L. Ottaviano

Thin and ultra-thin films of nickel phthalocyanine grown on highly oriented pyrolitic graphite : an XPS, UHV-AFM and air tapping-mode AFM study

Abstract Thin and ultra-thin films of nickel phthalocyanine have been deposited in ultra-high vacuum on highly oriented pyrolitic graphite. The growth mode and the interaction with the substrate have been studied in situ by X-ray photoelectron spectroscopy, scanning tunneling microscopy and atomic force microscopy, and ex situ by means of tapping-mode atomic force microscopy. We present photoelectron spectroscopy measurements at various film thicknesses which show no detectable interaction of the adsorbed organic layers with the substrate, but, varying the deposited film thickness by in-situ annealing the substrate, we show desorption effects semiquantitatively described. The XPS spectrum of the C 1s multiplet structure of NiPC obtained using a monochromatized source is also presented. The scanning probe images presented address morphological issues like the growth mode at initial stages of deposition and at higher coverages. Moreover, upon annealing of the PC films, we show high-resolution measurements consistent with the low-size stable α crystalline phase of phthalocyanine molecules. The potential of tapping-mode atomic force microscopy in imaging soft adlayers deposited over soft substrates is addressed throughout the paper.

Evidencing the mask effect of graphene oxide: a comparative study on primary human and murine phagocytic cells

Graphene oxide (GO) is attracting an ever-growing interest in different fields and applications. Not much is known about the possible impact of GO sheet lateral dimensions on their effects in vitro, especially on human primary cells. In an attempt to address this issue, we present a study to evaluate, how highly soluble 2-dimensional GO constituted of large or small flakes affects human monocyte derived macrophages (hMDM). For this purpose, the lateral size of GO was tuned using sonication and three samples were obtained. The non sonicated one presented large flakes (~1.32 μm) while sonication for 2 and 26 hours generated small (~0.27 μm) and very small (~0.13 μm) sheets of GO, respectively. Cell studies were then conducted to evaluate the cytotoxicity, the oxidative stress induction, the activation potential and the pro-inflammatory effects of these different types of GO at increasing concentrations. In comparison, the same experiments were run on murine intraperitoneal macrophages (mIPM). The interaction between GO and cells was further examined by TEM and Raman spectroscopy. Our data revealed that the GO sheet size had a significant impact on different cellular parameters (i.e. cellular viability, ROS generation, and cellular activation). Indeed, the more the lateral dimensions of GO were reduced, the higher were the cellular internalization and the effects on cellular functionality. Our data also revealed a particular interaction of GO flakes with the cellular membrane. In fact, a GO mask due to the parallel arrangement of the graphene sheets on the cellular surface was observed. Considering the mask effect, we have hypothesized that this particular contact between GO sheets and the cell membrane could either promote their internalization or isolate cells from their environment, thus possibly accounting for the following impact on cellular parameters.

Effects of oxygen annealing on gas sensing properties of carbon nanotube thin films

Carbon nanotubes (CNTs) thin films deposited by plasma enhanced chemical vapor deposition have been investigated as resistive gas sensors towards NO2 oxidizing gas. Effects of air oxidative treatment dramatically influence the nanotubes’ electrical resistance as determined by volt-amperometric measurements. In particular the electrical measurements show that electrical behavior of the CNT films can be converted from semiconducting to metallic through thermal treatments in oxygen. The electrical response was then measured exposing the films to sub-ppm NO2 concentrations (100 ppb in air) at 165 °C. Upon exposure to NO2, the electrical resistance of CNTs was found to decrease. The obtained results demonstrate that nanotubes could find use as a sensitive chemical gas sensor for (a) the fast response accompanied by a high sensitivity to sub-ppm NO2 exposure, and (b) the precise recover of the base resistance value in absence of NO2 at a fixed operating temperature, likewise indicating that intrinsic properties measured on as prepared nanotubes may be severely changed by extrinsic oxidative treatment effects.

Graphene oxide: from fundamentals to applications

In this review, we discuss the fundamental characterization of graphene oxide (GO) and its future application perspectives. Morphology is discussed through optical microscopy, fluorescence microscopy, scanning electron microscopy, and atomic force microscopy studies. Chemical, structural, and vibrational properties are discussed through x-ray photoemission spectroscopy and Raman spectroscopy studies. Two easy characterization strategies, based on the correlation between x-ray photoemission spectroscopy and contact angle/optical contrast measurements are reported. Sensing and nano-biotechnology applications are discussed with focus on practical gas sensing and optical sensing, on the one hand, and on the toxicity issue of GO, on the other hand. Synthesis and post-synthesis treatments are also discussed, these latter with emphasis on lithography.

Phase separation and dilution in implanted MnxGe1-x alloys

The structural and electronic properties of MnxGe1−x alloys (x⩽0.15) fabricated by ion implantation are investigated by means of x-ray diffraction and synchrotron radiation photoemission spectroscopy. The diffraction patterns point to the presence of ferromagnetic Mn5Ge3 nanoparticles; however, valence band spectra, interpreted by means of accurate ab initio calculations including Hubbard-like correlations, show clear fingerprints of an effective substitutional Mn dilution in the Ge semiconducting host.

The influence of air and vacuum thermal treatments on the NO2 gas sensitivity of WO3 thin films prepared by thermal evaporation

Abstract WO 3 films with thickness of 80 nm have been thermally evaporated onto Si 3 N 4 /Si substrates. The films have been initially treated in oxygen by a 24-h-long annealing at 300°C and 500°C. XPS measurements, to follow W 4f, O 1s peaks and the valence band, have been performed on these samples as prepared and after successive ultra high vacuum (UHV) thermal treatments. We observed that the UHV annealing procedure produces a lack of oxygen at the surface of the as deposited and 300°C annealed samples strongly modifying the W 4f peak and producing the increase of metallic states at the Fermi edge, while very few modifications have been observed in the 500°C sample. The films submitted to UHV thermal treatments have been also investigated as resistive gas sensors towards NO 2 . We observed a lowering of the base resistance and a decrease of the sensitivity properties with respect to the corresponding non-vacuum treated samples.

Structural, electrical, electronic and optical properties of melanin films

We present thick, uniform and rather flat melanin films obtained using spray deposition. The morphology of the films was investigated using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Temperature-dependent electrical resistance of melanin thin films evidenced a semiconductor-like character and a hysteretic behavior linked to an irreversible process of water molecule desorption from the melanin film. X-ray Photoelectron Spectroscopy (XPS) was carried out to analyze the role of the functional groups in the primary and secondary structure of the macromolecule, showing that the contribution of the 5,6-dihydroxyindole-2-carboxylic acid (DHICA) subunit to the molecule is about 35%. Comparison of the optical absorption of the thick (800nm) and thin (80nm) films showed a spectral change when the thickness increases. From in vacuum photoconductivity (PC) measured at controlled temperatures, we suggest that the melanin films exhibit a possible charge transport mechanism by means of delocalized

Core level and valence band investigation of WO3 thin films with synchrotron radiation

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X-ray photoemission spectroscopy and scanning tunneling spectroscopy study on the thermal stability of WO3 thin films

states along the stacked planar secondary structure.

Copper hexadecafluoro phthalocyanine and naphthalocyanine: The role of shake up excitations in the interpretation and electronic distinction of high-resolution X-ray photoelectron spectroscopy measurements

Abstract In this work, the electronic properties of the surface of WO3 films with thickness of 150 nm, thermally evaporated in high vacuum onto Si(100) substrates and pre-treated in air by a 24-h-long annealing at 300 °C and 500 °C (obtaining polycrystalline monoclinic samples) have been studied by surface and bulk sensitive core level (W 4f) and angle integrated valence band photoemission using synchrotron radiation (ELETTRA Synchrotron). The photon energy ranged from 50 eV to 200 eV. The line shape analysis of W 4f core level spectra has shown that the surface presents a sub-stoichiometric WO3 component assigned to oxygen vacancies ultimately responsible for the gas sensitivity of this material. Correspondingly, valence band spectra show well-defined metallic states W 5d in the gap and near the Fermi level. The variations of surface chemical composition caused by Ultra High Vacuum annealing, and prolonged exposure to UV beam has been monitored by changes in spectral line shape. A general consequence of annealing in vacuum is the segregation of oxygen from the bulk toward the surface as confirmed by independent scanning tunnelling spectroscopy measurements.