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Improved field emission properties of thiolated multi-wall carbon nanotubes on a flexible
carbon cloth substrate
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2007 Nanotechnology 18 395702
(http://iopscience.iop.org/0957-4484/18/39/395702)
Nanotechnology 18 (2007) 395702 (5pp) doi:10.1088/0957-4484/18/39/395702
Improved field emission properties of
thiolated multi-wall carbon nanotubes on
a flexible carbon cloth substrate
F T Chuang
1, P Y Chen
2, T C Cheng
2, C H Chien
3and B J Li
41Department of Material Science and Engineering, National Chiao-Tung University, Hsinchu 30050, Taiwan, Republic of China
2National Nano Device Laboratories, Hsinchu 30078, Taiwan, Republic of China 3Department of Electronics Engineering, National Chiao-Tung University, Hsinchu 30050, Taiwan, Republic of China
4Industrial Technology Research Institute (ITRI), Hsinchu 30050, Taiwan, Republic of China E-mail:[email protected]
Received 1 May 2007, in final form 22 July 2007
Published 4 September 2007
Online at
stacks.iop.org/Nano/18/395702
Abstract
In this paper we report the observation of enhanced field emission properties
from thiolated multi-wall carbon nanotubes (MWCNTs) produced by a
simple and effective two-step chemical surface modification technique. This
technique implements carboxylation and thiolation on the MWCNTs
synthesized by microwave plasma chemical vapor deposition (MPCVD) on
the flexible carbon cloth substrate. The resulting thiolated MWCNTs were
found to have a very low threshold field value of 1
.25 V μm
−1and a rather
high field enhancement factor of 1
.93 × 10
4, which are crucial for
applications in versatile vacuum microelectronics.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
Since carbon nanotubes (CNTs) were discovered by Iijima in 1991 [1], there has been increased interest in the study of electron field emission of CNTs, in particular for potential applications in field emission displays (FEDs), vacuum flat lamps, vacuum microelectronic sources, etc. [2–4]. Some unique physical properties, such as high aspect ratio, a whisker-like shape for optimum geometrical field enhancement, high electrical conductivity and extraordinary environmental stability, make CNTs promising candidates for cold cathode materials. In the past, field emission from patterned CNT thin films on silicon and various types of rigid substrates had been extensively studied [5–8]. However, these sorts of substrates are vulnerable to breaking. Therefore, the lighter, more durable and longer lifespan field emitters made on flexible substrates have recently attracted significant attention.
In 2004, Jo et al [9] and Banerjee et al [10] reported, respectively, the growth of CNTs or ZnO nanowires (ZnO NWs) on carbon cloth by thermal chemical vapor deposition (thermal CVD). We find that carbon cloth is intrinsically
a plastic and electrically conducting substrate, allowing the practical implementation of flexible electron sources for versatile vacuum microelectronics applications. In addition, carbon cloth is also considered to be a good cathode substrate due to its woven geometry, which provides an additional geometric enhancement effect to the overall field emission properties [9].
In this work, we report a study on surface chemical modifications on MWCNTs with the aim of improving the field emission properties of the MWCNTs grown on the flexible carbon cloth. Our experimental results obtained using a two-step chemical process show that thiolation of MWCNTs could significantly improve the field emission efficiency, and thus be very promising for the fabrication of low turn-on field and highly efficient electron emitters for flexible electronics.
2. Experimental details
All MWCNT samples were grown by MPCVD on the flexible carbon cloth substrate [11]. A Fe film of 20 nm was used as a catalyst and patterned with a size of 0.5×0.5 cm2. The
Nanotechnology 18 (2007) 395702 F T Chuang et al (a) 500µm (b) 10 µm 1 µm
Figure 1. SEM images of (a) the pristine carbon cloth, the inset showing the bent flexible MWCNT field emission cathode and (b) MWCNTs thin film grown on carbon cloth.
reactant gases were a mixture of H2 and CH4with a ratio of
90:30 sccm at a pressure of 10 Torr. The microwave power of 300 W was applied to generate the plasma. After heating at 550◦C for 20 min with a bias of−50 V, vertically aligned MWCNTs with similar height were uniformly grown over the carbon cloth. These as-grown MWCNTs then underwent a two-step chemical surface modification procedure. In step 1, MWCNTs were modified by 14 M HNO3 at 90◦C for 12 h
for the carboxylation of MWCNTs [12], i.e. caps of closed MWCNTs were removed, forming an open-ended shape. In step 2, the thiolation of MWCNTs was introduced by a method based on the pre-formation of carboxylic bonds of MWCNTs. After refluxing with deionized water to remove residual HNO3,
thiolated MWCNTs were obtained by reacting with 2 M 2-mercaptoethanol(C2H6OS)in a dehydration reaction [13]:
R–COOH+HS(CH2)2OH→R–COO(CH2)2SH+H2O,
(1) at 90◦C for 12 h. Finally, the residual C2H6OS was removed
by ethanol.
The morphologies of as-grown and chemically modi-fied MWCNTs were characterized by scanning electron mi-croscopy (SEM, JEOL JSM-6500F) and transmission electron microscopy (TEM, JEOL JEM-4000). X-ray photoelectron spectroscopy (XPS, PHI 1600) measurement nets were em-ployed to identify the chemical natures of MWCNTs before and after the carboxylation and thiolation processes.
Figure 2. XPS spectra of three types of MWCNTs. S 2s and S 2p emission peaks were revealed from the sample after surface thiolation.
The field emission properties of the three types of MWCNTs (as-grown, carboxylated and thiolated) were measured using a planar diode in the vacuum chamber at a pressure of∼5×10−7Torr at room temperature. All samples with the same emission area of 0.5×0.5 cm2were mounted on
a stainless-steel plate as a grounded cathode. The anode was another stainless-steel plate with a much larger area than the cathode, in order to collect all emitted electrons. The distance between the anode and cathode was fixed at 550μm during the field emission measurement. The voltage applied on the anode was varied from 0 to 1 kV, to supply an electric field for extraction of the electrons from the MWCNTs. The emission current was monitored using a Keithley 237 picoammeter.
3. Results and discussion
Shown in figures1(a) and (b) are the SEM images of a piece of the pristine carbon cloth composed of smooth textured carbon fibers and the MWCNT thin film grown on the carbon cloth, respectively. The inset of figure1(b) is a magnified SEM image at the apex of the MWCNTs. It is seen that dense, extremely long (approximately 20 μm) and well-aligned MWCNTs adhered very well to the carbon cloth. The inset of figure1(a) demonstrates the flexibility of MWCNT thin films grown on the carbon cloth, which was attached to the flexible indium– tin-oxide (ITO) template by the use of copper tape.
The effects of chemical modification were verified by the XPS measurements. Figure 2depicts the overall spectra of XPS from three types of MWCNT samples. Besides some common features relating to C 1s and O 1s emissions, as marked in the spectra, two distinct peaks assigned due to S 2p (∼163.94 eV) and S 2s (∼228.23 eV) emissions were revealed from the sample after thiolation [13].
It is a bit difficult to analyze the bond configurations from the O 1s peak, since as a known effect, physically absorbed oxygen molecules from ambient also contribute to all XPS spectra of MWCNTs. We therefore made a detailed analysis of the C 1s core-level peak shape on these three samples. Figure 3(a) shows that, for the as-grown samples, the C 1s emission was mainly composed of two components at 284.80 eV and 285.88 eV, respectively, as were obtained 2
(c) (b) (a) Intensity (a.u.) Intensity (a.u.) Intensity (a.u.)
Figure 3. C 1s core-level spectra of (a) as-grown, (b) carboxylated and (c) thiolated MWCNTs.
using deconvolution of the C 1s overall emission peak. These two components are know to be due to sp2single-bond (C–C)
and sp3 double-bond (C=C) carbon molecules, respectively,
with one eV energy separation [14]. After carboxylation, the measured C 1s peak depicted some low-count intensity at the high-energy side (figure 3(b)). These features could be fitted by adding contributions due to C=O (∼287.42 eV) and COOH (∼289.31 eV) in peak deconvolution, indicating that the carboxyl groups were successfully introduced to the surface of MWNTs via the oxidization of HNO3[15].
Finally, after thiolation, the COOH peak intensity became lower, as illustrated in figure3(c); at the same time, there was a slight decrease of the C=O peak intensity. Both observations imply that the chemical circumstance of C=O in the carboxyl
5 nm 5 nm
(a) (b)
(c)
Figure 4. TEM images of (a) as-grown MWCNT and (b) MWCNT after oxidation by HNO3(carboxylation), revealing an open-ended structure; (c) a schematic illustration of the sequence of chemical surface modifications on MWCNTs: as-grown, carboxylation and thiolation.
groups was changed due to the formation of thiol groups on top [13].
Figures4(a) and (b) are the TEM images of the as-grown and carboxylated MWCNTs. One can clearly observe that there was an amorphous layer outside the crystalline structures of as-grown MWCNTs. After oxidation by HNO3, the closed
caps were etched away, accompanied by the partial removal of amorphous layers of MWCNTs [16]. The MWCNTs remain open-ended after thiolation (not shown). Figure 4(c) is a schematic presentation of how carboxyl and thiol functional groups were attached on top of MWCNTS using the surface chemical modification technique.
Figure5(a) shows the measured emission current density versus the electric field (J–E curves) of three MWCNT samples before and after surface chemical modifications. Obviously, the thiolated MWCNTs depict the lowest turn-on field of 0.2 Vμm−1and threshold field of 1.25 Vμm−1, which are defined as the value of macroscopic fields producing the emission current density of 10μA cm−2and 1 mA cm−2, respectively.
The detailed field emission properties of the three samples in this study are summarized in table 1. Field emission characteristics are analyzed using the Fowler–Nordheim (FN) equation [9]:
J = Aβ 2E2
ϕ exp(−Bϕ
3/2/β E), (2)
where J is the emission current density (A cm−2), E is the applied field (V cm−1), ϕ is the work function of the MWCNTs,βis the field enhancement factor, which determines the amplification of the applied field, and A and B are
Nanotechnology 18 (2007) 395702 F T Chuang et al
(a)
(b)
Figure 5. (a) Dependences of the measured field emission current density on applied field for as-grown MWCNTs (), carboxylated MWCNTs (
◦
) and thiolated MWCNTs () grown on carbon cloth; (b) the corresponding Fowler–Nordheim plots.Table 1. Field emission characteristics of three MWCNTs studied in this work. Sample Morphological feature Turn-on field (V μm−1) Threshold field (V μm−1) Field enhancement factor As-grown MWCNTs Capped 0.5 — 0.7 × 104 Carboxylated MWCNTs Open-ended 0.34 1.52 1.37 × 104 Thiolated MWCNTs Open-ended 0.2 1.25 1.93 × 104
constants with values of 1.56×10−6(A eV V−2)and 6.83× 107(V eV−3/2cm−1).
The calculated FN plots of ln(J/E2) versus 1/E are
shown in figure5(b) together with the experimental data. The linear relationship implies that the field emission of all three MWCNT samples in this work was dominated by the FN tunneling mechanism. Assuming the work function of CNTs
Table 2. A comparison of the threshold field (the emission current density is defined as 1 mA cm−2) and field enhancement factor for various field emitters in the literature.
Emitters Threshold field (V μm−1) Field enhancement factor Ref. ZnO nanowires on carbon cloth 0.7 41 100 [10] SWNTs on silica 1.8 15 000 [20] CNTs films 2.3 6 800 [21] GaAs nanowires 6.5 3 500 [22] Si nanowires 3.4 — [23] ZnO nanoneedle 6.5 1 464 [24] ZnO nanocavity 11.6 1 035 [24] ZnO nanotubes 17.8 910 [25] ZnO nanorods 4.5 3 109 [26] As-grown MWNTS on carbon cloth — 7 000 This work Carboxylated MWNTs on carbon cloth 1.52 13 700 This work Thiolated MWNTs on carbon cloth 1.25 19 300 This work
is 5.0 eV, which seems appropriate for most MWCNTs [9], the field enhancement factor valuesβwere determined by fitting to the slope of the FN plot, which were 0.7×104, 1.36×104
and 1.93×104, respectively, for the as-grown, carboxylated
and thiolated MWCNTs. Again, the thiolated CNTs display the highest field enhancement factor among the three sample groups.
The fact that the field emission performance of the MWCNTs could be dramatically improved by the surface modification techniques employed in this work has been interpreted due to the combined effects of geometric enhancement of open-ended configuration and a lower electron affinity after surface thiolation. The effect that the circular sharp edge of open-ended MWCNTs could result in higher field amplification at the emission area was already demonstrated in [17–19]. Different from the previous work all using plasma post-treatment (PPT) with etchant gas (e.g. oxygen in [18, 19]), our approach is a wet chemical method that is very easy to process and cheaper in cost. In addition, the MWCNTs could be sharpened due to the removal of the surrounding amorphous carbon layer during the wet chemical process, which further improved the field emission properties. Moreover, the surface electron affinity in the thiol group is much smaller than that in C=C bonds [13]. Therefore, the electrons could be more readily emitted from the surface of the thiolated MWCNTs, which in turn leads to another significant enhancement of electronic field emissions. For a comparison, table 2 summarizes some results of the previously reported field emitters together with ours, at an emission current density level of 1 mA cm−2as the common reference. Our fabricated thiolated MWCNTs depict evidently a very low threshold field and a rather high field enhancement factor, compared with various field emitters in the literature.
4. Conclusion
In summary, we have reported a simple, but very effective, two-step chemical modification technique that could produce 4
MWCNTs with excellent field emission properties on carbon cloth by thiolation. Our fabricated thiolated MWCNTs reveal a very low threshold field value of 1.25 Vμm−1and a rather high field enhancement factor of 1.93×104. The improved field
emission properties of the thiolated MWCNTs were attributed to the combined effects of geometric enhancement of open-end MWCNTs and a lower electron affinity after thiolation, which have thus offered a useful solution with a large potential to fabricate low-threshold and high-efficiency field emitters for practical applications in flexible vacuum microelectronics.
Acknowledgments
We thank Professor C F Chen of the Material Science Engineering Department, National Chiao-Tung University, Taiwan, Republic of China and C Chiu of Engineering Physics, Queen’s University, Canada for fruitful discussions.
References
[1] Iijima S 1991 Nature354 56
[2] Choi W B et al 1999 Appl. Phys. Lett.75 3129
[3] Gangloff L et al 2004 Nano Lett.4 1575
[4] Pirio G, Legagneux P, Pribat D, Teo K B K, Chhowalla M, Amaratunga G A J and Milne W I 2002 Nanotechnology
13 1
[5] Rinzler A G, Hafner J H, Nikolaev P, Lou L, Kim S G, Tomanek D, Nordlander P, Colbert D T and Smalley R E 1995 Science270 1179
[6] Fan S, Chapline M G, Franklin N R, Tombler T W, Cassell A M and Dai H 1999 Science283 512
[7] Bonard J M, Kind H, Stockli T and Nilsson L O 2001
Solid-State Electron.45 893
[8] Rao A M, Jacques D, Haddon R C, Zhu W, Bower C and Jin S 2000 Appl. Phys. Lett.76 3813
[9] Jo S H, Wang D Z, Huang J Y, Li W Z, Kempa K and Ren Z F 2004 Appl. Phys. Lett.85 810
[10] Banerjee D, Jo S H and Ren Z F 2004 Adv. Mater.16 2028
[11] Chen C C, Chen C F, Hsu C H and Li I H 2005 Diamond Relat.
Mater.14 770
[12] Rajalakshmi N, Ryu H, Shaijumon M M and Ramaprabhu S 2005 J. Power Sources140 250
[13] Kim Y T and Mitani T 2006 J. Catal.238 394
[14] Okpalugo T I T, Papakonstantinou P, Murphy H, McLaughlin J, Brown N M D and McNally T 2005
Fullerenes, Nanotubes, Carbon Nanostruct.13 47
[15] Xie J, Zhang N, Guers M and Varadan V K 2002 Smart Mater.
Struct.11 575
[16] Chen C M, Chen M, Peng Y W, Lin C H, Chang L W and Chen C F 2005 Diamond Relat. Mater.14 798
[17] Pan Z W, Au F C K, Lai H L, Zhou W Y, Sun L F, Liu Z Q, Tang D S, Lee C S, Lee S T and Xie S S 2001 J. Phys.
Chem. B105 1519
[18] Saito Y and Uemura S 2000 Carbon38 169
[19] Tanaka H, Akita S, Pan L and Nakayama Y 2004 Japan. J.
Appl. Phys.43 L427
[20] Zhang L, Balzano L and Resasco D E 2005 J. Phys. Chem. B
109 14375
[21] Kumar A, Pushparaj V L, Kar S, Nalamasu O, Ajayan P M and Baskaran R 2006 Appl. Phys. Lett.89 163120
[22] Zhi C Y, Bai X D and Wang E G 2005 Appl. Phys. Lett.
86 213108
[23] Zeng B, Xiong G, Chen S, Jo S H, Wang W Z, Wang D Z and Ren Z F 2006 Appl, Phys. Lett.88 213108
[24] Zhao Q, Zhang H Z, Zhu Y W, Feng S Q, Sun X C, Xu J and Yu D P 2005 Appl. Phys. Lett.86 203115
[25] Wei A, Sun X W, Xu C X, Dong Z L, Yu M B and Huang W 2006 Appl. Phys. Lett.88 213102
[26] Li C, Fang G, Yuan L, Liu N, Ai L, Ziang Q, Zhao D, Pan C and Zhao X 2007 Nanotechnology18 155702
