Nanoscale Investigation of Mixed Arrays of Keggin-Type and Wells−Dawson-Type Heteropolyacids (HPAs) by Scanning Tunneling Microscopy (STM)

The nanoscale investigation of mixed arrays of Keggin-type (H 3 PW 12 O 40 ) and Wells−Dawson-type (H 6 P 2 W 18 O 62 ) heteropolyacids (HPAs) deposited on a graphite surface was carried out by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS). Pure H 3 PW 12 O 40 and H 6 P 2 W 18 O 62 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, H 3 PW 12 O 40 and H 6 P 2 W 18 O 62 exhibited negative differential resistance (NDR) behavior at applied voltages of −1.14 and −0.80V, respectively. Mixtures of H 3 PW 12 O 40 and H 6 P 2 W 18 O 62 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 H 3 PW 12 O 40 and H 6 P 2 W 18 O 62 . 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.