ABSTRACT
One of the most attractive commercial applications of semiconductor nanocrystals (NCs) is their use in lasers. Thanks to their high quantum yield, tunable optical properties, photostability, and wet-chemical processability, NCs have arisen as promising gain materials. Most of these applications, however, rely on incorporation of NCs in lasing cavities separately produced using sophisticated fabrication methods and often difficult to manipulate. Here, we present whispering gallery mode lasing in supraparticles (SPs) of self-assembled NCs. The SPs composed of NCs act as both lasing medium and cavity. Moreover, the synthesis of the SPs, based on an in-flow microfluidic device, allows precise control of the dimensions of the SPs, i.e. the size of the cavity, in the micrometer range with polydispersity as low as several percent. The SPs presented here show whispering gallery mode resonances with quality factors up to 320. Whispering gallery mode lasing is evidenced by a clear threshold behavior, coherent emission, and emission lifetime shortening due to the stimulation process.
ABSTRACT
Colloidal quantum dots (QDs) show great promise as LED phosphors due to their tunable narrow-band emission and ability to produce high-quality white light. Currently, the most suitable QDs for lighting applications are based on cadmium, which presents a toxicity problem for consumer applications. The most promising cadmium-free candidate QDs are based on InP, but their quality lags much behind that of cadmium based QDs. This is not only because the synthesis of InP QDs is more challenging than that of Cd-based QDs, but also because the large lattice parameter of InP makes it difficult to grow an epitaxial, defect-free shell on top of such material. Here, we propose a viable approach to overcome this problem by alloying InP nanocrystals with Zn(2+) ions, which enables the synthesis of InxZnyP alloy QDs having lattice constant that can be tuned from 5.93 Å (pure InP QDs) down to 5.39 Å by simply varying the concentration of the Zn precursor. This lattice engineering allows for subsequent strain-free, epitaxial growth of a ZnSezS1-z shell with lattice parameters matching that of the core. We demonstrate, for a wide range of core and shell compositions (i.e., varying x, y, and z), that the photoluminescence quantum yield is maximal (up to 60%) when lattice mismatch is minimal.
ABSTRACT
Semiconductors are indispensable as the active light-emitting element in many optoelectronic devices. However, even the purest bulk semiconductors suffer from considerable nonradiative recombination leading to low photoluminescence efficiencies. Zero-dimensional quantum dots show a much better carrier-to-photon conversion caused by confinement of the excitons but suffer from nonradiative recombination when assembled into a solid, due to exciton energy transfer. Here, we report on the shape-dependent optical properties of self-assembled supraparticles composed of CdSe/multishell nanocrystals. All supraparticles show stable and bright photoluminescence in ambient up to high excitation intensities. When the supraparticles are deposited on a silicon surface their spherical shape is deformed due to drying. In addition to single-exciton emission, we observe bright emission from multiexciton states at high excitation powers. In contrast, supraparticles that retain their perfectly spherical shape show a spectrum with sharp Mie whispering gallery modes, while multiexciton emission is absent.
ABSTRACT
Cores for thought! A new [Mn4 O4 ]6+ "cubane" core complex (L6 Mn4 O4 ) with six facially bridging phosphinate chelate ligands (L- =(MePh)2 PO2- ) was synthesized. Photo-excitation releases molecular O2 by intramolecular coupling of two core oxygen atoms and selective rearrangement to a [Mn4 O2 ]6+ "butterfly" core ([L5 Mn4 O2 ]+ ; see scheme). Thus the Mn4 O4 cubane core exhibits unique reactivity in O2 evolution which may account for its presence in the photosynthetic enzyme.
