www.elsevier.com/locate/carbon

Carbon 44 (2006) 2984–2989

Short carbon nanotubes produced by cryogenic crushing

Jeonghee Lee a,*, Taewon Jeong a, Jungna Heo a, Shang-Hyeun Park a, DongHun Lee a,Jong-Bong Park b, HyoukSoo Han b, YoungNam Kwon b, Igor Kovalev b,

Seon Mi Yoon c, Jae-Young Choi c, Yongwan Jin c, Jong M. Kim a,*,Kay Hyeok An d, Young Hee Lee d, SeGi Yu e

a The National Creative Research Initiatives Center for Electron Emission Source, Samsung Advanced Institute of Technology,

P.O. Box 111, Suwon 440–600, Republic of Koreab AE Center, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440–600, Republic of Korea

c Materials Center, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440–600, Republic of Koread Carbon Nanotube Research Laboratory, Department of Physics, SungKyunKwan University, Suwon, Republic of Korea

e Department of Physics, Hankuk University of Foreign Studies, 89 Wangsan-ri Mohyun, Yongin-si, Kyounggi-do 449–791, Republic of Korea

Received 29 December 2005; accepted 18 May 2006Available online 10 July 2006

Abstract

A cryogenic crushing method to produce short carbon nanotubes (CNTs) is described. Crushing CNTs at liquid nitrogen temperatureallows them to be shortened and make them appreciably soluble in a solvent without any dispersant. Typical lengths of less than 500 nmwere obtained from 30 min crushing. The CNTs were characterized using atomic force microscopy, thermogravimetric, and Ramananalyses.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Carbon nanotubes; Atomic force microscopy; Raman spectroscopy; Defects; Particle size

1. Introduction

Carbon nanotubes (CNTs) are a leading material innanotechnology because of their unique electronic, chemi-cal, and mechanical properties [1–3]. Their potential use indevices and systems often needs them to be purified andfunctionalized prior to further fabrication process. How-ever, the bundling nature of CNTs originating from hugeVan der Waals binding energy of �500 eV/lm of tube–tubecontact [4] makes them difficult to be soluble in solventseither aqueous or non-aqueous even with dispersingmaterials. While many applications in electronic, biological,and optical devices require short individual nanotubes

0008-6223/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2006.05.045

* Corresponding authors. Tel.: +82 31 280 9325; fax: +82 31 280 9349.E-mail addresses: jh24.lee@samsung.com (J. Lee), jongkim@samsung.

com (J.M. Kim).

20–500 nm in length, relatively long tubes 500–2000 nm inlength are required in field emission displays with the expec-tation of uniform emission performance. Together withthese demands of producing short CNTs, strong interestsin structural modification of CNTs to search for potentialhybrid materials such as foreign materials-encapsulatedCNTs and surface-functionalized nanoscale structures, havefurther directed to explore easily accessible and efficientmethods to cut CNTs.

A generally known method to cut CNTs is chemicaloxidation under ultra-sonication or reflux conditions usinga mixture of strong acids [5–8]. However, the acid treat-ment has several disadvantages such as loss of materials,increased number of defects, and difficulty in large scaleproduction. Recent reports from Smalley group describea progressive application of piranha solution at room-temperature for CNT cutting, which results in minimal

J. Lee et al. / Carbon 44 (2006) 2984–2989 2985

carbon loss, little sidewall damage, and no selectiveetching of the smaller diameter nanotubes [7,8]. Accord-ing to the report [9], shorter length nanotubes wereobtained by fluorination of the nanotubes followed bypyrolysis, but this leads to serious agglomeration of nano-tubes. Therefore, the fluorination process is better used toinduce carbon–carbon bond breakage, which createsvacancies after defluorination, prior to the etching processfor tube breaking in room-temperature piranha solution.This fluorination-based cutting process is reported as anefficient process, with an overall carbon yield of 70–80%[7,8].

Other reported methods to cut CNTs are the productionof short CNTs by ball-milling [10–13], and sometimes withaddition of polymers [14]. However, the treatment takeslong time (�2 h) and results in decreased quality. Withadditives to improve cutting efficiency, the process fur-ther causes the contamination of CNTs, leading to extraprocess to remove those additives. Application of sonicationand homogenization was also reported [15,16]. For singlewall carbon nanotubes (SWCNT), a mixture with a mono-chlorobenzene solution of polymethylmethacrylate has beenprocessed [17,18], but this method is not recommendabledue to damaging effect in sidewalls. As a more controlledapproach, lithographic techniques are reported with fineadjustment of uniform length and selective functionalizationonly at the ends of cut CNTs [19]. Critical disadvantages inthis technique are the use of expensive semiconductingprocessing equipments and the incapability for large scaleproduction.

Of difficulties in process and less effectiveness of gener-ally known cutting methods, we have searched a new wayof cutting CNTs in a large scale with minimum damageof CNT sidewalls. Here, we introduce a slightly modifiedmethod of ball-milling to produce short CNTs, namely,cryogenic crushing process, where the flexible tubes areexpected to be rigid due to the processing at extremelylow temperature (77 K). In this process, the CNT-contain-ing ball milling vessel is sunk in a liquid nitrogen bath, andthen the short CNTs are produced with crushing operation.The products are analyzed by atomic force microscopy(AFM), scanning electron microscopy (SEM), particle sizeanalyzer, thermal gravimetric analysis (TGA), and Ramanspectroscopy.

2. Experimental

Purified SWCNT powder by arc discharge was purchased from IljinNanotech (ASP-100F). Purified small diameter CNT powder was pur-chased from CNI Co. (601B). Both CNTs were used without further puri-fication. Typically �50 mg of CNTs, alone or mixed with 1 mL ofisopropanol, was placed in a 50 mL size cryogenic crushing vessel. Thecutting process was performed using Cryogenic Sample Crusher (ModelJFC-1500, JAI Co., Ltd.). The total crushing time for each batch was var-ied from 10 to 60 min. After finishing the crushing process, the vessel wasplaced in tap water flow to elevate the vessel temperature. In case of usingisopropanol as a crushing medium, the product in isopropanol was filteredthrough 0.1 lm anodic alumina membrane (Whatmann) and dried at50 �C overnight.

The lengths of CNTs after cryogenic crushing were observed byatomic force microscopy (AFM) and field emission-scanning electronmicroscopy (FE-SEM). The AFM samples were prepared as follows.Raw and shortened samples were dispersed in o-dichlorobenzene usingultra-sonication, and then centrifuged at 5000 rpm for 10 min. The upperpart of the centrifuged solution was spin-coated at 2000 rpm for 30 s on apolished Si substrate. AFM images were collected in tapping mode (DI-3100). For the preparation of SEM samples, the solvent for the sonica-tion was ethanol and the solution was natural-gravimetrically settleddown rather than using a forced centrifuge. Then, the upper part of solu-tion was drop-dried on a polished Si substrate. Average sizes of theCNTs were also measured by particle size analyzer (ELS-8000, OtsukaElectronics) for the dispersed solution containing 20 mg of the shortenedCNTs in 20 g of 1% Sodium dodecylbenzene sulfonate (NaDDBS) aque-ous solution after sonication for 6 h. The loss of materials convertedto amorphous carbon or unstable state of carbons during cryogeniccrushing was measured by thermogravimetric analysis (TGA) in air withheating rate of 5 �C/min–1000 �C. The measurement was performedafter pre-desiccation at 200 �C for 45 min for complete removal of resid-ual water prior to each data collection. Quantitative elemental analysisby Shimadzu ICPS-8100 sequential inductively coupled plasma (ICP)-atomic emission spectrometer was performed to check the possiblecontamination of the shortened CNTs during crushing process. Ramanspectra were obtained on a Renishaw Raman spectroscope with 785 nmexcitation, and then normalized to the G-band together with base linecorrection.

3. Results and discussion

The results in this report were obtained from the samplecrushed as an isopropanol mixed state. The shortenedCNTs after crushing are appreciably soluble in normalorganic solvents such as ethanol or dichlorobenzene with-out any dispersant. The cutting efficiency was confirmedby AFM images shown in Fig. 1. The shortened lengthsof CNTs are identified as less than 500 nm after 30 mincrushing, whose original length was approximately 2 lm.The SEM images with a series of crushing time variationare shown in Fig. 2 indicating that the nanotube lengthsdecrease with crushing time increase. The lengths in SEMimages look longer than those from the AFM, and it isascribed to the existence of entangled tubes due to less effi-cient dispersion in ethanol and less efficient separationfrom large aggregated particles.

In order to obtain more evidences of the cutting effi-ciency with respect to the crushing time, the average sizeof the shortened CNTs was measured by electrophoreticlight scattering (ELS) particle size analyzer. Despite ofthe difficulty in precise measurement for the size of onedimensional nanotube using light scattering analyzer, therelative size distribution by convolution of the scatteringcross-section could be roughly compared [20]. The mea-sured particle sizes of small diameter CNTs dispersed inaqueous NaDDBS solution at pH8 decrease as the crush-ing time, i.e., 3 lm for the untreated, 2.2 lm for 10 mincrushing, and 0.8 lm for 20 min crushing treatment.

TGA measures the amount of materials converted toamorphous or unstable carbon. According to the CNTanalysis protocol [21], the lower oxidation temperature isassociated with defective carbon, while the higher oxida-tion temperature with purer, less defective samples. Fig. 3

Fig. 2. SEM micrographs showing the shortened CNTs from Iljin SWCNTs with a variation of crushing time as (a) untreated, (b) 10 min, (c) 30 min and(d) 45 min.

Fig. 1. AFM images of spin-coated Iljin SWCNTs of: (a) untreated, (b) after 30 min treatment of cryogenic crushing process, onto Si substrate.

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shows the TGA analysis of the 30 min cryogenicallycrushed SWCNTs. For the DTG (derivative of thermo-gravimetry) curves in Fig. 3(b), we assign the low tempera-ture peak around 380 �C to defective carbon peak andthe higher temperature around 560 �C to highly orderednanotube peak. The occurrence of the low temperaturepeak at 380 C is expected from amorphous carbon orunstably too short SWCNTs as a result of crushingprocess. The sharp DTG peak at 560 �C obtained fromthe shortened SWCNTs seems to imply more uniformlydistributed cut powder rather than entangled aggregates.

From the weight difference of the untreated and treatedsamples in the temperature range of 350–450 �C, we cannotice that a partial demolition of the nanotubes stilloccurs up to 15% in this cryo-crushing process. Therefore,the estimated overall yield of carbon is approximately85%.

The shortened SWCNTs did not cause complete loss ofweight even up to 1000 �C in the heating process, whilethe untreated one gave a result of more weight losscompared to the shortened sample. This discrepancy mightbe explained by possible contamination of the shortened

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sample by the metal alloy components of ball and vessel,during crushing process, although it is not desired. Accord-ing to ICP analysis, the shortened sample contains majormetal impurities of Fe, Ni, and Co, together with Cr, Ca,Al, and Zn as minor elements. Since the ball and vesselare also composed of mainly Fe and Cr, it is plausible thatthe collision between ball and vessel is responsible for theextra metal contamination in the shortened sample.

Raman spectra of the shortened SWCNTs show that thedisorder (D) band at 1325 cm�1 in Fig. 4 increases withrespect of the crushing time. This result is reasonablebecause the CNTs having a shorter average length will beobtained by more crushing treatment, and then it will pro-duce more disordered carbons. We expect that the CNTs atliquid nitrogen temperature are rigid, not flexible, thus thecollisions inside vessel will execute the cutting of nanotubesmore efficiently, thus leading to produce less sidewall-dam-aged CNTs. In order to support this assumption, we areunder investigation to find clear evidence.

The radial breathing mode (RBM) of Raman spectrashows a broad band ranging from 180 to 220 cm�1

(Fig. 4(c)). Considering the inverse proportionality of

diameter with respect to the RBM frequency [22] and theexcitation laser of 785 nm in this spectra, the tubes areexpected to have the diameters of 1.1–1.4 nm with semicon-ducting character. Surely, however, the samples are mixedwith metallic tubes, as we observed two broad RBM bandsranging from 150 to 200 cm�1 with 633 nm laser excitation(not shown). The broad band in this RBM mode indicatesthat the SWCNTs produced by arc discharge have large dis-tribution of diameters. The RBM band shown in Fig. 4(c)largely decreases in relative intensity as the crushing timeincreases up to 30 min, then it does not change any morebeyond 30 min treatment. The decreasing tendency ofRBM band intensity may indicate a shortened length ofCNTs. According to literature [23], the intensity of RBMis proportional to the nanotube length. This is because theRBM frequencies correspond to long wavelength compara-ble to the length of nanotube and that the Raman intensitiesof these frequencies are proportional to the number ofatoms within the bond polarization theory. On the otherhand, it is strange that the RBM peak position of the short-ened CNTs shifts to lower frequency. However, this can bealso explained by more number of short tubes, since theshorter tubes vibrate at lower frequencies with standingwave modes consideration [23]. Therefore, both the inten-sity decrease and the shift to lower frequency of the RBMband obtained from our shortened CNTs are described wellwith theoretical expectation.

The same literature [23] also mentioned that finite-sizeeffect in short CNTs lowers the symmetry of nanotubes,thus many Raman-active modes appear in the intermedi-ate frequency region between 500 and 1000 cm�1 and at1217 cm�1. The relatively weak intensities of these interme-diate Raman modes are observed in Fig. 4(d) with indica-tion of arrow signs. The arrow with star positioned at1217 cm�1 could be assigned to the mode of an edge stateof breathing mode, whose vibration amplitude is knownto appear only at the two ends and does not depend onthe length of the nanotube. It is interesting that the60 min crushed sample has a peak at this position even veryweak, implying the abundance of nanotube ends.

In order to check the open-end property of the short-ened CNTs, we have produced peapods by simple sonica-tion of the shortened CNTs in C60 saturated toluenesolution. According to transmission electron microscopy(TEM) images, almost all CNTs with diameters larger than1.5 nm were converted to peapod structure (Fig. 5). Carrierdoping or element doping are attractive techniques fordrastically modifying the transport properties of materialsby introduction of various foreign materials [24–28]. Theshort CNTs are also useful to prepare flexible thin filmtransistor (TFT) [29,30]. Therefore, the cryogenic crushingmethod for the production of short CNTs is believed to bea fundamental base to convert long, entangled CNTs intomore potentially creative materials used in nano, optical,and bio electronic devices.

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Fig. 5. TEM image of fullerene encapsulated SWCNT using the shortenedCNTs prepared by cryogenic crushing method.

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4. Conclusion

We introduce the cryogenic crushing method for theproduction of short CNTs having open-ended tubes withminimum sidewall damages. This process is believed to bea very useful to change the structure of long, entangledCNTs to short nanotubes due to its simple applicationprocedure. The condition for the production of CNTs hav-ing diameters less than 500 nm is approximately 30 minwhen crushed as a solvent mixture state. The open-ended

shortened CNTs could be further modified to make newtype of hybrid materials. With potential applications ofshort CNTs in electronic, biological and optical nanoelec-tronics as well as flat panel display devices, the cryogeniccrushing method for CNT cutting will be a powerful wayto explore nano research areas.

Acknowledgements

This work was supported by the Korea Science andEngineering Foundation sponsoring the National CreativeResearch Initiative Program. Authors thank to Prof. Tourfor valuable discussion on CNT dispersion.

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