Self-assembled Spherical Metal Particles by Pulsed UV Laser Treatment of Thin Films
The fabrication of nanospheres by pulsed UV excimer laser treatment under ambient conditions is a fast fabrication technique. Nanoparticles can be generated as pure or multi-material systems. Gold, silver, and palladium are of special interest for plasmonics and catalysis. Using optimal irradiation parameters, spheres with a size distribution ranging from tens of nm up to several µm can be produced.
By G. Schmidl // G. Jia // J. Dellet // A. Gawlik // Z.-H. Lin // Y.-J- Chen // J.-S. Huang // J. Plentz
Nanoparticles and nanostructured surfaces play an increasingly important role in a wide variety of technology areas such as spectroscopy, photon management, biophotonics, and biomedicine. Their use in surface-enhanced Raman spectroscopy (SERS), in optical information storage, as metal nanoresonators, or for a higher yield of solar energy requires both special nanoparticle properties and manufacturing methods that allow a fast, cost-effective, large-area or spatially-resolved preparation of nanoparticles.
With the bottom-up technology – the chemical synthesis of nanoparticles – that is often used, the composition of materials is often limited. Multi-component particles or even alloys cannot be produced. Complex synthesis steps, which also include the shell creation for stabilization of the particles, make it possible to use them for bioanalysis only after successful removal of the shell. Top-down methods, on the other hand, are very costly due to the use of cleanroom methods. However, defined structures, in size and shape, can be prepared at special positions that do not require a chemical shell for stabilization.
With the development of laser-assisted nanoparticle production, a path has been taken which, under normal room conditions, allows the production of nanoparticles starting from a metallic thin layer with high throughput and without a chemical shell. In addition, the particle size can be specifically controlled via the laser parameters and the initial layer thickness.
The KrF excimer laser (λ=248 nm) used in the experiments emits ns pulses in the UV range with high energy and is, therefore, ideally suited for melting thin metal layers. Due to its pulse length in the ns range, the laser induces fast thermal effects from heating up to melting and ablation according to the laser energy, in contrast to furnace annealing. These fast processes have a favorable effect on the use of inexpensive glass substrates, since such substrates have a softening point of approx. 600°C and are, therefore, not suitable for higher temperatures over longer time scales.
This short melting process is followed by a rapid cooling process. This rapid solidification phase after melting can suppress phase separation (e.g., when using multi-metal layers). Thus, with the laser-induced production of nanoparticles, the generation of multi-component or alloy particles is possible. The first results were obtained for the palladium-gold system. The process kinetics mentioned can be influenced by such laser parameters as energy density, pulse number, and the substrate material or coating parameters.
The first experiments showed that for the Pd/Au bilayer system and optimal laser parameters round and homogeneously-distributed nanoparticles with a bimodal particle size distribution can be produced on a low-cost borofloat glass (see Figures (A) and (B)).
The extinction spectra of the laser-induced nanoparticles produced clearly show a resonant excitation of plasmons, in which the full width at half maximum and the resonance maximum are influenced by the laser parameters (see Figure (C)). This behavior speaks for an adjustable particle size and size distribution of the particles.
With this method of laser-induced nanoparticle production, not only can large areas of different substrates be coated with nanoparticles, but an automated process can also be used to start the self-organization process in a position-bound manner. Thus, it is possible to write structures or to place the nanoparticles at certain locations with a previously structured layer. The customized, plasmonically active nanoparticles open up possibilities in the field of spectroscopy, analytics, and catalysis.