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Large-scale Direct Patterning of Aligned Single-walled Carbon Nanotube Arrays using Dip-pen Nanolithography Jae-Hyeok Lee, Choolakadavil Khalid Najeeb, GwangHyeon Nam, Yonghun Shin, Jung-Hyurk Lim, and Jae-Ho Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01075 • Publication Date (Web): 04 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

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Chemistry of Materials

Large-scale Direct Patterning of Aligned Single-walled Carbon Nanotube Arrays using Dip-pen Nanolithography Jae-Hyeok Lee†*, Choolakadavil Khalid Najeeb‡, Gwang-Hyeon Nam‡ , Yonghun Shin§, Jung-Hyurk Lim∥, and Jae-Ho Kim‡* †

Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, 60208, USA Department of Molecular Science and Technology, Ajou University, Suwon, 443-749, Republic of Korea § Department of Electrical and Computer Engineering, Seoul National University, Seoul, 151-744, Republic of Korea ‡

∥Department

of Polymer Science and Technology, Korea National University of Transportation, Chungju 380-702, Republic

of Korea ABSTRACT: The strength of dip-pen nanolithography (DPN) is the ability to create nano- or micro-arrays of organic compounds and nanomaterials in a non-destructive and direct-write manner. However, transporting large-sized ink materials, such as carbon nanotubes (CNTs), has been a significant challenge. We report a direct-write patterning of aligned single-walled carbon nanotube (SWNT) arrays on silicon oxide using DPN. The patterned SWNT arrays show high degree of alignment and controllable width ranging from 2 µm down to 8 nm. Furthermore, field-effect transistors based on these SWNT arrays show p-type characteristic. High-throughput patterning of the aligned SWNTs over a large area was also achieved via polymer pen lithography (PPL). The reported technique will further expand the application of SWNTs to diverse nano-electronic devices.

INTRODUCTION Since their discovery in 1991, carbon nanotubes (CNTs) including single and multi-walled nanotubes (SWNTs and MWNTs) have drawn much attention as promising materials for a wide range of nanodevices such as field-effect transistors (FETs), field-emission display (FED), and chemical and bio sensors.1-5 For the fabrication of nanoelectronic devices, controlling the orientation of CNTs with precise alignment and desired architecture on various substrates is critical to exploit their unique structural, mechanical, electrical, and optical properties. Especially, high density and well-ordered nanotube arrays are required for electronic and photonic devices. 6 However, manipulating CNTs over large areas of diverse substrates with nanoscale accuracy has been challenging. For example, chemical vapor deposition (CVD) method has restricted substrate scope, because it requires high temperatures during the growth process.7 Mechanical stretching method was successfully used for the alignment of CNTs in polymer matrix, yet the presence of polymers limits its broad application to electronic devices.8,9 Additionally, crack-assisted CNT alignment over a large area was achieved via self-organization of the nanotubes during solvent evaporation. However, direction of the CNT alignment was uncontrollable using this method.10,11 To date, many efforts have been made to achieve properly aligned assembly of CNTs utilizing self-assembly,12-14 controlled capillary flow,15,16 Langmuir–Blodgett deposition,17-19

dielectrophoresis,20 applying magnetic fields,21,22 electrospinning,23,24 and dip-pen nanolithography (DPN) methods.11,25,26 Among those, the DPN method allows fabrication of highdensity arrays of nanomaterials. DPN is a lithography technique based on atomic force microscopy (AFM). DPN ink molecules are delivered from a scanning AFM probe tip onto a substrate through a water meniscus formed between the tip and the substrate.27 DPN-based patterning of various ink molecules, such as small organic molecules, deoxyribonucleic acids (DNA), proteins, viruses, bacteria, charged polymers, colloidal particles, metal ions, and sols, has been reported. 28-34 Several attempts have also been made to pattern CNTs on chemically modified substrates using indirect DPN methods.25,26,35,36 However, the reported techniques have limited substrate scope and require multiple pre-treatment steps on the substrate prior to the assembly of nanotubes. Given the advantage of direct DPN method (to produce highly ordered and precisely oriented arrays through a simple and efficient process), direct-write DPN patterning of CNTs can provide a powerful tool to fabricate CNT-based nanoelectronic devices. Developing such techniques is challenging due to the large size of CNTs and lack of driving forces that induce the deposition of CNTs onto the substrate. Here, we report a novel approach to pattern SWNTs on silicon oxide (SiO2) substrates at nano- and micro-scale using a direct-write DPN method. The patterned SWNTs show high density and high order of alignment with proper orientations. By combining the developed direct-write DPN method and photolithography, we successfully fabricated SWNT-based 1

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Figure 1. A schematic illustration of patterning SWNTs on SiO2 via direct-write DPN process using SWNT/SDBS/PEI composite ink.

FET device. Furthermore, large-area and high-throughput patterning of the SWNTs was achieved by a polymer pen lithography (PPL) technique. To our knowledge, this is the first to demonstrate the application of direct-write DPN patterned SWNTs to FET device and PPL.

EXPERIMENTAL SECTION Preparation of SWNT/DPN ink. SWNTs synthesized by arc-discharge method was used without further purification (95% purity, Hanwha Nanotech Co., Korea). 30 mg SWNTs was added to 10 mL aqueous solution containing 1% sodiumdodecylbenzene sufonate (SDBS, Aldrich). Then, the mixture was sonicated with a high power tip sonicator (75 W) for 30 minutes to disperse the SWNTs. The SWNT/SDBS solution was centrifuged in micro centrifuge tubes for 3 min to collect the dark black-colored supernatant. To each tube containing 1 mL of homogeneously dispersed SWNTs/SDBS solution, was added different volumes (0.1 ~ 0.5 mL) of polyethyleneimine (PEI, Aldrich, Mn ca. 423), followed by ultrasonication for 30 min at room temperature to prepare DPN ink solutions with different viscosities. We found that 1 : 0.2 v/v ratio of SWNT/SDBS solution and PEI resulted in the optimal viscosity and CNT mass fraction for the efficient patterning of SWNTs using DPN methods. Figure S1 illustrates the absorption of SWNT/SDBS/PEI composite ink (SWNT/DPN ink) onto the AFM probe. DPN Patterning and Imaging. Silicon AFM probe (M2N Inc., Korea, spring constant: 40 N/m, model: STP4) was treated with UV-ozone for 15 min (UVO cleaner model 42-220, Jelight Co.). Then, the probe was soaked in 50 μL SWNT/DPN ink solution for 30 min and dried under ambient temperatures (25 ± 2 ºC). All DPN patterning and imaging experiments described herein were conducted with the XE-100 AFM system (Park systems Inc., Suwon, Korea) under ambient conditions (15~30% relative humidity at room temperature (23~24 ºC)). Contact time (3 sec), scan speed (0.1 μm/s), and z-axis extension between the AFM probe and the substrate were controlled by the lithography software (XEL, Park Systems Inc., Korea).34 Polymer Removal by Chemical Vapor Etching Method. After the patterning of SWNTs/DPN ink, the SiO2 substrate was placed on top of a glass container with the SWNT patterned surface facing down. To the surface, vaporized methanol (100 mL) was slowly applied at room temperature for 3 h. Through this process, we removed PEI polymers from the patterned SWNTs/DPN ink composite.

FET Measurement. To investigate the electronic transport properties of the DPN patterned SWNTs, we fabricated SWNTs based FET (SWNT/FET) onto a 300 nm-thick SiO2/Si substrate using a conventional photolithography technique. This device has a channel length of 23 µm and a dielectric thickness of 300 nm with the under-lying heavily doped p-type Si as the back gate. The SWNTs were contacted with 100 nm thick gold (Au) electrodes. The device performance was measured using a Keithly 2612 semiconductor parameter analyzer. Equation was used to calculate carrier mobility:

eff 

Lg d WCoxVDS

where µ eff is the field-effect mobility, L is the channel length (obtained from scanning electron microscopy (SEM) image), gd is the transconductance, W is the channel width (obtained from SEM image), Cox is the oxide capacitance (11 nF·cm-2 for 300 nm thick SiO2), and VDS is the applied source-drain bias.

Characterization and Instrumentations. Characterization of the SWNTs/DPN ink was conducted by SEM (LEO Supra 55, Carl Zeiss SMT, Germany). The Fourier transforminfrared (FT-IR) spectra of PEI, SWNT/SDBS dispersion, and SWNT/DPN ink were recorded on a Nicolet OMNIC 6700 FT-IR spectrometer (Thermo scientific, USA). Raman spectra were recorded on a Dongwoo 320i instrument (Andor™ Technology, USA) using 785 nm diode laser beams with an acquisition time of 1 sec. The polarized micro-Raman spectra were obtained using an Ar+ laser at 514.5 nm as the excitation source (Renishaw inVia). All AFM images were obtained in tapping mode on a XE-100 AFM system (Park systems Inc., Suwon, Korea) under ambient air conditions. Conventional Si cantilevers (M2N, Inc., Korea, model; STP4) with typical spring constant (40 N/m) and resonance frequency (280~310 kHz) were used in the experiments.

RESULTS AND DISCUSSION Our effective patterning of SWNTs using a direct-write DPN method relies on three key strategies: 1) We used highly concentrated mono-dispersed SWNTs in the composite ink. 2) Polyethyleneimine was used as a carrier polymer to give a proper viscosity and hydrophilicity to the DPN ink. 3) We increased the wettability of the AFM probe surface by treating the probe with UV-ozone. Overall, a schematic representation of patterning SWNTs on SiO2 substrate via the direct-write DPN method using our unique ink composite is illustrated in Figure 1. 2


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Figure 2. Characterization of SWNT/DPN ink. (a) FI-IR spectra of PEI (black dotted line) and SWNT/DPN ink (red solid line). (b) Raman spectra of PEI (black dotted line), SWNT/DPN ink (red solid line) and SWNT/SDBS dispersion (black solid line). (c) SEM and (d) AFM image of SWNT/DPN ink deposited onto Si substrates. The inset in (c) shows a photography of the SWNT/DPN ink. (e) Height profile corresponding to the SWNTs across the blue line shown in the AFM image (d).

Mono-dispersion of highly concentrated SWNTs (3 mg/mL) in aquatic solution was achieved by using SDBS as an anionic surfactant. Subsequent addition of the cationic carrier polymer PEI to the SWNT solution elevated viscosity of the ink solution, which increased the loading amount of the ink onto the AFM tip. The elevated viscosity also served as a driving force for the deposition of the ink molecules to the substrate during DPN. It is important to note that the cationic PEI interacts with the anionic SDBS coated on SWNTs, resulting in the codeposition of SWNTs with the polymers. When nonionic polymers with high viscosity such as PEG and glycerol were used, only the polymers were transferred to the substrate without the SWNTs, highlighting the importance of using cationic matrix polymer. Furthermore, the electrostatic interaction between the cationic PEI in the SWNTs ink and the negatively charged SiO2 substrate may provide an additional driving force for the deposition. Finally, irradiation of the AFM probe with UVozone increases the hydrophilicity of the probe. This, in turn, increases wettability of the probe and thus the loading amount of the ink onto tip (Figure S1).37,38 In agreement with this, UVozone non-treated AFM probes showed inefficient inking of the DPN ink due to the low wettability (data not shown). Initially, we characterized the composition of our SWNT/DPN ink by FT-IR and Raman spectroscopy. The appearance of broad and weak absorption bands, which were originally sharp, indicated the interaction between the organic molecules (i.e. SDBS and PEI) and SWNTs (Figure 2a). A decrease in the peak intensity was observed due to the weak vibration energy of SWNTs. The interaction between SWNTs and PEI was further confirmed by the small shift of peaks to higher wave number. In the Raman spectra of the SWNT/SDBS solution, a weak diameter-dependent radial breathing mode (RBM band) at ca. 240 cm-1 as well as a strong tangential mode band (G-band) at ca.1573 cm-1 represented the presence of the nanotubes (Figure 2b). When PEI was added to the SDBS coated SWNTs solution, the width of the G band increased and the characteristic peaks of PEI

Figure 3. Characterization of the AFM probe loaded with SWNT/DPN ink. (a, b and c) SEM images of the AFM probe were taken after immersing the probe in the SWNT/DPN ink solution for 30 min. Inset in (b): an optical microscopy image of the AFM probe loaded with SWNT/DPN ink (d) Confocal Raman measurement of the marked area in (c) confirms the presence of SWNTs on the AFM probe. Note the typical Raman peaks of SWNTs: the disorder induced D-band (1350 cm−1) and tangential G-band (1590 cm−1).

appeared in the Raman spectra. Physical morphology of our SWNT/DPN ink composite was characterized by microscopic analyses. The SEM images of SWNT/DPN ink indicated that the nanotubes were homogeneously dispersed and exfoliated into smaller bundles (Figure 2c). Also, the originally flexible nanotubes converted to rigid rodlike structures after the addition of PEI. Such rigidity was important to efficiently pattern well-aligned SWNTs. The AFM images in Figure 2d further confirmed that the nanotubes were individually dispersed. The line profile indicated that the bundle diameter of SWNTs was 1.7 ± 0.2 nm (Figure 2e). Figure 3 shows that typical SEM images and Raman peak of SWNT coated AFM probe after immersing the UV-ozone treated AFM probe in the SWNT/DPN ink solution. Following the thorough characterization of our SWNT/DPN ink, direct-write DPN was conducted to fabricate hierarchically aligned SWNT arrays using the ink. We successfully deposited the ink from the AFM probe tip onto the substrate at relatively low humidity conditions (15~30%). Subsequent to the DPN patterning of the ink, excess PEI polymer was removed by methanol vapor, whereas the nanotubes remained on the substrate. Next, we evaluated the DPN pattered nanotubes. The SEM image showed that the SWNTs were arranged with a high degree of alignment parallel to the writing direction (Figure 4a). A polarized Raman spectroscopy is a powerful technique for alignment characterization because well-aligned SWNTs show strong anisotropic absorption of polarized radiation. More specifically, the intensity of the tangential Raman band (G-band) of the carbon atoms in the nanotube at 1580 cm-1 reaches the maximum when the tube axis is parallel to the polarization, and the intensity of the G-band is the minimum when the tube axis is perpendicular.39 To assess the degree of alignment, we collected the polarization-dependent Raman spectra of our DPN patterned 3



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Figure 4. SEM and AFM images of the SWNT arrays patterned via the direct-write DPN method. (a) SEM image of line patterned SWNTs on a silicon oxide substrate. (b) Polarized micro-Raman spectroscopy data of the SWNTs. (c) Representative images of the patterned SWNTs with controlled line width. (d) AFM height profile of the green line marked in (e). (e) A representative image of the patterned SWNTs with the minimum line with. When AFM tip conversion effect was considered, the line width (full width at half maximum) was 8 nm.

SWNTs using co-polarized and cross-polarized directions with respect to the SWNTs (Figure 4b). The G-band intensity ratio between the maximum and minimum values was 7. This result indicated that our DPN patterned SWNTs possess strikingly high degree of alignment in comparison with nanotubes prepared via self-assembly method40 which showed the maximum to the minimum intensity ratio of ~3.7. Finally, we were able to control the line width of the SWNT arrays from 2 μm down to 8 nm by adjusting the contact forces (~1200 nN to ~50 nN) of the AFM tip34 (Figure 4c). It is especially remarkable that we patterned SWNTs with an average height of 1.4 nm and a width of 8 nm (Figure 4d,e). Subsequently, we investigated the electrical property of the DPN patterned SWNTs using a semiconducting analyzer measurement system. For this purpose, SWNTs were patterned in between predefined gold source and drain electrodes (Figure 5a,b). The conductivity of the patterned SWNTs reached 1.4 × 103 S/m, and the current-voltage (IDS-VDS) curve showed quasi-linear characteristic (Figure 5c), which indicated the ohmic contact between the gold electrodes and SWNTs. We further studied electron transport property of the aligned SWNTs by fabricating FET. Insulating SiO2 is one of the essential substrates to make FET device, and our DPN method enabled direct patterning of SWNTs on top of 300 nm SiO2 with the underlying heavily doped p-type Si as the back gate. Interestingly, during the micro-electrode fabrication process, we observed that the SWNTs remained intact after the through washing processes with acetone, methanol and deionized water (Figure S2). As depicted in Figure 5d, the current of the SWNTs decreased when the gate voltage increased from - 4 V to + 4 V. According to the transfer and output curve, the hole mobility was 4.68 cm2/Vs, and the Ion/Ioff ratio was approximately 7 (applied bias voltage = 2 V, Figure 5e). This device showed a low on/off current ratio due to a mixture of metallic

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Figure 5. Electrical property of the DPN patterned SWNTs. (a) Optical image of a gold source and drain electrode with SWNTs patterned by DPN method. (b) SEM and AFM images of the SWNTs patterned electrode. (c) IDS-VDS curve of the SWNTsbased device. (d) IDS-VDS curves at different gate voltages (e) IDSVG curve of the DPN patterned SWNTs at a bias voltage of 2 V between source and drain electrodes. The electrical characteristics exhibit p-type FET behaviors.

and semiconducting nanotubes.41 The charge screening effect of the metallic SWNTs usually makes the devices unable to be turned off by gate.42 Altogether, we confirmed that our DPN patterned SWNT/FET device has a p-type characteristic. PEI is one of well-known n-type chemical dopant for SWNT FET.43 It is important to note that the p-type FET behavior of our device is an evidence of clear PEI removal on the FET. To the best of our knowledge, this is the first report on the SWNT-based FET device prepared via direct-write DPN. Also, efficiency of our method was demonstrated by patterning multiple arrays of SWNTs in several circuits across gap electrodes after a single inking (Figure S3). Finally, high-throughput and large-area patterning of the SWNTs using the developed ink composite was demonstrated. To this end, we applied PPL technique because the PPL has been shown effective in high-throughput fabrication of various bio and nanomaterials.44,45 A 1 cm2 polymer pen array having ~90,000 elastomeric polydimethylsiloxane (PDMS) tips with 50 μm separation was prepared using a silicon mold (Intaglio pyramid structure, Figure 6a,b). Then, the PDMS tips were treated with UV-ozone for 15 min to generate hydrophilic surfaces. 40 μL of our SWNT/DPN ink was dropped onto the UV-ozone treated PDMS tips and incubated for 10 min. Subsequently, excess of the ink was removed by spin coating (500 rpm, 20 sec). The PDMS tips coated with the SWNT ink was then attached to an AFM probe hand (XE 100, park systems Inc., Korea) that has a z-axis extension piezo scanner. With the contact time of 3 seconds and scan speed of 0.1 μm/s between line patterns, we were able to pattern uniform dots and lines of SWNTs on a SiO2 substrate (Figure 6c,d). One of the patterned dots showed a height of 6.4 nm and width of 8 μm. We also confirmed that the patterned SWNT lines were unidirectional, and the height was 22 nm. The overall results highlighted the advantage of the developed technique to rapidly pattern highly ordered SWNTs in large-scales with low-cost.

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Figure 6. High-throughput patterning of SWNTs dots and lines was achieved via PPL technique. (a) A schematic illustration of PPL-based direct patterning of SWNTs. (b) SEM images of pyramid-shaped PDMS tip. (c) SWNTs dot patterns confirmed by confocal and AFM images. Line profile of the dot patterned SWNT is shown at the bottom. (d) SWNTs line patterns confirmed by confocal and AFM images. Line profile is shown at the bottom. * [email protected] * [email protected]

CONCLUSION In this work, we demonstrated the nano-patterning of hierarchically aligned SWNTs on SiO2 substrates using a directwrite DPN technique. The use of anionic SDBS surfactant and cationic PEI polymer was the key to prepare SWNT/DPN ink with high SWNT mass fraction, viscosity, and hydrophilicity. Such properties facilitated the delivery of the nanotube ink to the substrate during the direct-write DPN process. Irradiation of the AFM tips with UV-ozone further assisted the inking process and the deposition of the ink from the tips. The electrical characterization of the SWNT patterns showed p-type semiconducting behavior. The controlled patterning of semiconducting SWNTs offers new possibilities to develop SWNTs-based electronic devices. Finally, the massive patterning of the SWNTs over a large area was demonstrated using PPL technique. We believe that the high-throughput patterning of SWNTs using PPL has not been reported before. We suggest that a use of highly enriched semiconducting SWNTs sorted via density-gradient ultracentrifugation (DGU)46 and chromatography separation methods47 will give a higher hole mobility and on/off current ratio of the SWNTs, enhancing the FET property of DPN patterned SWNTs/FET devices. Our direct-write DPN strategies will be applicable to other large-sized substances including 2D materials48 such as graphene (a conductor), MoS2 (a n-type semiconductor), WSe2 (a p-type semiconductor), and boron nitride (an insulator).

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2009-0093826). In addition, we thank the BK21 Program of Molecular Science & Technology at Ajou University and the Materials Research Science & Engineering Center (MRSEC) at Northwestern University.

ASSOCIATED CONTENT Supporting Information. Supporting Information is available free of charge on the ACS Publications website at DOI: Additional experimental procedure for DPN inking, Stepwise evaluation of DPN patterned SWNT device (optical and SEM images), and multi-arrays of DPN patterned SWNTs (PDF).

AUTHOR INFORMATION Corresponding Author

Notes The authors declare no competing financial interest.

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SYNOPSIS TOC. We report a direct-write patterning of aligned single-walled carbon nanotube (SWNT) arrays on silicon oxide using dip-pen nanolithography (DPN). The patterned SWNT arrays show high degree of alignment and controllable width ranging from 2 µm down to 8 nm. Furthermore, field-effect transistors based on these SWNT arrays show p-type characteristic. High-throughput patterning of the aligned SWNTs over a large area was achieved via polymer pen lithography. The reported technique will further expand the application of SWNTs to diverse nano-electronic devices.

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