Self-organized Silver Research Articles

Laser-induced linear dichroism in planar self-organized silver nanostructures

A method for obtaining metallic plasmonic nanostructures with linear dichroism, based on the method of burning out constant spectral dips, is proposed. Isotropic granular silver films obtained by physical vapor deposition in vacuum were irradiated with linearly polarized laser radiation in the spectral region of the plasmon resonance of their constituent nanoparticles. As a result of irradiation, the silver nanostructures change their sizes and shapes, and the films acquire a pronounced linear dichroism. Both the magnitude and the spectrum of linear dichroism depend on the state of the isotropic film before irradiation, which can be changed by heat treatment. For the unannealed films the dichroism does not change the sign over the entire spectral range studied and corresponds to the expected increase in the depth of the spectral dip for the light polarized parallel to the laser radiation polarization plane. For the annealed films, which consist of more distinctly formed and better separated nanoparticles, the dichroism value is greater, and the spectrum turns out to be sign-changing. The appearance of linear dichroism after laser irradiation is due to the differences in the change in the shape and size of the initially anisotropic nanoparticles that make up an isotropic film as a whole, depending on their orientation relative to the polarization plane of the laser beam. Keywords: plasmon resonance, silver nanostructures, linear dichroism, laser radiation.

Enhanced water splitting with silver decorated GaN photoelectrode

By means of a cost-effective approach, we demonstrate a GaN-based photoelectrode decorated with self-organized silver nano-islands employed for solar powered hydrogen generation, demonstrating 4 times increase in photocurrent compared with a reference sample without using any silver. Our photoelectrode exhibits a 60% incident photon-to-electron conversion efficiency. The enhanced hydrogen generation is attributed to a significantly increased carrier generation rate as a result of strongly localized electric fields induced by surface plasmon coupling effect. The silver coating also contributes to the good chemical stability of our photoelectrode in a strong alkali electrolyte. This work paves the way for the development of GaN and also InGaN based photoelectrodes with ultra-high solar hydrogen conversion efficiency.

Silver nanostructures formation in porous Si/SiO2 matrix

Self-organized silver nanostructures were grown in porous Si/SiO2 matrix fabricated by ion track technology. The different silver nanostructures with shapes like “sunflowers”, “azalea” or “corn” were realized by applying wet-chemical electroless deposition. We show that reproducible self-organized silver “sunflower” like nanostructures provide a high enhanced Raman signal of Nile blue dye molecules. Signal enhancement for a few or even just a single silver “sunflower” is demonstrated by analyzing the surface-enhanced Raman signature of Nile blue dye molecules. According to this, the silver nanostructures can act as efficient surfaces for surface enhanced Raman spectroscopy as well as (bio)-sensor applications.

Formation of Self-organized Silver Nanocup-Type Structures and Their Plasmonic Absorption

The present work reports on the formation of extremely low volume, silver nanocup-type structures on the surface by annealing of ultra-thin silver film on quartz in inert environment. Atomic force microscopy studies together with scanning electron microscopy confirmed the formation of Ag nanocup-type structures at the surface. A basic physical model for the formation of nanocups in terms of buckling and Oswald ripening due to surface-induced morphological instability and diffusional mass transport under thermal treatment is demonstrated. Surface plasmon resonance absorptions of nanocup structures are studied and preliminary experiment for observing the surface-enhanced Raman scattering of fullerene C70 molecules has been shown.

Conductive adhesives with self-organized silver particles

Common available conductive adhesives are filled with about 70-90 weight percent of silver particles. There is the technical need to lower the filler content for decreasing the viscosity and the pressure to finally decrease the costs for the highly expensive silver. Our approach to decrease the silver content with at the same time similar conductivity was to use nano-sized silver particles with different modified surfaces to obtain self-organized distribution of the silver particles in the liquid adhesive film followed by conventional curing techniques like temperature or UV curing. Through the self-organization, a bridging of the particles should be achieved which gives a similar conductivity as in highly filled commercial adhesives which follows percolation theory. The influence of the surface modification on the strength of the interaction between differently modified silver particles and the distribution in the liquid adhesive was simulated with Dissipative Particle Dynamics (DPD). Surface modification was verified with surface enhanced Raman spectroscopy (SERS) and X-ray photoelectron spectroscopy (XPS). The distribution of the silver particles in the cured adhesive film was investigated with scanning electron microscopy (SEM).

Surface-enhanced absorption by self-organized silver films with aciniform-like nanoaggregates at elevated temperatures

Morphological evolution of silver nanocomposite films prepared by the wet colloidal route and surface-enhanced phenomena on aggregate nanostructures evolved during annealing were investigated. Dramatic changes in morphologies of particles and pores incurred by rearragement, coarsening, premelting, and dewetting of the silver clusters at different concentrations (i.e., mass thicknesses). At a higher mass thickness, the morphological transitions from self-organized nanoaggregates with aciniform pattern at 300 °C to elongated and coarsened particles with circular holes at 400 °C to island clusters at 500 °C occurred in the films. The peculiar absorption with a much redder and broader surface plasmon feature, which gone far beyond the theoretical prediction, induced by the formation of aciniform nanoaggregates embedded in the porous polymer matrix at a critical mass thickness of 9.6 nm during partial degradation of the PVP polymer and rearrangement of silver clusters at 300 °C. The surface-enhanced absorption was dramatically reduced by the elemination of the aggregate nanostructures and the spontaneous formation of the silver nanoisland film at the dewetting temperature of 500 °C.

Self-Organized Silver Nanoparticles for Three-Dimensional Plasmonic Crystals

Metal nanostructures that support surface plasmons are compelling as plasmonic circuit elements and as the building blocks for metamaterials. We demonstrate here the spontaneous self-assembly of shaped silver nanoparticles into three-dimensional plasmonic crystals that display a frequency-selective response in the visible wavelengths. Extensive long-range order mediated by exceptional colloid monodispersity gives rise to optical passbands that can be tuned by particle volume fraction. These metallic supercrystals present a new paradigm for the fabrication of plasmonic materials, delivering a functional, tunable, completely bottom-up optical element that can be constructed on a massively parallel scale without lithography.

Vibrational coherence of self-organized silver nanocrystals in f.c.c. supra-crystals

Fabrication of devices from inorganic nanocrystals normally requires that they are self-organized into ordered structures. It has now been demonstrated that nanocrystals are able to self-organize in a 'supra'-crystal with a face-centred cubic (f.c.c.) structure. The physical properties of nanocrystals self-organized into compact arrays are quite different from those of both isolated nanocrystals and the bulk phase. The collective optical and magnetic properties of these nanocrystal assemblies are governed mainly by dipolar interactions. Here, we show that nanocrystals vibrate coherently when they are self-organized in f.c.c. supra-crystals. Hence, a phase relation exists between the vibrations of all of the nanocrystals in a supra-crystal. This vibrational coherence can be observed by a substantial change of the quadrupolar low-frequency Raman scattering peak. Although a change in electronic transport properties has previously been observed on self-organization of silver nanocrystals, vibrational coherence represents the first intrinsic property of f.c.c. supra-crystals.

Vibration of self-organized silver nanocrystals

Low-frequency Raman spectra are performed on silver nanocrystals self-organized on a hexagonal compact array on highly oriented pyrolytic graphite (HOPG) substrate. In addition to the well-known Raman scattering by the nanoparticle quadrupolar vibration mode, two lower intensity bands at larger frequency shifts are observed. They are attributed to acoustic phonons confined in the nanocrystals corresponding to the fundamental breathing mode and its first harmonic. Their observation was made possible because of the high crystallinity and the narrow size distribution of the silver nanoparticles. The coated particles organized in 2D superlattices appear thus to behave as isolated particles dispersed in matrix.

Fabrication and physical properties of self-organized silver nanocrystals

Abstract A simple method is used to prepare highly monodispersed silver nanoparticles in the liquid phase, which starts from an initial synthesis in functionalized AOT reverse micelles. To narrow the particle size distribution from 43 to 12.5% in dispersion, the particles are extracted from the micellar solution. The size-selected precipitation method is used. The nanocrystallites dispersed in hexane are deposited on a support. A monolayer made of nanoparticles with spontaneous compact hexagonal organization is observed. The immersion of the support on the solution yields to the formation of organized multilayers arranged as microcrystal in a face-centered-cubic structure. We compare the optical properties of spherical particles organized in a two- and three-dimensional structure with isolated and disordered particles. When particles, deposited on cleaved graphite, are arranged in a hexagonal array, the optical measurements under p-polarization show a new high-energy resonance, which is interpreted as a collective effect, resulting from optical anisotropy due to the mutual interactions between particles. We support this interpretation by numerical calculations performed for finite-size clusters of silver spheres. For disordered particles, a low-energy resonance appears instead of the high-energy resonance observed for spherical and organized particles. This is interpreted as optical shape anisotropy due to the asymmetrical arrangement of particles. The tip of a scanning tunneling microscope (STM) may be used as an extremely localized source of low-energy electrons to locally excite photon emission from a variety of metal films. The detection of locally excited luminescence at the junction of an STM tip provides access to electron dynamic properties at the surface, which makes it possible to study luminescence phenomena of nanometer-sized structures. The photon intensity emitted from electrically isolated silver nanoparticles self-organized as a 2D network on a gold (111) substrate is analyzed. We observed unexpectedly strong variations of photon-emission efficiency from isolated nanoparticles, depending on how tightly they are embedded within the network site. The quenching site observed in the STM photon emission map is interpreted as an enhanced interaction of electrons with surface photon modes.