- Email: [email protected]
Synthesis and field emission property of VO2 nanorods with a body-centered-cubic structure
ARTICLE IN PRESS Physica E 41 (2009) 548–551
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Synthesis and field emission property of VO2 nanorods with a body-centered-cubic structure Yuquan Wang, Zhengjun Zhang Advanced Materials Laboratory, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 20 August 2008 Accepted 15 October 2008 Available online 1 November 2008
Films consisting of vertically aligned VO2 nanorods were prepared on planar silicon substrate by thermally heating a sheet of vanadium in a rough vacuum. These nanorods were found to be of a bodycentered-cubic (BCC) structure with a lattice constant of 0.94 nm, which was not observed before for VO2. Due to their sharp tip of the nanometer scale, the BCC VO2 nanorods exhibited excellent field emission properties, which make them possible candidate materials for applications in field emission devices. & 2008 Elsevier B.V. All rights reserved.
PACS: 61.46.Km 79.70.+q 68.55.Nq Keywords: Nanorods VO2 Field emission
Vanadium oxides have aroused tremendous research interest in past years because of their excellent chemical and physical properties that are attractive to optical and electrical applications. For example, V2O3 is a paramagnetic metallic phase of corundum structure (Al2O3), and is an antiferromagnetic insulator of monoclinic structure at temperatures above or below 165 K, respectively [1,2]. V2O5 is a diamagnetic insulator with an orthorhombic structure and is an essential ingredient to heterogeneous catalysis widely used in chemical reactions [1]. VO2 is an interesting narrow band gap (i.e. 0.7 eV) semiconductor, which undergoes a semiconductor-to-metal phase transition (i.e. from monoclinic to tetragonal structure) at 68 1C, leading to giant drops in its electrical resistance and transmittance of infrared [1,3]. This makes VO2 a promising candidate material for applications in fast-switching, electrochromic and other applications [3]. Therefore, it is of great interest to investigate the possibility and properties of VO2 to crystallize into other forms, especially at the nanometer scale. Films of vanadium oxides can be prepared by various techniques, such as physical vapor deposition [4–7], sol–gel growth method [8], chemical vapor deposition [9,10], sputtering deposition [11,12], etc. An alternative approach to synthesize metal oxides is the thermal oxidation process, which has been
Corresponding author. Tel.: +86 10 62797033; fax: +86 10 62771160.
E-mail address: [email protected] (Z. Zhang). 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.10.006
proven powerful to synthesize large-scale nanostructures of metal oxides [13–16]. The advantage of this approach is its simplicity and relatively low substrate temperatures [13,15]. We thus employed this method in the present study to synthesize VO2 films. We report, in this letter, the synthesis of films of large-scale arrays of VO2 nanorods by a simple thermal oxidation method, and the observation that VO2 could crystallize into a bodycentered-cubic (BCC) structure, which was not observed before. The composition, chemical bonding and field emission properties of the BCC VO2 nanorods were also reported. A pure (99.99%) vanadium sheet of 5 mm 50 mm 0.7 mm in dimension was cleaned sequentially in acetone, alcohol and deionized water baths supersonically, and was connected to two copper electrodes in a vacuum chamber. The substrates used in this study were (0 0 1)-oriented silicon wafers. These were also cleaned supersonically in baths of acetone, alcohol and de-ionized water in sequence, and were put on a substrate holder 4 cm below the vanadium sheet. The chamber was first pumped down to a vacuum on the level of 5 102 Torr by a rotary pump, and then a voltage of 2 V was applied to the two electrodes, resulting in a current of 65 A passing through the vanadium sheet. The current heated up rapidly the sheet in this rough vacuum, leading to the deposition of vanadium oxides on the silicon substrate below the vanadium sheet. During the typical deposition time of 15 min, the maximum temperature of the vanadium sheet and the silicon substrate was measured to be 900 and 350 1C,
ARTICLE IN PRESS Y. Wang, Z. Zhang / Physica E 41 (2009) 548–551
549
respectively. After deposition, the growth morphology, structure and composition of the deposits were examined with scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area diffraction (SAD), electron energy loss spectrometer (EELS), X-ray and ultraviolet photoelectron spectrometer (XPS, UPS) and Rutherfold backscattered spectrometer (RBS). The field emission property of the deposits was also evaluated. Fig. 1(a) and (b) show SEM micrographs of the vanadium oxide deposited on the silicon substrate at the above conditions. The SEM images were taken with a JSM-4800 SEM working at 20 kV. One sees that after 15-min deposition a continuous film was formed on the substrate, which consisted of densely packed vertically aligned rods. The rods are 6 mm long, cone-shaped and have nanometer-size sharp tips, see Fig. 1(b). To determine the phase structure of these rods, we have carried out X-ray diffraction (XRD) of the films with a Rigaku X-ray diffractometer using Cu Ka radiation. However, the XRD pattern (not shown) showed a strong texture growth of the rods, and that peaks in the pattern did not match any known phases of vanadium oxides. We thus used SAD analysis of the rods to identify their phase structure. To determine the structure, we have done SAD analysis for over 20 rods, and for each rod we obtained three SAD patterns along different axes. By carefully indexing SAD patterns for each rod, we finally found that the rods are of a BCC structure with a lattice parameter of 0.94 nm, which was not observed previously for vanadium oxides. As an example, Fig. 1(c) shows a typical brightfield TEM image of the rods. Fig. 1(d) and (e) shows SAD patterns of the rod, taken along the axes of [1 0 0] and [3 1 0], respectively. Fig. 1(f) shows an HRTEM image of the rod corresponding to
Fig. 1. (a) A low-magnification and (b) a close-up SEM images of the BCC VO2 rods deposited on silicon substrates; (c) a bright-field TEM image and SAD patterns along the axis of (d) [1 0 0] and (e) [3 1 0] of the rods, respectively; and (f) an HRTEM image of the rods corresponding to (d).
5k
100.0k
Experimental V-L3
80.0k
Yield (counts)
V-L2
Yield (counts)
Simulated
4k
60.0k
40.0k
O-K
3k
2k
1k
20.0k
0 480
520 540 Energy (ev)
580
560
580
100
0
200 300 Channel
400
500
530.2
4000
2×105
(O1s)
Yield (counts)
Intensity
V2p3/2 516.9
3000 V2p1/2 524.1
2000
4.81 eV
21.22 eV
0
1000 540
1×105
535
530 525 520 Binding Energy (eV)
515
510
0
5
10 15 20 Binding Energy (eV)
Fig. 2. The (a) EELS, (b) RBS, (c) XPS and (d) UPS spectrum of the BCC VO2 rods, respectively.
25
30
ARTICLE IN PRESS Y. Wang, Z. Zhang / Physica E 41 (2009) 548–551
Emission Current Density (mA/cm2)
6 200
300 250
5
150
100
4 3 2 1 0 8
10
12
14 16 Field (V/μm)
18
20
-30.0 100 µm 150 µm
In(J/E2) ((A/m2)/(V/m)2)
200 µm
-30.5
250 µm 300 µm
-31.0
-31.5 6.0×10-8
7.0×10-8
8.0×10-8 9.0×10-8 1/E(m/V)
1.0×10-7
1.1×10-7
14 1400 β Turn-on Field
1200
12
1000
Turn-on Field (V/μm)
Fig. 1(d). The images and SAD patterns were taken with a JEM-2010F TEM working at 200 kV. It is seen that the rod is several hundred nanometers in diameter, and has a sharp tip at a size of several tens of nanometers, as shown also by Fig. 1(a)–(b). One sees that the BCC-structured rods are single crystalline, and were grown along the /0 0 2S direction. The composition of the rods was estimated by EELS analysis in a JEM-2010F TEM working at 200 kV. Fig. 2(a) shows a typical EELS spectrum of the BCC rods. It gave an estimation of 1:2 for the atomic ratio of V/O. This analysis has been done for more than 20 rods, and all gave an estimation of the ratio close to 1:2. This suggests that the BCC rods are VO2. Fig. 2(b) shows an RBS random spectrum of the films consisting of BCC rods with a 1.7 MeV He+ beam. One sees that the simulated spectrum using a composition of VO2 matched pretty well with the experimental curve in almost the whole depth range. The chemical state of vanadium atoms in the rods was also examined with XPS analysis. Fig. 2(c) shows an XPS spectrum of the BCC rods, obtained with a PHI Quantera SXM spectrometer using Al Ka (hn ¼ 1486.6 eV) as the excitation source, from a ‘‘clean’’ surface of the rods (sputtered with 4 keV Ar+). One sees from the figure that peaks of V2p3/2 and V2p1/2 of the BCC rods were at 516.9 and 524.1 eV, respectively, indicating an oxidation state of +4 for vanadium [17,18]. It is thus concluded that the vanadium oxide rods produced here are VO2 with a BCC structure that was not observed before. Since the BCC VO2 rods produced here have very fine tips (tens nanometers), they might be applied as electron source materials. For this purpose, the work function of the rods was measured by UPS analysis. Fig. 2(d) shows a typical UPS spectrum of BCC VO2 rods, obtained by a Kratos Axis Ultra electron spectrometer using He (I) ultraviolet light (21.22 eV) as the excitation source. The work function of the rods was estimated to be 4.81 eV. The field emission property of the BCC VO2 rods was evaluated using a Keithley 485 electrometer in a chamber with a vacuum level of 6 107 Pa, at different electrode distances. Fig. 3(a) shows the emission current from films (area of 0.1 cm2) of BCC VO2 rods as a function of the voltage applied, at electrode distances of 100, 150, 200, 250 and 300 mm, respectively. It is seen that the emission current increased rapidly when the applied voltage went up. For instance, at a distance of 100 mm, the current density was 1 mA/cm2 at a voltage of 1.6 kV, while at a voltage of 2 kV, it was increased to 8 mA/cm2 and was not saturated. This emission could be well fitted by the Fowler–Nordheim (F–N) formula of J ¼ (E2b2/f)exp(Bf3/2/Eb), where J is the current density, E is the applied field, f is the work function of the materials, B is a constant of 6.83 109 (eV3/2 V m1) and b is the field-enhancing factor [19]. Fig. 3(b) shows the F–N plots for the films measured at various electrode distances. The good linearity of these plots indicates that electrons were indeed driven by the field emission effect [19]. Using the work function of 4.81 eV measured by the UPS analysis (see Fig. 2(d)), the field-enhancing factor b was calculated from the F–N plots and was plotted in Fig. 3(c) as a function of electrode distance. In addition, the turn-on field (defined as the electrical field to get an emission current density of 10 mA/cm2) for different electrode distances was also derived and added in Fig. 3(c). One sees from the figure that the turn-on field of the films decreased with increasing the electrode distance, e.g., from 13 V/mm at 100 mm to 10 V/mm at 200 mm; meanwhile, the field-enhancing factor b increased from 850 to 1400, respectively. At all distances the field emission current density could reach as high as 5 mA/cm2, showing no tendency of saturation. This suggests that the BCC VO2 rods could be applied as possible electron emission materials. In conclusion, we found that VO2 could crystallize into a simple BCC structure when synthesized as nanorods, which was not observed before for the vanadium oxides family. The rods are
Enhancement Factor (β)
550
10 800 100
150
200 Distance (μm)
250
300
Fig. 3. (a) The field emission current density; (b) the corresponding F–N plots; and (c) the field-enhancing factor (b) and turn-on field for BCC VO2 rods measured at different electrode distances.
single crystalline and exhibited good field emission property, and might be used as electron emission materials in devices. The authors are grateful to the financial support by the National Natural Science Foundation of China (10675070, 10575061), and the National Basic Research Program of China (973 program, 2007CB936601).
ARTICLE IN PRESS Y. Wang, Z. Zhang / Physica E 41 (2009) 548–551
References [1] [2] [3] [4] [5] [6]
M. Abbate, Brazilian J. Phys. 24 (1994) 785. D. Adler, Rev. Mod. Phys. 40 (1968) 714. R. Heckingbottom, J.W. Linnett, Nature 194 (1962) 678. G.S. Nadkarni, V.S. Shirodkar, Thin Solid Films 105 (1983) 115. S. Nishio, M. Kakahana, Chem. Mater. 14 (2002) 3730. C. Julien, J.P. Guesdon, A. Gorenstein, A. Khelfa, I. Ivanov, Appl. Surf. Sci. 90 (1995) 389. [7] C.V. Ramana, O.M. Hussain, B.S. Naidu, P.J. Reddy, Thin Solid Films 305 (1997) 219. [8] N.T.B. Bay, P.M. Tien, S. Badilescu, Y. Djaoued, G. Bader, F.E. Girouard, V. Truong, L. Nguyen, J. Appl. Phys. 80 (1996) 7041.
551
[9] D. Barreca, L. Armelao, F. Caccavale, V.D. Noto, A. Gregori, G.A. Rizzi, E. Tondello, Chem. Mater. 12 (2000) 98. [10] H.L.M. Chang, H. You, J. Guo, D.J. Lain, J. Appl. Surf. Sci. 48–49 (1991) 12. [11] A. Talledo, C.G. Granqvist, J. Appl. Phys. 77 (1995) 4655. [12] A.Z. Moshfegh, A. Ignatiev, Thin Solid Films 198 (1991) 251. [13] J.G. Liu, Y. Zhao, Z.J. Zhang, J. Phys. Condens. Matter 15 (2003) L453. [14] J.G. Liu, Z.J. Zhang, Y. Zhao, X. Su, S. Liu, E.G. Wang, Small 1 (2005) 310. [15] Z.J. Zhang, Y. Zhao, M.M. Zhu, Appl. Phys. Lett. 88 (2006) 033101. [16] F. Galle´a, Z.C. Li, Z.J. Zhang, Appl. Phys. Lett. 89 (2006) 193111. [17] J. Mendialduaa, R. Casanovaa, Y. Barbauxb, J. Electron Spectros. Relat. Phenom. 71 (1995) 249. [18] G.A. Khan, C.A. Hogarth, J. Mater. Sci. 26 (1991) 597. [19] R.H. Fowler, L.W. Nordheim, Proc. R. Soc. London Ser. A 119 (1928) 173.
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Synthesis and field emission property of VO2 nanorods with a body-centered-cubic structure Yuquan Wang, Zhengjun Zhang Advanced Materials Laboratory, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 20 August 2008 Accepted 15 October 2008 Available online 1 November 2008
Films consisting of vertically aligned VO2 nanorods were prepared on planar silicon substrate by thermally heating a sheet of vanadium in a rough vacuum. These nanorods were found to be of a bodycentered-cubic (BCC) structure with a lattice constant of 0.94 nm, which was not observed before for VO2. Due to their sharp tip of the nanometer scale, the BCC VO2 nanorods exhibited excellent field emission properties, which make them possible candidate materials for applications in field emission devices. & 2008 Elsevier B.V. All rights reserved.
PACS: 61.46.Km 79.70.+q 68.55.Nq Keywords: Nanorods VO2 Field emission
Vanadium oxides have aroused tremendous research interest in past years because of their excellent chemical and physical properties that are attractive to optical and electrical applications. For example, V2O3 is a paramagnetic metallic phase of corundum structure (Al2O3), and is an antiferromagnetic insulator of monoclinic structure at temperatures above or below 165 K, respectively [1,2]. V2O5 is a diamagnetic insulator with an orthorhombic structure and is an essential ingredient to heterogeneous catalysis widely used in chemical reactions [1]. VO2 is an interesting narrow band gap (i.e. 0.7 eV) semiconductor, which undergoes a semiconductor-to-metal phase transition (i.e. from monoclinic to tetragonal structure) at 68 1C, leading to giant drops in its electrical resistance and transmittance of infrared [1,3]. This makes VO2 a promising candidate material for applications in fast-switching, electrochromic and other applications [3]. Therefore, it is of great interest to investigate the possibility and properties of VO2 to crystallize into other forms, especially at the nanometer scale. Films of vanadium oxides can be prepared by various techniques, such as physical vapor deposition [4–7], sol–gel growth method [8], chemical vapor deposition [9,10], sputtering deposition [11,12], etc. An alternative approach to synthesize metal oxides is the thermal oxidation process, which has been
Corresponding author. Tel.: +86 10 62797033; fax: +86 10 62771160.
E-mail address: [email protected] (Z. Zhang). 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.10.006
proven powerful to synthesize large-scale nanostructures of metal oxides [13–16]. The advantage of this approach is its simplicity and relatively low substrate temperatures [13,15]. We thus employed this method in the present study to synthesize VO2 films. We report, in this letter, the synthesis of films of large-scale arrays of VO2 nanorods by a simple thermal oxidation method, and the observation that VO2 could crystallize into a bodycentered-cubic (BCC) structure, which was not observed before. The composition, chemical bonding and field emission properties of the BCC VO2 nanorods were also reported. A pure (99.99%) vanadium sheet of 5 mm 50 mm 0.7 mm in dimension was cleaned sequentially in acetone, alcohol and deionized water baths supersonically, and was connected to two copper electrodes in a vacuum chamber. The substrates used in this study were (0 0 1)-oriented silicon wafers. These were also cleaned supersonically in baths of acetone, alcohol and de-ionized water in sequence, and were put on a substrate holder 4 cm below the vanadium sheet. The chamber was first pumped down to a vacuum on the level of 5 102 Torr by a rotary pump, and then a voltage of 2 V was applied to the two electrodes, resulting in a current of 65 A passing through the vanadium sheet. The current heated up rapidly the sheet in this rough vacuum, leading to the deposition of vanadium oxides on the silicon substrate below the vanadium sheet. During the typical deposition time of 15 min, the maximum temperature of the vanadium sheet and the silicon substrate was measured to be 900 and 350 1C,
ARTICLE IN PRESS Y. Wang, Z. Zhang / Physica E 41 (2009) 548–551
549
respectively. After deposition, the growth morphology, structure and composition of the deposits were examined with scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area diffraction (SAD), electron energy loss spectrometer (EELS), X-ray and ultraviolet photoelectron spectrometer (XPS, UPS) and Rutherfold backscattered spectrometer (RBS). The field emission property of the deposits was also evaluated. Fig. 1(a) and (b) show SEM micrographs of the vanadium oxide deposited on the silicon substrate at the above conditions. The SEM images were taken with a JSM-4800 SEM working at 20 kV. One sees that after 15-min deposition a continuous film was formed on the substrate, which consisted of densely packed vertically aligned rods. The rods are 6 mm long, cone-shaped and have nanometer-size sharp tips, see Fig. 1(b). To determine the phase structure of these rods, we have carried out X-ray diffraction (XRD) of the films with a Rigaku X-ray diffractometer using Cu Ka radiation. However, the XRD pattern (not shown) showed a strong texture growth of the rods, and that peaks in the pattern did not match any known phases of vanadium oxides. We thus used SAD analysis of the rods to identify their phase structure. To determine the structure, we have done SAD analysis for over 20 rods, and for each rod we obtained three SAD patterns along different axes. By carefully indexing SAD patterns for each rod, we finally found that the rods are of a BCC structure with a lattice parameter of 0.94 nm, which was not observed previously for vanadium oxides. As an example, Fig. 1(c) shows a typical brightfield TEM image of the rods. Fig. 1(d) and (e) shows SAD patterns of the rod, taken along the axes of [1 0 0] and [3 1 0], respectively. Fig. 1(f) shows an HRTEM image of the rod corresponding to
Fig. 1. (a) A low-magnification and (b) a close-up SEM images of the BCC VO2 rods deposited on silicon substrates; (c) a bright-field TEM image and SAD patterns along the axis of (d) [1 0 0] and (e) [3 1 0] of the rods, respectively; and (f) an HRTEM image of the rods corresponding to (d).
5k
100.0k
Experimental V-L3
80.0k
Yield (counts)
V-L2
Yield (counts)
Simulated
4k
60.0k
40.0k
O-K
3k
2k
1k
20.0k
0 480
520 540 Energy (ev)
580
560
580
100
0
200 300 Channel
400
500
530.2
4000
2×105
(O1s)
Yield (counts)
Intensity
V2p3/2 516.9
3000 V2p1/2 524.1
2000
4.81 eV
21.22 eV
0
1000 540
1×105
535
530 525 520 Binding Energy (eV)
515
510
0
5
10 15 20 Binding Energy (eV)
Fig. 2. The (a) EELS, (b) RBS, (c) XPS and (d) UPS spectrum of the BCC VO2 rods, respectively.
25
30
ARTICLE IN PRESS Y. Wang, Z. Zhang / Physica E 41 (2009) 548–551
Emission Current Density (mA/cm2)
6 200
300 250
5
150
100
4 3 2 1 0 8
10
12
14 16 Field (V/μm)
18
20
-30.0 100 µm 150 µm
In(J/E2) ((A/m2)/(V/m)2)
200 µm
-30.5
250 µm 300 µm
-31.0
-31.5 6.0×10-8
7.0×10-8
8.0×10-8 9.0×10-8 1/E(m/V)
1.0×10-7
1.1×10-7
14 1400 β Turn-on Field
1200
12
1000
Turn-on Field (V/μm)
Fig. 1(d). The images and SAD patterns were taken with a JEM-2010F TEM working at 200 kV. It is seen that the rod is several hundred nanometers in diameter, and has a sharp tip at a size of several tens of nanometers, as shown also by Fig. 1(a)–(b). One sees that the BCC-structured rods are single crystalline, and were grown along the /0 0 2S direction. The composition of the rods was estimated by EELS analysis in a JEM-2010F TEM working at 200 kV. Fig. 2(a) shows a typical EELS spectrum of the BCC rods. It gave an estimation of 1:2 for the atomic ratio of V/O. This analysis has been done for more than 20 rods, and all gave an estimation of the ratio close to 1:2. This suggests that the BCC rods are VO2. Fig. 2(b) shows an RBS random spectrum of the films consisting of BCC rods with a 1.7 MeV He+ beam. One sees that the simulated spectrum using a composition of VO2 matched pretty well with the experimental curve in almost the whole depth range. The chemical state of vanadium atoms in the rods was also examined with XPS analysis. Fig. 2(c) shows an XPS spectrum of the BCC rods, obtained with a PHI Quantera SXM spectrometer using Al Ka (hn ¼ 1486.6 eV) as the excitation source, from a ‘‘clean’’ surface of the rods (sputtered with 4 keV Ar+). One sees from the figure that peaks of V2p3/2 and V2p1/2 of the BCC rods were at 516.9 and 524.1 eV, respectively, indicating an oxidation state of +4 for vanadium [17,18]. It is thus concluded that the vanadium oxide rods produced here are VO2 with a BCC structure that was not observed before. Since the BCC VO2 rods produced here have very fine tips (tens nanometers), they might be applied as electron source materials. For this purpose, the work function of the rods was measured by UPS analysis. Fig. 2(d) shows a typical UPS spectrum of BCC VO2 rods, obtained by a Kratos Axis Ultra electron spectrometer using He (I) ultraviolet light (21.22 eV) as the excitation source. The work function of the rods was estimated to be 4.81 eV. The field emission property of the BCC VO2 rods was evaluated using a Keithley 485 electrometer in a chamber with a vacuum level of 6 107 Pa, at different electrode distances. Fig. 3(a) shows the emission current from films (area of 0.1 cm2) of BCC VO2 rods as a function of the voltage applied, at electrode distances of 100, 150, 200, 250 and 300 mm, respectively. It is seen that the emission current increased rapidly when the applied voltage went up. For instance, at a distance of 100 mm, the current density was 1 mA/cm2 at a voltage of 1.6 kV, while at a voltage of 2 kV, it was increased to 8 mA/cm2 and was not saturated. This emission could be well fitted by the Fowler–Nordheim (F–N) formula of J ¼ (E2b2/f)exp(Bf3/2/Eb), where J is the current density, E is the applied field, f is the work function of the materials, B is a constant of 6.83 109 (eV3/2 V m1) and b is the field-enhancing factor [19]. Fig. 3(b) shows the F–N plots for the films measured at various electrode distances. The good linearity of these plots indicates that electrons were indeed driven by the field emission effect [19]. Using the work function of 4.81 eV measured by the UPS analysis (see Fig. 2(d)), the field-enhancing factor b was calculated from the F–N plots and was plotted in Fig. 3(c) as a function of electrode distance. In addition, the turn-on field (defined as the electrical field to get an emission current density of 10 mA/cm2) for different electrode distances was also derived and added in Fig. 3(c). One sees from the figure that the turn-on field of the films decreased with increasing the electrode distance, e.g., from 13 V/mm at 100 mm to 10 V/mm at 200 mm; meanwhile, the field-enhancing factor b increased from 850 to 1400, respectively. At all distances the field emission current density could reach as high as 5 mA/cm2, showing no tendency of saturation. This suggests that the BCC VO2 rods could be applied as possible electron emission materials. In conclusion, we found that VO2 could crystallize into a simple BCC structure when synthesized as nanorods, which was not observed before for the vanadium oxides family. The rods are
Enhancement Factor (β)
550
10 800 100
150
200 Distance (μm)
250
300
Fig. 3. (a) The field emission current density; (b) the corresponding F–N plots; and (c) the field-enhancing factor (b) and turn-on field for BCC VO2 rods measured at different electrode distances.
single crystalline and exhibited good field emission property, and might be used as electron emission materials in devices. The authors are grateful to the financial support by the National Natural Science Foundation of China (10675070, 10575061), and the National Basic Research Program of China (973 program, 2007CB936601).
ARTICLE IN PRESS Y. Wang, Z. Zhang / Physica E 41 (2009) 548–551
References [1] [2] [3] [4] [5] [6]
M. Abbate, Brazilian J. Phys. 24 (1994) 785. D. Adler, Rev. Mod. Phys. 40 (1968) 714. R. Heckingbottom, J.W. Linnett, Nature 194 (1962) 678. G.S. Nadkarni, V.S. Shirodkar, Thin Solid Films 105 (1983) 115. S. Nishio, M. Kakahana, Chem. Mater. 14 (2002) 3730. C. Julien, J.P. Guesdon, A. Gorenstein, A. Khelfa, I. Ivanov, Appl. Surf. Sci. 90 (1995) 389. [7] C.V. Ramana, O.M. Hussain, B.S. Naidu, P.J. Reddy, Thin Solid Films 305 (1997) 219. [8] N.T.B. Bay, P.M. Tien, S. Badilescu, Y. Djaoued, G. Bader, F.E. Girouard, V. Truong, L. Nguyen, J. Appl. Phys. 80 (1996) 7041.
551
[9] D. Barreca, L. Armelao, F. Caccavale, V.D. Noto, A. Gregori, G.A. Rizzi, E. Tondello, Chem. Mater. 12 (2000) 98. [10] H.L.M. Chang, H. You, J. Guo, D.J. Lain, J. Appl. Surf. Sci. 48–49 (1991) 12. [11] A. Talledo, C.G. Granqvist, J. Appl. Phys. 77 (1995) 4655. [12] A.Z. Moshfegh, A. Ignatiev, Thin Solid Films 198 (1991) 251. [13] J.G. Liu, Y. Zhao, Z.J. Zhang, J. Phys. Condens. Matter 15 (2003) L453. [14] J.G. Liu, Z.J. Zhang, Y. Zhao, X. Su, S. Liu, E.G. Wang, Small 1 (2005) 310. [15] Z.J. Zhang, Y. Zhao, M.M. Zhu, Appl. Phys. Lett. 88 (2006) 033101. [16] F. Galle´a, Z.C. Li, Z.J. Zhang, Appl. Phys. Lett. 89 (2006) 193111. [17] J. Mendialduaa, R. Casanovaa, Y. Barbauxb, J. Electron Spectros. Relat. Phenom. 71 (1995) 249. [18] G.A. Khan, C.A. Hogarth, J. Mater. Sci. 26 (1991) 597. [19] R.H. Fowler, L.W. Nordheim, Proc. R. Soc. London Ser. A 119 (1928) 173.