Field emission vacuum electronic devices utilizing ultrathin carbon nanotube sheet
- PDF / 2,437,816 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 81 Downloads / 275 Views
130
Figure 4: Process of CNT FE lateral device fabrication. DISCUSSION SEM and TEM images provide more details about the CNT emitters produced by this method. Firstly, the fabricated emitters were uniform in term of the alignment and the length of emitters. Secondly, the CNTs synthesized by CVD were MWCNTs with 4-5 walls (Figure 5b). Under the irradiation of laser, emitter structure was modified into double-walled CNTs (Figure 5c). In addition, thermal effect bundled CNTs together, making it well separate, which further reduced the screening effect.
(a) (b) (c) Figure 5: (a) SEM image of emitter array cut by laser, (b) TEM image of as-synthesized MWCNTs with 5 walls, (c) TEM image of CNT tip with 2 graphitic walls. To verify the effect of the thickness on field emission performance, the devices were made with sheets stacked by 4 and 12 layers CNT sheets, and with buckypaper. The buckypaper was made by vacuum filtration of MWCNT suspension. The CNTs in the buckypaper were randomly distributed. The buckypaper was about 9 μm thick. The 4-layer and 12 layer CNT emitters did not show much difference although they were with different thicknesses (Figure 6a). The threshold fields corresponding to the device with the sheet thickness of 4 layers (210 nm), 12 layers, and with buckypaper (9 μm) are 0.67 V/μm, 0.68 V/μm, and 1.22 V/μm, respectively. The 4-layer and 12-layer sheet devices performed the field enhancement factor of 21687 and 20945, respectively. The reason was because after the laser cutting process, nanotubes along the edge of the sheet were bundled together (Figure 6b). Thus, the difference of number of layers did not affect the thickness of emitter array. On the other hand, buckypaper showed less effective emission behavior. Observing the SEM images shows that the edge of buckypaper shows layers of emitters, which are randomly aligned. Thus, the screening effect of buckypaper was worse than that of stacked CNT sheets.
131
(a)
(b) Figure 6: FE devices utilizing CNT sheets with different thicknesses: (a) current density performance, (b) SEM images of corresponding sheets. CNT sheets with different orientations were fabricated. Figure 7a shows that the more aligned CNTs to emission direction result in the better FE performance. The threshold fields corresponding to the orientation of 5o, 40o, and 60o are 0.67 V/μm, 0.86 V/μm, and 0.98 V/μm, respectively. Different alignments of the sheets created different alignment and distribution of emitter array. Under the electrostatic field, the emitters were aligned towards anode, which was parallel to the electrostatic force [3]. The smaller angles that nanotubes align relatively to the emission direction make that phenomenon less effective, resulting in the better FE performance.
(a)
(b) (c) (d) Figure 7: FE devices utilizing CNT sheets with different orientations: (a) current density performance, (b) SEM images of corresponding sheets.
132
The change of sheet structures over the time was tested with different levels of current density (Figure 8a). Figure 8b
Data Loading...
