Electric Fields and Interference Effects inside Noncentrosymmetric Multilayer Films at Electrode Surfaces from Electrochemically Modulated Surface Plasmon Resonance Experiments

Abstract

The electric field profile inside a self-assembled noncentrosymmetric zirconium phosphonate (ZP) multilayer film at a gold electrode is determined by the in situ optical technique of electrochemically modulated surface plasmon resonance (EM-SPR). In these experiments, changes in the index of refraction (Δn) of a ZP film during potential modulation are measured via the EM-SPR differential reflectivity curves. Modulated SPR experiments on the ZP films incorporated into air-gap capacitors are used to relate Δn to changes in the electric field strength (ΔE) inside the film. The noncentrosymmetric ZP films utilize the nonlinear optical chromophores [5-[4-[[4-[(6-hydroxyhexyl)sulfonyl]phenyl]azo]phenyl]pentoxy]phosphonic acid (HAPA) and [1-[4-[4-[(N-(2-hydroxyethyl)-N-methyl)amino]phenyl]azo](5-phosphonopentyl)]pyridinium bromide (PY-AZO). For a 6.7 nm ZP HAPA film, a change in electrode potential (Δφm) of 50 mV corresponds to a change in electric field strength (ΔE) of 1.4 × 104 V/cm. EM-SPR experiments on mixed ZP multilayers of HAPA and the centrosymmetric molecule 1,10-decanediylbis(phosphonate) are used to measure the spatial variations of the electric fields within the ultrathin films. Evidence for ion and solvent penetration into the ZP films is observed in the electrochemical environment. Additional studies of interference effects in mixed multilayers of HAPA and PY-AZO are used to verify the retention of directional order during the ZP multilayer deposition process.