- Email: [email protected]
Field emission characteristics of carbon nanofibers grown on copper micro-tips at low temperature
ARTICLE IN PRESS
Vacuum 82 (2008) 551–555 www.elsevier.com/locate/vacuum
Field emission characteristics of carbon nanofibers grown on copper micro-tips at low temperature Woo Yong Sunga, Wal Jun Kima, Ho Young Leeb, Yong Hyup Kima, a
School of Mechanical and Aerospace Engineering, Seoul National University, Sillim-dong, Gwanak-gu, Seoul 151-742, Republic of Korea b Advanced Materials and Process Research Center for IT, Sunkkyunkwan University, Suwon, Gyeonggi-do 440-746, Republic of Korea Received 31 October 2006; received in revised form 16 July 2007; accepted 16 July 2007
Abstract Carbon nanofibers (CNFs) were grown on copper micro-tips formed by electroplating. The nickel layer electroplated over the copper micro-tips was used as a catalyst. The CNFs were synthesized by using plasma-enhanced chemical vapor deposition (PECVD) of C2H2 and NH3 at 480 1C. The copper micro-tips were formed by high current pulse electroplating, which played a significant role in characterizing our CNFs. The CNFs grown on the copper micro-tips showed outstanding field emission performance and stability, whose turn-on field, defined as one at the current density of 10 mA/cm2, was 1.30 V/mm and the maximum current density reached 5.39 mA/cm2 at an electric field of 4.9 V/mm. r 2007 Elsevier Ltd. All rights reserved. Keywords: Field emission; Carbon nanofibers; PECVD; Low temperature
1. Introduction Carbon nanotubes (CNTs) [1] have recently emerged as an attractive cold cathode emitter due to their excellent field emission characteristics such as low turn-on field, high current density, and high physical and chemical stability. Interestingly, other forms of carbon nanostructures, carbon nanofibers (CNFs) [2], have also demonstrated comparably excellent field emission characteristics. Both CNTs and CNFs are suitable, in particular, for electron emitter applications such as field emission displays, etc. Chemical vapor deposition (CVD) has been widely used for the synthesis of carbon nanostructures. A catalyst plays an important role in growing CNF. Since chemical compositions, particle sizes, and geometries of the catalysts are important factors in the determination of the structures, properties, and dimensions of CNFs; various methods have been adopted to identify these factors of catalysts. Merkulov et al. [3] evaporated Ni on n-type Silicon by e-gun. Shyu and Hong [4] deposited Fe–Ni alloy Corresponding author. Tel.: +82 2 880 7385; fax: +82 2 887 2662.
E-mail address: [email protected] (Y.H. Kim). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.07.051
catalysts with various composition ratios by e-beam evaporation. Other groups have reported the synthesis of carbon nanostructures by using nickel catalysts formed by electroplating or electroless plating [5–8]. It has been also reported that the surface morphology of a substrate is an important factor in the growth of carbon nanostructures. Liu and Fan [9] presented improved field emission characteristics of CNTs grown on silicon nanowires formed by high-temperature thermal CVD. Sharma et al. [10] reported field emission characteristics of CNTs grown on a tungsten tip prepared by electrochemical etching. Sato et al. [11,12] presented CNTs grown on an arrays of pyramid-shaped protrusions on a silicon substrate by the lift-off process and density control of CNTs grown on cone-shaped nanoscale tips formed by reactive ion etching [13]. In the present study, we investigate CNFs grown on copper micro-tips and their field emission characteristics. The copper micro-tips were easily formed by high current pulse electroplating and had very sharp geometries. The geometry of the copper micro-tips provided a special morphology of the substrate. The high aspect ratio of the copper micro-tips induces the concentration of electric
ARTICLE IN PRESS W.Y. Sung et al. / Vacuum 82 (2008) 551–555
552
potential as well as facilitates the diffusion of source gas molecules. The CNFs were synthesized by using plasmaenhanced chemical vapor deposition (PECVD) at 480 1C, which is low enough for the use of glass substrates. The CNFs synthesized in the present study resembled flowers with bunch of fine branches formed around the tip and demonstrated high performance of field emission. 2. Experimental Before electroplating the copper micro-tips, a 30-nmthick Cr layer and a 300-nm-thick Cu layer were sequentially deposited on a silicon substrate by thermal evaporation. The Cr layer located between the Cu layer and the silicon substrate works as an adhesion layer, and the Cu layer works as a seed layer for the subsequent copper electroplating. The copper micro-tips were formed by high current pulse electroplating with a peak current of 850 mA, duty ratio of 60%, and frequency of 1000 Hz. The solution for the copper electroplating used in the present study contained copper sulfate (CuSO4 5H2O), sulfuric acid (H2SO4), and hydrochloric acid (HCl) in Ref. [16]. The copper micro-tips were coated with a 30-nm-thick Ti layer and a 20-nm-thick Cu layer. The Ti layer and the Cu layer work as a buffer layer to prevent the diffusion of the nickel catalyst and as an electrode layer for the subsequent Ni electroplating, respectively. Electroplated nickel layer was followed to form the catalyst for the synthesis of CNFs. The solution for the nickel electroplating used in the present study contained nickel sulfate (NiSO4 6H2O), nickel chloride (NiCl2 6H2O), and boric acid (H3BO3) [14]. The electroplating was performed under the conditions of room temperature and pH of 3.7. After the specimens were prepared with the catalysts, they were transferred to PECVD to synthesize CNFs. The temperature measured by using a thermocouple directly mounted on the specimens was maintained at 480 1C. For plasma deposition, a DC discharge between the heater stage (cathode) and the gas shower head (anode, 2.5 cm above the stage) was ignited by applying a fixed voltage of 450 V. A stable discharge current of 100 mA was main-
tained for the entire process time of 30 min. C2H2 and NH3 were introduced through separate mass flow controllers to synthesize CNFs. The ratio of C2H2:NH3 was kept constant (25 sccm:50 sccm) under the total pressure of 1.06 102 bar. The field emission characteristics of CNFs were investigated by using a diode-type configuration in a vacuum chamber with a base pressure of 5 1010 bar. The CNFs and a phosphor-coated indium–tin oxide (ITO) glass were used as a cathode and an anode, respectively, and a 610-mm-thick spacer was located between the cathode and anode. The emission currents were evaluated by averaging the measured currents over the area of a hole with a diameter of 6 mm, through which the currents were collected. The specimens were aged to remove contaminants and allowed to degas before taking reliability measurements. The field emission measurements were carried out several times while each of the result was recorded using LABVIEW. 3. Result and discussion Fig. 1(a) shows the sharp, vertically aligned geometry of the copper micro-tips. Each tip diameter ranged 30–50 nm, and the overall length was about 5–10 mm. The inset of Fig. 1(a) is an HR-TEM image of an individual copper micro-tip. Fig. 1(b) shows the electroplated Ni catalyst, and the inset of the figure shows the magnified images. The nickel catalysts crowd around the tip due to the concentration of electric potential at the micro-tips, as shown in the figure, and result in the selective growth of CNFs at the tip of the copper micro-structures in the subsequent process. The nickel catalysts are already shaped like particles and look like a sea squirt. Because of the morphology of the catalysts, the pretreatment process for making the catalytic particle can be skipped, and the activation level of the feeding gases can be increased. The images of the CNFs synthesized in the present study are shown in Fig. 2. The CNFs grown on the copper microtips have very unique geometry: a bunch of fine branches around the tips of the CNF backbones. The fine branches have a much smaller diameter than the backbones. The backbones have a diameter ranging 200–400 nm and a
Fig. 1. (a) Sharp, vertically aligned geometry of the copper micro-tips. (b) Electroplated Ni catalyst. Inset: magnified images.
ARTICLE IN PRESS W.Y. Sung et al. / Vacuum 82 (2008) 551–555
553
Fig. 2. Images of the CNFs synthesized in the present study.
Fig. 4. Raman spectra measured in the frequency range of 11001700 cm1.
Fig. 3. HR-TEM images of the CNFs.
length of 20 mm, and the branches have a diameter ranging 5–20 nm, as shown in the figure. HR-TEM images of the CNFs are shown in Fig. 3. The figure shows the tip part of the CNF backbone. As shown in the figure, there exist a large number of very small particles around the tip of the backbone. It is believed that they are catalytic nickel particles, from which the fine branches grow. We think that ion bombardment of PECVD sputtered the nickel catalyst at the tip of the backbone and produced a large number of very small nickel particles [15]. We cannot observe the fine branches in the TEM images because they are detached during the sonication process for the preparation of the TEM samples. The crystalline properties of the CNFs grown on copper micro-tips were inspected via Raman spectra analysis where the wavelength of the excitation laser was 514.53 nm. Fig. 4 illustrates the results of the Raman
spectra measured in the frequency range of 1100– 11700 cm1. The relative intensity ratio of D-band to G-band (ID/IG) was 0.85, indicating lower crystalline property, which is similar with that of the CNCs grown on the copper micro-tips [16]. Fig. 5 shows the field emission characteristics of the CNFs grown on the copper micro-tips. It was observed that the CNFs showed a turn-on field of 1.3 V/mm at the current density of 10 mA/cm2. The maximum current density was 5.39 mA/cm2 at an electric field of 4.9 V/mm. The inset of Fig. 5 indicates the Fowler–Nordheim (F–N) plot for the CNFs. Our emission results agreed well with the theoretical F–N characteristic. The field-enhancement factor b of the emitter can be evaluated from the wellknown equation b ¼ Bf3=2 d=S, where B ¼ 6:83 109 ðV eV3=2 m1 Þ, f is the work function, d is the distance between the emitter tip and anode, and S is the slope of the F–N plot [17]. The slope of the F–N plot was different in the low- and high-electric-field regimes, as revealed in Fig. 5. The slope of the F–N plot was steeper at the lowfield regime than at the high-field regime. Such a deviation of F–N behavior caused by the current saturation effect
ARTICLE IN PRESS W.Y. Sung et al. / Vacuum 82 (2008) 551–555
554
might be attributed to the detachment of adsorbates [18]. The adsorptive gas molecules enhanced the field emission current of CNFs at the low-electric-field range. When the current density grew as the electric field increased, the tip temperature of the CNFs rose via Joule heating. This rise in tip temperature resulted in the desorption of adsorbates on the CNFs and led to current saturation. We determined b to be about 5894 and 8053 in both the low- and high-field regimes, respectively, where the work function f was assumed to be 5 eV [19]. These values can be favorably compared with the previously reported values, which are in the range of 200–5000 [20]. From the intercepts and slopes of the F–N plots, we estimated that the total real emission area was 6.44 1013 for our CNFs. Fig. 6 represents the emission current stability over the test period of 10 h at the direct current (DC) field of 2.8 V/mm. The average emission current Iaverage, standard deviation DI, and fluctuation f,
10 C N Fs
Current density (mA / cm2)
Fig. 7. Emission image of the CNFs grown on the copper micro-tips.
where f ¼ DI/Iaverage, were determined for the studied CNFs. CNFs exhibited 12.6% fluctuations. Fig. 7 shows the emission image of the CNFs grown on the copper micro-tips. We observed a large number of emission sites as well as stable emission currents. It is believed that the fine branches of the CNFs contributed to the enhanced performance of field emission.
1
0.1
4. Conclusion 0
1
2
3
4
5
Applied voltage (V / µm)
Fig. 5. Field emission characteristics of the CNFs grown on the copper micro-tips.
CNFs were synthesized on copper micro-tips by using PECVD at 480 1C, which was low enough for use of glass substrates. The copper micro-tips were formed by high current pulse electroplating and had very sharp geometries. The high aspect ratio of the copper micro-tips led to the concentration of electric potential during the electroplating of the nickel catalysts and resulted in the formation of catalysts and, eventually, CNFs only around the tips. Because the catalysts had very rough morphology, the pretreatment process for making the catalytic particles can be skipped. The CNFs grown on the copper micro-tips had very unique geometries with a bunch of fine branches around the tips of the CNF backbones. The present method using the copper micro-tips allowed the synthesis of CNFs with high performance of field emission. We believe that the CNFs grown on the copper micro-tips have much potential in nanoelectronics applications. Acknowledgments
Fig. 6. Emission current stability over the test period of 10 h with a direct current (DC) field of 2.8V/um.
This work was supported by a Grant (code #: 05K150102430) from the Center for Nanostructured Materials Technology under the 21st Century Frontier R&D
ARTICLE IN PRESS W.Y. Sung et al. / Vacuum 82 (2008) 551–555
Programs of the Ministry of Science and Technology and the Brain Korea 21 Project in 2006, Korea. References [1] de Heer WA, Chatelain A, Ugarte D. Science 1995;270:1179. [2] Chen Y, Patel S, Ye Y, Shaw DT, Guo L. Appl Phys Lett 1998;73:2119. [3] Merkulov VI, Melechko AV, Guillorn MA, Lowndes DH, Simpson ML. Appl Phys Lett 2002;80:476. [4] Shyu YM, Hong FCN. Diamond Relat Mater 2001;10:1241. [5] Singh MK, Singh PP, Titus E, Misra DS, LeNormand F. Chem Phys Lett 2002;354:331. [6] Rasi T, Chuang C, Chao C, Liu W. Diamond Relat Mater 2003;12: 1453. [7] Park KH, Yim JH, Lee SI, Koh KH. J Vac Sci Technol 2005; B23(2):776. [8] Park KH, Lee SI, Koh KH. Diamond Relat Mater 2005;14:2094. [9] Liu Y, Fan S. Solid State Commun 2005;133:131.
555
[10] Sharma RB, Tondare VN, Joag DS, Govindaraj A, Rao CNR. Chem Phys Lett 2001;344:283. [11] Sato H, Takegawa H, Yamaji H, Miyake H, Hiramatsua K, Saito Y. J Vac Sci Technol 2004;B22:1335. [12] Sato H, Hata K, Miyake H, Hiramatsu K, Saito Y. J Vac Sci Technol 2005;B23(2):754. [13] Sato H, Matsubayashi M, Sakai T, Hata K, Miyake H, Hiramatsu K, et al. J Vac Sci Technol 2006;B24(2):1004. [14] Park KH, Yim JH, Lee SI, Koh KH. J Vac Sci Technol 2005; B23(2):776–80. [15] AuBuchon JF, Chen L-H, Daraio C, Jin S. Nano Lett 2006;6: 324. [16] Sung WY, Ok JG, Kim WJ, Lee SM, Yeon SC, Lee HY, et al. Nanotechnology 2007;18:245603. [17] Choi JH, Choi SH, Han JH, Yoo JB, Park CY, Jung T, et al. J Appl Phys 2003;94:487. [18] Dean KA, Chalamala BR. Appl Phys Lett 2000;76:375. [19] Tan TT, Sim HS, Laua SP, Yang HY, Tanemura M, Tanaka J. Appl Phys Lett 2006;88:103105. [20] Jung SM, Hahn JO, Jung HY, Suh JS. Nano Lett 2006;6:1569.
Vacuum 82 (2008) 551–555 www.elsevier.com/locate/vacuum
Field emission characteristics of carbon nanofibers grown on copper micro-tips at low temperature Woo Yong Sunga, Wal Jun Kima, Ho Young Leeb, Yong Hyup Kima, a
School of Mechanical and Aerospace Engineering, Seoul National University, Sillim-dong, Gwanak-gu, Seoul 151-742, Republic of Korea b Advanced Materials and Process Research Center for IT, Sunkkyunkwan University, Suwon, Gyeonggi-do 440-746, Republic of Korea Received 31 October 2006; received in revised form 16 July 2007; accepted 16 July 2007
Abstract Carbon nanofibers (CNFs) were grown on copper micro-tips formed by electroplating. The nickel layer electroplated over the copper micro-tips was used as a catalyst. The CNFs were synthesized by using plasma-enhanced chemical vapor deposition (PECVD) of C2H2 and NH3 at 480 1C. The copper micro-tips were formed by high current pulse electroplating, which played a significant role in characterizing our CNFs. The CNFs grown on the copper micro-tips showed outstanding field emission performance and stability, whose turn-on field, defined as one at the current density of 10 mA/cm2, was 1.30 V/mm and the maximum current density reached 5.39 mA/cm2 at an electric field of 4.9 V/mm. r 2007 Elsevier Ltd. All rights reserved. Keywords: Field emission; Carbon nanofibers; PECVD; Low temperature
1. Introduction Carbon nanotubes (CNTs) [1] have recently emerged as an attractive cold cathode emitter due to their excellent field emission characteristics such as low turn-on field, high current density, and high physical and chemical stability. Interestingly, other forms of carbon nanostructures, carbon nanofibers (CNFs) [2], have also demonstrated comparably excellent field emission characteristics. Both CNTs and CNFs are suitable, in particular, for electron emitter applications such as field emission displays, etc. Chemical vapor deposition (CVD) has been widely used for the synthesis of carbon nanostructures. A catalyst plays an important role in growing CNF. Since chemical compositions, particle sizes, and geometries of the catalysts are important factors in the determination of the structures, properties, and dimensions of CNFs; various methods have been adopted to identify these factors of catalysts. Merkulov et al. [3] evaporated Ni on n-type Silicon by e-gun. Shyu and Hong [4] deposited Fe–Ni alloy Corresponding author. Tel.: +82 2 880 7385; fax: +82 2 887 2662.
E-mail address: [email protected] (Y.H. Kim). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.07.051
catalysts with various composition ratios by e-beam evaporation. Other groups have reported the synthesis of carbon nanostructures by using nickel catalysts formed by electroplating or electroless plating [5–8]. It has been also reported that the surface morphology of a substrate is an important factor in the growth of carbon nanostructures. Liu and Fan [9] presented improved field emission characteristics of CNTs grown on silicon nanowires formed by high-temperature thermal CVD. Sharma et al. [10] reported field emission characteristics of CNTs grown on a tungsten tip prepared by electrochemical etching. Sato et al. [11,12] presented CNTs grown on an arrays of pyramid-shaped protrusions on a silicon substrate by the lift-off process and density control of CNTs grown on cone-shaped nanoscale tips formed by reactive ion etching [13]. In the present study, we investigate CNFs grown on copper micro-tips and their field emission characteristics. The copper micro-tips were easily formed by high current pulse electroplating and had very sharp geometries. The geometry of the copper micro-tips provided a special morphology of the substrate. The high aspect ratio of the copper micro-tips induces the concentration of electric
ARTICLE IN PRESS W.Y. Sung et al. / Vacuum 82 (2008) 551–555
552
potential as well as facilitates the diffusion of source gas molecules. The CNFs were synthesized by using plasmaenhanced chemical vapor deposition (PECVD) at 480 1C, which is low enough for the use of glass substrates. The CNFs synthesized in the present study resembled flowers with bunch of fine branches formed around the tip and demonstrated high performance of field emission. 2. Experimental Before electroplating the copper micro-tips, a 30-nmthick Cr layer and a 300-nm-thick Cu layer were sequentially deposited on a silicon substrate by thermal evaporation. The Cr layer located between the Cu layer and the silicon substrate works as an adhesion layer, and the Cu layer works as a seed layer for the subsequent copper electroplating. The copper micro-tips were formed by high current pulse electroplating with a peak current of 850 mA, duty ratio of 60%, and frequency of 1000 Hz. The solution for the copper electroplating used in the present study contained copper sulfate (CuSO4 5H2O), sulfuric acid (H2SO4), and hydrochloric acid (HCl) in Ref. [16]. The copper micro-tips were coated with a 30-nm-thick Ti layer and a 20-nm-thick Cu layer. The Ti layer and the Cu layer work as a buffer layer to prevent the diffusion of the nickel catalyst and as an electrode layer for the subsequent Ni electroplating, respectively. Electroplated nickel layer was followed to form the catalyst for the synthesis of CNFs. The solution for the nickel electroplating used in the present study contained nickel sulfate (NiSO4 6H2O), nickel chloride (NiCl2 6H2O), and boric acid (H3BO3) [14]. The electroplating was performed under the conditions of room temperature and pH of 3.7. After the specimens were prepared with the catalysts, they were transferred to PECVD to synthesize CNFs. The temperature measured by using a thermocouple directly mounted on the specimens was maintained at 480 1C. For plasma deposition, a DC discharge between the heater stage (cathode) and the gas shower head (anode, 2.5 cm above the stage) was ignited by applying a fixed voltage of 450 V. A stable discharge current of 100 mA was main-
tained for the entire process time of 30 min. C2H2 and NH3 were introduced through separate mass flow controllers to synthesize CNFs. The ratio of C2H2:NH3 was kept constant (25 sccm:50 sccm) under the total pressure of 1.06 102 bar. The field emission characteristics of CNFs were investigated by using a diode-type configuration in a vacuum chamber with a base pressure of 5 1010 bar. The CNFs and a phosphor-coated indium–tin oxide (ITO) glass were used as a cathode and an anode, respectively, and a 610-mm-thick spacer was located between the cathode and anode. The emission currents were evaluated by averaging the measured currents over the area of a hole with a diameter of 6 mm, through which the currents were collected. The specimens were aged to remove contaminants and allowed to degas before taking reliability measurements. The field emission measurements were carried out several times while each of the result was recorded using LABVIEW. 3. Result and discussion Fig. 1(a) shows the sharp, vertically aligned geometry of the copper micro-tips. Each tip diameter ranged 30–50 nm, and the overall length was about 5–10 mm. The inset of Fig. 1(a) is an HR-TEM image of an individual copper micro-tip. Fig. 1(b) shows the electroplated Ni catalyst, and the inset of the figure shows the magnified images. The nickel catalysts crowd around the tip due to the concentration of electric potential at the micro-tips, as shown in the figure, and result in the selective growth of CNFs at the tip of the copper micro-structures in the subsequent process. The nickel catalysts are already shaped like particles and look like a sea squirt. Because of the morphology of the catalysts, the pretreatment process for making the catalytic particle can be skipped, and the activation level of the feeding gases can be increased. The images of the CNFs synthesized in the present study are shown in Fig. 2. The CNFs grown on the copper microtips have very unique geometry: a bunch of fine branches around the tips of the CNF backbones. The fine branches have a much smaller diameter than the backbones. The backbones have a diameter ranging 200–400 nm and a
Fig. 1. (a) Sharp, vertically aligned geometry of the copper micro-tips. (b) Electroplated Ni catalyst. Inset: magnified images.
ARTICLE IN PRESS W.Y. Sung et al. / Vacuum 82 (2008) 551–555
553
Fig. 2. Images of the CNFs synthesized in the present study.
Fig. 4. Raman spectra measured in the frequency range of 11001700 cm1.
Fig. 3. HR-TEM images of the CNFs.
length of 20 mm, and the branches have a diameter ranging 5–20 nm, as shown in the figure. HR-TEM images of the CNFs are shown in Fig. 3. The figure shows the tip part of the CNF backbone. As shown in the figure, there exist a large number of very small particles around the tip of the backbone. It is believed that they are catalytic nickel particles, from which the fine branches grow. We think that ion bombardment of PECVD sputtered the nickel catalyst at the tip of the backbone and produced a large number of very small nickel particles [15]. We cannot observe the fine branches in the TEM images because they are detached during the sonication process for the preparation of the TEM samples. The crystalline properties of the CNFs grown on copper micro-tips were inspected via Raman spectra analysis where the wavelength of the excitation laser was 514.53 nm. Fig. 4 illustrates the results of the Raman
spectra measured in the frequency range of 1100– 11700 cm1. The relative intensity ratio of D-band to G-band (ID/IG) was 0.85, indicating lower crystalline property, which is similar with that of the CNCs grown on the copper micro-tips [16]. Fig. 5 shows the field emission characteristics of the CNFs grown on the copper micro-tips. It was observed that the CNFs showed a turn-on field of 1.3 V/mm at the current density of 10 mA/cm2. The maximum current density was 5.39 mA/cm2 at an electric field of 4.9 V/mm. The inset of Fig. 5 indicates the Fowler–Nordheim (F–N) plot for the CNFs. Our emission results agreed well with the theoretical F–N characteristic. The field-enhancement factor b of the emitter can be evaluated from the wellknown equation b ¼ Bf3=2 d=S, where B ¼ 6:83 109 ðV eV3=2 m1 Þ, f is the work function, d is the distance between the emitter tip and anode, and S is the slope of the F–N plot [17]. The slope of the F–N plot was different in the low- and high-electric-field regimes, as revealed in Fig. 5. The slope of the F–N plot was steeper at the lowfield regime than at the high-field regime. Such a deviation of F–N behavior caused by the current saturation effect
ARTICLE IN PRESS W.Y. Sung et al. / Vacuum 82 (2008) 551–555
554
might be attributed to the detachment of adsorbates [18]. The adsorptive gas molecules enhanced the field emission current of CNFs at the low-electric-field range. When the current density grew as the electric field increased, the tip temperature of the CNFs rose via Joule heating. This rise in tip temperature resulted in the desorption of adsorbates on the CNFs and led to current saturation. We determined b to be about 5894 and 8053 in both the low- and high-field regimes, respectively, where the work function f was assumed to be 5 eV [19]. These values can be favorably compared with the previously reported values, which are in the range of 200–5000 [20]. From the intercepts and slopes of the F–N plots, we estimated that the total real emission area was 6.44 1013 for our CNFs. Fig. 6 represents the emission current stability over the test period of 10 h at the direct current (DC) field of 2.8 V/mm. The average emission current Iaverage, standard deviation DI, and fluctuation f,
10 C N Fs
Current density (mA / cm2)
Fig. 7. Emission image of the CNFs grown on the copper micro-tips.
where f ¼ DI/Iaverage, were determined for the studied CNFs. CNFs exhibited 12.6% fluctuations. Fig. 7 shows the emission image of the CNFs grown on the copper micro-tips. We observed a large number of emission sites as well as stable emission currents. It is believed that the fine branches of the CNFs contributed to the enhanced performance of field emission.
1
0.1
4. Conclusion 0
1
2
3
4
5
Applied voltage (V / µm)
Fig. 5. Field emission characteristics of the CNFs grown on the copper micro-tips.
CNFs were synthesized on copper micro-tips by using PECVD at 480 1C, which was low enough for use of glass substrates. The copper micro-tips were formed by high current pulse electroplating and had very sharp geometries. The high aspect ratio of the copper micro-tips led to the concentration of electric potential during the electroplating of the nickel catalysts and resulted in the formation of catalysts and, eventually, CNFs only around the tips. Because the catalysts had very rough morphology, the pretreatment process for making the catalytic particles can be skipped. The CNFs grown on the copper micro-tips had very unique geometries with a bunch of fine branches around the tips of the CNF backbones. The present method using the copper micro-tips allowed the synthesis of CNFs with high performance of field emission. We believe that the CNFs grown on the copper micro-tips have much potential in nanoelectronics applications. Acknowledgments
Fig. 6. Emission current stability over the test period of 10 h with a direct current (DC) field of 2.8V/um.
This work was supported by a Grant (code #: 05K150102430) from the Center for Nanostructured Materials Technology under the 21st Century Frontier R&D
ARTICLE IN PRESS W.Y. Sung et al. / Vacuum 82 (2008) 551–555
Programs of the Ministry of Science and Technology and the Brain Korea 21 Project in 2006, Korea. References [1] de Heer WA, Chatelain A, Ugarte D. Science 1995;270:1179. [2] Chen Y, Patel S, Ye Y, Shaw DT, Guo L. Appl Phys Lett 1998;73:2119. [3] Merkulov VI, Melechko AV, Guillorn MA, Lowndes DH, Simpson ML. Appl Phys Lett 2002;80:476. [4] Shyu YM, Hong FCN. Diamond Relat Mater 2001;10:1241. [5] Singh MK, Singh PP, Titus E, Misra DS, LeNormand F. Chem Phys Lett 2002;354:331. [6] Rasi T, Chuang C, Chao C, Liu W. Diamond Relat Mater 2003;12: 1453. [7] Park KH, Yim JH, Lee SI, Koh KH. J Vac Sci Technol 2005; B23(2):776. [8] Park KH, Lee SI, Koh KH. Diamond Relat Mater 2005;14:2094. [9] Liu Y, Fan S. Solid State Commun 2005;133:131.
555
[10] Sharma RB, Tondare VN, Joag DS, Govindaraj A, Rao CNR. Chem Phys Lett 2001;344:283. [11] Sato H, Takegawa H, Yamaji H, Miyake H, Hiramatsua K, Saito Y. J Vac Sci Technol 2004;B22:1335. [12] Sato H, Hata K, Miyake H, Hiramatsu K, Saito Y. J Vac Sci Technol 2005;B23(2):754. [13] Sato H, Matsubayashi M, Sakai T, Hata K, Miyake H, Hiramatsu K, et al. J Vac Sci Technol 2006;B24(2):1004. [14] Park KH, Yim JH, Lee SI, Koh KH. J Vac Sci Technol 2005; B23(2):776–80. [15] AuBuchon JF, Chen L-H, Daraio C, Jin S. Nano Lett 2006;6: 324. [16] Sung WY, Ok JG, Kim WJ, Lee SM, Yeon SC, Lee HY, et al. Nanotechnology 2007;18:245603. [17] Choi JH, Choi SH, Han JH, Yoo JB, Park CY, Jung T, et al. J Appl Phys 2003;94:487. [18] Dean KA, Chalamala BR. Appl Phys Lett 2000;76:375. [19] Tan TT, Sim HS, Laua SP, Yang HY, Tanemura M, Tanaka J. Appl Phys Lett 2006;88:103105. [20] Jung SM, Hahn JO, Jung HY, Suh JS. Nano Lett 2006;6:1569.