Heteropolyacids
Heteropolyacids (HPAs) are early transition metal oxygen anion clusters (polyoxometalates) with well-defined molecular structures and tunable redox properties, which makes them useful as acid and oxidation catalysts and as model metal oxide molecules. The Keggin structure, for example [PW12O40]3-, has a central heteroatom tetrahedron surrounded by twelve WO6 octahedra, each carrying an outward-pointing terminal oxo group. The Wells-Dawson structure, for example [P2W18O62]6-, is an ellipsoidal anion built from two defect-Keggin fragments. On graphite, HPAs form ordered two-dimensional arrays, and their tunneling spectra show negative differential resistance at voltages that correlate with their reduction potentials. Related: Scanning tunneling microscopy, Negative differential resistance.
HPA molecules as STM tip modifiers (2002)
Song, Kitchin and Barteau used H3PW12O40 (about 11 to 12 Å across, twelve terminal W=O groups) to functionalize Pt/Ir STM tips. The rigid structure was proposed to present a single terminal oxygen as an atomically sharp, stable apex, giving atomic resolution of graphite and molecular resolution of H6P2W18O62 monolayers (primitive cell 14.6 Å, 35.2°). Because the NDR voltage depends on HPA structure and composition, the authors suggest HPAs as a tunable family of tip functionalization agents, extending the C60 approach to semiconducting oxides (Song et al. 2002). Summary: song2002.