DSpace at VNU: Growth of carbon nanotubes on stainless steel substrates by DC-PECVD

Applied Surface Science 256 (2009) 1065–1068 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Growth of carbon nanotubes on stainless steel substrates by DC-PECVD Dao Quang Duy a, Hyun Suk Kim b, Dang Mo Yoon a, Kang Jae Lee b, Jung Woong Ha a, Yong Gyoo Hwang a, Choong Hun Lee a,*, Bach Thanh Cong c a Division of Microelectronics and Display Technology, College of Natural Sciences, Wonkwang University, Iksan, 344-2 Shinyong Dong, Iksan, Jeonbuk 570-749, Republic of Korea Regional Innovation Center for Next Generation Industrial Radiation Technology, 208 College of Natural Sciences, Wonkwang University, Iksan, 344-2 Shinyong Dong, Iksan, Jeonbuk 570-749, Republic of Korea c Faculty of Physics, HaNoi University of Science, VietNam National University, 334 Nguyen Trai, Thanh Xuan, HaNoi, VietNam b A R T I C L E I N F O A B S T R A C T Article history: Available online June 2009 We report on the fabrication of carbon nanotubes (CNTs) on Ni-coated stainless steel (SUS) substrates by using dc plasma enhanced chemical vapor deposition The synthesized CNTs have the diameter of about 30 nm and the length of about 1.2 mm To verify the effects of SUS substrates on the growth of CNTs, CNTs had also been grown on Ni-coated Si substrates CNTs grown on the SUS substrates were more uniform compared with those grown on the Si substrates Field emission properties of the CNT films were measured in the diode configuration, and the turn-on electric field of 3.87 V/mm and field enhancement factor b of about 1737 were obtained from the synthesized CNTs at the gap of 500 mm between the SUS substrate and the anode These results have not only clarified the effects of the substrate on the growth of CNTs, but also shown the potential of CNTs in field emission applications, especially CNT-based coldcathode X-ray tubes ß 2009 Elsevier B.V All rights reserved PACS: 72.80 Keywords: Carbon nanotube DC-PECVD Pretreatment SUS Field emission Introduction Since the discovery of carbon nanotubes (CNTs) in 1991 [1], they have attracted considerable interest because of their unique physical properties and many potential applications With the nanometer-size diameter and very large aspect ratio, CNTs exhibit unique electronic properties such as excellent electron emission efficiency and extraordinary mechanical properties They are the potential building blocks for field emission displays, tips for scanning probe microscopy, X-ray sources using field emission cathode, hydrogen storage, chemical sensors, high-strength mechanical composites, etc [2–5] It is reported that X-ray sources with field emission cathodes have several intrinsic advantages over thermionic X-ray tubes, including low temperature, instantaneous response, and the potential for miniaturization Field emission X-ray tubes with CNT emitters [6] had recently been demonstrated to have significantly improved properties compared to those with metal [7] or diamond tips [8] For field emission X-ray tubes with CNT emitters, the synthesis of CNTs directly on metallic substrates will greatly simplify the preparation of cold cathodes However, it has been reported that the synthesis of CNTs on metallic or electrically conducting substrates is rather difficult compared to that on * Corresponding author Tel.: +82 63 850 6784; fax: +82 63 850 7138 E-mail address: chlee@wonkwang.ac.kr (C.H Lee) 0169-4332/$ – see front matter ß 2009 Elsevier B.V All rights reserved doi:10.1016/j.apsusc.2009.05.106 insulators such as glass or silicon wafers It has been attributed to the high mobility and lack of localization of carbon atoms on metallic surfaces, or to the difficulty of catalyst island formation due to the diffused reaction and interfacial bonding between the catalytic layer and the metallic surface at the growth temperature [9] Although metal substrates, in recent years, have been more frequently researched [10–12], their effects on growth of CNTs have not been fully understood In this research, we report the growth of carbon nanotubes on Ni-coated stainless steel (SUS) substrates by dc plasma enhanced chemical vapor deposition (DC-PECVD) The synthesized CNTs have the diameter of about 30 nm and the length of 1.2 mm, which are more uniform compared with those grown on Ni-coated Si substrates It is attributed to the higher uniformity of Ni catalyst particles on the SUS substrates after the pretreatment with NH3 plasma, compared to those on the Si substrates Field emission properties of the CNT films were measured in the diode configuration The turn-on electric field of 3.87 V/mm and the field enhancement factor b of about 1737 were obtained from the synthesized CNTs at the gap of 500 mm between the SUS substrate and the anode Experimental CNTs had been grown on Ni-coated SUS substrates with a TiN buffer layer by dc plasma enhanced chemical vapor deposition (DC-PEVCD) The Ni layer and the TiN buffer layer with a thickness 1066 D.Q Duy et al / Applied Surface Science 256 (2009) 1065–1068 of 50 and 1000 A˚, respectively, were deposited by using a radiofrequency magnetron sputtering system It has been reported that the formation of Ni grains during the pretreatment process plays a key role in growing CNTs; when Ni forms alloys such as NiFe, NiCr, etc (with SUS substrates) or NiSi2 (with Si substrates), other isomers of carbon such as carbon nanotips are formed instead of CNT [13], and therefore TiN buffer layers with excellent electrical conductivity (resistivity $25 mV cm) and high melting point ($3200 8C) [9] are usually added to prevent the reaction between catalyst layers and substrates [14] To create uniform Ni particles, NH3 gas was introduced for During this process, the cathode voltage, the temperature, and the flow rate were kept at À550 V, 600 8C, and 60 sccm, respectively The base pressure of the reactor was maintained 3.4 Â 10À6 Torr Before the CNT growth, we have performed annealing procedures at the temperature of 600 8C in H2 environments After the pretreatment and annealing processes, CNTs were grown at 600 8C for 15 using a mixture of acetylene and ammonia with the flow rates of 30 and 100 sccm, respectively To examine the effects of SUS substrates on the growth of CNTs, CNTs were grown also on Ni-coated Si substrates at the same synthesizing conditions as above The morphology, density, and quality of the CNTs were analyzed using field emission scanning electron microscope (FESEM, Hitachi S-4800), a high resolution transmission electron microscope (HRTEM, JEM 2200FS), and Raman spectroscopy (Ar+ laser 514 nm, 2.42 eV), respectively The morphology of Ni particles and Ni catalyst films were investigated using an atomic force microscope (AFM) The field emission properties of the CNT films were measured in the diode configuration in a vacuum chamber with pressure below 3.0 Â 10À7 Torr The anode was a Mo electrode, and the gap between the SUS substrate and the anode was 500 mm Results and discussion Fig 1(a) and (b) shows the AFM images of the Ni catalyst layers on SUS and Si substrates after the NH3 plasma pretreatment, respectively The catalyst layers on both substrates were observed to aggregate and form Ni nanoparticles However, Ni nanoparticles on SUS substrates (Fig 1(a)) seem to be more uniform in size than those on Si substrates: most nanoparticles on SUS substrates have diameter from 20 to 40 nm while those on Si substrates have from Fig Typical AFM images of Ni catalyst layers (a) on SUS substrates and (b) on Si substrates after the NH3 pretreatment process, and (c) on SUS substrates before the NH3 pretreatment process Fig Typical SEM images of CNTs grown (a) on SUS substrates and (b) on Si substrates D.Q Duy et al / Applied Surface Science 256 (2009) 1065–1068 1067 Table Distribution of the diameter of CNTs grown on SUS and Si substrates Diameter (nm) From From From From 10 30 60 90 to to to to 30 60 90 120 Fraction (%) SUS substrate Si substrate 83 17 0 55 31 7 10 to 140 nm, although the area uniformity of Ni particles on Si substrates is better compared with that on SUS substrates We suggest that because of floating voltage in DC-PECVD [15], etching of the Ni layer on the metal substrate could be more effective due to its low resistivity, and therefore the Ni nanoparticles were more uniform The morphology of surfaces of Ni catalyst layers on the SUS substrates (Fig 1(a)) seems to be inhomogeneous To interpret the phenomena, the Ni catalyst layers before the NH3 plasma pretreatment (Fig 1(c)) were also examined by AFM Before the NH3 plasma pretreatment, the morphology of the surface of Ni catalyst layers on SUS substrates was inhomogeneous (Fig 1(c)) and the inhomogeneity is attributed to the non-uniform surface of SUS substrates before the sputtering process The diameter of CNTs has been reported to be related to the size of the catalyst particles and the thickness of the catalytic layer [16], and a single CNT was grown on a single nanoparticle below a critical size [17] This means that the diameters of the CNTs can be controlled by changing the size of Ni nanoparticles during the pretreatment process Fig 2(a) and (b) shows SEM images of the CNTs grown on SUS and Si substrates, respectively The CNTs are well aligned perpendicular to the surface of both substrates, and have lengths of about 1.2 mm However, the diameters of the CNTs grown on the SUS (Fig 2(a)) are more uniform than those grown on the Si substrates (Fig 2(b)) Fig 3(a) and (d) shows TEM images of the CNTs grown on the SUS and Si substrates, respectively The diameter of the CNTs grown on the SUS substrates ranges from 10 to 50 nm, and most of them have the diameter of about 30 nm (Table 1) The diameters of the CNTs are much smaller than those previously reported [18] Although most of the CNTs grown on the Si substrates also have the diameter of about 30 nm, the diameters range from 10 to 140 nm (Table 1) It could be explained by the size uniformity of the Ni catalyst particles on the SUS substrates Fig 3(b) and (c) shows TEM images of the CNTs grown on the SUS substrates The CNTs are multiwalled with the number of walls around 31 The clear walls of CNTs (Fig 3(c)) show that the crystallinity of CNTs is as good as those previously reported [18] Fig shows the Raman spectra of the CNTs grown on both Si and SUS substrates The Raman spectra of the CNTs grown on both substrates are observed to have two prominent peaks at $1346 cmÀ1 (noted as D-band) and $1588 cmÀ1 (noted as Gband) with the intensity ratio ID/IG of 0.99, which is related to the size of the sp2 carbon clusters in the graphene sheet or the defect density [19] We suggest that the ID/IG ratio and therefore the quality of CNTs grown by PECVD not strongly depend on their diameters as well as details of individual CNTs [20] Fig 5(a) shows the field emission current density (J) as a function of the applied electric field (E) from the CNTs film grown on the SUS substrate The turn-on field, defined as the field for the emission current of mA, was 3.87 V/mm with the gap of 500 mm between the SUS substrate and the Mo anode According to previous works [6,21], the lowest turn-on electric field for multiwall CNTs was observed to be less than V/mm at the gap of less than 300 mm between substrate and anode Fig 5(b) shows the Fowler–Nordheim (F-N) plot of ln(I/V2) vs 1/V for the J–E Fig Typical TEM images of CNTs grown (a, b, and c) on SUS substrates and (d) on Si substrates curves shown in Fig 5(a) The plot shows a linear fit, indicating that the emission current of CNTs follows the F-N behavior Since the gap between the substrate and the anode was 500 mm, assuming a work function of 5.0 eV for CNT, the field emission factor b is estimated to be about 1737 The CNTs grown on the SUS substrates served as cold cathodes in the X-ray generation in our previous reports [5] 1068 D.Q Duy et al / Applied Surface Science 256 (2009) 1065–1068 the pretreatment with NH3 gas before the growth of CNTs, which were more uniform than those on the Si substrates Field emission properties of the CNT films were measured in the diode configuration, and turn-on electric fields of 3.87 V/mm and field enhancement factor b of about 1737 were obtained from the synthesized CNTs Those results have shown the potential of CNTs in field emission applications, especially CNT-based cold-cathode X-ray tubes Acknowledgements Fig Raman spectra for the CNTs grown on SUS substrates and on Si substrates This research was financially supported by the Ministry of Education, Science, and Technology (MEST) and Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Regional Innovation, and Regional Innovation Center For Next Generation Industrial Radiation Technology of Wonkwang University References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] Fig (a) Emission current density vs applied electric field and (b) F-N plot for the CNTs grown on SUS substrates [15] [16] Conclusion We had successfully grown carbon nanotubes on Ni-coated SUS substrates by DC-PECVD The synthesized CNTs have the diameter of about 30 nm and the length of about 1.2 mm, which were more uniform compared to those grown on Ni-coated Si substrates It is attributed to Ni catalyst particles on the SUS substrates formed by [17] [18] [19] [20] [21] S Iijima, Nature 354 (1991) 56 C.T Gibson, S Carnally, C.J Roberts, Ultramicroscopy 107 (2007) 1118 J Robertson, Mater Today 10 (2007) 36 R.B Rakhi, K Sethupathi, S Ramaprabhu, Int J Hydrogen Energy 33 (2008) 381 H.S Kim, D.Q Duy, J.H Kim, H.J Lee, D.M Yoon, S.S Shin, J.W Ha, K.J Lee, Y.G Hwang, C.H Lee, C.Y Park, J Korean Phys Soc 52 (2008) 1057 H Sugie, M Tanemura, V Filip, K Iwata, K Takahashi, F Okuyama, Appl Phys Lett 78 (17) (2001) 2578 S Hallenbeck, Radiology 117 (1974) C Ribbing, P Rangsten, K Hjort, Diamond Relat Mater 11 (2002) A.M Rao, D Jacques, R.C Haddon, W Zhu, C Bower, S Jin, 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2568 T Chen, Z Sun, Y.W Chen, L.L Wang, P.S Guo, W.X Que, Thin Solid Films 516 (2008) 1112  ... the CNTs grown on SUS substrates [15] [16] Conclusion We had successfully grown carbon nanotubes on Ni-coated SUS substrates by DC-PECVD The synthesized CNTs have the diameter of about 30 nm... (b) on Si substrates after the NH3 pretreatment process, and (c) on SUS substrates before the NH3 pretreatment process Fig Typical SEM images of CNTs grown (a) on SUS substrates and (b) on Si substrates. .. the effects of SUS substrates on the growth of CNTs, CNTs were grown also on Ni-coated Si substrates at the same synthesizing conditions as above The morphology, density, and quality of the CNTs