辉光放电对负偏压增强热丝化学气相沉积碳纳米管的准直生长作用研究_英文_

辉光放电对负偏压增强热丝化学气相沉积碳纳米管的准直生长作用研究_英文_ 第 33 卷 第 4 期 人工晶体学报 Vol . 33 4 No . JOURNAL OF SYNTHETIC CRYSTALS August ,2004 2004 年 8 月 Study of Glo w Discharge Effect on Al ignment Gro wth of Carbon Nanotubes by Negative Bia s2enhanced Hot Fila ment Chemical Va por Deposition 1 2 2 1WAN G Bi2ben, Lee S oonil, Kim J unghoi, HOU Bi2hui (1 . College of Applied Mathematics and Physics , Beijing University of Technology , Beijing 100022 ,China ; )2 . Department of Molecular Science and Technology , Ajou University , Suwon 4422749 , Korea ( )Received 9 February 2004 Abstract :Carbon nanotubes were synthesized on silicon substrates coated with NiFe films of different thickness by negative bia2s enhanced hot filament chemical vapor deposition at different negative bias and pressure , and their growth was investigated by scanning electron microscopy. It was found that the bent and aligned carbon nanotubes are respectively grown without and with glow discharge , indicating that glow discharge plays an important role for alignment growth of carbon nanotubes. Due to occurrence of glow discharge , a strong electric filed is established near the substrate , where its strength relative to the field without glow discharge can be enhanced two order of the magnitude . In the work , the effect of glow discharge on alignment growth of carbon nanotubes was studied in detail . Key words :aligned carbon nanotubes ; glow discharge ; negative bias CLC number :O484 Document code :A () Article ID :10002985X20040420629205 辉光放电对负偏压增强热丝化学气相沉积碳纳米管的准直生长作用研究 1 2 2 1王必本, Lee Soonil, Kim J unghoi, 侯碧辉 ( 1 . 北京工业大学应用数理学院 ,北京 100022 ; ) 2 . 亚洲大学分子科学与技术系 ,水源 4422749 , 韩国 摘要 :利用负偏压增强热丝化学气相沉积在不同的负偏压和压强下 ,在沉积有不同厚度 NiFe 膜的 Si 衬底上制备了 碳纳米管 ,并用扫描电子显微镜研究了它们的生长 。发现在无辉光放电的情况下 ,碳纳米管弯曲生长 ,而在辉光放 电时 ,碳纳米管准直生长 , 表 关于同志近三年现实表现材料材料类招标技术评分表图表与交易pdf视力表打印pdf用图表说话 pdf 明辉光放电对碳纳米管的准直生长起到了重要的作用 。由于辉光放电的产生 ,在衬底 表面附近形成很强的电场 。相对无辉光放电时的电场 ,场强提高了两个数量级 。本工作详细地研究了辉光放电对 碳纳米管的准直生长作用 。 关键词 :准直碳纳米管 ; 辉光放电 ; 负偏压 1 Introduction 1 ( ) Since their discovery by Iijima in 1991, carbon nanotubes CNTshave attracted much attention all over the word 2 3 owing to their unique characteristics such as high aspect ratios, high mechanical strength, and their electronic 4 behavior as either semiconductors or conductors depending on their diameters and helicity. It is proposed for CNTs that 5 there are wide applications in the future , for example , storing of hydrogen, used as anodes in lithium2ion secondary 6 7 batteriesand field electron emission sources in cold cathode flat panel displays and electron guns. It is well known () that the motion of the electrons in CNTs is limited in the direction of the axis , so aligned carbon nanotubes ACNTswill 8 ,9 be required for a number of applications and investigations. To date , ACNTs have been successfully synthesized by ( ) plasma2enhanced chemical vapor deposition CVD. However , the alignment mechanism of growing ACNTs has not been well understood. In the work , the bent CNTs and ACNTs were synthesized by negative bias2enhanced hot filament CVD Received date : 2004202209 ) (Biogra phy :WANG Bi2ben 19632,Male ,from Henan province ,Doctor . E2mail : bibenw @bjut . edu. cn without and with glow discharge , respectively. We studied the effect of glow discharge on alignment growth of CNTs by negative bias2enhanced CVD. 2 Experimental The system growing CNTs was described in reference 10 . Briefly , the filament is coiled W wire with a diameter of 1 mm , which was heated to about 1950 ?. The mesh between the filament and the substrate is used as the positive electrode of the bias circuit . The substrate is a silicon wafer , which was heated by thermal radiation from the heated filament , and the substrate temperature was 550 ?. The distances from the substrate to the filament and the mesh are 21mm and 6 mm , respectively. A negative bias relative to the mesh is applied to the substrate through the Mo holder . The reaction gas was a mixture of CH, NHand H, and the fluxes of CH, NH, and Hwere 20 sccm , 40 sccm to 40 4 3 2 4 3 2 3 3 sccm ,respectively. The working pressure changed from 2 ×10Pa to 4 ×10Pa . Prior to the growth of ACNFs , a Ta buffer film and NiFe catalyst film were successively deposited on the silicon substrate by RF2plasma sputtering. The thickness of Ta buffer is 100nm and the thickness of NiFe films is 10nm and 60nm , respectively. The Ta film was employed to improve the adhesion between the CNTs and the substrate . After the substrate was placed in the reaction chamber , Hwas introduced into the chamber . The filament started to 2 be heated when the pressure reached working value and became stable . NHwas introduced when the filament 3 temperature was about 1950 ?. CHwas introduced when the substrate temperature rose to 550 ?. Simultaneously , the 4 negative bias was applied to the substrate to begin the growth of CNTs. In the process of experiment , it was found that the current suddenly increased and there was a drop of the voltage when the negative bias was larger than 430V , meanwhile a blue glow on the Mo holder surface was observed , indicating the production of a glow discharge . CNTs were grown under different conditions. The time of deposition was 10min. 3 Results and discussion ( ) CNTs synthesized under different conditions were characterized by scanning electron microscopy SEM. Fig. 1 shows CNTs grown without glow discharge at a negativ e bias of 400V , where silicon wafer was deposited with 60nm NiFe 3 film and the pressure was 4 ×10Pa . As shown in Fig. 1 , only a few of bent CNTs with small diameter are grown. In the initial period of growing CNTs , NiFe film is firstly cracked to form NiFe particles due to different expansion 11 coefficients of silicon and NiFe and surface stress. Without glow discharge , the cracking of the film depended Fig. 1 The SEM image of CNTs grown without glow discharge on the heated silicon substrate so that low substrate temperature led to the formation of a few of small NiFe particles. Only these small NiFe particles are suitable for the growth of CNTs. The diameter of CNTs is determined by the 12 size of catalyst particles. Consequently , only a few of bent CNTs with small diameter can be synthesized. () ( ) Fig. 2 aand bare the SEM images of CNTs grown on silicon wafers coated with 60 nm NiFe films under glow 3 discharge at the negative bias of 450V and 500V when pressure was 4 ×10Pa , respectively. From Fig. 2 , the average μμdiameter of ACNTs is 241nm and 192nm and their average length is 4 . 52m and 7 . 04m at the negative bias of 450V and 500V , respectively. WANG Bi2ben et al : Study of Glow Discharge Effect of Alignment Growth of Carbon Nanotubes by Negative Bias2enhanced Hot Filament Chemical Vapor Deposition 第 4 期 631 () ( ) Fig. 2 The SEM images of CNTs grown with glow discharge at different bias. a2450V ; b2500V 3 3 () ( ) Fig. 3 The SEM images of CNTs grown with glow discharge at different pressure . a2 ×10Pa ; b4 ×10Pa () ( ) Fig. 3 aand bare the SEM images of CNTs grown on silicon wafer coated with 10 nm NiFe films under glow 3 3 discharge at the pressure of 2 ×10Pa and 4 ×10Pa when the negative bias was 450V , respectively. As shown in Fig. μ3 , the average diameter of ACNTs is hardly changed with pressure , it is about 60nm. Their average length is 2 . 15m 3 3 μand 1 . 68m when pressure is 2 ×10Pa and 4 ×10Pa , respectively. From Fig. 2 and Fig. 3 , the diameter of ACNTs in Fig. 3 is obviously smaller than that in Fig. 2 . It results from different thickness of NiFe films. The thickness of NiFe ( ) ( ) films employed in Fig. 2 and Fig. 3 are 60nm and 10nm and their SEM images are shown in Fig. 4 aand b. As () ( ) shown in Fig. 4 aand b, the size of NiFe grains in 10nm NiFe film is smaller than that in 60nm NiFe film ,resulting () ( ) Fig. 4 The SEM images of NiFe films with different thickness. a10nm ; b60nm in formation of smaller NiFe particles for 10nm NiFe film than those for 60nm NiFe film in the process of CNTs growth. Compared Fig. 1 with Fig. 2 and Fig. 3 , it is obvious that ACNTs can be synthesized under glow discharge . Simultaneously , their diameter and growth rate are related to the negative bias and pressure . The effects of the negative bias and pressure on the diameter and growth rate of ACNTs were studied in references13 , 14 . Here , the effect of glow discharge on the alignment growth of CNTs is discussed. Due to the production of glow discharge , a cathode sheath is formed near the substrate and a number of ions are produced in the sheath , and a strong electric filed builds up near the 15 substrate . The field is expressed by formula, 2 U c x ( )( )E = 1 - 1 l l ( ) where Uis the negative bias applied to the substrate Uis absolute value, l and x are the thickness of the sheath and c c 15the distance from the substrate surface , respectively. The thickness of the sheath is estimated by following formulas j g2 ) )( ( ? L 1 + 1nL2 jL L ()L = 3 l where jand jare respectively the current densities with and without glow discharge , and L is the distance between g L 16 cathode and anode . Before glow discharge does not occur , the electric field is basically uniform, the field strength is 4 2 617 ×10V/ m in our CVD system. In our experiment , jwas 0. 49A/ mbefore glow discharge did not appear. According to L the above formulas and data from our experiment , the calculated field strengths are shown in table 1. From table 1 , the field strength of the substrate surface increases two orders of the magnitude . Under the strong field , a number of ions produced by glow discharge bombarded with NiFe film to form more small NiFe particles than those depending heating. Therefore , the density of CNTs in Fig. 2 and Fig. 3 is larger than that in Fig. 1. Due to formation of small NiFe particles 17 and their high surface energy, they are easily sphered to reduce the contact area of the NiFe particles with Ta buffer and 18 the adhesive force of them with Ta buffer. As a result , they can separate from the substrate under the electrostatic force acting on them. From table 1 , the electric filed at the tops of CNTs is still very strong although the filed strength reduces with Table 1 The thickness of cathode sheath l , the f ield strength near the substrate E, s and the f ield strength at top of ACNTs , E t Parameters Value 3333( )Pressure Pa ×10 ×10 ×10 ×10 4 4 2 4 ()UV 450 500 450 450 c 2197195 273147 74186 207149 ()j A/ m ()01256 01207 01485 01248 l mm 6( 31516 41831 11856 31629 ) E×10V/ ms 6 ( 31454 41666 11847 31604 ) E×10V/ mt increment of the distance from the substrate surface , that is to say , a strong electrostatic force is applied to the NiFe particle so that it moves in the direction of the filed during growth of CNTs . Consequently , CNTs can be grown along the direction of the filed. Because of incidental fluctuation , the motion of the NiFe particle can instantaneously departs from the direction of the filed. So , CNTs are aligned rather than straight . 4 Summary In summary , CNTs were synthesized by negative bias2enhanced CVD without and with glow discharge , respectively. It is found that ACNTs can be grown under glow discharge . The effect of glow discharge on alignment growth of CNTs was studied . The results indicate that the growth of ACNTs results from the interaction of the strong electric field formed by glow discharge with the NiFe particles. References Iijima S. Helical Microtubles of Graphitic Carbon J . Nature , 1991 , 354 :56258. 2 Obraztsov A N , Pavlovsky I , Volkov A P , Obraztsova E D , Chuvilin 1 A L , Kuznetsov V L . Aligned Carbon Nanotube Films for Cold Cathode Applications J . J . Vac . Sci . Technol . ,2000 , B 18( 12) :105921063. WANG Bi2ben et al : Study of Glow Discharge Effect of Alignment Growth of Carbon Nanotubes by Negative Bias2enhanced Hot Filament Chemical Vapor Deposition 第 4 期 633 3 Treacy M M J , Ebbesen T W , Gibson J M. 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