Sol–Gel Encapsulation of Metal and Semiconductor Nanocrystals

Abstract The ability to encapsulate nanocrystals in glass matrices affords the opportunity to fabricate a variety of robust materials exhibiting properties tunable with the size, shape, or composition of the dopant. Among the various methods to these composites, sol–gel processing has the distinction of being a ‘wet chemistry’ approach, as it relies on high purity chemical reagents and mild reaction conditions to generate three‐dimensional metal‐oxide polymeric networks. Two general routes to noble metal and semiconductor nanocrystal‐sol–gel composites are explored. In one strategy, the synthesis of the nanocrystal occurs during the fabrication of the host matrix. This direct growth method has been investigated as an express route to nanocrystal‐doped glasses that typically involves the addition of metal salts to sol–gel precursors. Subsequent reduction or precipitation of the embedded metal salts, either with gases or other chemical reagents, leads to the formation of the desired nanocrystals. The second approach exploits the synthetic procedures already available for making high quality, crystalline nanoscale materials and focuses on a decoupled process in which the synthesis of the colloidal nanocrystals is independent of the fabrication of the composite network. In this way, the size, shape, and composition of the nanocrystals can be tailored for specific applications, promising a precise control over the dopant that is unavailable in the direct growth method. Improvements in these procedures have led to the preparation of uniformly dispersed nanocrystal‐sol–gel composites with high nanocrystal volume loadings and narrow nanocrystal size dispersities that have aroused considerable interest for their unique optical, catalytic, and electronic behaviors.