All-optical trion generation in single walled carbon nanotubes

We present evidence of all optical trion generation and emission in undoped single walled carbon nanotubes (SWCNTs). Luminescence spectra, recorded on individual SWCNTs over a large CW excitation intensity range, show trion emission peaks red-shifted with respect to the bright exciton peak. Clear chirality dependence is observed for 22 separate SWCNT species, allowing for determination of electron-hole exchange interaction and trion binding energy contributions. Luminescence data together with ultrafast pump probe experiments on chirality sorted bulk samples suggest that exciton-exciton annihilation processes generate dissociated carriers that allow for trion creation upon a subsequent photon absorption event.Comment: 13 pages, 4 figure

Optical gain observation on silicon nanocrystals embedded in silicon nitride under femtosecond pumping

We report the observation of positive optical gain in silicon nanocrystals (Si-nc) embedded in silicon nitride measured by the variable stripe length technique. We evidence the onset of stimulated emission and report gain coefficients up to 52 cm(-1) at the highest excitation power (6.5 W/cm(2)). Photoluminescence dynamics presents two distinct recombination lifetimes in the nanosecond and the microsecond ranges. This was interpreted in terms of fast carrier trapping in nitrogen-induced localized states in the Si-nc surface and subsequent slow radiative recombination, suggesting that carrier trapping in radiative surface states plays a crucial role in the optical gain mechanism of Si-nc. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3607276

Photoluminescence properties of rare earth (Nd, Yb, Sm, Pr)-doped CeO2 pellets prepared by solid-state reaction

Several structural and optical properties of ceria (band gap, refractive index and lattice parameter) make this material very promising for applications in optoelectronics and photovoltaics. In this paper, we show that CeO2 can be efficiently functionalized by doping with trivalent rare earth ions to give rise to photon management properties. The trivalent ions can be successfully inserted by solid-state reaction of the elementary oxide powders. By combining the information obtained from the absorbance spectra with that of the PL excitation spectra, we demonstrate the presence of the trivalent ions in CeO2 and provide insight in the electronic level structure and transfer mechanism. In particular, we prove that both the complex absorption spectra and the energy transfer mechanisms cannot be fully explained without considering the presence of isolated Ce3+ ions in CeO2

Luminescence of free-standing versus matrix-embedded oxide-passivated silicon nanocrystals: The role of matrix-induced strain:

We collect a large number of experimental data from various sources to demonstrate that free-standing (FS) oxide-passivated silicon nanocrystals (SiNCs) exhibit considerably blueshifted emission, by 200 meV on average, compared to those prepared as matrix-embedded (ME) ones of the same size. This is suggested to arise from compressive strain, exerted on the nanocrystals by their matrix, which plays an important role in the light-emission process; this strain has been neglected up to now as opposed to the impact of quantum confinement or surface passivation. Our conclusion is also supported by the comparison of low-temperature behavior of photoluminescence of matrix-embedded and free-standing silicon nanocrystals

Magnetic and luminescent coordination networks based on imidazolium salts and lanthanides for sensitive ratiometric thermometry

The synthesis and characterization of six new lanthanide networks [Ln(L)(ox)(H2O)] with Ln = Eu3+, Gd3+, Tb3+ , Dy3+ , Ho3+ and Yb3+ is reported. They were synthesized by solvo-ionothermal reaction of lanthanide nitrate Ln(NO3)(3)center dot xH(2)O with the 1,3-bis(carboxymethyl)imidazolium [HE] ligand and oxalic acid (H(2)ox) in a water/ethanol solution. The crystal structure of these compounds has been solved on single crystals and the magnetic and luminescent properties have been investigated relying on intrinsic properties of the lanthanide ions. The synthetic strategy has been extended to mixed lanthanide networks leading to four isostructural networks of formula [Tb1-xEux(L)(ox)(H2O)] with x = 0.01, 0.03, 0.05 and 0.10. These materials were assessed as luminescent ratiometric thermometers based on the emission intensities of ligand, Tb3+ and Eu3+ . The best sensitivities were obtained using the ratio between the emission intensities of Eu3+ (D-5(0) -> F-7(2) transition) and of the ligand as the thermometric parameter. [Tb0.97Eu0.03 (L)(ox)(H2O)] was found to be one of the best thermometers among lanthanide-bearing coordination polymers and metal-organic frameworks, operative in the physiological range with a maximum sensitivity of 1.38%.K-1 at 340 K

Site spectroscopy of Hf doping in Hf-doped LiNbO3 crystals

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