Electroluminescence of oxide-covered tantalum electrode as inorganic and organic probes

Electroluminescent (EL) emissions from the rare earth metal Tb3+, oxalate (C2O42−) and Ru(bpy)3+ incorporated into Ta/Ta2O5 electrodes in aqueous solutions were observed. Since the EL emission energies are smaller than the band gap of 4.1 eV of Ta2O5, the resulting emissions are considered to be generated from surface states in the oxide band gap region. To understand the origin and the role of the surface states at the thin oxide film, cyclic voltammetry (CV), scanning electrochemical microscopy (SECM) with ultramicroelectrode, Fourier transform infrared (FTIR) spectroscopy, photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) studies were performed. Oxalate and Ru(bpy)3+ were identified at the Ta/Ta2O5 surface. That is, incorporated inorganic and organic species can act as the luminescence centers for free electrons and holes. Since the emission spectra depend upon the concentration of intermediate surface states within the band gap, the EL might be useful for analytical sensing by the controlled introduction of the label materials to produce surface states.