Length-Sorted, Large-Diameter, PolyfluoreneWrapped Semiconducting Single-Walled Carbon Nanotubes for High-Density, ShortChannel Transistors Frank Hennrich,*,† Wenshan Li,†,§ Regina Fischer,‡ Sergei Lebedkin,† Ralph Krupke,*,†,§ and Manfred M. Kappes*,†,‡ †
Institute of Nanotechnology, Karlsruhe Institute of Technology, D-76021 Karlsruhe, Germany Institute of Physical Chemistry, Karlsruhe Institute of Technology, D-76128 Karlsruhe, Germany § Department of Materials and Earth Sciences, Technische Universität Darmstadt, D-64287 Darmstadt, Germany ‡
S Supporting Information *
ABSTRACT: Samples of highly enriched semiconducting SWCNTs with average diameters of 1.35 nm have been prepared by combining PODOF polymer wrapping with size-exclusion chromatography. The purity of the material was determined to be >99.7% from the transfer characteristics of short-channel transistors comprising densely aligned sc-SWCNTs. The transistors have a hole mobility of up to 297 cm2V−1 s−1 and an On/Off ratio as high as 2 × 108. KEYWORDS: carbon nanotubes, transistor, chromatography, polymer wrapping, dielectrophoresis
S
It is significantly less challenging to build a short-channel FET not from an individual sc-SWCNT but from an aligned and tightly packed array of them.8−11 These are of interest for SWCNT-based high-frequency power amplification, requiring merely a high linear density of parallel aligned sc-SWCNTs, all contacted to the same micron-scale source and drain electrodes for low-impedance matching. Proof-of-principle examples have recently been demonstrated,12,13 thus raising the hope that SWCNT-based high-frequency power amplification will be available much earlier than SWCNT-based high-performance logic. Nevertheless, it has so far been difficult to fully exploit the exceptional properties of such sc-SWCNT arrays due to restrictions both from materials and techniques. These include alignment and placement of nanotubes, as well as the difficulty of fully removing residual metallic SWCNTs14 and other impurities. Recently, FET arrays made with polyfluorene-sorted SWNT have been shown to have superior current density and high On/Off ratio.15 Indeed selective extraction of as-prepared SWCNT material by conjugated polymers has been demonstrated to be an effective enrichment and isolation method to obtain high purity sc-SWCNTs.16 The relative simplicity of the
ingle-walled carbon nanotubes (SWCNTs) exhibit unique physical and chemical properties with potential applications in a variety of fields including electronics, solar-energy harvesting, and photonics.1−4 This material can be prepared in a wide range of different structures, as expressed by a chiral index (n, m)5 which is strongly correlated with the electronic band structure. For example, SWCNTs can be either metallic (m-) or semiconducting (sc-). SWCNT raw materials typically comprise multi(n, m) mixtures of both m- and sSWCNTs. Therefore, accessing (n, m)-specific properties requires fractionating these mixtures. Sc-SWCNTs have been widely recognized as promising candidates for next-generation, high-performance field-effect transistors (FETs) because of their extraordinarily high intrinsic carrier mobility or carrier saturation velocity.6,7 Nevertheless, many challenges need to be overcome before the corresponding technology can be realized on an industrial scale. Most challenging is the implementation of SWCNT-based highperformance logic, which requires sc-SWCNTs to be integrated in individual devices at a device density comparable to existing or future MOSFET technology. Significant progress in this direction has been made over the years, but precise control in the fabrication of individual devices at ultrahigh density with reliable properties is still absent. Therefore, it appears rather unlikely that sc-SWCNT FETs will outperform MOSFETs in high-performance logic in the near future. © XXXX American Chemical Society
Received: September 4, 2015 Accepted: January 20, 2016
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75 SEC column. The chromatogram can roughly be divided in two regions: (i) elution time between ∼20−35 min during which only the polymer-wrapped SWCNTs elute and (ii) elution time ∼35−45 min during which both polymer-wrapped tubes and “free” polymer elutes. Fractions of 5−10 mL were sequentially collected from the first broad band eluting from the column (see Figure 1) and were then analyzed by measuring UV−vis−NIR absorption spectroscopy, photoluminescence excitation (PLE) maps, Raman spectroscopy, and AFM. Figure 2 shows absorption spectra in the wavelength range 300−2000 nm for the first four fractions. These spectra show
conjugated polymer extraction process, involving only a sonication and centrifugation step, distinguishes it as a costeffective method for the isolation of sc-SWCNTs. Measuring short-channel transistor device performance is an effective way to validate the high sc-purity of enriched sc-SWCNTs samples when simultaneously considering their mobility, On/Off current ratio and current density. In particular, polyfluorene-based copolymers have been shown to have one of the highest specific sorting abilities toward generating dispersed SWCNTs with >99% semiconducting content in one sonication/centrifugation step.17−19 But higher sc-SWCNTs purities are desirable. As reported,13 to further improve SWCNT-based high-frequency power transistors, devices need to have m-SWCNT contents of significantly less than 1% and the ultimate goal for digital logic is to reduce the m-SWCNT impurity level to less than partsper-million or even parts-per-billion. Besides, an additional constraint toward these applications is that the impurity level needs to be realized for large diameter SWCNTs with d > 1.2 nm, so as to reduce current-limiting Schottky barriers and to dramatically improve the On/Off ratios relative to smalldiameter SWCNTs.20,21 Reaching higher purities requires further fractionation. Here, we use size-exclusion chromatography (SEC), a method which separates molecules passing through the chromatography column according to differences in their size. SEC has been extensively used to size-separate SWCNTs suspended in water with the aid of surfactants.22 However, the SEC-based sizeseparation has not yet been demonstrated for separation of polymer-wrapped SWCNTs dispersed in organic solvents. In this work, we show that using SEC to separate polymer wrapped sc-SWCNTs with respect to length can significantly improve the sc-SWCNT purity. Furthermore, the length selection itself is of additional benefit for short-channel FET fabrications. Through investigating the SEC generated highpurity sc-SWCNT as the channel semiconductor in shortchannel, high-density transistors, we show that these devices possess a hole mobility of up to 297 cm2 V−1 s−1 and a current On/Off ratio as high as 2 × 108 at 800 nm channel length. This is the best performance reported so far for sc-SWCNTs in highdensity short-channel transistors and thus promising for future electronic applications.
Figure 2. Absorption spectra of SEC fractions of PODOF-wrapped PLV SWCNTs in toluene (fractions refer to those indicated in Figure 1). “Start” indicates starting suspension (diluted by 5× for better comparison).
mainly the first (S11), second (S22), and third (S33) interband transitions of sc-SWCNTs. Compared to aqueous suspensions of the same SWCNT raw materials with surfactants,23 the spectra are more richly structured. This reflects primarily (i) pronounced S11 and S22 interband transitions arising from a strongly reduced (n, m)-distribution and (ii) the essentially complete absence of the first metallic excitonic transitions, M11, otherwise visible in the spectral region between ∼500− 700 nm. Note, however, that quantitatively assessing the m-/scpurity of SWCNT suspensions by means of analyzing absorption spectra alone is difficult. Absorption features of side products such as fullerenes, amorphous carbon, and residual catalyst particles normally interfere in the M11 absorption region. Nevertheless, Figure 2 clearly shows a reduction in background absorption versus S22/S11 absorption when comparing the absorption spectra of the starting suspension with that of chromatographically purified fractions. We assume that SEC removes metallic tubes as well as side products. Figure 2 indicates that for fractions 2−4 the optical density (OD) of the PODOF-wrapped PLV SWCNTs in a 10 mm path length cuvette is greater than or close to 1.0 for the first (S11) and second (S22) semiconducting exciton peaks. This means that concentrations are quite high even after fractionation. Since absorption cross-section of sc-SWCNTs is in the range 1−2 × 10−17 cm2 per C-atom for the S11 absorption peak of scSWCNTs,24,25 the concentration of sc-SWCNTs in the resulting fractions can therefore be estimated to be in the range of several μg/mL. In addition to the absorption feature of SWCNTs there is also an absorption feature of the polymer at 384 nm. The intensity ratio of this peak to the SWCNT absorption is the same for fractions 1−3. We conclude (i) that this reflects the
RESULTS AND DISCUSSION Figure 1 contains the chromatogram obtained by fractionating PODOF-wrapped starting suspensions using a Toyopearl HW-
Figure 1. Chromatogram of PODOF-wrapped PLV SWCNTs obtained at 430 nm. Gray lines indicate fractions collected for analysis. B
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Figure 4 shows PLE maps obtained for the first four fractions. We have assigned chiral indices corresponding to local maxima in photoluminescence on the basis of ref 29. Besides the general observation that PODOF exclusively extracts close-to-armchair SWCNTs (i.e., (n, n − 1) and (n, n − 2)) there are also significant differences in the (n, m)composition of eluting fractions, as obtained from the PLE maps. Overall, the early fractions are enriched in tubes with larger diameters, while later fractions have smaller average diameters. AFM measurements on spin-coated samples show that this diameter trend correlates with mean length. For this, AFM images were acquired for 300−500 isolated tubes per fraction (representative AFM pictures obtained from spincoated SEC fractions 1, 2, 3, and 4 of PODOF-wrapped PLV SWCNTs can be found in the Supporting Information). These images were analyzed to yield mean lengths of 940 ± 385 nm for fraction 1, 590 ± 250 nm for fraction 2, 360 ± 200 nm for fraction 3, and 315 ± 170 nm for fraction 4 (Figure 5). In addition, from the AFM height analysis we have no indication of the presence of a significant amount of bundles. Consequently, it appears that larger diameter tubes are on average longer than the smaller diameter tubes. In earlier studies on length separation of surfactant suspended aqueous suspensions of SWCNTs it was speculated that sonication induced scission is the possible reason for analogous differences in (n, m)-distributions.22,23 A correlation between average diameter and mean length is consistent with present thinking concerning the mechanism by which SWCNTs are shortened during sonication.22,23 Electrical characterizations of the high-density short-channel transistor devices were performed to assess the purity of the scenriched large-diameter PODOF-wrapped SWCNTs. For this, we have deposited nanotubes from fraction 2 to closely match the SWCNT length distribution to the transistor channel length, taking into account that DEP promotes the deposition of longer nanotubes in mixtures of different tube lengths. The SEM image of a characteristic transistor device, shown in the Figure 6a, demonstrates that the sc-SWCNTs are highly aligned along the channel length with a high density of ∼30 SWCNTs per μm channel width. This deposition pattern is a result of the DEP conditions and of the electrode design and allows to more effectively identify residual m-SWCNTs from high purity dispersions as compared to single-tube devices. Also, such devices are promising for SWCNT-based radio frequency power amplification electronics since they promote a higher onstate conductance. Figure 6b shows the electrical charge transfer performance of all 12 devices prepared. As shown, all transistors demonstrate a uniform p-type switching behavior with a typical On-state current density of ∼10 μA/μm at 1 V bias and an On/Off ratio of up to 2 × 108. The hole mobility is up to 297 cm2 V−1 s−1 based on the standard model, described in the methods section. The average and standard deviation is 272 ± 30 cm2/(V s) for 6 out of 12 devices. The other half of the devices has an average mobility that is a factor of 2.5 less (106 ± 40 cm2/(V s)) and seems to be correlated with a higher nanotube density. The outstanding performance of these transistors can be attributed to both the high-density integration of nanotubes and their high degree of alignment using DEP and the highpurity enrichment of sc-SWCNTs through PODOF wrapping in combination with SEC. Since longer sc-SWCNTs are better aligned by DEP,30 one cannot distinguish the individual contributions of the length-sorting by SEC and the DEP to
amount of polymer adsorbed on the tubes (which moves through the SEC gel together with the SWCNTs) and (ii) that excess polymer can be removed from polymer-wrapped SWCNTs by SEC. The free polymer elutes significantly after the polymer-wrapped SWCNTs (see chromatogram in Figure 1). The free polymer would be expected to move much more slowly through the SEC medium as its molar mass (Mw ∼ 20000 amu as given by Sigma-Aldrich) is much lower than the molar mass estimated for ∼500 nm long SWCNTs (Mw ∼ 600000 amu). A further removal of residual metallic tubes by SEC is supported by Raman measurements of SWCNTs sedimented out of suspensions by using ultracentrifugation (direct Raman analysis of suspensions is hindered by the strong interference of the organic solvent). Figure 3 shows Raman spectra excited at
Figure 3. Raman spectra at 633 nm excitation of PODOF-wrapped PLV SWCNTs sedimented out of SEC fractions by ultracentrifugation. Different “Frac” refers to the corresponding fractions indicated in Figure 1, “Start” denotes the starting suspension before SEC. The spectra are intensity-normalized at the G+ band (at ca. 1594 cm−1) and vertically shifted for clarity.
633 nm in the low-frequency radial-breathing mode (RBM) and G-mode regions of tubes in SEC fractions 1−4 in comparison with the starting suspension. The above excitation wavelength well corresponds to the region of M11 absorption of metallic tubes and weak (off-resonance) absorption window between S33 and S22 bands of semiconducting tubes (Figure 2). As a result, the spectrum of SWCNTs in the starting suspension still clearly demonstrates features which can be attributed to residual metallic tubes−the characteristic broad shoulder (Breit−Wigner−Fano line shape)26,27 of the G-band at ca. 1535 cm−1 and the RBM band at ca. 190 cm−1. These features practically disappear in the spectra of fractionated tubes, thus indicating a further substantial reduction of metallic tubes. The RBM peaks observed at 253, 265, and 283 cm−1 may be attributed to (10,3), (7,6) and (7,5) semiconducting tubes, respectively,27,28 which are minor species in the PLV SWCNT material, but are resonantly excited at 633 nm. The major Raman signals of fractionated tubes, e.g. RBM bands within ∼140−200 cm−1, are apparently contributed by the offresonantly excited but abundant, larger diameter sc-SWCNTs (see Figure 2 and PLE maps below). We remark that the corresponding Raman spectra excited at 532 and 785 nm (not shown here), including those of SWCNTs in the starting suspension, are dominated by sc-SWCNTs and therefore poorly suited to follow the presence/removal of metallic tubes. C
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Figure 4. PLE maps of SEC fractions of PODOF wrapped PLV SWCNTs. Fractions refer to those indicated in Figure 1. Note that the chiral index distribution is somewhat dependent on fraction.
the enhanced degree of alignment. Likewise it is not possible to clarify how important the polymer removal by SEC is for the performance. Length sorting, polymer removal, and DEP alignment are a direct consequence of using SEC and cannot be studied separately. However, what can certainly be assumed is that PODOF-wrapped SWCNTs in toluene are better aligned by DEP than SWCNTs in aqueous surfactant solutions due to the low permittivity and conductance of toluene. For the physics we refer to ref 30. The purity of the dispersion can be estimated from the total number of contacted SWCNTs and the fact that none of the devices show the presence of a metallic SWCNT. Metallic SWCNTs would cause an off-state current on the order of 1−10 μA and, hence, orders of magnitude higher than observed.31 As counted by SEM, there are around 30 bridging SWCNTs per device in each of the 12 devices. Thus, the estimated purity of sc-SWCNTs should exceed 1 − 1/(12 × 30) ≈ 99.7%, taking into account that DEP preferentially deposits metallic SWCNTs32 and no metallic tubes have been detected at all. Strictly speaking, the purity that we derive is the purity of SWCNTs that are long enough to bridge the gap. However, since we have no indication that the semiconducting and the residual metallic SWCNTs have a different length distribution we generalize our result to the purity of the dispersion. The use of high-purity, length-sorted material significantly improves the reproducibility of devices. This becomes evident in Figure 7 when comparing the On-state conductance with the On/Off ratio: all 12 devices share a nearly identical On-state conductance of 10 μS/μm and the On/Off-ratios cluster within
1 order of magnitude. Further improvements in the On-state conductance may be expected when using either longer SWCNTs or shorter channel lengths to ensure that the contact length exceeds the charge transfer length.33 In comparison with recently reported transport data for devices prepared using scSWNCTs dispersed in water34−36 and toluene,15,31,37−40 shown in Figure 7, it becomes evident that the enrichment of scSWCNTs through PODOF wrapping in combination with SEC yields sc-SWCNT material that allows the fabrication of highdensity, short-channel transistors with high mobility, high On/ Off ratio and a high On-state conductance with a reproducibility within a factor of 2. Overall, we attribute the improved device performance characteristics not only to the high scpurity of SWCNTs in the ∼0.95−1.5 nm diameter range (as estimated from absorption spectra; see also the Supporting Information for fitted S22 region) but also to their length and crystallinity which allows for uniformly dense networks in the transistor channel. In general, this data set demonstrates improved performance with respect to mobility, On/Off ratio and reproducibility in On-state conductance when compared to the results reported thus far for conjugated polymer enriched sc-SWCNTs (Figure 7)
CONCLUSION We have prepared highly enriched samples of semiconducting SWCNTs by combining PODOF-wrapping with size-exclusion chromatography as confirmed by Raman spectroscopy. Lengthsorted fractions with a high optical density were obtained which D
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Figure 5. Length statistics extracted from AFM images obtained for spin-coated samples of SEC fractions of PODOF wrapped PLV SWCNTs. Fractions correspond to those indicated in Figure 1. Note the systematic decrease in average length with increasing fraction number.
Figure 6. (a) SEM image of a typical high-density, short-channel transistor made of sc-enriched large-diameter PODOF-wrapped SWCNTs, deposited by DEP. The scale bar equals 200 nm. (b) Electrical transfer characteristics for all 12 devices tested. The source-drain current per μm channel width ISD versus back-gate voltage VG was recorded at source−drain voltage VSD = 1 V.
the high reproducibility are promising for commercial SWCNT-based electronic applications in near future.
contain individually dispersed semiconducting nanotubes at a purity of >99.7%. The purity of the material was derived from the transfer characteristics of short-channel transistors, each of which comprises ca. 30 densely aligned sc-SWCNTs. The transistors with a hole mobility of up to 297 cm2 V−1 s−1 and On/Off ratios as high as 2 × 108 were fabricated by dielectrophoresis. These performances in combination with
METHODS The SWCNT raw material used for this study was prepared by pulsed laser vaporization (PLV) of carbon targets doped with 1 atom % Ni and Co catalyst in 0.5 bar argon atmosphere flowing at ca. 80 sccm through an oven operated at 1050 °C.41 For SWCNT suspensions 100 E
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available silicon cantilevers with fundamental resonance frequency of 320 kHz were used. 10 × 10 μm or 5 × 5 μm topographic (height) and amplitude images were collected simultaneously at a scan rate of 1 Hz with the parameters set point, amplitude, and feedback control optimized for each sample. Samples for AFM were prepared by dropping 0.5 μL of SWCNT solution onto a ∼1 cm2 silicon wafer and spin coating at 4000 rpm for 60 s. Transistor devices were fabricated by first pattering pairs of metal electrodes (5 nm Cr + 45 nm Pd) with channel dimensions of 800 nm in length LCh and 1 μm in width WCh on p-doped silicon with an 800 nm-thick thermal oxide layer using standard electron beam lithography and sputtering.30 All electrode pairs share one common electrode (drain). The site-selective integration of enriched sc-SWCNTs from an organic dispersion was achieved via dielectrophoresis (DEP).43,44 During the DEP process, a drop of 50 μL diluted dispersion (fraction 2, dilution ratio 1:5) was placed onto the electrode array under a nominal electric field of 5−10 Vrms/μm. After 5 min, the deposition was terminated by rinsing the surface repeatedly with toluene before switching the field off. All devices were subsequently annealed at 200 °C in air for 2 h before electrical testing. Electrical characterization was conducted using an Agilent 4155C semiconductor parameter analyzer system and a probe station with TRIAX probes with a low current detection limit of 30 fA. Transfer characteristics were measured with back gate voltage sweeps from VG = −80 to +80 V (step size 400 mV, scan rate 4 V/s) using source-drain voltages of 0.5, 1.0, and 1.5 V. The mobility μ has been calculated using the standard formula μ = (LCh/WCh)(1/CG·VSD)(dISD/dVG), with CG determined using a parallel plate capacitor model and ε = 3.9 for the relative permittivity of the silicon oxide layer.45 After electrical measurements, the devices were characterized with a Zeiss Ultra Plus scanning electron microscope under an 1 kV electron beam using the in-lens detector.
Figure 7. Red stars indicate on-state conductance per micron channel width (GON) versus On/Off ratio for all 12 short-channel, high-density sc-SWCNT transistors studied here (using SEC enriched PODOF-wrapped sc-SWCNTs (d = 1.35−1.72 nm) with VSD = 1 V). Also shown for comparison are recently reported data for devices prepared using aqueous sc-SWCNT dispersions [ref 34, d = 0.76 nm, and ref 35, d = 0.6−2.0 nm; ref 36, d = 1.5 nm] and for polymer-wrapped sc-SWCNTs dispersed in toluene [ref 31, d = 1.3−1.7 nm; and ref 15, d = 1.3−1.8 nm, ref 37, d = 1.3 nm; ref 38, d = 1.2−1.4 nm; ref 39, d = 1.3 nm; ref 40, d = 1.2−1.4 nm]. The region to the right of the dashed line is not accessible due to the low current detection limit of the setup. mg of the raw SWCNT soot and 100 mg of the polymer poly(9,9-di-ndodecylfluorenyl-2,7-diyl) (PODOF) (Sigma-Aldrich) were mixed in 100 mL of toluene and subjected to a sonication treatment for 2 h by using a titanium sonotrode (Bandelin, ∼20% power). During sonication, the suspension was placed in a water-circulation bath to aid cooling. After sonication, the suspension was then centrifuged for 2 h at 20000g. To generate the starting suspensions for SEC separation the supernatant was concentrated to ∼20 mL by evaporating ∼80 mL of toluene. Semipreparative chromatography was performed using Toyopearl HW-75 resin (Tosoh) filled into an HPLC steel column having 16 mm inner diameter and 20 cm length. Separation was performed with a commercial gel permeation chromatography system (SECcurity gel permeation chromatography (GPC) 1260 Infinity system, Agilent Technologies) that consisted of a quaternary pump (G1311B), the injector, the column, and a diode array detector (DAD) (G1315D). The GPC system was software controlled (WinGPC UniChrom v.8.1 Software, Polymer Standards Service GmbH). During an experimental run, the diode array detector was typically used to monitor five fixed wavelengths at 430, 450, 500, 550, and 600 nm. Alternatively, complete spectra could be measured from 190−950 nm with a bandwidth of 4 nm and a step width of 8 nm. After application of 5 mL of SWCNT starting suspension to the gel, the sample was pumped through the gel with a flow rate of 2 mL/min with toluene as eluent. Fractions were collected in 5−10 mL portions. UV−vis−NIR absorption spectra of the fractions were recorded on a Varian Cary 500 spectrophotometer. Raman spectra were obtained with Witec CRM200 (excitation at 633 nm) and Renishaw inVia (532 and 785 nm) Raman microscopes from flakes of SWCNTs which were practically completely sedimented out of suspensions after 1 h centrifugation at 150000g. Photoluminescence maps were measured in the emission range of ∼900−1700 nm and excitation range of 500− 950 nm (scanned in 3 nm steps) using a modified FTIR spectrometer (Bruker IFS66) equipped with a liquid-nitrogen-cooled Ge-photodiode and a monochromatized excitation light source as described elsewhere.42 Atomic force microscopy (AFM) was performed in an air environment with a multimode head and Nanoscope III controller (Digital Instruments), operating in tapping mode. Commercially
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b05572. epresentative AFM pictures obtained from spin coated SEC fractions (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
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[email protected]. Author Contributions
The experiments were conceived and designed by F.H., W.L., M.K., and R.K. F.H. and R.F. performed the synthesis, chromatographic separation, and characterization of nanotube dispersions. S.L. performed Raman spectroscopy. W.L. performed transistor fabrication and transport measurements. The manuscript was written by F.H. and W.L. with input from S.L., R.K., and M.K. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We gratefully acknowledge support by the Helmholtz Association through the program STN. REFERENCES (1) Avouris, P.; Freitag, M.; Perebeinos, V. Carbon Nanotube Photonics and Optoelectronics. Nat. Photonics 2008, 2, 341−350. F
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DOI: 10.1021/acsnano.5b05572 ACS Nano XXXX, XXX, XXX−XXX
