Langmuir monolayers of gold nanoparticles: from ohmic to rectifying charge transfer

The lateral electrical/electronic conductivity of alkanethiolate-protected gold nanoparticles was evaluated at the air/water interface by using the Langmuir method. For particles with short protecting monolayers (C4Au and C5Au), the current-voltage profiles exhibited ohmic behaviors with conductivity several orders of magnitude smaller than that of bulk gold. The conductivity is found to decrease exponentially with increasing interparticle spacing. This is interpreted on the basis of electron tunneling/hopping between neighboring particles where the tunneling coefficient (β) is found around 0.5 Angst-1. With longer alkyl protecting layers (C6 and above), the nanoparticle monolayers demonstrated rectifying charge-transfer characters. This transition from ohmic to diode-like responses can be attributable to the nanocomposite structure of the particle molecules, where the chemical nature of the core and the protecting monolayers, along with the interparticle environment and ordering, are found to play an important role in regulating the electrical/electronic properties of the nanoassemblies