Molecular rectification: characterisation of a dye sandwiched between gold electrodes

Langmuir-Blodgett (LB) films of the dye, 4-{2-[4-(N,N-dibutylaminophenyl]vinyl}-N-octadecylquinolinium octadecyl sulfate, are non-centrosymmetric and exhibit quadratic enhancement of the second-harmonic intensity with the number of layers when corrected for absorbance. The optimum effective susceptibility at 1.064 mum, monolayer thickness and refractive indices at the fundamental and harmonic frequencies are chi(eff)((2)) = 80 pm V-1, d = 2.46 +/- 0.05 nm, n(omega) = 1.51 +/- 0.02 and n(2omega) = 1.65 +/-0.02 respectively. In addition, metal/ (LB film)/metal devices of the dye exhibit asymmetric current-voltage characteristics and, under forward bias, the current increases exponentially with the electric field. The behaviour is attributed to molecular rectification, the assignment being unambiguous for oxide-free gold electrodes.