Investigation on the Plasma-Induced Emission Properties of Large Area Carbon Nanotube Array Cathodes with Different Morphologies
© Liao et al. 2010
Received: 22 June 2010
Accepted: 9 September 2010
Published: 28 September 2010
Large area well-aligned carbon nanotube (CNT) arrays with different morphologies were synthesized by using a chemical vapor deposition. The plasma-induced emission properties of CNT array cathodes with different morphologies were investigated. The ratio of CNT height to CNT-to-CNT distance has considerable effects on their plasma-induced emission properties. As the ratio increases, emission currents of CNT array cathodes decrease due to screening effects. Under the pulse electric field of about 6 V/μm, high-intensity electron beams of 170–180 A/cm2 were emitted from the surface plasma. The production mechanism of the high-intensity electron beams emitted from the CNT arrays was plasma-induced emission. Moreover, the distribution of the electron beams was in situ characterized by the light emission from the surface plasma.
In the past few years, carbon nanotubes (CNTs) have been extensively investigated due to their remarkable structures and excellent properties . They have also been identified as potential materials for a broad range of useful devices [2, 3], especially in the area of field emission devices [4–7]. CNT arrays always have attracted considerable attentions as ideal electron emitters for their excellent field emission properties [5–7]. Many new field emission devices based on CNT arrays were fabricated successfully. In the previous CNT-based devices studies, CNT arrays mainly were applied to the weak current devices under direct current (DC) electric fields. It is well known that plasma-flashover cathodes can generate intense-current electron beams under pulse electric fields and have been used extensively in high-power microwave tubes and accelerators [8–10]. As is mentioned above, CNT arrays have great potentials for the applications of plasma-flashover cathodes due to their excellent field emission properties [5–8]. Whereas the reports that focus on the plasma-induced emission properties of CNT arrays under the high-voltage pulse electric field are very few. Therefore, the studies on the field emission properties of CNTs under the pulse electric field are very important as well as under the DC electric field.
Here, we report the plasma-induced emission characteristics of CNT arrays under the high-voltage pulse electric field. The effects of the ratio of CNT height to CNT-to-CNT distance on the electron emission properties of the CNT arrays were investigated. Moreover, the distribution of electron beams was in situ characterized by light emissions from the plasmas. The production mechanism of the electron beams emitted from the CNT arrays was studied and explained.
Large area CNT arrays have been grown on substrates by a chemical vapor deposition method [5, 11], and 2-in. silicon wafers were used as the substrates. Briefly, a 10-nm Al2O3 layer acting as barrier layer was formed on the substrate surface by evaporation. Then, a 5-nm-thick Fe catalyst layer was e-beam evaporated onto the substrate surface. Finally, the substrates were inserted into the center of a quartz tube furnace. The furnace was heated to about 700°C in the mixed flow of the acetylene and hydrogen. Uniform well-aligned CNT arrays on the 2-in. silicon wafers can be obtained, and the heights of the arrays can be controlled by tuning growth conditions. The height of the as-grown CNT arrays depends on growth time. The growth times of different CNT arrays range from 10 to 80 min. Four kinds of arrays with different CNT heights in the range of 4–16 μm were employed in our experiment. The surface morphologies of the CNT arrays were analyzed by a field emission scanning electron microscopy (SEM). A high-resolution transmission electron microscope (HRTEM) was used to further characterize the synthesized CNTs.
The fabricated samples were placed on copper stages by electrically conductive glue and fixed by copper rings. The CNT arrays were adhere onto the copper stages and assembled into cathodes. Then, the CNT array cathodes were used to next plasma-induced emission tests under the pulse electric field. The high-voltage pulse emission experiments were performed in a diode powered by a pulse-forming network generator at background pressure of 5×10-4 Pa [12, 13]. The generator has an output double-pulse with about 100-ns duration, and the interval between two pulses was about 400 ns. The anode–cathode gap was 98 mm. During the emission process, the light emission from the CNT array cathode was in situ observed by a charge-coupled device (CCD) camera.
Results and discussions
Typical results of plasma-induced emission from the ZnO nanorod array cathode
1.06 × 10-6
8.75 × 10-6
6.75 × 10-7
5.33 × 10-6
5.91 × 10-7
3.51 × 10-6
3.11 × 10-7
1.50 × 10-6
In this study, large area well-aligned CNT arrays with different morphologies were fabricated. The plasma-induced emission properties of the CNT arrays with different CNT heights under the pulse electric field have been investigated. The ratios of CNT height to CNT-to-CNT distance have considerable effects on their electron emission properties. As the ratios increase, the emission currents of the CNT arrays decrease due to the screening effects. Plasmas formed on the array surface during the emission process, and high-intensity electron beams of about 170–180 A/cm2 were obtained from the CNT arrays. CNT arrays are excellent candidate as intense-current electron beam sources and can be applied to high-power vacuum electronic devices in the near future.
This work was supported by the National Basic Research Program of China (No. 2007CB936201), the National Natural Science Foundation of China (Nos. 10876001, 51002008), the Beijing Natural Science Foundation (No. 2082015), and the Major Project of International Cooperation and Exchanges (Nos. 50620120439, 2006DFB51000).
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