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CARBON NANOTUBE

1Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology,Department of Chemical Engineering, Tsinghua University, Beijing 100084, China.2Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China.3Key Laboratory for the Physics andChemistry of Nanodevices, PekingUniversity, Beijing100871, China. 4Department of Materials Science and Engineering, Stanford University,Stanford, CA 94305, USA. 5Center of Advanced Science and Engineering for Carbon(Case4Carbon), Department of Macromolecular Science and Engineering, Case WesternReserve University, Cleveland, OH 44106, USA. 6Institute of Advanced Materials forNano-Bio Applications, School of Ophthalmology and Optometry, Wenzhou MedicineUniversity, Wenzhou, Zhejiang 325027, China.*These authors contributed equally to this work.†Corresponding author. Email: wf-dce@tsinghua.edu.cn

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Acoustic-assisted assembly of an individualmonochromatic ultralong carbon nanotube for highon-current transistors

Zhenxing Zhu,1,2* Nan Wei,3* Huanhuan Xie,1,2* Rufan Zhang,1,4 Yunxiang Bai,1,2 Qi Wang,1

Chenxi Zhang,1,2 Sheng Wang,3 Lianmao Peng,3 Liming Dai,5,6 Fei Wei1†

Dow

nloade

Great effort has been applied to scientific research on the controllable synthesis of carbon nanotubes (CNTs) with highsemiconducting selectivity or high areal density toward themacroscale applications of high-performance carbon-basedelectronics. However, the key issue of compatibility between these two requirements for CNTs remains a challenge,blocking theexpectedperformanceboost of CNTdevices.We report an in situ acoustic-assisted assemblyof high-densitymonochromatic CNT tangles (m-CNT-Ts), consisting of one self-entangled CNT with a length of up to 100 mm andconsistent chirality. On the basis of a minimum consumed energy model with a Strouhal number of approximately 0.3,the scale could be controlled within the range of 1 × 104 to 3 × 104 mm2 or even a larger range. Transistors fabricated withone m-CNT-T showed an on/off ratio of 103 to 106 with 4-mA on-state current, which is also the highest on-state currentrecorded so far for single CNT–based transistors. This acoustic-assisted assembly of chiral-consistentm-CNT-Ts will providenewopportunities for the fabricationof high-performanceelectronics basedonperfect CNTswithhighpurity andhighdensity.

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INTRODUCTIONAlthough carbonnanotubes (CNTs) are themost viable option as a sub-stitute material for silicon for the next-generation transistors (1), the re-quirement of dense, aligned arrays of electronically pure CNTs is anenormous challenge for applications (2). Compared with post-treatmentsynthesis (3, 4), the in situ synthesis of CNTs with specific structures is adirect andnondestructivemethod, inwhich the composition anddispersionof catalysts have been demonstrated to be of central importance in control-ling the density (5), semiconducting selectivity (6), and chirality (7). How-ever, it is scarcely possible to directly synthesize pure semiconductingCNTsby an in situ catalytic reaction only, without any post-separation, andsmall amounts of metallic CNTsmay cause catastrophic device shortingfailures. Thus, the synthesis of CNTs with consistent chirality and highdensity is facing a bottleneck, and novel technical routes should be ex-plored to promote the further development of carbon-based electronics.

Most of us have been incautiously trapped by the technical standardsproposed by IBM in 2013 that have singled out the necessity of a highdensity of up to 125 CNTs mm−1 in tandemwith a high semiconductingselectivity of up to 99.9999% of CNT arrays for a larger output currentand a lower energy consumption in electronics (2). However, an impor-tant prerequisite for single-walled CNTs (SWNTs) with an averagediameter of 1 nmwas ignored, and few-walledCNTs, especially double-walled CNTs (DWNTs) and triple-walled CNTs (TWNTs), were notwell received because of the challenge of the reproducible synthesis ofall-semiconducting few-walled CNTs. If any inner wall of an individualfew-walled CNT is metallic, it will lead to leakage currents and a de-creased on/off ratio in CNT field effect transistors (CNTFETs). Recent

experimental reports havedemonstrated that ultralongCNTs synthesizedwith the assistance of water were mostly all-semiconducting DWNTs orTWNTs and that a representative TWNT with a perfect structure couldwithstand themaximum current of 17 mA, which is six times higher thanthat of SWNTs (8), and exhibited sensitive light responses (9). Consid-ering the standard proposed by IBMbased on the SWNTarrayswith anaverage diameter of 1 nm, a density of up to 125 SWNTs mm−1, and anoutput current of up to 3 mAper CNT, the total current drivability withinthe 1-mm range reaches 0.375 mA. In contrast, for TWNTs with an aver-age diameter of 3 nm and an output current of up to 17 mA per CNT, thedensity will decrease astonishingly to 22 TWNTs mm−1 for the same cur-rent drivability, markedly shortening the path toward the ultimate goal.On the other hand, half-meter-long CNTs with an atomically consistentperfect structure and superior properties could be synthesized using thefurnace-moving method (10); however, the as-synthesized horizontallyaligned ultralong CNTs usually have different chiralities and a tip-growth-mode–determined low density, resulting in difficulties in effec-tive separation. Thus, it will be easier to achieve themost difficult step ofshort CNT separation and manipulation for further applications if weassemble ultralong one-dimensional (1D) chiral-consistent CNTs intomacroscopic quasi-planar structures with a considerable density.

In actuality, tangling is a common phenomenon according to somerelated research on elastic rods immersed in glycerin (11), silk filamentsin a flowing soap film (12), and flagella of swimming bacteria (13)derived from fluid-body interactions. Furthermore, our previous studiesshowed that a coupling of the flow and a sound field could break theequilibrium state between the low-velocity gas flow and the ultralongCNT (14). These results suggest that the in situ assembly from an en-tangled ultralongCNT to a large-area planar structure through themeansof a sound field is feasible. We believe that this assembly of 1D electron-ically pure nanotubes will provide new approaches and routes for fabri-cating high-performance carbon-based electronics.

RESULTSIn situ assembly of monochromatic CNT tanglesFigure 1A shows a schematic illustration of the synthesis of mono-chromatic CNT tangles (m-CNT-Ts) by introducing acoustic waves

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in situ during the normal growth process of ultralong CNTs (seeMaterials and Methods for details). The synthesis of ultralong CNTsvia the chemical vapor deposition method relies on many factors,such as temperature, gas velocity, composition of reactants, and gas flow(10), of which gas flow stability is important for the control of thehorizontally aligned morphology. Therefore, we specifically designeda layered rectangular reactor (LRR) with a width-to-height aspect ratioof 120:12 (fig. S1), the structure of which led to a reduction of velocityfluctuation along the height direction and a significantly stabilized gasflow during the reaction. The gas flow will therefore be very sensitive toexternal interference, leading to easy entanglement of some floatingultralongCNTsunder the effect of acousticwaves.A typical as-synthesizedm-CNT-T, which consists of hundreds of small loops entangled by oneultralong CNT with a length of up to decimeters, is shown in Fig. 1B.The area of them-CNT-T can be as large as 104 mm2,whereas the averagedensity is hundreds of CNT segments per 100 mm, and the maximumdensity among the local areas can be up to a thousand segments per100 mm.Thismeans that half of the ultimate goal has been accomplished,and further advanceswill bepromoted through theusual post-densificationmethod (15) to significantly improve the areal density. The sphericalaberration–corrected transmission electron microscopy (Cs-correctedTEM) characterization, shown in Fig. 1C, reveals that the m-CNT-Tswere mainly formed from few-walled CNTs, and a single-walledm-CNT-T was transferred for high-resolution TEM (HRTEM) charac-terization assisted with a cellulose acetate film (fig. S2) (8). As observed,the m-CNT-T exhibits a loop-like secondary structure, with the loopdiameters ranging from 10 to 20 mm, as shown in Fig. 1D. Unlike thosepreviously reported coiled structures, the formation of theCNT loops in

Zhu et al. Sci. Adv. 2016;2 : e1601572 30 November 2016

the m-CNT-T, which strongly depends on the gas flow pattern, cannotbe attributed to the previous falling spaghetti mechanism (16), demon-strating a simultaneous combination of gas flow– and surface-directedgrowth. On the basis of our experimental observation and theoreticalcalculations, we developed an acoustic-induced vortex (AIV) mecha-nism for the fabrication of m-CNT-Ts.

AIV mechanism for the formation of m-CNT-TsThe growth flow for the formation of m-CNT-Ts in the LRR is a planePoiseuille flow (17), in which the initial parabolic pattern (fig. S3) wasabruptly disturbed with a periodic jet driven by a loudspeaker (fig. S4)from awall orifice, and thus, velocity fluctuations in tandemwith vorticeswere formed. Then, the vorticeswould develop under the effect of viscousdiffusion and pressure distribution (18). Because of the friction againstthe wall, the pressure gradient is smaller at the inlet and gradually in-creases toward the outlet (19). Thus, the forward vorticity is lower thanthe backward, which denotes the morphology of m-CNT-Ts with astraight segment in the upstream.As the vortices propagated downstream,they combined into larger vortices and interacted with each other,stretching and rotating and eventually consuming energy to form smallervortices. Because the interaction was spatially symmetric, these smallvortices became isotropic, with a round shape as the secondary struc-ture. Thus, the size of the secondary formed nanotube loops may beassociated with the minimum size of the vortex (see SupplementaryText for a detailed analysis).

To explore the principle of the formation of m-CNT-Ts and to testthe workingmechanism described above, we synthesizedm-CNT-Ts attwo different optimized gas velocities, determined by our previous work(10), for various acoustic wave frequencies. Depending on the growthconditions, the resultant m-CNT-Ts could be relatively large and denseor relatively small (area = 1 × 104 to 3 × 104 mm2; Fig. 2A). Furthermore,we performed a demonstration experiment with a hot wire to visualizethe phenomenon of AIVs (movie S1) and established a mathematicalmodel for a floating CNT under an acoustic wave to provide a methodfor further calculations (see Materials and Methods for details). More-over, the relationships between the acoustic wave frequency (f), gas flowvelocity (u), and average diameter of the secondary nanotube loops (D)were established using a Strouhal number (St; a dimensionlessparameter)model, St= fD/u, whichwas used to describe the tail or wingkinematics of swimming and flying animals (12, 20–22). Natural selec-tion is expected to favor high propulsive efficiency with minimally con-sumed energy over a narrow range of St, which usually peaks within theinterval 0.2 < St < 0.4 (21). As shown in Fig. 2B and table S1, them-CNT-Ts follow a similar natural selection, as shown by the as-defined St, whichalso exhibits distributions within the same range. Thus, the minimumdissipative energy principle in nature seems to work even at the nano-scale. Therefore, the average scale of an m-CNT-T can be optimized byadjusting the frequency of the acoustic wave under given growth con-ditions, as indicated in Fig. 2C. It is important to note that the frequencymust be confined to a narrow range between 10 and 40 Hz because ahigher frequency may cause CNT fracture, whereas a CNTmay still bestraight at a lower frequency (fig. S5). Atomic force microscopy charac-terization also demonstrated the CNT fracture because there was nocatalyst particle at the end of the CNT (fig. S6). Additionally, accordingto the Schulz-Flory distribution, for the growth of ultralong CNTs withthe catalyst activity probability above 92%, CNTs longer than 15 cmwillbe reduced by 66.7% compared to the number of those grown in thecatalyst region (10). The sparse arrays of long nanotubeswith their pitchwidely distributed in the 2- to 6-mmrange can ensure that each as-grown

Fig. 1. Synthesis of m-CNT-Ts. (A) Schematic illustration of the synthesis of m-CNT-Ts assisted with acoustic waves. For chiral characterization, under the illumination of asupercontinuum laser,m-CNT-Ts of different structures showdifferent colors. Unchangedcolor for each ultralong CNT indicates that the chirality is consistent. (B) Scanning elec-tron microscopy (SEM) characterization of a representative as-grown m-CNT-T (growthcondition: f = 30 Hz, u = 1.7mm s−1). (C) Cs-corrected TEM image of a triple-walled ultra-longCNT. An individual ultralong CNTwill curl to formmany small loops under the effectof acoustic-assisted gas flowbefore the ultimate formation of thewholem-CNT-T shownin (B). The loop diameter distribution was counted as shown in (D). Scale bars, 20 mm for(B) and 1 nm for (C).

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m-CNT-T with its width of up to hundreds of micrometers is made ofonly one ultralong CNT and will not entangle with other CNTs nearby,which is a prerequisite to producing single-chiralitym-CNT-T. The pitchdistances increase with the increasing length of the ultralong CNTs,providing a greater likelihood of producing larger m-CNT-Ts.

Chiral consistency verification of ultralong CNTsand m-CNT-TsIn addition to its effect of stabilizing the gas flow, LRR provides a facileroute for the large-scale synthesis of ultralong CNTs. By controlling thegas velocity distribution carefully and enlarging the invariable tempera-ture area (fig. S1), five 100-mm wafers of ultralong CNTs were synthe-sized simultaneously in anLRRheated in an enclosedmuffle furnacewithhearth dimensions of 1000 × 300 × 300 mm3. These wafer-scale ultra-long CNTs could be visible even by the naked eye under ambient con-ditions assisted with condensed vapor (23) (Fig. 3B and fig. S7), avoidingthe size constraints for the samples under a standard electron microscope.

Zhu et al. Sci. Adv. 2016;2 : e1601572 30 November 2016

The wafer-scale synthesis of ultralong CNTs will make it easier todirectly and widely identify the chiral structures with optical character-ization methods. For chiral identification of CNTs, resonance Rayleighscattering (RRS) is significantly more accurate and effective comparedto fluorescence excitation spectroscopy and Raman spectroscopy (24).A structure-property “atlas” for SWNToptical transitions (24) has beenestablished, and real-time true-color imaging of SWNTs (25) was alsoachieved, whereas RRS of few-walled CNTs can be more complicatedbecause of the interaction between thewalls (26). A 100-mm-longmono-chromatic ultralong CNTwas observed under an optical microscope bywide supercontinuum laser illumination (Fig. 3, A and B), and mono-chromatic serpentine CNTs with 11 segments, formed by one 400-mm-long ultralong CNT, show no color change or resonance peak shift,signifying the consistent chirality (fig. S8, C andD) (25). This is the firsttime that monochromatic CNTs with decimeter lengths have been ob-served under amicroscope. CNTswith different colors and chiral struc-tures are shown in Fig. 3 (C andD) and fig. S8 (A andB). Raman spectra

Fig. 2. AIV growth mechanism of m-CNT-Ts. (A) Representative statistical data for the diameter distribution of small loops composing each m-CNT-T. The left panel showsdiameter distributions for corresponding labeled m-CNT-Ts on the right panel. These m-CNT-Ts were synthesized under different conditions. Top to bottom: f = 35 Hz, u =1.2mm s−1; f= 25 Hz, u= 1.2mm s−1; f= 25Hz, u= 1.7mm s−1; f= 35 Hz, u= 1.7mm s−1. (B) Statistics about Strouhal number form-CNT-Ts synthesized under different conditionsand other natural things. f1 to f5: from 15 to 35 Hz in the 5-Hz step, u1 = 1.2mm s−1, u2 = 1.7mm s−1. For example, f1u1means that the experimental condition is f = 15Hz, u =1.2 mm s−1. Two dot-dashed lines indicate that St values for every m-CNT-T lie in the same interval as the reported natural things, and the dashed line indicates the reportedoptimal St. (C) Control law between the frequency of acoustic wave and the diameter of secondary nanotube loops. Three blue lines represent operation at u = 1.2 mm s−1,whereas red lines represent operation at u = 1.7 mm s−1. Scale bars from top to bottom in (A), 50, 50, 100, and 30 mm.

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further demonstrated that each monochromatic ultralong CNT ex-hibited the characteristics of perfect structure and consistent chirality.Double radial breathing mode (RBM) peaks shown in Fig. 3I indicatedthat this may be a DWNT, in agreement with the previously reportedresults (27). Moreover, if we consider the proposed standard (2) on awafer scale, SWNT arrays with a density of up to 125 SWNTs mm−1

and an average length of 7 nm weigh 2.35 mg when closely arrangedon a 100-mm silicon wafer. For TWNTs with an average length of10 cm and a diameter of 3 nm, the calculated density will decrease to21 TWNTs mm−1 for the equivalent mass. A recent breakthrough hasdemonstrated that we are closer to this target through the “in situ cat-alyst loading” approach (28).

Additionally, because of the polarization of CNTs, a complete pic-ture will not be observed if the CNT is curved, as shown in the bottompanel of Fig. 3A. Nevertheless, we could obtain a nearly complete imagefor an m-CNT-T (Fig. 4B) by rotating the objective table (Fig. 4A) andcombining snapshots taken from different angles. A red m-CNT-T wasthus observed completely (Fig. 4C; see original pictures in fig. S9), al-though some invisible CNT segments were evident through the com-

Zhu et al. Sci. Adv. 2016;2 : e1601572 30 November 2016

parison with the corresponding SEM image (Fig. 4D). Raman spectrarecorded from four different positions of the m-CNT-T with 633-nmexcitation (Fig. 4E) show a hardly noticeable D band; however, the sameRBM bands shift, indicating that the m-CNT-T is of high quality withconsistent chirality. Furthermore, RRS spectra with the same resonancepeak shifts (Fig. 4F) are clear evidence of the consistent chirality (25).

Photoelectronic properties of m-CNT-TsTo evaluate the performance of m-CNT-Ts, we fabricated back-gatedfield effect transistors with a representative schematic structure, withthe SEM images shown in Fig. 5 (A and B). The transfer characteristics(ID−VG)measured at variousVD biases are shown in Fig. 5C, indicatingthat the device consists of an all-semiconducting m-CNT-T. Moreover,these transistors exhibit an on/off ratio above 1000 anddeliver a respect-able on-state current of 4.4 mA at VD = 2 V and VG = −20 V, showingpromising compatibility between the high on/off ratio and high outputcurrent in CNTFETs. Meanwhile, this is also the highest output currentrecorded to date for single CNT–based transistors. The output charac-teristics (ID − VD) of the same device shown in Fig. 5D also reveal the

Fig. 3. Optical visualizationofwafer-scale ultralongCNTs. (A and B) A 100-mm-longCNT (dashed lines)with consistent color and chiral structurewas observedona 100-mmsilicon substrate on the basis of RRS. The three panels in (A) correspond to different positions shown in (B). Those wafer-scale ultralong CNTs assisted with condensed vapor,though with some visible dust particles, could be visualized. (C and D) Ultralong CNTs with different colors and chiral structures observed on the same wafer in (B). (E to G) RBMvibrations corresponding to the three panels in (A) from left to right. (H and I) RBMvibrations corresponding to twodifferent positions along themonochromatic CNT shown in (C)and (D), respectively. (J) Gbandof a representative ultralongCNT. There is noobviousDbandnear 1350 cm−1, suggesting theperfectness of structure. Scale bars, 20 mmfor (A) and5 mm for (C) and (D). a.u., arbitrary units.

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formation of ohmic contacts between the m-CNT-T and the metalcontacts. Furthermore, a sensitive photo response of the m-CNT-T toshort-wave infrared is also demonstrated by the photoconductancespectra shown in Fig. 5E, implying that the chirality is consistent andthat the m-CNT-Ts show great potential for use in optoelectronic de-vices. Furthermore, because of the special morphology of m-CNT-Tswith a straight segment next to the tangle, the areal density of m-CNT-Ts can be accurately and efficiently evaluated on the basis of the ratio ofcurrent intensity between the tangle and the straight segment only if theCNTs’ wall number has been confirmed.

Contrary to the previous prejudice against the use of few-walledCNTs in electronics, transistors based on few-walled ultralong CNTsexhibit superior performancewith an on/off ratio up to 107 and a higheroutput current than that in the usual SWNT-based devices (Fig. 5, F andG) (29). Additional representativemeasurement results indicate that theon/off ratios range from 105 to 107, whereas the output current can easilyexceed 15 mA (fig. S10), which corresponds to a high semiconductingratio of ultralong CNTs of up to 92.6% (fig. S11). The devices shownin Fig. 5B based on m-CNT-Ts demonstrate a lower on/off ratio that isbelieved to be due to the screening effect between the CNTs or the dirtysurface of substrates. More CNTFETs fabricated with m-CNT-Ts shownin fig. S12 demonstrate a higher on/off ratio of up to 103 to 106 at lowerVD biases. Thus, devices based on these m-CNT-Ts have achievedthe compatibility of the considerable current density and on/off ratiothat is more competitive than those of most existing CNT products(3, 5, 16, 29, 30) and have exhibited a promising potential for applica-tions in logic circuits and photodetector or radio frequency circuits(Fig. 5H). As the next step, we will synthesize more and larger m-CNT-Ts directly on silicon wafers; this will not only avoid the dirty matteroriginating from the cutting of the wafers but will also contribute tothe separation and applications of m-CNT-Ts at a larger scale. Further-more, the controlled synthesis of longer CNTs with reduced dia-meters is another key factor because the longer length probablyensures the all-semiconducting behavior through an individual few-walled CNT; additionally, a reduced diametermay have a positive effect

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on the turn-off performance of the transistors fabricated using a thingate dielectric. We believe that further steps must be taken to exploreand improve the device performances based on m-CNT-Ts.

DISCUSSIONExtremely high requirements have been proposed for carbon-basedelectronics. The areal density of aligned CNTs can reach 130 CNTs mm−1

through a catalytic reaction (5) or 500CNTs mm−1 by post-assembly (3).Thus, the density standard has not been the key problem, and it caneven be lowered by six times to 22 CNTs mm−1 for TWNTs. However,there exists an unavoidable obstacle that if the sample contains morethan 0.0001%metallic CNTs, the as-fabricated devices will face shortingfailure. Thus, it is necessary to explore new technical routes. The initialaimof assembling everydecimeter- tometer-longCNTinto single-chiralitymacroscopic tangles is aimed atmaking it easier to separate andmanipulateCNTs on the basis of RRS. However, this is only the first step of our roadmap for wafer-scale applications of m-CNT-Ts. We can synthesize moreand larger m-CNT-Ts directly on wafers so that the separation can beperformed on a larger scale because of the advantage of wafer-scale syn-thesis capability within the LRR. Then, m-CNT-Ts with the same colorcan be enriched on the designated substrate, even covering the wholesubstrate for ideal film formation. Because this film theoretically showssingle chirality and superlong length, transistors or otherdevices fabricatedon it are most likely to exhibit high performance. Furthermore, if we suc-ceed in aligning these enriched m-CNT-Ts on wafers, the CNTFETperformances will achieve another leap. Therefore, whereas the long-pursued CNTFET application is demanding and ultimately many tech-nological barriers will have to be overcome, the proposedmethod offersanother approach circumventing the purity and density problems, al-though the placement and uniformity problems need to be further ad-dressed in future studies.

In summary, we proposed an in situ assembly approach for the syn-thesis of large-area (1 × 104 to 3 × 104 mm2) quasi-planar m-CNT-Tsfrom decimeter-long CNTs with consistent chirality. This large-scale

Fig. 4. Characterization ofm-CNT-Ts. (A) Schematic illustration of RRS setupwith a rotating objective table. (B) Combined RRS image of a representative as-grownm-CNT-T.(C and D) RRS image and the corresponding SEM image of the same m-CNT-T. (E) Raman spectra of the as-grown m-CNT-T with 633-nm excitation (inset: RRS image of thetangle shoot from one angle). G, G band. (F) RRS spectra of the same m-CNT-T shown in (E). Scale bars, 20 mm for (B) to (D) and 10 mm for (E).

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m-CNT-T was assembled by introducing acoustic waves into thegrowth system of ultralong CNTs, and an innovative AIV mechanismwas proposed for the formation of m-CNT-Ts. The scale of m-CNT-Tscould be controlled on the basis of a natural selective model ofminimum consumed energy with a Strouhal number of approximately0.3. Furthermore, five 100-mm wafers of ultralong CNTs were firstsynthesized simultaneously in our homemade LRR and were visualizeddirectly by the naked eye under ambient conditions assisted with con-densed vapor. Additionally, chiral-consistent decimeter-long mono-chromatic CNTs and entangled CNTs were observed under a standardmicroscope basedonRRS,with theRaman spectra further demonstratingchiral consistency and structural perfection. CNTFETs fabricated withthe large-area m-CNT-Ts exhibited unique photoelectronic properties,showing a feasible method for compatibility between high semi-conducting purity and high areal density for CNTs. In addition to theirdirect use in the fabrication of chips or devices, these CNTs can also bestretched into a dense CNT array using the standard spinning treatmentfor millimeter-long fibers (15), offering a new route for high-density flex-ible electronicmaterials. Finally, we believe that the formationof a specific

Zhu et al. Sci. Adv. 2016;2 : e1601572 30 November 2016

architecture assisted by AIVs is also feasible for other flexible materialssynthesized in a gaseous environment. Additional techniques will be ex-plored to increase the density and area of these m-CNT-Ts, ultimatelyachieving effective separation based on RRS and large-scale applicationsin electronics, transparent display, sensors, superstrong fibers, aeronauticsand astronautics, and even space elevators.

MATERIALS AND METHODSSynthesis of m-CNT-Ts and serpentine CNTsm-CNT-Tswere synthesized in our designed LRRheated in customizedmuffle furnace (made byCNTFurnace Co. Ltd.). FeCl3 ethanol solution(0.03 M) was deposited as a catalyst precursor onto silicon substrateswith an 800-nm-thick SiO2 coating layer. The temperature wasincreased at a rate of 19.5°Cmin−1 at an atmosphere of H2/Ar and keptat 1005°C for 15 min. Then, CH4 and H2 (VCH4=VH2 ¼ 1:2:06; Ftotal =160 sccm, with 0.56% H2O) were inlet into the reactor continuouslyfor 30min (10). When t = 20 min during reaction, an acoustic wave(f= 25Hz) generated by a signal generator (RIGOLDG1022U, 1 mHz

Fig. 5. Photoelectrical characterization ofm-CNT-T. Schematic (A) and SEM images (B) of a back-gated transistor fabricatedon them-CNT-Twith a channel lengthof L=700nm.(C) Transfer characteristics (ID− VG) of the transistormeasured at various VD biases. (D) Output characteristics (ID− VD) of the samedevicemeasured at variousVG biases from−20 to20 V. (E) Photoconductance spectra of the same device. (F) Transfer characteristics (ID−VG) of the transistor fabricatedwith an individual ultralong CNTwith a channel length of L=2 mm. (G) Output characteristics (ID − VD) of the same devicemeasured at various VG biases from −40 to 20 V. (H) Standards preliminarily estimated for different CNT-based deviceapplications. The original goal was proposed by IBM in the study by Franklin (2), and then we substituted the requested parameters to another type that can be electricallymeasured, themetallic concentration to on/off ratio, and thequantity of CNTs to current density,withan assumption that eachCNT carries 10mAof on-current. Scale bar, 30 mmfor(B). RF, radio frequency.

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to 25MHz for sine wave, <40W) was introduced into the reactor forthe last 10 min.

The procedure for synthesizing serpentine CNTswas similar to that de-scribed above except without acoustic wave: T = 1005°C, u = 1.7 mm s−1,Ccat = 0.03 M, wH2O = 0.46%, and RH2/CH4 = 2.06. Another difference wasthat the substrates were quartz having been annealed in air for 8 hours.

A mathematical model about floating CNT underacoustic waveAccording to the propagation feature of longitudinal wave, when actingas a simple harmonic force at one position x = 0 on the CNT, the vibra-tion would transmit along the CNT. Suppose the deformation at x po-sition and t moment was d(t, x), and at x + dx position (dx denotes aminimum vibration) was d(t, x + dx); thus, the total deformation was

d t;x þ dxð Þ � d t; xð Þ ¼ ∂dðt; xÞ∂x

dx

Suppose the deformation was within elastic range, and according tothe law of the elastic

FxS¼ �E

∂dx∂

Fxþdx

S¼ �E

∂ðdx þ dxÞ∂x

Drag force acting on the CNT (31) was

FD ¼ 12

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2pmrkTN

p ffiffiffiffiffi4Sp

rWð1;1Þ*

sðdÞ Vdx

¼ P

ffiffiffiffiffi4Sp

rVdx P ¼ 1

2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2pmrkTN

pWð1;1Þ*

sðdÞ

� �

Thus, the resultant force acting on the CNT was

dFx ¼ Fx � Fxþdx � FD ¼ � ∂Fx∂x

dx � FD ¼ SE∂2d∂x2

dx � P

ffiffiffiffiffi4Sp

rVdx

According to Newton’s second law

SE∂2d∂x2

dx � P

ffiffiffiffiffi4Sp

rVdx ¼ rSdx

∂2d∂t2

Boundary condition was (L denoted length of CNT)

∂d∂x

� �x¼0

¼ � FaES

sinwt

dx¼L ¼ 0

Characterization of m-CNT-Ts and serpentine CNTsThe as-grown samples were inspected with SEM (JSM 7401F, 1.0 kV), aRaman spectrometer (Horiba HR 800, 532/633 nm), and TEM (JEM2010, 120.0 kV) to characterize the morphology and structure. Anoptical microscope (long working distance metallography microscope,FS 70Z) and a supercontinuum laser were used for RRS.

Zhu et al. Sci. Adv. 2016;2 : e1601572 30 November 2016

Visualization of wafer-scale ultralong CNTsA purchased humidifier was reformedwith ametal joint and a hosepipeto directly produce vapor. In a dark room with a lamp lighting nearby,the vaporwas blown to the surface of a silicon substrate, andCNTswerevisible with the naked eye. A bottle of liquid nitrogen was put under thesubstrates to maintain the image of ultralong CNTs to be shot with amicrolens. This method was also a further development derived fromthe reported vapor condensation–assisted optical microscopy.

Fabrication and measurements of electronic devicesPalladium (Pd) electrodes (70 nm thick) were patterned on Si/SiO2 sub-strates with an 800-nm SiO2 top layer using electron beam lithographyand deposited via electron beam evaporation under high vacuum. Thiswas preceded by the formation of titanium/gold (5/40 nm) source/drainmetal contacts with similar fabrication process as Pd contacts. After fab-rication, the device had a channel length of L = 700 nm. Electronicmea-surements were carried out with a Keithley 2612B sourcemeter at roomtemperature in air.

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/11/e1601572/DC1Supplementary Textfig. S1. Designed LRR for wafer-scale synthesis of ultralong CNTs.fig. S2. HRTEM characterization for a single-walled m-CNT-T.fig. S3. Velocity distribution of the designed LRR when synthesizing ultralong CNTs.fig. S4. Schematic illustration of acoustic wave generator.fig. S5. m-CNT-Ts synthesized at extreme frequencies.fig. S6. Characterization of CNT fracture.fig. S7. Characterization of wafer-scale ultralong CNTs on Si/SiO2 substrates.fig. S8. RRS characterization of serpentine CNTs.fig. S9. An m-CNT-T shoot from different angles under Rayleigh imaging microscopy.fig. S10. Additional representative devices based on ultralong CNTs.fig. S11. Electrical type of ultralong CNTs.fig. S12. Additional representative devices based on m-CNT-Ts.table S1. Statistical data and corresponding growth conditions related to Fig. 2 (B and C).movie S1. A demonstration experiment with hot wire to visualize the phenomenon of AIVs(f = 300 Hz).References (32–34)

REFERENCES AND NOTES1. J. Appenzeller, Carbon nanotubes for high-performance electronics—Progress and

prospect. Proc. IEEE 96, 201–211 (2008).2. A. D. Franklin, Electronics: The road to carbon nanotube transistors. Nature 498, 443–444

(2013).3. Q. Cao, S.-j. Han, G. S. Tulevski, Y. Zhu, D. D. Lu, W. Haensch, Arrays of single-walled

carbon nanotubes with full surface coverage for high-performance electronics. Nat.Nanotechnol. 8, 180–186 (2013).

4. S. H. Jin, S. N. Dunham, J. Song, X. Xie, J.-h. Kim, C. Lu, A. Islam, F. Du, J. Kim, J. Felts, Y. Li,F. Xiong, M. A. Wahab, M. Menon, E. Cho, K. L. Grosse, D. J. Lee, H. U. Chung, E. Pop,M. A. Alam, W. P. King, Y. Huang, J. A. Rogers, Using nanoscale thermocapillary flows tocreate arrays of purely semiconducting single-walled carbon nanotubes. Nat.Nanotechnol. 8, 347–355 (2013).

5. Y. Hu, L. Kang, Q. Zhao, H. Zhong, S. Zhang, L. Yang, Z. Wang, J. Lin, Q. Li, Z. Zhang,L. Peng, Z. Liu, J. Zhang, Growth of high-density horizontally aligned SWNT arrays usingTrojan catalysts. Nat. Commun. 6, 6099 (2015).

6. F. Zhang, P.-X. Hou, C. Liu, B.-W. Wang, H. Jiang, M.-L. Chen, D.-M. Sun, J.-C. Li, H.-T. Cong,E. I. Kauppinen, H.-M. Cheng, Growth of semiconducting single-wall carbon nanotubeswith a narrow band-gap distribution. Nat. Commun. 7, 11160 (2016).

7. F. Yang, X. Wang, D. Zhang, J. Yang, D. Luo, Z. Xu, J. Wei, J.-Q. Wang, Z. Xu, F. Peng, X. Li,R. Li, Y. Li, M. Li, X. Bai, F. Ding, Y. Li, Chirality-specific growth of single-walled carbonnanotubes on solid alloy catalysts. Nature 510, 522–524 (2014).

8. Q. Wen, W. Qian, J. Nie, A. Cao, G. Ning, Y. Wang, L. Hu, Q. Zhang, J. Huang, F. Wei,100 mm long, semiconducting triple-walled carbon nanotubes. Adv. Mater. 22,1867–1871 (2010).

7 of 8

SC I ENCE ADVANCES | R E S EARCH ART I C L E

on Nov

http://advances.sciencemag.org/

Dow

nloaded from

9. N. Wei, Y. Liu, H. Xie, F. Wei, S. Wang, L.-M. Peng, Carbon nanotube light sensors withlinear dynamic range of over 120 dB. Appl. Phys. Lett. 105, 073107 (2014).

10. R. Zhang, Y. Zhang, Q. Zhang, H. Xie, W. Qian, F. Wei, Growth of half-meter long carbonnanotubes based on Schulz–Flory distribution. ACS Nano 7, 6156–6161 (2013).

11. M. K. Jawed, N. K. Khouri, F. Da, E. Grinspun, P. M. Reis, Propulsion and instability of aflexible helical rod rotating in a viscous fluid. Phys. Rev. Lett. 115, 168101 (2015).

12. J. Zhang, S. Childress, A. Libchaber, M. Shelley, Flexible filaments in a flowing soap film asa model for one-dimensional flags in a two-dimensional wind. Nature 408, 835–839(2000).

13. K. Son, J. S. Guasto, R. Stocker, Bacteria can exploit a flagellar buckling instability tochange direction. Nat. Phys. 9, 494–498 (2013).

14. R. Zhang, Y. Zhang, Q. Zhang, H. Xie, H. Wang, J. Nie, Q. Wen, F. Wei, Optical visualizationof individual ultralong carbon nanotubes by chemical vapour deposition of titaniumdioxide nanoparticles. Nat. Commun. 4, 1727 (2013).

15. K. Jiang, Q. Li, S. Fan, Nanotechnology: Spinning continuous carbon nanotube yarns.Nature 419, 801 (2002).

16. N. Geblinger, A. Ismach, E. Joselevich, Self-organized nanotube serpentines. Nat.Nanotechnol. 3, 195–200 (2008).

17. M. Nishioka, M. Asai, Some observations of the subcritical transition in plane poiseuilleflow. J. Fluid Mech. 150, 441–450 (1985).

18. G. Buresti, Notes on the role of viscosity, vorticity and dissipation in incompressible flows.Meccanica 44, 469–488 (2009).

19. C. S. Chen, S. M. Lee, J. D. Sheu, Numerical analysis of gas flow in microchannels. Numer.Heat Transfer Part A 33, 749–762 (1998).

20. J. C. Liao, D. N. Beal, G. V. Lauder, M. S. Triantafyllou, The Kármán gait: Novel bodykinematics of rainbow trout swimming in a vortex street. J. Exp. Biol. 206, 1059–1073(2003).

21. G. K. Taylor, R. L. Nudds, A. L. R. Thomas, Flying and swimming animals cruise at aStrouhal number tuned for high power efficiency. Nature 425, 707–711 (2003).

22. M. S. Triantafyllou, G. S. Triantafyllou, D. K. P. Yue, Hydrodynamics of fishlike swimming.Annu. Rev. Fluid Mech. 32, 33–53 (2000).

23. J. Wang, T. Li, B. Xia, X. Jin, H. Wei, W. Wu, Y. Wei, J. Wang, P. Liu, L. Zhang, Q. Li, S. Fan,K. Jiang, Vapor-condensation-assisted optical microscopy for ultralong carbon nanotubesand other nanostructures. Nano Lett. 14, 3527–3533 (2014).

24. K. Liu, J. Deslippe, F. Xiao, R. B. Capaz, X. Hong, S. Aloni, A. Zettl, W. Wang, X. Bai,S. G. Louie, E. Wang, F. Wang, An atlas of carbon nanotube optical transitions. Nat.Nanotechnol. 7, 325–329 (2012).

25. W. Wu, J. Yue, X. Lin, D. Li, F. Zhu, X. Yin, J. Zhu, J. Wang, J. Zhang, Y. Chen, X. Wang, T. Li,Y. He, X. Dai, P. Liu, Y. Wei, J. Wang, W. Zhang, Y. Huang, L. Fan, L. Zhang, Q. Li, S. Fan,K. Jiang, True-color real-time imaging and spectroscopy of carbon nanotubes onsubstrates using enhanced Rayleigh scattering. Nano Res. 8, 2721–2732 (2015).

26. K. Liu, C. Jin, X. Hong, J. Kim, A. Zettl, E. Wang, F. Wang, Van der Waals-coupled electronicstates in incommensurate double-walled carbon nanotubes. Nat. Phys. 10, 737–742(2014).

Zhu et al. Sci. Adv. 2016;2 : e1601572 30 November 2016

27. K. Liu, X. Hong, M. Wu, F. Xiao, W. Wang, X. Bai, J. W. Ager, S. Aloni, A. Zettl, E. Wang,F. Wang, Quantum-coupled radial-breathing oscillations in double-walled carbonnanotubes. Nat. Commun. 4, 1375 (2013).

28. H. Xie, R. Zhang, Y. Zhang, Z. Yin, M. Jian, F. Wei, Preloading catalysts in the reactor forrepeated growth of horizontally aligned carbon nanotube arrays. Carbon 98, 157–161 (2016).

29. Z. Zhang, X. Liang, S. Wang, K. Yao, Y. Hu, Y. Zhu, Q. Chen, W. Zhou, Y. Li, Y. Yao, J. Zhang,L.-M. Peng, Doping-free fabrication of carbon nanotube based ballistic CMOS devicesand circuits. Nano Lett. 7, 3603–3607 (2007).

30. G. J. Brady, Y. Joo, S. S. Roy, P. Gopalan, M. S. Arnold, High performance transistors viaaligned polyfluorene-sorted carbon nanotubes. Appl. Phys. Lett. 104, 083107 (2014).

31. R. Y. M. Wong, C. Liu, J. Wang, C. Y. H. Chao, Z. Li, Evaluation of the drag force on single-walled carbon nanotubes in rarified gases. J. Nanosci. Nanotechnol. 12, 2311–2319 (2012).

32. L. Huang, B. White, M. Y. Sfeir, M. Huang, H. X. Huang, S. Wind, J. Hone, S. O’Brien,Cobalt ultrathin film catalyzed ethanol chemical vapor deposition of single-walled carbonnanotubes. J. Phys. Chem. B 110, 11103–11109 (2006).

33. J. Z. Wu, H. Y. Ma, M. D. Zhou, Introduction to Vorticity and Vortex Dynamics (HigherEducation Press, 1993).

34. M. S. Dresselhaus, G. Dresselhaus, A. Jorio, A. G. Souza Filho, R. Saito, Raman spectroscopyon isolated single wall carbon nanotubes. Carbon 40, 2043–2061 (2002).

Acknowledgments: We thank Z. Wu for theoretical assistance with hydromechanics, K. Jiangfor assistance with the setup of RRS characterization apparatus, and W. Qian for advice onthe manuscript. Funding: This work was supported by the Foundation for the NationalBasic Research Program of China (2016YFA0200101), National Natural Science Foundation ofChina (21636005), and Beijing Municipal Science and Technology Commission(D141100000614001). Author contributions: F.W. proposed and supervised the project.Z.Z. designed and performed the experiments and wrote the manuscript. N.W. fabricated andcharacterized electronic devices. H.X. and Y.B. participated in the synthesis of ultralongCNTs and serpentine CNTs. R.Z. helped test single CNT–based transistors. Q.W. helped designand set up LRR. C.Z., L.D., S.W., and L.P. participated in the manuscript preparation. Allauthors discussed the results and commented on the manuscript. Competing interests:The authors declare that they have no competing interests. Data and materials availability:All data needed to evaluate the conclusions in the paper are present in the paper and/orthe Supplementary Materials. Additional data related to this paper may be requestedfrom the authors.

Submitted 10 July 2016Accepted 26 October 2016Published 30 November 201610.1126/sciadv.1601572

Citation: Z. Zhu, N. Wei, H. Xie, R. Zhang, Y. Bai, Q. Wang, C. Zhang, S. Wang, L. Peng, L. Dai,F. Wei, Acoustic-assisted assembly of an individual monochromatic ultralong carbon nanotubefor high on-current transistors. Sci. Adv. 2, e1601572 (2016).

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high on-current transistorsAcoustic-assisted assembly of an individual monochromatic ultralong carbon nanotube for

Liming Dai and Fei WeiZhenxing Zhu, Nan Wei, Huanhuan Xie, Rufan Zhang, Yunxiang Bai, Qi Wang, Chenxi Zhang, Sheng Wang, Lianmao Peng,

DOI: 10.1126/sciadv.1601572 (11), e1601572.2Sci Adv 

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