Scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) studies of polyoxometalates (POMs) of the Wells–Dawson structural class

Abstract

Presented in this work is an extensive scanning tunneling microscopy (STM) study of self-assembled monolayers of polyoxometalates (POMs) belonging to the Wells–Dawson structural class. The effects of cation and framework atom substitutions in these POMs and their salts have been examined. The Wells–Dawson POMs and their salts formed self-assembled and well-ordered arrays on graphite surfaces. Tunneling spectroscopy (TS) measurements revealed that these POMs exhibited negative differential resistance (NDR) behavior in their tunneling spectra. The arrays of vanadium-substituted POMs, H 6+ x [P 2Mo 18− x V x O 62] ( x = 1, 2, 3), showed periodicities of ca. 11 Å × 14 Å. The shapes and periodicities of the corrugations in the STM images are consistent with the structure and the characteristic dimensions of the Wells–Dawson polyanion, [P 2Mo 18O 62] 6−, as determined from X-ray crystallography studies. Moreover, the STM results indicated that framework substitutions and increases in the number of charge-compensating protons had negligible effects on the array periodicities. Various salts of trisubstituted Wells–Dawson POMs, Q[P 2W 15Nb 3O 62] (Q = Na 9, Cs 9, (Bu 4N) 9, (Bu 4N) 5Na 3(Re(CO) 3)), also formed two-dimensional monolayer arrays with various periodicities. For this class of POM salts, the periodicities were greater than the anion dimensions, reflecting the size of the counter cation (Q). TS measurements revealed the positions of the counter cations in the POM salt arrays, and the positions were consistent with the cation positions found in the bulk crystal structures. For each class of Wells–Dawson anions, it was shown that the packing of the polyanions in the monolayers was not identical to, but resembled that found in bulk cleavage planes of the corresponding POM. A correlation between NDR peak voltage and reduction potential established for both POM series revealed that more reducible POMs showed NDR peaks at less negative applied voltage. In other words, a less negative NDR peak voltage corresponded to a higher reduction potential. This trend is consistent with those previously elucidated for POM catalysts with the Keggin structure.