The technological advantages of silicon make silicon nanopartides, which can be used as quantum dots in a tandem configuration, highly relevant for photovoltaics. However, producing a silicon quantum dot solar cell structure remains a challenge. Here we use a gas aggregation cluster source to produce silicon nanopartides. At a particular experimental condition, "cauliflower" silicon particles are formed as observed by high-resolution transmission electron microscopy. They are likely formed by aggregation of smaller silicon nanoparticles and are in an intermediate state in the evolution toward large crystalline particles. These "cauliflower" silicon particles consist of nanodomains of crystalline and amorphous silicon (a-Si) of which the latter exhibits photoluminescence. The luminescence decay times of the silicon "cauliflower" particles are increased a thousand times by isolating the particles. Here we present a detailed study of such "cauliflower" silicon nanopartides which is part of the quest for using silicon quantum dots in solar cells.

We report on the temperature dependence of the band-edge photoluminescence decay of organically capped colloidal ZnSe quantum dots (QDs) in the size range from 4.0 to 7.5 nm. A similar trend is observed for all investigated sizes: the decay time is short (∼5 ns) above 20 K and increases sharply below 20 K, eventually reaching a constant value (270–400 ns) at sufficiently low temperatures (<4 K). The temperature regime in which the decrease of lifetime occurs depends on the QD size and is lower for larger QDs. This behavior can be modeled by a Boltzmann distribution between a lower long-lived and a higher short-lived exciton states, with an energy separation ranging from 3.3 ± 0.2 to 1.5 ± 0.1 meV in the 4.0 ± 0.3 to 7.5 ± 0.5 nm size range. We show that this energy separation is consistent with coupling of the lowest exciton state to a confined acoustic phonon.

In this letter we report successful doping of ZnTe magic size nanocrystals (MSNCs) with Mn(2+). Colloidal ZnTe MSNCs are prepared via a hot-injection method and doped with Mn(2+) via cation exchange. The doped MSNCs show an emission band centered at 620 nm with a radiative decay time of 45 μs, characteristic of Mn(2+) in ZnTe. The excitation spectrum of the Mn(2+) emission shows narrow absorption bands corresponding to different sizes of ZnTe MSNCs providing further evidence that the 620 nm emission originates from Mn(2+) incorporated in the ZnTe host, rather than Mn(2+) bound to the surface. The Mn(2+)-doped ZnTe clusters may serve as nuclei for the growth of larger ZnTe quantum dots doped with a single Mn(2+) ion.

Downconversion of one visible photon to two near-infrared photons may increase the efficiency of c-Si solar cells by 30%. The lanthanide ion couple Er3+–Yb3+ is well known for efficient upconversion but for the reverse process, downconversion, fast multiphonon relaxation from the F47/2 level has been shown to compete with downconversion. Here we report efficient downconversion for the Er–Yb couple in Cs3Y2Br9. The low phonon energy in this bromide host suppresses multiphonon relaxation and efficient two step energy transfer from the F47/2 level of Er3+ is observed and results in strong 1000 nm emission from Yb3+. Based on emission spectra and luminescence life time measurements an intrinsic downconversion efficiency close to 200% is determined.