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Fabrication and characterization of a carbon nanotube-based nanoknife

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2009 Nanotechnology 20 095701

(http://iopscience.iop.org/0957-4484/20/9/095701)

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IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 20 (2009) 095701 (6pp) doi:10.1088/0957-4484/20/9/095701

Fabrication and characterization of acarbon nanotube-based nanoknifeG Singh1,2, P Rice1, R L Mahajan2,3,4 and J R McIntosh5

1 Department of Mechanical Engineering, University of Colorado at Boulder, CO 80309, USA2 Institute for Critical Technology and Applied Science, Virginia Polytechnic Institute andState University, Blacksburg, VA 24061, USA3 Department of Mechanical Engineering, Virginia Polytechnic Institute and State University,Blacksburg, VA 24061, USA4 Department of Engineering Science Mechanics, Virginia Polytechnic Institute and StateUniversity, Blacksburg, VA 24061, USA5 Department of Molecular, Cellular and Developmental Biology, University of Colorado atBoulder, CO 80309, USA

E-mail: gurpreet@vt.edu

Received 15 October 2008, in final form 16 January 2009Published 11 February 2009Online at stacks.iop.org/Nano/20/095701

AbstractWe demonstrate the fabrication and testing of a prototype microtome knife based on amultiwalled carbon nanotube (MWCNT) for cutting ∼100 nm thick slices of frozen–hydratedbiological samples. A piezoelectric-based 3D manipulator was used inside a scanning electronmicroscope (SEM) to select and position individual MWCNTs, which were subsequentlywelded in place using electron beam-induced deposition. The knife is built on a pair of tungstenneedles with provision to adjust the distance between the needle tips, accommodating variouslengths of MWCNTs. We performed experiments to test the mechanical strength of a MWCNTin the completed device using an atomic force microscope tip. An increasing force was appliedat the mid-point of the nanotube until failure occurred, which was observed in situ in the SEM.The maximum breaking force was approximately (8 × 10−7) N which corresponds well withthe typical microtome cutting forces reported in the literature. In situ cutting experiments wereperformed on a cell biological embedding plastic (epoxy) by pushing it against the nanotube.Initial experiments show indentation marks on the epoxy surface. Quantitative analysis iscurrently limited by the surface asperities, which have the same dimensions as the nanotube.

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1. Introduction

Three-dimensional (3D) cryo-electron microscopy (cryo-EM)of frozen–hydrated samples is important for structural andfunctional studies of cells. In this technique, samples arefrozen quickly to minimize crystallization damage; the resultis cellular material embedded in vitreous ice. Frozen–hydratedcell samples are sliced in a microtome and subsequentlyexamined in the cryo-EM, using a low temperature specimenholder. When compared with conventionally preparedsamples, the frozen–hydrated samples have a greater likelihoodof displaying cellular structures in their native state [1].However there is a problem intrinsic to cutting sections offrozen–hydrated samples with conventional diamond or glass

knives. The angle included at the knife’s cutting edge bendsthe sections sharply away from the block face, inducingcompressive stresses on the upper surface of the section relativeto the bottom, which leads to cracking of the sample surfacewhen it is laid flat [2]. A possible solution to this problem is touse a MWCNT in place of a conventional diamond knife. Thisdevice would reduce the angle by which the sample is bentduring cutting, due to the small diameter of a carbon nanotube.

Carbon nanotubes (CNTs) are promising materials forbuilding nanoscale mechanical structures. Ever since theirdiscovery in 1991, they have received much attention, due totheir extraordinary mechanical and electronic properties. Theirhigh tensile strength and well-defined cylindrical geometryoffer an attractive means to overcome the problem associated

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Nanotechnology 20 (2009) 095701 G Singh et al

with sectioning thin vitreous cell samples. A CNT shouldbe able to act as an effective compression-cutting tool thatcould slice through a block of vitreous cells in a mannersimilar to that of a steel wire cutting a block of cheese [3].CNTs are flexible and can have very high aspect ratios whichmake them suitable for 3D manipulation and the fabricationof nanodevices using a probe type manipulator. There havebeen a growing number of measurements of their mechanicalstrength. Wong et al [4] measured the bending strength ofa MWCNT using an atomic force microscope (AFM) tip tobe 28.5 GPa while Falvo et al [5] reported 100–150 GPa fortensile strength. In a direct tensile test by Yu et al [6] the tensilestrength of MWCNT was reported to be (11–63) GPa. In spiteof the significant progress in experiments on CNTs, there isstill considerable ambiguity and scatter in these mechanicaldata, due to the technical difficulties involved in manipulationsat nanometer scale; moreover, they are so small that there isno standard testing device that can be used to calibrate suchmeasurements [7, 8]. Applications that use CNTs are nowbeing realized in devices such as nanotweezers, nanobearingsand nano-oscillators that have been fabricated by manipulationof CNTs inside a SEM [9, 10]. In this paper, we describeinitial steps towards the development of a CNT-based knife wecall a nanoknife. This device is formed by welding a CNTacross two tungsten needles inside the SEM. Before using itas an actual cutting device we performed tests to investigateits mechanical strength and to identify the failure points ofindividual nanoknives.

2. Experimental details

2.1. Fabrication of the device

The device consists of two electrochemically sharpenedtungsten needles that extend over the edge of a glass support(figure 1). The needles, placed at an angle to each other, areglued on a glass substrate (20×15) mm2 using a 1:1 mixture ofinsulating varnish and n-propanol, which sustains high vacuumwithout out-gassing. The distance between the needle tips ismanipulated under an optical microscope. Once the desiredtip distance is obtained, the varnish is cured in an oven at80 ◦C for 30 min. The tips of the tungsten needles have aradius of curvature of approximately 100 nm. The device isobserved occasionally under the optical microscope as it cures,to see if varnish contraction has had any noticeable effect ontip distance.

Manipulation and attachment of the MWCNT to thedevice was a stepwise process achieved by using a 3-axismicromanipulator that moved an electrically isolated tungstenprobe. The manipulator sits on a custom-made aluminumplatform that fits on the SEM stage (Model: JEOL™ 6480 LV)allowing large linear movement of the manipulator andalignment with respect to the sample. In the first step,MWCNTs were fabricated using thermal chemical vapordeposition (CVD) at 725 ◦C using Fe as catalyst source, thesynthesized NT powder was then dried from toluene into athick mat. The MWCNT mat was cut into small pieces, and onepiece was gently rubbed over double-sided acrylic adhesive

Figure 1. Schematic of the device that was used for mechanicalcharacterization of MWCNTs and the proposed nanoknife. Twotungsten needles were used to support the MWCNT. The gapbetween the needles, where the nanotube is to be welded, wastypically half the length of the nanotube, approximately 20 μm. Insetis a SEM image of the aligned tungsten needles.

(This figure is in colour only in the electronic version)

tape attached to the edge of a sample block. This positionedthe nanotubes with their ends overhanging the edge from whichthey were readily accessible to the manipulator probe [11].

The block edge was aligned with respect to themanipulator, then the probe was brought into contact withthe exposed end of the nanotube of interest. The electronbeam was focused, and the image magnified at the point ofcontact. The electron beam irradiated residual hydrocarbons(usually present in the SEM on the sample surface) causingthem to decompose and then deposit on the sample, forming aweld. This process is called electron beam-induced deposition(EBID) [12, 13]. The CNT was then pulled and separated fromthe bundle by retracting the probe to its original position asshown in figure 2(a).

The next step involved replacing the CNT sample blockwith the glass slide device and realigning the manipulatorarm with respect to the two tungsten needles on the glassslide [14]. The CNT on the manipulator probe was then movedso the free end of the nanotube touched the tip of one of theneedles on the glass slide. It was subsequently welded to thatneedle using EBID. The manipulator arm was then moved atthe slowest possible speed (step size = 5 nm) to make theCNT touch near the tip of other needle, where it was alsowelded, so it would bridge the gap. Thus, the CNT waswelded at three different points (two on needles mounted onglass and one on the manipulator probe). The manipulatorarm was then moved laterally, breaking the CNT from themanipulator probe but, leaving it connected to the needles onthe glass substrate as shown in figure 2(b). More details onthe nanoknife are provided in the attached video file (availablefrom stacks.iop.org/Nano/20/095701) which is a collection ofSEM images at each fabrication step.

2.2. Mechanical characterization

The first step towards evaluating the nanoknife as an effectivecompression-cutting tool was to assess its mechanical strengthwith forces comparable to those required for preparing actualsamples. We have performed in situ mechanical tests onindividual nanoknives in the SEM by loading them in a

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Nanotechnology 20 (2009) 095701 G Singh et al

Figure 2. (a) SEM micrograph of MWCNTs on acrylic tape, hanging from the edge of the SEM Al block. (b) The nanotube has been weldedto the tungsten probe, and is now being taken away from the bundle to be later welded to the needles on the glass substrate.

(a) (b)

(c)

Figure 3. (a), (b) and (c) SEM micrographs showing the mechanical testing of the nanotube being pushed by the AFM tip for tests 1, 2 and 3respectively. Insets in the images are at the same magnification as the main images and correspond to the maximum deflection of the NT aswell as the AFM tip before breakage. The images are taken at faster scan rate to avoid creep effects.

transverse direction, similar to a rigidly supported beam withthe load acting at the center of the beam.

To determine the breaking force at maximum CNTdeflection, we employed an AFM tip with a nominal forceconstant value of 2.8 N m−1 (as provided by the manufacturer).The AFM tip was glued to a tungsten probe connected to

the manipulator. The AFM tip was then aligned with thenanoknife in a manner such that the tip touched the centerof the nanotube as shown in figure 3(a). Prior to testing, theAFM tip was slightly blunted by rubbing it against a zirconiasubstrate; this helped to prevent the AFM tip from slipping pastthe nanotube. Once the AFM tip was in contact with the CNT,

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Nanotechnology 20 (2009) 095701 G Singh et al

Figure 4. (I) Infra red camera view of the chamber showing arrangement used for cutting experiments with the nanoknife. (II)–(IV) SEMmicrographs of the cutting process being carried out at an oblique angle to the electron beam, providing easier alignment and greater visibility.Nanowelding (EBID) was performed using the SEM in spot-mode, and in this experiment it is clearly seen that the weld survives, while thenanotube breaks near the weld.

Table 1. Test data from mechanical testing of three nanoknives. The diameter of nanotubes and AFM tip deflection were measured in theSEM and a cumulative 10% error is assumed in their value because of nanotube vibration at higher magnifications. Weld area is assumed to bethe same as the cross section of the NT; variation arises in locating the exact size of the deposit. For test 1, the load test could not beconducted till the breakage point as the AFM tip started touching the tungsten needles on the support structure.

Number d (nm) δmax (nm) A (μm2) (×10−3) Fmax (nominal) N (×10−9) τ (GPa)

1 200 ± 20 150 ± 10 31 ± 6.2 420 ± 42 0.013 ± 0.0042 110 ± 11 200 ± 20 9.5 ± 1.9 560 ± 56 0.029 ± 0.0083 60 ± 6 290 ± 29 2.8 ± 0.5 812 ± 81.2 0.145 ± 0.043

the manipulator was set at its lowest speed with a step size of5 nm. The AFM tip then moved horizontally (in the field ofview of the SEM), pushing against the nanotube and deflectingit.

This deflection was recorded every ∼400 nm ofmanipulator movement by stopping the manipulator and takingan image (figures 3(b) and (c)). The load on the nanotubewas increased until it failed, which in this case occurred atthe weld. The AFM tip deflection at failure was measuredby image correlation, using commercial software (analySIS™)and stored SEM images. Figures 3(a)–(c) show the load testsbeing performed on different nanoknives fabricated in thisstudy. The data corresponding to these load tests are enlistedin table 1. In all of the tests we performed the welds failed,implying that the weld strength is weaker than the strength ofMWCNT. The total nanotube deflection for these tests rangedfrom (3 to 5) μm and there was no visible damage to theCNT after the welds broke. Data from our most successful

test showed a maximum deflection of the AFM tip at failure tobe 0.29 μm (where the most successful test corresponds to thetest in which we observed maximum tip deflection at the pointof failure).

The maximum force exerted by tip before weld breakageis given by

Fmax = K δmax (1)

where Fmax represents the maximum force exerted by the AFMtip, K corresponds to the nominal force constant of the AFMtip (N m−1) and δmax is the maximum deflection of the AFMtip (μm). The weld area was approximated to be the areascanned by the beam during deposition (roughly the same asthe cross section of nanotube in this case) [15, 16]. Assumingthe welds failure mode to be shear, the maximum weld strengthis obtained for test 3 as

τ = F/2A = 0.145 GPa. (2)

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Nanotechnology 20 (2009) 095701 G Singh et al

(b) (c)

(a)

Figure 5. (a) Top view of the Au-coated resin block just after the test. Indentations made by marks from the CNT and the tungsten needles arevisible. No deep or obviously visible cut was observed. (b) and (c) SEM images of the top surface of the Au-coated Epon resin showing theartifacts and surface roughness at the edge, which makes, it difficult to quantify any cutting or indentation marks from the nanoknife.

This value represents a weld strength that is less than the tensilestrength of MWCNTs reported by Yu et al [6], in which studythe tensile testing of MWCNTs was accomplished withoutbreaking the welds.

2.3. Cutting experiment on Epon resin

Epon resin is commonly used to embed cell biologicalsamples that have been fixed and dehydrated in preparationfor microtomy and then transmission electron microscopy [17].This resin had several advantages over vitreous ice for thetesting a nanoknife-based cutting process: (a) Epon can bepolymerized so it is reasonably soft, and its mechanicalproperties are fairly well known; (b) the sectioning of vitreousice would have required a cryo-environment, which was notattainable in our SEM. Moreover, ultra microtome cuttingexperiments on Epon resin by Asakura et al [18] havesuggested that the cutting force depends on section size and

section thickness. Based on their measurements, cutting asection 100 nm thick from a block of polymer whose face is(30 × 30) μm, would require a force of ∼1.8 μN. This valueis comparable to the maximum force observed in our tests, seetable 1, reinforcing our decision to try this resin for our firstcutting experiments with the nanoknife. A block of Epon resinwas trimmed to fit the cutting length of nanoknife. However,this polymeric material tends to become charged under thebeam of the SEM, blurring any images acquired. To reducethis problem and improve imaging at higher magnification, thesample was coated with a layer of Au a few nanometers thick.Hence, the optimum conditions to obtain high resolution andlarger working distance for our experiments were (15–30) kVacceleration voltage and minimum Au thickness value of∼20 nm. Lower acceleration voltage has other disadvantage;a recent study on CNTs has shown that the maximum physicaldamage to the nanotube is caused at lower SEM acceleration

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Nanotechnology 20 (2009) 095701 G Singh et al

voltages (close to 1 kV) [19]. This Epon specimen wasfixed to a tungsten probe, which in turn was connected to themanipulator arm. Figure 4(I) shows side view of the chamberobtained with an infra red camera as the specimen was beingaligned with respect to the nanotube inside the SEM. The blockwas pushed against the nanotube at an angle oblique to theelectron gun, allowing a better view of the nanotube as it madecontact with the specimen surface. Figures 4(II)–(IV) are acollection of images showing the stepwise movement of theresin block into the nanoknife. Images of the block surfacebefore and after the tests showed indentation marks from theCNT. It appears that the force exerted by the nanoknife isjust sufficient to leave a mark on the Au-coated surface ofthe block as shown in figure 5(a). The indentation marks onthe block are most likely due to plastic deformation of theEpon under the thin Au film. Since the gold coating is only afew nanometers thick, the CNT must pierce or deform the Aufilm to produce a mark that is visible in our SEM. However,it is impossible to make quantitative statements about themagnitude of the indentations in the plastic. Surface roughnessand the imaging characteristics of these specimens in ourelectron microscope limited the clarity of the before/afterdifferences, which hindered our interpretation of the resultsfrom these experiments, as highlighted in figures 5(b) and (c).

3. Summary and conclusion

We have demonstrated the feasibility of fabricating a nanoknife(compression-cutting tool) based on an individual CNT. Ananotube was stretched between two tungsten needles in amanner that allowed us to test the mechanical strength of theassembled device. A force test on the prototype nanoknifeindicated that failure was at the weld, while the CNT wasunaffected by the force we applied. In situ load tests on thenanoknife indicated maximum device strength of 0.145 GPa,corresponding to a weld breaking force of 0.81 ± 0.8 μN.

Cutting experiments performed on a sharp, Au-coatedEpon block showed indentation marks due to forces exerted bythe nanotube. Characterization of the cutting process has beenlimited by the lack of high resolution imaging of the polymericspecimen in our SEM, which made it difficult to locate a smallcut or mark. The surface roughness of the specimen, which isof the same order as the nanotube diameter, also contributed tothe problem.

Acknowledgments

This work was supported in part by RR000592 from the NIHand a generous gift from Bruce Holland to JRM. We also

thank Mary Morphew, Cindi Schwartz, and Mark Ladinsky forpreparing the epoxy resin specimens and Stefano Maggilinofor the software support. R L Mahajan would like to thankDudley Finch for initial help with the project. Authors arevery grateful to NIST-Boulder laboratories for allowing us touse the micromanipulator. The use of trade names is givenfor reference purposes only. The mention of commercialproducts in this manuscript does not represent an endorsementby NIST.

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