- Email: [email protected]
In situ fabrication of HfC-decorated carbon nanotube yarns and their field-emission properties
CARBON
4 8 ( 2 0 1 0 ) 5 3 1 –5 3 7
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
In situ fabrication of HfC-decorated carbon nanotube yarns and their field-emission properties Yuanchao Yang *, Liang Liu *, Yang Wei, Peng Liu, Kaili Jiang, Qunqing Li, Shoushan Fan Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history:
HfC-decorated carbon nanotube (CNT) yarns have been fabricated by pre-coating pure CNT
Received 14 May 2009
yarns with Hf and in situ current heating in vacuum. HfC nanocrystals were formed at
Accepted 25 September 2009
about 1600 K through reaction of the CNTs with Hf. The fabricated HfC-CNT yarns had a
Available online 2 October 2009
work function of 3.9 eV, lower than that of pure CNT yarns. For field-emission applications, HfC-CNT sharp tips made up of HfC nanorods were obtained by Joule-heating-induced electrical breakdown above 2100 K. Their emission current could reach 320 lA with a calculated density of 800 A/cm2 at an extraction voltage of 400 V. The emitters could operate under a rough vacuum of 102 Pa without obvious degradation. These excellent field-emission properties are attributed to the HfC nanorods, which have a low work function and are resistant to ion bombardment. 2009 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon nanotubes (CNTs) are promising one-dimensional (1D) nanomaterials due to their high-aspect-ratio, good electric and thermal conductivities, high tensile strength, and chemical inertness [1,2]. In 2002, we synthesized superaligned multiwalled CNT (MWCNT) arrays from which continuous CNT yarns could be directly dry spun like silk being drawn from a silkworm cocoon [3]. CNT sheets can also be drawn from them, and these have many applications, such as transparent conducting films and polarized light sources [4]. The raw CNT yarns and sheets can be processed to tight yarns by passing them through volatile solvents; this makes them easy to manipulate [5]. We have demonstrated that CNT yarn can be used as the filament of an incandescent lamp [3], and its cross section can be used as a field emitter [6]. In addition, CNT yarns can be current heated in situ in a vacuum to high temperature, which is indispensable for some chemical reactions. Therefore, it would be interesting to study the reaction of CNT
yarns with other materials by self-heating, which is a controllable, efficient, and energy-saving process. In this paper, we report the in situ reaction of CNT yarns with Hf. Using this method, Hf coatings are converted to HfC nanocrystals. HfC has an extremely high melting point (4163 K), a relatively low work function (3.3–3.8 eV), chemical inertness, and good resistance to ion bombardment [7,8]. It has been widely used in Spindt field-emission arrays to improve the stability of the emission current [9]. Recently, it has been shown that HfC-coated CNTs have better field-emission properties than pristine CNTs [10,11]. Consequently, the field emission of the fabricated HfC-decorated CNT yarns has also been studied using their cross sections made by electrical breakdown. The HfC nanocrystals formed at about 1600 K. The work function of the HfC-CNT yarn was measured to be 3.9 eV, 0.7 eV lower than that of pure CNT yarn [12]. By further heating the HfC-CNT yarn to breakdown, we obtained two sharp ends that could be used as field emitters. These HfC-CNT yarn emitters are made up of HfC nanorods and can provide a
* Corresponding authors: Fax: +86 10 62792457 (Y. Yang). E-mail addresses: [email protected] (Y. Yang), [email protected] (L. Liu). 0008-6223/$ - see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.09.074
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current of 320 lA with a calculated current density of about 800 A/cm2. Moreover, they can work stably under a rough vacuum of 102 Pa. These excellent field-emission properties are attributed to the HfC nanorods. HfC-CNT yarns could have potential applications as durable field emitters or thermionic electron sources.
2.
Experimental
2.1.
Fabrication of HfC-decorated CNT yarns
A 10 mm long and 200 lm wide CNT sheet was suspended between two nickel electrodes, which were fixed in a ceramic terminal block. The preparation process is schematically shown in Fig. 1a. A CNT sheet drawn from superaligned MWCNT arrays grown on a 4-in. silicon wafer was shrunk to a narrower sheet in air, and was put between the electrodes with the two ends fixed by silver paste. The CNT sheet fixed between the electrodes was then transferred into a magnetron sputtering system using a 99.99% pure Hf plate as the target, and a Hf film with a nominal thickness of 50 nm was deposited on it. The Hf-coated CNT sheet was then passed through ethanol, the surface tension of which made it shrink to a yarn about 34 lm in diameter [5]. After that, the Hf-CNT yarn was transferred to a high-vacuum chamber with a base vacuum higher than 1.0 · 105 Pa. A Keithley 2410 source meter was
used to supply power to heat the Hf-CNT yarn. Fig. 1b shows the uniform incandescent Hf-CNTyarn. A spectrometer (Konica-Minolta CS-1000) recorded the yarn’s incandescent light in the range 380–780 nm. The spectrum was a good fitted to the Planck black-body radiation law and the temperature could be derived from that [13]. The Hf-CNT yarn was heated to 1600 K for 30 min under a voltage of 14 V and 32 mA current. Finally, the HfC-decorated CNT yarn was obtained after the heating process through the reaction of CNTs with Hf. The microstructure of the HfC-CNT yarn was characterized by transmission electron microscopy (TEM). The thermionic emission (TE) of the HfC-CNT yarn was studied, from which the work function could be determined [12]. A stainless-steel plate was placed 5 mm from the hot yarn as the anode, and a Keithley 237 source meter was used to apply a voltage Ua to the anode and simultaneously record the TE current Ia.
2.2.
Field emitters made from HfC-CNT yarns
In order to study the field-emission properties of the HfCCNT yarns, it was necessary to obtain their cross sections. Recently, we fabricated high-quality field emitters from CNT yarns by the vacuum breakdown process [14]. Here, we adopted the same method to fabricate field emitters from HfC-CNT yarns. HfC-CNT yarn was heated to 2136 K under a DC bias of 21 V and a current of 47.6 mA in a
Fig. 1 – (a) Preparation of CNT sheet suspended between two nickel electrodes. (b) A uniform incandescent Hf-CNT yarn heated by current. (c) Schematic of field-emission measurement apparatus.
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vacuum of 1.0 · 105 Pa. It broke down at the hottest point, which was generally at the center of the yarn, in 5 min. Therefore, the 10 mm long HfC-CNT yarn turned to be two 5 mm long field emitters. The fabricated HfC-CNT yarn emitters were fixed with silver paste to nickel poles along their axial direction and assembled in a custom-made testing apparatus, which included a 1D manipulator. A stainless-steel plate was fixed to the 1D manipulator as the anode, and the gap between the HfCCNT yarn emitters and the anode was adjusted to 300 lm. Fig. 1c shows the schematic of the apparatus of field-emission measurement. The testing apparatus was placed inside a vacuum chamber with a base vacuum of 1.0 · 105 Pa, and a
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Keithley 2410 source meter was used to apply voltage and monitor the emission current.
3.
Results and discussion
3.1.
Characterization of HfC-CNT yarns
Fig. 2a and b shows the scanning electron microscopy (SEM) images of the Hf-CNT yarn before and after the heating process. The diameter of the raw Hf-CNT yarn was 34 lm, and it decreased to 28 lm after the heating process. This shrinking effect may be due to the surface tension of the liquid phase, which could be molten Hf. Although the melting point of bulk
Fig. 2 – SEM images of the Hf-CNT yarn (a) before and (b) after the heating process. Scale bar: 20 lm. (c) and (d) TEM images corresponding to (a) and (b). (e) EDS image of the nanoparticles shown in (d). (f) HRTEM image of the nanoparticles. The inset shows the diffraction pattern obtained by FFT with the assigned lattice plane index.
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Hf is as high as 2500 K, it is possible that nano-sized Hf coatings melt at 1600 K due to the size effect [15]. Fig. 2c shows the TEM image of the as-coated Hf-CNTyarn. It can be clearly seen that the CNTs are coated with uniform Hf films and have diameters of about 50 nm. After the heating process, the Hf coatings are converted to nanoparticles with about 20 nm diameter, as shown in Fig. 2d. These nanoparticles are attached to the outer surfaces of the CNTs. Fig. 2e is the energy-dispersive X-ray spectrum (EDS) image of the nanoparticles, which shows that the only elements present are Hf and C (the Cu peak is the signal from the copper TEM grid). To determine whether these nanoparticles were composed of HfC or Hf, their structure was characterized by high-resolution TEM (HRTEM), as shown in Fig. 2f. In the inset of Fig. 2f, the diffraction pattern obtained from the fast-Fourier transform (FFT) presents a square lattice, which indicates that the nanoparticles are fine crystallites. Moreover, the diffraction pattern is consistent with that of HfC crystals, which have a face-centered-cubic (fcc) lattice structure with a lattice constant of 0.464 nm [16]. This confirmed that the Hf coatings reacted with the CNTs and formed HfC nanocrystals during the heating process. Therefore, the Hf-CNT yarns become so-called HfC-CNT yarns, the CNTs being decorated with HfC nanocrystals.
3.2.
broken HfC-CNT yarns are needle-shaped with much smaller diameters at the ends. These should be attributed to the shrinking effect mentioned above, or to evaporation of the CNTs. Both of these processes can be induced by the local high temperature, which could be much higher than 2136 K. Fig. 4b shows a higher-magnification SEM image of one sharp end. This end is about 7 lm in diameter and is composed of much sharper tips with a morphology similar to that of the CNTs. The microstructure of the tips was further characterized by TEM. The tips retain high-aspect-ratio features similar to those of CNTs, as shown in Fig. 4c, but they are composed of nanocrystals with no CNTs. This is confirmed by Fig. 4d and e, which shows the higher-magnification TEM image of a single tip. In contrast to the HfC-CNT yarn obtained by the 1600 K heating process, which is shown in Fig. 2d and f, the tips formed at 2136 K have much denser nanocrystals and no graphite layers. Since the HfC nanocrystals have been obtained through the 1600 K heating process (Fig. 2f), it is plausible to suggest that the tips shown in Fig. 4c–e are HfC
Work function of HfC-CNT yarns
The work function of the HfC-CNT yarns was determined by the TE method [12]. For a thermionic electron source, the emission current follows the Richardson equation [17]: I0 / ¼ logðASÞ 0:4343 ; ð1Þ log kB T T2 where I0 is the zero-field current, A the Richardson constant, T the temperature in the Kelvin scale, S the effective area for emitting electrons, kB the Boltzmann constant, and / the work function of the cathode, which can be determined from the slope of logðI0 =T2 Þ 1=T. We actually measured the TE current Ia under an external voltage Ua in the accelerating field region, which can be described as [18] pffiffiffi pffiffiffiffiffiffi a Ua ; ð2Þ log Ia ¼ log I0 þ 1:906 T where a is a constant in cm1 that depends on the electrode pffiffiffiffiffiffi geometry. Fig. 3a shows typical log Ia Ua curves of the HfC-CNT yarn at various temperatures. Taking a linear fit on these curves, the zero-field current log I0 can be extrapolated from the intercept on the y axis. Fig. 3b shows the measured logðI0 =T2 Þ 1=T plots of the HfC-CNT yarn and pure CNT yarn for comparison. By linearly fitting the plots, the work function of the HfC-CNT yarn was determined to be 3.9 eV, and that of pure CNT yarn was 4.6 eV. The work function of the HfC-CNT yarn is very close to that of HfC (3.3–3.8 eV) [8], which indicates that most electrons are emitted from the HfC nanocrystals rather than from the CNTs. This lower work function has benefits for TE and field-emission applications.
3.3. Morphology and field-emission properties of HfC-CNT yarn emitters Fig. 4a shows the SEM image of HfC-CNT yarn at the breaking point; it can be seen that two sharp ends were formed. The
pffiffiffiffiffiffi Fig. 3 – Measurement of work function. (a) log Ia Ua curves of the HfC-CNT yarn at various temperatures. (b) Experimental data in logðI0 =T2 Þ 1=T plots of HfC-CNT yarn (solid squares) and pure CNT yarn (solid triangles). The straight lines are their linear fits. The work functions derived from the slopes are shown below the lines.
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nanorods. We have confirmed this idea from lattice information of the nanorods, which could provide direct evidence. HRTEM image of the nanorod in Fig. 4e is shown in Fig. 4f, which shows the lattice structure and a polycrystalline structure can be seen. The inset of Fig. 4f is the FFT patterns of the selected area (indicated by the white square in the image), which can be indexed to the (2 2 0) and (3 1 1) reflections of fcc HfC. The measured lattice spacings and angles are in good agreement with the parameters of HfC. Under the local extremely high temperature at the breaking point, the CNTs should be substantially converted to HfC nanocrystals or should evaporate, thus leaving HfC nanorods composed of
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continuous nanocrystals. Our results suggest that the formation of HfC nanorods involves a template mechanism in which the CNTs define the overall morphology [19–21]. Fig. 5a shows current–voltage (I–V) plots of the HfC-CNT yarn emitters. The threshold voltage for electron emission is about 150 V, and the maximum current reaches 320 lA under a voltage of 400 V. For a cross-sectional tip area of about 4 · 10 7 cm2, the current density can be calculated to be 800 A/cm2, which is better than that of pure CNT yarn emitters [6,14]. The corresponding Fowler–Nordheim (FN) plot (ln(I/V2) 1/V) is shown in the inset of Fig. 5a, the fitted straight line indicating the essential field-emission properties. Taking the work func-
Fig. 4 – (a) SEM image of the HfC-CNT yarn ends at the breaking point. Scale bar: 100 lm. (b) Higher-magnification SEM image of one end. Scale bar: 5 lm. (c) TEM image of the HfC tips. (d) and (e) shows higher-magnification TEM image of a single HfC tip. (f) HRTEM image of the HfC tip shown in (e). The inset is the FFT image of the selected lattice indicated by the white square.
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tion of the emitters as 3.9 eV, the field enhancement factor from the slope of the FN plot was 5097. This large value is attributed to the sharp ends of the yarn emitters. The most impressive feature of the HfC-CNT yarn emitters is their stability under a rough vacuum, which is indispensable for practical applications. Fig. 5b compares the emission currents of the HfC-CNT yarn emitters (solid squares) and the pure CNT yarn emitters (hollow squares) under various vacuum pressures over 12 h. The pure CNT yarn emitters were also fabricated by the vacuum breakdown process. The testing apparatus was the same for both types of emitters, and the anode voltage was fixed at 320 V. The base vacuum of the test chamber was 105 Pa, and it was changed to 104, 103, and 102 Pa for three periods by introducing N2 gas through a needle valve. The pure CNTyarn emitters presented serious current degradation when the pressure was higher than 103 Pa, the current finally decreasing by 90% from 20 lA to about 2 lA at 102 Pa. For the HfC-CNT yarn emitters, no obvious degradation was observed, and the current fluctuation was less than 10%.
Pure CNT yarn emitters are made up of MWCNTs, and the current degradation under rough vacuum is mainly caused by ion bombardment [22,23]. In contrast, the HfC-CNT yarn emitters consist of HfC nanorods, which are more resistant to ion bombardment due to their high melting point and good stability. They therefore present excellent emission stability under a rough vacuum. In addition, the large-current density provided by HfC-CNT yarn emitters is also attributed to the low work function and high melting point of the HfC nanorods. Failure of field-emitting CNTs is usually due to the local high temperature induced by Joule heating from the emission current [24–26]. The high melting point of the HfC nanorods could make them capable of withstanding a larger emission current. The HfC-CNT yarn emitters fabricated here could serve as large-current and durable field-electron sources, and could have many applications, such as in microwave devices [27], X-ray tubes [28], vacuum gauges [11,29], and display devices.
4.
Fig. 5 – (a) I–V plots of the HfC-CNT yarn emitters. The inset is the corresponding FN curve. (b) Stability of the emission current for HfC-CNT yarn emitters (solid squares) and pure CNT yarn emitters (hollow squares) under various vacuum conditions. The pressure was changed from 105 to 102 Pa by introducing N2 gas.
Conclusions
We have studied the reaction of CNT yarns with a Hf coating by self-electrical heating. The HfC nanocrystals were fabricated by heating them to 1600 K. The work function of the HfC-decorated CNT yarns was determined to be 3.9 eV by the TE method, 0.7 eV lower than that of pure CNT yarns. We obtained HfC-CNT yarn emitters by further heating to 2136 K and breakdown. These emitters are made up of HfC nanorods. They can provide a current density of 800 A/cm2 and can operate under a rough vacuum of 102 Pa without obvious degradation. These excellent field-emission properties make them promising for use as large-current and durable cold cathodes, which could have many potential applications, such as in microwave devices, X-ray tubes, vacuum gauges and display devices. The fabricated HfC-decorated CNTyarns are high yield and low cost. Superaligned MWCNT arrays grown on a single 4-in. wafer can produce over 100 m of CNT yarn. The wafer can be used repeatedly, which is both environmentally friendly and cost effective. Drawing the CNT sheets, sputtering the Hf films, shrinking to yarns, and the heating process could be integrated into an automatic system, thus making the mass production of HfC-CNT yarns possible. Moreover, this strategy for fabricating HfC-CNT yarns could also be used for synthesizing CNT yarns decorated with other carbides, such as ZrC, TiC, and TaC. Based on the mass production of HfC-CNT yarns, it is possible to apply the emitters to actual display devices. The display would consist of two glass sheets separated by a few millimeters, the rear sheet supporting the emitters and the front the phosphors. The rear sheet with HfC-CNT yarn emitters can be created by the following steps. First, a matrix of silver electrodes is formed on the glass using screen-printing technology, and then long HfC-CNT yarns are placed on top of that and further cut by laser to form a matrix. This procedure is recently realized by our group to develop an incandescent display using CNT film [30]. Next, each HfC-CNTyarn segment between a pair of electrodes is current heated to form emit-
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ters and electrical breakdown to form gaps. The display described here is similar to the surface-conduction electronemitter display (SED).
Acknowledgements The authors gratefully acknowledge of the help of Lina Zhang and Zhihua Han in taking the TEM images and helpful discussions with Professor Pijin Chen. This work was financially supported by the National Basic Research Program of China (2005CB623606 and 2007CB935301) and the NSFC (10704044, 50825201, and 10721404).
R E F E R E N C E S
[1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354(6348):56–8. [2] Saito R, Dresselhaus G, Dresselhaus MS. Physical properties of carbon nanotubes. Singapore: World Scientific Publishing Company; 1998. [3] Jiang KL, Li QQ, Fan SS. Spinning continuous carbon nanotube yarns: carbon nanotubes weave their way into a range of imaginative macroscopic applications. Nature 2002;419(6909):801. [4] Zhang M, Fang SL, Zakhidov AA, Lee SB, Aliev AE, Williams CD, et al. Strong, transparent multifunctional, carbon nanotube sheets. Science 2005;309(5738):1215–9. [5] Zhang XB, Jiang KL, Feng C, Liu P, Zhang L, Kong J, et al. Spinning and processing continuous yarns from 4-inch wafer scale super-aligned carbon nanotube arrays. Adv Mater 2006;18(12):1505–10. [6] Wei Y, Weng D, Yang YC, Zhang XB, Jiang KL, Liu L, et al. Efficient fabrication of field electron emitters from the multiwalled carbon nanotube yarns. Appl Phys Lett 2006;89(6):063101-1–3. [7] Zaima S, Adachi H, Shibata Y. Promising cathode materials for high brightness electron beams. J Vac Sci Technol B 1984;2(1):73–8. [8] Mackie WA, Morrissey JL, Hinrichs CH, Davis PR. Field emission from hafnium carbide. J Vac Sci Technol A 1992;10(4):2852–6. [9] Sato T, Yamamoto S, Nagao M, Matsukawa T, Kanemaru S, Itoh J. Fabrication and characterization of HfC coated Si field emitter arrays. J Vac Sci Technol B 2003;21(4):1589–93. [10] Zhang JH, Yang CR, Wang YJ, Feng T, Yu WD, Jiang J, et al. Improvement of the field emission of carbon nanotubes by hafnium coating and annealing. Nanotechnology 2006;17(1):257–60. [11] Yang YC, Qian L, Tang J, Liu L, Fan SS. A low-vacuum ionization gauge with HfC-modified carbon nanotube field emitters. Appl Phys Lett 2008;92(15):153105-1–3.
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[12] Liu P, Wei Y, Jiang KL, Sun Q, Zhang XB, Fan SS, et al. Thermionic emission and work function of multiwalled carbon nanotube yarns. Phys Rev B 2006;73(23):235412-1–5. [13] Li P, Jiang KL, Liu M, Li QQ, Fan SS, Sun JL. Polarized incandescent light emission from carbon nanotubes. Appl Phys Lett 2003;82(11):1763–5. [14] Wei Y, Jiang KL, Liu L, Chen Z, Fan SS. Vacuum-breakdowninduced needle-shaped ends of multiwalled carbon nanotube yarns and their field emission applications. Nano Lett 2007;7(12):3792–7. [15] Goldstein AN, Echer CM, Alivisatos AP. Melting in semiconductor nanocrystals. Science 1992;256(5062):1425–7. [16] Storms EK. The refractory carbides. New York: Academic; 1967. [17] Richardson OW. The emission of electricity from hot bodies. 2nd ed. London: Longmans, Green, and Co.; 1921. [18] Reimann AL. Thermionic emission. New York: Wiley; 1934. [19] Dai HJ, Wong EW, Lu YZ, Fan SS, Liber CM. Synthesis and characterization of carbide nanorods. Nature 1995;375(6534):769–72. [20] Wong EW, Maynor BW, Burns LD, Liber CM. Growth of metal carbide nanotubes and nanorods. Chem Mater 1996;8(8):2041–6. [21] Han WQ, Fan SS, Li QQ, Hu YD. Synthesis of gallium nitride nanorods through a carbon nanotube-confined reaction. Science 1997;277(5330):1287–9. [22] Bonard JM, Maier F, Stockli T, Chatelain A, de Heer WA, Salvetat JP, et al. Field emission properties of multiwalled carbon nanotubes. Ultramicroscopy 1998;73(1–4):7–15. [23] Dean KA, Chalamala BR. The environmental stability of field emission from single-walled carbon nanotubes. Appl Phys Lett 1999;75(19):3017–9. [24] Huang NY, She JC, Chen J, Deng SZ, Xu NS, Bishop H, et al. Mechanism responsible for initiating carbon nanotube vacuum breakdown. Phys Rev Lett 2004;93(7):075501-1–4. [25] Liang XH, Deng SZ, Xu NS, Chen J, Huang NY, She JC. On achieving better uniform carbon nanotube field emission by electrical treatment and the underlying mechanism. Appl Phys Lett 2006;88(11):111501-1–3. [26] Wei W, Liu Y, Wei Y, Jiang KL, Peng LM, Fan SS. Tip cooling effect and failure mechanism of field-emitting carbon nanotubes. Nano Lett 2007;7(1):64–8. [27] Teo KBK, Minoux E, Hudanski L, Peauger F, Schnell JP, Gangloff L, et al. Microwave devices: carbon nanotubes as cold cathodes. Nature 2005;437(7061):968. [28] Sugie H, Tanemura M, Filip V, Iwata K, Takahashi K, Okuyama F. Carbon nanotubes as electron source in an X-ray tube. Appl Phys Lett 2001;78(17):2578–80. [29] Choi IM, Woo SY, Song HW. Improved metrological characteristics of a carbon-nanotube-based ionization gauge. Appl Phys Lett 2007;90(2):023107-1–3. [30] Liu P, Liu L, Wei Y, Liu K, Chen Z, Jiang KL, et al. Fast hightemperature response of carbon nanotube film and its application as an incandescent display. Adv Mater 2009;21. doi:10.1002/adma.200900473.
4 8 ( 2 0 1 0 ) 5 3 1 –5 3 7
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
In situ fabrication of HfC-decorated carbon nanotube yarns and their field-emission properties Yuanchao Yang *, Liang Liu *, Yang Wei, Peng Liu, Kaili Jiang, Qunqing Li, Shoushan Fan Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history:
HfC-decorated carbon nanotube (CNT) yarns have been fabricated by pre-coating pure CNT
Received 14 May 2009
yarns with Hf and in situ current heating in vacuum. HfC nanocrystals were formed at
Accepted 25 September 2009
about 1600 K through reaction of the CNTs with Hf. The fabricated HfC-CNT yarns had a
Available online 2 October 2009
work function of 3.9 eV, lower than that of pure CNT yarns. For field-emission applications, HfC-CNT sharp tips made up of HfC nanorods were obtained by Joule-heating-induced electrical breakdown above 2100 K. Their emission current could reach 320 lA with a calculated density of 800 A/cm2 at an extraction voltage of 400 V. The emitters could operate under a rough vacuum of 102 Pa without obvious degradation. These excellent field-emission properties are attributed to the HfC nanorods, which have a low work function and are resistant to ion bombardment. 2009 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon nanotubes (CNTs) are promising one-dimensional (1D) nanomaterials due to their high-aspect-ratio, good electric and thermal conductivities, high tensile strength, and chemical inertness [1,2]. In 2002, we synthesized superaligned multiwalled CNT (MWCNT) arrays from which continuous CNT yarns could be directly dry spun like silk being drawn from a silkworm cocoon [3]. CNT sheets can also be drawn from them, and these have many applications, such as transparent conducting films and polarized light sources [4]. The raw CNT yarns and sheets can be processed to tight yarns by passing them through volatile solvents; this makes them easy to manipulate [5]. We have demonstrated that CNT yarn can be used as the filament of an incandescent lamp [3], and its cross section can be used as a field emitter [6]. In addition, CNT yarns can be current heated in situ in a vacuum to high temperature, which is indispensable for some chemical reactions. Therefore, it would be interesting to study the reaction of CNT
yarns with other materials by self-heating, which is a controllable, efficient, and energy-saving process. In this paper, we report the in situ reaction of CNT yarns with Hf. Using this method, Hf coatings are converted to HfC nanocrystals. HfC has an extremely high melting point (4163 K), a relatively low work function (3.3–3.8 eV), chemical inertness, and good resistance to ion bombardment [7,8]. It has been widely used in Spindt field-emission arrays to improve the stability of the emission current [9]. Recently, it has been shown that HfC-coated CNTs have better field-emission properties than pristine CNTs [10,11]. Consequently, the field emission of the fabricated HfC-decorated CNT yarns has also been studied using their cross sections made by electrical breakdown. The HfC nanocrystals formed at about 1600 K. The work function of the HfC-CNT yarn was measured to be 3.9 eV, 0.7 eV lower than that of pure CNT yarn [12]. By further heating the HfC-CNT yarn to breakdown, we obtained two sharp ends that could be used as field emitters. These HfC-CNT yarn emitters are made up of HfC nanorods and can provide a
* Corresponding authors: Fax: +86 10 62792457 (Y. Yang). E-mail addresses: [email protected] (Y. Yang), [email protected] (L. Liu). 0008-6223/$ - see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.09.074
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current of 320 lA with a calculated current density of about 800 A/cm2. Moreover, they can work stably under a rough vacuum of 102 Pa. These excellent field-emission properties are attributed to the HfC nanorods. HfC-CNT yarns could have potential applications as durable field emitters or thermionic electron sources.
2.
Experimental
2.1.
Fabrication of HfC-decorated CNT yarns
A 10 mm long and 200 lm wide CNT sheet was suspended between two nickel electrodes, which were fixed in a ceramic terminal block. The preparation process is schematically shown in Fig. 1a. A CNT sheet drawn from superaligned MWCNT arrays grown on a 4-in. silicon wafer was shrunk to a narrower sheet in air, and was put between the electrodes with the two ends fixed by silver paste. The CNT sheet fixed between the electrodes was then transferred into a magnetron sputtering system using a 99.99% pure Hf plate as the target, and a Hf film with a nominal thickness of 50 nm was deposited on it. The Hf-coated CNT sheet was then passed through ethanol, the surface tension of which made it shrink to a yarn about 34 lm in diameter [5]. After that, the Hf-CNT yarn was transferred to a high-vacuum chamber with a base vacuum higher than 1.0 · 105 Pa. A Keithley 2410 source meter was
used to supply power to heat the Hf-CNT yarn. Fig. 1b shows the uniform incandescent Hf-CNTyarn. A spectrometer (Konica-Minolta CS-1000) recorded the yarn’s incandescent light in the range 380–780 nm. The spectrum was a good fitted to the Planck black-body radiation law and the temperature could be derived from that [13]. The Hf-CNT yarn was heated to 1600 K for 30 min under a voltage of 14 V and 32 mA current. Finally, the HfC-decorated CNT yarn was obtained after the heating process through the reaction of CNTs with Hf. The microstructure of the HfC-CNT yarn was characterized by transmission electron microscopy (TEM). The thermionic emission (TE) of the HfC-CNT yarn was studied, from which the work function could be determined [12]. A stainless-steel plate was placed 5 mm from the hot yarn as the anode, and a Keithley 237 source meter was used to apply a voltage Ua to the anode and simultaneously record the TE current Ia.
2.2.
Field emitters made from HfC-CNT yarns
In order to study the field-emission properties of the HfCCNT yarns, it was necessary to obtain their cross sections. Recently, we fabricated high-quality field emitters from CNT yarns by the vacuum breakdown process [14]. Here, we adopted the same method to fabricate field emitters from HfC-CNT yarns. HfC-CNT yarn was heated to 2136 K under a DC bias of 21 V and a current of 47.6 mA in a
Fig. 1 – (a) Preparation of CNT sheet suspended between two nickel electrodes. (b) A uniform incandescent Hf-CNT yarn heated by current. (c) Schematic of field-emission measurement apparatus.
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vacuum of 1.0 · 105 Pa. It broke down at the hottest point, which was generally at the center of the yarn, in 5 min. Therefore, the 10 mm long HfC-CNT yarn turned to be two 5 mm long field emitters. The fabricated HfC-CNT yarn emitters were fixed with silver paste to nickel poles along their axial direction and assembled in a custom-made testing apparatus, which included a 1D manipulator. A stainless-steel plate was fixed to the 1D manipulator as the anode, and the gap between the HfCCNT yarn emitters and the anode was adjusted to 300 lm. Fig. 1c shows the schematic of the apparatus of field-emission measurement. The testing apparatus was placed inside a vacuum chamber with a base vacuum of 1.0 · 105 Pa, and a
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Keithley 2410 source meter was used to apply voltage and monitor the emission current.
3.
Results and discussion
3.1.
Characterization of HfC-CNT yarns
Fig. 2a and b shows the scanning electron microscopy (SEM) images of the Hf-CNT yarn before and after the heating process. The diameter of the raw Hf-CNT yarn was 34 lm, and it decreased to 28 lm after the heating process. This shrinking effect may be due to the surface tension of the liquid phase, which could be molten Hf. Although the melting point of bulk
Fig. 2 – SEM images of the Hf-CNT yarn (a) before and (b) after the heating process. Scale bar: 20 lm. (c) and (d) TEM images corresponding to (a) and (b). (e) EDS image of the nanoparticles shown in (d). (f) HRTEM image of the nanoparticles. The inset shows the diffraction pattern obtained by FFT with the assigned lattice plane index.
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Hf is as high as 2500 K, it is possible that nano-sized Hf coatings melt at 1600 K due to the size effect [15]. Fig. 2c shows the TEM image of the as-coated Hf-CNTyarn. It can be clearly seen that the CNTs are coated with uniform Hf films and have diameters of about 50 nm. After the heating process, the Hf coatings are converted to nanoparticles with about 20 nm diameter, as shown in Fig. 2d. These nanoparticles are attached to the outer surfaces of the CNTs. Fig. 2e is the energy-dispersive X-ray spectrum (EDS) image of the nanoparticles, which shows that the only elements present are Hf and C (the Cu peak is the signal from the copper TEM grid). To determine whether these nanoparticles were composed of HfC or Hf, their structure was characterized by high-resolution TEM (HRTEM), as shown in Fig. 2f. In the inset of Fig. 2f, the diffraction pattern obtained from the fast-Fourier transform (FFT) presents a square lattice, which indicates that the nanoparticles are fine crystallites. Moreover, the diffraction pattern is consistent with that of HfC crystals, which have a face-centered-cubic (fcc) lattice structure with a lattice constant of 0.464 nm [16]. This confirmed that the Hf coatings reacted with the CNTs and formed HfC nanocrystals during the heating process. Therefore, the Hf-CNT yarns become so-called HfC-CNT yarns, the CNTs being decorated with HfC nanocrystals.
3.2.
broken HfC-CNT yarns are needle-shaped with much smaller diameters at the ends. These should be attributed to the shrinking effect mentioned above, or to evaporation of the CNTs. Both of these processes can be induced by the local high temperature, which could be much higher than 2136 K. Fig. 4b shows a higher-magnification SEM image of one sharp end. This end is about 7 lm in diameter and is composed of much sharper tips with a morphology similar to that of the CNTs. The microstructure of the tips was further characterized by TEM. The tips retain high-aspect-ratio features similar to those of CNTs, as shown in Fig. 4c, but they are composed of nanocrystals with no CNTs. This is confirmed by Fig. 4d and e, which shows the higher-magnification TEM image of a single tip. In contrast to the HfC-CNT yarn obtained by the 1600 K heating process, which is shown in Fig. 2d and f, the tips formed at 2136 K have much denser nanocrystals and no graphite layers. Since the HfC nanocrystals have been obtained through the 1600 K heating process (Fig. 2f), it is plausible to suggest that the tips shown in Fig. 4c–e are HfC
Work function of HfC-CNT yarns
The work function of the HfC-CNT yarns was determined by the TE method [12]. For a thermionic electron source, the emission current follows the Richardson equation [17]: I0 / ¼ logðASÞ 0:4343 ; ð1Þ log kB T T2 where I0 is the zero-field current, A the Richardson constant, T the temperature in the Kelvin scale, S the effective area for emitting electrons, kB the Boltzmann constant, and / the work function of the cathode, which can be determined from the slope of logðI0 =T2 Þ 1=T. We actually measured the TE current Ia under an external voltage Ua in the accelerating field region, which can be described as [18] pffiffiffi pffiffiffiffiffiffi a Ua ; ð2Þ log Ia ¼ log I0 þ 1:906 T where a is a constant in cm1 that depends on the electrode pffiffiffiffiffiffi geometry. Fig. 3a shows typical log Ia Ua curves of the HfC-CNT yarn at various temperatures. Taking a linear fit on these curves, the zero-field current log I0 can be extrapolated from the intercept on the y axis. Fig. 3b shows the measured logðI0 =T2 Þ 1=T plots of the HfC-CNT yarn and pure CNT yarn for comparison. By linearly fitting the plots, the work function of the HfC-CNT yarn was determined to be 3.9 eV, and that of pure CNT yarn was 4.6 eV. The work function of the HfC-CNT yarn is very close to that of HfC (3.3–3.8 eV) [8], which indicates that most electrons are emitted from the HfC nanocrystals rather than from the CNTs. This lower work function has benefits for TE and field-emission applications.
3.3. Morphology and field-emission properties of HfC-CNT yarn emitters Fig. 4a shows the SEM image of HfC-CNT yarn at the breaking point; it can be seen that two sharp ends were formed. The
pffiffiffiffiffiffi Fig. 3 – Measurement of work function. (a) log Ia Ua curves of the HfC-CNT yarn at various temperatures. (b) Experimental data in logðI0 =T2 Þ 1=T plots of HfC-CNT yarn (solid squares) and pure CNT yarn (solid triangles). The straight lines are their linear fits. The work functions derived from the slopes are shown below the lines.
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nanorods. We have confirmed this idea from lattice information of the nanorods, which could provide direct evidence. HRTEM image of the nanorod in Fig. 4e is shown in Fig. 4f, which shows the lattice structure and a polycrystalline structure can be seen. The inset of Fig. 4f is the FFT patterns of the selected area (indicated by the white square in the image), which can be indexed to the (2 2 0) and (3 1 1) reflections of fcc HfC. The measured lattice spacings and angles are in good agreement with the parameters of HfC. Under the local extremely high temperature at the breaking point, the CNTs should be substantially converted to HfC nanocrystals or should evaporate, thus leaving HfC nanorods composed of
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continuous nanocrystals. Our results suggest that the formation of HfC nanorods involves a template mechanism in which the CNTs define the overall morphology [19–21]. Fig. 5a shows current–voltage (I–V) plots of the HfC-CNT yarn emitters. The threshold voltage for electron emission is about 150 V, and the maximum current reaches 320 lA under a voltage of 400 V. For a cross-sectional tip area of about 4 · 10 7 cm2, the current density can be calculated to be 800 A/cm2, which is better than that of pure CNT yarn emitters [6,14]. The corresponding Fowler–Nordheim (FN) plot (ln(I/V2) 1/V) is shown in the inset of Fig. 5a, the fitted straight line indicating the essential field-emission properties. Taking the work func-
Fig. 4 – (a) SEM image of the HfC-CNT yarn ends at the breaking point. Scale bar: 100 lm. (b) Higher-magnification SEM image of one end. Scale bar: 5 lm. (c) TEM image of the HfC tips. (d) and (e) shows higher-magnification TEM image of a single HfC tip. (f) HRTEM image of the HfC tip shown in (e). The inset is the FFT image of the selected lattice indicated by the white square.
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tion of the emitters as 3.9 eV, the field enhancement factor from the slope of the FN plot was 5097. This large value is attributed to the sharp ends of the yarn emitters. The most impressive feature of the HfC-CNT yarn emitters is their stability under a rough vacuum, which is indispensable for practical applications. Fig. 5b compares the emission currents of the HfC-CNT yarn emitters (solid squares) and the pure CNT yarn emitters (hollow squares) under various vacuum pressures over 12 h. The pure CNT yarn emitters were also fabricated by the vacuum breakdown process. The testing apparatus was the same for both types of emitters, and the anode voltage was fixed at 320 V. The base vacuum of the test chamber was 105 Pa, and it was changed to 104, 103, and 102 Pa for three periods by introducing N2 gas through a needle valve. The pure CNTyarn emitters presented serious current degradation when the pressure was higher than 103 Pa, the current finally decreasing by 90% from 20 lA to about 2 lA at 102 Pa. For the HfC-CNT yarn emitters, no obvious degradation was observed, and the current fluctuation was less than 10%.
Pure CNT yarn emitters are made up of MWCNTs, and the current degradation under rough vacuum is mainly caused by ion bombardment [22,23]. In contrast, the HfC-CNT yarn emitters consist of HfC nanorods, which are more resistant to ion bombardment due to their high melting point and good stability. They therefore present excellent emission stability under a rough vacuum. In addition, the large-current density provided by HfC-CNT yarn emitters is also attributed to the low work function and high melting point of the HfC nanorods. Failure of field-emitting CNTs is usually due to the local high temperature induced by Joule heating from the emission current [24–26]. The high melting point of the HfC nanorods could make them capable of withstanding a larger emission current. The HfC-CNT yarn emitters fabricated here could serve as large-current and durable field-electron sources, and could have many applications, such as in microwave devices [27], X-ray tubes [28], vacuum gauges [11,29], and display devices.
4.
Fig. 5 – (a) I–V plots of the HfC-CNT yarn emitters. The inset is the corresponding FN curve. (b) Stability of the emission current for HfC-CNT yarn emitters (solid squares) and pure CNT yarn emitters (hollow squares) under various vacuum conditions. The pressure was changed from 105 to 102 Pa by introducing N2 gas.
Conclusions
We have studied the reaction of CNT yarns with a Hf coating by self-electrical heating. The HfC nanocrystals were fabricated by heating them to 1600 K. The work function of the HfC-decorated CNT yarns was determined to be 3.9 eV by the TE method, 0.7 eV lower than that of pure CNT yarns. We obtained HfC-CNT yarn emitters by further heating to 2136 K and breakdown. These emitters are made up of HfC nanorods. They can provide a current density of 800 A/cm2 and can operate under a rough vacuum of 102 Pa without obvious degradation. These excellent field-emission properties make them promising for use as large-current and durable cold cathodes, which could have many potential applications, such as in microwave devices, X-ray tubes, vacuum gauges and display devices. The fabricated HfC-decorated CNTyarns are high yield and low cost. Superaligned MWCNT arrays grown on a single 4-in. wafer can produce over 100 m of CNT yarn. The wafer can be used repeatedly, which is both environmentally friendly and cost effective. Drawing the CNT sheets, sputtering the Hf films, shrinking to yarns, and the heating process could be integrated into an automatic system, thus making the mass production of HfC-CNT yarns possible. Moreover, this strategy for fabricating HfC-CNT yarns could also be used for synthesizing CNT yarns decorated with other carbides, such as ZrC, TiC, and TaC. Based on the mass production of HfC-CNT yarns, it is possible to apply the emitters to actual display devices. The display would consist of two glass sheets separated by a few millimeters, the rear sheet supporting the emitters and the front the phosphors. The rear sheet with HfC-CNT yarn emitters can be created by the following steps. First, a matrix of silver electrodes is formed on the glass using screen-printing technology, and then long HfC-CNT yarns are placed on top of that and further cut by laser to form a matrix. This procedure is recently realized by our group to develop an incandescent display using CNT film [30]. Next, each HfC-CNTyarn segment between a pair of electrodes is current heated to form emit-
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ters and electrical breakdown to form gaps. The display described here is similar to the surface-conduction electronemitter display (SED).
Acknowledgements The authors gratefully acknowledge of the help of Lina Zhang and Zhihua Han in taking the TEM images and helpful discussions with Professor Pijin Chen. This work was financially supported by the National Basic Research Program of China (2005CB623606 and 2007CB935301) and the NSFC (10704044, 50825201, and 10721404).
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