GeSn alloy quantum dots (QDs) were synthesized with sizes ranging from 1-3 nm exhibiting visible orange-red photoluminescence. Composition dependent optical properties were characterized and supported by theoretical calculations. Structural analysis suggests the QDs are diamond cubic phase, characteristic of GeSn thin films and nanocrystals (NCs) reported to date.

Hybrid nanomaterials composed of metal-semiconductor components exhibit unique properties in comparison to their individual counterparts, making them of great interest for optoelectronic applications. Theoretical and experimental studies suggest that interfacial interactions of individual components are of paramount importance to produce hybrid electronic states. The direct cross-linking of nanoparticles (NPs) via controlled removal of the surfactant ligands provides a route to tune interfacial interactions in a manner that has not been thoroughly investigated. Herein, we report the synthesis of CdSe/Ag heteronanostructures (aerogels) via oxidation induced self-assembly of thiol-coated NPs and the evolution of optical properties as a function of composition. Three hybrid systems were investigated, where the first and second excitonic energies of CdSe were matched with plasmonic energy of Au or Ag NPs and Ag hollow NPs. Physical characterization of the aerogels suggests the presence of an interconnected network of hexagonal CdSe and cubic Ag NPs. The optical properties of hybrids were systematically examined through UV-vis, photoluminescence (PL), and time-resolved (TR) PL spectroscopic studies that indicate the generation of alternate radiative decay pathways. A new emission (640 nm) from CdSe/Ag aerogels emerged at Ag loading as low as 0.27%, whereas absorption band tailing and PL quenching effects were observed at higher Ag and Au loading, respectively. The TRPL decay time of the new emission (∼600 ns) is markedly different from those of the band-edge (1.83 ± 0.03 ns) and trap-state (1190 ± 120 ns) emission maxima of phase pure CdSe, supporting the existence of alternate radiative relaxation pathways in sol-gel derived CdSe/Ag hybrids.

Optical transition energies and carrier dynamics in colloidally synthesized 2.0 ± 0.8 nm Ge1-xSnx quantum dots (x = 0.055-0.236) having visible luminescence were investigated using steady-state and time-resolved photoluminescence (PL) spectroscopy supported by first-principles calculations. By changing Sn content from x = 0.055 to 0.236, experimentally determined HOMO-LUMO gap at 15 K was tuned from 1.88 to 1.61 eV. Considering the size and compositional variations, these values were consistent with theoretically calculated ones. At 15 K, time-resolved PL revealed slow decay of luminescence (3-27 μs), likely due to the recombination of spin-forbidden dark excitons and recombination of carriers trapped at surface states. Increasing Sn concentration to 23.6% led to 1 order of magnitude faster recombination. At 295 K, PL decays were 3 orders of magnitude faster (9-28 ns) owing to the thermal activation of bright excitons and carrier detrapping from surface states.

The authors discuss a relatively comprehensive theoretical and experimental study aimed on unveiling the dominant efficiency loss mechanism at high injection levels in InGaN light-emitting diodes(LEDs), which still limits their application for general lighting despite the breathtaking performance demonstration. A large body of theoretical and experimental data ascribes the observed efficiency loss to overflow of hot electrons aggravated by nonuniform distribution of carriers in the active region as the primary origin of the efficiency droop-phenomenon, but Auger recombination has also been invoked as the genesis of the efficiency loss. The electron overflowand the associated efficiency loss can be reduced substantially by inserting, in the n-side of the InGaN active region, an InGaN stair-case electron injector (SEI) with a step-like increased indium composition to operate as an “electron cooler.” In contrast to electron-blocking layer usually employed to prevent the electron leakage from the active region, the SEI does not impede hole injection due to the absence of valence band offset with p-GaN. Moreover, SEI does not generate piezoelectric polarizationfield in addition to differential spontaneous polarization field that pulls down the conduction band at the AlGaN/GaN interface aggravating the electron rollover. In terms of the active region design, owing to their high three-dimensional density of states, it is argued that double heterostructures (DHs) are more attractive for general-lighting LEDs than necessarily quantum wells. The authors demonstrate that DH-based LED active regions,particularly wide ones and those composed of multiple DHs separated by thin (3 nm)In0.06Ga0.94N barriers of reduced barrier height, meant to allow efficient hole transport across the active regions, naturally act as an electron cooler, thus considerably reducing the electron overflow at high injection. However, a wide separation of electron and hole distribution functions in DHs wider than 6 nm substantially reduces the radiative recombination efficiency at injection current densities below∼200 A/cm2. Consequently, the LEDs with dual 6 nm and quad (4×) 3 nm DHs separated by 3-nm In0.06Ga0.94N barriers exhibit the highest external quantum efficiency with substantially reduced efficiency degradation at injection current densities of special interest for low-voltage general-lighting applications. The authors conclude that, for achieving the highest possible LED efficiency, it is imperative that optimum the SEI and the active region should be designed to operate in unison.

In this paper we report the synthesis, structure and Li ion conductivity of a new tetragonal garnet phase Nd 3 Zr 2 Li 7 O 12. In line with other tetragonal garnet systems, the Li is shown to be ordered in the tetrahedral and distorted octahedral sites, and the Li ion conductivity is consequently low. In an effort to improve the ionic conductivity of the parent material, we have also investigated Al doping to reduce the Li content, Nd 3 Zr 2 Li 5.5 Al 0.5 O 12 , and hence introduce disorder on the Li sublattice. This was found to be successful leading to a change in the unit cell symmetry from tetragonal to cubic, and an enhanced Li ion conductivity. Neutron diffraction studies showed that the Al was introduced onto the ideal tetrahedral garnet site, a site preference also supported by the results of computer modelling studies. The effect of moisture on the conductivity of these systems was also examined, showing significant changes at low temperatures consistent with a protonic contribution in humid atmospheres. In line with these observations, computational modelling suggests favourable exchange energy for the Li + /H + exchange process.