Improved field emission performance of carbon nanotube by introducing copper metallic particles
© Chen et al; licensee Springer. 2011
Received: 29 June 2011
Accepted: 3 October 2011
Published: 3 October 2011
To improve the field emission performance of carbon nanotubes (CNTs), a simple and low-cost method was adopted in this article. We introduced copper particles for decorating the CNTs so as to form copper particle-CNT composites. The composites were fabricated by electrophoretic deposition technique which produced copper metallic particles localized on the outer wall of CNTs and deposited them onto indium tin oxide (ITO) electrode. The results showed that the conductivity increased from 10-5 to 4 × 10-5 S while the turn-on field was reduced from 3.4 to 2.2 V/μm. Moreover, the field emission current tended to be undiminished after continuous emission for 24 h. The reasons were summarized that introducing copper metallic particles to decorate CNTs could increase the surface roughness of the CNTs which was beneficial to field emission, restrain field emission current from saturating when the applied electric field was above the critical field. In addition, it could also improve the electrical contact by increasing the contact area between CNT and ITO electrode that was beneficial to the electron transport and avoided instable electron emission caused by thermal injury of CNTs.
Carbon nanotubes (CNTs) have extensively been investigated since they were discovered by Iijima . The earliest research on CNTs as field emitter was conducted by de Heer et al. , which lifted the curtain on the field emission application of CNTs. Followed by a large number of studies on CNTs, the characters of high aspect ratio, small radius of curvature, good electric conductivity, and excellent chemical stability have contributed to the superior field emission behaviors such as lower turn-on voltage and larger emitting current density which have been considered as excellent and potential field emission electron sources used in field emission display (FED) [3–8].
Although CNTs have several advantages mentioned above, as the key component of FED, they must be deposited on a substrate (such as indium tin oxide (ITO) conductive glass) for their applications, low-cost, large-scale area, and homogeneous deposition become the primary targets of CNTs-based field emitters on the display panel . To meet these requirements, the electrophoretic deposition (EPD) technique was adopted. However, the problem of weak electrical contact has to be taken into account. As field emission electron sources, CNTs must have good adhesion and electric conductivity to the electrode so that they can exhibit excellent field emission performance, especially the stability of electron emission. Wang et al.  ascribed the instability in emission current to the structural damage during emission. Bonard et al.  attributed the failure of a CNT to the resistive heating at the contact to substrate. More investigations had revealed that emitting CNT would involve a self-heating process [12, 13], which might result in subliming and melting of a CNT and ultimately caused a failure in field emission. Xu and colleagues  addressed the physical mechanism responsible for the breakdown process because of the self-heating of CNTs. To sum up, how to avoid the instability of electron emission because of thermal injury of bad contact to substrate is an urgent issue to be overcome. It was reported that the presence of a charging agent could improve the adhesion of CNTs to substrate in the EPD process . Talin et al.  developed a method of EPD process with Mg (NO3)2 · 6H2O additive to precoat CNTs with Mg (OH)2, and then transformed the precoat into MgO by heat-treating that improved the adhesion of the CNTs. Above-mentioned method did not improve the electrical contact between the electrode and CNTs by reason that MgO was the dielectric material. It was also reported that nanostructured metal-CNT composites had a combination of high strength and good plasticity . In this article, a simple and low-cost way to improve field emission performance especially stability of carbon nanotube field emission display (CNT-FED) using copper metallic particle-carbon nanotube (Cu-CNT) assemblies through EPD technique is investigated.
Soon after finishing the deposition, the Cu-CNT film on ITO is rinsed in IPA and dried at 80°C for 10 min in the atmosphere, and then heated at 450°C in nitrogen atmosphere for half an hour to remove the residual solvents.
For the purpose of characterizing and analyzing, many kinds of test tools are adopted. The surface morphologies of samples are characterized using scanning electron microscope (SEM, Hitachi S-4800). To qualitatively analyze the presence of Cu element in Cu-CNT assembly cathode, the energy dispersive X-ray spectrum is measured by energy dispersive X-ray spectroscopy (EDS; Genesis 2000, EDAX, Inc.). The content of electrodeposited copper particles is identified using X-ray diffraction (XRD, Bruker D8 Focus, Bruker AXS, Inc.). For discussing the dispersion stability of suspension, zeta potential of the CNTs in electrophoresis solution is measured using Zetasizer 3000 (Malvern, Inc.). To evaluate the performance of CNTs by introducing Cu metallic particles, the field emission characteristic curve and electron emission stability curve are acquired by two Agilent 34401A sourcemeters (Agilent Technologies, Inc.) with one as ammeter and the other as voltmeter.
Results and discussion
The focus points of the experiment mentioned above can be described as follows: the adsorption of Cu2+ ions on the CNTs and the symmetrical dispersion of CNTs in suspension. The mechanism by which the Cu2+ ions are sorbed onto CNTs can be attributable to electrostatic attraction and chemical interaction between the Cu2+ ions and the surface functional groups of CNTs [18, 19]. On the surface of CNTs, it generally exists the defects such as pentagons and heptagons which can introduce a quantity of oxygen-containing functional groups like carboxyl (-COOH) and hydroxyl (-OH) in IPA solution . These functional groups, on the one hand, cause a rise in negative charge on surface of CNTs and absorb the Cu2+ ions by electrostatic attraction, on the other hand, supply protons of carboxyl and hydroxyl to exchange with the Cu2+ ions in solution.
Owing to the absorption of Cu2+ ions on CNTs in the IPA solution, the zeta potential, with a concentration of Cu (NO3)2 · 3H2O up to 5 × 10-4 mg/L, reaches +38.6 mV, meaning that the significantly electrostatic repulsion force induced by positive surface charges of CNTs is sufficient to prevent the agglomeration of CNTs in IPA solution which benefits to deposit.
where κ B is the Boltzmann constant, q the electric charge, A*Richardson constant, T the absolute temperature, S the contact area, and Δϕ is the difference of barrier height. Since the Cu metal has similar work function to CNT, the Δϕ of Cu-CNT assemblies to ITO is almost the same as the bare-CNT's. Thus, the contact area determines the contact resistance of interface between CNT and ITO.
In summary, the performance of introduction of Cu metallic particles to decorate CNTs field emitters by EPD method has been investigated. By means of SEM and EDS, we confirm that the EPD process is a simple and feasible way of fabricating Cu-CNT assembly cathode. The simulation of field distribution, field emission characteristic curve, and electron emission stability curve has been adopted to reveal the effect of Cu-CNT assemblies. The Cu-CNT assemblies have enhanced the electrical contact between CNTs and ITO electrode that the contact conductivity has greatly increased from 10-5 to 4 × 10-5 S and the turn-on field has been reduced from 3.4 to 2.2 V/μm. Meanwhile, the participation of Cu particles increases the field emitters. In addition, the field emission current tends to be undiminished over time. In contrast with the luminescence images, it is easy to find that the Cu-CNT assembly cathode indeed improves the field emission stability of CNT-FED. We expect that the introduction of Cu metallic particles to decorate the CNTs in this article will be an easy way to facilitate the improvement of emission stability of CNT-FED.
carbon nanotube field emission display
copper metallic particle-carbon nanotube
energy dispersive X-ray spectroscopy
field emission display
Fowler-Nordheim. IPA: isopropyl alcohol
indium tin oxide
scanning electron microscope
This study was supported by the National Basic Research Program of China (Grant No. 2010CB327705) and the National Natural Science Foundation of China (Grant No. 60877007).
- Iijima S: Helical microtubules of graphitic carbon. Nature 1991, 354: 56–58. 10.1038/354056a0View Article
- de Heer WA, Chatelain A, Ugarte D: A carbon nanotube field emission electron source. Science 1995, 270: 1179–1180. 10.1126/science.270.5239.1179View Article
- Treacy MMJ, Ebbesen TW, Gibson JM: Exceptionally high Young's modulus observed for individual carbon nanotubes. Nature 1996, 381: 678–680. 10.1038/381678a0View Article
- Wong EW, Sheehan PE, Lieber CM: Nanobeam mechanics: elasticity, strength, and toughness of nanorods and nanotubes. Science 1997, 277: 1971–1975. 10.1126/science.277.5334.1971View Article
- Dean KA, Chalamala BR: The environmental stability of field emission from single-walled carbon nanotubes. Appl Phys Lett 1999, 75: 3017–3019. 10.1063/1.125219View Article
- Wang QH, Setlur AA, Lauerhaas JM, Dai JY, Seelig EW, Chang RPH: A nanotube-based field-emission flat panel display. Appl Phys Lett 1998, 72: 2912–2913. 10.1063/1.121493View Article
- Choi WB, Jin YW, Kim HY, Lee SJ, Yun MJ, Kang JH, Choi YS, Park NS, Lee NS, Kim JM: Electrophoresis deposition of carbon nanotubes for triode-type field emission display. Appl Phys Lett 2001, 78: 1547–1549. 10.1063/1.1349870View Article
- Baughman RH, Zakhidov AA, de Heer WA: Carbon nanotubes--the route towards applications. Science 2002, 297: 787–792. 10.1126/science.1060928View Article
- Chen KF, Chen KC, Jiang YC, Jiang LY, Chang YY, Hsiao MC, Chan LH: Field emission image uniformity improvement by laser treating carbon nanotube powders. Appl Phys Lett 2006, 88: 193127. 10.1063/1.2203206View Article
- Wang ZL, Gao RP, de Heer WA, Poncharal P: In situ imaging of field emission from individual carbon nanotubes and their structural damage. Appl Phys Lett 2002, 80: 856–858. 10.1063/1.1446994View Article
- Bonard JM, Klinke C, Dean KA, Coll BF: Degradation and failure of carbon nanotube field emitters. Phys Rev B 2003, 67: 115406.View Article
- Dean KA, Burgin TP, Chalamala BR: Evaporation of carbon nanotubes during electron field emission. Appl Phys Lett 2001, 79: 1873–1875. 10.1063/1.1402157View Article
- Purcell ST, Vincent P, Journet C, Binh VT: Hot nanotubes: stable heating of individual multiwall carbon nanotubes to 2000 K induced by the field-emission current. Phys Rev Lett 2002, 88: 105502.View Article
- Huang NY, She JC, Chen J, Deng SZ, Xu NS, Bishop H, Huq SE, Wang L, Zhong DY, Wang EG, Chen DM: Mechanism responsible for initiating carbon nanotube vacuum breakdown. Phys Rev Lett 2004, 93: 075501.View Article
- Gao B, Yue GZ, Qiu Q, Cheng Y, Shimoda H, Fleming L, Zhou O: Fabrication and electron field emission properties of carbon nanotube films by electrophoretic deposition. Adv Mater 2001, 13: 1770–1773. 10.1002/1521-4095(200112)13:23<1770::AID-ADMA1770>3.0.CO;2-GView Article
- Talin AA, Dean KA, O'Rourke SM, Coll BF, Stainer M, Subrahmanyan R: FED cathode structure using electrophoretic deposition and method of fabrication. US Patent 2005, 6902658.
- Li HQ, Misra A, Horita Z, Koch CC, Mara NA, Dickerson PO, Zhu YT: Strong and ductile nanostructured Cu-carbon nanotube composite. Appl Phys Lett 2009, 95: 071907. 10.1063/1.3211921View Article
- Lu C, Liu C: Removal of nickel (II) from aqueous solution by carbon nanotubes. J Chem Technol Biotechnol 2006, 81: 1932–1940. 10.1002/jctb.1626View Article
- Rao GP, Lu C, Su F: Sorption of divalent metal ions from aqueous solution by carbon nanotubes: a review. Sep Purif Technol 2007, 58: 224–231. 10.1016/j.seppur.2006.12.006View Article
- Shim JW, Park SJ, Ryu SK: Effect of modification with HNO 3 and NaOH on metal adsorption by pitch-based activated carbon fibers. Carbon 2001, 39: 1635–1642. 10.1016/S0008-6223(00)00290-6View Article
- Cui JB, Teo KBK, Tsai JTH, Robertson J, Milne WI: The role of dc current limitations in Fowler-Nordheim electron emission from carbon films. Appl Phys Lett 2000, 77: 1831–1833. 10.1063/1.1310628View Article
- Chen LF, Song H, Cao LZ, Jiang H, Li DB, Guo WG, Liu X, Zhao HF, Li ZM: Effect of interface barrier between carbon nanotube film and substrate on field emission. J Appl Phys 2009, 106: 033703. 10.1063/1.3153279View Article
- Lim SC, Jang JH, Bae DJ, Han GH, Lee S, Yeo IS, Lee YH: Contact resistance between metal and carbon nanotube interconnects: effect of work function and wettability. Appl Phys Lett 2009, 95: 264103. 10.1063/1.3255016View Article
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