Russian-German beamline at BESSY II - Recent highlights
Recent highlights


ACS Nano 11 (2017) 368 Making Graphene Nanoribbons Photoluminescent

B.V. Senkovskiy, M. Pfeiffer, S.K. Alavi, A. Bliesener, J. Zhu, S. Michel, A.V. Fedorov, R. German, D. Hertel, D. Haberer, L. Petaccia, F.R. Fischer, K. Meerholz, P.H.M. van Loosdrecht, K. Lindfors, and A. Grüneis

We demonstrate the alignment-preserving transfer of parallel graphene nanoribbons (GNRs) onto insulating substrates. The photophysics of such samples is characterized by polarized Raman and photoluminescence (PL) spectroscopies. The Raman scattered light and the PL are polarized along the GNR axis. The Raman cross section as a function of excitation energy has distinct excitonic peaks associated with transitions between the one-dimensional parabolic subbands. We find that the PL of GNRs is intrinsically low but can be strongly enhanced by blue laser irradiation in ambient conditions or hydrogenation in ultrahigh vacuum. These functionalization routes cause the formation of sp3 defects in GNRs. We demonstrate the laser writing of luminescent patterns in GNR films for maskless lithography by the controlled generation of defects. Our findings set the stage for further exploration of the optical properties of GNRs on insulating substrates and in device geometries.

Figure 1.

(a) The PL of 7-AGNRs on quartz (peak at ~1.8 eV) brightens when the sample is exposed to blue light (440 nm). (b) The PL is partially polarized along the GNR axis demonstrating its origin in the GNR. Here the spectra for polarizer and analyzer both parallel to the ribbon axis (red solid line) and both perpendicular to the ribbon axis (blue solid line) are shown. The emission peaks are at a significantly lower photon energy (marked with D) than the excitons E11 and E22. (c) Scanning a 440 nm laser focus allows the writing of luminescent patterns on GNR monolayers. Here the letters "UoC" have been rendered bright. The four panels correspond to the four possible orientations of polarizer and analyzer with respect to the GNR axis and are indicated by arrows. The first (second) arrow denotes the polarization of the exciting (analyzed) light. The vertical direction corresponds to light polarization along the GNR axis according to the sketch of a GNR in the inset to b.

Nano Lett. 17 (2017) 4029.


ACS Nano 11 (2017) 368 Multiscale characterization of 13C-enriched fine-grained graphitic materials for chemical and electrochemical applications

V.O. Koroteev, W. Munchgesang, Yu.V. Shubin, Yu.N. Palyanov , P.E. Plyusnin, D.A. Smirnov, K.A. Kovalenko, M. Bobnar, R. Gumeniuk, E. Brendler, D.C. Meyer, L.G. Bulusheva, A.V. Okotrub, and A. Vyalikh

13C-enriched fine-grained graphitic material has been studied towards its potential for chemical and electrochemical applications. The structural and morphological modification of the material as results of pressure-assisted thermal treatment and gaseous BrF3 and/or Br2 room-temperature treatments has been investigated using a combination of the characterization tools: electron microscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron and near edge X-ray absorption fine structure spectroscopy, solid state nuclear magnetic resonance (NMR) spectroscopy and magnetic susceptibility measurements. It has been found that the starting material represents graphitized carbon with oxygen containing defects. The evidence of distorted sp2 hybridization of carbon was found in the Raman and the 13C NMR spectra. Under high pressure and temperature, some initially open graphitic edges are coupled that causes decreasing specific surface area and mean in-plane size of crystallites, and, generally, a higher degree of disorder. The Br2 treatment improves the material structure due to removal of tiny graphitic flakes and oxygenated carbon groups. The use of BrF3 results, in addition, in partial fluorination of graphitic material. Electrochemical characteristics along with a high degree of 13C isotope enrichment enable the application of these graphitic materials in operando studies using methods sensitive to 13C isotope, such as NMR.

Figure 1.

The structural and morphological modification of the 13C-enriched fine-grained graphitic material as results of pressure-assisted thermal treatment and gaseous BrF3 and/or Br2 room-temperature treatments.

Carbon 124 (2017) 161.


ACS Nano 11 (2017) 368 Spin-Orbit Coupling Induced Gap in Graphene on Pt(111) with Intercalated Pb Monolayer

I.I. Klimovskikh, M.M. Otrokov, V.Yu. Voroshnin, D. Sostina, L. Petaccia, G. Di Santo, S. Thakur, E.V. Chulkov, and A.M. Shikin

Graphene is one of the most promising materials for nanoelectronics owing to its unique Dirac cone-like dispersion of the electronic state and high mobility of the charge carriers. However, to facilitate the implementation of the graphene-based devices, an essential change of its electronic structure, a creation of the band gap should controllably be done. Brought about by two fundamentally different mechanisms, a sublattice symmetry breaking or an induced strong spin–orbit interaction, the band gap appearance can drive graphene into a narrow-gap semiconductor or a 2D topological insulator phase, respectively, with both cases being technologically relevant. The later case, characterized by a spin–orbit gap between the valence and conduction bands, can give rise to the spin-polarized topologically protected edge states. Here, we study the effect of the spin–orbit interaction enhancement in graphene placed in contact with a lead monolayer. By means of angle-resolved photoemission spectroscopy, we show that intercalation of the Pb interlayer between the graphene sheet and the Pt(111) surface leads to formation of a gap of ~200 meV at the Dirac point of graphene. Spin-resolved measurements confirm the splitting to be of a spin–orbit nature, and the measured near-gap spin structure resembles that of the quantum spin Hall state in graphene, proposed by Kane and Mele [ Phys. Rev. Lett. 2005, 95, 226801]. With a bandstructure tuned in this way, graphene acquires a functionality going beyond its intrinsic properties and becomes more attractive for possible spintronic applications.

Figure 1.

First derivative of the ARPES data measured in the ΓK direction of the (√3×√3)R30° domain for (a) graphene/Pt(111) and (b) graphene/Pb/Pt(111) using of a photon energy of 62 eV. Blue dashed lines highlight the spectral features originating from graphene rotational domains with periodicities different from (√3×√3)R30°. (c, left) LEED pattern of graphene/Pb/Pt(111) taken at Ep = 150 eV. Blue circles mark the reflexes coming from different rotational domains of graphene with respect to Pt(111). (c, right) 2D reciprocal lattices of graphene (√3×√3)R30°, Pb c(4 × 2) (with three equivalent rotational domains of Pb relative to Pt(111)) and Pt(111) (1 × 1) shown in black, yellow, and violet, respectively. (d) Sketch of the graphene/Pb/Pt(111) atomic structure deduced from the LEED measurements with black, yellow, and gray balls showing carbon, lead, and platinum atoms, respectively.

ACS Nano 11 (2017) 368.


Chem. Mater. 28 (2016) 8248 Tuning Surface Chemistry of TiC Electrodes for Lithium–Air Batteries

A.Ya. Kozmenkova, E.Yu. Kataev, A.I. Belova, M. Amati, L. Gregoratti, J. Velasco-Vélez, A. Knop-Gericke, B. Senkovsky, D.V. Vyalikh, D.M. Itkis, Y. Shao-Horn, and L.V. Yashina

One of the key problems hindering practical implementation of lithium–air batteries is caused by carbon cathode chemical instability leading to low energy efficiency and short cycle life. Titanium carbide (TiC) nanopowders are considered as an alternative cathode material; however, they are intrinsically reactive toward oxygen, and its stability is controlled totally by a surface overlayers. Using photoemission spectroscopy, we show that lithium–air battery discharge product, lithium peroxide (Li2O2), easily oxidizes clean TiC surface. At the same time, TiC surface, which was treated by molecular oxygen under ambient conditions, shows much better stability in contact with Li2O2 that can be explained by the presence of a surface layer containing a significant amount of elemental carbon in addition to oxides and oxycarbides. Nevertheless, such protective coatings produced by room temperature oxidation are not practically useful as one of its components, elemental carbon, is oxidized in the presence of lithium–air battery discharge intermediates. These results are of critical importance in understanding of TiC surface chemistry and in design of stable lithium–air battery electrodes. We postulate that dense, uniform, carbon-free titanium dioxide surface layers of 2–3 nm thickness on TiC will be a promising solution, and thus further efforts should be taken for developing synthetic protocols enabling preparation of TiO2/TiC core–shell structures.

Figure 1.

Ti 2p (a,c,e,g) and C 1s (b,d,f,h) photoemission spectra for clean (a,b) and oxidized (c-h) TiC (755) single crystal before (gray) and after (black) Li2O2 deposition measured at ultimate surface sensitivity provided by electron kinetic energy of 50 eV. A clean vicinal (755) TiC surface was obtained by annealing the single crystal in oxygen (10-7 mbar) at 1150 °C for 20 min and further "flashing" up to 1300 °C for several times. Such a procedure allowed one to eliminate oxygen from the surface; residual oxygen intensity never exceeded 5 at. % for the layer probed with a photoelectron kinetic energy of 200 eV. Fresh Li2O2 was deposited by evaporation of metallic Li under oxygen pressure of ~10-7 mbar in the preparation chamber of the spectrometer. Dots show experimental data, while solid lines represent fitted curves. Components are shown for spectra recorded after Li2O2 deposition. Insets demonstrate component fractions in spectra before (left bars) and after (right bars) Li2O2 deposition.

Chem. Mater. 28 (2016) 8248.


Scientific Reports 6 (2016) 25548 A curious interplay in the films of N-heterocyclic carbene PtII complexes upon deposition of alkali metals

A.A. Makarova, E.V. Grachova, D. Niedzialek, A.I. Solomatina, S. Sonntag, A.V. Fedorov, O.Yu. Vilkov, V.S. Neudachina, C. Laubschat, S.P. Tunik, and D.V. Vyalikh

The recently synthesized series of PtII complexes containing cyclometallating (phenylpyridine or benzoquinoline) and N-heterocyclic carbene ligands possess intriguing structures, topologies, and light emitting properties. Here, we report curious physicochemical interactions between in situ PVD-grown films of a typical representative of the aforementioned PtII complex compounds and Li, Na, K and Cs atoms. Based on a combination of detailed core-level photoelectron spectroscopy and quantum-chemical calculations at the density functional theory level, we found that the deposition of alkali atoms onto the molecular film leads to unusual redistribution of electron density: essential modification of nitrogen sites, reduction of the coordination PtII centre to Pt0 and decrease of electron density on the bromine atoms. A possible explanation for this is formation of a supramolecular system “Pt complex-alkali metal ion”; the latter is supported by restoration of the system to the initial state upon subsequent oxygen treatment. The discovered properties highlight a considerable potential of the PtII complexes for a variety of biomedical, sensing, chemical, and electronic applications.

Figure 1.

DFT-optimized molecular structure of [Pt(N^C)(NHC)Br] complex with 2 incorporated alkali metal atoms: Li (a), K (b). Color legend: platinum brown, bromine orange, nitrogen blue/light blue, carbon grey, hydrogen white, lithium lavender; potassium purpura.

Scientific Reports 6 (2016) 25548.


2D Materials 3 (2016) 025031 First-principles and angle-resolved photoemission study of lithium doped metallic black phosphorous

A. Sanna, A.V. Fedorov, N.I. Verbitskiy, J. Fink, C. Krellner, L. Petaccia, A. Chikina, D.Yu. Usachov, A. Grüneis, and G. Profeta

First principles calculations demonstrate the metallization of phosphorene by means of Li doping filling the unoccupied antibonding pz states. The electron–phonon coupling in the metallic phase is strong enough to eventually lead to a superconducting phase at Tc = 17 K for LiP8 stoichiometry. Using angle-resolved photoemission spectroscopy we confirm that the surface of black phosphorus can be chemically functionalized using Li atoms which donate their 2s electron to the conduction band. The combined theoretical and experimental study demonstrates the semiconductor-metal transition indicating a feasible way to induce a superconducting phase in phosphorene and few-layer black phosphorus.

Figure 1.

(a) Sketch of one-layer black phosphorus's (phosphorene) crystal structure, showing Li atom's (green atom) most stable adsorption site. Orange atoms represent the four atoms of the black phosphorus's primitive cell. (b) The Brillouin zone of black phosphorus. The ARPES spectra (referred to the chemical potential) of pristine black phosphorus recorded for (c) ZU and (d) ZT symmetry directions. ARPES spectra after Li doping along the (e) ZU and (f) ZT symmetry directions. (g) and (h) Depict the XPS spectra of the P2p and the Li1s core levels of doped black phosphorus, respectively. In panel (i), we report the x-ray absorption spectra for pristine and doped black phosphorus.

2D Materials 3 (2016) 025031.


Nano Lett. 16 ()2016) 4535 Large-Scale Sublattice Asymmetry in Pure and Boron-Doped Graphene

D.Yu. Usachov, A.V. Fedorov, O.Yu. Vilkov, A.E. Petukhov, A.G. Rybkin, A. Ernst, M.M. Otrokov, E.V. Chulkov, I.I. Ogorodnikov, M.V. Kuznetsov, L.V. Yashina, E.Yu. Kataev, A.V. Erofeevskaya, V.Yu. Voroshnin, V.K. Adamchuk, C. Laubschat, and D.V. Vyalikh

The implementation of future graphene-based electronics is essentially restricted by the absence of a band gap in the electronic structure of graphene. Options of how to create a band gap in a reproducible and processing compatible manner are very limited at the moment. A promising approach for the graphene band gap engineering is to introduce a large-scale sublattice asymmetry. Using photoelectron diffraction and spectroscopy we have demonstrated a selective incorporation of boron impurities into only one of the two graphene sublattices. We have shown that in the well-oriented graphene on the Co(0001) surface the carbon atoms occupy two nonequivalent positions with respect to the Co lattice, namely top and hollow sites. Boron impurities embedded into the graphene lattice preferably occupy the hollow sites due to a site-specific interaction with the Co pattern. Our theoretical calculations predict that such boron-doped graphene possesses a band gap that can be precisely controlled by the dopant concentration. B-graphene with doping asymmetry is, thus, a novel material, which is worth considering as a good candidate for electronic applications.

Figure 1.

(a) XPS spectra of graphene/Co(0001) system as a function of photon energy. (b) Calculated and measured intensity ratio Ct/Ch for differenet interface structures.

Nano Lett. 16 (2016) 4535.


Scientific Reports 5 (2015) 9379 Field emission luminescence of nanodiamonds deposited on the aligned carbon nanotube array

Yu.V. Fedoseeva, L.G. Bulusheva, A.V. Okotrub, M.A. Kanygin, D.V. Gorodetskiy, I.P. Asanov, D.V. Vyalikh, A.P. Puzyr, and V.S. Bondar

Detonation nanodiamonds (NDs) were deposited on the surface of aligned carbon nanotubes (CNTs) by immersing a CNT array in an aqueous suspension of NDs in dimethylsulfoxide (DMSO). The structure and electronic state of the obtained CNT–ND hybrid material were studied using optical and electron microscopy and Infrared, Raman, X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopy. A non-covalent interaction between NDs and CNT and preservation of vertical orientation of CNTs in the hybrid were revealed. We showed that current-voltage characteristics of the CNT–ND cathode are changed depending on the applied field; below ~3 V/μm they are similar to those of the initial CNT array and at the higher field they are close to the ND behavior. Involvement of the NDs in field emission process resulted in blue luminescence of the hybrid surface at an electric field higher than 3.5 V/μm. Photoluminescence measurements showed that the NDs emit blue-green light, while blue luminescence prevails in the CNT–ND hybrid. The quenching of green luminescence was attributed to a partial removal of oxygen-containing groups from the ND surface as the result of the hybrid synthesis.

Figure 1.

Scheme of set-up for field emission and electroluminescence measurements (a). I-V curves for CNTs (blue), NDs (red) and the CNT-ND hybrid material (black) measured at 50, 100, 300 and 500 μm (b). Arrows indicate the threshold field. Images of light emission from surface of the CNT-ND hybrid material at electric field from 0 to 15 V/μm (c).

Scientific Reports 5 (2015) 9379.


ACS Nano 9 (2015) 7314 Epitaxial B-Graphene: Large-Scale Growth and Atomic Structure

D.Yu. Usachov, A.V. Fedorov, A.E. Petukhov, O.Yu. Vilkov, A.G. Rybkin, M.M. Otrokov, A. Arnau, E.V. Chulkov, L.V. Yashina, M. Farjam, V.K. Adamchuk, B.V. Senkovskiy, C. Laubschat, and D.V. Vyalikh

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 CB sp2 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.

Figure 1.

B-graphene/Co(0001) system: (a,b) XPS spectra at 4.5 at. % of boron impurities, measured at photon energy of 380 eV, (c,d) LEED images, obtained at different boron content using electron energy of 70 eV, (e) ARPES spectrum at 15 at. % B, recorded near the K-point of the two-dimensional Brillouin zone in the direction, perpendicular to ΓK. Dashed line in panel (a) indicates C 1s BE value of the pure graphene/Co(0001).

ACS Nano 9 (2015) 7314.


Scientific Reports 5 8710 (2015) Insight into Bio-metal Interface Formation in vacuo : Interplay of S-layer Protein with Copper and Iron

A.A. Makarova, E.V. Grachova, V.S. Neudachina, L.V. Yashina, A. Blüher, S.L. Molodtsov, M. Mertig, H. Ehrlich, V.K. Adamchuk, C. Laubschat, and D.V. Vyalikh

The mechanisms of interaction between inorganic matter and biomolecules, as well as properties of resulting hybrids, are receiving growing interest due to the rapidly developing field of bionanotechnology. The majority of potential applications for metal-biohybrid structures require stability of these systems under vacuum conditions, where their chemistry is elusive, and may differ dramatically from the interaction between biomolecules and metal ions in vivo. Here we report for the first time a photoemission and X-ray absorption study of the formation of a hybrid metal-protein system, tracing step-by-step the chemical interactions between the protein and metals (Cu and Fe) in vacuo. Our experiments reveal stabilization of the enol form of peptide bonds as the result of protein-metal interactions for both metals. The resulting complex with copper appears to be rather stable. In contrast, the system with iron decomposes to form inorganic species like oxide, carbide, nitride, and cyanide.

Figure 1.

Schematic presentation of the redox process involving the carboxyl and hydroxyl functional groups of the protein (A) and the subsequent chemical bond reorganization (B, C) by the example of Aspartic acid and Serine side chains and copper.

Scientific Reports 5 (2015) 8710.


Nano Lett. (2015) 2396 Observation of Single-Spin Dirac Fermions at the Graphene/Ferromagnet Interface

D. Usachov, A. Fedorov, M.M. Otrokov, A. Chikina, O. Vilkov, A. Petukhov, A.G. Rybkin, Y.M. Koroteev, E.V. Chulkov, V.K. Adamchuk, A. Grüneis, C. Laubschat, and D.V. Vyalikh

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.

Figure 1.

ARPES insight into the electronic structure of graphene/Co interface. (a) ARPES data taken near the K-point from well-oriented graphene on Co(0001) at room temperature, using 40 eV photons with s + p polarization. (b) Structure of graphene in momentum space; the line schematically indicates the direction of measurements. (c) ARPES insight into the minicone at EF obtained with 28 eV photons. (d) ARPES constant-energy maps of the minicone.

Nano Lett. 15 (2015) 2396


ACS Nano 9 320 (2015) Oxygen Reduction by Lithiated Graphene and Graphene-Based Materials

E.Yu. Kataev, D.M. Itkis, A.V. Fedorov, B.V. Senkovsky, D.Yu. Usachov, N.I. Verbitskiy, A. Grüneis, A. Barinov, D.Yu. Tsukanova, A.A. Volykhov, K.V. Mironovich, V.A. Krivchenko, M.G. Rybin, E.D. Obraztsova, C. Laubschat, D.V. Vyalikh, and L.V. Yashina

Oxygen reduction reaction (ORR) plays a key role in lithium–air batteries (LABs) that attract great attention thanks to their high theoretical specific energy several times exceeding that of lithium-ion batteries. Because of their high surface area, high electric conductivity, and low specific weight, various carbons are often materials of choice for applications as the LAB cathode. Unfortunately, the possibility of practical application of such batteries is still under question as the sustainable operation of LABs with carbon cathodes is not demonstrated yet and the cyclability is quite poor, which is usually associated with oxygen reduced species side reactions. However, the mechanisms of carbon reactivity toward these species are still unclear. Here, we report a direct in situ X-ray photoelectron spectroscopy study of oxygen reduction by lithiated graphene and graphene-based materials. Although lithium peroxide (Li2O2) and lithium oxide (Li2O) reactions with carbon are thermodynamically favorable, neither of them was found to react even at elevated temperatures. As lithium superoxide is not stable at room temperature, potassium superoxide (KO2) prepared in situ was used instead to test the reactivity of graphene with superoxide species. In contrast to Li2O2 and Li2O, KO2 was demonstrated to be strongly reactive.

Figure 1.

Oxygen to reduced oxygen species conversion (a) and Li:C ratio (b) vs oxygen exposure dose for lithiated MLG (green) and CNW (red). α was calculated as α = (I – Imin)/(Imax – Imin), where I, Imin, and Imax correspond to O 1s peak integral intensity at the current exposure point, at the beginning, and at the end of oxygen exposure, respectively. Dotted lines are shown for eye guidance. (c–f) Maps of the background signal intensity (c, e) associated with morphology and of the background-corrected Li 1s intensity (d, f), which represents reaction product distribution of oxygen exposed lithiated MLG (c, d) and CNW (e, f) samples.

ACS Nano 9 (2015) 320.


Appl. Phys. Lett. 105 042407 (2015) Spin current formation at the graphene/Pt interface for magnetization manipulation in magnetic nanodots

A.M. Shikin, A.A. Rybkina, A.G. Rybkin, I.I. Klimovskikh, P.N. Skirdkov, K.A. Zvezdin, and A.K. Zvezdin

Spin electronic structure of the Graphene/Pt interface has been investigated. A large induced spin-orbit splitting (~80 meV) of graphene π states with formation of non-degenerated Dirac-cone spin states at the K-point of the Brillouin zone crossed with spin-polarized Pt 5d states at Fermi level was found. We show that this spin structure can be used as a spin current source in spintronic devices. By theoretical estimations and micromagnetic modeling based on the experimentally observed spin-orbit splitting, we demonstarte that the induced intrinsic magnetic field in such structure might be effectively used for induced remagnetization of the (Ni-Fe)-nanodots arranged atop the interface.

Figure 1.

Schematic spin electronic structure of the Dirac-cone graphene π states for the Graphene/Pt interface with a large spin-orbit splitting of the π states at the Fermi level near the K and K' points (a), and the difference between the contributions to the excited current of electrons with opposite spin orientations realized when setting up an electrical gradient (∇U) between the ends of the system (b). (c) Schematic presentation of the proposed device construction with the (Ni-Fe)-nanodots deposited atop the Graphene/Pt interface. The construction with thin Pt-stripes can be arranged on top of SiC.

Appl. Phys. Lett. 105 (2015) 042407.


Nano Lett. 13, 4697 (2013) The Chemistry of Imperfections in N-Graphene

D. Usachov, A. Fedorov, O. Vilkov, B. Senkovskiy, V.K. Adamchuk, L.V. Yashina, A.A. Volykhov, M. Farjam, N.I. Verbitskiy, A. Grüneis, C. Laubschat, and D.V. Vyalikh

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.

Figure 1.

N 1s photoemission spectra acquired after intercalation of different metals under N-graphene, followed by prolonged annealing; and concentrations of pyridinic and graphitic N impurities in each system. The peaks are marked as follows: NP, pyridinic; NG, graphitic. In the initial N-graphene/ Ni(111) system, the pyridinic peak is formed by the two components “1” and “2”, corresponding to different environments of pyridinic N; also two additional peaks can be identified: NA, atomic nitrogen bonded to the Ni substrate (0.16%) and NO, other types of nitrogen (0.15%).

Nano Lett. 14 (2014) 4982.


Scientific Reports 3, 2168 (2013) Observation of a universal donor-dependent vibrational mode in graphene

A.V. Fedorov, N.I. Verbitskiy, D. Haberer, C. Struzzi, L. Petaccia, D. Usachov, O.Y. Vilkov, D.V. Vyalikh, J. Fink, M. Knupfer, B. Büchner, and A. Grüneis

Electron–phonon coupling and the emergence of superconductivity in intercalated graphite have been studied extensively. Yet, phonon-mediated superconductivity has never been observed in the 2D equivalent of these materials, doped monolayer graphene. Here we perform angle-resolved photoemission spectroscopy to try to find an electron donor for graphene that is capable of inducing strong electron–phonon coupling and superconductivity. We examine the electron donor species Cs, Rb, K, Na, Li, Ca and for each we determine the full electronic band structure, the Eliashberg function and the superconducting critical temperature Tc from the spectral function. An unexpected low-energy peak appears for all dopants with an energy and intensity that depend on the dopant atom. We show that this peak is the result of a dopant-related vibration. The low energy and high intensity of this peak are crucially important for achieving superconductivity, with Ca being the most promising candidate for realizing superconductivity in graphene.

Figure 1.

The ARPES intensities of doped graphene. (a) ARPES spectra of maximally doped graphene for different dopants. The black dotted line denotes the ARPES intensity maxima. Upper row: Fermi surfaces and electrons transferred per C atom (values inside the contour). Data are acquired in s-polarization for Ca, Li, Na, Rb, Cs and in p-polarization for K doping. Lower row: ARPES scans along the ΓKM high symmetry direction in the vicinity of K point (at 1.7 Å1), summing the data for s and p polarization. The numbers give the energy of the Dirac points. (b) Iso-area contours for all dopants and magnification along the KM high symmetry direction. (c) High resolution ARPES data in the kink region for KM and ΓK directions measured in p- and s-polarization, respectively. The yellow lines give the bare-particle band structure.

Nature Communications 5 (2014) 3257.


Scientific Reports 3, 2168 (2013) Reactivity of Carbon in Lithium-Oxygen Battery Positive Electrodes

D.M. Itkis, D.A. Semenenko, E.Yu. Kataev, A.I. Belova, V.S. Neudachina, A.P. Sirotina, M. Hävecker, D. Teschner, A. Knop-Gericke, P. Dudin, A. Barinov, E.A. Goodilin, Y. Shao-Horn, and L.V. Yashina

Unfortunately, the practical applications of Li-O2 batteries are impeded by poor rechargeability. Here, for the first time we show that superoxide radicals generated at the cathode during discharge react with carbon that contains activated double bonds or aromatics to form epoxy groups and carbonates, which limits the rechargeability of Li-O2 cells. Carbon materials with a low amount of functional groups and defects demonstrate better stability thus keeping the carbon will-o’-the-wisp lit for lithium-air batteries.

Figure 1. Chemical transformations of superoxide species. The scheme
illustrates chemical processes that are being initiated right after oxygen reduction reactions.

Chemical transformations of superoxide species. The scheme illustrates chemical processes that are being initiated right after oxygen reduction reactions.

Nano Lett. 13 (2013) 4697.


Scientific Reports 3, 2168 (2013) Carbon nanowalls: the next step for physical manifestation of the black body coating

V.A. Krivchenko, S.A. Evlashin, K.V. Mironovich, N.I. Verbitskiy, A. Nefedov, C. Woöll, A.Ya. Kozmenkova, N.V. Suetin, S.E. Svyakhovskiy, D.V. Vyalikh, A.T. Rakhimov, A.V. Egorov, and L.V. Yashina

For almost ten years, graphene, a two-dimensional crystal of carbon atoms packed in a honeycomb structure, has been in the focus of intensive research. Its remarkable and promising electronic properties have raised high expectations regarding its potential use in the next generation electronics of a post-silicon era. However, a graphene-based technology first and foremost requires reliable and low-cost approaches to fabrication of high-quality and large-scale graphene layers on a variety of functional substrates. Here we show that graphene can be successfully integrated with the established metal-silicide technology. Starting from thin monocrystalline films of nickel, cobalt and iron, we were able to form metal silicides of high quality with a variety of stoichiometries under a CVD grown graphene layer. These graphene-capped silicides are reliably protected against oxidation and can cover a wide range of electronic materials/device applications. Most importantly, the coupling between the graphene layer and the silicides is rather weak and the properties of quasi-freestanding graphene are widely preserved.

Figure 1. SEM images of top view (a–e) and side view (a9–e9) of the CNW films on a Si (100) substrate.

SEM images of top view (a–e) and side view (a'–e') of the CNW films on a Si (100) substrate.

Scientific Reports 3 (2013) 3328.


Scientific Reports 3, 2168 (2013) The graphene/Au/Ni interface and its application in the construction of a graphene spin filter

A.A Rybkina, A.G. Rybkin, V.K. Adamchuk, D. Marchenko, A. Varykhalov, J. Sánchez -Barriga, and A.M. Shikin

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.

Figure 1. Schematic presentation of the modified construction of the graphene spin filter with monolayers of graphene and Au additionally
inserted between the graphene sheet and the Ni electrodes (a). Under application of a perpendicular magnetic field the orientation of the spin
precesses with moving electrons toward the second Ni electrode (b). When the spin moment of the electrons in the graphene aligns with that
in the second Ni electrode the electrons can be injected into the Ni, and a spin current flows through the system.

Schematic presentation of the modified construction of the graphene spin filter with monolayers of graphene and Au additionally inserted between the graphene sheet and the Ni electrodes (a). Under application of a perpendicular magnetic field the orientation of the spin precesses with moving electrons toward the second Ni electrode (b). When the spin moment of the electrons in the graphene aligns with that in the second Ni electrode the electrons can be injected into the Ni, and a spin current flows through the system.

Nanotechnology 24 (2013) 295201.


Scientific Reports 3, 2168 (2013) Controlled assembly of graphene-capped nickel, cobalt and iron silicides

O. Vilkov, A. Fedorov, D. Usachov, L.V. Yashina, A.V. Generalov, K. Borygina, N.I. Verbitskiy, A. Grüneis and D.V. Vyalikh

For almost ten years, graphene, a two-dimensional crystal of carbon atoms packed in a honeycomb structure, has been in the focus of intensive research. Its remarkable and promising electronic properties have raised high expectations regarding its potential use in the next generation electronics of a post-silicon era. However, a graphene-based technology first and foremost requires reliable and low-cost approaches to fabrication of high-quality and large-scale graphene layers on a variety of functional substrates. Here we show that graphene can be successfully integrated with the established metal-silicide technology. Starting from thin monocrystalline films of nickel, cobalt and iron, we were able to form metal silicides of high quality with a variety of stoichiometries under a CVD grown graphene layer. These graphene-capped silicides are reliably protected against oxidation and can cover a wide range of electronic materials/device applications. Most importantly, the coupling between the graphene layer and the silicides is rather weak and the properties of quasi-freestanding graphene are widely preserved.

Figure 1. Quasi-freestanding character of graphene on silicides. The sequence of ARPES data for graphene on Ni(111) (a), and at its detachment upon Si intercalation and silicide formation (b–d). The data were obtained
near the K-point of graphene Brillouin zone using 40 eV photons.

Quasi-freestanding character of graphene on silicides. The sequence of ARPES data for graphene on Ni(111) (a), and at its detachment upon Si intercalation and silicide formation (b–d). The data were obtained near the K-point of graphene Brillouin zone using 40 eV photons.

Scientific Reports 3 (2013) 2168.


Negligible Surface Reactivity of Topological Insulators Bi2Se3 and Bi2Te3 towards Oxygen and Water

L.V. Yashina, J. Sánchez-Barriga, M.R. Scholz, A.A. Volykhov, A.P. Sirotina, V.S. Neudachina, M.E. Tamm, A. Varykhalov, D. Marchenko, G. Springholz, G. Bauer, A. Knop-Gericke, and O. Rader

The long-term stability of functional properties of topological insulator materials is crucial for the operation of future topological insulator based devices. Water and oxygen have been reported to be the main sources of surface deterioration by chemical reactions. In the present work, we investigate the behavior of the topological surface states on Bi2X3 (X = Se, Te) by valence-band and core level photoemission in a wide range of water and oxygen pressures both in situ (from 10-8 to 0.1 mbar) and ex situ (at 1 bar). We find that no chemical reactions occur in pure oxygen and in pure water. Water itself does not chemically react with both Bi2Se3 and Bi2Te3 surfaces and only leads to slight p-doping. In dry air, the oxidation of the Bi2Te3 surface occurs on the time scale of months, in the case of Bi2Se3 surface of cleaved crystal, not even on the time scale of years. The presence of water, however, promotes the oxidation in air, and we suggest the underlying reactions supported by density functional calculations. All in all, the surface reactivity is found to be negligible, which allows expanding the acceptable ranges of conditions for preparation, handling and operation of future Bi2X3-based devices.

ASC Nano 7(6), 5181 (2013).
Figure 1. Optimized atomic geometry for structures Bi<sub>2</sub>Te<sub>3</sub>(OH)<sub>2</sub> (a) and Bi<sub>2</sub>Te<sub>3</sub>(OH)<sub>2</sub>O<sub>3</sub> (b).

Optimized atomic geometry for structures Bi2Te3(OH)2 (a) and Bi2Te3(OH)2O3 (b).

ASC Nano 7(6) (2013) 5181.


Self-Assembled Supramolecular Complexes with “Rods-in-Belt” Architecture in the Light of Soft X-rays

A.A. Makarova, E.V. Grachova, D.V. Krupenya, O. Vilkov, A. Fedorov, D. Usachov, A. Generalov, I.O. Koshevoy, S.P. Tunik, E. Rühl, C. Laubschat, and D.V. Vyalikh

One of the most important properties of the recently discovered “rods-in-belt” supramolecular complexes, containing Au-Cu or Au-Ag cluster core, is the possibility to tune their electronic and photophysical properties through modification of the ligand environment. This opens great perspectives for their applications in light emitting devices and in bio-imaging. The high structural ordering and self-assembling properties of these unique objects may be used to design artificial nanostructures with a complex topology which could become ideal building blocks for next generation electronics. Here we present a detailed experimental study of the electronic structure of the “rods-in-belt” supramolecular complexes. Applying X-ray photoemission and absorption spectroscopy we systematically unravel the structure of their occupied and unoccupied electronic states near the Fermi level. The major contribution to the highest occupied molecular orbitals is made by the triple bonded carbons hosted in the dialkynyl-gold “rods” and the copper (silver) atoms from the central cluster core of the heterometallic Au-Cu (Au-Ag) molecules. The lowest unoccupied molecular orbitals are located on the carbon skeleton of the complexes, including −C≡C− and −C=C− aromatic orbitals. The onset of the valence band in the Au-Ag systems starts at about 0.3 eV lower than that in the Au-Cu complexes, implying a slightly larger energy gap of the silver-based molecules. With increasing size the complexes become more and more sensitive to X-ray damage.

J. Phys. Chem. C 117 (23), 12385 (2013).
Figure 1. Side and top views on the “rods-in-belt” Au–Cu–S system. Color legend: gold, yellow; copper, blue; phosphorus, red; fluorine, green; carbon, gray; hydrogen, light gray.

Side and top views on the “rods-in-belt” Au–Cu–S system. Color legend: gold, yellow; copper, blue; phosphorus, red; fluorine, green; carbon, gray; hydrogen, light gray.

J. Phys. Chem. C 117 (23) (2013) 12385.


Scientific Reports 3, 1826 (2013). Large spin splitting of metallic surface-state bands at adsorbate-modified gold/silicon surfaces

L.V. Bondarenko, D.V. Gruznev, A.A. Yakovlev, A.Y. Tupchaya, D.Usachov, O. Vilkov, A. Fedorov, D.V. Vyalikh, S.V. Eremeev, E.V. Chulkov, A.V. Zotov and A.A. Saranin

Finding appropriate systems with a large spin splitting of metallic surface-state band which can be fabricated on silicon using routine technique is an essential step in combining Rashba-effect based spintronics with silicon technology. We have found that originally poor structural and electronic properties of the Au/Si(111)√3×√3 surface can be substantially improved by adsorbing small amounts of suitable species (e.g., Tl, In, Na, Cs). The resultant surfaces exhibit a highly-ordered atomic structure and spin-split metallic surface-state band with a momentum splitting of up to 0.052 A-1 and an energy splitting of up to 190 meV at the Fermi level. The family of adsorbate-modified Au/Si(111)√3×√3 surfaces, on the one hand, is thought to be a fascinating playground for exploring spin-splitting effects in the metal monolayers on a semiconductor and, on the other hand, expands greatly the list of material systems prospective for spintronics applications.

Figure 1. (a) Symmetrized Fermi surface of the Tl-adsorbed Au/Si(111)√3×√3 surface determined with ARPES. The k-space area where the ARPES measurements were carried out is outlined by a dashed blue line. (b) Constant energy contours. Arrows along the contours and their length indicate the in-plane spin component. The out-of-plane spin component is indicated by the colour with red and blue corresponding to the upward and downward directions, respectively. White colour indicates fully in-plane spin alignment. (c) Azimuthal dependence of the Fermi wave vector k<sub>F</sub> for the S<sub>1</sub><sup>A</sup> and S<sub>2</sub><sup>B</sup> bands. Experimental and calculated data are presented by dots and solid lines, respectively. (d) Azimuthal dependencies of the spin components, including in-plane components in the directions tangential and normal to the Fermi contour (upper panel) and out-of-plane component (lower panel).

(a) Symmetrized Fermi surface of the Tl-adsorbed Au/Si(111)√3×√3 surface determined with ARPES. The k-space area where the ARPES measurements were carried out is outlined by a dashed blue line. (b) Constant energy contours. Arrows along the contours and their length indicate the in-plane spin component. The out-of-plane spin component is indicated by the colour with red and blue corresponding to the upward and downward directions, respectively. White colour indicates fully in-plane spin alignment. (c) Azimuthal dependence of the Fermi wave vector kF for the S1A and S2B bands. Experimental and calculated data are presented by dots and solid lines, respectively. (d) Azimuthal dependencies of the spin components, including in-plane components in the directions tangential and normal to the Fermi contour (upper panel) and out-of-plane component (lower panel).

Scientific Reports 3 (2013) 1826.


Anisotropy of Chemical Bonding in Semifluorinated Graphite C2F Revealed with Angle-Resolved X-ray Absorption Spectroscopy

A.V. Okotrub, N.F. Yudanov, I.P. Asanov, D.V. Vyalikh, and L.G. Bulusheva

Highly oriented pyrolytic graphite characterized by a low misorientation of crystallites is fluorinated using a gaseous mixture of BrF3 with Br2 at room temperature. The golden-colored product, easily delaminating into micrometer-size transparent flakes, is an intercalation compound where Br2 molecules are hosted between fluorinated graphene layers of approximate C2F composition. To unravel the chemical bonding in semifluorinated graphite, we apply angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and quantum-chemical modeling. The strong angular dependence of the C-K and F-K edge NEXAFS spectra on the incident radiation indicates that room-temperature-produced graphite fluoride is a highly anisotropic material, where half of the carbon atoms are covalently bonded with fluorine, while the rest of the carbon atoms preserve π-electrons. Comparison of the experimental C-K edge spectrum with theoretical spectra plotted for C2F models reveals that fluorine atoms are more likely to form chains. This conclusion agrees with the atomic force microscopy observation of a chain-like pattern on the surface of graphite fluoride layers.

ACS Nano 7(1) 65 (2013).
(a) Models of fluorinated graphene with C<sub>2</sub>F composition. Left: Alternating zigzag chains of fluorinated carbon and bare carbon. Middle: Alternating armchair chains of fluorinated carbon and bare carbon. Right: Alternating double carbon bonds and CF–CF bonds. White spheres represent the bare carbon atoms; the change in color within the fluorinated unit indicates change of position (up or down) of fluorine atoms relative to the graphene sheet. (b) Fragments of fluorinated graphene with zigzag chain (model 1), armchair chain (model 2), and double bond (model 3) fluorine pattern, calculated at the B3LYP/6-31G level. The central atoms, which were replaced by nitrogen atoms to calculate the C<sub>95</sub>F<sub>42</sub>N<sup>+</sup> structures, are marked. (c) Comparison of experimental NEXAFS CK edge spectrum of the fluorinated HOPG with theoretical spectra of the models 1, 2, and 3. The components of the theoretical spectrum correspond to a contribution from a bare carbon atom (orange line) and fluorinated carbon atom (violet line).

(a) Models of fluorinated graphene with C2F composition. Left: Alternating zigzag chains of fluorinated carbon and bare carbon. Middle: Alternating armchair chains of fluorinated carbon and bare carbon. Right: Alternating double carbon bonds and CF–CF bonds. White spheres represent the bare carbon atoms; the change in color within the fluorinated unit indicates change of position (up or down) of fluorine atoms relative to the graphene sheet. (b) Fragments of fluorinated graphene with zigzag chain (model 1), armchair chain (model 2), and double bond (model 3) fluorine pattern, calculated at the B3LYP/6-31G level. The central atoms, which were replaced by nitrogen atoms to calculate the C95F42N+ structures, are marked. (c) Comparison of experimental NEXAFS CK edge spectrum of the fluorinated HOPG with theoretical spectra of the models 1, 2, and 3. The components of the theoretical spectrum correspond to a contribution from a bare carbon atom (orange line) and fluorinated carbon atom (violet line).

ACS Nano 7(1) (2013) 65.


Scientific Reports 3, 2168 (2013) Spin-dependent avoided-crossing effect on quantum-well states in Al/W(110)

A.G. Rybkin, A.M. Shikin, D. Marchenko, A. Varykhalov, and O. Rader

Despite their low atomic number, Al films show large spin-orbit splittings when grown on W(110). Our spin- and angle-resolved photoemission experiment reveals two types of spin-orbit split states: quantum-well states (QWSs) with small Rashba splitting proportional to the electron wave vector in the film plane k|| [Rashba parameter αR ~ 7 × 10-12 eVm for a 10-monolayer (ML) film] and substrate-derived interface states with large (~0.5-eV) splitting. The E(k||) dispersion of this pair of interface states changes only slightly up to 3 ML Al. At higher Al coverages, the QWSs and interface states show a remarkable avoided-crossing effect in their band dispersions. This avoided-crossing effect obeys symmetry as well as spin and, therefore, leads to a strongly enhanced spin-orbit splitting of Al QWSs. This is shown by E(k||) band dispersions and by spin- and angle-resolved spectra for several thicknesses up to 15 ML Al.

Figure 1. Angle-resolved photoemission data at (a) hν =
65 eV and (b)–(h) 62 eV as a function of binding energy and
emission angle for (a) a pure W(110) surface and (b)–(h) for
2–15 ML of Al on W(110). The labels I<sup>|</sup> and I<sup>||</sup> indicate interface
states formed at thicknesses of 2 and 3 ML. The label SS
corresponds to an Al(111)-derived surface state, and labels from
1 to 4 correspond to QWSs. The data are presented in the form
of the first derivative of the photoemission spectra. The borders of
the W surface-projected energy gap are marked by dotted lines,
and expected W 6<i>p</i> dispersions are marked by two straight dotted
lines.

Angle-resolved photoemission data at (a) hν = 65 eV and (b)–(h) 62 eV as a function of binding energy and emission angle for (a) a pure W(110) surface and (b)–(h) for 2–15 ML of Al on W(110). The labels I| and I|| indicate interface states formed at thicknesses of 2 and 3 ML. The label SS corresponds to an Al(111)-derived surface state, and labels from 1 to 4 correspond to QWSs. The data are presented in the form of the first derivative of the photoemission spectra. The borders of the W surface-projected energy gap are marked by dotted lines, and expected W 6p dispersions are marked by two straight dotted lines.

Phys. Rev. B 85 (2012) 045425.


Nano Lett. 11, 5401 (2011) Nitrogen-Doped Graphene: Efficient Growth, Structure, and Electronic Properties

D. Usachov, O. Vilkov, A. Grüneis, D. Haberer, A. Fedorov, V.K. Adamchuk, A.B. Preobrajenski, P. Dudin, A. Barinov, M. Oehzelt, C. Laubschat, and D. V. Vyalikh

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 timedependent 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 × 1012 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.

Figure 1. The procedure for the graphene synthesis on a weekly bonded monolayer of h-BN.

(left) Preparation of N-graphene samples; (right) ARPES of N-graphene/Au/Ni(111)/W(110), measured at the photon energy of 35 eV, through the K-point, few degrees off the direction, perpendicular to the ΓK. The Dirac cone is shifted by ~0.3 eV toward higher binding energies with respect to the π-band of undoped graphene/Au/Ni(111)/W(110), which has Dirac point directly at (or slightly above) the EF, and a band gap of ~0.2 eV appears at the K-point.

Nano Lett. 11 (2011) 5401.


Appl. Phys. Lett. 98, 122111 (2011) Atmospheric stability and doping protection of noble-metal intercalated graphene on nickel

D.E. Marchenko, A.Yu. Varykhalov, A.G. Rybkin, A.M. Shikin, and O. Rader

As it is well known, graphene grown on Ni(111) can be transformed to a so-called quasifreestanding state with the band dispersion identical to that of ideal graphene, when a layer of gold atoms is intercalated between the graphene layer and a nickel surface. Gold atoms under graphene block the chemical interaction between graphene and the nickel surface and restore the relativistic band structure and overall charge neutrality while graphene remains an epitaxial overlayer on the metal. In this system, the Dirac cone becomes spin polarized and spin split due to a Rashba phenomenon induced by the strong spin-orbit interaction in the Au.

Figure 1. Directions of the angle-resolved photoelectron spectroscopy (ARPES) measurements (a) for wide angle overview scans in GK and GM directions of the graphene Brillouin zone at this figure and (b) for the region near the K-point in Figure 2. ARPES spectra for the Graphene/Au/Ni system for (c) as prepared sample and (d) a sample exposed to atmosphere and subsequently annealed.

Figure 1. Directions of the angle-resolved photoelectron spectroscopy (ARPES) measurements (a) for wide angle overview scans in ΓK and ΓM directions of the graphene Brillouin zone at this figure and (b) for the region near the K-point in Figure 2. ARPES spectra for the Graphene/Au/Ni system for (c) as prepared sample and (d) a sample exposed to atmosphere and subsequently annealed.

In the present work we show that the linear relativistic dispersion of Au-intercalated graphene/Ni(111) persists without energy shift after exposure to atmosphere which means that graphene and the Au monolayer protect the Ni against carbidization and oxidation and that doping of the graphene does not occur.

Figure 1 shows angle resolved photoelectron intensity in an E(k) representation measured at 62 eV photon energy (c) before and (d) after exposure to air for 10 min and short annealing at 800 K. As we can see, at this stage no difference in the electronic structure caused by the exposure. In particular, (i) no carbon of the graphene has become carbidized which would lead to extra nondispersive states, (ii) no immediate graphene/Ni(111) interface is created which would shift the E(k) dispersion by 2 eV to higher binding energy, (iii) no apparent band gap opens at EF, and (iv) no apparent doping occurs.

Figure 2. Measurements in the vicinity of the K-point for Graphene/Au/Ni(111) (a) as prepared, (b) after exposure to atmosphere, and (c) after annealing. No change in the doping is observed within 20 meV.

Figure 2. Measurements in the vicinity of the K-point for Graphene/Au/Ni(111) (a) as prepared, (b) after exposure to atmosphere, and (c) after annealing. No change in the doping is observed within 20 meV.

In order to investigate the doping state of the graphene π band in details, Figure 2 displays measurements in the "perpendicular to ΓK" geometry near the K-point (a) as prepared, (b) after exposure to air for ~1 h, and (c) after subsequent annealing. The cut through the Dirac cone shows the Dirac point in Figure 2(a) located at ΔEF~100 meV above the Fermi level, according to our fit. The fit results give the same values within 20 meV after exposure and after annealing. No detectable energy shift means that the doping state of graphene is not affected by the exposure to air. No changes were detected with low-energy electron diffraction and XPS also, except the peaks to background intensity ratio change, even after direct transfer of the sample from air to measurements without additional heating. It was shown that Au-intercalated graphene/Ni(111) is self-protective against carbidization and oxidation of the Ni and against disruption of the spin-dependent electronic structure of graphene. The protection of ferromagnets could be interesting for nanostructures such as the separate Co islands on W(110) which can be graphene covered while maintaining their topology. Moreover, the graphene is self-protective against doping and random doping. The former could mean that doping of graphene by nonmetals requires open edges which are rare on graphene/Au/Ni(111) while the latter is possibly related to the contact to the Au. It provides for the metallic screening which for freestanding graphene is very weak due to the vanishing density of states at the Dirac point.

Appl. Phys. Lett. 98 (2011) 122111.


Phys. Rev. B 82, 075415 (2010) Quasifreestanding single-layer hexagonal boron nitride as a substrate for graphene synthesis

D. Usachov, V.K. Adamchuk, D. Haberer, A. Grüneis, H. Sachdev, A. B. Preobrajenski, C. Laubschat, and D. V. Vyalikh

Graphene is one of the amazing recent developments in modern sciences and seems to be one of the most promising pathways towards implementation of the next generation electronic devices. Due to graphene's unique properties, devices based on mechanisms alternative to classical charge transport come into reach, that would allow for unprecedented speed of the graphene-based transistors. Despite a variety of unique properties of graphene, the challenge of how to bring graphene-based devices into real life use remains. The development of a reliable technological protocol for effective, large-scale synthesis of high-quality graphene flakes on insulating substrates is one of the main long-term goals in graphene technology.

Figure 1. The procedure for the graphene synthesis on a weekly bonded monolayer of h-BN.

Figure 1. The procedure for the graphene synthesis on a weekly bonded monolayer of h-BN.

A new approach for synthesizing graphene layers on insulating and quasifreestanding hexagonal boron nitride (h-BN) that has been originally formed on nickel substrates is schemat-ically shown in Figure 1. In the beginning, a clean surface of Ni(111) was prepared (step 1). Instead of using a bulk single crystal as the substrate, thin (~100 Å) Ni films have been epitaxially grown on W(110) resulting in Ni(111) layers. Using W(110) enables removing Ni from the W(110) by short "flashes" and repeating the preparation sequence for verification purposes. Next, a high-quality h-BN layer was produces on top of Ni(111) by cracking borazine vapor (B3N3H6) at a pressure of 10-7 mbar and a temperature of 750 °C for 10 min (step 2). The reaction is known to be self-limited to a single monolayer. At the next stage the ML-h-BN was liberated from a tight chemical interaction with Ni atoms by depositing Au on top (step 3) and its subsequent intercalation (step 4) by annealing the system at ~500 °C for 5 min. At the final stage (step 5), a graphene layer was formed by chemical vapor deposition (CVD) of acetylene (C2H2) on the quasifreestanding ML h-BN at a temperature of ~750 °C and acetylene gas pressure of ~3×10-4 mbar for ~90 min. The exposure has been chosen to obtain graphene coverage of nearly 0.5 ML, which ensures absence of graphene multilayers. Therefore, the measured electronic structure corresponds to a single-atom thick layer of graphene. Each step of the above approach has been cross-checked by means of angle-resolved photoelectron spectroscopy (ARPES), x-ray photoemission, and absorption techniques.

Figure 2. The electron-energy band structure of the graphene/ML h-BN/ML Au/Ni(111)/(110) system measured by ARPES at the room temperature. The dashed lines denote the PE maxima of ML h-BN/Au and graphene/Au.

Figure 2. The electron-energy band structure of the graphene/ML h-BN/ML Au/Ni(111)/(110) system measured by ARPES at the room temperature. The dashed lines denote the PE maxima of ML h-BN/Au and graphene/Au.

Figure 2 shows an overview band-structure map taken after the CVD of acetylene on the quasifreestanding ML h-BN (step 5). A new electron band is detected, which is approaching EF at the K point of the Brillouin zone. It evidently exhibits linear dispersion at the K point that allows us to attribute it to the π band of graphene. The interesting region at the K point, framed by the dotted rectangle in Figure 2, is depicted as an inset. The sharpness of this band and its linear behavior confirm our assumption that the quasifreestanding ML h-BN has a positive effect on the orientation of the graphene layer. The second branch of Dirac cone is hardly visible due to the geometry of the ARPES experiment. However, comparison of the energy position of the π band of graphene/h-BN/Au/Ni(111) with that of graphene/Au/Ni(111) suggests that the apex of the Dirac cone is located close to the Fermi level. Apparently, the structural order of the graphene layer synthesized on the quasifreestanding ML h-BN allows exploring its electronic structure and, in particular, the behavior of the π band, which obviously exhibits linear dispersion near the Fermi level.

It is believed that 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.

Phys. Rev. B 82 (2010) 075415 (2010).


Nature Chemistry 2, 1084 (2010) Three-dimensional chitin-based scaffolds from Verongida sponges (Demospongiae: Porifera)

H. Ehrlich, M. Ilan, M. Maldonaldo, , G. Bavestrello, Z. Klajic, J. L. Carballo, S. Schiaparelli, A. Ereskovsky, P. Schupp, R. Born, H. Worch, V.V. Bazhenov, D. Kurek, V. Varlamov, D.V. Vyalikh, K. Kummer, V.V. Sivkov, S.L. Molodtsov, H. Meissner, G. Richter, E. Steck, W. Richter, S. Hunoldt, M. Kammer, S. Paasch, V. Krasotkin, G. Patzke, and E. Brunner

The formation of extended skeletal structures often involves hierarchical processing: assemblies of organic molecules are used to build the basic framework blocks which predetermine the organization of subsequent inorganic deposits. The resulting inorganic materials may in turn be used as building blocks for the production of more complex structures of higher order. Animal tissues use a variety of skeletal structures. The two most abundant systems make use of collagen or chitin as major framework constituents. The collagenous system is based on the association between collagen - a unique fibrous protein - and varying quantities of non-collagenous proteins. In the chitin system, the aminopolysaccaride chitin is combined with non-collagenous proteins. Both, collagen and chitin are extracellular secretions, usually with a conspicuous fibrous organization at different hierarchical levels (nanofibrils - microfibrils - fibers). Likewise, collagen and chitin structures may serve as scaffolds for amorphous or/and crystalline inorganic deposits.

Figure 1. Aplysina aerophoba – a typical representative of Verongida sponges, in its natural underwater environment (Kotor Bay, Montenegro, scale bar: 50 cm).

Aplysina aerophoba - a typical representative of Verongida sponges, in its natural underwater environment (Kotor Bay, Montenegro, scale bar: 50 cm).

The chitin systems are usually of ectodermal origin and chitin itself is involved in the exoskeleton formation. In contrast, the collagen systems are almost exclusively of mesodermal origin and are involved in endoskeletons. Phylogenetic studies suggest that the chitin systems of fungi and animals are related and appeared before collagen systems evolved. Because of their different developmental and evolutionary origin, the chitin and the collagen systems are usually considered to be independent. However, it was recently discovered that chitin is incorporated into the spongin-based skeletal fibers of the sponges (Phylum Porifera) Verongula gigantea and Ianthella basta which belong to the order Verongida (class Demospongiae). There are 14 taxonomic orders within the class Demospongiae. This order encompasses about 95% of extant sponges. Three of these orders - Verongida, Dictyoceratida, and Dendroceratida - exhibit skeletons without siliceous spicules. Instead, the skeletons consist of spongin fiber networks. Spongin is a protein resulting from a super-compaction of collagen fibrils and filaments. Recent phylogenetic and embryological studies have shown that Verongida, Dictyoceratida, and Dendroceratida - although being characterized by fibrous sponging skeletons - do not make up a cohesive phylogenetic unit. The recently discovered chitin/collagen composite fiber skeletons found in the aforementioned Verongida sponge species have apparently evolved independently from Dictyoceratida and Dendroceratida.

However, a systematic study of the numerous other species belonging to the order Verongida has not yet been performed. Sponges are probably the earliest branching animals with a fossil record dating back to the Precambrian. This is the reason why the presence of chitin in sponges is also of evolutionary interest. But the elucidation of the three dimensional organization of their chitin structures may not only be of biological/evolutionary interest.

A detailed and comprehensive study of the structural and physico-chemical properties of three dimensional skeletal scaffolds of the marine sponges Aiolochroia crassa, Aplysina aerophoba (see Fig. 1), Aplysina cauliformis, Aplysina cavernicola, and Aplysina fulva (Verongida: Demospongiae) was made applying 13C solid-state NMR, NEXAFS and FTIR spectroscopies. It was demonstrated that alkali-resistant, fibrous material remaining after demineralization, consists of α-chitin for all species under study. There may be a huge potential of the three-dimensional chitin-based scaffolds for application in the tissue engineering.

Nature Chemistry 2 (2010) 1084.
International Journal of Biological Macromolecules 47 (2010) 132.
International Journal of Biological Macromolecules 47 (2010) 141.


Organic electronics 11, 1461 (2010) Electronic properties of potassium-doped FePc

V.Yu. Aristov, O.V. Molodtsova, V.V. Maslyuk, D.V. Vyalikh, T. Bredow, I. Mertig, A.B. Preobrajenski, and M. Knupfer

It is widely believed that magnetic transition metal phthalocyanines (MTM-Pc's) like FePc and CoPc can be implemented as building blocks for new generation electronic devices. Their potential usage in various technological applications like optical switches, information storage and nonlinear optics is widely studied and hotly discussed. MTM-Pc's exhibit a good compatibility with ultrahigh vacuum (UHV) and can be grown as thin, ultra clean, well-ordered films on solid substrates. Nowadays these materials are also discussed for development of low dimensional molecular magnets, which are possible candidates for implementation in information storage media, spintronics and quantum computers. To make further progress in the development of next generation electronics both understanding and control of the physical, chemical and transport properties of organic semiconductors are required. Here, we comprehensively explore the evolution of the electronic structure of thin molecular layers of FePc (Figure 1) upon electron doping induced by embedding potassium atoms in the system.

Figure 1. Representation of the Fe phthalocyanine  molecule, where the non equivalent positions of the nitrogen atoms are labeled as N1 and N2. The elaborated scenario suggests that the attachment of the potassium atoms can be only occurred at the four N2 sites.

Figure 1. Representation of the Fe phthalocyanine molecule, where the non equivalent positions of the nitrogen atoms are labeled as N1 and N2. The elaborated scenario suggests that the attachment of the potassium atoms can be only occurred at the four N2 sites.

Figure 2 shows the NEXAFS spectra of FePc taken at the K edges of nitrogen and carbon atoms as a function of K doping. The rather sharp features A, B and C can be attributed to N 1s → π* and C 1s → π* excitations. The A, B, and C empty states detected for both N and C K-edge spectra are essentially associated with the same π* states, which have finite probability to be located at the pyrrole carbon and nitrogen atoms. This does not exclude, however, contributions from excitations into C-C - π* orbitals to these peaks.

Figure 2. The evolution of the nitrogen (a) and carbon (b) K-edge NEXAFS spectra upon deposition of potassium on the thin molecular layer of FePc.

Figure 2. The evolution of the nitrogen (a) and carbon (b) K-edge NEXAFS spectra upon deposition of potassium on the thin molecular layer of FePc.

The profile of feature A in Figure 2(a) changes upon doping mostly by a decrease of spectral weight at the low energy side (feature A1 in Figure 2(a)). This becomes especially evident from a direct comparison of the N K-edge spectra with different K doping shown in the inset of Figure 2(a). Apparently, the intensity of the A1 feature in both figures substantially reduces upon doping. This is a direct result of the filling of the lowest unoccupied molecular orbits (LUMO) by charge transfer from the potassium ions. The intensity reduction as seen in Figure 2(a) and Figure 2(b) is in good correlation with the doping level. The spectral features B and C at higher photon energies show a shift to lower energies as a function of doping, similar to what is observed for potassium-doped C60, and can be associated with the relaxation of the molecular structure and bond lengths of the FePc molecules upon adding charge to the LUMO, which is of antibonding π* character.

With increasing potassium quantity the intensity of spectral feature A1 in Figure 2(b), which can be attributed to transitions from C 1s core levels of the benzene carbon atoms (C2, C3, C4) into to LUMO, decreases. This behavior becomes more evident in inset of Figure 2, where the C K-edge spectra for several potassium doping levels are superimposed. We can conclude that the density of states of the LUMO decreases with K doping as a consequence of filling this molecular orbital. Assuming that the LUMO wave function has a finite probability at both carbon and nitrogen sites in FePc, it is obvious that the filling of this molecular orbital becomes observable at both nitrogen and carbon K edges. Thus, all observations made in this study suggest that the filling of the LUMO of FePc correlates well with the quantity of potassium atoms, which indeed are located closely to pyrrole nitrogen atoms of the FePc molecule.

Organic electronics 11 (2010) 1461.


Nano Lett. 10, 3360 (2010) Tunable Band Gap in Hydrogenated Quasifreestanding Graphene

D. Haberer, D.V. Vyalikh, S. Taioli, B. Dora, M. Farjam, J. Fink, D.E. Marchenko, T. Pichler, K. Ziegler, S. Simonucci, M. S. Dresselhaus, M. Knupfer, B. Büchner, and A. Grüneis

The discovery of graphene has led to a dramatic increase in research effort due to its remarkable physical properties. Graphene is a zero gap semiconductor with a linear energy band dispersion around the Fermi energy (EF), so that the charge carriers mimic massless Dirac fermions. This allows one to address basic questions of quantum electrodynamics in a benchtop experiment. Nevertheless, the application of graphene in semiconductor devices requires a band gap in order to switch the conductivity between an on and off state. Physical and chemical approaches for opening a gap are under discussion. First, size quantization of about 1 nm induces band gaps of ~1 eV in graphene nanoribbons, nanotubes, and quantum dots. However, in the case of nanotubes, the preparation of samples with Ohmic contacts is still challenging. Similarly, in the case of nanoribbons, the electronic properties are determined by the edges, rendering this approach technologically very demanding. An alternative strategy is the chemical functionalization of graphene which induces band gaps and can even be reversed.

In this work a tunable band gap in hydrogenated quasifreestanding graphene on Au (EF is the Dirac point) is demonstrated. Pristine graphene samples were prepared under ultrahigh vacuum conditions by chemical vapor deposition on Ni(111) films. Hereafter one monolayer (ML) of Au was deposited on graphene and intercalated into the graphene/Ni interface by annealing. This procedure liberates graphene from the strong substrate interaction rendering it quasifreestanding. Hydrogenation of graphene/Au was performed by exposing graphene to a beam of atomic H that was produced by cracking H2 at 2800 K in a W capillary at 2×10-9 mbar for 10-100 s. X-ray photoemission spectroscopy (XPS) of the C 1s core level and near edge X-ray absorption spectroscopy (NEXAFS) at the K-edge of carbon were performed to study the changes in the chemical bonding of graphene for each of the functionalization steps.

Figure 1. (a) From top to bottom: graphene on nickel, gold intercalated between the graphene/Ni interface, and hydrogenated graphene on gold. The hydrogen atoms are marked in violet. C1 and C2-C3 indicate chemical environments for graphene/gold and hydrogenated graphene, respectively. (b) ARPES spectrum of graphene/Au along with a 3NN TB calculation and (c) the raw photoemission data. (d) ARPES spectra around the K point of hydrogenated graphene with increasing H-coverages as denoted. The last image (bottom right) corresponds to graphene after thermal annealing.

(a) From top to bottom: graphene on nickel, gold intercalated between the graphene/Ni interface, and hydrogenated graphene on gold. The hydrogen atoms are marked in violet. C1 and C2-C3 indicate chemical environments for graphene/gold and hydrogenated graphene, respectively. (b) ARPES spectrum of graphene/Au along with a 3NN TB calculation and (c) the raw photoemission data. (d) ARPES spectra around the K point of hydrogenated graphene with increasing H-coverages as denoted. The last image (bottom right) corresponds to graphene after thermal annealing.

Figure 1(a) illustrates the Au intercalation and hydrogenation procedure. Hydrogen induces a local sp3 bonding thereby breaking double bonds to the neighboring carbon atoms. Atoms C1 - C3 denote the chemical environments of unhydrogenated carbons, C atoms next to an sp3 site, and C atoms in an sp3 environment, respectively.

An analysis of the quasi-particle dispersion and a discussion of the electronic band structure of hydrogenated graphene should be made. This is vital for unraveling changes close to EF that determine optical and transport properties. For this purposes, ARPES experiments, which is the most powerful technique to study electronic energy band dispersions and gives access to graphene's spectral function, were made. Figure 1(b) shows the spectral function of graphene intercalated with Au in the ΓK direction. From the momentum dispersion of the π-band, a Fermi velocity of vF = 1.05×106 m/s, identical to that of graphite, was obtained. Interestingly, the third nearest neighbor (3NN) tight-binding (TB) calculations employing the previously reported TB parameters from graphite yield a band structure that is fully consistent with the measured ARPES maxima of graphene on Au. Au intercalated graphene is therefore an ideal model system to study hydrogenation. Figure 1(d) displays the spectral function of graphene at the K point after in situ exposure to a beam of atomic hydrogen. It is clear that already at low H/C ratios of 0.5%, we observe a dramatic decrease of the photoemission intensity of the π band close to EF accompanied by a general broadening of the π-band. The most striking effect is the opening of a gap of reduced ARPES intensity between the π-band and EF. The π band maxima are therefore not crossing EF anymore but appear at lower energies as hydrogenation proceeds. The last graph of Figure 1(d) shows the fully recovered π-band after annealing at 600 K, demonstrating the reversibility of this functionalization procedure.

The functionalization of quasifreestanding graphene with atomic hydrogen was investigated. The XPS measurements of graphene have shown a C 1s core level shift of 0.5 eV toward lower binding energy upon Au intercalation in between the graphene/Ni(111) interface, resulting in a substantial reduction of the substrate interaction. NEXAFS measurements indicate a rehybridization from sp2 to sp3 and the formation of C-H bonds perpendicular to the graphene layer. Most importantly, the ARPES spectra of hydrogenated graphene clearly show the downshift of the π band's spectral function to lower energies and also a broadening. Calculations support sublattice symmetry breaking as the reason for the observed changes in the ARPES upon hydrogenation. Since the energy difference between the onset of the integrated PE intensity (proportional to the DOS) and EF is related to half the electronic band gap value, tunable optical properties for hydrogenated graphene are expected. Results also unraveled an interesting connection between the hydrogenation of graphene and amorphous carbon. On the one hand, the origin for the tunable optical band gap in a-C:H might also be understood in terms of sublattice symmetry breaking. On the other hand, hydrogenation might as well also lead to functional optical devices based on graphene because the electronic band gap can be tuned with hydrogen coverage. Results are therefore also relevant to optical absorption, photoluminescence, and resonance Raman spectroscopy of graphene.

Nano Lett. 10 (2010) 3360.

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