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  • 1. Abou-Hamad, E.
    et al.
    Babaa, M. -R
    Bouhrara, M.
    Kim, Y.
    Saih, Y.
    Dennler, S.
    Mauri, F.
    Basset, J. -M
    Goze-Bac, C.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Structural properties of carbon nanotubes derived from (13)C NMR2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 16, 165417- p.Article in journal (Refereed)
    Abstract [en]

    We present a detailed experimental and theoretical study on how structural properties of carbon nanotubes can be derived from 13C NMR investigations. Magic angle spinning solid state NMR experiments have been performed on single-and multiwalled carbon nanotubes with diameters in the range from 0.7 to 100 nm and with number of walls from 1 to 90. We provide models on how diameter and the number of nanotube walls influence NMR linewidth and line position. Both models are supported by theoretical calculations. Increasing the diameter D, from the smallest investigated nanotube, which in our study corresponds to the inner nanotube of a double-walled tube to the largest studied diameter, corresponding to large multiwalled nanotubes, leads to a 23.5 ppm diamagnetic shift of the isotropic NMR line position d. We show that the isotropic line follows the relation d = 18.3/D + 102.5 ppm, where D is the diameter of the tube and NMR line position d is relative to tetramethylsilane. The relation asymptotically tends to approach the line position expected in graphene. A characteristic broadening of the line shape is observed with the increasing number of walls. This feature can be rationalized by an isotropic shift distribution originating from different diamagnetic shielding of the encapsulated nanotubes together with a heterogeneity of the samples. Based on our results, NMR is shown to be a nondestructive spectroscopic method that can be used as a complementary method to, for example, transmission electron microscopy to obtain structural information for carbon nanotubes, especially bulk samples.

  • 2. Abou-Hamad, E.
    et al.
    Kim, Y.
    Bouhrara, M.
    Saih, Y.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Luzzi, D. E.
    Goze-Bac, C.
    NMR strategies to study the local magnetic properties of carbon nanotubes2012In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 407, no 4, 740-742 p.Article in journal (Refereed)
    Abstract [en]

    The local magnetic properties of the one dimensional inner space of the nanotubes are investigated using C-13 nuclear magnetic resonance spectroscopy of encapsulated fullerene molecules inside single walled carbon nanotubes. Isotope engineering and magnetically purified nanotubes have been advantageously used on our study to discriminate between the different diamagnetic and paramagnetic shifts of the resonances. Ring currents originating from the pi electrons circulating on the nanotube, are found to actively screen the applied magnetic field by -36.9 ppm. Defects and holes in the nanotube walls cancel this screening locally. What is interesting, that at high magnetic fields, the modifications of the NMR resonances of the molecules from free to encapsulated can be exploited to determine some structural characteristics of the surrounding nanotubes, never observed experimentally. (C) 2011 Elsevier B.V. All rights reserved.

  • 3.
    Abou-Hamad, Edy
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Goze-Bac, Christophe
    Université Montpellier II, France.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Schmid, Michael
    Physikalisches Institut, Universität Stuttgart, Germany.
    Aznar, Robert
    Université Montpellier II, France.
    Mehring, Michael
    Physikalisches Institut, Universität Stuttgart, Germany.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Electronic properties of Cs-intercalated single-walled carbon nanotubes derived from nuclear magnetic resonance2011In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 13, 053045 (1)-(9) p.Article in journal (Refereed)
    Abstract [en]

    We report on the electronic properties of Cs-intercalated singlewalled carbon nanotubes (SWNTs). A detailed analysis of the 13C and133Cs nuclear magnetic resonance (NMR) spectra reveals an increased metallization of the pristine SWNTs under Cs intercalation. The ‘metallization’ of CsxC materials where x =0–0.144 is evidenced from the increased local electronic density of states (DOS) n(EF)at the Fermi level of the SWNTs as determined from spin–lattice relaxation measurements. In particular, there are two distinct electronic phases called α and β and the transition between these occurs around x = 0.05. The electronic DOS at the Fermi level increases monotonically at low intercalation levels x <0.05 (α-phase), whereas it reaches a plateau in the range 0.05 < x < 0.143 at high intercalation levels (β-phase). The new β-phase is accompanied by a hybridization of Cs(6s) orbitals with C(sp2)orbitals of the SWNTs. In both phases, two types of metallic nanotubes are found with a low and a high local n(EF), corresponding to different local electronic band structures of the SWNTs.

  • 4.
    Abou-Hamad, Edy
    et al.
    Universite Montpellier II.
    Kim, Y
    University of Pennsylvania.
    Talyzin, Alexandr
    Umeå University, Faculty of Science and Technology, Physics.
    Goze-Bac, Christophe
    Universite Montpellier II.
    Luzzi, David
    University of Pennsylvania.
    Rubio, Angelo
    University of Basque Country.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Hydrogenation of C-60 in Peapods: Physical Chemistry in Nano Vessels2009In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, no 20, 8583-8587 p.Article in journal (Refereed)
    Abstract [en]

    Hydrogenation of C-60 molecules inside SWNT was achieved by direct reaction with hydrogen gas at elevated pressure and temperature. Evidence for the C-60 hydrogenation in peapods is provided by isotopic engineering with specific enrichment of encapsulated species and high resolution C-13 and H-1 NMR spectroscopy with the observation of characteristic diamagnetic and paramagnetic shifts of the NMR lines and the appearance of sp(3) carbon resonances. We estimate that approximately 78% of the C-60 molecules inside SWNTs are hydrogenated to an average degree of 14 hydrogen atoms per C-60 molecule. As a consequence, the rotational dynamics of the encapsulated C60Hx molecules is clearly hindered. Our successful hydrogenation experiments open completely new roads to understand and control confined chemical reactions at the nano scale

  • 5.
    Abou-Hamad, Edy
    et al.
    Universite Montpellier 2, France.
    Kim, Y.
    University of Pennsylvania, Philadelphia.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boesch, D.
    University of California at Berkeley, and Materials Sciences Division, Lawrence Berkeley National Laboratory.
    Aloni, S.
    University of California at Berkeley, and Materials Sciences Division, Lawrence Berkeley National Laboratory.
    Zettl, Alex
    University of California at Berkeley, and Materials Sciences Division, Lawrence Berkeley National Laboratory.
    Rubio, Angelo
    Universidad del Pas Vasco UPV/EHU.
    Luzzi, David E.
    University of Pennsylvania, Philadelphia.
    Goze-Bac, Christophe
    CNRS Universit Montpellier 2.
    Molecular dynamics and phase transition in one-dimensional crystal of C60 encapsulated inside single wall carbon nanotubes2009In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 3, no 12, 3878-3883 p.Article in journal (Refereed)
    Abstract [en]

    One-dimensional crystals of 25% 13C-enriched C60 encapsulated inside highly magnetically purified SWNTs were investigated by following the temperature dependence of the 13C NMR line shapes and the relaxation rates from 300 K down to 5 K. High-resolution MAS techniques reveal that 32% of the encapsulated molecules, so-called the C60α, are blocked at room temperature and 68%, labeled C60β, are shown to reversly undergo molecular reorientational dynamics. Contrary to previous NMR studies, spin−lattice relaxation time reveals a phase transition at 100 K associated with the changes in the nature of the C60β dynamics. Above the transition, the C60β exhibits continuous rotational diffusion; below the transition, C60β executes uniaxial hindered rotations most likely along the nanotubes axis and freeze out below 25 K. The associated activation energies of these two dynamical regimes are measured to be 6 times lower than in fcc-C60, suggesting a quiet smooth orientational dependence of the interaction between C60β molecules and the inner surface of the nanotubes.

  • 6.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    In situ, high pressure differential thermal analysis and ionic conductance of PMMA-based gels with and without TiO2 nano-particle filler2006In: Electrochimica Acta, Vol. 51, 4537- p.Article in journal (Refereed)
  • 7.
    Barzega, Hamid Reza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Univ Calif Berkeley, Dept Phys, Berkeley, CA 94720 USA.
    Gracia Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    C60/Collapsed Carbon Nanotube Hybrids: A Variant of Peapods2015In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 15, no 2, 829-834 p.Article in journal (Refereed)
    Abstract [en]

    We examine a variant of so-called carbon nanotube peapods by packing C60 molecules inside the open edge ducts of collapsed carbon nanotubes. C60 insertion is accomplished through a facile single-step solution-based process. Theoretical modeling is used to evaluate favorable low-energy structural configurations. Overfilling of the collapsed tubes allows infiltration of C60 over the full cross-section of the tubes and consequent partial or complete reinflation, yielding few-wall, large diameter cylindrical nanotubes packed with crystalline C60 solid cores.

  • 8.
    Barzegar, Hamid R.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ramstedt, Madeleine
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Tai, Cheuk W.
    Malolepszy, Artur
    Stobinski, Leszek
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Simple Dip-Coating Process for the Synthesis of Small Diameter Single-Walled Carbon Nanotubes-Effect of Catalyst Composition and Catalyst Particle Size on Chirality and Diameter2012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 22, 12232-12239 p.Article in journal (Refereed)
    Abstract [en]

    We report on a dip-coating method to prepare catalyst particles (mixture of iron and cobalt) with a controlled diameter distribution on silicon wafer substrates by changing the solution's concentration and withdrawal velocity. The size and distribution of the prepared catalyst particles were analyzed by atomic force microscopy. Carbon nanotubes were grown by chemical vapor deposition on the substrates with the prepared catalyst particles. By decreasing the catalyst particle size to below 10 nm, the growth of carbon nanotubes can be tuned from few-walled carbon nanotubes, with homogeneous diameter, to highly pure single-walled carbon nanotubes. Analysis of the Raman radial breathing modes, using three different Raman excitation wavelengths (488, 633, and 785 nm), showed a relatively broad diameter distribution (0.8-1.4 nm) of single-walled carbon nanotubes with different chiralities. However, by changing the composition of the catalyst particles while maintaining the growth parameters, the chiralities of single-walled carbon nanotubes were reduced to mainly four different types, (12, 1), (12, 0), (8, 5), and (7, 5), accounting for about 70% of all nanotubes.

  • 9.
    Barzegar, Hamid Reza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitrogen Doping Mechanism in Small Diameter Single-Walled Carbon Nanotubes: Impact on Electronic Properties and Growth Selectivity2013In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 48, 25805-25816 p.Article in journal (Refereed)
    Abstract [en]

    Nitrogen doping in carbon nanostructures has attracted interest for more than a decade, and recent implementation of such structures in energy conversion systems has boosted the interest even more. Despite numerous studies, the structural conformation and stability of nitrogen functionalities in small diameter single-walled carbon nanotubes (SWNTs), and the impact of these functionalities on the electronic and mechanical properties of the SWNTs, are incomplete. Here we report a detailed study on nitrogen doping in SWNTs with diameters in the range of 0.8?1.0 nm, with well-defined chirality. We show that the introduction of nitrogen in the carbon framework significantly alters the stability of certain tubes, opening for the possibility to selectively grow nitrogen-doped SWNTs with certain chirality and diameter. At low nitrogen concentration, pyridinic functionalities are readily incorporated and the tubular structure is well pertained. At higher concentrations, pyrrolic functionalities are formed, which leads to significant structural deformation of the nanotubes and hence a stop in growth of crystalline SWNTs. Raman spectroscopy is an important tool to understand guest atom doping and electronic charge transfer in SWNTs. By correlating the influence of defined nitrogen functionalities on the electronic properties of SWNTs with different chirality, we make precise interpretation of experimental Raman data. We show that the previous interpretation of the double-resonance G?-peak in many aspects is wrong and instead can be well-correlated to the type of nitrogen doping of SWNTs originating from the p- or n-doping nature of the nitrogen incorporation. Our results are supported by experimental and theoretical data.

  • 10.
    Barzegar, Hamid Reza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsen, Christian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Jia, Xueen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Palladium nanocrystals supported on photo-transformed C-60 nanorods: effect of crystal morphology and electron mobility on the electrocatalytic activity towards ethanol oxidation2014In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 73, 34-40 p.Article in journal (Refereed)
    Abstract [en]

    We report on the synthesis and decoration of high-aspect-ratio crystalline C-60 nanorods (NRs) by functionalized palladium nanoparticles with an average size of 4.78 +/- 0.66 nm. In their pristine form, C-60 NRs suffer from partial damage in the solution-based decoration process resulting in poor crystallinity. However, by modifying the NR surface via in situ photochemical transformation in the liquid state, we are able to prepare highly stable NRs that retain their crystalline structure during the decoration process. Our method thus opens up for the synthesis of highly crystalline nanocomposite hybrids comprising Pd nanoparticles and C-60 NRs. Bys measuring the electron mobility of different C-60 NRs, we relate both the effect of electron mobility and crystallinity to the final electrocatalytic performance of the synthesized hybrid structures. We show that the photo-transformed C-60 NRs exhibit highly advantageous properties for ethanol oxidation based on both a better crystallinity and a higher bulk conductivity. These findings give important information in the search for efficient catalyst support.

  • 11.
    Barzegar, Hamid Reza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsen, Christian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Solution-Based Phototransformation of C-60 Nanorods: Towards Improved Electronic Devices2013In: Particle & particle systems characterization, ISSN 0934-0866, E-ISSN 1521-4117, Vol. 30, no 8, 715-720 p.Article in journal (Refereed)
    Abstract [en]

    A modified liquid-liquid interface precipitation synthesis of C-60 nanorods, effects and opportunities following an in situ photochemical transformation in the liquid state, and an electronic characterization using a field-effect transistor (FET) geometry are reported. The nanorods feature a high aspect ratio of approximate to 10(3) and a notably small average diameter of 172 nm. Interestingly, it is found that a decreased nanorod diameter appears to correlate with distinctly improved electronic properties, and an average electron mobility of 0.30 cm(2) V-1 s(-1), as measured in a FET geometry, is reported for as-grown nanorods, with the peak value being an impressive 1.0 cm(2) V-1 s(-1). A photoexposure using green laser light ( = 532 nm) is demonstrated to result in the formation of a polymer-C-60 shell encapsulating a monomer-C-60 bulk; such photo-transformed nanorods exhibit an electron mobility of 4.7 x 10(-3) cm(2) V-1 s(-1). It is notable that the utilized FET geometry only probes the polymer-C-60 nanorod surface shell, and that the monomer-C-60 bulk is anticipated to exhibit a higher mobility. Importantly, photoexposed nanorods can be conveniently processed as a stabile dispersion in common hydrophobic solvents, and this finding is attributed to the insoluble character of the polymer-C-60 shell.

  • 12.
    Barzegar, Hamid Reza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Malolepszy, Artur
    Stobinski, Leszek
    Tai, Cheuk-Wai
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Water assisted growth of C-60 rods and tubes by liquid-liquid interfacial precipitation method2012In: Molecules, ISSN 1420-3049, E-ISSN 1420-3049, Vol. 17, no 6, 6840-6853 p.Article in journal (Refereed)
    Abstract [en]

    C-60 nanorods with hexagonal cross sections are grown using a static liquid-liquid interfacial precipitation method in a system of C-60/m-dichlorobenzene solution and ethanol. Adding water to the ethanol phase leads instead to C-60 tubes where both length and diameter of the C-60 tubes can be controlled by the water content in the ethanol. Based on our observations we find that the diameter of the rods/tubes strongly depends on the nucleation step. We propose a liquid-liquid interface growth model of C-60 rods and tubes based on the diffusion rate of the good C-60 containing solvent into the poor solvent as well as on the size of the crystal seeds formed at the interface between the two solvents. The grown rods and tubes exhibit a hexagonal solvate crystal structure with m-dichlorobenzene solvent molecules incorporated into the crystal structure, independent of the water content. An annealing step at 200 degrees C at a pressure <1 kPa transforms the grown structures into a solvent-free face centered cubic structure. Both the hexagonal and the face centered cubic structures are very stable and neither morphology nor structure shows any signs of degradation after three months of storage.

  • 13. Bouhrara, M.
    et al.
    Abou-Hamad, E.
    Alabedi, G.
    Al-Taie, I.
    Kim, Y.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Goze-Bac, C.
    Electromagnetic Properties of Inner Double Walled Carbon Nanotubes Investigated by Nuclear Magnetic Resonance2013In: Journal of Nanomaterials, ISSN 1687-4110, E-ISSN 1687-4129, Vol. 2013, 713475- p.Article in journal (Refereed)
    Abstract [en]

    The nuclear magnetic resonance (NMR) analytical technique was used to investigate the double walled carbon nanotubes (DWNTs) electromagnetic properties of inner walls. The local magnetic and electronic properties of inner nanotubes in DWNTs were analyzed using 25% (13) C enriched C-60 by which the effect of dipolar coupling could be minimized. The diamagnetic shielding was determined due to the ring currents on outer nanotubes in DWNTs. The NMR chemical shift anisotropy (CSA) spectra and spin-lattice relaxation studies reveal the metallic properties of the inner nanotubes with a signature of the spin-gap opening below 70 K.

  • 14. Bouhrara, M.
    et al.
    Saih, Y.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Goze-Bac, C.
    Abou-Hamad, E.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    High-resolution (13)C nuclear magnetic resonance evidence of phase transition of Rb,Cs-intercalated single-walled nanotubes2011In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 110, no 5, 054306- p.Article in journal (Refereed)
    Abstract [en]

    We present 13 C high-resolution magic-angle-turning (MAT) and magic angle spinning nuclear magnetic resonance data of Cs and Rb intercalated single walled carbon nanotubes. We find two distinct phases at different intercalation levels. A simple charge transfer is applicable at low intercalation level. The new phase at high intercalation level is accompanied by a hybridization of alkali (s) orbitals with the carbon (sp2) orbitals of the single walled nanotubes, which indicate bundle surface sites is the most probable alkali site.

  • 15. Buga, Sergei G.
    et al.
    Blank, Vladimir D.
    Dubitsky, Gennadii A.
    Serebryanaya, Nadejda R.
    Fransson, Åke
    Umeå University, Faculty of Science and Technology, Applied Physics and Electronics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Synthesis of superhard 3D-polymeric C60 fullerites from rhombohedral 2D-polymer by high-pressure-high-temperature treatment.2003In: High Pressure Research vol. 23, issue 3: Proceedings of the 40th European High Pressure Research Group Meeting on Advances in High Pressure Research (EHPRG'40), London: Taylor & Francis , 2003, 259-264 p.Conference paper (Refereed)
    Abstract [en]

    Rhombohedral C60 polymer was subjected to high-pressure-high-temperature treatment at P =13 GPa, T =620-1620 K. After quenching, crystalline and disordered structures with densities in the range of 2.1-2.9 g cm-1 were obtained. The structures of the samples have been investigated by powder X-ray diffraction and Raman scattering. DSC analysis showed a transformation of the polymeric structure into monomeric on annealing in the range 400-640 K. The temperature dependence of the electrical resistance of samples with disordered structure was measured in the range 2.5-300K. For different samples, the conductivity was proportional to T1/2, T3/2, T4 and exp(-1/T1/4).

  • 16.
    Dzwilewski, Andrzej
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Matyba, Piotr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Moons, Ellen
    Karlstad universitet, Fysik.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Facile fabrication of organic CMOS circuits: understanding and optimization of the processManuscript (Other academic)
  • 17.
    Dzwilewski, Andrzej
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Physics.
    C60 Field-Effect Transistors: Effects of Polymerization on Electronic Properties and Device Performance.2007In: Extended abstracts – 9th European Conference on Molecular Electronics, 2007Conference paper (Other academic)
  • 18.
    Dzwilewski, Andrzej
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Physics.
    C60 Field-Effect Transistors: Effects of Polymerization on electronic Properties and Device Performance.2007In: Physical Review B, Vol. 75, no 7, 075203- p.Article in journal (Refereed)
    Abstract [en]

    We have investigated thin-film field-effect transistors (TFTs) with C60 as the active material, and we report the effects of photo-induced polymerization of the C60 film. We find that the effects of a complete polymerization for a typical top-contact C60 TFT is as follows: the electron mobility (μn) at room temperature drops slightly from 0.074 to 0.068 cm2/Vs, the activation energy of μn decreases from 0.10 meV to 0.09 meV, and the threshold voltage for TFT operation decreases markedly by ~15 %. The latter observation suggests that the effective number of electron traps in the C60 film decreases following polymerization. Considering that the polymerization was achieved with a low-energy HeNe laser, it is conceivable that the polymerization approach could be of interest for applications, e.g., organic bulk-heterojunction solar cells, where a stabilized C60 morphology attained with benign means is desired

  • 19.
    Dzwilewski, Andrzej
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Photo-induced and resist-free imprint patterning of fullerene materials for use in functional electronics2009In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 131, no 11, 4006-4011 p.Article in journal (Refereed)
    Abstract [en]

    We report a novel and potentially generic method for the efficient patterning of films of organic semiconductors and demonstrate the merit of the method on the high-solubility fullerene [6,6]-phenyl C61- butyric acid methyl ester (PCBM). The patterning technique is notably straightforward as it requires no photoresist material and encompasses only two steps: (i) exposure of select film areas to visible laser light during which the PCBM mononer is photochemically converted into a dimeric state, and (ii) development via solvent washing after which the nonexposed portions of the PCBM film are selectively removed. Importantly, the method is highly benign in that it leaves the electronic properties of the remaining patterned material intact, which is directly evidenced by the fact that we fabricate fully functional arrays of micrometersized field-effect transistors with patterned PCBM as the active material.

  • 20.
    Edman, Ludvig
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Matyba, Piotr
    Umeå University, Faculty of Science and Technology, Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Shin, Joon Ho
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Mats
    Light-emitting Electrochemical Cells with mm-sized Electrode Gap: Controlling Light at Low Voltage and Identification of Degradation Mechanism.2008In: SPIE Photonics Europe, 2008Conference paper (Other academic)
  • 21.
    Gracia-Espino, Eduardo
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Yan, Aiming
    Zettl, Alex
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fabrication of One-Dimensional Zigzag [6,6]-Phenyl-C-61-Butyric Acid Methyl Ester Nanoribbons from Two-Dimensional Nanosheets2015In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 9, no 10, 10516-10522 p.Article in journal (Refereed)
    Abstract [en]

    One-dimensional (10) zigzag [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanoribbons are produced by folding two-dimensional ultrathin PCBM nanosheets in a simple solvent process. The unique 1D PCBM nanostructures exhibit uniform width of 3.8 +/- 0.3 nm, equivalent to four PCBM molecules, and lengths of 20-400 nm. These nanoribbons show well-defined crystalline structure, comprising PCBM molecules in a hexagonal arrangement without trapped solvent molecules. First-principle calculations and detailed experimental characterization provide an insight into the structure and formation mechanism of the 1D PCBM nanoribbons. Given their dimensions and physical properties, we foresee that these nanostructures should be ideal as acceptor material in organic solar cells.

  • 22.
    Gracia-Espino, Eduardo
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Understanding the Interface of Six-Shell Cuboctahedral and Icosahedral Palladium Clusters on Reduced Graphene Oxide: Experimental and Theoretical Study2014In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 136, no 18, 6626-6633 p.Article in journal (Refereed)
    Abstract [en]

    Studies on noble-metal-decorated carbon nanostructures are reported almost on a daily basis, but detailed studies on the nanoscale interactions for well-defined systems are very rare. Here we report a study of reduced graphene oxide (rGOx) homogeneously decorated with palladium (Pd) nanoclusters with well-defined shape and size (2.3 +/- 0.3 nm). The rGOx was modified with benzyl mercaptan (BnSH) to improve the interaction with Pd clusters, and N,N-dimethylformamide was used as solvent and capping agent during the decoration process. The resulting Pd nanoparticles anchored to the rGOx-surface exhibit high crystallinity and are fully consistent with six-shell cuboctahedral and icosahedral clusters containing similar to 600 Pd atoms, where 45% of these are located at the surface. According to X-ray photoelectron spectroscopy analysis, the Pd clusters exhibit an oxidized surface forming a PdOx shell. Given the well-defined experimental system, as verified by electron microscopy data and theoretical simulations, we performed ab initio simulations using 10 functionalized graphenes (with vacancies or pyridine, amine, hydroxyl, carboxyl, or epoxy groups) to understand the adsorption process of BnSH, their further role in the Pd cluster formation, and the electronic properties of the graphene-nanoparticle hybrid system. Both the experimental and theoretical results suggest that Pd clusters interact with fiinctionalized graphene by a sulfur bridge while the remaining Pd surface is oxidized. Our study is of significant importance for all work related to anchoring of nanoparticles on nanocarbon-based supports, which are used in a variety of applications.

  • 23.
    Gracia-Espino, Eduardo
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Physics.
    Jia, Xueen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Improved oxygen reduction performance of Pt–Ni nanoparticles by adhesion on nitrogen-doped graphene2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 5, 2804-2811 p.Article in journal (Refereed)
    Abstract [en]

    Graphene and its derivatives hold great potential as support for nanocatalyst in various energy applications, such as fuel cells, batteries, and capacitors. In this work, we used density functional theory to analyze substrate effect on the electrocatalytic activity of Pt–Ni bimetallic nanoparticles for oxygen reduction reaction (ORR). The dissociative mechanism is used to evaluate the ORR performance (energy barrier for O2 dissociation, free energy of intermediates, d-band center, overpotential, and electrochemical activity) for a Pt–Ni core–shell-like nanoparticle (PtNiCS) deposited on nondefective graphene (GS) or nitrogen-doped graphene (N-GS). The electronic and catalytic properties of PtNiCS on N-GS designate N-doped graphene as the best substrate to use for ORR, showing better interaction with the bimetallic cluster, improved charge transfer between constitutes, and a superior ORR performance when compared to PtNiCS on GS. The N-GS has a significant effect in reducing the energy barrier for O2 dissociation and decrease the energetic stability of HO* intermediates, resulting in enhanced ORR activity compared with the PtNiCS on GS. In addition, the strong interaction between PtNiCS cluster and N-GS substrate may lead to an improved long-term stability of the catalytic particle during ORR cycles.

  • 24.
    Hedenström, Mattias
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Johnels, Dan
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Characterization of Hydrogenated Fullerenes by NMR Spectroscopy2010In: Fulleranes: The Hydrogenated Fullerenes / [ed] Franco Cataldo, Susana Iglesias-Groth, Dordrecht: Springer Netherlands, 2010, Vol. 2, 171-202 p.Chapter in book (Other academic)
    Abstract [en]

    NMR spectroscopy is so far the only analytical technique that has been used to get a detailed structural characterization of hydrogenated fullerenes. A substantial amount of information derived from different NMR experiments can thus be found in the literature for a number of fullerenes hydrogenated to various degrees. These studies have benefitted from the fact that chemical shifts of H-1 and C-13 and in some cases also He-3 can be used to obtain structural information of these compounds. Such results, together with discussions about different NMR experiments and general considerations regarding sample preparations, are summarized in this chapter. The unique information, both structural and physicochemical, that can be derived from different NMR experiments ensures that this technique will continue to be of central importance in characterization of hydrogenated fullerenes.

  • 25. Hedman, Daniel
    et al.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics. Department of Physics, University of California and the Lawrence Berkeley National Laboratory, USA.
    Rosen, Arne
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsson, J. Andreas
    On the Stability and Abundance of Single Walled Carbon Nanotubes2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, 16850Article in journal (Refereed)
    Abstract [en]

    Many nanotechnological applications, using single-walled carbon nanotubes (SWNTs), are only possible with a uniform product. Thus, direct control over the product during chemical vapor deposition (CVD) growth of SWNT is desirable, and much effort has been made towards the ultimate goal of chirality-controlled growth of SWNTs. We have used density functional theory (DFT) to compute the stability of SWNT fragments of all chiralities in the series representing the targeted products for such applications, which we compare to the chiralities of the actual CVD products from all properly analyzed experiments. From this comparison we find that in 84% of the cases the experimental product represents chiralities among the most stable SWNT fragments (within 0.2 eV) from the computations. Our analysis shows that the diameter of the SWNT product is governed by the well-known relation to size of the catalytic nanoparticles, and the specific chirality is normally determined by the product's relative stability, suggesting thermodynamic control at the early stage of product formation. Based on our findings, we discuss the effect of other experimental parameters on the chirality of the product. Furthermore, we highlight the possibility to produce any tube chirality in the context of recent published work on seeded-controlled growth.

  • 26.
    Horvath, Istvan
    et al.
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Jia, Xueen
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics. Umeå University, Faculty of Science and Technology, Department of Physics.
    Johansson, Per
    Wang, Chao
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Moskalenko, Roman
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics. Department of Pathology, Sumy State University, Sumy 40000, Ukraine.
    Steinau, Andreas
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Forsgren, Lars
    Umeå University, Faculty of Medicine, Department of Pharmacology and Clinical Neuroscience, Clinical Neuroscience.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Svensson, Johan
    Zetterberg, Henrik
    Morozova-Roche, Ludmilla A
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Pro-inflammatory S100A9 Protein as a Robust Biomarker Differentiating Early Stages of Cognitive Impairment in Alzheimer's Disease2016In: ACS Chemical Neuroscience, ISSN 1948-7193, E-ISSN 1948-7193, Vol. 7, no 1, 34-39 p.Article in journal (Refereed)
    Abstract [en]

    Pro-inflammatory protein S100A9 was established as a biomarker of dementia progression and compared with others such as Aβ1-42 and tau-proteins. CSF samples from 104 stringently diagnosed individuals divided into five subgroups were analyzed, including nondemented controls, stable mild cognitive impairment (SMCI), mild cognitive impairment due to Alzheimer's disease (MCI-AD), Alzheimer's disease (AD), and vascular dementia (VaD) patients. ELISA, dot-blotting, and electrochemical impedance spectroscopy were used as research methods. The S100A9 and Aβ1-42 levels correlated with each other: their CSF content decreased already at the SMCI stage and declined further under MCI-AD, AD, and VaD conditions. Immunohistochemical analysis also revealed involvement of both Aβ1-42 and S100A9 in the amyloid-neuroinflammatory cascade already during SMCI. Tau proteins were not yet altered in SMCI; however their contents increased during MCI-AD and AD, diagnosing later dementia stages. Thus, four biomarkers together, reflecting different underlying pathological causes, can accurately differentiate dementia progression and also distinguish AD from VaD.

  • 27.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Laboratory of Environmental Science and Technology, The Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, China.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sandström, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Cheng, Shaodong
    Shen, Hangjia
    Wang, Chuanyi
    Guo, Shaojun
    Yang, Guang
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Atomistic understanding of the origin of high oxygen reduction electrocatalytic activity of cuboctahedral Pt3Co-Pt core-shell nanoparticles2016In: Catalysis Science & Technology, ISSN 2044-4753, E-ISSN 2044-4761, Vol. 6, no 5, 1393-1401 p.Article in journal (Refereed)
    Abstract [en]

    PtM-based core-shell nanoparticles are a new class of active and stable nanocatalysts for promoting oxygen reduction reaction (ORR); however, the understanding of their high electrocatalytic performance for ORR at the atomistic level is still a great challenge. Herein, we report the synthesis of highly ordered and homogeneous truncated cuboctahedral Pt3Co-Pt core-shell nanoparticles (cs-Pt3Co). By combining atomic resolution electron microscopy, X-ray photoelectron spectroscopy, extensive first-principles calculations, and many other characterization techniques, we conclude that the cs-Pt3Co nanoparticles are composed of a complete or nearly complete Pt monolayer skin, followed by a secondary shell containing 5-6 layers with similar to 78 at% of Pt, in a Pt3Co configuration, and finally a Co-rich core with 64 at% of Pt. Only this particular structure is consistent with the very high electrocatalytic activity of cs-Pt3Co nanoparticles for ORR, which is about 6 times higher than commercial 30%-Pt/Vulcan and 5 times more active than non-faceted (spherical) alloy Pt3Co nanoparticles. Our study gives an important insight into the atomistic design and understanding of advanced bimetallic nanoparticles for ORR catalysis and other important industrial catalytic applications.

  • 28.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mikolajczuk, Ania
    Polish Acad Sci, Inst Phys Chem, PL-01224 Warsaw, Poland.
    Tai, Cheuk-Wai
    Stockholm Univ, Arrhenius Lab, Dept Mat & Environm Chem, S-10691 Stockholm, Sweden.
    Borodzinski, Andrzej
    Polish Acad Sci, Inst Phys Chem, PL-01224 Warsaw, Poland.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Palladium nanocrystals supported on helical carbon nanofibers for highly efficient electro-oxidation of formic acid, methanol and ethanol in alkaline electrolytes2012In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 209, 236-242 p.Article in journal (Refereed)
    Abstract [en]

    We present the synthesis of palladium nanocrystals self-assembled on helical carbon nanofibers functionalized with benzyl mercaptan (Pd-S-HCNFs) and their electrocatalytic activity toward the oxidation of formic acid, methanol and ethanol. Helical carbon nanofibers (HCNFs) were first functionalized with benzyl mercaptan based on the pi-pi interactions between phenyl rings and the graphitic surface of HCNFs. Palladium nano crystals (PdNC) were fixed on the surface of functionalized HCNF by Pd-S bonds in a simple self-assembly method. The as-prepared materials were characterized by high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), cyclic voltammetry (CV), and fuel cell tests. CV characterization of the as-prepared materials shows a very high electrocatalytic activity for oxidation of formic acid, ethanol and methanol in strong alkaline electrolyte. In comparison to commercial catalyst Vulcan XC-72 decorated with Pd nanoparticles, the proposed Pd-S-HCNFs nano composite material shows oxidation currents for formic acid, ethanol and methanol at the Pd-S-HCNF-modified electrode that are higher than that at the Pd/XC-72 modified electrode with a factor of 2.0, 1.5, and 2.3, respectively. In a formic acid fuel cell the Pd-S-HCNF modified electrode yields equal power density as commercial Pd/XC-72 catalyst. Our results show that Pd-decorated helical carbon nanofibers with diameters around 40-60 nm have very high potential as active material in fuel cells, electrocatalysts and sensors. (C) 2012 Elsevier B.V All rights reserved.

  • 29.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Ma, Jingyuan
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Jia, Xueen
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Lu, Lu
    Ma, Chuansheng
    Yang, Guang
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Small palladium islands embedded in palladium-tungsten bimetallic nanoparticles form catalytic hotspots for oxygen reduction2014In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 5, Article number: 5253- p.Article in journal (Refereed)
    Abstract [en]

    The sluggish kinetics of the oxygen reduction reaction at the cathode side of proton exchange membrane fuel cells is one major technical challenge for realizing sustainable solutions for the transportation sector. Finding efficient yet cheap electrocatalysts to speed up this reaction therefore motivates researchers all over the world. Here we demonstrate an efficient synthesis of palladium-tungsten bimetallic nanoparticles supported on ordered mesoporous carbon. Despite a very low percentage of noble metal (palladium: tungsten = 1:8), the hybrid catalyst material exhibits a performance equal to commercial 60% platinum/Vulcan for the oxygen reduction process. The high catalytic efficiency is explained by the formation of small palladium islands embedded at the surface of the palladium-tungsten bimetallic nanoparticles, generating catalytic hotspots. The palladium islands are similar to 1 nm in diameter, and contain 10-20 palladium atoms that are segregated at the surface. Our results may provide insight into the formation, stabilization and performance of bimetallic nanoparticles for catalytic reactions.

  • 30.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Chalmers University of Technology.
    Jia, Xueen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Reduction free room temperature synthesis of a durable and efficient Pd/ordered mesoporous carbon composite electrocatalyst for alkaline direct alcohols fuel cell2014In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 4, no 2, 676-682 p.Article in journal (Refereed)
    Abstract [en]

    The development of easy and environmentally benign synthesis methods of efficient electrocatalysts for use in energy conversion applications motivates researchers all over the world. Here we report a novel and versatile method to synthesize well-dispersed palladium-functionalized ordered mesoporous carbons (Pd/OMCs) at room temperature without any reducing agent by one-pot mixing of tri(dibenzylideneacetone)palladium(0) (Pd2DBA3) and OMCs together in a common N,N-dimethylformamide (DMF) solution. The formation of Pd nanoparticles and their crystallization on the OMC is catalyzed by protons in the solution and can thus be controlled by the solution pH. The complete process and the as-prepared nanocomposite was characterized by UV-spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (HTEM), X-ray photoelectron spectrum (XPS), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The electrocatalytic property of the as-decorated material was examined with cyclic voltammetry (CV). The Pd/OMC composite shows up to two times higher electrocatalytic ability with a significantly better durability towards ethanol and methanol oxidation in alkaline media compared to commercial high surface area conductive carbon black Vulcan XC-72 decorated with equivalent Pd nanoparticles. Our described method provides new insight for the development of highly efficient carbon based nanocatalysts by simple and environmentally sound methods.

  • 31.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Self-assembled palladium nanocrystals on helical carbon nanofibers as enhanced electrocatalysts for electro-oxidation of small molecules2012In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 22, no 17, 8541-8548 p.Article in journal (Refereed)
    Abstract [en]

    We present a novel approach to prepare helical carbon nanofibers homogeneously functionalized with single crystal palladium nanoparticles via a phase-transfer method. The materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA) and electrochemical measurements. We find that homogeneous and small single-crystal Pd nanoparticles can be easily functionalized with phenyl mercaptan, transferred into the toluene phase from the dimethyl sulfoxide (DMSO) phase and then non-covalently self-assembled onto the surface of helical carbon nanofibers with a very good dispersion and homogeneous diameters of 4.5 +/- 0.6 nm. The palladium-helical carbon nanofiber composite exhibits significantly higher electrochemical active area and electrocatalytic activity towards the electrooxidation of formic acid, ethanol and methanol than the commercial electrocatalyst Pd/Vulcan XC-72. Our results show that the prepared material can be potentially used as an advanced nano-electrocatalyst in a direct alkaline fuel cell system.

  • 32.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tai, Cheuk-Wai
    Stockholm Univ, Dept Mat & Environm Chem, Stockholm, Sweden.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Phase-transfer synthesis of amorphous palladium nanoparticle-functionalized 3D helical carbon nanofibers and its highly catalytic performance towards hydrazine oxidation2012In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 543, 96-100 p.Article in journal (Refereed)
    Abstract [en]

    Amorphous palladium nanoparticles functionalized helical carbon nanofibers (ApPd-HCNFs) were synthesized using a phase-transfer method. Palladium nanoparticles (Pd-NP) were first prepared using n-dodecyl sulfide as reducing agent and stabilizing ligands in ethanol. The Pd-NPs were then modified with benzyl mercaptan and transferred into a toluene solution with HCNFs which were decorated with amorphous palladium. The materials were characterized with high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and cyclic voltammetry showing that amorphous palladium nanoparticles were uniformly anchored at the HCNFs surface and that the ApPd-HCNFs exhibit high electrocatalytic activity towards hydrazine oxidation. (C) 2012 Elsevier B.V. All rights reserved.

  • 33.
    Iwasiewicz-Wabnig, Agnieszka
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Makarova, Tatiana L.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    No insulator-metal transition in Rb4C60 under pressure below 2 GPaIn: Physical Review BArticle in journal (Refereed)
  • 34.
    Iwasiewicz-Wabnig, Agnieszka
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Makarova, Tatiana
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Absence of an insulator-metal transition in Rb4C60 up to 2 GPa2008In: Physical Review B, ISSN 0163-1829, Vol. 77, no 8, 085434- p.Article in journal (Refereed)
    Abstract [en]

    We present the results of direct resistance measurements on Rb4C60 under pressures up to 2 GPa. At all pressures covered by this study and over the temperature range of 90–450 K Rb4C60 is a semiconductor with a weakly pressure dependent band gap near 0.7 eV. We do not observe the insulator-to-metal transition previously reported to occur below 1.2 GPa, although we cannot rule out the possibility that such a transition might occur at some significantly higher pressure. The measured resistivity is surprisingly low and is dominated by carriers excited over a 0.1 eV gap. Because the corresponding conductivity increases with deformation of the sample, we assign these states to structural or orientational defects. The known structural transformation below 0.5 GPa leads to a decrease in resistivity under high pressure, but the material remains semiconducting. A Rb6C60 control sample showed a similar behavior, also being a semiconductor under all conditions studied. At temperatures above 460 K, Rb was partially lost from our samples, resulting in metallization by a transformation into Rb3C60.

  • 35.
    Iwasiewicz-Wabnig, Agnieszka
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Electrical resistance of Rb4C60 under pressure2008In: The Journal of Physics and Chemistry of Solids, vol. 69, issues 5-6: Proceedings of the 14th International Symposium on Intercalation Compounds - ISIC 14, Amsterdam: Elsevier B.V. , 2008, 1218-1220 p.Conference paper (Refereed)
    Abstract [en]

    We report the results of direct in situ resistance measurements of Rb4C60 under high pressure up to 2 GPa, in the temperature range 90–400 K. The resistance changes smoothly with pressure and temperature without sharp anomalies, and all data sets can be fitted to the same theoretical semiconductor model. We find no signs of the insulator-to-metal transition previously reported in this range, but the fitted band gap decreases with pressure and such a transition may possibly take place above 5 GPa.

  • 36.
    Jia, Xueen
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tai, Cheuk-Wai
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Synthesis of Palladium/Helical Carbon Nanofiber Hybrid Nanostructures and Their Application for Hydrogen Peroxide and Glucose Detection2013In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 5, no 22, 12017-12022 p.Article in journal (Refereed)
    Abstract [en]

    We report on a novel sensing platform for H2O2 and glucose based on immobilization of palladium-helical carbon nanofiber (Pd-HCNF) hybrid nanostnictures and glucose oxidase (GOx) with Nafion on a glassy carbon electrode (GCE). HCNFs were synthesized by a chemical vapor deposition process on a C-60-supported Pd catalyst. Pd-HCNF nanocomposites were prepared by a one-step reduction free method in dimethylformamide (DMF). The prepared materials were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. The Nafion/Pd-HCNF/GCE sensor exhibits excellent electrocatalytic sensitivity toward H2O2 (315 mA M-1 cm(-2)) as probed by cyclic voltammetry (CV) and chronoamperometry. We show that Pd-HCNF-modified electrodes significantly reduce the overpotential and enhance the electron transfer rate. A linear range from 5.0 mu M to 2.1 mM with a detection limit of 3.0 mu M (based on the S/N = 3) and good reproducibility were obtained. Furthermore, a sensing platform for glucose was prepared by immobilizing the Pd-HCNFs and glucose oxidase (GOx) with Nafion on a glassy carbon electrode. The resulting biosensor exhibits a good response to glucose with a wide linear range (0.06-6.0 mM) with a detection limit of 0.03 mM and a sensitivity of 13 mA M-1 cm(-2). We show that small size and homogeneous distribution of the Pd nanoparticles in combination with good conductivity and large surface area of the HCNFs lead to a H2O2 and glucose sensing platform that performs in the top range of the herein reported sensor platforms.

  • 37. Jiang, Linhai
    et al.
    Yao, Mingguang
    Liu, Bo
    Li, Quanjun
    Liu, Ran
    Lv, Hang
    Lu, Shuangchen
    Gong, Chen
    Zou, Bo
    Cui, Tian
    Liu, Bingbing
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Controlled Synthesis of CeO2/Graphene Nanocomposites with Highly Enhanced Optical and Catalytic Properties2012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 21, 11741-11745 p.Article in journal (Refereed)
    Abstract [en]

    In this paper, CeO2 nanocubes with the (200)-terminated surface/graphene sheet composites have been prepared successfully by a simple hydrothermal method. It is found that the CeO2 nanocubes with high crystallinity and specific exposed surface are well dispersed on well-exfoliated graphene surface. The (200)-terminated surface/graphene sheet composites modified electrode showed much higher sensitivity and excellent selectivity in its catalytic performance compared to a CeO2 nanoparticle-modified electrode. The photoluminescence intensity of the CeO2 anchored on graphene is about 30 times higher than that of pristine CeO2 crystals in air. The higher oxygen vacancy concentration in CeO2 is supposed to be an important cause for the higher photoluminescence and better electrochemical catalytic performance observed in the (200)-terminated surface/graphene sheet composites. Such ingenious design of supported well-dispersed catalysts in nanostructured ceria catalysts, synthesized in one step with an exposed high-activity surface, is important for technical applications and theoretical investigations.

  • 38. Kim, Y
    et al.
    Abou-Hamad, E
    Rubio, A
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Talyzin, Alexandr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boesch, D
    Aloni, S
    Zettl, A
    Luzi, D E
    Goze-Bac, C
    Communications: nanomagnetic shielding: high-resolution NMR in carbon allotropes2010In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 132, no 2, 021102- p.Article in journal (Refereed)
    Abstract [en]

    The understanding and control of the magnetic properties of carbon-based materials is of fundamental relevance in applications in nano- and biosciences. Ring currents do play a basic role in those systems. In particular the inner cavities of nanotubes offer an ideal environment to investigate the magnetism of synthetic materials at the nanoscale. Here, by means of 13 C high resolution NMR of encapsulated molecules in peapod hybrid materials, we report the  largest diamagnetic shifts (down to -68.3 ppm) ever observed in carbon allotropes, which is connected to the enhancement of the aromaticity of the nanotube envelope upon doping. This diamagnetic shift can be externally controlled by in situ modifications such as doping or electrostatic charging. Moreover, defects such as C-vacancies, pentagons, and chemical functionalization of the outer nanotube quench this diamagnetic effect and restore NMR signatures to slightly paramagnetic shifts compared to nonencapsulated molecules. The magnetic interactions reported here are robust phenomena independent of temperature and proportional to the applied magnetic field. The magnitude, tunability, and stability of the magnetic effects make the peapod nanomaterials potentially valuable for nanomagnetic shielding in nanoelectronics and nanobiomedical engineering.

  • 39.
    Kim, Y
    et al.
    Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania.
    Abou-Hamad, Edy
    CNRS Université Montpellier 2, France .
    Rubio, Angelo
    European Theoretical Spectroscopy Facility (ETSF), Universidad del País Vasco UPV/EHU, Spain .
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Talyzin, Alexandr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boesch, D
    Department of Physics, University of California, Berkeley, California 94720 USA and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, USA .
    Aloni, S
    Department of Physics, University of California, Berkeley, California 94720 USA and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, USA .
    Zettl, Alex
    Department of Physics, University of California, Berkeley, California 94720 USA and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, USA .
    Luzzi, David E.
    Department of Materials Science and Engineering, University of Pennsylvania, USA .
    Goze-Bac, Christophe
    CNRS Université Montpellier 2, France .
    Nanomagnetic shielding: High-resolution NMR in carbon allotropes2010In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 132, 021102- p.Article in journal (Refereed)
    Abstract [en]

    Theunderstanding and control of the magnetic properties of carbon-based materialsis of fundamental relevance in applications in nano- and biosciences.Ring currents do play a basic role in those systems.In particular the inner cavities of nanotubes offer an idealenvironment to investigate the magnetism of synthetic materials at thenanoscale. Here, by means of 13C high resolution NMR ofencapsulated molecules in peapod hybrid materials, we report the largestdiamagnetic shifts (down to −68.3 ppm) ever observed in carbonallotropes, which is connected to the enhancement of the aromaticityof the nanotube envelope upon doping. This diamagnetic shift canbe externally controlled by in situ modifications such as dopingor electrostatic charging. Moreover, defects such as C-vacancies, pentagons, andchemical functionalization of the outer nanotube quench this diamagnetic effectand restore NMR signatures to slightly paramagnetic shifts compared tononencapsulated molecules. The magnetic interactions reported here are robust phenomenaindependent of temperature and proportional to the applied magnetic field.The magnitude, tunability, and stability of the magnetic effects makethe peapod nanomaterials potentially valuable for nanomagnetic shielding in nanoelectronicsand nanobiomedical engineering

  • 40.
    Krstic, Vojislav
    et al.
    Univ Dublin Trinity Coll, Sch Phys.
    Ewels, Christopher P
    Univ Nantes, CNRS.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ferreira, Mauro S
    Univ Dublin Trinity Coll, Sch Phys.
    Janssens, Anne M
    Delft Univ Technol.
    Stephan, Odile
    Univ Paris 11, LPS.
    Glerup, Marianne
    Univ Oslo, Dept Chem.
    Indirect magnetic coupling in light-element-doped single-walled carbon nanotubes2010In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 4, no 9, 5081-5086 p.Article in journal (Refereed)
    Abstract [en]

    Single-walled carbon nanotubes substitutionally doped with the light-element phosphorus are synthesized and are investigated by electrical and nuclear magnetic resonance measurements. Decreased spin lattice relaxation times compared to undoped tubes point toward enhanced spin-sensitive scattering. Temperature dependence of the zero-bias conductance shows step-like features, a signature of scattering from a very low density (few sites per nanotube) of localized spin moments at oxidized phosphorus sites, consistent with density functional calculations. This supports recent predictions that localized magnetic moments must be indirectly magnetically coupled through the nanotube conduction electrons.

  • 41.
    Larsen, Christian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    On the fabrication of crystalline C-60 nanorod transistors from solution2012In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 23, no 34, 344015- p.Article in journal (Refereed)
    Abstract [en]

    Flexible and high-aspect-ratio C-60 nanorods are synthesized using a liquid-liquid interfacial precipitation process. As-grown nanorods are shown to exhibit a hexagonal close-packed single-crystal structure, with m-dichlorobenzene solvent molecules incorporated into the crystalline structure in a C-60:m-dichlorobenzene ratio of 3.2. An annealing step at 200 degrees C transforms the nanorods into a solvent-free face-centred-cubic polycrystalline structure. The nanorods are deposited onto field-effect transistor structures using two solvent-based techniques: drop-casting and dip-coating. We find that dip-coating deposition results in a preferred alignment of non-bundled nanorods and a satisfying transistor performance. The latter is quantified by the attainment of an electron mobility of 0.08 cm(2) V-1 s(-1) and an on/off ratio of >10(4) for a single-crystal nanorod transistor, fabricated with a solution-based and low-temperature process that is compatible with flexible substrates.

  • 42. Liu, Bingbing
    et al.
    Cui, Qiliang
    Yu, Miao
    Zou, Guangtian
    Carlsten, Jan
    Umeå University, Faculty of Science and Technology, Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Raman study of bromine-doped single-walled carbon nanotubes under high pressure2002In: Journal of Physics: Condensed Matter vol. 14 issue no. 44: Proceedings of the Joint 18th Conference on High Pressure Science and Technology (AIRAPT 18) and the 11th High Pressure Conference of China (HPCC-11), Beijing 2001, Bristol: Institute of Physics , 2002, 11255-11259 p.Conference paper (Refereed)
    Abstract [en]

    Raman results for different single-walled carbon nanotube bundles doped withBr2 were studied both at ambient pressure and under high pressure up to 6 GPa.Our study indicates that bromine resides in the interstitial channel of nanotubebundles as a form of polymer.

  • 43.
    Liu, Bingbing
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Physics.
    Olsson, Eva
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Nyeanchi, Emmanuel B.
    Umeå University, Faculty of Science and Technology, Physics.
    Zhu, Xiaomei
    Umeå University, Faculty of Science and Technology, Physics.
    Zou, Guangtian
    Electric resistance of single-walled carbon nanotubes under hydrostatic pressure2001In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 118, no 1, 31-36 p.Article in journal (Refereed)
    Abstract [en]

    The electric resistance of single-walled nanotube mats has been studied systematically under both ambient and high hydrostatic pressures up to 1.5 GPa. Both the temperature dependence of the resistance and the magnetoresistance indicate that electrical transport occurs by variable range hopping, apparently in 2D. We suggest that this unexpected dimensionality arises from a fractal network of tubes and bundles. Under hydrostatic pressure (HP) the resistance still shows 2D variable range hopping and decreases with increasing HP. An irreversible increase in localization length and DOS is induced below 0.5 GPa. The behavior is reversible and due to strong interaction of tubes from 0.5 GPa up to 1.05 GPa. These results indicate that 2D variable range hopping occurs within bundles.

  • 44.
    Liu, Bingbing
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Zou, Guangtian
    Transport properties of single-walled nanotube mats under hydrostatic pressure2000In: Electronic Properties of Novel Materials: Science and Technology of Molecular nanostructures: Proceedings of the XIV Winter School on Electronic Properties of Novel Materials, Kirchberg 2000, Melville: American Institute of Physics (AIP) , 2000, 344-348 p.Conference paper (Refereed)
    Abstract [en]

    The study of electrical transport properties of bulk single-walled carbon nanotube (SWNT) materials attracts a lot of attention. However, the transport results differ from sample to sample[l]. Some results show semiconducting behavior with different laws and whether the behavior is dominated by on-tube or on-bundle effects or by interbundle contacts is still unknown. Therefore, further investigation is still meaningful. The effect of hydrostatic pressure(HP), which is expected to significantly change the interaction between tubes or bundles, gives us a further possibility to understand the characteristic properties. Under HP, Raman spectra and electric resistance show pronounced changes at 1.5GPa[2]. In this study, we systematically studied the electric resistance up to 1.5 GPa, giving a different image of the transport behavior.

  • 45.
    Liu, Bingbing
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Nyeanchi, Emmanuel B.
    Umeå University, Faculty of Science and Technology, Physics.
    Makarova, Tatiana L.
    Umeå University, Faculty of Science and Technology, Physics.
    Zhu, Xiaomei
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Yang, R.
    Li, D.
    Gao, C.
    Yang, H.
    Zou, Guangtian
    Li, H.-D.
    Olsson, Eva
    Synthesis and characterization of single-walled nanotubes produced with Ce as catalyst2000In: Molecular Materials: Molecular Crystals and Liquid Crystals Section C, vol. 13: Proceedings of the 4th International Workshop on Fullerenes and Atomic Clusters (IWFAC'99), St. Petersburg 1999, Overseas Publishers Association N.V. , 2000, 75-80 p.Conference paper (Refereed)
    Abstract [en]

    We have studied the effect of using rare-earth elements C (typically +4 oxide state) and Eu (typically +2 oxide state) together with nanometer Ni as catalysts to synthesize SWNT in high yield by arc evaporation. A black collar deposit containing mainly SWNT was formed in the presence of Ce/Ni while only nanometer carbon particles were formed using Eu/Ni, as identified by SEM, TEM and Raman spectra. The Raman spectra of our SWNT mats show difference from spectra shown in the literature in a low frequency range, indicating that our mats contain chiral tubes. The temperature dependent resistance of SWNT mats was also measured. A semiconducting behaviour was observed with a negative dR/dT and no resistance minimum from 2 to 350 K. The resistance follows a 2D variable range hopping behaviour.

  • 46.
    Liu, Bingbing
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Olsson, Eva
    Yang, Ruseng
    Li, Hongdong
    Zhang, Shulin
    Yang, Haibin
    Zou, Guangtian
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Synthesis and characterization of single-walled nanotubes produced with Ce/Ni as catalysts.2000In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 320, no 3-4, 365-372 p.Article in journal (Refereed)
    Abstract [en]

    SWNTs were synthesized in high yield in the presence of Ce/Ni as catalyst. Raman and SEM results show that a fraction of the tubes are chiral and have small diameters (the smallest one is 0.79 nm), indicating that Ce plays an important role in the formation. Acid treatment is effective in eliminating impurities but changes the morphology and the electric resistance of mats with increasing treatment time. The resistance of as-grown and short time acid-treated mats follow a 2D VRH behaviour while mats treated for a long time follow a 3D model. The results indicate that hopping conduction occurs on bundles or on tubes and that there are intrinsic links between tubes in as-grown mats.

  • 47. Liu, Dedi
    et al.
    Yao, Mingguang
    Li, Quanjun
    Cui, Wen
    Zou, Bo
    Cui, Tian
    Lui, Bingbing
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    High pressure and high temperature induced polymerization of C60 nanotubes2011In: CrystEngComm, ISSN 1466-8033, E-ISSN 1466-8033, Vol. 13, no 10, 3600-3605 p.Article in journal (Refereed)
    Abstract [en]

    C60 nanotubes with outer diameters ranging from 400–800 nm were polymerized at 1.5 GPa, 573 K and 2.0 GPa, 700 K, respectively. Raman and photoluminescence spectroscopy were employed to characterize the polymeric phases of the treated samples. Both Raman and photoluminescence spectra showed that the C60 nanotubes transformed into the dimer and orthorhombic phases under the two different conditions, respectively. The photoluminescence peaks were tuned from visible to near infrared range. Comparative studies indicated that C60 nanotubes were more difficult to polymerize than bulk C60 material under the same conditions due to the nanoscale size effect in the C60 nanotubes.

  • 48. Liu, Dedi
    et al.
    Yao, Mingguang
    Wang, Lin
    Li, Quanjun
    Cui, Wen
    Liu, Bo
    Lui, Ran
    Zou, Bo
    Cui, Tian
    Liu, Bingbing
    Liu, Jing
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pressure-induced phase transitions of C70 nanotubes2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 18, 8918-8922 p.Article in journal (Refereed)
    Abstract [en]

    Single crystalline C70 nanotubes having a face-centered-cubic (fcc) structure with diameters on a nanometer scale were synthesized by a facile solution method. In situ high pressure Raman spectroscopy and X-ray diffraction have been employed to study the structural stability and phase transitions of the pristine sample. We show that the molecular orientation-related phase transition from the fcc structure to a rhombohedral structure occurs at about 1.5 GPa, which is 1 GPa higher than in bulk C70. Also, the C70 molecules themselves are more stable in the nanotubes than in bulk crystals, manifested by a partial amorphization at 20 GPa. The crystal structure of C70 nanotubes could partially return to the initial structure after a pressure cycle above 30.8 GPa, and the C70 molecules were intact up to 43 GPa. The bulk modulus of C70 nanotubes is measured to be 50 GPa, which is twice larger than that of bulk C70.

  • 49. Ma, Honglei
    et al.
    Zhang, Xuemei
    Liu, Bingbing
    Li, Quanjun
    Zeng, Qifeng
    Yu, Shidan
    Zou, Bo
    Cui, Tian
    Zou, Guangtian
    Liu, Zhenxian
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Noreus, Dag
    Synchrotron X-ray diffraction and infrared spectroscopy studies of C60H18 under high pressure2010In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 1, no 4, 714-719 p.Article in journal (Refereed)
    Abstract [en]

    In situ high-pressure angle-dispersive synchrotron X-ray diffraction and high-pressure mid-infrared (IR) spectrum measurements of C60H18 were carried out up to 32 and 10.2 GPa, respectively. Our diffraction data indicated that the fcc structure of C60H18 was stable up to 32 GPa. The bulk modulus B0 was determined to be 21 ± 1.16 GPa, about 40% higher than that of C60. The C−H vibrations still existed up to 10.2 GPa, and the vibrational frequencies decreased with increasing pressure. IR-active vibrational frequencies and their corresponding eigenvectors of C60H18 were simulated by DMOL3. The effects of the hydrogen atoms attached to the fullerene molecular cage on the stability of the structure under high pressure are discussed.

  • 50. Mahfouz, R.
    et al.
    Bouhrara, M.
    Kim, Y.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Goze-Bac, C.
    Abou-Hamad, E.
    Properties of K,Rb-intercalated C60 encapsulated inside carbon nanotubes called peapods derived from nuclear magnetic resonance2015In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 118, no 11, 114305Article in journal (Refereed)
    Abstract [en]

    We present a detailed experimental study on how magnetic and electronic properties of Rb, K-intercalated C-60 encapsulated inside carbon nanotubes called peapods can be derived from C-13 nuclear magnetic resonance investigations. Ring currents do play a basic role in those systems; in particular, the inner cavities of nanotubes offer an ideal environment to investigate the magnetism at the nanoscale. We report the largest diamagnetic shifts down to -68.3 ppm ever observed in carbon allotropes, which is connected to the enhancement of the aromaticity of the nanotube envelope upon intercalation. The metallization of intercalated peapods is evidenced from the chemical shift anisotropy and spin-lattice relaxation (T-1) measurements. The observed relaxation curves signal a three-component model with two slow and one fast relaxing components. We assigned the fast component to the unpaired electrons charged C-60 that show a phase transition near 100 K. The two slow components can be rationalized by the two types of charged C-60 at two different positions with a linear regime following Korringa behavior, which is typical for metallic system and allow us to estimate the density of sate at Fermi level n(E-F).

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