160 research outputs found
Quasi-freestanding and single-atom thick layer of hexagonal boron nitride as a substrate for graphene synthesis
We demonstrate that freeing a single-atom thick layer of hexagonal boron
nitride (hbn) from tight chemical bonding to a Ni(111) thin film grown on a
W(110) substrate can be achieved by intercalation of Au atoms into the
interface. This process has been systematically investigated using
angle-resolved photoemission spectroscopy, X-ray photoemission and absorption
techniques. It has been demonstrated that the transition of the hbn layer from
the "rigid" into the "quasi-freestanding" state is accompanied by a change of
its lattice constant. Using chemical vapor deposition, graphene has been
successfully synthesized on the insulating, quasi-freestanding hbn monolayer.
We anticipate that the in situ synthesized weakly interacting graphene/hbn
double layered system could be further developed for technological applications
and may provide perspectives for further inquiry into the unusual electronic
properties of graphene.Comment: in print in Phys. Rev.
Induced magnetism of carbon atoms at the graphene/Ni(111) interface
We report an element-specific investigation of electronic and magnetic
properties of the graphene/Ni(111) system. Using magnetic circular dichroism,
the occurrence of an induced magnetic moment of the carbon atoms in the
graphene layer aligned parallel to the Ni 3d magnetization is observed. We
attribute this magnetic moment to the strong hybridization between C and
Ni 3d valence band states. The net magnetic moment of carbon in the graphene
layer is estimated to be in the range of per atom.Comment: 10 pages, 3 figure
The helicity amplitudes A and A for the D resonance obtained from the reaction}
The helicity dependence of the reaction
has been measured for the first time in the photon energy range from 550 to 790
MeV. The experiment, performed at the Mainz microtron MAMI, used a
4-detector system, a circularly polarized, tagged photon beam, and a
longitudinally polarized frozen-spin target. These data are predominantly
sensitive to the resonance and are used to determine its
parameters.Comment: 5 pages, 4 figure
First measurement of the Gerasimov-Drell-Hearn integral for Hydrogen from 200 to 800 MeV
A direct measurement of the helicity dependence of the total photoabsorption
cross section on the proton was carried out at MAMI (Mainz) in the energy range
200 < E_gamma < 800 MeV. The experiment used a 4 detection system, a
circularly polarized tagged photon beam and a frozen spin target.
The contributions to the Gerasimov-Drell-Hearn sum rule and to the forward
spin polarizability determined from the data are 226 \pm 5 (stat)\pm
12(sys) \mu b and -187 \pm 8 (stat)\pm 10(sys)10^{-6} fm^4, respectively, for
200 < E_\gamma < 800 MeV.Comment: 6 pages, 3 figures, 3 table
In Situ Observations during Chemical Vapor Deposition of Hexagonal Boron Nitride on Polycrystalline Copper.
Using a combination of complementary in situ X-ray photoelectron spectroscopy and X-ray diffraction, we study the fundamental mechanisms underlying the chemical vapor deposition (CVD) of hexagonal boron nitride (h-BN) on polycrystalline Cu. The nucleation and growth of h-BN layers is found to occur isothermally, i.e., at constant elevated temperature, on the Cu surface during exposure to borazine. A Cu lattice expansion during borazine exposure and B precipitation from Cu upon cooling highlight that B is incorporated into the Cu bulk, i.e., that growth is not just surface-mediated. On this basis we suggest that B is taken up in the Cu catalyst while N is not (by relative amounts), indicating element-specific feeding mechanisms including the bulk of the catalyst. We further show that oxygen intercalation readily occurs under as-grown h-BN during ambient air exposure, as is common in further processing, and that this negatively affects the stability of h-BN on the catalyst. For extended air exposure Cu oxidation is observed, and upon re-heating in vacuum an oxygen-mediated disintegration of the h-BN film via volatile boron oxides occurs. Importantly, this disintegration is catalyst mediated, i.e., occurs at the catalyst/h-BN interface and depends on the level of oxygen fed to this interface. In turn, however, deliberate feeding of oxygen during h-BN deposition can positively affect control over film morphology. We discuss the implications of these observations in the context of corrosion protection and relate them to challenges in process integration and heterostructure CVD.P.R.K. acknowledges funding from the Cambridge Commonwealth Trust and the Lindemann
Trust Fellowship. R.S.W. acknowledges a research fellowship from St. John’s College,
Cambridge. S.H. acknowledges funding from ERC grant InsituNANO (no. 279342), EPSRC
under grant GRAPHTED (project reference EP/K016636/1), Grant EP/H047565/1 and EU FP7
Work Programme under grant GRAFOL (project reference 285275). The European Synchrotron
Radiation Facility (ESRF) is acknowledged for provision of synchrotron radiation and assistance
in using beamline BM20/ROBL. We acknowledge Helmholtz-Zentrum-Berlin Electron storage
ring BESSY II for synchrotron radiation at the ISISS beamline and continuous support of our
experiments.This is the final version. It was first published by ACS at http://pubs.acs.org/doi/abs/10.1021/cm502603
Transfer-free electrical insulation of epitaxial graphene from its metal substrate
High-quality, large-area epitaxial graphene can be grown on metal surfaces
but its transport properties cannot be exploited because the electrical
conduction is dominated by the substrate. Here we insulate epitaxial graphene
on Ru(0001) by a step-wise intercalation of silicon and oxygen, and the
eventual formation of a SiO layer between the graphene and the metal. We
follow the reaction steps by x-ray photoemission spectroscopy and demonstrate
the electrical insulation using a nano-scale multipoint probe technique.Comment: Accepted for publication in Nano Letter
Helicity dependence of the γ→p→→nπ+π0 reaction in the second resonance region
The helicity dependence of the total cross section for the reaction has been measured for the first time at incident photon energies from 400 to 800 MeV. The measurement was performed with the large acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. This channel is found to be excited predominantly when the photon and proton have a parallel spin orientation, due to the intermediate production of the D13 resonance.
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