2.1 Fabrication of 2D superlattices of large Au nanoparticles

Needless to say, precise control of particle size and shape is required to fabricate their well-defined superlattices without mosaic structures. In 2001, we developed a simple and quite useful method to manipulate the size of Au nanoparticles by using the heat-treatment of small Au nanoparticles not in solution [40,41] but in the solid state [42,43], which is far from the conventional techniques [44–48]. A preparation of dodecanethiol-protected Au nanoparticles (DT-Au) as a source for the heat treatment followed the Brust’s two-phase (toluene/water) reaction procedure [49]. The organic (toluene) phase was then separated, evaporated completely in a rotary evaporator at 40 °C, and dried in vacuo at 30 °C for a day. The crude solid obtained was heat-treated at 150–250 °C at the heating rate of 2 °C min^–1, and held for 30 min. The heat-treated product was dissolved in a small amount of toluene and mixed with methanol to remove excess free dodecanethiol and TOAB. The dark brown precipitate was filtered off, washed with methanol, and redispersed in toluene. Octadecanethiol (ODT) was also used as a protective agent in place of dodecanethiol. This heat-treatment of as-prepared DT-Au nanoparticles at 150, 190 and 230 °C led to the thermodynamical particle growth, the particle sizes becoming 3.4 ± 0.3, 5.4 ± 0.7, and 6.8 ± 0.5 nm, respectively. As shown in Fig. 2 , their UV–vis spectra displayed surface plasmon absorptions at around 520 nm. These results indicate that the smaller nanoparticle has the lower melting point and the nanoparticles grow uniformly until they become stable at the heat-treatment temperature. As Maye et al. pointed out [41], an evolution of the small Au nanoparticles proceeds via sequential DT desorption, Au core coalescence, and DT re-protection. During the heat-treatment process, TOAB melted is thought to serve as a solvent in the particle growth process, because the uniform growth of nanoparticles was not observed without TOAB molecules. The Au contents increased from 79.7 to 94.7 wt% with an increase in the heat-treatment temperature, i.e., with the size, which is originated from a decrease in the surface area of Au nanoparticles.

Fig. 3 presents the TEM images of DT-Au nanoparticles heat-treated at 150, 190, and 230 °C, respectively. The particle sizes are proportional to the heat-treatment temperatures in this region. These particles form 2D superlattices with almost completely hexagonal packing on flat carbon-coated copper grids for TEM. The formation area of 2D superlattices is extended with an increase in the concentration of Au nanoparticles in toluene. The thickness of ligand layer estimated from the mean diameters calculated from TEM and AFM images was 1.25 nm, which is smaller than the DT ligand length (~1.8 nm) [50]. This result suggests that the DT ligands are interpenetrating each other in 2D superlattices of DT-Au nanoparticles [17]. Using ODT in place of DT gives the similar results except that the Au nanoparticles of 9.7 ± 0.9 nm are obtained by the heat-treatment at 250 °C [43]. Thus we can fabricate the self-assembled 2D superlattices of Au nanoparticles ranging 3.4–9.7 nm in size with hexagonal packing. The formation of self-assembled 2D superlattices of Au nanoparticles protected by quaternary ammonium bromide salts [32,51] or amines [52] has been investigated as well.

2.2 Electron transport property of 2D superlattice of 5.4-nm DT-Au nanoparticles

Recently, the electronic properties of metal nanoparticles have been investigated within the context of decreasing electronic device size features to the nanoscopic level [13]. Applications of individual nanoparticles as computer transistors, electrometers, chemical sensors, and in wireless electronic logic and memory schemes have been described and in some cases demonstrated [53], albeit somewhat crudely at this point. A conventional MOSFET (metal oxide semiconductor field effect transistor), which use ~10⁵ electrons for switching function, will no longer be able to control the flow of electrons as its size reaches the sub-50-nm regime. Thus the ordered metal nanoparticles with well-defined 2D configuration have been focused on to produce the novel nanoelectronic devices like a single electron transistor (SET) which works with one electron at room temperature, allowing the drastic reduction of electric power [1,2,53,54]. The monodisperse metal nanoparticles have potentiality to be the building blocks for nanoelectronic devices using the single electron tunneling effect – the correlated transfer of electrons one-by-one through the object [55].

Gorter and others argued that, provided the energy to charge a metal with a single electron, e/2 C (e is electron charge, C is metal capacitance), was larger than k_B T, electrons would be forced to flow through the metal in discrete integer amounts rather than in fluid-like quantities normally associated with transport in macroscopic materials [56]. Further reasoning led to the prediction that current-voltage (I–V) curves of a nanoscopic metal should be distinctly non-ohmic; that is, current steps should appear corresponding to the transport of 1 e^–, 2 e^–, 3 e^–, etc., currents through the metal. In order to avoid thermally induced tunneling processes at room temperature, the metal island of any single-electron device must be less than 10 nm in diameter. This dimension is difficult to reach with electron beam lithography or scanning probe microscopies, although it is now easily achieved by chemists using solution-phase nanoparticle synthesis methods.

Based on this background, we have fabricated the large-scale 2D superlattice of 5.4-nm dodecanethiol-protected Au nanoparticles on air-water interface by the LB method and transferred it onto the 500-nm gap Au electrodes on GaAs [57]. Fig. 4 presents the preliminary I–V curve of this 2D superlattice at 4.2 K. It was found that for large particles of ~5 nm in diameter the electron behavior was dominated by the Coulomb blockade effect at quite low temperature while the I–V response was ohmic at room temperature. To develop the single electron tunneling devices, the Coulomb blockade phenomenon should be observed at room temperature.

2.3 Fabrication of 2D superlattices of small Au nanoparticles

When one uses 2D superlattices of metal nanoparticles as nanoelectronic devices, making use of Coulomb blockade phenomenon at room temperature, metal nanoparticles smaller than 2 nm in diameter are required. Since the formation of ordered structures of such small nanoparticles is difficult mainly due to the weak attractive capillary force between the nanoparticles, one should make use of weak interaction between the protective agents or between the protective agent and the substrate (Fig. 5 ). If these interactions are too strong to be reversible, precipitates of nanoparticles are formed in solution or no rearrangement of metal nanoparticles on substrates occurs, respectively. First research on 2D superlattice formation consisting of metal nanoparticles smaller than 2 nm was reported by Schmid and co-workers early in 2000 [58], where they used an interaction between the protective ligand and the modified substrate to generate 2D superlattices of 1.4-nm Au nanoparticles. The 2D hexagonal and square lattices of Au₅₅ nanoparticles on polymer films were successfully prepared by dipping a poly(ethyleneimine) (PEI)-covered carbon-coated copper grid (for TEM) into an aqueous solution of [Au₅₅(Ph₂PC₆H₄SO₃H)₁₂Cl₆] nanoparticles. Classical acid-base reactions took place because of the NH functional groups on the PEI. Most of the nanoparticles were found in ordered arrangements of up to a few μm².

Recently, we succeeded in 2D self-assembly of 1.5–1.9 nm Au nanoparticles by using a newly synthesized protective ligand, 2,6-bis(1′-(n-thioalkyl)benzimidazol-2-yl)pyridine (TC_nBIP; see inset in Fig. 6a ) [59]. This ligand was designed to have two functional groups, a disulfide group and 2,6-bis-(benzimidazol-2-yl)pyridine (BIP) group, the former serving to produce small Au nanoparticles and the latter inducing the interaction between the ligands. The small Au nanoparticles were prepared by the NaBH₄ reduction of HAuCl₄$\dot{}$4 H₂O in the presence of a series of TC_nBIP in DMF/H₂O mixed solution. In UV–vis spectra of the resulting solutions, little plasmon resonance is observed, which means the formation of very small Au nanoparticles [60]. Fig. 6a,b presents the TEM image of TC₈BIP-protected Au nanoparticles of 1.9 nm in size and the schematic illustration of the interdigitation of ligands. The mean interparticle spacing is estimated to be ca. 3.8 nm, meaning the thickness of TC₈BIP layers between the Au nanoparticles to be 1.9 nm. Taking the length of TC₈BIP molecule to be ~1.5 nm into consideration, this result suggests that each Au nanoparticle is stabilized by one TC₈BIP layer, and presumably the TC₈BIP ligands are interdigitating each other as a result of a π–π interaction of BIP groups, which is also suggested by the formation of perfectly-ordered 2D superlattices on water (Fig. 6c). Tuning the ligand length could control the interparticle spacing of 2D self-assembled superlattices from 1.9 to 2.6 nm [61]. Furthermore, making use of an interligand complexation of TC_nBIP with ferrous ions could expand the interparticle spacing of 2D superlattices to 3.6 nm [62]. Now the electron transport properties of 2D superlattices of 1.5–1.9 nm TC_nBIP-protected Au nanoparticles using 50-nm gap and 500-nm interdigitated array electrodes.