G. Dumpich

Tailored nanoparticle films from monosized tin oxide nanocrystals: Particle synthesis, film formation, and size-dependent gas-sensing properties

In order to investigate the change of gas-sensitive properties of undoped tin oxide nanoparticle films depending on particle size, a thin film synthesis technique has been developed. Well-defined tin oxide nanoparticles have been prepared using a gas-phase condensation method. Pure SnO was used as starting material and was evaporated at T=820 °C. The resulting particles were sintered and crystallized in-flight at T=650 °C. Size-selected nanoparticles ranging from 10 to 35 nm were produced to form a nanoparticle film by means of electrostatic precipitation or low pressure impaction. The effect of in-flight oxidation, sintering, and crystallization on the structure, size, and size distribution of nanoparticles have been studied in detail. The samples show n-type semiconductors’ behavior like bulk SnO2. The influence of particle size on gas sensitivity and response behavior is investigated for C2H5OH at operating temperatures 200–300 °C using silicon substrates having an interdigitated contact pattern and an...

Quantitative determination of effective dipole and monopole moments of magnetic force microscopy tips

We present experimental results on the characterization of commercially available magnetic force microscopy (MFM) thin film tips as a function of an external magnetic field. Well defined magnetic stray fields are produced using current carrying rings with radii ranging between 603 and 2369 nm fabricated by electron-beam lithography directly imaged by MFM. Treating the MFM tip as a point probe, the analysis of the image contrast as a function of both the magnetic stray field and the lift height allows for a quantitative determination of effective magnetic dipole and monopole moments of the tip as well as their imaginary location within the real physical tip. Our systematic study gives a quantitative relationship on how absolute values of the magnetic dipole and monopole moments and their location within the tip depend on a characteristic decay length of the z component of the magnetic field being detected. From this we can estimate the effective tip volume of the real physical thin film tip relevant in MFM...

Epitaxial Fe16N2 films grown by sputtering

We have been able to obtain α‘‐Fe16N2 films using an underlayer template to induce the epitaxial growth of this metastable phase. They are epitaxial in the (001) direction and show single crystallinity in plane. Furthermore, they are deposited by simple reactive nitrogen sputtering. They have an average magnetic moment of 250 emu/g, considerably larger than the moment (217 emu/g) for pure bcc iron. Conversion electron Mossbauer spectroscopy gives three hyperfine fields corresponding to three different iron sites, as expected for this structure.

Electromigration in self-organized single-crystalline silver nanowires

We present electromigration experiments on single-crystalline silver nanowires. The wires were grown on 4° vicinal silicon (100) substrates by self-organization and were contacted by electron beam lithography. The electromigration experiments were performed in situ in a scanning electron microscope at room temperature with constant dc conditions. In contrast to other experiments we observe void formation at the anode side of the wires. If the current is reversed, the electromigration behavior is also reversed.

Structural and magnetic properties of NixFe1-x evaporated thin films

Abstract Thin Ni x Fe 1- x alloy films of various compositions ( x x > 0.46) were vacuum deposited onto quartz crystal substrates by electron beam evaporation. Structural properties of the Ni x Fe 1- x films were obtained by electron microprobe analysis (EPMA), X -ray diffraction and TEM-investigation. Magnetic properties were determined by M ( H , T )- and conversion electron Mossbauer spectroscopic (CEMS) measurements. These gave the concentration dependence of the saturation magnetization M s ( x ) and the mean hyperfine field B hf (x) of the evaporated films. The results are compared to those found for bulk Ni x Fe 1- x alloys and sputtered Ni x Fe 1- x alloys films, which had been investigated by other authors . The comparison of the magnetic behavior of the evaporated and sputtered Ni x Fe 1- x films yields the possibility to discuss more intensively the influence of Ar which had been suggested to be decisive in changing the magnetic properties of sputtered Ni x Fe 1- x films.

Magnetoresistance and magnetization reversal process of Co nanowires covered with Pt

The magnetoresistance (MR) of single Co nanowires of various width is investigated at low temperatures in magnetic fields up to 5 T. The in-plane longitudinal MR shows pronounced features at coercive fields indicating the magnetization reversal process. The nanowires are prepared by electron beam lithography. Some of the wires are covered with a 2 nm thick Pt layer, the others are uncovered. The MR behavior of the Pt covered Co nanowires shows significantly different behavior from that of the uncovered nanowires. This is interpreted mainly as arising from an oxidation of the Co surface leading to a formation of an antiferromagnetic CoO layer on top of the uncovered Co nanowires. The CoO layer hinders the magnetization reversal process by domain wall pinning, which is reflected by the MR behavior of these samples. In contrast, the MR behavior of the Pt covered Co nanowires shows no pinning effects. Thus, we conclude that covering the Co nanowires in situ with a 2 nm thick Pt layer prevents oxidation effects.

Observation of ultrafast lattice heating using time resolved electron diffraction

We use ultrafast electron diffraction to study lattice heating of 20nm noble metal films after femtosecond optical excitation with moderate excitation fluences. Using the Debye–Waller effect, the rise times of the lattice temperature were measured to be 1.1ps in copper (5.9mJ∕cm2 incident fluence) and 4.7ps in gold (0.9mJ∕cm2).

Evidence for a high-spin Fe state in fcc Fe1-xNix thin films in the invar region

Fe1-xNix alloy films with compositions in the Invar region (0.32 < x < 0.39) were prepared by simultaneous co-deposition of Ni and Fe under uhv conditions onto quatrz crystal substrates. The structure of the films was obtained by X-ray diffraction, the composition by electronprobe microanalysis (EPMA). The temperature and field dependence of the magnetization M(B, T) was measured for 0 T < B < 1 T between 4.2 and 300 K. After investigating the structural and magnetic properties of the as-prepared Fe1-xNix electron Mossbauer spectra (CEMS) were taken at room temperature for each Fe1-xNix film as-prepared and after annealing It is possible to achieve Fe1-xNix films as-prepared with pure fcc-structure and also mixtures of bcc and fcc phases. As-prepared films show, in contrast to the behavior of bulk Fe-xNix samples, high magnetic saturation moments Ms and and large mean hyperfine-fields Bhf which follow the Slater-Pauling curve. After annealing the films exhibit bulk behavior. We conclude that Fe1-xNix films as prepared are in a magnetic state of the fcc-phase which is probably related to the occurence of γ2-Fe. By annealing the films these states convert into the metastable γ-state of the fcc phase with corresponding changes of the magnetic moment and the mean hyperfine field BBhf.

Composition control in metal-organic vapor-phase epitaxy grown InGaAs nanowhiskers

InGaAs nanowhiskers were grown by metal-organic vapor-phase epitaxy on (111)B GaAs substrates using the vapor-liquid-solid growth mode. The diameter of nanowhiskers was defined by monodisperse gold nanoparticles deposited on the GaAs substrate from the liquid phase. By adjusting the triethylgallium to trimethylindium flow ratio, InxGa1−xAs whiskers with various compositions were realized. The composition characterization of the grown whiskers was done by high-resolution x-ray diffractometry. A detailed analysis of measured spectra allowed resolving the presence of an InGaAs three-dimensional layer between whiskers. High-resolution transmission electron microscopy investigation revealed the lattice constant of the grown whisker structures, which agrees with the whisker composition defined by x-ray diffractometry. Finally, low-temperature photoluminescence measurements of the realized InGaAs whiskers were carried out.

Electron-beam lithography with metal colloids: Direct writing of metallic nanostructures

We report on the fabrication of metallic nanostructures in the sub-100 nm region by means of electron-beam lithography with metal colloids. A thin organometallic film consisting of surfactant stabilized Pd-colloids (⩽3 nm) is directly patterned by electron-beam irradiation. Non-exposed colloids are easily removed by rinsing the sample with appropriate dissolvers. The metallic character of the nanostructures is checked by resistance measurements. We find the morphology and the resistance behavior of the present nanostructured Pd-colloids to be similar to those of granular thin Pd/C films. Accordingly, the metal content of the nanostructures fabricated with Pd-colloids can be estimated.