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Vertically aligned ZnO nanowires produced by a catalyst-free thermal evaporation method and their field emission properties
Chemical Physics Letters 404 (2005) 69–73 www.elsevier.com/locate/cplett
Vertically aligned ZnO nanowires produced by a catalyst-free thermal evaporation method and their field emission properties Heon Ham, Guozhen Shen *, Jung Hee Cho, Tae Jae Lee, Sung Ho Seo, Cheol Jin Lee
*
Department of Nanotechnology, Hanyang University, 17-Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea Received 20 December 2004; in final form 21 January 2005
Abstract Vertically aligned ZnO nanowires have been synthesized on Si substrate by catalyst-free thermal evaporating metallic zinc powder at a low temperature of 600 C. Studies found that the ZnO nanowires are single-crystalline wurtzite structures with 70 nm in diameter and 10 lm in length. The turn on field of the ZnO nanowires was about 6.2 V/lm at a current density of 0.1 lA/cm2, and the emission current density reached 1 mA/cm2 at an applied field of about 15.0 V/lm. Field emission property from the ZnO nanowires was enough high level to be applicable to field emission displays and vacuum microelectronic devices. 2005 Elsevier B.V. All rights reserved.
1. Introduction Fabrication of commercial field emission microcathode array is based on thin film technology and semiconductor processing methods. Although present commercial technology continues to be the structure choice for current field emission display applications, one-dimensional (1-D) vertically aligned nanostructures have been attracting much attention in recent years to be fabricated as the field emission display (FED) because of their high efficiency, reduction of cost and device size compared to conventional thermionic emitters [1,2]. Emitters with large aspect ratios, such as carbon nanotubes (CNTs), show much better FE properties than the flat samples due to the enhancement of the local field and the prototype of CNTs-based flat panel display (FPD) has been fabricated [3]. Compared with CNTs, oxide emitters are more stable in harsh environment and controllable in electrical properties [2]. *
Corresponding authors. Fax: +82 2 2290 0768. E-mail addresses: [email protected] (G. Shen), [email protected] (C.J. Lee). 0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.01.084
As a wide band-gap semiconductor of 3.37 eV with a large exciton binding energy (60 meV), ZnO-based 1-D nanostructures have received increasing attention over the past few years due to their potential applications in optoelectronic switches, high-efficiency photonic devices, near-UV lasers and assembling complex threedimensional nanoscale systems [4–7]. Various kinds of ZnO nanostructures such as nanorods/nanowires, nanobelts, nanotubes, nanobridges, nanonails, nanowalls, nanorings etc., have been realized using several techniques including vapor phase methods (thermal evaporation and condensation [8–13], metalorganic chemical vapor deposition, [14–16] laser ablation [17], etc.) and solution phase methods [18–21]. In this Letter, we reported the synthesis of vertically aligned ZnO nanowires on Si substrate at 600 C by simple vapor deposition technique without the use of catalyst. In this process, metallic zinc powder was used as the zinc source since Zn vapor pressure can reach up to 10.8 Pa at a very low temperature of 400 C. The structures and optical properties of ZnO nanowires are investigated using SEM, TEM, and PL. And field emission from the ZnO nanowires was also investigated.
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H. Ham et al. / Chemical Physics Letters 404 (2005) 69–73
2. Experimental A (0 0 1) oriented n-type silicon wafer of 10 · 10 mm2 in size was used as the deposited substrate. Before used, the wafer was etched using hydrofluoric acid for about 30 s and cleaned ultrasonically with ethanol and deionized water in sequence, and then dried at room temperature. The Si substrate was put with face down direction on a quartz boat filled with zinc powder (100 mesh, 99.998%, Aldrich). The quartz boat was then loaded in a horizontal quartz tube with an inner diameter of 20 cm. After loading, the quartz tube was heated up to 600 C under Ar gas (500 sccm) for 30 min. The vertical distance between the zinc source and the substrate was about 3–5 mm. After evaporation, it was found that a layer of white wax-like material was deposited on the Si substrate. The structure of the products was analyzed using XRD (Rigaku DMAX 2500). The morphology and microstructure of the products were analyzed by scanning electron microscopy (SEM, Hitachi S-4700) and high-resolution transmission electron microscopy (HRTEM, Tecnai F20). Photoluminescence measurement was performed at room temperature using a He–Cd laser line of 325 nm as the excitation source. Field electron emission from the ZnO nanowires was investigated. A Ti film deposited n-type silicon substrate was used as the cathode layer. Field emission was measured at room temperature in the vacuum ambient of 1.7 · 10 6 Torr. The distance between the Cu plate anode and the tip of ZnO nanowires was 200 lm and the measured emission area was 0.05 cm2. The emission current was monitored with Keithley 6517A electrometer and recorded at 1.0 s intervals by applying a sweep step of 20 V.
3. Results and discussion Fig. 1a is the SEM image, which gives the general morphology of the deposited ZnO product. It shows that a film of wire-like nanostructures vertically aligned on the surface of the whole silicon substrate. A magnified SEM shown in Fig. 1b shows that the average diameter of the ZnO nanowires is about 70 nm and the length is about 3 lm. It should be mentioned that the distance between the zinc powder and the substrate has great influence on the final product. Longer distance between zinc powder and silicon substrate resulted in the formation of ZnO nanowires with small diameters but poor alignment. XRD was used to get the structural information about the ZnO nanowires. Fig. 2a is the typical XRD pattern of the vertically aligned ZnO nanowires. All peaks in this pattern can be indexed to wurtzite hexago˚ and nal ZnO phase with cell constants a = 3.248 A
Fig. 1. SEM images of ZnO nanowires vertically aligned on silicon substrate.
˚ , which are in good agreement with the rec = 5.206 A ˚ ported data (JCPDS card No. 36-1451: a = 3.249 A ˚ ). No peaks of metallic Zn were detected and c = 5.206 A indicating the purity of the sample in the experimental error range. Energy dispersive X-ray fluorescence (EDX) was used to get the chemical composition of the vertically aligned ZnO nanowires grown on the Si substrate and the spectra were shown in Fig. 2b. The spectra show only peaks of Zn and O and the detected Si peak is due to the Si substrate. Detailed structural characterization of the ZnO nanowires was investigated using HRTEM, as shown in Fig. 3a. It indicates that the obtained ZnO nanowires are quite straight and have uniform diameters along their lengths. The diameters are about 70 nm in accordance to the SEM observations. Fig. 3b is the lattice-resolved HRTEM image of a single ZnO nanowire. The lattice fringe of the ZnO nanowire is about 0.52 nm, corresponding to the (0 0 1) fringes perpendicular to the growth direction, which is consistent with that of the bulk wurtzite ZnO crystal. The inset selected area
H. Ham et al. / Chemical Physics Letters 404 (2005) 69–73
71
Fig. 3. TEM images of ZnO nanowires: (a) Low-resolution TEM image; (b) high-resolution TEM image. Inset shows the corresponding SAED pattern. Fig. 2. (a) XRD pattern and (b) EDX spectra of ZnO nanowires on silicon substrate.
diffraction (SAED) pattern indicates that the ZnO nanowire grows along the [0 0 1] direction, which is the general growth direction for 1-D wurtzite ZnO nanostructures [22]. Since no metal catalyst was employed in our synthesis, the growth mechanism of vertically aligned ZnO nanowires cannot be explained using the conventional vapor–liquid–solid (VLS) model. During the process, zinc powders are evaporated to generate zinc vapors and they are transferred to the low temperature region by Ar gas and deposit on silicon substrate to form a thin layer of Zn film. And then small ZnO nanorods homogeneously epitaxially grow on the newly formed ZnO film [14,16,28] and finally resulted in the formation of vertically aligned ZnO nanowires. Photoluminescence of the obtained ZnO nanowires was also investigated at room temperature and the result is shown in Fig. 4. The PL spectra consist of a sharp and strong emission band located at 374 nm and a weak and broad emission band centered at 500 nm. The near-UV
Fig. 4. PL spectra of ZnO nanowires measured at room temperature.
emission at 374 nm agrees well with the band gap of bulk ZnO [23], which is originated from the recombination of free excitons [24]. Careful studies found that the green emission at 500 nm is asymmetric (as shown in the
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H. Ham et al. / Chemical Physics Letters 404 (2005) 69–73
inset of Fig. 4) and the intensity of this emission decreased with the increase of oxygen content. As previous reports, the green emission is related to the singly ionized oxygen vacancy and it results from the recombination of a photo-generated hole with a singly ionized charge state of the specific defects [13,20]. Recently, one-dimensional material such as CNTs and nanowires has attracted much attention due to their good field emission properties. It is well known that CNTs generally indicate low turn-on field about 0.5– 2 V/lm and high emission current density about several mA/cm2 [25–27]. However, CNTs can be easily degraded in oxygen ambient even though they show good emission property as a field emitter. More recently, semiconductor nanowires including ZnO nanowires showed good emission properties to be used to a field emitter due to their high aspect ratio and high conductivity [28–30]. Moreover, ZnO nanowires exhibit strong endurance to oxygen ambient compared with CNTs. Therefore, in this work, we tried to evaluate field emission property from ZnO nanowires. Fig. 5 illustrates the emission current density from ZnO nanowires. Before the emission current measurement, we conducted an electrical annealing to get stable field emission properties. After electrical annealing process at the current density about 0.1 mA/cm2 during 1 h, the current density was more stabilized and improved about two and half times. The bias voltage sweeps (from 100 to 3500 V) were conducted several times. In this work, we could find stable emission property from ZnO nanowires regardless of measurement numbers. The turn-on (corresponding to the current density of 0.1 lA/cm2) electric field was about 6.2 V/ lm and the emission current density was about 1.0 mA/cm2 at an applied field of 15.0 V/lm.
In the previous work, we reported the field emission properties from ZnO nanowires grown on Co catalyst particles in which the turn-on field was about 6.0 V/lm at 0.1 lA/cm2, and the emission current density was about 1.0 mA/cm2 at 11.0 V/lm [2]. Recently, Jo et al. [28] showed similar field emission properties from thin films of ZnO nanowires grown on Au catalyst nanoparticles, which indicated the current density of about 1 mA/cm2 at the electric field of about 15 V/lm. On the other hands, Zhu et al. showed that ZnO nanoneedle arrays achieved the low turn-on field of about 2.4 V/lm and the high emission current density of about 2.4 mA/cm2 at 7 V/lm from [6]. They suggested that good field emission properties was caused by the small geometry of ZnO nanowires. It is well known that field emission mainly depends on tip morphology and density of nanowires. We consider that our ZnO nanowires can be enough used to field emission displays even though the emission current density is not high level compared with CNTs. The Fowler–Nordheim plot usually presented in the literature is also shown in the inset of Fig. 5. It exhibits two linear behaviors in the measurement range. The field enhancement factor b can be calculated from the slope of Fowler–Nordheim plot if the work function of the emitter is known. The measurement value for the work function of ZnO nanowires was about 5.3 eV [31]. In the case of high applied electric field, b value revealed about 1500 from the inset of Fig. 5. On the other hand, it showed much lower b value at lower applied electric field, resulting from unstable local field and emission current from ZnO nanowires during sweeping an applied voltage [32].
4. Conclusion In conclusion, vertically aligned ZnO nanowires were successfully synthesized on Si substrate via a simple catalyst-free thermal evaporation process at low temperature. The obtained ZnO nanowire is about 70 nm in diameter and several micrometers in length. Field electron emission of ZnO nanowires was investigated. The turn on field for the ZnO nanowires was about 6.2 V/ lm at a current density of 0.1 lA/cm2. The emission current density from the ZnO nanowires reached 1 mA/cm2 at an applied field of about 15.0 V/lm. The emission current from ZnO nanowires synthesized on a Si substrate without the presence of a catalyst can enough be used to field emission displays and vacuum microelectronic devices.
Acknowledgement Fig. 5. Field emission properties from ZnO nanowires grown on n-Si substrate. The turn-on electric field was about 6.2 V/lm, and the emission current density reached about 1 mA/cm2 at 15.0 V/lm.
This work was supported by Korea Research Foundation Grant (KRF-2003-037-C00017).
H. Ham et al. / Chemical Physics Letters 404 (2005) 69–73
References [1] B.R. Chalamala, R.H. Reuss, K.A. Dean, E. Sosa, D.E. Golden, J. Appl. Phys. 91 (2002) 6141. [2] C.J. Lee, T.J. Lee, S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, Appl. Phys. Lett. 81 (2002) 3648. [3] C. Adessi, M. Devel, Phys. Rev. B 65 (2002) 075418. [4] Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [5] M.S. Arnod, P. Avouris, Z.W. Pan, Z.L. Wang, J. Phys. Chem. B 107 (2002) 113. [6] Y.W. Zhu, H.Z. Zhang, X.C. Sun, S.Q. Feng, J. Su, Q. Zhang, B. Xiang, R.M. Wang, D.P. Yu, Appl. Phys. Lett. 83 (2003) 144. [7] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [8] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [9] S.C. Lyu, Y. Zhang, C.J. Lee, H. Ruh, H.J. Lee, Chem. Mater. 15 (2003) 3294. [10] G.Z. Shen, J.H. Cho, C.J. Lee, Chem. Phys. Lett. 401 (2005) 414. [11] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Lett. 3 (2003) 1249. [12] H.T. Ng, J. Li, M.K. Smith, P. Nguyen, A. Cassell, J. Han, M. Meyyappan, Science 300 (2003) 1249. [13] J.Q. Hu, Y. Bando, Appl. Phys. Lett. 82 (2003) 1401. [14] J.J. Wu, S.C. Liu, Adv. Mater. 14 (2002) 215. [15] B.P. Zhang, N.T. Binh, Y. Segawa, K. Wakatsuki, N. Usami, Appl. Phys. Lett. 83 (2003) 1635.
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[16] W.I. Park, Y.H. Jun, S.W. Jung, G. Yi, Appl. Phys. Lett. 82 (2003) 964. [17] Y. Sun, G.M. Fuge, M.N.R. Ashfold, Chem. Phys. Lett. 396 (2004) 21. [18] L. Vayssieres, Adv. Mater. 15 (2003) 464. [19] J. Zhang, L. Sun, C. Liao, C. Yan, Chem. Commun. (2002) 262. [20] Y. Li, G.S. Cheng, L.D. Zhang, J. Mater. Res. 15 (2000) 2305. [21] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430. [22] Z.L. Wang, Materialstoday (2004) 26. [23] E.M. Wong, P.C. Searson, Appl. Phys. Lett. 74 (1999) 2939. [24] V. Stikant, D.R. Clarke, J. Appl. Phys. 83 (1998) 5447. [25] W. Zhu, C. Bower, O. Zhou, G. Kochanski, S. Jin, Appl. Phys. Lett. 75 (1999) 873. [26] C.J. Lee, J. Park, S.Y. Kang, J.H. Lee, Chem. Phys. Lett. 326 (2000) 175. [27] J.M. Bonard, N. Weiss, H. Kind, T. Stokli, L. Forro, K. Kern, A. Chatelain, Adv. Mater. 13 (2001) 184. [28] S.H. Jo, J.Y. Lao, Z.F. Ren, R.A. Farrer, T. Baldacchini, J.T. Fourkas, Appl. Phys. Lett. 83 (2003) 4821. [29] K.W. Wong, X.T. Zhou, F.C.K. Au, H.L. Lai, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 75 (1999) 2918. [30] H.M. Kim, T.W. Kang, K.S. Chung, J.P. Hong, W.B. Choi, Chem. Phys. Lett. 377 (2003) 491. [31] T. Minami, T. Miyata, T. Yamamoto, Surf. Coat. Technol. 108 (1998) 583. [32] M. Sveningsson, R.E. Morjan, O.A. Nerushev, Y. Sato, J. Backstrom, E.E.B. Chambell, F. Rohmund, Appl. Phys. A 73 (2001) 409.
Vertically aligned ZnO nanowires produced by a catalyst-free thermal evaporation method and their field emission properties Heon Ham, Guozhen Shen *, Jung Hee Cho, Tae Jae Lee, Sung Ho Seo, Cheol Jin Lee
*
Department of Nanotechnology, Hanyang University, 17-Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea Received 20 December 2004; in final form 21 January 2005
Abstract Vertically aligned ZnO nanowires have been synthesized on Si substrate by catalyst-free thermal evaporating metallic zinc powder at a low temperature of 600 C. Studies found that the ZnO nanowires are single-crystalline wurtzite structures with 70 nm in diameter and 10 lm in length. The turn on field of the ZnO nanowires was about 6.2 V/lm at a current density of 0.1 lA/cm2, and the emission current density reached 1 mA/cm2 at an applied field of about 15.0 V/lm. Field emission property from the ZnO nanowires was enough high level to be applicable to field emission displays and vacuum microelectronic devices. 2005 Elsevier B.V. All rights reserved.
1. Introduction Fabrication of commercial field emission microcathode array is based on thin film technology and semiconductor processing methods. Although present commercial technology continues to be the structure choice for current field emission display applications, one-dimensional (1-D) vertically aligned nanostructures have been attracting much attention in recent years to be fabricated as the field emission display (FED) because of their high efficiency, reduction of cost and device size compared to conventional thermionic emitters [1,2]. Emitters with large aspect ratios, such as carbon nanotubes (CNTs), show much better FE properties than the flat samples due to the enhancement of the local field and the prototype of CNTs-based flat panel display (FPD) has been fabricated [3]. Compared with CNTs, oxide emitters are more stable in harsh environment and controllable in electrical properties [2]. *
Corresponding authors. Fax: +82 2 2290 0768. E-mail addresses: [email protected] (G. Shen), [email protected] (C.J. Lee). 0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.01.084
As a wide band-gap semiconductor of 3.37 eV with a large exciton binding energy (60 meV), ZnO-based 1-D nanostructures have received increasing attention over the past few years due to their potential applications in optoelectronic switches, high-efficiency photonic devices, near-UV lasers and assembling complex threedimensional nanoscale systems [4–7]. Various kinds of ZnO nanostructures such as nanorods/nanowires, nanobelts, nanotubes, nanobridges, nanonails, nanowalls, nanorings etc., have been realized using several techniques including vapor phase methods (thermal evaporation and condensation [8–13], metalorganic chemical vapor deposition, [14–16] laser ablation [17], etc.) and solution phase methods [18–21]. In this Letter, we reported the synthesis of vertically aligned ZnO nanowires on Si substrate at 600 C by simple vapor deposition technique without the use of catalyst. In this process, metallic zinc powder was used as the zinc source since Zn vapor pressure can reach up to 10.8 Pa at a very low temperature of 400 C. The structures and optical properties of ZnO nanowires are investigated using SEM, TEM, and PL. And field emission from the ZnO nanowires was also investigated.
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H. Ham et al. / Chemical Physics Letters 404 (2005) 69–73
2. Experimental A (0 0 1) oriented n-type silicon wafer of 10 · 10 mm2 in size was used as the deposited substrate. Before used, the wafer was etched using hydrofluoric acid for about 30 s and cleaned ultrasonically with ethanol and deionized water in sequence, and then dried at room temperature. The Si substrate was put with face down direction on a quartz boat filled with zinc powder (100 mesh, 99.998%, Aldrich). The quartz boat was then loaded in a horizontal quartz tube with an inner diameter of 20 cm. After loading, the quartz tube was heated up to 600 C under Ar gas (500 sccm) for 30 min. The vertical distance between the zinc source and the substrate was about 3–5 mm. After evaporation, it was found that a layer of white wax-like material was deposited on the Si substrate. The structure of the products was analyzed using XRD (Rigaku DMAX 2500). The morphology and microstructure of the products were analyzed by scanning electron microscopy (SEM, Hitachi S-4700) and high-resolution transmission electron microscopy (HRTEM, Tecnai F20). Photoluminescence measurement was performed at room temperature using a He–Cd laser line of 325 nm as the excitation source. Field electron emission from the ZnO nanowires was investigated. A Ti film deposited n-type silicon substrate was used as the cathode layer. Field emission was measured at room temperature in the vacuum ambient of 1.7 · 10 6 Torr. The distance between the Cu plate anode and the tip of ZnO nanowires was 200 lm and the measured emission area was 0.05 cm2. The emission current was monitored with Keithley 6517A electrometer and recorded at 1.0 s intervals by applying a sweep step of 20 V.
3. Results and discussion Fig. 1a is the SEM image, which gives the general morphology of the deposited ZnO product. It shows that a film of wire-like nanostructures vertically aligned on the surface of the whole silicon substrate. A magnified SEM shown in Fig. 1b shows that the average diameter of the ZnO nanowires is about 70 nm and the length is about 3 lm. It should be mentioned that the distance between the zinc powder and the substrate has great influence on the final product. Longer distance between zinc powder and silicon substrate resulted in the formation of ZnO nanowires with small diameters but poor alignment. XRD was used to get the structural information about the ZnO nanowires. Fig. 2a is the typical XRD pattern of the vertically aligned ZnO nanowires. All peaks in this pattern can be indexed to wurtzite hexago˚ and nal ZnO phase with cell constants a = 3.248 A
Fig. 1. SEM images of ZnO nanowires vertically aligned on silicon substrate.
˚ , which are in good agreement with the rec = 5.206 A ˚ ported data (JCPDS card No. 36-1451: a = 3.249 A ˚ ). No peaks of metallic Zn were detected and c = 5.206 A indicating the purity of the sample in the experimental error range. Energy dispersive X-ray fluorescence (EDX) was used to get the chemical composition of the vertically aligned ZnO nanowires grown on the Si substrate and the spectra were shown in Fig. 2b. The spectra show only peaks of Zn and O and the detected Si peak is due to the Si substrate. Detailed structural characterization of the ZnO nanowires was investigated using HRTEM, as shown in Fig. 3a. It indicates that the obtained ZnO nanowires are quite straight and have uniform diameters along their lengths. The diameters are about 70 nm in accordance to the SEM observations. Fig. 3b is the lattice-resolved HRTEM image of a single ZnO nanowire. The lattice fringe of the ZnO nanowire is about 0.52 nm, corresponding to the (0 0 1) fringes perpendicular to the growth direction, which is consistent with that of the bulk wurtzite ZnO crystal. The inset selected area
H. Ham et al. / Chemical Physics Letters 404 (2005) 69–73
71
Fig. 3. TEM images of ZnO nanowires: (a) Low-resolution TEM image; (b) high-resolution TEM image. Inset shows the corresponding SAED pattern. Fig. 2. (a) XRD pattern and (b) EDX spectra of ZnO nanowires on silicon substrate.
diffraction (SAED) pattern indicates that the ZnO nanowire grows along the [0 0 1] direction, which is the general growth direction for 1-D wurtzite ZnO nanostructures [22]. Since no metal catalyst was employed in our synthesis, the growth mechanism of vertically aligned ZnO nanowires cannot be explained using the conventional vapor–liquid–solid (VLS) model. During the process, zinc powders are evaporated to generate zinc vapors and they are transferred to the low temperature region by Ar gas and deposit on silicon substrate to form a thin layer of Zn film. And then small ZnO nanorods homogeneously epitaxially grow on the newly formed ZnO film [14,16,28] and finally resulted in the formation of vertically aligned ZnO nanowires. Photoluminescence of the obtained ZnO nanowires was also investigated at room temperature and the result is shown in Fig. 4. The PL spectra consist of a sharp and strong emission band located at 374 nm and a weak and broad emission band centered at 500 nm. The near-UV
Fig. 4. PL spectra of ZnO nanowires measured at room temperature.
emission at 374 nm agrees well with the band gap of bulk ZnO [23], which is originated from the recombination of free excitons [24]. Careful studies found that the green emission at 500 nm is asymmetric (as shown in the
72
H. Ham et al. / Chemical Physics Letters 404 (2005) 69–73
inset of Fig. 4) and the intensity of this emission decreased with the increase of oxygen content. As previous reports, the green emission is related to the singly ionized oxygen vacancy and it results from the recombination of a photo-generated hole with a singly ionized charge state of the specific defects [13,20]. Recently, one-dimensional material such as CNTs and nanowires has attracted much attention due to their good field emission properties. It is well known that CNTs generally indicate low turn-on field about 0.5– 2 V/lm and high emission current density about several mA/cm2 [25–27]. However, CNTs can be easily degraded in oxygen ambient even though they show good emission property as a field emitter. More recently, semiconductor nanowires including ZnO nanowires showed good emission properties to be used to a field emitter due to their high aspect ratio and high conductivity [28–30]. Moreover, ZnO nanowires exhibit strong endurance to oxygen ambient compared with CNTs. Therefore, in this work, we tried to evaluate field emission property from ZnO nanowires. Fig. 5 illustrates the emission current density from ZnO nanowires. Before the emission current measurement, we conducted an electrical annealing to get stable field emission properties. After electrical annealing process at the current density about 0.1 mA/cm2 during 1 h, the current density was more stabilized and improved about two and half times. The bias voltage sweeps (from 100 to 3500 V) were conducted several times. In this work, we could find stable emission property from ZnO nanowires regardless of measurement numbers. The turn-on (corresponding to the current density of 0.1 lA/cm2) electric field was about 6.2 V/ lm and the emission current density was about 1.0 mA/cm2 at an applied field of 15.0 V/lm.
In the previous work, we reported the field emission properties from ZnO nanowires grown on Co catalyst particles in which the turn-on field was about 6.0 V/lm at 0.1 lA/cm2, and the emission current density was about 1.0 mA/cm2 at 11.0 V/lm [2]. Recently, Jo et al. [28] showed similar field emission properties from thin films of ZnO nanowires grown on Au catalyst nanoparticles, which indicated the current density of about 1 mA/cm2 at the electric field of about 15 V/lm. On the other hands, Zhu et al. showed that ZnO nanoneedle arrays achieved the low turn-on field of about 2.4 V/lm and the high emission current density of about 2.4 mA/cm2 at 7 V/lm from [6]. They suggested that good field emission properties was caused by the small geometry of ZnO nanowires. It is well known that field emission mainly depends on tip morphology and density of nanowires. We consider that our ZnO nanowires can be enough used to field emission displays even though the emission current density is not high level compared with CNTs. The Fowler–Nordheim plot usually presented in the literature is also shown in the inset of Fig. 5. It exhibits two linear behaviors in the measurement range. The field enhancement factor b can be calculated from the slope of Fowler–Nordheim plot if the work function of the emitter is known. The measurement value for the work function of ZnO nanowires was about 5.3 eV [31]. In the case of high applied electric field, b value revealed about 1500 from the inset of Fig. 5. On the other hand, it showed much lower b value at lower applied electric field, resulting from unstable local field and emission current from ZnO nanowires during sweeping an applied voltage [32].
4. Conclusion In conclusion, vertically aligned ZnO nanowires were successfully synthesized on Si substrate via a simple catalyst-free thermal evaporation process at low temperature. The obtained ZnO nanowire is about 70 nm in diameter and several micrometers in length. Field electron emission of ZnO nanowires was investigated. The turn on field for the ZnO nanowires was about 6.2 V/ lm at a current density of 0.1 lA/cm2. The emission current density from the ZnO nanowires reached 1 mA/cm2 at an applied field of about 15.0 V/lm. The emission current from ZnO nanowires synthesized on a Si substrate without the presence of a catalyst can enough be used to field emission displays and vacuum microelectronic devices.
Acknowledgement Fig. 5. Field emission properties from ZnO nanowires grown on n-Si substrate. The turn-on electric field was about 6.2 V/lm, and the emission current density reached about 1 mA/cm2 at 15.0 V/lm.
This work was supported by Korea Research Foundation Grant (KRF-2003-037-C00017).
H. Ham et al. / Chemical Physics Letters 404 (2005) 69–73
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