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Carbon Nanotubes Cathode of Field Emission Lamp Prepared by Electrophoretic Deposition
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Energy Procedia
Energy Procedia 00 (2011) Energy Procedia 16000–000 (2012) 240 – 243 www.elsevier.com/locate/procedia
2012 International Conference on Future Energy, Environment, and Materials
Carbon Nanotubes Cathode of Field Emission Lamp Prepared by Electrophoretic Deposition Gang Chena，Lan Zhangb , Huizhong Mab , Ning Yaoa, Binglin Zhanga,a* b
a Department of Physics, Zhengzhou University, Zhengzhou 450052, China Department of Engineering Mechanics, Zhengzhou University, Zhengzhou 450001
Abstract Field emission lamp is a kind of green and low energy loss light source. Carbon nanotube cathode of field emission lamp was prepared by electrophoretic deposition. Two types of solute salt were added in electrophoretic solution separately, which were used to prepare two types of carbon nanotubes films on identical aluminum sheets by electrophoretic deposition method. Scanning electron microscopy(SEM) were used to observe the surface morphology of the films. The field emission properties of the films were tested by using a diode structure. The turnon field of the film deposited with CsNO3 / Ca(NO3)2·4H2O solution was 0.7V/μm. The current density of the film was about 4000mA/cm2 at an electric field of 1.8V/μm. While corresponding data by using Mg(NO3)2·6H2O solution was 0.8V/μm and 3800mA/cm2. It is concluded that the field emission properties of the deposited film cathode by using CsNO3/ Ca(NO3)2·4H2O solution better than that using Mg(NO3)2·6H2O solution. © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society.
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer]
Open access under CC BY-NC-ND license.
Keywords: Field emission; Carbon Nanotubes; Electrophoretic Deposition
1. Introduction Carbon nanotubes (CNTs) films have attracted much attention for their unique characteristics, such as nanometer scale, high aspect ratio, superior mechanical strength, good conductance and high chemical stability[1,2]. All of these properties make them possess stable emission, long life times, and low emission threshold potentials suitable for field electron emission[3,4], and can be applied to field emission displays (FEDs)[5,6], and field emission lamp to be a cold cathode. Especially, the field emission lamp with green
* Corresponding author. E-mail address: [email protected].
1876-6102 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society . Open access under CC BY-NC-ND license. doi:10.1016/j.egypro.2012.01.040
GangChen, ChenLan et al. / Energy Procedia 16 (2012) 240 –000–000 243 Gang Zhang/ Energy Procedia 00 (2011)
and low energy loss property have bright future, and the emission cold cathode is the most important part in the field emission lamp. Electrophoretic deposition (EPD)[6,7] is suitable for the formation of the cathode layer due to the advantages of short formation time, little restriction in the shape of substrates, simple deposition apparatus and low-cost manufacturing. With the method a large sized nanotube film can be fabricated at room temperature. In the EPD process, the presence of salts solution can play an important role in increasing the deposition rate and in improving the adhesion of CNTs to substrates. Thus select a type of suitable solution salt can improve the property of field emission . Mg(NO3)2·6H2O was a traditional salt to be used in electrophoretic deposition [6,8]. In this paper, CsNO3 with Ca(NO3)2·4H2O together were also used to be electrophoretic salts because cesium has low work function . It can probably lower the turn-on field of the emission for improving the field emission properties. The field electron emission properties of the prepared films by using the two types of salt solution prepared films were investigated , respectively. 2. Experiments In this work, CNTs prepared by chemical vapor deposition technique were used. They tend to form aggregates due to their high van der Waals interactions and highly entangled structure. To form uniform emission sites, well-distributed CNTs film were required. CNTs aggregates were grinded into smaller units first, then dispersed in two beakers, each contains certain volume IPA solvent. After that salt Mg(NO3)2·6H2O, CsNO3 with Ca(NO3)2·4H2O were added into each beaker, respectively, to charge the CNTs aggregates positively and to form adhesive material. Then stirred for 24h to form stabilized suspension. During the EPD process, carbon nanotubes were deposited onto an aluminium sheet cathode at room temperature. While a stainless steel was used as anode. The morphology of the two types of CNTs-deposited films were studied by scanning electron microscopy (SEM). Field electron emission properties were measured by a diode type structure in a vacuum chamber. The two electrodes were separated by a 500μm insulating sheet, and the measured field electron emission area was about 1.0 cm2. The deposited CNTs films were used as cathode emitter, and a piece of indium tin oxide (ITO) glass coated with phosphor as anode. Phosphor screen images were taken by a CCD camera. 3. Results and discussion
a
Fig.1 SEM image of CNTs films deposited on Al substrate ( a) Mg(NO3)2·6H2O ( b) CsNO3 +Ca(NO3)2·4H2O
b
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Gang Lan ChenZhang/ et al. /Energy EnergyProcedia Procedia 00 16 (2011) (2012) 000–000 240 – 243 Gang Cheng,
3
Fig.1 shows scanning electron microscope (SEM) of two cathodes films deposited uniformly on 2×2.5 cm2 of Al substrate. Both cathodes with randomly oriented CNTs, identical morphology exist as aggregates and a single nanotube diameter about 10nm. Then field electron emission measurement were carried out when base pressure of vacuum chamber were maintained at 1×10-5Pa. Fig.2(a) presents the current density versus electric field (J–E curve) characteristics of the CNTs cathodes. The turn-on field of the sample with Mg(NO3)2·6H2O was 0.8V/μm and the current density of 3800μA/cm2 was obtained at 1.8V/μm. While corresponding data of the sample with CsNO3 and Ca(NO3)2·4H2O was 0.7V/μm and 4000μA/cm2 at 1.8V/μm, and the current density was increased to 8000μA/cm2 at 2.0V/μm.This meant that CsNO3 and Ca(NO3)2·4H2O had lower turn-on field and higher current density than that with Mg(NO3)2·6H2O solution. -24 -25
Cs+Ca Mg
-26
Cs+Ca Mg
6000
ln(J/E2) (A/V2)
Current density(μA/cm2)
8000
4000 2000 0
-27 -28 -29 -30 -31 -32
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1/E (μm/V)
Electric field (V/μm)
Fig.2 ( a) J–E characteristics and ( b)F–N plot of CNTs film on Al substrates
Fig.2.show J–E characteristics and the corresponding Fowler–Nordheim (F–N) plot of two types films. The plotting ln(I/E2) vs. 1/E yields a straight line in both samples, which indicates that the emission follows Fowler–Nordheim model. The linearity of the F–N plots, suggests that the measured emission current came from field emission by electron tunneling behavior.
b
a
Fig.3 Field emission luminescence images of phosphor screen ( a) Mg(NO3)2·6H2O
( b) CsNO3 +Ca(NO3)2·4H2O at 1.8V/μm
GangChen, ChenLan et al. / Energy Procedia 16 (2012) 240 –000–000 243 Gang Zhang/ Energy Procedia 00 (2011)
Fig.3 shows the corresponding emission performance of two types cathodes under a phosphor anode screen at an applied field of 1.8 V/μm. The field emission luminescence images results reveal that CsNO3 +Ca(NO3)2·4H2O cathode have better light uniformity and higher emission dot density. When increasing the electric field, it disclosed that there were scarcely any carbon nanotubes on the substrate at electric field exceeded 1.9V/μm for Mg(NO3)2·6H2O samples, and field emission failure. For CsNO3 with Ca(NO3)2·4H2O samples ,when electric field higher than 2.2V/μm still have CNTs on the substrate, and field emission occurred. This means that the deposited carbon nanotubes film by using CsNO3/ Ca(NO3)2·4H2O solution had larger adhesive force on the substrate, improved the adhesion of CNTs to substrate. These results indicate that the CsNO3 +Ca(NO3)2·4H2O cathode exhibited excellent field emission properties, i.e. low turn-on emission field, high emission current density, and high emission light spot density and well distributed over the whole CNT cathode. It can be a good cold cathode for field emission lamp. 4. Conclusions In summary, two types of carbon nanotubes-deposited films were prepared on identical aluminium sheets by electrophoretic deposition method. The turn-on field of the deposited film with CsNO3 /Ca(NO3)2·4H2O salt solution was 0.7V/μm. The current density of the film was about 4000mA/cm2 at an electric field of 1.8V/μm. While corresponding data of the sample deposited with Mg(NO3)2·6H2O salt solution were 0.8V/μm and 3800mA/cm2. The experimental results indicate that the deposited CNTs film by using CsNO3 / Ca(NO3)2·4H2O solution had low turn on field, could endure higher electric field and improved the adhesion of CNTs to substrate under the same conditions. It can be a good cold cathode for field emission lamp. Acknowledgements This work was supported by Excellent Youth Foundation of He’an Scientific Committee (104100510023) and by Henan province university innovation talents of science and technology support program (2009HASTIT013) References [1] Sumio Iijima. Helical microtubules of graphitic carbon. Nature 1991;354: 56 – 58. [2] Yahachi Saito, Sashiro Uemura. Field emission from carbon nanotubes and its application to electron sources. Carbon 2000; 38:169-182. [3]Rinzler A G,Hafner J H,Lou L, et.al. UnraveIIingnanotubes: fieId emission from an atomic wire. Science 1995; 269:15501553. [4]De Heer WA, ChatelainA, Ugarte D. A carbon nanotube field-emission electron source. Science 1995;270: 1179-1180. [5] N. S. Lee, D. S. Chung, I. T. Han, J. H. Kang, Y. S. Choi, H. Y. Kim, et al.. Application of carbon nanotubes to field emission displays. Diamond and Related Materials 2001;10 : 265-270. [6] W. B. Choi, Y. W. Jin, H. Y. Kim, S. J. Lee, M. J. Yun, J. H. Kang, et al..Electrophoresis deposition of carbon nanotubes for triode-type field emission display. Applied Physics Letters 2001;78, 1547-1549. [7]Chunsheng Du, D.Heldebrant, Ning Pan. Preparation of carbon nanotubes composite sheet using electrophoretic deposition process. Journal of Materials Science Letters 2002;21:565– 568. [8]Huizhong Ma , Lan Zhang , Junjie Zhang , Liwei Zhang, Ning Yao, Binglin Zhang. Electron field emission properties of carbon nanotubes-deposited flexible film. Applied Surface Science 2005;251:258–261.
243
Energy Procedia
Energy Procedia 00 (2011) Energy Procedia 16000–000 (2012) 240 – 243 www.elsevier.com/locate/procedia
2012 International Conference on Future Energy, Environment, and Materials
Carbon Nanotubes Cathode of Field Emission Lamp Prepared by Electrophoretic Deposition Gang Chena，Lan Zhangb , Huizhong Mab , Ning Yaoa, Binglin Zhanga,a* b
a Department of Physics, Zhengzhou University, Zhengzhou 450052, China Department of Engineering Mechanics, Zhengzhou University, Zhengzhou 450001
Abstract Field emission lamp is a kind of green and low energy loss light source. Carbon nanotube cathode of field emission lamp was prepared by electrophoretic deposition. Two types of solute salt were added in electrophoretic solution separately, which were used to prepare two types of carbon nanotubes films on identical aluminum sheets by electrophoretic deposition method. Scanning electron microscopy(SEM) were used to observe the surface morphology of the films. The field emission properties of the films were tested by using a diode structure. The turnon field of the film deposited with CsNO3 / Ca(NO3)2·4H2O solution was 0.7V/μm. The current density of the film was about 4000mA/cm2 at an electric field of 1.8V/μm. While corresponding data by using Mg(NO3)2·6H2O solution was 0.8V/μm and 3800mA/cm2. It is concluded that the field emission properties of the deposited film cathode by using CsNO3/ Ca(NO3)2·4H2O solution better than that using Mg(NO3)2·6H2O solution. © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society.
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer]
Open access under CC BY-NC-ND license.
Keywords: Field emission; Carbon Nanotubes; Electrophoretic Deposition
1. Introduction Carbon nanotubes (CNTs) films have attracted much attention for their unique characteristics, such as nanometer scale, high aspect ratio, superior mechanical strength, good conductance and high chemical stability[1,2]. All of these properties make them possess stable emission, long life times, and low emission threshold potentials suitable for field electron emission[3,4], and can be applied to field emission displays (FEDs)[5,6], and field emission lamp to be a cold cathode. Especially, the field emission lamp with green
* Corresponding author. E-mail address: [email protected].
1876-6102 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society . Open access under CC BY-NC-ND license. doi:10.1016/j.egypro.2012.01.040
GangChen, ChenLan et al. / Energy Procedia 16 (2012) 240 –000–000 243 Gang Zhang/ Energy Procedia 00 (2011)
and low energy loss property have bright future, and the emission cold cathode is the most important part in the field emission lamp. Electrophoretic deposition (EPD)[6,7] is suitable for the formation of the cathode layer due to the advantages of short formation time, little restriction in the shape of substrates, simple deposition apparatus and low-cost manufacturing. With the method a large sized nanotube film can be fabricated at room temperature. In the EPD process, the presence of salts solution can play an important role in increasing the deposition rate and in improving the adhesion of CNTs to substrates. Thus select a type of suitable solution salt can improve the property of field emission . Mg(NO3)2·6H2O was a traditional salt to be used in electrophoretic deposition [6,8]. In this paper, CsNO3 with Ca(NO3)2·4H2O together were also used to be electrophoretic salts because cesium has low work function . It can probably lower the turn-on field of the emission for improving the field emission properties. The field electron emission properties of the prepared films by using the two types of salt solution prepared films were investigated , respectively. 2. Experiments In this work, CNTs prepared by chemical vapor deposition technique were used. They tend to form aggregates due to their high van der Waals interactions and highly entangled structure. To form uniform emission sites, well-distributed CNTs film were required. CNTs aggregates were grinded into smaller units first, then dispersed in two beakers, each contains certain volume IPA solvent. After that salt Mg(NO3)2·6H2O, CsNO3 with Ca(NO3)2·4H2O were added into each beaker, respectively, to charge the CNTs aggregates positively and to form adhesive material. Then stirred for 24h to form stabilized suspension. During the EPD process, carbon nanotubes were deposited onto an aluminium sheet cathode at room temperature. While a stainless steel was used as anode. The morphology of the two types of CNTs-deposited films were studied by scanning electron microscopy (SEM). Field electron emission properties were measured by a diode type structure in a vacuum chamber. The two electrodes were separated by a 500μm insulating sheet, and the measured field electron emission area was about 1.0 cm2. The deposited CNTs films were used as cathode emitter, and a piece of indium tin oxide (ITO) glass coated with phosphor as anode. Phosphor screen images were taken by a CCD camera. 3. Results and discussion
a
Fig.1 SEM image of CNTs films deposited on Al substrate ( a) Mg(NO3)2·6H2O ( b) CsNO3 +Ca(NO3)2·4H2O
b
241
242
Gang Lan ChenZhang/ et al. /Energy EnergyProcedia Procedia 00 16 (2011) (2012) 000–000 240 – 243 Gang Cheng,
3
Fig.1 shows scanning electron microscope (SEM) of two cathodes films deposited uniformly on 2×2.5 cm2 of Al substrate. Both cathodes with randomly oriented CNTs, identical morphology exist as aggregates and a single nanotube diameter about 10nm. Then field electron emission measurement were carried out when base pressure of vacuum chamber were maintained at 1×10-5Pa. Fig.2(a) presents the current density versus electric field (J–E curve) characteristics of the CNTs cathodes. The turn-on field of the sample with Mg(NO3)2·6H2O was 0.8V/μm and the current density of 3800μA/cm2 was obtained at 1.8V/μm. While corresponding data of the sample with CsNO3 and Ca(NO3)2·4H2O was 0.7V/μm and 4000μA/cm2 at 1.8V/μm, and the current density was increased to 8000μA/cm2 at 2.0V/μm.This meant that CsNO3 and Ca(NO3)2·4H2O had lower turn-on field and higher current density than that with Mg(NO3)2·6H2O solution. -24 -25
Cs+Ca Mg
-26
Cs+Ca Mg
6000
ln(J/E2) (A/V2)
Current density(μA/cm2)
8000
4000 2000 0
-27 -28 -29 -30 -31 -32
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1/E (μm/V)
Electric field (V/μm)
Fig.2 ( a) J–E characteristics and ( b)F–N plot of CNTs film on Al substrates
Fig.2.show J–E characteristics and the corresponding Fowler–Nordheim (F–N) plot of two types films. The plotting ln(I/E2) vs. 1/E yields a straight line in both samples, which indicates that the emission follows Fowler–Nordheim model. The linearity of the F–N plots, suggests that the measured emission current came from field emission by electron tunneling behavior.
b
a
Fig.3 Field emission luminescence images of phosphor screen ( a) Mg(NO3)2·6H2O
( b) CsNO3 +Ca(NO3)2·4H2O at 1.8V/μm
GangChen, ChenLan et al. / Energy Procedia 16 (2012) 240 –000–000 243 Gang Zhang/ Energy Procedia 00 (2011)
Fig.3 shows the corresponding emission performance of two types cathodes under a phosphor anode screen at an applied field of 1.8 V/μm. The field emission luminescence images results reveal that CsNO3 +Ca(NO3)2·4H2O cathode have better light uniformity and higher emission dot density. When increasing the electric field, it disclosed that there were scarcely any carbon nanotubes on the substrate at electric field exceeded 1.9V/μm for Mg(NO3)2·6H2O samples, and field emission failure. For CsNO3 with Ca(NO3)2·4H2O samples ,when electric field higher than 2.2V/μm still have CNTs on the substrate, and field emission occurred. This means that the deposited carbon nanotubes film by using CsNO3/ Ca(NO3)2·4H2O solution had larger adhesive force on the substrate, improved the adhesion of CNTs to substrate. These results indicate that the CsNO3 +Ca(NO3)2·4H2O cathode exhibited excellent field emission properties, i.e. low turn-on emission field, high emission current density, and high emission light spot density and well distributed over the whole CNT cathode. It can be a good cold cathode for field emission lamp. 4. Conclusions In summary, two types of carbon nanotubes-deposited films were prepared on identical aluminium sheets by electrophoretic deposition method. The turn-on field of the deposited film with CsNO3 /Ca(NO3)2·4H2O salt solution was 0.7V/μm. The current density of the film was about 4000mA/cm2 at an electric field of 1.8V/μm. While corresponding data of the sample deposited with Mg(NO3)2·6H2O salt solution were 0.8V/μm and 3800mA/cm2. The experimental results indicate that the deposited CNTs film by using CsNO3 / Ca(NO3)2·4H2O solution had low turn on field, could endure higher electric field and improved the adhesion of CNTs to substrate under the same conditions. It can be a good cold cathode for field emission lamp. Acknowledgements This work was supported by Excellent Youth Foundation of He’an Scientific Committee (104100510023) and by Henan province university innovation talents of science and technology support program (2009HASTIT013) References [1] Sumio Iijima. Helical microtubules of graphitic carbon. Nature 1991;354: 56 – 58. [2] Yahachi Saito, Sashiro Uemura. Field emission from carbon nanotubes and its application to electron sources. Carbon 2000; 38:169-182. [3]Rinzler A G,Hafner J H,Lou L, et.al. UnraveIIingnanotubes: fieId emission from an atomic wire. Science 1995; 269:15501553. [4]De Heer WA, ChatelainA, Ugarte D. A carbon nanotube field-emission electron source. Science 1995;270: 1179-1180. [5] N. S. Lee, D. S. Chung, I. T. Han, J. H. Kang, Y. S. Choi, H. Y. Kim, et al.. Application of carbon nanotubes to field emission displays. Diamond and Related Materials 2001;10 : 265-270. [6] W. B. Choi, Y. W. Jin, H. Y. Kim, S. J. Lee, M. J. Yun, J. H. Kang, et al..Electrophoresis deposition of carbon nanotubes for triode-type field emission display. Applied Physics Letters 2001;78, 1547-1549. [7]Chunsheng Du, D.Heldebrant, Ning Pan. Preparation of carbon nanotubes composite sheet using electrophoretic deposition process. Journal of Materials Science Letters 2002;21:565– 568. [8]Huizhong Ma , Lan Zhang , Junjie Zhang , Liwei Zhang, Ning Yao, Binglin Zhang. Electron field emission properties of carbon nanotubes-deposited flexible film. Applied Surface Science 2005;251:258–261.
243