Effect of ammonia gas etching on growth of vertically aligned carbon nanotubes/nanofibers

藏当leonlineatwww.sciencedirect.comScience DirectNonferrous MetalsSociety of chinaElsEvier PressTrans. Nonferrous Met. Soc. China 21(2011)s130-s134www.nmsccnEffect of ammonia gas etching on growth oftically aligned carbon nanotubes/nanofibersSang-Gook KIM, Sooh-Yung KIM, Hyung-Woo LEE.1. Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave,Cambridge MA 02139, USA;2. National Core Research Center for Hybrid Materials Solution, Pusan National UniversityReceived 21 April 2010; accepted 10 September 2010Abstract: The etching effect of ammonia(NH,)on the growth of vertically aligned nanotubes/nanofibers( CNTs)was investigatedby direct-current plasma enhanced chemical vapor deposition(DC-PECVD). NH, gas etches Ni catalyst layer to form nanoscaleislands while NH3 plasma etches the deposited amorphous carbon. Based on the etching effect of NH, gas on Ni catalyst, thedifferences of growing bundles of CNTs and single strand CNTs were discussed; specifically, the amount of optimal NH3 gas etchingis different between bundles of CNTs and single strand CNTs. In contrast to the CNT carpet growth. the single strand CNT growthrequires shorter etching time (5 min)than large catalytic pattems (10 min) since nano dots already form catalyst islands for CNTgrowth. Through removing the plasma pretreatment process, the damage from being exposed at high temperature substrate occurringuring the plasma generation time is minimized. High resolution transmission electron microscopy (HTEM)shows fishbone structureof CNTs grown by PECVDKey words: carbon nanotube; ammonia etching: nickel catalyst; plasma enhanced chermical vapor deposition(PECVD)nano devices and technologies [7]. However, the vertical1 IntroductionCnT growth mechanism remains unclear although thereare many studies on the CNT growth by PECVD[8-9Carbon nanotube (CNT)is one of the mostNH3 pretreatment before plasma generation may beanticipated nano-materials because of its small the key control parameter in the Cnt growth with thedimension and excellent mechanical, electrical, thermal, form factors of islands in catalyst layers[ 10]. In this work,and chemical properties[ 1-3]. Various synthesis methods the effect of NH3 gas etching on the growth of CNTs inwere developed, such as laser ablation, arc discharge, forest form and a single strand array was investigated,and chemical vapor deposition (CVD)[(4-5]. Each and different roles of NH3 gas in both forms of CNTsynthesis method has its own advantages over other growth were discussedmethods, and finds its own potentials in terms of quality2 CNT growth in PECVDIn the recent decade, plasma enhanced chemicalvapor deposition( PECVD), has been widely used toVertically aligned CNTs are grown on a patternedgrow CNTs vertically[6]. PECVD's capability of Ni sample using a home- built PECVD shown in Fig. I(a)growing aligned CNTs at relatively low temperature It is generally accepted that CNTs can be grown by threemakes it possible to grow aligned CNTs in an array and steps: the decomposition of the hydrocarbon gas(in thissingle strand configuration together with various work, acetylene( C2H2) was used over a catalytic metalatalytic pattern generation techniques. The controlled (nickel (Ni)), the diffusion of the carbon atoms throughsynthesis of carbon nanotubes/nanofibers(CNTs) by the bulk and the surface of the islands of catalyst layer,PECVD is an essential step for realizing many functional and中国煤化工 rbon atoms beneathFoundation item: Project supported by Intelligent Microsystem Center(IMC). ProjectCN MH Gal Core Research Centerrough the National Research Foundation of Korea funded bychnology, Project( 2010)ported by Pusan National UniversityCorresponding author: Hyung-Woo LEE: Tel: +82-51-510-3160, E-mail: LHW2010@pusan. ac krSang-Gook KIM. et al/Trans. Nonferrous Met. Soc. China 21(2011)s130-s134s131depositing a nickel(Ni) catalytic25 nm-thicktitanium (Ti)layer was depositedSi wafer topromote the adhesion of Ni to the Si wafer and preventcatalyst silicide formation at the high temperature duringthe CNT growth. To investigate the effect of NH; etchingon single strand CNTs, Ni dots(100-200 nm in diameter)were patterned on a p-type B doped Si wafer(100)Each prepared sample was loaded in a PECVDchamber evacuated by rotary and turbo-molecular pumps<1.33x10Pa)to eliminateimpurities and water vapors. When the desired pressure(b) Anodereached, the sample was heated to 580C, and at thistemperature only NH, gas without igniting plasma wasintroduced at 160 cm'i'min to make the islands of Nicatalyst. Then, C2H, was introduced into the chamber forma was ignited forsimultaneously while NH3 was introduced. CNTs weregrown at a gas mixture of highly purified (99.99% purity)Fig. I Home-built PECVD (a) and schematic of CNT growth by C2H, and NH,, 50 cm/min: 1 60 cm/min After the CNTplasma(b)growth time, the plasma was turned off, and the heaterwas cooled down slowly to prevent the CNT damagethe islands in the tube form of graphene layers(see from the sudden change of temperatureFig.l(b)10-121With the mechanism of the CNT growth by electric 3 Results and discussionfield, CNTs were grown in PECVD. To investigate theeffect of NH3 etching, two types of Ni catalyst sampleTo verify the etching effect of the NH, gas,were prepared: one for a forest of CNTs and the other for surface of Ni catalyst layer with 15 nm in thicknesssingle strand CNTs. For growing a bundle of CNTs, a NH, introduction for different time was examinedrectangular marker was patterned in to 30 umx300 umFig 2(a) shows a FESEM image before NH,size through the photolithography processes. Before introduction, and Figs. 2(b)-(d)show the results after200nm中国煤化工CNMHGFig 2 FESEM images of Ni catalyst layer with thickness of 15 nm before N, introduction (a)and after NH, introduction for 5 min(b). 10 min(c)and 15 min(d)s132Sang-Gook KIM. et al/Trans. Nonferrous Met. Soc. China 21(2011)s130-sl34NH, introduction for 5, 10, and 15 min. As the NH, bundle of CNTs as individual nano dots work like theintroduction time increases, it is found that catalyst islands formed by NH; etching process. With referenceislands are formed by the NH3 etching process. As shown to several previous papers for film thickness to growin Fig. 2(b), a wide excavated area is formed after 5 min. CNT forest and single strand CNTs[ 13-14]. a thin NiAfter NH; introduction for 10 min, islands for the CNTs film (15 nm)is applied with NH, for 5 min beforegrowth are formed on the catalyst layer. Over 15 min, Ni growing CNTs. However, there is no CNT on Ni catalystislands still existfilm. Fig 4(a) shows FESEM image of CNT growth inTo check the role of the substrate temperaturecarpet film with a thickness of 15 nm. As shown, CNTscatalyst layer was annealed for 10 min without NH3 gas. are grown sparsely on carpet film. After increasing NH,However, there is no Ni island even with I h heating. etching time to 10 min, CNT forest and single strandThese experimental results confirm the role of NH, gas CNTs are grown. Fig 4(b)shows the well grown CNTsas an etchant. Without heating substrate, however, the on the carpet film. From these results, it can be seen thatcatalyst islands are not formed even during NH, gas the balance between the thickness of catalysts and theintroduction. From this phenomenon, it is expected that NH etching time is one of the most importanthigh substrate temperature leads to the increase of the parameters in growing CNTs by PECVDsurface energy and the mobility of Ni atoms. Finally, itwill help Ni thin film to be cracked for forming islandsFig3 shows CNT growth results from differentetching times before generating the plasma to grow CNT.As shown in Fig 3(b), CNTs are grown well afterforming islands by NH3 etching effect. This result furtherconfirms that the island formation by NH3 etching isessential for the CNT growth. In general, NH;pretreatment for catalyst island formation by etching isachieved by NH: plasma for 10-40 min[ 12-13high substrate temperature occurringduring plasma exposing for a long time can give damageto the other component when CNT applied devices aremade. By reducing the plasma ignitiontheselection of materials will be more flexible in the designof a deviceGrowing single strand CNTs differs from growing aFig 4 FESEM images of grown CNTs after NH, etching foUnder the same conditions, single strand CNT wasgrown on single nano dot. Fig. 5(a)shows the verticallyaligned single strand CNT on single nano dot with athickness of 15 nm. In this case, the shorter NH3 gasetching time of 5 min is required as the Ni dots arealready formed before the growth process. As shown inFig 5(b), however, well grown single strand CNT cannotbe obtained. In the middle of the white dot circle, thetrace of Ni dot is shown instead of CNTs. It is theevidence of over-etching by NH,. To investigate thenecessity of NH, etching in single nano dots, the growth中国煤化工 dots without NHally aligned singlestranCNMHGFig3 CNTs growth results from different etching time beforeFrom this experimental result, it is expected thatgenerating plasma to grow CNT: (a)5 min: (b)10 minNH3 pretreatment in gas phase has not only the roleSang-Gook KIM, et al/Trans. Nonferrous Met. Soc. China 21(2011)sl30-s134s133generating catalyst islands by etching effect, but also theunknown role in growing single strand CNTs on single 4 Conclusionsnano dots. High resolution transmission electronmicroscopy(HRTEM) images of CNT's top and body1)With different NH, gas inserting time into theshow fishbone structures with multiple layers parallel to chamber, Ni catalyst islands are formed over 10 min NH3he outer surface of the Ni catalyst surrounded by introduction time. Under this condition, well verticallydisordered outer layers along the CNT axis, as shown in aligned grown CNTs are obtained. By removing theplasma pretreatment process, the damage of the substratefrom being exposed at high temperature for a long timeby plasma ignition can be minimized and flexiblesubstrate choices in making CNTs integrated devices canbe obtained2)Through the comparison with a large areapatterned sample and single nano dot, it is found that asingle nano dot needs shorter NH, etching time than alarge area pattemed one to grow CNTs as a single dotforms catalyst island already3)Different NH, etching times are optimized for abundle of CNTs and single strand CNTs, and successfulgrowth of vertically aligned single strand CNTs isachieved repeatedly4)NH, in the gas state, not in plasma state, has theetching effect and is one of the most important factorsthat should be considered in CNTs growth by PECVDThis finding of NH, etching effect in the gas phaseintroduces the well grown CNTs to a wide range ofopportunity for the integration into MEMS and NEMS250nmFig 5 FESEM images of grown CNTs after NH, etching for 5 Referencesmin(a) and 10 min(b)on single nano dot[U RUECKES T KIM K, JOSELEVICH E, TSENG G Y, CHEUNG C.LIEBER C M Carbon nanotube-based nonvolatile random accessmemory for molecular computing [I Science, 2000, 289. 94-97.121 KONG J. 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