See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/343480271 Vertically Aligned Nanopatterns of Amine‐Functionalized Ti 3 C 2 MXene via Soft Lithography Article in Advanced Materials Interfaces · August 2020 DOI: 10.1002/admi.202000424 CITATIONS READS 14 855 10 authors, including: Tae-Eun Song Hee-Tae Jung National NanoFab Center Korea Advanced Institute of Science and Technology 10 PUBLICATIONS 40 CITATIONS 286 PUBLICATIONS 10,957 CITATIONS SEE PROFILE SEE PROFILE Yury Gogotsi Chi Won Ahn Drexel University Korea Advanced Institute of Science and Technology 1,362 PUBLICATIONS 189,619 CITATIONS 155 PUBLICATIONS 4,222 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Textile Supercapacitors - Thesis View project MXene for supercapacitors View project All content following this page was uploaded by Tae-Eun Song on 14 August 2020. The user has requested enhancement of the downloaded file. SEE PROFILE Full Paper www.advmatinterfaces.de Vertically Aligned Nanopatterns of Amine-Functionalized Ti3C2 MXene via Soft Lithography Tae-Eun Song, Hwajin Yun, Yong-Jae Kim, Ho Seung Jeon, Kyungryul Ha, Hee-Tae Jung, Hee Han, Yury Gogotsi, Chi Won Ahn,* and Yonghee Lee* ion intercalation to their full potential.[8] When considering stacking 2D MXene flakes, parallel alignment is preferred over vertical alignment with respect to the supporting substrate because of the flakes’ large lateral size compared to its ultrathin layer thickness of ≈1 nm. MXene films with vertical alignment are warranted for specific applications such as ones that require high throughput with extremely narrow pathways for ion transport, as well as sensors and plasmonic devices. Inducing vertical alignment of 2D sheets remains quite challenging, but was done by using liquid-crystalline MXene.[9] Nanoscale patterning technology of MXenes is necessary for microminiaturization, device integration, and maximization of device performance, beyond what can be achieved using inkjet or other printing methods.[10,11] Previous studies exploring microscale thin-film patterning of MXenes have been conducted;[12] however, studies based on nanopatterns with aligned MXene stacking have not yet been reported. Because MXenes are difficult to etch as a consequence of their composition; technology has not been developed for patterning MXene at the micro­meter to nanometer scale. High-resolution and high-aspect-ratio nanostructure patterning technology is valuable in various fields, such as organic electronics, optoelectronics, biosensors, energy storage, display devices, and plasmonics.[13–24] Thus, alignment control of stacked MXene with nanoscale patterning is critical for tuning the properties of MXene films for specific applications. Recently, Zheng’s group published a method for microscale patterning of MXenes with vertical alignment.[25] They fabricated anti-T shaped random micropatterns with vertically aligned MXene flakes via vacuum filtration utilizing a metal screen mesh. Through this process they dramatically enhanced the electrochemical properties of the MXene film but were unable to fabricate high-resolution nanopatterns with vertically aligned MXene flakes. “Soft lithography” refers to nonphotolithographic methods when forming high-quality microstructures and nanostructures. This process is becoming an attractive approach to fabricating microstructures and nanostructures that cannot be prepared photolithographically. Polydimethylsiloxane (PDMS) has emerged as the material of choice for the rapid, low-cost fabrication of microfluidic channels because of its numerous advantages which include high transparency and Thin films of well-stacked two-dimensional MXene flakes have been used in various applications, especially in sensors and microscale energy storage devices, such as micro-supercapacitors. Miniaturization and integration of devices, as well as maximization of device performance require nanoscale patterning of MXene, beyond what can be achieved using inkjet or screen printing. However, nanoscale patterning technology for MXene is yet to be developed. In the present work, a simple fabrication method is demonstrated for manufacturing Ti3C2Tx MXene films with vertically aligned nanopatterns via soft lithography. This process involves polydimethylsiloxane (PDMS) stamping with line-patterned PDMS molds. The feature size of the vertical line patterning of MXene is controlled with the nanometers accuracy by swelling of the PDMS mold by toluene, which also guides vertical alignment of MXene flakes. As a result, vertically aligned MXene nanopatterns are fabricated with a width of ridges less than 200 nm and 2-µm regular spacing between the ridges. The oleylamine-functionalized MXene flakes are also developed for better dispersion in toluene, providing a general protocol to fabricate MXene dispersions in nonpolar solvents. 1. Introduction MXenes, a large family of two-dimensional (2D) transitionmetal carbides and nitrides, are increasingly attracting attention because of their excellent performance in energy storage,[1–3] electromagnetic interference (EMI) shielding,[4] heating,[5] and sensing applications.[6,7] MXene films perform well within energy applications such as batteries and capacitors, especially where well-formed interlayer spaces can be utilized for Dr. T.-E. Song, H. Yun, Y.-J. Kim, Dr. H. S. Jeon, K. Ha, Dr. H. Han, Dr. C. W. Ahn, Dr. Y. Lee National Nano Fab Center (NNFC) Daejeon 34141, South Korea E-mail: cwahn@nnfc.re.kr; yhlee@nnfc.re.kr Y.-J. Kim, Prof. H.-T. Jung, Prof. Y. Gogotsi Department of Chemical and Biomolecular Engineering (BK-21 Plus) Korea Advanced Institute of Science and Technology (KAIST) Daejeon 34141, South Korea Prof. Y. Gogotsi Department of Materials Science and Engineering and A.J. Drexel Nanomaterials Institute Drexel University Philadelphia, PA 19104, USA The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202000424. DOI: 10.1002/admi.202000424 Adv. Mater. Interfaces 2020, 2000424 2000424 (1 of 8) © 2020 Wiley-VCH GmbH www.advancedsciencenews.com www.advmatinterfaces.de biocompatibility. Micromachining,[26] microcontact printing,[27] microtransfer molding,[28] micromolding in capillaries (MIMIC),[29] and solvent-assisted micromolding (SAMIM)[30] are examples of methods used to fabricate structures on the submicrometer scale. Recently, solute–solvent separation soft lithography (3S soft lithography) has been reported.[31] Furthermore, Whitesides et al. demonstrated that the extent of PDMS swelling in solvents is determined by the solubility of the solvent in PDMS and, in particular, by the Hildebrand solubility parameters of the solvents and PDMS.[32] Swelling associated with PDMS is commonly considered an undesirable characteristic in many applications. Nonpolar solvents such as toluene and hexane swell PDMS substantially and induce deformation of the PDMS structure, degrade device performance, and induce the use of PDMS in solvent-manipulation applications. This swelling behavior is a noteworthy disadvantage to using PDMS for soft lithography. However, we hypothesized that swelling and swelling-induced deformation of a PDMS mold could offer an opportunity to control the orientation of 2D flakes and produce a vertical line pattern of MXene, offering a new approach to fabricating nanopatterns. In the present work, we demonstrate a simple and comprehensive method for fabricating high-resolution nanopatterns with vertical alignment of Ti3C2Tx (T stand for surface terminations, such as O, OH, and F) MXene flakes. The developed method is not based on photolithography and thus does not require an etchant; etchants are undesirable because they can oxidize Ti3C2Tx. Soft lithography via stamping of PDMS molds was used to produce MXene patterns. Toluene was used to disperse MXene, enabling the MXene solution to infiltrate line patterns on the PDMS mold and induce PDMS swelling to guide the vertical alignment of the MXene flakes into the nanoscale space. However, because the pristine Ti3C2Tx flakes have OH−, F−, or O-terminations on their surface,[33] they are hydrophilic and do not disperse in toluene.[34] Besides, MXene flakes may oxidize in aqueous solution.[5] Therefore, hydrophobic MXene flakes that can be dispersed in nonpolar solvents are required for nanoscale patterning by soft lithography using hydrophobic PDMS. For this purpose, we functionalized Ti3C2Tx with oleylamine (OAm, C18H35NH2), making dispersible in nonpolar solvents such as toluene and chloroform. As a result, vertically aligned nanopatterns of MXene were successfully produced by PDMS-based soft lithography. During the molding process, the PDMS absorbs toluene and expands, resulting in a nanoscale guide pattern in which MXene flakes are present. Because of this guide pattern on a PDMS mold, vertically aligned nanopatterns of MXenes replicated the periodic patterns of the PDMS mold after drying on the substrate. 2. Results and Discussion Figure 1 illustrates the overall procedure, including the synthesis of the OAm-functionalized MXene dispersion and the fabrication of vertically aligned nanopatterns of MXene flakes. A soft lithography method via stamping of PDMS molds was used to produce the highly periodic and high-resolution nanopatterns. To prepare the MXene solution for PDMS stamping, we modified the surface properties of the MXene flakes from Adv. Mater. Interfaces 2020, 2000424 hydrophilic to hydrophobic by functionalizing them with the hydrophobic OAm ligand (Figure 1a); the method used to prepare the OAm-modified MXene is detailed in the Experimental Section (Supporting Information). TEM images and an electron diffraction (ED) pattern clearly show that the 2D morphology of MXene was maintained after OAm modification (Figure S1, Supporting Information). The size of MXene flakes ranged from 500 nm to 5 µm before sonication (Figure S1a, Supporting Information). Bigger flakes than the inner space of PDMS mold (1 µm) are not suitable to form nanoscale patterns, and we were not able to obtain clean vertically aligned nanopatterns with those large flakes. Thus, we sonicated the solution for 3 h to obtain MXene flakes smaller than 1 µm (Figure S1d, Supporting Information). After functionalization of the MXene surface, OAm-functionalized MXene (OAm@Ti3C2Tx) flakes were dispersed in toluene. The OAm@Ti3C2Tx solution in toluene was dropped onto a Si wafer, and the PDMS mold was then placed on top (Figure 1b). Here, we used PDMS molds with a height of 1 µm and line patterns with a 1 µm width. The PDMS stamp was sufficiently soft to completely adhere to the substrate under its own weight; no additional force was needed to achieve conformal contact. Toluene drove the MXene into the line patterns on the PDMS mold via capillary and pressure effects after the PDMS stamp was covered (Figure 1c, left). Toluene dissolved or swelled the surface of the polymer of the PDMS mold. Thereafter, the grooves of line patterns shrank to the nanoscale dimensions as the PDMS mold pushed the MXene film horizontally (Figure 1c, center). The MXene film dried as the toluene evaporated, and the PDMS mold maintained conformal contact with the substrate (Figure 1c, right). After the PDMS mold was dried and removed, highly periodic and high-aspectratio MXene nanopatterns were formed on the Si wafer. The overall morphology of the MXene patterns represents a nanostructured negative replica of the relief patterns on the PDMS mold (Figure 1d). As expected, these MXene patterns included a vertical alignment of MXene flakes, as shown in Figure 1e. To prepare the toluene-dispersible MXene flakes, we produced OAm-functionalized MXene and confirmed its dispersibility in several organic solvents (Figure 2 and Figure S2, Supporting Information). OAm@Ti3C2Tx was synthesized via an electrostatic adsorption of positively charged amine on the negatively charged MXene surface.[35] Pristine MXene and 10 mL of OAm were placed in a glass container and stirred at 450 rpm for 24 h at room temperature. After adsorption, excess of OAm was washed away through centrifugation using ethanol. Toluene was then added to the OAm@Ti3C2Tx sediment. Finally, we obtained a well-dispersed OAm@Ti3C2Tx solution in toluene. Pristine MXene and OAm@Ti3C2Tx films used for measurement of contact angles were prepared by drop-casting a pristine MXene solution and OAm@Ti3C2Tx solution onto a Si wafer, which was subsequently dried. Ten microliters of water was then dropped onto the pristine MXene- and OAm@Ti3C2Tx -coated wafer, and contact angles of the water drops were measured, showing a change in the hydrophobicity of MXene after functionalization with OAm (53.1° for pristine MXene and 106.4° for OAm@Ti3C2Tx, Figure 2a,b). Pristine MXene flakes disperse in water (Figure 2c) and polar organic solvents because 2000424 (2 of 8) © 2020 Wiley-VCH GmbH www.advancedsciencenews.com www.advmatinterfaces.de Figure 1. Schematic of the vertical line patterns of OAm-functionalized hydrophobic 2D MXene. a) MXene combined with OAm is modified from hydrophilic to hydrophobic because of the long carbon chains of the OAm-functionalized MXene. b) The solution of MXene dispersed in toluene is dropped onto the substrate and the polydimethylsiloxane (PDMS) mold is placed on top. c) The PDMS mold is swollen by the toluene organic solvent and shrinks when the toluene evaporates. The vertical MXene line patterns represent a nanostructure of MXene. d) Scanning electron microscopy (SEM) image of the vertical MXene line patterns of the nanostructures. e) Cross-sectional schematic of the vertical MXene line patterns shown in (d). of the hydrophilic functional groups (e.g., =O, –OH, and –F) on their surface.[34] By contrast, OAm@Ti3C2Tx flakes became hydrophobic because of the long hydrocarbon chains of OAm. They disperse in toluene and chloroform (i.e., nonpolar organic solvents), but not in water (Figure 2d, Figure S2, Supporting Information). To verify the properties of MXene flakes after surface modification with OAm, we characterized their surface chemistry before fabrication of the MXene patterns (Figure 3). First, the change of the MXene surface characteristics after functionalization was confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy. These measurements were conducted on MXene films prepared from pristine MXene and OAm@Ti3C2Tx solutions. A comparison of the XPS survey spectra of MXene and OAm@ Ti3C2Tx reveals that nitrogen (N1s 396 eV ≈403 eV) was present only in OAm@Ti3C2Tx (Figure 3a),[36] meaning that the N of OAm was adsorbed on the surface of Ti3C2Tx MXene. N1s intensity of OAm@Ti3C2Tx increased compared to the Adv. Mater. Interfaces 2020, 2000424 pristine MXene (Figure 3b). It is also seen that the ratio of N1s over other elements of OAm@Ti3C2Tx increased, while the ratio of F1s decreased (Figure S4, Supporting Information). After modification of MXene with OAm, CH2 and CH3 bonds increased in the C1s peak (Figure S3a,b, Supporting Information) and Ti-OH and Ti-H2O contributions to the O1s peak decreased, suggesting the intercalation of OAm ligands between MXene layers (Figure S3c,d, Supporting Information). It is also confirmed that the F1s intensity of OAm@ Ti3C2Tx over other elements decreased compared to the pristine MXene, indicating that detachment of fluorine occurred during functionalization with OAm (Figures S3e,f and S4, Supporting Information). The changes of TiO2 and Ti-X (Ti-C, TixOy, Ti-N) bonds are seen in the Ti2p peak before and after modification of MXene with OAm (Figures S3g,h and S4, Supporting Information). A peak around 459 eV assigned to TiO2 slightly increased after OAm functionalization, however, as seen in TEM images (Figure S1, Supporting Information), the oxidation state of the surface of MXene has not changed 2000424 (3 of 8) © 2020 Wiley-VCH GmbH www.advancedsciencenews.com www.advmatinterfaces.de Figure 2. Contact angle of pristine MXene (a) and MXene modified with OAm (OAm@Ti3C2Tx) (b). Dispersion test of pristine MXene and OAm@Ti3C2Tx in water (c) and toluene (d). much. It is known that MXene can degrade in the aqueous dispersion, however, the oxidation process is retarded in organic solvents. Also, passivation of the MXene surface by OAm ligand can contribute to the protection of MXenes from oxidation.[37] Adsorption of OAm on the Ti3C2Tx MXene surface was confirmed by FT-IR analysis (Figure 3c).[38] The intensities of the absorption bands corresponding to –OH groups on the MXene surface at 3485 and 3644 cm−1 are high in the spectrum of the pristine MXene, but decrease in the OAm-functionalized MXene. In addition, the absorption band corresponding to N–H at 3300 cm−1, which is not observed in the spectrum of the pristine MXene, is prominent in the spectrum of OAm@Ti3C2Tx, showing intermolecular interactions between OAm and active terminations on the surface of MXene by hydrogen bonding and/or van der Waals forces.[39,40] Lastly, X-ray diffraction (XRD) was performed to confirm the effect of the OAm on MXene flakes on the interlayer spacing of stacked MXene films (Figure 3d). The XRD patterns also show the result of the functionalization of MXene with OAm.[41,42] The shift of the (002) peak from 7.08° to 6.12° indicates that the interlayer spacing of MXene increased from 1.25 to 1.45 nm, due to adsorption of OAm. The OAm, which has a long hydrocarbon chain, is bound to the Ti3C2 MXene surface, leading to a larger gap between the MXene layers. After producing toluene-dispersible OAm@Ti3C2Tx, we attempted fabrication of MXene patterns via stamping with line-patterned PDMS molds. After patterning via PDMS stamping, analyses by optical microscopy (OM), atomic force microscopy (AFM), and scanning electron microscopy (SEM) Figure 3. X-ray photoelectron spectroscopy (XPS) survey spectra (a), N1s region (b), Fourier transform infrared (FT-IR) spectra (c), and X-ray diffraction (XRD) patterns (d) of MXene and OAm@Ti3C2Tx. Interlayer spacing increased from 1.25 to 1.45 nm after functionalization with OAm. Adv. Mater. Interfaces 2020, 2000424 2000424 (4 of 8) © 2020 Wiley-VCH GmbH www.advancedsciencenews.com www.advmatinterfaces.de Figure 4. Morphological analysis of MXene nanopatterns constructed on the Si substrate using polydimethylsiloxane (PDMS) stamping: a) optical microscopy image; b) atomic force microscopy (AFM) image; c) low-magnification, and d) high-magnification scanning electron microscopy (SEM) images of MXene nanopatterns with the 2 µm periodicity obtained by dropping and pressing; and e) low-magnification and f) high-magnification cross-sectional SEM images of MXene nanopatterns with a height of ≈600 nm. were conducted to confirm the morphology of the patterned films and structural features of the MXene patterns (Figure 4). The OM images show MXene nanopatterns on a large area (Figure 4a). We obtained height and width profiles from the AFM image in Figure 4b; these profiles provide information about the feature size of the MXene nanopatterns. The height and periodicity of the patterns are approximately 600 nm and 2 µm, respectively. SEM images were obtained using a dualbeam focused ion beam (FIB) SEM system for cross-sectional SEM images. In the top-view observations, dark-gray regions are the MXene film on the Si wafer and relatively bright lines are MXene nanopatterns formed by PDMS stamping. These images clearly show highly periodic MXene line patterns with ≈2 µm periodicity (Figure 4c) and a high-resolution MXene pattern (Figure 4d). We obtained the corresponding SEM crosssectional images to further characterize the structural features of the MXene nanopatterns. A Pt layer was deposited to protect the MXene patterns during the FIB process (Figure 4e,f). Dark regions representing the MXene pattern are observed between the Pt layer and the substrate. The cross-sectional SEM image clearly shows MXene nanopatterns constructed on MXene films with closely packed MXene flakes (Figure 4f). However, we could not confirm the stacking directions of the MXene flakes in each pattern because of the limited SEM resolution. Interestingly, the constructed MXene patterns show 100–200 nm wide ridges with 2 µm periodicity from peak to peak in the pattern, even though we used a 1 × 1 µm line pattern of the PDMS mold with 2 µm of pitch from center to center of the lines. Moreover, the overall morphology of the patterned MXene structures represents a nanostructured, negative replica of the relief pattern Adv. Mater. Interfaces 2020, 2000424 on the PDMS mold. From these results, we inferred that the shape resulting from swelling-induced deformation of the line patterns in the PDMS mold was transferred to MXene nanopattern on a silicon wafer in a single step. However, no distinct nanopattern is formed in the case of the pristine MXene dispersed in water and most of the MXene solution flows out of the PDMS mold when the PDMS mold is placed on the wafer. Water poorly wets PDMS[32] and the transferred pattern of MXene is therefore irregular (Figure S5, Supporting Information). To further investigate how MXene flakes are stacked in vertically aligned MXene patterns, TEM observations were conducted using cross-sectional thin-film samples of vertically aligned MXene patterns (Figure 5). We prepared this crosssectional thin-film sample using a dual-beam FIB system after cross-sectional SEM observation. A low-magnification TEM image shows a vertically aligned MXene pattern (Figure 5a and Figure S6, Supporting Information). It exhibits a ≈2 µm pitch size between two peaks of the aligned MXene patterns and a height of ≈500 nm. A high-magnification TEM image (Figure 5b) shows that the upper region of the vertically aligned MXene patterns have well-stacked layers, whereas bent layers and empty spaces are formed at the bottom region of the vertically aligned MXene pattern. Well-stacked MXene flakes are parallel to the substrate at the bottom of the vertically aligned MXene pattern, near the Si-wafer substrate. High-magnification TEM images clearly show these well-stacked MXene layers with bent MXene layers at the top of each ridge (Figure 5c) and stacked parallel MXene layers at the bottom (Figure 5d). Well-stacked layers (black dashed lines in Figure 5c) and bent 2000424 (5 of 8) © 2020 Wiley-VCH GmbH www.advancedsciencenews.com www.advmatinterfaces.de Figure 5. Cross-sectional TEM analysis of a vertically aligned MXene pattern. a) Low-magnification TEM image of cross-sectional MXene pattern prepared by FIB. b) High-magnification TEM image of a vertically aligned MXene pattern corresponding to the white boxed region in Figure 5a (inset is a schematic illustration). c,d) High-magnification TEM images of MXene layers in the vertically aligned MXene pattern; the images correspond to the boxed areas in Figure 5b. e) Selected-area TEM image and f) electron diffraction (ED) pattern corresponding to the circled area in Figure 5a. layers (red dashed lines in Figure 5c) show interlayer spacings of approximately 3.6 and 18 nm, respectively. We assume that the larger layer spacing than that in pristine stacked MXene was a consequence of the long chain length of OAm (Figure S6, Supporting Information).[43] In addition, different orientations of stacked layers were confirmed by analysis of the ED pattern in the TEM image in Figure 5e. We observed tilted peaks along the azimuthal angle (red arrow) of the peak in the red circle; they likely arise from the different orientation of the MXene layers in the vertically aligned pattern (Figure 5f). It was expected that the ED pattern would have a circular arc shape due to various orientations of stacked MXene layers, but we were not able to obtain it, probably due to non-random orientation of MXene flakes relative to the beam. The patterned region was too small for achieving good alignment between electron beam and sample, and the TEM sample was bent during thinning by FIB (Figure S7, Supporting Information). We inferred that the variation of the interlayer spacing in well-stacked layers was induced by the relatively long chain length of OAm on the MXene flakes[35] and that variation of the interlayer spacing in bent layers was induced by the formation of empty space as a consequence of (1) the MXene flakes being larger than the pattern size of the PDMS mold and (2) rapid solvent permeation from the MXene solution to the PDMS mold. After the MXene Adv. Mater. Interfaces 2020, 2000424 solution was dropped onto the substrate, large MXene flakes were deposited; these deposited MXene flakes and dispersible solvent infiltrated the PDMS pattern through rapid solvent permeation from the MXene solution to the contacted PDMS mold. We assume that the shape of the grooves in PDMS changed from square to triangular in cross-section as a result of PDMS swelling in toluene. Because of this shape change, we obtained a vertically aligned pattern with decreasing ridge width from the bottom (≈200 nm) to the top (less than 100 nm). The height of the vertical ridges was approximately 600 nm, which is less than the PDMS groove depth of 1 µm. This approach can replace masking and etching when manu­ facturing arrays of posts and other patterns for plasmonic devices,[44] EMI shielding, current collectors for batteries and supercapacitors and other devices. It may also be applicable to other 2D materials. Various MXene films patterned by PDMSbased soft lithography using the swelling-induced deformation effect can be realized various shapes of PDMS stamps. Moreover, we expect that the vertically aligned patterns of MXenes can provide a high-ionictransport channels. Thus, we anticipate that vertical MXene layers on nanoscale active materials may lead to more efficient electrodes, potentially opening a path for the fabrication of microelectronic circuits using MXene solutions and accelerating the commercialization of MXene-based technology. 2000424 (6 of 8) © 2020 Wiley-VCH GmbH www.advancedsciencenews.com www.advmatinterfaces.de 3. Conclusion Ti3C2Tx was functionalized with nonpolar OAm ligands and subsequently dispersed in toluene to prepare ink suitable for soft lithography. This ink was used for fabrication of nanopatterns by PDMS stamping under ambient conditions. PDMS stamping enables the formation of patterns and micro/nanostructures with 2D MXene flakes without photolithography, thus eliminating the use of etchants and eliminating possibility of oxidation and degradation of MXene. Regularly spaced ridges with vertical alignment of MXene flakes were produced using the swelling-assisted PDMS stamping. The developed method should be applicable to other 2D materials. (MSIT), and this research was also supported by NRF-2015M3A7B6027973, and NRF- 2015M3A7B7046618 of the National Research Foundation (NRF) of Korea funded by the Ministry of Science and ICT, Korea. Conflict of Interest The authors declare no conflict of interest. Keywords MXene, nanopatterns, oleylamine, polydimethylsiloxane (PDMS), soft lithography, swelling, vertical alignment Received: March 9, 2020 Revised: July 4, 2020 Published online: 4. Experimental Section Functionalization of MXene with OAm: First, pristine MXene was synthesized by solid–liquid reaction.[45,46] The surface modification of MXene was performed by reacting MXene with OAm. MXene (10 mg) was dispersed in OAm (10 mL), and the mixture was vortexed for 3 min. This solution was then stirred continuously (450 rpm) for 24 h at room temperature. After modification of MXene, the obtained OAm@Ti3C2Tx was washed with ethanol (30 mL) to remove residual OAm. The washing solution was centrifuged at 4000 rpm for 5 min, and the OAm@Ti3C2Tx product was collected after the supernatant was discarded. The OAm@Ti3C2Tx was then dispersed in toluene (3 mL) under ultrasonication for 3 h. Preparation of Polydimethylsiloxane (PDMS) Mold Pattern: To prepare a master pattern, we fabricated an array of line shapes with a periodicity of 2 µm and width of 1 µm in Si using e-beam lithography. A PDMS mold was replicated from the Si master. The PDMS was prepared by mixing a PDMS prepolymer (Sylgard 184A/B = 10:1, Dow Corning) and pouring the mixed PDMS onto the Si master. After bubbles were removed from the mixture, the PDMS mold was cured at 80 °C for 2 h. Fabrication of Patterned MXene Films Using Soft Lithography: The solution was prepared by mixing the MXene with toluene as a solvent. The solution was then dropped onto the substrate. The cured PDMS mold with the aforementioned topographic features was placed and pressed onto the MXene solution. Conformal contact was maintained under additional pressure for 2 h, which induced the formation of MXene nanopatterned structures. Characterization: A Helios NanoLab (FEI) dual-beam FIB/SEM system was used for SEM imaging (surface and cross-section) and cross-sectional TEM samples preparation. TEM images were obtained as bright field images using a JEOL JEM-3010 microscope operating at 300 kV. Contact angle images were obtained with a Phoenix 300 Plus (SEO Co., Ltd.). FT-IR spectra were acquired with a Bruker Alpha spectrometer. XPS was carried out with a Kratos Axis-Supra, and XRD patterns were obtained with a RIGAKU Smartlab. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements T.-E.S., H.Y., and Y.-J.K. contributed equally to this work. 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