Characterisation of electrically-stimulated nano-spots on diamond for electrochemical sensing
Final Report Abstract
This project was aimed to investigate the basic electrochemical characteristics of nano-spot areas on diamond surface with respect to their potential application in bio/electrochemical sensing. The actual work was focused on theoretical modelling of semiconducting electrodes with active nano-spots in electrolytes, and experimental investigation of the structured surfaces. Here we explored a novel electrode structure which incorporated a low-doped cap layer of diamond on highly boron-doped diamond substrate, where the surface of the cap layer is decorated by chemically active nano-spots. These electrode structures were fabricated on single crystal and on nano-crystalline diamond (NCD) substrates. We propose and describe a model of lateral depletion of the junctions between nano-spots on semiconducting substrates in electrolytes. This model predicts that the activity of the nanospots can be suppressed by lateral pinch-off of the junction depending on the spot size, semiconductor doping and the surface characteristics of the cap layer. The results of the model were confirmed experimentally by characterising cap-layer electrodes on single crystal diamond decorated by gold nano-particles. The proposed depletion effect might be useful to suppress parasitic activity in order to resolve the targeted signal in a neighbouring potential range. It has been confirmed that the presence of a cap layer reduces significantly the background activity of single crystal and NCD substrates. At the same time, the conductivity across the grain boundaries across the undoped NCD cap provides a low-resistance contact to individual nano-particles on the surface. Following this concept, we have proposed and fabricated an alldiamond electrode array with a surface pn-junction by boron/nitrogen doping and chemically active areas formed by local etching through the cap layer. The fabricated structures showed typical behaviour of a microelectrode array, like a saturated current expected for hemispherical diffusion. In the range of ethanol oxidation, the fabricated arrays showed higher sensitivity and larger separation of the signal from the hydrolysis regime in comparison to a large-area electrode. Besides, the fabricated arrays demonstrated a high stability at high anodic potentials in strong electrolytes. The results of this project demonstrated that diamond electrodes with a cap layer and chemically active nano-spots on the surface are very promising for chemical sensing. The characteristics of such sensor devices can be controlled either by varying the geometry of the active areas (to control the surface diffusion processes), or the electronic characteristics of the cap layer (to control the background noise and to suppress the surface activity by lateral depletion). The proposed equivalent circuits and the numerical models provide a tool to optimise the cap-layer electrodes for specific applications. In particular, one can develop a hybrid sensor which combines the functions of electrochemical electrodes and an ionsensitive FET. In addition, the active areas of such device can be decorated with biological objects for bio-sensing. All these aspects might be the subject of a future research project.
Publications
- A. Denisenko, C. Pietzka, M. Dipalo and E. Kohn, Diamond electrodes with gold nanoparticles: The role of surface depletion layer, Proc. of the 2nd Conf. on New Diamond & Nano Carbon NDNC 2008, 26 ¿ 29 May 2008, Taipei, Taiwan, paper P2-070.
- C. Pietzka, M. Dipalo, A. Denisenko, Z. Gao, E. Kohn, "O-Terminated boron doped NCD electrode with low background current" Proc. of Hasselt Diamond Workshop 2008 SBDD XIII February 25 ¿ 27, 2008, Hasselt, paper 10.5.
