Browsing by Author "Romppainen, Henrik"
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Item Carbon nanotube modified pyrolyzed carbon 3D microelectrodes(2021-05-17) Romppainen, Henrik; Jokinen, Ville; Shoja, Yalda; Sähkötekniikan korkeakoulu; Franssila, SamiPyrolytic carbon (PyC) is an often used electrode material due to its stability, inertness and biocompatibility. However, bare PyC has some limitations; its electrical conductivity does not compare to metals, its surface is not remarkably electrochemically active on its own, and it is difficult to pattern via conventional tools. One potential approach for addressing all these limitations is to utilize a polymer photoresist as PyC precursor material, and integrate carbon nanotubes (CNTs) into the PyC. The idea is that in the composite, CNTs inside the PyC would provide a conductivity enhancement, while CNTs protruding outside of the PyC would increase electrode surface area and thus the electrochemical activity. However, not enough attention has been directed towards the negative effect that adding CNTs has on lithographic quality, while striving towards these improvements. A balance must be found between the improvements and the loss in resolution. Thus, the objective of this thesis was to optimize the integration of CNTs into SU-8 photoresist derived PyC in order to improve its conductivity and electrochemical activity. This includes analyzing the effect of CNT concentration, UV exposure time and film thickness on the conductivity, patternability and electrochemical activity of PyC films. To meet these objectives, simple PyC microelectrodes with varying concentrations of CNTs, UV exposures and film thicknesses were fabricated. The fabrication process involved mixing CNTs into liquid SU-8, patterning the composite with photolithography, and subsequently pyrolyzing the films to form the final carbon-only structures. After fabrication, the lithographic quality was inspected and the film thickness, resistivity and electrochemical activity were measured and evaluated. The CNTs agglomerated considerably during the fabrication process, affecting lithographic resolution in a negative way, thinner films more severely than thicker ones. The CNTs tended to strongly concentrate on edges and sidewalls of PyC features, being mostly absent from top surfaces and insides of the features. Resistivity of the CNT-embedded films was measured to be around 4 mohm-cm, regardless of CNT concentration, providing roughly 20 percent lower resistivity than CNT-free films, which is rather modest reduction compared to previous reports. Cyclic voltammetry measurements both with hexaammineruthenium and dopamine showed that PyC electrodes with 0.3 weight-percent CNTs showed better electrochemical response than bare PyC. This can be attributed to enhanced effective surface area provided by the CNTs. The sensitivity of the 0.3 percent CNT-embedded electrodes was approximately 0.36 mA/(nM-cm2), roughly 300 times the sensitivity of previous reports for bare PyC.Item Fotolitografia valomikroskoopilla(2018-01-05) Romppainen, Henrik; Selin, Jorma; Sähkötekniikan korkeakoulu; Turunen, Markus
