Application of Carbon Nanotubes for Displays Jin Won Cha May 10, 2006 MatE 297 Application of Nano Materials San Jose State University Instructor: Dr. Zhen Guo 1. Introduction With the development of resolution of broadcasting, internet, and movie industries, many fabricators and customers both require high-definition displays and devices to develop the quality of telecommunication, such as resolution. Applied to displays such as TVs, this high- definition technology will satisfy users’ desire for the vivid picture, which is now adopted by CRT, LCD, Plasma and DLP display. Until now, unlike cathode ray tubes (CRTs), first generation field emission displays (FEDs), called flat panel displays, in which the electron source consists of a matrix-addressed array of millions of cold emitters, have been based on thin film technology and semiconductor processing methods . While FEDs have had many difficulties to resolve, specifically adequate color purity, brightness, lifetime, and scalability, substantial investments are being made to develop a second generation FEDs, which will be larger, less costly, or both. For these improvements of FEDs, new alternatives are required such as (1) replacing the field emitter with an alternative material and (2) producing field emitter designs for high efficiency. Among the numerous materials that have been investigated as improved field emission sources for FEDs, it was discovered that carbon-based materials are excellent emitters. Recently, carbon nanotube (CNT) structures have attention because of their unique electrical properties and potential applications. CNTs, with high chemical stability, thermal conductivity, and high mechanical strength, give the anode an electric field enhancement as an alternative material. As a result, CNTs require lower electric fields, can operate in larger geometries, and can be integrated into FEDs in thick film form . 2. How to be applied to displays First of all, as a field emission structure formed by various semiconductor micro- fabrication techniques, CNTs have good properties that are related to aspect ratio and shape. The emission tips of existing FEDs are usually fabricated by photolithography and thin film methods, and are coated with metal into emitter wells, as shown in Figure 1. The electrons emitted from these tips impinge against a phosphor-coated screen, creating the image in a FED. A. Structure of CNTs CNTs are wires of pure carbon with diameters as low as nanometer and lengths of many microns. According to various synthesized methods and conditions, the structure of CNTs can be separated as single- or multi-wall CNT, as shown in Figure 2. Each multi-wall structure includes one or several single-wall CNTs. Depending on the number of concentric subshells, its diameter usually ranges from a few nanometers to about 40 nanometers. According to the arrangement of hexagons of carbon atoms, each single CNT is rolled up into a cylinder, with its unique pattern, as shown in Figure 3. Depending on the arrangement of hexagons of carbon atom, there are various structures of nanotubes: (a) armchair, (b) zigzag, and (c) chiral. Among the structures of nanotubes, the first two structures have high symmetry, which is expressed by Ch (chiral vector). As shown in Figure 4, each point to produce each independent nanotube is written with (n,m) where n and m each are integer. An armchair nanotube corresponds to the case of n=m, which is Ch=(n,n), and a zigzag nanotube corresponds to the case of m=0, or Ch=(n,0). All other (n,m) chiral vectors correspond to chiral nanotubes . In addition to symmetry of hexagons, the end of nanotubes is finished like a semispherical cap, which is composed of several carbon atoms. As a result, CNT has not only a high aspect ratio but also one-dimensional properties as a sharp tip that is useful for field emitter. B. Field emission properties and operation of CNTs For field emission, the surface of materials such as CNTs should be conductive under the action of a high electrostatic field. When the field is applied to the cathode, the electrons emitted from the surface of a condensed phase radiate into another phase, usually vacuum, overcoming the energy barrier to escape the vacuum level. Thus, rather than semiconductor, conductor such as metal is useful for field emitter. According to the arrangement of hexagons, CNTs are either metallic or semiconducting, depending on their chirality and diameter, described in Figure 4. In case of armchair nanotubes, they are always metallic; however, the zigzag and chiral can be either metallic or semiconducting, as shown in Figure 4. Thus, depending on the alignments of CNT unit cells, each CNT has different barrier energies and band gap for electrons to overcome, as shown in Figure 5. Since a cathode tip requires high conducting property and low barrier energy, when adopted as an emitter, CNTs also require the same requirement as metal-coated cathode tip, as well as high chemical stability and high mechanical strength. According to the Figure 4, all armchairs, zigzags with a multiple of 3, and chiral vector in which n-m is a multiple of 3 are metallic; however, the other zigzags and chiral vectors, in which n-m is not a multiple of 3, are semiconducting . In case of CNTs cathode materials fabricated by DC arc discharge of 20 ampere, the characteristic of emission current density versus applied voltage was observed as shown in Figure 6. The emission in FEDs is also associated with the voltages, which can offer important advantages such as lower power consumption and improved beam focusing. With respect to the CNTs, this anode tip work is necessary for achieving stable operation, as addressed, and is also related to the characteristic of emission current versus time. As regarding the lifetime characteristic of emission current, compared to the emission current by CNTs, the emission current of established FEDs fabricated by thin film methods does not show better result, as in Figure 7. According to the alignments of CNTs unit cell and the applied voltages on each CNT electrode, CNTs-FED demonstrates various characteristics on emission current and on brightness which is propositional to the current density collected from a single pixel dot with relation to the applied voltage, shown in Figure 8. 3. How to be manufactured To control the properties required as a cathode tip, both various fabrication of CNTs and the alignment of these synthesized CNTs are important parameters for stable operation and good reliability. Since it is not easy to control the fabrication of the CNTs property, there have been many experiments and trials using the existing thin film processing technologies, overcoming limitation of controlling the CNTs unit cell under fabrication processing. Among these trials, some techniques will be introduced briefly. An effective method for the synthesis of bundles of single-wall CNTs with a narrow diameter distribution deals with the laser vaporization of a graphite target. In the early technique, a Co-Ni/graphite composite laser vaporization target was used, which was composed of 1.2 atom percent of Co-Ni alloy with equal amounts of Co and Ni added to the graphite. Thus, single- walled CNTs produced in a quarts tube heated to 1200°C by the laser vaporization method, using a graphite target and a cooled collector for nanotubes. The carbon arc provides a simple and conventional method, generating the high temperatures, usually more than 3000°C, needed for the vaporization of carbon atoms into plasma. When the arc is operated, CNTs form on the negative electrode. In the middle of processing, no catalyst need be used and CNTs are found in bundles in the inner region of the cathode where the temperature is a maximum. CNTs can grow at the same time as conventional vapor-grown carbon fibers, and in this case, most of these CNTs are multi-wall. Fe, Co, and Ni particles are known to be catalysts, with which hydrocarbons (CH4, C6H6) and H2 gases are reacted. In a reaction tube at 1100°C, CNTs from the vapor phase grow and appear after breakage of the vapor-grown carbon fiber. Another method relates to the use of carbon ion bombardment to make carbon whiskers, known as a graphite material with high crystallinity. In this method, carbon is vaporized in vacuum through ion or electron irradiation. Along with a cold other surface structure, the resulting deposit including CNTs is accumulated. As an other example, the method using solar energy for the synthesis of single-wall CNTs has been reported, using an experimental chamber where solar energy is focused on the crucible to achieve a temperature of 3000 K on a clear day . 4. Tendency of CNT-FED The electron field emission behavior of CNTs is dependant on many factors, such as CNT paste preparation, special sintering technology and post-treatment . Because of the demand of high-resolution and image quality, HDTV such as flat and wide panel screen is very popular, and make our life style more sensitive to vivid picture. For this reason, display- manufacturing technique using CNTs will be concerned as next generation alternation. References  A.A. Talin, K.A. Dean and J.E. Jaskie, “Field emission displays: a critical review,” Solid- State Electronics, 45, pp. 963-976 (2001).  L. Yukui, Z. Changchun and L. Xinghui, “Field emission display with carbon nanotubes cathode: prepared by a screen-printing process,” Diamond and Related Materials, 11, pp. 1845- 1847 (2002).  R. Saito and G. Dresselhaus, Physical Properties of Carbon Nanotubes, (ICP, London, 1998) pp. 4, 36, 61.  J.M. Kim and W.B. Choi, “Field emission from carbon nanotubes for displays,” Diamond and Related Materials, 9, pp. 1184-1189 (2000).  Y.S. Shi, C.C. Zhu and W. Qikun, “Large area screen-printing cathode of CNT for FED,” Diamond and Related Materials, 12, pp. 1449-1452 (2003).  F.G. Zeng, C.C. Zhu and W. Liu, “The fabrication and operation of fully printed Carbon nanotube field emission displays,” Microelectronics Journal, 20, pp. 1-5 (2005). List of Figures Figure 1. Spindt type field emission tips (a) a top down view of an array of tips and (b) a cross-section of one emitter . Figure 2. Multi-wall Carbon Nanotubes  Figure 3. (a) armchair (b) zigzag (c) chiral  (0,0) (1,0) (2,0) (3,0) (4,0) (5,0) (6,0) (7,0) (8,0) (9,0) (10,0) (11,0) Zigzag (1,1) (2,1) (3,1) (4,1) (5,1) (6,1) (7,1) (8,1) (9,1) (10,1) (2,2) (3,2) (4,2) (5,2) (6,2) (7,2) (8,2) (9,2) (10,2) (3,3) (4,3) (5,3) (6,3) (7,3) (8,3) (9,3) (4,4) (5,4) (6,4) (7,4) (8,4) (9,4) (5,5) (6,5) (7,5) (8,5) y (6,6) (7,6) (8,6) a1 (7,7) a2 Armchair x Figure 4. Unit lattice expressed by (n,m): red circles – metallic, empty circles – semiconducting . Figure 5 Dispersion relations in nanotubes (a)armchair(5,5) (b)zigzag(9,0) (c)zigzag(10,0) Figure 6. Emission current density vs. applied voltage of FED device . Figure 7. Lifetime of emission current: Left – monochrome FED, Right – SWNTs-FED [1,4]. Figure 8. Brightness vs. applied voltage of a single pixel of fully printed CNT-FEDs .
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