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Graphene is a promising material both as a coating for existing neural electrodes as well as for transparent electrodes made exclusively from graphene. We studied graphene‐based microelectrodes by investigating their recording and stimulation properties in order to evaluate their suitability for neuronal implants. In this work, we compare three different electrode material compositions. Microelectrode arrays (MEA) with an electrode size of about 700 μm2 were prepared of gold, graphene on gold, and plain graphene on glass substrate. In order to reduce polymer contamination during graphene transfer, we employed a polymer‐free transfer and lift‐off process. Impedance studies revealed a value of 2.3 MΩ at 1 kHz for plain, and 0.88 MΩ for graphene on gold. Neuronal recording experiments showed a sufficient SNR for both graphene‐based materials and a stable impedance, unaffected by surface degradation metal electrodes are known for. Stimulation measurements yielded a charge injection capacity of 0.15 mC/cm2 using biphasic pulses of 1 ms and 1 μA transparent graphene electrodes. Cyclic voltammetry revealed a large voltage range of −1.4 V to +1.6 V before water electrolysis occurs. Graphene‐coated gold microelectrodes show enhanced recording properties, whereas plain graphene electrodes might be better suited for stimulation applications.
Electrodes based on carbon nanotubes are a promising approach to manufacture highly sensitive sensors with a low limit of signal detection and a high signal-to-noise ratio. This is achieved by dramatically increasing the electrochemical active surface area without increasing the overall geometrical dimensions. Typically, carbon nanotube electrodes are nearly planar and composed of randomly distributed carbon nanotube networks having a limited surface gain for a specific geometrical surface area. To overcome this limitation, we have introduced vertically aligned carbon nanotube (VACNT) networks as electrodes, which are arranged in a microelectrode pattern of 60 single electrodes. Each microelectrode features a very high aspect ratio of more than 300 and thus a dramatically increased surface area. These microelectrodes composed of VACNT networks display dramatically decreased impedance over the entire frequency range compared to planar microelectrodes caused by the enormous capacity increase. This is experimentally verified by electrochemical impedance spectroscopy and cyclic voltammetry.
A high-frequency photocathode based on carbon nanotube (CNT) blocks on semiisolating GaAs or low-temperature grown GaAs was fabricated and used for electron emission in a diode configuration. The CNT blocks can achieve high currents and current densities of I=0.5 mA and J=28 mA/cm 2 at E=2.1 V/μm, respectively. The photomodulation was achieved with a GaAs photoswitch electrically connected to the bottom of the CNT block emitter. This photocathode can find applications in high frequency tubes and optically driven X-ray sources.
Niedrigtemperatur-Transfer von vertikal ausgerichteten Kohlenstoff Nanoröhren auf Mikrostrukturen
(2011)
Vertikal angeordnete Kohlenstoff Nanoröhren (engl. vertically aligned carbon nanotubes; VA-CNT) können seit einigen Jahren mit hoher Reproduzierbarkeit durch CVD-Verfahren bei 600 °C - 800 °C auf Katalysatorschichten aus Aluminium und Eisen hergestellt werden. Sie zeigen sehr gute Eigenschaften im Bereich der Feldemission, aber auch in sub-µm Vias. Allerdings sind die relativ hohen Temperaturen oft nicht prozesskompatibel und müssen vermieden werden um keine Degradation darunterliegender Strukturen in Kauf zu nehmen. Da die Reduzierung der CVD-Temperaturen beim Wachstum der VA-CNTs unter 600 °C zu hohen Defektraten führt, schlagen wir ein neues Verfahren zum Transfer der VA-CNTs bei niedrigen Temperaturen vor. In einer Kombination aus Flip-Chip- und Stempel-Technik werden hoch geordnete, mikrostrukturierte VA-CNTs auf temperaturempfindliche Substrate aus Kalk-Natron-Glas, Gold oder Kupfer bei Temperaturen von 100 °C transferiert. Die vertikale Struktur der Nanoröhren bleibt dabei erhalten. Eine leitende Verbindung kann durch ein leitfähiges, bio-kompatibles Polymer gewährleistet werden, was eine Erweiterung des Einsatzbereichs auf Life-Science Applikationen erlauben würde.