21 research outputs found

    All-optical trion generation in single walled carbon nanotubes

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    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

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    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

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    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

    Stellar populations in NGC 5128 with the VLT: evidence for recent star formation

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    We resolve stars of the nearest giant elliptical galaxy NGC 5128 using VLT with FORS1 and ISAAC. We construct deep U, V and Ks color-magnitude and color-color diagrams in two different halo fields (in the halo and in the north-eastern diffuse shell). In the outer, shell field, at ~14 kpc from the center of the galaxy, there is a significant recent star formation with stars as young as 10 Myr, approximately aligned with the prominent radio and x-ray jet from the nucleus of the host AGN. Ionized gas filaments are evident in ultraviolet images near the area where neutral HI and CO molecular gas was previously observed. The underlying stellar population of the halo of the giant elliptical is predominantly old with a very broad metallicity distribution. The presence of an extended giant branch reaching M_bol=-5 mag suggests the existence of a significant intermediate-age AGB population in the halo of this galaxy.Comment: 18 pages, 18 figures; to be published in A&A; high res. version at http://www.eso.org/~mrejkuba/cenA_starsVLT.p

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

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    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

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    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

    Thickness-dependent optical band gap in one-dimensional Ca3Co2O6 nanometric films

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    Recent studies on the physical properties of Ca3CO2O6 nanometric samples have shown that their properties are significantly different from those of the bulk samples. The origin of this change is not trivial. We have carried out optical measurements on Ca3CO2O6 thin films with different thicknesses in order to characterize their electronic structure using optical spectroscopy measurements. The absorption spectra show a dependence on the film thickness that is correlated to the grain size in the polycrystalline layers. We found that the optical band gap increases from 1.3 to 1.55 eV when the thickness changes from 35 to 100 nm. The change in the band gap evolution with the film thickness is discussed in terms of both the amorphous effect and the grain size in the Ca3CO2O6 thin films. Finally, we show that these results are consistent with recent measurements concerning magnetic and electrical properties of Ca3Co2O6 nanometric samples. (C) 2011 Elsevier B.V. All rights reserved

    Optical properties of self-assembled InGaN/GaN quantum dots

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    Optical spectroscopy under varying temperature is used to investigate samples containing planes of self-assembled Ga1-xInxN quantum dots (0.15 < x < 0.20), embedded in a GaN matrix. The samples have been grown by molecular beam epitaxy on sapphire substrates and the nano-islands have been obtained by the Stranski-Krastanov growth mode transition. Half-widths at half maximum as small as 0.05 eV are obtained for photoluminescence (PL) lines at T = 2 K. Increasing the growth time decreases the PL energy and drastically increases the PL decay time, as a result of the increasing of the average dot height. Time-resolved PL measurements with variable temperature allow us to observe the competitive influence of several mechanisms, namely the usual radiative and nonradiative recombination processes, plus the carrier feeding from random fluctuations, which plays a crucial role in the case of the larger dots. (C) 2001 Elsevier Science B.V. All rights reserved

    High internal electric field in a graded-width InGaN/GaN quantum well: Accurate determination by time-resolved photoluminescence spectroscopy

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    Time-resolved photoluminescence (PL), at T=8 K, is used to study a graded-width InGaN/GaN quantum well. Across the sample, the well width continuously varies from similar to5.5 to 2.0 nm corresponding to PL peak energies varying between 2.0 and 2.9 eV and to PL decay rates covering four orders of magnitude. The plot of decay times versus PL energies is very well fitted by a calculation of the electron-hole recombination probability versus well width. The only fitting parameter is the electric field in the well, which we find equal to 2.45 +/-0.25 MV/cm, in excellent agreement with experimental Stokes shifts for this type of samples. (C) 2001 American Institute of Physics

    Optical properties of GaN epilayers and GaN/AlGaN quantum wells grown by molecular beam epitaxy on GaN(0001) single crystal substrate.

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    International audienceGaN epilayers and GaN/AlGaN quantum wells(QWs) were grown by molecular beam epitaxy on GaN(0001) single crystal substrates. Transmission electron microscopy(TEM) was used to assess the crystal quality of the homoepitaxial layers. A dislocation density of less than 105 cm−2 is deduced from TEM imaging. Low temperature (1.8 K) photoluminescence(PL) of homoepitaxial GaN reveals PLlinewidths as low as 0.3 meV for bound excitons. The PL integrated intensity variation between 10 and 300 K is compared to that observed on a typical heteroepitaxialGaN/Al2O3 layer. A 2 nm thick GaN/Al0.1Ga0.9NQW has been studied by time-resolved and continuous wave PL. The decay time is close to a purely radiative decay, as expected for a low defect density. Finally, the built-in polarization field measured in a homoepitaxialQW is shown to be comparable to that measured on heteroepitaxialQWsgrown either on sapphire or silicon substrates
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