Neuroscientists use neural electrodes to explore the working mechanisms of the nervous system. Therefore, ideal electrodes should have a small size and the ability to record and stimulate at a single cell resolution with low noise. Materials used for fabrication should be flexible and stable for a long period in the biological media. However, conventional recording and stimulation techniques do not have sufficient spatiotemporal resolution for neuroscience research. Combining electrical and optical modalities into one device helps overcome the resolution limits and record more detailed information. For this application, transparent conductive materials are needed. Graphene is a potential solution due to its advantageous combination of properties, such as high conductivity, transparency, and flexibility. However, important characteristics of recording and stimulation electrodes, such as the impedance and charge injection capacity of graphene electrodes, do not reach the levels of conventional materials. The electrical characteristics of graphene could be improved further with surface modification, chemical doping, or stacking. Each method has been shown to improve the conductivity of graphene, although some affect the transparency of the layer. In this work, three methods were used to improve the electrical characteristics of multilayer graphene neural electrode without losing transparency or flexibility. These methods include growing a thicker layer of graphene, adding metal nanoparticles to the surface of the electrode, and nitric acid doping of graphene. For that purpose, graphene electrodes were fabricated on a silicon wafer. The electrical characteristics of these electrodes were assessed with electrochemical impedance spectroscopy, cyclic voltammetry and 4-point probe measurements. Furthermore, the optical transmittance was measured. The improvement methods were then tested on these electrodes, and the performance was evaluated. Adding metal nanoparticles to the surface of the electrode showed the most promising results. With gold nanoparticles, the impedance at 1 kHz was lowered 82%, and charge storage capacity increased 529%. However, at the same time, 30% of the optical transmittance was lost. With lower nanoparticle density, 6% of transmittance was lost, and 7% of impedance gained. Nitric acid doping did not improve the impedance, but the charge storage capacity was increased up to 66%. Thicker layers of graphene displayed a lower sheet resistance. However, impedance or charge storage capacity were not improved.

Numerous advancements have been made in transparent electrode technologies that can complement optogenetics and imaging modalities. However, several obstacles restrict the design and material of electrode devices, including the required flexibility, transparency, low impedance, high charge storage capacity (CSC), and high charge injection capacity (CIC), among others. The impedance of transparent graphene arrays is higher, and its CIC is significantly lower than platinum, a metal typically employed for electrophysiological recording and stimulation. It is possible to enhance the electrochemical properties of planar, transparent graphene electrodes by functionalizing their surfaces with platinum nanoparticles (Pt NPs), effectively increasing the electrode surface area. Existing research on platinum nanoparticles on transparent graphene electrodes has solely focused on simultaneous electrical recording and optical imaging of neuronal activity. There is currently no quantitative evidence of the extent to which platinum nanoparticles can impact the stimulating properties of transparent graphene electrodes and any indication of the stability of the coating. Therefore, material and electrochemical device characterizations were conducted to compare the recording and stimulating properties of graphene versus graphene with Pt NPs of varying surface densities.

Treatment of diseases, illnesses or disorders are always sought without any undesired side-effects and complications. Neurological disorders, such as Epilepsy and Parkinson’s, are currently treated using pharmaceuticals. However, drugs have a low specificity and lose their effectiveness after several years. In addition, prolonged drug usage often leaves patients with many side-effects and complications. Development in neuroscience indicate the treatment of such neurological disorders via neuro-stimulation. Currently, research in understanding the mechanisms behind neuronal stimulation is carried out by research institutes all around the globe. There are three modes for neuronal stimulation, electrical, optical and lastly the use of ultrasonic waves. As optogenetics provides a more specific technique in neuronal stimulation and electrical recording can provide high spatial resolution data, it is sought to combine this two modes. This work reports the development of a monolithically fabricated active implant, with optogenetic compatibility using transfer-free graphene electrodes. To achieve this desired goal, a microfabrication process is developed, which is reproducible and scalable with modern day microfabrication technology. In order to be compatible with optogenetics, graphene electrodes are used, as these are transparent and allow for neuronal stimulation using light. The graphene electrodes are grown on a pre-defined molybdenum catalyst, allowing for a transfer-free chemical vapor depostion (CVD) of graphene. Microelectrode-arrays (MEAs) are developed, using graphene electrodes, that include different sized electrodes and different amount of electrodes. The amount being constrained by the available space on the cortex of a mouse. The aim of this work was to be able to control or read out these MEAs using electronics developed alongside the electrodes. As graphene is grown at a temperature above the melting point of aluminium, conventional CMOS technology can not be used in combination with graphene electrodes. The process developed, did not include any materials which would be damaged during the graphene growth. The metal gate is replaced by a polysilicon gate and the metal interconnects connecting the active devices and the MEAs are defined after the growth of graphene. Resulting in an active implant which is monolithically fabricated in combination with a transfer-free graphene process. Monolithically fabricating an active graphene based implant, comes with complications. Delamination of the passivisation layer occurred whilst trying to open contact openings to and from the active devices. This was resolved using a more appropriate chemical etchant. A fully monolithically fabricated active graphene-based implant was obtained. Electrical measurements showed that the active devices did not behave as expected. Revisiting simulations, it was established that there was leakage of dopants from the gate to the channel. This results in the NMOS devices to be always on and the PMOS devices to have a higher threshold voltage. However, this particular wafer, had many high temperature steps after the doping of the polysilicon. Reduction of these high temperature steps result in less doping getting into the channel. Alternatively, a thicker gate oxide can be used, to serve as a more robust barrier. A last proposed solution is the reduction of the doping concentration and doping energy of the polysilicon, which would result in less dopant being near the gate oxide interface and thus less dopant leaking into the channel. There are wafers included in this work with less high temperature steps. However, due to time limitation it was not possible to finish production of these wafer in order to confirm it experimentally. Nevertheless, it is believed that with minor adjustments, this research project is a viable foundation for active graphene based implants and structures.