Shape and Doping Enhanced Field Emission Properties of

Apr 15, 2010 - ... University of Technology, Ningbo, 315016, People's Republic of China ... Large-Scale Growth of Well-Aligned SiC Tower-Like Nanowire...
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J. Phys. Chem. C 2010, 114, 8251–8255

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Shape and Doping Enhanced Field Emission Properties of Quasialigned 3C-SiC Nanowires Xinni Zhang,†,‡ Youqiang Chen,§ Zhipeng Xie,*,† and Weiyou Yang*,| State Key Lab of New Ceramics and Fine Processing, Tsinghua UniVersity, Beijing, 100084, People’s Republic of China, College of Physics Science, Qingdao UniVersity, Qingdao, 266071, People’s Republic of China, Department of Chemistry, Tsinghua UniVersity, Beijing, 100084, People’s Republic of China, and Institute of Materials, Ningbo UniVersity of Technology, Ningbo, 315016, People’s Republic of China ReceiVed: February 04, 2010

We have reported the enhanced field emission properties of quasialigned 3C-SiC nanowires synthesized via catalyst assisted pyrolysis of polysilazane. The as-synthesized Al-doped SiC nanowires possess a tapered and bamboo-like structure with clear and tiny tips sized in several to tens of nanometers. The fabricated SiC nanowires have extremely low turn-on fields of 0.55-1.54 V µm-1 with an average of ∼1 V µm-1, which is the lowest one ever reported for any type of SiC emitters. The field-enhancement factor has been calculated to be 2983. The superior FE properties can be clearly attributed to the significant enhancements of the tapered and bamboo-like unique morphology and Al doping of SiC nanowires. Density functional theory calculations suggest that Al dopants in 3C-SiC nanowires could favor a more localized state near the Fermi energy, which improves the electron field emissions. We strongly believe that the present work will open a new insight in the fabrication of field emission sources with ultralow turn-on fields enhanced by both shape and doping. 1. Introduction

2. Experimental Methods

Field emission (FE) is one of the main features of nanostructures, and is attracting extensive interest in displays and other electronic devices.1 For practical applications, it is vital to improve the FE properties of the nanostructures (e.g., to have low turn-on and threshold fields). There are three effective efforts to meet this challenge: reducing the tip size of the nanostructures,2 increasing the density of the emitting sites,3 and tailoring the band gap of established nanomaterials via doping strategy.4

Commercially available polysilazane (Ceraset, Kion, USA) was used as the raw material. The polysilazane was first solidified by heat-treatment at 260 °C for 30 min and then ground into powders. Then 3 wt % Al(NO3)3 was introduced as the catalyst by adding an ethanol solution of Al(NO3)3 at 0.2 mol/L concentration into the powders. The obtained powder mixture was pyrolyzed at 1550 °C for 30 min in a conventional furnace with a graphite resistance heater under ultrahigh purity Ar (99.99%) of 0.1 MPa at a flowing rate of 200 sccm, followed by furnace cooling to ambient temperature. A graphite sheet with a thickness of ∼1 mm was utilized as the substrate and located on the top of the alumina crucible. The resulting products were characterized with field emission scanning electron microscopy (FESEM, JSM-6301F, JEOL, Tokyo, Japan), X-ray diffraction (XRD, Automated D/Max-RB, Rigaku, Japan) with Cu KR radiation (λ ) 1.54178 Å), and transmission electron microscopy (HRTEM, JEOL-2011, Tokyo, Japan) with an energy-dispersive spectrum (EDS). The FE properties of SiC nanowire arrays at room temperature were measured in a vacuum chamber at a pressure of ∼7.0 × 10-5 Pa. The current-voltage (I-V) curves were recorded on a Keithley 237 unit with a detection resolution of 1 pA. The distance between the surface of SiC nanowires and the anode was fixed at 300-850 µm.

Silicon carbide (SiC) is one of the most important wide bandgap semiconductors with a high electron mobility, breakdown field strength, as well as high thermal conductivity, excellent mechanical properties, and chemical stability,5 making its useful for widespread applications in high-temperature/highvoltage electronics and short-wavelength optics.6 Numerous reported works revealed that SiC is an excellent candidate for field emission materials. Thus, to date, tremendous efforts have been devoted to the synthesis of SiC nanostructures via various methods.7,8 However, most of the obtained nanostructures are pure SiC without tailored morphologies.9 In this paper, we report the fabrication of quasialigned 3CSiC nanowires with tailored shapes and Al doping. It is found that the FE performance of SiC nanowires can be substantially enhanced by the shape and Al dopants. The as-synthesized nanostructures exhibit an extremely low emission turn-on field with an average of ∼1 V µm-1, which is the lowest value ever reported for 1D SiC nanostructures. * To whom correspondence should be addressed. E-mail: xzp@ mail.tsinghua.edu.cn (Z.P.) and [email protected] (W.Y.). † State Key Lab of New Ceramics and Fine Processing, Tsinghua University. ‡ College of Physics Science, Qingdao University. § Department of Chemistry, Tsinghua University. | Institute of Materials, Ningbo University of Technology.

3. Results and Discussion Panels a and b of Figure 1 are typical SEM images of the obtained nanostructures under different magnifications, displaying bunches of nanowires grown on a graphite substrate. The length of nanowires can be up to 30 µm with an aspect ratio of over 100. Figure 1c is a representative SEM image of a tip of the nanowire under high magnification, disclosing the tapered shape of the nanostructure with a tiny and clear tip. It is worth noting that almost all the nanowires possess the tapered shape with sharp and clear tips (Supporting Information, Figure S1).

10.1021/jp101067f  2010 American Chemical Society Published on Web 04/15/2010

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Figure 1. (a, b) Typical SEM imagrs of as-synthesized SiC nanowires under low magnification. (c) A typical SEM image displaying the tapered shape of the SiC nanowires with a clear and tiny tip. (d) A representative SEM image showing the quasialigned growth of SiC nanowires. (e) A typical XRD pattern of the product.

Figure 2. (a, b) Typical TEM images of the tapered SiC nanowires. (c) A typical TEM image displaying the bamboo-like structure of the SiC nanowires. The inset is the corresponding SAED pattern. (d) A representative EDS spectrum of the SiC nanowires. (e) Al element mapping within the SiC nanowires.

Closer observations (Figure 1b,d) suggest the quasialigned growth of the nanowires. Figure 1e presents a typical XRD pattern of the resulting products, suggesting that β-SiC is the only crystalline phase (JCPSD Card No. 29-1129). The strong and sharp peaks indicate that the obtained nanowires are well crystallized.

Further characterization of the nanowires was performed with TEM. Figure 2a is a typical TEM image of 3C-SiC nanowires under a low magnification, clearly suggesting the tapered shapes of the synthesized nanowires. The observations of more than 20 nanowires suggest that the tips of the tapered wires are clear without any attachments of catalytic droplets, and sized in

Quasialigned 3C-SiC Nanowires

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Figure 3. (a) A typical transparent anode image of electron emission from the quasialigned SiC nanowires. (b) Field-emission current density versus electric field (J-E) for quasialigned SiC nanowires. The inset shows the corresponding Fowler-Nordheim relationship (ln(J/E2)-1/E plot). (c) A schematic model for electron emission of the tapered and bamboo-like SiC nanowires. (d) Total density states of pure and Al-doped 3C-SiC. The Fermi energy is 0 eV.

several to tens of nanometers (Figure 2b). Such SiC nanowire tips could be promising materials for atomic force microscopy and/or scanning tunneling microscopy.10 Figure 2c shows a representative TEM image of the nanowire body under a high magnification, exhibiting an interesting bamboo-like morphology (Supporting Information, Figure S2). The SAED pattern (inset in Figure 2c), which is identical over the entire wire, suggests the wires are 3C-SiC with its single-crystalline nature. The wires possess a perfect structure without defects (Supporting Information, Figure S3). Both TEM image and SAED pattern suggest that the nanowires grow along the [111] direction. Figure 2d is a typical EDS spectrum recorded from the nanowires, revealing that the wires consist of Si, C, Al, and Cu, with a small amount of O. The detected O is from the amorphous layer on the surfaces of the nanowires (Supporting Information, Figure S3), while 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 nanostructure is SiC. Along with the XRD anyalysis (Figure 1e), it implies that synthesized nanostructures are Al-doped 3C-SiC nanowires. The Al concentration is measured to be ∼5 atom % with a uniformly spatial distribution within the wires (Figure 2e). Nanowires with tailored morphology and doping could be ideal objects for electron FE. Figure 3a is the recorded transparent anode image of the emitting surface of the SiC nanowire cathode with an emission area of ∼1.6 cm2 when the applied voltage and the distance between nanowires and anode were fixed at 2 V µm-1 and 300 µm, respectively. It suggests that the electron emission is homogeneous. Figure 3b shows the FE current density as a function of the applied field as a current density versus electric field (J-E) plot, and the inset presents the ln(J/E2)-1/E plot of the SiC nanowires. The J-E curve was obtained after sweeping the voltage several times until the electron emission was stable. The relatively smooth and consistent curves indicate the stability of electron emission. The turn-on field (Eto, defined to be the electric field required to generate a current density of 10 µA cm-2) and threshold field

(Ethr, defined to be the electric field required to generate a current density of 1 mA cm-2) for the SiC nanowires are found to be ∼1.25 and 1.88 V µm-1, respectively. The measurements on more than ten samples with different distances (300-850 µm) between the surface of nanowires and the anode disclose that the Eto of our SiC nanowires is in the 0.55-1.54 V µm-1 range with an average of ∼1 V µm-1 (Supporting Information, Figures S4-6). To the best of our knowledge, such an extremely low turn-on field emission has been little achieved for any type of SiC emitters (see Table 1), suggesting that our SiC nanowires could be an excellent candidate in applications for vacuum micro/nano electronic devices. Considering the relative low density and quasialigned growth of current nanowires (Figure 1a,b), another point should be noted that the FE properties can be further improved easily once the growth of SiC nanowires is well-aligned with a higher density. To understand the FE behavior, the J-E data have been analyzed by the FN equation:24

J ) (Aβ2E2 /Φ) exp[-BΦ3/2(βE)-1] where J is the current density, E is the applied electric field, and Φ is the work function. A and B are constants, corresponding to 1.56 × 10-10 A eV V-2 and 6.83 × 103 eV-3/2 µm-1, respectively. Then the FN plot is shown as the inset in Figure 3b. The linear relationship of the FN plot implies that the electron emission from the SiC nanowires follows the conventional field emission mechanism. Using the work function value of SiC (4.0 eV), the field-enhancement factor, β, has been calculated to be 2983, which can greatly enhance the FE properties of SiC nanowires.25 The high β-value can be mainly attributed to the small radii of curvature26 and the high aspect ratio of our nanowires.4 The superior FE properties from current SiC nanowires can be attributed to the following reasons. The first is the shape enhancement. Our synthesized SiC nanowires nearly meet all

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TABLE 1: Emission Turn-On Fieldsa for Various SiC Emitter Materials emitters BN coated SiC nanowires carbon coated SiC nanowires needle-shaped SiC nanowires β-SiC nanowires SiC nanowires SiC nanowires orient SiC nanowires β-SiC nanobelts R-SiC nanorods β-SiC nanorods SiC/Si heterostructures aligned SiC porous nanowire arrays core-shell SiC-SiO2 nanotructures β-SiC nanoarchitectures quasialigned β-SiC nanowires a

turn-on fields (Eto)/V µm-1 6 4.2 5 10.1 3.33-9.97 20 0.7-1.5 3.2 27 13-17 2.6 at 1 µA cm-2 2.3-2.9 3.3-4.5 12 0.55-1.54

ref 11 12 13 14 15 16 17 18 19 20 21 8 22 23 this work

Field required to generate an emission current density of 10 µA/cm2; if other values are used, it will be mentioned separately.

the requirements to be an ideal electron emitter. One is the long and tapered structures with tiny tips of the wires. It can greatly benefit the electron emission due to the strong local electric field at the tips and the unique direction of electron emission, owing to their geometries of small curvature radius.27 The clear tips of our wires also have a profound contribution to the electron emission because the alloy drops on the tips limit the electron emission and reduce the current density,28 leading to weakening of the FE behavior.29 Meanwhile, the bamboo-like nanostructures can enhance the FE behavior, since the sharp corners of the nanowires may act as efficient electron emitting sites.30 That is to say, the unique bamboo-like structure of SiC nanowires has greatly increased the density of the emitting sites, as illustrated by a schematic model for the electron emission in Figure 3c. The second is the improvement of Al doping. It has been found that doping can reduce the work function, in turn, considerably enhancing the FE properties of the nanostructures4,31 This is caused by the presence of electronic states in the valence and conduction bands within the SiC nanowires, which are close to the Fermi energy. To investigate the effect of Al doping on the FE property of SiC nanowires, plane-wave self-consistent field (PWSCF) calculations32 were performed to illustrate the electronic states. This is a plane-wave and pseudopotential method based on the density functional theory (DFT). The generalized gradient approximation (GGA) in the Wang and Perdew (PW91) form33 was used to describe the exchangedcorrelation. Doping in 3C-SiC is modeled by a 16-atom supercell constructed by stacking two cubic conventional cells with an Al atom replacing a Si atom.34 The corresponding doping concentration of Al:Si used in the computation is 12.5% (close to the setup in experimental EDS). The calculated total density states of pure and Al-doped 3C-SiC are plotted against energies in Figure 3d. This demonstrate clearly that doping has greatly enhanced the localized density states around the Fermi energy region (energy ) 0 eV). The states near the Fermi level are responsible for electron field emission, implying the significantly enhanced field emission properties of the Al-doped SiC nanowires. 4. Conclusions In summary, we have reported the FE properties of the quasialigned 3C-SiC nanowires with a tapered and bamboolike structure. FE measurements show that the synthesized SiC nanowires exhibit extremely low turn-on fields of 0.55-1.54 V µm-1 with an average of ∼1 Vµm-1, which is the lowest one ever reported for any type of SiC emitters. The field-

enhancement factor of current SiC nanowires has been calculated to be 2983. We attribute the superior FE properties to the significant enhancements of the tapered and bamboo-like unique morphology and Al doping of the synthesized SiC nanowires. It is strongly believed that current work will open a new insight in the fabrication of field emission sources with ultralow turnon fields enhanced by both shape and doping. Acknowledgment. The authors are thankful for financial support from the National Natural Science Foundation of China (NSFC, Grant Nos. 50572049 and 50872058) and the International Cooperation Project of Ningbo Municipal Government (Grant No. 2008B10044). We gratefully appreciate the help from Dr. Peng Liu for the measurements of FE properties. Supporting Information Available: Structure characterization and FE property measurements of quasialigned 3C-SiC nanowires. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) De Heer, W.; Chatelain, A.; Ugarte, D. Science 1995, 270, 1179. (b) Fan, S.; Chapline, M.; Franklin, N.; Tombler, T.; Cassell, A.; Dai, H. Science 1999, 283, 512. (c) Yin, L. W.; Bando, Y.; Zhu, Y. C.; Li, M. S.; Li, Y. B.; Golberg, D. AdV. Mater. 2005, 17, 110. (2) Tang, Y.; Cong, H.; Chen, Z.; Cheng, H. Appl. Phys. Lett. 2005, 86, 233104. (3) (a) He, J. H.; Yang, R. S.; Chueh, Y. L.; Chou, L. J.; Chen, L. J.; Wang, Z. L. AdV. Mater. 2006, 18, 650. (b) Lo, H.; Das, D.; Hwang, J.; Chen, K.; Hsu, C.; Chen, C.; Chen, L. Appl. Phys. Lett. 2003, 83, 1420. (4) Fang, X. S.; Bando, Y.; Gautam, U. K.; Ye, C.; Golberg, D. J. Mater. Chem. 2008, 18, 509. (5) (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. (6) (a) Morkoc, H.; Strite, S.; Gao, G.; Lin, M.; Sverdlov, B.; Burns, M. J. Appl. Phys. 1994, 76, 1363. (b) Neudeck, P. J. Electron. Mater. 1995, 24, 283. (c) Zhang, L.; Yang, W.; Jin, H.; Zheng, Z.; Xie, Z.; Miao, H.; An, L. Appl. Phys. Lett. 2006, 89, 14. (7) (a) Hu, J. Q.; Lu, Q. K.; Tang, K. B.; Deng, B.; Jiang, R. R.; Qian, Y. T.; Yu, W. C.; Zhou, G. E.; Liu, X. M.; Wu, J. X. J. Phys. Chem. B 2000, 104, 5251. (b) Sun, X. H.; Li, C. P.; Wong, W. K.; Wong, N. B.; Lee, C. S.; Lee, S. T.; Teo, B. K. J. Am. Chem. Soc. 2002, 124, 14464. (c) Yang, W.; Miao, H.; Xie, Z.; Zhang, L.; An, L. Chem. Phys. Lett. 2004, 383, 441. (d) Li, Z. J.; Zhang, J. L.; Meng, A.; Guo, J. Z. J. Phys. Chem. B 2006, 110, 22382. (e) Fre´chette, J.; Carraro, C. J. Am. Chem. Soc. 2006, 128, 14774. (f) Wu, R. B.; Pan, Y.; Yang, G. Y.; Gao, M. X.; Wu, L. L.; Chen, J. J.; Zhai, R.; Lin, J. J. Phys. Chem. C 2007, 111, 6233. (g) Li, J. Y.; Zhang, Y. F.; Zhong, X. H.; Yang, K. Y.; Meng, J.; Cao, X. Q. Nanotechnology 2007, 18, 245606. (h) Yang, Y. J.; Meng, G. W.; Liu, X. Y.; Zhu, X. G.; Kong, M. G.; Han, F. M.; Zhao, X. L.; Xu, Q. L.; Zhang, L. Cryst. Growth Des. 2008, 8, 1818.

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