Constructing Functional Mesostructured Materials from Colloidal Nanocrystal Building Blocks

Abstract

Through synthesizing colloidal nanocrystals (NCs) in the organic phase, chemists gain fine control over their composition, size, and shape. Strategies for arranging them into ordered superlattices have followed closely behind synthetic advances. Nonetheless, the same hydrophobic ligands that help their assembly also severely limit interactions between adjacent nanocrystals. As a result, examples of nanocrystal-based materials whose functionality derives from their mesoscale structure have lagged well behind advances in synthesis and assembly. In this Account, we describe how recent insights into NC surface chemistry have fueled dramatic progress in functional mesostructures. In these constructs, intimate contact between NCs as well as with heterogeneous components is key in determining macroscopic behavior. The simplest mesoscale assemblies we consider are networks of NCs constructed by in situ replacement of their bulky, insulating surface ligands with small molecules. Transistors are a test bed for understanding conductivity, setting the stage for new functionality. For instance, we demonstrated that by electrochemically charging and discharging networks of plasmonic metal oxide NCs, the transmittance of near infrared light can be strongly and reversibly modulated. When we assemble NCs with heterogeneous components, there is an even greater potential for generating complex functionality. Nanocomposites can exhibit favorable characteristics of their component materials, yet the interaction between components can also have a strong influence. Realizing such opportunities requires an intimate linking of embedded NCs to the surrounding matrix phase. We accomplish this link by coordinating inorganic anionic clusters directly to NC surfaces. By exploiting this connection, we found enhanced ionic conductivity in Ag2S-in-GeS2 nanocrystal-in-glass electrodes. In another example, we also found enhanced optical contrast when linking electrochromic niobium oxide to embedded tin-doped indium oxide (ITO) NCs. These dramatic effects emerge from reconstruction of the inorganic glass immediately adjacent to the NC interface. When co-assembling NCs with block copolymers, direct coordination of the polymer to NC surfaces again opens new opportunities for functional mesoscale constructs. We strip NCs of their native ligands and design block copolymers containing a NC tethering domain that bonds strongly, yet dynamically, to the resulting open coordination sites. This strategy enables their co-assembly at high volume fractions of NCs and leads to well-ordered mesoporous NC networks. We find these architectures to be exceptionally stable under chemical transformations driven by cation insertion, removal, and exchange. These developments offer a modular toolbox for arranging NCs deliberately with respect to heterogeneous elements and open space. We have control over metrics that define such architectures from the atomic scale (bonding and crystal structure) through the mesoscale (crystallite shapes and sizes and pore dimensions). By ...

Amino Acid-Based Stabilization of Oxide Nanocrystals in Polar Media: From Insight in Ligand Exchange to Solution 1H NMR Probing of Short-Chained Adsorbates

“…Such differences might play a role in the interaction with structure directing agents in the synthesis of mesoporous materials. 9 In addition, most amino acids contain only C, H, O and N, which burn off easily, and do not introduce new elements in, e.g., a nanocomposite of the high temperature NCs already suggested that Gln is directly bound to the NC surface. However, this is an indirect deduction and does not give any information about the fate of the original ligand, dodecanoic acid (DDAc).…”

“…Examples include (i) capacitor 5 or superconducting 6,7 nanocomposite thin films formed using a methanol-based precursor solution containing both molecular precursors for the matrix and dispersed metal oxide NCs (ii) nanocrystal-in-glass composites, fabricated from aqueous dispersions of indium tin oxide 8 or (iii) mesoporous mate-rials, constructed from NC building blocks by electrostatic interaction with a structure directing block-copolymer. 9 De-aggregation of NCs in nonpolar solvents is often attained by providing a steric stabilizer. [10][11][12][13][14] In order to achieve colloidal stability in polar solvents, the hydrophobic, steric ligands need to be exchanged for organic or inorganic polar ligands.…”

Colloidal Nanocrystal Frameworks

“…[ 24,27,30,[58][59][60][61] In these early studies, CNFs already demonstrated remarkable stability during both chemical and electrochemical transformations. For example, CdSe CNFs undergo sequential cation exchange to Cu 2 Se, Ag 2 Se, and PbSe CNFs without signifi cant framework degradation or loss of order.…”

“…To date, the most successful CNF assembly schemes have employed electrostatics as an interfacing strategy. [ 21,22,24,25,[27][28][29][30] Here, the surfaces of the nanocrystal components are ionic in character; both ligandcoated [ 31 ] and ligand-stripped nanocrystals [66][67][68] are suitable in this regard.…”

“…Colloidal nanocrystal frameworks (CNFs) have therefore emerged as an exciting new class of mesostructured porous materials that are conveniently assembled using polymeric ADAs ( Figure 1 ). [21][22][23][24][25][26][27][28][29][30] One important advantage in using colloidal nanocrystals as CNF building units, rather than molecular precursors, is that their physiochemical properties can be precisely controlled through synthesis in advance of the framework assembly scheme. Modern synthetic methods now provide access to an impressive selection of colloidal nanocrystals of controlled composition, [31][32][33] size, [ 34 ] shape, [ 35,36 ] and polydispersity.…”