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We present synthesis, structure and field emission properties of a novel hybrid carbon nanomaterial, NanoBudsTM, combining fullerenes and single-walled carbon nanotubes (CNTs) . NanoBuds consist of fullerenes attached to the outside surface of CNTs, i.e. nanotubes are functionalized with fullerenes. Two floating catalyst methods for their synthesis were developed, using pre-made iron catalyst particles by a hot wire based PVD method or grown in situ via ferrocene vapour decomposition in the presence of CO and trace amounts of H2O and CO2. TEM images show spherical structures i.e. fullerenes at the surface of the tube. Statistical size measurements on the basis of HR-TEM images revealed that the majority of fullerenes consists of C42 and C60. Interestingly, evidence of C20 fullerenes, the smallest possible dodecahedra is found. Raman spectra show a pronounced G-band at 1600 cm-1 associated with CNTs, and only a weak D-band at 1320-1350 cm-1. The main peaks in MALDI-TOF spectrum are attributed to C60 (C60H2, C60H2O) and C42 (C42COO) fullerenes. Accordingly, fullerenes are attached to CNTs via either oxygen (preferable for fullerenes larger than C54) or carboxylic (for smaller fullerenes) bridges, which was confirmed by FT-IR measurements. EELS observations with TEM also showed the existence of oxygen. Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements of samples deposited on Au(111) substrate confirmed the covalent nature of fullerene bonding to the tube. Atomistic density-functional-theory based calculations showed that systems composed of fullerenes and nanotubes with single vacancies covalently functionalized through ester groups can indeed exist. In-situ deposited i.e. non-purified planar NanoBud mats showed stable cold electron field emission with a current density of 189 µA/cm2 at 1.26 V/µm. The threshold voltage was about 0.6 V/µm, compared to over 2 V/µm for similarly produced planar nanotube mats.
1. A. G. Nasibulin, P. V. Pikhitsa, H. Jiang, D. P. Brown, A. V. Krasheninnikov, A. S. Anisimov, P. Queipo, A. Moisala, D. Gonzalez, G. Lientschnig, A. Hassanien, S. D. Shandakov, G. Lolli, D. E. Resasco, M. Choi, D. Tománek, and E. I. Kauppinen, (2007) A Novel Hybrid Nanomaterial, Nature Nanotechnology 2(3), 156-161.
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