Mimicking Axons by Conductive Metal Lines Formed by Photocatalytically Grown Gold
Authors: Abshari Fatemeh , Veziroglu Salih , Vahl Alexander , Gerken Martina 
Published: August 2024

Nanoionic memristive devices based on the field−driven migration of metal cations and oxygen vacancies have gathered high interest in the scientific community for their capability to technically mimic synaptic connections. Less attention has been given to long−range global plasticity in artificial neuronal networks. The aim of our research is to use light−stimulated growth of gold metal lines to technically mimic the growth of long−range axonal connections for neuromorphic computing architectures. For this purpose, we investigate the formation of metal lines by UV−light−stimulus−driven photocatalytic deposition of gold on titanium dioxide layers. In this study, we present a significant contribution that stems from our active involvement in CRC 1461- Neurotronics, a pivotal initiative funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation). Our research falls under Project-ID 434434223 – SFB 1461, within the broader context of Bio-inspired information pathways.
0.8-mm wide photoactive titanium dioxide lines are defined by photolithography on a Sodalime glass substrate. We deposited a 70 nm thick titanium dioxide line on a 10 nm thick indium tin oxide adhesion layer by DC sputtering. The substrate was then heat treated for 90 minutes at 400°C and subsequently cooled on a metal plate to achieve the photocatalytically active anatase phase of titanium dioxide [1]. The growth process is conducted in a beaker by reduction from an aqueous gold chloride precursor solution (HAuCl4). In each step of the growth process, the substrate is placed in a beaker containing 15 ml of the precursor. The solution is obtained by dissolving 15 mg of HAuCl4 powder in 60 ml of deionized water. We utilize a UV LED with the peak wavelength of 365 nm and an approximate intensity of 1.8 - 2.7 mW/cm² to illuminate the sample in the beaker. The illumination time and UV intensity are crucial for this dynamic process. Here, we illuminated the sample for 12 hours and refreshed the solution every 2 hours. After each refreshment of the solution, the substrate is washed thoroughly with deionized (DI) water and is dried under nitrogen gas. We analyze the surface properties of grown gold utilizing scanning electron microscopy (SEM) and measure the conductance of the grown lines by applying voltage via the needles of a hard contact probe station.
In the first experiments the growth of gold clusters on the surface is observed instead of the homogeneous gold coverage of the photolithographically defined titanium dioxide lines, which are needed for long-range electrical connections. Then, we coincidentally observed that the presence of a scratch on the titanium dioxide line led to the growth of continuous gold lines on the damaged area. Subsequently, deliberate scratches were introduced manually using tweezers, varying in both size and thickness. SEM images depict islands and covered areas, with notable growth observed on the scratched regions. Remarkably, wider scratches exhibit a proportionally increased width of the grown lines, and a uniform line devoid of any gaps is distinctly visible on broader scratches. We observed particle growth on thinner lines. However, unlike the uniform alignment observed in wider scratches, these particles did not coalesce into a continuous line. Instead, they remained separated from each other, exhibiting small gaps between individual particles. The morphological analysis of the grown gold line using SEM reveals a crystalline morphology of particles.
As the primary objective of this research is to mimic axons with gold lines, ensuring conductivity is crucial for obtaining long−range electrical connections mimicking axons. To achieve this, voltage was applied using the needles of a hard contact probe station, and the resulting current was measured. The linear behavior observed in the I–V diagram of the wider line serves as a confirmation of its conductance (≈0.003 S).
References:
[1] Veziroglu, S., Paulsen, M., Schardt, J., Adejube, B., Aktas, C., Vahl, A., & Gerken, M. (2023). Photocatalytic Deposition for Metal Line Formation. In Bio-Inspired Information Pathways: From Neuroscience to Neurotronics (pp. 241-263). Cham: Springer International Publishing.

Figure 1