Chemical modification of carbon nanotube for improvement of field emission property

Accelarated PublicationChemical modification of carbon nanotube for improvement of field emission propertySunwoo Lee a ,Tetsuji Oda a ,Paik-Kyun Shin b,*,Boong-Joo Lee caElectronic Engineering,The University of Tokyo,113-8656Hongo,Tokyo,JapanbSchool of Electrical Engineering,Inha University,#253Yonghyun-Dong,Nam-Gu,Incheon Metropolitan City 402-751,Republic of Korea cElectronic Engineering,Namseoul University,21Maeju-ri,Seounghwan-Eup,Cheonan City,Choongnam 330-707,Republic of Koreaa r t i c l e i n f o Article history:Received 17November 2008Received in revised form 31December 2008Accepted 17February 2009Available online 25February 2009Keywords:Chemical modification Carbon nanotube CNTField emission Tunnelinga b s t r a c tIn the present work,chemical modification of carbon nanotube was proposed for improvement of field emission property.Multi-wall carbon nanotubes (MWCNTs)were grown vertically on silicon substrate using catalytic chemical vapor deposition.Tips of grown MWCNTs were chemically modified using oxy-gen plasma,nitric acid,and hydrofluoric acid.Surface state and morphology of the chemically modified CNTs were T tips were opened and defects working as trap sites were generated on the CNT surface by the chemical modification process leading to improvement of field emission property.We suggest that two main factors determining the field enhancement factor are geometric factor and surface state of the CNT tips.Ó2009Elsevier B.V.All rights reserved.1.IntroductionCarbon nanotubes (CNTs)have attracted much attention be-cause of their unique electrical properties and their potential appli-cations [1,2].Large aspect ratios of CNTs with high chemical stability,thermal conductivity,and high mechanical strength are advantageous for applications to the field emitter [3].Since CNTs are grown directly on a substrate by CVD,the CNT emitter can be fabricated simply.Many researchers have devoted efforts to the artificial control of alignment,number density,and aspect ratio of CNTs [4–7].Although it is essential for FED application to eluci-date the correlation between the structural properties and field electron emission properties of CNTs,systematic experiments on the field emission property regarding the change of surface state of CNTs by chemical modification have not been carried out Ts having strong covalent bonds are very stable against to chemical attacks.Breaking these strong covalent bonds and chang-ing surface state would be expected to change the CNT’s physical property as well as chemical property [8,9].As field emission behavior takes place at the tip of the CNT,one could control the field emission property by changing the structure and surface state of the CNT tips.In this study,the correlation between the field emission prop-erty and structural property or surface state of CNTs was investi-gated as a function of the chemical modification.Although the field emission properties of CNTs were improved with increasing the aspect ratio of the CNT,the field enhancement factor obtained from the Fowler–Nordheim plot was found to be much larger than that obtained from the geometric factors.These results suggest that the field emission from CNTs is strongly influenced by the sur-face states induced by surface defects and attached functional groups,rather than by their geometric factors.2.ExperimentalIn our experiment,the nickel catalyst films were prepared by sputtering method on silicon substrate using low power and long time (at 10W for 1h)to minimize size and distribution of the nick-el catalyst particles.MWCNTs used in this work were grown in a thermal CVD system with C 2H 2source gas and Ar carrier gas with a flow rate of 30/100sccm at 700°C on the nickel catalyst.The CNTs were chemically modified by oxygen plasma,nitric acid (HNO 3),and hydrofluoric acid (HF).The modified samples were named as O 2–CNT,HNO 3–CNT,and HF–CNT,respectively.The oxygen plasma treatment was done with a gas flow rate of O 2:Ar =20:200sccm at 500°C for 5min.The HNO 3treatment was done in 20vol%HNO 3solution at room temperature for 1h,and the samples was subsequently rinsed in distilled water,and dried at room temperature for 1h.The HF treatment was done in 20vol%HF solution at room temperature for 1h,and the sample was rinsed and dried.0167-9317/$-see front matter Ó2009Elsevier B.V.All rights reserved.doi:10.1016/j.mee.2009.02.021*Corresponding author.Tel.:+82328607393;fax:+82328635822.E-mail address:shinsensor@inha.ac.kr (P.-K.Shin).Microelectronic Engineering 86(2009)2110–2113Contents lists available at ScienceDirectMicroelectronic Engineeringjournal homepage:www.else v i e r.c o m /l o c a t e /m eeThe field emission characteristics of the grown CNT film was measured by digital multimeter in a vacuum chamber with a base pressure of 1.5Â10À8Torr.A flat parallel diode type configuration was used in the setup as shown in Fig.1.Both electrodes were glass plated with a conductive indium tin oxide (ITO)coating,and the cathode contained the grown CNT film.The distance between the anode and the CNT film surface was 100l m as separated by spacers.The surface morphology and internal structure of the CNTs were characterized by scanning electron microscopy (SEM)and trans-mission electron microscopy (TEM).3.Results and discussionsSEM images and TEM images (right side of each image)of the as-grown CNTs and the chemically modified CNTs are shown in Ts grown in this work are bamboo type multi-wall carbon nanotubes,which are vertically aligned to the substrate.The length of chemically modified CNTs is slightly shorter than that of as-grown CNTs due to the chemical etching during the chemical mod-ification processes.In case of the HNO 3–CNT,length was drastically reduced,because CNTs were partly delaminated and remained CNTs were fallen down during the chemical modification process.Tip of as-grown CNT is typically closed,while those of chemi-cally modified CNTs are opened as shown in Fig.2(right side of each image).The most parts of CNT consist of stable hexagonal car-bon structure,while the tip of CNT has pentagonal structure to close the tube end [10].The pentagonal carbon structure is easily broken by the chemical attack relative to the hexagonal structure [11].Relatively weak bonds at the CNT tip might be broken and opened by the chemical modification.Since the bond breaking might be started from the outer shell of the MWCNT used in this work and propagated into the inner shell,the shape of CNT tips be-came sharp.Furthermore,the chemical modification process might result in changing the surface state by the bond breaking as well as the structural change.In order to confirm the above mentioned surface state change,X-ray photoelectron spectroscopy (XPS)using the monochrome Al Ka X-ray was carried out.Wide scan spectra for as-grown and chemically modified CNTs are shown in Fig.3.In all cases,carbon peak (C1s,284.5eV)and oxygen peak (O1s,530eV)are observed [12].The oxygen peak stronger than that of the as-grown CNT film for the O 2–CNT,the weak nitrogen peak for HNO 3–CNT,and fluo-rine peak for HF–CNT are observed.This result correspondstoFig.1.Schematic drawing of the setup for measurement of the field emissioncurrent.Fig.2.SEM (left)and TEM (right)images of as-grown and chemically modified CNTs.S.Lee et al./Microelectronic Engineering 86(2009)2110–21132111the previous TEM results that the chemical modification processes could change the surface states of the CNT tips.The chemical modification dependence on the field emission property was investigated.Fig.4a shows emission current density as a function of applied electric field for the as-grown CNTs and the chemically modified CNTs.It is found that the chemically modified CNTs exhibit a better field emission property than that for the as-grown CNTs.If we define the threshold electric field (E th )as the ap-plied electric field that produces an emission current of 1mA/cm 2,it can be clearly seen from Fig.4b that threshold electric field is chemical modification dependent.The Fowler–Nordheim (F–N)equation can be described as,J ¼1:56Â10À6ðb E Þ2/exp À6:83Â109/3=2b E!;where J (A/cm 2)is the emission current density,E (V/l m)is theapplied electric field,b is the field enhancement factor,and /(eV)is the work function of the emitter [13].The experimental value b can be estimated on the basis of the slope of the F–N plot as shown in Fig.4c.Although there is no distinguishable difference in geo-metric factors such as diameter and length of each CNTs,the field emission property for chemically modified CNTs is better than that for as-grown CNTs.We estimated the field enhancement factors for each CNTs using geometric factors from SEM images and the FN plot of the experimental field emission data.The field enhance-ment factor estimated from the FN plot (b $1000s)was two or-ders greater than that estimated from the geometric factors (b $10s).This result implies that the field enhancement factor estimated from the F–N plot includes another factor for the improvement of field emission.Another factor affecting field emis-sion more dominantly might be correlated with the surface state of the CNT tips.TEM results and XPS results strongly imply that defects working as trap sites might be on the CNT surfaces.As shown in Fig.4c,there are two different kinds of tunneling mechanism from the slope of J /E 2vs.1/E plots.The slope at low field regime is quite dif-ferent from that at high field regime.Trap sites play a dominant role in tunneling mechanism at lower field than FN tunneling re-gime,so called trap assisted tunneling (TAT)[14].Tunneling gov-erned by TAT mechanism at low field regime affect the threshold electric field,and is related to trap sites on CNT tips.The tunneling model is based on a two-step tunneling process via traps on CNT surface which incorporates energy loss by phonon emission [15].Fig.4d shows the basic two-step process of an electron tunneling from a region with higher Fermi energy (the cathode)to a region with lower Fermi energy (the anode).Electrons could be emitted at relatively low electric field with an aid of trap sites.Finally,we suggest that two main factors determining the field enhance-ment factor are geometric factor and surface state.Therefore gen-eration of trap sites on CNT surface is strongly required to improve the field emission property,as well as the geometricfactor.Fig.3.XPS wide scan spectra of the as-grown CNTs and the chemically modified CNTs.Since some parts of CNTs are delaminated during HNO 3chemical modification process as shown in Fig.2c,strong oxygen and silicon signals are detected from the naturally oxidized Si substrate.2112S.Lee et al./Microelectronic Engineering 86(2009)2110–21134.SummaryWe have found that CNT tips were opened and defects working as trap sites were generated on the CNT surface by the chemical modification process leading to improvement of field emission property.Trap sites play a dominant role in tunneling mechanism at lower field than FN tunneling regime.We found that another factor affecting the field emission might be correlated with the sur-face state of the CNT tips.Therefore generation of trap sites on CNT surface is strongly required to improve the field emission property,as well as the geometric factor.References[1]W.A.de Heer,A.Chatelain,D.Ugarte,Science 270(1995)1179.[2]B.I.Yakobson,R.E.Smalley,Am.Sci.85(1997)324.[3]T.W.Ebbesen,Carbon Nanotubes,CRC Press,Boca Raton,FL,1997.[4]M.Chhowalla,K.B.K.Teo,C.Ducati,N.L.Rupesinghe,G.A.J.Amaratunga,A.C.Ferrari,D.Roy,J.Robertson,ne,J.Appl.Phys.90(2001)5308.[5]Y.Y.Wei,G.Eres,V.I.Merkulov,D.H.Lowndes,Appl.Phys.Lett.78(2001)1394.[6]V.I.Merkulov,D.H.Lowndes,Y.Y.Wei,G.Eres,E.Voelkl,Appl.Phys.Lett.76(2000)3555.[7]M.Katayama,K.-Y.Lee,S.Honda,T.Hirao,K.Oura,Jpn.J.Appl.Phys.43(2004)L774.[8]W.K.Hong,H.C.Shin,S.H.Tsai,et al.,Jpn.J.Appl.Phys.39(2000)L925.[9]U.D.Weglikowska,J.M.Benoit,P.W.Chiu,et 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