Single Molecular Bridging of Au Nanogap Using Aryl Halide Molecules

Single molecular junctions of benzene dihalide molecules (<i>para</i>-X–(C₆H₄)–X, X = Cl, Br, I) binding to Au electrodes were systematically studied by using the scanning tunneling microscopy break junction (STM-BJ) technique. The STM-BJ characterization revealed that the single molecular junction was formed only with 1,4-diiodobenzene, which was due to its ability to form particularly stable halogen bonds with Au electrodes for the iodide anchoring group. The conductance and strength of the metal–molecule bond of the single 1,4-diiodobenzene molecular junction were compared with that of 1,4-benzenediamine (<i>para-</i>H₂N–(C₆H₄)–NH₂). The conductance of a single 1,4-diiodobenzene molecular junction was 3.6 × 10^–4 <i>G</i>₀ (<i>G</i>₀ = 2e²/h), which was smaller than 1 × 10^–2 <i>G</i>₀ measured for 1,4-benzenediamine. The distances to break single molecular junctions were 0.05 and 0.03 nm for single 1,4-diiodobenzene and 1,4-benzenediamine molecular junctions, respectively. The longer breakdown distance of the single 1,4-diiodobenzene molecular junctions indicated that the Au–I bond was stronger than that of the Au–NH₂ bond. The present work demonstrates that an iodide group can be utilized as an anchoring group for the single molecular junction