Electronic properties and atomic structure of graphene oxide membranes

We have performed a near-edge X-ray absorption fine-structure (NEXAFS) and a transmission electron microscopy (TEM) investigation of freely suspended graphene oxide (GO) sheets. We utilized a photoemission electron microscope to identify GO membranes and to acquire C K and 0 K absorption spectra. The overall line shape of C K-edge spectra demonstrates that the honeycomb carbon network of graphene is the scaffold of GO. However, the intensity ratio of pi* and sigma* resonances, and a broad feature at about 20 eV from the edge, indicate the presence of new carbon bonds. The 0 K-edge spectra show that oxidized regions are made of carbonyl, epoxide, and hydroxyl groups attached to the plane of graphene, while carboxyl groups might also be present at the edges. Further, our study indicates the presence of ordered arrangements of oxygen atoms in GO sheets. Our investigation provides a new and efficient route to study the electronic structure of suspended membranes. (C) 2010 Elsevier Ltd. All rights reserved.

Formation Mechanisms and Structural Properties of Aluminium-Chromium Based Oxynitride Thin Films Synthesized by Physical Vapor Deposition

Hossein Najafi

Oxynitride thin films, a novel class of mixed anionic solid structures, have gained an intense interest during recent years related to the unique interplay of oxygen (O) and nitrogen (N) with metals in the material structure. This matter can result in specific physical and structural properties. Accordingly, by varying the anionic composition (O/(O+N)), it is possible to tune the properties of oxynitrides, and particularly to enhance mechanical strength and phase stability. Consequently, the understanding of the role of O and N in such structures is of great interest. Taking this into account, we studied the formation mechanisms and properties of AlCr-based oxynitride films synthesized by the reactive magnetron sputtering (650◦C) and cathodic arc evaporation (550◦C) with an emphasis for application in hard coat- ings. Our results revealed that the variation of the anionic composition (O/(O+N)) disclosed the existence of three distinct growth regimes including: (i) a nitride regime O/(O+N)≤ 0.6, (ii) a transition regime 0.6 < O/(O+N) < 0.97, and (iii) an oxide regime O/(O+N)≥ 0.97, which were correlated to the structural features, chemical environ- ment, physical and mechanical properties of the films within each regime. For the oxygen fraction up to 60%, transmission electron microscopy (TEM) and x-ray diffrac- tion (XRD) alongside indicate the development of films in the nitride-type structure with a continues anionic substitutions, i.e. solid solution of AlCrOx N1−x . With fur- ther increase of the oxygen content, the structure of the layers is that of a poorly crystallized phase, and a near-amorphous film was favored in accordance with the observed low nanohardness, i.e. transition regime. Finally, occurring at the highest oxygen content, oxide regime (O/(O+N)≥ 0.97) was assigned to the films with oxide- type structure; however, with some probable oxygen substitution by nitrogen. The composition/structure variation was consistent with the chemical environment of the involved atoms, evaluated by x-ray photoelectron spectroscopy (XPS), where the electronic properties of the films exhibited substantial changes from regime to regime. Moreover, the film properties observed within each regime were also strongly in- fluenced by the cationic composition and deposition conditions. Al-rich layers (Cr/Al ≤ 0.1) were found to crystallize in the hexagonal wurtzite (B4) structure in the nitride regime (O/(O+N) ≤ 0.55) and transformed to a fully amorphous network and cubic γ-(Al,Cr)2O3 in the transition and oxide regimes, respectively. Conversely, Cr-rich films (Cr/Al≃1) were crystallized in the cubic (B1) structure within the nitride regime and preserved their structure in the transition regime despite the large pro- portion of oxygen. Such coatings in the oxide regime consisted of a solid solution of α-(Al,Cr)2(O0.97,N0.03)3 with corundum lattice and favorable protective proper- ties. Furthermore, it was observed that the structure of the films deposited at lower temperatures (400◦C) remains in the transition regime, and the oxide structure is not developed. The presence of silicon (Si, 3-4 at.%) and yttrium (Y≤ 1 at.%) in the oxynitride films was found to change the thresholds and structural features within the growth regimes. Oxynitride layers exhibited an enhanced phase stability (i.e. cubic-B1) by the incorporation of both Si and Y. As a result, the nitride regime was extended in the range of O/(O+N)≤ 0.8. However, on the other hand, Si and Y prohibit the formation of corundum phase in the oxide regime suggesting retarded diffusional processes with the presence of such alloying elements. Considering the preparation conditions of the oxide coatings, we found a previously unobserved formation of cubic crystalline fcc-(Al,Cr)2O3 and its transition to the well-known corundum phase (α- (Al,Cr)2O3) at higher film thickness beyond 2 μm by employing an extra-high substrate bias during arc evaporation. The TEM, XRD, and XPS results alongside indicated that the cubic (B1) layer acquires the properties of a metastable monoxide (M1−x O), which is kinetically stabilized by the occurrence of vacancies. It was proposed that there is a thickness-dependent gradient in point defect density, and its modification leads to the destabilized fcc-(Al,Cr)2O3 and a cubic-to-corundum transformation occurs. Favorable phase stability and oxidation resistance together with the low wear rates and friction behavior observed for the oxynitride films underline their potential for hard coating of tools used under severe dry cutting conditions.

EPFL2012

Toxicity study of nanostructures

Lenke Horváth

The great achievement of nanotechnology is the controlled synthesis of a large variety of nanometer-size materials like needle-formed nanotubes, nanowires and two-dimensional graphene flakes. Due to their unique physico-chemical properties, these nanostructures are considered to be of great benefit for many applications in engineering, electronics, alternative energies and nanomedicine. Since the expectations for the improvement of our everyday life through engineered nanomaterials are high, there is rapid expansion of their manufacturing, which makes it likely that intentional and unintentional human and environmental exposure will increase in the near future. Consequently, the concern grows, related to their possible health hazards, as some of them strongly resemble asbestos. Motivated by this issue, we have investigated the in vitro acute cellular toxicity associated with four model nanomaterials: carbon nanotubes, boron nitride nanotubes, titanium dioxide nanofilaments and graphene oxide. This study focused on the toxic effect these nanomaterials had on cell types found in the respiratory system, where exposure to these materials is most prominent. These are lung epithelial cells, macrophages, and fibroblasts, but also other cell types like kidney cells were tested. The cytotoxicity was assessed by using MTT, DNA and FMCA assays, which measure different endpoints such as metabolic activity, cell proliferation and viable cell number. The cell death was determined by the Annexin V assay. We employed various microscopic techniques: light microscopy to reveal the morphological alterations associated with the nanomaterial toxicity at the cellular level; scanning electron microscopy to study the cell-nanomaterial interactions on the surface of the cell membrane; transmission electron microscopy to examine the uptake and subsequent localization of the nanomaterials within the cytosol and cell organelles. In addition, the generation of intracellular reactive oxygen species induced by graphene oxide was detected by the DCF assay. Last but not least, the pro-inflammatory potential and the biochemical perturbations in cells exposed to boron nitride nanotubes were investigated by Western blot and Synchrotron Infrared Microspectroscopy (SIRMS), respectively. All these techniques point to the adverse effects of the investigated nanostructures: i) The toxic effect of carbon nanotubes and carbon nanoparticles showed a time- and dose-dependent impairment in the metabolic activity of the cells characteristic for each cell type. Moreover, distinct morphological alterations typical for cell death were particularly apparent in macrophages. ii) The toxic potential of boron nitride nanotubes exhibited more pronounced adverse effects than carbon nanotubes. This was demonstrated by cell viability assays combined with cytopathological and biochemical analyses, showing induction of serious morphological changes, particularly in macrophages and fibroblasts, and biochemical processes characteristic for cell death. A higher acute toxicity was determined for boron nitride nanotubes when compared to crocidolite asbestos and their pro-inflammatory potential was demonstrated by the secretion of mature IL-1β cytokine in macrophages. iii) Titanium dioxide nanofilaments were also shown to impair the metabolic activity of the studied cells and induce morphological changes pointing to cell insult. iv) Graphene oxide exhibited a mild cytotoxic action in comparison to carbon nanotubes on epithelial cells and macrophages. The interaction of the nanomaterial with the cell surface generated reactive oxygen species during the initial phase of the exposure and transmission electron microscopy showed that graphene oxide flakes are taken up via the endocytic pathway. In summary, our findings highlight important physico-chemical parameters, which are important in relation to the toxic effect of nanomaterials. These are: i) the chemical composition; ii) the surface modification including functionalized groups and structural defects; iii) the geometry: length, diameter and tortuosity. In addition to the identified nanomaterial characteristics, we pinpoint that the target cells ́ response depends on their type, which is likely to be linked to their physiological function.

EPFL2012

Near-edge x-ray absorption fine-structure investigation of graphene

Marco Grioni, Daniela Pacilè

We report the near-edge x-ray absorption fine-structure (NEXAFS) spectrum of a single layer of graphite (graphene) obtained by micromechanical cleavage of highly ordered pyrolytic graphite on a SiO2 substrate. We utilized a photoemission electron microscope to separately study single-, double-, and few-layers graphene samples. In single- layer graphene we observe a splitting of the pi* resonance and a clear signature of the predicted interlayer state. The NEXAFS data illustrate the rapid evolution of the electronic structure with the increased number of layers.

American Physical Society2008

Formation Mechanisms and Structural Properties of Aluminium-Chromium Based Oxynitride Thin Films Synthesized by Physical Vapor Deposition

Hossein Najafi

EPFL2012

Toxicity study of nanostructures

Lenke Horváth

EPFL2012