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Observation of field emission from carbon nanoparticles film coating on top of vertically aligned carbon nanotubes on silicon substrate
Vacuum 167 (2019) 113–117
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Observation of field emission from carbon nanoparticles film coating on top of vertically aligned carbon nanotubes on silicon substrate
T
Xiao Hui Yanga, Hua Ii Mab, Fan Guang Zengb,∗ a b
School of Physics and Electronics, North China University of Water Resources and Electric Power, Zhengzhou, 450011, China Zhengzhou Institute of Aeronautical Industry Management, Zhengzhou, 450015, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon nanoparticles film Vertically aligned carbon nanotubes Low turn-on and threshold electric fields High emission density Good field emission stability
The carbon nanoparticles film coating on top of vertically aligned carbon nanotubes on silicon substrate (CNPs/ CNTs/silicon) has been prepared by the pyrolysis of FePC at 960 °C. The surface morphologies and structure of the prepared sample are characterized by Scanning Electron Microscope, X-ray energy dispersive spectroscopy and Raman Spectroscopy. The field electron emission (FEE) properties of the CNPs/CNTs are also measured. A low turn-on electric field of 1.3 V / μm at a current density of 10 μA/ cm2 , low threshold electric field of 1.8 V / μm at a current density of 1 mA/ cm2 , a high emission density and good field emission stability are obtained. The above results make CNPs/CNTs/silicon an effective field emitter.
1. Introduction The carbon nanotubes (CNTs) have been considered as one of the most promising materials for field electron emitters owing to their prominent structural and physics properties, such as high aspect ratio, small radius of curvature, good electric conductivity, and excellent chemical stability [1–7]. The success applications in CNT-based field emission devices require the emitter to have low turn-on electric field, low threshold field, high emission density, and excellent stability at a large current density. To achieve high-performance from CNTs emitters, coating the surface of CNTs with other materials has been used as one of candidates to improve field emission properties [8–12]. Carbon nanoparticles (CNPs) have low work function and high electric conductivity. Therefore, it is expected to exhibit good field emission behaviors as carbon nanotubes. Will the field emission properties be improved If CNPs and CNTs are combined? To our best knowledge, very few previous observations of field emission from CNPs/CNTs have been reported [13], despite several researches on CNPs [14,15] and some studies on field emission from CNTs [2–7]. In this paper, we combine these two materials together in the form of carbon nanoparticles film coating on top of vertically aligned carbon nanotubes (CNPs/CNTs) on silicon substrate by pyrolysis of iron phthalocyanine (FePc) under Ar/H2 atmosphere at 960 °C. The method not only prepares the CNPs/CNTS/silicon with excellent morphologies but significantly improves their field emission properties. The CNPs/CNTs/silicon exhibits superior field emission characteristics
∗
with lower turn-on and threshold fields, high emission density and good emission stability, which make the CNPs/CNTs/silicon promising applications in vacuum electronics devices. 2. Experimental procedure Carbon nanoparticles film coating on top of vertically aligned carbon nanotubes on silicon substrate (CNPs/CNTs/silicon) are prepared by pyrolysis of iron phthalocyanine (FePc) under Ar/H2 atmosphere at a predetermined temperature in a flow reactor comprising of a quartz glass tube and a dual furnace fitted with independent temperature controllers (XD-1200NT). The n-type silicon wafer is used as a substrate, which is cleaned successively in an ultrasonic bath with ethanol and de-ionized water and dried in air. Micro-sized diamond particles are dispersed in polyvinyl alcohol (PVA) solution by mixing 0.02g diamond particles with the diameter of 4–8 μm and 2g PVA powder in deionized water. The solution was spun-coated onto the surface of silicon wafer. After diamond particles are deposited on silicon substrate, the growth of CNPs/CNTs/silicon is carried out. FePc and silicon substrate deposited with diamond particles are placed in the first and second furnaces respectively in the quartz glass reactor. After the temperature in the second furnace reaches 960 °C, the first furnace is heated at 500°C-750 °C for ca. 60 min and the second furnace is kept at the 960°C for ca. 60 min to complete the pyrolysis process. After the growth is finished, the furnace is cooled naturally to room temperature under the protection of Ar.
Corresponding author. E-mail addresses: [email protected] (X.H. Yang), [email protected] (F.G. Zeng).
https://doi.org/10.1016/j.vacuum.2019.06.001 Received 23 April 2019; Received in revised form 1 June 2019; Accepted 1 June 2019 Available online 02 June 2019 0042-207X/ © 2019 Published by Elsevier Ltd.
Vacuum 167 (2019) 113–117
X.H. Yang, et al.
Fig. 1. Plan-view SEM images of the silicon deposited with Micro-sized diamond particles.
The microstructure and elemental compositions of the prepared CNPs/CNTs/silicon are characterized by scanning electron microscopy (SEM; JSM-6700F), X-ray energy dispersive spectroscopy (EDS) and Raman spectroscopy (Renishaw in Via Raman microscope). The field emission tests are performed in DC mode with a diode-type electron emission structure in a vacuum chamber with a low pressure of 4 × 10−4Pa at room temperature. The as-grown CNPs/CNTs/silicon, acting as field emitter, is electrically connected to a 5.3 × 5.3cm2 movable stainless steel plate. The area of the CNPs/CNTs/silicon is about 0.2cm2 . The same stainless steel plate serves as the anode. The field emission tests are conducted by applying variable DC voltages on the anode in the vacuum chamber. The emission current is measured using a Keithley Model 2450 System Source Meter. The distance between anode and cathode CNPs/CNTs/silicon is 5 mm. To evaluate the emission uniformity, a phosphor-coated indium-tin-oxide glass substrate is used as an anode to observe the electron emission patterns.
Three patent features are the D peak at 1353 cm−1 induced by the disorder and defect [17], the G band at 1589 cm−1 manifesting the presence of graphitic carbon and 2D peak at 2703cm−1 indicating longrange order in the sample [17,18]. The ratio of the D peak to G peak implies graphitization degree of carbon nanoparticles. Fig. 4 shows that the intensity of D band is less than G band, suggesting that the crystallization of the CNPs is better [19], which provides a possibility for CNPs to be a good field emitter. Field electron emission tests of the CNPs/CNTs/silicon are conducted, shown in Fig. 5. Fig. 5a is the J-E curve of the sample. The turnon electric field is approximately 1.3V / μm corresponding to the current density of 10 μA/ cm2 . The threshold electric field is about 1.8V / μm where the current density reaches 1mA/cm2 . To easily check the turn-on and threshold electric fields, the current density on lg scale versus the electric field curve of the sample is shown in Fig. 5b. To further investigate the emission properties of as-grown sample， We refer to the Fowler-Nordheim (F-N) equation
3. Results and discussion
⎞ 1 E [1], where J is the emission current ⎠ density, E is the applied electric field, ϕ is the work function determined by the material,a = 1.54 × 10−6AeVV −2 and 3 b = 6.830890 × 109eV − 2Vm−1. Current density J is defined as J=I/A, where I is the measured emission current and A is the area of the CNPs/CNTs/silicon. The field enhancement factor β , which depends on the morphology, can be determined by fitting the slope of the F-N plot and taking a reasonable ϕ . The F-N plot is achieved by re-plotting the J-E data as Ln (J/ E2) versus 1/E, as shown in inset of Fig. 5a. The approximate linear characteristics of the F-N plot clearly indicates that the electron emission is attributed to the tunneling effect. By taking 4.0 eV as the work function value for the CNPs [13,14], the field enhancement factor (β ) is calculated to be about 3.6 × 103 from the F-N plot slope. In addition to the field emission I-V characteristics, emission uniform is also observed, shown in Fig. 6. The image is taken at the electric field of 1.8V / μm . The image shows that the emission sites are almost uniform at most of the emission area. But it can be obviously observed that the dark parts exist on the emission image. It is thought that it is aroused by the uneven growth of the CNPs, which is consistent with the images in Fig. 2. Due to the limits of experiment condition, it is difficult to accurately determine the emission density. Whereas, it can be seen that the density of the emission sites is high. It could be due to the low resistance of the carbon nanotubes under the CNPs film that guarantees an abundant electron source. The superior field emission performances of CNPs/CNPs/silicon are mainly attributed to the following reasons. Firstly, the low work ln
Fig. 1 is SEM images of the silicon wafer deposited with diamond particles. Images show that the micro-sized diamond particles with irregular shape are loosely distributed on the silicon substrate. The morphologies of the CNPs/CNTs/silicon are presented in Fig. 2. It can be seen from Fig. 2a–c that the vertically aligned CNTs with about 12 μm long are grown on silicon substrate. Moreover, it can be found that the top of CNTs is covered by a layer of film with about 100 nm thick and the film splits from the sites around the diamond particles and then propagates. The film crack is induced probably because of the film shrinking faster than the inner materials during cooling naturally to room temperature. Fig. 2d presents that the film is consisted of densely particles. The film elements are given by EDS later. In addition, Fig. 2a shows that the CNTs twist together on the diamond particles and very few quantities of twisted CNTs scatter on the film. Fig. 3 is the SEM-EDS image of film. The corresponding elemental compositions (atomic %) are given in Table 1. The value of C atom equals to 78.41% and that of Si atom equals to 16.06%, which suggests that the film is a layer of compact carbon nanoparticles. The size of CNPs is not uniform and the average particle diameter is about 50 nm. These carbon nanoparticles are not strict balls, which benefits the field electron emission. Theories and experimental results reveal that materials with tips or sharp edges pointing to the electrical field direction can result in a high enhancement factor and easily render field emission under a low macroscopic electric field [3–7,16]. Fig. 4 is the Raman scattering spectra of the CNPs/CNTs/silicon. 114
( ) = ln ( ) − ⎛⎝ J
E2
aβ2
ϕ
3
bϕ 2
β
( )
Vacuum 167 (2019) 113–117
X.H. Yang, et al.
Fig. 2. Plan-view SEM images of CNPs/CNTs/silicon.
Fig. 3. SEM image for the EDS experiment CNPs/CNTs/silicon.
Fig. 4. Raman spectrum of CNPs/CNTs/silicon.
function of the CNPs could lower the height of potential barrier that makes electrons emitting from it more easily. Secondly, the high field enhancement factor contributes to the low turn-on and threshold electric fields. As evident from the SEM images (Fig. 2), the carbon nanoparticles film coats on top of the vertically aligned CNTs, which gives large enhancement factor to the composite. The large enhancement factor enhances the electric field on the surface of the CNPs, allowing for electrons tunneling from CNPs into vacuum under a relative low macroscopic electric field. Thirdly, according to the Raman spectra
Table 1 Elemental composition (atomic %) of CNPs/CNTs/silicon. Element
C
Si
O
Fe
Atom (%) Total (%)
78.41 100.00
16.06
4.77
0.76
115
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X.H. Yang, et al.
Fig. 7. Field emission stability of CNPs/CNTs/silicon.
applications in vacuum electronic devices. The emission stability test of CNPs/CNTs/silicon is performed by recording the average emission current every second at an average emission current density of approximately 1mA/cm2 for 7.6 h. As shown in Fig. 7, it can be seen that the current density shows a gradually increase at the beginning and then fluctuates around current density of 1mA cm−2 within the first 5 h. The stability of emission current of the sample is probably due to the excellent thermal conductivity and the thermal stability of the diamond particles. The diamond particles are deposited on silicon before the sample is grew. However, it should be noted that the average current density decays slowly after 5 h. The reason of emission current decay could be ascribed to the following possible factors. The first one is the electrostatic force that can peel off the CNTs from substrate as the macroscopic electric field is continually applied, which causes current to decay. Though the turn-on and threshold fields are low, the high voltage is needed due to the large distance between cathode and anode. Therefore, the second one could be Joule heating arising from the applied high voltages, which will “burn” the CNTs at the point where the temperature reaches a value. Fig. 5. (a) The current density versus the electric field plot of CNPs/CNTs/ silicon emitter. The inset shows the F-N plot, (b)the current density on lg scale versus the electric field plot of CNPs/CNTs/silicon emitter.
4. Conclusion We synthesize the carbon nanoparticles film coating on top of vertically aligned CNTs on silicon wafer by the pyrolysis of FePC at 960 °C and measure their field emission characteristics. The results present that CNPs/CNTs/silicon exhibits good emission properties. The turn-on electric field is 1.3V / μm at an emission current density of 10 μA/ cm2 and the threshold electrical field is 1.8V / μm at an emission current density of 1mA/cm2 . Besides, CNPs/CNTs/silicon has a high emission density at around 1 mA/cm2 and long-time field emission stability. The excellent field emission properties of CNPs/CNTs/silicon are derived from the structure of CNPs/CNTs/silicon and the high degree of graphitization of the CNPs. The results show that CNPs/CNTs/silicon will be an excellent field emitter. Acknowledgements This work is supported by Innovation Scientists and Technicians Troop Construction Projects of Henan Province (No. 164200510006), Aeronautical Science Foundation of China (No. 2015 ZF 55013) and Henan key Laboratory of aeronautical material and application technology.
Fig. 6. Emission pattern from CNPs/CNTs/silicon with 1 mA/cm2.
References
characteristics, the high degree of graphitization of CNPs makes it having high conductivity, which ensures the sufficient electrons and enlarges emission current [13,14,16]. The stability is also a major factor that affects its potential
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Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Observation of field emission from carbon nanoparticles film coating on top of vertically aligned carbon nanotubes on silicon substrate
T
Xiao Hui Yanga, Hua Ii Mab, Fan Guang Zengb,∗ a b
School of Physics and Electronics, North China University of Water Resources and Electric Power, Zhengzhou, 450011, China Zhengzhou Institute of Aeronautical Industry Management, Zhengzhou, 450015, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon nanoparticles film Vertically aligned carbon nanotubes Low turn-on and threshold electric fields High emission density Good field emission stability
The carbon nanoparticles film coating on top of vertically aligned carbon nanotubes on silicon substrate (CNPs/ CNTs/silicon) has been prepared by the pyrolysis of FePC at 960 °C. The surface morphologies and structure of the prepared sample are characterized by Scanning Electron Microscope, X-ray energy dispersive spectroscopy and Raman Spectroscopy. The field electron emission (FEE) properties of the CNPs/CNTs are also measured. A low turn-on electric field of 1.3 V / μm at a current density of 10 μA/ cm2 , low threshold electric field of 1.8 V / μm at a current density of 1 mA/ cm2 , a high emission density and good field emission stability are obtained. The above results make CNPs/CNTs/silicon an effective field emitter.
1. Introduction The carbon nanotubes (CNTs) have been considered as one of the most promising materials for field electron emitters owing to their prominent structural and physics properties, such as high aspect ratio, small radius of curvature, good electric conductivity, and excellent chemical stability [1–7]. The success applications in CNT-based field emission devices require the emitter to have low turn-on electric field, low threshold field, high emission density, and excellent stability at a large current density. To achieve high-performance from CNTs emitters, coating the surface of CNTs with other materials has been used as one of candidates to improve field emission properties [8–12]. Carbon nanoparticles (CNPs) have low work function and high electric conductivity. Therefore, it is expected to exhibit good field emission behaviors as carbon nanotubes. Will the field emission properties be improved If CNPs and CNTs are combined? To our best knowledge, very few previous observations of field emission from CNPs/CNTs have been reported [13], despite several researches on CNPs [14,15] and some studies on field emission from CNTs [2–7]. In this paper, we combine these two materials together in the form of carbon nanoparticles film coating on top of vertically aligned carbon nanotubes (CNPs/CNTs) on silicon substrate by pyrolysis of iron phthalocyanine (FePc) under Ar/H2 atmosphere at 960 °C. The method not only prepares the CNPs/CNTS/silicon with excellent morphologies but significantly improves their field emission properties. The CNPs/CNTs/silicon exhibits superior field emission characteristics
∗
with lower turn-on and threshold fields, high emission density and good emission stability, which make the CNPs/CNTs/silicon promising applications in vacuum electronics devices. 2. Experimental procedure Carbon nanoparticles film coating on top of vertically aligned carbon nanotubes on silicon substrate (CNPs/CNTs/silicon) are prepared by pyrolysis of iron phthalocyanine (FePc) under Ar/H2 atmosphere at a predetermined temperature in a flow reactor comprising of a quartz glass tube and a dual furnace fitted with independent temperature controllers (XD-1200NT). The n-type silicon wafer is used as a substrate, which is cleaned successively in an ultrasonic bath with ethanol and de-ionized water and dried in air. Micro-sized diamond particles are dispersed in polyvinyl alcohol (PVA) solution by mixing 0.02g diamond particles with the diameter of 4–8 μm and 2g PVA powder in deionized water. The solution was spun-coated onto the surface of silicon wafer. After diamond particles are deposited on silicon substrate, the growth of CNPs/CNTs/silicon is carried out. FePc and silicon substrate deposited with diamond particles are placed in the first and second furnaces respectively in the quartz glass reactor. After the temperature in the second furnace reaches 960 °C, the first furnace is heated at 500°C-750 °C for ca. 60 min and the second furnace is kept at the 960°C for ca. 60 min to complete the pyrolysis process. After the growth is finished, the furnace is cooled naturally to room temperature under the protection of Ar.
Corresponding author. E-mail addresses: [email protected] (X.H. Yang), [email protected] (F.G. Zeng).
https://doi.org/10.1016/j.vacuum.2019.06.001 Received 23 April 2019; Received in revised form 1 June 2019; Accepted 1 June 2019 Available online 02 June 2019 0042-207X/ © 2019 Published by Elsevier Ltd.
Vacuum 167 (2019) 113–117
X.H. Yang, et al.
Fig. 1. Plan-view SEM images of the silicon deposited with Micro-sized diamond particles.
The microstructure and elemental compositions of the prepared CNPs/CNTs/silicon are characterized by scanning electron microscopy (SEM; JSM-6700F), X-ray energy dispersive spectroscopy (EDS) and Raman spectroscopy (Renishaw in Via Raman microscope). The field emission tests are performed in DC mode with a diode-type electron emission structure in a vacuum chamber with a low pressure of 4 × 10−4Pa at room temperature. The as-grown CNPs/CNTs/silicon, acting as field emitter, is electrically connected to a 5.3 × 5.3cm2 movable stainless steel plate. The area of the CNPs/CNTs/silicon is about 0.2cm2 . The same stainless steel plate serves as the anode. The field emission tests are conducted by applying variable DC voltages on the anode in the vacuum chamber. The emission current is measured using a Keithley Model 2450 System Source Meter. The distance between anode and cathode CNPs/CNTs/silicon is 5 mm. To evaluate the emission uniformity, a phosphor-coated indium-tin-oxide glass substrate is used as an anode to observe the electron emission patterns.
Three patent features are the D peak at 1353 cm−1 induced by the disorder and defect [17], the G band at 1589 cm−1 manifesting the presence of graphitic carbon and 2D peak at 2703cm−1 indicating longrange order in the sample [17,18]. The ratio of the D peak to G peak implies graphitization degree of carbon nanoparticles. Fig. 4 shows that the intensity of D band is less than G band, suggesting that the crystallization of the CNPs is better [19], which provides a possibility for CNPs to be a good field emitter. Field electron emission tests of the CNPs/CNTs/silicon are conducted, shown in Fig. 5. Fig. 5a is the J-E curve of the sample. The turnon electric field is approximately 1.3V / μm corresponding to the current density of 10 μA/ cm2 . The threshold electric field is about 1.8V / μm where the current density reaches 1mA/cm2 . To easily check the turn-on and threshold electric fields, the current density on lg scale versus the electric field curve of the sample is shown in Fig. 5b. To further investigate the emission properties of as-grown sample， We refer to the Fowler-Nordheim (F-N) equation
3. Results and discussion
⎞ 1 E [1], where J is the emission current ⎠ density, E is the applied electric field, ϕ is the work function determined by the material,a = 1.54 × 10−6AeVV −2 and 3 b = 6.830890 × 109eV − 2Vm−1. Current density J is defined as J=I/A, where I is the measured emission current and A is the area of the CNPs/CNTs/silicon. The field enhancement factor β , which depends on the morphology, can be determined by fitting the slope of the F-N plot and taking a reasonable ϕ . The F-N plot is achieved by re-plotting the J-E data as Ln (J/ E2) versus 1/E, as shown in inset of Fig. 5a. The approximate linear characteristics of the F-N plot clearly indicates that the electron emission is attributed to the tunneling effect. By taking 4.0 eV as the work function value for the CNPs [13,14], the field enhancement factor (β ) is calculated to be about 3.6 × 103 from the F-N plot slope. In addition to the field emission I-V characteristics, emission uniform is also observed, shown in Fig. 6. The image is taken at the electric field of 1.8V / μm . The image shows that the emission sites are almost uniform at most of the emission area. But it can be obviously observed that the dark parts exist on the emission image. It is thought that it is aroused by the uneven growth of the CNPs, which is consistent with the images in Fig. 2. Due to the limits of experiment condition, it is difficult to accurately determine the emission density. Whereas, it can be seen that the density of the emission sites is high. It could be due to the low resistance of the carbon nanotubes under the CNPs film that guarantees an abundant electron source. The superior field emission performances of CNPs/CNPs/silicon are mainly attributed to the following reasons. Firstly, the low work ln
Fig. 1 is SEM images of the silicon wafer deposited with diamond particles. Images show that the micro-sized diamond particles with irregular shape are loosely distributed on the silicon substrate. The morphologies of the CNPs/CNTs/silicon are presented in Fig. 2. It can be seen from Fig. 2a–c that the vertically aligned CNTs with about 12 μm long are grown on silicon substrate. Moreover, it can be found that the top of CNTs is covered by a layer of film with about 100 nm thick and the film splits from the sites around the diamond particles and then propagates. The film crack is induced probably because of the film shrinking faster than the inner materials during cooling naturally to room temperature. Fig. 2d presents that the film is consisted of densely particles. The film elements are given by EDS later. In addition, Fig. 2a shows that the CNTs twist together on the diamond particles and very few quantities of twisted CNTs scatter on the film. Fig. 3 is the SEM-EDS image of film. The corresponding elemental compositions (atomic %) are given in Table 1. The value of C atom equals to 78.41% and that of Si atom equals to 16.06%, which suggests that the film is a layer of compact carbon nanoparticles. The size of CNPs is not uniform and the average particle diameter is about 50 nm. These carbon nanoparticles are not strict balls, which benefits the field electron emission. Theories and experimental results reveal that materials with tips or sharp edges pointing to the electrical field direction can result in a high enhancement factor and easily render field emission under a low macroscopic electric field [3–7,16]. Fig. 4 is the Raman scattering spectra of the CNPs/CNTs/silicon. 114
( ) = ln ( ) − ⎛⎝ J
E2
aβ2
ϕ
3
bϕ 2
β
( )
Vacuum 167 (2019) 113–117
X.H. Yang, et al.
Fig. 2. Plan-view SEM images of CNPs/CNTs/silicon.
Fig. 3. SEM image for the EDS experiment CNPs/CNTs/silicon.
Fig. 4. Raman spectrum of CNPs/CNTs/silicon.
function of the CNPs could lower the height of potential barrier that makes electrons emitting from it more easily. Secondly, the high field enhancement factor contributes to the low turn-on and threshold electric fields. As evident from the SEM images (Fig. 2), the carbon nanoparticles film coats on top of the vertically aligned CNTs, which gives large enhancement factor to the composite. The large enhancement factor enhances the electric field on the surface of the CNPs, allowing for electrons tunneling from CNPs into vacuum under a relative low macroscopic electric field. Thirdly, according to the Raman spectra
Table 1 Elemental composition (atomic %) of CNPs/CNTs/silicon. Element
C
Si
O
Fe
Atom (%) Total (%)
78.41 100.00
16.06
4.77
0.76
115
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Fig. 7. Field emission stability of CNPs/CNTs/silicon.
applications in vacuum electronic devices. The emission stability test of CNPs/CNTs/silicon is performed by recording the average emission current every second at an average emission current density of approximately 1mA/cm2 for 7.6 h. As shown in Fig. 7, it can be seen that the current density shows a gradually increase at the beginning and then fluctuates around current density of 1mA cm−2 within the first 5 h. The stability of emission current of the sample is probably due to the excellent thermal conductivity and the thermal stability of the diamond particles. The diamond particles are deposited on silicon before the sample is grew. However, it should be noted that the average current density decays slowly after 5 h. The reason of emission current decay could be ascribed to the following possible factors. The first one is the electrostatic force that can peel off the CNTs from substrate as the macroscopic electric field is continually applied, which causes current to decay. Though the turn-on and threshold fields are low, the high voltage is needed due to the large distance between cathode and anode. Therefore, the second one could be Joule heating arising from the applied high voltages, which will “burn” the CNTs at the point where the temperature reaches a value. Fig. 5. (a) The current density versus the electric field plot of CNPs/CNTs/ silicon emitter. The inset shows the F-N plot, (b)the current density on lg scale versus the electric field plot of CNPs/CNTs/silicon emitter.
4. Conclusion We synthesize the carbon nanoparticles film coating on top of vertically aligned CNTs on silicon wafer by the pyrolysis of FePC at 960 °C and measure their field emission characteristics. The results present that CNPs/CNTs/silicon exhibits good emission properties. The turn-on electric field is 1.3V / μm at an emission current density of 10 μA/ cm2 and the threshold electrical field is 1.8V / μm at an emission current density of 1mA/cm2 . Besides, CNPs/CNTs/silicon has a high emission density at around 1 mA/cm2 and long-time field emission stability. The excellent field emission properties of CNPs/CNTs/silicon are derived from the structure of CNPs/CNTs/silicon and the high degree of graphitization of the CNPs. The results show that CNPs/CNTs/silicon will be an excellent field emitter. Acknowledgements This work is supported by Innovation Scientists and Technicians Troop Construction Projects of Henan Province (No. 164200510006), Aeronautical Science Foundation of China (No. 2015 ZF 55013) and Henan key Laboratory of aeronautical material and application technology.
Fig. 6. Emission pattern from CNPs/CNTs/silicon with 1 mA/cm2.
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