H₃PW₁₂O₄₀-functionalized tip for scanning tunneling microscopy

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   Figure 1

   (A) Molecular structure of the pseudospherical Keggin-type [PW₁₂O₄₀]³⁻ heteropolyanion. Oxygen atoms are represented as spheres. A sphere projecting outward in each WO₆ octahedron represents terminal oxygen species. (B) A simplified representation of the W⩵O projection.

   
   
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   Figure 2

   (A) A typical I–V spectrum obtained on bare graphite by using a Pt/Ir-H₃PW₁₂O₄₀ tip. (B) Distribution of NDR peak voltages of H₃PW₁₂O₄₀ on graphite obtained with a normal Pt/Ir tip (total number of tunneling spectra = 24). (C) Distribution of NDR peak voltages measured on bare graphite with a Pt/Ir-H₃PW₁₂O₄₀ tip (total number of tunneling spectra = 62).

   
   
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   Figure 3

   (A) Atomic resolution STM image of graphite obtained with a Pt/Ir-H₃PW₁₂O₄₀ tip. (B) STM image of H₆P₂W₁₈O₆₂ monolayer on graphite obtained with a Pt/Ir-H₃PW₁₂O₄₀ tip. (C) Schematic representation of unit cell of the H₆P₂W₁₈O₆₂ array. (D) Molecular structure of the ellipsoidal Wells–Dawson-type [P₂W₁₈O₆₂]⁶⁻ heteropolyanion. The polyanion consists of two defect-Keggin-type fragments, [PW₉O₃₄]⁹⁻. Each fragment consists of a central PO₄ tetrahedron sharing corners with nine WO₆ octahedra—the octahedra are somewhat distorted from an ideal octahedron. Three WO₆ octahedra form a compact group by sharing edges, whereas the remaining six octahedra in each of the [PW₉O₃₄]⁹⁻ fragments form a zigzag ring by alternately sharing edges and corners. The two fragments are linked by six nearly linear W—O—W bonds.

   
   
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   Figure 4

   (A-C) A set of unusual STM images of graphite obtained with a Pt/Ir-H₃PW₁₂O₄₀ tip with varying scan size. (D) A schematic representation of unit cells of the superperiodic structure and underlying graphite arrays for φ = 27°. φ represents an azimuthal angle between lattice vectors, a₁ and b₁. Superperiodic lattice vectors can be expressed in terms of the graphite lattice vectors by b₁ = 18a₁ + 15a₂ and b₂ = −15a₁ + 33a₂ for φ = 27°, and b₁ = 15a₁ + 18a₂ and b₂ = −18a₁ + 33a₂ for φ = 33°.

   
   
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   Figure 5

   Another set of unusual STM images of graphite obtained with a Pt/Ir-H₃PW₁₂O₄₀ tip with varying scan size, showing a superimposed hexagonal superperiodic structure (β = 60°, b₁ = b₂ = 14.97 Å) and underlying real-size graphite (α = 60°, a₁ = a₂ = 2.46 Å). φ is either 25.3° or 34.7°. Superperiodic lattice vectors can be expressed in terms of the graphite lattice vectors by b₁ = 4a₁ + 3a₂ and b₂ = −3a₁ + 7a₂ for φ = 25.3°, and b₁ = 3a₁ + 4a₂ and b₂ = −4a₁ + 7a₂ for φ = 34.7°.