Electrical Breakdown of Aliphatic and Aromatic Self-Assembled Monolayers Used as Nanometer-Thick Organic Dielectrics

This paper describes a new type of metal−insulator−metal junction based on self-assembled monolayers (SAMs). The junction consists of a drop of liquid mercury supporting a SAM, in contact with a flat metal surface (M'), also supporting a SAM: that is, a Hg−SAM/SAM−M' junction. This junction is stable, easy to assemble, and reproducible: it can generate areas of contact down to 0.01 mm2 without photolithography. These properties suggest this junction as a useful "test bed" for experiments in molecular electronics. The junction allowed measurements of electrical properties of SAMs on different metals: M' = Ag, Au, Cu, and Hg. The work described here focused on the electrical breakdown voltage (BDV, the maximum voltage sustained by the junction). The BDV depends on M' (BDVAg = 3.2 ± 0.5 V; BDVHg = 3.1 ± 0.4 V; BDVCu = 3.0 ± 0.3 V; BDVAu = 1.5 ± 0.2 V for SAMs formed from hexadecanethiol) and correlates with the organizational parameters of the SAM on M': it increases as the packing density increases and the tilt angle decreases. The BDV also depends on the chain length of the alkanethiol forming the SAM for the same metal surface (M'). Alkanethiol SAMs on Ag having carbon chain lengths longer than C14 can sustain a constant electrical field up to 8 ± 1 × 108 V/m. This value for the BDV is similar to that of bulk polyethylene. A survey of SAMs with different chemical structures shows that the BDV correlates overall with the thickness of the densely packed hydrocarbon portion of the SAM: aliphatic and aromatic SAMs of the same thickness show similar BDVs.