Schematic of the RF-DC plasma CVD system. RF plasma and DC plasma are generated between RF electrode and grounded electrode and between DC cathode and grounded electrode, respectively. 

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... vertically aligned CNTs have a variety of applications like field electron emitters for cathode ray lighting tubes (Saito et al., 1998), field emission displays (FED) (Sohn et al., 2001; Wang et al., 2001), backlight flat lamps in liquid-crystal displays (Bonard et al., 2001; Yoo et al., 2007), X-ray sources (Yue et al., 2002; Haga et al., 2004), as well as large surface- area electrodes for super-capacitors (Frackowiak et al., 2000; Futaba et al., 2006; McDonough et al., 2009), because of their higher aspect ratio and longer lifetime. A high electric field in the sheath generated in plasma enables self-standing and vertically aligned growth of carbon nanotubes (CNT) on substrates during plasma enhanced-chemical vapor deposition (PE-CVD). Power sources of direct-current (DC) with and without hot-filaments (Ren et al., 1998; Huang et al., 1998; Hayashi et al., 2001; Chhowalla et al., 2001), microwave (Sung et al., 1999; Murakami et al., 2000; Hayashi et al., 2002; Kojima et al., 2005), and radio-frequency (RF) (Hirao et al., 2001; Delzeit et al., 2002; Honda et al., 2003; Sato et al., 2006) have been utilized for generating plasma in vertically aligned growth of CNT by PE-CVD. DC plasma can generate the sheath of high electric field on the cathode. High electric filed sheath can also be generated on RF electrode by self-bias, but only under lower gas pressure (Hirao et al., 2001; Kaneko et al., 2005). Both DC and RF plasmas have the potential for large-area growth of CNT. Higher density plasma can be generated by DC or microwave power sources. Vertically aligned CNTs are grown owing to a pull-up force exerted by the high electric field in the sheath formed on a substrate.Increasing the gas pressure makes the sheath thinner to form a higher electric field on the substrate without increasing ion bombardment energy (Hayashi et al., 2010). Therefore, DC plasma has the potential to grow vertically aligend CNTs on a large-area substrate under high gas pressure. The problem of applying DC plasma for the growth of CNTs is the instability of DC glow discharge by occasionally generated arcing. Hence, a new plasma CVD method called RF-DC plasma CVD was developed (Hayashi et al., 2006; Hayashi et al., 2010). This method applies DC plasma under assistance of RF plasma for discharge stabilization. It was demonstrated that, with an increase in the RF power, the firing potential of DC discharge decreases and the DC discharge current for the same discharge voltage increases. These features of RF-DC plasma mean that the impedance of DC discharge decreases under RF plasma generation. Arcing occurs occasionally at the cusped points on the electrodes, where high electric fields exist. Stable glow discharge can continue with less frequent arcing under the condition of low impedance. Thus, with the generation of RF plasma, the DC glow discharge is more stable. The perpendicular alignment of CNTs grown by PE-CVD on a substrate surface is more than that of CNTs oriented electrically after dispersion on the substrate. However, the density of aligned CNTs grown by PE-CVD as well as thermal CVD is too high to decrease the electric field at their tips. The density by PE-CVD is of the order of 10 9 /cm 2 , which corresponds to an average spacing between CNTs of a few hundred nm. In comparison, the density of CNTs formed by thermal CVD is of the order of 10 10 /cm 2 . The enhancement factor of electric field at the tip of CNT decreases with the increase in spacing between them because of the field-screening effect. It was calculated that the maximum density of field electron emission is obtained when the spacing between CNTs is comparable to their height (Nilsson, 2000; Suh, 2002; Jo, 2003). The spacing for CNTs grown by PE-CVD is generally much larger than the height. Therefore, to increase the density of field electron emission from aligned CNTs, the density of CNTs should be reduced by controlling the growth or by the method of post-treatment. For example, the growth of pattered arrays was controlled using electron beam lithography of catalyst (Teo, 2002), or by post-treatment carried out by a process with energetic plasma ions (Weng, 2004). This chapter first shows the features of RF-DC PE-CVD plasma obtained through the diagnostics by the method of Langmuir probe, and the result of CNT growth analysis. Second, it shows the increase in density of field electron emission by the dip-dry method of post treatment, which is simple and suitable for vertically aligned CNTs grown over large-area. Figure 1 shows the schematic of the RF-DC plasma CVD system. Three electrodes (i. e., a 13.56 MHz RF electrode, a grounded electrode, and a DC cathode) are vertically fixed parallel to each other in a vacuum chamber. The distance between the RF and the grounded electrode is 10 mm, and that between the DC cathode and grounded electrode is 15 mm. The DC cathode, which is round in shape with 110 mm in diameter, having a hole at the center, also plays the role of a substrate holder. A substrate up to 75×75 mm 2 can be mounted on the cathode. An RF plasma is generated between the RF electrode and the grounded electrode, while a DC plasma is generated between the DC cathode and the grounded electrode. The grounded electrode is ring-shaped to facilitate passage of a part of the RF generated plasma into the space of DC discharge. The inside of the ring is covered with a mesh or wires, or is left without them. RF power induces not more than 500 W to the RF electrode. a negative bias not more than 650 V is applied to the cathode electrode. Substrate temperature is measured with a pyrometer at the backside of the substrate or on the surface of the substrate through the center hole of the RF electrode. Figure 2 shows the relationship between the voltage and the current of DC discharge with and without RF plasmas in pure hydrogen, measured in the RF-DC plasma CVD system. The current shows average values measured with increasing and decreasing voltage. The firing potential without RF plasma was approximately 300 V, which decreased to a voltage of around 100 V with RF plasma. The discharge voltage decreased from 350 V to 270 V Iron foil substrates of 0.2 mm thickness, which had been ultrasonically cleaned in ethanol, were fixed onto the cathode electrode. They were pretreated before CNT growth in a pure hydrogen plasma under a DC voltage of 350 V with RF power of 500 W and pressure of 2200 Pa for 15 min. Methane gas was added to hydrogen by 20% under the same operating pressure during the growth of CNTs for 15 min. The DC voltage was increased from 470 V to 620 V to maintain the substrate temperature at 700 °C throughout the pretreatment and growth process. For comparing the effect of RF plasma, CNT growth was also carried out with a lower RF power of 300 W at the same substrate temperature. Figure 4 shows scanning electron microscopy (SEM) images of vertically aligned CNTs, grown with an RF power of 500 W. Multi-walled CNTs of several tens of nm in diameter and several microns in length are observed. The density of the CNTs is of the order of 10 9 /cm 2 . All of the CNTs have catalyst iron particles at their tips. Individual CNT grew self- standing without sticking to each other by the van der Waals attraction. Such vertically aligned CNTs were observed to grow all over the surface of the iron foil substrate of 75 mm × 75 mm dimension (Hayashi et al., 2010). CNTs grown with RF power of 300 W showed similar alignment in growth as shown in Fig.4. The length of CNTs was found to be about 6  m. Raman analysis of the CNTs showed the peak height ratio of Graphite to Defect (G/D) vary between 1.1 and 1.3. To analyze the early stages of vertically aligned growth of CNTs, the substrate surface was observed by SEM. The SEM images of substrate surface after 1 min growth of CNTs under the condition of RF power of 300 W (a) and 500 W (b) are shown in Fig.5. They show isolated or strung particles of more or less 100 nm and smaller particles of a few to several tens of nm in sizes. The smaller particles are observed more remarkably for the 500 W RF power (Fig.5(b)). Some short fibers of few tens of nm in diameter, which should be the sprouts of CNTs, are observed in magnified oblique views. The fibers grown with 500 W RF power are longer than those with 300 W RF power. The result means that the initial growth of CNT with 500 W RF power is faster than that with 300 W RF power. As was discussed in section 2.2, because electron temperature increases with the increase of RF power, more active species for the growth of CNTs are produced under the condition of 500 W RF power. However, the diameter of the short fibers is smaller than that of the vertically grown CNTs in Fig.4. The growth of CNT can be explained by the vapor-liquid-solid (VLS) growth model (Tibbetts, Field emission properties of vertically b = - 6.83 × aligned 10 7 d 3/2 CNTs / β ( V grown ) by RF-DC PE-CVD were (1) ,where investigated d,  under and β the are application electrode spacing of high (in voltage cm), the in the work base function pressure (in eV) of 4 and × 10 -5 field- Pa. Negative enhancement bias factor, voltage respectively was applied (Ishikawa to the CNTs et al., 1993; on an Forbes, iron substrate 1999). When with d a = grounded 240 μ m counter and  = 4.7 electrode. eV (Gao The et al., two 2001) electrodes β = 1.7 × were 10 3 is spaced obtained at for 240 the μ m slope apart. in Fig.6(b), Figure 6 by shows eq. (1). the dependence of the field ...