Nanoscale investigation of mixed arrays of Keggin-type and Wells-Dawson-type heteropolyacids (HPAs) by scanning tunneling microscopy (STM)
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abstract
The nanoscale investigation of mixed arrays of Keggin-type (H3PW12O40) and Wells-Dawson-type (H6P2W18O62) heteropolyacids (HPAs) deposited on a graphite surface was carried out by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS). Pure H3PW12O40 and H6P2W18O62 formed self-assembled and well-ordered two-dimensional arrays, and their molecular dimensions were in good agreement with values determined by XRD. In tunneling spectroscopy, H3PW12O40 and H6P2W18O62 exhibited negative differential resistance (NDR) behavior at applied voltages of -1.14 and -0.80 V, respectively. Mixtures of H3PW12O40 and H6P2W18O62 deposited from aqueous solution formed less ordered two-dimensional arrays than did the pure components. The mixed arrays exhibited a bimodal distribution of NDR peak voltages which was represented as a combination of the monomodal distributions of NDR peak voltages of pure H3PW12O40 and H6P2W18O62. Individual molecules in the mixed array were successfully identified by differences in molecular size (STM image) and electronic properties (NDR peaks). The ability to probe sites on a heterogeneous surface on a site-by-site basis is of potential importance in the design, characterization, and utilization of surfaces molecularly tailored with HPAs or other reactive complexes.
