In this contribution, we present new tailor-made substrates for surface-enhanced Raman scattering (SERS). They are based on precise control of the mean shape and the average diameter of nanoparticles prepared by self-assembly of atoms on dielectric supports. Tailoring of the SERS substrates have been achieved by precise tuning of the localized surface plasmon polariton resonance of silver nanoparticles to the vicinity of the laser wavelength used for SERS excitation. The
underlying method relies on control of the growth kinetics of supported metal nanoparticles which causes a pronounced shape change as a function of particle size. Additionally, the strong dependence of the energetic position of surface plasmon excitation on the shape of the particles is exploited. With this preparation method, SERS substrates with optimized plasmon resonances and field enhancement can be easily produced for specific excitation wavelengths and detection ranges. The nanoparticles have been characterized by optical spectroscopy and atomic force microscopy (AFM). Silver nanoparticles with a plasmon resonance at about 2.4 eV were prepared with and without a protective CaF2 coating. SERS spectra of pyrene were obtained with excitation at 514.5 nm. They exhibit a good reproducibility. Furthermore, the substrates did not show degradation during the measurements and those with protective coating still yielded 70% of the SERS intensity of uncoated substrates, indicating their potential usefulness for an analytical detection of specific molecules. Further tailoring of supported metal nanoparticles for SERS applications by laser irradiation will be discussed.
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.