Bias effect on the growth of carbon nanotips using microwave plasma chemical vapor deposition

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Bias effect on the growth of carbon nanotips using microwave plasma chemical vapor

deposition

C. L. Tsai, C. F. Chen, and L. K. Wu

Citation: Applied Physics Letters 81, 721 (2002); doi: 10.1063/1.1494839

View online: http://dx.doi.org/10.1063/1.1494839

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Bias effect on the growth of carbon nanotips using microwave plasma

chemical vapor deposition

C. L. Tsai,a) C. F. Chen, and L. K. Wu

Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30050, Taiwan, Republic of China

共Received 15 March 2002; accepted for publication 24 May 2002兲

Carbon nanotips with a high-aspect ratio were directly grown on Pt films. Carbon nanotips grew up to 5.4 ␮m length and 64 nm diameter under a⫺120 V bias. Compared to the hollow structure of carbon nanotubes, transmission electron microscopy images indicate its solid body, which is made of graphite. Carbon nanotips possess good field emission characteristics, that is, a turn-on field of 1.5 V/␮m and 761 ␮A/cm2 under 2.2 V/␮m. The Pt films provide a good conduction path for electron transport from the cathode to the emission site and do not act as catalysts. © 2002

American Institute of Physics. 关DOI: 10.1063/1.1494839兴

A good candidate for use in field emission must have a high-aspect-ratio structure, a low work function, and chemi-cal stability. Carbon nanotubes, since their first discovery in 1991,1have been considered for many different applications. Their small dimension, strength, and the remarkable physical properties of these materials make them the most promising emitters for field emission devices. Due to the size effect and structural diversity of nanomaterials, the physical properties strongly depend on their atomic-size structure, size, and chemistry.2Applications of nanomaterials in nanotechnology are focused on four fields: material preparation, property characterization, device fabrication, and system integration.

In this study, new carbon nanomaterials are developed in field emission. Well-aligned carbon nanotips were grown on the platinum共Pt兲 films in the chemical vapor deposition sys-tem. Basically, carbon nanotubes were synthesized with metal catalysts such as Fe, Co, Ni, and others.3–5 These metal catalysts play a key role in the deposition. However, a Pt film which is highly chemically inert provides only a good conduction path for electron transport from the cathode to the emission sites instead of catalysts. Therefore, the intrinsic properties of carbon nanotips are different from carbon nano-tubes.

Five nanometer Ti 共the improvement of adhesion be-tween Si and Pt兲 and 20 nm Pt films were pre-coated sequen-tially on Si by using electron beam evaporation. The reactive gas mixture was CH4/H2 with a flow rate of 10/10 sccm.

The applied microwave power and the pressure during the growth of carbon nanotips were 400 W and 15 Torr, respec-tively. An optical pyrometer was used to monitor the sub-strate temperature, that was maintained at about 700 °C. The growth time was 45 min, but nanotips grown on Si under a ⫺120 V bias only lasted for 30 min.

It is considered that the carbon active species in the plasma are accelerated to the substrate by the negative bias to form s p2and noncrystalline clusters in the nucleation period. Some clusters are then transformed into s p3clusters through the collision of carbon species in the growth period. Mean-while, the accelerated active hydrogen radicals will remove

the other s p2 clusters with lower active energy. In this situ-ation, the competition between etching and deposition is re-peated. Nevertheless, biasing the samples can cause their rate of deposition to exceed the rate of etching. Many reports have presented the method to enhance the nucleation density of diamond by applying negative bias.6 – 8This study focuses mainly on determining the optimum negative bias to synthe-size the carbon nanotips. Figures 1 and 2 present the scan-ning electron microscope共SEM兲 pictures of carbon nanotips grown under various biases and substrates. The photograph on the right of each figure is the enlarged image. Figure 1共a兲 displays that only a low density of tiny nanotips can be grown under ⫺80 V. This also implies that samples grown under a bias less negative than ⫺80 V cause little carbon materials to be deposited on the Pt films. Increasing the bias to ⫺120 V, it generates the high-density carbon nanotips. Figure 2共a兲 shows these well-aligned carbon nanotips grown upward to 5.4␮m length and 64 nm diameter under⫺120 V. Sharper nanotips have a higher-aspect ratio, indicating good

a兲_{Electronic mail: lun@ms15.url.com.tw} FIG. 1. SEM photographs of carbon nanotips grown on Pt under_and_{共b兲 ⫺150 V.} 共a兲 ⫺80 V

APPLIED PHYSICS LETTERS VOLUME 81, NUMBER 4 22 JULY 2002

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characteristics for field emission. Figure 1共b兲, however, indi-cates that tips will grow to a submicrometer diameter under a higher bias共more negative than ⫺120 V兲, revealing that bias can enhance the growth of carbon nanotips on Pt films. Hence, the optimal bias for growing carbon nanotips on Pt films is⫺120 V.

Figure 2共b兲 also shows good results of carbon nanotips grown on Si under⫺120 V, but the deposition time is shorter than those grown on Pt films. This is due to the fact that it is easier to form carbon materials on Si than on Pt.9

Figure 3共a兲 displays the transmission electron micros-copy 共TEM兲 images of an end section of an individual

nan-otip grown under⫺120 V. Unlike hollow carbon nanotubes, carbon nanotips are solid. The main feature of note is the tip’s somewhat irregular shape, with one primary protrusion. The diffraction pattern 共DP兲 indicates that the end section is graphite. Moreover, Fig. 3共b兲 displays the lateral section of the same tip, showing a well-organized microcrystalline graphite section. The DP of Fig. 3共b兲 also confirms the exis-tence of well-organized microcrystalline graphite, proving that the carbon nanotips are made of graphite.

Figure 4 exhibits the Raman spectra of carbon nanotips grown under various biases and substrates. All of them have two sharp peaks located on about 1345 and 1580 cm⫺1, re-spectively. The first-order Raman spectrum of aligned carbon nanotips shows strong sharp peaks at 1581 cm⫺1 共G line兲, which is the high-frequency E_2g first-order mode and 1350 cm⫺1 共roughly corresponding to the D line associated with disorder-allowed zone-edge modes of graphite兲. The peaks

FIG. 5. The current density vs electric field and FN plot of carbon nanotips grown under⫺120 V on 共a兲 Pt and 共b兲 Si.

FIG. 2. SEM photographs of carbon nanotips grown under⫺120 V on 共a兲 Pt and共b兲 Si.

FIG. 3. TEM images and diffraction pattern of共a兲 the end section and 共b兲 lateral section of an individual tip.

FIG. 4. Raman spectra of carbon nanotips grown under various biases and substrates.

722 Appl. Phys. Lett., Vol. 81, No. 4, 22 July 2002 Tsai, Chen, and Wu

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imply that the nanotips are characteristic of microcrystalline graphite. The relative intensities of the two peaks depend on the type of graphitic material. Normally, the intensity of the 1350 cm⫺1peak increases共i兲 with an increase in the amount of unorganized carbon in the samples and共ii兲 with a decrease in the graphite crystal size.10

The most conspicuous feature of carbon nanotips共grown under ⫺120 V兲 is that their Raman spectra show an addi-tional weak peak at about 1618 cm⫺1共D

⬘

line兲. The origin of the D and D

⬘

lines in other forms of carbon materials has been explained as disorder-induced features, caused by the finite particle size effect or lattice distortion.11–13Besides, a sample grown under⫺120 V with a narrow bandwidth of the

G line and the D line has well-organized carbon. The

previ-ous TEM image clearly displays the existence of well-organized graphite in the sample.

The field emission tests are performed on a diode struc-ture, in which the carbon nanotips are separated from the anode, indium-tin-oxide glass, using 500 ␮m glass as spac-ers. The current–voltage (I – V) properties are measured and analyzed through the Fowler–Nordheim共FN兲 model, via the ln(I/V2) vs 1/V plot. Figure 5 characterizes carbon nanotips grown under ⫺120 V on Si and Pt. The current densities at 2.2 V/␮m of nanotips under⫺120 V grown on Pt and Si are 761 and 617 ␮A/cm2, respectively. The threshold voltage (VT) is defined as the intersection of the slope of FN plots

with abscissa. According to the FN analysis, the emission behavior of the sample grown on the Pt films is better than

that grown on Si (lower turn-on field⫽1.5 V/␮m). It is at-tributed to the presence of Pt layers, which provide a good conduction path for electron transport from the cathode to the emission sites.14

The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 90-2216-E-009-036.

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