The development of optical gain in chemically synthesized semiconductor nanoparticles (nanocrystal quantum dots) has been intensely studied as the first step toward nanocrystal quantum dot lasers. We examined the competing dynamical processes involved in optical amplification and lasing in nanocrystal quantum dots and found that, despite a highly efficient intrinsic nonradiative Auger recombination, large optical gain can be developed at the wavelength of the emitting transition for close-packed solids of these dots. Narrowband stimulated emission with a pronounced gain threshold at wavelengths tunable with the size of the nanocrystal was observed, as expected from quantum confinement effects. These results unambiguously demonstrate the feasibility of nanocrystal quantum dot lasers.

Advances in colloidal quantum dots: The confinement found in colloidal semiconductor quantum dots enables the design of materials with tunable properties. García de Arquer et al . review the recent advances in methods for synthesis and surface functionalization of quantum dots that enable fine tuning of their optical, chemical, and electrical properties. These important developments have driven the commercialization of display and lighting applications and provide promising developments in the related fields of lasing and sensing. —MSL

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today’s strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.

Because of the strong spatial confinement of electronic wave functions and reduced dielectric screening, the effects of carrier-carrier Coulomb interactions are greatly enhanced in semiconductor nanocrystals (NCs) compared with those in bulk materials. These interactions open a highly efficient decay channel via Auger recombination, which represents a dominant recombination pathway for multiexcitons in NCs. Furthermore, strong Coulomb coupling between charge carriers leads to extremely efficient direct photogeneration of multiexcitons by single photons via carrier (or exciton) multiplication. This review focuses on spectral and dynamical properties of multiexcitons in semiconductor NCs. The specific topics discussed here include the structure of NC electronic states, spectral signatures of multiexcitons in transient absorption and photoluminescence, exciton-exciton interaction energies, Auger recombination, and carrier multiplication. This chapter also briefly reviews the implications of multiexciton effects for practical technologies, such as NC lasing and photovoltaics.