Coaxial Nanocable: Silicon Carbide and Silicon Oxide Sheathed with Boron Nitride and Carbon
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Iijima S., Ichihashi T., ibid. 363, 603 (1993).
Terrones M., et al., ibid. 257, 576 (1996).
; Y. Miyamoto A. Rubio S. G. Louie M. L. Cohen ibid. p. 18360; X. Blase
M. S. Dresselhaus G. Dresselhaus P. Eklund Science of Fullerenes and Carbon Nanotubes (Academic Press New York 1996). Experimental verification with scanning tunneling microscope has been reported recently by J. W. G. Wildöer L. C. Venema A. G. Rinzler R. E. Smalley and C. Dekker [ Nature 391 59 (1998)] and T. W. Odom J.-L. Huang P. Kim and C. M. Lieber [ ibid. p. 62].
Ajayan P. M., et al., ibid. 362, 522 (1993);
Han W., et al., ibid. 265, 374 (1997);
Guerret-Piécourt C., Le Bouar Y., Loiseau A., Pascard H., ibid. 372, 761 (1994);
In the experiment the secondary harmonics beam (wavelength of 532 nm) from a pulsed Nd:yttrium-aluminum-garnet laser (Qanta-Ray GCR-200; Spectra-Physics Mountain View CA) was focused on the target surface to get an energy density of about 3 J/cm 2 per shot. The target was placed at the center of a quartz reactor tube heated to 1200°C. The flow rate of nitrogen carrier gas was 300 standard cubic centimeters per minute and the system pressure was 500 torr. At high temperature SiO vapor can be produced by a reaction between Li + ions and the quartz tube during laser ablation. Thus we could still obtain a similar result using a target without SiO powder. We also found that the result is not sensitive to the concentration of carbon in the target.
Microscopic observation was performed with a high-resolution transmission electron microscope (Topcon 002B) working at 200 kV.
Although the diameter is homogeneous throughout a wire the size distribution is diverse among different wires. The core has a lateral dimension in the same order with the thickness of the amorphous layer which varies from 3 to 20 nm depending on the diameter of the wire. The outer graphitic sheath usually contains 4 to 15 shells which are 1.4 to 5.1 nm in thickness independent of the wire diameter.
A scanning transmission electron microscope [VG-HB501 (VG Microscopes Sussex England)] working at 100 kV equipped with a parallel energy-loss spectrometer [Gatan (Warrendale PA) PEELS 666] was used. The spatial resolution was about 0.5 nm. See
R. F. Egerton Electron Energy-Loss Spectroscopy in the Electron Microscope (Plenum New York ed. 2 1996).
A slight disaccord between the B and N profiles at the center point is due to the quantification problem of the B K edge spectra. The long tail of L edge from Si present at the center of tubes overlaps the B K edge and affects the background subtraction there.
The reference spectra were provided by D. Imhoff and N. Brun.
SiC is a large gap (3 eV) semiconductor SiO 2 and BN tubes are insulating and carbon nanotubes are metallic or semiconducting (7).
T. S. Bartnitskaya V. I. Lyashenko A. V. Kurdyumov N. F. Ostroskaya I. G. Rogovaya Powder Metallurgy Metal Ceram. 33 (nos. 7 to 8) 335 (1994).
This work was partially supported by the Special Coordination Funds of the Science and Technology Agency of the Japanese Government and a New Energy and Industrial Technology Development Organization international research grant. The Sumitomo Foundation is acknowledged by K.S.