Complex High-Aspect-Ratio Metal Nanostructures by Secondary Sputtering Combined with Block Copolymer Self-Assembly

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1 Complex High-Aspect-Ratio Metal Nanostructures by Secondary Sputtering Combined with Block Copolymer Self-Assembly Hwan-Jin Jeon,* Ju Young Kim, Woo-Bin Jung, Hyeon-Su Jeong, Yun Ho Kim, Dong Ok Shin, Seong-Jun Jeong, Jonghwa Shin,* Sang Ouk Kim,* and Hee-Tae Jung* The creation of complex nanopatterns with high resolution, high aspect ratio, high areal density, and tunable features from a soft template presents an important challenge in the fields of materials science, nanotechnology, and biotechnology. [1 10] This challenge is particularly difficult to achieve using block copoly mer (BCP) self-assembly due to the thermodynamic limitations on the BCP morphologies and the nature of the ultrafine pattern dimensions used in conventional pattern-transfer methods. [11 16] In this study, we introduced a new BCP lithography approach that involves a simple secondary sputtering technique [15 18] during ion bombardment with BCP assembly. A target material is etched and deposited simultaneously onto the side surface of a BCP template with a high spatial resolution, followed by removal of the prepatterned BCP. These steps result in the fabrication of various nanopatterns with highly enhanced feature dimensions (10 nm), a high aspect ratio (>5), and a high areal density ( line ea mm 1 ) using a single conventional BCP template with a 70 nm scale resolution, an aspect ratio of 1, and an areal density of 7000 line ea mm 1. Also, the controlled angle of the rebounded particles during the secondary sputtering step provides morphological flexibility, such as the formation of a double wall, nanotube, or nanocrescent, which has been difficult to achieve using previous BCP lithographic techniques. This approach provides a foundation for the design of new routes to BCP lithography. The procedures used to fabricate a variety of complex nanopatterns with high resolution, high aspect ratio, and high areal density from a single conventional BCP structure are summarized in Figure 1a c. First, a target material was deposited onto a substrate (SiO 2 or glass) using e-beam- or thermal-deposition techniques. We primarily used gold in this study as a representative target material. Different types of metals or semiconductor patterns may be prepared, provided that the target layers may be deposited onto a substrate. A common polystyrene-b-poly(methyl methacrylate) (PSb-PMMA) BCP layer with a lamellar or cylindrical structure (lamellae: kg mol 1 ; cylinder: kg mol 1 ) was used, and other BCPs with different chemistries may be used as well. PS-b-PMMA thin films were spin-cast onto a neutrally modified surface, followed by thermal or chemical annealing of the BCP films. [19] The well-ordered BCP template was prepared by modifying the surface energy of the substrate using a well-established PS-r-PMMA random copolymer brush treatment. [20 24] Balance between the interfacial tension of the PS and PMMA mediated the vertical alignment of the nanoscale lamellar or cylindrical nano domains on the surface above the PS-b-PMMA thin films. After a sufficient thermal annealing period, the PMMA component was selectively removed using reactive ion etching (RIE) to form a template with vertically aligned PS lamellae or nanoporous cylinders (Supporting Information Figure S1). The Au substrate exposed within the PMMA-etched regions as a result of RIE was then etched by Ar-ion bombardment at a relatively low power. The etched Au particles were then deposited at the side walls of the PS template (Figure 1b). We recently found that polycrystalline particle sputtering resulted in large-angle deflections (with Dr. H.-J. Jeon, W.-B. Jung, Prof. H.-T. Jung National Research Laboratory for Organic Opto-Electronic Materials Department of Chemical and Biomolecular Engineering (BK-21 plus) Korea Advanced Institute of Science and Technology (KAIST) hjjeon@nnfc.re.kr; heetae@kaist.ac.kr Dr. H.-J. Jeon Department of Nano-structured Materials Research Korea National Nanofab Center Dr. J. Y. Kim, Dr. S.-J. Jeong, Prof. S. O. Kim National Creative Research Initiative Center for Multi-Dimensional Directed Nanoscale Assembly Department of Materials Science & Engineering Korea Advanced Institute of Science and Technology (KAIST) sangouk.kim@kaist.ac.kr Dr. H. S. Jeong Soft Innovative Materials Research Center Korea Institute of Science and Technology (KIST) Wanju-gun, Jeollabuk-do , Republic of Korea Dr. Y. H. Kim Advanced Functional Materials Research Group Korea Research Institute of Chemical Technology (KRICT) Daejeon , Republic of Korea Dr. J. Y. Kim, Dr. D. O. Shin Power Control Device Research Section Electronics and Telecommunications Research Institute (ETRI) Daejeon , Republic of Korea Prof. J. Shin Advanced Photonic Materials and Devices Laboratory Department of National Research Laboratory Department of Materials Science & Engineering Korea Advanced Institute of Science and Technology (KAIST) qubit@kaist.ac.kr 1

2 Figure 1. a c) Schematic diagram of the procedure used during advanced BCP lithography and secondary sputtering to obtain the 10 nm complex nanostructure array with a high aspect ratio. d h) SEM images and EDX data obtained from the complex, high-aspect ratio wall nanostructures prepared from the single lamellar BCP template. d) SEM images of the perpendicularly aligned lamellae of the BCP template. e) SEM image of the Au material that covered the side walls of the PS pattern and the energy dispersive X-ray (EDX) spectroscopic data. f) SEM images of the 10 nm highaspect ratio double-wall nanostructures. g) U-shaped patterns obtained by secondary sputtering. h) L-shaped asymmetric long bench nanostructure obtained by tilting the substrate during ion bombardment. respect to normal incidence) of the particles at a low bombarding energy, whereas the particles were deflected along the normal direction at high ion-bombardment energies.[17,18,25 28] After complete removal of the PS template by RIE, the Au nanopattern remained on the substrate (Figure 1c). The aspect ratio and spacing of the resulting Au nanopatterns were determined by the thickness and spacing of the pristine BCP assembly, whereas the lateral size of the Au nanopatterns was m due to equilibration during the side-deposition and etching steps of the ion bombardment process. 2 Figure 1d h shows scanning electron microscopy (SEM) images of the BCP lamellar template after PMMA removal. The high resolution and high aspect ratio of the Au nanopatterns resulted from secondary sputtering during ion bombardment of the single conventional BCP lamellar phase. Figure 1d shows the vertically aligned PS lamellae with a 140 nm pitch (70 nm lamellar width and spacing) formed on the 20 nm thick Au surface. After Ar-ion bombardment at a low energy (500 ev), the Au layer was selectively etched and redeposited onto the PS side surface (Figure 1e). The thin bright regions in the SEM

3 images represent Au layers on the PS side walls. Energy-dispersive X-ray (EDX) spectroscopy further verified that thin Au layers were generated on the side surfaces of the PS features (see the inset of Figure 1e). Secondary sputtering during ion bombardment etched Au from the bottom substrate and simultaneously deposited Au onto the side walls of the PS template. In other words, the Au particles on the unprotected Au bottom layer were etched and the etched Au particles were resputtered onto the side walls of the PS prepatterns during ion bombardment. [17,18,25,26] After removing PS, a vertically aligned residual Au thin layer 10 nm in width and 60 nm in height was generated over a large area (Figure 1f h). A Au lamellar structure with two side walls on the edge of the bottom layer (U-shaped lamellae) was generated, as shown in Figure 1g. Additional Au etching processes removed the bottom Au layer, and only the side Au walls remained (Figure 1f). Different Au pattern shapes could be prepared by changing the Ar-ion-bombardment tilt angle (Figure 1h). Similarly, asymmetric L-shaped Au patterned structures were prepared simply by tilting the Ar-ion-bombardment direction. The accelerated Ar ions with a 5 10 tilt angle etched the unprotected Au bottom layer and interrupted the attachment of Au particles onto the exposed side walls of the prepatterned PS. The redeposited Au particles etched from the bottom Au layer formed a 10 nm wide Au wall on one side of the prepatterned PS. Well-aligned Au line patterns 10 nm in width could be prepared from a well-aligned PS PMMA lamellar template 70 nm in width using our new BCP lithographic technique [29] (Figure S5 and S6, Supporting Information). These gold line patterns are tilted about 5 off-center. This tilting is attributed to the tilted BCP PS prepattern, which stood facing each other with 85. Several significant advances have been made using this approach. Complex nanostructures with various shapes and sizes have been fabricated from a single conventional BCP template. The patterned line density ( line ea mm 1 ) of a target material, such as Au, may be nearly doubled relative to the density obtained from conventional prepatterned PS (7000 ea mm 1 ) without the need for additional deposition or etching processes. High-resolution (10 nm) nanopatterns may be prepared using a conventional 70 nm wide BCP pattern. The aspect ratio of a nanostructure may be significantly increased to 5, and the aspect ratio may be further increased by increasing the film thickness of the prepatterned BCP. A variety of Au lamellar patterns (simple, L-shaped, and U-shaped lines) with a high resolution (10 nm), high areal density ( line ea mm 1 ), and a high aspect ratio (5) may be created from a single conventional PS-PMMA lamellar structure (width = 70 nm, spacing = 70 nm, and areal density = 7000 line ea mm 1 ) through the use of a simple ion bombardment process. Similarly, a variety of Au hole cylinder patterns with a high resolution and high aspect ratio may be fabricated from conventional BCP cylinders using this approach. Figure 2a shows a PS hole cylinder pattern with features 40 nm in diameter and 50 nm in depth, prepared using conventional PS-PMMA cylinders formed on a Au substrate. After ion bombardment for 120 s, a 10 nm thick Au layer formed on the side walls of the prepatterned PS cylinder. After removing the PS, Au tubes with 10 nm thick walls, 30 nm height, and 40 nm outer diameter were fabricated over a large area (Figure 2b). The feature size in the Au hole cylinder pattern could be further controlled simply by tuning the cylindrical PS hole etching time. SEM images and the corresponding size distributions of the Au hole cylinders as a function of the RIE time revealed that the hole size in the PS hole cylinder pattern increased as the RIE time increased using 40 sccm O 2 /100 sccm CF 4 (Figure 2c). As the RIE time increased, the hole cylinder gap spacing decreased. Each central peak in the deviation curve obtained at a given RIE time yielded the diameters of the hole cylinder features at RIE times of 5, 25, 45, and 65 s of 27, 36, 45, and 52 nm, respectively. The RIE process increased the hole cylinder diameter by etching the inner wall of the prepatterned PS prior to ion bombardment. The advanced BCP lithography technique is not limited to Au. Cu, Pd, Pt, and ITO cylindrical patterns with a high resolution, high aspect ratio, and high areal density may be prepared by depositing different target materials (Figure 2d). Different nanopattern shapes, including nanocrescent rings, may be fabricated from PS-b-PMMA cylinders by changing the ion-bombardment direction. A nanocrescent ring forms from a tilted cone (asymmetric tilting cylinder) shape in three dimensions to create a one-sided opened ring from the top view. The cylinder is cut from the top of the edge to the bottom plane. High-resolution nanocrescent structures are very difficult to fabricate using currently available lithographic techniques, although this structure is in high demand in the field of nanophotonics. [30 32] The nanocrescent shape may be achieved by depositing Au particles onto the side surfaces of cylindrical PS structures such that the screening effects of the PS walls during the tilting process retard etching of the redeposited particles. This process forms asymmetric nanocrescent structures with a tapered height (Figure 3a). SEM images (Figure 3b) revealed the generation of nanocrescent ring-shaped nanostructures 70 nm in diameter and with a variety of tapering heights, from 30 to 60 nm over a large area. A side-view SEM image, shown in Figure 3b, revealed that these structures formed tilting cones (asymmetric tilting cylinders) with a tapered circular wall, indicating that the right side surface of the cylinder was exposed to accelerated ions and was partially etched during the tilting process. The top-view SEM image shown in Figure 3b reveals that right-opened ring structures were produced with a high uniformity over a large area. The open angle of the crescent ring could be precisely controlled by varying the Ar-ion-bombardment tilt angle (Figure 3c). Varying the tilt angle from 40 to 50 produced nanocrescent ring structures with a variety of open angles, Tilting at 50 yielded a 64 open angle for the nanocrescent ring. As the tilting angle decreased to 40, the open angle of the nanocrescent ring increased to 90 because the portion of the cylinder s bottom edge exposed by tilting to ions increased during the small-angle ion-bombardment process. A diversity of 3D nanostructure shapes could be fabricated by modifying the side features of the BCP prepattern. This method enabled the fabrication of nanotube structures as well as a variety of shapes achieved by varying the feature shape of the prepatterned side surfaces. High-resolution, high-aspect ratio, and high-areal density patterns are important for a variety of applications. These factors control the surface anchoring energy of a target substrate and introduce preferred alignment among functional organic materials. Figure 4 shows the vertical alignment of 3

4 Figure 2. a,b) SEM images of high-resolution 15 nm hexagonal hole cylinder gold patterns obtained from the BCP PS hole pattern. c) The size of the nanotube gold patterns could be controlled simply using a PS pretrimming process during RIE. d) Nanotube patterns obtained using a variety of materials. a) SEM images of the cylindrical BCP nanotemplate. b) Hole cylinder nanostructures with 10 nm walls and a high aspect ratio, obtained by secondary sputtering. Schematic illustration of the method used to control the size of the hole cylinder pattern via a PS pretrimming process during RIE. c) SEM images of hole cylinder gold patterns prepared with various diameters, depending on the RIE time during the PS pretrimming process. d) Ultrathin hexagonal nanotube patterns prepared using Cu, Pd, Pt, or ITO. liquid-crystal (LC) molecules on a patterned ITO substrate surface without the need for additional alignment tools. All previous approaches required an additional rubbing layer to vertically align the LC molecules. 4 The LC alignment was prepared by fabricating a dense array of hole cylinder ITO patterns with walls 10 nm thick, 40 nm diameters, and 50 nm heights, over a large area (4 cm 4 cm) using our new BCP lithography (Figure 4a). It is important to

5 Figure 3. a,b) Fabrication of a nanocrescent ring-shaped structure with a tapered height via tilting ion bombardment, and c) control over the crescent ring as a function of the tilting angle during the ion-bombardment process. note that the high resolution and high aspect ratio of the ITO pattern yielded optoelectronic properties (transmittance and conductivity) comparable to those of pristine ITO glass (Supporting Information Figure S7) due to the high resolution and high aspect ratio of the ITO pattern fabricated by this approach. A negative LC (T iso = 90 C, Δε = 4.2, Δn = , MLC-6608, Merck) was injected into the sandwiched ITO nanopatterns to obtain a 5 μm gap in the isotropic phase. The conoscopic interference pattern revealed a centered black cross superimposed on circular bands of interference colors, as is generally observed among homeotropic alignment modes (Figure 4b). The interference pattern remained unchanged upon rotation of the sample, confirming that the optical axis of the LC cell was aligned perpendicular to the substrate. The vertical alignment among LC molecules was further verified by collecting polarized optical microscopy (POM) images, which revealed a black texture over the entire substrate surface under all rotation angles (Figure 4c). This pattern indicated that the optical axis of the LC molecules coincided with the direction of light propagation. Figure 4d shows the electro-optic response of a homeotropic LC cell to a 120 Hz AC electric field applied normal to the cell surface. LC molecules with a negative dielectric anisotropy (Δε = ε // ε < 0) underwent a Fredericks transition from homeotropic (dark OFF mode) to homogeneous (bright ON mode) alignment at a critical electric field of 2.3 V. The threshold voltage of our LC cell was similar to the values reported by others using the same negative liquid crystal (MLC-6608, Merck). The vertical alignment was attributed to an increase in the elastic energy and an increase in the hydrophobic properties induced by the high-aspect-ratio dense arrays of vertical nanotubes with sharply pointed honeycomb shapes. This elastic energy enhancement overwhelmed the interfacial tension, leading to homeotropic alignment. The hydrophobic character of the patterned surface was responsible for forming a homeotropic alignment. [33 35] As shown in Figure 4d, the water contact angles on the nanopatterned ITO and Au substrates exceeded 120, in clear contrast to those obtained on the nonpatterned ITO surface (45 ). In summary, we have created high-resolution, high-areal density, high-aspect-ratio, and morphologically complex nanopatterns from a single conventional BCP structure. This finding presents several significant advances in the area of BCP self-assembly. First, a variety of complex patterns was generated from a single BCP template using a simple process. The controlled angle of the rebounded particles during the secondary sputtering step provided morphological flexibility, such as the formation of a double wall, nanotube, or nanocrescent, which has been difficult to achieve using previous BCP lithographic techniques. Second, it was possible to create a high aspect ratio of 5 simply by controlling the thickness of the BCP films. Third, the feature width was reduced to 15 nm and the areal density of the nanostructures was doubled relative to the areal densities obtained using conventional BCP templates with 70 nm feature dimensions. Finally, our method may be applied 5

6 Figure 4. Homeotropic alignment (vertical alignment) of nematic BCP-SSL liquid-crystal molecules on the dense hole cylinder-patterned ITO, for use in liquid-crystal-display devices without an alignment layer. a) Schematic diagram showing the fabrication of the LC-based device cell using dense hole cylinder-patterned ITO glass. b,c) The conoscopic interference patterns, obtained using a Bertrand lens, and wide-view polarized optical microscopy (POM) images of the homeotropic alignment of LC molecules on the dense ITO hole cylinder pattern, as a function of the cell rotation. d) Wide-view POM images collected before and after applying an electric field to the patterned ITO (inset), and electro-optical response of the VA cell. e) Contact angle measurement of the pristine and patterned ITO substrate. to most metal/oxides/semiconducting materials. A high aspect ratio of 5 with a 10 nm lateral resolution and the use of diverse target materials, such as Au, Pt, Ag, and ITO, could be achieved simply by depositing the target material onto the side walls of the BCP template in a controlled manner. The observations made during this effort provide a foundation for the design of new routes to BCP lithography. Taking advantage of the universality of the secondary sputtering phenomenon, our patterning method may be applied to the fabrication of other functional materials, including metals, ceramics, and semiconductors. A large degree of freedom with respect to target material choice may be exploited for the preparation of a variety of electronic and optoelectronic devices. Experimental Section Preparation of the PS-b-PMMA Nanotemplates: The inorganic target substrates were deposited by e-beam or thermal evaporator. A perpendicular orientation of PS-b-PMMA at the substrate surface was achieved by introducing a neutral surface modification layer consisting of PS-r-PMMA brushes. The PS-b-PMMA thin films nm thick were then spin-coated and annealed via thermal or solvent annealing. An annealing temperature of 250 was used for thermal annealing and tetrahydrofuran was used for solvent annealing. PS-b-PMMA with a number average molecular weight of kg mol 1 was used to prepare the lamellar nanotemplates, and cylinder nanotemplates were prepared using kg mol 1 PS and PMMA blocks. Vertical nanodomains were formed by annealing, and the PMMA nanodomains in the nanotemplate were selectively removed by a wet-etching process 6

7 in acetic acid or a dry etching process using RIE. Dry etching of PMMA and the O 2 /Ar mixture gas stream in a flow rate of 10 sccm/40 sccm was used under a radio frequency (RF) power of 50 W. Photoresist Patterning for Directed Self-Assembly of the BCP Thin Films: A negative tone SU8 photoresist (MicroChem Corp. US) was used to prepare the photoresist patterns for the directed self-assembly of the BCP films. A photoresist layer 300 nm thick was spin-cast onto a neutrally modified silicon substrate and soft baked at 90 C for 2 min to remove residual solvent. The photoresist film was exposed to an I-line source (Midas/MDA-6000 DUV, KR; wavelength: 365 nm; 9.5 mw cm 2 ) through a pattern mask and postbaked at 100 C for 60 s to selectively crosslink the exposed portion. Pattern development was performed by immersing the film into propylene glycol methyl ether acetate solvent for 60 s, followed by thorough rinsing with 2-propanol. Preparation of Complex Nanostructures from the Single BCP Templates: Ten nanometer nanowalls in the target material were generated on the side surfaces of the PS BCP nanostructures using an ion bombardment plasma process (Ar 20 sccm, 500 V, 60 s). After the PS prepatterns had been removed by RIE under O sccm, 10 nm nanostructures of the target material were fabricated over a large area. The PS prepattern could be removed using acetone, toluene, or other solvents. LC Cell Fabrication Using the Patterned ITO Substrates: Electro-optic LC cells were fabricated by assembling ITO glass supporting ITO nanotube structures. The distance between the ITO glasses was controlled using microbead spacers, and the cells were sealed using a UV-curable adhesive (NOA 63, Norland Products Inc., Cranbury, NJ). The fabricated empty cell was placed on a temperature-controlled hot plate (FP82HT Mettler Toledo Korea, Seoul, Korea), and 5CB was loaded by capillary action at 40 C to the isotropic phase. After the filling process, the cell was cooled to room temperature at a rate of 2 C min 1. Characterization: Scanning electron microscopy (Sirion FE-SEM, FEI, NNFC in KAIST) images were obtained by collecting secondary electrons produced by bombarding the sample at an acceleration voltage between 3 and 10 kv. LC alignment was probed using polarized optical microscopy. The contact angles of a water droplet were measured using a contact angle measurement system at room temperature. Water droplet images were captured using the camera in the contact angle measurement system. The electric conductance of the patterned ITO substrate was measured using a probe station (4200-SCS Keithley, Keithley Instruments, Inc., Cleveland, OH), and the optical transmittance was measured by UV vis absorption spectroscopy (92-570, JASCO, Tokyo, Japan). Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements H.-J.J., J.Y.K., and W.-B.J. contributed equally to this work. This work was supported by the National Creative Research Initiative (CRI) Center for Multi-Dimensional Directed Nanoscale Assembly (2015R1A3A ) and the Leading Foreign Research Institutes Recruitment Program (NRF-2015K1A4A ) and (NRF-2015M3A7B ), funded by the Ministry of Science, ICT, and Future Planning, Korea (MSIP). Received: May 12, 2016 Revised: June 20, 2016 Published online: [1] G. M. Whitesides, J. P. Mathias, C. T. Seto, Science 1991, 254, [2] G. M. Whitesides, B. Grzybowski, Science 2002, 295, [3] I. W. Hamley, Angew. Chem., Int. Ed. 2003, 42, [4] J. Y. Cheng, A. M. Mayes, C. A. Ross, Nat. Mater. 2004, 3, 823. [5] N. Vogel, M. Retsch, C.-A. Fustin, A. del Campo, U. Jonas, Chem. Rev. 2015, 115, [6] L. Gao, Y. Zhang, H. Zhang, S. Doshay, X. Xie, H. Luo, D. Shah, Y. Shi, S. Xu, H. Fang, J. A. Fan, P. Nordlander, Y. Huang, J. A. Rogers, ACS Nano 2015, 9, [7] B. H. Kim, M. S. Onses, J. B. Lim, S. Nam, N. Oh, H. Kim, K. J. Yu, J. W. Lee, J.-H. Kim, S.-K. Kang, C. H. Lee, J. Lee, J. H. Shin, N. H. Kim, C. Leal, M. Shim, J. A. Rogers, Nano Lett. 2015, 15, 969. [8] N. Vogel, R. A. Belisle, B. Hatton, T.-S. Wong, J. Aizenberg, Nat. Commun. 2013, 4, [9] Y.-C. Tseng, Q. Peng, L. E. Ocola, J. W. Elam, S. B. Darling, J. Phys. Chem. C 2011, 115, [10] S. B. Darling, Prog. Polym. Sci. 2007, 32, [11] L. Leibler, Macromolecules 1980, 13, [12] G. H. Fredrickson, F. S. Bates, Annu. Rev. Mater. Sci. 1996, 26, 501. [13] M. Park, C. Harrison, P. M. Chaikin, R. A. Register, D. H. Adamson, Science 1997, 276, [14] W. Van Zoelen, G. ten Brinke, Soft Matter 2009, 5, [15] T. Thurn-Albrecht, J. Schotter, G. A. Kastle, N. Emley, T. Shibauchi, L. Krusin-Elbaum, K Guarini, C. T. Black, M. T. Tuominen, T. P. Russell, Science 2000, 290, [16] X. Gu, Z. Liu, I. Gunkel, S. T. Chourou, S. W. Hong, D. L. Olynick, T. P. Russell, Adv. Mater. 2012, 24, [17] H.-J. Jeon, K. H. Kim, Y.-K. Baek, D. W Kim, H.-T. Jung, Nano Lett. 2010, 10, [18] H.-J. Jeon, H. S. Jeong, Y. H. Kim, W. B. Jung, J. Y. Kim, H.-T. Jung, ACS Nano 2014, 8, [19] E. Kim, H. Ahn, S. Park, H. Lee, M. Lee, S. Lee, T. Kim, E.-A. Kwak, J. H. Lee, X. Lei, J. Huh, J. Bang, B. Lee, D. Y. Ryu, ACS Nano 2013, 7, [20] P. Mansky, Y. Liu, E. Huang, T. P. Russell, C. J. Hawker, Science 1997, 275, [21] S.-J. Jeong, G. Xia, B. H. Kim, D. O. Shin, S.-H. Kwon, S.-W. Kang, S. O. Kim, Adv. Mater. 2008, 20, [22] B. H. Kim, J. Y. Kim, S.-J. Jeong, J. O. Hwang, D. H. Lee, D. O. Shin, S. Y. Choi, S. O. Kim, ACS Nano 2010, 4, [23] B. H. Kim, D. H. Lee, J. Y. Kim, D. O. Shin, H. Y. Jeong, S. Hong, J. M. Yun, C. M. Koo, H. Lee, S. O. Kim, Adv. Mater. 2011, 23, [24] B. H. Kim, K. J. Byeon, J. Y. Kim, J. Kim, H. M. Jin, J. Y. Cho, S. J. Jeong, J. Shin, H. Lee, S. O. Kim, Small 2014, 10, [25] S.-Y. Cho, H.-J. Jeon, J.-S. Kim, J. M. Ok, H.-T. Jung, Adv. Funct. Mater. 2014, 24, [26] S.-Y. Cho, H.-J. Jeon, H.-W. Yoo, K. M. Cho, W.-B. Jung, J.-S. Kim, H.-T. Jung, Nano Lett. 2015, 15, [27] R. Behrisch, G. Maderlechner, B. M. U. Scherzer, M. T. Robinson, App. Phys. 1979, 18, 391. [28] P. Sigmund, Phys. Rev. 1969, 184, 383. [29] S.-J. Jeong, J. E. Kim, H.-S. Moon, B. H. Kim, S. M. Kim, J. B. Kim, S. O. Kim, Nano Lett. 2009, 9, [30] R. Bukasov, J. S. Shumaker-Parry, Nano Lett. 2007, 7, [31] L. Y. Wu, B. M. Ross, L. P. Lee, Nano Lett. 2009, 9, [32] V. E. Bochenkov, D. S. Sutherland, Nano Lett. 2013, 13, [33] W. Van Zoelen, G. ten Brinke, Soft Matter 2009, 5, [34] Y. Yi, M. Nakata, A. R. Martin, N. A. Clark, Appl. Phys. Lett. 2007, 90, [35] Y. W. Yi, V. Khire, C. N. Bowman, J. E. Maclennan, N. A. Clark, J. Appl. Phys. 2008, 103,