Field emission properties of carbon nanotubes with different morphologies
Introduction
Since the discovery of carbon nanotubes (CNTs) [1], several methods for synthesizing two types of CNTs (i.e., base model and tip model), such as thermal chemical vapor deposition (CVD) [2], [3], and arc discharge and laser vaporization [4], [5], [6], have been examined and reported on by previous research groups. The plasma-enhanced CVD (PECVD) method [7] constitutes one of the most promising candidates for the synthesis of multiwall CNTs (MWNTs), which obtains good vertical alignment on a large area at low temperature [8], [9]. Considerable interest has evolved in potential applications of the PECVD technique for the production of MWNTs in vacuum microelectronics [10], [11]. In addition, a great deal of effort is being devoted to the use application of CNTs in the fabrication of a wide range of field emitters for field emission displays [12], [13], [14] due to their high aspect ratio, mechanical strength, and remarkable thermal conductivity. However, the close proximity of the CNTs' tips reduces the field enhancement compared to that of an isolated tube due to electrical screening [15], [16]. The mechanism of field emission of CNTs, however, remains an open question to date. Previous investigation has studied the effects of the essential parameters of CNTs, such as differences in nanotube orientation [17], tip morphologies [18], and the length and density of nanotubes [19]. Less attention, however, has been paid to the effect of the different morphologies of CNTs. This paper reports on the field emission results of the base-model well-aligned multiwall carbon nanotubes (Base-CNTs), curled multiwall carbon nanotubes (Curled-CNTs), and tip-model well-aligned carbon nanotubes (Tip-CNTs), all of which were prepared by direct current PECVD. This study also describes an effective method for improving the geometrical structure of Curled-CNTs and enhancing their emission through an argon ion irradiation treatment.
Section snippets
Experiment
MWNTs were grown on Ni-catalyzed Si substrates by means of DC PECVD with different pre-treatment ammonia (NH3) plasma currents. Acetylene (C2H2) and NH3 were used as the carbon source and etchant gas, respectively. During the growth of the MWCNTs, the gas flow rates of NH3 and C2H2 were observed to be 180 and 60 sccm, respectively. The substrate temperature was about 600 °C. The nickel (Ni) catalyst was evaporated on an n-type silicon (Si) wafer via DC magnetron sputter deposition. The
Results and discussion
Fig. 1 shows SEM images corresponding to the Base-CNTs, Curled-CNTs, and 44Tip-CNTs prepared using NH3 plasma currents of 60, 40, and 20 mA, respectively, during the NH3 pre-treatment. It was observed that the morphology parameters of the MWNTs were governed by the NH3 plasma current. Table 1 summarizes the growth conditions and raw data pertaining to the tube length and diameter of each sample, which were used for the SEM and TEM measurements. An in-situ monitoring method confirmed that the
Conclusions
MWNTs with different morphologies can be synthesized by adjusting the plasma current during NH3 pre-treatment, using DC PECVD. The field emission behavior of Base-CNTs, Curled-CNTs, and Tip-CNTs were compared and analyzed. The Base-CNTs exhibited a higher emission current and lower turn-on voltage than the Curled-CNTs and Tip-CNTs. Similarly, the field enhancement factor γ calculated from an F–N plot was higher in the case of the Base-CNTs than the Curled-CNTs and Tip-CNTs. The enhanced
Acknowledgment
This work was supported by KERI (Korea Electrotechnology Research Institute) and also supported by grant No. RTI04-03-02 from the Regional Technology Innovation program of the ministry of commerce, industry and energy(MOCIE).
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