Rectification in molecular assemblies of donor–acceptor monolayers
Introduction
The concept that organic molecules can be utilized in electronic or opto-electronic devices has gained a tremendous attention in the past decades. A lot of researches are being carried out on molecular rectification to validate the early prediction by Aviram and Ratner (AR) back in 1974 [1]. They proposed that a D–σ–A molecule connecting an electron donor moiety (D) with low ionization potential to an electron acceptor moiety (A) with high electron affinity through a saturated ‘sigma’ bridge (σ) might act as a rectifier when sandwiched between two metallic contacts. Inelastic electron transfer from high lying LUMO of acceptor to the low lying HOMO of donor through the sigma bridge was proposed to the reason behind the asymmetry in the current–voltage (I–V) characteristics. Metal/molecule/metal sandwich structures based on Langmuir–Blodgett (LB) monolayers [2], [3], [4], [5] or self-assembled monolayer (SAM) [6], [7], [8] were fabricated to study single-molecule properties. Vacuum-evaporated noble metals or Hg electrode [6], [9], [10] as top contacts were used to characterize the I–V characteristics of the organic monolayer. Recent studies have employed scanning tunneling microscopy to probe the I–V of molecular junctions with more limited contact areas [5], [8]. The asymmetry in the molecules was introduced by synthesis, connecting electron donor and acceptor moieties, sometimes with additional alkyl chains [7], [11].
Instead of a unimolecular D–σ–A pair, we aimed at D/A assembly with one donor and one acceptor monolayer electrostatically bound through an inert polymer monolayer. In this Letter, we report molecular rectification based on a D/A assembly, which provides many advantages. Instead of complex chemical synthesis, a variety of D/A assemblies can now be imagined and build via layer-by-layer (LbL) technique [12], [13]. Moreover, the donor and acceptor components in a D–σ–A molecule are not always sufficiently isolated, so that their molecular orbitals are sometimes delocalized over the entire molecule. In the molecular D/A case, on the other hand, a variety of assemblies can be build from the known donor and acceptor molecules. The components’ molecular orbitals also remain localized within the respective moieties of the D/A assemblies, which is a prerequisite to observe unimolecular rectification. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the individual components can be kept in mind in building the D/A assemblies. Moreover, electrostatic binding of donor and acceptor molecules amongst themselves and with the bottom electrode provides stable contact when top electrode (Hg drop) is brought into contact. In this work, monolayer of copper (II) phthalocyanine (CuPc), which is a donor and monolayer of Rose Bengal dye in the Xanthane class (as an acceptor) have been used to form the D/A assembly.
Section snippets
Experimental
Copper(II) phthalocyanine (CuPc), rose bengal (RB) and poly(allylamine hydrochloride) (PAH) were purchased from Aldrich Chemical Co. and used without further purification. The molecular structures of RB and CuPc are shown in Fig. 1. LbL films of CuPc and RB were deposited by electrostatic adsorption with PAH as the polycation. Since the LbL deposition relies on surface charge reversal during each layer adsorption, deposition of one molecular layer or monolayer was also assured. Monolayer films
Results and discussion
I–V characteristics were recorded for several CuPc/RB molecular assemblies, CuPc monolayer, and RB monolayer with Si and Hg as the two electrodes. A typical I–V plot from CuPc/RB based D/A assembly is shown in Fig. 2a. The figure shows that rectification is observed in the molecular CuPc/RB junction. Higher current is observed in the negative bias regime. A range of I–V characteristics were taken at various spots on the films with different maximum voltage (VMax). Rectification is observed in
Conclusions
In conclusion, we have demonstrated molecular rectification in donor/acceptor electrostatic assemblies. Instead of complex synthesis of asymmetric unimolecular D–σ–A pair, donor and acceptor moieties were bounded by electrostatic self-assembly. The results presented here substantiate that the rectifying behavior is intrinsic to the asymmetry of the D/A assemblies and is truly a molecular property of the supramolecule that bridges the metal electrodes. Electron flow was favored in the
Acknowledgment
B.M. acknowledges CSIR Junior Research Fellowship No. F.NO. 2-56/2002(I) EU.II (Roll No. 506120).
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