Directed Assembly of Single Wall Carbon Nanotube Field Effect Transistors Erika Penzo,†,∥ Matteo Palma,†,⊥ Daniel A. Chenet,‡ Geyou Ao,§ Ming Zheng,§ James C. Hone,‡ and Shalom J. Wind*,† †
Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States Department of Mechanical Engineering, Columbia University, New York, New York 10027, United States § National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ‡
S Supporting Information *
ABSTRACT: The outstanding electronic properties of single wall carbon nanotubes (SWCNTs) have made them prime candidates for future nanoelectronics technologies. One of the main obstacles to the implementation of advanced SWCNT electronics to date is the inability to arrange them in a manner suitable for complex circuits. Directed assembly of SWCNT segments onto lithographically patterned and chemically functionalized substrates is a promising way to organize SWCNTs in topologies that are amenable to integration for advanced applications, but the placement and orientational control required have not yet been demonstrated. We have developed a technique for assembling length sorted and chirality monodisperse DNA-wrapped SWCNT segments on hydrophilic lines patterned on a passivated oxidized silicon substrate. Placement of individual SWCNT segments at predetermined locations was achieved with nanometer accuracy. Three terminal electronic devices, consisting of a single SWCNT segment placed either beneath or on top of metallic source/drain electrodes were fabricated. Devices made with semiconducting nanotubes behaved as typical p-type field effect transistors (FETs), whereas devices made with metallic nanotubes had a finite resistance with little or no gate modulation. This scalable, high resolution approach represents an important step forward toward the potential implementation of complex SWCNT devices and circuits. KEYWORDS: carbon nanotubes, directed assembly, DNA-wrapped SWCNT, carbon nanotube FETs and chirality.15,16 Chromatography separation has produced 99.9% pure semiconductor SWCNT solutions,17 and it is expected that improvements will continue to be realized until the required 0.0001% purity8 is attained. Growth of single chirality SWCNTs by catalyst18 or precursor19 engineering has also been recently demonstrated. Research in SWCNT patterning has not been quite as successful. Aligned growth of SWCNTs on mis-cut quartz20−23 and sapphire24,25 substrates, along with transfer onto oxidized silicon substrates,26,27 may provide a viable route to circuit creation if sufficient density can be achieved8 (this was, in fact, the technique employed by Shulaker et al. in their impressive demonstration of a carbon nanotube computer1). However, this method places severe constraints on circuit layout and, indeed, it has not yet produced complex circuit topologies. Directed assembly techniques, which organize nanotubes from solution to desired locations on a surface, can take full advantage of the solution purifications methods, producing arrays of nanotubes,
T
wo decades of research on single wall carbon nanotubes (SWCNTs) have proven their outstanding electronic properties, culminating with the recent demonstration of all the critical elements of a SWCNT-based computer.1 Early observations of electronic switching2,3 in SWCNTs were quickly followed by examples of high transconductance transistors. 4−7 Even relatively crude SWCNT field effect transistors (FETs) outperformed advanced devices built in silicon,6,8,9 demonstrating the potential of SWCNTs for high performance nanoelectronics. Despite this, further advances, including high speed switching based on complex SWCNT circuits, have been elusive. There are two main challenges to the development of a SWCNT based nanoelectronics technology: nanotubes need to have uniform electronic properties, and they need to be organized into circuit-amenable topologies on surfaces. Great progress has been made in purifying SWCNTs by type, diameter and even individual chirality by such means as densitygradient ultracentrifugation,10 column chromatography,11 and aqueous two-phase extraction.12 In particular, DNA wrapping promotes efficient solvation of SWCNT in water13 and enables the purification of SWCNT segments with uniform length14 © XXXX American Chemical Society
Received: January 15, 2016 Accepted: January 25, 2016
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Figure 1. Scheme of the patterning of hydrophilic lines on PEG passivated SiO2 substrates and of the mechanism of DNA-wrapped SWCNT binding. The patterning is done by e-beam lithography of PMMA. The PEG passivating layer is selectively removed by reactive ion etching (RIE), which also renders the underneath surface hydrophilic. The inset in the center shows an SEM image of a patterned line on PMMA after development. The line is about 10 nm thick. The PMMA is coated with Ti for imaging purposes. Magnesium ions in the SWCNT solution bridge the negative charges on the post-O2 plasma SiO2 surface and the ones on the DNA sugar−phosphate backbone, allowing for the selective deposition of the DNA-wrapped SWCNTs on the patterned SiO2 lines.
RESULTS AND DISCUSSION Highly doped silicon substrates with a 300 nm thick thermal oxide and, in some cases, prepatterned Au electrodes were coated with a layer of poly(ethylene glycol) (PEG). A mixture of PEG-silane (2-[methoxy(polyethyleneoxy)propyl]trimethoxysilane) and PEG-thiol (HS−(CH2)11−(C2H6O2)3− OH) in anhydrous toluene allowed for the simultaneous passivation of the SiO2 surface and of the Au electrodes. The PEG layer was coated with poly(methyl methacrylate) (PMMA) resist, 60 nm thick. A Nanobeam nB4 electron beam lithography system was used to pattern narrow lines in the PMMA (Figure 1). The width of the lines was varied between 10 and 40 nm, whereas the length was kept constant at 200 nm, slightly longer than the average length of the SWCNT segments (see nanotube segments length distribution in Figure S1, Supporting Information). The patterned PMMA was used as a mask to selectively remove the PEG coating from the patterned lines with a 24 s long oxygen plasma exposure (Diener Plasma Etch System). The oxygen plasma treatment renders the surface hydrophilic, probably due to the formation of silanol groups on the SiO240 and to the oxidation of the Au electrodes.41 DNA-wrapped SWCNT segments were dispersed in a buffered solution optimized to promote binding to the hydrophilic lines (approximately 1.2 nM nanotubes in 0.1X TAE, 0.25X DPBS, and with 12.5 mM MgCl2). Adsorption of the nanotubes was enabled by the formation of a charge inversion layer on the patterned hydrophilic line due to the presence of magnesium ions in the buffer, thus promoting the binding of the negatively charged DNA-wrapped SWCNTs.40 Solutions that did not contain Mg ions did not produce any SWCNT deposition. Pristine SWCNTs are hydrophobic (indeed, they do not dissolve in water without the aid of surfactants). Single stranded DNA wraps around CNTs due to π stacking between the DNA base carbon rings and the ones constituting the CNT sidewalls, leaving on the outside the negatively charged DNA sugar−phosphate backbone.16 The resulting structure is composed of alternating hydrophilic− hydrophobic domains a few nanometers in size. As a
all with the same electronic properties, yet most directed assembly techniques28−30 to date generally yield ensembles of SWCNTs with large angular dispersion and little to no control over the number of SWCNTs deposited at a particular site. Single nanotube control has been achieved by dielectrophoretic assembly, such as in the associated works by Krupke et al.31−34 and others,35−38 but this technique introduces complexity to the fabrication process, and there are potential limitations, both in terms of circuit topology and ultimate achievable density. We have also recently reported single nanotube control using bivalent assembly of end-functionalized, fixed length SWCNT segments,39 although improvements are need in order to attain the desired yield required for high performance applications. In this work, we describe a technique that combines ultrahigh resolution lithographic patterning with selective chemistry to direct the assembly of individual SWCNT segments from solution onto desired locations on a substrate, with nanometer resolution. Hydrophilic lines, as narrow as 10 nm, were lithographically patterned on an oxidized silicon substrate coated with a passivating film. DNA-wrapped SWCNT dispersed in a buffered solution were physisorbed onto the hydrophilic lines with a high degree of selectivity. The small width of the lines resulted in the adsorption of one nanotube segment per line, enabling the fabrication of single nanotube electronic devices. Devices made with semiconducting SWCNTs displayed typical switching properties of SWCNTs FETs, whereas devices made with metallic SWCNTs showed little or no gate modulation. Two different approaches were taken for the directed self-assembly: in one, the SWCNTs were placed at selected locations on the surface, and electrodes were patterned on top of them; in the other, the electrodes were prepatterned on the substrate, and the SWCNTs were placed on top of the electrodes, bridging them. Both approaches produced working devices with currents in the nanoampere regime, limited by the small diameter of the SWCNTs. B
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whereas misplaced tubes were not observed for the 500 nm pattern (Figure 2b). When the lines are close, the PEG coated regions between adjacent lines are probably not large enough to completely repel the nanotubes, and occasionally SWCNT segments were observed to position themselves diagonally across multiple lines. Lines wider than 10 nm resulted in the absorption of multiple nanotubes and/or in a larger angular dispersion (Figure S2, Supporting Information). No difference was observed in the binding yield between substrates with or without prepatterned electrodes. Three solutions of SWCNT segments wrapped in ssDNA and dissolved in DI water were used for the assembly. One solution consisted of mixed chirality SWCNTs; another one was (6,5) enriched,16 thus prevalently composed of semiconducting SWCNTs (as shown in the NIR fluorescence map in Figure S3, Supporting Information); the third solution was a mixture of (5,5) and (6,6) metallic SWCNTs.42 In this last case, the nanotubes were separated in a polymer aqueous two-phase system, so that the final solution of metallic SWCNTs contained ∼20 wt % of polyacrylamide and ∼2 wt % of poly(ethylene glycol). The mixed chirality SWCNT solution and the semiconducting SWCNT solution had undergone some length purification via size exclusion chromatography (SEC).14 These two solutions had the same chemical composition (water, DNA, SWCNTs) and showed the same assembly behavior (Figure 2). The presence of the additional polymers in the third solution (metallic SWCNTs) resulted in a loss of selectivity in the assembly process, as shown in Figure 3. The two SEM images show samples passivated with PEG and patterned with two hydrophilic regions: one 10 nm wide, 200 nm long line placed between the electrodes, and one square (1 μm side) placed next to the electrodes and used as a control feature. Figure 3b shows the result of the assembly of metallic SWCNTs. Clearly, the binding was not very selective and the nanotubes physisorbed on the PEG and did not preferentially deposit on the hydrophilic regions. Figure 3a shows the result of the assembly of semiconducting SWCNTs. In this case, the binding was highly selective and the nanotubes deposited exclusively on the patterned hydrophilic regions: the line between the electrodes and the square feature on the side. No nonspecific binding was observed. Removal of the added polymers by ultracentrifugation (Millipore Amicon 100 K tubes) improved the binding selectivity of the metallic tubes, but it did not reach the high specificity of the two solutions in
consequence of this structure, DNA-wrapped SWCNTs preferentially physisorb onto surfaces within a specific hydrophilicity range, which is also affected by the particular buffer conditions. In our case, the PEG surface is sufficiently hydrophobic to prevent physisorption of the DNA-wrapped nanotubes, hence its passivating role. In order to retain the adsorbed SWCNT segments in the desired positions and to minimize salt precipitation from the buffer, a solvent exchange, from buffer to a mixture of DI water and ethanol, was performed prior to drying. The best binding yield was found for 10 nm wide lines, resulting in the placement of one SWCNT segment per line with 95% yield (i.e., a single SWCNT was positioned on 95% of the patterned lines; 121 lines were counted). Determination of the angular dispersion of the nanotubes was limited by the resolution of the SEM, coupled with the short length of the SWCNT segments; nonetheless, an upper bound of 5° can conservatively be placed on the dispersion (Figure 2 and Supporting Information Figures S4, S5, and S6). Lines were patterned at spacings of 100 and 500 nm or at isolated device sites. In the case of the 100 nm patterns, we found an average of one misplaced nanotube every 10 patterned lines (Figure 2a),
Figure 2. SEM images of DNA-wrapped SWCNT segments assembled from solution (mixed chirality solution) onto substrates with prepatterned Au electrodes. The SiO2 surface and the Au electrodes are passivated with a PEG layer. The PEG is selectively removed by e-beam patterning of 10 nm wide lines in PMMA and reactive ion etching. When lines are patterned as close as 100 nm (a), some tubes are observed to cross over the lines, but when lines are patterned farther apart (500 nm in (b)), no nanotube crossing is observed. The lateral shift in the position of the nanotube segments respect to the electrodes is due to registration inaccuracy in the e-beam patterning.
Figure 3. SEM images of samples passivated with PEG and patterned with hydrophilic regions (via selective PEG removal). The hydrophilic regions are one 10 nm wide, 200 nm long line placed between the electrodes, and one square (1 μm side) placed next to the electrodes and used as a control feature. (a) Assembly of semiconducting SWCNTs. In this case, the binding is selective and the nanotubes selectively deposit on the line between the electrodes and on the squared feature on the side. No nonspecific binding is observed. (b) Assembly of metallic SWCNTs. The binding is not selective and the nanotubes physisorb to the PEG and do not preferentially deposit on the hydrophilic regions. C
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performed before electrode patterning. Samples were immersed in boiling DI water, hydrochloric acid, and a commercial DNA removing/cleaning product (DNA away, Molecular BioProducts), followed by annealing in forming gas at 350 °C. Patterning of lines and electrodes was then done by e-beam lithography. Pd electrodes, capped with Au, were formed by physical vapor deposition and lift-off. Following electrodes deposition on top of the SWCNTs, samples were annealed in forming gas at 350 °C to remove any residual PMMA. Figure 4 shows the transfer characteristics of devices made with a SWCNT segment from the (6,5) enriched solution; it displays switching behavior typical for a carbon nanotube FET. The inset SEM image shows one representative device made with a single SWCNT segment positioned in the center of the two electrodes, exactly at the location of the patterned hydrophilic line. The saturation current for the devices is low, ranging from 100 nA. This low output conductance can be attributed to the small diameter of the tubes (∼7−8 Å), which dictates both an increased bandgap (for semiconducting SWCNTs) and reduced current carrying capabilities.43,44 The variation in the output current from device to device may be due to differences in contact resistance, possibly resulting from DNA or buffer residue on the SWCNTs. Figure 5 shows the current vs gate voltage characteristics of devices made with metallic SWCNTs, and it displays the expected conductive properties. One of the devices (shown in the inset SEM image) comprised two SWCNT segments assembled on top of the electrodes; the positioning was not perfect due to the presence of the added polymers in the solution, as discussed above. New techniques have recently been developed to fully remove these polymers, and it is expected that solutions of metallic SWCNTs processed with these new purification techniques would assemble with the same high specificity of the mixed-chirality and semiconducting SWCNT solutions (i.e., with no added polymers). This work was motivated by the needs of high performance carbon nanotube electronics, where, as stated above, the inability to deterministically control the location of each and every SWCNT has been a major barrier to progress. Our results demonstrate that this barrier can be overcome by combining high resolution lithographic patterning with selective surface chemistry. This approach is particularly compatible with solution-based methods for the selection of SWCNTs by diameter and chirality and, as we have shown, can be used with both semiconducting and metallic SWCNTs. Additional work is required, however, before this technique can be applied to the formation of advanced circuits. First, high performance SWCNT electronics requires maximal output conductance. As noted above, the small diameter of the tubes used in this study placed severe constraints on their potential for performance. Small diameter tubes were used primarily because of the selectivity of the DNA wrapping (solubilization and separation of SWCNTs by DNA wrapping is effective only for small diameter tubes16,43,44) and the fact that these tubes are easily sorted by length. Newer methods that are capable of highly efficient separation of larger diameter (≥1 nm) tubes in nearly monochiral solutions have recently been developed.45 The selective placement technique presented herewith could very likely be adapted to the surfactants used in those separation methods; however, it would still be necessary to employ some length sorting process in order to maximize the probability of a single SWCNT being deposited at each site. Second, we have thus far demonstrated the formation of SWCNT arrays with a
Figure 4. Source-drain current (Isd) versus gate voltage (Vg) at 100 mV source-drain bias for five devices assembled from a (6,5) enriched DNA-wrapped SWCNT solution with Pd electrodes patterned on top of the nanotube segment. The inset SEM image shows one representative device; the SWCNT segment is positioned exactly in the center of the electrodes, in the location of the hydrophilic line patterned to direct the assembly.
Figure 5. Source-drain current (Isd) versus gate voltage (Vg) at 100 mV source-drain bias. The devices are directed assembled from a metallic(5,5) and (6,6)DNA-wrapped SWCNT solution and the SWCNTs are deposited on top of the Au electrodes. The solution contains polyacrylamide and poly(ethylene glycol) that prevent the successful outcome of the directed assembly process (nonspecific binding is observed). The inset SEM image shows one representative device.
which the additional polymers were absent (see inset in Figure 5). After nanotube assembly some cleaning procedures were necessary in order to eliminate salt residue that may have precipitated from the buffer solution and to remove the DNA wrapping around the tubes, because they could affect the nanotubes’ electronic properties. In the case of electrodes patterned on top of the SWCNTs, the cleaning steps were D
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blown dry with nitrogen and the PEG was removed from the exposed lines by mean of a 24 s long oxygen plasma (Diener Plasma Etch System). The remaining PMMA was then stripped by ultrasonic agitation (Branson 3510) in a N-methylpyrrolidone (NMP, Sigma-Aldrich) bath for 30 min followed by acetone and ethanol rinses. SWCNT Assembly. Three solutions of SWCNT segments wrapped in ssDNA and dissolved in DI water were used for the assembly. One solution consisted of mixed chirality SWCNT (concentration ∼40 μg/mL, calculated from the E11 optical transition). Another other one was (6,5) enriched,16 thus prevalently made of semiconducting SWCNT (concentration ∼6 μg/mL, calculated from the E11 optical transition). The third solution was made of a mixture of (5,5) and (6,6) metallic SWCNT42 (concentration ∼29 μg/mL, calculated from the E11 optical transition). In this last case, the nanotubes were separated in a polymer aqueous two-phase system, so that the final solution of metallic SWCNTs contained ∼20 wt % of polyacrylamide and ∼2 wt % of poly(ethylene glycol) (PEG). Prior deposition, the metallic SWCNT solution was centrifuged in Millipore Amicon 100 K tubes to remove the added polymers. The mixed chirality and the semiconducting SWCNT solutions had undergone some length purification via size exclusion chromatography (SEC).14 The segments length distribution was quantified by tapping mode AFM and software analysis using ImageJ software (see Figure S1, Supporting Information). Knowing the average length of the CNT segments allowed the estimation of the molar concentration (70 nM for the unsorted solution, 10 nM for the (6,5) enriched solution, 48 nM for the metallic SWCNTs solution). Immediately after PMMA removal, a 40 μL solution of SWCNTs, optimized to promote binding to the hydrophilic lines, was dropped on top of the patterned samples. This solution was made of DNA-wrapped SWCNT segments in DI water (approx 1.2 nM total concentration) with 0.1X TAE buffer, 0.25X DPBS, and with 12.5 mM MgCl2. Samples were incubated in a humidified chamber for 3 h. In order to minimize salt deposition from the buffer solution onto the SWCNTs, a solvent exchange was performed prior to drying. Each buffered sample was dipped for 10 s in DI water, followed by a 20 s dip in a 10 mL mixture of ethanol (50%) and DI water (50%). It was then transferred to a well containing 10 mL of an 80% ethanol 20% DI water solution, where it was allowed to sit for 50 min. Samples were subsequently air-dried and dipped for 10 s in “DNA away” (Molecular BioProducts). They were then rinsed in DI water, placed in boiling DI water for 10 min and then left overnight to cool down. Finally, samples were dipped in HCl for 10 s, in the case of SWCNTs with electrodes on top, or for 1 h, in the case of SWCNTs on top of electrodes; they were rinsed in DI water and dried with N. They were then annealed at 350 °C for 4 h in forming gas (Ar and H). If the SWCNTs were deposited first, electrodes were patterned following the annealing. Electrodes Patterning. Electrodes were patterned by ebeam lithography (Nanobeam nB4 electron beam lithography system) using a bilayer resist. The bottom layer was made of EL copolymer (Microchem) about 100 nm thick; the top layer was made of PMMA (495 K, 2% in anisole, Microchem) about 50 nm thick. If electrodes were patterned before PEG passivation and SWCNT assembly, prior resist spinning, substrates were rinsed with acetone and IPA and exposed to an oxygen plasma for 1 min (Diener Plasma Etch System). If electrodes were
pitch of 100 nm. High performance SWCNT circuits require a density of ∼50 SWCNTs/μm.9 Thus, further improvement is needed in this area as well. On the basis of our results, it might be expected that increasing the pitch could result in an increase in defects, but we believe this can be managed by a combination of controlling the thickness of the PEG layer (or possibly using a different passivation) and the SWCNT concentration in solution. It should also be pointed out that even if these remaining challenges toward the creation of complex circuits are not overcome, our technique still has potential applications in SWCNT-based sensors and transducers.
CONCLUSIONS We have presented a technique for the directed assembly from solution of DNA-wrapped SWCNT segments with uniform electronic properties onto lithographically patterned lines on a surface. Individual nanotube control was achieved with spacing as close as 100 nm. This is the first demonstration of such high resolution and high density ordered arrays of SWCNT with uniform and controlled electronic properties. Our directed assembly method was applied to the fabrication of single nanotube electronic devices, resulting in the expected electronic behavior for the types of nanotubes used in the assembly. We believe this work represents a major step toward the implementation of SWCNT complex circuits for future nanoelectronics technologies. MATERIALS AND METHODS Patterning of Hydrophilic Regions. Samples made of silicon with 300 nm of thermally grown silicon oxide and, in some cases, prepatterned gold electrodes, were passivated with a layer of poly(ethylene glycol) (PEG). Substrates were first cleaned in an aged (1.5 h old) piranha solution (3:1 H2SO4:H2O2) followed by a deionized (DI) water rinse, an ethanol rinse, and blown dry with an inert gas (Ar or N2). Samples were incubated for 48 h to 1 week to allow a uniform PEG layer to form in order to prevent nonspecific binding of DNA-wrapped SWCNTs. Longer incubation times resulted in better passivation. The dry samples were put in an UV−ozone cleaner for 5 min at 18 W, then immediately incubated in a solution of PEG-silane (2-[methoxy(polyethyleneoxy)propyl]trimethoxysilane, Mw = 460−590, Gelest) in anhydrous toluene (300 μL of PEG-silane in 30 mL of toluene), with 100 μM PEG-thiol (HS−(CH2)11−(C2H6O2)3−OH, Prochemia) if the substrates had prepatterned Au electrodes (PEG-silane passivated the SiO2 surface, whereas PEG-thiol passivated the Au). Acetic acid (30 μL for 30 mL of toluene-PEG solution) was added as a catalyst to the solution directly before immersing the samples. Samples were incubated for 48 h to 1 week to allow a uniform PEG layer to form so that it then successfully prevented nonspecific binding of DNA-wrapped SWCNTs. After passivation samples were rinsed with acetone and ethanol and blown dry with Ar. They were coated with 60 nm of PMMA (495 K, 2% in anisole, Microchem) and soft-baked for at least 30 min at 170 °C before being patterned by e-beam (Nanobeam nB4 electron beam lithography system). The pattern consisted of lines 200 nm long and 10 nm, 20 nm, 30 nm, and 40 nm thick. Development was done for 45 s in a 3:1 mixture of isopropyl alcohol (IPA) and MIBK kept at 4 °C with ultrasonic agitation (Branson 3510) and then for an additional 15 s in IPA at 4 °C with ultrasonic agitation. Samples were E
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(3) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. RoomTemperature Transistor Based on a Single Carbon Nanotube. Nature 1998, 393, 49−52. (4) Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. Ballistic Carbon Nanotube Field-Effect Transistors. Nature 2003, 424, 654− 657. (5) Rosenblatt, S.; Yaish, Y.; Park, J.; Gore, J.; Sazonova, V.; McEuen, P. L. High Performance Electrolyte Gated Carbon Nanotube Transistors. Nano Lett. 2002, 2, 869−872. (6) Wind, S. J.; Appenzeller, J.; Martel, R.; Derycke, V.; Avouris, P. Vertical Scaling of Carbon Nanotube Field-Effect Transistors Using Top Gate Electrodes. Appl. Phys. Lett. 2002, 80, 3817−3819. (7) Wind, S. J.; Appenzeller, J.; Martel, R.; Derycke, V.; Avouris, P. Fabrication and Electrical Characterization of Top Gate Single-Wall Carbon Nanotube Field-Effect Transistors. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 2002, 20, 2798. (8) Franklin, A. D. Electronics the Road to Carbon Nanotube Transistors. Nature 2013, 498, 443−444. (9) Franklin, A. D.; Luisier, M.; Han, S. J.; Tulevski, G.; Breslin, C. M.; Gignac, L.; Lundstrom, M. S.; Haensch, W. Sub-10 Nm Carbon Nanotube Transistor. Nano Lett. 2012, 12, 758−762. (10) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Sorting Carbon Nanotubes by Electronic Structure Using Density Differentiation. Nat. Nanotechnol. 2006, 1, 60−65. (11) Hersam, M. Progress Towards Monodisperse Single-Walled Carbon Nanotubes. Nat. Nanotechnol. 2008, 3, 387−394. (12) Gui, H.; Streit, J. K.; Fagan, J. A.; Hight Walker, A. R.; Zhou, C.; Zheng, M. Redox Sorting of Carbon Nanotubes. Nano Lett. 2015, 15, 1642−1646. (13) Zheng, M.; Jagota, A.; Semke, E.; Diner, B.; McLean, R.; Lustig, S.; Richardson, R.; Tassi, N. DNA-Assisted Dispersion and Separation of Carbon Nanotubes. Nat. Mater. 2003, 2, 338−342. (14) Huang, X.; McLean, R.; Zheng, M. High-Resolution Length Sorting and Purification of DNA-Wrapped Carbon Nanotubes by SizeExclusion Chromatography. Anal. Chem. 2005, 77, 6225−6228. (15) Ao, G.; Khripin, C. Y.; Zheng, M. DNA-Controlled Partition of Carbon Nanotubes in Polymer Aqueous Two-Phase Systems. J. Am. Chem. Soc. 2014, 136, 10383−10392. (16) Tu, X.; Manohar, S.; Jagota, A.; Zheng, M. DNA Sequence Motifs for Structure-Specific Recognition and Separation of Carbon Nanotubes. Nature 2009, 460, 250−253. (17) Tulevski, G. S.; Franklin, A. D.; Afzali, A. High Purity Isolation and Quantification of Semiconducting Carbon Nanotubes Via Column Chromatography. ACS Nano 2013, 7, 2971−2976. (18) Yang, F.; et al. Chirality-Specific Growth of Single-Walled Carbon Nanotubes on Solid Alloy Catalysts. Nature 2014, 510, 522− 524. (19) Sanchez-Valencia, J. R.; Dienel, T.; Groning, O.; Shorubalko, I.; Mueller, A.; Jansen, M.; Amsharov, K.; Ruffieux, P.; Fasel, R. Controlled Synthesis of Single-Chirality Carbon Nanotubes. Nature 2014, 512, 61−64. (20) Ding, L.; Yuan, D. N.; Liu, J. Growth of High-Density Parallel Arrays of Long Single-Walled Carbon Nanotubes on Quartz Substrates. J. Am. Chem. Soc. 2008, 130, 5428−5429. (21) Ismach, A.; Kantorovich, D.; Joselevich, E. Carbon Nanotube Graphoepitaxy: Highly Oriented Growth by Faceted Nanosteps. J. Am. Chem. Soc. 2005, 127, 11554−11555. (22) Jeon, S.; Lee, C.; Tang, J. Y.; Hone, J.; Nuckolls, C. Growth of Serpentine Carbon Nanotubes on Quartz Substrates and Their Electrical Properties. Nano Res. 2008, 1, 427−433. (23) Kocabas, C.; Hur, S. H.; Gaur, A.; Meitl, M. A.; Shim, M.; Rogers, J. A. Guided Growth of Large-Scale, Horizontally Aligned Arrays of Single-Walled Carbon Nanotubes and Their Use in ThinFilm Transistors. Small 2005, 1, 1110−1116. (24) Ago, H.; Nakamura, K.; Ikeda, K.; Uehara, N.; Ishigami, N.; Tsuji, M. Aligned Growth of Isolated Single-Walled Carbon Nanotubes Programmed by Atomic Arrangement of Substrate Surface. Chem. Phys. Lett. 2005, 408, 433−438.
patterned after PEG passivation and SWCNT assembly, hence on top of the SWCNT segments, prior resist spinning substrates did not receive any treatment (other than the washing and drying process done after the SWCNT assembly). In both cases, the first resist layer was soft-baked for 1 h at 155 °C before spinning the second layer; the second resist layer was soft-baked for 1 h at 170 °C before being patterned by e-beam (Nanobeam nB4 electron beam lithography system). Development was done in a 1:3 DI water:IPA solution, at room temperature, for 1 min. For samples without PEG and SWCNT, a short oxygen plasma (15 s, Diener Plasma Etch System) was performed after development, follow by the ebeam evaporation (Angstrom EvoVac Deposition System) of 1 nm of Ti followed by 50 nm of Au. Lift-off was done overnight in remover PG (Microchem). For samples with PEG passivation and assembled SWCNT segments, a stack of 0.8 nm of Ti, 15 nm of Pd, 35 nm of Au, was deposited directly after development by e-beam evaporation (Angstrom EvoVac Deposition System). In this case, lift-off was done overnight in acetone because NMP based solvents have been found to highly dope SWCNTs. Following lift-off, samples were annealed at 350 °C for 4 h in forming gas (Ar and H).
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b00353. Length distribution of mixed chirality SWCNT segments; SWCNT assembly on 30 nm wide hydrophilic lines; NIR fluorescence map of semiconducting SWCNTs; additional SEM and AFM images of the assembled SWCNT segments. (PDF)
AUTHOR INFORMATION Corresponding Author
*[email protected]. Present Addresses ∥
The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA; ⊥ Department of Chemistry and Biochemistry, School of Biological and Chemical Sciences, Queen Mary University of London, London, United Kingdom. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS The authors thank Profs. C. Nuckolls and M. Sheetz for resource support, as well as the staff and facilities of the Columbia Nano Initiative cleanroom, where much of the fabrication work was performed. The authors also gratefully acknowledge financial support from the Office of Naval Research under Award No. N00014-09-1-1117. REFERENCES (1) Shulaker, M. M.; Hills, G.; Patil, N.; Wei, H.; Chen, H. Y.; Wong, H. S.; Mitra, S. Carbon Nanotube Computer. Nature 2013, 501, 526− 530. (2) Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, P. Single- and Multi-Wall Carbon Nanotube Field-Effect Transistors. Appl. Phys. Lett. 1998, 73, 2447−2449. F
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ACS Nano (25) Han, S.; Liu, X. L.; Zhou, C. W. Template-Free Directional Growth of Single-Walled Carbon Nanotubes on a- and R-Plane Sapphire. J. Am. Chem. Soc. 2005, 127, 5294−5295. (26) Jin, S.; et al. Using Nanoscale Thermocapillary Flows to Create Arrays of Purely Semiconducting Single-Walled Carbon Nanotubes. Nat. Nanotechnol. 2013, 8, 347−355. (27) Meitl, M. A.; Zhu, Z. T.; Kumar, V.; Lee, K. J.; Feng, X.; Huang, Y. Y.; Adesida, I.; Nuzzo, R. G.; Rogers, J. A. Transfer Printing by Kinetic Control of Adhesion to an Elastomeric Stamp. Nat. Mater. 2006, 5, 33−38. (28) Bardecker, J. A.; Afzali, A.; Tulevski, G. S.; Graham, T.; Hannon, J. B.; Jen, A. K. Y. Directed Assembly of Single-Walled Carbon Nanotubes Via Drop-Casting onto a Uv-Patterned Photosensitive Monolayer. J. Am. Chem. Soc. 2008, 130, 7226−7227. (29) Park, H.; Afzali, A.; Han, S. J.; Tulevski, G. S.; Franklin, A. D.; Tersoff, J.; Hannon, J. B.; Haensch, W. High-Density Integration of Carbon Nanotubes Via Chemical Self-Assembly. Nat. Nanotechnol. 2012, 7, 787−791. (30) Xu, P. F.; Noh, H.; Lee, J. H.; Cha, J. N. DNA Mediated Assembly of Single Walled Carbon Nanotubes: Role of DNA Linkers and Annealing. Phys. Chem. Chem. Phys. 2011, 13, 10004−10008. (31) Krupke, R.; Hennrich, F.; Löhneysen, H. v.; Kappes, M. M. Separation of Metallic from Semiconducting Single-Walled Carbon Nanotubes. Science 2003, 301, 344−347. (32) Krupke, R.; Hennrich, F.; Weber, H. B.; Kappes, M. M.; v. Löhneysen, H. Simultaneous Deposition of Metallic Bundles of SingleWalled Carbon Nanotubes Using Ac-Dielectrophoresis. Nano Lett. 2003, 3, 1019−1023. (33) Vijayaraghavan, A.; Blatt, S.; Weissenberger, D.; Oron-Carl, M.; Hennrich, F.; Gerthsen, D.; Hahn, H.; Krupke, R. Ultra-Large-Scale Directed Assembly of Single-Walled Carbon Nanotube Devices. Nano Lett. 2007, 7, 1556−1560. (34) Vijayaraghavan, A.; et al. Toward Single-Chirality Carbon Nanotube Device Arrays. ACS Nano 2010, 4, 2748−2754. (35) An, L.; Friedrich, C. Dielectrophoretic Assembly of Carbon Nanotubes and Stability Analysis. Prog. Nat. Sci. 2013, 23, 367−373. (36) Banerjee, S.; White, B. E.; Huang, L. M.; Rego, B. J.; O’Brien, S.; Herman, I. P. Precise Positioning of Single-Walled Carbon Nanotubes by Ac Dielectrophoresis. J. Vac. Sci. Technol., B 2006, 24, 3173−3178. (37) Duchamp, M.; Lee, K.; Dwir, B.; Seo, J. W.; Kapon, E.; Forró, L.; Magrez, A. Controlled Positioning of Carbon Nanotubes by Dielectrophoresis: Insights into the Solvent and Substrate Role. ACS Nano 2010, 4, 279−284. (38) Sorgenfrei, S.; Meric, I.; Banerjee, S.; Akey, A.; Rosenblatt, S.; Herman, I. P.; Shepard, K. L. Controlled Dielectrophoretic Assembly of Carbon Nanotubes Using Real-Time Electrical Detection. Appl. Phys. Lett. 2009, 94, 053105. (39) Penzo, E.; Palma, M.; Wang, R.; Cai, H.; Zheng, M.; Wind, S. J. Directed Assembly of End-Functionalized Single Wall Carbon Nanotube Segments. Nano Lett. 2015, 15, 6547. (40) Kershner, R. J.; et al. Placement and Orientation of Individual DNA Shapes on Lithographically Patterned Surfaces. Nat. Nanotechnol. 2009, 4, 557−561. (41) Ron, H.; Matlis, S.; Rubinstein, I. Self-Assembled Monolayers on Oxidized Metals. 2. Gold Surface Oxidative Pretreatment, Monolayer Properties, and Depression Formation. Langmuir 1998, 14, 1116−1121. (42) Khripin, C.; Fagan, J.; Zheng, M. Spontaneous Partition of Carbon Nanotubes in Polymer-Modified Aqueous Phases. J. Am. Chem. Soc. 2013, 135, 6822−6825. (43) Zhang, L.; Tu, X. M.; Welsher, K.; Wang, X. R.; Zheng, M.; Dai, H. J. Optical Characterizations and Electronic Devices of Nearly Pure (10,5) Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2009, 131, 2454. (44) Zhang, L.; Zaric, S.; Tu, X. M.; Wang, X. R.; Zhao, W.; Dai, H. J. Assessment of Chemically Separated Carbon Nanotubes for Nanoelectronics. J. Am. Chem. Soc. 2008, 130, 2686−2691.
(45) Fagan, J. A.; et al. Isolation of > 1 Nm Diameter Single-Wall Carbon Nanotube Species Using Aqueous Two-Phase Extraction. ACS Nano 2015, 9, 5377−5390.
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Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States Department of Mechanical Engineering, Columbia University, New York, New York 10027, United States § National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ‡
S Supporting Information *
ABSTRACT: The outstanding electronic properties of single wall carbon nanotubes (SWCNTs) have made them prime candidates for future nanoelectronics technologies. One of the main obstacles to the implementation of advanced SWCNT electronics to date is the inability to arrange them in a manner suitable for complex circuits. Directed assembly of SWCNT segments onto lithographically patterned and chemically functionalized substrates is a promising way to organize SWCNTs in topologies that are amenable to integration for advanced applications, but the placement and orientational control required have not yet been demonstrated. We have developed a technique for assembling length sorted and chirality monodisperse DNA-wrapped SWCNT segments on hydrophilic lines patterned on a passivated oxidized silicon substrate. Placement of individual SWCNT segments at predetermined locations was achieved with nanometer accuracy. Three terminal electronic devices, consisting of a single SWCNT segment placed either beneath or on top of metallic source/drain electrodes were fabricated. Devices made with semiconducting nanotubes behaved as typical p-type field effect transistors (FETs), whereas devices made with metallic nanotubes had a finite resistance with little or no gate modulation. This scalable, high resolution approach represents an important step forward toward the potential implementation of complex SWCNT devices and circuits. KEYWORDS: carbon nanotubes, directed assembly, DNA-wrapped SWCNT, carbon nanotube FETs and chirality.15,16 Chromatography separation has produced 99.9% pure semiconductor SWCNT solutions,17 and it is expected that improvements will continue to be realized until the required 0.0001% purity8 is attained. Growth of single chirality SWCNTs by catalyst18 or precursor19 engineering has also been recently demonstrated. Research in SWCNT patterning has not been quite as successful. Aligned growth of SWCNTs on mis-cut quartz20−23 and sapphire24,25 substrates, along with transfer onto oxidized silicon substrates,26,27 may provide a viable route to circuit creation if sufficient density can be achieved8 (this was, in fact, the technique employed by Shulaker et al. in their impressive demonstration of a carbon nanotube computer1). However, this method places severe constraints on circuit layout and, indeed, it has not yet produced complex circuit topologies. Directed assembly techniques, which organize nanotubes from solution to desired locations on a surface, can take full advantage of the solution purifications methods, producing arrays of nanotubes,
T
wo decades of research on single wall carbon nanotubes (SWCNTs) have proven their outstanding electronic properties, culminating with the recent demonstration of all the critical elements of a SWCNT-based computer.1 Early observations of electronic switching2,3 in SWCNTs were quickly followed by examples of high transconductance transistors. 4−7 Even relatively crude SWCNT field effect transistors (FETs) outperformed advanced devices built in silicon,6,8,9 demonstrating the potential of SWCNTs for high performance nanoelectronics. Despite this, further advances, including high speed switching based on complex SWCNT circuits, have been elusive. There are two main challenges to the development of a SWCNT based nanoelectronics technology: nanotubes need to have uniform electronic properties, and they need to be organized into circuit-amenable topologies on surfaces. Great progress has been made in purifying SWCNTs by type, diameter and even individual chirality by such means as densitygradient ultracentrifugation,10 column chromatography,11 and aqueous two-phase extraction.12 In particular, DNA wrapping promotes efficient solvation of SWCNT in water13 and enables the purification of SWCNT segments with uniform length14 © XXXX American Chemical Society
Received: January 15, 2016 Accepted: January 25, 2016
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Figure 1. Scheme of the patterning of hydrophilic lines on PEG passivated SiO2 substrates and of the mechanism of DNA-wrapped SWCNT binding. The patterning is done by e-beam lithography of PMMA. The PEG passivating layer is selectively removed by reactive ion etching (RIE), which also renders the underneath surface hydrophilic. The inset in the center shows an SEM image of a patterned line on PMMA after development. The line is about 10 nm thick. The PMMA is coated with Ti for imaging purposes. Magnesium ions in the SWCNT solution bridge the negative charges on the post-O2 plasma SiO2 surface and the ones on the DNA sugar−phosphate backbone, allowing for the selective deposition of the DNA-wrapped SWCNTs on the patterned SiO2 lines.
RESULTS AND DISCUSSION Highly doped silicon substrates with a 300 nm thick thermal oxide and, in some cases, prepatterned Au electrodes were coated with a layer of poly(ethylene glycol) (PEG). A mixture of PEG-silane (2-[methoxy(polyethyleneoxy)propyl]trimethoxysilane) and PEG-thiol (HS−(CH2)11−(C2H6O2)3− OH) in anhydrous toluene allowed for the simultaneous passivation of the SiO2 surface and of the Au electrodes. The PEG layer was coated with poly(methyl methacrylate) (PMMA) resist, 60 nm thick. A Nanobeam nB4 electron beam lithography system was used to pattern narrow lines in the PMMA (Figure 1). The width of the lines was varied between 10 and 40 nm, whereas the length was kept constant at 200 nm, slightly longer than the average length of the SWCNT segments (see nanotube segments length distribution in Figure S1, Supporting Information). The patterned PMMA was used as a mask to selectively remove the PEG coating from the patterned lines with a 24 s long oxygen plasma exposure (Diener Plasma Etch System). The oxygen plasma treatment renders the surface hydrophilic, probably due to the formation of silanol groups on the SiO240 and to the oxidation of the Au electrodes.41 DNA-wrapped SWCNT segments were dispersed in a buffered solution optimized to promote binding to the hydrophilic lines (approximately 1.2 nM nanotubes in 0.1X TAE, 0.25X DPBS, and with 12.5 mM MgCl2). Adsorption of the nanotubes was enabled by the formation of a charge inversion layer on the patterned hydrophilic line due to the presence of magnesium ions in the buffer, thus promoting the binding of the negatively charged DNA-wrapped SWCNTs.40 Solutions that did not contain Mg ions did not produce any SWCNT deposition. Pristine SWCNTs are hydrophobic (indeed, they do not dissolve in water without the aid of surfactants). Single stranded DNA wraps around CNTs due to π stacking between the DNA base carbon rings and the ones constituting the CNT sidewalls, leaving on the outside the negatively charged DNA sugar−phosphate backbone.16 The resulting structure is composed of alternating hydrophilic− hydrophobic domains a few nanometers in size. As a
all with the same electronic properties, yet most directed assembly techniques28−30 to date generally yield ensembles of SWCNTs with large angular dispersion and little to no control over the number of SWCNTs deposited at a particular site. Single nanotube control has been achieved by dielectrophoretic assembly, such as in the associated works by Krupke et al.31−34 and others,35−38 but this technique introduces complexity to the fabrication process, and there are potential limitations, both in terms of circuit topology and ultimate achievable density. We have also recently reported single nanotube control using bivalent assembly of end-functionalized, fixed length SWCNT segments,39 although improvements are need in order to attain the desired yield required for high performance applications. In this work, we describe a technique that combines ultrahigh resolution lithographic patterning with selective chemistry to direct the assembly of individual SWCNT segments from solution onto desired locations on a substrate, with nanometer resolution. Hydrophilic lines, as narrow as 10 nm, were lithographically patterned on an oxidized silicon substrate coated with a passivating film. DNA-wrapped SWCNT dispersed in a buffered solution were physisorbed onto the hydrophilic lines with a high degree of selectivity. The small width of the lines resulted in the adsorption of one nanotube segment per line, enabling the fabrication of single nanotube electronic devices. Devices made with semiconducting SWCNTs displayed typical switching properties of SWCNTs FETs, whereas devices made with metallic SWCNTs showed little or no gate modulation. Two different approaches were taken for the directed self-assembly: in one, the SWCNTs were placed at selected locations on the surface, and electrodes were patterned on top of them; in the other, the electrodes were prepatterned on the substrate, and the SWCNTs were placed on top of the electrodes, bridging them. Both approaches produced working devices with currents in the nanoampere regime, limited by the small diameter of the SWCNTs. B
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whereas misplaced tubes were not observed for the 500 nm pattern (Figure 2b). When the lines are close, the PEG coated regions between adjacent lines are probably not large enough to completely repel the nanotubes, and occasionally SWCNT segments were observed to position themselves diagonally across multiple lines. Lines wider than 10 nm resulted in the absorption of multiple nanotubes and/or in a larger angular dispersion (Figure S2, Supporting Information). No difference was observed in the binding yield between substrates with or without prepatterned electrodes. Three solutions of SWCNT segments wrapped in ssDNA and dissolved in DI water were used for the assembly. One solution consisted of mixed chirality SWCNTs; another one was (6,5) enriched,16 thus prevalently composed of semiconducting SWCNTs (as shown in the NIR fluorescence map in Figure S3, Supporting Information); the third solution was a mixture of (5,5) and (6,6) metallic SWCNTs.42 In this last case, the nanotubes were separated in a polymer aqueous two-phase system, so that the final solution of metallic SWCNTs contained ∼20 wt % of polyacrylamide and ∼2 wt % of poly(ethylene glycol). The mixed chirality SWCNT solution and the semiconducting SWCNT solution had undergone some length purification via size exclusion chromatography (SEC).14 These two solutions had the same chemical composition (water, DNA, SWCNTs) and showed the same assembly behavior (Figure 2). The presence of the additional polymers in the third solution (metallic SWCNTs) resulted in a loss of selectivity in the assembly process, as shown in Figure 3. The two SEM images show samples passivated with PEG and patterned with two hydrophilic regions: one 10 nm wide, 200 nm long line placed between the electrodes, and one square (1 μm side) placed next to the electrodes and used as a control feature. Figure 3b shows the result of the assembly of metallic SWCNTs. Clearly, the binding was not very selective and the nanotubes physisorbed on the PEG and did not preferentially deposit on the hydrophilic regions. Figure 3a shows the result of the assembly of semiconducting SWCNTs. In this case, the binding was highly selective and the nanotubes deposited exclusively on the patterned hydrophilic regions: the line between the electrodes and the square feature on the side. No nonspecific binding was observed. Removal of the added polymers by ultracentrifugation (Millipore Amicon 100 K tubes) improved the binding selectivity of the metallic tubes, but it did not reach the high specificity of the two solutions in
consequence of this structure, DNA-wrapped SWCNTs preferentially physisorb onto surfaces within a specific hydrophilicity range, which is also affected by the particular buffer conditions. In our case, the PEG surface is sufficiently hydrophobic to prevent physisorption of the DNA-wrapped nanotubes, hence its passivating role. In order to retain the adsorbed SWCNT segments in the desired positions and to minimize salt precipitation from the buffer, a solvent exchange, from buffer to a mixture of DI water and ethanol, was performed prior to drying. The best binding yield was found for 10 nm wide lines, resulting in the placement of one SWCNT segment per line with 95% yield (i.e., a single SWCNT was positioned on 95% of the patterned lines; 121 lines were counted). Determination of the angular dispersion of the nanotubes was limited by the resolution of the SEM, coupled with the short length of the SWCNT segments; nonetheless, an upper bound of 5° can conservatively be placed on the dispersion (Figure 2 and Supporting Information Figures S4, S5, and S6). Lines were patterned at spacings of 100 and 500 nm or at isolated device sites. In the case of the 100 nm patterns, we found an average of one misplaced nanotube every 10 patterned lines (Figure 2a),
Figure 2. SEM images of DNA-wrapped SWCNT segments assembled from solution (mixed chirality solution) onto substrates with prepatterned Au electrodes. The SiO2 surface and the Au electrodes are passivated with a PEG layer. The PEG is selectively removed by e-beam patterning of 10 nm wide lines in PMMA and reactive ion etching. When lines are patterned as close as 100 nm (a), some tubes are observed to cross over the lines, but when lines are patterned farther apart (500 nm in (b)), no nanotube crossing is observed. The lateral shift in the position of the nanotube segments respect to the electrodes is due to registration inaccuracy in the e-beam patterning.
Figure 3. SEM images of samples passivated with PEG and patterned with hydrophilic regions (via selective PEG removal). The hydrophilic regions are one 10 nm wide, 200 nm long line placed between the electrodes, and one square (1 μm side) placed next to the electrodes and used as a control feature. (a) Assembly of semiconducting SWCNTs. In this case, the binding is selective and the nanotubes selectively deposit on the line between the electrodes and on the squared feature on the side. No nonspecific binding is observed. (b) Assembly of metallic SWCNTs. The binding is not selective and the nanotubes physisorb to the PEG and do not preferentially deposit on the hydrophilic regions. C
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performed before electrode patterning. Samples were immersed in boiling DI water, hydrochloric acid, and a commercial DNA removing/cleaning product (DNA away, Molecular BioProducts), followed by annealing in forming gas at 350 °C. Patterning of lines and electrodes was then done by e-beam lithography. Pd electrodes, capped with Au, were formed by physical vapor deposition and lift-off. Following electrodes deposition on top of the SWCNTs, samples were annealed in forming gas at 350 °C to remove any residual PMMA. Figure 4 shows the transfer characteristics of devices made with a SWCNT segment from the (6,5) enriched solution; it displays switching behavior typical for a carbon nanotube FET. The inset SEM image shows one representative device made with a single SWCNT segment positioned in the center of the two electrodes, exactly at the location of the patterned hydrophilic line. The saturation current for the devices is low, ranging from 100 nA. This low output conductance can be attributed to the small diameter of the tubes (∼7−8 Å), which dictates both an increased bandgap (for semiconducting SWCNTs) and reduced current carrying capabilities.43,44 The variation in the output current from device to device may be due to differences in contact resistance, possibly resulting from DNA or buffer residue on the SWCNTs. Figure 5 shows the current vs gate voltage characteristics of devices made with metallic SWCNTs, and it displays the expected conductive properties. One of the devices (shown in the inset SEM image) comprised two SWCNT segments assembled on top of the electrodes; the positioning was not perfect due to the presence of the added polymers in the solution, as discussed above. New techniques have recently been developed to fully remove these polymers, and it is expected that solutions of metallic SWCNTs processed with these new purification techniques would assemble with the same high specificity of the mixed-chirality and semiconducting SWCNT solutions (i.e., with no added polymers). This work was motivated by the needs of high performance carbon nanotube electronics, where, as stated above, the inability to deterministically control the location of each and every SWCNT has been a major barrier to progress. Our results demonstrate that this barrier can be overcome by combining high resolution lithographic patterning with selective surface chemistry. This approach is particularly compatible with solution-based methods for the selection of SWCNTs by diameter and chirality and, as we have shown, can be used with both semiconducting and metallic SWCNTs. Additional work is required, however, before this technique can be applied to the formation of advanced circuits. First, high performance SWCNT electronics requires maximal output conductance. As noted above, the small diameter of the tubes used in this study placed severe constraints on their potential for performance. Small diameter tubes were used primarily because of the selectivity of the DNA wrapping (solubilization and separation of SWCNTs by DNA wrapping is effective only for small diameter tubes16,43,44) and the fact that these tubes are easily sorted by length. Newer methods that are capable of highly efficient separation of larger diameter (≥1 nm) tubes in nearly monochiral solutions have recently been developed.45 The selective placement technique presented herewith could very likely be adapted to the surfactants used in those separation methods; however, it would still be necessary to employ some length sorting process in order to maximize the probability of a single SWCNT being deposited at each site. Second, we have thus far demonstrated the formation of SWCNT arrays with a
Figure 4. Source-drain current (Isd) versus gate voltage (Vg) at 100 mV source-drain bias for five devices assembled from a (6,5) enriched DNA-wrapped SWCNT solution with Pd electrodes patterned on top of the nanotube segment. The inset SEM image shows one representative device; the SWCNT segment is positioned exactly in the center of the electrodes, in the location of the hydrophilic line patterned to direct the assembly.
Figure 5. Source-drain current (Isd) versus gate voltage (Vg) at 100 mV source-drain bias. The devices are directed assembled from a metallic(5,5) and (6,6)DNA-wrapped SWCNT solution and the SWCNTs are deposited on top of the Au electrodes. The solution contains polyacrylamide and poly(ethylene glycol) that prevent the successful outcome of the directed assembly process (nonspecific binding is observed). The inset SEM image shows one representative device.
which the additional polymers were absent (see inset in Figure 5). After nanotube assembly some cleaning procedures were necessary in order to eliminate salt residue that may have precipitated from the buffer solution and to remove the DNA wrapping around the tubes, because they could affect the nanotubes’ electronic properties. In the case of electrodes patterned on top of the SWCNTs, the cleaning steps were D
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blown dry with nitrogen and the PEG was removed from the exposed lines by mean of a 24 s long oxygen plasma (Diener Plasma Etch System). The remaining PMMA was then stripped by ultrasonic agitation (Branson 3510) in a N-methylpyrrolidone (NMP, Sigma-Aldrich) bath for 30 min followed by acetone and ethanol rinses. SWCNT Assembly. Three solutions of SWCNT segments wrapped in ssDNA and dissolved in DI water were used for the assembly. One solution consisted of mixed chirality SWCNT (concentration ∼40 μg/mL, calculated from the E11 optical transition). Another other one was (6,5) enriched,16 thus prevalently made of semiconducting SWCNT (concentration ∼6 μg/mL, calculated from the E11 optical transition). The third solution was made of a mixture of (5,5) and (6,6) metallic SWCNT42 (concentration ∼29 μg/mL, calculated from the E11 optical transition). In this last case, the nanotubes were separated in a polymer aqueous two-phase system, so that the final solution of metallic SWCNTs contained ∼20 wt % of polyacrylamide and ∼2 wt % of poly(ethylene glycol) (PEG). Prior deposition, the metallic SWCNT solution was centrifuged in Millipore Amicon 100 K tubes to remove the added polymers. The mixed chirality and the semiconducting SWCNT solutions had undergone some length purification via size exclusion chromatography (SEC).14 The segments length distribution was quantified by tapping mode AFM and software analysis using ImageJ software (see Figure S1, Supporting Information). Knowing the average length of the CNT segments allowed the estimation of the molar concentration (70 nM for the unsorted solution, 10 nM for the (6,5) enriched solution, 48 nM for the metallic SWCNTs solution). Immediately after PMMA removal, a 40 μL solution of SWCNTs, optimized to promote binding to the hydrophilic lines, was dropped on top of the patterned samples. This solution was made of DNA-wrapped SWCNT segments in DI water (approx 1.2 nM total concentration) with 0.1X TAE buffer, 0.25X DPBS, and with 12.5 mM MgCl2. Samples were incubated in a humidified chamber for 3 h. In order to minimize salt deposition from the buffer solution onto the SWCNTs, a solvent exchange was performed prior to drying. Each buffered sample was dipped for 10 s in DI water, followed by a 20 s dip in a 10 mL mixture of ethanol (50%) and DI water (50%). It was then transferred to a well containing 10 mL of an 80% ethanol 20% DI water solution, where it was allowed to sit for 50 min. Samples were subsequently air-dried and dipped for 10 s in “DNA away” (Molecular BioProducts). They were then rinsed in DI water, placed in boiling DI water for 10 min and then left overnight to cool down. Finally, samples were dipped in HCl for 10 s, in the case of SWCNTs with electrodes on top, or for 1 h, in the case of SWCNTs on top of electrodes; they were rinsed in DI water and dried with N. They were then annealed at 350 °C for 4 h in forming gas (Ar and H). If the SWCNTs were deposited first, electrodes were patterned following the annealing. Electrodes Patterning. Electrodes were patterned by ebeam lithography (Nanobeam nB4 electron beam lithography system) using a bilayer resist. The bottom layer was made of EL copolymer (Microchem) about 100 nm thick; the top layer was made of PMMA (495 K, 2% in anisole, Microchem) about 50 nm thick. If electrodes were patterned before PEG passivation and SWCNT assembly, prior resist spinning, substrates were rinsed with acetone and IPA and exposed to an oxygen plasma for 1 min (Diener Plasma Etch System). If electrodes were
pitch of 100 nm. High performance SWCNT circuits require a density of ∼50 SWCNTs/μm.9 Thus, further improvement is needed in this area as well. On the basis of our results, it might be expected that increasing the pitch could result in an increase in defects, but we believe this can be managed by a combination of controlling the thickness of the PEG layer (or possibly using a different passivation) and the SWCNT concentration in solution. It should also be pointed out that even if these remaining challenges toward the creation of complex circuits are not overcome, our technique still has potential applications in SWCNT-based sensors and transducers.
CONCLUSIONS We have presented a technique for the directed assembly from solution of DNA-wrapped SWCNT segments with uniform electronic properties onto lithographically patterned lines on a surface. Individual nanotube control was achieved with spacing as close as 100 nm. This is the first demonstration of such high resolution and high density ordered arrays of SWCNT with uniform and controlled electronic properties. Our directed assembly method was applied to the fabrication of single nanotube electronic devices, resulting in the expected electronic behavior for the types of nanotubes used in the assembly. We believe this work represents a major step toward the implementation of SWCNT complex circuits for future nanoelectronics technologies. MATERIALS AND METHODS Patterning of Hydrophilic Regions. Samples made of silicon with 300 nm of thermally grown silicon oxide and, in some cases, prepatterned gold electrodes, were passivated with a layer of poly(ethylene glycol) (PEG). Substrates were first cleaned in an aged (1.5 h old) piranha solution (3:1 H2SO4:H2O2) followed by a deionized (DI) water rinse, an ethanol rinse, and blown dry with an inert gas (Ar or N2). Samples were incubated for 48 h to 1 week to allow a uniform PEG layer to form in order to prevent nonspecific binding of DNA-wrapped SWCNTs. Longer incubation times resulted in better passivation. The dry samples were put in an UV−ozone cleaner for 5 min at 18 W, then immediately incubated in a solution of PEG-silane (2-[methoxy(polyethyleneoxy)propyl]trimethoxysilane, Mw = 460−590, Gelest) in anhydrous toluene (300 μL of PEG-silane in 30 mL of toluene), with 100 μM PEG-thiol (HS−(CH2)11−(C2H6O2)3−OH, Prochemia) if the substrates had prepatterned Au electrodes (PEG-silane passivated the SiO2 surface, whereas PEG-thiol passivated the Au). Acetic acid (30 μL for 30 mL of toluene-PEG solution) was added as a catalyst to the solution directly before immersing the samples. Samples were incubated for 48 h to 1 week to allow a uniform PEG layer to form so that it then successfully prevented nonspecific binding of DNA-wrapped SWCNTs. After passivation samples were rinsed with acetone and ethanol and blown dry with Ar. They were coated with 60 nm of PMMA (495 K, 2% in anisole, Microchem) and soft-baked for at least 30 min at 170 °C before being patterned by e-beam (Nanobeam nB4 electron beam lithography system). The pattern consisted of lines 200 nm long and 10 nm, 20 nm, 30 nm, and 40 nm thick. Development was done for 45 s in a 3:1 mixture of isopropyl alcohol (IPA) and MIBK kept at 4 °C with ultrasonic agitation (Branson 3510) and then for an additional 15 s in IPA at 4 °C with ultrasonic agitation. Samples were E
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ACS Nano
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patterned after PEG passivation and SWCNT assembly, hence on top of the SWCNT segments, prior resist spinning substrates did not receive any treatment (other than the washing and drying process done after the SWCNT assembly). In both cases, the first resist layer was soft-baked for 1 h at 155 °C before spinning the second layer; the second resist layer was soft-baked for 1 h at 170 °C before being patterned by e-beam (Nanobeam nB4 electron beam lithography system). Development was done in a 1:3 DI water:IPA solution, at room temperature, for 1 min. For samples without PEG and SWCNT, a short oxygen plasma (15 s, Diener Plasma Etch System) was performed after development, follow by the ebeam evaporation (Angstrom EvoVac Deposition System) of 1 nm of Ti followed by 50 nm of Au. Lift-off was done overnight in remover PG (Microchem). For samples with PEG passivation and assembled SWCNT segments, a stack of 0.8 nm of Ti, 15 nm of Pd, 35 nm of Au, was deposited directly after development by e-beam evaporation (Angstrom EvoVac Deposition System). In this case, lift-off was done overnight in acetone because NMP based solvents have been found to highly dope SWCNTs. Following lift-off, samples were annealed at 350 °C for 4 h in forming gas (Ar and H).
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b00353. Length distribution of mixed chirality SWCNT segments; SWCNT assembly on 30 nm wide hydrophilic lines; NIR fluorescence map of semiconducting SWCNTs; additional SEM and AFM images of the assembled SWCNT segments. (PDF)
AUTHOR INFORMATION Corresponding Author
*[email protected]. Present Addresses ∥
The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA; ⊥ Department of Chemistry and Biochemistry, School of Biological and Chemical Sciences, Queen Mary University of London, London, United Kingdom. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS The authors thank Profs. C. Nuckolls and M. Sheetz for resource support, as well as the staff and facilities of the Columbia Nano Initiative cleanroom, where much of the fabrication work was performed. The authors also gratefully acknowledge financial support from the Office of Naval Research under Award No. N00014-09-1-1117. REFERENCES (1) Shulaker, M. M.; Hills, G.; Patil, N.; Wei, H.; Chen, H. Y.; Wong, H. S.; Mitra, S. Carbon Nanotube Computer. Nature 2013, 501, 526− 530. (2) Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, P. Single- and Multi-Wall Carbon Nanotube Field-Effect Transistors. Appl. Phys. Lett. 1998, 73, 2447−2449. F
DOI: 10.1021/acsnano.6b00353 ACS Nano XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsnano.6b00353 ACS Nano XXXX, XXX, XXX−XXX
