Attosecond streaking of photoelectron emission from disordered solids

Attosecond streaking of photoelectrons emitted by extreme ultraviolet light has begun to reveal how electrons behave during their transport within simple crystalline solids. Many sample types within nanoplasmonics, thin-film physics, and semiconductor physics, however, do not have a simple single crystal structure. The electron dynamics which underpin the optical response of plasmonic nanostructures and wide-bandgap semiconductors happen on an attosecond timescale. Measuring these dynamics using attosecond streaking will enable such systems to be specially tailored for applications in areas such as ultrafast opto-electronics. We show that streaking can be extended to this very general type of sample by presenting streaking measurements on an amorphous film of the wide-bandgap semiconductor tungsten trioxide, and on polycrystalline gold, a material that forms the basis of many nanoplasmonic devices. Our measurements reveal the near-field temporal structure at the sample surface, and photoelectron wavepacket temporal broadening consistent with a spread of electron transport times to the surface

Temporal broadening of attosecond photoelectron wavepackets from solid surfaces

The response of solids to electromagnetic fields is of crucial importance in many areas of science and technology. Many fundamental questions remain to be answered about the dynamics of the photoexcited electrons that underpin this response, which can evolve on timescales of tens to hundreds of attoseconds. How, for example, is the photoexcited electron affected by the periodic potential as it travels in the solid, and how do the other electrons respond in these strongly correlated systems? Furthermore, control of electronic motion in solids with attosecond precision would pave the way for the development of ultrafast optoelectronics. Attosecond electron dynamics can be traced using streaking, a technique in which a strong near-infrared laser field accelerates an attosecond electron wavepacket photoemitted by an extreme ultraviolet light pulse, imprinting timing information onto it. We present attosecond streaking measurements on the wide-bandgap semiconductor tungsten trioxide, and on gold, a metal used in many nanoplasmonic devices. Information about electronic motion in the solid is encoded on the temporal properties of the photoemitted electron wavepackets, which are consistent with a spread of electron transport times to the surface following photoexcitation

Intermetallic GaPd2_{2} Thin Films for Selective Hydrogenation of Acetylene

The preparation of single‐phase and catalytically active GaPd2 coatings was accomplished via DC magnetron sputtering using an intermetallic sputter target. Thin and uniform layers were deposited on borosilicate glass, Si(111) and planar as well as micro‐structured stainless steel foils. The specimens were examined regarding their phase composition, film morphology and microstructure. Thin films of different layer thickness were catalytically characterized in the semi‐hydrogenation of acetylene, which was conducted at 473 K and a feed gas composition of 0.5 vol.% C2H2, 5 vol.% H2 as well as 50 vol.% C2H4 in helium. Pre‐reduction of the catalyst was found to be essential to enhance the catalytic selectivity. Sputtered GaPd2 showed a high selectivity of 73 % for the hydrogenation to ethylene at conversion levels above 80 %. The surface‐specific activity was strongly increased to 8.97 molacetylene· (A 0· h)–1 compared to bulk‐ or nanoscale GaPd2 (1.93 and 0.30 molacetylene· (A 0· h)–1, respectively) caused by the high specific surface area of the thin films

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

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

Nanoparticle functionalized laser patterned substrate: an innovative route towards low cost biomimetic platforms

Integration of nanotechnology and advanced manufacturing processes presents an attractive route to produce devices for adaptive biomedical device technologies. However, tailoring biological, physical, and chemical properties often leads to complex processing steps and therefore to high manufacturing cost impeding further scalability. Herein, a novel laser-based approach is introduced to manufacture low cost biocompatible polymer substrates functionalized with ultrapure nanoparticles. Laser direct writing was performed to create micron-sized patterns on 188 μm-thick cyclic olefin polymer (COP) substrates using a picosecond pulsed 1064 nm Nd:YAG laser. The Pulsed Laser Ablation in Liquids (PLAL) technique was exploited in this work to prepare colloidal solutions of ultrapure nanoparticles to impart bio-functionality onto laser patterned surfaces. Combining the laser patterns and their modification with PLAL-nanoparticles resulted in a functional and biocompatible substrate for biosensing applications. Our in vitro cell viability studies using a model cell line (human skin keratinocyte, HaCaT) suggest that these nanoparticles immobilized on the surfaces function as a biomimetic platform with the ability to interact with different biological entities (e.g. DNA, antibodies etc.)

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