Peter Siffalovic

Laser-based apparatus for extended ultraviolet femtosecond time-resolved photoemission spectroscopy

A novel laser-based apparatus is presented utilizing high harmonic radiation for visible pump–EUV probe experiments on ultrafast processes. True femtosecond temporal resolution is achieved by a monochromator making use of dedicated narrowband multilayer mirrors rather than gratings for selection of single harmonic orders in the photon energy range between 66 and 73 eV. First applications of this new light source for electron spectroscopy on gas phase helium and xenon demonstrate the selection of a single high harmonic order with the intensity ratio between the selected and its adjacent harmonic not exceeding 10:1. A pump–probe study of hot electron production on a solid Pt(110) surface yields a cross-correlation corresponding to a temporal system resolution of 100 fs.

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.

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.

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.

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.

Real-time tracking of superparamagnetic nanoparticle self-assembly.

The spontaneous self-assembly process of superparamagnetic nanoparticles in a fast-drying colloidal drop is observed in real time. The grazing-incidence small-angle X-ray scattering (GISAXS) technique is employed for an in situ tracking of the reciprocal space, with a 3 ms delay time between subsequent frames delivered by a new generation of X-ray cameras. A focused synchrotron beam and sophisticated sample oscillations make it possible to relate the dynamic reciprocal to direct space features and to localize the self-assembly. In particular, no nanoparticle ordering is found inside the evaporating drop and near-surface region down to a drop thickness of 90 microm. Scanning through the shrinking drop-contact line indicates the start of self-assembly near the drop three-phase interface, in accord with theoretical predictions. The results obtained have direct implications for establishing the self-assembly process as a routine technological step in the preparation of new nanostructures.

Measurement of nanopatterned surfaces by real and reciprocal space techniques

Measurement of nanopatterned surfaces by real and reciprocal space techniques A newly developed laboratory grazing-incidence small-angle X-ray scattering GISAXS system capable of statistical measurements of surface morphology at the nanometer scale was developed. The potential of the GISAXS system is compared to the AFM technique for a nanopatterned silicon surface produced by ion-beam erosion. The characteristic period of the ion-beam induced ripples and their lateral correlation length were estimated from AFM. Using GISAXS the reciprocal space map of surface morphology was measured and analyzed. The two microfocus X-ray sources emitting radiation at the Cu-Kα and Cr-Kα were used. The lateral periods of ripples obtained by the reciprocal space mapping techniques match the results of real space techniques. The setup has the potential to monitor and control the deposition process and formation of nanostructures with sufficient temporal and spatial resolution.

Annealing behaviour of structural and magnetic properties of evaporated Co thin films

Cobalt thin films of 50 nm nominal thickness were e-beam evaporated on silicon substrates covered with thermal oxide. Two series of independent and cumulative vacuum annealings up to 600 ◦ C and 650 ◦ C, respectively, were performed. The x-ray diffraction, specular and non-specular x-ray reflectivity and longitudinal magneto-optical Kerr effect measurements were applied to probe the annealing behaviour of the film structure and magnetic properties. A gradual transition from the hexagonal close-packed (hcp) to the face-centred cubic (fcc) structure was observed. Evolution of the in-plane magnetic anisotropy is dominated by residual stresses which relax during the structural transformation. The coercivity follows the stress behaviour in the hcp phase up to 300 ◦ C and increases abruptly above 400 ◦ C due to improving the magneto-crystalline anisotropy in the growing fcc crystallites and enhanced surface/interface roughness. (Some figures in this article are in colour only in the electronic version)

GISAXS analysis of 3D nanoparticle assemblies?effect of vertical nanoparticle ordering

We report on grazing-incidence small-angle x-ray scattering (GISAXS) study of 3D nanoparticle arrays prepared by two different methods from colloidal solutions-layer-by-layer Langmuir-Schaefer deposition and spontaneous self-assembling during the solvent evaporation. GISAXS results are evaluated within the distorted wave Born approximation (DWBA) considering the multiple scattering effects and employing a simplified multilayer model to reduce the computing time. In the model, particular layers are represented by nanoparticle chains where the positions of individual nanoparticles are generated following a model of cumulative disorder. The nanoparticle size dispersion is considered as well. Three model cases are distinguished-no shift between the neighboring chains (AA stacking), a shift equal to half of the mean interparticle distance (AB stacking) and random shift between the chains. The first two cases correspond to vertically correlated nanoparticle positions across different chains. A comparison of the experimental GISAXS patterns with the model cases enabled us to distinguish important differences between the 3D arrays prepared by the two methods. In particular, laterally ordered layers without vertical correlation of the nanoparticle positions were found in the nanoparticle multilayers prepared by the Langmuir-Schaefer method. On the other hand, the solvent evaporation under particular conditions produced highly ordered 3D nanoparticle assemblies where both laterally and vertically correlated nanoparticle positions were found.