Papers

AVS2001 Session NT+EL+NS-FrM: Nanotubes: Field Emission

Friday, November 2, 2001 8:20 AM in Room 133

Friday Morning

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8:20 AM		NT+EL+NS-FrM-1 Nanotube Electronic and Transport Properties J. Bernholc (North Carolina State University) Carbon nanotubes can be either metallic or semiconducting. Their properties change significantly with doping and strain, and they are excellent candidates for nanoscale electronic devices and sensors. We have investigated the electronic and quantum transport properties of bent, deformed and tapered nanotubes, as well as nanotube-metal contacts, which will likely form components of future nanotube-based devices. Bent armchair nanotubes keep their metallic character up to fairly high angles, while metallic chiral nanotubes open a sizable gap at the Fermi level, indicating that they can be used as nanoscale strain sensors. Tapered armchair tubes remain metallic, while their zigzag counterparts are semiconducting, as expected. Ballistic transmission is very sensitive to interactions with the substrate and coupling to the contacts. Our ab initio calculations for NT/Al structures show substantial charge transfer and rehybridization effects, which strongly affect the quantum conductance. We have also investigated BN nanotubes that are intrinsically polar. For BN tubes in chiral or zigzag structures, the symmetry permits a pyroelectric field along the tube axis, which is of the order of kV/cm per nanotube. The pyro- and piezo-effects will likely be useful in nanoscale switches, resonators, actuators, and transducers. Another important aspect of nanotubular structures is their high Li uptake, with potential applications in high performance batteries. Our quantum simulations show that Li cannot penetrate nanotube walls unless large defect structures are present, and that high concentration of Li leads to aggregates in the interstitial channels. In collaboration with M. Buongiorno Nardelli, J.-L. Fattebert, V. Meunier, D. Orlikowski, C. Roland and Q. Zhao.
9:00 AM		NT+EL+NS-FrM-3 Laser Irradiation Effect on Field Emission of Carbon Nanotubes T.W. Jeong, S. Yu, J.H. Lee, J.N. Heo (Samsung Advanced Institute of Technology, Korea); W.S. Kim (Sungkyunkwan University, Korea); W. Yi (Samsung Advanced Institute of Technology, Korea); J.B. Yoo (Sungkyunkwan University, Korea); J.M. Kim (Samsung Advanced Institute of Technology, Korea) Recently, field emission properties of carbon nanotubes (CNTs) have been intensivly studied as electron emission sources in field emission displays, since CNTs exhibit a good emission stability, low operating voltages, and high saturation electric field for field emission. Through current-voltage measurment, we have investigated the effect of laser irradiation on the field emission characteristics of CNT films. Multi-walled CNT thin layers were grown on the Ni/TiN sputtered alumina substrate by thermal chemical vapor deposition (CVD) process, and single-walled CNT layers were fabricated by screen printing of CNT paste onto the glass and subsequent heat treatment. In case of the CNT film grown by the thermal CVD, laser irradiation does not influence the field emission characteristics significantly. On the while, in case of the CNT film prepared by paste process, the field emission current was enhanced up to 10 times after the irradiation. Main reason for the current increase is considered as thermal heating by the laser, since organic materials existing in the paste-processed CNTs are detached out by the heat exposure leading to a clean surface of electron emitting CNTs. When the intensity of the laser was increased over a certain level, the emission current was reduced due to CNT deterioration by the heat. Therefore, laser irradiation onto the paste-processed CNTs is worthwhile for further study since currently available CNT based FEDs are fabricated mainly by paste process.
9:20 AM		NT+EL+NS-FrM-4 Saturated Emission Near E_F from Multiwall Carbon Nanotubes J.L. Shaw, D.S.Y. Hsu (Naval Research Laboratory) Energy distributions of the beam emitted from gated multi-wall carbon nanotube arrays¹ are measured in UHV as a function of gate voltage (emission current), temperature, and gas exposure. The emitters were grown by catalyzed CVD on silicon. At low currents (~1nA), the shape of the energy distribution was similar to that of metal emitters. At moderate current, emission could be detected at energies up to several volts below the Fermi level. The spectral shape at lower energies varied among specimens, after degassing, and after exposure to water. The intensity at the lower energies was reduced after degassing and increased by water adsorption. Operating the emitters removed the water at a rate related to the emission current. At increasing voltage, the fraction of the current emitted at the lower energies increased, while the emission current near the Fermi level saturated. That is, the additional current emitted after increasing the gate voltage occurred only below a specific energy, and that specific energy moved further below the Fermi level at higher voltages. The saturation effect was reduced by elevated temperatures (up to 600C) but still dominated the spectra, indicating that transport through the silicon substrate or at the silicon-nanotube interface was not responsible. Instead, we believe the emission at a specific energy is limited by the ability of the nanotube to replace electrons near the Fermi level, leaving only lower energy electrons available for emission. ¹ ¹ D.S.Y Hsu, these proceedings
9:40 AM		NT+EL+NS-FrM-5 Integrally Gated In-situ Grown Carbon Nanotube Field Emitter Arrays D.S.Y. Hsu, J.L. Shaw (Naval Research Laboratory) We report on the fabrication of two different configurations of integrally gated carbon nanotube field emitter arrays and the associated field emission. Local control of field emitter arrays, usually by gating, is necessary for most applications. Although several designs of gated carbon nanotube field emitter arrays, all based on a nanotube paste or slurry technology, have been demonstrated, to date no integrally-gated microfabricated carbon nanotube field emitter arrays based on in-situ-grown carbon nanotubes have been reported. We grew carbon nanotubes directly on gated cell structures in a cold-walled hot-filament CVD reactor at 650- 700 C using ammonia and acetylene or ethylene reactant gases and Ni or Fe catalysts. In one configuration, multi-walled carbon nanotubes with 20-30nm average diameter were grown on the tips of standard gated silicon tip-on-post field emitter arrays with 2.5 micron diameter gate apertures. Typical threshold voltages were less than 20V. For a 37 thousand cell array our highest collected anode current to date was 1.1 mA at a gate voltage of 41V. Stable emission even at 600 C has been obtained. Exposure to water vapor shifted the current-voltage curve to lower voltages, consistent with work of Dean and Chalamala¹ on ungated single SWNT emitters. We also investigated electron energy distributions under various experimental conditions.² In a second configuration, we grew multi-walled carbon nanotubes on the bottoms of hole structures with horizontal gate apertures having 1.5 - 2.5 micron diameters. Average emission currents up to 60 nA per cell with anode to gate current ratios greater than 10 have been measured. Economical manufacture of both configurations is projected because the first configuration does not require a sharp tip and the second requires only a much reduced number of steps. ¹ K.A. Dean and B.R. Chalamala, App. Phys. Lett. 76, 375 (2000). ² J.L. Shaw and D.S.Y. Hsu, this proceedings.
10:00 AM		NT+EL+NS-FrM-6 Field Emission in Cylindrical Geometry with Carbon Nanotube Cathodes: Characterization and Application to Luminescent Tubes J.-M. Bonard, M. Croci, O. Noury, T. Stöckli, A. Chatelain (Ecole Polytechnique Fédérale de Lausanne, Switzerland); I. Arfaoui (IPE, Switzerland) There have been in the past some attemps to realize tubular lighting elements with field emitters to offer a mercury-free alternative to incandescent or fluorescent lamps. To ensure a uniform light emission from the cylindrical anode, the cathode itself must be cylindrical, and the problem of depositing field emitters on a non-planar surface has up-to-now prevented the demonstration of such devices. We report here on the successful realization of a field emission diode in a cylindrical geometry, with multiwall carbon nanotubes deposited on the metallic cathode as the electron sources. The deposition was carried out by chemical vapor deposition of acetylene over metallic wires, and we observed a significant influence of the wire material. The field emitters show excellent performances: electron emission is observed below 1 kV and currents up to 1 A cm^-2 can be obtained. The cathodes can be used to realize luminescent mercury-free tubes: our most recent tube has a length of 30 cm and a diameter of 4 cm, and reaches luminances equivalent to those of commercial fluorescent elements. We show also that the cylindrical geometry allows one to gain information on the different parameters involved in the field emission. Space-charge related problems and the influence of residual gas pressure have been addressed. Field emission microscopy measurements show also that modifications of the structure of the nanotubes can occur during the emission, especially during or following heating. Spectacular observations of the failure of single nanotube emitters also offer direct clues to the degradation mechanism.
10:20 AM		NT+EL+NS-FrM-7 Fabrication and Electron Field Emission Properties of Carbon Nanotube Films by Electrophoretic Deposition G.Z. Yue, B. Gao, Q. Qiu, Y. Cheng, H. Shimoda, L. Fleming, O. Zhou (University of North Carolina) Carbon nanotubes have recently attracted considerable interests as electron field emitters for potential vacuum microelectronic applications such as field emission flat panel displays. Compared to the conventional field emitters, they show lower emission threshold fields and enhanced high current capabilities. However, their utilization in practical devices has been hindered, in part, by difficulties in processing. In this paper, we demonstrate that the electrophoretic deposition (EPD) method can be utilized effectively to deposit pre-formed carbon nanotubes on various substrates with good homogeneity and packing density. The films show very good emission stability at high current densities.
10:40 AM		NT+EL+NS-FrM-8 Field Emission Energy Distribution for an Undergate Type Triode CNT-FED S.H. Jin, S. Yu, J.H. Kang, W. Yi, T.W. Jeong, Y.S. Choi, J.H. Lee, J.N. Heo (Samsung Advanced Institute of Technology, Korea); W.S. Kim, Y.H. Lee (Sungkyunkwan University, Korea); J.M. Kim (Samsung Advanced Institute of Technology, Korea) We have measured I-V characteristics and field emission energy distribution (FEED) for an undergate-type triode carbon nanotube (CNT) field emission display (FED), which has gate electrodes located under the cathode electrodes. The emitters of an undergated CNT-FED were fabricated by the printing method on the glass plate with single walled CNT paste, which was mostly consisted of CNTs and glass powders. For the diode emission, the FEED peaks shifted from the Fermi level with the ratio of 67 meV/V by the change of the applied voltage, and the field enhancement factor was found to be 26,000 cm^-1 from the I-V measurement. On the while, the peaks for the triode emission shift with 270 ~ 500 meV/V, and it is expected mainly due to the strong electric field induced by the gate electrodes. Several subpeaks were also observed below the main FEED peaks, which indicated existence of other minor electron transition mechanism within CNTs. The full width at half maximum of FEED peaks for the diode and triode emissions were broader than those reported by others, i.e., 0.7 ~ 1.9 eV, which may be caused by field emission from various geometry of CNTs. In summary, we have measured the FEED of the undergate type triode CNT-FED for the first time, and the FEED peaks are shifted as the applied voltage increased, which may be ascribed to the energy band bending of dielectric materials on CNTs.
11:00 AM		NT+EL+NS-FrM-9 Integrated Field Emission Devices with Single Carbon Nanofiber Cathodes M.A. Guillorn, V.I. Merkulov (Oak Ridge National Laboratory); A.V. Melechko, E.D. Ellis (University of Tennessee); G.J. Bordonaro (Cornell University); L.R. Baylor, J.H. Whealton, M.L. Simpson, D.H. Lowndes (Oak Ridge National Laboratory) The fabrication of robust integrated field emission (FE) devices is currently an area of intense research. FE devices require an electron source with a low threshold field and stable operating characteristics. Recently, we have focused on vertically aligned carbon nanofibers (VACNFs)¹ as FE sources for these reasons.² In this paper we will present field emission data on isolated VACNFs measured by a scanned probe technique and device performance of integrated gated cathode and triode structures that use a single VACNF as the FE element. Isolated high aspect ratio VACNFs were grown on Si substrates using a DC acetylene/ammonia PECVD process from catalyst sites defined with electron beam lithography (EBL). FE properties of individual VACNFs were measured with a scanned probe system capable of positioning a 1-micron diameter probe tip above an individual VACNF. Gated cathode and triode structures using individual VACNFs as FE elements were fabricated using a combination of traditional micro- and nanofabrication techniques.³ EBL was used to define the first layer of features consisting of catalyst sites for VACNF growth and alignment marks for subsequent photolithography steps. Alternating layers of PECVD silicon dioxide and metal were then deposited onto the substrate and patterned using photolithography creating electrostatic extractor and focus electrodes aligned with the catalyst site. The dielectric layers were reactive ion etched until the buried catalyst sites were released. To complete the devices the growth of the VACNF was performed as described above. The FE current from these devices was measured using a micro channel plate system. ¹V.I. Merkulov, D.H. Lowndes, Y.Y. Wei, G. Eres, E. Voelkl, Appl. Phys. Lett., 76, 3555 (1999) ²V.I. Merkulov, D.H. Lowndes, L.R. Baylor, J. Appl. Phys, 89, 1933 (2001) ³M.A. Guillorn, V.I. Merkulov, G.J. Bordonaro, et al, J. Vac. Sci and Tech. B, 19, 573 (2001).
11:20 AM		NT+EL+NS-FrM-10 Advances in Conception of Flat Panel Displays Based on Carbon Nanotube Field Emitters I. Arfaoui, J.-M. Bonard, D. Sarangi (EPFL, Switzerland); J. Dijon (CEA, France); A. Chatelain (EPFL, Switzerland) Because of their unusual sharpness and conducting properties, Carbon NanoTubes (CNT) are an ideal candidate material for field emitting tips. In order to develop a technology allowing the fabrication of low cost large area Field Emission Displays (15 - 40), we therefore used CNT as field emissive materials. For that, it is important to have an optimised deposition method of well-aligned and well-distributed carbon nanotubes on large area substrate at low or moderate temperature. Our work deals with the growth and FE properties of CNT films on Si substrates. These patterned emitting films are made by Chemical Vapour Deposition: the CNT grow preferentially on Si substrates where a metallic catalyst (i.e. Ni, Co, Fe...) is deposited by sputtering followed by standard lithography, by microcontact printing or ink jet printing. We have optimised growth parameters (catalyst, temperature, ...) in order to obtain the lowest threshold emission fields and the highest emitting site density. However, we have shown that emitting site density is the most important parameter for Flat Panel Displays. Two particular points are discussed: structural and FE properties of diode and triode structures and a model of the emission process. In order to improve the cost issue as well as the reliability of FED, we have studied innovative triode structures. Finally, a microscopic model of the emission process is used to predict emission life and spatial uniformity of CNT films.
11:40 AM		NT+EL+NS-FrM-11 Huge Secondary Electron Emission from Coated Carbon Nanotubes J.N. Heo (Samsung Advanced Institute of Technology, Korea); W.S. Kim (Sungkyunkwan University, Korea); T.W. Jeong (Samsung Advanced Institute of Technology, Korea); Y.M. Shin, H.J. Jeong (Sungkyunkwan University, Korea); S. Yu, J.H. Lee, W. Yi (Samsung Advanced Institute of Technology, Korea); Y.H. Lee (Sungkyunkwan University, Korea); J.M. Kim (Samsung Advanced Institute of Technology, Korea) Carbon nanotubes (CNTs) have been reported to show very attractive properties such as high geometric anisotropy, high mechanical strength, and chemical stability, which leads to potential applications in the areas of display, nanoelectronics, and nano-sensors. According to the recent report by W.Yi and et al.(J. Appl. Phy. 89, 4091(2001)), huge secondary electron emission (SEE), i.e., value higher than 10⁴, was obtained for the MgO coated CNTs. In order to understand this high SEE feature systematically, a seres of samples have been fabricated by varying several parameters such as the thickness of the MgO layer, openness of the CNT tip, selectively growing of CNTs on patterned catalytic layer. CNTs were grown vertically by a thermal chemical vapor evaporator on the Si substrates with Ni and TiN thin layer. MgO or other dielectric layers were deposited on CNTs by an electron beam evaporator. Among the sample grown parameters, especially MgO thickness variation resulted in significant SEE yield change. From the thickness variation of coated CNTs from 60 to 450 nm, the maximum SEE yield was obtained for 150nm. From the energy distribution of secondary electrons, this huge SEE yield for a coated CNT is illustrated with the help of field enhanced SEE due to high geometrical aspect ratio. In addition, experimental results for different dielectric layers such as SiO₂ or LaF₃ will also be presented.