Hydrogenation and Its Effects on the Field Emission Characteristics of

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Hydrogenation and Its Effects on the Field Emission Characteristics of Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Jun Yu, Yuan Mei Foong, Angel T. T. Koh, and Daniel H. C. Chua* Department of Materials Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574, Republic of Singapore ABSTRACT: Field emission (FE) properties of a core/shell multiwalled carbon nanotube/tetrahedral amorphous carbon (CNT/ta-C) structure both with and without hydrogenation post-treatment were studied. It was found that the FE properties of the pristine vertically aligned CNTs improved with the ta-C coating and were further enhanced with a slight hydrogenation post-treatment. The thickness effect of the ta-C coating film and the hydrogenation duration effect on the FE characteristics of the specimens were investigated, and we report that the thickness of the ta-C coating film does not influence the FE properties of the emitters. Instead, the duration of hydrogenation treatment plays a key role in modifying their FE characteristics.

1. INTRODUCTION Carbon nanotubes (CNTs) have been widely considered as a promising candidate in field emission (FE) applications for their superior properties, such as the nanoscale diameter, structural integrity, high electrical and thermal conductivity, and chemical stability.1
3 The FE properties of CNTs were experimentally investigated as early as 1995 when Rinzler et al. demonstrated laser-irradiation-induced electron FE from an individual CNT.4 The CNTs used were not aligned, and the surface coverage density was also low. de Heer et al. reported using arrays of carefully aligned CNTs to produce FE sources, but unfortunately, the tubes were not well-distributed.5 For better utilization for FE applications, the CNTs should be highly oriented and well-distributed; thus, Wang et al. fabricated buckybundle CNTs with aligned arrays via an arc deposition technique, resulting in an onset field of 0.8 V/μm. This was a much lower value than that those reported previously.6 Subsequently, Saito et al. manufactured cathode ray tubes equipped with field emitters composed of multiwalled CNTs.7 This manufacture is the first known practical attempt of CNTs on an industrial scale. Despite this attempt, the commercial use of CNTs in FE devices is still far from actual implementation due to its many unsolved problems. For instance, FE devices require emitters to be grown perpendicular to the substrate and also require very high current densities (>500 mA cm
2), indicating the need for a high-density growth of emitters. To address these problems, Ren et al. fabricated large-scale well-aligned CNTs on nickel-coated glass below 666 °C via plasma-enhanced hot filament chemical vapor deposition.8 Chhowalla’s group also successfully obtained high-density vertically aligned CNTs using a direct current plasma-enhanced chemical vapor deposition.9 This group performed a detailed parametric study of various factors influencing the growth of r 2011 American Chemical Society

aligned CNTs and came up with specific CNT growth recipes. These recipes were later widely regarded as a CNT growth manual to access various morphologies of CNTs. From the known growth process of CNTs, the fabrication of CNT emitters with the required morphology was easily achieved. However, it is obvious that CNTs with very high density are not ideal for FE applications because the close packing of tubes will screen the local electric field, thus reducing the field enhancement advantages of the 2D emitters.10,11 This means that a desirable current density cannot be obtained by a limitless increase in the coverage of CNTs on the surface of the substrate. Other approaches should also be considered based on appropriate mutual CNT distance. Fortunately, the growth of CNTs with appropriate neighboring distance can be well-controlled with the aid of photolithography technique.12 Regardless of mutual CNT distance, considerable effort has been made in order to enhance the FE characteristics of CNTs over the past several years. One method is to modify the characteristics of CNTs themselves. Zhao et al. synthesized amorphous CNTs using anodized aluminum oxide (AAO) as a base template to form cylindrical aligned tubes. Without any grain boundaries, the electronic transport properties of these tubes were reported.13 However, most of the efforts focus on a simpler approach of modifying the surface of each and every CNT. Some success has been reported using this method. For example, Nagatsu et al. coated the CNTs with an amorphous carbon thin film approximately 0.6
1 μm thick and observed an improved FE performance. This improvement was observed with the reduction of the ignition voltage for electron emission, from Received: October 27, 2010 Revised: April 13, 2011 Published: May 24, 2011 11336

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Figure 1. Illustration of the preparation procedures of the specimens: (a) CNT growth on the silicon substrate, (b) ta-C coating on the CNTs, and (c) hydrogen plasma post-treatment on the surface of ta-C coated CNTs.

240 to 110 V.14 Jin et al. successfully lowered the work function of the CNTs from 4.5 to 1.9 eV through covering the surface of CNTs with a thin layer of barium strontium oxide while preserving their geometry.15 Some other studies reported a lower turnon field and higher field enhancement by modifying the CNTs with wide-band-gap materials (WBGMs), such as SiO2, MgO, and BN.16,17 Our group has reported coating pristine vertically aligned CNTs with a capping layer of molybdenum oxide, which is also a WBGM. This has been successful in lowering the turn-on field of the emitters.18 In this work, we attempt to modify pristine CNTs with a wide-band-gap and high sp3 content amorphous carbon material, which is sometimes known as tetrahedral amorphous carbon (ta-C) or amorphous diamond. Coating the CNTs using ta-C material has many advantages when it is used for electron emission applications. First, ta-C is a form of diamond-like carbon material, thus possessing superior mechanical properties, such as high Young’s modulus (∼800 GPa), high hardness (∼70 GPa), high wear resistance, and chemical inertness.19,20 Thus, the surface of the CNTs can be protected against ion-bombardment corrosion effects in the high-vacuum working environment. As such, the thermal stability can be enhanced while prolonging the lifetime. Second, ta-C has also been widely reported for its desirable FE properties that arise from its low or negative electron affinity,21,22 and there have been studies reporting on a reduced voltage threshold for ta-C coated molybdenum metal tips.23 Thus, the lowered threshold field and enhanced FE characteristics have been speculated if ta-C can be directly coated on vertically aligned CNTs. Very recently, our group reported that surface hydrogenation can further reduce the work function of diamond-like carbon by 1 eV, proving that the ta-C surface emission could be further enhanced by postdeposition hydrogenation treatment.24 In this work, we will study the FE characteristics of the ta-C coated pristine vertically aligned multiwalled CNTs both with and without hydrogenation post-treatment. Furthermore, a systematic study of the thickness effect of the ta-C coating material and the hydrogenation duration effect on their FE characteristics will also be investigated.

2. EXPERIMENTAL SECTION Several process steps are required to prepare the samples, as schematically demonstrated in Figure 1. High-density vertically aligned CNTs were used as the base template for electron emission. Fe catalyst was first deposited onto the Si(100) silicon substrate. A plasma-enhanced chemical vapor deposition (PECVD) technique operating at 700 °C with a 1.2 Torr C2H2 feed gas was used as the growth process. The aligned CNTs have

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an average length of approximately 7
8 μm after 10 min of growth. A detailed preparation process of the CNTs is reported in our previous work.17 Subsequently, a 248 nm KrF Lambda Physik excimer pulsed laser deposition (PLD) system was used to deposit ta-C onto the CNT substrates for 5 and 10 min. During the deposition process, the chamber pressure was maintained below 5  10
6 Torr and the distance between the substrate and target was maintained at 50 mm. The target was rotated with a speed of around 6 rpm while being ablated by the laser with the energy density of 20 J/cm2. The carbon target was prepared with high-purity carbon powder (99.5%) with the particle size of 325 mesh. The process conditions have been previously shown to give high sp3 content ta-C films.25 After deposition, both the 5 and the 10 min ta-C coated on CNT samples were treated in hydrogen plasma for 10, 20, and 30 s via a microwave plasma CVD facility coupled with a 2.45 GHz microwave power supply unit. A microwave power of 500 W was applied for the hydrogenation. The chamber pressure was set to be 15 Torr, and the H2 flow rate was set to be 300 sccm. This process has been previously reported to be successful in incorporating hydrogen onto the surface of ta-C.24,25 The samples before and after hydrogen treatment were analyzed for comparison. The FE properties of the triple-layered specimen were measured using a parallel plate geometry custom-designed FE system. A base pressure of ∼10
7 Torr was achieved before the test. The emitter-to-anode distance was maintained at 100 μm by inserting a polymer film spacer, on which a hole with a fixed area was fashioned to define the total emission area. The FE current
voltage (I
V) relationship was obtained by applying a dc voltage between the sample and anode. The emission current was measured using a Keithley 237 source measurement unit. The surface morphology of the obtained samples was characterized by a PHILIPS XL30-FEG scanning electron microscope (SEM). A high-resolution transmission electron microscope (HRTEM) (JEOL-2010) operating at 200 kV was used to study the nanostructure of the specimens. X-ray photoelectric spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) analyses were carried out using a Kratos DLD Ultra UHV spectrometer. A monochromatized Al KR X-ray source (1486.6 eV photons) with a spot size of about 1 mm was used for XPS measurements. Core-level XPS spectra were obtained by photoelectrons at a takeoff angle of 90°, measured with respect to the sample surface at a vacuum of 5  10
9 Torr. XPS spectra were collected in a concentric hemispherical analyzer in a constant energy mode, with a constant pass energy Ep of 20 eV. For UPS analysis, photons of the He (I) resonance line (21.2 eV) through a helium cold discharge were used. The analyzer energy was set to 120 meV with the acquisition time per spectrum of ∼150 s. A summary of these samples with different preparation processes is listed in Table 1.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Samples by SEM and TEM. SEM images in Figure 2 show the top view and cross-sectional view (inset) of the samples. Figure 2a shows that the pristine CNTs are vertically aligned and are very dense over the Si(100) substrates. On the tip of CNTs, some particles can be observed, which are believed to be catalyst particles. Compared to the 11337

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Table 1. Labels of the Specimens with Different Preparation Processes no hydrogenation

10 s hydrogenation post-treatment

20 s hydrogenation post-treatment

30 s hydrogenation post-treatment

CNTs with 5 min ta-C

sample A0

sample A10

sample A20

sample A30

CNTs with 10 min ta-C

sample B0

sample B10

sample B20

sample B30

Figure 3. TEM images of (a) sample A0, (b) sample A10, and (c) sample B0.

Figure 2. SEM images of (a) pristine CNTs, (b) sample A0, (c) sample A10, (d) sample A30, and (e) sample B0.

pristine CNTs, sample A0 shows a layer of whiskers formed with diameters larger than that of the pristine CNTs, and the whiskers extended from the tips of the CNTs (Figure 2b). Sample A10 possesses a very similar surface morphology (Figure 2c) with sample A0. In other words, it is not easy to distinguish the difference of the samples with and without the 10 s duration hydrogen plasma postdeposition treatment process. However, when the hydrogenation treatment goes to 30 s (and beyond), sample A30 shows thinner whiskers (Figure 2d), whose formation is highly likely due to the more severe etching effect by the hydrogen plasma. Figure 2e shows the images of sample B0, whose whiskers exhibit slightly larger diameters with the 10 min ta-C coating than that with the 5 min ta-C coating. High-resolution TEM images of the specimens are shown in Figure 3. It can be observed from Figure 3a that the CNT approximately 10 nm in diameter is coated with ta-C, confirming the core
shell nanostructures of the CNT and ta-C composite. The diameter of the taC
CNT whisker is around 40 nm. It can be further observed that a slightly nonuniform off-centered ta-C coating was obtained at the CNT core. This off-centered formation is most probably due to the angle between the target and the sample during deposition. After a 10 s hydrogen plasma treatment, the sidewalls of these whiskers were slightly etched, as shown in Figure 3b. Figure 3c shows that, with longer deposition duration, that is, a 10 min duration, a much thicker ta-C coating

can be observed for sample B0 than sample A0, though it is still off-centered. The total diameter of the 10 min taC
CNT whisker is much thicker at about 127 nm. 3.2. Field Emission Studies of the Samples. Thickness Effect of the ta-C Coating Film. The current density
electric field (J
E) curves of the pristine CNTs, sample A0, and sample B0 are shown in Figure 4a. The threshold field, which can be obtained from the J
E curve, is defined as the electric field where the emission current density is at 1 mA cm
2.14 The threshold field value for the pristine CNTs, sample A0, and sample B0 are 4.45, 4.02, and 3.76 V/μm, respectively. It means that both the 5 and the 10 min ta-C coated samples have an enhanced effect, allowing easier election emission over that of pristine CNTs. However, it is not easy to determine whether the thickness of the ta-C coatings can affect the FE properties. This is because the J
E curves of the 5 and 10 min ta-C coated samples overlap each other at higher emission current. However, due to the off-centered coating of the ta-C films on the CNT, this may allow electrons to emit from the tips and on both the thinner and/or the thicker side of the composite coated emitters. As such, the FE results presented here show the statistical average values of the entire emission area of the samples. The Fowler
Nordheim (F
N) plots for the three kinds of samples are shown in the inset of Figure 4a. The straight lines indicate that the emission currents obey the conventional F
N equation ! RA Bφ3=2 2 ðβEÞ exp
ð1Þ J ¼ φ βE where J represents the FE current density (A m
2), φ is the emission barrier height of the emitters (eV), R denotes the effective emission area, and β refers to the shape factor. The universal constants are A = 1.54  10
6 A eV V
2 and B = 6.83  103 eV
3/2 V μm
1. It was noted that the F
N plot of sample B0 comprises two linear regions with a knee point in between, one in the lower-field region and the other in the higher-field region. The deviation of the F
N plot in the high electric field region is commonly observed for semiconductor field emitters and is probably due to overheating of the emitter tips and/or a space charge effect.26
29 More specifically, as the ambient temperature increased during the emission process, the work function of the emitter could change, resulting in the emitter exhibiting enhanced FE 11338

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Figure 4. (a) The field emission J
E characteristics of the pristine CNTs, sample A0, and sample B0. The corresponding Fowler
Nordheim plot is shown in the inset. (b) UPS spectra of the pristine CNTs, sample A0, and sample B0.

performance. On the other hand, space charge could be generated around the emitter tips during emission, which reduced the actual electric field at the emission sites. As ta-C consists mainly of sp3 carbon bonds, whereas CNTs are rich in sp2 bonds, deposition of ta-C onto the CNT surface could result in a decrease of the conductivity. This can explain more severe space charge effects in the presence of ta-C coated samples during the emission process. According to the F
N equation, as shown in eq 1, the emission barrier height φ is one of the parameters that will affect the emission current. However, during calculation, the value of φ is usually assumed to be similar to the work function value. Therefore, in order to further investigate the FE properties of these samples, UPS was used to measure their work function values. Figure 4b shows the UPS spectra of the pristine CNTs, sample A0, and sample B0. The work function values for the pristine CNTs, sample A0, and sample B0 are 4.67, 4.41, and 4.46 eV, respectively. The φ values for the 5 and 10 min ta-C samples are roughly the same. Interestingly, these results are consistent with the respective J
E plots of these samples. Because of the shallow penetration of UPS equipment, generally

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Figure 5. Field emission J
E characteristics of (a) samples A0
A30 and (b) samples B0
B30. The corresponding Fowler
Nordheim plots are shown in the inset.

several nanometers deep, it can be determined that the φ values obtained for ta-C coated CNT specimens actually reflect the work function for the ta-C surface coating material. This explains why the 5 and 10 min ta-C samples exhibit similar work function values. As these φ values for the ta-C coated samples are lower than that for pristine CNTs, we believe that the electron emission is ejected from the surface of ta-C rather than the interface between ta-C and CNTs or anywhere else. Therefore, the 5 and 10 min samples exhibit roughly the same FE properties because of having similar work functions. This further suggests that the thickness of the ta-C coating film does not have a significant effect on the FE performance of these composite emitters. However, this finding contradicts those reported by Zhao et al. and Duan et al.30,31 Zhao et al. claimed that the surface work function of ultrathin films correlated with the thickness of the film. Below a certain point, an increment in the film thickness would lead to a significant decrease in the surface work function due to substrate-induced effects. However, this does not imply that the thickness can be increased without any upper limit. Duan et al. proposed that, if the thickness was larger than a certain value, the electron transport within the film would be affected due to the electron scattering caused by impurities or boundary 11339

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Figure 6. Carbon 1s core-level XPS spectra of (a) sample A0 and (b) sample A10, indicating a decreased sp2 content after hydrogen plasma treatment.

of the quantum barrier. Therefore, it was assumed that the ta-C coated CNTs may exhibit an optimum FE performance with a ta-C film coated between 5 and 10 min. This translates to an optimal window of thickness in the range of 50
100 nm where a similar emission profile is observed. Hydrogenation Duration Effect. Figure 5 shows the FE characteristic plots of ta-C coated samples with and without hydrogenation treatment. It is obvious that the 5 and 10 min ta-C coated samples exhibit the same trend that a slight hydrogenation treatment (10 s duration) would enhance their FE properties, but longer duration of hydrogenation treatments would gradually reduce this enhancement (20 s hydrogenation) or make it worse than the original pristine samples (30 s hydrogenation). One advantage in using ta-C is its amorphous nature, which means that the hydrogen plasma does not preferentially edge any grain boundaries as the latter are nonexistent. However, it may prefer to etch the lower concentration of sp2 bonds, which thus leaves behind the high sp3 surface and forming C
H dipoles on the surface. The FE enhancement mechanism of the slightly hydrogenated ta-C coated samples may lie in this C
H dipole formed at the ta-C surface.32,33 As hydrogen possesses a lower electronegativity than carbon, the C
H bond would be polarized with a positive charge on the H atom, resulting in a C
H dipole pointing from the surface toward the film. The generated potential would help to pull the vacuum level down and to lower the electron

Figure 7. UPS spectra of (a) samples A0
A30 and (b) samples B0
B30.

affinity or emission barrier height of the sample surface. The FE enhancement may also result from the increase of the surface conductivity that is caused by the charge transfer between the ta-C and the absorbed aqueous phase on the surface.34
36 With the surface hydrogenation treatments longer than 10 s, the FE performance of the samples deteriorated as the postdeposition treatment duration increased. The severe damage of the nanostructures is probably due to the plasma etching effect, as shown in the TEM images. It is well known that a CNT is an excellent route for electron transport, which is fundamentally good for FE. However, sp2 carbon bonds possess a lower etching resistance than sp3 carbon bonds. Pristine CNTs are essentially composed of sp2 carbon bonds; hence, longer hydrogen plasma treatments tend to etch CNTs, as shown in Figure 6. The sp2 content detected on the surface of the composite emitters decreases from around 52% to 46% after the 10 s hydrogen plasma treatment. This finding is consistent with that previously reported by Hong et al.37 The etching of CNTs makes the electron transport along the emitter core increasingly difficult due to a decrease in conductivity, thereby resulting in poor FE performances. Figure 7 shows the corresponding UPS spectra of these samples. From the spectra, it can be found that the work function 11340

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The Journal of Physical Chemistry C values of ta-C coated samples are lowered with a 10 s hydrogenation treatment and further reduced after a 20 s hydrogenation process. However, with 30 s hydrogenation, the work function values increased. This trend is not consistent with the trend of J
E plots. This implies that, other than the work function, there could be other parameters influencing their FE properties. This parameter is believed to be the shape factor of the emitters. Although the 20 s hydrogenation samples show the smallest work function values among the entire set of samples for both the 5 and the 10 min samples, the shape of the composite emitter may already have been damaged. The damage extent of the 20 s hydrogenation surpasses the contribution to the work function of the emitter, resulting in the best FE performance of the 10 s hydrogenation sample.

4. CONCLUSIONS In summary, FE characteristics of the pristine CNTs, ta-C coated CNTs, and the hydrogenated ta-C coated CNTs samples were studied. The thickness effect of the ta-C coating film and the hydrogenation duration effect on their FE characteristics were investigated as well. Results show that the FE properties of the pristine CNTs were improved with the ta-C coating and were further enhanced with a slight hydrogenation treatment. The thickness of the ta-C coating film did not influence their FE properties, but the duration of hydrogenation treatment was a key factor affecting their FE performance. The FE enhancement mechanism of the slightly hydrogenated ta-C samples may be due to the C
H dipole formed at the ta-C surface as well as the increase of the surface conductivity that is caused by the charge transfer between the ta-C and the absorbed aqueous phase on the surface. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: (65) 6516 8933. Fax: (65) 6776 3604.

’ ACKNOWLEDGMENT The authors would like to acknowledge the financial support from the grant of NUS R284000087112. ’ REFERENCES (1) Saito, Y.; Hamaguchi, K.; Hata, K.; Uchida, K.; Tasaka, Y.; Ikazaki, F.; Yumura, M.; Kasuya, A.; Nishina, Y. Nature 1997, 389, 554. (2) Wang, Q. H.; Setlur, A. A.; Lauerhaas, J. M.; Dai, J. Y.; Seeling, E. W.; Chang, R. P. H. Appl. Phys. Lett. 1998, 72, 2912. (3) Kim, D. H.; Kim, C. D.; Lee, H. R. Carbon 2004, 42, 1807. (4) Rinzler, G.; Hafner, J. H.; Nikolaev, P.; Lou, L.; Kim, S. G.; Tomanek, D.; Nordlander, P.; Colbert, D. T.; Smalley, R. E. Science 1995, 269, 1550. (5) de Heer, W. A.; Chatelain, A.; Ugarte, D. Science 1995, 270, 1179. (6) Wang, Q. H.; Corrigan, T. D.; Dai, J. Y.; Chang, R. P. H.; Krauss, A. R. Appl. Phys. Lett. 1997, 70, 3308. (7) Saito, Y.; Uemura, S.; Hamaguchi, K. Jpn. J. Appl. Phys. 1998, 37, L346. (8) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Provencio, P. N. Science 1998, 282, 1105. (9) Chhowalla, M.; Teo, K. B. K.; Ducati, C.; Rupesinghe, N. L.; Amaratunga, G. A. J.; Ferrari, A. C.; Roy, D.; Robertson, J.; Milne, W. I. J. Appl. Phys. 2001, 90, 5308.

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