Influence of field evaporation treatment on the field emission properties of carbon nanotubes array

Applied Surface Science 256 (2010) 3912–3916

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Influence of field evaporation treatment on the field emission properties of carbon nanotubes array§ Xin Bai a,*, Wen-Jing Zhang b, Gengmin Zhang b a Institute of Optoelectronics, Shenzhen University, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education, Nanhai Ave. 3688, Shenzhen 518060, Guangdong, China b Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 23 September 2009 Received in revised form 15 January 2010 Accepted 18 January 2010 Available online 25 January 2010

Field evaporation was used in the post-fabrication treatment of a carbon nanotubes (CNTs) array and effectively modified the CNTs morphology in favor of the field emission under a moderate field. After the field evaporation treatment, the uniformity of the emission site distribution improved but the onset voltage rose. Using the Fowler–Nordheim theory, the actual onset field and the evaporation field around the CNT were calculated to be 4.6–5 and 9–12 V/nm, respectively. These values are close to those obtained from the individual CNT samples. The above results have provided an alternative to modify the configuration of an array sample and demonstrated the feasibility of tackling the problem of the disparity in the field emission capability of different CNTs in an array. Crown Copyright ß 2010 Published by Elsevier B.V. All rights reserved.

Keywords: Carbon nanotubes Field evaporation Field emission

1. Introduction Ever since it was discovered, carbon nanotubes (CNTs) have been the subject of extensive and in-depth studies for its peculiar physical and chemical properties. CNTs are characterized by small curvature radius, an extremely high aspect ratio, stable chemical properties, ultra-high mechanical strength, and excellent thermal and electrical conductivity, enabling carbon nanotubes to be highly suitable for use as field emission material [1,2]. The application of CNTs field emission electron source can be used for electron microscopy, microwave devices [2–4], field emission flat panel display [5–8], etc. CNTs, when used in field emission display, are characterized by their low working voltage and low power dissipation; moreover, CNTs themselves are so stable that they do not readily react with other materials. Its high mechanical strength can help prolong the service life and enhance the stability of displaying devices. The successful production of large-area wellaligned CNTs film has made it possible to use CNTs in flat panel display, such as wall-hanging television, and the weight and volume of traditional displaying devices are expected to shrink further. Hence, field emission display is likely to be a strong contender for cathode ray tubes (CRT) and liquid crystal displays (LCD) in the future market, while CNTs are the best prospective material for field emission cathode arrays.

§ Project supported by the National Natural Science Foundation of China (Grant No. 60771004) and the MOST of China (Nos. 2006CB932402 and 2007CB936204). * Corresponding author. Tel.: +86 755 267 33319. E-mail address: [email protected] (X. Bai).

In November 1995, de Heer et al. [6] reported the research results of the field emission of CNTs film, and fabricated a crude flat field emission prototype using CNTs for the first time. Soon afterwards, by employing the Spindt tip model [9], a number of experimental groups successfully created prototype CNTs field emission display in succession [5,7,8,10]. Notwithstanding the current field-emission laboratory products, there still remain several unsolved problems in flat field emission. During field emission, the adsorption and desorption of the tip have significant effect on emission currents; when in use, the tip end may be subjected to degradation and failure [11]. Furthermore, the tip end restructures [12,13] or blunts [14] under an intense field; the challenge still remains to find out how to create a field emission tip array of optimal consistency or how to give the tip array a consistency treatment [15,16]. Therefore, despite the fact that Samsung released its 4.5-inch field emission display in as early as 1999 [7], no commercial product is available today in the market. Apart from manufacturing techniques and the cost factor, the main reason lies in our inadequate knowledge of the uniformity of largearea flat field emission, as well as the CNTs emission mechanism. Indeed, the uniformity of flat field emission is a worldwide problem, which is yet to be neatly solved. In this paper, the research work is to solve this problem for the efforts and attempts. That is, in order to improve the uniformity of the field emission of CNTs array, the field evaporation technique was used on the CNTs array sample and the stand out tips were evaporated, then the ability of the field emission tips is basically the same, hence the emission sites distribution (ESD) carry out uniformity. As early as 1995, Rinzler et al. [17] announced a controversial result by claiming that some carbon atom chains had been pulled

0169-4332/$ – see front matter . Crown Copyright ß 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.01.049

X. Bai et al. / Applied Surface Science 256 (2010) 3912–3916

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Table 1 Treatment process and parameters of the field evaporation of the sample. Treatments

V (kV) Time (min) V(I = 500 mA)

1

2

3

4

5

6

7

8

9

10

11

12

2.11 10 1838

2.08 9 1923

2.11 27 1880

2.42 33 1952

2.72 20 1931

2.78 15 2172

2.77 37 2202

3.08 15 2182

3.51 3.5 h 2495

3.51 5 2502

3.72 28 2554

3.93 41 2621

V in the first line refers to the positive voltage applied to the field evaporation, with a unit of kV; the unit of time is minute, except that the ninth treatment is timed by hour; VI = 500 mA indicates the value of negative voltage when the field-emission current stands at 500 mA.

out from the CNTs during field emission. Four years later, Hata et al. [18] observed for the first time the field evaporation of singlewalled CNTs in the field ion microscopy with a threshold electric field of about 10 V/nm. After that, the study of CNTs field evaporation has been relatively slow, only one of the few published theoretical results [19,20]. It deserves special note that in recent years ‘‘field evaporation’’ is often used to refer to the evaporation of nanometer materials under intense field electron emission currents, of which the major reason is Joule heat. In this case, the voltage applied to the nanometer materials is negative [21–23]. To discriminate from the said conception, we still follow the traditional definition of ‘‘field evaporation’’ [24]. When positive voltage, which is high enough, is applied to the sample, positive ions will be emitted from its surface. When foreign atoms deposited on the surface of the sample break away from it in the form of ions, it is called as field desorption; when the atoms of the sample break away from it in the form of ions, it is called as field evaporation. Through in situ observation of the TEM-nanofactory system, Wang et al. [25] found that the field evaporation could render individual CNT end atomically flat and even cut the CNT short when the evaporation lasts comparatively long. We have also found that the work function of the individual samples is changed [26] after field evaporation, which further affects the CNT field emission properties. In view of the requirement of plane array sample field emission for the consistency of tips and uniformity of ESD, we hypothesize whether it is feasible to transfer the field evaporation technique to planar samples by exploiting its ability to modify the CNTs morphology. This paper will introduce the efforts and attempts made by this laboratory in this regard. Experimental results show that the ESD uniformity is improved by field evaporation treatment, and also increases the onset voltage. However, from the point of view of the practical applications, we believe that the increase of turn-on voltage to a certain degree for ESD uniformity is worth it. 2. Experiment Hot filament CVD method is applied to fabricate the planar array CNTs samples on Si(1 0 0) substrate, all of which are multiwalled CNTs with diameter varying from 60 to 100 nm and approximately 20 mm in length, the size of the substrate is about 0.8 cm  0.6 cm. See Fig. 1(a) for the scanning electron microscopy (SEM) photos of samples fabricated with orderly and extremely intensive CNTs array. The field evaporation and emission experiment were conducted in the home-made field emission microscopy (FEM) and the base vacuum of the FEM approximates 1  106 Pa; the distance d between the anode and the cathode can be regulated consecutively between 0 and 2 mm and is usually adjusted as d = 500 mm. When conducting the field emission I–V test, as cathode, the CNTs array sample links to the negative high voltage with 0 to 6 kV power supply; anode is the fluorescent screen, which is used to collect field-emission current and observe field emission ESD by being earthed after connecting an amperemeter in series. Reverse the negative and positive, so the

CNTs sample becomes anode and can do the experiment field evaporation treatment. First of all, the field emission I–V curve was tested, and the I–V curve is used to determine the threshold voltage of field emission. According to the aforementioned experimental results of an individual CNT field evaporation, the threshold voltage of field evaporation is about 3 times [25,26] as that of the threshold voltage of field emission (because the anode–cathode distance is fixed, for simplification, the voltage can be regarded as field). In order to find the transformation of the field emission properties between before and after field evaporation treatment, the I–V curves were measured before and after field evaporation. See Table 1 for parameters of the field evaporation treatment process of the samples. 3. Result and argumentation The distribution of the field emission sites of the sample before field evaporation treatment is shown in Fig. 1(c), due to the shielding effect; the field emission derives almost from the array edge only. And see Fig. 1(d), the distribution undergoing 12 consecutive evaporation treatments lasting a total of around 7.5 h, this situation improved, ESD becomes more uniformity. Meanwhile, the turn on electric field of the samples (here, this refers to the average field strength between the anode and the cathode when the field-emission current density reaches 1 mA/cm2 instead of the actual field at the tip of the CNTs. It can be expressed by F*, i.e., F* = V/d, where V is the applied voltage, while d is the anode– cathode distance) demonstrates an uptrend. When the emission current reaches 500 mA, the corresponding voltage rose to 2621 V from its original 1838 V. See Fig. 2(a) for I–V curves, Fig. 2(b) for the F–N plots, and Fig. 2(c) for the turn on electric field of the samples. Fig. 1(b) refers to the SEM images of the samples after the field evaporation treatment. From the abovementioned images, it can be noticed that the CNTs arrays appear to be disordered and unsystematic after field evaporation treatments; even some CNTs are directly dragged away from the base by the strong field. Although based on some oversimplified assumptions, such as the negligence of the emitter-surface curvature and any electronband structures other than the Fermi sea, the Fowler–Nordheim (F–N) theory is still extensively used as a standard model in the study of the I–V behaviors of a field emitter [9,27]: 3=2

J ¼ 1:54  106

y ¼ 3:79  104

F2 f exp 6:87  107 vðyÞ F ft 2 ðyÞ E1=2

f

! (1)

(2)

where J is the field-emission current density in A/cm2, f is work function of the emitter in eV; and t and v are two slowly varying functions that are generally approximate to the constant 1. F is the actual local-electric field around the sample surface in V/cm (F* = V/d, refers to the average field strength between anode–

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Fig. 1. SEM images and ESD of the CNTs array sample, (a) and (c) denote before, and (b) and (d) denote after field evaporation treatment. The voltages applied on the sample to measure the ESD are (b) 1.67 kV and (d) 3.04 kV.

Fig. 2. The variety of the field emission properties during the field evaporation treatment process, (a) I–V curves, (b) F–N plots, (c) the average electric field F*, and (d) the actual electric field F near the sample surface. The values beside the legends in (a) and (b) and the values overlapped on the plot (c) are the field evaporation treatment voltages.

X. Bai et al. / Applied Surface Science 256 (2010) 3912–3916

cathode). F cannot be measured directly in the experiment and has to be related to the applied voltage V and anode–cathode distance through a coefficient. In a configuration with the electrodes quite near to each other, F is usually expressed as follows: F¼

bV

(3)

d

The value of b has no dimension and is called the field enhancement factor [24]. In our experiment, the anode–cathode distance d is settled 500 mm. Also, it is the field-emission current, I, instead of J, that is directly measurable. By adopting trigonometric approximation, that means t = 1 and v ¼ 1, from formula (1) it is easy to find that:     I 1 3=2 d ln 2 ¼ 6:87  107 f  b V V ! 2 þ ln 1:54  106  A 

b

2

d f

(4)

where A is the effective area of the field emission. This is a linear equation concerning lnðI=V 2 Þ and V 1 with its slope coefficient 3=2 decided by (f d=b). The field enhance factor b can be easily deduced by fitting the F–N plots (it should be noted that the field enhancement factor b is the average field enhancement factor in allusion to the overall sample surface); thus, the actual field strength F near the sample surface can further be deduced. Fig. 2(c) and (d) shows the trend of the threshold field, in which (c) refers to F*, and (d) refers to F. During the overall measurement process, F* witnesses an uptrend from the original 2.3 to 3.5 V/mm. But the field enhancement factor declined from 2.2  103 to 1.5  103, and F maintains comparative stability, fluctuating between 4.6 and 5.0 V/nm. A comparison between the patterns in Fig. 1(c) and (d) shows that the emission sites were no longer restricted to the edge area after the field evaporation treatment. That is, the previously dominating emitters at the edge were shortened by the field evaporation so that the CNTs in the interior of the array had a chance to also participate in the emission. As shown in the SEM image in Fig. 1(b), the CNTs were not packed to each other closely any more after the field evaporation treatment and the screening effect could be effectively avoided. Hence the CNTs at the interior of the array could experience stronger electric field and also contribute to the emission. The comparative stability F means the actual threshold field of field emission does not experience tremendous changes, which demonstrates the field evaporation treatment may only improve ESD and cause threshold voltage rising, but exert little influence upon the actual threshold field of field emission. Judging from the statistics in Table 1 and Fig. 2(c), which correspond to the 3-step-like rise of the field evaporation voltage, voltage VI = 500 mA also increases in the same way, while the emission current stays at 500 mA. In other words, under each staircase-type field evaporation treatment voltage, F* maintains relative stability rather than varying drastically along with the extension of the field evaporation treatment period. It could be understood in this way: upon each application of positive voltage, the parts of most protruding, that have the strongest emissive ability, were evaporated first, which leads to the declined field enhancement factor. The declined field enhancement factor reduced the actual field near the sample surface, so it lowered and even stopped the evaporation process that makes no major changes occur to the sample’s morphology. The note process indicated why the field emission ability remains almost unchanged after approximate evaporation voltages treatment. With b ¼ ð2:21:5Þ  103 and field evaporation voltage ranging

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from 2.1 to 3.9 kV, the actual evaporation field on the sample’s surface approximates 9.2–12 V/nm, close to that of a single CNT [26]. The F–N plots in Fig. 2(b) illustrate that emission becomes more stable after the positive electric field operation. As the field evaporation proceeds, the I–V curves shift from left to right, while the F–N plots shift from right to left. Inconsistent with the aforementioned F–N theory (i.e., the ln(I/V2) and V1 amount to the linear equation), the far right curve in Fig. 2(b) and its corresponding F–N curve at the primary field evaporation stage distinctly deviate from the linear equation. 4. Conclusion The field evaporation technique was transferred to deal with the CNTs planar sample and realized its morphology re-treatment. This provides an alternative for controlling large-scale CNTs morphology so as to solve the problem of disparity in field emission of array samples. Experiments show that the fieldevaporation phenomenon takes place under a positive electric field within 9–12 V/nm, and the sample ESD treated by the field evaporation undergoes certain improvement. Moreover, the field emission onset voltage tends to ascend in the field-evaporation process; however, the actual field strength of the sample’s surface fluctuates between 4.6 and 5.0 V/nm, which illustrates the little influence of the field evaporation on the actual onset field of the sample’s surface. Acknowledgements We are grateful to professors Zengquan Xue and Zhaoxiang Zhang for illuminating discussions and making numerous valuable comments. References [1] Y. Saito, R. Mizushima, T. Tanaka, et al., Synthesis, structure and field emission of carbon nanotubes, Fullerene Sci. Technol. 7 (1999) 653–664. [2] N. de Jonge, Y. Lamy, K. Schoots, et al., High brightness electron beam from a multi-walled carbon nanotube, Nature 420 (2002) 393–395. [3] H. Schmid, H.W. Fink, Carbon nanotubes are coherent electron sources, Appl. Phys. Lett. 70 (1997) 2679–2680. [4] C. Oshima, K. Mastuda, T. Kona, et al., Young’s interference of electrons in field emission patterns, Phys. Rev. Lett. 88 (2002) 038301–138301. [5] M.Q. Ding, X.H. Li, G.D. Bai, et al., Fabrications of Spindt-type cathodes with aligned carbon nanotube emitters, Appl. Surf. Sci. 251 (2005) 201–204. [6] W.A. de Heer, A. Chatelain, D. Ugarte, A carbon nanotube field-emission electron source, Science 270 (1995) 1179–1180. [7] W.B. Choi, D.S. Chung, J.H. Kang, et al., Fully sealed, high-brightness carbonnanotube field-emission display, Appl. Phys. Lett. 75 (1999) 3129–3131. [8] R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Carbon nanotubes – the route toward applications, Science 297 (2002) 787–792. [9] C.A. Spindt, I. Brodie, L. Humphrey, et al., Physical properties of thin-film field emission cathodes with molybdenum cones, J. Appl. Phys. 47 (1976) 5248– 5263. [10] D. Normile, Technology – nanotubes generate full-color displays, Science 286 (1999) 2056–2057. [11] J.M. Bonard, C. Klinke, K.A. Dean, et al., Degradation and failure of carbon nanotube field emitters, Phys. Rev. B 67 (2003). [12] K. Tunvir, A. Kim, S.H. Nahm, The effect of two neighboring defects on the mechanical properties of carbon nanotubes, Nanotechnology 19 (2008) 065703. [13] N. de Jonge, M. Doytcheva, M. Allioux, et al., Cap closing of thin carbon nanotubes, Adv. Mater. 17 (2005) 451–455. [14] X. Bai, M.S. Wang, G.M. Zhang, et al., Field emission of individual carbon nanotubes on tungsten tips, J. Vac. Sci. Technol. B 25 (2007) 561–565. [15] J.H. Song, M.Y. Sun, Q. Chen, et al., Field emission from carbon nanotube arrays fabricated by pyrolysis of iron phthalocyanine, J. Phys. D 37 (2004) 5–9. [16] Y.Q. Fu, A. Colli, A. Fasoli, et al., Deep reactive ion etching as a tool for nanostructure fabrication, J. Vac. Sci. Technol. B 27 (2009) 1520–1526. [17] A.G. Rinzler, J.H. Hafner, P. Nikolaev, et al., Unraveling nanotubes – field-emission from an atomic wire, Science 269 (1995) 1550–1553. [18] K. Hata, M. Ariff, K. Tohji, et al., Selective formation of C-20 cluster ions by field evaporation from carbon nanotubes, Chem. Phys. Lett. 308 (1999) 343– 346.

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[19] N. Nakaoka, K. Tada, K. Watanabe, Theory of field evaporation and field emission from carbon nanotubes, Phys. B: Condens. Matter 323 (2002) 214–215. [20] N. Ohmae, N. Matsumoto, T. Ohata, et al., Atomic structure and field evaporation of carbon nanotube studied by atomprobe field ion microscopy, Diam. Relat. Mater. 16 (2007) 1179–1182. [21] K.A. Dean, T.P. Burgin, B.R. Chalamala, Evaporation of carbon nanotubes during electron field emission, Appl. Phys. Lett. 79 (2001) 1873–1875. [22] M.S. Wang, L.M. Peng, J.Y. Wang, et al., Electron field emission characteristics and field evaporation of a single carbon nanotube, J. Phys. Chem. B 109 (2005) 110–113.

[23] A.G. Umnov, V.Z. Mordkovich, Field-induced evaporation of carbon nanotubes, Appl. Phys. A: Mater. 73 (2001) 301–304. [24] .R. Gomer, Field Emission and Field Ionization, Harvard University Press, MA, 1961 [25] M.S. Wang, Q. Chen, L.-M. Peng, Grinding a nanotube, Adv. Mater. 20 (2008) 724. [26] X. Bai, M.S. Wang, Y. Liu, et al., Field evaporation of the end of a carbon nanotube, Acta Phys. Sin. 57 (2008) 638 (in Chinese). [27] H. Fowler, Nordheim, Electron emission in intense electric fields, Proc. R. Soc. Lond. Ser. A 119 (1928) 173–181 (Containing Papers of a Mathematical and Physical Character).