2009-11-12
Nanocrystal Solids: A Modular Approach to Materials Design
Colloidal nanocrystals are considered promising building blocks for electronic and optoelectronic devices. Potentially, they can combine the advantages of crystalline inorganic semiconductors with sizetunable electronic structure and inexpensive solution-based device fabrication. Nanoparticles of different metals, semiconductors and magnetic materials can self-assemble from colloidal solutions into long range ordered periodic structures (superlattices). Combining two types of nanoparticles yields binary nanoparticle superlattices (BNSLs) exhibiting very rich phase diagrams with a multitude of close-packed and non-closepacked phases. Through a series of systematic studies of self-assembly phenomena in single- and multicomponent nanoparticle assemblies we demonstrate that observed structural diversity is a result of the intricate interplay of entropy-driven crystallization with isotropic and anisotropic interparticle interactions, such as van der Waals, Coulombic and dipolar forces.
The electronic properties of nanocrystal solids are determined by concentration of mobile carriers and electronic communication between individual nanocrystals. We developed several approaches to electronic doping of nanocrystal solids based on the formation of inter- and intra-nanocrystal charge transfer complexes. For example, hydrazine molecules adsorbed at the nanocrystal surface behave as n-type charge-transfer dopant whereas Au-PbS core-shell nanocrystals show stable p-type doping due to electron transfer from PbS shell into the Au core. The bulky and insulating nature of conventional organic capping ligands typically results in poor electronic coupling in the nanocrystal solids. To address this problem we demonstrated that molecular metal chalcogenide complexes can serve as versatile ligands for a broad range of colloidal nanocrystals. This new class of nanocrystal colloids provides a set of advantages such as all-inorganic design, small (<0.5 nm) interparticle spacing, easy thermal ligand-to-semiconductor conversion, and diverse compositional tunability for both nanocrystal and ligand constituents. As the model systems, we show electron mobility of ~1.5 cm2/Vs in arrays of CdSe nanocrystals and very high (~200 S/cm) conductivity in 5 nm gold nanocrystal solids capped with [Sn2S6]4- Zintl ions.