Three-Dimensionally Ordered Gold Nanocrystal/Silica Superlattice Thin Films Synthesized via Sol-Gel Self-Assembly

Nanocrystals (NCs) exhibit unique size-dependent optical, electronic, and chemical properties. The ability to adjust properties by controlling size, shape, composition, crystallinity, and structure has led to a wide range of potential applications for NCs in areas such as optics, electronics, catalysis, magnetic storage, and biological labeling. Furthermore, NC assembly into 2D and 3D superlattices is of interest for development of “artificial solids” with collective optical and electronic properties that can be further tuned by the NC spacing and arrangement. To date, there have been three approaches to the fabrication of superlattice solids and thin films. The most commonly used approach involves evaporating a drop of NC organic solution on a solid support (e.g., a transmission electron microscopy (TEM) grid, mica, highly ordered pyrolytic graphite (HOPG), etc.), forming face-centered cubic (fcc), body-centered cubic (bcc), or hexagonal close-packed (hcp) superlattice films. Heath and co-workers reported a method to form 2D hcp superlattice films of silver NCs by Langmuir–Blodgett deposition. In a third approach, a homogenous precipitation of superlattice solids was prepared by the slow addition of nonsolvents into an NC organic solution. Over a long aging time, well-shaped crystal superlattice solids with fcc structure were finally obtained. The common problem in these approaches is that the NCs that have been used are alkane-chain stabilized, and are therefore hydrophobic and soluble only in organic solvents. The formation of an ordered NC superlattice is limited to organic solvents, and relies on the van der Waals interactions between interdigitated alkane chains surrounding the NCs. The thermally defined, interdigitated alkane chains result in weak mechanical stability of the superlattice. In such materials, charge transport is conducted through the interdigitated organic media between NCs. Recent results indicate that it is desirable to incorporate NCs in inorganic thin film matrices such as silica or titania for achieving chemical and mechanical robustness and enhanced device functionality. Early efforts focused on the encapsulation of metal NCs inside sol–gel matrices by introduction of metal nanoparticles or metal precursors followed by either thermal decomposition or reduction. Recently, mesoporous materials have been used as templates to create hybrid silica materials infiltrated with metal or semiconducting NCs. There are several disadvantages of the above methods, which include: 1) the NC arrays inside the final materials exhibit a poorly defined or less-ordered structure. This is problematic for fundamental studies of charge transport, where repeatable/reproducible results are required.