Molecules that mimic Schottky diodes

Abstract

Self-assembled monolayers of cationic donor–(π-bridge)–acceptor dyes coupled with anionic donors exhibit asymmetric current–voltage (I–V) characteristics when contacted by Au or PtIr probes. Rectification ratios of 3000 at ±1 V are obtained from Au–S–C₁₀H₂₀–A⁺–π–D|D^–|Au structures in which the cationic moiety is 5-(4-dimethylaminobenzylidene)-5,6,7,8-tetrahydro-isoquinolinium and the counterion is copper phthalocyanine-3,4′,4″,4‴-tetrasulfonate(SAM 1). Similar behaviour, with a high rectification ratio of 700–900 at ±1 V, is also obtained for the CuPc(SO₃^–)₄ salt of 4-[2-(4-dimethylaminonaphthalen-1-yl)-vinyl]-quinolinium (SAM 2). The properties are dependent upon the D–π–A⁺ moieties which, for these highly rectifying salts, have sterically locked non-planar structures causing the conjugation to be effectively broken. Its effect on the electrical asymmetry is less spectacular when the cationic species is sterically unhindered: the rectification ratio decreases to 15–70 at ±1 V for films of the 4-[2-(4-dimethylaminophenyl)-vinyl]-pyridinium salt (SAM 3), which has single-ring substituents on opposite sides of the –CHCH– bridge and an almost planar D–π–A⁺ structure. Rectification ratios from the sterically hindered structures are on a par with electrical asymmetries from metal–insulator–metal (MIM) devices where oxide-induced Schottky barriers dominate the behaviour.