinely used in the catalytic growth of carbon nanotubes. The experiments reveal that crystalline particles form from the gel-like film already around 300C. At usual carbon nanotube growth temperatures of 700C, the particles are single-crystalline with a log-normal size distribution. The observations furthermore show that in spite of the crystalline structure of the particles there is a high mobility. The experiments allow to obtain detailed information about the chemistry and the crystallinity of the catalyst film, which provides valuable information for the interpretation of the carbon nanotube growth by chemical vapor deposition.

cond-mat.mtrl-sci↗

arXiv2005

Thermodynamic calculations on the catalytic growth of multiwall carbon nanotubes

We have developed a thermodynamic model of the catalytic growth of multiwall carbon nanotubes from hydrocarbon precursors at elevated temperature. Using this model we have computed the heat distribution, and carbon concentration in the catalyst. Calculations delivered a analytical formula for the growth time and growth rate. We find that the growth is mainly driven by a concentration gradient within the catalyst, rather than a temperature gradient.

cond-mat.mtrl-sci↗

arXiv2005

Comparative study of the catalytic growth of patterned carbon nanotube films

Three different catalysts (Fe, Ni, Co nitrates dissolved in ethanol) were patterned on a SiO2/Si substrate and multi-wall carbon nanotubes were grown by catalytic decomposition of acetylene. We compare the growth of the carbon nanostructures in the temperature range between 580C and 1000C. With our experimental set-up the catalyst solutions of cobalt and nickel were found to be less efficient than the one of iron. An optimal production of multi-wall nanotubes was observed at temperatures between 650C and 720C with the iron solution as catalyst. We found a tendency towards thicker structures with higher temperatures. Finally, we suggest a mechanism for the growth of these carbon structures.

cond-mat.mtrl-sci↗

arXiv2005

Raman spectroscopy and field emission measurements on catalytically grown carbon nanotubes

We used microcontact printing to pattern a silicon surface with an iron-containing catalytic solution. Multi-wall carbon nanotubes were subsequently grown on the patterned areas by chemical vapor deposition at temperatures between 650 and 1000C. We demonstrate that the diameter of the catalytically grown multi-wall nanotubes increases with the deposition temperature. Raman spectroscopy has been used to investigate the crystalline character of the obtained structures and it is found that the fraction of the nano-crystalline shell increases with the temperatures. The measurement of the field emission properties shows a correlation between the tube diameter and the emission field values.

cond-mat.mtrl-sci↗

arXiv2005

Giant Spin-splitting in the Bi/Ag(111) Surface Alloy

Surface alloying is shown to produce electronic states with a very large spin-splitting. We discuss the long range ordered bismuth/silver(111) surface alloy where an energy bands separation of up to one eV is achieved. Such strong spin-splitting enables angular resolved photoemission spectroscopy to directly observe the region close to the band edge, where the density of states shows quasi-one dimensional behavior. The associated singularity in the local density of states has been measured by low temperature scanning tunneling spectroscopy. The implications of this new class of materials for potential spintronics applications as well as fundamental issues are discussed.

cond-mat.mtrl-sci↗

arXiv2005

Growth and surface alloying of Fe on Pt(997)

The growth of ultra-thin layers of Fe on the vicinal Pt(997) surface is studied by thermal energy He atom scattering (TEAS) and Auger electron spectroscopy (AES) in the temperature range between 175K and 800K. We find three distinct regimes of qualitatively different growth type: Below 450K the formation of a smooth first monolayer, at and above 600K the onset of bulk alloy formation, and at intermediate temperature 500K - 550K the formation of a surface alloy. Monatomic Fe rows are observed to decorate the substrate steps between 175K and 500K. The importance of the high step density is discussed with respect to the promotion of smooth layer growth and with respect to the alloying process and its kinetics.

cond-mat.mtrl-sci↗