Molecular shapes, orientation, and packing of polyoxometalate arrays imaged by scanning tunneling microscopy

Reported here are both STM images and spatially resolved tunneling spectra of four different polyoxometalate (POM) structural class members: Keggin structure, H-3[PW12O40] (spherical); Finke-Droege (FD) structure, Na-16[CU4(H2O)(2)(P2W15O56)(2)] (prolate spheroidal); Wells-Dawson (WD) structure, H-7[P2MO17VO62] (prolate spheroidal); and Pope-Jeannin-Preyssler (PJP) structure, K12.5Na1.5[NaP5W30O110] and (NH4)(14) [NaP5W30O110] (oblate spheroidal). In all four cases, the results demonstrate the formation of well-ordered 2-D inorganic POM anion arrays (composed of catalytically active molecular constituents) on graphite. Importantly, the image shapes and lattice spacings accurately reflect the POM anisotropies, permitting the determination of anion orientation with respect to the surface plane.