V. K. Adamchuk

Nitrogen-Doped Graphene: Efficient Growth, Structure, and Electronic Properties

A novel strategy for efficient growth of nitrogen-doped graphene (N-graphene) on a large scale from s-triazine molecules is presented. The growth process has been unveiled in situ using time-dependent photoemission. It has been established that a postannealing of N-graphene after gold intercalation causes a conversion of the N environment from pyridinic to graphitic, allowing to obtain more than 80% of all embedded nitrogen in graphitic form, which is essential for the electron doping in graphene. A band gap, a doping level of 300 meV, and a charge-carrier concentration of ∼8×10(12) electrons per cm2, induced by 0.4 atom % of graphitic nitrogen, have been detected by angle-resolved photoemission spectroscopy, which offers great promise for implementation of this system in next generation electronic devices.

Commissioning results and performance of the high-resolution Russian-German Beamline at BESSY II

Excellent results were obtained during commissioning of the Russian–German Beamline at bending-magnet D16-1A of the electron-storage ring BESSY II. The beamline is based on a Petersen-type plane-grating monochromator covering the XUV and soft X-ray photon-energy range. Photoionization spectra taken from the 2,–13 resonance of doubly excited He reveal a resolving power of E=DED100,000 at hn=64.12 eV. To our knowledge, this represents the best energy resolution achieved up to now on a bending-magnet beamline in the grazing-incidence photon-energy range. We present results on photon flux and dimensions of the light spot at the sample position, and show representative photoionization spectra of several gases studied in the course of the characterization of the beamline at various photon energies. The high stability of the beamline is demonstrated by the first observation of the weak 4,� 24 resonance of doubly excited He, which had not been observed before. r 2003 Elsevier Science B.V. All rights reserved.

Evidence for a New Two-Dimensional C4H-Type Polymer Based on Hydrogenated Graphene

www.MaterialsViews.com C O M M U N IC A IO N Danny Haberer , * Cristina E. Giusca , Ying Wang , Hermann Sachdev , * Alexander V. Fedorov , Mani Farjam , S. Akbar Jafari , Denis V. Vyalikh , Dmitry Usachov , Xianjie Liu , Uwe Treske , Mandy Grobosch , Oleg Vilkov , Vera K. Adamchuk , Stephan Irle , * S. Ravi P. Silva , Martin Knupfer , Bernd Büchner , and Alexander Grüneis * Evidence for a New Two-Dimensional C 4 H-Type Polymer Based on Hydrogenated Graphene

Quasifreestanding single-layer hexagonal boron nitride as a substrate for graphene synthesis

We demonstrate that freeing a single-atom thick layer of hexagonal boron nitride (h-BN) 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 h-BN 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 h-BN monolayer. We anticipate that the in situ synthesized weakly interacting graphene/h-BN double layered system could be further developed for technological applications and may provide perspectives for further inquiry into the unusual electronic properties of graphene.

The Chemistry of Imperfections in N‑Graphene

Many propositions have been already put forth for the practical use of N-graphene in various devices, such as batteries, sensors, ultracapacitors, and next generation electronics. However, the chemistry of nitrogen imperfections in this material still remains an enigma. Here we demonstrate a method to handle N-impurities in graphene, which allows efficient conversion of pyridinic N to graphitic N and therefore precise tuning of the charge carrier concentration. By applying photoemission spectroscopy and density functional calculations, we show that the electron doping effect of graphitic N is strongly suppressed by pyridinic N. As the latter is converted into the graphitic configuration, the efficiency of doping rises up to half of electron charge per N atom.

Formation of quasi-free graphene on the Ni(111) surface with intercalated Cu, Ag, and Au layers

This paper reports on a study by angle-resolved photoelectron and low-energy electron energy loss spectroscopy of graphene monolayers, which are produced by propylene cracking on the Ni(111) surface, followed by intercalation of Cu, Ag, and Au atoms between the graphene monolayer and the substrate, for various thicknesses of deposited metal layers and annealing temperatures. It has been shown that the spectra of valence-band π states and of phonon vibrational modes measured after intercalation become similar to those characteristic of single-crystal graphite with weak interlayer coupling. Despite the strong coupling of the graphene monolayer to the substrate becoming suppressed by intercalation of Cu and Ag atoms, the π state branch does not reach at the K point of the Brillouin zone the Fermi level, with the graphene coating itself breaking up partially to form graphene domains. At the same time after intercalation of Au atoms, the electronic band structure approaches the closest to that of isolated graphene, with linear π-state dispersion near the K point of the Brillouin zone, and the point of crossing of the filled, (π), with empty, (π*), states lying in the region of the Fermi level, which makes this system a promising experimental model of the quasi-free graphene monolayer.

Intercalation of silver atoms under a graphite monolayer on Ni(111)

The process of silver intercalation under a graphite monolayer (GM) grown on the (111) nickel single-crystal face, GM/Ni(111), is studied. The experiments were conducted in ultrahigh vacuum. The systems were formed in situ in a vacuum chamber under direct monitoring of each stage in the formation of the systems by angle-resolved UV photoelectron spectroscopy and LEED. The possibility of silver intercalation in the GM/Ni(111) system was studied in the course of deposition of various amounts of the metal on the given subject with subsequent heat treatment. It was established that the process occurs optimally under cyclic alternation of the operations of adsorbate (Ag) deposition on the GM/Ni(111) surface and subsequent annealing of the system. In the intermediate stages of GM/Ag/Ni(111) formation, the GM on Ni(111) was found to exist in two phases. Ag intercalation under a graphite monolayer on Ni(111) at room temperature was verified.

Observation of Single-Spin Dirac Fermions at the Graphene/ Ferromagnet Interface

With the discovery and first characterization of graphene, its potential for spintronic applications was recognized immediately. Since then, an active field of research has developed trying to overcome the practical hurdles. One of the most severe challenges is to find appropriate interfaces between graphene and ferromagnetic layers, which are granting efficient injection of spin-polarized electrons. Here, we show that graphene grown under appropriate conditions on Co(0001) demonstrates perfect structural properties and simultaneously exhibits highly spin-polarized charge carriers. The latter was conclusively proven by observation of a single-spin Dirac cone near the Fermi level. This was accomplished experimentally using spin- and angle-resolved photoelectron spectroscopy, and theoretically with density functional calculations. Our results demonstrate that the graphene/Co(0001) system represents an interesting candidate for applications in devices using the spin degree of freedom.

Epitaxial B-Graphene: Large-Scale Growth and Atomic Structure

Embedding foreign atoms or molecules in graphene has become the key approach in its functionalization and is intensively used for tuning its structural and electronic properties. Here, we present an efficient method based on chemical vapor deposition for large scale growth of boron-doped graphene (B-graphene) on Ni(111) and Co(0001) substrates using carborane molecules as the precursor. It is shown that up to 19 at. % of boron can be embedded in the graphene matrix and that a planar C-B sp(2) network is formed. It is resistant to air exposure and widely retains the electronic structure of graphene on metals. The large-scale and local structure of this material has been explored depending on boron content and substrate. By resolving individual impurities with scanning tunneling microscopy we have demonstrated the possibility for preferential substitution of carbon with boron in one of the graphene sublattices (unbalanced sublattice doping) at low doping level on the Ni(111) substrate. At high boron content the honeycomb lattice of B-graphene is strongly distorted, and therefore, it demonstrates no unballanced sublattice doping.

The graphene/Au/Ni interface and its application in the construction of a graphene spin filter.

A modification of the contact of graphene with ferromagnetic electrodes in a model of the graphene spin filter allowing restoration of the graphene electronic structure is proposed. It is suggested for this aim to intercalate into the interface between the graphene and the ferromagnetic (Ni or Co) electrode a Au monolayer to block the strong interaction between the graphene and Ni (Co) and, thus, prevent destruction of the graphene electronic structure which evolves in direct contact of graphene with Ni (Co). It is also suggested to insert an additional buffer graphene monolayer with the size limited by that of the electrode between the main graphene sheet providing spin current transport and the Au/Ni electrode injecting the spin current. This will prevent the spin transport properties of graphene from influencing contact phenomena and eliminate pinning of the graphene electronic structure relative to the Fermi level of the metal, thus ensuring efficient outflow of injected electrons into the graphene. The role of the spin structure of the graphene/Au/Ni interface with enhanced spin-orbit splitting of graphene π states is also discussed, and its use is proposed for additional spin selection in the process of the electron excitation.