Many potential applications of semiconductor nanocrystals are hindered by nonradiative Auger recombination wherein the electron-hole (exciton) recombination energy is transferred to a third charge carrier. This process severely limits the lifetime and bandwidth of optical gain, leads to large nonradiative losses in light-emitting diodes and photovoltaic cells, and is believed to be responsible for intermittency ("blinking") of emission from single nanocrystals. The development of nanostructures in which Auger recombination is suppressed has recently been the subject of much research in the colloidal nanocrystal field. Here, we provide direct experimental evidence that socalled "giant" nanocrystals consisting of a small CdSe core and a thick CdS shell exhibit a significant (orders of magnitude) suppression of Auger decay rates. As a consequence, even multiexcitons of a very high order exhibit significant emission efficiencies, which allows us to demonstrate optical amplification with an extraordinarily large bandwidth (>500 meV) and record low excitation thresholds. This demonstration represents an important milestone toward practical lasing technologies utilizing solution-processable colloidal nanoparticles.Colloidal semiconductor nanocrystals (NCs) have been the subject of intense research due to potential applications in low-threshold lasers, biological tags, third-generation photovoltaics, and light-emitting diodes (LEDs). 1,2 All of these technologies can benefit from the unique properties of NCs such as a size-tunable energy gap, high photoluminescence (PL) quantum yields, good stability, and chemical processability. However, many of these potential applications are hindered by Auger recombination, wherein the energy of one electron-hole pair (exciton) is nonradiatively transferred to another charge carrier. 3 In NCs, this process occurs on subnanosecond time scales and reduces optical gain lifetime, 4 restricts the available time to extract multiple excitons generated via carrier multiplication, 5 limits LED brightness due to the build-up of charged NCs, 6 and leads to PL intermittency ("blinking") that is typically observed in single-NC studies. 7,8 While the physics underlying Auger recombination in NCs is still not fully understood, general considerations suggest that the rate of this process is directly dependent upon the strength of carrier-carrier Coulomb coupling and the degree of spatial overlap between the electron and hole wave functions involved in the Auger transition. 9-11 Previous approaches to reducing Auger recombination rates have utilized the manipulation of both of these parameters. For example, using elongated NCs (quantum rods), one can separate interacting excitons along the rod axis, which leads to decreased exciton-exciton Coulomb coupling. 12 Also, one can reduce the rate of Auger transitions by separating electrons and holes between the core and the shell

A strong electron–hole exchange interaction (EI) in semiconductor nanocrystals (NCs) gives rise to a large (up to tens of meV) splitting between optically active ('bright') and optically passive ('dark') excitons. This dark–bright splitting has a significant effect on the optical properties of band-edge excitons and leads to a pronounced temperature and magnetic field dependence of radiative decay. Here we demonstrate a nanoengineering-based approach that provides control over EI while maintaining nearly constant emission energy. We show that the dark–bright splitting can be widely tuned by controlling the electron–hole spatial overlap in core–shell CdSe/CdS NCs with a variable shell width. In thick-shell samples, the EI energy reduces to <250 μeV, which yields a material that emits with a nearly constant rate over temperatures from 1.5 to 300 K and magnetic fields up to 7 T. The EI-manipulation strategies demonstrated here are general and can be applied to other nanostructures with variable electron–hole overlap.

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Biexciton photoluminescence (PL) quantum yields (Q(2X)) of individual CdSe/CdS core-shell nanocrystal quantum dots with various shell thicknesses are derived from independent PL saturation and two-photon correlation measurements. We observe a near-unity Q(2X) for some nanocrystals with an ultrathick 19-monolayer shell. High Q(2X)'s are, however, not universal and vary widely among nominally identical nanocrystals indicating a significant dependence of Q(2X) upon subtle structural differences. Interestingly, our measurements indicate that high Q(2X)'s are not required to achieve complete suppression of PL intensity fluctuations in individual nanocrystals.

In this work we propose and demonstrate a solution to the problems which arise when SiO2 monodisperse nanospheres of diameters under 300 nm or over 550 nm are used to obtain opal-based photonic crystals. If the nanospheres are too small, the sedimentation rate is very slow or even may not occur; if they are large enough, no significant order can be achieved because the velocity is too high. This method, based on the electrophoretic phenomenon, allows us to control the sedimentation velocity. Furthermore, other species of importance in this field, such as SiO2 spheres covered with a thick layer of TiO2, do profit from this method.