Experimental observation of the role of countercations in modulating the electrical conductance of Preyssler-type polyoxometalate nanodevices

Abstract

Polyoxometalates are nanoscale molecular oxides with promising properties that are currently explored for molecule-based memory devices. In this work, we synthesize a series of Preyssler polyoxometalates (POMs), [Na⊂P₅W₃₀O₁₁₀]¹⁴⁻, stabilized with four different counterions, H⁺, K⁺, NH₄⁺, and tetrabutylammonium (TBA⁺). We study the electron transport properties at the nanoscale (conductive atomic force microscopy, C-AFM) of molecular junctions formed by self-assembled monolayers (SAMs) of POMs electrostatically deposited on the ultraflat gold surface prefunctionalized with a positively charged SAM of amine-terminated alkylthiol chains. We report that the electron transport properties of P₅W₃₀-based molecular junctions depend on the nature of the counterions; the low-bias current (in the voltage range [−0.6 V; 0.6 V]) gradually increases by a factor of ∼100 by changing the counterion in the order: K⁺, NH₄⁺, H⁺ and TBA⁺. From a statistical study (hundreds of current–voltage traces) using a simple analytical model for charge transport in nanoscale devices, we show that the energy position of the lowest unoccupied molecular orbital (LUMO) of P₅W₃₀ with respect to the Fermi energy of the electrodes increases from ∼0.4 eV to ∼0.7 eV and that the electrode coupling energy also increases from ∼0.05 to 1 meV in the same order from K⁺, NH₄⁺, H⁺ to TBA⁺. We discuss several hypotheses on the possible origin of these features, such as a counterion-dependent dipole at the POM/electrode interface and counterion-modulated molecule/electrode hybridization, with, in both cases, the largest effect in the case of TBA⁺ counterions.