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H3PW12O40-functionalized tip for scanning tunneling microscopy

Recent reports of C60-functionalized metal tips [Kelly, K. F., Sarkar, D., Hale, G. D., Oldenburg, S. J. & Halas, N. J. (1996) Science 273, 1371–1373] and carbon nanotube tips [Dai, H., Hafner, J. H., Rinzler, A. G., Colbert, D. T. & Smalley, R. E. (1996) Nature (London) 384, 147–151] demonstrate the potential of controlling the chemical identity and geometric structure of tip atoms in scanning tunneling microscopy (STM). This work reports the performance of a heteropolyacid (HPA)-functionalized Pt/Ir tip, which was formulated by contacting a mechanically formed tip with a solution of H3PW12O40 molecules. Attachment of an H3PW12O40 molecule on the metal tip was confirmed by observing the characteristic negative differential resistance (NDR) behavior of H3PW12O40 in tunneling spectroscopy. Atomic resolution images of bare graphite as well as of H6P2W18O62 HPA monolayers on graphite were successfully obtained with a Pt/Ir-HPA tip. In the H3PW12O40 molecule on a metal tip, it is likely that a terminal oxygen of W⩵O (an oxygen species projecting outward from the pseudospherical H3PW12O40 molecule) serves as an atomically sharp and stable tip. Additionally, superimposed superperiodic structures commensurate with the underlying graphite lattice were regularly observed with the modified tips. This result suggests that tip functionalization with these metal oxide molecules may enhance resolution in a fashion analogous to functionalization with C60.