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Scanning probe microscopies have revealed their great potential to image the sample surface with atomic scale resolution. It has been recognized that the property of probe apex, which is a gateway of interaction with the sample, is very important and the obtained images are greatly modulated by the 'states of the tip'. In the case of atomic force microscopy (AFM), the overall shape of the tip, presence of electric charge, chemical species, and unsaturated bonds affect the force to be used for imaging.
Recently, we have designed and synthesized a tripodal single molecule which can be used as a probe tip and whose function is controllable by light irradiation [1,2]. The molecule consists of rigid tripod part to be fixed on the Au-coated cantilever by formation of three Au-S bonds and a functional group that specifically interacts with target molecules. These two parts are connected by azobenzene moiety that shows reversible photo-isomerization to the cis-form by UV light (360 nm) and the trans-form by visible light (450 nm) irradiation. With this special molecular tip combined with the frequency-modulation detection AFM (FM-AFM), one can image exactly the same area by two kinds of tips (the trans-tip senses the surfaces with the functional group and the cis-tip traces the surface topography), which has a great advantage to extract the chemical interaction from the total interactions working between the tip and sample. Reversible photo-switching of the tripod molecule adsorbed on an Au-coated cantilever and imaging of a surface by the SINGLE molecule have been achieved [1]. With a tip molecule that has carboxyl group (-COOH) on its head, total interaction increased with a trans-tip at the sites where hydrogen bonding interaction is possible.
The tipodal molecule is firmly fixed on the Au surface even in the liquid. It was confirmed by tapping mode AFM measurements of the molecule adsorbed on Au(111) in milliQ water. FM-AFM measurements using the molecular tip in solution are in progress, where the molecular tip is expected to work as electron donor/acceptor for manipulation of an electron.
[1] D. Takamatsu, Y. Yamakoshi, K. Fukui, J. Phys. Chem. B 110, 1968@(2006).
[2] D. Takamatsu, Y. Yamakoshi, K. Fukui, e-J. Surf. Sci. Nanotechnol. 4, 249(2006).
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