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Studies On Preparation, Physical Properties And Related Devices Of One-Dimensional Carbon/Carbon Nitride Nanostructures

Posted on:2009-07-21Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y C ZhaoFull Text:PDF
GTID:1101360245464658Subject:Condensed matter physics
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In this thesis, we have optimized the parameters of floating catalytic chemical vapor deposition (FCCVD) technique, and prepared single-walled carbon nanotubes (SWNTs) with several different morphologies. Their related physical properties have also been investigated. Meanwhile, we have fabricated suspended SWNT devices, and measured the electrical transport properties of the devices. Moreover, we have reported the large-scale synthesis of the nitrogen-rich carbon nitride micro/nanofibers. The main results of this thesis include the following four parts.Firstly, by using FCCVD method, we synthesized SWNTs with three different morphologies: individual SWNTs, SWNT networks and SWNT films. The morphology, structure and diameter distribution of the as-prepared SWNT products were characterized by using several techniques. Individual SWNTs were directly deposited on Au-patterned Si substrates, which allowed us to carry out comparative studies on surface-enhanced Raman scattering (SERS) and normal Raman scattering (NRS) from the different segments of an individual SWNT. It suggested that SERS is powerful for detecting Raman signals from individual and isolated SWNTs.Secondly, we directly deposited individual SWNTs on the Si/SiO2 substrates covered with a ~100-nm-thick polymethylmethacrylate (PMMA) film, and then used them to fabricate suspended SWNT devices. Electrical transport properties of the SWNT devices were investigated at both room temperature and low temperature. Ambipolar behavior was measured in some of the semiconducting SWNT devices, and Coulomb oscillations were also observed when experimental temperature was below 60 K. Our results indicated that Schottky barrier between nanotube and metal electrodes was significant for the electrical properties of the suspended SWNT devices. Moreover, we used focused ion beam (FIB) to open both ends of the SWNT in the nanotube device, and thus water molecules can enter the inner channel of SWNT when the device is exposed to the water vapor. As a voltage/current was applied on one part of the SWNT, an electromotive force was clearly detected along the other part of the same nanotube. We suggested the electromotive force can be induced by the mutual coupling between the free charge carriers of the SWNT and the water dipoles inside the nanotube channel. Thus, individual water-filled SWNTs can be used as hydroelectric power converters.Thirdly, the prepared SWNT films were post-treated by using a series of diamond wire drawing dies, and the obtained SWNTs were highly dense and perfectly aligned. X-ray diffraction (XRD) indicated that the highly dense and perfectly aligned SWNTs (HDPA-SWNTs) formed a two-dimensional triangular lattice with the lattice constant of 19.62 ?. A sharp (002) diffraction peak was also observed. Raman spectra revealed that the radial breathing modes (RBMs) of nanotubes with larger diameters in the HDPA-SWNTs were remarkably suppressed. The HDPA-SWNTs have large density, low resistivity and excellent photoconductance properties.Finally, we reported the first synthesis of the nitrogen-rich carbon nitride micro/nanofibers via a thermal evaporation method. Our successful synthesis is attributed to both the high thermal stability of the carbon nitride precursor and the lower experimental temperature, which allowed the CNx (x > 1) nucleus in the precursor remain stable during vapor transfer. They acted as the basic units for one-dimensional structure assembling, and thus made the produced micro/nanofibers nitrogen-rich. The products had graphitic structure, and exhibited sp2 hybridized bonding feature. Blue photoluminescence was observed with a broad full width at half maximum (FWHM), which was closely related to the high nitrogen content and weekly ordered structure of the micro/nanofibers. Our development of a vapor-phase synthesis should be helpful for synthesizing other nitrogen-rich nanostructures and nitrogen-doped carbon nanotubes.
Keywords/Search Tags:single-walled carbon nanotubes, floating catalytic chemical vapor deposition, Raman scattering, individual SWNT device, Schottky barrier, Coulomb oscillation, water-filled SWNTs, energy converter, aligned SWNTs, carbon nitride, micro/nanofibers
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