Publications of the IFF

Saturation Field of Frustrated Chain Cuprates: Broad Regions of Predominant Interchain Coupling

A thermodynamic method to extract the interchain coupling (IC) of spatially anisotropic 2D or 3D spin-1/2 systems from their empirical saturation field Hs (T=0) is proposed. Using modern theoretical methods we study how Hs is affected by an antiferromagnetic (AFM) IC between frustrated chains described in the J1-J2-spin model with ferromagnetic 1st and AFM 2nd neighbor in-chain exchange. A complex 3D-phase diagram has been found. For Li2CuO2 and Ca2Y2Cu5O10, we show that Hs is solely determined by the IC and predict Hs≈61  T for the latter. With Hs≈55  T from magnetization data one reads out a weak IC for Li2CuO2 close to that obtained from inelastic neutron scattering.
S. Nishimoto, S.-L. Drechsler, R. O. Kuzian, J. van den Brink, J. Richter, W. E. A. Lorenz, Y. Skourski, R. Klingeler and B. Büchner Physical Rev. Lett 107 (2011) 097201 1-4 URL

Evidence for a New Two-Dimensional C 4 H-Type Polymer Based on Hydrogenated Graphene

A new polymer with C4H stoichiometry based on graphene is synthesized in situ using template-induced polymerization of self-organizing hydrogen adsorbates on graphene. The polymerization is observed “live” on the surface of graphene by photoemission spectroscopy. Photoemission spectroscopy allows for an accurate determination of the carbon/hydrogen stoichiometry, an aspect that is extremely important for understanding functionalized graphene.
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 Advanced Functional Materials (2011), XX, 1-7, URL

Direct imaging of Joule heating dynamics and temperature profiling inside a carbon nanotube interconnect

Understanding resistive (or Joule) heating in fundamental nanoelectronic blocks, such as carbon nanotubes, remains a major challenge, particularly in regard to their structural and thermal variations during prolonged periods of electrical stress. Here we show real-time imaging of the associated effects of Joule heating in the channel of carbon nanotube interconnects. First, electrical contacts to nanotubes entirely filled with a sublimable material are made inside a transmission electron microscope. On exposure to a high current density, resistive hotspots are identified on (or near) the contact points. These later migrate and expand along the carbon nanotube, as indicated by the localized sublimation of the encapsulated material. Using the hotspot edges as markers, it is possible to estimate the internal temperature profiles of the nanotube. Simple and direct, our method provides remarkable spatial and temporal insights into the dynamics of resistive hotspots and millisecond-paced thermal variations occurring inside nanoscaled tubular interconnects.
Pedro M.F.J. Costal, Ujjal K. Gautam, Yoshio Bando & Dmitri Golberg Nature Communications(2011), 2-6, URL

Graphene: Piecing it Together

Graphene has a multitude of striking properties that make it an exceedingly attractive material for various applications, many of which will emerge over the next decade. However, one of the most promising applications lie in exploiting its peculiar electronic properties which are governed by its electrons obeying a linear dispersion relation. This leads to the observation of half integer quantum hall effect and the absence of localization. The latter is attractive for graphenebased fi eld effect transistors. However, if graphene is to be the material for future electronics, then signifi cant hurdles need to be surmounted, namely, it needs to be mass produced in an economically viable manner and be of high crystalline quality with no or virtually no defects or grains boundaries. Moreover, it will need to be processable with atomic precision. Hence, the future of graphene as a material for electronic based devices will depend heavily on our ability to piece graphene together as a single crystal and defi ne its edges with atomic precision. In this progress report, the properties of graphene that make it so attractive as a material for electronics is introduced to the reader. The focus then centers on current synthesis strategies for graphene and their weaknesses in terms of electronics applications are highlighted.
Mark H. Rümmeli, Claudia G. Rocha, Frank Ortmann, Imad Ibrahim, Haldun Sevincli, Felix Börrnert, Jens Kunstmann, Alicja Bachmatiuk, Markus Pötsche, Masashi Shiraishi, M. Meyyappan, Bernd Büchner, Stephan Roche and Gianaurelio Cuniberti, Advanced Functional Materials(2011) XX, 1-20 URL

CCVD Synthesis of Carbon-Encapsulated Cobalt Nanoparticles for Biomedical Applications

Carbon-encapsulated ferromagnetic Cobalt nanoparticles (Co@C) have been synthesized by catalytic chemical vapour deposition (CCVD). The nanoparticles, mainly ranging between 10 and 15 nm, are tightly encapsulated by 2–3 concentric graphitic carbon shells and protected from oxidation. Because of their magnetic properties (saturation magnetization of 106 emu/g and a coercivity HC of 250 Oe), Co@C nanoparticles have been investigated for hyperthermia application. Although the observed values of the specifi c absorption rate (28.7 W/gCo@C at 30 kA/m and 215.4 W/gCo@C at 70 kA/m) are slightly lower than required in actual hyperthermia therapies, the observed strong heating effect provides a very promising starting point for future clinical application. It is also demonstrated that these nanoparticles can at the same time be used for magnetic resonance imaging (MRI) with an effi ciency comparable to commercially available T2 contrast agents. 
Petar Lukanov, Vijay K. Anuganti, Yulia Krupskaya, Anne-Marie Galibert, Brigitte Soula, Carmen Tilmaciu, Aldrik H. Velders, Ruediger Klingeler, Bernd Büchner and Emmanuel Flahaut, Advanced Functional Materials(2011) XX, 1-6 URL

Saturation Field of Frustrated Chain Cuprates: Broad Regions of Predominant Interchain Coupling

A thermodynamic method to extract the interchain coupling (IC) of spatially anisotropic 2D or 3D spin-1/2 systems from their empirical saturation field H_s (T=0) is proposed. Using modern theoretical methods we study how Hs is affected by an antiferromagnetic (AFM) IC between frustrated chains described in the J₁-J₂-spin model with ferromagnetic 1st and AFM 2nd neighbor in-chain exchange. A complex 3D-phase diagram has been found. For Li₂CuO₂ and Ca₂Y₂Cu₅O₁₀, we show that Hs is solely determined by the IC and predict H_s≈61  T for the latter. With H_s≈55  T from magnetization data one reads out a weak IC for Li₂CuO₂ close to that obtained from inelastic neutron scattering. 
S. Nishimoto, S.-L. Drechsler, R. O. Kuzian, J. van den Brink, J. Richter, W. E. A. Lorenz, Y. Skourski, R. Klingeler, and B. Büchner, Physical Review Letters 107 (2011) 097201 URL

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

Observation of Electronic Ferroelectric Polarization in Multiferroic YMn₂O₅

We report the observation of a magnetic polarization of the O 2p states in YMn₂O₅ through the use of soft x-ray resonant scattering at the oxygen K edge. Remarkably, we find that the temperature dependence of the integrated intensity of this signal closely follows the macroscopic electric polarization, and hence is proportional to the ferroelectric order parameter. This is in contrast with the temperature dependence observed at the Mn L₃ edge, which reflects the Mn magnetic order parameter. First-principles calculations provide a microscopic understanding of these results and show that a spin-dependent hybridization of O 2p and Mn 3d states results in a purely electronic contribution to the ferroelectric polarization, which can exist in the absence of lattice distortions.
S. Partzsch, S. B. Wilkins, J. P. Hill, E. Schierle, E. Weschke, D. Souptel, B. Büchner, J. Geck Phys. Rev. Lett 107 (2011) 057201 URL

Spin Gap in the Zigzag Spin-1/2 Chain Cuprate Sr_0.9Ca_0.1CuO₂

We report a comparative study of ⁶³Cu nuclear magnetic resonance spin lattice relaxation rates T1^-1 on undoped SrCuO₂ and Ca-doped Sr_0.9Ca_0.1CuO₂ spin chain compounds. A temperature independent T1^-1 is observed for SrCuO₂ as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T1^-1 for T<90  K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J1-J2 Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J2-exchange coupling as a possible cause of the spin gap.
F. Hammerath, S. Nishimoto, H.-J. Grafe, A. U. B. Wolter, V. Kataev, P. Ribeiro, C. Hess, S.-L. Drechsler, B. Büchner, Phys. Rev. Lett. 107 (2011)  017203 URL

A New Family of 1D Exchange Biased Heterometal Single Molecule Magnets: Observation of Pronounced Quantum Tunneling Steps in the Hysteresis Loops of Quasi-Linear {Mn₂Ni₃} Clusters

First members of a new family of heterometallic Mn/Ni complexes [Mn₂Ni₃X₂L₄(LH)₂(H₂O)₂] (X = Cl: 1; X = Br: 2) have been synthesized and characterized. The molecular structures feature a quasi-linear MnIII-NiII-NiII-NiII-MnIII core with six-coordinate metal ions, where elongated axes of all the distorted octahedral coordination polyhedra are aligned parallel and are fixed with respect to each other by intramolecular hydrogen bonds. 1 and 2 exhibit quite strong ferromagnetic exchange interactions throughout (J_Mn-Ni ≈ 44 K (1) or 42 K (2); J_Ni-Ni ≈ 19 K (1) or 18 K (2)) that lead to an S_tot = 7 ground state. Frequency-tunable high-field ESR, measured on an oriented powder sample of 1 revealed a substantial magnetic anisotropy gap with a negative axial anisotropy D = – 0.55 K which gives evidence for a bistable (easy axis) magnetic ground state of the molecule. The analysis of the ESR data by means of a minimal effective spin Hamiltonian enables an accurate description of the energy level scheme and the spin states of the complex. The observed high spin ground state and the negative axial anisotropy ensure necessary prerequisites for the realization of a single molecular magnet. Indeed, slow relaxation of the magnetization at low temperatures for both 1 and 2 is evident from frequency-dependent peaks in the out-of-phase AC susceptibility and hysteresis loops in magnetization versus DC field measurements. Moreover, pronounced quantum tunnelling steps are observed in the hysteresis loops which are also very characteristic for single molecular magnets.
A. Das, K. Gieb, Y. Krupskaya, S. Demeshko, S. Dechert, R. Klingeler, V. Kataev, B. Büchner, P. Müller, F. Meyer, J. Am. Chem. Soc. 133 (2011) 3433–3443 URL

Manifestation of New Interference Effects in a Superconductor-Ferromagnet Spin Valve

Superconductor/ferromagnet (S/F) spin valve effect theories based on the S/F proximity phenomenon assume that the superconducting transition temperature Tc of F1/F2/S or F1/S/F2 trilayers for parallel magnetizations of the F1- and F2-layers Tc{P} are smaller than for the antiparallel orientations Tc{AP}. We report for CoOx/Fe1/Cu/Fe2/In multilayeres with varying Fe2-layer thickness the sign-changing oscillating behavior of the spin valve effect DeltaTc=Tc{AP}-Tc{P}. We observe the full direct effect with Tc{AP}>Tc{P} for Fe2-layer thickness d{Fe2}<1 nm and the full inverse (Tc{AP}= 1 nm. Interference of Cooper pair wave functions reflected from both surfaces of the Fe2-layer appear as the most probable reason for the observed behavior of DeltaTc. 
P. V. Leksin, N. N. Garif'yanov, I. A. Garifullin, J. Schumann, V. Kataev, O. G. Schmidt, B. Büchner, Phys. Rev. Lett. 106 (2011) 067005 URL  

Sexithiophene encapsulated in single walled carbon nanotube: An in situ Raman spectroelectrochemical study of a peapod

The interaction of singlewalled carbon nanotubes (SWCNTs) and a-sexithiophene (6T) was studied by Raman spectroscopy and by in situ Raman spectroelectrochemistry. The encapsulation of 6T in SWCNT and its interaction causes a bleaching of its photoluminescence, and also small shifts of its Raman bands. The Raman features of the SWCNT with embedded 6T (6T-peapods) change in both intensity and frequency compared to those of pristine SWCNT, which is a consequence of a change of the resonant condition. Electrochemical doping demonstrated that the electrode potential applied to the SWCNT wall causes changes in the embedded 6T. The effects of electrochemical charging on the Raman features of pristine SWCNT and 6T@SWCNT were compared. It is shown that the interaction of SWCNT with 6T also changes the electronic structure of SWCNT in its charged state. This change of electronic structure is demonstrated both for semiACHTUNGTRENUNGconducting and metallic tubes. M. Kalbáč, L. Kavan, S. Gorantla, T. Gemming, L. Dunsch, Chem. Eur. J. 16:38 (2010) 11753–11759 URL

Latent Porosity in Potassium Dodecafluoro-closo-dodecaborate(2−). Structures and Rapid Room Temperature Interconversions of Crystalline K₂B₁₂F₁₂, K₂(H₂O)₂B₁₂F₁₂, and K₂(H₂O)₄B₁₂F₁₂ in the Presence of Water Vapor

Structures of K(2)(H(2)O)(2)B(12)F(12) and K(2)(H(2)O)(4)B(12)F(12) were determined by X-ray diffraction. They contain [K(μ-H(2)O)(2)K](2+) and [(H(2)O)K(μ-H(2)O)(2)K(H(2)O)](2+) dimers, respectively, which interact with superweak B(12)F(12)(2-) anions via multiple K···F(B) interactions and (O)H···F(B) hydrogen bonds (the dimers in K(2)(H(2)O)(4)B(12)F(12) are also linked by (O)H···O hydrogen bonds). DFT calculations show that both dimers are thermodynamically stabilized by the lattice of anions: the predicted ΔE values for the gas-phase dimerization of two K(H(2)O)(+) or K(H(2)O)(2)(+) cations into [K(μ-H(2)O)(2)K](2+) or [(H(2)O)K(μ-H(2)O)(2)K(H(2)O)](2+) are +232 and +205 kJ mol(-1), respectively. The calculations also predict that ΔE for the gas-phase reaction 2 K(+) + 2 H(2)O → [K(μ-H(2)O)(2)K](2+) is +81.0 kJ mol, whereas ΔH for the reversible reaction K(2)B(12)F(12 (s)) + 2 H(2)O((g)) → K(2)(H(2)O)(2)B(12)F(12 (s)) was found to be -111 kJ mol(-1) by differential scanning calorimetry. The K(2)(H(2)O)(0,2,4)B(12)F(12) system is unusual in how rapidly the three crystalline phases (the K(2)B(12)F(12) structure was reported recently) are interconverted, two of them reversibly. Isothermal gravimetric and DSC measurements showed that the reaction K(2)B(12)F(12 (s)) + 2 H(2)O((g)) → K(2)(H(2)O)(2)B(12)F(12 (s)) was complete in as little as 4 min at 25 °C when the sample was exposed to a stream of He or N(2) containing 21 Torr H(2)O((g)). The endothermic reverse reaction required as little as 18 min when K(2)(H(2)O)(2)B(12)F(12) at 25 °C was exposed to a stream of dry He. The products of hydration and dehydration were shown to be crystalline K(2)(H(2)O)(2)B(12)F(12) and K(2)B(12)F(12), respectively, by PXRD, and therefore these reactions are reconstructive solid-state reactions (there is also evidence that they may be single-crystal-to-single-crystal transformations when carried out very slowly). The hydration and dehydration reaction times were both particle-size dependent and carrier-gas flow rate dependent and continued to decrease up to the maximum carrier-gas flow rate of the TGA instrument that was used, demonstrating that the hydration and dehydration reactions were limited by the rate at which H(2)O((g)) was delivered to or swept away from the microcrystal surfaces. Therefore, the rates of absorption and desorption of H(2)O from unit cells at the surface of the microcrystals, and the rate of diffusion of H(2)O across the moving K(2)(H(2)O)(2)B(12)F(12 (s))/K(2)B(12)F(12 (s)) phase boundary, are even faster than the fastest rates of change in sample mass due to hydration and dehydration that were measured. The exchange of 21 Torr H(2)O((g)) with either D(2)O or H(2)(18)O in microcrystalline K(2)(D(2)O)(2)B(12)F(12) or K(2)(H(2)(18)O)(2)B(12)F(12) at 25 °C was also facile and required as little as 45 min to go to completion (H(2)O((g)) replaced both types of isotopically labeled water at the same rate for a given starting sample of K(2)B(12)F(12), demonstrating that water molecules were exchanging, not protons. Significant portions of mass (m) vs time (t) plots for the (1,2)H(2)O((g))/K(2)((2,1)H(2)O)(2)B(12)F(12 (s)) exchange reactions fit the equation m ∝ e(-kt), with 10(3)k = 1.9 s(-1) for one particle size distribution and 10(3)k = 0.50 s(-1) for another. Finally, K(2)(H(2)O)(2)B(12)F(12) was not transformed into K(2)(H(2)O)(4)B(12)F(12) after prolonged exposure to 21 Torr H(2)O((g)) at 25 °C, 37 Torr H(2)O((g)) at 35 °C, or 55 Torr H(2)O((g)) at 45 °C.  D. Peryshkov, A. A. Popov, S. H. Strauss, Water Vapor, J. Am .Chem. Soc. 2010, 132 (39), 13902-13913. DOI: 10.1021/ja105522d URL

A Facile Route to Metal Nitride Clusterfullerenes Using Guanidinium Salts: A Selected Organic Solid as the Nitrogen Source

Using guanidinium salts 1 and 2 as the new nitrogen sources, metal nitride clusterfullerenes (NCFs) based on a variety of metals (Dy, Sc, Y, Gd, Lu, and mixed metals Sc/Dy, Sc/Gd, Sc/Lu, and Lu/Ce) have been synthesized based on a new “selective organic solid” (SOS) route. The synthesis of Dy-NCFs by using Dy/1 was studied in detail, and the optimum molar ratio of 1/Dy/C has been determined to be 2.5:1:10. For several representative metals such as Sc, Y, Gd, Dy, and Sc/Dy, we quantitatively compared the yield of M3N@C80 synthesized by the SOS route with the reported “reactive gas atmosphere” route, thereby indicating that the yield of M3N@C80 by using 1 could be comparable to that obtained by the reactive gas atmosphere route. Three other nitrogen sources (3–5) were also studied for comparison, which were mixed with Dy metal but did not result in the formation of Dy-NCF. A possible reaction scheme for the solid-state reaction of 1, metal, and graphite is proposed. The SOS route appears to be a general route for the synthesis of NCFs that promises both high selectivity of NCFs and high reproducibility of the fullerene yield. Another advantages of the SOS route compared to the reported “trimetallic nitride template” (TNT) process and the reactive gas atmosphere route is that no additional heating pretreatment is needed, thus simplifying the procedure and being much more facile. Shangfeng Yang, Lin Zhang, Wenfeng Zhang, Lothar Dunsch, Chem. Eur. J. 16 (2010) 16:41 (2010) 12398-12405 URL

High-Performance Field Effect Transistors from Solution Processed Carbon Nanotubes

Nanoelectronic field effect transistors (FETs) are produced using solution processed individual carbon nanotubes (CNTs), synthesized by both arc discharge and laser ablation methods. We show that the performance of solution processed FETs approaches that of CVD-grown FETs if the nanotubes have minimal lattice defects and are free from surface contamination. This is achieved by treating the nanotubes to a high-temperature vacuum annealing process and using 1,2-dichloroethane for dispersion. We present CNT FETs with mobilities of up to 3546 cm2/(V s), transconductance of 4.22 µS, on-state conductance of 9.35 µS and on/off ratios as high as 106. High-resolution transmission electron microscopy is used to examine the presence of catalyst particles and amorphous carbon on the surface and Raman spectroscopy is used to examine the lattice defects, both of which lead to reduced device performance. H. Wang, J. Luo, A. Robertson, Y. Ito, W. Yan, V. Lang, M. Zaka, F. Schaeffel, M. H. Ruemmeli, G. A. D. Briggs, J. H. Warner, ACS nano vol. 4, no. 11, 6659–6664 (2010) URL

Full spin switch effect for the superconducting current in a superconductor/ferromagnet thin film heterostructure

Using the spin switch design F1/F2/S theoretically proposed by Oh et al., (Appl. Phys. Lett. 71, 2376 (1997)), that comprises a ferromagnetic bilayer as a ferromagnetic component, and an ordinary superconductor as the second interface component, we have realized a full spin switch effect for the superconducting current. An experimental realization of this spin switch construction was achieved for the CoOx /Fe1/Cu/Fe2/ In multilayer. P. V. Leksin, N. N. Garif’yanov, I. A. Garifullin, J. Schumann, H. Vinzelberg, V. Kataev, R. Klingeler, O. G. Schmidt, B. Büchner, Appl. Phys. Lett. 97, 102505 (2010); doi:10.1063/1.3486687 URL
  

Tunable Band Gap in Hydrogenated Quasi-Free-Standing Graphene

We show by angle-resolved photoemission spectroscopy that a tunable gap in quasi-free-standing monolayer graphene on Au can be induced by hydrogenation. The size of the gap can be controlled via hydrogen loading and reaches 1.0 eV for a hydrogen coverage of 8%. The local rehybridization from sp2 to sp3 in the chemical bonding is observed by X-ray photoelectron spectroscopy and X-ray absorption and allows for a determination of the amount of chemisorbed hydrogen. The hydrogen induced gap formation is completely reversible by annealing without damaging the graphene. Calculations of the hydrogen loading dependent core level binding energies and the spectral function of graphene are in excellent agreement with photoemission experiments. Hydrogenation of graphene gives access to tunable electronic and optical properties and thereby provides a model system to study hydrogen storage in carbon materials. D. Haberer, D. V. Vyalikh, S. Taioli, B. Dora, M. Farjam, J. Fink, D. Marchenko, T. Pichler, K. Ziegler, S. Simonucci, M. S. Dresselhaus, M. Knupfer, B. Büchner and A. Grüneis, Nano Lett., DOI: 10.1021/nl101066m URL

Unraveling the electron spin resonance pattern of nonsymmetric radicals with 30 fluorine atoms: electron spin resonance and vis-near-infrared spectroelectrochemistry of the anion radicals and dianions of C60(CF3)(2n) (2n = 2-10) derivatives and density functional theory-assisted assignment.

The charged states of C(60)(CF(3))(2n) (2n = 2-10) derivatives have been studied by electron spin resonance (ESR) and vis-near-infrared (NIR) spectroelectrochemistry. The anion radicals and diamagnetic dianions were furthermore described by theoretical calculations. The ESR spectra of anion radicals exhibit complex patterns due to multiple CF(3) groups. Their interpretation is accomplished by DFT calculations with B3LYP functional. It is shown that calculations provide reliable results when the extended aug-cc-pCVTZ basis set is used for fluorine atoms; however, specially tailored basis sets such as EPR-III also give very similar results with only a fraction of the computational cost. Absorption spectra of the anions exhibit NIR absorption bands, whose assignment is provided by time-dependent DFT calculations. A.A. Popov, I.E. Kareev, N.B. Shustova, S.H. Strauss, O.V. Boltalina, L. Dunsch, Journal of the American Chemical Society (2010) Volume: 132  Issue: 33  Pages: 11709-11721 URL

Al₃Li₄(BH₄)(₁₃): A Complex Double-Cation Borohydride with a New Structure

The new double-cation Al-Li-borohydride is an attractive candidate material for hydrogen storage due to a very low hydrogen desorption temperature (approximately 70 degrees C) combined with a high hydrogen density (17.2 wt%). It was synthesised by high-energy ball milling of AlCl(3) and LiBH(4). The structure of the compound was determined from image-plate synchrotron powder diffraction supported by DFT calculations. The material shows a unique 3D framework structure within the borohydrides (space group=P-43n, a=11.3640(3) A). The unexpected composition Al(3)Li(4)(BH(4))(13) can be rationalized on the basis of a complex cation [(BH(4))Li(4)](3+) and a complex anion [Al(BH(4))(4)](-). The refinements from synchrotron powder diffraction of different samples revealed the presence of limited amounts of chloride ions replacing the borohydride on one site. In situ Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG) and thermal desorption measurements were used to study the decomposition pathway of the compound. Al-Li-borohydride decomposes at approximately 70 degrees C, forming LiBH(4). The high mass loss of about 20 % during the decomposition indicates the release of not only hydrogen but also diborane. I. Lindemann, R. Domènech Ferrer, L. Dunsch, Y. Filinchuk, R. Cerný, H. Hagemann, V. D'Anna, L.M. Lawson;Daku, L. Schultz, O. Gutfleisch, Chemistry-A European Journal (2010) Volume: 16 Issue: 29 Pages: 8707-8712 URL

Spin-Flow Vibrational Spectroscopy of Molecules with Flexible Spin Density: Electrochemistry, ESR, Cluster and Spin Dynamics, and Bonding in TiSc₂N@C₈₀

The recently isolated TiSc₂N@C₈₀ was used to study the spin state of a Ti³⁺ ion in a mixed metal nitride cluster in a fullerene cage. The electronic state of the new clusterfullerene is characterized starting with the redox behavior of this structure. It differs markedly from that of homometallic nitride clusterfullerenes in giving reversible one-electron transfers even on the cathodic scale. Both oxidation and reduction of TiSc₂N@C₈₀ occur at the endohedral cluster changing the valence state of Ti from Ti(II) in anion to Ti(IV) in cation. The unpaired electron in TiSc₂N@C₈₀ is largely fixed at the Ti ion as shown by low temperature ESR measurements. Isotropic g-factor 1.9454 points to the significant spin-orbit coupling with an unquenched orbital momentum of the 3d electron localized on Ti. Measurements with the frozen solution also point to the strong anisotropy of the g-tensor. DFT computations show that the cluster can adopt several nearly isoenergetic configurations. DFT-based Born-Oppenheimer molecular dynamics (BOMD) simulations reveal that, unlike in Sc₃N@C₈₀, the cluster dynamics in TiSc₂N@C₈₀ cannot be described as a 3D rotation. The cluster rotates around the Ti-N axis, while the Ti atom oscillates in one position around the pentagon/hexagon edge. Evolution of the spin populations along the BOMD trajectory has shown that the spin distribution in the cluster is very flexible, and both an intracluster and clustercage spin flows take place. Fourier transformation of the time dependencies of the spin populations results in the spin-flow vibrational spectra, which reveal the major spin-flow channels. It is shown that the cluster-cage spin flow is selectively coupled to one vibrational mode, thus, pointing to the utility of the clusterfullerene for the molecular spin transport. Spin-flow vibrational pectroscopy is thus shown to be a useful method for characterization of the spin dynamics in radicals with flexible spin density distribution. A. A. Popov, Ch. Chen, S. Yang, F. Lipps, L. Dunsch, ACS Nano (2010) Vol. 4, No 8, 4857–4871 URL

Influence of the Cage Size on the Dynamic Behavior of Fullerenes: A Study of 13C NMR Spin-Lattice Relaxation

A detailed study on the relaxation mechanisms of higher cage fullerene sizes is done as a prerequisite for studies of the influence of the endohedral structures on fullerene cage carbon relaxation. Recent studies of the dynamic behavior of C60 and C70 in aromatic solvents and CS2 solution show the influence of the shape and the symmetry of the cage to be highly important as well as the influence of the solvent to be negligible. As higher fullerene cages have more than one stable isomer, the isolation of isomeric pure structures is of high importance for a detailed study of the dynamic behavior of such fullerenes. Here we investigated the three higher fullerene cage isomers D2-C76, C2v(3)-C78, and D2-C80 with respect to the relaxation rate of the carbons measured in their temperature dependence. Thus, we study the influence on the relaxation of the carbons and the dynamic behavior of these fullerenes in solution. Besides the diffusion dependence on the shape of the carbon cage, the relaxation behavior at lower temperatures is found to be dependent on the difference in chemical shift anisotropy within the carbon cage. This difference is originated from the changes of symmetry and results in polarization of electron density. Furthermore, the mobility of the carbons is influenced by their pyramidalization. S. Klod, L. Dunsch, ACS Nano vol 4, no. 6 URL

Redox-Tuning Endohedral Fullerene Spin States: From the Dication to the Trianion Radical of Sc₃N@C80(CF₃)₂ in Five Reversible Single-Electron Steps

The first endohedral trianion captured: Sc₃N@C80(CF₃)₂ (see figure) exhibits three reversible reductions and two reversible oxidations and affords the facile generation of the monocation, monoanion, and trianion in solution, which can be characterized by ESR and absorption spectroscopies. This is the first time that such a broad range of charged states of any endohedral fullerene has been spectroscopically characterized. A. A. Popov, N. B. Shustova, A. L. Svitova, M. A. Mackey, C. E. Coumbe, J. P. Phillips, S. Stevenson, S. H. Strauss, O. V. Boltalina, L. Dunsch Chemistry – A European Journal (2010) Vol 16, Issue 16, pages 4721–4724 URL

Saturnene Revealed: X-ray Crystal Structure of D_5d-C₆₀F₂₀ Formed in Reactions of C₆₀ with A_xMF_y Fluorinating Agents (A=Alkali Metal; M=3d Metal)

Saturnene has four moons: Reactions of C₆₀ with ternary metal fluorides yielded fluorofullerenes from C₆₀F₂ to C₆₀F₄₈, including elusive saturnene, C₆₀F₂₀, which has now been characterized by X-ray crystallography. Four benzene molecules “hover” over this D_5d molecule at the corners of a square inscribed in the idealized body-centered-cubic unit cell (see structure; F yellow). The tight unit-cell packing explains the very low solubility of saturnene. N. B. Shustova, Z. Mazej, Y.-S. Chen, A. A. Popov, S. H. Strauss, O. V. Boltalina, Angew. Chem. Int. Ed. 2010, 49, 812 –815 URL

Direct Low-Temperature Nanographene CVD Synthesis over a Dielectric Insulator

Graphene ranks highly as a possible material for future high-speed and flexible electronics. Current fabrication routes, which rely on metal substrates, require post-synthesis transfer of the graphene onto a Si wafer, or in the case of epitaxial growth on SiC, temperatures above 1000 °C are required. Both the handling difficulty and high temperatures are not best suited to present day silicon technology. We report a facile chemical vapor deposition approach in which nanographene and few-layer nanographene are directly formed over magnesium oxide and can be achieved at temperatures as low as 325 °C. M. H. Rümmeli, A. Bachmatiuk, A. Scott, F. Börrnert, J. H. Warner, V. Hoffman, J.-H. Lin, G. Cuniberti, B. Büchner, ACS Nano, 2010, 4 (7), pp 4206–4210 URL

An endohedral redox system in a fullerene cage: the Ce based mixed-metal cluster fullerene Lu₂CeN@C₈₀

Redox reactions of endohedral fullerenes, and especially their oxidation, usually result in a change of the redox state of the carbon cage. Here we demonstrate that an oxidation of the endohedral species is possible bypassing the fullerene cage in an unprecedented reversible cascade electron transfer under anodic conditions. The first Ce-based non-scandium mixed-metal nitride clusterfullerene (NCF) Lu₂CeN@C₈₀(Ih) was synthesized and isolated. The electronic and vibrational properties of Lu₂CeN@C₈₀ are characterized by UV-vis-NIR and FTIR spectroscopies and the cage structure of Lu₂CeN@C₈₀ is determined by ¹³C NMR spectroscopy to be C₈₀(/_h). At room temperature the NMR peak positions are shifted from the normal values of the diamagnetic M₃N@C₈₀ NCFs because of the unpaired f-electron localized on the Ce atom in the Ce³⁺ state. The variable-temperature NMR study enabled the estimation of the diamagnetic terms in the ¹³C chemical shifts, which were found to be close to those of diamagnetic M₃N@C₈₀ NCFs. The electrochemical properties of Lu₂CeN@C₈₀ were investigated by cyclic and square wave voltammetry, revealing two electrochemically irreversible but chemically reversible reduction steps and one reversible oxidation step. As the oxidation potential is significantly less positive than that in all other M₃N@C₈₀ NCFs, we conclude that an oxidation of the endohedral Ce occurs with the formation of Lu₂CeIVN@C₈₀+, the first endohedral metallofullerene species with the tetra-valent cerium atom. This hypothesis is also supported by DFT calculations. Lin Zhang, Alexey A. Popov, Shangfeng Yang, Sabrina Klod, Peter Rapta and Lothar Dunsch, Phys. Chem. Chem. Phys., 2010, 12, 7840 - 7847, DOI: 10.1039/c002918a URL

Metal Sulfide in a C₈₂ Fullerene Cage: A New Form of Endohedral Clusterfullerenes

The row of endohedral fullerenes is extended by a new type of sulfur-containing clusterfullerenes: the metal sulfide (M₂S) has been stabilized within a fullerene cage for the first time. The new sulfur-containing clusterfullerenes M₂S@C₈₂-C₃v(8) have been isolated for a variety of metals (M = Sc, Y, Dy, and Lu). The UV−vis−NIR, electrochemical, and FTIR spectroscopic characterization and extended DFT calculations point to a close similarity of the M2S@C82 cage isomeric and electronic structure to that of the carbide clusterfullerenes M₂C₂@C_2n. The bonding in M₂S@C₈₂ is studied in detail by molecular orbital analysis as well as with the use of quantum theory of atom-in-molecules (QTAIM) and electron localization function (ELF) approaches. The metal sulfide cluster formally transfers four electrons to the carbon cage, and metal−sulfur and metal−carbon cage bonds with a high degree of covalency are formed. Molecular dynamics simulations show that Sc₂S cluster exhibits an almost free rotation around the C₃ axis of the carbon cage, resulting thus in a single line ⁴⁵Sc NMR spectrum. L. Dunsch, S. Yang, L. Zhang, A. Svitova, S. Oswald, A. A. Popov, J. Am. Chem. Soc., 2010, 132 (15), pp 5413–5421, URL

Direct Arc-Discharge Assisted Synthesis of C₆₀H₂(C₃H₅N): A cis-1-Pyrrolino C₆₀ Fullerene Hydride with Unusual Redox Properties

By direct arc synthesis of C₆₀H₂(C₃H₅N) using a modified Krtschmer−Huffman method, it is demonstrated for the first time that exohedral fullerenes with large side groups can be formed under the arc and reactive gas atmosphere conditions. The thus formed novel pyrrolino fullerene hydride was comprehensively characterized by UV−vis, infrared (IR), Raman and nuclear magnetic resonance (NMR) and studied by means of electrochemistry and in situ electron spin resonance/visible-near infrared (ESR/Vis-NIR) spectroelectrochemistry. The detailed NMR and absorption spectroscopic studies show C₆₀H₂(C₃H₅N) as the fullerene hydride with a pyrrolino ring attached on the vicinal position of two hydrogen atoms. This first cycloaddition adduct of C₆₀H₂ gives rise to an unusual reversible dimerization of its anion radicals and a loss of cage hydrogen atoms at the third electron transfer. The spectroelectrochemical study confirms the formation of a [C₆₀H₂(C₃H₅N)]− radical and the diamagnetic state of [C₆₀H₂(C₃H₅N)] ²−. This study reveals a unique function of a carbide structure like Al₄C₃ and the presence of a reactive gas atmosphere with NH₃ in the formation of such an exohedral pyrrolino structure C₆₀H₂(C₃H₅N). N. Chen, S. Klod, P. Rapta, A. A. Popov, L. Dunsch, Chem. Mater., 2010, 22 (8), pp 2608–2615, URL

Surface acoustic wave mediated dielectrophoretic alignment of rolled-up microtubes in microfluidic systems

The alignment behavior of solution dispersed rolled-up microtubes by surface acoustic waves (SAW) is demonstrated. In contrast to the random alignment of rolled-up insulated silicon oxide tubes, metallic chromium tubes can be effectively aligned and assembled into “tube-chains” parallel to the SAW propagation direction. The experiments suggest that the tube orientation is mainly determined by the dielectrophoresis (DEP) forces acting on the tubes. The DEP forces arise from the induced dipole moment of the tubes in the SAW generated piezoelectric field on the LiNbO3 substrate. X. H. Kong, Ch. Deneke, H. Schmidt, D. J. Thurmer, H. X. Ji, M. Bauer, O. G. Schmid, Applied Physics Letters 96, 134105 (2010) URL

Confinement of Fractional Quantum Number Particles in a Condensed Matter System

The idea of confinement states that in certain systems constituent particles can be discerned only indirectly being bound by an interaction whose strength increases with increasing particle separation. Though the most famous example is the confinement of quarks to form baryons and mesons in (3+1)-dimensional Quantum Chromodynamics, confinement can also be realized in the systems of condensed matter physics such as spin-ladders which consist of two spin-1/2 antiferromagnetic chains coupled together by spin exchange interactions. Excitations of individual chains (spinons) carrying spin S=1/2, are confined even by an infinitesimal interchain coupling. The realizations studied so far cannot illustrate this process due to the large strength of their interchain coupling which leaves no energy window for the spinon excitations of individual chains. Here we present neutron scattering experiments for a weakly-coupled ladder material. At high energies the spectral function approaches that of individual chains; at low energies it is dominated by spin 0,1 excitations of strongly-coupled chains. B. Lake, A. M. Tsvelik, S. Notbohm, D. A. Tennant, T. G. Perring,.M. Reehuis, Ch. Sekar, G. Krabbes, B. Büchner Nature Physics, Nature Physics 6, 50 - 55 (2010) URL

Nanoscale Electronic Order in Iron Pnictides

The charge distribution in RFeAsO_1-xF_x (R ¼ La;Sm) iron pnictides is probed using As nuclear quadrupole resonance. Whereas undoped and optimally doped or overdoped compounds feature a single charge environment, two charge environments are detected in the underdoped region. Spin-lattice relaxation measurements show their coexistence at the nanoscale. Together with the quantitative variations of the spectra with doping, they point to a local electronic order in the iron layers, where low- and highdoping- like regions would coexist. Implications for the interplay of static magnetism and superconductivity are discussed. G. Lang, H.-J. Grafe, D. Paar, F. Hammerath, K. Manthey, G. Behr, J. Werner, B. Büchner, Phys Rev Lett 104, 097001 (2010) URL

Self-assembly of neutral hexanuclear circular copper(II) meso-helicates: topological control by sulfate ions

Bis-pyridylimine ligands with different linking elements are capable of forming unique hexanuclear circular Cu(II) mesohelicates; the self-assembly is controlled by coordination of sulfate ions to the metal centres. H. B. Tanh Jeazet, Kerstin Gloe, Th. Doert, O. N. Kataeva, A. Jäger, G. Geipel, G. Bernhard, B. Büchner, Karsten Gloe, Chem. Commun. (2010) 46, 2373–237; URL

Examining Co-based nanocrystals on graphene using low-voltage aberration-corrected transmission electron microscopy

We present a method to produce graphene and few layer graphene sheets using solution phase chemistry, which are used as ultrathin support membranes for enhanced imaging of nanomaterials using transmission electron microscopy. We demonstrate this by decorating the surface of the graphene sheets with Co-based nanocrystals (CoCl₂ and hcp Co). Low-voltage aberration-corrected high resolution transmission electron microscopy at 80 kV is used to image the nanocrystals on the thin graphene supports. We show that electron beam irradiation causes the CoCl₂ nanocrystals to become mobile on the graphene surface and exhibit both rotational and translational motion. We provide real-time in situ monitoring with atomic resolution of the coalescence of two CoCl₂ nanocrystals on the graphene surface, driven by electron beam irradiation. The CoCl₂ nanocrystals are then annealed in vacuum and transform into Co nanocrystals with hcp crystal structure. We show that these Co nanocrystals are catalytic and electron beam irradiation leads to the etching of the graphene surface, not observed for the CoCl₂ nanocrystals.
J. Warner, M. Ruemmeli, A. Bachmatiuk, M. Wilson, B. Buechner, ACS Nano, (2010) 4 (1) pp 470-47, URL

Atomic Resolution Imaging and Topography of Boron Nitride Sheets Produced by Chemical Exfoliation

Here, we present a simple method for preparing thin few-layer sheets of hexagonal BN with micrometer-sized dimensions using chemical exfoliation in the solvent 1,2-dichloroethane. The atomic structure of both few-layer and monolayer BN sheets is directly imaged using aberration-corrected high-resolution transmission electron microscopy. Electron beam induced sputtering effects are examined in real time. The removal of layers of BN by electron beam irradiation leads to the exposure of a step edge between a monolayer and bilayer region. We use HRTEM imaging combined with image simulations to show that BN bilayers can have AB stacking and are not limited to just AA stacking. Jamie H. Warner, Mark H. Rümmeli, Alicja Bachmatiuk, Bernd Büchner, ACS Nano 2010 4(3), pp 1299–1304 URL

Graphene Synthesis on Cubic SiC/Si Wafers. Perspectives for Mass Production of Graphene-Based Electronic Devices

The outstanding properties of graphene, a single graphite layer, render it a top candidate for substituting silicon in future electronic devices. The so far exploited synthesis approaches, however, require conditions typically achieved in specialized laboratories and result in graphene sheets whose electronic properties are often altered by interactions with substrate materials. The development of graphene-based technologies requires an economical fabrication method compatible with mass production. Here we demonstrate for the fist time the feasibility of graphene synthesis on commercially available cubic SiC/Si substrates of >300 mm in diameter, which result in graphene flakes electronically decoupled from the substrate. After optimization of the preparation procedure, the proposed synthesis method can represent a further big step toward graphene-based electronic technologies. V. Yu. Aristov, G. Urbanik, K. Kummer, D. V. Vyalikh, T. Hänke, B. Büchner, I. Vobornik, J. Fujii, G. Panaccione, Y. A. Ossipyan, M. Knupfer, Nano Letters 10, 992–995 (2010) URL

Investigating the outskirts of Fe and Co catalyst particles in alumina-supported catalytic CVD carbon nanotube growth

Using thermal CVD, the synthesis of multi-walled carbon nanotubes exhibiting roots anchored directly onto alpha-alumina supports, rather than the catalyst particle, is reported. At such roots, the alignment of the graphitic planes with the support lattice fringes depends on the support crystal structure and orientation. Surface defects may alter the reactivity of the surface or control the anchoring of supported atoms or nanoparticles. We argue this surface defect is provided by the catalyst particle’s edge interaction with the support, in other words its circumference. The development of oxide-based catalysts is attractive in that they potentially provide an appropriate solution to directly integrate the synthesis of carbon nanotubes and graphene into silicon-based technology. M.H. Ruemmeli, F. Schaeffel, A. Bachmatiuk, D. Adebimpe, G. Trotter, F. Boerrnert, A. Scott, E. Coric, M. Sparing, B. Rellinghaus, P.G. McCormick, G. Cuniberti, M. Knupfer, L. Schultz, B. Buechner, ACS nano 4 (2010) Nr. 2, S. 1146-1152 URL

Addition of Carbene to the Equator of C70 To Produce the Most Stable C71H2 Isomer: 2aH-2(12)a-Homo(C70-D5h(6))[5,6]fullerene

Unlike C60, in which all carbon-atom environments are identical, C70 has five distinct carbon-atom environments, which give rise to eight distinct C-C bond types. Hence, the addition chemistry of C70 involves both chemo- and regioselectivity. The synthetic chemistry of C70 is centered on the areas near the poles,[1,2] as these areas have the highest curvature and hence high bond strain.[3, 4] This relatively high bond strain in turn makes the polar regions the most reactive sites of the molecule. The equatorial region of C70, on the other hand, has little curvature and hence lower bond strain. Thus, the carbon atoms at the equator are much less reactive, as there is a much higher activation barrier to be overcome before reactions can occur. For example, carbene (CH2) has been added to the polar region of C70, and several isomers of C71H2 have been synthesized and fully characterized.[5–8] However, the addition of carbene to the equatorial bond of C70 (to form C2v-C71H2) has not been detected. B. Li, C. Shu, X. Lu, L. Dunsch, Z. Chen, T.J.S. Dennis, Z. Shi, L. Jiang, T. Wang, W. Xu, C. Wang, Angew. Chem. Int. Ed. 2010, 49, 962 –966, URL

A Pseudoatom in a Cage: Trimetallofullerene Y3@C80 Mimics Y3N@C80 with Nitrogen Substituted by a Pseudoatom

Y3C80 obtained in the synthesis of nitride clusterfullerenes Y3N@C2n (2n=80-88) by the reactive atmosphere method is found to be a genuine trimetallofullerene, Y3@C80, with low ionization potential and divalent state of yttrium atoms. DFT studies of the electronic structure of Y3@C80 show that this molecule mimics Y3N@C80 with the pseudoatom (PA) instead of the nitrogen atom. Topology analysis of the electron density and electron localization function show that yttrium atoms form Y-PA bonds rather than direct Y-Y bonds. Molecular dynamics simulations show that the Y3PA cluster is as rigid as Y3N and rotates inside the fullerene cage as a single entity. A. Popov, L. Zhang, L. Dunsch, ACS Nano Vol. 4, No. 2 (2010) 795–802 URL