Low-frequency modes in the Raman spectrum of sp-sp2 nanostructured carbon

A novel form of amorphous carbon with sp-sp2 hybridization has been recently produced by supersonic cluster beam deposition showing the presence in the film of both polyynic and cumulenic species [L. Ravagnan et al. Phys. Rev. Lett. 98, 216103 (2007)]. Here we present a in situ Raman characterization of the low frequency vibrational region (400-800 cm-1) of sp-sp2 films at different temperatures. We report the presence of two peaks at 450 cm-1 and 720 cm-1. The lower frequency peak shows an evolution with the variation of the sp content and it can be attributed, with the support of density functional theory (DFT) simulations, to bending modes of sp linear structures. The peak at 720 cm-1 does not vary with the sp content and it can be attributed to a feature in the vibrational density of states activated by the disorder of the sp2 phase.Comment: 15 pages, 5 figures, 1 tabl

Multi-wavelength Raman scattering of nanostructured Al-doped zinc oxide

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 and morphology and synthesized by pulsed laser deposition (PLD) under different oxygen pressure conditions. The comparison of Raman data for pure ZnO and AZO films with similar morphology at the nano/mesoscale allows to investigate the relation between Raman features (peak or band positions, width, relative intensity) and material properties such as local structural order, stoichiometry and doping. Moreover Raman measurements with three different excitation lines (532, 457 and 325 nm) point out a strong correlation between vibrational and electronic properties. This observation confirms the relevance of a multi-wavelength Raman investigation to obtain a complete structural characterization of advanced doped oxide materials.Comment: 27 pages, 7 figures, submitted to the Journal of Applied Physic

Excitation Wavelength- and Medium-Dependent Photoluminescence of Reduced Nanostructured TiO2 Films

The performance of TiO2 nanomaterials in solar energy conversion applications can be tuned by means of thermal treatments in reducing atmospheres, which introduce defects (such as oxygen vacancies), allowing, for instance, a better charge transport or a higher photocatalytic activity. The characterization of these defects and the understanding of their role are pivotal to carefully engineer the properties of TiO2, and, among various methods, they have been addressed by photoluminescence (PL) spectroscopy. A definitive framework to describe the PL properties of TiO2, however, is still lacking. In this work, we report on the PL of nanostructured anatase TiO2 thin films, annealed in different atmospheres (oxidizing and reducing), and consider the effects of different excitation energies and different surrounding media on their PL spectra. A broad PL signal centered around 1.8–2.0 eV is found for all the films with UV excitation in air as well as in vacuum, while the same measurements in ethanol lead to a blueshift and to intensity changes in the spectra. On the other hand, measurements with different sub-bandgap excitations show PL peaking at 1.8 eV, with an intensity trend only dependent on the thermal treatment and not on the surrounding medium. The results of PL spectroscopy, together with electron paramagnetic resonance spectroscopy, suggest the critical role of oxygen vacancies and Ti3+ ions as radiative recombination centers. The complex relationship between thermal treatments and PL data in the explored conditions is discussed, suggesting the importance of such investigations for a deeper understanding on the relationship between defects in TiO2 and photoactivity

The key role of interband transitions in hot-electron-modulated TiN films

Titanium nitride (TiN) is an emerging new material in the field of plasmonics, both for its linear and nonlinear optical properties. Similarly to noble metals, like, e.g., gold (Au), the giant third-order optical nonlinearity of TiN following excitation with fs-laser pulses has been attributed to the generation of hot electrons. Here we provide a numerical study of the Fermi smearing mechanism associated with photogenerated hot carriers and subsequent interband transitions modulation in TiN films. A detailed comparison with Au films is also provided, and saturation effects of the permittivity modulation for increasing pump fluence are discussed

Scanning tunneling microscopy and Raman spectroscopy of polymeric sp-sp2 carbon atomic wires synthesized on the Au(111) surface

Long linear carbon nanostructures based on sp-hybridization can be synthesized by exploiting on-surface synthesis of halogenated precursors evaporated on Au(111), thus opening a way to investigations by surface-science techniques. By means of an experimental approach combining scanning tunneling microscopy and spectroscopy (STM and STS) with ex situ Raman spectroscopy we investigate the structural, electronic and vibrational properties of polymeric sp-sp2 carbon atomic wires composed by sp-carbon chains connected through phenyl groups. Density-functional-theory (DFT) calculations of the structure and the electronic density of states allow us to simulate STM images and to compute Raman spectra. The comparison of experimental data with DFT simulations unveil the properties and the formation stages as a function of the annealing temperature. Atomic-scale structural information from STM complement the Raman sensitivity to the single molecular bond to open the way to detailed understanding of these novel carbon nanostructures

Unfolding the Origin of the Ultrafast Optical Response of Titanium Nitride

Ultrafast plasmonics is driving growing interest for the search of novel plasmonic materials, overcoming the main limitations of noble metals. In this framework, titanium nitride (TiN) is brought in the spotlight for its refractory properties combined with an extremely fast electron-lattice cooling time (<100 fs) compared to gold (approximate to 1 ps). Despite the results reported in literature, a clear-cut explanation of the origin of the ultrafast and giant optical response of TiN-based materials upon excitation with femtosecond laser pulses is still missing. To address this issue, an original model is introduced, capable of unfolding the modulation of TiN optical properties on a broad bandwidth, starting from the variations of electronic and lattice temperatures following ultrafast photoexcitation. The numerical analysis is validated on ultrafast pump-probe spectroscopy experiments on a simple structure, a TiN film on glass. This approach enables a complete disentanglement of the interband and intraband contributions to the permittivity modulation. Moreover, it is also shown that, varying the synthesis conditions of the TiN film, not only the static, but also the dynamical optical response can be efficiently tuned. These findings pave the way for a breakthrough in the field: the design of TiN-based ultrafast nanodevices for all-optical modulation of light