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Investigation of Field Emission and Photoemission Properties of High-Purity Single-Walled Carbon Nanotubes Synthesized by Hydrogen Arc Discharge Byeongchul Ha,† Jeunghee Park,*,§ Shin Young Kim,§ and Cheol Jin Lee*,‡ Department of Nano Science, Cheongju UniVersity, Cheongju 360-764, Korea, Department of Chemistry, Korea UniVersity, Jochiwon 339-700, Korea, and School of Electrical Engineering, Korea UniVersity, Seoul 136-713, Korea ReceiVed: June 3, 2006; In Final Form: September 19, 2006
Single-walled carbon nanotubes (SWCNTs) were directly synthesized by a hydrogen arc-discharge method by using only Fe catalyst. The synthesized carbon materials indicated high-purity SWCNTs without amorphous carbon materials from SEM observation. The SWCNTs had diameters of 1.5-2.0 nm from TEM and Raman observation. After a simple purification, TGA indicated that SWCNTs had a purity of ca. 90.1 wt %. Field emission from the SWCNT emitters which were fabricated by using a spray method was measured by a diode structure. The vertically aligned SWCNT emitters showed the low turn-on voltage of 0.86 V/µm and a high emission-current density of 3 mA/cm2 at an applied field of about 3 V/µm. From a Fowler-Nordheim plot, the vertically aligned SWCNT revealed a high field enhancement factor of 2.35 × 104. The photoemission measurements, excited by a photon energy of 360 eV, showed significantly delocalized graphite-π states at the purified SWCNTs. Here, we investigated that the field-emission properties of SWCNTs would be attributed to the high electronic density of states near Fermi energy, including the delocalized graphite-π states.
1. Introduction Since the first observation of single-walled carbon nanotubes (SWCNTs),1 the distinguished mechanical, chemical, and physical properties of SWCNTs have been studied by several research groups.1-3 Furthermore, many potential applications such as field emission displays,4-12 secondary batteries,13 various transistors,14,15 and AFM/STM tips16,17 have been extensively studied. Mass production of high-purity SWCNTs is strongly desired to realize many kinds of applications. Currently, many researchers have focused their attention on the production of large quantities of high-purity SWCNTs using various techniques. Until now, diverse methods, including arc discharge,18-29 laser ablation,30,31 and catalytic chemical vapor deposition (CCVD),32-38 have been developed for the synthesis of SWCNTs aiming to produce high-purity and high-yield SWCNTs with low cost in a large scale. Among various growth techniques, the arc-discharge method is relatively uncomplicated and inexpensive, compared with other methods. It has been widely used because of its potential merits to make the massive production of SWCNTs. Moreover, the SWCNTs synthesized by arc discharge showed high crystallinity and selectivity in contrast with the CVD method.18-36 In past years, Journet et al. had already reported large-scale synthesis of SWCNTs using nickel and yttrium catalysts filled inside a graphite rod as anode in a He atmosphere.18 They obtained a high yield of SWCNTs mainly from the “collar” area of cathode. Saito et al. reported the synthesis of SWCNTs with various diameters dependent on He pressure using a Rh/Pt catalyst by arc discharge.19 Ando et al. announced the mass production of SWCNTs using Ni/Y catalysts and He buffer * Address correspondence to these authors. E-mail:
[email protected] (C.J.L.);
[email protected] (J.P.). † Department of Nano Science, Cheongju University. § Department of Chemistry, Korea University. ‡ School of Electrical Engineering, Korea University.
gases by the arc plasma jet method.21 Li et al. reported the synthesis of SWCNT fibers using Ni/Y and FeS catalysts in a He atmosphere by an arc-discharge method.24 Sugai et al. reported the production of SWCNTs using Ni/Co and Ni/Y catalysts in a He, Ar, or Kr atmosphere by a pulsed arc-discharge method.25 Huang et al. announced the synthesis of SWCNTs using Ni and Y2O3 catalysts in a bowl-like cathode by arc discharge.26 Shi et al. reported the production of SWCNTs with Y/Ni alloy or CaC2/Ni at high He pressure by the dc arcdischarge method.27 Until now, arc-discharge SWCNTs have been mainly produced by bimetallic catalysts or a He atmosphere. Even though most groups have used various catalysts and buffer gases, there are few reports about the arc-discharge synthesis of SWCNTs with only Fe catalyst and hydrogen buffer gases. The combination of simplest catalyst and buffer gas is probably desirable to commercialize the mass production. Here, we synthesized SWCNTs using Fe catalysts and hydrogen buffer gases by arc discharge. Generally, the products of arc discharge synthesis contain unavoidably other carbon materials and metal catalyst particles as impurities.18-29 It is well-known that hydrogen selectively etches impurities such as amorphous carbon from the synthesized carbon product during the reaction. Therefore, hydrogen arc discharge can promise production of high-purity SWCNTs, compared to conventional inert gas arc discharge. Recent studies show that the SWCNTs synthesized by arc discharge were better suitable for field emission than those synthesized by a CVD method because of their high crystallinity and selectivity.37,38 Moreover, due to the difficulty of the direct growth of SWCNTs on substrates, the SWCNT paste emitter is a promising method for a large area field emission display.37-40 One expects instinctively that the vertically aligned SWCNTs are better emitters than the randomly directed SWCNTs because most electrons are emitted from the tips of the SWCNTs. Interestingly, however, there were few reports about field
10.1021/jp0634407 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/04/2006
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emission of vertically aligned SWCNTs synthesized by arc discharge although there were some reports about the field emission of vertically aligned MWCNTs synthesized on substrates by CVD methods. Therefore, in this work we investigated field emission properties from the vertically aligned SWCNTs on the substrates. Lovall et al. reported that the field emission energy distribution (FEED) of carbon nanotubes (CNTs) is attributed to the density of states (DOS) of CNTs.41 Besides, Suzuki et al. reported the electronic DOS of carbon nanotube tips by photoemission spectroscopy.42 However, there was no report on the relationship between the field emission and DOS of SWCNTs yet. Here, we measured the X-ray photoemission to observe the electronic structure of the SWCNTs because we considered that the emitted electrons from field emission originate from the valence band electronic structure, and thus the photoemission would show a strong correlation with their field emission properties. In this work, we tried to understand the relationship between the field emission and DOS of SWCNTs. According to our experimental results, the field emission of SWCNTs can be closely related to the DOS of SWCNTs near the Femi level. 2. Experimental Section High-purity SWCNTs were produced by a dc arc-discharge method in a stainless steel chamber. The anode was a graphite rod (6 mm in diameter and 300 mm in length) with a drilled hole (3 mm in diameter) filled with a mixture of Fe catalyst and a graphite powder, and the fixed cathode was a pure graphite rod. The stainless steel chamber and the cathode area were always circulated by cooling water. To generate arc plasma, a gap between two electrodes had been maintained with a constant distance of 1 mm and hydrogen buffer gas was filled with a pressure in the range of 50-450 Torr, and the DC discharge current was in the range of 30-70 A. After the evaporation for 20 min, the soot materials were collected from three parts such as the cathode, the chamber wall, and the weblike materials around the cathode. In this work, as-synthesized SWCNTs were purified by using both a thermal oxidation and a chemical treatment method. The thermal oxidation was performed at 450 °C for 30 min to remove amorphous carbon materials while a chemical treatment was conducted with 10% HCl for 10 min to eliminate the catalyst particles. To measure the field emission from the SWCNTs, the entangled SWCNTs were dipped in ethanol solution and dispersed by sonication. Then, the SWCNT suspension was sprayed on the Ag/Ti (1000 nm)/n-Si substrate for 8 min with a spray gun. SWCNTs on the Ag/Ti (1000 nm)/n-Si substrate were vertically aligned by the mechanical method of taping. The substrate was dried in air, followed by baking at 400 °C for 20 min, using a rapid thermal annealing in an Ar atmosphere to maintain good mechanical adhesion and ohmic contact property between the SWCNTs and the Ag/Ti (1000 nm)/n-Si substrate. Field electron-emission measurements were performed in a vacuum chamber at pressure of less than 2 × 10-7 Torr. The cathode consisted of the Ag/Ti film on the silicon substrate, the anode was a copper plate positioned about 300 µm above the SWCNTs, and the measured emission area was 7 mm2. Emission current was monitored with a Keithley 6517 A and DC power was supplied with a constant power voltage and current controller (HCN140-3500 of Hochspannungs-Netzgera¨t). The as-synthesized and purified SWCNTs were characterized by scanning electron microscopy (SEM) (Hitachi, S-4700),
Figure 1. The optical image of produced carbon materials after the reaction: (a) near the cathode and (b) in the chamber.
transmission electron microscopy (TEM) (JEOL, JEM-3011), and thermogravimetric analysis (TGA). Raman spectroscopy (Rainshow, RM1000-invia) was performed with Ar laser excitation (laser beam wavelength: 514.7 nm). The XPS measurements were performed at the U7 beam line of the Pohang Light Source (PLS). The U7 beam line was designed to provide soft X-rays in the energy range of 501500 eV. The XPS data were collected at 360 eV. The spectral resolving power (E/∆E) of incident photons is 5000. The binding energy C 1s peak is fixed to 284.6 eV. The experiment was performed in an ultrahigh vacuum chamber with a base pressure of e5 × 10-10. The photoelectrons emitted from the surface of the sample were collected and their energy was analyzed with an electron energy analyzer (Physical Electronics: Model PHI 3057 with a 16-channel detector). The analyzer was located 55° from the surface normal. The XPS peak was curve-fitted by Voigt profiles. 3. Results and Discussion We synthesized SWCNTs by a hydrogen arc-discharge method using not bimetallic catalyst but only Fe catalyst. After evaporation of a graphite rod, we could find a lot of soot materials in the reactor chamber. As-synthesized soot showed a little different yield and purity according to the position in the reactor. In this work, we distinguished soot materials as three parts such as the cathode, chamber wall, and weblike soot at the chamber as shown in Figure 1. We could obtain the product yield of soot materials dependent on the position (chamber wall > cathode > web). However, the purity of carbon nanotubes from soot materials followed the web > cathode > chamber wall sequence. According to our observations, the morphology and structure of the SWCNTs were almost the same regardless of the positions.
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Figure 2. SEM micrographs of the as-synthesized SWCNTs: (a) in the web, (b) in the cathode, (c) in the chamber wall, and (d) in the purified SWCNTs.
Figure 2 shows the SEM images of carbon soot material produced by a hydrogen arc-discharge method using 2 atom % Fe catalyst at a hydrogen atmosphere of 450 Torr in the current of 60 A. Panels a, b, and c of Figure 2 are the SEM images of the as-synthesized SWCNTs obtained from the three parts before purification. All soot materials indicate high-purity carbon filaments regardless of sample position unlike the previous results. There is no amorphous carbon particle in the soot material even though some catalyst particles appear on the carbon filaments.20-29 The carbon filaments show an entangled network having average diameters of about 20 nm. We confirmed that the carbon filaments reveal SWCNT bundles from TEM observation and Raman analysis as shown in Figures 4 and 5. From the SEM images, we observed that the yield of assynthesized soot material was steadily enhanced as the amount of catalyst increases from 0.7 to 2.5 atom %. However, the highest purity (90%) of carbon materials was obtained when 2 atom % Fe catalyst was used. As the Fe-catalyst content increases to be >2 atom %, a lot of spherical carbon particles encapsulating the Fe-catalyst nanoparticles are produced besides the carbon filaments. Figure 2d is the SEM image of SWCNTs after a simple purification process, in which the purified SWCNTs have especially clean surfaces without damage. Our purification process is very effective to produce the high-quality SWCNTs, without fatal damage, due to a short purification time and a mild acid treatment. In contrast, the existing purification process induced the fatal damage on the final product because it took a long time and was performed by various acid treatments. We can produce high-quality SWCNTs using a brief purification method because our as-synthesized SWCNTs reveal very high purity. We also investigated the synthesis of SWCNTs using other catalysts (Ni, Co), and found that the Fe catalysts exhibit a superior reactivity compared to them. Fe catalysts are easily vaporized into the graphite at an arcing region, and the activation of Fe catalysts is quite increased by a hydrogen atmosphere.44 In past years, helium was largely used as a buffer gas to synthesize SWCNTs by arc discharge. Recently, hydrogen has
been used to synthesize SWCNTs instead of helium in arc discharge. Hydrogen buffer gases contribute to the high purity and yield of SWCNTs because hydrogen selectively etches the amorphous carbon attached on the surface of Fe-catalyst nanoparticles or SWCNTs, resulting in a hindrance of impurity growth like amorphous carbon material. Thus, we confirmed that SWCNTs synthesized in a hydrogen atmosphere had higher purity and better crystallinity than those in a helium atmosphere through the characterization of TEM and Raman spectra. Table 1 lists a series of synthesis condition, and the yield and purity of the products. The synthesis condition is varied by using three parameters; hydrogen pressure, arc-discharge currents, and the content of Fe catalysts. To obtain the high-purity and high-yield SWCNTs, we had to optimize those three parameters. We also discovered that as-synthesized SWCNTs have various morphologies depending on the reaction parameters. The purity and yield of SWCNTs were mainly affected by hydrogen gas pressure and currents. Generally, the yields of carbon products were increased when the intensity of current or the pressure of hydrogen was elevated. On the other hand, the purity of SWCNTs indicated a poor level when the hydrogen gas pressure was very low. The highest yield and purity of SWCNTs were achieved at the process condition of 450 Torr hydrogen pressure, 60 A current, and 2.0 atom % Fe. Figure 3 shows the TEM images of the SWCNTs synthesized under optimized process conditions. The carbon products shown in Figure 3 were collected from the web. Each carbon filament consists of a SWCNT bundle, and SWCNTs reveal good crystallinity even though a lot of Fe particles appear on the surface of as-synthesized SWCNTs as shown in Figure 3a. Interestingly, the Fe particles are surrounded by several graphitic sheets, as shown in Figure 3b. In general, the discontinuity of the graphitic shells helps to eliminate the iron particles effectively, because acid could easily permeate into them. The diameter of the individual SWCNTs in the bundles observed in the HRTEM image is in the range of 1.5-2.0 nm with significant homogeneity, as shown in Figure 3c. Sometimes, we could find a few isolated SWCNTs with a larger diameter
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TABLE 1: The Synthesis Conditions, Yield, and Purity of Arc-Discharge SWCNTs web soot
cathode soot
chamber wall soot
hydrogen (pressure in Torr)
current (A)
contents of Fe catalyst (atom %)
yielda
purityb
yielda
purityb
yielda
purityb
450 450 450 450 250 250 250 250 250 250 250 150 50 50
60 60 60 70 30 60 60 60 60 60 70 70 60 70
0.7 1.3 2 2 2 0.7 1.3 2 2.5 3 2 2 2 2
H M VH M L VL VL H VH H H M L VL
H H VH H H H H VH M L H L L L
L M VH M L L L H H H H H H L
H H H H VH M H H L M H L L L
M M VH VH M VL L VH VH H VH VH H L
M M H M H L L H M M M VL VL VL
a Yield, which indicates the total amount of carbon products (SWCNTs, carbonaceous particles, amorphous carbon, metal particles surrounded graphitic shells), is graded into five classes: VH (very high), H (high), M (medium), L (low), and VL (very low). b Purity, which indicates the ratio of SWCNTs to the whole amount of carbon products, is graded into five classes: VH (very high), H (high), M (medium), L (low), and VL (very low).
compared with the SWCNTs in the bundles as shown in Figure 3d. Figure 3e shows the purified SWCNT bundles, indicating high purity of SWCNTs without catalysts and amorphous carbon materials. It shows that our simple purification method is very effective to obtain high-purity SWCNTs. Figure 4 shows the Raman spectrum of the as-synthesized SWCNTs. The peak position at 1590.6 cm-1 is referred to as the G band with a triplet of A1g, Elg, and E2g modes. In this work, the G band has a narrow and strong spectrum peak, indicating a good arrangement of the hexagonal lattice of graphite, whereas the other peak at 1565.2 cm-1, appearing at the left of the G band, is considered to be characteristic of the Raman spectrum for SWCNTs. The weak D band at the peak of 1340.8 cm-1 in the spectrum reveals the high purity of assynthesized SWCNTs. The diameter of an individual SWCNT can be determined by the radial breathing mode (RBM) from 100 to 300 cm-1. Generally, the van der Waals interaction appears between the tubes because individual SWCNTs are packed into bundles. According to the following correlation between frequency ω (cm-1) and diameter d (nm), d ) 223.75/ (ω - 6.5),36 the peaks at 123.5, 146.5, 165.9, 174.7, and 185.2 cm-1 correspond to the SWCNTs with diameters of 1.91, 1.59, 1.4, 1.33, and 1.25 nm, respectively. It is considered that the main peak of 146.5 cm-1 in RBM modes corresponds to an Elg symmetry mode in a (8,8) armchair SWCNT.43 From Raman analysis, the average diameter of SWCNTs is about 1.5 nm, and this result is in good agreement with HRTEM observation. We performed a thermogravimetric analysis (TGA) for the as-synthesized and purified SWCNTs. TGA shows that the assynthesized and purified products consist of 37 and 90.1 wt % SWCNTs, respectively, as shown in Figure 5. The product weight percent of as-synthesized SWCNTs increases with increasing temperature and arrives at 110 wt % at 400 °C. But, the weight percent of as-synthesized SWCNTs reduces over 400 °C and drops to 63 wt % at 820 °C, resulting in the 37 wt % purity of SWCNTs. We consider that the total weight of assynthesized SWCNTs increases with the temperature increment at the initial stage due to the oxidization of Fe catalysts. On the other hand, the weight percent of the purified SWCNTs drops from 300 to 840 °C, remaining 9.9 wt %. As a result, the SWCNTs reveals the purity of 90.1 wt %. The above results indicate that the SWCNTs produced by a hydrogen arc-discharge method with Fe catalysts indicate higher purity compared with previous works.18-29
Figure 6 shows the SEM images of vertically aligned SWCNTs on an Ag/Ti/Si substrate after a taping treatment. The SWCNTs were mechanically aligned upward promptly by a raising tape. In this work, we attached a conventional tape on SWCNT film softly, and then detached the tape from the SWCNT film. From SEM images, we consider that a lot of the prudent SWCNTs will contribute to emission performance effectively. The field-emission characteristics of purified SWCNTs were measured over 3 times in each sample as shown in Figure 7. Vertically aligned SWCNTs show the low turn-on voltage of 0.86 V/µm, high field enhancement factor of 23529, and high emission-current density of 3 mA/cm2 at an applied field of about 3V/µm. On the other hand, the purified and randomly oriented SWCNTs show the turn-on voltage of 2.3 V/µm, and emission-current density of 1.0 µA/cm2 at an applied field of about 3V/µm, while as-synthesized and randomly oriented SWCNTs show the turn-on voltage of 2.91/µm, and emissioncurrent density of 0.2 µA/cm2 at an applied field of about 3V/ µm. The purified SWCNTs indicate lower turn-on voltage than as-synthesized SWCNTs because amorphous carbon materials, which can hinder the electron emission from the surface of the SWCNTs, were removed after purification. The vertically aligned SWCNTs by a mechanical taping method show much lower turn-on voltage and higher field enhancement factors than randomly oriented SWCNTs, indicating that most electrons are emitted from the tips of SWCNTs and the electronic DOS of tips near the Fermi energy is different from that of side walls.45 These measured turn-on voltages and high emission current density reveal that the vertically aligned SWCNTs can be use as highly effective field emitters. The Fowler-Nordheim plot is shown in the inset of Figure 7. To calculate the field enhancement factor (β), we use the Fowler-Nordheim equation
J ) A(β2V2/φd2) exp(-Bφ3/2d/βV) where J is the current density, A ) 4.76 × 10-18 (A V-2 eV), B ) 6.83 × 109 (V eV-3/2 V m-1), β is a field enhancement factor, φ is the work function, d is a distance between the anode and the cathode, and V is the applied voltage. When assuming the work function to be 5.0 eV, the randomly oriented SWCNTs indicated nearly the same the field enhancement factor (β) of 4101 regardless of purification, while the vertically aligned SWCNTs showed the higher field enhancement factors (β) of
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Figure 3. TEM micrographs of the as-synthesized SWCNTs: (a) low magnification TEM image of as-synthesized SWCNTs, (b) HRTEM image of Fe particles surrounded by several graphitic layers, (c) HRTEM image of a SWCNT bundle, (d) HRTEM image of an isolated SWCNT, and (e) HRTEM image of the purified SWCNTs.
Figure 4. Raman spectra of the as-synthesized SWCNTs. The inset shows the RBM modes.
23529 compared with the randomly oriented SWCNTs. Here, we suggest that deviation from Fowler-Nordheim behavior as
Figure 5. Thermogravimetric properties of as-synthesized and purified SWCNTs.
shown in the inset of Figure 7 is mainly attributed to two kinds of emission sites due to an uneven height of SWCNTs. Partly, we consider that this deviation may be caused by electronic
Field-Emission Properties of SWCNTs
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Figure 6. SEM micrographs of the vertically aligned SWCNTs on an Ag/Ti/Si substrate by a taping method: (a) low magnification SEM image and (b) high magnification SEM image.
Figure 7. Field emission from the SWCNTs: (a) randomly oriented SWCNTs, as-synthesized; (b) randomly oriented SWCNTs, purified; and (c) vertically aligned SWCNTs, purified. The inset shows the corresponding Fowler-Nordheim plots.
properties of the SWCNTs such as a nonmetallic density of states at the tip of the tubes.45 The vertically aligned SWCNTs indicate lower turn-on voltage and a higher field enhancement factor β than those of the previously reported SWCNTs, whereas the turn-on voltage of the randomly directed SWCNTs is a little higher, and the field enhancement factor (β) is lower.39-40,46 However, compared with the reported MWCNT measurements, both the vertically aligned and the randomly oriented SWCNTs show lower turn-on voltage, and a higher field enhancement factor (β).46 Figure 8a shows the fine-scanned C 1s peak of the X-ray photoemission spectrum of as-synthesized and purified SWCNT samples, using photon energy 360 eV. The peaks exhibit an asymmetric shape, and can be deconvoluted into two bands at 284.6 (PC1) and 285.3 (PC2) by curve fitting. The strong PC1 can be assigned to the C atoms binding to the graphite network, and the weaker and broader PC2 corresponds to the C atoms at the defect sites, which is consistent with the previous work of the CNTs.48-52 The fwhm of PC1 is 0.60 and 0.66 eV for assynthesized and purified SWCNTs samples, respectively. The respective fwhm of PC2 is 1.4 and 2.4 eV for as-synthesized and purified SWCNTs samples. The area ratio of PC1 and PC2 is about 2:1. The purification would inevitably produce some dangling bonds, but still leave the great crystalline perfection of the graphite sheets. The valence band (VB) spectrum has been measured as shown in Figure 8b. The zero energy is chosen at the Fermi level EF, which is the threshold of the emission spectrum. The VB electrons are responsible for bonding and thus significant change
Figure 8. Photoemission from as-synthesized and purified SWCNTs, corresponding to (a) C 1s peak and (b) the valence band. Fermi energy, EF, is chosen at zero binding energy.
in the DOS.41,47-54 The spectrum is normalized by using the intensity at 13 eV. The spectrum in the region between 2 and 8 eV (marked by “B”) is assigned to 2p-π, which overlaps with the top of 2p-σ, and is contributed by the graphite 2p-π electrons. The purified SWCNTs show significantly enhanced intensity of the “B” region compared with the as-synthesized SWCNTs, indicating a greater delocalization of 2p-π electrons. It indicates that the elimination of amorphous carbon and/or Fe nanoparticle impurities would result in a stronger van der Waals interaction with the neighboring nanotubes inside the bundles, so their electronic states can be more dispersed in energy. The VB spectrum in the range 0-1 eV (denoted by “A”) marks another significant feature, whereby the intensity of the as-synthesized
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Figure 9. A schematic diagram of the field emission process. The figure illustrates that the electronic DOS of metallic SWCNTs contributes to the emission current.41 The emitted distribution of electrons is referred to as the FEED. The shaded region shows the DOS below the Fermi energy (EF), which is occupied and contributes to the field-emission current (j(E - EF)).
SWCNTs rises more gradually from zero, compared to that of the purified SWCNTs. The rise is directly related to the increased DOS near EF. The elimination of the metallic Fe nanoparticles may decrease the DOS near EF. It is known that the field emission properties of CNTs can be influenced by the DOS of valence bands near the Fermi level.39,41 The excellent field-emission properties of SWCNTs would be attributed to the high electronic DOS near the Fermi energy, due to the delocalized graphite-π states in the “B” region. It is well-known that field emission is largely dependent on the shape of CNT tips. However, besides the shape of CNT tips, the electronic DOS of valence bands near the Fermi level can influence the field emission properties of CNTs.39,41,49 In this work, we tried to investigate the effect of the electronic DOS to the field emission properties. Especially, we aimed to explain the relationship between the field emission properties and photoemission properties from SWCNT materials. The emission current and emission stability of SWCNTs can be attributed to the density of states near the Fermi energy since the occupied level near the Fermi energy will almost contribute the emitted electrons. As shown in Figure 8b, there is the possibility that in the binding energy of 0.47-2.26 eV, the presence of Fe metal cages existing in the as-synthesized SWCNTs contributes to the higher DOS compared with the purified SWCNTs.49 Thus, as-synthesized SWCNTs illustrate metallic electronic states below the Fermi level. From the magnified region “A” as shown in the inset of Figure 8b, in the range of binding energy of 0-0.47 eV, the purified SWCNTs show higher DOS than the as-synthesized SWCNTs. Thus, the purified SWCNTs can promise the lower turn-on voltage than the as-synthesized SWCNTs as shown in Figure 7. The low turn-on voltage might be caused by the higher DOS near the Fermi level besides the removal of amorphous carbons. Moreover, in the range of 2.267.9 eV, the DOS of purified SWCNTs is higher than that of as-synthesized SWCNTs, noticeably revealing the delocalized graphite π band at 3.16 eV. As shown in Figure 9, the field-emission properties of SWCNTs are attributed to the electronic DOS of SWCNT valence bands near the Fermi energy.39,41,52-54 The FEED is directly correlated with the electronic DOS of SWCNT valence bands near the Fermi energy. Considering the FEED,39,53,54 we
Ha et al. suggest that our high applied voltage (>700 V) is enough to emit the π state electrons of SWCNTs in the photoemission spectra. Dean et al. addressed that the FEED showed the electron emission spectra up to 6 eV below the Fermi energy at an applied bias of 760 V. Regarding their result, we conclude that the emitted electrons at our high applied voltage include the emitted electrons from the delocalized graphite π band at the binding energy of 3.16 eV.54 The tunneling probability below the Fermi level increases with high applied voltages because a tunneling barrier (-eEz) decreases with high applied voltages. The decrease of a tunneling barrier with high applied voltages induces more emitted electrons from the electronic DOS of SWCNTs to the vacuum level and increases the emission current. Consequently, the emission current is directly correlated with the electronic DOS below the Fermi level and applied voltages, inferring that the FEED is a convolution between the tunneling barrier and the electronic DOS of the SWCNTs.39,41 The current density j(E - EF) is expected to depend on the energy relative to the Fermi energy as41,55
j(E - EF) ) (J0/d)e(E-EF)/dD(E - EF) with D(E - EF) being the electronic density of states below the Fermi energy. The factor d is related to the applied electric field E and work function φ. Therefore, it is suggested that the electronic DOS near the Fermi level including the π state of SWCNTs strongly contributes to field emission current because the FEED is consistent with tunneling from the electronic DOS near the Fermi level.54 4. Conclusion We synthesized high-yield and high-purity SWCNTs using Fe catalysts by a hydrogen arc-discharge method. We confirmed that the synthesis of high-purity SWCNTs was achievable on the optimized condition of Fe concentrations, discharge currents, and hydrogen pressure. The produced SWCNTs indicated the diameters of 1.5-2.0 nm in a bundle but the diameters of 3.3 nm in an isolated SWCNT shape. The as-synthesized SWCNTs were effectively purified by using a brief purification method. TGA showed that the purified carbon materials consisted of 90.1 wt % SWCNTs. For field-emission properties, the vertically aligned SWCNTs by a mechanical taping method indicated a much lower turnon voltage, higher emission-current densities, and higher fieldenhancement factors than randomly oriented SWCNTs. We suggest that the field emission of SWCNTs originated from the electronic DOS near the Fermi level. We also observed that SWCNTs have the delocalized graphite-π states below the Fermi energy, which contribute to the field-emission performance. Acknowledgment. This work was supported by the Center for Nanotubes and Nanostructured Composites at Sungkyunkwan University, by the National R&D project for Nano Science and Technology, by the Ministry of Commerce, Industry, and Energy of Korea through a components and Materials Technology Development Project (No. 0401-DD2-0162), and by the research fund of Korea University. References and Notes (1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (2) Ouyang, M.; Huang, J.; Lieber, C. M. Acc. Chem. Res. 2002, 35, 1018. (3) Dai, H.; Kong, J.; Zhou, C.; Franklin, N.; Tombler, T.; Cassell, A.; Fan, S.; Chapline, M. J. Phys. Chem. B 1999, 103, 11246.
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