Session:

Nanoionic memristive devices that are 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. To mimic the growth of axons we investigate the formation of metal lines by UV-light-stimulus-driven photocatalytic deposition of gold on titanium dioxide layers. The growth process is conducted in a beaker by reduction from aqueous gold chloride precursor (HAuCl₄) and is observed by transmission optical measurements in an inverted-microscope setup. Surface properties are analyzed by 3D laser interference microscopy and scanning electron microscopy (SEM) and electrical conduction measurements are taken.
We fabricated the photoactive titanium dioxide layers with the thickness of 40-100 nm on quartz glass by DC sputtering. The substrates were then heat treated for 1h at 300C and subsequently cooled on a metal plate to achieve the photocatalytically active anatase phase of titanium dioxide. In the growth process the substrates are placed in a beaker containing 15 ml of the precursor. The solution is obtained by dissolving 15 mg of HAuCl₄crystalline powder in 60 ml of deionized water. We utilize a UV LED with the wavelength peak of 365 nm to illuminate the sample in the beaker. The illumination time and UV-intensity are crucial for this dynamic process. Here we illuminated for 3h and replaced the solution every hour. Furthermore, we repeated this experiment by adding a surfactant to the solution to achieve faster growth and better surface coverage as proposed by literature.
In-situ characterization of the growth process is possible by placing the beaker in an optical microscope with transmission configuration. While the sputtered titanium dioxide and the quartz glass substrate show high transparency, the grown gold will absorb and reflect the transmitted light and thus yield an observable change in transmission. We also analyzed the morphology of the grown gold layers by 3D laser interference microscopy and scanning electron microscopy (SEM) after the growth procedure. The measurements show the formation of gold clusters on the surface of the titanium dioxide layers. Sufficient growth time is needed to reach the percolation threshold and attain a conductive gold layer which is crucial for our aim of mimicking axonal connections. We recorded electrical current-voltage characteristics to investigate the electrical conductivity of the grown gold. Our measurements revealed a value of electrical conductivity ??=0.015 S for areas with high surface coverage. It is obvious from 3D laser interference microscopy that the thickness of gold clusters is not uniform throughout the surface of titanium dioxide. Therefore, the conductivity across the 2D surface of the TiO₂/Au composite varies. With its potential capability to locally control the growth of long-range metallic connections, photocatalysis is a promising approach towards mimicking the dynamic formation of long-range axonal connections in neural networks.

Co-Author(s): Moritz Paulsen (Christian-Albrechts-Universit zu Kiel (CAU)), Jan Schardt (Christian-Albrechts-Universit zu Kiel (CAU)), Blessing Adejube (Christian-Albrechts-Universit zu Kiel (CAU)), Salih Veziroglu (Christian-Albrechts-Universit zu Kiel (CAU)), Alexander Vahl (Christian-Albrechts-Universit zu Kiel (CAU)), Martina Gerken (Christian-Albrechts-Universit zu Kiel (CAU))

Fatemeh Abshari

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