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Well-aligned MoO2 nanowires arrays: Synthesis and field emission properties
ARTICLE IN PRESS Physica B 404 (2009) 1901–1904
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Well-aligned MoO2 nanowires arrays: Synthesis and field emission properties Fan Wang , Bingqiang Lu School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China
a r t i c l e in fo
abstract
Article history: Received 4 November 2008 Received in revised form 26 November 2008 Accepted 22 December 2008
Crystalline MoO2 nanowires arrays were synthesized by electrochemical deposition from Na2MoO4 aqueous solution using pre-annealed anodic aluminum oxide (AAO) template, then followed by annealed in a tube furnace at 650 1C under an atmosphere of a 10% hydrogen/90% nitrogen mixture. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Field emission measurements demonstrated that the turn-on voltage of the MoO2 nanowires arrays was 3.0 V/mm and the threshold voltage was 4.6 V/mm. The MoO2 nanoarrays provide an alternative substitute for the manufacturing of Mo-based field emitters because of the convenient preparation of nanoarrays. & 2009 Published by Elsevier B.V.
PACS: 82.45.Qr 85.45.Db Keywords: Crystal growth Nanomaterials Oxide Field emission
1. Introduction One-dimension (1D) nanostructured materials have been intensively investigated as building components in nanoscale devices. The high aspect ratio and the special apex structure of 1D nanomaterials makes them genuine candidates of field emission. Mo oxides, mainly including MoO3 and MoO2, have excellent field emission properties [1,2]. To obtain high brightness and focusing sources on large areas, the emitter arrays should be vertically aligned and grown with suitable density. Zhou et al. prepared well-aligned MoO2 nanowires arrays by thermal evaporation in a vacuum chamber [2]. Thermal evaporation is a mature preparation technique and many aligned nanoarrays patterns had been reported, but most of them were not very homogeneous, which might be crucial to the rational design of nanoemitters. To achieve highly ordered MoO2 nanowires arrays with large area, we used a simple one-step method by electrochemical deposition in a porous anodic aluminum oxide (AAO) template. The preparation of nanowires using AAO template ensures the homogeneous properties of each nanowire by the uniformity of each AAO’s channel. The nanowires are separated and their length can be tuned easily by the template’s thickness. For synthesizing metal and semiconductor nanowires arrays, electrochemical deposition with the AAO template is a simple yet versatile method in controlling structure and composition by tuning Corresponding author. Tel.: +86 771 3237022; fax: +86 771 3233718.
E-mail address: [email protected] (F. Wang). 0921-4526/$ - see front matter & 2009 Published by Elsevier B.V. doi:10.1016/j.physb.2008.12.025
applied potentials and electrolyte ingredients. However, several single crystalline oxide nanomaterials were directly synthesized with AAO template, including ZnO [3], TiO2 [4], and iron oxide [5]. In these syntheses, the electroreduction of nitrate ions produced hydroxyl ions [3,5], which resulted in the hydrolyzation of metal ions and formed hydroxide colloidal particles. Further electrochemical reaction caused bonds formation between sol particles and resulted in oxide nanowires. Needless to say, this procedure cannot be performed with incomplete elimination of the excessive hydroxyl ions, which may lead to severe corrosion of AAO template. Using the oxoanions with high oxidation states as precursor, transition-metal oxides nanostructures were formed. Penner and co-workers had demonstrated the growth of MoO2 [6] and MnO2 [7] nanowires by templating against the steps presented on a HOPG surface using electrochemical deposition. In the case of MoO2 deposition, the electrolyte solution contains the precursor MoO2 4 , and deposition involves the following reduction: MoO2 4 þ 2H2 O þ 2e ! MoO2 þ 4OH
(1)
As mentioned above, this approach fails to prepare MoO2 nanowires using AAO template without any treatments. It is pivotal to enhance the performance of alkali resistance of AAO template if using metal oxoanions as precursor. AAO template consists of both amorphous and crystalline g-Al2O3. Hightemperature (900 1C) annealing can converse the Al oxides partly into a-Al2O3, which help enhance the performance of alkali resistance and maintain the porous structure of AAO template
ARTICLE IN PRESS 1902
F. Wang, B. Lu / Physica B 404 (2009) 1901–1904
during the growth. We noticed that the preparation of nanowires in the pores of template with small diameters (o30 nm) was still scarce, and the synthesis of MoO2 nanowires using AAO template had not reported yet. It is worthwhile noting that the diameter and density of the nanoemitters plays a crucial role for the field emission properties [8]. Therefore, the synthesis of MoO2 nanowires with small diameter is also of interest for the research of field emission.
ultrasonic treatment in concentrated HF acid to dissolve the AAO template [10], and a drop of suspension was placed on the carbon-coated copper grid. Field emission measurements were conducted in a vacuum chamber under a pressure of 107 Pa and room temperature. Those J–E characteristics were analyzed by applying the Fowler–Nordheim (FN) equation for field emission:
2. Experiment
3. Result and discussion
The fabrication of AAO templates with pore diameter of about 30 nm has been reported in previous works [9]. After removing the remaining Al and barrier layer, the templates were heated in an oven at 900 1C for 6 h. Then a layer of Au film was sputtered onto one side of the AAO templates to serve as the working electrode in a three-electrode system for electrochemical deposition. A saturated calomel electrode (SCE) was used as reference electrode and a platinum slice was as the counter electrode. The electrolyte contained a mixture of 0.1 mol/L Na2MoO4, 1.0 mol/L NH4Cl solution, and the pH value was adjusted to 8.5–9.5 using aqueous ammonium hydroxide [6]. Electrochemical deposition was carried out under potentiostatic conditions for 1–2 h with the deposition potential of 1.0 V. Then, the samples were annealed in a tube furnace at 650 1C under an atmosphere of a 10% hydrogen/ 90% nitrogen mixture to obtain high-quality nanowires arrays. The morphology and structure of the as-prepared MoO2 nanowires were characterized by XRD (Thermo ARL SCINTAG X’TRA), SEM (Hitachi S-3400N, 25 kV), TEM (JEM200CX, 120 kV) and high-resolution TEM (HRTEM, Hitachi, H-9000). For SEM observations, ion milling of samples was carried out to remove the over-growth MoO2 layer and expose the nanowires. For TEM characterization, the as-obtained samples were subjected to
According to Penner’s work [6], the potential of deposition plays an important role in the growth of MoO2 nanowires. A more negative potential is necessary for the convenient rate of deposition. In our experiments, an electrochemical deposition at 1.0 V produced continuous growth of MoO2 nanowires in AAO template. The framework of annealed template had properly maintained during whole deposition process. Instead, the corrosion of templates caused by the hydroxyl ions produced from the reduction of MoO2 4 , which increased the local pH in the AAO’s pores, was found for the un-annealed templates. These results were consistent with the hypothesis that annealing the template could help reduce the corrosion. Fig. 1a shows the XRD pattern of MoO2 nanostructures obtained by the electrochemical deposition and then annealed. All of the reflection peaks can be readily indexed to pure MoO2 (JCPDS card 86-0135, a ¼ 5.69 A˚, b ¼ 4.857 A˚, c ¼ 5.625 A˚). In addition, the prominent peaks on XRD pattern mean the achieving of high crystalline MoO2 nanostructures. Owing to the continuous and suitable rate of deposition, the nanowires had similar length and the smooth overgrowth layer was formed on the exposed face of the template (Fig. 1b). After removing the overgrowth layer by ion milling, we found that vertically aligned nanowires with high
2
J ¼ ðE2 b =FÞ expðBF3=2 =EbÞ
Fig. 1. (a) XRD patterns of the MoO2 nanowires with AAO template and SEM images of the MoO2 nanowires (b) before and (c) after removed the over-growth layer by ion milling.
ARTICLE IN PRESS F. Wang, B. Lu / Physica B 404 (2009) 1901–1904
1903
Fig. 2. TEM images of MoO2 nanowires: (a) before and (b) after annealing; (c) The EDS spectrum of the prepared of MoO2 nanowires; (d) The HRTEM image of a MoO2 nanowire. The inset is the corresponding SAED pattern.
density were grown inside the template pores. Typical SEM images of the as-obtained MoO2 nanoarrays are presented in Fig. 1c. The nanowires have sharp and pointed tips, which might be favorable for the field emission. Fig. 2 shows the TEM images and the corresponding EDX analysis. The diameters of nanowires are about 30 nm and the lengths up to several ten microns, which accords to the pore diameters and the template’s thickness. The nanowires after annealed reveal singlecrystal structure by both the diffraction contrast and the diffraction patterns (Fig. 2b inset). EDX analysis (Fig. 2c) indicates that these nanowires only consists of the Mo and O element. The detailed information of nanowire’s structure obtained from HRTEM (Fig. 2d) also reveals the single crystalline MoO2 nanowires. The interlanar spacing perpendicular to the nanowire axis is 0.24 nm, which corresponds to (2 0 0) and (0 0 2) planes of MoO2. Investigation on the field emission performance of MoO2 nanowires arrays was performed in a chamber with a base pressure of 107 Pa. To get the convenient emitter arrays, the sample was cleaned by ion milling to make sure the exposure of nanowires out of the template background. After washing in deion water for several times and dried at 120 1C, the sample was attached to the stainless steel stand with carbon glue and served as cathode. A molybdenum microprobe of 1 mm2 at the top was acted as anode. The spacing of two electrodes was 600 mm. The typical curve of emission current density versus applied voltage (J–E) is illustrated in Fig. 3. The result indicates good emission properties of these nanoarrays. The turn-on voltage, corresponding to current density of 10 mA/cm2, was 3.0 V/mm for MoO2 nanoarrays. The threshold voltage was 4.6 V/mm, where the current density reaches to 1 mA/cm2. A maximal current density of 2.9 mA/cm2 was achieved at an applied field of 5.2 V/mm. Further increase of the applied field led to arc discharge and a sudden dropping of the emission current. It is interesting that the value of turn-on voltage is close to those of freestanding MoO2 nanoarrays though the diameters and shapes of MoO2 array are different [2,11], which shows that the alignment of nanoarrays plays a significant role in the field emission properties. By plotting ln(J/E2) versus 1/E, a straight line is obtained (see inset, Fig. 3). The linearity of this curve shows that a conventional F–N mechanism is responsible for the field emission from MoO2 nanowires. These results may be attributed to very good
Fig. 3. Field emission current density (J) as a function of applied voltage (E) for the MoO2 nanowires arrays. The inset shows the corresponding FN plots.
uniformity in the height and diameter of the nanowires and the separation between nanowires. Mo metal has been the emitter material of choice because it has good thermal, mechanical, and electrical properties and is easy to achieve high aspect ratios. However, most of the employed fabrication technologies for uniform and stable Mo field emitter arrays are not time-efficient and cost-effective [12,13]. In this study, Mo oxide provides an alternative substitute for the manufacturing of Mo-based field emitters because of the convenient preparation of nanoarrays. This novel 1D nanomaterial is indeed very promising candidate for future cold cathode materials.
4. Conclusion The well-aligned MoO2 nanowires were prepared by electrochemical process using AAO template and annealed under a mixture of hydrogen/ nitrogen gas. The emission properties of MoO2 nanowires arrays were investigated. The results showed such arrays exhibited low turn-on voltage and large currents in high field. It is suggested that well-aligned MoO2 nanoarrays synthesized on AAO template can be used as emission materials
ARTICLE IN PRESS 1904
F. Wang, B. Lu / Physica B 404 (2009) 1901–1904
for field emission displays and microelectronic devices. We also suggest an alternative method for synthesizing other metal oxide nanowires, which should be of interest in many fields.
Acknowledgment This research is supported by the Guangxi Science Foundation (no. 0832010). References [1] J. Zhou, S.Z. Deng, N.S. Xu, J. Chen, J.C. She, Appl. Phys. Lett. 83 (2003) 2653. [2] J. Zhou, N.S. Xu, S.Z. Deng, J. Chen, J.C. She, Z.L. Wang, Adv. Mater. 15 (2003) 1835.
[3] L. Li, S.S. Pan, X.C. Dou, Y.G. Zhu, X.H. Huang, Y.W. Yang, G.H. Li, L.D. Zhang, J. Phys. Chem. C 111 (2007) 7288. [4] Z. Miao, D.S. Xu, J.H. Ouyang, G.L. Guo, X.S. Zhao, Y.Q. Tang, Nano Lett. 2 (2002) 717. [5] D.X. Xue, L.Y. Zhang, A.B. Gui, X.F. Xu, Appl. Phys. A 80 (2005) 439. [6] M.P. Zach, K. Inazu, K.H. Ng, J.C. Hemminger, R.M. Penner, Chem. Mater. 14 (2002) 3206. [7] Q.G. Li, J.B. Olson, R.M. Penner, Chem. Mater. 16 (2004) 3402. [8] S.H. Jo, Y. Tu, Z.P. Huang, D.L. Carnahan, D.Z. Wang, Z.F. Ren, Appl. Phys. Lett. 82 (2003) 3520. [9] A.P. Li, F. Muller, A. Birner, K. Nielsch, U. Gosele, J. Appl. Phys. 84 (1998) 6023. [10] F. Wang, Y. Wang, J.F. Yu, Y.C. Xie, J.L. Li, K. Wu, J. Phys. Chem. C 112 (2008) 13121. [11] J.G. Liu, Z.J. Zhang, C.Y. Pan, Y. Zhao, X. Su, Y. Zhou, D.P. Yu, Mater. Lett. 58 (2004) 3812. [12] A.A. Talin, K.A. Dean, J.E. Jaskie, Solid-State Electron. 45 (2001) 963. [13] H. Kim, B.K. Ju, Y.H. Lee, J. Jang, M.H. Oha, J. Electrochem. Soc. 147 (2000) 4705.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Well-aligned MoO2 nanowires arrays: Synthesis and field emission properties Fan Wang , Bingqiang Lu School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China
a r t i c l e in fo
abstract
Article history: Received 4 November 2008 Received in revised form 26 November 2008 Accepted 22 December 2008
Crystalline MoO2 nanowires arrays were synthesized by electrochemical deposition from Na2MoO4 aqueous solution using pre-annealed anodic aluminum oxide (AAO) template, then followed by annealed in a tube furnace at 650 1C under an atmosphere of a 10% hydrogen/90% nitrogen mixture. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Field emission measurements demonstrated that the turn-on voltage of the MoO2 nanowires arrays was 3.0 V/mm and the threshold voltage was 4.6 V/mm. The MoO2 nanoarrays provide an alternative substitute for the manufacturing of Mo-based field emitters because of the convenient preparation of nanoarrays. & 2009 Published by Elsevier B.V.
PACS: 82.45.Qr 85.45.Db Keywords: Crystal growth Nanomaterials Oxide Field emission
1. Introduction One-dimension (1D) nanostructured materials have been intensively investigated as building components in nanoscale devices. The high aspect ratio and the special apex structure of 1D nanomaterials makes them genuine candidates of field emission. Mo oxides, mainly including MoO3 and MoO2, have excellent field emission properties [1,2]. To obtain high brightness and focusing sources on large areas, the emitter arrays should be vertically aligned and grown with suitable density. Zhou et al. prepared well-aligned MoO2 nanowires arrays by thermal evaporation in a vacuum chamber [2]. Thermal evaporation is a mature preparation technique and many aligned nanoarrays patterns had been reported, but most of them were not very homogeneous, which might be crucial to the rational design of nanoemitters. To achieve highly ordered MoO2 nanowires arrays with large area, we used a simple one-step method by electrochemical deposition in a porous anodic aluminum oxide (AAO) template. The preparation of nanowires using AAO template ensures the homogeneous properties of each nanowire by the uniformity of each AAO’s channel. The nanowires are separated and their length can be tuned easily by the template’s thickness. For synthesizing metal and semiconductor nanowires arrays, electrochemical deposition with the AAO template is a simple yet versatile method in controlling structure and composition by tuning Corresponding author. Tel.: +86 771 3237022; fax: +86 771 3233718.
E-mail address: [email protected] (F. Wang). 0921-4526/$ - see front matter & 2009 Published by Elsevier B.V. doi:10.1016/j.physb.2008.12.025
applied potentials and electrolyte ingredients. However, several single crystalline oxide nanomaterials were directly synthesized with AAO template, including ZnO [3], TiO2 [4], and iron oxide [5]. In these syntheses, the electroreduction of nitrate ions produced hydroxyl ions [3,5], which resulted in the hydrolyzation of metal ions and formed hydroxide colloidal particles. Further electrochemical reaction caused bonds formation between sol particles and resulted in oxide nanowires. Needless to say, this procedure cannot be performed with incomplete elimination of the excessive hydroxyl ions, which may lead to severe corrosion of AAO template. Using the oxoanions with high oxidation states as precursor, transition-metal oxides nanostructures were formed. Penner and co-workers had demonstrated the growth of MoO2 [6] and MnO2 [7] nanowires by templating against the steps presented on a HOPG surface using electrochemical deposition. In the case of MoO2 deposition, the electrolyte solution contains the precursor MoO2 4 , and deposition involves the following reduction: MoO2 4 þ 2H2 O þ 2e ! MoO2 þ 4OH
(1)
As mentioned above, this approach fails to prepare MoO2 nanowires using AAO template without any treatments. It is pivotal to enhance the performance of alkali resistance of AAO template if using metal oxoanions as precursor. AAO template consists of both amorphous and crystalline g-Al2O3. Hightemperature (900 1C) annealing can converse the Al oxides partly into a-Al2O3, which help enhance the performance of alkali resistance and maintain the porous structure of AAO template
ARTICLE IN PRESS 1902
F. Wang, B. Lu / Physica B 404 (2009) 1901–1904
during the growth. We noticed that the preparation of nanowires in the pores of template with small diameters (o30 nm) was still scarce, and the synthesis of MoO2 nanowires using AAO template had not reported yet. It is worthwhile noting that the diameter and density of the nanoemitters plays a crucial role for the field emission properties [8]. Therefore, the synthesis of MoO2 nanowires with small diameter is also of interest for the research of field emission.
ultrasonic treatment in concentrated HF acid to dissolve the AAO template [10], and a drop of suspension was placed on the carbon-coated copper grid. Field emission measurements were conducted in a vacuum chamber under a pressure of 107 Pa and room temperature. Those J–E characteristics were analyzed by applying the Fowler–Nordheim (FN) equation for field emission:
2. Experiment
3. Result and discussion
The fabrication of AAO templates with pore diameter of about 30 nm has been reported in previous works [9]. After removing the remaining Al and barrier layer, the templates were heated in an oven at 900 1C for 6 h. Then a layer of Au film was sputtered onto one side of the AAO templates to serve as the working electrode in a three-electrode system for electrochemical deposition. A saturated calomel electrode (SCE) was used as reference electrode and a platinum slice was as the counter electrode. The electrolyte contained a mixture of 0.1 mol/L Na2MoO4, 1.0 mol/L NH4Cl solution, and the pH value was adjusted to 8.5–9.5 using aqueous ammonium hydroxide [6]. Electrochemical deposition was carried out under potentiostatic conditions for 1–2 h with the deposition potential of 1.0 V. Then, the samples were annealed in a tube furnace at 650 1C under an atmosphere of a 10% hydrogen/ 90% nitrogen mixture to obtain high-quality nanowires arrays. The morphology and structure of the as-prepared MoO2 nanowires were characterized by XRD (Thermo ARL SCINTAG X’TRA), SEM (Hitachi S-3400N, 25 kV), TEM (JEM200CX, 120 kV) and high-resolution TEM (HRTEM, Hitachi, H-9000). For SEM observations, ion milling of samples was carried out to remove the over-growth MoO2 layer and expose the nanowires. For TEM characterization, the as-obtained samples were subjected to
According to Penner’s work [6], the potential of deposition plays an important role in the growth of MoO2 nanowires. A more negative potential is necessary for the convenient rate of deposition. In our experiments, an electrochemical deposition at 1.0 V produced continuous growth of MoO2 nanowires in AAO template. The framework of annealed template had properly maintained during whole deposition process. Instead, the corrosion of templates caused by the hydroxyl ions produced from the reduction of MoO2 4 , which increased the local pH in the AAO’s pores, was found for the un-annealed templates. These results were consistent with the hypothesis that annealing the template could help reduce the corrosion. Fig. 1a shows the XRD pattern of MoO2 nanostructures obtained by the electrochemical deposition and then annealed. All of the reflection peaks can be readily indexed to pure MoO2 (JCPDS card 86-0135, a ¼ 5.69 A˚, b ¼ 4.857 A˚, c ¼ 5.625 A˚). In addition, the prominent peaks on XRD pattern mean the achieving of high crystalline MoO2 nanostructures. Owing to the continuous and suitable rate of deposition, the nanowires had similar length and the smooth overgrowth layer was formed on the exposed face of the template (Fig. 1b). After removing the overgrowth layer by ion milling, we found that vertically aligned nanowires with high
2
J ¼ ðE2 b =FÞ expðBF3=2 =EbÞ
Fig. 1. (a) XRD patterns of the MoO2 nanowires with AAO template and SEM images of the MoO2 nanowires (b) before and (c) after removed the over-growth layer by ion milling.
ARTICLE IN PRESS F. Wang, B. Lu / Physica B 404 (2009) 1901–1904
1903
Fig. 2. TEM images of MoO2 nanowires: (a) before and (b) after annealing; (c) The EDS spectrum of the prepared of MoO2 nanowires; (d) The HRTEM image of a MoO2 nanowire. The inset is the corresponding SAED pattern.
density were grown inside the template pores. Typical SEM images of the as-obtained MoO2 nanoarrays are presented in Fig. 1c. The nanowires have sharp and pointed tips, which might be favorable for the field emission. Fig. 2 shows the TEM images and the corresponding EDX analysis. The diameters of nanowires are about 30 nm and the lengths up to several ten microns, which accords to the pore diameters and the template’s thickness. The nanowires after annealed reveal singlecrystal structure by both the diffraction contrast and the diffraction patterns (Fig. 2b inset). EDX analysis (Fig. 2c) indicates that these nanowires only consists of the Mo and O element. The detailed information of nanowire’s structure obtained from HRTEM (Fig. 2d) also reveals the single crystalline MoO2 nanowires. The interlanar spacing perpendicular to the nanowire axis is 0.24 nm, which corresponds to (2 0 0) and (0 0 2) planes of MoO2. Investigation on the field emission performance of MoO2 nanowires arrays was performed in a chamber with a base pressure of 107 Pa. To get the convenient emitter arrays, the sample was cleaned by ion milling to make sure the exposure of nanowires out of the template background. After washing in deion water for several times and dried at 120 1C, the sample was attached to the stainless steel stand with carbon glue and served as cathode. A molybdenum microprobe of 1 mm2 at the top was acted as anode. The spacing of two electrodes was 600 mm. The typical curve of emission current density versus applied voltage (J–E) is illustrated in Fig. 3. The result indicates good emission properties of these nanoarrays. The turn-on voltage, corresponding to current density of 10 mA/cm2, was 3.0 V/mm for MoO2 nanoarrays. The threshold voltage was 4.6 V/mm, where the current density reaches to 1 mA/cm2. A maximal current density of 2.9 mA/cm2 was achieved at an applied field of 5.2 V/mm. Further increase of the applied field led to arc discharge and a sudden dropping of the emission current. It is interesting that the value of turn-on voltage is close to those of freestanding MoO2 nanoarrays though the diameters and shapes of MoO2 array are different [2,11], which shows that the alignment of nanoarrays plays a significant role in the field emission properties. By plotting ln(J/E2) versus 1/E, a straight line is obtained (see inset, Fig. 3). The linearity of this curve shows that a conventional F–N mechanism is responsible for the field emission from MoO2 nanowires. These results may be attributed to very good
Fig. 3. Field emission current density (J) as a function of applied voltage (E) for the MoO2 nanowires arrays. The inset shows the corresponding FN plots.
uniformity in the height and diameter of the nanowires and the separation between nanowires. Mo metal has been the emitter material of choice because it has good thermal, mechanical, and electrical properties and is easy to achieve high aspect ratios. However, most of the employed fabrication technologies for uniform and stable Mo field emitter arrays are not time-efficient and cost-effective [12,13]. In this study, Mo oxide provides an alternative substitute for the manufacturing of Mo-based field emitters because of the convenient preparation of nanoarrays. This novel 1D nanomaterial is indeed very promising candidate for future cold cathode materials.
4. Conclusion The well-aligned MoO2 nanowires were prepared by electrochemical process using AAO template and annealed under a mixture of hydrogen/ nitrogen gas. The emission properties of MoO2 nanowires arrays were investigated. The results showed such arrays exhibited low turn-on voltage and large currents in high field. It is suggested that well-aligned MoO2 nanoarrays synthesized on AAO template can be used as emission materials
ARTICLE IN PRESS 1904
F. Wang, B. Lu / Physica B 404 (2009) 1901–1904
for field emission displays and microelectronic devices. We also suggest an alternative method for synthesizing other metal oxide nanowires, which should be of interest in many fields.
Acknowledgment This research is supported by the Guangxi Science Foundation (no. 0832010). References [1] J. Zhou, S.Z. Deng, N.S. Xu, J. Chen, J.C. She, Appl. Phys. Lett. 83 (2003) 2653. [2] J. Zhou, N.S. Xu, S.Z. Deng, J. Chen, J.C. She, Z.L. Wang, Adv. Mater. 15 (2003) 1835.
[3] L. Li, S.S. Pan, X.C. Dou, Y.G. Zhu, X.H. Huang, Y.W. Yang, G.H. Li, L.D. Zhang, J. Phys. Chem. C 111 (2007) 7288. [4] Z. Miao, D.S. Xu, J.H. Ouyang, G.L. Guo, X.S. Zhao, Y.Q. Tang, Nano Lett. 2 (2002) 717. [5] D.X. Xue, L.Y. Zhang, A.B. Gui, X.F. Xu, Appl. Phys. A 80 (2005) 439. [6] M.P. Zach, K. Inazu, K.H. Ng, J.C. Hemminger, R.M. Penner, Chem. Mater. 14 (2002) 3206. [7] Q.G. Li, J.B. Olson, R.M. Penner, Chem. Mater. 16 (2004) 3402. [8] S.H. Jo, Y. Tu, Z.P. Huang, D.L. Carnahan, D.Z. Wang, Z.F. Ren, Appl. Phys. Lett. 82 (2003) 3520. [9] A.P. Li, F. Muller, A. Birner, K. Nielsch, U. Gosele, J. Appl. Phys. 84 (1998) 6023. [10] F. Wang, Y. Wang, J.F. Yu, Y.C. Xie, J.L. Li, K. Wu, J. Phys. Chem. C 112 (2008) 13121. [11] J.G. Liu, Z.J. Zhang, C.Y. Pan, Y. Zhao, X. Su, Y. Zhou, D.P. Yu, Mater. Lett. 58 (2004) 3812. [12] A.A. Talin, K.A. Dean, J.E. Jaskie, Solid-State Electron. 45 (2001) 963. [13] H. Kim, B.K. Ju, Y.H. Lee, J. Jang, M.H. Oha, J. Electrochem. Soc. 147 (2000) 4705.