F. Stietz

Laser-based method for fabricating monodisperse metallic nanoparticles

An experimental method is presented for fabrication of almost monodisperse metal nanoparticles on substrate surfaces. It relies on substantial narrowing of broad size distributions through irradiation with short laser pulses by exploiting the size dependent optical absorption coefficient of the metal particles. Successive irradiation by applying two laser wavelengths completely removes the smallest clusters of the distribution and causes a size reduction of the largest particles. Finally, only clusters with diameters in a very narrow size interval remain on the surface. By using this method, Ag clusters with mean diameters of 〈d〉=10 nm and size distributions with standard deviations of Δd/〈d〉=0.13 have been prepared.

The role of surface defects in laser-induced thermal desorption from metal surfaces

Abstract Laser-induced thermal desorption of Na dimers from rough sodium surfaces adsorbed on quartz substrates has been studied. For this purpose, laser pulses with λ = 532 and 1064 nm and 7 ns duration were used. The kinetic energy distribution and the integral desorption signal of the dimers were determined as a function of the laser fluence. Measurements were also performed after reducing the surface roughness by annealing at different temperatures. The fluence dependence of the integral desorption rate exhibits a plateau which follows and precedes a sharp increase. The results indicate that the Na dimers come off preferentially from special sites of low binding energy and low coordination number. Two of these sites can be distinguished. They exhibit different annealing behavior, different binding energies and can be depleted selectively by choosing the laser fluence appropriately.

Quantitative optical determination of the shape of Cu nanocrystals in a composite film

We demonstrate that optical extinction spectroscopy can be used to determine the effective shape of Cu nanocrystals ~NCs! embedded in a transparent amorphous Al2O3 host both produced by pulsed laser deposition. The axial ratio of the NCs was extracted from the positions of the surface plasmon modes of the optical extinction spectra of the nanocomposite film. Comparison to the results obtained by grazing incidence small angle x-ray scattering shows excellent agreement. Thus, optical spectroscopy can be used as a simple, easily accessible, and versatile tool for the characterization of the NCs that form nanocomposite films.

Fundamental reactions in laser ablation of metals : defect-initiated bond breaking

Abstract The role of surface defects in laser stimulated ablation has been investigated. For this purpose, rough Na surfaces served as a model system. They were prepared by deposition of Na atoms on quartz substrates under ultrahigh vacuum conditions and exposed to laser pulses with λ =532 nm and 7 ns duration. In addition to atoms, Na dimers are detached in large quantities. The time-of-flight distributions and the integral desorption signal of these dimers were measured as a function of the laser fluence, and the desorption yield was determined for increasing numbers of successive laser pulses. Measurements have also been performed after reducing the surface roughness by annealing at different temperatures. We find that desorption of Na dimers occurs as a thermal process for the fluence range and wavelength used in the experiments. The fluence dependence of the integral desorption rate exhibits a plateau which follows and precedes a sharp increase. For constant laser fluence the integral desorption rate decreases as a function of the number of laser pulses. The results indicate that the Na dimers come off preferentially from special sites of low binding energy and low coordination number. Two of these sites, the reservoir of which is limited, can be distinguished. They have different annealing behavior, different binding energies and can be depleted selectively by appropriately choosing the laser fluence. The sites with the lowest binding energy seem to be dimers which are adsorbed on terraces of the metal surface.

Theoretical foundations for size- and shape-selective laser-based manipulation of supported metal nanoparticles

We demonstrate that laser-based thermal processing of an ensemble of metal nanoparticles on a transparent substrate can be highly selective with regard to the dimensions of the particles. The selectivity originates from the resonant dependence of the absorption cross section for surface plasmon excitation of a metal nanoparticle on its size and shape. This makes possible resonant heating by selective absorption and subsequent rapid quenching of the deposited energy by electron-phonon coupling. As a result, the temperature rise of a nanoparticle is determined by the absorbed photon energy and by the thermal properties of the substrate rather than by the heat flow between the particles, provided their number density and the laser pulse duration are properly chosen. Finally, desoprtion and diffusion activated by the temperature rise cause substantial changes of the particle size and shape. These laser-induced modifications are even more selective than laser-stimulated heating due to a threshold-like dependence of the thermally activated processes on the temperature of an individual particle. Altogether this can be exploited in a novel technique to control the size and shape distribution of supported metal nanoparticles through laser illumination in a very precise manner. Here, we present a detailed theoretical treatment of all aspects of selective laser- induced thermal processing of nanoparticles.

Laser manipulation of the size and shape of supported metal nanoparticles

Laser manipulation of the size and shape of metal nanoparticles generated by self-assembly of atoms on dielectric substrates is discussed. Techniques are presented that allow one to prepare nanoparticles with a narrow size distribution and with well-defined shape by using laser irradiation after and during particle growth. Optical spectroscopy of supported nanoparticles is demonstrated to be a very versatile tool for characterizing the particles in addition to direct imaging by scanning probe microscopy. We also show that laser manipulation of the size or shape of nanoparticles can be used to determine the homogeneous linewidth of surface plasmon excitation and thus examine the ultrafast decay time of this collective electron oscillation in nanoparticles. Prospects for future experiments in this field and applications of monodisperse nanoparticles are outlined.

Site-selective, resonant photochemical desorption of metal atoms with laser light: manipulation of metal surfaces on the atomic scale

Abstract We have investigated non-thermal desorption of metal atoms using laser light in a wide wavelength range, i.e. from 355 to 1064 nm. Rough Na and K films served as model systems for bond breaking of Na and K atoms. The results demonstrate that localized electronic excitations on metal surfaces can be exploited to desorb atoms from a well-defined binding site. The desorption rate even depends resonantly on the applied photon energy making detachment site-selective. We also find that the reservoir from which atoms are released can be depleted and refilled repeatedly. Furthermore, the binding site depleted by desorption could be identified as fourfold coordinated adatoms on terraces by comparing the experimental data to the results of relativistic density functional calculations. The conclusions of this paper are not only of interest for the understanding of localized electronic states at metal surfaces but should also have essential impact for reducing the roughness of metal surfaces by manipulating and smoothing the topography with laser light on the atomic scale.

Ultrafast electron dynamics in metal nanoparticles: decay times of surface plasmon excitation

Summary form only given. We present results obtained by a recently developed technique to determine the homogeneous linewidths of surface plasmon resonances of metal nanoparticles and thus measure T/sub 2/. The method is based on spectral hole burning in the inhomogeneously broadened absorption profiles of metal nanoparticles and has been applied to supported oblate Ag clusters with radii of only few nanometers. From the experimental results and a theoretical model of hole burning, decay times of as little as 3.5 fs have been obtained. This value is shorter than expected for damping by bulk electron scattering. We conclude that additional damping mechanisms, in particular surface scattering, come into play if the electrons are confined in particles with sizes below 10 nm. Furthermore, an influence of the shape of the clusters on the decay time was observed.

Shaping metallic nanostructures and their optical properties through irradiation with short laser pulses

We present experiments which demonstrate that post growth laser irradiation of metallic nanostructures, prepared by deposition of atoms onto substrate surfaces, can be used to narrow their size distributions considerably and to fabricate monodisperse particles with high number density on surfaces. For this purpose, the size dependence of the plasmon frequency, which dominates the optical absorption of the nanoparticles, is exploited for size selective laser evaporation of atoms from the cluster surfaces. If laser irradiation is not performed after growth but during preparation, particles with well defined and predetermined shape can be prepared by exploiting the pronounced shape dependence of the plasmon frequency. Both methods have been applied to Ag particles on quartz with radii between 0.5 and 5 nm. The drastic narrowing of their size distributions and the fabrication of well defined shapes is not only visible in scanning force microscopy images but is also reflected in the optical spectra. Tailoring the size and shape of the clusters implies tailoring the width and the maximum frequency of their optical absorption.

Decay times of surface plasmon excitation in metal nanoparticles determined by laser-induced persistent spectral hole burning

We describe a novel technique to determine the homogeneous linewidths of surface plasmon resonances of metal nanoparticles in the presence of inhomogeneous broadening and thus measure the decay time T/sub 2/. The method is based on spectral hole burning in the inhomogeneously broadened absorption profiles of metal nanoparticles and has been applied to supported oblate Ag clusters with radii of 7.5 nm. From the experimental results and a theoretical model of hole burning the linewidth of 260 meV corresponding to a decay time of 4.8 fs was extracted. This value is shorter than expected for damping by bulk electron scattering. We conclude that additional damping mechanisms, in particular surface scattering, come into play if the electrons are confined in particles with sizes below 10 nm. Furthermore, an influence of the shape of the clusters on the decay time was observed.