Architectonic Quantum Dot Solids

Three things largely determine the electronic properties of a crystal: the energy levels of the atoms or lattice sites, the coupling between adjacent sites, and the symmetry of the solid. Imagine being able to control each of these properties separately, and therefore being able to “design” a solid with a prescribed set of electronic properties. For an atomic solid, this would amount to being able to tune the electronegativity of an atom, to control the strength of the covalent interactions within the lattice, and to choose the crystal structure. While the chemistry of the periodic table does not permit such control, the chemistry of “artificial atoms”,1 or quantum dots (QDs), does. In this Account, we will discuss one example of “designing” a unique electronic property into a QD solid. This property, which is the ability of the solid to reversibly pass through a metal−insulator (MI) transition under ambient conditions, is something that has not been previously demonstrated for a more traditional solid. A two-dimensional (2D) hexagonal superlattice of organically passivated silver QDs was fabricated as a Langmuir monolayer. Selecting a particular size of QD controlled the (super)lattice site energies. The coupling between adjacent QDs was coarsely controlled by selection of the organic surface passivant, and precisely controlled by compressing the superlattice using the Langmuir technique. By using the title “Architectonic2 Quantum Dot Solids”, we emphasize that many of the collective properties of the superlattice were rationally designed into the material.