Papers

AVS2012 Session GR+AS+EM+MI+MN-TuM: Optical, Magnetic, Mechanical and Thermal Properties

Tuesday, October 30, 2012 8:00 AM in Room 13

Tuesday Morning

Start	Invited?	Item
8:00 AM		GR+AS+EM+MI+MN-TuM-1 Characterization of Magnetically Tunable Iron Nanorod Coated Graphene Nanoplatelets Scooter Johnson, Madhu Gowda, Shu-Fan Cheng, Nelson Garces, Boris Feigelson, Francis Kub, Charles Eddy, Jr. (U.S. Naval Research Laboratory) Composites made from iron coated graphene nanoplatelets (GNPs) show promise for applications such as, magnetic switches, electromagnetic interference shielding, and electromagnetic waveguides due to the large conductivity of GNPs combined with the magnetism of iron. Additionally, this composite can be easily formed into millimeter thick sheets making it a promising composite for other applications. We report a novel method to synthesize iron oxide compound onto GNP using microwave hydrothermal synthesis at 60° C and reaction times between 10 and 120 minutes. Scanning electron microscopy imaging reveals iron oxide nanorods approximately 100 nm long adhered to the GNPs for reaction times as short as 10 minutes. X-ray photoemission spectroscopy reveals that the iron/carbon ratio remains constant across these reaction times. The resistivity of the composite increases with reaction time from 0.2 to 0.6 ohm-cm. Saturation magnetization and coercive field values follow a decreasing trend with increasing reaction time. From 10 to 120 minutes saturation magnetization decreases by 70% from 170 emu/g and coercive field decreases by 40% from 52 Oe. Remnant magnetization of around 0.7 memu/g remains constant throughout. We also report the temperature-dependent magnetic response of the compound across the Morin transition, which for submicron particles of α-Fe₂O₃ is near 250 K. Preliminary results suggest that while the nanorod size and quantity remains constant with reaction time, the resistive and magnetic properties change. This may suggest that we are tuning the magnetism of the system by changing the iron structure between the ferromagnetic γ -Fe₂O₃and the antiferromagnetic α -Fe₂O₃.
8:20 AM		GR+AS+EM+MI+MN-TuM-2 Dynamical Origin of Blue Photoluminescence from Graphene Oxide Annemarie Exarhos, Michael Turk, Patrick Vora, James Kikkawa (University of Pennsylvania) The tunable broadband emission from graphene oxide (GO) has sparked significant interest in research regarding its potential for band gap engineering. Here, we use polarization sensitive time-resolved optical spectroscopy to study the spectral diffusion and sub-picosecond dynamics of the excited carriers in GO and photo-exposed GO, where photo-exposure has been demonstrated to constitute a reducing condition. In steady state measurements, a significant blueshifting of the photoluminescence (PL) is observed with photo-exposure. This blueshift correlates with a marked difference in the temporal behavior of the PL from GO and photo-exposed GO. The PL spectra are very similar at short delay times, but an increased non-radiative recombination rate in the exposed GO leads to a decreased lifetime in the material. Utilizing in-plane polarization memory measurements, we examine the electron-hole polarization in these systems which can probe excitonic effects and help to provide a better understanding of the role of the sp² graphene lattice in GO and exposed GO. We further discuss the relevance of our data to the origins of PL in these systems. A.L.E. gratefully acknowledges the support of NSF DMR-0907226. M.E.T., P.M.V., and the construction of a Kerr gate system are supported by the Department of Energy Office of Basic Energy Sciences Award DE-SC0002158.
8:40 AM		GR+AS+EM+MI+MN-TuM-3 Spin-Transport and Magnetism in Graphene Roland Kawakami (University of California, Riverside) Graphene is an attractive material for spintronics due to its high mobility and the low intrinsic spin-orbit and hyperfine coupling, which should lead to excellent spin transport properties. In 2007, graphene became the first material to exhibit gate tunable spin transport and spin precession at room temperature. However, the spin injection efficiency was low and the spin lifetime was much shorter than predicted theoretically. In this talk, I will report on our progress in this area. The low spin injection efficiency into graphene is due to the conductivity mismatch between the ferromagnetic metal (Co) spin injector and the single layer graphene (SLG). To alleviate this problem and enhance the spin injection efficiency, we developed atomically smooth MgO tunnel barriers by utilizing a TiO₂ seed layer. With tunneling contacts, the non-local spin signal is found to be as high as 130 ohms at room temperature, with a spin injection efficiency of 30%. In addition to improving the spin injection efficiency, the tunneling contacts were found to improve the spin lifetime as well. This indicates that the short spin lifetimes reported before are due to the contact-induced spin relaxation from the ferromagnetic electrodes. Using tunneling contacts, we investigate spin relaxation in single layer graphene (SLG) and bilayer graphene (BLG). At low temperatures, contrasting behaviors of gate voltage dependence of the spin lifetime are observed between SLG and BLG, which suggest different mechanisms for spin relaxation in SLG and BLG. A final topic of interest is magnetism and the formation of magnetic moments in graphene. While there is substantial theoretical work on magnetic moments generated by hydrogen adatoms and lattice vacancies, the experimental situation is less clear. We have developed a new method for detecting magnetic moment formation based on scattering of pure spin currents in graphene spin valves. We will report the progress on our efforts to identify magnetism with this approach.
10:00 AM		BREAK - Complimentary Coffee in Exhibit Hall
10:40 AM		GR+AS+EM+MI+MN-TuM-9 A “How To” for Magnetic Carbon Hendrik Ohldag (SLAC National Accelerator Laboratory); Elke Arenholz, Tolek Tyliszczak (Lawrence Berkeley National Laboratory); Daniel Spemann, Roland Hoehne, Pablo Esquinazi, Magda Ungureneau, Tilman Butz (University of Leipzig, Germany) While conventional wisdom says that magnetic materials have to contain some metallic atoms, the confirmation of intrinsic magnetic order in pure metal free carbon represents an ultimate and general scientific breakthrough because of the fundamental importance of carbon as an elemental building block of organic as well as inorganic matter. The common controversy raised across all disciplines is whether the magnetism of carbon is intrinsic or induced by other elements. We address this controversy by providing clear experimental evidence that metal free carbon can be ferromagnetic at room temperature using dichroism x-ray absorption spectro-microscopy. For this purpose we acquired soft x-ray microscopy images of magnetic structures on a thin carbon film that have been produced by irradiation with a focused 2.25MeV proton beam. Our element specific magnetic probe shows no indication of magnetically ordered Fe, Co or Ni impurities in these samples. In a second step we investigate the particular electronic states that are involved in carbon magnetism and find that the carbon p-states as well as C-H bonds show a magnetic moment, indicating that hydrogenation plays a crucial role in developing the ferromagnetic order. Our surface sensitive approach reveals that the magnetism at the surface of the irradiated graphite samples is much larger than in the bulk of the sample. We observe a surface magnetic moment similar to what is typically present in classical ferromagnetic 3d transition metals. REFERENCES P.Esquinazi et al., Magnetic order in graphite: Experimental evidence, intrinsic and extrinsic difficulties, Journal of Magnetism and Magnetic Materials, Vol 322, 1156 (2010). H. Ohldag et al.,p-Electron ferromagnetism in metal free carbon probed by soft x-ray dichroism,Phys. Rev. Lett. 98, 187204 (2007) H. Ohldag et al., The role of hydrogen in room temperature ferromagnetism at graphite surfaces, New J. Phys. 12 123012 (2010)
11:00 AM		GR+AS+EM+MI+MN-TuM-10 From Graphene to Amorphous Carbon by Sublimation and Condensation Brad Steele, Romain Perriot, Vasily Zhakhovsky, Ivan Oleynik (University of South Florida) The mechanisms of the non-equilibrium melting process of graphene and the structure of the liquid phase of carbon was studied by molecular dynamics (MD). Graphene undergoes a non-equilibrium melting process at high temperature and low pressure as the carbon chains are formed out of the graphene sheet, thus making up a transient liquid phase of carbon. As the chains expand the material sublimates to a low dense gas of carbon chains. Under higher pressure the gas phase will condense to an intermediate porous phase of carbon with a significant sp² fraction of atoms, followed by the liquid phase, and finally an amorphous phase. Mechanisms of melting of graphene, including formation of topological and Stone Wales (SW) defects in two and three dimensions will be discussed.