V. Kempter

Synthesis and Characterization

As compared to bulk materials, magnetic nanoparticles possess distinct magnetic properties and attempts have been made to exploit their beneficial properties for technical and biomedical applications, e.g. for magnetic fluids, high-density magnetic recording, or biomedical diagnosis and therapy. Early magnetic fluids (MFs) were produced by grinding magnetite with heptane or long chain hydrocarbon and a grinding agent, e.g. oleic acid [152]. Later procedures for MFs precipitated Fe 3+/Fe 2+ of an aqueous solution with a base, coated the particles by oleic acid, and dispersed them in carrier liquid [161]. However, besides the elemental composition and crystal structure of the applied magnetic particles, particle size and particle size distribution determine the properties of the resulting MF. Many methods for nanoparticle synthesis including the preparation of metallic magnetic particles have been described in the literature. However, there still remain important questions, e.g. concerning control of particle size, shape, and monodispersity as well as their stability towards oxidation. Moreover, peptization by suitable surfactants or polymers into stable MFs is an important issue since each application in engineering or biomedicine needs special MFs with properties adjusted to the requirements of the system.

Study of the surface electronic structure of MgO bulk crystals and thin films

The electronic structures of the surfaces of MgO single crystals, oxidized Mg polycrystals and oxidized Mg films grown by molecular beam epitaxy on Si(100) surfaces were studied using several techniques. These include metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS (He I)), and X-ray photoelectron spectroscopy (XPS). Spectra of oxidized Mg layers on Si(100) show additional features to those obtained for cleaved MgO crystals. These spectral features are attributed to dissociative adsorption of oxygen at bulk oxygen sites. Weak heating of the oxidized Mg layers removes these features and the electronic spectra for all three studied systems become similar. However, the experimental MIES and UPS spectra, both arising mainly from the ionization of the O 2p orbitals, have different structures. They are interpreted on the basis of ab initio Hartree-Fock and density functional calculations of the electronic structures of the ideal MgO(100) surface. It is shown, that the differences in the spectra can be understood by taking into account that UPS spectra reflect the density of electronic states within several surface layers, whereas MIES probes the surface states which are the most extended into the vacuum.

Atomic and electronic structure of the corundum (0001) surface : comparison with surface spectroscopies

Abstract The electronic structure and geometry of the Al-terminated corundum (0001) surface were studied using a slab model within the ab-initio Hartree-Fock technique. The distance between the top Al plane and the next O basal plane is found to be considerably reduced on relaxation (by 0.57 A, i.e. by 68% of the corresponding interlayer distance in the bulk). An interpretation of experimental photoelectron spectra (UPS He I) and metastable impact electron spectra (MIES) is given using the calculated total density of states of the slab and the projections to the atoms, atomic orbitals, and He 1s floating atomic orbital at different positions above the surface. Calculated projected densities of states exhibit a strong dependence on the relaxation of surface atoms. The good agreement of simulated and experimental UPS and MIES spectra supports the correctness of calculated surface relaxation.

Coadsorption of Cs and hydrogen on W(110) studied by metastable impact electron spectroscopy

Abstract The adsorption of hydrogen and Cs alone as well as the coadsorption of Cs and hydrogen on W(110) was studied by metastable impact electron spectroscopy (MIES) and supplemented by UPS, AES, and work function measurements. The main conclusions are: (a) ionization of the Cs(6s) level is not observed for Cs coverages θ Cs ⩽ 0.4 ML, (b) the formation of a H(1S)-metal bond occurs at a binding energy of E B = 4.7 eV , (c) upon Cs and hydrogen coadsorption an independent layer of hydrogen is formed between the substrate and the Cs adlayer. The charge density of the Cs adlayer is lowered by about 25% upon the formation of the hydrogen intermediate layer.

CO2 chemisorption at Mg and MgO surfaces: a study with MIES and UPS (He I)

Abstract The interaction of CO2 with Mg and MgO films (on Si substrates) is studied with MIES in conjunction with UPS (He I) at room temperature. On Mg surfaces the presence of a carbonate (CO2−3) species is detected on top of an oxide layer. On MgO surfaces chemisorption does not take place at regular sites, but presumably at low-coordinated O2− ions as found at step sites. Again a CO2−3 species can be identified with MIES. Coadsorbed alkali atoms (Li) increase the rate of carbonate formation on MgO by about a factor of three at room temperature.

Investigations with Infrared Spectroscopy on Films of the Ionic Liquid [EMIM]Tf2N

The reflection-absorption infrared (RAIRS) spectra of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]Tf 2N) are presented as a function of temperature between 114 and 292 K. A comparison is made with the corresponding infrared spectra (obtained with transmission spectroscopy) from bulk [EMIM]Tf 2N. The liquid and amorphous films show rather similar spectra, indicating that the film structure is similar in both cases. On the other hand, these spectra differ considerably from those of crystalline films. Characteristic differences seen between the film and bulk spectra are attributed to the different structures of the respective networks. There are, however, indications that under all studied conditions the cation-anion interaction is between the C-H groups of the [EMIM] ring and the SO 2 groups of the anion.

Mg clusters on MgO surfaces : study of the nucleation mechanism with MIES and ab initio calculations

We combined experimental studies using ultraviolet photoelectron spectroscopy (UPS), metastable impact electron spectroscopy (MIES) and temperature programmed desorption (TPD) with abinitio calculations of metal adsorption on the perfect MgO surface and at defect sites in order to elucidate the role of surface defects in the initial stages of nucleation and growth of metal clusters at oxide surfaces. MgO films (2 nm thick) grown on Mo and W substrates were used as a prototype system. The MIES and UPS (HeI) spectra were collected insitu, and the growth of Mg clusters was observed by monitoring the dynamics of additional MIES peaks during Mg deposition. TPD experiments were made in order to monitor the surface coverage by Mg clusters and to determine the Mg desorption energies. Interpretation of the results was made on the basis of theoretical modelling using density functional theory (DFT) calculations in both periodic and embedded cluster models. The geometric and electronic structures of the surface terrace, F-centre, positively charged anion vacancy, and step edge at the MgO(001) surface were calculated, and their role in adsorption and clustering of Mg atoms on this surface was studied. The absolute position of the top of the surface valence band of MgO with respect to the vacuum was calculated and compared with the MIES results. The MIES spectra were modelled on the basis of surface density of states (SDOS). The calculated SDOS predicted the location of additional peaks in the band gap and their shift as a function of Mg concentration on the surface in agreement with the MIES data. The desorption energies of Mg atoms from small Mg clusters formed at step edges are found to be about 1.3 eV atom-1. Comparison between the theoretical results and the experimental data suggests preferential initial adsorption of Mg atoms at steps and kinks, rather than at charged and neutral vacancies. At larger exposures these Mg atoms serve as the nucleation sites.

Alkali-metal-affected adsorption of CO on W(110) studied by metastable impact electron spectroscopy

Abstract The adsorption of CO on clean and alkali-covered W(110) is studied by Metastable Impact Electron Spectroscopy (MIES) and UPS. Moleculary adsorbed CO is not observed for adsorption on clean W(110) neither by MIES nor by UPS at room temperature, but its existence cannot be excluded by the present results. From a comparison with data for the coadsorption of oxygen and K it is concluded that for K precoverages up to 0.8 ML features from the molecular adsorption of CO are seen in the MIE spectra. K precoverages beyond 0.8 ML, however, show spectral features corresponding to oxygen adsorption only. The exposure of a CO saturated W(110) surface to various amounts of K confirms that the complete disappearance of CO induced molecular spectral features occurs around 0.8 ML K coverage, and is mediated by a direct transfer of the K(4s) electron to the CO molecule.

Excitation of Li(2p) in slow collisions of Li+ ions with cesiated W(110) surfaces

The excitation of the Li(2p) state in low-energy collisions of Li+ ions with low work function surfaces is studied for impact energies below 1.0 keV. The yields of excited atoms and electrons are measured for the scattering from cesiated and oxidized cesiated W(110) surfaces characterized by AES, LEED, and Δφ. It is concluded that Auger deexcitation of the 2p state populated by resonant charge transfer between the Li+ projectile and the partially cesiated surface strongly influences the photon yield. For the oxidized surface it is shown that resonant electron exchange between the solid and the Li(2p) state is not the mechanism for projectile excitation. It is proposed that projectile excitation is caused by a direct transition between the 2s and 2p states of the Li+ projectile which is neutralized on its way towards the surface.

Surface engineering of Co and FeCo nanoparticles for biomedical application

Monodisperse Co, Fe, and FeCo nanoparticles are prepared via thermal decomposition of metal carbonyls in the presence of aluminium alkyls, yielding air-stable magnetic metal nanoparticles after surface passivation. The particles are characterized by electron microscopy (SEM, TEM, ESI), electron spectroscopy (MIES, UPS, and XPS) and x-ray absorption spectroscopy (EXAFS). The particles are peptized by surfactants to form stable magnetic fluids in various organic media and water, exhibiting a high volume concentration and a high saturation magnetization. In view of potential biomedical applications of the particles, several procedures for surface modification are presented, including peptization by functional organic molecules, silanization, and in situ polymerization.