Laser-Induced, Polarization Dependent Shape Transformation of Au/Ag Nanoparticles in Glass
Bimetallic, initially spherical Ag/Au nanoparticles in glass prepared by ion implantation have been irradiated with intense femtosecond laser pulses at intensities still below the damage threshold of the material surface. This high-intensity laser processing produces dichroism in the irradiated region, which can be assigned to the observed anisotropic nanoparticle shapes with preferential orientation of the longer particle axis along the direction of laser polarization. In addition, the particle sizes have considerably been increased upon processing.
KeywordsAlloy nanoparticles Glass Laser irradiation Femtosecond laser processing Dichroism
Nano-sized metal particles embedded in glass are of great interest because of their potential application as non-linear material for photonic devices [1, 2]. The non-linear properties of nanocomposite glasses equipped with such particles are induced by the surface plasmon resonance at the interface between particles and glass matrix. This means that the optical effects in the spectral region around the surface plasmon resonance result from an electric field enhancement or a quantum confinement. Thus, applications are possible as in integrated photonic networks, in nanoelectronics, for surface enhanced Raman scattering, for up-conversion processes and laser materials. Recently, the preparation of specific bimetallic nanoparticles like core-shell structures has been intensively investigated because of the far-reaching possibilities to modify the macroscopic properties [3–6]. A special way to extend the range of manipulating the optical properties of such nanocomposite glasses can be achieved by a development of central voids within the particles. There are known some first examples for hollow nanoparticles in glass that were prepared by sequential implantation of two different metal ions [6–8].
A further degree of freedom introducing anisotropic optical properties is the method of femtosecond laser pulse-induced shape transformation of the nanoparticles which has been studied intensively in recent years [9–11]. Depending on the actual irradiation parameters, uniformly oriented prolate or oblate nanoparticles can be prepared, whose orientation is controlled by the laser polarization . So far these effects have been demonstrated for Ag nanoparticles at low concentration. In this Letter, we will demonstrate that a similar shape modification of initially spherical bimetallic and hollow Ag/Au nanoparticles in soda-lime glasses is possible by irradiation with femtosecond laser pulses at high intensity, but below damage threshold; in particular, it will be shown that anisotropic particle shapes or nearly linear arrangements of nanoparticles with preferential orientation along the direction of laser polarization can be fabricated with the help of this irradiation technique.
The samples used for this study were sheets of soda-lime glass containing (in mol%) 72.4% SiO2 and 14.4% Na2O as main components, which were exposed subsequently to Au+ (150 keV) and Ag+ (100 keV) ion implantation at room temperature. By this sequential high-dose ion implantation of Ag+ and Au+, metal particles have been formed in a surface-near region of the soda-lime silicate glass. The dose of implanted ions was 4 × 1016 ions/cm2 for each type of ions (for further details see ). To characterize the surface plasmon resonance due to the metal nanoparticles formed in the implanted areas the optical density of glass samples was recorded by means of a Perkin-Elmer spectrometer in the wavelength range of 250–900 nm. These samples were irradiated by linearly polarized laser pulses of 150 fs temporal width at a wavelength λ = 550 nm. This wavelength is the sum frequency of a 1 kHz repetition rate Ti:Sapphire laser at λ = 800 nm and the idler (λ = 1,760 nm) of a Travelling-wave Optical Parametric Amplifier of Superfluorescence (TOPAS). The laser beam was focused on the sample to a spot size of ~100 μm. Moving the sample continuously on a motorized X–Y translation stage, several parallel lines of ~1.5 mm length and 150 μm lateral distance have been inscribed in the glass at a velocity of 0.5 mm/s, corresponding to, on average, 200 laser pulses hitting each spot within the lines. The polarization direction of the laser was parallel to the lines (writing direction).
Finally, we want to discuss briefly which physical mechanisms may lead to the observed shape changes of bimetallic nanoparticles upon intense fs laser irradiation, in particular explaining the preferential orientation of elongated particles more or less arranged parallel to the laser polarization. The shape changes observed here can be compared to similar previous results obtained on two quite different types of metal-dielectric nanocomposites. The first type of material is glass containing low concentration of Ag nanoparticles. For such systems it has been found that only the individual particle and its immediate surroundings are affected by laser-induced modifications. The sequence of processes there starts with field-driven electron emission from the particle, followed by electron trapping in the glass matrix, ion emission, their local recombination with trapped electrons, and diffusion and precipitation of Ag atoms at the poles of the particle in the transiently heated nearest shell of the matrix . The second type of material studied previously is plasma polymer embedding a quasi 2-dimensional metal island film. Irradiating these systems with similar parameters as in this work, a grating-like superstructure oriented along the laser polarization with a typical period of 2/3 of the laser wavelength has been observed, where stripes of unchanged metal nanostructure (percolation region) are alternating with stripes of coagulated larger, spherical particles . The explanation for these self-organized structures comprises spatially modulated energy input in the metal layer by interference of the incoming laser light with the scattered surface wave, where statistical inhomogeneities of the sample provide a feedback, so that the structures are becoming more pronounced and regular shot by shot.
The shape changes reported here are somehow intermediate between the abovementioned limiting cases. Comparing the situation with the plasma polymer samples, we here also observe a large increase of particle sizes, but no regular superstructure. Comparing with isolated Ag nanoparticles in glass, we also find individual non-spherical shapes with the long axes oriented preferentially along the laser polarization; but, in addition, particles are growing and sometimes merging, preferentially in situations where together they form a coagulated particle with longer axis along the laser polarization.
From these considerations we conclude that the laser-induced shape changes of bimetallic nanoparticles are initiated by the same mechanisms as in the case of low-concentrated Ag nanoparticles, i.e., directional electron emission and capture in the glass matrix, followed by the processes listed above. The main difference appears to come from the higher metal concentration leading to spatially and temporally more extended regions of high temperature around metal particles in the matrix, enabling much larger diffusion distances of electrons and metal ions or atoms. It cannot be decided at present if, in addition to the basic mechanism of particle reshaping during laser irradiation (migration of individual atoms or ions), also processes like migration and coalescence of very small particles contribute to the observed particle growth. The reason is that all processes are started by an ~100 fs laser pulse; then the surrounding glass is heated within a few ps and cools down again by heat conduction within a few ns. Particle formation or growth under such strongly non-equilibrium conditions has, to the best of our knowledge, so far not been modeled theoretically.
Still, however, the local trapping sites for electrons in glass are obviously a necessary prerequisite for shape anisotropy of the particles. This is confirmed by the lack of similar shape anisotropy of the metal particles after fs irradiation in plasma polymers. Furthermore, the temperature increase during laser irradiation within the particle regions explains the transformation of hollow particles into solid ones because of their thermal instability [17, 18]. The vacancies leave the central void toward the outer surface and the hollow region disappears at elevated temperatures. The laser-induced changes of shape and configuration of nanoparticles described above can also explain the slight blue shift of the surface plasmon resonance observed for perpendicularly polarized light as well as the lacking red-shift for parallel polarization (as shown in Fig. 3). From the reduced total optical density after laser treatment, one can conclude a reduced concentration of precipitated particles compared with the nanocomposites before irradiation. That is, obviously the recombination of emitted ions with trapped electrons is not completed. While, however, the probability for emission during interaction with an ultrashort laser pulse is the same for Au and Ag ions, the mobility of Au in the glass matrix is considerably less than that of Ag species. This difference should also affect the amount of both elements being incorporated into the particles again; so in the end the concentration ratio in the particles will be shifted toward Ag atoms, and this will shift their plasmon resonances toward shorter wavelengths.
In conclusion, we have shown that spherical, bimetallic Au/Ag nanoparticles in glass at high concentration can be transformed to anisotropic shapes (accompanied by size increase) preferentially oriented along the direction of the linear laser polarization. The mechanism appears to be similar to that observed for silver nanoparticles in glass at low concentration, but with additional effects like coalescence caused by the close proximity of the particles. Overall, the demonstrated high-intensity laser processing is a promising and flexible technique to design the linear and non-linear optical properties of metal-glass nanocomposites.
The authors would like to thank the Institute of Solid State Physics of the Friedrich Schiller University of Jena for implantation of glass samples.