Precise cutting of single-walled carbon nanotubes This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2007 Nanotechnology 18 055301 (http://iopscience.iop.org/0957-4484/18/5/055301) Download details: IP Address: 128.197.27.9 The article was downloaded on 12/04/2013 at 15:40 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Precise cutting of single-walled carbon nanotubes
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
2007 Nanotechnology 18 055301
(http://iopscience.iop.org/0957-4484/18/5/055301)
Download details:
IP Address: 128.197.27.9
The article was downloaded on 12/04/2013 at 15:40
Please note that terms and conditions apply.
View the table of contents for this issue, or go to the journal homepage for more
Home Search Collections Journals About Contact us My IOPscience
Precise cutting of single-walled carbonnanotubesShiren Wang1, Zhiyong Liang1,3, Ben Wang1, Chuck Zhang1 andZia Rahman2
1 High-Performance Materials Institute (HPMI), Department of Industrial and ManufacturingEngineering, FAMU-FSU College of Engineering, Florida State University, Tallahassee,FL 32310-6046, USA2 Materials Characterization Facility, AMPAC, University of Central Florida, 12443 ResearchParkway, Suite 304, Orlando, FL 32826, USA
Received 19 September 2006, in final form 12 November 2006Published 9 January 2007Online at stacks.iop.org/Nano/18/055301
AbstractWe precisely cut single-walled carbon nanotubes (SWNTs) to create shortnanotubes with controlled length and open ends using an ultra-microtomeand magnetically aligned SWNT membranes. At −60 ◦C, multiple layers ofSWNT membranes were stacked and frozen together, and then cut at lengthsof 50 and 200 nm, respectively. Transmission electron microscopy (TEM)and Raman characterizations clearly indicated that nanotubes weremechanically chopped to create open ends without notable sidewall damage.Atomic force microscopy (AFM) characterization and statistical analysisshowed that the length distributions of the cut SWNTs can be controlled.Short SWNTs are very promising for applications in biomoleculartransportation, field emission, field effect transistor and nanocomposites.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
Short SWNTs with open ends are very attractive materials forvarious applications, including drug delivery, DNA analysis,molecular sensors, high-efficiency electron emitters, field ef-fect transistors (FET) and high-performance nanocomposites.Here, we report results of an investigation which presents anovel and effective method to precisely section SWNTs to pro-duce short nanotubes with controlled length distribution, openends and limited sidewall damage. SWNTs are molecular-scale tubes of graphitic carbon with outstanding properties,such as excellent mechanical properties, exceptional thermaland electrical conductivity [1]. SWNTs show diameters inthe range of 0.6–1.4 nm [2] while multi-walled carbon nan-otubes (MWNTs) are in the range of 1.4–100 nm [3]. Usu-ally, these SWNTs are capped at the ends. However, oncecapped ends were removed and SWNTs were turned into open-end fullerene pipes, they showed a capillarity-induced filling,making themselves very promising candidates for biologicaltransportation tools for DNA, protein and drug delivery [4, 5].
3 Author to whom any correspondence should be addressed.
Open-end SWNTs have already been shown to shuttle variousmolecular cargos inside living cells including proteins, shortpeptides and nucleic acids, demonstrating a capability to ef-fectively breach the cell-membrane barrier for delivering andenabling functionality of extracellular agents [6–8]. Releaseof oligodeoxynucleotides from SWNT carriers in vitro hasbeen demonstrated by near-infrared (NIR) excitation of nan-otubes that were located inside living cells [6]. A recent studyalso suggested a promising way to cure cancers by targetinginternalized SWNTs into cancer cells that expressed specificcell-surface receptors and were subsequently used as NIR ab-sorbing agents for cancer cell destruction without hurting nor-mal cells [8]. The utilization of the intrinsic physical proper-ties of open-end SWNTs provides a new means to transportbiomolecules and bring in new possibilities in disease ther-apy. Open-end nanotubes can also be used to improve inter-face bonding for high-performance nanocomposites. Polymermolecules can fill in the SWNTs during mixing, resulting inmechanical locks between SWNTs and polymer chains whenpolymers cross-link to form 3D networks during curing reac-tion [9]. In addition, SWNTs showed amazing ballistic trans-port properties. The sub-50 nm FET, which used a sub-50 nm
SWNT as a channel, reached a current of about 20 μA at alow bias voltage of about 0.4 V. In contrast, devices using a1.1 μm SWNT as a channel only reached similar currents ata bias voltage of 2 V [10]. This suggests that the 50 nm nan-otube field-effect transistors are more ballistic than long chan-nel devices. Hence, shortened sub-50 nm nanotubes will bevery useful in fabricating ballistic nanotube electronic devicesand ultra-sensitive molecular sensors. Zhu et al also foundthat open-end carbon nanotubes can improve the field emis-sion characteristic with a field enhancement factor as high as4540 [11].
Many efforts have been directed toward cutting nanotubesfor short lengths and open ends. Smalley’s group initiallydeveloped a chemical method to cut long, tangled, purifiednanotubes (1.1–1.3 nm in diameter) using concentrated,strong acids [12, 13]. The same technique was adaptedto shorten SWNTs produced from electric arc-producednanotube material (1.3–1.5 nm in diameter) [14]. Theresulting shortened SWNTs can be chemically functionalized,solubilized and chromatographically purified [15]. Chenfound that the acid cutting technique may not be appliedto small-diameter nanotubes (0.7–0.8 nm in diameter), whilesmall diameter nanotubes are more attractive materials forstudying nanotube chemistry because of their expected higherchemical reactivity [16, 17]. Other methods have alsobeen tried. Smalley and co-workers demonstrated thatchemical fluoridation followed by pyrolysis provided themodified and cleaved nanotube fragments with broad lengthdistributions [18, 19]. The ultrasonic process is a typicalprocedure in dispersing SWNTs into a solvent and resinmatrix. This process was also found to shorten SWNTs [20].However, sonication sometimes creates holes along the SWNTsidewalls, resulting in ‘worm-eaten’ damage [21, 22]. Solidstate processes such as grinding or milling with diamond ballscan also shorten nanotubes, but these milling methods coulddamage sidewalls and create problems in separating mixturecomponents [23, 24]. Chen developed an extending soft cuttingmethod by grinding in cyclodextrins, which successfullyavoided long-time ultrasonic processing or sonicating in strongacids and oxidants [16]. Lustig et al reported a lithographicallycut method to chop SWNTs, which included lithography toplace protective photo-resist patterns over the nanotubes andconducted reactive ion etching to destroy the unprotectedparts of the nanotubes [25]. Zettl’s group reported a cuttingmethod using a low-energy electron beam, which can cutthe nanotubes either partially through (creating hinge-likegeometries) or fully through (creating size-selected nanotubesegments) [26]. However, most of the reported methods,such as mechanical grinding, acid cutting, fluorine cutting,etc., have difficulty in the length control, and the reportedliterature did not provide detailed information of lengthcontrol. For the lithography method, it is limited in the lengthcontrol because there was no nanotube alignment. Electron-beam cutting can control the cutting length, but it is oflimited interest because this operation can only be conductedwithin the SEM or TEM instrument; scale-up applicationis very difficult. Considering these issues, we present amethod to precisely cut SWNTs using aligned nanotubemembranes through an ultra-microtome. Magnetically alignedSWNT membranes (Buckypapers) were fabricated through the
Figure 1. SEM images of aligned SWNT Buckypaper using a 17.3 Tmagnetic field. (The arrow shows the alignment direction.)
filtration of aqueous surfactant-suspended nanotubes in a highmagnetic field. Multiple layers of frozen aligned membraneswere sectioned into desired lengths with an ultra-microtomeinstrument, which can cut nanotubes as short as 5 nm [27, 28].The chopped carbon nanotubes were characterized using TEM,AFM and Raman techniques. The mechanism of the lengthdistribution was also discussed.
2. Experimental details
2.1. Magnetic alignment of carbon nanotubes
The pristine nanotubes were dispersed into water and stabilizedwith surfactant. Then the carbon nanotubes’ suspension waspassed through a nylon filter film, which was kept in the boreof a resistive coil magnet with a magnetic field of 17.3 T atthe National High Magnetic Field Laboratory. The filtrationpump pressure was about 138–172 kPa. After a desired amountof suspension passed through the filter, isopropyl alcohol waspassed through the filter to wash off the surfactant. The filterwas then removed from the magnetic field and kept in a vacuumoven at full vacuum pressure. This facilitated the drying ofthe nanotube membranes (Buckypaper). After drying, thefilter was removed and the magnetically aligned nanotubeBuckypaper was obtained.
2.2. Sectioning aligned Buckypaper
About 20–40 magnetically aligned Buckypaper strips werecut from the large aligned Buckypaper film and stacked uptogether with the same nanotube alignment direction, thenfrozen into a rigid block for preparing the cutting samples. At atemperature of −60 ◦C, using a cryo-diamond knife (ultra-thin45◦, Diatome Inc.) with a cutting edge of about 1–2 nm radius,the frozen block of multiple layers of aligned Buckypaperstrips were sectioned at 50 and 200 nm, respectively, usinga Leica EM UC6/FC6 ultra-microtome. In order to keep theconsistency and eliminate the pre-cut length distribution effect,we used the same batch of SWNTs for all cases in this research.
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Figure 2. TEM images of cut nanotubes with open ends. (Scale bars in (A) and (B) are 10 and 5 nm respectively.)
Figure 3. TEM and AFM images of sectioned SWNT bundles. (A) AFM image of the cut nanotube bundles. (B) TEM image of the cutnanotube bundles.
3. Results and discussions
3.1. Magnetically aligned Buckypaper
Single-walled carbon nanotubes are predicted to have a weakmagnetic susceptibility. Their susceptibility can be eitherdiamagnetic or paramagnetic, depending upon the helicityof the carbon nanotubes and Fermi energy [29]. Smalley’sgroup first utilized high magnetic fields to align SWNTsand produced magnetically aligned nanotube membranes orBuckypaper [30]. We built upon their process to produce largemagnetically aligned SWNT Buckypaper films of 60 in2 and10–15 μm thick using a 17.3 T magnetic field. SEM analysisrevealed that the nanotubes were aligned toward the magneticdirection in the resultant Buckypapers, as shown in figure 1,which provides us the possibility to precisely section SWNTsand control the nanotubes’ length distributions.
3.2. Observations of cut nanotube open ends
In the experiment, the aligned Buckypaper film samples weresectioned at lengths of 50 and 200 nm, respectively. Thesectioned SWNTs were dispersed into an aqueous suspensionusing slight sonication (12–15 W m−2) and surfactant toprepare the samples for AFM and TEM examination. TEMresults revealed that a majority of the SWNTs’ ends were openafter cutting, as shown in figures 2(A) and (B).
Low sonication power allows us to disperse the sectionedSWNTs into small bundles for AFM and TEM analysis asshown in figure 3. The mechanical cutting was proven by thesharp cut cross section. Since the cutting feeding directionwas perpendicular to the tube alignment direction, perfectlyaligned (alignment angle θ = 0◦) nanotube bundles will havea flat cutting section, which is vertical to the axis. Partiallyaligned tubes may have sharp but tilted section surfaces asfigure 3(A) shows. The AFM analysis indicates that themechanical cutting deformed nanotube bundles. The TEMimage clearly illustrates that nanotube bundles were cut almostperpendicular to the axis of nanotubes, as shown in figure 3(B).During cutting, the diamond knife provided large pressure tothe local zone of carbon nanotubes. A continuous increasein pressure led to local deformation, leading to nanotubescracking and these local cracks propagated under increasingpressure until the nanotube was broken.
3.3. Nanotube length distributions
A drop of the pristine (uncut) SWNT suspension was used toprepare the AFM samples for measuring length distribution.Figure 4(A) shows the AFM image of the dispersed uncutnanotubes. SIMAGIS software was used to quantify thenanotube length [31] and the results were extracted infigure 5(A), which indicated that the SWNT lengths of thepristine sample were centred at a mean of 580 nm with a
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Figure 4. Length distribution observations of (A) pristine nanotubes; (B) 200 nm cut nanotubes; (C) 50 nm cut nanotubes.
Figure 5. Statistical analysis of the length distribution. The greenline is the calculated empirical probability density function.(A) Pristine nanotubes; (B) 200 nm cut nanotubes; (C) 50 nm cutnanotubes.
standard deviation of 320 nm. About 55% of nanotubes werelonger than 500 nm and no observation was below 50 nm.
Precise control of nanotube cutting length can be achievedby the instrument’s ability to precisely control each cuttingstep length and good nanotube alignment in the samples.Firstly the cutting length was set to 200 nm and then theBuckypaper sample was sectioned. Figure 4(B) shows the200 nm cutting nanotube image characterized by AFM. TheAFM image was quantitatively analysed by SIMAGIS softwareand length information was extracted in figure 5(B).
The length distribution shown in figure 5(B) indicatedthat the 200 nm cut nanotubes were centred at a mean of246 nm with a standard deviation of 136 nm. Compared withuncut nanotubes, the variance of the 200 nm cutting nanotubes’length distribution was reduced. About 60% of the lengths fellin the range of 100–300 nm, according to cumulated counting.
Similarly, we cut the nanotubes with feeding lengths of50 nm and characterized the 50 nm cut tubes using AFM.The AFM image was quantitatively analysed by SIMAGISsoftware in figure 4(C) and length information was extractedin figure 5(C). Based upon the statistical analysis, the lengthdistributions of the 50 nm cut nanotubes were quite skewedand had long tails. The mean length was about 87 nm. Longernanotubes show lower probability in the statistical analysis.About 60% of nanotube lengths ranged from 50–150 nm.
As figure 5 shown, considerable differences exist inthe length distributions among the uncut, 200 nm cut and50 nm cut nanotubes. The mean length shifts between thesetubes were obvious. Differences among the variance of thelength distributions were also outstanding and the varianceaccordingly decreased as cutting feed length diminished.These statistical results suggest that microtome cutting
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Wavenumber (cm-1)
Figure 6. Raman spectra: (A) acid-cut SWNTs by sulfuric acid;(B) microtome-cut SWNTs and (C) pristine SWNTs.
provided an effective way to precisely control the nanotubelength distribution.
In addition, there are some impurities shown in the cutSWNTs samples. These impurities possibly arise from theresidual surfactant. All of these nanotube samples for the AFMexperiments were dispersed in the aqueous solution with theaid of surfactant. Then a drop of nanotube suspension wasdripped on a silicon wafer surface, dried and then washed withisopropanol to remove surfactant for AFM analysis. However,for the uncut tubes, the surfactant was easy to wash out, butit is difficult to remove surfactant from cut tubes. This isbecause the surfactant has more interactions with nanotubeopen ends and even can partially fill into the open ends ofthe tubes. Therefore, more surfactant can still wrap on the cuttubes after we washed the samples. As a result, we see moreimpurities appeared in the images of the cut tubes. Due to theun-removed surfactant, the cut nanotubes also seem to becomelarger bundles.
It also needs to be addressed that, in the AFM images,most of the observations are of ropes rather than individualtubes. We generally distinguish the individual tubes versusbundles of SWNTs based on their diameters. If the diameteris less than 2 nm, we considered the observations asindividual nanotubes, otherwise bundles. However, sinceit is very difficult to achieve individual tubes, we haveto count the length of both individual and bundle SWNTs
together. This could compromise the length analysis results ata certain degree, but we treated all cases identically to ensureconsistency.
We can also see that the length distributions of cutnanotubes are not uniform. One possible reason may arisefrom the length distribution of pre-cut SWNTs. Since thepre-cut SWNTs are discontinuous and do not have the samelength, then the length of pre-cut SWNTs would contributeto the final non-uniform length distribution of cut nanotubes.Another possible reason may stem from the partial alignmentof nanotubes in the aligned Buckypaper films. Ideally, carbonnanotubes would be perfectly aligned and cut nanotube lengthswould match the cutting set lengths with a small meanshift toward the left due to the discontinuous tube length.Actually, not all nanotubes achieved perfect alignment inthe magnetically aligned Buckypaper films. For instance,nanotubes may be aligned within the range of 0◦–10◦ or0◦–20◦. If the nanotubes were randomly oriented, somenanotubes would not be cut since they could possibly bealmost parallel to the cutting direction. Due to this reason,the experimental value of the mean lengths of the cut tubesshifted right slightly compared to the expected mean lengths,showing that alignment is important for controlling nanotubes’length distribution. The better the tube alignment was, themore precisely the cutting length was achieved. If thereexists an alignment angle, then the real cutting length canbe computed based on the expected cutting length, and itshould be slightly larger than the expected length. The lengthstatistical distribution results supported this analysis. Forexample, 200 nm cut nanotubes had a mean length of 246 nm,which was slightly longer than the expected 200 nm.
3.4. Sidewall damage analysis
Sidewall damage of the microtome-cut SWNTs was comparedto the acid-cut and pristine SWNTs. The acid-cut tubes wereprepared by dispersing SWNTs in sulfuric acid (98%, FisherScientific Inc.) at 70 ◦C for 2 h. Figure 6 shows the resultsof Raman analysis. D-band of the microtome-cut tube wasalmost the same as the pristine SWNTs, but the acid-cutSWNTs showed an enlarged D-band. Since the D-band wasproportional to the defects of the tubes, the result suggestsminimal damage of the SWNTs by microtome cutting. TEMobservation of the acid- and microtome-cut tubes, showingin figure 7, also clearly shows the difference between theirsidewalls. The sidewalls of the acid-cut tubes appear
Figure 7. TEM images of (A) acid-cut SWNTs (2 nm scale bar); (B) microtome-cut SWNTs (5 nm scale bar).
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worm-eaten damage, while the microtome-cut tube sidewallswere almost intact [32].
4. Conclusion
The method of precisely sectioning carbon nanotubes providesan effective approach to cut carbon nanotubes with controllablelength, open ends and minimum sidewall damage. Thisapproach avoids the drawbacks of other chopping methods andstraightforwardly shortens carbon nanotubes with controlledlength distribution. Short nanotubes with open ends andcontrolled lengths would bring wide applications in medicine,electronic devices and composite materials.
Acknowledgments
This work was partially supported by Army ResearchLaboratory NOLES program. The authors would like to thankMr Walter Roy and Dr Shawn Walsh for their support andmanagement of this project. This research was also supportedby AFOSR grant no. FA9550-05-1-0271. The management ofthis program by Dr Charles Lee is greatly appreciated. We alsowould like to thank National High Magnetic Field Laboratory(NHMFL) and AMPAC of University of Central Florida forallowing us to use their facilities.
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