Charge carrier generation in a conjugated polymer studied via ultrafast pump-push-probe experiments

Conjugated polymers find rapidly growing application in electroluminescent displays and are extensively studied for use in photovoltaics and laser diodes. For a wide range of conjugated materials ultrafast pump-probe experiments have revealed the excited state dynamics of singlet and triplet excitons as well as positively and negatively charged polarons. Charge carriers play a key role in all the above mentioned applications. However, there is yet no clear picture of the mechanisms which lead to their generation. Photocurrent excitation cross-correlation measurement on methyl-substituted ladder-type poly(para)phenyl (m-LPPP), a prototypical conjugated polymer with very appealing properties for the above mentioned applications, have suggested that charge carrier generation occurs preferentially from higher lying states during energy migration. Our approach to examining this mechanism consists of an innovative modification of the ultrafast time-resolved pump-probe technique

Versatile Synthesis of Nanofoams through Femtosecond Pulsed Laser Deposition

Nanofoam materials are gaining increasing interest in the scientific community, thanks to their unique properties such as ultralow density, complex nano‐ and microstructure, and high surface area. Nanofoams are attractive for multiple applications, ranging from advanced catalysis and energy storage to nuclear fusion and particle acceleration. The main issues hindering the widespread use of nanofoams are related to the choice of synthesis technique, highly dependent on the desired elemental composition and leading to a limited control over the main material properties. Herein, femtosecond pulsed laser deposition is proposed as a universal tool for the synthesis of nanofoams with tailored characteristics. Nanofoams made by elements with significantly different properties—namely, boron, silicon, copper, tungsten, and gold—can be produced by suitably tuning the deposition parameters. The effect of the background pressure is studied in detail, in relation to the morphological features and density of the resulting nanofoams and nanostructured films. This, together with the analysis of the specific features shown by nanofoams made of different elements, offers fresh insights into the aggregation process and its relation to the corresponding nanofoam properties down to the nanoscale, opening new perspectives toward the application of nanofoam‐based materials

Convergence of particle schemes for the Boltzmann equation.

We show the convergence of a certain family of Markov chains, defined on the state space of a N-particle system (as the Bird's method), to the solutions of the (regularized) Boltzmann equation

Plasmonics in heavily-doped semiconductor nanocrystals

Heavily-doped semiconductor nanocrystals characterized by a tunable plasmonic band have been gaining increasing attention recently. Herein, we introduce this type of materials focusing on their structural and photo physical properties. Beside their continuous-wave plasmonic response, depicted both theoretically and experimentally, we also review recent results on their transient, ultrafast response. This was successfully interpreted by adapting models of the ultrafast response of gold nanoparticles.Comment: 20 pages review paper, 15 figure

The coherent dynamics of photoexcited green fluorescent proteins

The coherent dynamics of vibronic wave packets in the green fluorescent protein is reported. At room temperature the non-stationary dynamics following impulsive photoexcitation displays an oscillating optical transmissivity pattern with components at 67 fs (497 cm-1) and 59 fs (593 cm-1). Our results are complemented by ab initio calculations of the vibrational spectrum of the chromophore. This analysis shows the interplay between the dynamics of the aminoacidic structure and the electronic excitation in the primary optical events of green fluorescent proteins.Comment: accepted for publication in Physical Review Letter

Evidence for the Band-Edge Exciton of CuInS2 Nanocrystals Enables Record Efficient Large-Area Luminescent Solar Concentrators

AbstractTernary I‐III‐VI2 nanocrystals (NCs), such as CuInS2, are receiving attention as heavy‐metals‐free materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bio‐imaging. The origin of the optical properties of CuInS2 NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokes‐shifted and long‐lived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviours in defect‐free NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by low‐bandgap NCs would be accompanied by increased re‐absorption losses. Here, by studying stoichiometric CuInS2 NCs, it is revealed for the first time the spectroscopic signatures predicted for the free band‐edge exciton, thus supporting the VB‐structure model. At very low temperatures, the NCs also show dark‐state emission likely originating from enhanced electron‐hole spin interaction. The impact of the observed optical behaviours on LSCs is evaluated by Monte Carlo ray‐tracing simulations. Based on the emerging device design guidelines, optical‐grade large‐area (30×30 cm2) LSCs with optical power efficiency (OPE) as high as 6.8% are fabricated, corresponding to the highest value reported to date for large‐area devices

Disclosing Early Excited State Relaxation Events in Prototypical Linear Carbon Chains

One-dimensional (1D) linear nanostructures comprising sp-hybridized carbon atoms, as derivatives of the prototypical allotrope known as carbyne, are predicted to possess outstanding mechanical, thermal, and electronic properties. Despite recent advances in the synthesis, their chemical and physical properties are still poorly understood. Here, we investigate the photophysics of a prototypical polyyne (i.e., 1D chain with alternating single and triple carbon bonds), as the simplest model of finite carbon wire and as a prototype of sp-carbon-based chains. We perform transient absorption experiments with high temporal resolution (<30 fs) on monodispersed hydrogen-capped hexayne H$-$(C$\equiv$C)$_6-$H synthesized by laser ablation in liquid. With the support of detailed computational studies based on ground state density functional theory (DFT) and excited state time-dependent (TD)-DFT calculations, we provide a comprehensive description of the excited state relaxation processes at early times following photoexcitation. We show that the internal conversion from a bright high-energy singlet excited state to a low-lying singlet dark state is ultrafast and takes place with a 200-fs time constant, followed by thermalization on the picosecond timescale and decay of the low-energy singlet state with hundreds of picoseconds time constant. We also show that the timescale of these processes does not depend on the end groups capping the sp-carbon chain. The understanding of the primary photo-induced events in polyynes is of key importance both for fundamental knowledge and for potential optoelectronic and light-harvesting applications of low dimensional nanostructured carbon-based materials.Comment: 24 pages, 6 figure

Amplified spontaneous emission and efficient tunable laser emission from a substituted thiophene-based oligomer

We investigated gain and lasing in spin-coated films of a soluble substituted oligothiophene. With increasing excitation power, the photoluminescence spectra show a clear line narrowing due to amplified spontaneous emission. We measure a low threshold (20 μJ cm−2) for line narrowing and a large gain cross section (6×10−16 cm2), indicating that this molecule is a promising active material for organic solid-state lasers. As a demonstrator, we realize a transverse electromagnetic (TEM00) single-mode laser with tunable emission from the yellow to the red (a range of 37 nm), with a pump threshold as low as 18 μJ cm−2 and efficiency of 1.9%. These results are among the best so far reported for organic lasers

Monolithic polymer microcavity lasers with on-top evaporated dielectric mirrors

We report on a monolithic polymeric microcavity laser with all dielectric mirrors realized by low-temperature electron-beam evaporation. The vertical heterostructure was realized by 9.5 TiOx∕SiOx pairs evaporated onto an active conjugated polymer, that was previously spincast onto the bottom distributed Bragg reflector (DBR). The cavity supports single-mode lasing at 509nm, with a linewidth of 1.8nm, and a lasing threshold of 84μJ∕cm2. We also report on the emission properties of the polymer we used, investigated by a pump-probe technique. These results show that low-temperature electron-beam evaporation is a powerful and straightforward fabrication technique for molecular-based fully integrable microcavity resonators