Temperature-Dependent Field Emission Properties of 3C-SiC

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Temperature-Dependent Field Emission Properties of 3C-SiC Nanoneedles Guang Wei,†,‡ Haiyun Liu,† Changkuai Shi,‡ Fengmei Gao,‡ Jinju Zheng,‡ Guodong Wei,‡,* and Weiyou Yang‡,* † ‡

College of Material Science and Engineering, Taiyuan University of Technology, Taiyuan City, 030024, P.R. China Institute of Materials, Ningbo University of Technology, Ningbo City, 315016, P.R. China ABSTRACT: In this work, we report the temperaturedependent field emission properties of 3C-SiC nanoneedles (SiCNNs) in the range of room temperature (RT) to 500 °C. SiCNNs are synthesized via catalyst-assisted pyrolysis of a polyaluminasilazane precursor. The obtained SiC nanostructures are needlelike shaped with numerous sharp corners around the tiny tips. Field emission characteristics show that turn-on field (Eto) of as-synthesized SiCNNs are ranged in 1.30 to 0.66 V/μm with the temperature raised from RT to 500 °C. At a fixed electric field of 1.37 V/μm, about a three-order-ofmagnitude increase of the emission current level has been observed. We attribute the significant reduction of Eto and the remarkable increase of emission current to the decrease of work function induced by the raise of temperatures.

1. INTRODUCTION Silicon carbide (SiC) 1D nanostructure has been considered as one of the most promising candidates with excellent properties for field emitters, not only because of its outstanding mechanical properties, high thermal conductivity and chemical stabilities, and electron affinity1,2 but also because of its intrinsic properties of high aspect ratio and high field enhancement factor.2,3 Up to date, many efforts have been devoted to the investigation of field emission (FE) properties related to various SiC nanostructures. The reported turn-on fields (Eto, defined to be the electric field required to generate a current density of 10 μA/cm2) of SiC nanowires were ranged in 3.33
20 Vμm
1,4 and that of SiC nanorods and nanobelts exhibited to 13
17 Vμm
1,5 and 3.2 Vμm
1,6 respectively; the FE properties could be further enhanced up to 0.7
2.9 Vμm
1 by using aligned SiC nanowires as the emitters;3,7 Most recently, Zhang et al. have obtained Eto of quasi-aligned β-SiC nanowires as low as to 0.55
1.54 Vμm
1.8 To improve the FE properties of SiC nanostructures, one of the effective strategies is to reduce the tip size of nanostructures,9 namely, to obtain a needlelike nanostructures, which could favor a high electron emission current density due to the strong local electric field at the tips and the unique emission direction owing to their geometries of small curvature radius.10 Even much progress has been made to the FE properties of SiC 1D nanostructures during the past decades as mentioned above, most of the reported works have been carried out just at RT. A study of field emission from nanostructures at higher temperatures is very interesting for many reasons:11,12 i) can disclose the changes of the electron emission characteristics under various conditions and provide additional insight into the physical properties of nanostructures; ii) for investigation of direct thermal r 2011 American Chemical Society

electric conversion; iii) benefit to obtaining a high emission current densities required for many unique applications for example electron microscopes. In this communication, we report the temperature-dependent FE properties of 3C-SiC SiCNNs, which are synthesized via pyrolysis of a polymeric precursor. FE properties measurements disclose that the Eto of as-synthesized SiCNNs are ranged in 1.30 to 0.66 V/μm with the temperature raised from RT to 500 °C, suggesting that the as-synthesized SiC nanoneedles could be an excellent candidate for field emitters.

2. EXPERIMENTAL PROCEDURE SiC nanoneedles were synthesized by catalyst-assisted pyrolysis of polyaluminasilazane precursors. The precursors were obtained by reaction of polyureamethylvinylsilazane (Ceraset, Kion Corporation, USA) and aluminum isopropoxide (AIP, Beijing Bei Hua Fine Chemicals Company, Beijing, China) with a weight ratio of polysilazane-to-AIP = 16:1 mixed by ball milling for 12 h.13 The obtained polyaluminasilazane was first solidified by heat-treatment at 260 °C for 30 min under Ar atmosphere, and then subjected to be ball-milled into powders. Graphic papers were dipped into the ethanol solution of Co(NO3)2 with a concentration of 0.05 mol/L for 1 min and used as the substrates for the growth of SiCNNs (an effective area of 0.25 cm2 used for followed characteristic of FE properties). After being dried in air at RT, the graphic paper was put onto the top of a high-purity alumina crucible containing powder mixtures. The crucible with substrate was Received: March 12, 2011 Revised: May 2, 2011 Published: June 14, 2011 13063

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The Journal of Physical Chemistry C then placed in a graphite-heater furnace and pyrolysis at 1400 °C for 30 min under Ar (99.9%, 0.1 MPa) at a flowing rate of 200 sccm, followed by furnace cooling to ambient temperature. The obtained products were characterized using field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan), transmission electron microscopy (TEM, JEM-2100, JEOL, Japan), and X-ray diffraction (XRD, D8 Advance, Bruker, Germany) with Cu KR radiation (λ = 1.5406 Å). The FE properties of SiCNNs grown on the graphite substrate have been performed on a home-built high vacuum field emission setup with a base pressure of ∼5.0  10
7 Pa (Figure 1) at temperatures of RT to 500 °C. The current
voltage (I
V) curves were recorded on a Keithley 248 unit with a detection resolution of 0.1 fA. The distance between the surface of SiCNNs substrate and the anode of vacuum chamber was fixed at 800 μm.

3. RESULTS AND DISCUSSION The pyrolysis products grown on the graphite paper substrate were first observed under SEM. It seems that the products with high-density wirelike nanomaterials are deposited homoge-

Figure 1. Schematic diagram of the experimental setup used for the measurement of temperature-dependent field emission properties of SiCNNs.

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neously on the whole graphite paper substrate suggesting the large-scale synthesis of SiCNNs (part a of Figure 2). The nanoneedles are up to dozens of micrometers in length and several hundred nanometers in diameter. Parts b and c of Figure 2 are representative SEM images of the top part of SiCNNs under high magnifications in different view visions, which display a typical tapered shape rather than a cylinder one. The formation of this specific structure can be attributed to the size change of the liquid catalytic droplets related to the pyrolysis process, in which the growth of nanostructures is dominant by vapor
liquid
solid mechanism.14 Part d of Figure 2 is the typical XRD patterns recorded from the SiCNNs þ substrate and substrate, which correspond to the red curve and black one, respectively. By compared with these two XRD patterns, the red curve reveals that the SiCNNs are well crystallized and can be indexed the only phase of 3C-SiC (JCPDS Card No. 29
1129). Further characterization of the SiCNNs was performed by using TEM. Part a of Figure 3 is a typical TEM image of the obtained nanostructures under low magnification clearly suggesting the SiCNNs possess a typical tapered tip. It implies that the diameter of the nanostructure first increases with the growth process and decreases sharply near the growth end. Such SiCNNs could be a promising candidate for scanning tunneling microscopy and atomic force microscopy.15 Part b of Figure 3 is a typical selected area electron diffraction (SAED) pattern recorded from the marked area A in part a of Figure3, which is identical over the entire nanoneedle implying that it is 3C-SiC with its single-crystal nature. Part c of Figure 3 is a representative EDS spectrum recorded from the marked area B in part a of Figure3, disclosing that the nanoneedle consists of Si, C, Al, and Cu, with a small amount of O. More than 10 analyses on different single SiC nanoneedles suggest that the average Al concentration is ∼0.7 at.%, which could benefit to enhancing the field emission due to a more localized state near the Fermi energy induced by

Figure 2. (a
c) Typical SEM images of as-synthesized SiCNNs under different magnifications, (d) representative XRD patterns recorded from SiCNNsþsubstrate and substrate. 13064

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Figure 3. (a) Typical TEM image of SiCNNs under a low magnification. (b) Corresponding SAED pattern of SiCNNs taken from marked area A in (a). (c) A representative EDS spectrum of SiCNNs recorded from the marked area B in (a). (d) A typical HRTEM image of SiCNNs. (e) A typical TEM image showing the tip structure of SiCNNs. (f
g) Respective HRTEM images recorded from the areas of A and B in (e).

the Al dopants.8 The detected O is from the amorphous layer on the surfaces of the nanoneedle (as shown in part d of Figure 3), whereas the Cu comes from the copper grid used to support the TEM sample. The atomic ratio of Si to C, within the experimental limit, is close to 1:1 suggesting the nanoneedle is SiC. Part d of Figure 3 presents a representative HRTEM image of the assynthesized nanostructure implying its perfect structure with few defects. It displays two sets of fringes with the d-space of 0.25 nm corresponding to the (111) plane of 3C-SiC. Both the SAED pattern and the HRTEM image suggest that SiCNNs grow along [111] direction, as indexed in parts a and f of Figure 3. Part e of Figure 3 is a typical enlarged TEM image of the tip of SiCNNs exhibiting numerous sharp corners around the tip. Parts f and g of Figure 3 are respective HRTEM images recorded from the marked areas of A and B in part e of Figure 3, which display the detailed structure of the tip of SiCNNs. They suggest that tip is clear, and numerous sharp corners around the tip are sized in

several to tens of nanometers. These specific structures can greatly enhance the FE behavior because both tiny tips and sharp corners can simultaneously act as efficient electron emitting sites due to the effect of local electric field.8,9,16 The FE characteristics of the resultant SiCNNs are revealed by emission current density (J) versus applied electric field (E) (J
E) plot. Part a of Figure 4 shows J
E curves of the SiCNNs under different temperatures in the range of RT to 500 °C. These relatively smooth and consistent curves indicate the stable electron emission. Interestingly, these curves shows the same characteristics: i) all the curves remain horizontal with emission current density of zero at the beginning stage; ii) when the applied electric field is beyond a certain value, the current density increased nearly exponentially with the increase of applied electric field. At RT, the Eto of SiCNNs is 1.30 V/μm, which is lower than most works ever reported8 and other inorganic semiconductor nanostructures,2,17 such as ZnO nanoneedles 13065

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Figure 5. Schematic diagram of surface energy band structure of SiCNNs.

three-order-of-magnitude higher than that at RT. This might be attributed to insufficient carrier supply at RT due to a large depletion region.25 To explain the FE behaviors, J
E data have been analyzed by the F
N equation:26 J ¼ ðAβ2 E2 =ΦÞ exp½
BΦ3=2 ðβEÞ
1 

ð1Þ

where β is the field-enhancement factor, E is the applied electric field calculated from the external voltage applied (V) divided by the anode-sample spacing (d), and Φ is the effective work function of SiCNNs, A and B are constants, corresponding to 1.56  10
6 AeVV
2 and 6.83  103 (eV)
3/2Vμm
1, respectively. Then the F
N plots under different temperatures are shown in part c of Figure 4. All of the plots can be approximately fitted to straight lines. The linear relationship of the F
N plot implies that the electron emission from the SiCNNs in the range of RT to 500 °C follows the conventional field emission mechanism through a deformed surface potential barrier at FE sites. Because of the fact that there is no significant influence of temperature (RT to ∼500 °C) on the crystal structure of SiCNNs, the field-enhancement factor can be set as a constant regardless of the temperature increase. The value of β is estimated ∼4203 by considering the slope of FN plot recorded at RT, which can be mainly attributed to the small radii of curvature27 and the high aspect ratio of our nanowires.2 Then determining of Φ3/2 from the measurements is to trace the F
N plot, whose slope k can be expressed as: Figure 4. (a) J
E plots of SiCNNs under different ambient temperatures. (b) The variation of turn-on field and emission current density with temperatures for SiCNNs. (c) Corresponding F
N plots of SiCNNs under different ambient temperatures.

(2.5 V/μm),18 ZnS nanobelts (3.47 V/μm),19 WO3 nanotips (2.0 V/μm),20 AlN nanoneedles (2.0 V/μm),21 Si nanotip arrays (8.5 V/μm),22 C nanoparticle on carbon fabric (1.33 V/μm),23 and beaklike SnO2 nanorods (8.5 V/μm).24 When increasing the temperature from RT to 500 °C, the turn-on field obviously decreases from 1.30 to 0.66 V/μm (shown as the black curve in part b of Figure 4) suggesting that Eto of SiCNNs decreases with the increase of temperature. The current variations related to the temperatures at a fixed electric field of E = 1.37 V/μm are also measured, shown as the blue plot in part b of Figure 4, suggesting the increase of current densities with the raise of temperatures. It shows that, as the applied field is fixed at E = 1.37 V/μm, the SiCNNs can only generate emission current density of J = 20 μA/cm2 at RT; whereas, at 400 °C, the SiCNNs can produce emission current density of J = 2447 μA/cm2, which is at least

k ¼
BΦ3=2 =β

ð2Þ

Thus, the work function of SiCNNs under various temperatures can be calculated, and shows decrease from 4.0 to 3.1 eV with the temperatures increasing from RT to 500 °C, which well interprets the observed thermo-enhanced effect of field emission from SiCNNs emitters. This can be attributed to the quantum mechanical tunneling mechanism, namely, the Fermi energy determines the field emission current. The work function can be defined by:28 Φ ¼ E0
EF

ð3Þ

where E0 and EF are respective fixed vacuum level and the Fermi level, as schematically shown in Figure 5. It implies that a lower EF would result in a larger Φ. The relationship between EF and temperatures (T) can be expressed as following equation:29     ðEc þ Ev Þ kB TL Nv þ EF ¼ ð4Þ ln 2 2 Nc where Ec and Ev refer to the respective energies of the conduction and valence band, KB is the Boltzmann constant, TL is the lattice 13066

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The Journal of Physical Chemistry C temperature, Nv and Nc the effective density-of-state of electrons and holes, respectively. At RT, the second term is small and can be negligible. With increasing of temperatures, EF becomes larger due to the enhanced contribution to the second term in eq 4 and leads to the reduce of Φ. Thus, for low-temperature emission, the Fermi level is lower and the electrons have to transmit through a much higher barrier; and, in turn, the emission current under the same field intensity will be larger for high-temperature field emission.30 In an other word, the significant reduction of Eto and the remarkable increase of emission current of SiCNNs are due to the decrease of work function induced by the increase of temperature.31 Recently, Choueib et al.25,32 reported the saturation effects in the emitted current based on the temperaturedependent current
voltage (I
V
T) characteristics by observing FE of individual single-crystal SiC nanowires. They found that strong saturation resulted in highly nonlinear FN plots due to the formation of a depletion layer near the nanowire ends and consists well with the Poole
Frenkel (PF) transport equation. Their work might open a door to improve the uniformity, stability, and photon control of mass-produced planar nanowire FE cathodes. Owning to the electrons at high temperature mainly originating from thermal excitation of carriers trapped within a potential well into the conduction band, the number of free carriers increases strongly with temperature and thermal activation energies of FE sites decrease with increasing voltage, which can also make the contribution for the FE at high temperatures.12 Although the detailed carrier transportation mechanism needs to be further investigated extensively, the temperature-dependent FE behavior of our SiCNNs might be caused by the following possible reasons: one is that the emission sites could exhibit an increase in emission site intensity at elevated temperatures due to increase of the free carrier concentration;25,33 the other is related to the presence of defects and/or surface states in nanomaterials,34 which might have small active energies. That is to say, when the temperature is increased, the carriers trapped in these states are activated into the conduction band leading to a higher emission current density.

4. CONCLUSIONS In summary, we have reported the temperature-dependent field emission properties of 3C-SiC SiCNNs in the range of RT to 500 °C. The obtained SiC nanostructures, synthesized via catalyst-assisted pyrolysis of polyaluminasilazane precursors, are needlelike shaped with numerous sharp corners around the tiny tips. Field emission characteristics show that Eto of assynthesized SiCNNs are ranged in 1.30 to 0.66 V/μm with the temperature raised from RT to 500 °C. At a fixed field of 1.37 V/μm, about a three-order-of-magnitude increase of the emission current level can been observed. The FE behavior of SiCNNs in the range of RT to ∼500 °C follows the classical F
N theory. We attribute the significant reduction of Eto and the remarkable increase of emission current to the decrease of work function induced by the increase of temperature. Current work suggests that our SiCNNs could be an excellent candidate for field emitters. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (G.W.); [email protected] (W.Y.), Tel: þ86-574-87080966.

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’ ACKNOWLEDGMENT The project was supported by the Program of Science and Technology Innovation for Undergraduates in Zhejiang Province (Grant No. 2009R422018), Zhejiang Provincial Science Foundation for Distinguished Young Scholars (Grant No. R4100242), Zhejiang Provincial Nature Science Foundation (Grant No. Y4110529), National Natural Science Foundation of China (NSFC, Grant Nos. 50872058 and 50572083), the International Cooperation Project of Ningbo Municipal Government (Grant No. 2008B10044). ’ REFERENCES (1) (a) Casady, J.; Johnson, R. Solid-State Electron. 1996, 39, 1409. (b) Wong, E.; Sheehan, P.; Lieber, C. Science 1997, 277, 1971. (c) Fan, J.; Wu, X.; Chu, P. Prog. Mater. Sci. 2006, 51, 983. (2) Fang, X. S.; Bando, Y.; Gautam, U. K.; Ye, C.; Golberg, D. J. Mater. Chem. 2008, 18, 509. (3) Pan, Z. W.; Lai, H. L.; Au, F. C. K.; Duan, X. F.; Zhou, W. Y.; Shi, W. S.; Wang, N.; Lee, C. S.; Wong, N. B.; Lee, S. T.; Xie, S. S. Adv. Mater. 2000, 12, 1186. (4) (a) Wong, K. W.; Zhou, X. T.; Au, F. C. K.; Lai, H. L.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 1999, 75, 2918. (b) Shen, G. Z.; Bando, Y.; Ye, C. H.; Liu, B. D.; Golberg, D. Nanotechnology 2006, 17, 3468. (c) Wu, Z. S.; Deng, S. Z.; Xu, N. S.; Chen, J.; Zhou, J.; Chen, J. Appl. Phys. Lett. 2002, 80, 3829. (d) Deng, S. Z.; Li, Z. B.; Wang, W. L.; Xu, N. S.; Jun, Z.; Zheng, X. G.; Xu, H. T.; Jun, C.; She, J. C. Appl. Phys. Lett. 2006, 89, 23118. (e) Ryu, Y.; Park, B.; Song, Y.; Yong, K. J. J. Cryst. Growth 2004, 271, 99. (f) Tang, C. C.; Bando, Y. Appl. Phys. Lett. 2003, 83, 659. (5) Zhou, X. T.; Lai, H. L.; Peng, H. Y.; Au, F. C. K.; Liao, L. S.; Wang, N.; Bello, I.; Lee, C. S.; Lee, S. T. Chem. Phys. Lett. 2000, 318, 58. (6) Wei, G.; Qin, W.; Kim, R.; Sun, J.; Zhu, P.; Wang, G.; Wang, L.; Zhang, D.; Zheng, K. Chem. Phys. Lett. 2008, 461, 242. (7) Yang, Y. J.; Meng, G. W.; Liu, X. Y.; Zhang, L. D.; Hu, Z.; He, C. Y.; Hu, Y. M. J. Phys. Chem. C 2008, 112, 20126. (8) Zhang, X.; Chen, Y.; Xie, Z.; Yang, W. J. Phys. Chem. C 2010, 114, 8251. (9) Tang, Y.; Cong, H.; Chen, Z.; Cheng, H. Appl. Phys. Lett. 2005, 86, 233104. (10) Liu, C.; Hu, Z.; Wu, Q.; Wang, X. Z.; Chen, Y.; Sang, H.; Zhu, J. M.; Deng, S. Z.; Xu, N. S. J. Am. Chem. Soc. 2005, 127, 1318. (11) (a) Klein, R.; Leder, L. B. Phys. Rev. 1961, 124, 1046. (b) Shin, S.; Fisher, T.; Walker, D.; Strauss, A.; Kang, W.; Davidson, J. J. Vac. Sci. Technol. B 2003, 21, 587. (c) Wan, C. S.; Li, Z. H.; Fan, K. N.; Zheng, X.; Chen, G. H. Phys. Rev. B 2006, 73, 165422. (d) Liao, L.; Zhang, W.; Lu, H.; Li, J.; Wang, D.; Liu, C.; Fu, D. Nanotechnology 2007, 18, 225703. (e) Zhang, Q. Y.; Xu, J. Q.; Zhao, Y. M.; Ji, X. H.; Lau, S. P. Adv. Funct. Mater. 2009, 19, 742. (12) Zhu, H.; Masarapu, C.; Wei, J.; Wang, K.; Wu, D.; Wei, B. Phys. E 2009, 41, 1277. (13) Dhamne, A.; Xu, W.; Fookes, B. G.; Fan, Y.; Zhang, L.; Burton, S.; Hu, J.; Ford, J.; An, L. J. Am. Ceram. Soc. 2005, 88, 2415. (14) Wang, H. T.; Xie, Z. P.; Yang, W. Y.; Fang, J. Y.; An, L. N. Cryst. Growth Des. 2008, 8, 3893. (15) (a) Zhang, G.; Jiang, X.; Wang, E. Science 2003, 300, 472. (b) Mavrandonakis, A.; Froudakis, G.; Andriotis, A.; Menon, M. Appl. Phys. Lett. 2006, 89, 123126. (16) Saito, Y.; Hata, K.; Murata, T. Japan. J. Appl. Phys. 2 2000, 39, L271. (17) Fang, X.; Zhai, T.; Gautam, U. Prog. Mater. Sci. 2011, 56, 175. (18) Li, Y.; Bando, Y.; Golberg, D. Appl. Phys. Lett. 2004, 84, 3603. (19) Fang, X.; Bando, Y.; Shen, G.; Ye, C.; Gautam, U. K.; Costa, P. M. F. J.; Zhi, C.; Tang, C.; Golberg, D. Adv. Mater. 2007, 19, 2593. (20) Zhou, J.; Gong, L.; Deng, S. Z.; Chen, J.; She, J. C.; Xu, N. S.; Yang, R.; Wang, Z. L. Appl. Phys. Lett. 2005, 87, 223108. (21) Zhao, Q.; Xu, J.; Xu, X.; Wang, Z.; Yu, D. Appl. Phys. Lett. 2004, 85, 5331. 13067

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