Papers

ICMCTF2008 Session D3-1: Carbon-Based Nanostructured Composite and Nanolaminated Films

Wednesday, April 30, 2008 1:30 PM in Room Royal Palm 4-6

Wednesday Afternoon

Start	Invited?	Item
1:30 PM		D3-1-1 Defects in Graphene: Electronic Structure, Scattering Mechanisms, and Magnetism M. Katsnelson (Radboud University Nijmegen, Netherlands) Electronic transport in graphene are determined by different kinds of defects and irregularities of its structure. The most important effects are probably connected with the bending fluctuations (formation of ripples) and with adsorbed molecules. A review is done of our current understanding of graphene corrogations. The results of atomistic Monte Carlo simulations of structure of free graphene sheets are presented. The effects of corrogations on the electronic prioperties of graphene, e.g., on Quantum Hall Effect and on charge carrier transport, are discussed. The characteristics of molecular doping in graphene are calculated and discussed, using nitrogen dioxide as an example. The corresponding experimental data and perspectives of use graphene for chemical sensors are reviewed, as well as the results of first-principle electronci structure calculations. Peculiarities of defect-induced magnetism of sp electrons in comparison with conventional itinerant-electron magnets are discussed; it is shown that, in principle, the sp-electron magnets can have much higher Curie temperatures than conventional magnetic semiconductors. Perspectives of carbon-based magnetic materials are reviewed.
2:10 PM		D3-1-3 Elastic Properties of Cr₂AlC Thin Films Probed by Nanoindentation and ab Initio Molecular Dynamics D.P. Sigumonrong, D. Music (RWTH Aachen University, Germany); C. Walter (University of Leoben, Austria); J. Emmerlich, R. Iskandar, J. Mayer, J.M. Schneider (RWTH Aachen University, Germany) The reported experimentally determined bulk modulus for Cr₂AlC is 40% to 28% lower than the value obtained by ab initio calculations. To identify the origin of this extensive difference between theory and experiment the elastic modulus of thin Cr₂AlC films was measured by nanoindentation and the temperature dependence thereof was studied by ab initio molecular dynamics. Our experimental and theoretical elastic modulus data are within the expected error margin and hence consistent with the published ab initio data.
2:30 PM		D3-1-4 Solid Phase Reactions Between Al₂O₃ Substrates and TiC Thin Films Resulting in the Formation of the Ti₂AlC MAX Phase P.O.. Persson, J. Rosén, M.M.M. Bilek (University of Sydney, Australia) Depositions of Ti and C on Al₂O₃ substrates were performed using a high current pulsed cathodic arc fitted with three cathodes. The depositions were performed at a substrate temperature of 900°C. The ratios between Ti and C were controlled by means of alternating plasma pulses from two cathodes, with final film compositions ranging from C rich to Ti rich. Microstructure, composition and chemical bonding was investigated using a combination of XRD and (S)TEM methods. It is shown that for Ti rich conditions, a solid phase reaction occurs during deposition between the deposited film and the Al₂O₃ substrate, in which Al from the substrate reacts with the deposited Ti_xC_y layer to form epitaxially oriented Ti₂AlC MAX phase and residual TiC. O and Al from the sapphire substrate are uniformly distributed in the film except close to the surface where an increased level of O is detected. Using high resolution electron energy loss spectroscopy (EELS) it is shown that the O is bound to Ti in the structure and that this chemical bond changes with increasing O content. The results are spectacular in two ways. First, this shows how MAX phases are formed to stabilize non-stoichiometric films. Secondly the results question the appropriateness of Al₂O₃ as a substrate material for thin film growth of MAX phases.
2:50 PM		D3-1-5 Differences in Basal Plane Dislocation Motion in MAX-Phases S. Korte (University of Cambridge, United Kingdom); D. Music, J.M. Schneider (RWTH Aachen University, Germany); W.J. Clegg (University of Cambridge, United Kingdom) Recent ab initio calculations have shown considerable differences in the bonding between atoms of the same type (M, A or X), giving rise to differences in the aspect ratio of the unit cell, elastic properties and Fermi levels. In this paper the effect of composition, in particular the valence electron concentration of the transition metal on the resistance of the crystal lattice to dislocation flow has been studied. Using observation of basal plane dislocations made by transmission electron microscopy, estimates of the Peierls stress have been made using ab initio estimates of the unstable stacking fault energy and compared with a very simple reformulation of the Peierls method. Both approaches allows for both in-plane elastic energy changes and dynamic changes in dislocation width. These estimates are compared with those of the related, similar compounds such as TiC.
3:10 PM		D3-1-6 Thin Film Synthesis and Characterization of M-Al-C (M=Cr, Nb, Ti, V) Compounds T.H. Scabarozi, M.W. Barsoum (Drexel University); J. Roche, P. Hann, T. Flemming, B. Adamson, A. Rosenfeld, C. Gennaoui, K. Wittenberger, S.E. Lofland, J.D. Hettinger (Rowan University) We present the synthesis and characterization of thin film MAX-phase compounds. Thin film synthesis was performed by magnetron sputtering from elemental target materials on c-axis sapphire and MgO wafers. Films were grown either directly on substrate, or on a VC, V₂C, or TiC seedlayers. Multi-phase and single phase films were grown along the (000l). Synthesis temperature range was 450-900°C, the lowest belonging to chromium. Films were characterized by Raman spectroscopy, electron microscopy, X-ray diffraction, and atomic force microscopy. Tribological properties, such as surface friction, were studied with a Hysitron Triboscope. Electronic transport was also investigated and compared to their bulk counterpart. The vanadium system revealed the presence of two new phases not seen in bulk, V₃AlC₂ and V₄AlC₃.
3:30 PM		D3-1-7 Formation of (Ti,Al)N/Ti₂AlN Multilayers by Annealing of TiN/TiAl Multilayers T. Cabioch, F. Pailloux, M. Jaouen (University of Poitiers, France) TiN/TiAl multilayers of various modulation wavelengths (8, 13 and 32 nm) were deposited at room temperature onto Si(100) substrates by Ion Beam Assisted Deposition and then annealed in vacuum between 600 C and 900 C. The as-deposited multilayers as well as those transformed by the thermal annealing were characterized by X-Ray diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM), Energy Filtered Transmission Electron Microscopy imaging (EFTEM) and X-Ray Photoelectron Spectroscopy (XPS) experiments. The results so obtained reveal that one keeps the multilayer structure after the annealing process. (Ti,Al)N/TiAlN multilayers can be obtained when the modulation wavelength is fixed at 13 nm and the annealing temperature is 600°C. The different characterization techniques used for this study also revealed that the Ti₂AlN growth occurred in what were the as-deposited TiAl sublayers. XPS experiments also reveal that these as-deposited sublayers contain a noticeable amount of nitrogen atoms. This is attributed to a poising effect of the TiAl target during the assisted deposition of the TiN sublayers. The nitrogen oversaturation of the TiAl layers prevents their crystallisation making them mostly constituted of an amorphous solid solution. Depending on the nitrogen concentration C_N inside TiAl layers, and thus of the modulation wavelengths; the annealing induces a solid state transformation which involves a complex diffusion mechanism of Al and N atoms that will be mostly discussed on the basis of in situ XRD and TEM experiments.