Investigation of Wells-Dawson heteropoly oxofluorotungstates by scanning tunneling microscopy and tunneling spectroscopy
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abstract
Ammonium salts of Wells-Dawson heteropoly oxofluorotungstates with different framework compositions, (NH4)9Mn(II)W17O56F 6NaH49H2O, (NH4)9Cu(II)W17O56 F6NaH49H2O, (NH4)8Mn(III)W17O56 F6NaH48H2O, and (NH4)8Fe(III)W17O56 F6NaH48H2O, were deposited on highly oriented pyrolytic graphite surfaces and imaged by scanning tunneling microscopy. All samples formed monolayers of two-dimensional, well-ordered, close-packed arrays. The lattice constants of these arrays in each case were roughly ca. 13 16 these values were consistent with the molecular dimensions of typical Wells-Dawson heteropolyanions (10.98 14.48 ), plus charge-compensating NH4+ cations (ionic radius of 1.43 ) between the heteropolyanions. The presence and positions of these cations in the surface arrays were subsequently confirmed by tunneling spectroscopy (TS), and the positions were consistent with those found in the bulk crystal structures of these salts. TS measurements also showed that the oxofluorotungstates exhibited negative differential resistance (NDR) behavior in the negative sample bias regime of their current-voltage spectra. Their NDR peak voltages varied depending on the identity of the monosubstituted polyatom, Mn2+, Cu2+, Mn3+, and Fe3+. The reduction potentials of heteropoly oxofluorotungstates increased and NDR peak voltages appeared at less negative applied values with the decreasing electronegativity of the monosubstituted polyatom. The correlation between reduction potentials and NDR peak voltages established for these heteropolyacids (HPAs) was consistent with trends observed in the cation-exchanged and heteroatom-substituted HPAs; the more reducible HPAs showed NDR behavior at less negative applied voltages. However, the trends in NDR peak voltage and reduction potential with respect to the electronegativity of the monosubstituted polyatom were opposite of those observed for the cation-exchanged and heteroatom-substituted HPAs.
