Ordered array formation and negative differential resistance behavior of cation-exchanged heteropoly acids probed by scanning tunneling microscopy

Cation-exchanged 12-molybdophosphates deposited on a graphite surface were imaged by scanning tunneling microscopy (STM), and their current-voltage behavior in tunneling spectroscopy (TS) measurements was examined. The periodicity of the polyanion arrays in the STM images was ca. 11-14 Angstrom, and the spacing was dependent on the identity of the countercation. It was also observed that the characteristic negative differential resistance (NDR) behavior of 12-molybdophosphates, observed in TS measurements, is closely related to the reduction potential of the polyoxometalate and to the electronegativity of the countercation. The applied potential (sample to tip) at which NDR behavior was observed decreased with increases in the reduction potential of the HPA as well as with increases in electronegativity of the countercation. More electronegative countercations facilitated electron transfer between the cation and the heteropoly anion, apparently by acting as electron reservoirs, leading to NDR behavior at lower applied voltages and generating higher reduction potentials. This work demonstrates the first correlation between tunneling spectra measured by STM and the chemical properties of polyoxometalates.