From silver nanoparticles to thin films: Evolution of microstructure and electrical conduction on glass substrates

doi:10.1016/j.jpcs.2008.11.012

Journal of Physics and Chemistry of Solids

Volume 70, Issue 2, February 2009, Pages 459-465

https://doi.org/10.1016/j.jpcs.2008.11.012 Get rights and content

Abstract

Silver nanoparticles embedded in a dielectric matrix are investigated for their potential as broadband-absorbing optical sensor materials. This contribution focuses on the electrical properties of silver nanoparticles on glass substrates at various morphological stages. The electrical current through thin films, consisting of silver nanoparticles, was characterized as a function of film thickness. Three distinct conductivity zones were observed. Two relatively flat zones (“dielectric” for very thin films and “metallic” for films thicker than 300–400 Å) are separated by a sharp transition zone where percolation dominates. The dielectric zone is characterized by isolated particle islands with the electrical conduction dominated by a thermally activated tunneling process. The transition zone is dominated by interconnected silver nanoclusters—a small increase of the film thickness results in a large increase of the electrical conductivity. The metallic conductivity zone dominates for thicknesses above 300–400 Å.

Introduction

Nanostructured silver particles exhibit unique optical characteristics. In contrast to their corresponding bulk counterparts, metallic nanoparticles can absorb electromagnetic radiation, resulting in surface plasmon polaritons at the metal–dielectric interface. The resonance wavelength of metallic nanoparticles is strongly dependent on the metal and the particle size and particle shape [1], [2], [3], [4]. As initially isolated metallic nanoparticles form a fractal structure during the deposition process, the interconnection leads to nanoclusters with more irregular shapes and broader size distributions [5], [6], [7], [8], [9]. Such fractal structures can greatly extend the absorption from the visible wavelength band into the infrared (IR) wavelength region [10], [11], [12], [13]. By adjusting the particle size and shape distribution, it is possible to tailor the optical properties of these materials to specific applications such as surface-enhanced Raman scattering [14], [15], color filters [16], [17], [18], and all optical switching [19]. Our interest in exploring the effects of microstructure and applied external electrical fields on the conduction of silver thin films near the percolation threshold is due to their potential use as multispectral optical sensors, which are sensitive to visible and infrared radiation [5].

We recently reported on the tailored plasmonic behavior of Ag/Teflon^® AF nanocomposite materials. We were able to show a large broadband visible to infrared absorption spectrum [5] suitable for multispectral sensor applications. Also, we demonstrated a nanocomposite with an absorption spectrum that closely matched the solar radiation spectrum [20]. Westphalen et al. [21] and Tian and Tatsuma [22] showed that the excitation of surface plasmons in metal clusters can lead to the generation of photoelectrons. Pillai et al. [23] showed that solar cells containing metallic nanoparticles can dramatically enhance the near infrared absorption due to the presence of surface plasmons.

The synthesis of such nanocomposites typically involves the deposition of metallic nanoparticles into a dielectric matrix. The electrical properties of such metal/dielectric nanocomposites are determined by the embedded metallic nanostructures [24]. Thus, the model to describe the electrical properties of discontinuous metal films on a dielectric substrate can be used to describe the conduction mechanism in metal/dielectric nanocomposites [24], [25], [26]. The metallic film structures can be described as a system of metallic nanoparticles embedded in a combined dielectric medium of glass substrate and vacuum spacing between particles.

Generally, the electrical conductivity of metallic films can be divided into three zones [25], [26], [27], [28], [29], [30]. Due to the isolated nature of discontinuous particles, extremely thin films (dielectric zone) show a very low conductivity. As the film thickness increases, the electrical conductivity rapidly increases as isolated particles start to coalesce (percolation zone). At film thicknesses close to the electron mean free path (EMFP), the film exhibits near metallic conductivity (metallic zone).

Section snippets

Experimental details

Silver was deposited onto glass and silicon substrates. Plain soda-lime glass microscope slides (75×25×1 mm³) were purchased from Fisher Scientific and cut in half. Prime grade Si(1 0 0) wafers with a diameter of 2 in were obtained from Silicon Quest International, cut into small squares about 8×8 mm², and used for scanning electron microscopy (SEM) imaging. Both types of substrates were sonicated in methanol and rinsed with copious amounts of deionized water before use. Silver wires of 99.999%

Results and discussion

Fig. 1 shows a series of FESEM images of silver films at various thicknesses deposited at a constant rate of 0.067 Å/s. As the thickness increases, a transition from isolated island particles to interconnected clusters, and finally to a complete continuous film is observed. Typically, metals do not wet dielectric surfaces and their growth obeys the Volmer–Weber mechanism (island growth) [32]. At the initial deposition stage, the metal atoms tend to coalesce into individual isolated islands to

Conclusion

The electrical conductance of silver films on glass substrates was investigated as a function of film thickness and under various applied external electrical fields. Three conductivity zones with distinct microstructures were observed: dielectric (film consists of isolated particle islands), transition (film consists of percolated metallic network), and metallic (film consists of a metallic continuum). The electron transport in the dielectric zone is governed by an activated tunneling process,

Acknowledgments

This work was supported by ARO Grant W911NF-06-1-0295 and by ONR Grant N00014-03-1-0247.

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View full text

From silver nanoparticles to thin films: Evolution of microstructure and electrical conduction on glass substrates

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusion

Acknowledgments

Surf. Sci.

Thin Solid Films

Thin Solid Films

Thin Solid Films

Opt. Commun.

Sol. Energy Mater. Sol. Cells

Phys. Rep.

Vacuum

Vacuum

Microelectron. Eng.

Thin Solid Films

Thin Solid Films

Shape-controlled synthesis and surface plasmonic properties of metallic nanostructures

MRS Bull.

The bakerian lecture: experimental relations of gold (and other metals) to light

Philos. Trans. R. Soc. London

Contributions to the optics of turbid media, particularly of colloidal metal solutions

Ann. Phys. (Leipzig)

Large broadband visible to infrared plasmonic absorption from Ag nanoparticles with a fractal structure embedded in a Teflon AF (R) matrix

Appl. Phys. Lett.

Fractal aggregation and optical absorption of copper nanoparticles prepared by in situ chemical reduction within a Cu2+–polymer complex

Appl. Phys. Lett.

Electrical conduction in ultra thin metal films. II. Experimental

Proc. R. Soc. London A

Optical Properties of Nanostructured Random Media

Optical-properties of thin semicontinuous gold-films over a wavelength range of 2.5 to 500 μM

Phys. Rev. B

Controlled assembly of SERS substrates templated by colloidal crystal films

J. Mater. Chem.

Surface-enhanced Raman spectroscopy: a brief retrospective

J. Raman Spectrosc.

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