Single-Electron Transfer in Nanoparticle Solids

Monolayer-protected nanoparticles exhibit unique electronic conductivity properties, which can be tailored by the combined effects of the conductive inorganic cores and the insulating organic shells. In scanning tunneling spectroscopic (STS) studies of isolated particles, the resulting current– potential (I–V) profile generally exhibits a Coulomb blockade in the central region, beyond which a Coulomb staircase (single-electron transfer; SET) may be identified. Such unique characteristics are the fundamental basis for the development of single-electron transistors. By contrast, in studies of nanoparticle ensembles that form (sub)micrometer-thick solid films, typically only linear (Ohmic) I–V behavior is observed, especially at a relatively high voltage bias, because of rampant structural defects within these particle solids that facilitate interparticle charge transfer (e.g., percolation effects). Fundamentally, the collective conductivity properties of organized assemblies of particles are found to be determined not only by the particle chemical structure (core size, shape, and surface ligands), but by the specific chemical environments and interparticle interactions as well. Whereas the electrochemical analogue of the Coulombstaircase phenomenon has been observed in studies of particles dissolved in an electrolyte solution, quantized charge transfer in nanoparticle solids has remained elusive. Thus, an immediate question arises—can single-electron transfer be realized with nanoparticle solids? The fact that nanoparticle solid thin films (monolayers or more complicated organized assemblies) can be readily fabricated by using the Langmuir– Blodgett (LB) or self-assembly technique means that achieving solid-state single-electron transfer will offer a significant advance towards the development of nanoparticle-based single-electron transistors without the necessity of sophisticated instrumentation (e.g., a scanning tunneling microscope). Herein we report a recent breakthrough using monolayers of moderately disperse gold nanoparticles, in which well-defined single-electron transfer is observed for the first time in the solid state. Our primary goal here is to identify key parameters that are important to realize SET in nanoparticle solid films. As the structural intermediate between isolated particles and thick particle films, particle monolayers exhibit unique electronic conductivity properties. For instance, Heath and co-workers observed an insulator–metal transition of a Langmuir monolayer of alkanethiolate-protected silver (AgSR) nanoparticles when the interparticle spacing was sufficiently small. Such a transition was also manifested in electrochemical impedance measurements, and in scanning electrochemical microscopy (SECM) studies. However, in these early studies, the particle-conductivity profiles did not exhibit the characteristics of quantized charging of the particle molecular capacitance, most probably because the particles used were too big and/or too polydisperse. In the present study, the gold nanoparticles were protected by a hexanethiolate monolayer (denoted as C6Au), and were synthesized by the Brust protocol. The particles then underwent careful fractionation by using a binary solvent–nonsolvent mixture of toluene and ethanol, and thermal annealing in toluene at 110 °C for 8 h in an oil bath in order to reduce the core-size dispersity. The fraction with an average core diameter of 2.0 nm and core-size dispersity of ca. 20 % (as determined by transmission electron microscopy measurements, with the particle composition approximated as Au314(C6)91 ) was used in the subsequent measurements. A monolayer of the C6Au nanoparticles was then deposited by using the LB technique (i.e., vertical deposition) at controlled interparticle separation (calculated by assuming a hexagonal close-packed structure) onto an interdigitated array (IDA) electrode, in order to take conductivity measurements. In a typical experiment, a known amount of the particle solution, typically 1 mg mL in hexane, was first spread onto the water surface (water resistance > 18 MX, from a Barnstead Nanopure Water System) in an LB trough (NIMA 611D) and at least 30 min was allowed for solvent evaporation before the first compression and between compression cycles. A representative isotherm is included in the Supporting Information (Fig. S1). The particle monolayer was then deposited onto an IDA electrode (25 pairs of gold fingers of dimensions 3 mm × 5 lm × 5 lm, from Abtech) whose surface was coated beforehand by a self-assembled monolayer of butanethiols to render it hydrophobic (the dipper speed was set at 1 mm min). Once deposition was complete, the IDA electrode with the particle monolayer was kept under vacuum (Cryogenic Equipment; JANIS Co.) overnight for solvent (water) evaporation. Electrochemical measurements were then carried out in vacuo at different temperatures (Lakeshore 331 temperature controller) with an EG&G PARC 283 C O M M U N IC A IO N