Elsevier

Physica B: Condensed Matter

Volume 405, Issue 21, 1 November 2010, Pages 4517-4522
Physica B: Condensed Matter

Single layer porous gold films grown at different temperatures

https://doi.org/10.1016/j.physb.2010.08.029Get rights and content

Abstract

Large area porous gold films can be used in several areas including electrochemical electrodes, as an essential component in sensors, or as a conducting material in electronics. Here, we report on evaporation induced crystal growth of large area porous gold films at 20, 40 and 60 °C. The gold films were grown on liquid surface at 20 °C, while the films were grown on the wall of beakers when temperature increased to 40 and 60 °C. The porous gold films consisted of a dense network of gold nanowires as characterized by TEM and SEM. TEM diffraction results indicated that higher temperature formed larger crystallites of gold wires. An in situ TEM imaging of the coalescence of gold nanoparticles mimicked the process of the growth of these porous films, and a plotting of the coalescence time and the neck radius showed a diffusion process. The densities of these gold films were also characterized by transmittance, and the results showed film grown at 20 °C had the highest density, while the film grown at 60 °C had the lowest consistent with SEM and TEM characterization. Electrical measurements of these gold films showed that the most conductive films were the ones grown at 40 °C. The conductivities of the gold films were related to the amount of contamination, density and the diameter of the gold nanowires in the films. In addition, a gold film/gold nanoparticle hybrid was made, which showed a 10% decrease in transmittance during hybridization, pointing to applications as chemical and biological sensors.

Introduction

Thin metal films with or without porous structures have unique properties that are different from bulk materials. These films have many different nanoscale morphologies such as island films, hillocks films and holey films [1]. The optical and electronic behavior of thin metal films is associated with the film morphology [2], for example, thin gold films have excellent tailorable surface plasmon resonances, surface enhanced Raman scattering and highly enhanced fluorescence properties, which allows ultrahigh sensitivity down to single molecule level due to this dramatic enhancement [3], [4].

To fabricate thin porous films, methods like dealloying [5], [6], [7], [8], templating [9], [10], [11], wet chemical [12], [13], electrochemical deposition [14] and evaporation induced self-assembly are used [15], [16], [17]. Among these methods, evaporation induced film growth from nanoparticles are of interest due to its simple method. Generally, the nanoparticles are self-assembled and form ordered 3D structures [18], [19], [20], [21]. But, for gold nanoparticles, the self-assembled nanoparticles could form disordered networks of nanowires with [16] or without [17] surfactants at room temperature.

We reported earlier about the fabrication of porous gold films using evaporation induced crystal growth at room temperature [17]. However, the growth mechanism is not clear, in particular the temperature dependency needs to be investigated, since it is an important parameter during evaporation induced growth of films. A coalescence process of the gold nanoparticles [22] was discussed in Ref. [17] as an essential part of the growth mechanism, but more work is needed to show or mimic the process.

Here, we describe a study of single layer porous gold films at different growth temperature, as well as their optical and electrical properties. Moreover, the crystal growth of gold nanoparticles was investigated in situ by TEM, which showed the coalescence of gold nanoparticles. Transmittance measurements indicated that the evaporation speed at different temperature caused the different film density, and the electrical measurements showed that the conductivity of the gold film was due to contamination in the film, the film density and the diameter of the gold wires in the film. Moreover, a gold film/gold nanoparticle hybrid was fabricated using hexamethylene diamine (HD) as a linking molecule, suggesting potential application of these gold films as sensors.

Section snippets

Synthesis of gold nanoparticles

1.0 ml 1.0 wt% HAuCl4 (Sigma) solution was added into 99.0 ml doubly distilled water and heated to boil while stirring, then, 4.0 ml 1.0 wt% sodium citrate (Sigma) was added [23]. The solution was kept boiling for 5 to 10 min, until the solution turned to a red wine color.

Self-assembly of gold film

The synthesized colloid gold solution was stored in 100 ml beakers with an internal diameter of 6 cm, separately. The beakers were then covered by a 1000 ml beaker in order to avoid dust from air. To grow gold films, the beakers were

Results and discussion

In an earlier report [17], we found that the gold film floated on surface of the solution with an area up to several square centimeters at about 21 °C. This floating film was usually deposited on a substrate by a simple subtracting procedure. For example, a glass slide was dipped into the solution and a floating film was then picked up by retracting the slide.

However, when the evaporation temperature increased to 40 and 60 °C, no such floating film was present, instead the films were deposited on

Conclusions

In summary, we investigated the behavior of the growth of porous single layer gold films at different temperature. The densities of these films were highest for the lowest growth temperatures, as revealed by transmittance and electron microscopy. To mimic the growth process, which was hard to observe in the real process, we studied the coalescence of nanoparticles in situ by electron irradiation in a TEM. The in situ TEM measurement showed a linear relationship between time and (x/r)6 as

Acknowledgements

We thank the Sundsvall Community for financial support. We also thank Mr. Staffan Palovaara for helping with SEM-EDX characterization.

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