Electrical and Electrochemical Properties of Nanocomposite Thin Films Formed by Exchange-Precipitation Route from Nanocr
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y be exploited for nanostructure designs according to sizes, facets or shapes.
Scheme 1: A schematic illustration of the exchange -crosslinking-precipitation route for the nanocrystal thin film formation.
EXPERIMENT Synthesis. The 2-nm Au nanoparticles encapsulated with alkanethiolate shell (Au2-nm ) were synthesized by the standard two-phase method [2-3]. The 5-nm Au nanoparticles (Au5-nm ) were evolved by heating treatment of the presynthesized Au2-nm particles, details of which were recently reported [9]. Preparations. In a typical experiment, 1,9-nonanedithiols (NDT) were mixed in an organic solvent (hexane or toluene) with the decanethiolate (DT)encapsulated nanoparticles in the concentration ranges of 0.1~10 µM nanoparticles and 0.5~5.0 mM dithiols. The substrate was immersed into the solution for the film deposition. This process involved an exchange and crosslinking reactions between the DT shell and the solution NDT and a consequent precipitation of the linked film on the substrate. The film thickness was controlled by immersion time. Most substrates were gold thin films on Cr-primed glass slides and polycrystalline gold disks. Other substrates included glassy carbon disks and glass slides. The resulting films were thoroughly rinsed with pure solvent before characterizations. Instrumentation. Conductivity measurements were performed using an interdigitated microelectrode (IME, Microsensor Systems) as the substrate for thin film deposition. The resistance measurement used a computer-interfaced multimeter. The temperature dependence was determined in a sealed glass tube purged with N2, and immersed in a bath of different organic solvents mixed with dry ice. Electrochemical measurements were performed using a computer-interfaced potentiostat (EG&G Model 273A). A conventional threeelectrode cell was employed, with a platinum coil as the auxiliary electrode and a Ag/AgCl (sat’d KCl) as the reference electrode. The geometric electrode area was 0.34 cm2. All solutions were deaerated with ultrahigh purity argon before the electrochemical experiment. UV-Visible (UV-Vis) spectra were acquired with a HP 8453 spectrophotometer. Transmission electron microscopy (TEM) was performed on Hitachi H-7000 Microscope (100 kV). RESULTS AND DISCUSSION Morphological Properties. Figure 1 shows two representative TEM images for thin films of NDT-Au2-nm and NDT-Au5-nm deposited on carbon-
Figure 1:TEM images of NDT -Au2-nm (A, left) and NDT-Au5-nm (B, right) films.
coated copper grids upon short immersion time. The preparation ensured submonolayer coverages for both nanoparticle films. The films display features of clustered or networked particles with individually-identifiable sizes. Such features were observed for films ranging from sub-monolayer to multilayer coverage. The image clearly reveals the absence of aggregations of the linked particles to larger core sizes. For the NDT-Au5-nm film (B), the average separation distance of the particles, estimated by measuring the edge-to-edge distances of the particles in the clustered
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