Single layer porous gold films grown at different temperatures
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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