Large-area fluidic assembly of single-walled carbon nanotubes through dip-coating and directional evaporation
© The Author(s) 2017
Received: 7 December 2016
Accepted: 15 March 2017
Published: 20 March 2017
We present a simple and scalable fluidic-assembly approach, in which bundles of single-walled carbon nanotubes (SWCNTs) are selectively aligned and deposited by directionally controlled dip-coating and solvent evaporation processes. The patterned surface with alternating regions of hydrophobic polydimethyl siloxane (PDMS) (height ~ 100 nm) strips and hydrophilic SiO2 substrate was withdrawn vertically at a constant speed (~3 mm/min) from a solution bath containing SWCNTs (~0.1 mg/ml), allowing for directional evaporation and subsequent selective deposition of nanotube bundles along the edges of horizontally aligned PDMS strips. In addition, the fluidic assembly was applied to fabricate a field effect transistor (FET) with highly oriented SWCNTs, which demonstrate significantly higher current density as well as high turn-off ratio (T/O ratio ~ 100) as compared to that with randomly distributed carbon nanotube bundles (T/O ratio ~ <10).
Controlled positioning of nanomaterials (e.g., nanotubes, nanowires, and nanoparticles) is an essential process for advances in nanoelectrical devices and photonic system [1–4]. In particular, selective and addressable deposition of SWCNTs is important for high-performance CNT based-electronic devices such as light-emitting diode, flexible display, conductive film, and transistor [5–10]. Therefore, extensive efforts have been made towards integration of individual SWCNTs into microscale devices with particular emphasis on efficiency and scalability. Several methods have been proposed to align SWCNTs, networks, or films using various principles including solution/flow based patterning [11–15], and patterned catalytic growth [16, 17]. There still remain, however, substantial challenges that need to be overcome in order to achieve a large-area assembly in a simple, cost-effective, and reproducible manner.
A typical bottom-up approach can be found in chemical vapor deposition (CVD) process, in which CNTs are grown on specific locations with the aid of local heating or catalytic activation [18, 19]. Although the CVD process allows for a precise assembly of nanomaterials for CNT-based electronic or flexible devices in a geometry-controllable fashion, such bottom-up approach is inherently facing challenges such as low density, high thermal budget, and high costs. Alternatively, a solution-based top-down approach has been introduced for scalable SWCNTs-based integrated device, which exploits site-selective attachment of SWCNTs onto patterned self-assembled monolayers (SAMs) formed via microcontact printing or dip-pen nanolithography [20–23]. The technique offers a high-throughput, low-expertise route to assembling SWCNTs without an external stimulus (e.g., electric field or local heating). Nonetheless, it is difficult or slow to attain uniform SAM patterns at nanoscale resolution that are needed for large-scale integration and precise positioning. It is also problematic that the SAM patterns are prone to contamination or undesirable chemical reaction during solution-phase coating process. Moreover, the nanotubes are usually covalently-functionalized with chemical groups to interact with a predefined SAM pattern [1, 24]. In this case, the functionalized nanotubes located at defect sites have limited utility for electrical applications such as transistor and semiconducting device, since the nanotubes may not preserve the sp2 carbon structure and thus their electronic characteristics [25, 26].
In this work, we demonstrate a simple fluidic-assembly method for highly oriented SWCNTs patterns, using a dip-coating process onto a topographically heterogeneous interface of alternating non-polar (hydrophobic) PDMS strips on the polar (hydrophilic) SiO2 substrate. Well-defined PDMS nano-strips were created by solvent-assisted decal transfer lithography (sa-DTL) by utilizing irreversible bonding and anisotropic swelling . The transferred PDMS skin layer acts as a non-wetting region against non-specific deposition of SWCNTs and induces a spontaneous, large-area assembly after selective wetting and drying. One important aspect of the current strategy, as compared to previous chemical patterning methods , is the use of a hydrophobic, uniformly defined physical structure (~100 nm height) instead of chemically patterned monolayer or multilayer of SAMs. Such physically defined patterns exhibit chemical and mechanical stability for a long time. Another key aspect is the introduction of a dip-coating process as a scalable and reproducible deposition method. Unlike previous deposition processes such as direct dipping and spin coating, the dip-coating process allows us to achieve precise and selective integration of highly aligned SWCNTs onto hydrophilic locations of a topographically modified surface. As described shortly, the positioning process was width-dependent, such that the alignment accuracy was increased with decreasing the pattern width below ~1 μm. We further demonstrate that the aligned arrays of SWCNTs can be used for a high-performance SWCNTs-FET device with high current density and turn-off ratio.
Preparation of PDMS stamps
PDMS stamps were prepared by replica molding from silicon masters that had been prepared by photolithography. A mixture of base and curing agent (10:1 w/w) of Sylgard 184 silicone elastomer was poured onto the patterned masters and cured at 70 °C for 2 h.
Solvent-assisted decal transfer lithography (sa-DTL)
First, both PDMS stamp and substrate were treated by short O2 plasma treatment (200 mTorr, 60 W, 20 s) for irreversible bonding. Immediately after the plasma treatment, the PDMS stamp was carefully placed on the substrate. The assembly was subsequently immersed in polar solvents such as THF or 1,2-dichlorobenzene (1.2-DCB) for 1 min.
Preparation of SWCNTs colloidal solution
In order to obtain uniform SWCNTs, it is necessary to prepare a well dispersed colloidal solution. We conducted the following treatment for purification and dispersion. SWCNTs (ASP-100F produced by Il-jin Nanotech, Korea) were stirred in nitric acid solution at 50 °C for 30 min to purify and simultaneously exfoliate from bundles. Then SWCNTs were neutralized with deionized (DI) water and trapped on a membrane filter (Millipore, 0.2 μm pore size, 47 mm diameter) using vacuum filtration. The SWCNTs on the filter were dried in a vacuum oven chamber at 80 °C for 48 h. We used 1,2-dichlorobenzene (1,2-DCB) as a solvent which is one of the well-known mediums appropriate for dissolution of SWCNTs 34-35 with the concentration of 0.1 mg/ml. Finally, an ultrasonication process was performed for 20 h.
Dip coating was performed by using a custom-built dip coater with controllable withdrawal velocity. The step motor was mounted on the dip coater to allow movement of the platform upward or downward with a controlled speed, and the manipulator controlled the position of the substrate. The PDMS patterned substrate was held on the hanger by a holder and the beaker filled with SWCNTs colloidal solution was set on the platform. The substrate was immersed into the prepared SWCNTs colloidal solution with the concentration of 0.1 mg/ml, and slowly pulled out at the constant withdrawal velocity of 3 mm/min.
Device fabrication and characterization
To evaluate electrical characteristics of fluidic assembled SWCNTs, we fabricated a field effect transistor with two Au electrodes (source/drain) and a back-gate. A conventional lift-off process was used to form the electrodes on the gate-oxide. A highly conductive 4 in. P-type silicon substrate (100) resistivity: 3–6 Ωcm was used as the back-gate with a gate-oxide thickness of 10 nm. The channel length (Lc) between the electrodes was varied from 2 to 20 μm with an interval of 2 μm (ten different spacing in total). I–V characteristics and gating effects of the devices were measured using Agilent 4156C parameter analyzer.
Scanning electron microscopy (SEM)
Images were taken using high-resolution SEM (S4800, Hitachi, Japan) at an acceleration voltage higher than 5 kV.
Atomic force microscopy (AFM)
AFM measurements were performed using a commercial AFM (NanoScope IV MultiMode AFM, Veeco Metrology LLC, Santa Barbara, CA). The scan rate was 0.5 Hz and 256 lines were scanned per sample. Tapping mode tips, OMCL_AC240TM-B2 with spring constant 0.9–2.2 mN, were obtained from OLY0MPUS (Japan). Data were processed using Nanoscope III 4.31r6 software (Veeco Instruments Inc.)
Results and discussion
Formation of stripe-patterned substrate and aligned bundles of SWCNTs
In the dip-coating process presented here, the polarity difference between PDMS and SiO2 regions can induce selective localization of SWCNTs along the strip direction. Specifically, when such a surface is drawn vertically from the SWCNTs solution, a thin meniscus is formed selectively on the hydrophilic SiO2 substrate at the three phase contact line (solid–liquid–vapor interface) with a contact angle θ (Fig. 1b). As the pulling velocity is sufficiently slow, the solvent starts to evaporate mostly from the pattern edges, leading to convective transport of the SWCNTs that is similar to the well-known coffee-ring flow . In addition, the confined geometry of a topographically patterned surface strongly influences the direction of evaporation, so that the liquid layer dries and forms aligned bundles of SWCNTs along the nano-strips . This directional evaporation significantly enhances align accuracy and pattern fidelity as compared to other solution-based methods (e.g., spin-coating, dipping). Recently, this edge pinning and drying has been successfully applied to align carbon nanotubes films  and assemble nanoparticles on pre-patterned surfaces .
Construction of phase map on the alignment morphology
Fabrication of SWCNTs-FET device
In this paper, we have presented a simple yet widely applicable fluidic assembly method by utilizing dip coating and directional evaporation. The topographically patterned surface in the form of alternating hydrophobic PDMS strips on the hydrophilic SiO2 substrate was used to provide a precise control over selective wetting and drying along the strip direction, completing the deposition of highly aligned bundles of SWCNTs. The method involves a low-expertise and scalable dip coating process and does not require additional surface modification or external stimulus. Furthermore, the simple FET device with aligned nanotube bundles demonstrated significantly higher current density and high turn-off ratio (T/O ratio ~ 100) as compared to that with randomly distributed bundles (T/O ratio ~ <10). The same approach could be applied to obtain highly oriented nanomaterial arrays including nanowires or nanoparticles.
PK and TJK conducted the experiments, and analysed the results. Both authors finalized the drafted manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
This research was supported from the National Research Foundation of Korea (NRF) (Grants NRF-2015H1A2A1030560 and NRF-2014M3C1B2048201). This work was also supported by INHA university Research Grant (INHA-54472).
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