Fabrication and spectroscopic investigation of branched silver nanowires and nanomeshworks
© Zhang et al.; licensee Springer. 2012
Received: 3 September 2012
Accepted: 21 October 2012
Published: 27 October 2012
Wide wavelength ranges of light localization and scattering characteristics can be attributed to shape-dependent longitude surface plasmon resonance in complicated nanostructures. We have studied this phenomenon by spectroscopic measurement and a three-dimensional numerical simulation, for the first time, on the high-density branched silver nanowires and nanomeshworks at room temperature. These nanostructures were fabricated with simple light-induced colloidal method. In the range from the visible to the near-infrared wavelengths, light has been found effectively trapped in those trapping sites which were randomly distributed at the corners, the branches, and the junctions of the nanostructures in those nanostructures in three dimensions. The broadened bandwidth electromagnetic field enhancement property makes these branched nanostructures useful in optical processing and photovoltaic applications.
Noble metal nanostructures supporting surface plasmons arising from the coherent oscillations of the conduction electrons under the radiation of the incident light have been widely investigated [1–24]. The optical spectral signatures of the plasmonic nanostructures are mainly dependent on the distribution of the electromagnetic field on the surface of the metal nanostructures. When the size of the metal nanostructures is smaller than the wavelength of the incident light, they support localized surface plasmon resonance (LSPR) at particular resonance wavelengths . The LSPR properties of the plasmonic nanostructures are size and shape dependent [1–3]. With the increase of the size of the metal nanostructures, high-order resonance occurs owing to the surface plasmon polariton (SPP) mode interferences. For example, metal nanowires support the Fabry-Perot (FP) modes , micron-size metal nanoplates support the whispering gallery modes , and the flag type of metal nanostructure supports competitive LSPR and FP modes simultaneously . These metal nanostructures with regular shapes usually exhibit narrow-band resonance characteristic seen from their extinction spectra.
As well known, on the contrary to those elemental nanoparticles, metal nanostructures with complicated geometries, such as nanochains and branched nanowires assembled or fused by the elemental nanoparticles, show enhanced light-trapping ability [8–24]. When the metal nanoparticles are in close proximity or jointed together, the intensity of the localized electromagnetic field at the corner area and the junctions of these nanostructures are dramatically enhanced owing to the existing plasmon coupling between different propagation modes or resonance modes [8, 9]. It is usually accompanied by obvious redshift and extension of their resonance bands [3, 9]. The overall optical properties including obvious light-trapping and scattering enhancement in a wide wavelength range make these complicated metal nanostructures highly desirable in improving the performance of bio-chemical sensors [21, 22], solar cells [24, 25], and wide-bandwidth light-emitting diodes . Although considerable progress has been made in the fabrication methods of such complicated metal nanostructures, such as laser welding [8–10], sintering triggered thermally  or chemically , electrodeposition , microwave irradiation , and chemical synthesis methods [15–22], it is still a challenge to fabricate high-yield metal nanostructures with excellent light-trapping property in a wide wavelength range to meet the practical demands for a large-scale production.
In this study, we present a simple and facile light-induced colloidal method to fabricate high-density branched silver nanowires and nanomeshworks. These complicated nanostructures showed significant light-trapping and scattering properties in the wide range from the visible to the near-infrared wavelengths, arising from the shape-dependent longitude surface plasmon resonance. The broadened bandwidth electromagnetic field enhancement characteristic makes these nanostructures useful in optical processing and photovoltaic applications.
The fabrication process of the branched silver nanowires and nanomeshworks includes two steps.
Synthesis of the silver seeds
The first step is the synthesis of the silver seeds. A 5-mL amount of 0.5 M trisodium citrate, 0.5 mL of 0.05 M l-arginine, 0.15 mL of 0.5 M polyvinylpyrolidone (PVP), and 0.2 mL of 0.5 M silver nitrite were added to 7 mL of deionized water while stirring slowly. Then, 2 mL of 0.4 M sodium borohydride (NaBH4) was added to the solution slowly drop by drop. The color of the silver seed solution changed to dark brown rapidly.
Fabrication of the branched silver nanowires and nanomeshworks
Results and discussion
Compared with the conventional silver nanowire solution, silver solution prepared in our way containing branched nanostructures shows greatly enhanced light-trapping and scattering characteristics in a wide wavelength range. The two-step light-induced chimerical colloidal method used here was first reported by Pietrobon and Kitaev for the synthesis of decahedral silver nanoparticles . The scheme of the light exposure process is shown in Figure 1a. Surprisingly, silver nanostructures with distinct morphology and optical spectral signatures were found when we increased the material concentration by 100 times higher. Figure 1b shows the extinction spectra of the silver solution measured by a fiber-optic spectrometer (PG2000, Ideaoptics Technology Ltd., Shanghai, China) and the photographs of the solution samples at different exposure times. The first step is the synthesis of the silver seed solution. As shown in the inset of Figure 1b, the color of the diluted silver seed solution (exposure time is 0 h) is light yellow. As shown in the extinction spectrum, a narrow resonance band at 420 nm corresponding to the LSPR wavelength of typical silver nanospheres with a diameter of several nanometers  can be observed. The second step is the light-induced regrowth process of the silver nanostructures which is highly dependent on the material concentration. At a low concentration as reported in , decahedral silver nanoparticles with uniform sizes and shapes are the ultimate product. However, when the material concentration was increased by 100 times, the ultimate products containing mainly the branched nanowires and nanomeshworks can be observed. As seen from the extinction spectra, the resonance band at 420 nm decreased, and the absorption ratio in the longer wavelength increased obviously with the increase of the exposure time. As the welded metal nanobranches and nanomeshworks increased, the color of the solution darkened rapidly with the extension of the exposure time, which is shown in the inset of Figure 1b. After a 5-h exposure procedure, the color of the solution turned to black which is obviously different from silver nanoparticles with regular shapes, such as bared nanowires  or nanoplates . The optical spectra and the photographs of the solution are strong evidences to show that the ultimate silver nanostructures can trap light effectively in a broad wavelength range from the visible to the near-infrared wavelengths.
To investigate the light-trapping property of these nanostructures, we use three-dimensional FEM to simulate the electric field distribution of the branched nanostructures. Rectangular wave incoming from the upper x y plane was set as the source boundary. The other outer boundaries were set as the scattering boundary condition. The cladding layer surrounding the silver nanostructures is water with a refractive index n = 1.33. The dispersion coefficient of water is neglected in the simulation. The dielectric constant of silver is taken from .
The above simulation results indicate that such randomly constructed silver nanostructures with rough surfaces and multi-branches have high light-trapping efficiency, in accordance with the measured extinction spectra.
In this study, we introduce a simple chemical method for the fabrication of high-density branched silver nanostructures. Both experimental measurement and relative three-dimensional numerical simulation results show that these nanostructures have significant properties as follows: First, these branched silver nanostructures have significant light-trapping and scattering properties in a broad wavelength range. Second, light can propagate in a longer distance along the waveguide-like plasmonic nanostructures to improve the interaction between the surrounding materials and light. Third, the fabrication routes with easy operation and high yields meet the satisfaction of the large-scale production. These properties make such nanostructures useful in practice applications, such as photovoltaics, bio-chemical sensing, and optical processing.
This work is supported by NSFC under grant number 60977038, Doctoral Fund of Ministry of Education of China under grant number 20110092110016, the National Basic Research Program of China (973 Program) under grant number 2011CB302004, the Scientific Research Foundation of Graduate School of Southeast University under grant number YBPY1104, and the Foundation of Key Laboratory of Micro-Inertial Instrument and Advanced Navigation Technology, Ministry of Education, China.
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