Molecular-Level Switching of Polymer/Nanocrystal Non-Covalent Interactions and Application in Hybrid Solar Cells

Hy brid composites obtained upon blending conjugated polymers and colloidal inorganic semiconductor nanocrystals are regarded as attractive photo-active materials for optoelectronic applications. Here we demonstrate that tailoring nanocrystal surface chemistry permits to exert control on non-covalent bonding and electronic interactions between organic and inorganic components. The pendant moieties of organic ligands at the nanocrystal surface do not merely confer colloidal stability while hindering charge separation and transport, but drastically impact morphology of hybrid composites during formation from blend solutions. The relevance of our approach to photovoltaic applications is demonstrated for composites based on poly(3-hexylthiophene) and Pbs nanocrystals, considered as inadequate before the submission of this manuscript, which enable the fabrication of hybrid solar cells displaying a power conversion efficiency that reaches 3 %. Upon (quasi)steady-state and time-resolved analisys of the photo-induced processes in the nanocomposites and their organic and inorganic components, we ascertained that electron transfer occurs at the hybrid interface yielding long-lived separated charge carriers, whereas interfacial hole transfer appears slow. Here we provide a reliable alternative aiming at gaining control over macroscopic optoelectronic properties of polymer/nanocrystal composites by acting at the molecular-level via ligands' pendant moieties, thus opening new possibilities towards efficient solution-processed hybrid solar cells

Improved photovoltaic performance of bilayer heterojunction photovoltaic cells by triplet materials and tetrapod-shaped colloidal nanocrystals doping

The aim of this work is to investigate the photovoltaic properties of indium tin oxide/poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) / poly(3-hexylthiophene-2,5-diyl) /fullerene/aluminum bilayer heterojunction solar cells when the active polymer layer is doped with triplet organic molecules (a platinum porphyrin complex) and tetrapod-shaped colloidal CdTe nanocrystals. In both cases, the device photovoltaic responses are greatly improved due to the enhanced triplet exciton population, in the case of molecular doping, and due to the improved charge transport and charge separation characteristics, for nanocrystal doping. The latter are related both to the relatively large nanostructured interface and to the high intrinsic carrier mobilities of nanocrystals

Probe Tips Functionalized with Colloidal Nanocrystal Tetrapods for High-Resolution Atomic Force Microscopy Imaging

The performance and resolution of atomic force microscopy (AFM) imaging depends mainly on the quality and shape of the probe tip, since the obtained AFM image is a convolution of the tip profile and the sample structure. Therefore, tip radii that are smaller and aspect ratios that are higher than the sample features are desirable in order to obtain good images. Progress in the ability to design, fabricate, and assemble nanostructures in the size range of a few nanometers has raised the demand for probe tips with a corresponding resolution. Standard commercially available tips made of Si or SiN have a pyramidal shape with a tip radius of the order of 10 nm or larger and therefore do not image nanostructures with features in the few nanometer range adequately. One solution to this problem is the commercially available super-sharp Si probes with tip radius of 2 nm, which, however, obtain their high resolution at a price: the sharp tip can break easily during an experiment. These limitations have stimulated many efforts to enhance the resolution of AFMby functionalizing the probe tips with high-aspect-ratio nanostructures. Carbon nanotubes have demonstrated excellent properties in this respect. Different approaches for the attachment of the carbon nanotubes to the AFM cantilever have been developed, and a spatial resolution of only a few nanometers has been demonstrated. However, the attachment of carbon nanotubes to theAFM tip is still a time consuming and very difficult task, and often results in non-reproducible nanotube configuration and placement. The optimal attachment geometry, with the tip perpendicular to the sample under investigation, is particularly hard to realize. Also, the inherent thermal vibration of long nanotubes can cause difficulties when they are used for AFM imaging. Recent approaches to overcome these difficulties comprise the growth of multiwalled carbon nanotubes and the electron beam induced deposition of carbon nanocones on tipless cantilevers. For a recent review on AFM probes see elsewhere. Shape-controlled semiconductor nanocrystals are another very interesting family of nanostructures that can enhance the spatial resolution of AFM. Tetrapod-shaped nanocrystals are especially appealing for functionalizing AFM tips. Their ability to align on a surface with three supporting base arms, and the fourth arm pointing straight up, resembles an optimal geometry for the sensing of topography with the fourth, vertical arm. Recent advances in colloidal chemical synthesis have led to tetrapod samples with arm lengths of the order of several hundred nanometers and a diameter at the arm extremity well below 10 nm. Moreover, the optoelectronic properties of shape-controlled nanocrystals can extend the functionality of AFM beyond the probing of topography. Banin and coworkers, for example, showed that AFM probes functionalized with spherical core/shell nanocrystals can be used for near field optical imaging. Here, we report the positioning of single CdTe tetrapods on flattenedAFM tips and demonstrate the feasibility of these tips, via the vertical tetrapod arm, for high resolution AFM imaging. Withour tippreparationweachieve anoptimal probingangle of 908, due to the use of contactmode scanning for the preparation of the tip flat. This inherently leads to a tip geometrywith the flat parallel to thesampleplane,which, combinedwiththecapability of tetrapods to self-align with three arms contacting the surface and the fourth pointing vertically upward, results in a geometry where the vertical arm probes the topography at a 908 angle to the sample surface. The high aspect ratio shape of the tetrapod arms, with diameters ranging from 5 to 10nm and lengths ranging from 100 to 300 nm, provides excellent properties for high-resolution topography scanning. In particular, we find that the tetrapod-functionalized tips work very well for imaging surfaces that are covered with nanocrystal samples. Furthermore, our tip fabrication technique could open the way for the fabrication of high aspect ratio optically and electronically sensitive probe tips due to the semiconductor properties of the tetrapods. Large aspect ratio colloidal nanocrystal CdTe tetrapods with arm lengths ranging from 100 to 300 nm and diameters around 10 nm were fabricated by chemical synthesis as reported elsewhere and dissolved in toluene (see Supporting Information Fig. S2 for a TEM image of these very large tetrapods). The rapid growth of the tetrapod arms led to a pointed shape (i.e., to a decreasing arm diameter toward the arm extremity), which is advantageous for our purpose of high spatial resolution imaging (see Fig. 1b). Figure 1(b and c) show transmission electron microscopy (TEM) images of tetrapods deposited by drop casting onto a carbon coated TEM grid. The images show that the tetrapods self-align, with three arms contacting the substrate and the fourth arm pointing straight upward, appearing as a dark circular spot in the image. A sketch of the tetrapod-functionalized AFM probe is shown in Figure 1a. [!] Dr. R. Krahne, C. Nobile, A. Fiore, R. Mastria, Prof. R. Cingolani, Dr. L. Manna National Nanotechnology Laboratory of CNR-INFM Distretto Tecnologico ISUFI Via per Arnesano, Lecce 73100 (Italy) E-mail: [email protected]

An Insight into Chemistry and Structure of Colloidal 2D-WS2 Nanoflakes: Combined XPS and XRD Study

The surface and structural characterization techniques of three atom-thick bi-dimensional 2D-WS2 colloidal nanocrystals cross the limit of bulk investigation, offering the possibility of simultaneous phase identification, structural-to-morphological evaluation, and surface chemical description. In the present study, we report a rational understanding based on X-ray photoelectron spectroscopy (XPS) and structural inspection of two kinds of dimensionally controllable 2D-WS2 colloidal nanoflakes (NFLs) generated with a surfactant assisted non-hydrolytic route. The qualitative and quantitative determination of 1T’ and 2H phases based on W 4f XPS signal components, together with the presence of two kinds of sulfur ions, S22− and S2−, based on S 2p signal and related to the formation of WS2 and WOxSy in a mixed oxygen-sulfur environment, are carefully reported and discussed for both nanocrystals breeds. The XPS results are used as an input for detailed X-ray Diffraction (XRD) analysis allowing for a clear discrimination of NFLs crystal habit, and an estimation of the exact number of atomic monolayers composing the 2D-WS2 nanocrystalline samples

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

Tetrapod-shaped colloidal nanocrystals of II-VI semiconductors prepared by seeded growth.

We report a general synthetic approach to tetrapod-shaped colloidal nanocrystals made of various combinations of II-VI semiconductors. Uniform tetrapods were prepared using preformed seeds in the sphalerite structure, onto which arms were grown by coinjection of the seeds and chemical precursors into a hot mixture of surfactants. By this approach, a wide variety of core materials could be chosen (in practice, most of the II-VI semiconductors that could be prepared in the sphalerite phase, namely, CdSe, ZnTe, CdTe); in contrast, the best materials for arm growth were CdS and CdTe. The samples were extensively characterized with the aid of several techniques

Photoluminescence Emission Induced by Localized States in Halide Passivated Colloidal Two-Dimensional WS2 Nanoflakes

Engineering physicochemical properties of two-dimensional transition metal dichalcogenide (2D-TMD) materials by surface manipulation is essential for their practical and large-scale application especially for colloidal 2D-TMDs that are plagued by the unintentional formation of structural defects during the synthetic procedure. However, the available methods to manage surface states of 2D-TMDs in solution-phase are still limited hampering the production of high quality colloidal 2D-TMD inks to be straightforwardly assembled into actual devices. Here, we demonstrate an efficient solution-phase strategy to passivate surface defect states of colloidally synthetized WS2 nanoflakes with halide ligands, resulting in the activation of the photoluminescence emission. Photophysical investigation and density functional theory calculations suggest that halide atoms enable the suppression of non-radiative recombination through the elimination deep gap trap states, and introduce localized states in the energy band structure from which excitons efficiently recombine. Halide passivated WS2 nanoflakes importantly preserve colloidal stability and photoluminescence emission after several weeks of storing in ambient atmosphere, corroborating the potential of our developed 2D-TMD inks

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..

Improved Photovoltaic Performance of Heterostructured Tetrapod-Shaped CdSe/CdTe Nanocrystals Using C60 Interlayer

[*] Prof. G. Gigli, Dr. Y. Q. Li, R. Mastria, A. Fiore, Dr. C. Nobile, Dr. L. X. Yin, Dr. M. Biasiucci, Dr. G. Cheng, Prof. R. Cingolani, Dr. L. Manna National Nanotechnology Lab of CNR-INFM (NNL) University of Salento Via Arnesano, 73100 Lecce (Italy) E-mail: [email protected] Dr. M. Biasiucci, Prof. A. M. Cucolo Physical Department ''E. R. Caianiello'' University of Salerno Via S. Allende, 84081 Baronissi (SA) (Italy

Room temperature polariton condensation from Whispering gallery modes in CsPbBr3 microplatelets

Room temperature (RT) polariton condensate holds exceptional promise for revolutionizing various fields of science and technology, encompassing optoelectronics devices to quantum information processing. Using perovskite materials like all-inorganic CsPbBr3 single crystal provides additional advantages, such as ease of synthesis, cost-effectiveness, and compatibility with existing semiconductor technologies. In this work, we show the formation of whispering gallery modes (WGM) in CsPbBr3 single crystals with controlled geometry, synthesized using a lowcost and efficient capillary bridge method. Through the implementation of microplatelets geometry, we achieve enhanced optical properties and performance thanks to the presence of sharp edges and a uniform surface, effectively avoiding non-radiative scattering losses caused by defects. This allows us not only to observe strong light matter coupling and formation of whispering gallery polaritons, but also to demonstrate the onset of polariton condensation at RT. This investigation not only contributes to the advancement of our knowledge concerning the exceptional optical properties of perovskite-based polariton systems, but also unveils prospects for the exploration of WGM polariton condensation within the framework of a 3D perovskite-based platform, working at RT. The unique characteristics of polariton condensate, including low excitation thresholds and ultrafast dynamics, open up unique opportunities for advancements in photonics and optoelectronics devices