Optical and Electrical Properties of Three-Dimensional\nInterlinked Gold Nanoparticle Assemblies

Abstract

The optical and electrical properties of 11−20 nm thick films composed of ∼4 nm gold\nnanoparticles (Au-NPs) interlinked by six organic dithiol or bis-dithiocarbamate derivatives were compared\nto investigate how these properties depend on the core of the linker molecule (benzene or cyclohexane)\nand its metal-binding substituents (thiol or dithiocarbamate). Films prepared with the thiol-terminated linker\nmolecules, (1,4-bis(mercaptomethyl)benzene, 1,4-bis(mercaptomethyl)cyclohexane, 1,4-bis(mercaptoacetamido)benzene, and 1,4-bis(mercaptoacetamido)cyclohexane), exhibit thermally activated charge transport. The activation energies lie between 59 and 71 meV. These films show distinct plasmon absorption\nbands with maxima between 554 and 589 nm. In contrast, the film prepared with 1,4-cyclohexane-bis(dithiocarbamate) has a significantly red-shifted plasmon band (∼626 nm) and a pronounced absorbance\nin the near infrared. The activation energy for charge transport is only 14 meV. These differences are\nexplained in terms of the formation of a resonant state at the interface due to overlap of the molecular\norbital and metal wave function, leading to an apparent increase in NP diameter. The film prepared with\n1,4-phenylene-bis(dithiocarbamate) exhibits metallic properties, indicating the full extension of the electron\nwave function between interlinked NPs. In all cases, the replacement of the benzene ring with a cyclohexane\nring in the center of the linker molecule leads to a 1 order of magnitude decrease in conductivity. A linear\nrelationship is obtained when the logarithm of conductivity is plotted as a function of the number of\nnonconjugated bonds in the linker molecules. This suggests that nonresonant tunneling along the\nnonconjugated parts of the molecule governs the electron tunneling decay constant (β_N_-_CON), while the\ncontribution from the conjugated parts of the molecule is weak (corresponding to resonant tunneling). The\nobtained value for β_N_-_CON is ∼1.0 (per non-conjugated bond) and independent of the nanoparticle-binding\ngroup. Hence, the molecules can be viewed as consisting of serial connections of electrically insulating\n(nonconjugated) and conductive (conjugated) parts.