We report on the temperature dependence of the photoluminescence ͑PL͒ spectrum and of the PL relaxation dynamics for colloidal CdSe/ ZnS core/shell quantum dots ͑QDs͒ embedded in an inert polystyrene matrix. We demonstrate that the confinement energy in the QDs is independent of the temperature. The coupling with both acoustic and optical phonons is also studied. Quantum confinement results in a strong increase of the excitonacoustic-phonon coupling constant, up to 71 eV/ K, and in a reduced exciton-longitudinal-optical ͑LO͒-phonon coupling constant, down to 21 meV, with respect to bulk CdSe. In addition, we demonstrate that the main nonradiative process that limits the quantum efficiency of the QD at room temperature is the thermal escape from the dot assisted by scattering with four LO phonons. Thermally activated trapping in surface states is also observed at low temperature, with an activation energy of about 15 meV.

A recently proposed technique is described that provides improved nearsie-farside (NF) decompositions of elastic scattering amplitudes. The technique, involving a new resummation formula for Legendre partial wave series, reduces the importance of unphysical contributions to NF subamplitudes, which can appear in more conventional NF decompositions. Applications are made to a strong absorption model that arises in chemical and nuclear physics, as well as to a 16 O + 12 C optical potential at E lab = 132 MeV. ìÑä 539.188 ùãÖåÖçíÄêçõÖ îàáàäé-ïàåàóÖëäàÖ èêéñÖëëõ ïàåàóÖëäÄü îàáàäÄ ÚÓÏ 23 ‹ 2 2004 IMPROVED NEARSIDE-FARSIDE DECOMPOSITION 7

We report on the temperature and size dependence of the photoluminescence of core CdTe colloidal quantum dots (QDs). We show that at temperatures lower than 170 K a thermally activated transition between two different states separated by about 12-20 meV takes place. At temperatures higher than 170 K, the main nonradiative process is thermal escape assisted by multiple longitudinal optical (LO) phonons absorption. Moreover, we show that quantum confinement affects both the exciton-acoustic phonons and the exciton-LO phonons coupling. The coupling constant with acoustic phonons is strongly enhanced in QDs (up to 31µeV/K) with respect bulk CdTe (0.7µeV/K). On the contrary, the exciton-LO phonons coupling constant decreases as the dot size decreases (down to 14 meV with respect 24.5 meV in the bulk).

No abstract

Using low temperature single molecule spectroscopy on rigid-rod conjugated polymers we are able to identify homogeneously broadened, strongly polarized emission from individual chromophore units on a single chain. Gated fluorescence spectroscopy allows real time imaging of intramolecular energy transfer as the chain behaves as a series of weakly interacting chromophores. Energy transfer is controlled by the chromophoric spectral linewidth, which depends on temperature. Linewidths exceeding intramolecular disorder lead to incoherent chromophore coupling and collective fluorescence phenomena.

In the last few years, white-light emission from organic compounds has been the subject of increasing interest due to its potential impact on the lighting industry and backlight applications. In order to obtain white light from organic lightemitting devices (OLEDs), the simultaneous excitation of different molecular species emitting at different primary colors is required. [1,2] So far, the most exploited approach has been the fabrication of multilayer devices by consecutive evaporations or co-evaporation of different emitting compounds.[1±6]However, this technique requires complex technological processes and a large amount of wasted organic materials, resulting in relatively high fabrication costs. Spin-coating of a blend of different soluble emitters in a single layer seems to be a more cost-effective technique. [7,8] Though cheaper, this approach has the drawback that customized color combinations are not always possible due to Förster transfer from the highenergy emitting material (donor) to the low-energy one (acceptor), which induces emission only from the lower-gap compound. [9,10] An alternative approach which overcomes such a problem is to blend two blue-light-emitting organic molecules of different electron affinities, whose interaction gives rise to exciplex states. [11,12] The combination of the exciplex emission with the blue-light emission of the individual donor molecule results in the generation of white light. However, in both of these approaches the purity of the color emission is strongly dependent on the relative concentration of the different molecular species and, generally, on the applied voltage. This is a problem for lighting applications in which the source intensity (but not the color) has to be varied by changing the applied electrical power. [13] In this frame, the synthesis of a soluble compound showing white-light emission in the solid state is strongly desirable because it would enable the fabrication of a new class of devices which combine the simplicity and low cost of the single-layer spin-coated structures without the problems connected with the material concentration and bias.In this work, we demonstrate a bright single-layer white OLED realized by spin-coating a single emitting molecular material, namely 3,5-dimethyl-2,6-bis(dimesitylboryl)-dithieno[3,2-b:2¢,3¢-d]thiophene (compound 1 in Fig. 1). In 1, white electroluminescence is achieved by the superposition of the intrinsic blue±green-light emission (BGE) of the single molecule with a red-shifted emission (RSE) that occurs only in the solid state. The origin of the RSE peak is due to the formation of cross-like dimers between the molecules. This has been demonstrated by optical measurements and theoretical calculations, and also performed on similar compounds functionalized with different substitution patterns (2, 3, and 4 in Fig. 1) in order to control the self-assembling of the molecules. By virtue of the excellent properties of compound 1, namely the good electron-acceptor characteristics of the dimesitylboryl moieties...

Semiconductor nanoplatelets (NPLs) have emerged as a very promising class of colloidal nanocrystals for light-emitting devices owing to their quantum-well-like electronic and optical characteristics. However, their lower photoluminescence quantum yield (PLQY) and limited stability have hampered the realization of their outstanding luminescent properties in device applications. Here, to address these deficiencies, we present a two-step synthetic approach that enables the synthesis of core/shell NPLs with precisely controlled shell composition for engineering their excitonic properties. The proposed CdSe colloidal quantum wells possess a graded shell, which is composed of a CdS buffer layer and a Cd x Zn1–x S gradient layer, and exhibit bright emission (PLQY 75–89%) in the red spectral region (634–648 nm) with a narrow emission line width (21 nm). These enhanced optical properties allowed us to attain low thresholds for amplified spontaneous emission (down to ∼40 μJ/cm2) under nanosecond laser excitation. We also studied the electroluminescent performance of these NPLs by fabricating solution-processed light-emitting diodes (LEDs). In comparison to NPL-LEDs with CdSe/CdS core/shell NPLs, which exhibit an external quantum efficiency (EQE) value of only 1.80%, a significantly improved EQE value of 9.92% was obtained using graded-shell NPLs, the highest value for colloidal NPL-based-LEDs. In addition, the low efficiency roll-off characteristics of NPL-LEDs enabled a high brightness of up to ∼46 000 cd/m2 with an electroluminescence peak centered at 650 nm. These findings demonstrate the paramount role that heterostructure engineering occupies in enhancing the optoelectronic characteristics of semiconductor NPLs toward practically relevant levels.

Photoinduced electron transfer processes in dye-semiconductor systems with different spacer groups J. Chem. Phys. 137, 22A529 (2012) Raman scattering and anti-Stokes luminescence in poly-paraphenylene vinylene/carbon nanotubes composites J. Appl. Phys. 111, 083109 (2012) Fabrication, charge carrier transport, and application of printable nanocomposites based on indium tin oxide nanoparticles and conducting polymer 3,4-ethylenedioxythiophene/polystyrene sulfonic acid J. Appl. Phys. 110, 104301 (2011) Ni filled flexible multi-walled carbon nanotube-polystyrene composite films as efficient microwave absorbers Appl. Phys. Lett. 99, 113116 (2011) Organic bistable memory based on Au nanoparticle/ZnO nanorods composite embedded in poly (vinylpyrrolidone) layer Appl. Phys. Lett. 99, 023303 (2011) Additional information on Appl. Phys. Lett.