In this work we have investigated the temperature-dependence of the band-edge photoluminescence decay of efficiently luminescing organically capped CdSe quantum dots ͑QDs͒ with diameters ranging from 1.7 to 6.3 nm over a broad temperature range ͑1.3-300 K͒. The overall trend is similar for all the investigated sizes, consisting of different temperature regimes. The low-temperature regime ͑below ϳ50 K͒ is characterized by purely radiative decay and can be modeled by a thermal distribution between a lower dark and a higher bright exciton state, with a size-dependent energy separation ͑viz., from 0.7 to 1.7 meV͒ and dark exciton lifetime ͑viz., from 0.3 to 1.4 s for QDs ranging from 6.3 nm to 1.7 nm in diameter͒. Nonradiative relaxation processes become increasingly important above ϳ50 K until the temperature antiquenching regime is reached, leading to a decrease in the nonradiative contributions and photoluminescence intensity recovery above ϳ200 K.

The influence of exchange interaction between paramagnetic ions on the luminescence properties is investigated for Mn 2ϩ-Mn 2ϩ , Cr 3ϩ-Cr 3ϩ , Cr 3ϩ-Gd 3ϩ , and Mn 2ϩ-Gd 3ϩ pairs. Two effects are reported: shortening of the lifetime of the spin-forbidden emission and a shift of emission lines or bands to a longer wavelength. The shortening of the lifetime of the spin-forbidden emission is important in Mn 2ϩ doped phosphors. For application of Mn 2ϩ-doped phosphors in displays the long lifetime of the Mn 2ϩ emission is a problem. Shortening of the lifetime is possible by exchange coupling with Mn 2ϩ neighbors, or possibly other paramagnetic neighbors. The results reported here show that the lifetime shortening depends on the exchange-coupling parameter J. The strongest coupling is observed in Cr 3ϩ-Cr 3ϩ pairs and the lifetime shortening is strong ͑e.g., in LaAlO 3 :Cr 3ϩ J ϭ 63 cm Ϫ1 and the lifetime is about 30 times shorter for the pair emission͒. The coupling in Mn 2ϩ-Mn 2ϩ and Cr 3ϩ-Gd 3ϩ pairs is weak. Typically, values for J are around 1 cm Ϫ1 and the lifetime is reduced by a factor of 2-10 ͑e.g., J is Ϫ1.0 cm Ϫ1 for exchange coupling between Cr 3ϩ and Gd 3ϩ ; the lifetime of the spin-forbidden Cr 3ϩ emission is 14 ms in GdAlO 3 compared to 56 ms in LaAlO 3 ͒. The lifetime shortening for the Mn 2ϩ emission is not related to the observed red shift for the pair emission. For the Mn 2ϩ-Gd 3ϩ pair the exchange interaction is very weak and no significant lifetime shortening could be measured for Gd 3ϩ-Mn 2ϩ pairs ͑24 ms for the Mn 2ϩ emission in YF 3 vs. 22 ms in GdF 3 ͒.

We have performed ultrafast absorption bleach recovery and fluorescence upconversion measurements ( approximately 100 fs time resolution) for three CdSe samples, with nanoparticle diameters of 2.7, 2.9, and 4.3 nm. The two types of experiments provide complementary information regarding the contributions of the different processes involved in the fast relaxation of electrons and holes in the CdSe quantum dots. Transient absorption and emission experiments were conducted for the 1S [1Se-1S3/2(h)] transition, 1S(e) and 1S3/2(h) representing the lowest electron (e) and hole (h) levels. The bleach recovery of the 1S transition shows a approximately 400-500 fs initial rise, which is followed by a size-dependent approximately 10-90 ps decay and finally a long-lived (approximately ns) decay. The fluorescence upconversion signal for the 1S transition shows quite different temporal behavior: a two times slower rise time (approximately 700-1000 fs) and, when the fluorescence upconversion signal has risen to about 20% of its maximum intensity, the signal displays a slight leveling off (bend), followed by a continued rise until the maximum intensity is reached. This bend is well reproducible and power and concentration independent. Simulations show that the bend in the rise is caused by a very fast decay component with a typical time of about 230-430 fs. Considering that the 1S quantum dot excitation is comprised of five exciton substates (F=+/-2, +/-1L, 0L, +/-1U, and 0U), we attribute the disparity in the rise of the bleaching and emission transients to the results from the dynamics of the different excitons involved in respectively the bleaching and fluorescence experiments. More specifically, in transient absorption, population changes of the F=+/-1U excitons are probed, in emission population effects for the F=+/-2 ("dark") and the F=+/-1L ("bright") exciton states are monitored. It is discussed that the fast (approximately 400-500 fs) rise of the bleach recovery is representative of the feeding of the F=+/-1U exciton (by filling of the 1S(e) electron level) and that the slower (approximately 700-1000 fs) feeding of the emissive +/-2, +/-1L excitons is determined by the relaxation of the hole levels within the 1S3/2 fine structure. Finally, the approximately 230-430 fs component, typical of the bend in the fluorescence transient, is attributed to the thermalization of the close-lying +/-2 ("dark") and +/-1L ("bright") excitons.

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The project FULLSPECTRUM -an Integrated Project (IP) in the terminology of the European Commission -pursues a better exploitation of the FULL solar SPECTRUM by (1) further developing concepts already scientifically proven but not yet developed and (2) by trying to prove new ones in the search for a breakthrough in photovoltaic (PV) technology. More specific objectives are the development of: (a) III-V multijunction cells (MJC), (b) solar thermo-photovoltaic (TPV) converters, (c) intermediate band (IB) materials and cells (IBC), (d) molecular-based concepts (MBC) for full PV utilisation of the solar spectrum and (e) manufacturing technologies (MFG) for novel concepts including assembling. MJC technology towards 40% efficiency will be developed using lower cost substrates and high light concentration (up or above 1000 suns). TPV is a concept with a theoretically high efficiency limit because the entire energy of all the photons is used in the heating process and because the non-used photons can be fed back to the emitter, therefore helping in keeping it hot. In the IBC approach, sub-bandgap photons are exploited by means of an IB. Specific IB materials will be sought by direct synthesis suggested by material-band calculations and using nanotechnology in quantum dot (QD) IBCs. In the development of the MBC, topics such as the development of two-photon dye cells and the development of a static global (direct and diffuse) light concentrator by means of luminescent multicolour dyes and QDs, with the radiation confined by photonic crystals, will be particularly addressed. MFG include optoelectronic assembling techniques and coupling of light to cells with new-optic miniconcentrators. r

The physical mechanism of photostimulated luminescence (PSL) of RbBr:Tl, resulting from x-ray irradiation, has been investigated. Spectroscopic methods have been utilized to identify the bromine F center as the occupied electron trap. By means of absorption studies the charge state of the dopant Tl was found to be monovalent. The spectral emission showed the characteristic Jahn–Teller splitting of the 3P1 state of the Tl+ and its known temperature dependence. PSL lifetime and efficiency experiments have been conducted in the temperature regime from liquid helium to 500 K to understand the charge transport from the F center to the activator and have been compared with corresponding data obtained from direct optical excitation into the A band of the Tl+. The observed temperature dependencies of both lifetimes could be explained by the properties of the s2 ion Tl+. As a resulting model for the PSL process a tunneling from the relaxed excited state of the F center to an excited state of the Tl2+ ion following photostimulation is suggested. The temporal, thermal, and spectral behavior of the subsequent radiative fraction of the deexcitation process is solely determined by the properties of the Tl+ ion in the RbBr matrix.

A comparative study of ultraviolet emission with peak wavelengths around 350 nm from oxidized porous silicon and that from SiO2 powder Interdiffusion of lateral composition modulated (GaP)2/(InP)2 shortperiod superlattices with different encapsulants

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