S. Luby

Towards strain gauges based on a self-assembled nanoparticle monolayer--SAXS study.

An in situ small-angle x-ray scattering study of the nanoparticle displacement in a self-assembled monolayer as a function of a supporting membrane strain is presented. The average nanoparticle spacing is 6.7 nm in the unstrained state and increases in the applied force direction, following linearly the membrane strain which reaches the maximum value of 11%. The experimental results suggest a continuous mutual shift of the nanoparticles and their gradual separation with the growing stress rather than nanoparticle islands formation. No measurable shift of the nanoparticles was observed in the direction perpendicular to the applied stress.

Fe3O4/γ-Fe2O3 nanoparticle multilayers deposited by the Langmuir-Blodgett technique for gas sensors application.

Fe3O4/γ-Fe2O3 nanoparticles (NPs) based thin films were used as active layers in solid state resistive chemical sensors. NPs were synthesized by high temperature solution phase reaction. Sensing NP monolayers (ML) were deposited by Langmuir-Blodgett (LB) techniques onto chemoresistive transduction platforms. The sensing ML were UV treated to remove NP insulating capping. Sensors surface was characterized by scanning electron microscopy (SEM). Systematic gas sensing tests in controlled atmosphere were carried out toward NO2, CO, and acetone at different concentrations and working temperatures of the sensing layers. The best sensing performance results were obtained for sensors with higher NPs coverage (10 ML), mainly for NO2 gas showing interesting selectivity toward nitrogen oxides. Electrical properties and conduction mechanisms are discussed.

The optical band gap of semiconducting iron disilicide thin films

Abstract Optical transmittance and reflectance measurements have been carried out on semiconducting β-iron disilicide layers formed by annealing of iron films evaporated onto silicon substrates and capped with amorphous silicon or SiOx thin overlayers. The dependence of the absorption coefficient on the energy of photons favours direct allowed transitions with the forbidden energy gap of 0.87 ± 0.04 eV.

Electron paramagnetic resonance and the ageing process in amorphous germanium

Abstract Thin films of a-Ge were prepared by evaporation in oil-pumped high vacuum and in ultrahigh “dry” vacuum equipments, respectively. The ageing process, i.e. the time increase of the resistivity and the EPR have been studied with these samples. The results achieved show that the ageing is connected with the presence of reactive gases, absorbed or diffused into the volume of the film. The resistivity increases linearly with the logarithm of the time variable in the first phase, then after the time 100–500 h the dependences saturate. EPR lines of width ≈ 80 G, g -factor value of 2.021 and the intensity corresponding to the concentration of free spins ≈ 10 19 /cm 3 were ascertained. The results agree with preceding papers and confirm the hypothesis that the origin of the EPR signal in a-Ge is the presence of unsaturated bonds. The dependence of the concentration of these bonds upon the annealing and deposition conditions and the concentration of impurities in the source material is studied. The similarity in the dependences of the number of unsaturated bonds and the resistivity upon the concentration of doping impurities supports the idea that the unsaturated (or free) bonds may also contribute to the electrical conductivity of a-Ge.

Metal oxide/silicon oxide multilayer with smooth interfaces produced by in situ controlled plasma-enhanced MOCVD

Molybdenum oxide/silicon oxide and tungsten oxide/silicon oxide: multilayer with 24 periods and a period thickness of 9.2 nm were fabricated with plasma-enhanced MOCVD. The layer thickness was controlled by an in situ soft X-ray reflectivity measurement. For the deposition of the SiO2 layers, a new silicon organic precursor, pentamethylcyclopentadienyldisilane (Me5C5Si2H5) was used in an O-2 remote: plasma process. The high quality of multilayer interfaces was observed by cross-section transmission electron microscopy (TEM), the interface toughness was measured by hard X-ray reflectivity and diffuse scattering at grazing incidence experiments to be about 0.1 nm. Auger electron spectroscopy (AES) gives the information, that the silicon oxide is practically carbon free, and the carbon content of the metal oxides is low (<5%)

Modified Langmuir-Blodgett deposition of nanoparticles - measurement of 2D to 3D ordered arrays

Modified Langmuir-Blodgett deposition of nanoparticles - measurement of 2D to 3D ordered arrays The ordered nanoparticle monolayers and multilayers over macroscopic areas were prepared by the modified Langmuir-Blodgett method. Using this approach, the nanoparticle monolayer is formed on the water surface by compression and subsequently it is transferred onto the substrate by a controlled removal of the water subphase. The ordering and homogeneity of the prepared mono- and multilayers was studied by scanning electron microscope (SEM), grazing-incidence small-angle X-ray scattering (GISAXS) and X-ray reflectivity (XRR) techniques. From the results it follows that an ordered nanoparticle monolayer was formed over a large area. For the multilayer, the layering and lateral ordering of each layer was confirmed by XRR and SEM performed after the deposition of each nanoparticle layer.

Pulsed laser deposition of Co- and Fe-based amorphous magnetic films and multilayers

Abstract Magnetic films and multilayers were prepared by pulsed laser ablation of Co- and Fe-based amorphous magnetic ribbons with compositions Co 67 Cr 7 Fe 4 Si 8 B 14 and Fe 73.5 Nb 3 Cu 1 Si 13.5 B 9 , respectively. Targets were ablated in vacuum (∼10 −5 Pa) by KrF excimer laser pulses at fluences from 3 to 7 J/cm 2 . Films were deposited on oxidized silicon wafers, placed 80 mm apart from the target. From X-ray diffraction spectra it follows that all the films are amorphous, while Rutherford backscattering spectrometry analyses confirm that their composition is close to the respective target when they are deposited at the lower fluence. In the case of higher fluence, the concentrations of both Si and B in the films decrease. From ferromagnetic resonance studies it results that the effective magnetizations 4 πM eff and g -factors of the films are close to the values of the respective ribbon only in the case of Fe-dominating composition. Nanometric multilayers (4-nm magnetic alloy/6 nm Cu) ×5 were also deposited and electrically and magnetically characterized.

Nonequilibrium Phases of Nanoparticle Langmuir Films

We report on an in-situ observation of the colloidal silver nanoparticle self-assembly into a close-packed monolayer at the air/water interface followed by a 2D to 3D transition. Using the fast tracking GISAXS technique, we were able to observe the immediate response to the compression of the self-assembled nanoparticle layer at the air/water interface and to identify all relevant intermediate stages including those far from the equilibrium. In particular, a new nonequilibrium phase before the monolayer collapse via the 2D to 3D transition was found that is inaccessible by the competing direct space imaging techniques such as the scanning and transmission electron microscopies due to the high water vapor pressure and surface tension.

Coplanar and non-coplanar x-ray reflectivity characterization of lateral W/Si multilayer gratings

Structural characterization of a fully etched amorphous W/Si multilayer grating with a lateral periodicity of 800 nm is performed by x-ray reflectivity in the coplanar and non-coplanar modes using a scintillation detector and a two-dimensional gas-filled detector, respectively. Three-dimensional reciprocal space constructions were used to explain the scattering features recorded in both geometries. Coplanar coherent grating truncation rods were fitted by a dynamical theory for rough gratings. Comparison of the reflectivity from the reference planar multilayer completes the study.

Superconductivity of tungsten-silicon multilayers

Abstract Tungsten-silicon multilayers were prepared by electron beam deposition in ultrahigh vacuum. The number of bilayers and their thicknesses were 10 and 2–24 nm, respectively. Structural properties were analyzed by low and large angle X-ray diffraction and TEM. Tungsten films in multilayers were amorphous for the layer thickness dw≦4 nm. Multilayers with dw=1 and 2 nm were superconducting with Tc = 2.72–4.21 K. Superconductivity was preserved in samples annealed up to 650°C for 40 s. At higher annealing temperatures the formation of crystalline tungsten silicides was observed, simultaneously the periodicity of multilayers was destroyed and superconductivity was lost.