In situ and real-time monitoring of structure formation during non-reactive sputter deposition of lanthanum and reactive sputter deposition of lanthanum nitride.

Lanthanum and lanthanum nitride thin films were deposited by magnetron sputtering onto silicon wafers covered by natural oxide. In situ and real-time synchrotron radiation experiments during deposition reveal that lanthanum crystallizes in the face-centred cubic bulk phase. Lanthanum nitride, however, does not form the expected NaCl structure but crystallizes in the theoretically predicted metastable wurtzite and zincblende phases, whereas post-growth nitridation results in zincblende LaN. During deposition of the initial 2-3 nm, amorphous or disordered films with very small crystallites form, while the surface becomes smoother. At larger thicknesses, the La and LaN crystallites are preferentially oriented with the close-packed lattice planes parallel to the substrate surface. For LaN, the onset of texture formation coincides with a sudden increase in roughness. For La, the smoothing process continues even during crystal formation, up to a thickness of about 6 nm. This different growth behaviour is probably related to the lower mobility of the nitride compared with the metal. It is likely that the characteristic void structure of nitride thin films, and the similarity between the crystal structures of wurtzite LaN and La2O3, evoke the different degradation behaviours of La/B and LaN/B multilayer mirrors for off-normal incidence at 6.x nm wavelength.

Thermal dynamics of pulsed-laser excited gold nanorods in suspension.

Photothermal reactions of metallic nanostructures, such as gold nanorods show appealing structural relaxations, such as bubble formation or particle modification. We have employed a pump-probe method to record the structural relaxations of a suspension of gold nanorods upon femtosecond laser excitation by pulsed X-ray scattering both with wide-angle and small-angle sensitivity. Single-pulse reactions include transient bubble formation at 20 J m-2 and irreversible nanorod reshaping at 30 J m-2. Thus the window for reversible excitation is very narrow. Additionally we could map the time-domain and fluence behaviour in a wide range to characterize the relaxations comprehensively. The polarized laser pulse first selectively excites nanorods aligned with the laser electric field, but at higher fluence non-aligned rods are also transformed. At low fluence this transformation happens in the solid state, while at higher fluence the rods melt.

Pulsed laser ablation in liquids: Impact of the bubble dynamics on particle formation.

Pulsed laser ablation in liquids (PLAL) is a multiscale process, involving multiple mutually interacting phenomena. In order to synthesize nanoparticles with well-defined properties it is important to understand the dynamics of the underlying structure evolution. We use visible-light stroboscopic imaging and X-ray radiography to investigate the dynamics occurring during PLAL of silver and gold on a macroscopic scale, whilst X-ray small angle scattering is utilized to deepen the understanding on particle genesis. By comparing our results with earlier reports we can elucidate the role of the cavitation bubble. We find that symmetry breaking at the liquid-solid interface is a critical factor for bubble motion and that the bubble motion acts on the particle distribution as confinement and retraction force to create secondary agglomerates.

Direct Observation of the Thickness-Induced Crystallization and Stress Build-Up during Sputter-Deposition of Nanoscale Silicide Films.

The kinetics of phase transitions during formation of small-scale systems are essential for many applications. However, their experimental observation remains challenging, making it difficult to elucidate the underlying fundamental mechanisms. Here, we combine in situ and real-time synchrotron X-ray diffraction (XRD) and X-ray reflectivity (XRR) experiments with substrate curvature measurements during deposition of nanoscale Mo and Mo1-xSix films on amorphous Si (a-Si). The simultaneous measurements provide direct evidence of a spontaneous, thickness-dependent amorphous-to-crystalline (a-c) phase transition, associated with tensile stress build-up and surface roughening. This phase transformation is thermodynamically driven, the metastable amorphous layer being initially stabilized by the contributions of surface and interface energies. A quantitative analysis of the XRD data, complemented by simulations of the transformation kinetics, unveils an interface-controlled crystallization process. This a-c phase transition is also dominating the stress evolution. While stress build-up can significantly limit the performance of devices based on nanostructures and thin films, it can also trigger the formation of these structures. The simultaneous in situ access to the stress signal itself, and to its microstructural origins during structure formation, opens new design routes for tailoring nanoscale devices.

A hierarchical view on material formation during pulsed-laser synthesis of nanoparticles in liquid.

Pulsed-laser assisted nanoparticle synthesis in liquids (PLAL) is a versatile tool for nanoparticle synthesis. However, fundamental aspects of structure formation during PLAL are presently poorly understood. We analyse the spatio-temporal kinetics during PLAL by means of fast X-ray radiography (XR) and scanning small-angle X-ray scattering (SAXS), which permits us to probe the process on length scales from nanometers to millimeters with microsecond temporal resolution. We find that the global structural evolution, such as the dynamics of the vapor bubble can be correlated to the locus and evolution of silver nanoparticles. The bubble plays an important role in particle formation, as it confines the primary particles and redeposits them to the substrate. Agglomeration takes place for the confined particles in the second bubble. Additionally, upon the collapse of the second bubble a jet of confined material is ejected perpendicularly to the surface. We hypothesize that these kinetics influence the final particle size distribution and determine the quality of the resulting colloids, such as polydispersity and modality through the interplay between particle cloud compression and particle release into the liquid.

A portable ultrahigh-vacuum system for advanced synchrotron radiation studies of thin films and nanostructures: EuSi2 nano-islands.

A portable ultrahigh-vacuum system optimized for in situ variable-temperature X-ray scattering and spectroscopy experiments at synchrotron radiation beamlines was constructed and brought into operation at the synchrotron radiation facility ANKA of the Karlsruhe Institute of Technology, Germany. Here the main features of the new instrument are described and its capabilities demonstrated. The surface morphology, structure and stoichiometry of EuSi2 nano-islands are determined by in situ grazing-incidence small-angle X-ray scattering and X-ray absorption spectroscopy. A size reduction of about a factor of two of the nano-islands due to silicide decomposition and Eu desorption is observed after sample annealing at 1270 K for 30 min.

Dynamics of silver nanoparticle formation and agglomeration inside the cavitation bubble after pulsed laser ablation in liquid.

The formation of nanoparticles within the laser-induced cavitation bubble is studied in situ using small angle X-ray scattering with high spatiotemporal resolution. Directly after laser ablation, two different particle fractions consisting of compact primary particles of 8-10 nm size and agglomerates of 40-60 nm size are formed. The abundance of these species is strongly influenced by the dynamics of the oscillating cavitation bubble. Primary particle mass is most abundant during maximal expansion of the first bubble and reappears a little weaker in the rebound. In contrast to this, the mass abundance of agglomerates is relatively low in the first bubble but strongly increases during first bubble collapse and following rebound. Although most of the ablated material is trapped inside the bubble and follows its oscillation, a minor fraction of both species could be detected outside the cavitation bubble even before its final collapse.

Asynchronous sampling for ultrafast experiments with low momentum compaction at the ANKA ring.

A high-repetition-rate pump-probe experiment is presented, based on the asynchronous sampling approach. The low-α mode at the synchrotron ANKA can be used for a time resolution down to the picosecond limit for the time-domain sampling of the coherent THz emission as well as for hard X-ray pump-probe experiments, which probe structural dynamics in the condensed phase. It is shown that a synchronization of better than 1 ps is achieved, and examples of phonon dynamics of semiconductors are presented.

Ultrafast structural dynamics of the photocleavage of protein hybrid nanoparticles.

Protein-coated gold nanoparticles in suspension are excited by intense laser pulses to mimic the light-induced effect on biomolecules that occur in photothermal laser therapy with nanoparticles as photosensitizer. Ultrafast X-ray scattering employed to access the nanoscale structural modifications of the protein-nanoparticle hybrid reveals that the protein shell is expelled as a whole without denaturation at a laser fluence that coincides with the bubble formation threshold. In this ultrafast heating mediated by the nanoparticles, time-resolved scattering data show that proteins are not denatured in terms of secondary structure even at much higher temperatures than the static thermal denaturation temperature, probably because time is too short for the proteins to unfold and the temperature stimulus has vanished before this motion sets in. Consequently the laser pulse length has a strong influence on whether the end result is the ligand detachment (for example drug delivery) or biomaterial degradation.