Carbon Nanotube Field Emitters for Display Applications Using Screen Printing

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1 Materials Science Forum Online: ISSN: , Vols , pp doi:1.428/ 25 Trans Tech Publications, Switzerland Carbon Nanotube Field Emitters for Display Applications Using Screen Printing Hyuk Jung 1,Duck-Jin Lee 1, Hyun-Tae Chun 1, Nam-Je Koh 2, Young-Rae Cho 3, and Dong-Gu Lee 1,a 1 School of Advanced Materials & Systems Engineering, Kumoh National Institute of Technology, Gumi, Korea 2 Device Research Lab., LG.PHILIPS Displays, Gumi, Korea 3 Division of Materials Science and Engineering, Pusan National University, Pusan, Korea 1,a dglee@kumoh.ac.kr Keywords: Carbon nanotubes, field emission display, screen printing, post-treatment Abstract. In this study, a 1"-sized panel with novel tetrode structure was tried to prevent broadening of electrons emitted from CNTs. The structure of the novel tetrode is composed of CNT emitters on a cathode electrode, a gate electrode, an extracting electrode coated on the top of a hopping electron spacer (HES), and an anode. HES contains funnel-shaped holes whose inner surfaces are coated with MgO. Electrons extracted through the gate are collected inside the funnel-shaped holes and hop along the hole surface to the top extracting electrode. The effects of HES on emission characteristics of field emission display (FED) were investigated. An active ozone treatment for the complete removal of residues of organic binders in the emitter devices was applied to the FED panel as a post-treatment Introduction Flat panel display devices have been significantly developed for the overcoming the large and bulky CRT display [1-3]. Among them, FED device is very similar to CRT in principle. Recently, carbon nanotubes (CNTs) have been a good candidate as a field emitter material for the application of FED because of sharp tips of CNTs. The screen printing method is mostly employed in major worldwide companies for the fabrication of a large FED panel using CNTs owing to its large scalability with low cost and simple process [4]. In this study, FED panels using CNTs were fabricated by screen printing and were characterized for different processing conditions such as additives in CNT pastes, post-ozone treatment, and kinds of CNTs (MWNTs or SWNTs). Device Fabrication Figure 1 shows the schematic view of CNT field emitters having a triode and a tetrode structure. As shown in Fig. 1, 1 -sized panels (152 gate line 27 cathode line) were fabricated and the emission characteristics of the panels were investigated in a vacuum chamber at a pressure of ~1-8 Torr. For the fabrication of the field emission triode (Fig. 1(a)), a borosilicate glass is prepared with patterned periodic grooves for maintaining constant distance of cathode-gate electrode. CNT emitters as well as cathode electrodes were formed by screen printing inside the grooves and then dried and fired at 15 o C for 2 mins. and 4 o C for 6 mins., respectively. Gate sheet metal electrodes having 9 holes/subpixel were placed on the glass at the cathode-gate distance of 4 µm and gate-anode distance of 29 µm. For the tetrode structure shown in Fig. 1(b), an insulating plate, which is called a hopping electron spacer (HES), is placed over the gate electrode. On top side of HES, a conductive metal film is All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-21/2/16,6:18:52)

2 189 PRICM-5 formed as an extraction electrode for the extraction of electrons. HES contains also funnel-shaped holes whose inner surfaces are coated with MgO. The MgO material was adjusted to make its secondary electron yield value one. Electrons extracted through a gate are collected inside the holes and hop along the hole surface to the top extracting electrode. Through this HES structure, electrons are more collected and evenly distributed in each subpixel. The CNT pastes for screen printing were formulated with.4-1. wt% of CNTs (MWNTs or SWNTs) and -1 wt% of frit glass powders in an ethyl cellulose-terpineol binder. Then UV-ozone treatment using a mercury vapor lamp (λ=185 nm & 256 nm) was applied to FED panels for -3 mins. for the complete removal of the residues of organic binders in the emitter devices. Results and Discussion For the better and more uniform emission of FED panels, an electrical aging treatment (anode aging and gate aging) was applied. Anode voltage was increased to 2.4 kv at a rate of 4 V/hr and kept at 2.4 kv for 18 hrs with no voltage to gate electrode. Gate voltage was also applied up to 24 V at a rate of 5 V/hr with a pulse mode of 1 Hz and duty of 2%. I-V characteristics of FED panel were measured at an anode voltage of 2 kv and with sweep of gate voltage from to ~2 V at a rate of 5 V/sec. Figure 2 shows I-V curves of 1"-sized FED panels with or without an addition of frit glass powders. With an addition of frits, the turn-on field was increased from 3.3 V/µm 2 to 4.2 V/µm 2 and the emission current density was decreased from 436 µa/cm 2 to 117 µa/cm 2 at a turn-on field of 5 V/µm 2. And the emission uniformity of cathode lines as shown in Fig. 3 was decreased from 72% to 28% in case of the frit addition. Although frit addition provides CNT powders with good adhesion to cathode electrode, the emission characteristics of the panels were worsened. Figure 4 shows the emission images of the 1" FED triode panels with or without frit addition. For the prevention of broadening of electron beams from CNT emitters and collection of electrons without loss, HES having a funnel-shaped holes was placed just over the gate holes [4-5]. Figure 5 shows the gate, anode, and HES currents as a function of gate voltage at different HES voltages. The emission efficiency of this tetrode structure was found to be around 12-14% depending upon the HES voltage. With an increase in HES voltage, more electrons are extracted from the gate. It is noted that HES current is negative. It may that electrons emitted from CNTs collide with MgO inside HES holes, producing secondary electrons. The production of secondary electrons may cause the MgO surface to be positively charged. The top electrode of HES may supply electrons back to the MgO surface for the charge neutralization. It is not clear yet and should be further studied. Figure 6 shows the emission images of lines with or without HES. With use of HES, the broadened electron beams were focused to each subpixel and each subpixel was clearly defined. Because of the use of CNT paste for the screen printing, it is quite difficult to organic binder residues from CNT paste only by firing at 4 o C. Therefore, UV-ozone treatment was tried for -3 mins. to remove contaminants (Fig. 7). The ozone treatment was effective to the improvement of emission characteristics of CNTs compared to non-treated CNTs. However, the increase in treatment time gave damages to the CNT crystallinity, leading to lower emission from the observation of Raman spectra. The short time treatment (1 min.) of ozone was found to good for the emission characteristics of the panels. Conclusion The fabrication of a 1"-sized FED panel using CNTs has been successfully demonstrated. The originality of this study is the forming grooves on the glass substrate for the constant cathode-gate

3 Materials Science Forum Vols distance and the applying the HES structure to the conventional FED triode to collect and focus the emitted electrons from CNTs. Finally, the ozone treatment on a FED panel helped the removal of contaminants of the CNT surface, leading to better emission results.. Acknowledgment The authors would like to acknowledge the financial supports of the Center for Research of High Quality and Automated Processes in Electronic Parts Industry in KIT that is assigned by Korea Science Foundation.. References [1] P. G. Collins and A. Zettl: Phys. Rev., B55(1997), p [2] H. Schmid, and H. W. Fink: Appl. Phys. Lett., 7(1997), p2679. [3] A. G. Rinzler, J. H. Hafner, P. Nikolaev, L. Lou, S. G. Kim, D. Tomanek, P. Nordlander, D. Colbert, and R. E. Smalley: Science, 269(1995), p.269. [4] J. J. Scholtz, D. Dijkkamp, and R. W. A. Schmitz: Philips J. Res., 5 (1996), p [5] R. Balkenede, A. A. M. B. Bogaerts, J. J. Scholotz, R. R. M. Tijburg, and H. X. Willems: Philips J. Res., 5 (1996), p. 365 a) b) Fig. 1, Schematic view of field emitters with (a) triode and (b) tetrode(addition of HES) structure. Current density(µa/cm 2 ) frit no frit Electric field(v/µm) Fig. 2, I-V curves of 1" FED triode panels with or without frit (.4 wt%) additions. (.8 wt% SWNT in ethyl cellulose-terpineol binder) Current(µΑ) frit no frit 4LINE 6LINE 12LINE 18LINE 2LINE Line numbers Fig. 3, Emission current variation of some cathode lines in FED triode panel depending upon the frit (.4 wt%) addition. (.8 wt% SWNT in ethyl cellulose-terpineol binder)

Line with HES Line with HES Line without HES Line without HES b) Current (I G, Current I A,, µa) (I G, I A,, µa) Current density(µa/cm Current 2 ) density(µa/cm 2 ) 8 6 8 4-6 16 18 2 22 24 26 28 6-2

4 1892 PRICM-5 Efficiency a) ( ) HES Voltage Gate Voltage=28 V 4 V 5 V Gate Current, I G (μ A) 8,796 8,917 (87.8%)(86.5%) Anode Current, I A (μ A) 1,258 1,437 (12.5%)(13.9%) HES Current, (μ A) Total Current (μ ( A) 1,28 1,36 Fig. 4, Emission images of 1"FED triode panels (a) with or (b) without frit (.4 wt%) addition. Line with HES Line with HES Line without HES Line without HES b) Current (I G, Current I A,, µa) (I G, I A,, µa) Current density(µa/cm Current 2 ) density(µa/cm 2 ) I I G I G A I I G I G A HES 18Voltage= V HES Voltage=5 V I A 28 HES Voltage=4 V + HES Voltage=5 Gate Voltage V (V G, V) Fig. 16 5, I-V 18curves 2 of 22a FED 24 tetrode Gate Voltage (V G, V) panel depending upon the HES voltage. (.4 wt% SWNT in ethyl cellulose terpineol binder, no frit) m in 1m in 1m in 3m in m in 1m in 1m in 3m in Appiled Electric field(v /µ m ) 5 m in 1 m in 2 b) 1 m in m in Appiled Electric field(v /µ m ) a) I A 5 Fig. 6, Emission images of cathode lines in FED tetrode panel with or without HES. (.4 wt% SWNT in ethyl Fig. cellulose-terpineol 6, Emission images binder, of no cathode frit) lines in FED tetrode panel with or without HES. (.4 wt% SWNT in ethyl cellulose-terpineol binder, no frit) Appiled Electric field (V /µ m ) Fig. 7,I-V curves of (a)mwnts (2 wt% MWNT ethyl-cellulose+terpineol binder, no frit) and (b) SWNTs (.4wt% SWNT in ethyl-cellulose+terpineol binder, no frit) for different ozone treatment time.

5 PRICM / Carbon Nanotube Field Emitters for Display Applications Using Screen Printing 1.428/ DOI References [4] J. J. Scholtz, D. Dijkkamp, and R. W. A. Schmitz: Philips J. Res., 5 (1996), p /S (97)

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