Field emission from carbon nanotube emitters fabricated by the metal ...

JOURNAL OF APPLIED PHYSICS 100, 064308 共2006兲

Field emission from carbon nanotube emitters fabricated by the metal intermediation layer Taewon Jeong,a兲 Jungna Heo, Jeonghee Lee, Sanghyun Park, Yongwan Jin, and J. M. Kimb兲 NCRI, Samsung Advanced Institute of Technology, Yongin 449-712, Korea

Taesik Oh, Chongwyun Park, and Ji-Beom Yoo Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, Suwon 440-746, Korea

Byoungyun Gong and Naesung Lee Department of Nano Science and Technology, Sejong University, Seoul 143-747, Korea

SeGi Yu Department of Physics, Hankuk University of Foreign Studies, Yongin 449-791, Korea

共Received 29 October 2005; accepted 25 May 2006; published online 22 September 2006兲 Multiwalled carbon nanotube 共MWNT兲 emitters fabricated by the metal intermediation process were studied. This was intended to allow strong adhesion and high electrical contact between the cathode electrode and MWNT emitters. The process was performed by hot-pressed bonding of a metal layer to a MWNT film surface, where the metal layer was deposited on a main substrate. Through this process, MWNTs have open and sharp ends, and the metal layer and the MWNTs have strong electrical contact. Together with unchanged crystallinity of MWNTs as before the process, these effects improve the field emission properties, resulting in 64% reduction of turn on field and two to three orders of magnitude increase of current density. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2234543兴 I. INTRODUCTION

II. EXPERIMENTS

Recently, field emission from carbon nanotubes 共CNTs兲 has been widely studied to develop a flat panel display device such as field emission display 共FED兲.1 FED has been expected to be a potential display device due to high efficiency, low energy consumption, and high resolution. Electron emitters in FED are required to possess a low driving voltage and durability for long lifetime. CNTs as field emitters have a very low turn-on field due to their high field enhancement factor originated from the high geometric aspect ratio and small radii of curvatures at the tips. In addition, CNTs have advantageous properties as emitters such as high electrical conductivity, high mechanical strength, and good chemical inertness.2–4 In this paper, we introduce a process to fabricate vertically aligned CNT emitters having a good metallic contact between CNTs and electron supplying substrate. This process allows an easy approach to make devices under 100 ° C for the realization of characteristics of high field enhancement factor and high electrical current flow. The morphology of CNTs and the adhesion state between CNTs and the substrate were examined by scanning electron microscopy 共SEM兲 and transmission electron microscopy 共TEM兲. The crystallinity of CNTs was characterized by Raman spectroscopy. Then, the field emission characteristics were measured to evaluate the validity of this process.

The CNT film was grown on a supplementary substrate to carry out the process shown in Fig. 1共a兲, where the film was composed of vertically aligned multiwalled carbon nanotubes 共MWNTs兲 produced by thermal chemical-vapor deposition 共CVD兲 using a C2H2 gas as a carbon source at 650 ° C on a Si substrate. Metal intermediation 共MI兲 was first performed by depositing Ag over the surface of CNT film placed on the supplementary substrate. Typically, the deposited Ag layer covers whole CNT film 关Fig. 1共b兲兴. Employing a shadow mask made by polyimid tape with 3 ⫻ 5 mm2 opening area, the emitter area of the objective substrate is defined 关Fig. 1共c兲兴. The Ag layers deposited on both supplementary and objective substrates were formed by dc magnetic sputtering system and their thickness was 1000 Å. Then, the Ag-coated CNT layer was transferred to the Agcoated objective substrate by applying the pressure of 3 MPa at 100 ° C for 30 s with a contact of two Ag surfaces together. By simply removing the supplementary substrate, the Ag metal-intermediated CNT emitters were produced on the objective substrate, as shown in Fig. 1共d兲. Field emission measurement was carried out in a diodetype configuration with 200 ␮m spacing between the cathode and anode, where the cathode was the prepared sample and the anode was a indium-doped tin oxide 共ITO兲 coated glass substrate or a phosphor screened substrate. III. RESULTS AND DISCUSSION

a兲

Also at Center for Nanotubes and Nanostructured Composites; electronic mail: [email protected] b兲 Author to whom correspondence should be addressed; electronic mail: [email protected] 0021-8979/2006/100共6兲/064308/4/$23.00

The MI process was performed at 100 ° C for the transfer of CNTs to the substrate. This processing temperature is remarkably low compared to the melting point of the silver

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© 2006 American Institute of Physics

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FIG. 3. SEM images of MWNT emitters 共a兲 before and 共b兲 after the MI process. Insets are the high resolution TEM images of CNT tips. 共c兲 SEM image of the interface between MWNTs and Ag-coated substrate and 共d兲 TEM image of the interface between the MWNT and Ag layer after the MI process.

FIG. 1. Schematics illustrating the metal intermediation process.

metal 共about 961 ° C兲. According to the reports of Castro et al.5 and Cuenot et al.,6 melting point decreases in proportion to the reduction of the cluster size for silver and it is represented in Fig. 2 by the equation of Castro et al.5 for the surface tension of 3.09 J / m2,6 where Tm and T0 are the values of the melting temperature at the surface and bulk, respectively. If the cluster radius is less than 4.8 nm, the melting point can be decreased below 100 ° C. The Ag layer deposited on CNTs seemed to have nanosized morphology

FIG. 2. The plot of the relative melting point as a function of the inverse cluster radius.

due to the formation of Ag over nanometer size diameters of CNTs. Therefore, the Ag layer on CNTs could be melted and bonded with Ag on the substrate below 100 ° C in our experiments. Figure 3 shows the SEM images of the as-grown MWNT films and the transferred MWNT films after the MI process. The as-grown MWNT film has a typical characteristic of MWNTs grown by thermal CVD. The average length of MWNTs is about 8 ␮m and the average diameter is about 50 nm 关Fig. 3共a兲兴. In case of the transferred MWNTs, the average length is shortened to 6 ␮m with the average diameter of 50 nm 关Fig. 3共b兲兴. The insets show the TEM images of the as-grown MWNTs and the transferred MWNTs by the MI process, respectively. In the case of the as-grown MWNTs, the end of the CNT has a closed tip with a catalyst metal capped structure, while that of the transferred MWNT has an opened tip structure with the catalytic metal particle removed. Figures 3共c兲 and 3共d兲 show the SEM and TEM images of interface between MWNTs and Ag layer on the transferred MWNTs. The structure of interface indicates that there is no evidence of the alloying carbon and silver 关Fig. 3共d兲兴, but CNTs are tightly bonded to the Ag layer. Figure 4 shows the Raman spectra of the as-grown MWNT film and the transferred film. Raman spectroscopy can be used to expect the degree of graphitization of carbon nanotubes. The strong band at around 1580 cm−1 is known as the G mode, which is the tangential C–C stretching mode indicating good graphite crystallinity. The band at around 1350 cm−1 is due to the D mode, which is observed due to the existence of disordered carbon structure and sp3 carbons.


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FIG. 4. The Raman spectra of the as-grown MWNTs and the transferred MWNTs by the MI process.

The relative intensity of the D band to the G band is used to figure out the degree of graphitization of CNTs. The relatively weak intensity of the D to G band indicates that the carbon nanotubes have a good graphitized structure. In the case of this work, the intensity ratios of the D to G peak are larger than one and their values are almost the same for the both of the as-grown MWNTs and the transferred MWNTs, except a slight increase of the D peak for the transferred

FIG. 6. 共a兲 Simplified band diagram and theoretical FEED spectrum and 共b兲 FEED of the transferred MWNTs by the MI process.

FIG. 5. Field emission characteristics showing 共a兲 the current density vs electric field relationship and 共b兲 FN plot of the MWNT emitters before and after the MI process.

MWNTs. It is considered that the slight increase of the D band intensity is caused by disordered carbons, which were possibly generated from the process of opening and sharpening the end of CNT tips. However, Raman spectra indicate still a good crystallinity with subtle change of peak intensities after the MI process. The field emission properties of the as-grown MWNT film and the transferred MWNT film were compared to see the effects of the morphological change of MWNT tip ends, the metal contact between MWNTs and substrate, and the crystalline state’s consistency of MWNTs. Figure 5共a兲 shows the plot of current density versus electric field. At the current density of 1 ␮A / cm2, the turn-on field of the as-grown MWNT film is 3.72 V / ␮m, while that of the transferred


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MWNTs is remarkably lowered to 2.37 V / ␮m, which is the 64% of the turn-on field of the as-grown one. At the field of 4 V / ␮m, the current density of the as-grown MWNT film is 6.82 ␮A / cm2, while that of the transferred MWNTs is 4281 ␮A / cm2 which is 628 times of the as-grown one. From the intercept A and gradient B of the Fowler-Nordheim 共FN兲 plot,7 we can estimate emission area 共␣兲 and field enhancement factor 共␤兲关Fig. 5共b兲兴. The values of ␣ are 3.00 ⫻ 10−10 and 1.75⫻ 10−7 cm2 for the as-grown MWNTs and the transferred MWNTs, respectively. The values of ␤ are 1170 and 1210, respectively. Here, ␣ = 共1.66⫻ 10−10 ⫻ AB2 / ␸兲exp共−10.4/ ␸1/2兲 and ␤ = 共6.5265⫻ 107 ⫻ ␸3/2d兲 / B, where ␸ is the work function of CNTs 共⬃5 eV兲 and d is a distance between anode and cathode. After the MI process, about 3% increase of the field enhancement factor was obtained, probably due to the opening and sharpening of tip ends. On the other hand, about 600 times increase of the emission area was obtained, and this factor seemed to induce a dominant cause of field emission improvement. It is considered that the metallic contact between MWNTs and substrate has a role of positive effect to the increase of emission area. The electrical characteristics of the metallic contact between CNTs and substrate can be verified by the measurement of the field emission electron energy distribution 共FEED兲 analysis, which measures energy spectra of field emitted electrons using an electron analyzer.8 Figure 6共a兲 illustrates the relationship between the measured kinetic energy Ekin of the electrons emitted from the energy level E, the potential V applied between the cathode and the analyzer, and the analyzer work function ⌽A. Describing shortly, a negative voltage V is applied to the emitter with respect to the analyzer. Electrons are emitted from the energy level E below the Fermi level EF, which serves as a reference energy in the FEED spectrum. Here, Ekin stands for the measured kinetic energy of field emitted electrons, ⌽A for analyzer work function, ⌽ for emitter work function, and VL for vacuum level. The Fermi level of the CNT, EF, serves as a reference energy level if its energy relative to the Fermi level of the analyzer depends only on the applied voltage. Hence, it is convenient to plot all FEED spectra in terms of 共E − EF兲 = Ekin − eV + ⌽A. This energy scale implicitly takes into account any changes in electron energy 共Ekin兲 due to changes in the extraction voltage V. The work function of the emitter, ⌽, does not influence the amount of energy shift 共E − EF兲 of emitted electrons but determines the tunneling probability and thus the field emission current.

Figure 6共b兲 displays the FEED of emitted electrons from the transferred MWNTs by the MI process, where FEED was measured with the extraction voltages ranging from 1300 to 1500 V with a 20 V step, where a stainless-steel mesh grid instead of the ITO anode was used with the same diode configuration as the previous FE measurement system. It is found that the FEED peak positions of the transferred MWNTs are fixed at the Fermi level and independent of applied voltage. Thus, it indicates that CNTs have a good electrical contact with the substrate because there is no voltage drop between CNTs and the substrate. It further indicates that there is no impurity on the tip ends, since some kind of insulating impurity over the emitter tip induces a voltage drop by band bending effect.8 IV. CONCLUSION

Through MI process, the field emission properties for CNTs were improved with the effects of self-vertical alignment of emitters, modified tip structure, and improved electrical contact between CNTs and substrate. These effects resulted in the lowering of turn-on field and the increase of emission current. This improvement is attributed to the good metallic contact between CNTs and a substrate as well as the increased field enhancement factor resulting from opening and sharpening of emitter tips. The FEED measurements of the transferred MWNT emitters support the good electrical contact between MWNTs and substrate, since no voltage drop between CNTs and the substrate was observed. ACKNOWLEDGMENTS

This work was supported in part by the Korean Ministry of Science and Technology through the National Creative Research Initiative Program. The work done by one of the authors 共S. Y.兲 was supported by the Korea Research Foundation grant 共KRF-2003-015-C00160兲. W. B. Choi et al., Appl. Phys. Lett. 75, 3129 共1999兲. W. A. de Heer, A. Chatelain, and D. Ugarte, Science 270, 1179 共1995兲. 3 Y. Saito, K. Hamaguchi, S. Uemura, K. Uchida, Y. Tasaka, M. Yumura, A. Kasuya, and N. Nishina, Appl. Phys. A: Mater. Sci. Process. 67, 95 共1998兲. 4 K. A. Dean and B. R. Chalamala, Appl. Phys. Lett. 75, 3017 共1999兲. 5 T. Castro, R. Reifenberger, E. Choi, and R. P. Andres, Phys. Rev. B 42, 8548 共1990兲. 6 S. Cuenot, C. Frétigny, S. Demoustier-Champagne, and B. Nysten, Phys. Rev. B 69, 165410 共2004兲. 7 R. H. Fowler and L. W. Nordheim, Proc. R. Soc. London, Ser. A 119, 173 共1928兲. 8 R. Schlesser, M. T. McClure, B. L. McCarson, and Z. Sitar, J. Appl. Phys. 82, 5673 共1997兲. 1 2
