Identification of chemical species and specific ligands is important for studies on catalysis and mesoscopic molecules. In this presentation, we report on characteristics of a novel tripod molecule with an azobenzene arm as a single molecular tip for noncontact atomic force microscopy (NC-AFM). It consists of a rigid adamantane core, three linear legs with thiols at the end to be adsorbed strongly on Au surfaces by three Au-S bonds. The sharp shape of the molecule has an advantage for reducing van der Waals force that works as background force against chemical interaction working at short range. Azobenzene is a famous molecule which can change its structure between trans- and cis-form by UV and visible light irradiation, respectively. So our molecule is expected to show photoisomerization that enables in-situ change of the tip apexes, leading to novel chemical analysis on sub-molecular scale by NC-AFM.
We have successfully confirmed photoisomerization of the synthesized molecule adsorbed on the apex of a Au-coated cantilever as well as on a flat Au(111) surface. We have clarified that our molecule can act as a photo-switching tip that enables observation of the same area with different tip apexes with different structures [1].
To examine the concept of the high spatial resolution chemical analysis, NC-AFM measurements of a TiO2(110) surface were performed by using a molecule whose tip-part can generate temporal hydrogen-bonding with the surface hydroxyl species in its trans-form. We have succeeded in obtaining NC-AFM images of hydrogen adatoms on TiO2(110) with atomic resolution by using carboxyl-terminated single molecular tip. By comparing the images obtained by the trans-form (visible light irradiation) and the cis-form (UV light irradiation), the frequency shift was apparently enhanced at hydrogen adatoms with the trans-form tip, indicating that temporal hydrogen bonding was detected. We believe it is a great progress on the use of a specially designed single molecule for controlling the interaction by external stimulus to identify chemical species and specific ligands on sub-molecular scale.
[1] D. Takamatsu, Y. Yamakoshi, and K. Fukui, J. Phys. Chem. B., 110, 1968 (2006).
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