Scanning tunneling microscopy of lead dioxide

Abstract

Scanning tunneling microscopy (STM) has become a superb tool for in situ electrochemical studies, because it provides extremely high resolution, real-space images of surface structure. We are using STM to study the growth morphology of lead dioxide, and to develop methods for determining the internal microstructure of the material. We have found that lead dioxide has a self-similar (fractal) grain structure from the nanometer to the micrometer scale. Knowledge of the effects of chemical environment and electrochemical deposition conditions on this morphology will lead to better understanding of the properties of lead dioxide as battery electrode material. To determine internal microstructure, STM is used to record structural features which emerge during chemical or electrochemical etching of a thick (hundreds of monolayers) deposit. These features represent internal layers which are successively ‘peeled’ back by the etching process. The images can be combined to form a three-dimensional tomographic volume from which two-dimensional slices can be taken in any orientation. This gives spatial information with subnanometer resolution about the number density and connectivity of structural voids, which affect electrolyte diffusion and electrode strength. Our STM instrument has been optimized regarding image acquisition rate. This minimizes the effects of microscope drift and facilitates observing time-dependent surface phenomena. Fast image acquisition rates are important for depth profiling, because a large number of depth ‘slices’ are required, with low distortion from microscope drift; otherwise, it would be impossible to obtain the image registration needed for constructing tomographic etching profiles.