From silver nanoparticles to thin films: Evolution of microstructure and electrical conduction on glass substrates
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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2021, Applied Surface ScienceCitation Excerpt :We can see that for 1.5% covered area, the NP size distribution derived from the TEM analysis coincides with the TOF size distribution obtained during deposition, reinforcing our monitoring method (Fig. 4 (d)). Increasing the percentage of covered area shifts the NPs mean size and broadens the distribution towards bigger NPs, as shown in Figs. 4(e) and (f), as already reported by H. Wei and H. Eilers [4]. Considering the capacitive component of the impedance, in the previous section, we have proposed a model to calculate the evolution of the capacitance as a function of the covered area.