End-to-End Assembly of Shape-Controlled Nanocrystals via a Nanowelding Approach Mediated by Gold Domains.

[*] Dr. A. Figuerola, I. R. Franchini, A. Fiore, Dr. S. Kudera, Prof. R. Cingolani, Dr. L. Manna National Nanotechnology Laboratory of CNR-INFM, Unita di Ricerca IIT Distretto Tecnologico ISUFI, via per Arnesano km 5, I-73100 Lecce (Italy) Fax: (þ39) 0832298237 E-mail: [email protected] Dr. A. Figuerola, A. Fiore, R. Mastria, Prof. R. Cingolani Scuola Superiore ISUFI; University of Salento Distretto Tecnologico ISUFI, via per Arnesano km 5, I-73100 Lecce (Italy

Temperature and Size Dependence of the Optical Properties of Tetrapod-Shaped Colloidal Nanocrystals Exhibiting Type-II Transitions

We have investigated the optical properties of colloidal seed-grown CdSe (seed)/CdTe (arms) nanotetrapods both experimentally and computationally. The tetrapods exhibit a type-II transition arising from electrons localized in the CdSe seed region and holes delocalized in the CdTe arms, along with a residual type-I recombination in long-arm tetrapods. Experiments and theory helped to identify the origin of both types of transitions and their size dependence. In particular, time-resolved experiments performed at 10 K evidenced a size-dependent, long living type-II radiative emission arising from the peculiar electron–hole wave function localization. Temperature-dependent photoluminescence (PL) studies indicate that, at high temperature (>150 K), the main process limiting the PL quantum efficiency of the type-I PL is thermal escape of the charge carriers through efficient exciton-optical phonon coupling. The type-II PL instead is limited both by thermal escape and by the promotion of electrons from the condu..

Strongly enhanced light-matter coupling of a monolayer WS2 from a bound state in the continuum

Optical bound states in the continuum (BIC) allow to totally prevent a photonic mode from radiating into free space along a given spatial direction. Polariton excitations derived from the strong radiation-matter interaction of a BIC with an excitonic resonance inherit an ultralong radiative lifetime and significant nonlinearities due to their hybrid nature. However, maximizing the light-matter interaction in these structures remains challenging, especially with 2D semiconductors, thus preventing the observation of room temperature nonlinearities of BIC polaritons. Here we show a strong light-matter interaction enhancement at room temperature by coupling monolayer WS2 excitons to a BIC, while optimizing for the electric field strength at the monolayer position through Bloch surface wave confinement. By acting on the grating geometry, the coupling with the active material is maximized in an open and flexible architecture, allowing to achieve a 100 meV photonic bandgap with the BIC in a local energy minimum and a record 70 meV Rabi splitting. Our novel architecture provides large room temperature optical nonlinearities, thus paving the way to tunable BIC-based polariton devices with topologically-protected robustness to fabrication imperfections.Comment: 23 pages, 10 figur

Understanding natural scenes: Contributions of image statistics

Visual processing of natural scenes is carried out in a hierarchical sequence of stages that involve the analysis of progressively more complex features of the visual input. Recent studies have suggested that the semantic content of natural stimuli (e.g., real world photos) can be categorized based on statistical regularities in their appearance, which can be detected early in the visual processing stream. Here we review the studies which have investigated the role of scene statistics in the perception of natural scenes, focusing on both basic visual processing and specific tasks (visual search, expert categorization, emotional picture viewing). Visual processing seems to be adapted to visual regularities in the visual input, such as the amplitude-frequency relationship. Moreover, scene statistics can aid performance in specific tasks such as distinguishing animals from artifactual scenes, possibly by modulating early visual processing stages

Elucidating the effect of the lead iodide complexation degree behind the morphology and performance of perovskite solar cells

The inclusion of iodide additives in hybrid perovskite precursor solutions has been successfully exploited to improve the solar cell efficiency but their impact on perovskite formation, morphology and photovoltaic performance is still not clear. Here an extensive analysis of the effect of iodide additives in the solution-phase and during the perovskite film formation, as well as their effect on device performance is provided. The results demonstrate that in the solution-phase the additives promote the formation of lead poly-iodide species resulting in the disaggregation of the inorganic lead iodide framework and in the formation of smaller nuclei inducing the growth of uniform and smooth perovskite films. Most importantly, the complexation capability of different iodide additives does not only directly affect film morphology but also influences the density of defect states by varying the stoichiometry of precursors. These findings demonstrate that the fine control of the interactions of the chemical species in the solution-phase is essential for the precise control of the morphology at the nanoscale and the growth of the perovskite films with a reduced density of defect states. Therefore, the in-depth understanding of all the processes involved in the solution-phase is the first step for the development of a facile and reproducible approach for the fabrication of hybrid perovskite solar cells with enhanced photovoltaic performance

Rod-coil block copolymer as nanostructuring compatibilizer for efficient CdSe NCs/PCPDTBT hybrid solar cells

In this work the performance improvement of hybrid solar cells (HSCs), constituted by polycyclopentadithiophene-benzothiadiazole (PCPDTBT) and CdSe nanocrystals (NCs), achieved thanks to the use as additive of an on-purpose designed rod-coil block copolymer (BCP), is evaluated. The rod-coil BCP, namely polycyclopentadithiophene-benzothiadiazole-block-poly(4-vinylpyridine) (PCPDTBT-b-P4VP), is synthesized with a chain growth-like procedure starting from a PCPDTBT macroinitiator suitably tailored in order to achieve molecular similarity with the commercial PCPDTBT homopolymer here used in the HSCs, in order to optimize the interactions between the two materials in the device. Nitroxide-mediated radical polymerization (NMRP) of 4-vinylpyridine generates the rod-coil flexible chain which is maintained short to limit the insertion of insulating moieties in the additive structure. The employment of the rod-coil BCP as additive is demonstrated to be an effective alternative to the standard post-deposition thermal treatment. The device with 1% of additive performs better than the thermal annealed one and shows an improvement of 60% in power conversion efficiency (PCE) if compared to the pristine CdSe NCs/PCPDTBT cell. The optical and morphological analysis of the CdSe NCs/PCPDTBT films with and without additive elucidates the relation between the device performance and the active layer microstructure and clearly highlights how the improvement of the miscibility between the polymer and the inorganic NC species can be associated to the increased efficiency in HSCs. A future development of this room-temperature processing approach of the active layer, not requiring any additional post-fabrication annealing steps, could implement HSCs fabrication by common printing technologies for a cost-effective fabrication of devices onto large-area and flexible substrates