- Email: [email protected]
Enhancement of field emission of carbon nanotubes using a simple microwave plasma method
CARBON
4 9 ( 2 0 1 1 ) 4 8 4 –4 8 6
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Enhancement of field emission of carbon nanotubes using a simple microwave plasma method Pejman Hojati-Talemi, George P. Simon
*
Department of Materials Engineering, Monash University, Clayton Victoria 3800, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
A microwave plasma-based method for the enhancement of electron field emission behav-
Received 13 June 2010
iour of carbon nanotubes is presented. This method is shown to be able to unzip and exfo-
Accepted 11 September 2010
liate carbon nanotubes, thereby significantly increasing the field emission current due to
Available online 29 September 2010
the formation of more emitting tips. Crown Copyright 2010 Published by Elsevier Ltd. All rights reserved.
Due to their high electrical conductivity and small radius of curvature, carbon nanotubes (CNTs) are promising candidates for the development of electron field emission (FE) sources [1–4] which play a crucial role in devices such as flat panel displays, vacuum microelectronic devices, or even efficient light sources. To enhance the field emission properties of CNTs, a variety of methods such as coating with tetrahedral amorphous carbon, plasma treatment, preparation of hybrid structures or thermal treatment [1–4] have been reported. In this letter we report the effect of a simple, microwavebased method on field emission of CNTs with random alignment. Unlike other plasma-based methods which require purpose-designed devices, this method makes use of a domestic microwave oven to produce plasma inside a quartz tube. We have reported this method before as a technique for mild functionalization of CNTs [5]. The materials, methods and details of Raman and XPS studies are described in detail elsewhere [5]. In order to compare the effect of this treatment on CNTs, two samples, treated for 30 and 240 s were prepared. As a point of comparison, the effect of standard acid treatment of CNTs was also studied. For preparation of samples for FE study, CNTs (as-received CNT: NT1, acid-treated CNT: aNT, MW-plasma treated CNT: mpNT) were dispersed in ethanol in a sonic bath, and then droplets of the dispersion were applied and dried over a polished graphite substrate for several times till the whole surface was covered with a 0.5 mm layer of CNTs. The FE tests were performed using
a parallel electrode system at a vacuum of 1 · 10 4 Pa. Fig. 1a shows the variation of emitted current density (J) as a function of applied field (E) of samples. It was observed that both microwave treated samples exhibit higher electron emission, in comparison to the NT1, whilst in contrast the aNT shows a much weaker level of electron emission. The turn-on electric field (defined as the field value corresponding to a current density of 10 lA/cm2) of samples are shown in Table 1. Compared to the NT1, acid treatment has resulted in a two times higher turn-on field, whilst the MW-plasma method leads to a significantly lower turn-on field, which is a good indication of the considerable improvement in the FE properties of these tubes. By using the slopes of the Fowler–Nordheim plots (Fig. 1b), and assuming a work function of 5 eV for CNTs, the mean field enhancement factors (b) of the samples can be estimated. The calculated b values (Table 1) show that the acid treatment of samples has resulted in lowering of b, whilst MW-plasma treatment for 30 s has resulted in a more than two times enhancement. Further increasing the MW-plasma processing time to 4 min led to a b, three times higher than as-received CNTs. Fig. 1c shows the stability of the emission of mpNT240. Due to field-induced alignment of nanotubes in the first minutes, an increase in emission is observed. With regards to longer time behaviour, the degradation of some nanotubes protruding from the surface can lead to a drop in emission of CNTs which usually occurs in the first 2 h [6].
* Corresponding author: Fax: +61 3 99054934. E-mail address: [email protected] (G.P. Simon). 0008-6223/$ - see front matter Crown Copyright 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.09.045
CARBON
a
8.E-04
-7
NT1 mpNT30
6.E-04
mpNT240
5.E-04 4.E-04
c
1.E-04
-9
ln(JV-2 )
7.E-04
J(A/cm2)
b
-5 aNT
3.E-04
J(A/cm2)
9.E-04
485
4 9 (2 0 1 1) 4 8 4–48 6
-11 -13 -15
5.E-05
-17
2.E-04
-19
1.E-04
0.E+00
-21
0.E+00 0
1
E(V/m)
2
3
0
1
2
1/E
3
0
200
400
600
Time (min)
Fig. 1 – (a) Current density vs. applied field, (b) the FN plot of the same data and (c) the field emission stability after 10 h, under a field of 0.45 V/lm.
Table 1 – Oxygen content and field emission parameters for the treated and as-received nanotubes. Sample NT1 aNT mpNT30 mpNT240
Turn-on field (V/lm) 1.06 2.12 0.62 0.47
b
Oxygen content (%)
6403 3078 1429 19,864
0.7 36.54 1.06 1.04
We have seen this phenomenon after 1 h followed by a stable emission for the next 9 h. Comparison of these results with other papers reporting FE properties of nanotubes [1–4] or recent developments of graphene-based emitters [7] shows the superior FE property of these samples, which is much better than emitters based on aligned CNTs [4]. TEM images of the treated CNTs provide an interesting explanation for such changes in the FE properties observed for these tubes. Whilst acid treatment significantly damages
Fig. 2 – TEM images of (a) NT1, (b) aNT, (c) mpNT240 and (d) HRTEM image of the edges of the carbon sheets observed in mpNT240.
486
CARBON
4 9 ( 2 0 1 1 ) 4 8 4 –4 8 6
the structure of CNTs, in case of MW-plasma treatment some type of sheet-like materials forms between the tubes. This material was assumed to be amorphous carbon sheets arising from the degradation of CNTs, but HRTEM micrographs of these materials (Fig. 2d) revealed that they are in fact graphitic nanosheets, with a diameter of less than 10 nm, and consist of 10–30 graphene layers with a d-spacing of 0.34 nm. It has recently been shown that growing such graphitic structures over CNTs by the PECVD method can also enhance the field emission property of CNTs by providing more emitting edges [1]. The sp3 hybridization of carbon atoms on these edges can also result in lowering the energy barrier for emission of electrons, and consequently a better emission [3]. Since the only carbon source in the MW-plasma process was CNTs, the possible mechanism for such a change in morphology is the unzipping of CNTs. Theoretical studies have demonstrated the possibility of unzipping of CNTs [8], and recently researchers have reported unzipping of nanotubes by Ar plasma [9] or KMnO4 solution [10]. However, we have shown here that this can be readily achieved by an easy, direct MW-plasma technique, resulting in a CNT/graphene composite which demonstrates superior FE properties. XPS analysis of samples showed that the acid treatment has increased the oxygen content of CNTs significantly (Table 1), but in case of mpNT30, there is only a slight increase in oxygen content. Further lengthening the MW-plasma process time to 4 min, leads to no additional change in the oxygen content of the CNTs. Raman spectra of the samples show that, the ratio of intensity of G (1580 cm 1) and D (1310 cm 1) peaks (IG/ID) of MW-plasma treated samples has decreased from 0.71 for the NT1, to 0.61 and 0.55 for mpNT30 and mpNT240, respectively. This decrease in IG/ID can be due to formation of functional groups or cutting of the tubes and the formation of new edges or other structural defects. The absence of any change in oxygen content between 30 and 240 s of process, removes the possibility of functionalization as a possible explanation for changes in IG/ID, and supports the concept based on unzipping of CNTs. It has also been shown in the literature that there is a direct relation between the width of the D band in Raman spectra and the diameter of CNTs [11]. Comparison of the width of D band in our samples also shows a decrease from 73 cm 1 for NT1 to 60 and 49 cm 1 for mpNT30 and mpNT240, respectively, further confirming the unzipping and exfoliation of CNTs. In summary, a novel and simple method for preparation of CNT/graphene composite is developed. The observation of
graphitic nanosheets and the decrease in IG/ID without formation of new functional groups (as characterised by XPS) confirms that unzipping and exfoliation of CNTs is the source for formation of carbon nanosheets. Formation of this new type of nanostructure is found to considerably enhance the electron field emission of these CNTs compared to other related methods.
R E F E R E N C E S
[1] Yang GM, Yang CC, Xu Q, Zheng WT, Li S. Enhancement mechanism of field electron emission properties in hybrid carbon nanotubes with tree- and wing-like features. J Solid State Chem 2009;182(12):3393–8. [2] Zhang J, Feng T, Yu W, Liu X, Wang X, Li Q. Enhancement of field emission from hydrogen plasma processed carbon nanotubes. Diamond Relat Mater 2004;13(1):54–9. [3] Yu K, Zhu Z, Xu M, Li Q, Lu W, Chen Q. Soluble carbon nanotube films treated using a hydrogen plasma for uniform electron field emission. Surf Coat Technol 2004;179(1):63–9. [4] Chen Y, Sun Z, Chen J, Xu NS, Tay BK. Field emission properties from aligned carbon nanotube films with tetrahedral amorphous carbon coating. Diamond Relat Mater 2006;15(9):1462–6. [5] Hojati-Talemi P, Cervini R, Simon G. Effect of different microwave-based treatments on multi-walled carbon nanotubes. J Nanopart Res 2010;12(2):393–403. [6] Umnov AG, Mordkovich VZ. Field-induced evaporation of carbon nanotubes. Appl Phys A Mater Sci Process 2001;73(3):301–4. [7] Wu Zhong-Shuai, Pei Songfeng, Ren Wencai, Tang Daiming, Gao Libo, Liu Bilu, et al. Field emission of single-layer graphene films prepared by electrophoretic deposition. Adv Mater 2009;21(17):1756–60. [8] Lu G, Scudder H, Kioussis N. Hydrogen-induced unzipping of single-walled carbon nanotubes. Phys Rev B 2010;68(20):205416-1–5. [9] Jiao L, Zhang L, Wang X, Diankov G, Dai H. Narrow graphene nanoribbons from carbon nanotubes. Nature 2009;458(7240):877–80. [10] Kosynkin DV, Higginbotham AL, Sinitskii A, Lomeda JR, Dimiev A, Price BK, et al. Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons. Nature 2009;458(7240):872–6. [11] Antunes EF, Lobo AO, Corat EJ, Trava-Airoldi VJ. Influence of diameter in the Raman spectra of aligned multi-walled carbon nanotubes. Carbon 2007;45(5):913–21.
4 9 ( 2 0 1 1 ) 4 8 4 –4 8 6
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Enhancement of field emission of carbon nanotubes using a simple microwave plasma method Pejman Hojati-Talemi, George P. Simon
*
Department of Materials Engineering, Monash University, Clayton Victoria 3800, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
A microwave plasma-based method for the enhancement of electron field emission behav-
Received 13 June 2010
iour of carbon nanotubes is presented. This method is shown to be able to unzip and exfo-
Accepted 11 September 2010
liate carbon nanotubes, thereby significantly increasing the field emission current due to
Available online 29 September 2010
the formation of more emitting tips. Crown Copyright 2010 Published by Elsevier Ltd. All rights reserved.
Due to their high electrical conductivity and small radius of curvature, carbon nanotubes (CNTs) are promising candidates for the development of electron field emission (FE) sources [1–4] which play a crucial role in devices such as flat panel displays, vacuum microelectronic devices, or even efficient light sources. To enhance the field emission properties of CNTs, a variety of methods such as coating with tetrahedral amorphous carbon, plasma treatment, preparation of hybrid structures or thermal treatment [1–4] have been reported. In this letter we report the effect of a simple, microwavebased method on field emission of CNTs with random alignment. Unlike other plasma-based methods which require purpose-designed devices, this method makes use of a domestic microwave oven to produce plasma inside a quartz tube. We have reported this method before as a technique for mild functionalization of CNTs [5]. The materials, methods and details of Raman and XPS studies are described in detail elsewhere [5]. In order to compare the effect of this treatment on CNTs, two samples, treated for 30 and 240 s were prepared. As a point of comparison, the effect of standard acid treatment of CNTs was also studied. For preparation of samples for FE study, CNTs (as-received CNT: NT1, acid-treated CNT: aNT, MW-plasma treated CNT: mpNT) were dispersed in ethanol in a sonic bath, and then droplets of the dispersion were applied and dried over a polished graphite substrate for several times till the whole surface was covered with a 0.5 mm layer of CNTs. The FE tests were performed using
a parallel electrode system at a vacuum of 1 · 10 4 Pa. Fig. 1a shows the variation of emitted current density (J) as a function of applied field (E) of samples. It was observed that both microwave treated samples exhibit higher electron emission, in comparison to the NT1, whilst in contrast the aNT shows a much weaker level of electron emission. The turn-on electric field (defined as the field value corresponding to a current density of 10 lA/cm2) of samples are shown in Table 1. Compared to the NT1, acid treatment has resulted in a two times higher turn-on field, whilst the MW-plasma method leads to a significantly lower turn-on field, which is a good indication of the considerable improvement in the FE properties of these tubes. By using the slopes of the Fowler–Nordheim plots (Fig. 1b), and assuming a work function of 5 eV for CNTs, the mean field enhancement factors (b) of the samples can be estimated. The calculated b values (Table 1) show that the acid treatment of samples has resulted in lowering of b, whilst MW-plasma treatment for 30 s has resulted in a more than two times enhancement. Further increasing the MW-plasma processing time to 4 min led to a b, three times higher than as-received CNTs. Fig. 1c shows the stability of the emission of mpNT240. Due to field-induced alignment of nanotubes in the first minutes, an increase in emission is observed. With regards to longer time behaviour, the degradation of some nanotubes protruding from the surface can lead to a drop in emission of CNTs which usually occurs in the first 2 h [6].
* Corresponding author: Fax: +61 3 99054934. E-mail address: [email protected] (G.P. Simon). 0008-6223/$ - see front matter Crown Copyright 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.09.045
CARBON
a
8.E-04
-7
NT1 mpNT30
6.E-04
mpNT240
5.E-04 4.E-04
c
1.E-04
-9
ln(JV-2 )
7.E-04
J(A/cm2)
b
-5 aNT
3.E-04
J(A/cm2)
9.E-04
485
4 9 (2 0 1 1) 4 8 4–48 6
-11 -13 -15
5.E-05
-17
2.E-04
-19
1.E-04
0.E+00
-21
0.E+00 0
1
E(V/m)
2
3
0
1
2
1/E
3
0
200
400
600
Time (min)
Fig. 1 – (a) Current density vs. applied field, (b) the FN plot of the same data and (c) the field emission stability after 10 h, under a field of 0.45 V/lm.
Table 1 – Oxygen content and field emission parameters for the treated and as-received nanotubes. Sample NT1 aNT mpNT30 mpNT240
Turn-on field (V/lm) 1.06 2.12 0.62 0.47
b
Oxygen content (%)
6403 3078 1429 19,864
0.7 36.54 1.06 1.04
We have seen this phenomenon after 1 h followed by a stable emission for the next 9 h. Comparison of these results with other papers reporting FE properties of nanotubes [1–4] or recent developments of graphene-based emitters [7] shows the superior FE property of these samples, which is much better than emitters based on aligned CNTs [4]. TEM images of the treated CNTs provide an interesting explanation for such changes in the FE properties observed for these tubes. Whilst acid treatment significantly damages
Fig. 2 – TEM images of (a) NT1, (b) aNT, (c) mpNT240 and (d) HRTEM image of the edges of the carbon sheets observed in mpNT240.
486
CARBON
4 9 ( 2 0 1 1 ) 4 8 4 –4 8 6
the structure of CNTs, in case of MW-plasma treatment some type of sheet-like materials forms between the tubes. This material was assumed to be amorphous carbon sheets arising from the degradation of CNTs, but HRTEM micrographs of these materials (Fig. 2d) revealed that they are in fact graphitic nanosheets, with a diameter of less than 10 nm, and consist of 10–30 graphene layers with a d-spacing of 0.34 nm. It has recently been shown that growing such graphitic structures over CNTs by the PECVD method can also enhance the field emission property of CNTs by providing more emitting edges [1]. The sp3 hybridization of carbon atoms on these edges can also result in lowering the energy barrier for emission of electrons, and consequently a better emission [3]. Since the only carbon source in the MW-plasma process was CNTs, the possible mechanism for such a change in morphology is the unzipping of CNTs. Theoretical studies have demonstrated the possibility of unzipping of CNTs [8], and recently researchers have reported unzipping of nanotubes by Ar plasma [9] or KMnO4 solution [10]. However, we have shown here that this can be readily achieved by an easy, direct MW-plasma technique, resulting in a CNT/graphene composite which demonstrates superior FE properties. XPS analysis of samples showed that the acid treatment has increased the oxygen content of CNTs significantly (Table 1), but in case of mpNT30, there is only a slight increase in oxygen content. Further lengthening the MW-plasma process time to 4 min, leads to no additional change in the oxygen content of the CNTs. Raman spectra of the samples show that, the ratio of intensity of G (1580 cm 1) and D (1310 cm 1) peaks (IG/ID) of MW-plasma treated samples has decreased from 0.71 for the NT1, to 0.61 and 0.55 for mpNT30 and mpNT240, respectively. This decrease in IG/ID can be due to formation of functional groups or cutting of the tubes and the formation of new edges or other structural defects. The absence of any change in oxygen content between 30 and 240 s of process, removes the possibility of functionalization as a possible explanation for changes in IG/ID, and supports the concept based on unzipping of CNTs. It has also been shown in the literature that there is a direct relation between the width of the D band in Raman spectra and the diameter of CNTs [11]. Comparison of the width of D band in our samples also shows a decrease from 73 cm 1 for NT1 to 60 and 49 cm 1 for mpNT30 and mpNT240, respectively, further confirming the unzipping and exfoliation of CNTs. In summary, a novel and simple method for preparation of CNT/graphene composite is developed. The observation of
graphitic nanosheets and the decrease in IG/ID without formation of new functional groups (as characterised by XPS) confirms that unzipping and exfoliation of CNTs is the source for formation of carbon nanosheets. Formation of this new type of nanostructure is found to considerably enhance the electron field emission of these CNTs compared to other related methods.
R E F E R E N C E S
[1] Yang GM, Yang CC, Xu Q, Zheng WT, Li S. Enhancement mechanism of field electron emission properties in hybrid carbon nanotubes with tree- and wing-like features. J Solid State Chem 2009;182(12):3393–8. [2] Zhang J, Feng T, Yu W, Liu X, Wang X, Li Q. Enhancement of field emission from hydrogen plasma processed carbon nanotubes. Diamond Relat Mater 2004;13(1):54–9. [3] Yu K, Zhu Z, Xu M, Li Q, Lu W, Chen Q. Soluble carbon nanotube films treated using a hydrogen plasma for uniform electron field emission. Surf Coat Technol 2004;179(1):63–9. [4] Chen Y, Sun Z, Chen J, Xu NS, Tay BK. Field emission properties from aligned carbon nanotube films with tetrahedral amorphous carbon coating. Diamond Relat Mater 2006;15(9):1462–6. [5] Hojati-Talemi P, Cervini R, Simon G. Effect of different microwave-based treatments on multi-walled carbon nanotubes. J Nanopart Res 2010;12(2):393–403. [6] Umnov AG, Mordkovich VZ. Field-induced evaporation of carbon nanotubes. Appl Phys A Mater Sci Process 2001;73(3):301–4. [7] Wu Zhong-Shuai, Pei Songfeng, Ren Wencai, Tang Daiming, Gao Libo, Liu Bilu, et al. Field emission of single-layer graphene films prepared by electrophoretic deposition. Adv Mater 2009;21(17):1756–60. [8] Lu G, Scudder H, Kioussis N. Hydrogen-induced unzipping of single-walled carbon nanotubes. Phys Rev B 2010;68(20):205416-1–5. [9] Jiao L, Zhang L, Wang X, Diankov G, Dai H. Narrow graphene nanoribbons from carbon nanotubes. Nature 2009;458(7240):877–80. [10] Kosynkin DV, Higginbotham AL, Sinitskii A, Lomeda JR, Dimiev A, Price BK, et al. Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons. Nature 2009;458(7240):872–6. [11] Antunes EF, Lobo AO, Corat EJ, Trava-Airoldi VJ. Influence of diameter in the Raman spectra of aligned multi-walled carbon nanotubes. Carbon 2007;45(5):913–21.