New-Type Planar Field Emission Display with Superaligned Carbon

Apr 11, 2012 - Carbon Nanotubes for Displaying. K. Jiang. 2017, 101-127 ... Li-Hu Huang , Jyun-An Gong , Yuan-Yao Li. Vacuum 2016 125, 13-20 ... M. Ca...
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Letter pubs.acs.org/NanoLett

New-Type Planar Field Emission Display with Superaligned Carbon Nanotube Yarn Emitter Peng Liu, Yang Wei, Kai Liu, Liang Liu, Kaili Jiang, and Shoushan Fan* Tsinghua-Foxconn Nanotechnolgy Research Center, Department of Physics, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: With the superaligned carbon nanotube yarn as emitter, we have fabricated a 16 × 16 pixel field emission display prototype by adopting screen printing and laser cutting technologies. A planar diode field emission structure has been adopted. A very sharp carbon nanotube yarn tip emitter can be formed by laser cutting. Low voltage phosphor was coated on the anode electrodes also by screen printing. With a specially designed circuit, we have demonstrated the dynamic character display with the field emission display prototype. The emitter material and fabrication technologies in this paper are both easy to scale up to large areas. KEYWORDS: Carbon nanotube yarn, planar type structure, field emission display, laser cutting, screen printing uperaligned carbon nanotube (CNT) yarn and film have supplied the possibility for the fabrication of CNT devices at large area scale with high uniformity.1−3 For the advantages such as fast response, monochromatic, high brightness, and low power consumption,4−7 field emission is a very hot topic for the application of nanomaterials, especially for one-dimensional materials8−10 due to their high aspect ratio. Field emission of CNT has been widely studied for their exceptional field emission properties,8,9,11−19 and it has been reported that superaligned CNT yarn has shown excellent field emission properties.20−22 The application of superaligned CNT yarn in field emission is possible to facilitate the fabrication of large area field emission display (FED) with improved performance, which requires a low cost fabrication process for mass production. The CNT yarn is difficult to integrated into the conventional vertical FED structures, which is adopted by the CNT paste15−19 and Spindt tip.6,7,23 Itoh and et al. have fabricated a planar type field emission structure in ref 24. Canon and Toshiba have fabricated surface-conduction electron-emission display (SED),25,26 which is also a planar type structure. Invoked by the planar type design, we have also proposed a planar type FED with CNT yarn emitter that can be fabricated with the state of art of screen printing and laser cutting technologies for CNT film devices.27,28 Both of the technologies are low cost, high efficiency, and easy to scale up to large area samples. We have fabricated a simple diode FED structure. The emission properties can be adjusted by varying the structure geometry parameters. For the sample shown in this paper, the emission current and brightness of the 16 × 16 emitter structure are centered at 0.5 μA and 30 cd/m2, respectively. The power consumption at all-on is only 30 mW. Uniformity can be also adjusted for the planar structure even after vacuum sealing. The grayscale of the diode FED can be modulated by the voltage amplitude or pulse width. It was

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shown that the FED prototype can display Chinese characters dynamically. Figure 1a shows the traditional vertical triode FED structure that the Spindt type emitter adopted. Several CNT FEDs have also adopted this type of structure.15−19 However, the fabrication process is complex and the post-treatment of the CNT emitters under the gate electrode is difficult.15−19 As has been shown, superaligned CNT yarn shows excellent field emission ability.20−22 However, it is difficult to integrate the CNT yarn emitter into the conventional vertical field emission structure. We have designed a planar FED structure. It is a diode structure. The difference between our structure and the several previous planar structures24−26 is that we have coated phosphor on the electrode countered to the emitter. The schematic illustration was shown in Figure 1b. CNT yarn emitters are almost in the same plane with the anode. This structure can utilize almost all the electrons to impinge phosphor and emit visible light. The structure has a high mechanical stability due to the high Young’s modulus29,30 and small mass density of the CNT yarn. When the electrons are emitted from the CNT yarn after applying the electric field between the anode and cathode, they will impinge the phosphor coated on the anode. Visible light is emitted from the phosphor and can be used as a display. Compared with conventional vertical FED, the fabrication process can be simplified. The structure can also be adopted by other kinds of FED with one-dimensional or two-dimensional emitter materials. Figure 1b shows the three subpixels of red-greenblue (RGB) colors. Received: January 29, 2012 Revised: March 29, 2012

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Figure 1. (a) Traditional vertical triode FED structure. The dielectric part has not been labeled out. (b) Planar type FED structure. RGB subpixels were also shown. (c) Schematic illustration of fabrication process of planar type CNT yarn FED structure. From top to bottom: screen printing electrodes, placing the CNT yarn, fixing the yarn and printing the phosphor, laser cutting to form emission gaps. (d) Model of laser cutting to form the emission gap.

Figure 2. (a) SEM of CNT yarn FED structure, scale bar 100 μm. The inset is the enlarged image of the tip, scale bar 5 μm. (b) TEM images of the yarn tip, scale bar 2 μm. The inset is the enlarged image of a separated CNT tip, scale bar 20 nm. (c) Statistics distribution of the gap width. (d) Statistics distribution of the yarn tip cone angles.

Figure 1c shows the schematic illustration of the fabrication process for the planar FED. Two low-cost fabrication technologies (screen printing and laser cutting) were mainly adopted. The electrode was first formed on a substrate; then the CNT yarn was placed on it, and a top layer was used to fix the yarn. The formation of the electrodes, insulator, phosphor, and the fixing layer of the CNT yarn emitter can all be fulfilled with screen printing. Phosphor was also coated by screen printing. After that, a scanning laser was used to burn out the emission gap, as shown in Figure 1d. The formation of the emitter tip and gap was attributed to burning process due to

laser heating in atmosphere ambience. The detailed mechanism of emission gap formation was analyzed in Supporting Information. The shape of the tip can be controlled by the laser cutting parameters. The scanning speed of laser beam can be as fast as several meters per second, thus the high efficiency of fabrication process can be ensured. Precise position control can be achieved by the precise movement of the laser beam or the movement of station table. As is well-known, the sharp tip can focus the electric field and thus facilite the electron emission. Parts a and b of Figure 2 are the scanning electron microscopy (SEM) and transmission B

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Figure 3. (a) Microscopy image of an individual pixel. (b) I−V and F−N curves of the field emission. (c) Brightness changes with emission voltage. (d) Brightness changes with pulse width at the pulse mode.

Figure 4. Field emission properties variation with the gap geometry parameters. (a) On-voltage and on-field for different gap width. (b) Simulated electric field magnitude distribution for different gap width. The inset is the enlarged images of the electric field distribution at the tips. (c) Electric field in front of the cathode 100 nm away at the different electrode distance and the electric field enhancement factor. (d) On-voltage and on-field for different gap position. The position is defined as the distance from the tip to and anode. (e) Simulated electric field magnitude distribution for different gap position. The inset is the enlarged images of the electric field distribution at the tips. (f) Electric field in front of the cathode 100 nm away for different gap position and the electric field enhancement factor.

electron microscope (TEM) image of the CNT yarn emitter. The yarn has formed to a very sharp tip. The sharp tip can lower the field emission voltage. The inset of Figure 2b shows a magnified TEM image of tips. It can be seen that there are still many tips even at the micro dimension, and the CNTs in the tip are mostly close-ended. Therefore, better emission stability can be expected according to the reports.8,31 A uniform tip shape distribution can be achieved by controlling on the laser cutting parameters. Parts c and d of Figure 2 show the statistics of the gap width and the yarn tip cone angles. They are centrally distributed. Figure 3 shows the field emission of an individual pixel. Figure 3a is the microscopy image of a pixel during emission.

The phosphor emitted bright light due to the electron impingement. Figure 3b is the I−V curve. Inset shows the Fowler−Nordheim (F−N) (LnI/V2 − 1/V) curve. The straight F−N line validates the field emission. We have shown that the brightness can be adjusted by the amplitude of emission voltage or the pulse width. Figure 3c is the brightness variation with voltage amplitude. Figure 3d is the brightness of a pixel under pulse drive of different pulse width. It shows the display can also be used in grayscale display even at the simple diode state. The field emission properties of the structure varied with the geometry parameters such as the gap width and position. Figure 4a shows the field emission on-voltage and on-field at different gap width. The on-voltage was defined as the voltage where C

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Figure 5. Chinese characters displayed by the planar type CNT yarn FED prototype. These characters are the Chinese characters of “TsinghuaFoxconn Nanotechnology Research Center”.

Figure 6. (a) CNT yarn emitter before and after laser treatment. The tip after treatment becomes sharper than the original. (b) Field emission of the identical pixel before and after laser treatment. (c) and (d) are the field emission statistics of the 16 × 16 pixels before and after laser treatment. The emission properties such as the F−N slope and emission brightness show a more centralized distribution.

be seen that the field enhancement factor increases with the gap width. The result is accordant with the theoretical prediction32−34 due to the field enhancement factor increase with the gap. We have also studied the on-voltage and on-electric field variations with the gap position. The gap position was defined as the distance from the CNT yarn tip to anode. The results were shown in Figure 4d. The gaps are about 19.6 ± 3.2 μm width. We can see that the on-voltage and on-field both increase with the distance to anode. Figure 4e shows the simulated electric field magnitude distribution for different gap position. The simulated parameters are same with those in Figure 4b. The inset also shows the enlarged the electric field distribution at the tips. However, the electric field variation is not as obvious as for different gap widths. Figure 4f shows the electric field amplitude at the position 100 nm in front of the tip and the field enhancement factor for different gap position.

there is abrupt current increase. The on-field was defined as the on-voltage over the gap width. For a different sample, the threshold current to define an abrupt increase varies from 10 to 30 nA. The on-voltage increases with the gap width, whereas the on-field decreases with the gap width. A simulation about the electric field distribution for different gap width can explain this phenomenon. Figure 4b shows the simulated electric field magnitude distribution at different gap width. The left side is the cathode, and the right is the anode. The cathode was set to 0 V, and the anode was set at 200 V. The inset of Figure 4b shows the enlarged electric field distribution at the tips. The electric field variation can be clearly seen. Figure 4c shows the electric field amplitude at the position 100 nm in front of the tip and the field enhancement factor at different gap width. The field enhancement factor was defined as the local electric field over the value that the voltage divided by the gap width. It can D

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Figure 7. Superaligned CNT film planar field emitter. (a) SEM of an individual superaligned CNT film emitter. The CNT film was cut into the sawtooth shape, scale bar 500 μm. The inset is the enlarged image of one tip, scale bar 20 μm. (b) Field emission I−V and F−N curves of the CNT film field emitter.

the emission area are both improved to 1.2 times and 4.5 times of the original based on the F−N theory by substituting the work function of CNT.36,37 For the FED of 16 × 16 pixels, such a further laser treatment can improve the uniformity by enhance the emission of the original poor emission pixels. Parts c and d of Figure 6 show the distribution comparison before and after laser treatment. We can see that both of the F−N slopes and the brightness of each pixel after the laser treatment are more centrally distributed than the original one. Because the yarns are very light and thin, the large area sample fabrications can be easily realized with a yarn-placing machine (it is being designed now). Furthermore, except the CNT yarn FED, the CNT film can also be used to fabricate FED, which can be easily scaled up to large area. The CNT film emitter structure and emission properties are shown in Figure 7. The detailed result with CNT film emitter has been shown in the Supporting Information. Therefore, the scale-up of the FED with the superaligned CNT emitter will be easy. However, there are still many problems that need to be solved for the real application of our sample. As we have mentioned above, the most difficult and most important one is how to improve the emission uniformity. Except for that, the lifetime of the device is also a key problem. The elongation of lifetime not only requires better emitter and structure but also needs the improvement of supporting technologies, such as a better vacuum. In our planar type structure, there is no ultrahigh voltage. It is also good to the long lifetime. A further problem is the efficiency of the phosphor. Because the emission voltage is only 1−300 V (Figure 4a,d), the high voltage phosphor does not have the best efficiency. We have adopted the low voltage phosphor, but the color representation of the low voltage phosphor is not as good as the high voltage ones. In future applications, full color display may need extra attempts in phosphor selection. However, as with any new technology, although there are still many problems at present, we believe it is still worth to try. In conclusion, we have fabricated a new type of planar field emission display with the screen printing and laser cutting technologies with the superaligned CNT yarn as emitters. With a suitable laser cutting parameter, we can get very sharp tip yarn emitters with uniform distribution. The field emission properties of different geometries were studied. The emission uniformity can be further improved by laser treatment even after vacuum sealing. A 16 × 16 pixels FED prototype can dynamically display Chinese characters.

It can be found that both the on-field and on-voltage decrease with the distance to anode. The result is accordant with the experiment result in Figure 4d. We have fabricated a 16 × 16 pixels planar type CNT yarn FED prototype. The prototype is evacuated and sealed. Evaporable getters were used to sustain the vacuum. For the sample shown in Figure 2a, the gap width is 50 μm and the distance from tip to anode is 100 μm, and most of the pixels show emission current and brightness of 0.5 μA and 30 cd/m2, respectively. The all-on power consumption is only 30 mW. With a homemade drive circuit, Figure 5 shows the characters displayed by the panel. A dynamic display was also shown in the Supporting Information. The drive voltage is about 280 V. The phosphor is the low-voltage green phosphor (ZnO:Zn). The colorful display can be to realize by using RGB phosphors. However, it can be found that there are still nonuniformity and crosstalk for the sample. The emission nonuniformity of each pixel may be the most direct reason. It is also the most difficult one to solve. As for other kinds of field emission display, a negative feedback resistive layer may be a possible solution for this problem.6 The nonuniformity of the phosphor and the pixel structure geometry may be also the reason for the image nonuniformity. They can be solved by improved fabrication technology and process. A correction in drive circuit may be also helpful to improve the uniformity.35 The crosstalk in our sample is different from that in other kinds of display. Only the pixels to the right of selected one are emitting light. For the sample shown in Figure 5, the left part in one pixel is the emitter, and the right is the phosphor-coated anode. Therefore, there may be some electrons which have overcome the pixel boundary. We have found that a further shielding electrode is useful to alleviate this phenomenon. The crosstalk in our structure may be easy to solve. A problem of the device production is the performance optimization after the sample fabrication. Compared with the vertical FED structure, the planar CNT yarn FED can be easily post-treated even after vacuum-sealing with the laser. Such a further laser treatment can enhance the field emission of the poor pixel. Figure 6a shows the yarn tip morphology before (upper) and after (lower) laser treatment. The tip becomes much sharper than the original one. The laser can trim the abundant CNTs by the high temperature process in vacuum. Figure 6b shows the field emission properties comparison before and after the laser treatment. The emission voltage is greatly lowered. The inset shows the correspondent F−N curves. It can be derived that the field enhancement factor and E

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ASSOCIATED CONTENT

S Supporting Information *

Description of the bending deflection of the carbon nanotube (CNT) yarn in the planar field emission structure with related figures and a table of laser parameters. Description of the CNT film emitter and FED with a related figure. Two movie files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-10-62772769. Fax: 8610-62772457. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Duanliang Zhou, Xuewei Guo, and Chao Xu for their help in sample fabrication, drive circuit design, and vacuum sealing. This work is financed by the National Basic Research Program of China (2012CB932301), NSFC (51102144, 51102147, 50825201), and the China Postdoctoral Science Foundation (project 20070420026, 200801077).

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