Recent Research Developments

Index of Recent Research News

Feb 5, 2003

    Scanning Tunneling Microscopy With Chemically Modified Gold Tips: In-Situ Reestablishment of Chemical Contrast

    Scanning tunneling microscopy (STM) is a powerful method of surface analysis that generally provides the spatial disposition of atoms and molecules with high resolution. However, it often does not provide direct discrimination of chemical species. Therefore, recent work has been directed toward chemically selective imaging by various means. One method for the chemical discrimination of specific atoms and functional groups in STM images is based on the control of interactions between samples and chemically modified tips, in particular, gold tips modified with self-assembled monolayers (SAMs) of thiols. The SAMs are selected in such a way that their terminal functionality interacts with a chemical functionality of interest on the surface, for example, by hydrogen bond formation or by metal–ligand coordination. A drawback of SAM-modified gold tips has been the relatively low rate of successful tip fabrication. So far, only a limited fraction of all prepared tips has shown contrast enhancement. And for those that did, the useable lifetime was no more than a few hours before contrast enhancement was lost. Presumably, the disappearance of contrast enhancement is due to the loss of SAM species from the STM tip apex.

    Figure Caption: STM images of 1-octadecanoic acid monolayers on a highly oriented pyrolytic graphite (HOPG) substrate. The image to the left was collected with a bare gold tip. The dark rows are due to the carboxyl groups of the 1-octadecanoic acid. Individual methylene groups are easily discernible in this image. The image to the right was collected with a gold tip modified with 4-mercaptopyridine. The terminal lone pair on the 4-mercaptopyridine can hydrogen bond to the acidic hydrogen on the surface, thus the rows of carboxyl groups now appear as bright, or "contrast enhanced" regions.

    A recent discovery by Dr. Joel A. Olson, a postdoctoral research associate, and Professor Philippe Buhlmann has served to increase the useable lifetime of SAM modified gold tips from hours to months. The method utilizes the brief application of a high bias voltage (e.g. +1.9V) between the sample and the tip, while in tunneling, to cause SAM molecules to reoccupy the tip apex, thereby allowing the tips to display selective chemical contrast in imaging. Such a bias treatment is applied once a tip has lost its ability to provide contrast enhancement, and may be applied repeatedly. Additionally, tips that did not initially display selective chemical contrast could be treated to do so, thus increasing the rate of successful tip fabrication to ~50%. Furthermore, the bias can be reversed to cause the egress of SAM molecules from the apex of the tip, and thus cause it to behave as a conventional STM tip. This technique, then, provides significantly increased experimental flexibility; it allows direct comparison of the same region of a surface both with and without selective chemical contrast. The described behavior is consistent with the authors’ proposed mechanism whereby the SAM species migrate laterally toward or away from the tip apex (depending on the polarity of the bias) due to the inhomogeneous electric field that is generated between the sample and the tip; especially convincing in this regard is the reversibility of the effect. Field induced lateral movement of surface adsorbed species is another interesting aspect of this discovery.

    This work, reported in Analytical Chemistry (web release date 01-24-03), may serve to expand the use of chemically modified gold tips into such disciplines as UHV STM, where previously the limited success rate of tip fabrication caused such experiments to be impractical.

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Next scheduled update: Feb. 19, 2003.

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