M. Kiel

Structural characterization of a spin-assisted colloid-polyelectrolyte assembly: stratified multilayer thin films.

The assembly of polyelectrolytes and gold nanoparticles yields stratified multilayers with very low roughness and high structural perfection. The films are prepared by spin-assisted layer-by-layer self-assembly (LbL) and are characterized by X-ray reflectivity (XRR), UV-vis spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Typical structures have four repeat units, each of which consists of eight double layers (DL) of poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride), one monolayer of gold nanoparticles (10 nm diameter), and another layer of poly(allylamine hydrochloride). XRR scans show small-angle Bragg peaks up to seventh order, evidencing the highly stratified structure. Pronounced Kiessig fringes indicate a low global roughness, which is confirmed by local AFM measurements. TEM images corroborate the layered structure in the growth direction and nicely show the distinct separation of the individual particle layers. An AFM study reveals the lateral gold particle distribution within one individual particle layer. Interestingly, the spin-assisted deposition of polyelectrolytes reduces the roughness induced by the particle layers, leading to self-healing of roughness defects and a rather perfect stratification.

Measuring the range of plasmonic interaction.

When gold nanoparticles are covered with nanometric layers of transparent polyelectrolytes, the plasmon absorption spectrum A(λ) increases by a factor of approximately three and shifts to the red. These modifications of dissipative experimental observables stop when the cover layer thickness approaches the particle diameter. Spectral modifications of dispersive parameters like the reflection R, however, keep changing with increasing cover layer thickness. The shift of the plasmon resonance caused by two interacting particle layers is studied as a function of the separating distance between the two layers. We discuss these observations in the context of an effective medium theory and conclude that it can only be applied for a layer thickness on the order of the particle diameter.

Time-resolved x-ray scattering

Ultrafast x-ray diffraction (UXRD) has become more and more prevalent in various scientific disciplines that are interested in directly observing atomic motion in real time. The timescale, amplitude and phase of collective atomic motion can be determined with high accuracy, even when the induced amplitude is smaller than thermal fluctuations. The structural rearrangements induced by an ultrafast stimulus (charge carriers excitation or heat deposition by a laser pulse) can be recorded in real time. Here we report on a new laser-driven plasma-x-ray source (PXS) and discuss different applications which will be addressed in UXRD experiments.

Ultrafast structure and polarization dynamics in nanolayered perovskites studied by femtosecond X-ray diffraction

The polarization and lattice dynamics in a metal/ferroelectric/metal nanolayer system is studied by femtosecond x-ray diffraction. Two Bragg reflections provide information on the coupled dynamics of the two relevant phonon modes for ferroelectricity in perovskites, the tetragonal distortion and the ion displacement. Optical excitation of the metallic SrRuO3(SRO) layers generates giant stress, compressing the ferroelectric PbTr0.2Ti0.8O3(PZT) layers by up to 2%. The maximum elongation of the tetragonal mode is reached after 1.3 ps. With a slight delay the ferroelectric polarization P is reduced by up to 100 percent that is due to the anharmonic coupling of the two modes.

Ultrafast structure and polarization dynamics in nanolayered perovskites studied by femtosecond x-ray diffraction

The polarization and lattice dynamics in a metal/ferroelectric/metal nanolayer system is analyzed by femtosecond x-ray diffraction. Optically induced giant stress in the metal layers can switch off the ferroelectric polarization within 2 ps.

Probing strain propagation in nanolayered perovskites by diffraction of femtosecond x-rays

Propagating strain waves in SrTiO3, launched from PbZr0.2Ti0.8O3 films are measured by time-resolved x-ray diffraction. X-ray interference among contributions from differently strained regions allow to determine absolute transient strain amplitudes down to Δ a/a = 2 · 10 −5..

Probing strain propagation in nanostructured perovskites by ultrafast x-ray diffraction

Propagating strain waves in SrTiO₃ with amplitudes of Deltaa(t)/a₀ ap 10^-4 launched from a PbZr_0.2Ti_0.8O₃ film are measured by X-ray diffraction. The shape of the picosecond transients depends nonlinearly on the strain amplitude due to X-ray interference.

Probing Strain Propagation in Nanolayered Perovskites by Ultrafast X-ray Diffraction

Propagating strain waves in SrTiO3 launched from PbZr0.2Ti0.8O3 films are measured by time-resolved x-ray diffraction. X-ray interference among contributions from differently strained regions allow to determine absolute transient strain amplitudes down to Δa(t)/a0 ≈ 2 · 10-5.