Photoinduced Rotation of Colloidal Semiconductor Nanocrystals in an Electric Field.

We demonstrate that solution-phase semiconductor nanocrystals (NCs) undergo photoinduced rotation in an external electric field. Present measurements backed by theoretical calculations show that the rotation of colloidal NCs is driven by the excited-state dipole moment, which is counterbalanced by the solvent viscosity drag. Corresponding angular velocities range from 0.5°/ns for cubic CsPbBr3 NCs to 3°/ns for nanoparticles with a large photoinduced charge separation (CdSe/CdS core-shell and dot-in-a-rod NCs). Because of photoinduced rotation, solution-phase semiconductor NCs exhibited an order-of-magnitude increase in the spectral changes caused by the quantum confined Stark effect (QCSE), compared to solid NC assemblies. The enhanced QCSE of colloidal NCs reflected their global alignment in solution, which could be retained in a solid environment by slow crystallization. Overall, we expect that the demonstrated phenomenon of the colloidal nanocrystal rotation in an electric field will open up new avenues for developing electro-optical and voltage-sensitive applications.

Tuning the Dimensionality of Excitons in Colloidal Quantum Dot Molecules.

Electrically coupled quantum dots (QDs) can support unique optoelectronic properties arising from the superposition of single-particle excited states. Experimental methods for integrating colloidal QDs within the same nano-object, however, have remained elusive to the rational design. Here, we demonstrate a chemical strategy that allows for the assembling of colloidal QDs into coupled composites, where proximal interactions give rise to unique optoelectronic behavior. The assembly method employing "adhesive" surfactants was used to fabricate both homogeneous (e.g., CdS-CdS, PbS-PbS, CdSe-CdSe) and heterogeneous (e.g., PbS-CdS, CdS-CdSe) nanoparticle assemblies, exhibiting quasi-one-dimensional exciton fine structure. In addition, tunable mixing of single-particle exciton states was achieved for dimer-like assemblies of CdSe/CdS core-shell nanocrystals. The nanoparticle assembly mechanism was explained within the viscoelastic interaction theory adapted for molten-surface colloids. We expect that the present work will provide the synthetic and theoretical foundation needed for building assemblies of many inorganic nanocrystals.

Low-threshold laser medium utilizing semiconductor nanoshell quantum dots.

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 enhancing the lifetime of biexciton populations. This usually requires the employment of charge-delocalizing particle architectures, such as core/shell NCs, nanorods, and nanoplatelets. Here, we report on a two-dimensional nanoshell quantum dot (QD) morphology that enables a strong delocalization of photoinduced charges, leading to enhanced biexciton lifetimes and low lasing thresholds. A unique combination of a large exciton volume and a smoothed potential gradient across interfaces of the reported CdSbulk/CdSe/CdSshell (core/shell/shell) nanoshell QDs results in strong suppression of Auger processes, which was manifested in this work though the observation of stable amplified stimulated emission (ASE) at low pump fluences. An extensive charge delocalization in nanoshell QDs was confirmed by transient absorption measurements, showing that the presence of a bulk-size core in CdSbulk/CdSe/CdSshell QDs reduces exciton-exciton interactions. Overall, present findings demonstrate unique advantages of the nanoshell QD architecture as a promising optical gain medium in solid-state lighting and lasing applications.