Electrical rectification from aligned diodesbased on the donor-(π-bridge)-acceptor molecules

Abstract

As traditional devices containing silicon transistors begin to approach their physical limits, new systems composed of organic molecules are being considered for molecularscale devices of the future. The present work reports on the electrical properties of molecular diodes, especially observations of electrical rectification from molecular systems based on donor-(π-bridge)-acceptor molecules. For this purpose three types of molecular assembly were incorporated and their growth was observed with the quartz crystal microbalance (QCM) technique. Covalent self-assembly proved to be the most efficient method of forming well-ordered molecular films compared to those obtained via LB and ESA techniques. SAMs of Q3CNQ molecules yielded higher rectification than their LB analogues and achieved rectification ratio of 30 at ± 1V for every sample. On the other hand ESA films, in which molecular alignment of the physisorbed cationic dye was controlled by selfassembly of the anionic component, were probably more disordered, but exhibited higher (and sample-dependent) rectification ratios with a maximum of 450 at ± 1V. QCM also showed the phenomena of trapped water molecules within the physisorbed ESA monolayer that affected molecular order and also the electrical properties of the samples. Scanning tunnelling microscopy (STM), incorporated for obtaining current-voltage (I-V) characteristics from samples, showed that stearic hindrance has to be taken into consideration when designing donor-(π-bridge)-acceptor rectifiers. Sufficient isolation of donor and acceptor groups by the π-bridge is essential in order to prevent delocalisation of molecular orbitals over the entire molecule. Therefore, implementation of the Aviram-Ratner model of molecular rectification became possible although molecules investigated here did not possess the proposed σ-bridge. Additionally, the rectification effect arising from geometrical asymmetry induced by electrode-linking alkyl chains was shown to be negligible here, which is contrary to other theories of molecular rectification.