Rosanna Mastria

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.

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: liberato.manna@unile.it 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)

Dynamic Microscopy Study of Ultrafast Charge Transfer in a Hybrid P3HT/Hyperbranched CdSe Nanoparticle Blend for Photovoltaics.

We present a spectroscopic investigation on a new hyperbranched cadmium selenide nanocrystals (CdSe NC)/poly(3-hexylthiophene) (P3HT) blend, a potentially good active component in hybrid photovoltaics. Combined ultrafast transient absorption spectroscopy and morphological investigations by means of an ultrafast confocal microscope reveal a strong influence of the complex local structure on the photogenerated carrier dynamics. In particular, we map the electron-transfer process across the hybrid NC/polymer interface, and we reveal that charge separation occurs through a preferential pathway from the CdSe nanobranches to the P3HT chains. Efficient charge generation at the distributed heterojunction is also confirmed by scanning kelvin probe force microscopy measurements.

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: roman.krahne@unile.it

Improved photovoltaic performances by post-deposition acidic treatments on tetrapod shaped colloidal nanocrystal solids

The ligand exchange reaction with pyridine is the standard procedure for the integration of colloidal semiconductor nanocrystals (NCs) in photovoltaic devices; however, for large sized and irregularly shaped branched NCs, such as CdSe@CdTe tetrapods, this procedure can lead to a considerable waste of materials and the aggregation of NCs in the colloidal solution, therefore resulting in the formation of an inhomogeneous film and low device performances. Here, we report on alternative post-deposition treatments with carboxylic acids on films of CdSe@CdTe tetrapod shaped NCs. This approach guarantees the removal of the insulating surfactant, necessary to obtain good charge transport among NCs, while preserving the film integrity. We perform a complete characterization of the nanocrystalline films treated with different carboxylic acids and demonstrate the successful integration of such films in photovoltaic devices, showing a doubled efficiency with respect to the standard ligand exchange procedure. Our approach represents a general route towards the development of NC based devices with improved performances and minimized waste of material.

Morphological Study of CdSe Nanocrystals Passivated with a Low Band Gap Rod-Coil Diblock Copolymer for Hybrid Solar Cells

Different nanocomposite materials consisting of semiconducting CdSe nanocrystals (NCs) and a low band gap copolymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3.4-b’]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) were prepared, morphologically characterized, and tested in hybrid solar cells. In addition, a PCPDTBT-based rod-coil diblock copolymer was synthesized through a grafting-onto approach and a preliminary evaluation of the morphology of the hybrid nanocomposites with CdSe NCs was performed.