Hierarchical or one-dimensional architectures are among the most exciting developments in material science these recent years. We present a nanostructured TiO(2) assembly combining these two concepts and resembling a forest composed of individual, high aspect-ratio, treelike nanostructures. We propose to use these structures for the photoanode in dye-sensitized solar cells, and we achieved 4.9% conversion efficiency in combination with C101 dye. We demonstrate this morphology beneficial to hamper the electron recombination and also mass transport control in the mesopores when solvent-free ionic liquid electrolyte is used.

A systematic study of the shift and linewidth of the Eg Raman peak at 144 cm−1 of anatase TiO2 nanopowders, produced by a flame aerosol technique, is here presented. The analysis was performed as a function of the crystal domain size and of the degree of oxidation. In the nanopowders, a clear contribution of the stoichiometry defects to the peak shift was evidenced, while the peak width seems to be less affected by the oxygen content. The Raman peak behavior due to size reduction has been interpreted in the framework of a phonon quantum confinement model. A critical review of the different approaches to this model, adopted in the literature to explain the behavior of the anatase Raman spectra as a function of the domain size, is presented. In particular, the hypothesis of a three-dimensional isotropic model for the dispersion relations is discussed. This analysis evidences general limits in the application of the phonon confinement model to the study and characterization of nanoparticles and nanostructured materials, showing how an uncritical use of the confinement theory can yield distorted results

A template-free process for the synthesis of nanocrystalline TiO2 hierarchical microstructures by reactive pulsed laser deposition (PLD) is here presented. By a proper choice of deposition parameters a fine control over the morphology of TiO2 microstructures is demonstrated, going from classical compact/columnar films to a dense forest of distinct hierarchical assemblies of ultrafine nanoparticles (<10 nm), up to a more disordered, aerogel-type structure. Correspondingly, the film density varies with respect to bulk TiO2 anatase, with a degree of porosity going from 48% to over 90%. These structures are stable with respect to heat treatment at 400 degrees C, which results in crystalline ordering but not in morphological changes down to the nanoscale. Both as deposited and annealed films exhibit very promising photocatalytic properties, even superior to standard Degussa-P25 powder, as demonstrated by the degradation of stearic acid as a model molecule. The observed kinetics are correlated to the peculiar morphology of the PLD grown material. We show that the 3D multiscale hierarchical morphology enhances reaction kinetics and creates an ideal environment for mass transport and photon absorption, maximizing the surface area-to-volume ratio while at the same time providing readily accessible porosity through the large inter-tree spaces that act as distributing channels. The reported strategy provides a versatile technique to fabricate high aspect ratio 3D titania microstructures through a hierarchical assembly of ultrafine nanoparticles. Beyond photocatalytic and catalytic applications, this kind of material could be of interest for those applications where high surface-to-volume and efficient mass transport are required at the same time.

In this work we present a detailed Raman scattering investigation of zinc oxide and aluminum-doped zinc oxide (AZO) films characterized by a variety of nanoscale structure

The operation of four basic two-input logic gates fabricated with a single graphene transistor is demonstrated. Single-transistor operation is obtained in a circuit designed to exploit the charge neutrality point of graphene to perform Boolean logic. The type of logic function is selected by offset of the input digital signals. The merits and limitations of the fabricated gates are assessed by comparing their performance with that of conventional logic gates

One-dimensional carbon atomic wires displaying sp hybridization have an appealing electronic and vibrational structure which profoundly affects their optical and transport properties. Here we investigated charge transfer in alternating triple–single bond carbon atomic wires (polyynes) terminated by phenyl rings and its effects on the structure of the system. The occurrence of a charge transfer between carbon wires and metal nanoparticles (both in liquids and supported on surfaces) is evidenced by Raman and surface enhanced Raman scattering (SERS) as a softening of the vibrational stretching modes. This is interpreted, with the support of density functional theory (DFT) calculations of the Raman modes, as a modification of the bond length alternation of carbon atoms in the wire. As a consequence of the charge transfer, carbon wires rearrange their structure toward a more equalized geometry which corresponds to a tendency toward a cumulenic structure (i.e., all double bonds). These observations open potential perspectives for developing carbon-based atomic devices with tunable electronic properties.

The operation of a digital logic inverter consisting of one pp- and one nn-type graphene transistor integrated on the same sheet of monolayer graphene is demonstrated. Both transistors initially exhibited pp-type behavior at low gate voltages, since air contamination shifted their Dirac points from zero to a positive gate voltage. Contaminants in one transistor were removed by electrical annealing, which shifted its Dirac point back and therefore restored nn-type behavior. Boolean inversion is obtained by operating the transistors between their Dirac points. The fabricated inverter represents an important step toward the development of digital integrated circuits on graphene

SummaryGraphene, nanotubes and other carbon nanostructures have shown potential as candidates for advanced technological applications due to the different coordination of carbon atoms and to the possibility of π-conjugation. In this context, atomic-scale wires comprised of sp-hybridized carbon atoms represent ideal 1D systems to potentially downscale devices to the atomic level. Carbon-atom wires (CAWs) can be arranged in two possible structures: a sequence of double bonds (cumulenes), resulting in a 1D metal, or an alternating sequence of single–triple bonds (polyynes), expected to show semiconducting properties. The electronic and optical properties of CAWs can be finely tuned by controlling the wire length (i.e., the number of carbon atoms) and the type of termination (e.g., atom, molecular group or nanostructure). Although linear, sp-hybridized carbon systems are still considered elusive and unstable materials, a number of nanostructures consisting of sp-carbon wires have been produced and characterized to date. In this short review, we present the main CAW synthesis techniques and stabilization strategies and we discuss the current status of the understanding of their structural, electronic and vibrational properties with particular attention to how these properties are related to one another. We focus on the use of vibrational spectroscopy to provide information on the structural and electronic properties of the system (e.g., determination of wire length). Moreover, by employing Raman spectroscopy and surface enhanced Raman scattering in combination with the support of first principles calculations, we show that a detailed understanding of the charge transfer between CAWs and metal nanoparticles may open the possibility to tune the electronic structure from alternating to equalized bonds.