Molecular Signature of Polyoxometalates in Electron Transport of Silicon-based Molecular Junctions

TL;DR

This study demonstrates the covalent immobilization of polyoxometalates on silicon substrates and characterizes their electron transport properties, revealing their potential for molecular electronic applications.

Contribution

It reports the synthesis and covalent attachment of novel POM hybrids with remote diazonium groups and analyzes their electron transport behavior in silicon-based junctions.

Findings

Polyoxometalates show distinct tunneling barriers of 1.6-1.8 eV.

Homogeneous monolayers confirmed by electron transport measurements.

Molecular electronic signatures are evident in solid-state electrical properties.

Abstract

Polyoxometalates (POMs) are unconventional electro-active molecules with a great potential for applications in molecular memories, providing efficient processing steps onto electrodes are available. The synthesis of the organic-inorganic polyoxometalate hybrids [PM$_{11}$O$_{39}$(Sn(C$_6$H$_4$)C$\equiv$C(C$_6$H$_4$)N$_2$)]$^{3-}$ (M = Mo, W) endowed with a remote diazonium function is reported together with their covalent immobilization onto hydrogenated n-Si(100) substrates. Electron transport measurements through the resulting densely-packed monolayers contacted with a mercury drop as a top electrode confirms their homogeneity. Adjustment of the current-voltage curves with the Simmons equation gives a mean tunnel energy barrier of 1.8 eV and 1.6 eV, for the Silicon-Molecules-Metal (SMM) junctions based on the polyoxotungstates (M = W) and polyoxomolybdates (M = Mo), respectively. This follows the trend observed in the electrochemical properties of POMs in solution, the polyoxomolybdates being easier to reduce than the polyoxotungstates, in agreement with lowest unoccupied molecular orbitals (LUMOs) of lower energy. The molecular signature of the POMs is thus clearly identifiable in the solid-state electrical properties and the unmatched diversity of POM molecular and electronic structures should offer a great modularity.