The lateral electrical/electronic conductivity of alkanethiolate-protected nanoparticles was evaluated by using the Langmuir method with an interdigitated arrays (IDA) electrode perpendicularly aligned at the air|water interface where a particle ensemble was trapped between the IDA fingers. For gold 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 is found around 0.5 A-1. With longer alkyl protecting layers (>C6), 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.
More importantly, using sensitive pulse voltammetric techniques, we also observed quantized charge transfer across a gold nanoparticle monolayer at the air|water interface. Differential pulse voltammetry revealed a series of well-defined voltammetric peaks, which are ascribed to the single electron transfer of the particle ensemble. This observation was interpreted on the basis of relatively weak electronic coupling between neighboring particles where the particles behave more individually.
The experimental protocol was also used to examine the conductivity properties of semiconductor nanoparticles. PbS and CdTe nanoparticles were used as the illustrating examples. It was found that the effective bandgap energies determined by the voltammetric methods were very consistent with those measured by spectroscopic techniques and decreased with increasing surface pressures. In addition, the ensemble conductivity properties were very sensitive to photoexcitation, providing an additional degree of manipulation of charge transfer at nanoscale interfaces.
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