Carbon nanotubes grown on cobalt-containing amorphous carbon composite films

Transcription

1 Diamond & Related Materials 15 (2006) Carbon nanotubes grown on cobalt-containing amorphous carbon composite films Y.B. Zhang, S.P. Lau *, L. Huang, B.K. Tay School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore Received 19 August 2004; received in revised form 27 July 2005; accepted 22 September 2005 Available online 26 October 2005 Abstract Carbon nanotubes (CNTs) have been produced on silicon wafer by filtered cathodic vacuum arc technique using cobalt-containing graphite targets followed by thermal chemical vapor deposition. The Co-containing amorphous carbon (a-c:co) composite films have various contents of Co as a catalyst for CNTs growth. It is found that dense and random CNTs were grown on the a-c:co composite film deposited using a 2 at.% Cocontaining graphite target and nanoforest CNTs on the composite films using 5, 10 and 15 at.% Co-containing targets. The nanoforest CNTs using a 15 at.% Co-containing target have very good field emission properties with a low threshold field of 1.6 V/Am and a high and stable current density of 2.1 ma/cm 2 at 3 V/Am, which may result from the smaller diameter of CNTs. It is found that the field emission properties of the CNTs are significantly affected by the diameter of CNTs rather than its orientation. D 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Co-containing amorphous carbon; Field emission 1. Introduction Carbon nanotubes (CNTs) have attracted wide attention all over the world because of their superior mechanical strength [1], varied thermal properties [2], large surface area for hydrogen storage [3], and high aspect ratio for field emission [4]. Chemical vapor deposition (CVD) of CNTs has been reported to possess good field emission characteristics [5]. The CVD CNTs field emitters can directly grow on a catalyst layer and are easily patterned [6]. Many efforts have been concentrated on the preparation of aligned CNTs for field emission applications. Unfortunately, some aligned CNTs with a high density exhibited poorer field emission properties than those of randomly-oriented CNTs due to a screening effect [7,8]. Recent studies showed that after annealing, cobalt nanocrystals could be produced in the carbon matrix of cobalt-containing carbon films [9]. The size of the cobalt grains may be dependent on the cobalt content. It is known that the catalyst particle size is closely related to the diameter of nanotube [10]. This provides a way to controllably grow CNTs * Corresponding author. Tel.: ; fax: address: esplau@ntu.edu.sg (S.P. Lau). with an expected diameter and site density through a proper Co catalyst content in the carbon matrix. The cobalt-containing amorphous carbon (a-c:co) composite films with varying Co contents can be deposited by the filtered cathodic vacuum arc (FCVA) technique [11]. It was reported that randomly-oriented and low-density CNTs with a diameter of nm were grown on amorphous carbon films with a small fraction of Co catalyst using low-temperature thermal CVD, and good field emission properties were obtained from the CNTs film [12]. In this article, we report randomly-oriented and nanoforest CNTs grown by thermal CVD method on a-c:co composite films with various Co contents at a relatively low temperature of 580 -C and present their field emission properties. It is found that nanoforest CNTs grown on a-c:co films deposited using a 15 at.% Co-containing graphite target, have a low field emission threshold field and a high emission current density. 2. Experimental The a-c:co composite films were deposited using an industrial FCVA system from Nanofilm Technologies International Pte Ltd. (FS1-04 FCVA Carbon system). The FCVA system has been described elsewhere [13]. The cathode is a 60- mm-diameter graphite cylinder with a Co content of 2, 5, 10 or /$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi: /j.diamond

2 172 Y.B. Zhang et al. / Diamond & Related Materials 15 (2006) at.%. The a-c:co composite film was deposited at room temperature on a highly doped <100>p ++ -type silicon wafer. The wafer substrate was pre-cleaned with acetone, alcohol and de-ionized water followed by a nitrogen blow-dry using a static neutralizing blow-off gun. Prior to deposition, argon ion beams (800 ev and 45 ma) were used for 5 min to sputter clean the native oxide layer on the surface of the silicon substrate. During deposition the substrate was negatively biased at 80 V, which corresponds to 100 ev of impinging C + ion energy. As the Co content of the cathode increased from 2, 5, 10 to 15 at.%, the arc current changed correspondingly to 60, 80, 100 and 100 A to maintain arc stability. CNTs were grown by thermal CVD using Co in the as-deposited a-c:co composite films as a catalyst. One batch of the composite films on silicon wafer was put in a vertical tube furnace with a base pressure of 10 3 Torr. Before introducing acetylene gas, the furnace was ramped to 580 -C in 10 min with the flow of 30 sccm nitrogen and 40 sccm hydrogen. Then acetylene gas with a mass flow of 5 sccm was introduced into the furnace and a negative dc bias of 400 V was applied to the a-c:co composite films. The total pressure was about 100 Torr and the temperature of the furnace was kept at 580 -C. After CNTs were grown for 5 min, the furnace was cooled down to room temperature with the flow of nitrogen only. The cooling time was about 30 min. Another batch of the as-deposited a-c:co composite films was annealed with the same process under nitrogen only. The surface morphologies of the annealed a-c:co composite films were observed using atomic force microscopy (AFM) (Digital Instruments) in tapping mode. The CNTs were observed using scanning electron microscopy (SEM) (JEOL, JSM-5910LV). The structure of CNTs was characterized by employing micro-raman spectroscopy. The visible Raman spectra were excited using nm Ar + laser (Spectra- Physics, Model 160-series) on Renishaw micro-raman System 1000 spectrometer. The spectral resolution is 1.5 cm 1. Electron field emission properties were measured at room temperature in a parallel plate configuration with an indium tin oxide (ITO)-coated glass anode and an anode cathode spacing of 250 Am. A test chamber was maintained at Torr by a mechanical and a turbo-molecular pump. The tested emission area was 5 mm in diameter. A voltage of up to 2.5 kv was applied to the cathode using a computer-controlled power supply while the emission current was measured automatically as the voltage ramped. 3. Results and discussion Fig. 1 shows the AFM images of the a-c:co composite films deposited using various Co-containing graphite targets after annealing. The surface morphologies are found to change significantly with the Co content in the target. As the a-c:co composite film is deposited using a 2 at.% Co-containing target, a uniform surface morphology is found after annealing. No Co grains are formed on the surface. When the Co content in the target increases from 5 at.% to 15 at.%, Co nanocrystalline grains are clearly found on the surface. The density of Co grains increases extraordinarily in the a-c:co composite film deposited using a 15 at.% Co-containing target. As the Co content in the target is 5 at.%, the size of Co grains is around 40 nm estimated from its AFM surface image. When the Co Fig. 1. AFM images of the a-c:co composite films deposited using (a) 2, (b) 5, (c) 10 and (d) 15 at.% Co-containing graphite targets after annealing.

3 Y.B. Zhang et al. / Diamond & Related Materials 15 (2006) content increases to 10 at.%, two kinds of nanocrystals are formed. One is around 60 nm and the other is clustered with a size of about 20 nm. It is interesting to note that the Co nanocrystals become much smaller with a grain size of about 10 nm in the a-c:co composite film deposited using a 15 at.% Co-containing target. From the measurement of X-ray photoelectron spectroscopy, the surface Co/C ratio is determined to be 0.52, 0.55, 0.49 and 1.11 for the annealed a-c:co composite films deposited using 2, 5, 10 and 15 at.% Co-containing targets, respectively. Although the Co content is similar for the first three a-c:co composite films, after annealing the Co structure state in the carbon matrix is different. It should result from various Co distribution states in the as-deposited a-c:co composite films due to different deposition conditions, such as the Co content in the target and arc current. After annealing the a-c:co composite film deposited using a 15 at.% Co-containing target has higher surface Co/C ratio and smaller Co nanocrystalline grains. It suggests that the size of Co nanocrystals may be related to the Co content and Co distribution state in the as-deposited carbon matrix, which needs further investigation. Fig. 2 shows the SEM micrographs of CNTs grown on a- C:Co composite films. After thermal CVD, a black-colored CNTs layer (confirmed by transmission electron microscopy) is formed on the surface of the a-c:co composite films. When the a-c:co composite film is deposited using a 2 at.% Cocontaining graphite target, the CNTs are grown randomly. Continuous dense distribution of tubes and random tangles of CNTs are observed on the surface. Co particles can be found on the tip of CNTs. The diameter of tubes is around 50 nm. However, nanoforest CNTs are observed on the other three a- C:Co composite films deposited using a higher Co-containing target. They show a spontaneous tendency to have their tips pointing upwards while the rest is tangled. When the a-c:co composite film is deposited using a 5 at.% Co-containing (a) target, the nanoforest tubes are shorter in length than the randomly-oriented CNTs grown on the a-c:co composite film deposited using a 2 at.% Co-containing target in spite of similar diameter and density. Co particles are clearly observed on the tube tip. When the a-c:co composite film is deposited using a 10 at.% Co-containing target, two kinds of nanoforest CNTs are observed on the surface of the a-c:co composite film. One kind of CNTs with a diameter of nm is dense and the other with a diameter of nm is sparse. This should result from two Co nanocrystals with different sizes. Nanoforest CNTs with a thin diameter of about 10 nm are grown on the a- C:Co composite film deposited using a 15 at.% Co-containing target. It is interesting to note that after annealing the microstructure of Co in the a-c:co composite film dominates the growth orientation of CNTs. The Co nanocrystalline grains help to grow nanoforest CNTs and its size is similar to the diameter of the CNTs. Therefore, the microstructure of Co and crystal size in the a-c:co composite film play an important role in determining the properties of CNTs. Fig. 3 shows the visible Raman spectra of the as-deposited a-c:co composite film, CNTs grown on the a-c:co composite film, and pyrolytic graphite. In general, carbon films show common features in their Raman spectra: a G peak at around 1580 cm 1 and a D peak at around 1350 cm 1 for visible excitation. The G peak is due to the bond-stretching mode of all pairs of sp 2 atoms in both rings and chains and the D peak is due to the breathing mode of sp 2 atoms in aromatic rings [14]. A similar broad carbon peak, which can be fitted with a set of G and D peaks, is found in all the a-c:co composite films. As a representative, the spectrum of the a-c:co composite film deposited using a 2 at.% Co-containing target is shown in Fig. 3, which is similar to the spectra of the a-c:co composite films obtained by Li et al. [15]. The Raman spectra of all the annealed a-c:co composite films are similar to those of the asdeposited films, which verifies an amorphous carbon structure (b) (c) (d) Fig. 2. SEM micrographs of the CNTs grown on the a-c:co composite films deposited using (a) 2, (b) 5, (c) 10 and (d) 15 at.% Co-containing targets.

4 174 Y.B. Zhang et al. / Diamond & Related Materials 15 (2006) Intensity (a.u.) Raman Shift (cm -1 ) pyrolytic graphite CNT-15%Co CNT-2%Co a-c:2%co Fig. 3. Visible (514.5 nm) Raman spectra of the a-c:co composite film deposited using a 2 at.% Co-containing target, the CNTs grown on the a-c:co composite films deposited using 2 and 15 at.% Co-containing targets, and pyrolytic graphite. in the annealed films and no graphitic nanocrystals are formed after annealing. In the spectra of all the CNTs grown on the a- C:Co composite films, well-separated G and D peaks appear clearly. The positions of the strong G and D peaks are centered at 1579 and 1355 cm 1, respectively. All Raman spectra look similar although the intensity ratio of D and G peaks changes slightly. As an example, the typical Raman spectra of CNTs grown on the a-c:co composite films deposited using 2 and 15 at.% Co-containing targets are shown in Fig. 3, which are similar to the typical Raman spectra of graphitic nanocrystals with a dimension around a few nanometers [16 18]. As a reference the Raman spectra of submicro pyrolytic graphites are also shown in Fig. 3. It can be seen that the G and D peaks become broader due to the submicro size [13]. Fig. 4(a) shows the emission current density vs. applied electric field (J E curve) of the CNTs grown on the a-c:co composite films. The J E curves are obtained with the first three rounds of ramping. The data in the ramping rounds show similar characteristics. It can be found that field emission properties of CNTs strongly depend on the Co content in the target. With the ramping of applied electric field in field emission measurement, a threshold field of 4.2, 3.2, 3.0 and 1.6 V/Am is obtained for the CNTs grown on the a-c:co composite film deposited using a 2, 5, 10 and 15 at.% Co-containing target, respectively. The threshold field is defined as the electric field at which the current density of 0.1 AA/cm 2 is obtained. As the electric field increases to a certain value, the emission current tends to saturate due to the current limited resistor and the resistance of the films [19]. The maximum emission current density of several ma/cm 2 can be achieved from the CNTs grown on the a-c:co composite films. At 6 V/Am, an average emission current density of 0.2, 0.3, 0.5 and 7 ma/cm 2 is obtained from the CNTs grown on the a-c:co composite film deposited using a 2, 5, 10 and 15 at.% Co-containing target, respectively. It can be seen that the threshold field for the CNTs grown on the a-c:co composite films decreases and the emission current density increases as the Co content in the target increases. It is interesting to note that the CNTs grown on the a-c:co composite film deposited using a 15 at.% Cocontaining target exhibited very good electron field emissions with a low threshold field and a high current density. Fig. 4(b) shows the field emission lifetime measurement of the nanoforest CNTs using a 15 at.% Co-containing target at 3 V/Am. In the test one copper plate replaces the ITO-coated glass anode since it may be damaged during long time emission. Mild stability at a high emitting current density is exhibited by the robust nanoforest CNTs. In the initial 15 h, the emission current density of 2 ma/cm 2 is quite stable with marginal fluctuation possibly from accidental arcing. Then the current density decreases slightly due to some CNTs degradation and failure [20]. Fig. 5 shows the corresponding Fowler Nordheim (FN) plots of the J E curves of the CNTs grown on the a-c:co composite films. FN plots of Ln(J/E 2 ) vs. 1/E are linear, indicating that emission is governed by electronic tunneling from CNTs with metallic conduction characteristics [21]. At the high fields, the saturated emission current deviates from the FN law. From the deviation a saturation field of 6.8, 6.6, 6.4 and 3 V/Am and the corresponding current density of 0.5, 0.5, 0.7 and 2.1 ma/cm 2 are obtained for the CNTs grown on the a- C:Co composite film deposited using a 2, 5, 10 and 15 at.% Co-containing graphite target, respectively. Taking an approx- Emission Current Density (A/cm 2 ) Current Density (A/cm 2 ) 1e-2 1e-3 1e-4 1e-5 1e-6 1e-7 1e-1 1e-2 1e-3 1e-4 1e-5 1e-6 1e-7 2% 5% 10% 15% (a) Electric Field (V/µm) (b) Time (min) Fig. 4. (a) Electron field emission J E curves of the CNTs grown on the a- C:Co composite films deposited using 2, 5, 10 and 15 at.% Co-containing targets. (b) A curve of emission current density vs. time recorded at a field of 3 V/Am for the nanoforest CNTs using a 15 at.% Co-containing target.

5 Y.B. Zhang et al. / Diamond & Related Materials 15 (2006) Ln(J/E 2 ) (10-8 A/V 2 ) % 5% 10% 15% Fitting /E (µm/v) Fig. 5. Fowler Nordheim (FN) plots of the J E curves of the CNTs grown on the a-c:co composite films deposited using 2, 5, 10 and 15 at.% Co-containing targets. imate work function of 5 ev for CNTs [22], field enhancement factors of 1168, 1119, 1900 and 2699 are derived, respectively. According to the random distribution of CNTs in Fig. 2(a), even the nanoforest CNTs in Fig. 2(b), it is impossible to obtain a high geometric enhancement factor over This indicates that there is a possible decrease of work function on the surface of CNTs. The effect of Co particles on the tips of CNTs can be ruled out since the work function of Co is also around 5 ev. It may be due to a small radius of curvature of CNTs and an electrostatic effect. Furthermore, the CNTs grown on the a-c:co composite films deposited using 2 and 5 at.% Co-containing targets present similar field emission properties since there is no significant difference in their J E curves and field enhancement factors. Both kinds of CNTs have similar density and diameter. However, one is randomlyoriented and the other tends to have their tips pointing upwards. This shows that orientation is not the most critical factor to influence the field emission properties of CNTs. It is interesting to note that the CNTs with smallest diameter exhibit very good field emission properties with a low threshold field of 1.6 V/Am and a high emission current density of 2.1 ma/cm 2 at 3 V/Am. This indicates that the diameter of the CNTs grown on the a-c:co composite films may dominate the field emission properties. It is found that two kinds of CNTs with different diameters are grown on the a-c:co composite film using a 10 at.% Co-containing target. The diameter of one kind of CNTs with major amount is similar to that of CNTs using a 5 at.% Co-containing target and the other is similar to that of CNTs using a 15 at.% Cocontaining target. It is found that the field emission properties of the mixed CNTs are between them and closer to those of CNTs using the 5 at.% Co-containing target. Therefore, it can be concluded that the field emission properties of the CNTs grown on the a-c:co composite films may be strongly influenced by the diameter of the CNTs and the effect of the CNTs orientation is not significant. 4. Conclusions In summary, the field emission properties of the CNTs grown on a-c:co composite films are strongly influenced by their diameter and the orientation of CNTs is not a critical factor. When the a-c:co composite film is deposited using a 2 at.% Co-containing graphite target, the grown CNTs are randomly oriented and densely tangled. 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