Auger decay of multiple excitons represents a significant obstacle to photonic applications of semiconductor quantum dots (QDs). This nonradiative process is particularly detrimental to the performance of QD-based electroluminescent and lasing devices. Here, we demonstrate that semiconductor quantum shells with an “inverted” QD geometry inhibit Auger recombination, allowing substantial improvements to their multiexciton characteristics. By promoting a spatial separation between multiple excitons, the quantum shell geometry leads to ultralong biexciton lifetimes (>10 ns) and a large biexciton quantum yield. Furthermore, the architecture of quantum shells induces an exciton–exciton repulsion, which splits exciton and biexciton optical transitions, giving rise to an Auger-inactive single-exciton gain mode. In this regime, quantum shells exhibit the longest optical gain lifetime reported for colloidal QDs to date (>6 ns), which makes this geometry an attractive candidate for the development of optically and electrically pumped gain media.

Low-dimensional semiconductors hold strong promise for future energy applications. These nanomaterials are inexpensive to process and offer a broad spectrum of attractive quantum-mechanical properties. The notorious problem of low-dimensional nanostructures, however, lies in their limited performance under high energetic loads when more than one exciton per particle is created. Multiple excitons undergo fast annihilation, causing efficient roll-off in energy-intensive applications, including high-brightness light-emitting diodes, X-ray scintillators, and solar cells. In this Focus Review, we highlight an emerging type of low-dimensional semiconductors that make it possible to avoid such multiexciton (MX) energy losses. Recently demonstrated colloidal quantum shells benefit from the spatial separation of multiple excitons, which leads to extraordinary improvements to MX lifetimes and MX quantum yield. This makes the quantum shell morphology attractive for solution-processed optical and electrical devices. Here, we compare the optoelectronic properties of quantum shells against other low-dimensional semiconductors and discuss their emerging opportunities in solid-state lighting and energy-harvesting applications.

Multiple-exciton (MX) generation is beneficial to many applications of semiconductors, including photoinduced energy conversion, stimulated emission, and carrier multiplication. The utility of MX processes is generally enhanced in small-size semiconductor nanocrystals exhibiting the quantum confinement of photoinduced charges. Unfortunately, a reduced particle volume can also accelerate the nonradiative Auger decay of multiple excitations, greatly diminishing the MX feasibility in nanocrystal-based photovoltaic, laser, and photoelectrochemical devices. Here, we demonstrate that such Auger recombination of biexcitons could be suppressed through the use of a quantum-well (QW) nanoshell architecture. The reported nanoscale geometry effectively reduces Coulomb interactions between photoinduced charges underlying Auger decay. This leads to increased biexciton lifetimes, as was demonstrated in this work through ultrafast spectroscopy methods. In particular, we observed that the biexciton lifetime of CdSe-based QW nanoshells (CdS/CdSe/CdS) was increased more than 30 times relative to that of zero-dimensional CdSe NCs. The slower biexciton decay in QW nanoshells was attributed to a large confinement volume, which compared favorably to other existing MX architectures.

The present work evaluates the feasibility of quantum computing with exciton qubits in coupled colloidal semiconductor nanocrystals (NCs). A strategy for manipulating twoqubit states of colloidal NC hetero-dimers is described. We show that a sequence of laser pulses with the same photon energy can bring excitonic states of a nanocrystal hetero-dimer into entanglement and perform arbitrary qubit rotations (quantum gates). Our simulations of a realistic two-particle assembly of CdSe/CdS core/shell NCs demonstrate that such two-qubit gate operations can be driven by optical parametric oscillators with a theoretical error of 0.1%. A strategy for upscaling two-qubit hetero-dimers to N-qubit exciton gates in semiconductor NC assemblies is discussed.

Colloidal semiconductor nanocrystals (NCs) represent a promising class of nanomaterials for lasing applications. Currently, one of the key challenges facing the development of high-performance NC optical gain media lies in...