- Email: [email protected]
Change of surface morphology and field emission property of carbon nanotube films treated using a hydrogen plasma
Applied Surface Science 225 (2004) 380–388
Change of surface morphology and field emission property of carbon nanotube films treated using a hydrogen plasma Ke Yua,c,*, Ziqiang Zhua,*, Yongsheng Zhanga, Qiong Lia, Weiming Wanga, Laiqiang Luoa, Xianwen Yub, Honglei Mac, Zhenwen Lid, Tao Fenge a
Department of Electric Engineering, East China Normal University, 3663 Zhong Shan North Road, Shanghai 200062, PR China b School of Mathematics and Physics, Zhejiang Normal University, Jinhua 321004, PR China c School of Physics and Microelectronics, Shandong University, Jinan 250061, PR China d Shenzhen Nanotech Port Co., Ltd, Shenzhen 518057, PR China e Ion Beam Laboratory, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, PR China Received 27 May 2003; received in revised form 25 October 2003; accepted 25 October 2003
Abstract Large-scaled homogeneous multi-wall carbon nanotube (MWNT) films have been obtained at room temperature by a transfer technique. The surface morphology and microstructure features of the films before and after hydrogen plasma (HP) treatment were investigated by scanning electron microscopy, high-resolution transmission electron microscopy, and micro-Raman spectra. The electrical resistance in the temperature range 5–296 K and field emission in diode structure for the MWNT films were examined. Electron microscopy results demonstrated that the surface of treated nanotubes had become open palpus-like graphite layers or were covered by the onion-like carbon nanoparticles. The temperature dependence on electric resistance for MWNT films, which was fitted by three-dimensional variable range hopping conduction, was observed; and the slope and scope of the temperature dependence for the treated film were larger than that of the untreated one. The field emission tests indicated that the turn-on field (at 0.1 mA/cm2) decreases from 3.9 to 1.5 V/mm after the sample is treated with hydrogen plasma for 30 min. High emission spot density of about 1:5 104 cm2 is achieved at an applied electric field 5.2 V/mm. The conversion mechanism of the treated carbon nanotubes(CNTs) microstructure and the reason for the change of electronic properties and field emission of MWNT films were discussed. # 2003 Elsevier B.V. All rights reserved. PACS: 81.07.De; 82.33.Xj; 85.45.Db Keywords: Carbon nanotubes; Hydrogen plasma process; Field emission
1. Introduction * Corresponding authors. Tel.: þ86-21-62233780; fax: þ86-21-62225754. E-mail addresses: [email protected] (K. Yu), [email protected] (Z.Q. Zhu).
Since the discovery of carbon nanotubes (CNTs) in 1991 [1], much effort has been made in the study of their potential applications, especially in field
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.10.029
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
emission [2,3]. Compared with metal and semiconductor microtip field emission sources, carbon nanotubes can operate in less stringent vacuum conditions because of their high mechanical strength and high chemical stability. Up to now, many methods for synthesizing CNTs have been reported, including arc discharge [4], laser vaporization [5], pyrolysis [6] and plasma-enhanced chemical vapor deposition (CVD or PECVD) [7–9]. However, It is hard to fabricate the large-scaled homogeneous CNT film on the substrate with growth method in processing. In addition, low emission site density (ESD) and non-uniform emission remain the key issues to be addressed for display. In this paper, we report a transfer technique about CNT films fabrication, which involves printing technique, hydrogen plasma treatment, and some other processes. The change of morphology, microstructures, electronic properties and field emission of carbon nanotube films treated by hydrogen plasma (HP) were discussed. As a result, we have found that HP treated CNTs films with low turnon field and high emission site density are promising material for developing CNTs field emitter displays.
2. Experimental Multi-wall carbon nanotubes (MWNTs) were obtained by catalytic decomposition of CH4 on a La2NiO4 catalyst [10]. Raw MWNTs were first placed on an alumina plate, then heated to 500 8C for 20 min under normal atmosphere. After initial firing, the powders were removed from the furnace, and then separated using ultrasonic cleaner in ethanol. In order to remove catalyst particles attached at the extremities of the nanotubes, the MWNT powders were immersed in a low concentration of nitric acid solution, then followed by a washing by deionized water and dried at baking box. Through the temperature-programmed oxidation (TPO) method and the electron microscopy examination, it was proved that the purity of carbon nanotube was greater than 90%. A mechanical grinder mixed purified MWNT powder with ethanol, break powders, and disperse nanotubes uniformly in the paste. After having been dried, MWNTs (3 g) were mixed with the organic solvent (2 g), which was composed of ethyl cellulose and C10H18O with 3:97 mass ratio, and the mixture formed the black slurry
381
Fig. 1. Schematic diagram of the equipment used for HP treatment.
screen-printed on nickel or stainless steel substrates. A MWNTs film with thickness and area of about 10 mm and 15 cm2 was obtained. After deposition, the films were first annealed at atmospheric pressure of nitrogen at 500 8C for 10 min to remove the binders and solution, and to improve the contact between MWNTs and substrate, then the samples were handled through hydrogen plasma treatment. Fig. 1 is the schematic diagram of the plasma treatment equipment. The area of radio-frequency (rf) plate electrode is approximately 100 cm2, and the spacing of the electrode and samples is 5 cm. The radio-frequency source hereby was operated at 13.6 MHz with a rf power of 250 W. Planar capacitor coupling transferred energy into plasma. The conditions for hydrogen plasma treatment were as follows: hydrogen flow rate of 10 sccm, operating pressure of 200 Pa, a bias of 150 V, process duration of 10, 20, 30,and 40 min, and the average temperature of samples of about 60, 90, 120, 150 8C during the HP treatment, respectively. The surface morphology and microstructures of MWNT films were examined by scanning electron microscopy (SEM)(JEOL JSM-6700F, 10 KV), high-resolution transmission electron microscopy (HRTEM)(Hitachi H-9000, NAR 300 KV), and micro-Raman spectra (Jobin Yvon LabRam INFINITY) at excitation of 514.5 nm (Arþ-laser), respectively. The field emission test was preformed with diode structure under 105 Pa. The MWNT film of 1 cm 1 cm in size on Ni substrate (as a cathode) was separated from a phosphors/ITO/glass anode by two spacers, the spacers were composed of a Teflon film with a thickness of 200 mm. Through a window of the vacuum chamber, the distribution of the field emission
382
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
sites could easily be observed and recorded with a camera from the light points on fluorescent anodes. Meanwhile, current versus voltage curves were measured with standard electronic instruments. Electrical resistance of the rectangular MWNT films (1 cm in length; 0.5 cm in width; 10 mm in thickness) formed by the silk screen-printing on the bare glass substrate, were measured with the conventional fourprobe method in the temperature range of 5–296 K.
3. Results and discussion 3.1. Electron microscopy and Raman spectroscopy Fig. 2a-d are top-view SEM images of the samples with HP treatment for 0, 10, 20, 30 min, respectively. Fig. 2a shows an original MWNT film. The outer diameter of the tubes ranged in 30–40 nm and the typical length of tubes was about 1–5 mm. The MWNT surfaces were smooth and tidy. However, after the HP treatment, the configuration of nanotubes in the top
layer of film was totally changed. The smooth MWNT surfaces became rough (Fig. 2b) and were covered by ball nanoparticles with a size of 20–40 nm (Fig. 2c and d). Three-dimensional multiple-way connected webs were formed in local area by ‘‘nanoparticle-welded’’ MWNTs. These webs contained numerous ‘‘Y’’, ‘‘H’’, and ‘‘T’’-junction (see the inset of Fig. 2d). With the disapperance of the short and tiny MWNTs, the spaces between tubes became larger, and the nanotubes got thicker. HRTEM investigations were performed to study the treated and untreated nanotube microstructure in detail. Fig. 3a shows a TEM image of original MWNTs, which indicates the 90% original material is true MWNTs. As is shown in the inset of Fig. 3a, original MWNT was composed of many cylindrical graphite layers arranged regularly in the inner wall, and the ceatre part of MWNT is empty. Some amorphous carbon layers coating the outer periphery of carbon nanotube was observed. HRTEM images of MWNTs with HP treatment for 30 min revealed that some nanotube graphite layers were bent or wavy;
Fig. 2. SEM images of the MWNT films: (a) untreated, (b) HP treated for 10 min, (c) HP treated for 20 min, and (d) HP treated for 30 min. The inset is an image of three-dimensional multiple-way connected MWNT webs.
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
383
Fig. 3. HRTEM images of MWNTs: (a) the original MWNTs, (b) the HP treated MWNT with graphite layers that were bent, shaped into wave structure and became palpiform. The inset is an enlarged image that is pointed out by arrow, (c) the HP treated MWNT wall became open palpus-like graphite layers, (d) the MWNT was cut off, (e) onion-like carbon nanoparticles grew on the MWNT wall, (f) the HP treated MWNT with graphite layers that are bent and shaped into nanoparticles. The inset is an enlarged image that is pointed out by arrow.
some nanotube walls had become open palpus-like graphite layers. These changes indicated that there were many defects and dangling bonds on the surface of the tubes (Fig. 3b and c). The nanotube, which was cut off with opened tip, was observed as well (Fig. 3d). Fig. 3e shows that the onion-like carbon nanoparticles grew on the MWNT wall. The image of the bent nanotube graphite layers with irregular graphitic structures is shown in Fig. 3f. Palpiform of the nanotube wall and nanoparticles would contribute to electron emission. It is suggested that the formation of the microstructure of treated nanotubes is a process of structure reconstruction. During hydrogen plasma treatment, the amorphism of structure was produced on the MWNT graphite layers due to the bombardment of high energetic ions from the plasma, quite a
few defects such as dangling bonds, interlayer crosslinkings, sp3 defects and open palpus-like graphite layers were formed, and these interlayer crosslinkings and sp3 defects can lead to bent or irregular graphitic structures. Carbon etched from short and defective MWNTs as well as from remnant amorphous carbon was released in the plasma gas phase and re-deposited in the form of onion-like nanoparticles of different sizes. Accompanied by the destruction of some tubes, MWNTs became shorter and thicker on average; relative broader spaces between tubes were left. Raman spectroscopy has been extensively used to characterize various carbon materials, including nanotubes, and this technique shows a high sensitivity to disorder on the surface based on the optical skin depth [11]. Fig. 4 shows Raman spectra of the MWNT films
384
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
3.2. Electronic properties and field emission
Intensity (arb. units)
1350 1578
2700 2945 3242
2441
(b) 1568
4280
2691
1342
(a)
1000
2438
2000
2925
3220
3000
4263
4000
Raman shift (cm-1) Fig. 4. Raman spectra of the MWNT films: (a) untreated, and (b) HP treated for 30 min.
with non-treatment and HP treatment for 30 min. In the case of the untreated MWNT film, the G peak at ð2Þ 1568 cm1 was the E2g mode corresponding to the movement in opposite direction of two neighboring carbon atoms in a graphitic sheet and it indicates the presence of crystalline graphitic carbon in the MWNT film. The D peak approximately 1342 cm1 was a A1g breathing mode. This mode was generally attributed to defects in the curved graphite sheet, sp3 carbon or other impurities [12]. The R ¼ ID /IG ratio, where I corresponds to the peak area of the Lorentzian functions, allowed us to estimate the crystallite size and relative extent of structural defects. For the original MWNT film, this ratio was equal to 0.65. It was estimated that the in-plane graphite crystallite size was L ¼ 4:4 nm ðID =IG Þ1 ¼ 6:77 nm. For the MWNT film with hydrogen plasma treatment for 30 min, R ¼ 1:98 and L ¼ 2:22 nm. The appearance of the relatively strong D peak of treated film can be interpreted as being attributed to the wave structure of carbon sheets in tubes and the existence of onion-like carbon. Compared with original film, blue shift of each peak position of the treated MWNT film took place and these peaks were considerably weaker and broader. These can be resulted from the increased disorder and defect density in the treated MWNTs. Raman spectroscopy shows that HP treatment is able to produce radiation damage and various kinds of defects in the MWNT wall. This is in accordance with HRTEM results.
Electrical resistances of the MWNT films with nontreatment and HP treatment for 30 min were measured by the standard four-probe arrangement in a He cryostat. Silver paste was used to join contact the samples with Pt wires (0.05 mm in diameter). The ohmic contact was confirmed by checking the linear relationship between voltage and current in the measured temperature range 5 K < T < 296 K. The resistance R and the temperature T were measured with the Keithley 220 programmable current source (current level was about 100 mA), 181 nm and 439 Rh–Fe thermometer, respectively. Fig. 5 shows the temperature dependence on the normalized resistance for the MWNT films with and without HP treatment. For two samples, Negative dR/dT is observed over the entire range. However, compared with original MWNT film, R increased slowly with T decreasing for the treated film at high T. It is suggested that the treated MWNT film possesses more metallic properties whereas original MWNTs are much closer to a semiconductor. A steep increase of R at low temperature is characteristic of a weakly localized system. The resistances of two samples in logarithmic scale indicated linear temperature dependence when the abscissa represented T1/4 (see the inset of Fig. 5), which suggested the electron transport associated with three-dimensional variable range hopping [13]. The resistance R was directly proportional to exp½ð10:7=TÞ1=4 for the original film in the range 5 K < T < 40 K, to exp½ð59:5=TÞ1=4 for HP treated film below about 17 K, and to exp½ð13:6=TÞ1=4 between 17 K < T < 162 K. The resistivity r was about 2:2 104 Om for both of samples at 296 K. This change of electronic properties may be due to reconstruction of the nanotube structures during H-plasma treatment. The formation of three-dimensional multiple-way connected MWNT webs increased conductive connectivity, and this will reduce resistance R and slow down its increasing with T decreasing at high T. The large-scaled temperature dependence observed on electric resistance for the treated MWNT film is probably a result of the scattering of conduction electrons due to the increased defect density and disorder in the nanotube graphite layers [14]. The improvement of electric conductivity of the HP treated MWNT film is of benefit field emission,
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
1.6
a b
Ln [R /R (296K)]
R / R (296 K)
5
385
4
3
1.2 T=40K
0.8
0.4
0.0 0.2
T=17K
T=162K
0.3
2
0.4
0.5
0.6
0.7
T -1/4 (K -1/4)
1 0
50
1 00
150
200
250
300
T (K) Fig. 5. Resistance vs. temperature, normalized to 296 K, for the MWNT films: (a) untreated, and (b) HP treated for 30 min. The inset is the logarithmic plots of the electric resistance against T1/4.
which can lower heat of the emitter at the high field. Therefore it can enhance emission stability and energy availability. Field emission is one of the most promising applications for carbon nanotubes. Fig. 6 illustrates the typical curves of electron emission current density versus electric field from MWNT emitter films with different duration of HP treatment. The inset shows the corresponding Fowler–Nordheim (F–N) plots, and these approximately straight lines indicated that the emitting electrons were mainly resulted from field emission. It is evident that the field emission properties of the MWNT films are improved with the increase of duration of HP treatment within 30 min. The turn-on field, at which emission current density reaches 0.1 mA/cm2, decreases from 3.9 to 1.5 V/mm after the sample is treated with hydrogen plasma for 30 min. The current density of the MWNT emitter film with HP treatment for 30 min could reach 1 mA/cm2 with the fluctuation below 4% at an applied electric field
around 5.2 V/mm, which would meet the requirement for field emitter display application. Fig. 7 shows the emission images on the fluorescent screen of the MWNT emitter films with different duration of HP treatment at an applied electric field around 5.2 V/mm. It is found that with the increase of duration of HP treatment within 30 min, the EDS increased gradually and the light spots on the fluorescent screen became more uniform, fine, and dense. Roughly counted from a magnified emission photograph of the MWNT film with HP treatment for 30 min in Fig. 7d, the emission site density reached about 1:5 104 cm2. Based on our experimental observations, the improvement of field emission can be explained briefly as follows: According to FN theory, field emission properties depend on three parameters, i.e., the work function (emission energy barrier) of the emitter surface, F, the field enhancement factor, b, which is in first approximation proportional to the reciprocal of the emitter radius of curvature, and the
386
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
Fig. 6. Emission current density vs. electric field curves of the MWNT films: (a) untreated, (b) HP treated for 10 min, (c) HP treated for 20 min, (d) HP treated for 30 min, (e) HP treated for 40 min. The inset shows Fowler–Nordheim plots correspondingly.
emission site density. The slope of the F–N plot is proportional to F3=2 /b. We could find from the inset in Fig. 6 that the slope absolute value of the F–N plot decreased with the increase of duration of HP treatment. The decrease in the slope means a lowering in the work function and/or an increase in the field enhancement factor. Here, we believe that the HP treatment effects mainly increased the field enhancement factor; at the same time it might also lower the work function of emitter surface. The HP treatment can change the surface microstructure of MWNTs and produce open palpus-like graphite layers and carbon nanoparticles as the emitters on the MWNT walls, which decreased the curvature radius of the original emitter and increased the field enhancement factor as well as local electron field. In addition, many defects were produced on the surface of MWNTs due to the bombardment of H ions, it is thought that H atoms are more likely to bond on the defect sites and form C–H dipoles; hence the Hterminated regions alternate with non-H-termination regions on the surface of MWNTs. In the absence of external electrical field, the C–H dipoles will bring about the surface potential fluctuations between these
regions with different H-termination saturation so that a potential step occurs at the interface of these regions. The potential steps separate charges on MWNTs’ surface, so the positive and negative space charges collect on each side of the interface along the surface of the MWNTs, which resulted in a lateral surface electrical field. When the external field is applied, the surface electrical field can locally enhance the external field, which is equal to increase of the enhancement factor. Furthermore, the C–H bonds on the surface of MWNTs could lower the work function of emitter, which is also well documented in the literature [15]. The main reason why the homogeneity of electron emission and ESD were improved through HP treatment is the increase of the emitting sites. In general, electrons transport along the tube and can only emit from MWNT tip, however, if there are sharp geometric defects on the outer wall of the nanotubes, electrons can also emit from these defect regions in which there exist a high field enhancement. Therefore, the open palpus-like graphite layers, irregular graphitic structures and the carbon nanoparticles as emitters on the treated MWNTs wall can emit electrons, which
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
387
Fig. 7. Emission images of the MWNT films on the fluorescent screen: (a) untreated, (b) HP treated for 10 min, (c) HP treated for 20 min, (d) HP treated for 30 min, (e) HP treated for 40 min.
resulted in the increase of ESD and emission current of treated MWNT films. A prolonged HP treatment would reduce both emission current and ESD, as shown in Fig. 6(e) and Fig. 7(e), which is due to the dilution of MWNTs of the treated film.
emitter and the H-adsorption on the surface of MWNTs. In our experiment, the formation of the 3D multipleway connected nanotube webs may have potential application in nano-scale mechanical or electrical devices field.
4. Conclusions Acknowledgements Large-scaled homogeneous carbon nanotube films have been obtained at room temperature by a transfer technique. The effects of HP treatment for the MWNT films on the electronic properties and field emission were investigated. The surface morphology and microstructure characteristic of MWNT films were examined by SEM, HRTEM, and Raman spectroscopy. The electronic properties change of the HP treated film is due to the MWNT structure reconstruction. The field emission improvement is mainly resulted from the increase of emitting sites, sharpening of the
The authors acknowledge the financial support from the NSF of China (No. 69925409), a Key Basic Research and Development Program of China Grant (973) and the PSF of Shandong Province, China (No. Y2001 G02). References [1] S. Iijima, Nature (London) 354 (1991) 56. [2] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179.
388
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
[3] W.B. Choi, D.S. Chung, J.H. Kang, H.Y. Kim, Y.W. Jin, I.T. Han, et al., Appl. Phys. Lett. 75 (1999) 3129. [4] S. Iijima, T. Ichihashi, Nature (London) 363 (1993) 603. [5] A. Thess, R. Lee, P. Nikolaev, H.J. Dai, P. Petit, J. Robert, et al., Science 273 (1996) 483. [6] M. Terrones, N. Grobert, J. Olivarers, J.P. Zhang, H. Terrones, K. Kordatos, et al., Nature (London) 388 (1997) 52. [7] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegal, et al., Science 282 (1998) 1105. [8] O.M. Ku¨ ttel, O. Groening, C. Emmenegger, L. Schlapbach, Appl. Phys. Lett. 73 (1998) 2113. [9] S.L. Sung, S.H. Tsai, C.H. Tseng, F.K. Chiang, X.W. Liu, H.C. Shih, et al., Appl. Phys. Lett. 74 (1999) 197.
[10] Q. Liang, L.Z. Gao, Q. Li, S.H. Tang, B.C. Liu, Z.L. Yu, Carbon 39 (2001) 897. [11] M. Sveningsson, R.-E. Morjan, O.A. Nerushev, Y. Sato, J. Ba¨ ckstro¨ m, E.E.B. Campbell, F. Rohumund, Appl. Phys. A. 73 (2001) 409. [12] W.S. Bacsa, D. Ugarte, A. Chatelain, W.A. De Heer, Phys. Rev. B 50 (1994) 15473. [13] Y. Yoshida, I. Oguro, J. Appl. Phys. 83 (1998) 4985. [14] J.E. Fischer, H. Dai, A. Thess, R. Lee, N.M. Hanjani, D.L. Dehaas, R.E. Smalley, Phys. Rev. B 55 (1997) R4921. [15] A. Hart, B.S. Satyanarayana, W.I. Milne, J. Robertson, Appl. Phys. Lett. 74 (1999) 1594.
Change of surface morphology and field emission property of carbon nanotube films treated using a hydrogen plasma Ke Yua,c,*, Ziqiang Zhua,*, Yongsheng Zhanga, Qiong Lia, Weiming Wanga, Laiqiang Luoa, Xianwen Yub, Honglei Mac, Zhenwen Lid, Tao Fenge a
Department of Electric Engineering, East China Normal University, 3663 Zhong Shan North Road, Shanghai 200062, PR China b School of Mathematics and Physics, Zhejiang Normal University, Jinhua 321004, PR China c School of Physics and Microelectronics, Shandong University, Jinan 250061, PR China d Shenzhen Nanotech Port Co., Ltd, Shenzhen 518057, PR China e Ion Beam Laboratory, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, PR China Received 27 May 2003; received in revised form 25 October 2003; accepted 25 October 2003
Abstract Large-scaled homogeneous multi-wall carbon nanotube (MWNT) films have been obtained at room temperature by a transfer technique. The surface morphology and microstructure features of the films before and after hydrogen plasma (HP) treatment were investigated by scanning electron microscopy, high-resolution transmission electron microscopy, and micro-Raman spectra. The electrical resistance in the temperature range 5–296 K and field emission in diode structure for the MWNT films were examined. Electron microscopy results demonstrated that the surface of treated nanotubes had become open palpus-like graphite layers or were covered by the onion-like carbon nanoparticles. The temperature dependence on electric resistance for MWNT films, which was fitted by three-dimensional variable range hopping conduction, was observed; and the slope and scope of the temperature dependence for the treated film were larger than that of the untreated one. The field emission tests indicated that the turn-on field (at 0.1 mA/cm2) decreases from 3.9 to 1.5 V/mm after the sample is treated with hydrogen plasma for 30 min. High emission spot density of about 1:5 104 cm2 is achieved at an applied electric field 5.2 V/mm. The conversion mechanism of the treated carbon nanotubes(CNTs) microstructure and the reason for the change of electronic properties and field emission of MWNT films were discussed. # 2003 Elsevier B.V. All rights reserved. PACS: 81.07.De; 82.33.Xj; 85.45.Db Keywords: Carbon nanotubes; Hydrogen plasma process; Field emission
1. Introduction * Corresponding authors. Tel.: þ86-21-62233780; fax: þ86-21-62225754. E-mail addresses: [email protected] (K. Yu), [email protected] (Z.Q. Zhu).
Since the discovery of carbon nanotubes (CNTs) in 1991 [1], much effort has been made in the study of their potential applications, especially in field
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.10.029
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
emission [2,3]. Compared with metal and semiconductor microtip field emission sources, carbon nanotubes can operate in less stringent vacuum conditions because of their high mechanical strength and high chemical stability. Up to now, many methods for synthesizing CNTs have been reported, including arc discharge [4], laser vaporization [5], pyrolysis [6] and plasma-enhanced chemical vapor deposition (CVD or PECVD) [7–9]. However, It is hard to fabricate the large-scaled homogeneous CNT film on the substrate with growth method in processing. In addition, low emission site density (ESD) and non-uniform emission remain the key issues to be addressed for display. In this paper, we report a transfer technique about CNT films fabrication, which involves printing technique, hydrogen plasma treatment, and some other processes. The change of morphology, microstructures, electronic properties and field emission of carbon nanotube films treated by hydrogen plasma (HP) were discussed. As a result, we have found that HP treated CNTs films with low turnon field and high emission site density are promising material for developing CNTs field emitter displays.
2. Experimental Multi-wall carbon nanotubes (MWNTs) were obtained by catalytic decomposition of CH4 on a La2NiO4 catalyst [10]. Raw MWNTs were first placed on an alumina plate, then heated to 500 8C for 20 min under normal atmosphere. After initial firing, the powders were removed from the furnace, and then separated using ultrasonic cleaner in ethanol. In order to remove catalyst particles attached at the extremities of the nanotubes, the MWNT powders were immersed in a low concentration of nitric acid solution, then followed by a washing by deionized water and dried at baking box. Through the temperature-programmed oxidation (TPO) method and the electron microscopy examination, it was proved that the purity of carbon nanotube was greater than 90%. A mechanical grinder mixed purified MWNT powder with ethanol, break powders, and disperse nanotubes uniformly in the paste. After having been dried, MWNTs (3 g) were mixed with the organic solvent (2 g), which was composed of ethyl cellulose and C10H18O with 3:97 mass ratio, and the mixture formed the black slurry
381
Fig. 1. Schematic diagram of the equipment used for HP treatment.
screen-printed on nickel or stainless steel substrates. A MWNTs film with thickness and area of about 10 mm and 15 cm2 was obtained. After deposition, the films were first annealed at atmospheric pressure of nitrogen at 500 8C for 10 min to remove the binders and solution, and to improve the contact between MWNTs and substrate, then the samples were handled through hydrogen plasma treatment. Fig. 1 is the schematic diagram of the plasma treatment equipment. The area of radio-frequency (rf) plate electrode is approximately 100 cm2, and the spacing of the electrode and samples is 5 cm. The radio-frequency source hereby was operated at 13.6 MHz with a rf power of 250 W. Planar capacitor coupling transferred energy into plasma. The conditions for hydrogen plasma treatment were as follows: hydrogen flow rate of 10 sccm, operating pressure of 200 Pa, a bias of 150 V, process duration of 10, 20, 30,and 40 min, and the average temperature of samples of about 60, 90, 120, 150 8C during the HP treatment, respectively. The surface morphology and microstructures of MWNT films were examined by scanning electron microscopy (SEM)(JEOL JSM-6700F, 10 KV), high-resolution transmission electron microscopy (HRTEM)(Hitachi H-9000, NAR 300 KV), and micro-Raman spectra (Jobin Yvon LabRam INFINITY) at excitation of 514.5 nm (Arþ-laser), respectively. The field emission test was preformed with diode structure under 105 Pa. The MWNT film of 1 cm 1 cm in size on Ni substrate (as a cathode) was separated from a phosphors/ITO/glass anode by two spacers, the spacers were composed of a Teflon film with a thickness of 200 mm. Through a window of the vacuum chamber, the distribution of the field emission
382
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
sites could easily be observed and recorded with a camera from the light points on fluorescent anodes. Meanwhile, current versus voltage curves were measured with standard electronic instruments. Electrical resistance of the rectangular MWNT films (1 cm in length; 0.5 cm in width; 10 mm in thickness) formed by the silk screen-printing on the bare glass substrate, were measured with the conventional fourprobe method in the temperature range of 5–296 K.
3. Results and discussion 3.1. Electron microscopy and Raman spectroscopy Fig. 2a-d are top-view SEM images of the samples with HP treatment for 0, 10, 20, 30 min, respectively. Fig. 2a shows an original MWNT film. The outer diameter of the tubes ranged in 30–40 nm and the typical length of tubes was about 1–5 mm. The MWNT surfaces were smooth and tidy. However, after the HP treatment, the configuration of nanotubes in the top
layer of film was totally changed. The smooth MWNT surfaces became rough (Fig. 2b) and were covered by ball nanoparticles with a size of 20–40 nm (Fig. 2c and d). Three-dimensional multiple-way connected webs were formed in local area by ‘‘nanoparticle-welded’’ MWNTs. These webs contained numerous ‘‘Y’’, ‘‘H’’, and ‘‘T’’-junction (see the inset of Fig. 2d). With the disapperance of the short and tiny MWNTs, the spaces between tubes became larger, and the nanotubes got thicker. HRTEM investigations were performed to study the treated and untreated nanotube microstructure in detail. Fig. 3a shows a TEM image of original MWNTs, which indicates the 90% original material is true MWNTs. As is shown in the inset of Fig. 3a, original MWNT was composed of many cylindrical graphite layers arranged regularly in the inner wall, and the ceatre part of MWNT is empty. Some amorphous carbon layers coating the outer periphery of carbon nanotube was observed. HRTEM images of MWNTs with HP treatment for 30 min revealed that some nanotube graphite layers were bent or wavy;
Fig. 2. SEM images of the MWNT films: (a) untreated, (b) HP treated for 10 min, (c) HP treated for 20 min, and (d) HP treated for 30 min. The inset is an image of three-dimensional multiple-way connected MWNT webs.
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
383
Fig. 3. HRTEM images of MWNTs: (a) the original MWNTs, (b) the HP treated MWNT with graphite layers that were bent, shaped into wave structure and became palpiform. The inset is an enlarged image that is pointed out by arrow, (c) the HP treated MWNT wall became open palpus-like graphite layers, (d) the MWNT was cut off, (e) onion-like carbon nanoparticles grew on the MWNT wall, (f) the HP treated MWNT with graphite layers that are bent and shaped into nanoparticles. The inset is an enlarged image that is pointed out by arrow.
some nanotube walls had become open palpus-like graphite layers. These changes indicated that there were many defects and dangling bonds on the surface of the tubes (Fig. 3b and c). The nanotube, which was cut off with opened tip, was observed as well (Fig. 3d). Fig. 3e shows that the onion-like carbon nanoparticles grew on the MWNT wall. The image of the bent nanotube graphite layers with irregular graphitic structures is shown in Fig. 3f. Palpiform of the nanotube wall and nanoparticles would contribute to electron emission. It is suggested that the formation of the microstructure of treated nanotubes is a process of structure reconstruction. During hydrogen plasma treatment, the amorphism of structure was produced on the MWNT graphite layers due to the bombardment of high energetic ions from the plasma, quite a
few defects such as dangling bonds, interlayer crosslinkings, sp3 defects and open palpus-like graphite layers were formed, and these interlayer crosslinkings and sp3 defects can lead to bent or irregular graphitic structures. Carbon etched from short and defective MWNTs as well as from remnant amorphous carbon was released in the plasma gas phase and re-deposited in the form of onion-like nanoparticles of different sizes. Accompanied by the destruction of some tubes, MWNTs became shorter and thicker on average; relative broader spaces between tubes were left. Raman spectroscopy has been extensively used to characterize various carbon materials, including nanotubes, and this technique shows a high sensitivity to disorder on the surface based on the optical skin depth [11]. Fig. 4 shows Raman spectra of the MWNT films
384
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
3.2. Electronic properties and field emission
Intensity (arb. units)
1350 1578
2700 2945 3242
2441
(b) 1568
4280
2691
1342
(a)
1000
2438
2000
2925
3220
3000
4263
4000
Raman shift (cm-1) Fig. 4. Raman spectra of the MWNT films: (a) untreated, and (b) HP treated for 30 min.
with non-treatment and HP treatment for 30 min. In the case of the untreated MWNT film, the G peak at ð2Þ 1568 cm1 was the E2g mode corresponding to the movement in opposite direction of two neighboring carbon atoms in a graphitic sheet and it indicates the presence of crystalline graphitic carbon in the MWNT film. The D peak approximately 1342 cm1 was a A1g breathing mode. This mode was generally attributed to defects in the curved graphite sheet, sp3 carbon or other impurities [12]. The R ¼ ID /IG ratio, where I corresponds to the peak area of the Lorentzian functions, allowed us to estimate the crystallite size and relative extent of structural defects. For the original MWNT film, this ratio was equal to 0.65. It was estimated that the in-plane graphite crystallite size was L ¼ 4:4 nm ðID =IG Þ1 ¼ 6:77 nm. For the MWNT film with hydrogen plasma treatment for 30 min, R ¼ 1:98 and L ¼ 2:22 nm. The appearance of the relatively strong D peak of treated film can be interpreted as being attributed to the wave structure of carbon sheets in tubes and the existence of onion-like carbon. Compared with original film, blue shift of each peak position of the treated MWNT film took place and these peaks were considerably weaker and broader. These can be resulted from the increased disorder and defect density in the treated MWNTs. Raman spectroscopy shows that HP treatment is able to produce radiation damage and various kinds of defects in the MWNT wall. This is in accordance with HRTEM results.
Electrical resistances of the MWNT films with nontreatment and HP treatment for 30 min were measured by the standard four-probe arrangement in a He cryostat. Silver paste was used to join contact the samples with Pt wires (0.05 mm in diameter). The ohmic contact was confirmed by checking the linear relationship between voltage and current in the measured temperature range 5 K < T < 296 K. The resistance R and the temperature T were measured with the Keithley 220 programmable current source (current level was about 100 mA), 181 nm and 439 Rh–Fe thermometer, respectively. Fig. 5 shows the temperature dependence on the normalized resistance for the MWNT films with and without HP treatment. For two samples, Negative dR/dT is observed over the entire range. However, compared with original MWNT film, R increased slowly with T decreasing for the treated film at high T. It is suggested that the treated MWNT film possesses more metallic properties whereas original MWNTs are much closer to a semiconductor. A steep increase of R at low temperature is characteristic of a weakly localized system. The resistances of two samples in logarithmic scale indicated linear temperature dependence when the abscissa represented T1/4 (see the inset of Fig. 5), which suggested the electron transport associated with three-dimensional variable range hopping [13]. The resistance R was directly proportional to exp½ð10:7=TÞ1=4 for the original film in the range 5 K < T < 40 K, to exp½ð59:5=TÞ1=4 for HP treated film below about 17 K, and to exp½ð13:6=TÞ1=4 between 17 K < T < 162 K. The resistivity r was about 2:2 104 Om for both of samples at 296 K. This change of electronic properties may be due to reconstruction of the nanotube structures during H-plasma treatment. The formation of three-dimensional multiple-way connected MWNT webs increased conductive connectivity, and this will reduce resistance R and slow down its increasing with T decreasing at high T. The large-scaled temperature dependence observed on electric resistance for the treated MWNT film is probably a result of the scattering of conduction electrons due to the increased defect density and disorder in the nanotube graphite layers [14]. The improvement of electric conductivity of the HP treated MWNT film is of benefit field emission,
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
1.6
a b
Ln [R /R (296K)]
R / R (296 K)
5
385
4
3
1.2 T=40K
0.8
0.4
0.0 0.2
T=17K
T=162K
0.3
2
0.4
0.5
0.6
0.7
T -1/4 (K -1/4)
1 0
50
1 00
150
200
250
300
T (K) Fig. 5. Resistance vs. temperature, normalized to 296 K, for the MWNT films: (a) untreated, and (b) HP treated for 30 min. The inset is the logarithmic plots of the electric resistance against T1/4.
which can lower heat of the emitter at the high field. Therefore it can enhance emission stability and energy availability. Field emission is one of the most promising applications for carbon nanotubes. Fig. 6 illustrates the typical curves of electron emission current density versus electric field from MWNT emitter films with different duration of HP treatment. The inset shows the corresponding Fowler–Nordheim (F–N) plots, and these approximately straight lines indicated that the emitting electrons were mainly resulted from field emission. It is evident that the field emission properties of the MWNT films are improved with the increase of duration of HP treatment within 30 min. The turn-on field, at which emission current density reaches 0.1 mA/cm2, decreases from 3.9 to 1.5 V/mm after the sample is treated with hydrogen plasma for 30 min. The current density of the MWNT emitter film with HP treatment for 30 min could reach 1 mA/cm2 with the fluctuation below 4% at an applied electric field
around 5.2 V/mm, which would meet the requirement for field emitter display application. Fig. 7 shows the emission images on the fluorescent screen of the MWNT emitter films with different duration of HP treatment at an applied electric field around 5.2 V/mm. It is found that with the increase of duration of HP treatment within 30 min, the EDS increased gradually and the light spots on the fluorescent screen became more uniform, fine, and dense. Roughly counted from a magnified emission photograph of the MWNT film with HP treatment for 30 min in Fig. 7d, the emission site density reached about 1:5 104 cm2. Based on our experimental observations, the improvement of field emission can be explained briefly as follows: According to FN theory, field emission properties depend on three parameters, i.e., the work function (emission energy barrier) of the emitter surface, F, the field enhancement factor, b, which is in first approximation proportional to the reciprocal of the emitter radius of curvature, and the
386
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
Fig. 6. Emission current density vs. electric field curves of the MWNT films: (a) untreated, (b) HP treated for 10 min, (c) HP treated for 20 min, (d) HP treated for 30 min, (e) HP treated for 40 min. The inset shows Fowler–Nordheim plots correspondingly.
emission site density. The slope of the F–N plot is proportional to F3=2 /b. We could find from the inset in Fig. 6 that the slope absolute value of the F–N plot decreased with the increase of duration of HP treatment. The decrease in the slope means a lowering in the work function and/or an increase in the field enhancement factor. Here, we believe that the HP treatment effects mainly increased the field enhancement factor; at the same time it might also lower the work function of emitter surface. The HP treatment can change the surface microstructure of MWNTs and produce open palpus-like graphite layers and carbon nanoparticles as the emitters on the MWNT walls, which decreased the curvature radius of the original emitter and increased the field enhancement factor as well as local electron field. In addition, many defects were produced on the surface of MWNTs due to the bombardment of H ions, it is thought that H atoms are more likely to bond on the defect sites and form C–H dipoles; hence the Hterminated regions alternate with non-H-termination regions on the surface of MWNTs. In the absence of external electrical field, the C–H dipoles will bring about the surface potential fluctuations between these
regions with different H-termination saturation so that a potential step occurs at the interface of these regions. The potential steps separate charges on MWNTs’ surface, so the positive and negative space charges collect on each side of the interface along the surface of the MWNTs, which resulted in a lateral surface electrical field. When the external field is applied, the surface electrical field can locally enhance the external field, which is equal to increase of the enhancement factor. Furthermore, the C–H bonds on the surface of MWNTs could lower the work function of emitter, which is also well documented in the literature [15]. The main reason why the homogeneity of electron emission and ESD were improved through HP treatment is the increase of the emitting sites. In general, electrons transport along the tube and can only emit from MWNT tip, however, if there are sharp geometric defects on the outer wall of the nanotubes, electrons can also emit from these defect regions in which there exist a high field enhancement. Therefore, the open palpus-like graphite layers, irregular graphitic structures and the carbon nanoparticles as emitters on the treated MWNTs wall can emit electrons, which
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
387
Fig. 7. Emission images of the MWNT films on the fluorescent screen: (a) untreated, (b) HP treated for 10 min, (c) HP treated for 20 min, (d) HP treated for 30 min, (e) HP treated for 40 min.
resulted in the increase of ESD and emission current of treated MWNT films. A prolonged HP treatment would reduce both emission current and ESD, as shown in Fig. 6(e) and Fig. 7(e), which is due to the dilution of MWNTs of the treated film.
emitter and the H-adsorption on the surface of MWNTs. In our experiment, the formation of the 3D multipleway connected nanotube webs may have potential application in nano-scale mechanical or electrical devices field.
4. Conclusions Acknowledgements Large-scaled homogeneous carbon nanotube films have been obtained at room temperature by a transfer technique. The effects of HP treatment for the MWNT films on the electronic properties and field emission were investigated. The surface morphology and microstructure characteristic of MWNT films were examined by SEM, HRTEM, and Raman spectroscopy. The electronic properties change of the HP treated film is due to the MWNT structure reconstruction. The field emission improvement is mainly resulted from the increase of emitting sites, sharpening of the
The authors acknowledge the financial support from the NSF of China (No. 69925409), a Key Basic Research and Development Program of China Grant (973) and the PSF of Shandong Province, China (No. Y2001 G02). References [1] S. Iijima, Nature (London) 354 (1991) 56. [2] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179.
388
K. Yu et al. / Applied Surface Science 225 (2004) 380–388
[3] W.B. Choi, D.S. Chung, J.H. Kang, H.Y. Kim, Y.W. Jin, I.T. Han, et al., Appl. Phys. Lett. 75 (1999) 3129. [4] S. Iijima, T. Ichihashi, Nature (London) 363 (1993) 603. [5] A. Thess, R. Lee, P. Nikolaev, H.J. Dai, P. Petit, J. Robert, et al., Science 273 (1996) 483. [6] M. Terrones, N. Grobert, J. Olivarers, J.P. Zhang, H. Terrones, K. Kordatos, et al., Nature (London) 388 (1997) 52. [7] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegal, et al., Science 282 (1998) 1105. [8] O.M. Ku¨ ttel, O. Groening, C. Emmenegger, L. Schlapbach, Appl. Phys. Lett. 73 (1998) 2113. [9] S.L. Sung, S.H. Tsai, C.H. Tseng, F.K. Chiang, X.W. Liu, H.C. Shih, et al., Appl. Phys. Lett. 74 (1999) 197.
[10] Q. Liang, L.Z. Gao, Q. Li, S.H. Tang, B.C. Liu, Z.L. Yu, Carbon 39 (2001) 897. [11] M. Sveningsson, R.-E. Morjan, O.A. Nerushev, Y. Sato, J. Ba¨ ckstro¨ m, E.E.B. Campbell, F. Rohumund, Appl. Phys. A. 73 (2001) 409. [12] W.S. Bacsa, D. Ugarte, A. Chatelain, W.A. De Heer, Phys. Rev. B 50 (1994) 15473. [13] Y. Yoshida, I. Oguro, J. Appl. Phys. 83 (1998) 4985. [14] J.E. Fischer, H. Dai, A. Thess, R. Lee, N.M. Hanjani, D.L. Dehaas, R.E. Smalley, Phys. Rev. B 55 (1997) R4921. [15] A. Hart, B.S. Satyanarayana, W.I. Milne, J. Robertson, Appl. Phys. Lett. 74 (1999) 1594.