Self-organization of mesoscopic silver wires by electrochemical deposition

Abstract

Long, straight mesoscale silver wires have been fabricated from AgNO3 electrolyte via electrodeposition without the help of templates, additives, and surfactants. Although the wire growth speed is very fast due to growth under non-equilibrium conditions, the wire morphology is regular and uniform in diameter. Structural studies reveal that the wires are single-crystalline, with the [112] direction as the growth direction. A possible growth mechanism is suggested. Auger depth profile measurements show that the wires are stable against oxidation under ambient conditions. This unique system provides a convenient way for the study of self-organization in electrochemical environments as well as for the fabrication of highly-ordered, single-crystalline metal nanowires.

Keywords: crystal growth; electrochemistry; electrodeposition; mesowires; nanoelectrochemistry; nanowires; self-organization; silver nanowires; silver nitrate; stability.

Figure 1

Schematic diagrams of the experimental procedure. (a) By slowly freezing the silver nitrate electrolyte a thin aqueous layer of electrolyte forms between the glass slide and the ice. The concentration of electrolyte is higher than the initial concentration due to the segregation effect. (b) Applying a constant voltage across the two electrodes let the silver grow from the cathode into the electrolyte. (c) Cooling is stopped and the temperature rises after deposition. After melting the ice the silver wires can be taken out of the deposition cell and rinsed with deionized water (d). After drying they are ready for further TEM- and SEM-analysis.

Figure 2

SEM images of silver wires: (a) Overview: low-magnification image. (b) Zoom-in of (a). (c) Image of one silver wire, illustrating the homogeneous thickness and smooth surface. (d) Image of several long silver wires.

Figure 3

TEM analysis of thin silver wires and corresponding EDX information. (a) Bright-field image of typical silver wires with the axes which should be oriented in [112] direction and (b) the corresponding SAED patterns recorded from the marked region, the growth direction is mostly like [112]. The zone axis is <111>. (c) HAADF-STEM image of a silver wire and (d) the corresponding EDX spectrum taken at the labelled position on the wire of Figure 3c. Strong silver element peaks can be identified. Very weak gold, iron, carbon, and cobalt peaks were also found. Gold and carbon come from the substrate. The cobalt and iron signal comes from the pole pieces of the objective lens; no peak can be attributed to sulfur, nitrogen or oxygen, the latter demonstrating that the silver wires are not oxidized at ambient conditions in air.

Figure 4

Auger depth profile curves of freshly prepared and aged silver wires. The full and dotted curves correspond to data obtained from two different wires. The relative atomic concentration is plotted versus the sputter depth; (a) freshly-prepared sample taken out of the electrodeposition cell and washed by deionized water and immediately investigated by SAMS. (b) Sample aged by exposure to ambient conditions for four months. Freshly-prepared and aged samples show similar depth profiles, indicating that the single-crystalline silver wires are stable under ambient conditions on this time scale.

Figure 5

TEM analysis of thin comb-like silver dendrites (dark areas in the image left) and corresponding SAED patterns. Patterns (1)–(5) (right) were recorded at different spots marked by the corresponding number in the TEM image (left). The zone axis is <111>. This suggests that the comb structure is single-crystalline. All SAED patterns display the same hexagonal pattern. This suggests that the comb structure is single- crystalline. Each branch should grow along one of the <112> directions.