Modification of metal nanoparticles with laser light has been a well-known technique for several years. Still, selective tailoring of certain sizes or shapes of nanoparticles has remained a challenge. In this paper, we present recent studies on tailoring the size and shape of supported nanoparticles with continuous-wave and femtosecond
pulsed laser light and compare them to our results obtained with ns pulsed laser light. The underlying method is based on the size and shape dependent plasmon resonance frequencies of the nanoparticles. In principle, irradiation with a given laser photon energy excites and heats nanoparticles of certain sizes or/and shapes and leads to diffusion and evaporation of surface atoms. Thus, tailoring the dimensions of the nanoparticles can be accomplished. In our experiments, gold and silver nanoparticles were prepared under ultrahigh vacuum conditions by deposition of atoms and subsequent diffusion and nucleation, i.e. Volmer-Weber growth. This gives particle ensembles with size and shape distributions of approximately 30% - 40%. The nanoparticle ensembles were irradiated with laser light either during or after growth. It turns out, that irradiation with cw or ns laser light makes possible selective modification of the nanoparticles. In contrast, application of fs laser pulses results in non-selective modification. For example, post-grown irradiation of supported gold nanoparticles with ns laser pulses (photon energy = 1.9 eV) causes a clear reduction of the width of the surface plasmon resonance from 0.52 eV to 0.20 eV (HWHM). Similar experiments were carried out with fs pulsed laser light (photon energy = 1.55 eV), which result in a slightly reduced line width but also, to an overall decrease of the extinction. A
comparison of all experiments revealed, that for size or shape tailoring of supported metal nanoparticles best results have been achieved with ns pulsed laser light.
Surface plasmon excitation in metal nanoparticles has found great interest in the past. This collective oscillation of the conduction electrons can be stimulated with light and is characterized by resonances, whose positions depend on the material, the dimensions and the dielectric surrounding of the particles. Recently, the investigation of the ultrafast dephasing time T2 and of the decay mechanisms of surface plasmon excitation have become of particular importance, an essential reason being that T2 is proportional to the enhancement factor of the electric field in the vicinity of the nanoparticle surface. This enhancement plays a key role in a great variety of applications. The present paper presents an overview of our recent experiments on the ultrafast decay of surface plasmon excitation, in particular by using a technique that allows us to measure the homogeneous line widths of surface plasmon resonances in the presence of inhomogeneous broadening and thus determine T2. The method is based on persistent spectral hole burning in the absorption profiles of supported metal nanoparticles by nanosecond laser pulses. The technique has been systematically applied to silver and, more recently, to gold nanoparticles on different substrates. Size and shape dependent dephasing times ranging from 2.6 to 15 fs have been extracted from the experimental results using a theoretical model. The values reflect the reduced dimensions of the nanoparticles and we conclude that additional damping mechanisms, in particular surface scattering and chemical interface damping, come into play.
In this contribution, we present measurements of the ultrafast dephasing time T2 of surface plasmon polariton excitation in gold nanoparticles by means of persistent spectral hole burning. T2 is an essential parameter that does not only reflect the role of different dephasing and deexcitation mechanisms but also allows one to determine the field enhancement factor that is of great importance for many applications of nanoparticles. In our experiments gold nanoparticles were first fabricated in ultrahigh vacuum on sapphire substrates by deposition of atoms, followed by diffusion and nucleation, i.e. Volmer-Weber growth. Subsequently, systematic measurements of T2 in the size range between r = 7 nm and 14 nm were carried out. The most essential among the numerous results is the observation of the influence of the reduced dimension on the dephasing time. While T2 = 14 fs has been measured for r = 12 nm which is, within the error bars, consistent with the damping
contained in the bulk dielectric function, the value of T2 shrinks to, for example, T2 = 11 fs for r = 7 nm. This reduction of T2 can be attributed to surface scattering of the electrons. Further experiments are in progress to confirm the predicted 1/r law for the variation of T2.
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 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.
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