Colloidal nanocrystal quantum dot assemblies as artificial solids

Abstract

The prospect of designing novel materials with electrical, optical, and magnetic properties by design has intrigued scientists and engineers for years. Building blocks for such “artificial solids” have emerged from recent advances in nanomaterial synthesis, characterization, and emerging understanding of their size-dependent properties. Colloidal semiconductor nanocrystal quantum dots (NQDs) stand out as an intellectually intriguing and experimentally advantageous system for the fundamental study of artificial solids and their technological development. The authors review the rapid evolution of artificial solids from an early theoretical concept towards the refined control of metamaterials with programmable electronic structure and their potential commercial applications, in particular, in next-generation energy technologies. The review is organized around the three independently adjustable parameters of artificial solids: (i) the electronic structure of NQD as artificial atom by tailoring the quantum confinement of the wave function, (ii) the interdot coupling as an artificial bond, and (iii) the self-assembly of NQDs into ordered superstructures as artificial crystals. The authors review elementary aspects of colloidal NQD synthesis as well as pertinent advances which have led to refined control over the NQD size, shape, and composition. Coupling between NQDs is reviewed in the context of an artificial bond; we summarize chemical and physical approaches to address the seemingly contradictory requirements of coupling nanostructures while preserving the effects of quantum-confinement. The authors review the self-assembly of NQDs into ordered superstructures in analogy to atomic crystal growth and discuss fundamental interactions between NQD and how they can be modulated to direct the growth of superlattices with predefined structures. Collectively, the experimental control over the properties of the artificial atom, bond, and crystal enable the systematic exploration of the electronic phase diagram of NQD solids. From an applied perspective, these advances have created an immensely fertile opportunity space technological applications of artificial solids in optoelectronic devices. The authors conclude with a perspective on three specific unresolved challenges ahead: (i) knowledge gaps concerning the detailed physiochemical nature of the NQD surface, (ii) limitations posed by the inherent inhomogeneity within the ensemble of NQDs, (iii) the true electronic structure of NQD solids, and (iv) the connection between NQD model systems in the laboratory and commercially deployable NQD technologies.