Field emission of carbon nanotubes grown on carbon cloth

Transcription

1 Field emission of carbon nanotubes grown on carbon cloth S. H. Jo, J. Y. Huang, S. Chen, G. Y. Xiong, D. Z. Wang, and Z. F. Ren a Department of Physics, Boston College, 140 Commonwealth Avenue, Chestnut Hill, Massachusetts Received 14 April 2005; accepted 6 September 2005; published 31 October 2005 Field emission of carbon nanotubes CNTs grown on carbon cloth under various growth conditions has been studied. Iron sulfate or stainless steel was used as the catalyst, acetylene C 2 H 2 or methane CH 4 was used as the carbon source, and argon Ar, hydrogen H 2, or ammonia NH 3 was used as the carrier gas. It is found that the morphology of CNTs is strongly dependent on the growth conditions: temperature, gas combination, and growth time. In general, higher temperature produces better field emitters than lower temperature, C 2 H 2 /Ar is better than C 2 H 2 /NH 3, but CH 4 /H 2 is the best, and longer growth time normally yields longer CNTs leading to better field emitters. The best sample was made from 0.18 mol/l iron sulfate catalyst with CH 4 /H 2 at 860 C for 2 h: an emission current density of 1 ma/cm 2 was obtained at 0.4 V/ m corresponding to a field enhancement factor of American Vacuum Society. DOI: / I. INTRODUCTION Since the demonstration of field emission of carbon nanotubes CNTs in 1995, 1,2 CNTs have been considered as one of the best electron-emitting materials and extensive research has been carried out. 3,4 The excellent field emission properties of CNTs result certainly from the large field enhancement factors due to the high aspect ratio of CNTs, i.e., the small diameter and the elongated shape of CNTs. 3,4 Therefore, it is very important to increase the field enhancement factor in order to fabricate a field emission electron source with low operation voltage and high current. Recently, it was reported that CNTs and zinc oxide nanowires grown on carbon cloth show very high field enhancement factor and, as a result, very low turn-on electric field. 5 7 Since CNTs can be easily grown on a large area carbon cloth and the carbon cloth is flexible, large area field emission cathodes with nonflat surface can be easily fabricated, which can lead to an easy fabrication of electron sources with complex geometry. This improved field emission characteristics can be explained by the fractal geometry of CNTs grown on carbon cloth, where the field enhancement factor is given by the product of the field enhancement factors of CNTs and the carbon fiber. 8 Since various CNTs with different morphology can be grown by thermal chemical vapor deposition CVD, the field emission characteristics from CNTs grown on carbon cloth at different growth conditions can be very different. In this study, we report the studies on growth and field emission of CNTs on carbon cloth with different catalysts, temperatures, growth times, and gas combinations by thermal CVD. Other parameters such as gas flow rate, pressure, and catalyst reduction time for each gas combination were not changed in this experiment, and optimum values for these parameters of each gas combination were used which were obtained from previous experiments carried out with other substrates a Author to whom all correspondence should be addressed; electronic mail: renzh@bc.edu II. CARBON NANOTUBE GROWTH The catalysts for the growth of CNTs were formed on commercially available carbon cloth samples by two different methods in this study. In the first method, carbon cloth samples were soaked in the iron II sulfate FeSO 4 water solution for 5 min and dried at room temperature hereafter designated by Fe-catalyst samples. In the second method, carbon cloth samples were coated with a film of 30 nm stainless steel type 304 by rf magnetron sputtering hereafter designated by SS-catalyst samples. Thermal CVD method was used to grow CNTs in a tube furnace. Three different gas combinations were used. In the first combination, the catalyst was first heat treated in 0.8 Torr of flowing NH 3 80 sccm for 10 min, then C 2 H 2 20 sccm was added to grow CNTs hereafter designated by C 2 H 2 /NH 3. In the second combination, the catalyst was first heat treated in 50 Torr of flowing mixture of H 2 10 sccm and Ar 100 sccm for 30 min, then the pressure was adjusted to 0.8 Torr by controlling the exhaust valve and the hydrogen was replaced with C 2 H 2 10 sccm for CNTs growth hereafter designated by C 2 H 2 /Ar. In the third combination, the catalyst was first heat treated in 150 Torr of flowing mixture of H sccm and Ar 100 sccm for 10 min, then the hydrogen flow rate was reduced to 20 sccm and CH sccm was added to grow CNTs hereafter designated by CH 4 /H 2. The growth temperatures were 650, 690, 730, and 760 C for C 2 H 2 /NH 3 and C 2 H 2 /Ar, and 780, 820, 860, and 900 C for CH 4 /H 2 due to a higher dissociation temperature. These process conditions are summarized in Table I. The temperature was measured by using a thermocouple inserted in the tube furnace in the separate test experiments without gas flow. All six possible combinations of the catalyst and the gas were used, leading to multiwalled carbon nanotubes MWNTs with different diameters and lengths. The combination of SS catalyst with CH 4 /H 2 did not produce CNTs J. Vac. Sci. Technol. B 23 6, Nov/Dec X/2005/23 6 /2363/6/$ American Vacuum Society 2363

2 2364 Jo et al.: Field emission of carbon nanotubes grown on carbon cloth 2364 TABLE I. CNTs growth conditions of three gas combinations using thermal CVD. C 2 H 2 /NH 3 C 2 H 2 /Ar CH 4 /H 2 Reduction condition Gas flow sccm NH 3 :80 H 2 :10+Ar:100 H 2 :300+Ar:100 Pressure Torr Time min Growth condition Gas flow sccm C 2 H 2 :20+NH 3 :80 C 2 H 2 :10+Ar:100 CH 4 :700+H 2 :20+Ar:100 Pressure Torr Growth temperature C III. FIELD EMISSION CHARACTERISTICS Field emission measurements were carried out in a high vacuum Torr chamber using the simple diode configuration with a5mmdiameter cylindrical anode. The gap between the anode and the carbon cloth on cathode electrode was 2.2 mm. The size of the cathode, i.e., the carbon cloth with CNTs is approximately 1.5 cm 1.5 cm. The result of the first voltage sweep shows usually an unstable current. However, after the first sweep, a high voltage corresponding to 1 ma/cm 2 is applied to the sample for 10 min. After this electrical aging, the field emission current is quite stable and reproducible. The measurements were carried out up to the fourth sweep, and the results of the third and fourth sweeps were essentially identical to the second sweep result, as shown in Fig. 1. Since the total field emission current over 5 mm diameter anode is about A, we estimate the emission site density to be about 10 3 /cm 2 if we assume that each emitting CNT contribute 1 A to the total field emission current. 1 A per emitting CNT is assumed because CNT carrying more than 1 A is easily degraded. 14 Figures 2 a, 2 b, and 2 c show the field emission characteristics of the Fe-catalyst samples grown with C 2 H 2 /NH 3, C 2 H 2 /Ar, and CH 4 /H 2 at different temperatures, respectively. The concentration of FeSO 4 solution was 0.36 mol/l, and the growth time was fixed to 1 h for all the three gas combinations. The field emission characteristics are represented by the emission current density versus macroscopic electric field J-F. It can be seen for the samples grown with C 2 H 2 /NH 3 and C 2 H 2 /Ar that, up to the growth temperature of 730 C, CNTs grown at higher temperature shows better field emission than those grown at lower temperature, as shown in Figs. 2 a and 2 b. However, if the growth temperature is further increased to 760 C, the field emission is deteriorated. The same trend can be seen from the samples grown with CH 4 /H 2 though the difference is not significant, as shown in Fig. 2 c. It can also be seen that, if the growth temperature is the same, the field emission of CNTs grown with C 2 H 2 /Ar is better than those with C 2 H 2 /NH 3. The further improvement of field emission of CNTs grown with CH 4 /H 2 can be attributed to the change of gas combination and the higher growth temperature. The same dependence of field emission on the gas combination and the growth temperature is observed from SS-catalyst samples, as shown in Fig. 3. To compare the field emission of different samples, the electric field corresponding to an emission current density of 1mA/cm 2 for each sample is extracted from the J-F plot, and shown in Fig. 4. From Fig. 4, the dependence of field emission on the gas combination and the growth temperature can be seen more clearly. This dependence can be explained by the morphologies observed with a scanning electron microscope SEM, JEOL JSM-6340F. Figure 5 shows some of the SEM images of CNTs grown with Fe catalyst and different gases at different growth temperatures. From Fig. 5, it can be seen that, when the gas is the same, the protruding CNTs grown at higher temperature right column are generally longer than those grown at lower temperature left column except Fig. 5 g. In addition, it can be seen that the change of gas from C 2 H 2 /NH 3 the first row to C 2 H 2 /Ar the second row results in longer protruding CNTs, while the change of gas to CH 4 /H 2 the third row results in even longer and smaller diameters. Since the field enhancement factor of CNTs is proportional to the aspect ratio, 15 this mor- FIG. 1. Example of the change of field emission characteristics of CNTs grown on carbon cloth during consecutive voltage sweep. J. Vac. Sci. Technol. B, Vol. 23, No. 6, Nov/Dec 2005

3 2365 Jo et al.: Field emission of carbon nanotubes grown on carbon cloth 2365 FIG. 3. Field emission characteristics of various CNTs grown on carbon cloth with SS catalyst. a C 2 H 2 /NH 3 and b C 2 H 2 /Ar at different growth temperatures. FIG. 2. Field emission characteristics of various CNTs grown on carbon cloth with Fe catalyst 0.36 mol/l. a C 2 H 2 /NH 3, b C 2 H 2 /Ar, and c CH 4 /H 2 at different growth temperatures. phological change of protruding CNTs grown with Fe catalyst can explain the dependence of field emission on the gas combination and temperature shown in Fig. 4 a. However, if the growth temperature is 760 C, the diameter of CNT is excessively increased to several hundred nanometers in the case of C 2 H 2 /NH 3, as shown in Fig. 5 g, so that the aspect ratio is decreased rather than increased. In case of CNTs grown with SS catalyst, a similar morphological change of CNTs was observed, as shown in Fig. 6, and this morphological change can also explain the dependence of field emission shown in Fig. 4 b. From Fig. 4 a, it can be seen that the field emission from the sample grown with C 2 H 2 /Ar at 730 C is very close to that from the sample grown with CH 4 /H 2 at 860 C, even though the morphology of protruding CNTs is very much different, as shown in Figs. 5 d and 5 f. The diameter of CNTs grown with CH 4 /H 2 is generally much smaller than that of those with C 2 H 2 /Ar. In order to see the individual CNT in high magnification, a transmission electron microscope TEM, JEOL JEM-2010F was used. It was shown that the CNTs grown with C 2 H 2 /Ar had smooth ends and the range of diameter of CNTs is quite wide. Some of the CNTs have a diameter of less than 20 nm, as shown in Fig. 7 a. On the other hand, the size of the catalyst at the end of the CNTs grown with CH 4 /H 2 was much bigger than the diameter of the CNTs themselves, as shown in Fig. 7 b. Although the average diameter of CNTs grown with C 2 H 2 /Ar is larger than that of those with CH 4 /H 2, a small number of CNTs grown with C 2 H 2 /Ar have a diameter less than 20 nm, which is comparable to the diameter of the end catalyst of CNTs JVST B-Microelectronics and Nanometer Structures

4 2366 Jo et al.: Field emission of carbon nanotubes grown on carbon cloth 2366 FIG. 4. Electric field required to obtain field emission current density of 1mA/cm 2 for various CNTs grown on carbon cloth with different growth temperatures and gas combinations. a Fe-catalyst samples and b SScatalyst samples. FIG. 5. SEM micrographs of various CNTs grown on carbon cloth with Fe catalyst. a C 2 H 2 /NH 3 at 650 C, b C 2 H 2 /NH 3 at 730 C, c C 2 H 2 /Ar at 650 C, d C 2 H 2 /Ar at 730 C, e CH 4 /H 2 at 780 C, f CH 4 /H 2 at 860 C, and g C 2 H 2 /NH 3 at 760 C. The scale bar represents 1 m. grown with CH 4 /H 2. There exists catalyst at the end of CNTs because the CNTs are grown according to the tip-growth model. Therefore, some of the CNTs grown with C 2 H 2 /Ar can have the similar field enhancement factor to the CNTs grown with CH 4 /H 2, because the field enhancement factor is related not to the diameter of the CNT body but to the diameter of the end of CNT. It is well known that not all of CNTs are participating in the field emission. 16,17 Only a very small number of actively emitting CNTs with the highest field enhancement factor are participating in the field emission. Therefore, even though only a small number of CNTs grown with C 2 H 2 /Ar have the similar diameter to the CNTs grown with CH 4 /H 2, both samples can show similar field emission characteristics. However, this explanation is not conclusive because the actively emitting CNTs with the highest field enhancement factor are usually not observed in the SEM or TEM observation. Since the length of CNTs is proportional to the growth time, the effect of growth time on the field emission was investigated. The growth times of samples with SS catalyst and C 2 H 2 /Ar at 730 C and with Fe catalyst and CH 4 /H 2 at 860 C were chosen to be 0.5, 1, and 2 h, respectively. The concentration of the FeSO 4 solution for the Fe-catalyst samples was 0.36 mol/l. Figure 8 shows the morphologies of the CNTs grown at different growth time. It is clear that the length increases with the growth time. Figure 9 shows the dependence of the electric field at a current density of 1mA/cm 2 on the CNT growth time. It seems that the field emission capability is saturated after growth time of 1 h, which means that the field enhancement factor is saturated even though the length is further increased. This saturation can be explained by the same screening effect that was observed and modeled in previous work on field emission of the CNTs grown by thermal CVD Ref. 18 and by plasma enhanced CVD. 19 Finally, the effect of concentration of FeSO 4 on the field emission of CNTs grown with Fe catalyst and CH 4 /H 2 was investigated. The concentrations of FeSO 4 solution were chosen to be 0.18, 0.36, and 0.72 mol/l, and the growth temperature and time were 860 C and 1 h, respectively. Figure 10 shows the morphologies of CNTs grown with 0.18 and 0.72 mol/l FeSO 4 solutions, and no significant difference J. Vac. Sci. Technol. B, Vol. 23, No. 6, Nov/Dec 2005

5 2367 Jo et al.: Field emission of carbon nanotubes grown on carbon cloth 2367 FIG. 6. SEM micrographs of various CNTs grown on carbon cloth with SS catalyst. a C 2 H 2 /NH 3 at 650 C, b C 2 H 2 /NH 3 at 730 C, c C 2 H 2 /Ar at 650 C, and d C 2 H 2 /Ar at 730 C. The scale bar represents 1 m. FIG. 9. Electric field required to obtain field emission current density of 1mA/cm 2 for CNTs grown on carbon cloth with different growth time. Fe-catalyst and SS-catalyst samples were grown with C 2 H 2 /Arat730 C and CH 4 /H 2 at 860 C, respectively. FIG. 7. TEM micrographs of CNTs grown on carbon cloth with a C 2 H 2 /Ar at 730 C and b CH 4 /H 2 at 860 C. FIG. 10. SEM micrographs of various CNTs grown on carbon cloth using CH 4 /H 2 at 860 C with a 0.18 mol/l and b 0.72 mol/l FeSO 4 solution. The scale bar represents 1 m. FIG. 8. SEM micrographs of various CNTs grown on carbon cloth with SS catalyst and C 2 H 2 /Ar at 730 C for a 0.5 h and b 2 h, with Fe catalyst and CH 4 /H 2 at 860 C for c 0.5 h and d 2 h. The scale bar represents 1 m. FIG. 11. Electric field required to obtain field emission current density of 1mA/cm 2 for CNTs grown on carbon cloth using CH 4 /H 2 at 860 C with different concentrations. JVST B-Microelectronics and Nanometer Structures

6 2368 Jo et al.: Field emission of carbon nanotubes grown on carbon cloth 2368 IV. CONCLUSIONS Various CNTs are grown on carbon cloth with a combination of different catalysts and growth conditions, and the field emission characteristics of the grown CNTs are measured and compared. First, higher growth temperature results, in general, in better field emission because of the longer protruding CNTs. However, the increment of the field emission is getting smaller, i.e., saturated as the growth temperature is increased because of the screening effect. Second, the field emission characteristics are improved by changing gas combination from C 2 H 2 /NH 3 to C 2 H 2 /Ar, and to CH 4 /H 2. Third, the dependence of field emission on the growth time shows saturation characteristics, and the field emission is slightly improved as the concentration of catalyst solution decreases. The best sample, which was grown with Fe catalyst 0.18 mol/l and CH 4 /H 2 at 860 C for 1 h, shows the field enhancement factor of , and emits a current density of 1 ma/cm 2 at 0.4 V/ m. FIG. 12. Fowler Nordheim F-N plot for the best sample grown with Fe catalyst 0.18 mol/l and CH 4 /H 2 at 860 C. can be seen in this SEM observation. However, the field emission characteristics show a slight improvement as the concentration decreases, as shown in Fig. 11. This improvement is thought to result from the slight relief of the screening effect as the density of CNTs decreases slightly due to the decrease of concentration. An emission current density of 1mA/cm 2 was obtained at 0.4 V/ m from this sample. Figure 12 shows the Fowler Nordheim F-N plot log J/F 2 1/F for the best sample grown with Fe catalyst and CH 4 /H 2. The concentration of FeSO 4 solution was 0.18 mol/ l, and the growth temperature and time were 860 C and 1 h, respectively. The field enhancement factor can be calculated from the slope of F-N plot since log J/F 2 =log A 2 / B 3/2 / F, where A= AeVV 2, B= ev 3/2 Vm 1, is the field enhancement factor, and is the work function. 20 Assuming =5 ev as for graphite, 21 the field enhancement factor of this sample is calculated to be The deviation of measured data from the straight line in the F-N plot is thought to be caused by the adsorbate-enhanced field emission mechanism 14 because no treatment was carried out to remove the adsorbates before the measurement. ACKNOWLEDGMENTS The work was supported by DOE under Grant No. DE- FG02-00ER45805 S.H.J., G.Y.X., and Z.F.R., by NSF under Grant No. NIRT Z.F.R., and by the US Army Research Development and Engineering Command, Natick Soldier Center, under Grant No. DAAD16-03-C-0052 Z.F.R.. 1 A. G. Rinzler, J. H. Hafner, P. Nikolaev, L. Lou, S. G. Kim, D. Tománek, P. Nordlander, D. T. Colbert, and R. E. Smalley, Science 269, W. A. De Heer, A. Châtelain, and D. Ugarte, Science 270, J.-M. Bonard, H. Kind, T. Stöckli, and L.-O. Nilsson, Solid-State Electron. 45, J.-M. Bonard, M. Croci, C. Klinke, R. Kurt, O. Noury, and N. Weiss, Carbon 40, S. H. Jo, D. Z. Wang, J. Y. Huang, W. Z. Li, K. Kempa, and Z. F. Ren, Appl. Phys. Lett. 85, S. H. Jo, D. Banerjee, and Z. F. Ren, Appl. Phys. Lett. 85, D. Banerjee, S. H. Jo, and Z. F. Ren, Adv. Mater. Weinheim, Ger. 16, J. Y. Huang, K. Kempa, S. H. Jo, S. Chen, and Z. F. Ren, Appl. Phys. Lett. 87, W. Z. Li, D. Z. Wang, S. X. Yang, J. G. Wen, and Z. F. Ren, Chem. Phys. Lett. 335, W. Z. Li, J. G. Wen, Y. Tu, and Z. F. Ren, Appl. Phys. A: Mater. Sci. Process. 73, W. Z. Li, J. G. Wen, and Z. F. Ren, Appl. Phys. A: Mater. Sci. Process. 74, W. Z. Li, J. G. Wen, and Z. F. Ren, Chem. Phys. Lett. 368, G. Y. Xiong, Y. Suda, D. Z. Wang, J. Y. Huang, and Z. F. Ren, Nanotechnology 16, K. A. Dean and B. R. Chalamala, Appl. Phys. Lett. 76, R. G. Forbes, C. J. Edgcombe, and U. Valdrè, Ultramicroscopy 95, L. Nilsson, O. Groening, O. Kuettel, P. Groening, and L. Schlapbach, J. Vac. Sci. Technol. B 20, J.-M. Bonard, K. A. Dean, B. F. Coll, and C. Klinke, Phys. Rev. Lett. 89, L. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J. M. Bonard, and K. Kern, Appl. Phys. Lett. 76, S. H. Jo, Y. Tu, Z. P. Huang, D. L. Carnahan, D. Z. Wang, and Z. F. Ren, Appl. Phys. Lett. 82, R. G. Forbes, Solid-State Electron. 45, J.-M. Bonard, J.-P. Salvetat, T. Stöckli, W. A. de Heer, L. Forró, and A. Châtelain, Appl. Phys. Lett. 73, J. Vac. Sci. Technol. B, Vol. 23, No. 6, Nov/Dec 2005