J. Phys. Chem. C 2008, 112, 95-98
95
Growth and Field Emission Properties of Cactus-like Gallium Oxide Nanostructures Chuanbao Cao,* Zhuo Chen, Xiaoqiang An, and Hesun Zhu Research Center of Materials Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ReceiVed: May 19, 2007; In Final Form: October 13, 2007
Cactus-like -β-Ga2O3 nanostructures were synthesized by a simple carbon thermal reduction process. Cactuslike -β-Ga2O3 nanostructures are composed of a hollow microsphere and numerous -β-Ga2O3 nanowires grown from the surface. The structure and morphology of the as-synthesized Ga2O3 nanostructures were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and selected area electron diffraction. The field emission properties of the products were investigated, and the turn-on fields of 12.6 V/µm and the threshold fields of 23.2 V/µm of the product are reported for the first time. The growth mechanism was proposed through observing the morphologies of different growth stages.
Introduction Because of their unique electrical, optical, magnetic, and mechanical properties, nanomaterials have attracted considerable attention, and many nanoscale devices based on them have been fabricated.1 Among various nanomaterials, metal oxides stand out as one of the most versatile materials, owing to their diverse properties and functionalities. Their low dimensional structures not only inherit fascinating properties from their bulk form such as piezoelectricity, chemical sensing, and photodetection but also possess unique properties associated with their highly anisotropic geometry and size confinement.2 Therefore, recently many efforts have focused on the synthesis and characterization of functional metal oxide nanostructures as well as their applications.3 A variety of metal oxide nanostructures has been obtained, including one-dimensional (1-D) tubes,4 rods,5 wires,6 belts7, and ribbons7 and two-dimensional (2-D) sheets,8 diskettes,9 etc. To further realize functional nanodevices, the integration of these 1-D and 2-D building blocks into complex functional architectures should be fulfilled. A self-assembly technique provides an effective and inexpensive approach for achieving this goal. Although significant progress has been made in the synthesis of 1-D and 2-D nanostructures, the fabrication of complex nanostructures by the self-assembly method remains challenging. Gallium oxide -(β-Ga2O3) is a wide- band gap compound with a band gap of approximately 4.9 eV at room temperature.10 The magnetism due to conduction electron spins in this material can exhibit an original memory effect within a temperature range of 4-293 K.11 It exhibits conduction12 and luminescence properties13 and thus has potential applications in optoelectronic devices including flat-panel displays, solar energy conversion devices, optical limiters for ultraviolet light, and high-temperature stable gas sensors.14,15 1-D crystalline -β-Ga2O3 nanostructures, including nanowires, nanorods, nanobelts, and nanoribbons, have been synthesized by various methods, such as thermal evaporation,8,16-18 laser ablation,19 arc discharge,20,21 microwave plasma,22,23 and metal-catalyzed growth.24,25 However, there are few reports concerning the complex Ga2O3 nanostructures.23 * Corresponding author. Fax: 861068913937.
In this paper, we report a simple carbon thermal reduction process to grow cactus-like -β-Ga2O3 architectures composed of many curled -β-Ga2O3 nanowires standing on a hollow -βGa2O3 sphere without the aid of a catalyst. Since the nanowires on the spheres point toward different directions in the 3-D space, it may be possible to design a nanodevice that utilizes this feature such that electron or photon emission from the nanowires occurs in different directions. The structures of -β-Ga2O3 were investigated, and a growth mechanism was proposed. Field emission properties for the -β-Ga2O3 architectures were also studied. Experimental Procedures Cactus-like -β-Ga2O3 nanostructures were synthesized by a simple carbon thermal reduction process under an atmosphere of Ar. A total of 0.5 g of Ga2O3 (AR reagent, the Beijing Chemical Factory) and 0.1 g of graphite powder was mixed mechanically. The powders were loaded in an alumina boat, and then the boat was positioned in the center of the alumina tube. A silicon substrate was ultrasonically cleaned in ethanol for 5 min and rinsed with distilled water and then was placed downstream of the tube. The tube was put in a horizontal tube furnace. After purging with Ar gas for 5 min, the furnace was heated to 1150 °C under Ar gas and then kept at that temperature for 20 to ∼60 min under Ar at a rate of 100 mL/min. After the furnace was cooled to room temperature, the silicon wafer was covered with a layer of white product. The as-synthesized products were examined using an X-ray diffractometer (Philips X’pert Pro diffractometer with Cu KR radiation, λ ) 0.15418 nm), a scanning electron microscope (Hitachi S-4800), and a high-resolution transmission electron microscope (Tecnai F30) equipped with an X-ray energy dispersive spectrometer and selected area electron diffractometer. Field emission (FE) properties for the as-synthesized products were investigated in a vacuum chamber with a pressure of 1.2 × 10-6 Pa at room temperature. A rod-like stainless steel probe (1 mm in diameter) 0.78 mm2 in area was used as an anode. The sample was used as the cathode. The spacing between these two electrodes was 100 µm. The emission current was measured
10.1021/jp0738762 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/12/2007
96 J. Phys. Chem. C, Vol. 112, No. 1, 2008
Figure 1. (a and b) Low-magnification SEM images of the cactuslike -β-Ga2O3 nanostructures. (c) SEM image of high-density Ga2O3 nanowires aligned on the sphere surface. (d) Local- magnification SEM image of the tip of the nanowire.
by a picoameter (Keithley 485). A ballast resistor of 10 MV was used to protect the apparatus against circuit shorting. Results and Discussion The cactus-like -β-Ga2O3 nanostructures were synthesized by a simple carbon thermal reduction method. After the reaction, the silicon substrate was covered with a layer of white product. The morphology of the product obtained after growth for 60 min is shown in Figure 1. Figure 1a,b depicts two different magnification SEM images that show that the whole silicon substrate is covered with Ga2O3 microspheres with diameters ranging from several micrometers to several tens of micrometers. It can be clearly seen that numerous 1-D nanowires with a very high density grew on the surface of the sphere, as shown in Figure 1c. The diameters of these nanowires ranged from a base of ∼240 nm to a top of ∼50 nm. Notably, most of the nanowires are curled, and many nanowires have several branch nanorods at their tip as fingers as shown in Figure 1c,d. The -β-Ga2O3 hierarchical nanostructures formed by self-assembly have potential applications for future nanoscale devices. Further structural characterization of the cactus-like -βGa2O3 nanostructures was investigated by TEM, SAED, and high-resolution TEM (HRTEM). Figure 2a shows the lowmagnification TEM image of the Ga2O3 nanowire broken from the microsphere. This indicates that the surface of the Ga2O3 nanowire is not smooth and that its diameter decreases gradually from base to top, which is consistent with the SEM results. Regretfully, the nanowires with several branched nanorods at their tip could not be observed by TEM, which was probably due to the breaking of branched nanorods from the nanowire during ultrasonic dispersion for the preparation of the TEM specimen. The chemical composition of the nanowires was investigated by EDS analysis, as shown in the inset of Figure 2a. This confirms that the nanowire is only made of Ga and O elements and that the ratio of Ga to O is close to 0.68:1. The C and Cu peaks originate from the carbon film and the copper grid, respectively. The SAED pattern confirmed the singlecrystalline nature of the Ga2O3 nanowire grown along the direction, as shown in Figure 2b. The HRTEM image shown in Figure 2c gives a lattice fringe of about 0.564 and 0.593 nm, corresponding to the d001 and d200 spacings of the bulk monoclinic Ga2O3 phase, respectively. The XRD pattern of the products is shown in Figure 3. All diffraction peaks in the pattern can be indexed to the monoclinic Ga2O3 phase except the (400) reflection of the single-crystalline silicon substrate. The lattice constants calculated from the XRD
Cao et al. data are a ) 12.23 Å, b ) 3.04 Å, and c ) 5.80 Å, which are in agreement with those of the standard pattern (JCPDS Card No. 41-1103). The absence of other peaks demonstrates the high purity of the products. To better understand the -β-Ga2O3 nanostructure’s growth process, we performed some comparative experiments to identify the effect of growth time on the formation of the -β-Ga2O3 nanostructures. SEM studies were carried out for the products obtained at different growth times. Figure 4a shows the lowmagnification SEM image of the product obtained after 20 min. It can be seen that many spheres with a rough surface are deposited on the silicon substrate, some of which are broken. From these broken spheres, one can clearly see that the spheres are hollow. There are only a few nanowires on the surface of the microspheres, which can be seen more clearly from the enlarged SEM image of a single sphere in Figure 4b. With the growth time increased to 35 min, the number of nanowires increased obviously, as shown in Figure 4c. From the local magnification image shown in Figure 4d, it can be seen that many little Ga2O3 islands sprouted from the surface of the sphere and that furthermore the Ga2O3 nanowire grew on the island. When the growth time was kept at 60 min, a number of 1-D β-Ga2O3 nanowires with a high density were obtained on the surface of the microsphere (i.e., the formation of the cactuslike -β-Ga2O3 nanostructures), as shown in Figure 1. On the basis of the previous experimental results, we proposed a reasonable explanation for the growth mechanism, which is as follows. At the first stage, the starting material, Ga2O3 powder, will be reduced gradually by graphite and decompose to produce Ga2O vapor as the temperature increases. The reduction process can be described by the following reactions:
Ga2O3 + 2C f Ga2O + 2CO
(1)
Ga2O3 + 2CO f Ga2O + 2CO2
(2)
where Ga2O is in the vapor phase.19,26 The formed Ga2O vapor will be transported to the surface of the silicon substrate by the Ar carrier gas. Meanwhile, the Ga2O vapor will also react with CO simultaneously at a desired temperature26
Ga2O + 3CO f 2Ga + C + 2CO2
(3)
and the liquid Ga droplets may be formed on the substrate and then solidify to form Ga microspheres. In the second stage, the surface of the Ga spheres can be easily oxided by the residual oxygen in the system, which results in the formation of a thin Ga2O3 layer on the Ga sphere surface. The Ga2O3 layer becomes thicker gradually during the process of oxidization. In addition, due to the low melting point of metal Ga (about 29.8 °C),27 the Gibbs-Thomson effect, and the fact that the vapor of gallium will be oxidized or carried by Ar gas, the actual vapor pressure will be lower than the equilibrium vapor pressure. The inner gallium will evaporate continuously, which leads to the formation of the hollow Ga2O3 microsphere. Meanwhile, homogeneous nuclei are formed first on the surface of the microsphere, and then the epitaxial growth of nanowires takes place on these nuclei (i.e., a number of Ga2O3 nanowires grows from the surface of the sphere, and finally, the hollow cactus-like -βGa2O3 nanostructures are formed, similarly to hollow ZnO urchins’ growth).28 The whole process is represented schematically in Figure 5. The growth mechanism as mentioned previously is only a qualitative explanation, and further research is needed for an accurate understanding of the cactus-like Ga2O3 nanostructures’ growth.
Cactus-like Gallium Oxide Nanostructures
J. Phys. Chem. C, Vol. 112, No. 1, 2008 97
Figure 2. (a) TEM image of a Ga2O3 nanowire. (b) SAED pattern taken along the [010] zone axis. (c) HRTEM image of the nanowire. The inset in panel a is the EDS spectrum of the nanowire.
Figure 3. XRD pattern of the -β-Ga2O3 nanostructures.
Figure 4. SEM images of the products grown for 20 min (a and b) and 35 min (c and d), respectively.
Field emission measurements of the cactus-like -β-Ga2O3 nanostructures were conducted in a vacuum chamber with a pressure of 1.2 × 10-6 Pa at room temperature. A rod-like
stainless steel probe (1 mm in diameter) 0.78 mm2 in area was used as an anode. The cactus-like -β-Ga2O3 nanostructures were used as the cathode. The spacing between these two electrodes was 100 µm in our experiment. The curve of emission current density versus applied field (J-E) is depicted in Figure 6. By definition, the turn-on field and the threshold field are the electronic fields required to generate an emission current density of 10 µA/cm2 and 10 mA/cm2, respectively. The turn-on fields of 12.6 V/µm and the threshold fields of 23.2 V/µm were obtained for the products, higher than the previously reported turn-on fields of 7.73 V/µm and the threshold fields of 8.45 V/µm for Ga2O3-C nanocables.29 However, the low turn-on and threshold fields reported by Zhan et al.29 should be attributed to the excellent electron emission property of carbon due to their Ga2O3 nanowires being coated with carbon. To the best of our knowledge, there are no reports concerning the field emission intrinsic property of Ga2O3 nanomaterials presently. In addition, there are many reports concerning the field emission properties of the nanostructures of various metal oxides, such as the turn-on field of 18 V/µm for ZnO nanowires,30 2.4 V/µm for ZnO nanoneedle arrays,31 1.3 V/µm for ZnO nanobelts,32 0.6 to ∼0.8 V/µm for ZnO nanowires,33 13.85 V/µm for tungsten oxide nanowire networks,34 2.6 V/µm for quasi-aligned tungsten oxide nanowires,35 5.6 V/µm for IrO2 nanorods,36 and 10.3 V/µm for RuO2 nanorods.37 The present value of the turn-on field is moderate as compared to these values as mentioned previously. Clearly, the turn-on field of these metal oxides depends on the synthesis technique, crystalline phase, and morphology. The inset in Figure 6 shows the Fowler-Nordheim (F-N) plot of the field emission of the cactus-like -β-Ga2O3 nanostructures. The F-N plot exhibits linear dependence at high fields, which reveals that the emission current is really caused by the quantum tunneling effect. According to the F-N theory,38 the relationship between current density J and applied electric field E can be described as follows:
J ) (Aβ2E2/φ) exp[-Bφ3/2(βE)-1]
(4)
where A ) 4.43 × 10-22 (A V-2 eV), B ) 6.83 × 109 (V m-1 eV-3/2), and φ is the work function, which is estimated to be
98 J. Phys. Chem. C, Vol. 112, No. 1, 2008
Cao et al. References and Notes
Figure 5. Schematic illustration of the formation of hollow cactuslike -β-Ga2O3 nanostructures. The whole process includes the formation of Ga spheres, oxidation of the spheres, evaporation of the inner Ga in the spheres and nanowire growth from the surface of spheres, and finally the formation of hollow cactus-like -β-Ga2O3 nanostructures.
Figure 6. Field emission J-E curve. The inset is the corresponding F-N plot.
4.15 eV for Ga2O3.39 Here, β is the field enhancement factor, which can be determined using the slope of the ln(J/E2) - 1/E plot. The field enhancement factor, β, has been calculated to be about 38.2, which is lower than other metal oxides. The factor β in the F-N equation is introduced to reflect the degree of the FE enhancement of any tip over a flat surface. It represents the true value of the electric field at the tip as compared to its average macroscopic value. The β value of the Ga2O3 nanostructure is related to the geometry, crystal structure, conductivity, work function, and nanostructure density.40 Owing to the uniform directional distribution of nanowires gown on the microsphere, it may be possible to design a field emission nanodevice that meets the requirement that electron emission from the nanowires occurs in different directions. Conclusion In summary, the cactus-like -β-Ga2O3 nanostructures were synthesized by a simple carbon thermal reduction process on a silicon substrate. The cactus-like -β-Ga2O3 nanostructures are composed of a hollow microsphere and numerous -β-Ga2O3 nanowires grown from the surface. The complex functional architectures formed by self-assembly can serve as promising candidates for future nanoscale device applications. Field emission measurements show that the cactus-like -β-Ga2O3 nanostructures have turn-on fields of 12.6 V/µm and threshold fields of 23.2 V/µm.
(1) Huang, Y.; Duan, X.; Cui, Y.; Lauhon, L. J.; Kim, K. H.; Lieber, C. M. Science (Washington, DC, U.S.) 2001, 294, 1313. (2) Lu, J. G.; Chang, P.; Fan, Z. Mater. Sci. Eng., A 2006, 52, 49. (3) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. AdV. Funct. Mater. 2003, 13, 9. (4) Liu, Y.; Liu, M. AdV. Funct. Mater. 2005, 15, 57. (5) Manna, L.; Scher, E. C.; Alivisators, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (6) Morales, A. M.; Lieber, C. M. Science (Washington, DC, U.S.) 1998, 279, 208. (7) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science (Washington, DC, U.S.) 2001, 291, 1947. (8) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 902. (9) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. J. Am. Chem. Soc. 2002, 124, 8673. (10) Binet, L.; Gourier, D. J. Phys. Chem. Solids 1998, 59, 1241. (11) Binet, L.; Gourier, D. J. Phys. Chem. 1996, 100, 17630. (12) Yamaga, M.; Villora, E.; Shimamura, K.; Ichinose, N.; Honda, M. Phys. ReV. B: Condens. Matter Mater. Phys. 2003, 68, 155207. (13) Song, Y. P.; Zhang, H. Z.; Lin, C.; Zhu, Y. W.; Li, G. H.; Yang, F. H.; Yu, D. P. Phys. ReV. B: Condens. Matter Mater. Phys. 2004, 69, 075304. (14) Edwards, D. D.; Mason, T. O.; Goutenoir, F.; Poeppelmeimer, K. R. Appl. Phys. Lett. 1997, 70, 1706. (15) Ogita, M.; Saika, N.; Nakanishi, Y.; Hatanaka, Y. Appl. Surf. Sci. 1999, 142, 188. (16) Chun, H. J.; Chio, Y. S.; Bae, S. Y.; Seo, H. W.; Hong, S. J.; Park, J.; Yang, H. J. Phys. Chem. B 2003, 107, 9042. (17) Gao, H. Y.; Bando, Y.; Sato, T.; Zhang, Y. F. Appl. Phys. Lett. 2002, 81, 2267. (18) Yu, D. P.; Bubendorff, J. L.; Zhou, J. F.; Wang, Y. L.; Troyon, M. Solid State Commun. 2002, 124, 417. (19) Hu, J. Q.; Li, Q.; Meng, X. M.; Lee, C. S.; Lee, S. T. J. Phys. Chem. B 2002, 106, 9536. (20) Han, W. Q.; Kohler, R. P.; Ernst, F.; Ru¨hle, M. Solid State Commun. 2000, 115, 527. (21) Park, G. S.; Choi, W. B.; Kim, J. M.; Choi, Y. C.; Lee, Y. H.; Lim, C. B. J Cryst. Growth 2000, 220, 494. (22) Graham, U. M.; Sharma, S.; Sunkara, M. K.; Davis, B. H. AdV. Funct. Mater. 2003, 13, 576. (23) Sharma, S.; Sunkara, M. K. J. Am. Chem. Soc. 2002, 124, 12288. (24) Chang, K. W.; Wu, J. J. AdV. Mater. 2004, 16, 545. (25) Liang, C. H.; Meng, G. W.; Wang, G. Z.; Wang, Y. W.; Zhang, L. D.; Zhang, S. Y. Appl. Phys. Lett. 2001, 78, 3202. (26) Xu, L.; Su, Y.; Li, S.; Chen, Y. Q.; Zhou, Q. T.; Yin, S.; Feng, Y. J. Phys. Chem. B 2007, 111, 760. (27) Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 2005. (28) Shen, G. Z.; Bando, Y.; Lee, C. J. J. Phys. Chem. B 2005, 109, 10578. (29) Zhan, J.; Bando, Y.; Hu, J.; Li, Y.; Golberg, D. Chem. Mater. 2004, 16, 5158. (30) Tseng, Y.-K.; Huang, C.-J.; Cheng, H.-M.; Lin, I.-N.; Liu, K-S.; Chen, I.-C. AdV. Funct. Mater. 2003, 13, 811. (31) Zhu, Y. W.; Zhang, H. Z.; Sun, X. C.; Feng, S. Q.; Xu, J.; Zhao, Q.; Xiang, B.; Wang, R. M.; Yu, D. P. Appl. Phys. Lett. 2003, 83, 144. (32) Wang, W. Z.; Zeng, B. Q.; Yang, J.; Poudel, B.; Huang, J. Y.; Naughton, M. J.; Ren, Z. F. AdV. Mater. 2006, 18, 3275. (33) Debasish, B.; Sung, H. J.; Zhi, F. R. AdV. Mater. 2004, 16, 2028. (34) Zhou, J.; Ding, Y.; Deng, S. Z.; Gong, L.; Xu, N. S.; Wang, Z. L. AdV. Mater. 2005, 17, 2107. (35) Li, Y.; Bando, Y.; Golberg, D. AdV. Mater. 2003, 15, 1294. (36) Chen, R. S.; Huang, Y. S.; Liang, Y. M.; Hsieh, C. S.; Tsai, D. S.; Tiong, K. K. Appl. Phys. Lett. 2004, 84, 1552. (37) Cheng, C. L.; Chen, Y. F.; Chen, R. S.; Huang, Y. S. Appl. Phys. Lett. 2005, 86, 103104. (38) Fowler, R. H.; Nordheim, L. W. Proc. R. Soc. London, Ser. A 1928, 119, 173. (39) Robertson, J.; Falabretti, B. Mater. Sci. Eng., B 2006, 135, 267. (40) Xu, C. X.; Sun, X. W.; Chen, B. J. Appl. Phys. Lett. 2004, 84, 1540.
