Thin Solid Films 435 (2003) 318–323
Density control of carbon nanotubes using NH3 plasma treatment of Ni catalyst layer Jong Hyung Choia, Tae Young Leea, Sun Hong Choia, Jae-Hee Hana, Ji-Beom Yooa,*, Chong-Yun Parka, Taewon Jungb, SeGi Yub, Whikun Yib, In Taek Hanb, J.M. Kimb a
Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, 300, Chunchun-Dong, Jangan-Gu, Suwon 440-746, South Korea b FED projects, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Korea
Abstract The effect of NH3 plasma pre-treatment on the growth characteristics of CNTs was investigated. We observed that NH3 plasma pre-treatment etched and conglomerated Ni catalyst film, resulting in the formation of Ni nanoparticles. The aligned CNTs from the Ni nanoparticles were grown by plasma enhanced chemical vapor deposition (PECVD). As Ni film thickness decreased from ˚ the size of Ni nanoparticles decreased about from 140 to 90 nm and the average diameter of CNTs became smaller. 300 to 30 A, As the NH3 plasma power was increased, the density of Ni nanoparticles was decreased, leading to the decrease in the density of CNTs. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Carbon nanotubes; NH3 plasma pre-treatment; Ni nanoparticles; Density control
1. Introduction CNTs have attracted great attention because of their unique physical properties such as high mechanical strength and thermal stability and electronic properties w1,2x. Because CNTs have high electron emission efficiency at low voltage due to high aspect ratio, CNTs have been regarded as the most efficient electron emitter among various materials. For application as an electron emitter, the density control of CNTs including the control of length and diameter of CNTs is required because of the screening effect. Although, the diameter and length can be easily changed by the catalyst layer thickness, the control of CNTs density is reported to be very difficult w3,4x. A method such as an electron-beam lithography and photolithography technique can be used to control the CNT. However, high cost hinders the application of these techniques to large area field emission display. There are many methods for synthesis of CNTs. Especially, plasma enhanced chemical vapour deposition (PECVD) method has been reported as one of the most *Corresponding author. Tel.: q82-31-290-7413; fax: q82-31-2907410. E-mail address:
[email protected] (J.-B. Yoo).
promising candidates for the synthesis of CNTs due to the low temperature growth, vertical alignment and large area growth w5,6x. Plasma etching of catalytic layer in PECVD system can be applied to the density control of CNTs through the density control of catalytic particle. In this study, we used NH3 plasma pre-treatment to form Ni nanoparticles. NH3 plasma pre-treatment of Ni layer was adoped to control the size and density of Ni nanoparticles. CNTs were grown on the Ni nanoparticles. The density and growth characteristics of CNTs were investigated. 2. Experiments NH3 plasma pre-treatment was used to make a Ni nanoparticles. Ni catalytic layer was deposited on the Cr coated substrate using magnetron sputtering. The ˚ thickness of Ni layer was changed from 30 to 300 A. ˚ was deposited on the The Cr buffer layer of 1500 A substrate by magnetron sputtering to prevent a reaction of the metal catalyst with the Si substrate. The metal coated substrate was transferred to the reaction chamber and pumped down below 2=10y3 Torr by a rotary pump. After the pressure was stabilised and substrate
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090Ž03.00341-9
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˚ and pre-treatment Fig. 1. SEM micrographs showing change in the morphology of Ni films with NH3 pre-treatment time: (a) Ni layer of 30 A ˚ and pre-treatment for 10 min and (d) 40 min; (e) Ni layer of 300 A ˚ and pre-treatment for 10 for 10 min and (b) 40 min; (c) Ni layer of 100 A min and (f) 40 min.
was heated to the desired temperature, NH3 was introduced into the chamber. For the pre-treatment of Ni layer, 30 and 70 W of dc NH3 plasma was applied for 10 to 40 min at 390 8C. The CNT growth was performed at 550 oC for 15 min by the PECVD. A C2H2 gas was used as the carbon source. We grew CNTs film in the pressure of 3.25 Torr by maintaining the flow of acetylene and ammonia gases with a total flow rate of 42.9– 40.3 sccm. The volume ratio of acetylene and ammonia
was 1:4. Detailed processes for the growth of CNTs were described in our previous reports w7,8x. Field Emission Scanning Electron Microscopy (FESEM) was employed for the analysis of the morphology and density of CNTs. To investigate crystallization and defects in CNTs with NH3 pre-treatment, we used Raman spectroscopy. The emission characteristics of CNTs were measured in a vacuum chamber with a parallel diodetype configuration at 3=10y6 Torr.
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Fig. 2. SEM images of CNTs grown for 15 min at 550 8C after NH3 plasma pre-treatment for 20 min. Plasma power for the pre-treatment and ˚ (b) Ni layer of 100 A; ˚ (c) Ni layer of 300 A. ˚ Inset of (a), (b) and (c) correspond growth are 30 and 70 W, respectively: (a) Ni layer of 30 A; to Ni catalyst layer after NH3 plasma pre-treatment.
3. Results and discussion Fig. 1 shows a change in the surface morphology of Ni film with NH3 plasma pre-treatment time. Applied plasma power was 30 W and the temperature during pre-treatment was 390 8C. and the pressure was approximately 2.82 Torr. We observed that Ni film was trans˚ the formed into nanoparticles. For the Ni layer of 30 A average size of nanoparticles increased from 55 to 94 nm as the pre-treatment time increase from 10 to 40 min (Fig. 1a and b). But the density of particles was not remarkably reduced. Although, NH3 plasma pretreatment provided an etching precursors, an etching effect of NH3 plasma was negligible because volume of Ni nanoparticles was not reduced compared with that of as-deposited Ni film (we calculated volume of Ni nanoparticles based on the diameter of Ni nanoparticles through image analyzer and height of Ni nanoparticles through SEM cross-sectional view). It was suggested that NH3 plasma pre-treatment has a heating effect on Ni film rather than an etching effect under the given condition. With an increase in Ni film thickness, time for the formation of nanoparticle increased but a trend in change of surface morphology was similar to that of
˚ (Fig. 1d and f). The size of particles Ni layer of 30 A was increased from 90 to 140 nm with an increase in ˚ Ni film thickness from 30 to 300 A. After the Ni nanoparticles were formed using NH3 plasma pre-treatment for 20 min, CNTs were grown from the Ni nanoparticles by DC-PECVD for 15 min ˚ average size of (Fig. 2). In case of Ni layer of 30 A, Ni nanoparticles was 60 nm after NH3 plasma pretreatment. But Ni film was not completely transformed ˚ (Inset of into particles in Ni layer of 100 and 300 A Fig. 2). As the thickness of Ni film increased from 30 ˚ the average diameter of grown CNTs was to 300 A increased from 20 to 70 nm due to thicker catalytic layer (Fig. 2a and c). This result agrees well with previous report w9x. If the size of Ni nanoparticles were less than 100 nm, CNTs were directly grown on each Ni particles and the diameter of CNTs had almost the same size of catalytic particles w4x. However, in this study, the diameter of grown CNTs was smaller than that of Ni nanoparticles. There are several reasons for smaller diameter of CNTs than that of nanoparticle. First, it may be likely that the particle was etched and divided into smaller parts during the CNTs growth, because plasma intensity during the CNTs growth was
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Fig. 3. SEM micrographs showing the morphology of Ni film and CNTs with increase in NH3 plasma power from 30 to 70 W. Pre-treatment and ˚ after pre-treatment; (b) morphology of growth time of all samples were fixed 20 and 15 min, respectively: (a) morphology of Ni layer of 30 A ˚ after pre-treatment; (c) morphology of Ni layer of 300 A ˚ after pre-treatment; (d) CNTs grown on Ni 30 A ˚ after pre-treatment; Ni layer of 100 A ˚ after pre-treatment; (f) CNTs grown on Ni 300 A ˚ after pre-treatment. (e) CNTs grown on Ni 100 A
70 W which is larger than the power for NH3 pretreatment. Second, it was reported that several CNTs grew on the catalytic particle if the size of the catalytic particle was larger than the critical size w10x. In this study, formed Ni nanoparticle may be larger than the critical size of catalytic particle, resulting in the growth of several CNTs from one Ni nanoparticle. Reduction of Ni nanoparticle size through NH3 pre-treatment at higher plasma power than 30 W was investigated. Fig. 3a, b and c is SEM images of surface morphology of Ni catalytic layer after pre-treatment at the increased power of 70 W for 20 min. SEM images of CNTs grown on Ni nanoparticles are shown in Fig. 3d, e and f.
Because the etching effect became dominant factor at the increased NH3 plasma intensity, Ni film was partially transformed into a nanoparticles and etched off by NH3 plasma at the same time. The size of remained Ni nanoparticles was smaller than that of Fig. 1a and the density of particles was reduced because smaller particles was etched by NH3 plasma. Consequently, density of CNTs was decreased from 2=109 cmy2 (Fig. 2a) to 2=106 cmy2 as shown in Fig. 3d, but only short and curved CNTs were grown due to the small diameter of Ni nanoparticle (-50 nm) w11x. The diameter of grown CNTs (approx. 40 nm) was small compared to that of CNTs (approx. 70 nm) shown in Fig. 2, suggesting that
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Fig. 4. Raman spectra of CNTs grown under different conditions: (a) pre-treatment for 10 min with 30 W; (b) pre-treatment for 20 min with 30 W; (c) pre-treatment for 30 min with 30 W; (d) pre-treatment for 20 min with 70 W. For all sample, pre-treatment temperature was fixed at 390 8C and growth time of CNTs and temperature were 15 ˚ min and 550 8C, respectively. Ni layer thickness was 100 A.
Ni catalytic film was etched by NH3 plasma. Although, we did not obtained the reduction of density of the ˚ we confirm CNTs grown of Ni layer of 100 and 300 A, that density control of CNTs can be achieved by control of NH3 plasma intensity, pre-treatment time and catalyst thickness.
To investigate crystallization and defects in CNTs grown on NH3 pre-treated substrate, we used Raman spectroscopy and result is shown in Fig. 4. Raman spectra using the argon laser excitation wavelength of 514.5 nm (Renishaw-3000) indicate the difference in peak intensity and width according to various NH3 plasma pre-treatment conditions. Strong intensity at approximately 1350 cmy1 (D line) indicates disorder and defects in CNTs and intensity at approximately 1596 cmy1 (G line) was usually interpreted as a peak from highly oriented pyrolytic graphite w12,13x. An increase in DyG with NH3 plasma pre-treatment time and power indicates an increase in disorder in CNTs, which may be due to atomic nitrogen on the surface of the Ni nanoparticle. As NH3 plasma pre-treatment time was increased, amount of nitrogen which came from the decomposition of NH3 might increase and leading to an increase in the amount of nitrogen implanted into the Ni surface w4x. Because nitrogen and carbon have a strong binding force, implanted nitrogen interrupt diffusion of carbon atoms on the Ni surface resulting in an increase in disorder in CNTs. It may be suggested that an increase in DyG with NH3 plasma pre-treatment time was attributed to nitrogen incorporation. Fig. 5 shows the emission current grown on NH3 pretreated substrate as a function of an applied voltage (IV). The turn-on electric field, Eto, was defined as the electric field at 1 mAycm2 of the current density. Eto of Reference, a, b and c in Fig. 5 were 6.2, 9.3, 4.6 and 6.8 Vymm, respectively. The turn-on electric field was
Fig. 5. Current density (mAycm2 ) vs. applied electric field curves of CNTs with different pre-treatment condition: (Ref) NH3 plasma pre-treatment ˚ (a) NH3 plasma pre-treatment for 20 min with 30 W, Ni layer of 300 A; ˚ (b) NH3 plasma pre-treatment for 20 min with 30 W, Ni layer of 30 A ˚ (c) NH3 plasma pre-treatment for 20 min with 70 W, Ni layer of 30 A. ˚ For all sample, pre-treatment for 30 min with 30 W, Ni layer of 30 A; temperature was fixed at 390 8C and growth time of CNTs and temperature were 15 min and 550 8C, respectively.
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increased as plasma power of NH3 pre-treatment increased from 30 to 70 W and thickness of Ni layer ˚ But Eto was decreased with increased from 30 to 300 A. an increase in NH3 pre-treatment time from 20 to 30 min. It was reported that the turn-on electric field closely depends on density of CNTs w14x. CNTs of low density yield low currents essentially because of the low emission site (Fig. 5c). For high density of CNTs screening effects reduced the field enhancement, resulting in the low emission current (Fig. 5a) w14x. Because, CNTs that ˚ after NH3 pre-treatment were grown on Ni layer of 30 A for 30 min with 30 W have a medium density, it has relatively high current saturation and low turn-on electric field than the other samples. 4. Conclusion The NH3 plasma did not only etched, but also conglomerated Ni catalyst layer. Below a certain plasma power, conglomeration effect due to the heating was dominant. Ni nanoparticles were formed, but the density of Ni nanoparticles was not remarkably decreased. As the NH3 plasma pre-treatment time increased, the defects and disorder of CNTs increased. As the NH3 plasma power increased, an etching became a dominant factor rather than conglomeration effect, resulting in a decrease in the density of CNTs. As Ni film thickness was decreased, the average diameter of CNTs became smaller and the size of Ni nanoparicles and the diameter of CNTs decreased.
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