Langmuir - blodgett films constructed from a charge-transfer complex and gold nanoparticles

A charge-transfer complex between an amphiphilic bis(tetrathiafulvalene) (bis-TTF) annulated macrocycle (1) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p- quinodimethane (F₄-TCNQ^-), (1²⁺)(F ₄-TCNQ^-)₂, which can form molecular-assembly nanowires by application of the Langmuir-Blodgett (LB) method, was hybridized with gold nanoparticles with an average diameter of 13 nm. LB films of (1 ²⁺)(F₄-TCNQ)₂ were transferred onto a substrate surface from a subphase containing gold nanoparticles at concentrations ranging from 10^-6 to 10^-4 M, based on the number of gold atoms in the subphase (c_Au). One-dimensional nanowires and zero-dimensional gold nanoparticles were found to coexist in films transferred onto mica by a single withdrawal from a subphase with c_Au less than 1 × 10^-5 M, whereas films transferred from a subphase with c_Au greater than 1 × 10^-4 M formed network structures of gold nanoparticles, which was confirmed by AFM measurements as well as electronic spectroscopy. The room-temperature electrical conductivities of the latter LB films (c_Au > 1 × 10^-4 M) were 2 or 3 orders of magnitude higher than those of the former (c_Au < 1 × 10 ^-5 M). The films, which contained nanoparticle network structures, showed two types of conducting behavior depending on the temperature range (above and below ∼150 K). At higher temperatures, conduction was dominated by (1²⁺)(F₄-TCNQ^-)₂, which is an intrinsic semiconductor, whereas collective tunneling charge transport in the gold nanoparticle network structure was observed at lower temperatures. In the prepared films, the gold nanoparticles were randomly connected to each other, and a random single electron tunneling array yielded a finite threshold voltage (V_th) at lower temperatures due to Coulomb blockade behavior and three-dimensional correlation of the conducting paths.