Recent Advances in Potentiometric Scanning Electrochemical Microscopy

Abstract

Since the invention of Scanning Tunnelling Microscopy (STM) in 1981 by Binnig and Rohrer, surface analysis has seen tremendous growth. The fact that they received the Nobel Prize in 1986, only five years later, is an indication of the importance of their pioneering work. STM was but the first of a family of techniques, called Scanning Probe Microscopy (SPM), with many more to come in the following years. Their basic element is a local experiment, which is repeated sequentially at the pre-defined points of a raster grid. Then, the gathered information is presented by plotting the measured parameter as a function of their coordinate. The most important advantage of them over the conventional optical microscopy is their incredible resolution. Even individual atoms can be ”seen”, because they are not limited by Abbes’ formula. Modifications of the original STM followed quickly. For instance, Atomic Force Microscopy was invented in 1982 by the same researchers. In 1989, not long after the introduction of the STM, electrochemists invented the Scanning Electrochemical Microscope (SECM), the electrochemical version of SPM. It is based on the same concept, except the scanning probe is a microelectrode. With this technique, highly resolved chemical information can be gathered about a wide range of surfaces. One of the biggest disadvantages of the SPM techniques in general is their low speed, due to the scanning process. The entire image is recorded with the same measuring tip, as opposed to optical techniques, where there is usually a sensor array. As a consequence of this, the more data points are in an image, the longer it will take to record it. This is especially a problem in the potentiometric operation mode of the SECM. The response time of the measuring cell is determined by the RC time constant, which in turn, depends mainly on the resistance of the measuring microelectrode. Due to the small size of the microelectrodes, their resistance can even reach the GΩ range, resulting in imaging times that can be measured in minutes. Other SPM techniques have received significant improvement during the last few decades, and their imaging speed can even reach video framerates. Low speed, however, is an often overlooked limitation of the SECM, and prevents the quick recording of highly resolved images. That is, one has to choose between high resolution and quick imaging. The image will either be quickly completed but distorted, or high quality but asynchronous, because the points of the image will not only have different spatial, but different temporal coordinates as well.

My thesis is mostly devoted to the investigation of this problem, and three possible solutions to it.