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Characteristics of flat panel display using carbon nanotubes as electron emitters
Diamond and Related Materials 9 (2000) 1270–1274 www.elsevier.com/locate/diamond
Characteristics of flat panel display using carbon nanotubes as electron emitters J.L. Kwo a, Meiso Yokoyama a, W.C. Wang b, F.Y. Chuang b, I.N. Lin c, * a Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, 701 ROC b Electronic Research & Service Organization, ITRI, Hsinchu, Taiwan, 300 ROC c Materials Science Center, National Tsing-Hua University, Hsinchu, Taiwan, 300 ROC
Abstract The fabrication of carbon nanotube emitters with excellent emission properties is described. Carbon nanotube (CNT ) clusters synthesized by arc discharge were crushed, mixed with conductive pastes and then screen-printed. The measurements carried out using a diode structure reveal that the electron field emission of the as-grown CNT clusters can be turned on with a field as low as 1.45 V/mm and can attain a current density as large as 3 mA/cm2. By contrast, the electron field emission of the CNT thick films can be turned on at 1.34 V/mm, attaining a emission current density of 7 mA/cm2 at 4 V/mm. No significant degradation of these performances is observed with these electron emitters operated at 3 V/mm under a current density of 0.4 mA/cm2 for 1500 min. A flat panel display using CNT thick films as electron emitters, which possessed a brightness as high as 800 cd/m2 under an applied field of 5 V/mm, has been demonstrated. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Carbon nanotubes; Flat panel display
1. Introduction Carbon nanotubes grown on the cathode of an arc discharge were first observed by Iijima in 1991 [1], in which the needle-like tubes consist of coaxial tubes of graphite and range in size from 4 to 30 nm in diameter and (about 1 mm in length). The discovery of carbon nanotubes has been attracting considerable attention because of their own unique physical properties and their potential for a variety of applications [2–4]. Due to their geometrical properties, high aspect ratios, small tip radii of curvature, and conductivity, CNT exhibit excellent field emission characteristics. A high field emission current density of 10 mA/cm2 and a low turn-on electric field of 0.8 V/mm have been demonstrated [5,6 ]. The single-wall nanotubes (SWNT ) have a lower turn-on field, while the multi-wall nanotubes (MWNTs) exhibit a better lifetime [7]. Recently, Ise Electronic Corporation and Samsung Corporation have made an attempt to demonstrate the CRT-lighting-elements and prototype display using MWNT as cold emitters [8,9]. * Corresponding author. Tel.: +886-3-5742574; fax: +886-3-5716977. E-mail address: [email protected] (I.N. Lin)
These results indicate a great potential of carbon nanotubes for applications in vacuum fluorescent display ( VFD) technology, particularly in the field emission display ( FED). Nowadays, CNT can be synthesized by arc discharge [10,11], laser ablation [12], and chemical vapor deposition [13–15], etc. Among these techniques, the arcdischarge process is the more promising process to use with a screen-printing process, due to its simplicity and large production capacity. Therefore, it was adopted in this work for synthesizing the CNT material. The electron field emission measurements were carried out on both as-grown CNT clusters and screen-printed CNT thick films. A field emission display made of CNT thick films will be demonstrated.
2. Experimental The anode used in the d.c. arc discharge was a pure graphite rod 150 mm in length (9 mm in diameter), and the cathode was a graphite disc (25 mm in diameter, 5 mm in thickness) mounted on a water-cooled copper block. The chamber was maintained at 500 mbar in a helium atmosphere. The current source was a d.c. power
0925-9635/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 9 25 - 9 63 5 ( 9 9 ) 00 3 5 3- 2
J.L. Kwo et al. / Diamond and Related Materials 9 (2000) 1270–1274
supply operated in constant current mode, and the arc current was set to 150 A. The gap between the anode and cathode was kept at 2–3 mm, while the voltage applied across the two electrodes was 20–24 V. For the CNT-printing process, CNT slurry was made by crushing the CNT clusters in a ball mill for several hours with suitable binders, followed by screen printing of the slurry. A layer of silver paste was first printed on glass to serve as bottom electrode. The screen-printed CNT-thick films was heat-treated to remove the binder material and then etched by an Ar plasma generated by an ECR plasma etcher at a power of 100 W for several minutes. The morphology and structure of CNT clusters were examined using scanning electron microscopy (SEM ) and Raman spectroscopy. Field emission characteristics of the CNT clusters and CNT thick films were measured by a diode technique (under a vacuum of 10−7 Torr) using an electrometer. In this measurement, the anode plate, a glass plate coated with an ITO layer and a P22 phosphor, was separated from the cathode using 350 mm spacers. The electron field emission properties were analyzed using the Fowler–Nordheim model [16 ], viz.
A
I=aV2 exp −
B
bW3/2 e , V
where a and b are constants. The turn-on voltage was estimated as the voltage, at which the log(I/V2)−1/V curve deviates from the Fowler–Nordheim plot, and the effective work function (W =W/b) of the films was e calculated from the slope of the Fowler–Nordheim plot, where b is the field enhancement factor and W is the
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true work function of the materials. A prototype carbon nanotube field emission display (CNT FED) was fabricated. All the interfaces between the anode plate, spacer bar, and CNT emitter plate were sealed using glass frits, followed by thermal annealing to react the glass frits with the glass plates.
3. Results and discussion SEM examinations revealed that the CNT clusters are composed almost entirely of fibre bundles, with each bundle consisting of about 65% multi-wall nanotubes, several microns in length. The carbonaceous particles attached to the CNT could be removed easily by a posttreatment process (500°C in air). The proportion of CNT in the as-grown clusters varied considerably with the chamber’s parameters. For the purpose of optimizing the growth conditions, the electron field emission properties of the as-grown CNT clusters were examined directly using the diode measurement set-up. The CNT clusters were fixed to a metal substrate using Ag paste after polishing (using emery paper) to ensure that the two surfaces were plane parallel. The anodes, which were glass plates pre-coated with P22 red phosphor and an ITO layer, were separated from the cathodes using a 350 mm glass spacer. The electrical measurements were performed at 5×10−7 Torr. The typical electron emission characteristics and the corresponding Fowler– Nordheim plots of the CNT clusters grown using optimized conditions are illustrated in Fig. 1 and the insert, respectively, indicating that the turn-on field was around
Fig. 1. I–V characteristics of CNT clusters synthesized by arc discharge with 150 A arc current and 20 V arc voltage, under 500 mbar (He); the inset shows the corresponding Fowler–Nordheim plot.
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1.5 V/mm and the effective work function was around 0.20 eV. The emission current density of the CNT clusters changed markedly with the growth conditions, implying that the proportion of the CNT in the clusters varied pronouncedly with growth parameters such as arc current, arc voltage and helium pressure. The emission current density obtained was about 2 mA/cm2 under a 3 V/mm applied field and was around 7.3 mA/cm2 under a 5.0 V/mm applied field (not shown). The brightness attained was as high as 700 cd/m2 in the latter case.
The CNT clusters were then crushed and mixed with conductive pastes to form slurries. This was followed by a screen-printing process to form CNT thick films. The deposited CNT thick films were next cured at 500°C in air for 1 h. To improve the electron emission properties of the CNT thick films, a surface treatment for removing the fillers (or particles) and exposing the CNT on the films’ surfaces was necessary. As shown in Fig. 2, the microwave plasma etching process markedly modified the electron emission characteristics of CNT thick
Fig. 2. I–V characteristics of CNT thick films, which were plasma-etched for 30 or 600 s, with the inset showing the corresponding Fowler–Nordheim plot (the turn-on fields and the effective work functions are: E =1.30 V/mm, w =0.22 eV for no-etched films; E =1.40 V/mm, w =0.21 eV for 30 s 0 e 0 e etched films; and E =1.30 V/mm, w =0.21 eV for 300 s etched films). 0 e
Fig. 3. I–V characteristics of several CNT thick films screen-printed using the same paste; the inset shows the stability of emission current operated at constant electric field of 3 V/mm.
J.L. Kwo et al. / Diamond and Related Materials 9 (2000) 1270–1274
films. Surface treatment under a 100 W Ar plasma for 30 s on the CNT thick films increased their emission current density and lowered the turn-on electric field. However, the emission current density decreased dramatically for the films etched for 300 s, which was attributed to the over-etching of the CNT embedded in filler. The electron emission characteristics of CNT-thick films prepared by screen-printing technique were similar to those of the CNT clusters grown by arc discharge. Fig. 3 shows the electron emission properties of CNT thick films prepared by the same printing process, using the same pastes. This figure indicates that the deviation in these properties is small. The electric fields necessary to induce an emission current density equivalent to Je=7 mA/cm2 are in the range of E =3.5−4.5 V/mm. a The inset in Fig. 3 illustrates that no significant degradation of the field emission capacity for the CNT thick film was observed when the films were operated continuously at 3 V/mm (Je=0.4 mA/cm2) for up to 1500 min. The results described above imply that the CNT possess marvelous electron field emission characteristics,
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irrespective of whether they are in the form of CNT clusters or CNT thick films. The feasibility of using CNT as electron emitters is further demonstrated by a structure shown in Fig. 4a, which is a diode structure, with 350 mm spacers used to separate the phosphor screen from the CNT thick film emitter. The emission image of such a 4 inch fully sealed CNT FED panel, containing red, green and blue P22 phosphor, is shown in Fig. 4b. The measured brightness of 800 cd/m2 was achieved at E=5 V/mm, using an addressing scheme with a 1/60 duty cycle. The CNT FED panel has subsequently been operated for more than 500 h without any significant degradation.
4. Summary The electron field emission properties of CNT materials synthesized by the arc-discharge technique were evaluated. Both CNT clusters and CNT thick films were found to show excellent electron emission characteristics.
(a)
(b) Fig. 4. (a) Schematics of the structure of a 4 inch fully sealed CNT field emission display and (b) the emission image of the CNT-FED with red, green and blue P22 phosphors, operated at 5 V/mm applied electric field.
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The electron field emission can be turned on at a field as low as 1.50 V/mm and attains a current density as large as 7.3 mA/cm2. A luminance of approximately 700 cd/m2 can be obtained at an applied field of 5 V/mm. No significant degradation of emission current was observed with these electron emitters up to 1500 min of continuous operation. The 4 inch fully sealed CNT-FED panel has been demonstrated and operated for 500 h without any significant degradation.
References [1] [2] [3] [4]
S. Iijima, Nature 354 (1991) 56. S. Susumu, Science V278 (1997) 77. P.G. Collins, A. Zettle, R.E. Smalley, Science 278 (1997) 100. W.A. de Haas, A. Chatelain, D. Ugarte, Science 270 (1995) 1179.
[5] J.M. Bonard, J.P. Salvetat, T. Sto¨ckli et al., Appl. Phys. Lett. 73 (1998) 918. [6 ] Q.H. Wang, T..D. Corrigan, J.Y. Dai, R.P.H. Chang, Appl. Phys. Lett. 70 (1997) 3308. [7] P.G. Collins, A. Zettl, Appl. Phys. Lett. 69 (1997) 1969. [8] S. Uemura, T. Nagasako, J. Yotani, T. Shimojo, in: SID 98 DIGEST (1998) 1052. [9] W.B. Choi, D.S. Chung, S.H. Park, J.M. Kim, in: SID 99 DIGEST (1999) 1134. [10] J.M. Lauerhaas, J.Y. Dai, A.A. Setlur, R.P.H. Chang, J. Mater. Res. 12 (1997) 1536. [11] P.M. Ajayan, Ph. Redlich, M. Ruhle, J. Mater. Res. 12 (1997) 244. [12] A.G. Rinzler, J. Liu, R.E. Smalley, Appl. Phys. A 67 (1998) 29. [13] J.M. Mao, L.F. Sun, L.X. Qian, Z.W. Pan, B.H. Chang, W.Y. Zhou, G. Wang, S.S. Xie, Appl. Phys. Lett. 72 (1998) 3297. [14] G. Che, B.B. Lakshmi, E.R. Fisher, Chem. Mater. 10 (1998) 260. [15] H. Jantoljak, J.P. Salvetat, C. Thomsen, Appl. Phys. A 67 (1998) 113. [16 ] A. Vander Ziel, in: Solid State Physical Electronics, Prentice-Hall, Englewood Cliffs, NJ, 1968, p. 144.
Characteristics of flat panel display using carbon nanotubes as electron emitters J.L. Kwo a, Meiso Yokoyama a, W.C. Wang b, F.Y. Chuang b, I.N. Lin c, * a Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, 701 ROC b Electronic Research & Service Organization, ITRI, Hsinchu, Taiwan, 300 ROC c Materials Science Center, National Tsing-Hua University, Hsinchu, Taiwan, 300 ROC
Abstract The fabrication of carbon nanotube emitters with excellent emission properties is described. Carbon nanotube (CNT ) clusters synthesized by arc discharge were crushed, mixed with conductive pastes and then screen-printed. The measurements carried out using a diode structure reveal that the electron field emission of the as-grown CNT clusters can be turned on with a field as low as 1.45 V/mm and can attain a current density as large as 3 mA/cm2. By contrast, the electron field emission of the CNT thick films can be turned on at 1.34 V/mm, attaining a emission current density of 7 mA/cm2 at 4 V/mm. No significant degradation of these performances is observed with these electron emitters operated at 3 V/mm under a current density of 0.4 mA/cm2 for 1500 min. A flat panel display using CNT thick films as electron emitters, which possessed a brightness as high as 800 cd/m2 under an applied field of 5 V/mm, has been demonstrated. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Carbon nanotubes; Flat panel display
1. Introduction Carbon nanotubes grown on the cathode of an arc discharge were first observed by Iijima in 1991 [1], in which the needle-like tubes consist of coaxial tubes of graphite and range in size from 4 to 30 nm in diameter and (about 1 mm in length). The discovery of carbon nanotubes has been attracting considerable attention because of their own unique physical properties and their potential for a variety of applications [2–4]. Due to their geometrical properties, high aspect ratios, small tip radii of curvature, and conductivity, CNT exhibit excellent field emission characteristics. A high field emission current density of 10 mA/cm2 and a low turn-on electric field of 0.8 V/mm have been demonstrated [5,6 ]. The single-wall nanotubes (SWNT ) have a lower turn-on field, while the multi-wall nanotubes (MWNTs) exhibit a better lifetime [7]. Recently, Ise Electronic Corporation and Samsung Corporation have made an attempt to demonstrate the CRT-lighting-elements and prototype display using MWNT as cold emitters [8,9]. * Corresponding author. Tel.: +886-3-5742574; fax: +886-3-5716977. E-mail address: [email protected] (I.N. Lin)
These results indicate a great potential of carbon nanotubes for applications in vacuum fluorescent display ( VFD) technology, particularly in the field emission display ( FED). Nowadays, CNT can be synthesized by arc discharge [10,11], laser ablation [12], and chemical vapor deposition [13–15], etc. Among these techniques, the arcdischarge process is the more promising process to use with a screen-printing process, due to its simplicity and large production capacity. Therefore, it was adopted in this work for synthesizing the CNT material. The electron field emission measurements were carried out on both as-grown CNT clusters and screen-printed CNT thick films. A field emission display made of CNT thick films will be demonstrated.
2. Experimental The anode used in the d.c. arc discharge was a pure graphite rod 150 mm in length (9 mm in diameter), and the cathode was a graphite disc (25 mm in diameter, 5 mm in thickness) mounted on a water-cooled copper block. The chamber was maintained at 500 mbar in a helium atmosphere. The current source was a d.c. power
0925-9635/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 9 25 - 9 63 5 ( 9 9 ) 00 3 5 3- 2
J.L. Kwo et al. / Diamond and Related Materials 9 (2000) 1270–1274
supply operated in constant current mode, and the arc current was set to 150 A. The gap between the anode and cathode was kept at 2–3 mm, while the voltage applied across the two electrodes was 20–24 V. For the CNT-printing process, CNT slurry was made by crushing the CNT clusters in a ball mill for several hours with suitable binders, followed by screen printing of the slurry. A layer of silver paste was first printed on glass to serve as bottom electrode. The screen-printed CNT-thick films was heat-treated to remove the binder material and then etched by an Ar plasma generated by an ECR plasma etcher at a power of 100 W for several minutes. The morphology and structure of CNT clusters were examined using scanning electron microscopy (SEM ) and Raman spectroscopy. Field emission characteristics of the CNT clusters and CNT thick films were measured by a diode technique (under a vacuum of 10−7 Torr) using an electrometer. In this measurement, the anode plate, a glass plate coated with an ITO layer and a P22 phosphor, was separated from the cathode using 350 mm spacers. The electron field emission properties were analyzed using the Fowler–Nordheim model [16 ], viz.
A
I=aV2 exp −
B
bW3/2 e , V
where a and b are constants. The turn-on voltage was estimated as the voltage, at which the log(I/V2)−1/V curve deviates from the Fowler–Nordheim plot, and the effective work function (W =W/b) of the films was e calculated from the slope of the Fowler–Nordheim plot, where b is the field enhancement factor and W is the
1271
true work function of the materials. A prototype carbon nanotube field emission display (CNT FED) was fabricated. All the interfaces between the anode plate, spacer bar, and CNT emitter plate were sealed using glass frits, followed by thermal annealing to react the glass frits with the glass plates.
3. Results and discussion SEM examinations revealed that the CNT clusters are composed almost entirely of fibre bundles, with each bundle consisting of about 65% multi-wall nanotubes, several microns in length. The carbonaceous particles attached to the CNT could be removed easily by a posttreatment process (500°C in air). The proportion of CNT in the as-grown clusters varied considerably with the chamber’s parameters. For the purpose of optimizing the growth conditions, the electron field emission properties of the as-grown CNT clusters were examined directly using the diode measurement set-up. The CNT clusters were fixed to a metal substrate using Ag paste after polishing (using emery paper) to ensure that the two surfaces were plane parallel. The anodes, which were glass plates pre-coated with P22 red phosphor and an ITO layer, were separated from the cathodes using a 350 mm glass spacer. The electrical measurements were performed at 5×10−7 Torr. The typical electron emission characteristics and the corresponding Fowler– Nordheim plots of the CNT clusters grown using optimized conditions are illustrated in Fig. 1 and the insert, respectively, indicating that the turn-on field was around
Fig. 1. I–V characteristics of CNT clusters synthesized by arc discharge with 150 A arc current and 20 V arc voltage, under 500 mbar (He); the inset shows the corresponding Fowler–Nordheim plot.
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1.5 V/mm and the effective work function was around 0.20 eV. The emission current density of the CNT clusters changed markedly with the growth conditions, implying that the proportion of the CNT in the clusters varied pronouncedly with growth parameters such as arc current, arc voltage and helium pressure. The emission current density obtained was about 2 mA/cm2 under a 3 V/mm applied field and was around 7.3 mA/cm2 under a 5.0 V/mm applied field (not shown). The brightness attained was as high as 700 cd/m2 in the latter case.
The CNT clusters were then crushed and mixed with conductive pastes to form slurries. This was followed by a screen-printing process to form CNT thick films. The deposited CNT thick films were next cured at 500°C in air for 1 h. To improve the electron emission properties of the CNT thick films, a surface treatment for removing the fillers (or particles) and exposing the CNT on the films’ surfaces was necessary. As shown in Fig. 2, the microwave plasma etching process markedly modified the electron emission characteristics of CNT thick
Fig. 2. I–V characteristics of CNT thick films, which were plasma-etched for 30 or 600 s, with the inset showing the corresponding Fowler–Nordheim plot (the turn-on fields and the effective work functions are: E =1.30 V/mm, w =0.22 eV for no-etched films; E =1.40 V/mm, w =0.21 eV for 30 s 0 e 0 e etched films; and E =1.30 V/mm, w =0.21 eV for 300 s etched films). 0 e
Fig. 3. I–V characteristics of several CNT thick films screen-printed using the same paste; the inset shows the stability of emission current operated at constant electric field of 3 V/mm.
J.L. Kwo et al. / Diamond and Related Materials 9 (2000) 1270–1274
films. Surface treatment under a 100 W Ar plasma for 30 s on the CNT thick films increased their emission current density and lowered the turn-on electric field. However, the emission current density decreased dramatically for the films etched for 300 s, which was attributed to the over-etching of the CNT embedded in filler. The electron emission characteristics of CNT-thick films prepared by screen-printing technique were similar to those of the CNT clusters grown by arc discharge. Fig. 3 shows the electron emission properties of CNT thick films prepared by the same printing process, using the same pastes. This figure indicates that the deviation in these properties is small. The electric fields necessary to induce an emission current density equivalent to Je=7 mA/cm2 are in the range of E =3.5−4.5 V/mm. a The inset in Fig. 3 illustrates that no significant degradation of the field emission capacity for the CNT thick film was observed when the films were operated continuously at 3 V/mm (Je=0.4 mA/cm2) for up to 1500 min. The results described above imply that the CNT possess marvelous electron field emission characteristics,
1273
irrespective of whether they are in the form of CNT clusters or CNT thick films. The feasibility of using CNT as electron emitters is further demonstrated by a structure shown in Fig. 4a, which is a diode structure, with 350 mm spacers used to separate the phosphor screen from the CNT thick film emitter. The emission image of such a 4 inch fully sealed CNT FED panel, containing red, green and blue P22 phosphor, is shown in Fig. 4b. The measured brightness of 800 cd/m2 was achieved at E=5 V/mm, using an addressing scheme with a 1/60 duty cycle. The CNT FED panel has subsequently been operated for more than 500 h without any significant degradation.
4. Summary The electron field emission properties of CNT materials synthesized by the arc-discharge technique were evaluated. Both CNT clusters and CNT thick films were found to show excellent electron emission characteristics.
(a)
(b) Fig. 4. (a) Schematics of the structure of a 4 inch fully sealed CNT field emission display and (b) the emission image of the CNT-FED with red, green and blue P22 phosphors, operated at 5 V/mm applied electric field.
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J.L. Kwo et al. / Diamond and Related Materials 9 (2000) 1270–1274
The electron field emission can be turned on at a field as low as 1.50 V/mm and attains a current density as large as 7.3 mA/cm2. A luminance of approximately 700 cd/m2 can be obtained at an applied field of 5 V/mm. No significant degradation of emission current was observed with these electron emitters up to 1500 min of continuous operation. The 4 inch fully sealed CNT-FED panel has been demonstrated and operated for 500 h without any significant degradation.
References [1] [2] [3] [4]
S. Iijima, Nature 354 (1991) 56. S. Susumu, Science V278 (1997) 77. P.G. Collins, A. Zettle, R.E. Smalley, Science 278 (1997) 100. W.A. de Haas, A. Chatelain, D. Ugarte, Science 270 (1995) 1179.
[5] J.M. Bonard, J.P. Salvetat, T. Sto¨ckli et al., Appl. Phys. Lett. 73 (1998) 918. [6 ] Q.H. Wang, T..D. Corrigan, J.Y. Dai, R.P.H. Chang, Appl. Phys. Lett. 70 (1997) 3308. [7] P.G. Collins, A. Zettl, Appl. Phys. Lett. 69 (1997) 1969. [8] S. Uemura, T. Nagasako, J. Yotani, T. Shimojo, in: SID 98 DIGEST (1998) 1052. [9] W.B. Choi, D.S. Chung, S.H. Park, J.M. Kim, in: SID 99 DIGEST (1999) 1134. [10] J.M. Lauerhaas, J.Y. Dai, A.A. Setlur, R.P.H. Chang, J. Mater. Res. 12 (1997) 1536. [11] P.M. Ajayan, Ph. Redlich, M. Ruhle, J. Mater. Res. 12 (1997) 244. [12] A.G. Rinzler, J. Liu, R.E. Smalley, Appl. Phys. A 67 (1998) 29. [13] J.M. Mao, L.F. Sun, L.X. Qian, Z.W. Pan, B.H. Chang, W.Y. Zhou, G. Wang, S.S. Xie, Appl. Phys. Lett. 72 (1998) 3297. [14] G. Che, B.B. Lakshmi, E.R. Fisher, Chem. Mater. 10 (1998) 260. [15] H. Jantoljak, J.P. Salvetat, C. Thomsen, Appl. Phys. A 67 (1998) 113. [16 ] A. Vander Ziel, in: Solid State Physical Electronics, Prentice-Hall, Englewood Cliffs, NJ, 1968, p. 144.