Cation-exchanged 12-molybdophosphates deposited on a graphite surface were imaged by scanning tunneling microscopy (STM), and their current-voltage behavior in tunneling spectroscopy (TS) measurements was examined. The periodicity of the polyanion arrays in the STM images was ca. 11-14 Å, and the spacing was dependent on the identity of the countercation. It was also observed that the characteristic negative differential resistance (NDR) behavior of 12-molybdophosphates, observed in TS measurements, is closely related to the reduction potential of the polyoxometalate and to the electronegativity of the countercation. The applied potential (sample to tip) at which NDR behavior was observed decreased with increases in the reduction potential of the HPA as well as with increases in electronegativity of the countercation. More electronegative countercations facilitated electron transfer between the cation and the heteropoly anion, apparently by acting as electron reservoirs, leading to NDR behavior at lower applied voltages and generating higher reduction potentials. This work demonstrates the first correlation between tunneling spectra measured by STM and the chemical properties of polyoxometalates.

Reported here are both STM images and spatially resolved tunneling spectra of four different polyoxometalate (POM) structural class members: Keggin structure, H(3)[PW(12)O(40)] (spherical); Finke-Droege (FD) structure, Na(16)[Cu(4)(H(2)O)(2)(P(2)W(15)O(56))(2)] (prolate spheroidal); Wells-Dawson (WD) structure, H(7)[P(2)Mo(17)VO(62)] (prolate spheroidal); and Pope-Jeannin-Preyssler (PJP) structure, K(12.5)Na(1.5)[NaP(5)W(30)O(110)] and (NH(4))(14) [NaP(5)W(30)O(110)] (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.

Images and tunneling spectra of H 3 PMo 12 O 40 and Cs 3 PMo 12 O 40 deposited on graphite were obtained in air using scanning tunneling microscopy before and after pyridine adsorption, and the effects of pyridine adsorption on the HPA arrays and on their tunneling spectra were examined. Fresh H 3 PMo 12 O 40 formed well-ordered two-dimensional arrays with a nearly square symmetry on the graphite surface, with a characteristic lattice spacing of about 10.8 Å, as determined from STM images. Upon interaction with pyridine, the STM images showed not only a change in array symmetry from square to hexagonal but also an increase in the lattice constant to ca. 16.5 Å, as well. This indicated that even in the two-dimensional array of H 3 PMo 12 O 40 , the ∼6 Å pyridine molecules were mainly adsorbed on the acid sites in the interstices between the polyanions, causing an increase in the interstitial spacing. In contrast, STM images of both fresh Cs 3 PMo 12 O 40 and pyridineadsorbed Cs 3 PMo 12 O 40 showed similar two-dimensional ordered array formation with hexagonal symmetry and a lattice constant of about 13.7 Å (reflecting the larger size of Cs + than H + ). This similarity of the surface arrays of Cs 3 PMo 12 O 40 with and without pyridine exposure is due to the absence of acid adsorption sites for pyridine in the two-dimensional structure of Cs 3 PMo 12 O 40 . Confirmation of pyridine adsorption by H 3 PMo 12 O 40 and not by Cs 3 PMo 12 O 40 was also obtained by infrared spectroscopy and by spatially resolved tunneling spectroscopy; the latter technique distinguished the anion, cation, and void positions in these arrays.

PMo 12 O 40 , x ) 1, 2, 2.5, 3) deposited on highly oriented pyrolytic graphite (HOPG) surfaces were successfully imaged by scanning tunneling microscopy (STM). All of these heteropoly acids (HPAs) formed clear twodimensional ordered arrays on graphite, and their periodicities were in good agreement with values determined by X-ray crystallography. Spatially resolved tunneling spectroscopy was used to demonstrate that the arrays imaged represented HPA monolayers, as the characteristic tunneling spectrum of graphite was obtained at high-symmetry interstitial sites in these arrays. The periodicity of the ordered arrays of the water-soluble Keggintype HPAs was ca. 10-11 Å. However, ordered arrays of the water-insoluble K 3 PMo 12 O 40 and H 3-x Cs x PMo 12 O 40 showed periodicities of ca. 12-14 Å, values somewhat larger than that of the pure acid form. The array spacing increased monotonically with increasing substitution of larger (Cs) cations, as expected. However, the nearly invariant periodicity of 2-D arrays of the water-soluble HPAs with changes in the number of protons suggests that there is little or no water of crystallization in these arrays, in contrast to the bulk compounds. STM permitted observation of the ordered structures of HPAs when deposited on a chemically inert surface, as well as the determination of the effects of changing framework and chargecompensating cations on these structures. The ubiquity of ordered array formation of these HPAs suggests that they can be utilized to create well-defined surfaces with more complex chemical functions than one typically encounters in studies of metal or oxide single-crystal surfaces.