Photopatterned Single-Walled Carbon Nanotube Films Utilizing the

Oct 10, 2016 - First, a SWNT/photofunctional dispersant film is prepared on a substrate by casting, using SWNT aqueous dispersions (0.035 wt %, weight...
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Photopatterned Single-Walled Carbon Nanotube Films Utilizing the Adsorption/Desorption Processes of Photofunctional Dispersants Yoko Matsuzawa,* Yuko Takada, Hirokuni Jintoku, Hideyuki Kihara, and Masaru Yoshida* Research Institute for Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5-2 1-1-1 Higashi Tsukuba 305-8565 Japan S Supporting Information *

ABSTRACT: We describe the application of photodetachable and recyclable dispersants for single-walled carbon nanotubes (SWNTs) in the fabrication of photopatterned SWNT thin films. Because adsorption and desorption of the dispersants on the SWNT surfaces affect not only their dispersibility in water but also their solubility, SWNT photopatterns were obtained on glass substrates in only three steps, i.e., casting the SWNT/ dispersant solution, UV-light exposure of the casted SWNT/ dispersant films through a photomask, and subsequent rinsing with neutral water. This patterning procedure is simple and scalable and will enable us to prepare microfabricated SWNT thin films. KEYWORDS: carbon nanotubes, patterned thin films, photoisomerization, surfaces, adsorption/desorption atterned films of single-walled carbon nanotubes (SWNTs) have been employed in a variety of electronics applications,1 including capacitors,2,3 field-emitting transistors (FETs),4,5 and electronic junctions.6,7 The efficient preparation of such films for SWNT-based devices requires precise patterning over controlled areas using a simple and reliable method. Several processes including chemical vapor deposition,8 growth patterning,9 microfluidic patterning,10 contact transfer,11,12 chemically anchored deposition,13,14 various etching procedures,15,16 and inkjet printing17,18 have been developed for the fabrication of well-patterned SWNT films. In particular, wet processes that utilize SWNT dispersions as inks are quite promising to simplify the manufacturing scheme, in contrast to dry processes that usually necessitate plasma-etching systems, harmful alkaline developers, and other complex procedures. To improve these wet processes, readily dispersible SWNTs and tunable dispersibility will be important keys for the successful fabrication of patterned SWNT films because the film properties are strongly influenced by the dispersibility of the individual SWNTs. The facile preparation of coating films and patterns is also essential to simplifying the whole patterning process. We have developed a photofunctional dispersant that can modulate the dispersibility of SWNTs in water using UVlight irradiation (SB4 in Figure 1a).19 The molecular structure of SB4, comprising a stilbene core unit, benzamide linker groups, and terminal ammonium groups, is suitable for adsorption on the surface of the SWNTs through π−π-stacking interactions. SWNTs “wrapped” with SB4 exhibit high solubility in water. Upon light exposure, the molecular structure of SB4 changes from planar to bent, reducing the adsorption ability of SB4 on the SWNT surfaces because of steric hindrance. Consequently, the “unwrapped” SWNTs lose their

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solubility and form aggregates and precipitates. We have previously confirmed complete detachment of the dispersants after the photoreaction without any structural decomposition of the SWNT surfaces by thermogravimetric analysis and Raman spectroscopy of the SWNT precipitates.20 Recently, investigations on stimuli-responsive materials have extensively been performed, where photoinduced functions of thin films composed of nanofibers or nanofilms with photochromic molecules have been attracting attention as smart materials.21,22 Inspired by the controllable dispersibility of the SWNTs (i.e., their change in water solubility due to photoinduced adsorption/desorption of the dispersant), we demonstrate a “smart” approach for the preparation of patterned SWNT films using a photopatterning process. The patterning procedure in this study is shown in Figure 1b. First, a SWNT/photofunctional dispersant film is prepared on a substrate by casting, using SWNT aqueous dispersions (0.035 wt %, weight concentration calculated by the mixing ratio of SWCNT and water) as inks (i). The film is exposed to UV light (385 nm) with a photomask (ii). UV-light irradiation causes a structural change of the dispersant in the film state that allows the reacted dispersant to be removed by rinsing with water (iii), whereas the unexposed regions composed of SWNTs with unreacted dispersants also remain well soluble in water. After rinsing with water (iv), only the photopatterned SWNT films without dispersants stay on the substrate as negative patterns. The structure of the stilbene-core photofunctional dispersant SB4 is shown in Figure 1a. After photoisomerization, the cisReceived: May 24, 2016 Accepted: October 10, 2016

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DOI: 10.1021/acsami.6b06169 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Chemical structures of photofunctional dispersants and their transformed structures. (b) Illustration of our concept for the fabrication of patterned SWNT films.

reduces the thermal stability of cis-AB2 and accelerates the thermal back reaction from cis to trans.26 Consequently, the observed value of the thermal reversion rate kc−t for cis-AB2 is ca. 4 × 10−3 s−1, which is 100 times faster than that observed for ordinary azobenzenes.27 Unfortunately, the thermal stability of cis-AB2 is so low that NMR and high-performance liquid chromatography measurements to determine the cis content in the photostationary state could not be carried out. To evaluate the dispersibility properties of AB2, a SWNT dispersion prepared with AB2 by sonication and a subsequent ultracentrifugation was subjected to two-dimensional photoluminescence (2D PL) mapping, ultraviolet−visible−nearinfrared (UV−vis−NIR) absorption spectroscopy, and atomic force microscopy (AFM) imaging (Figures S4 and S5, SI). As shown in Figure S4, the presence of high-intensity peaks was observed in the 2D PL mapping, and also in the NIR spectrum, well-defined absorption peaks were confirmed to be present. Because aggregation of SWNTs into bundles quenches PL and broadens the NIR absorption peaks,28 the spectroscopic results clearly suggest that the dispersibility of AB2 is as high as that of SB4, resulting in isolated SWNTs from the bundles.19 An AFM image also indicates that SWNTs are present as single tubes because the height profiles from cross-sectional analysis of the AMF image show the sizes of the diameters of SWNTs. We noticed that an excess amount of free dispersant in the solution disturbed the photoreaction of those attached on the SWNT surfaces through a filtering effect.20 Therefore, after removal of excess AB2 from the dispersion by dialysis (Figure S6, SI), exposure to UV light with smooth stirring caused precipitation of the SWNTs (Figure 2). According to our previous results using SB4 as the photodetachable dispersant,19,20 AB2 also detaches from the SWNT surface in the same manner as SB4 does. It is notable that precipitation is

stilbene unit is converted to a phenanthrene one through cyclization and oxidation steps. There are several unique properties with this stilbene-type dispersant: (1) it is possible to disperse the SWNTs by adding relatively small amounts (at least 3 mg of SB4 to 1 mg of SWNT in 3 mL of deionized water) of SB4, in comparison to ordinary surfactants such as sodium deoxycholate (DOC) and sodium dodecyl sulfate (SDS).20 (2) Upon UV-light exposure, the photoreacted dispersants are desorbed from the surfaces of the SWNTs in a stirred solution without any other treatments. However, SB4 is not without flaws: control of the SWNT dispersibility takes a long time (more than several hours) in solution. Furthermore, the stilbene-type dispersant is not recyclable because the solubility change is based on irreversible reactions. To circumvent these issues, we designed and synthesized a dispersant in which the stilbene moiety is replaced by an azobenzene motif with reversible photoisomerization reactivity (AB2; Figure 1a). Although several studies on SWNT dispersions using azobenzene-containing dispersants have been reported,23−25 their application to the photopatterning of SWNT films has not yet been investigated. First, we evaluated the dispersibility of SWNTs using AB2 as an alternative dispersant. Then, we examined the patterning process utilizing adsorption and desorption control by both of the proposed photofunctional dispersants (AB2 and SB4) on the surfaces of the SWNTs. AB2 was synthesized using 4,4′-diaminoazobenzene instead of 4,4′-diaminostilbene (Figure S1, Supporting Information, SI). In aqueous solution, AB2 undergoes trans−cis photoisomerization upon UV-light irradiation (Figure S3, SI). After several minutes in darkness, cis-AB2 immediately reverts to trans-AB2. It is known that substitution of electron-donating groups at both positions para to the azo moiety significantly B

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amine groups. The molecular interaction between the stilbene units of SB4 in the solid film is thus expected to be weak, resulting in a small blue shift of λmax (6 nm). In the case of AB2, the spectral change observed in the film should be similar to that for SB4 (9 nm blue shift of λmax). The broadening of the absorption bands may be due to the differences in the microenvironmental conditions of the chromophores.29 Upon UV-light exposure, the photoreactions of both SB4 and AB2 progress in the films. For SB4, the absorption band at 350 nm eventually disappears after 30 min of UV-light exposure, and an alternative band appears at around 280 nm due to the phenanthrene moiety, although we cannot yet explain why the apparent photoreaction time of SB4 is drastically reduced from the order of hours in the solution phase to 30 min in the solid state (Figure S8, SI). The absorption maximum at 360 nm in the AB2 film also decreases with light exposure and reaches a photostationary state after 30 min, in line with the photoreactivity of AB2 in solution. After the solution is left overnight in darkness, the initial absorbance at around 360 nm is partially recovered because of thermal back-reaction (cis−trans isomerization) of AB2. Hybrid films composed of SWNTs and the photoresponsive dispersants were prepared by casting the dispersions on fusedsilica substrates (see the SI). As shown in Figure 4a, monitoring of the absorption peak of SB4 in the films indicates the decrement of the π−π* absorption band at around 370 nm due to the photoreaction of the dispersants with UV-light exposure for 1 h. On the other hand, there are no significant changes in the absorption peaks of the SWNTs in the NIR region. This is promising the result that only the photoreacted dispersant with the phenanthrene structure can be desorbed from the SWNT surfaces by rinsing with water. The desorption behavior of the dispersants in the hybrid films was monitored by following the change in absorbance at around 280 nm. It was found that removal of the dispersant is achieved by washing the films with distilled water, while the absorption bands of the SWNTs in the NIR region are still observable. The spectral changes in the hybrid film composed of AB2 and SWNTs are similar to those observed for SB4 (Figure 4c). The peak due to the π−π* absorption band of AB2 decreases with UV-light exposure and diminishes after rinsing with water, whereas the absorption peaks due to the SWNTs are mostly unchanged. In contrast to the results for the exposed films, the hybrid films without UV-light exposure are easily dissolved in water. After immersion in distilled water for several minutes, the hybrid films are washed away from the substrates. As shown in Figure 4b,d, not only the absorption bands of the dispersants but also those of the SWNTs completely disappear in the UV− vis−NIR spectra. Thus, the photoreaction of the dispersants in

Figure 2. UV−vis−NIR absorption spectra (black, before trans−cis photoisomerization; blue, after trans−cis photoirradiation; red, after cis−trans thermal isomerization), photographs, and schematic diagrams indicating dispersibility changes of SWNTs by the reversible structural change of AB2.

complete within 90 min, although 3 h is required for the appearance of photoinduced precipitation in the SB4/SWNTs system with the same light source. We also observed, after allowing the solution with SWNT precipitates to stand for a few minutes at room temperature, that the SWNTs could be redispersed in water by applying sonication. The reversible dispersibility was clearly monitored by NIR absorption spectroscopy (Figure 2). As a benefit of replacing the stilbene core with the azobenzene unit in the molecular structure, we improved the dispersant properties in terms of fast photoreaction response and reversible dispersibility. To clarify the photoreactivities of SB4 and AB2 in the solid state, neat thin films of the respective compounds were prepared on fused-silica substrates (see the SI). The films were exposed to UV light, and the photoisomerization processes were followed by UV−vis absorption spectroscopy. The results are shown in Figure 3a,b. The π−π* absorption bands of both SB4 and AB2 in the films show a broadening with a slight blue shift for λmax in comparison with the solution spectra. According to the X-ray structure analysis of a single crystal of SB4 (Figure S7, SI), the intermolecular interaction between the stilbene cores of SB4 is likely hindered by both electrostatic repulsion forces and the bulkiness of the terminal quaternary

Figure 3. Changes in the UV−vis absorption spectra of the solid films of (a) SB4 and (b) AB2. C

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Figure 4. Changes in the UV−vis−NIR absorption spectra of the solid films composed of SB4/SWNTs [(a) with light exposure and (b) without light exposure] and AB2/SWNTs [(c) with light exposure and (d) without light exposure]: initial spectra (black); after UV-light exposure (blue); after rinsing with distilled water (red). Asterisks indicate an absorption peak assigned to fused-silica substrates. (e) Microscopic image of micropatterned SWNT films on fused-silica substrates.

the hybrid films implies the on−off switching property of the SWNTs’ water solubility. The unexposed areas of the hybrid films maintain their water solubility, whereas the photoreacted dispersants in the exposed areas lose their adsorption ability on the SWNT surfaces. Consequently, only the photoreacted dispersants are dissolved in water, while the “unwrapped” SWNT patterns remain on the surface. Patterned SWNT films were prepared by exploiting the solubility control achieved through the dispersants’ photoreactivity. A solution of SWNTs with a dispersant (mixing ratios and preparing prodecures are shown in Figure S10, SI) was cast onto a cleaned fused-silica substrate. Following the scheme shown in Figure 1b, a pattern was photochemically defined by exposing the substrates to UV irradiation through a photomask. During the water rinse, only the photoreacted dispersants and unirradiated SWNTs were dissolved, leaving the insoluble SWNTs in ordered patterns on the substrate. The microscopic images and surface morphology of the patterned SWNT films are shown in Figures 4e and S9 and S10

in the SI. The typical resolution in the structural pattern is not only several tens of microns but also as little as 5 μm as a lower limit. In the case of screen-printing as a typical wet-processing method for SWNT thin film fabrication, one can also produce patterned structures on a scale of several tens of microns. However, resolution below 10 μm has thus far been difficult. In contrast, our method provides a SWNT pattern with a size of less than 10 μm by the direct photopatterning of the SWNT films by a combination of UV-light exposure and subsequent washing with plain water. Because the neutral water rinse would not damage most substrates, we expect that patterned SWNTs could be prepared on various media, including poly(ethylene terephthalate) (PET), poly(methyl methacrylate) (PMMA), silicons, and glasses, via the SWNT/dispersant hybrid films. Preliminary measurements of the conductivity of the patterned films are not yet satisfactory (4 × 106 Ω/□ for the SWNT/SB4 system and 1 × 105 Ω/□ for the SWNT/AB2 system), probably because of the physical scission of the SWNTs during the sonication procedure used to prepare the dispersions. Other issues remain with patterning resolution. It is well-known that D

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ACKNOWLEDGMENTS The authors thank the A-STEP program (AS2621398M) of the Japan Science and Technology Agency for funding. The authors also thank Drs. D. Nishide and H. Kataura for measurements of 2D PL mapping and Dr. M. Goto for X-ray crystallography.

the conductivity and resolution are affected by many experimental conditions, i.e., the intrinsic nature of the SWNTs, their dispersibility, the mixing ratio of the SWNTs with the dispersants, the dispersing methods, the light source, and the photomask alignment.30 With consideration of these parameters, our procedure should be optimized in the near future. In conclusion, we have presented a concept for patterned SWNT film fabrication based on the photoinduced solubility changes of SWNTs in water. The key mechanism of patterning is the change in the solubility, which is affected by the degree of the molecular interaction between the photofunctional dispersing agents and the surfaces of the SWNTs. The adsorption and desorption of the dispersing agents on the SWNT surfaces affect not only their dispersibility in water but also their solubility. Through the combination of light exposure via a photomask and rinsing with neutral water, we obtained patterned SWNT structures on a scale of below 10 μm. With respect to the existing methods such as controlled deposition on templated surfaces, stamps, and photolithography, this approach is more convenient. Further, this concept could be developed to prepare various patterned nanocarbon films by using a suitable combination of the nanocarbon materials and photodetachable dispersants. We emphasize that the characteristic features of our patterning procedures are simple and versatile. Because improvement of the film conductivity and resolution remains a challenge, we continue to fine-tune our patterning procedures for the application of flexible microfabricated SWNT thin films for solar cells, sensors, and FETs as comb-shaped electrodes.

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ABBREVIATIONS SWNTs, single-walled carbon nanotubes; DOC, sodium deoxycholate; SDS, sodium dodecyl sulfate; 2D PL, twodimensional photoluminescence; UV−vis−NIR, ultraviolet− visible−near-infrared; PET, poly(ethylene terephthalate); PMMA, poly(methyl methacrylate)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06169. General methods containing experimental details, synthetic procedures of AB2, UV−vis absorption spectra of AB2 in deionized water, PL mapping and UV−vis− NIR spectra of the SWNT/AB2 dispersion, crystalpacking diagram of SB4, UV−vis absorption spectra of SB4 in deionized water, dialysis effect on UV−vis−NIR absorption spectra of the SWNT/AB2 dispersion, preparing methods for solid films, microscopic images and an SEM observation of patterned films, AFM images of the films, schematic illustration of the procedure of patterned SWNT films, and microscopic images of patterned SWNT films on various substrates (PDF) X-ray crystallographic data of SB4 in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

Y.M. and M.Y. conceived the project. Y.M. designed and supervised the experiments. Y.M. and Y.T. performed the experiments. Y.M. and M.Y. interpreted the results and wrote the paper. H.J. and H.K. contributed to the manuscript preparation. Notes

The authors declare no competing financial interest. E

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