Electronic conductivity of semiconductor nanoparticle monolayers at the air|water interface

The electronic conductivity of PbS and CdTe nanoparticle monolayers was examined voltammetrically by using interdigitated array (IDA) electrodes at the air/water interface. Their band gap energies were estimated from the I-V responses and were very consistent with results obtained from optical measurements as well as solution electrochemistry. For CdTe nanoparticles, the I-V responses were analogous to those of a molecular diode with reproducible voltammetric behavior after repeated potential cycling. Interestingly, there appeared to be indications of particle surface trap states in the voltammetric responses that correlated with spectroscopic measurements. In addition, the band gap of the nanoparticle monolayers could be manipulated by the interparticle interactions, red shifting with decreasing interparticle separation. In contrast, the electroactive nature of the PbS particles led to the decomposition of the nanoparticles and hence deposition onto the electrode surface. The resulting voltammetric responses evolved from those typical of the faradaic reactions to a rectifying feature of much larger current scales, which finally became linear (ohmic) because of shorting between neighboring IDA fingers. In these studies, it was found that photoexcitation played an important role in regulating the current responses, providing a mechanistic basis on which to manipulate the electronic/electrical properties of semiconductor nanomaterials. The conductivity of the final interfinger deposits was about 2 orders of magnitude smaller than that for pure metallic lead, indicating some surface contamination and/or less than perfect crystalline structure.