Electron-phonon coupling and vibrational properties of size-selected linear carbon chains by resonance Raman scattering.

UV resonance Raman spectroscopy of size-selected linear sp-carbon chains unveils vibrational overtones and combinations up to the fifth order. Thanks to the tunability of the synchrotron source, we excited each H-terminated polyyne (HCnH with n = 8,10,12) to the maxima of its vibronic absorption spectrum allowing us to precisely determine the electronic and vibrational structure of the ground and excited states for the main observed vibrational mode. Selected transitions are shown to enhance specific overtone orders in the Raman spectrum in a specific way that can be explained by a simple analytical model based on Albrecht's theory of resonance Raman scattering. The determined Huang-Rhys factors indicate a strong and size-dependent electron-phonon coupling increasing with the sp-carbon chain length.

Vibrational properties of graphdiynes as 2D carbon materials beyond graphene.

Two-dimensional (2D) hybrid sp-sp2 carbon systems are an appealing subject for science and technology. For these materials, topology and structure significantly affect electronic and vibrational properties. We investigate here by periodic density-functional theory (DFT) calculations the Raman and IR spectra of 2D carbon crystals belonging to the family of graphdiynes (GDYs) and having different structures and topologies. By joining DFT calculations with symmetry analysis, we assign the IR and Raman modes in the spectra of all the investigated systems. On this basis, we discuss how the modulation of the Raman and IR active bands depends on the different interactions between sp and sp2 domains. The symmetry-based classification allows identifying the marker bands sensitive to the different peculiar topologies. These results show the effectiveness of vibrational spectroscopy for the characterization of new nanostructures, deepening the knowledge of the subtle interactions that take place in these 2D materials.

Structure modulated charge transfer in carbon atomic wires.

sp-Hybridized carbon atomic wires are appealing systems with large property tunability. In particular, their electronic properties are intimately related to length, structure, and type of functional end-groups as well as to other effects such as the intermolecular charge transfer with metal nanoparticles. Here, by a combined Raman, Surface Enhanced Raman Scattering (SERS) investigation and first principles calculations of different N,N-dimethylanilino-terminated polyynes, we suggest that, upon charge transfer interaction with silver nanoparticles, the function of sp-carbon atomic wire can change from electron donor to electron acceptor by increasing the wire length. In addition, the insertion into the wire of a strong electrophilic group (1,1,4,4-tetracyanobuta-1,3-diene-2,3-diyl) changes the electron-accepting molecular regions involved in this intermolecular charge transfer. Our results indicate that carbon atomic wires could display a tunable charge transfer between the sp-wire and the metal, and hold promise as active materials in organic optoelectronics and photovoltaics.

Pulsed laser deposition of two-dimensional ZnO nanocrystals on Au(111): growth, surface structure and electronic properties.

Two-dimensional (2D) ZnO structures have been deposited on the Au(111) surface by means of the pulsed laser deposition technique. In situ scanning tunneling microscopy and scanning tunneling spectroscopy measurements have been performed to characterize morphological, structural and electronic properties of 2D ZnO at the nanoscale. Starting from a sub-monolayer coverage, we investigated the growth of ZnO, identifying different atomic layers (up to the fifth). At low coverage, we observed single- and bi-layer nanocrystals, characterized by a surface moiré pattern that is associated to a graphene-like ZnO structure. By increasing the coverage, we revealed a morphological change starting from the fourth layer, which was attributed to a transition toward a bulk-like structure. Investigation of the electronic properties revealed the semiconducting character of 2D ZnO. We observed a dependence of the density of states (DOS) and, in particular, of the conduction band (CB) on the ZnO thickness, with a decreasing of the CB onset energy for increasing thickness. The CB DOS of 2D ZnO shows a step-like behaviour which may be interpreted as due to a 2D quantum confinement effect in ZnO atomic layers.

Carbon-atom wires: 1-D systems with tunable properties.

This review provides a discussion of the current state of research on linear carbon structures and related materials based on sp-hybridization of carbon atoms (polyynes and cumulenes). We show that such systems have widely tunable properties and thus represent an intriguing and mostly unexplored field for both fundamental and applied sciences. We discuss the rich interplay between the structural, vibrational, and electronic properties focusing on recent advances and the future perspectives of carbon-atom wires and novel hybrid sp-sp(2)-carbon architectures.

Note: Fabrication and characterization of molybdenum tips for scanning tunneling microscopy and spectroscopy.

We present a method for the preparation of bulk molybdenum tips for Scanning Tunneling Microscopy and Spectroscopy and we assess their potential in performing high resolution imaging and local spectroscopy by measurements on different single crystal surfaces in UHV, namely, Au(111), Si(111)-7 × 7, and titanium oxide 2D ordered nanostructures supported on Au(111). The fabrication method is versatile and can be extended to other metals, e.g., cobalt.

Structure-dependent optical and electrical transport properties of nanostructured Al-doped ZnO.

The structure-property relation of nanostructured Al-doped ZnO thin films has been investigated in detail through a systematic variation of structure and morphology, with particular emphasis on how they affect optical and electrical properties. A variety of structures, ranging from compact polycrystalline films to mesoporous, hierarchically organized cluster assemblies, are grown by pulsed laser deposition at room temperature at different oxygen pressures. We investigate the dependence of functional properties on structure and morphology and show how the correlation between electrical and optical properties can be studied to evaluate energy gap, conduction band effective mass and transport mechanisms. Understanding these properties opens up opportunities for specific applications in photovoltaic devices, where optimized combinations of conductivity, transparency and light scattering are required.

Island organization of TiO2 hierarchical nanostructures induced by surface wetting and drying.

We report on the reorganization and bundling of titanium oxide nanostructured layers, induced by wetting with different solvents and subsequent drying. TiO(2) layers are deposited by pulsed laser deposition and are characterized by vertically oriented, columnar-like structures resulting from assembling of nanosized particles; capillary forces acting during evaporation induce bundling of these structures and lead to a micrometer-size patterning with statistically uniform islands separated by channels. The resulting surface is characterized by a hierarchical, multiscale morphology over the nanometer-micrometer length range. The structural features of the pattern, i.e., characteristic length, island size, and channel width, are shown to depend on properties of the liquid (i.e., surface tension) and thickness and density of the TiO(2) layers. The studied phenomenon permits the controlled production of multiscale hierarchically patterned surfaces of nanostructured TiO(2) with large porosity and large surface area, characterized by superhydrophilic wetting behavior without need for UV irradiation.

Hierarchical TiO2 photoanode for dye-sensitized solar cells.

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.

Hierarchically organized nanostructured TiO2 for photocatalysis applications.

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.

Nanostructured Ag(4)O(4) films with enhanced antibacterial activity.

Ag(4)O(4) (i.e. silver(I)-silver(III) oxide) thin films with tailored structure and morphology at the nanoscale have been grown by reactive pulsed laser deposition (PLD) in an oxygen-containing atmosphere and they are shown to exhibit a very strong antibacterial activity towards Gram-negative bacteria (E. coli) and to completely inhibit the growth of Gram-positive bacteria (S. aureus). The formation of this particular high-valence silver oxide is explained in terms of the reactions occurring during the expansion of the ablated species in the reactive atmosphere, leading to the formation of low-stability Ag-O dimers and atomic oxygen, providing reactive species at the substrate where the film grows. PLD is shown to allow control of the structure (i.e. crystallinity and grain size) and of the morphology of the films, from compact and columnar to foam-like, thus allowing the deposition of nanocrystalline films with increased porosity and surface area. The antibacterial action towards E. coli is demonstrated and is shown to be superior to that of nanostructured Ag-based medical products. This can be related to the release of Ag ions with high oxidation number, which are known to be very reactive towards bacteria, and to the peculiar morphology at the nanoscale resulting in a large effective surface area.

Influence of cumulenic chains on the vibrational and electronic properties of s p-s p2 amorphous carbon.

We report the production and characterization of a form of amorphous carbon with s p-s p(2) hybridization (atomic fraction of sp hybridized species > or =20%) where the predominant sp bonding appears to be (=C=C=)(n) cumulene. Vibrational and electronic properties have been studied by in situ Raman spectroscopy and electrical conductivity measurements. Cumulenic chains are substantially stable in high vacuum conditions for temperatures lower than 250 K and they influence the electrical transport properties of the s p-s p(2) carbon through a self-doping mechanism by pinning the Fermi level closer to one of the mobility gap edges. Upon heating above 250 K the cumulenic species decay to form graphitic nanodomains embedded in the s p(2) amorphous matrix thus reducing the activation energy of the material. This is the first example of a pure carbon system where the s p hybridization influences bulk properties.

Cluster-beam deposition and in situ characterization of carbyne-rich carbon films.

Nanostructured carbon films produced by supersonic cluster beam deposition have been studied by in situ Raman spectroscopy. Raman spectra show the formation of a sp2 solid with a very large fraction of sp-coordinated carbyne species with a long-term stability under ultrahigh vacuum. Distinct Raman contributions from polyyne and cumulene species have been observed, as well as different stabilities under gas exposure. Our experiments confirm theoretical predictions and demonstrate the possibility of producing a carbyne-rich pure carbon solid. The stability of the sp2-sp network has important implications for astrophysics and for the production of novel carbon-based systems.