E. Majkova

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.

The study of phase transitions in amorphous bilayers prepared by rapid quenching

Abstract Amorphous bilayers with layers of FeNiB and CoFeCrSiB have been prepared by planar flow casting from a single crucible with two nozzles close to each other and with a partition between them forming two separate vessels. Such an arrangement has allowed us to obtain ribbons with two homogeneous layers, one on top of the other, along the whole ribbon length with high quality surface and with contact interlayer having submicron thickness. The character of the interlayer has been investigated by SEM, EMA, CS-TEM, AES and resistometry in the as-quenched state and after annealing below and after crystallization. From the results it seems evident that the process of connection of the two layers takes place below the crystallization temperature by mutual interdiffusion of component atoms, thus giving rise to mechanically solid connection.

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.

Kinetics of nanoparticle reassembly mediated by UV-photolysis of surfactant.

Real-time reassembly of an ordered nanoparticle monolayer due to UV-photolysis of the surfactant shell of nanoparticles was observed. The technique of grazing-incidence small-angle X-ray scattering provided the possibility to track in situ the nanoparticle pair correlation function of the sample processed in a UV-ozone reactor. The analysis revealed a total shift of approximately 1 nm of the nanoparticle nearest-neighbor distance. The temporal evolution of the interparticle distance proved to be the first-order process governed by the UV-photolysis and described by a single-exponential decay function. The nanoparticles tend to agglomerate into a labyrinth-like structure with a typical length scale of some 30 nm.