SCANNING TUNNELING MICROSCOPY AND TUNNELING SPECTROSCOPY OF ORDERED HETEROPOLYANION AND ISOPOLYANION ARRAYS ON GRAPHITE
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Four polyanion derivatives, H3PMo12O40, Na4SiW12O40, H7SiW9V3O40, and (NH4)6V10O286H2O, deposited on graphite, were imaged by scanning tunneling microscopy (STM) in air. STM images of these samples revealed the formation of ordered two-dimensional arrays on graphite having periodicites consistent with the molecular dimensions of the molecules as determined from X-ray crystallographic data. Spatially resolved current-voltage (I-V) spectroscopy was shown to differentiate between the isopolyacid arrays of (NH4)6V10O286H2O and the underlying graphite substrate, the spectrum for the latter being obtained at interstitial positions in the polyanion array. Current maxima and negative differential resistance (NDR) were also noted in the I-V spectra of the polyanions. NDR has been explained previously by several different tunnel junction models, all of which involve localized electronic states of the sample. Thus potential applications of tunneling through these ordered polyanion arrays range from adsorbate identification, to correlations of electronic properties with oxide reactivity, to fabrication of nanoscale devices. 1992, American Chemical Society. All rights reserved.