Controlled Segmentation of Metal Nanowire Array by Block Copolymer Lithography and Reversible Ion Loading

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1 Nanowires Controlled Segmentation of Metal Nanowire Array by Block Copolymer Lithography and Reversible Ion Loading Jeong Ho Mun, Seung Keun Cha, Ye Chan Kim, Taeyeong Yun, Young Joo Choi, Hyeong Min Jin, Jae Eun Lee, Hyun Uk Jeon, So Youn Kim,* and Sang Ouk Kim* Spatial arrangement of 1D nanomaterials may offer enormous opportunities for advanced electronics and photonics. Moreover, morphological complexity and chemical diversity in the nanoscale components may lead to unique properties that are hardly anticipated in randomly distributed homogeneous nanostructures. Here, controlled chemical segmentation of metal nanowire arrays using block copolymer lithography and subsequent reversible metal ion loading are demonstrated. To impose chemical heterogeneity in the nanowires generated by block copolymer lithography, reversible ion loading method highly specific for one particular polymer block is introduced. Reversibility of the metal ion loading enables area-selective localized replacement of metal ions in the self-assembled patterns and creates segmented metal nanowire arrays with different metallic components. Further integration of this method with shear aligning process produces high aligned segmented metal nanowire array with desired local chemical compositions. 1. Introduction 1D metallic nanostructures such as nanoscale fibers, tubes, and wires have attracted a great deal of research interest owing to their unique electronic and optical properties arising from the quantum confinement and shape anisotropy. [1,2] For such nanostructures, template-directed synthesis using nanoporous crystalline materials is widely used Dr. J. H. Mun, S. K. Cha, T. Yun, Y. J. Choi, H. M. Jin, J. E. Lee, H. U. Jeon, Prof. S. O. Kim Department of Materials Science and Engineering National Creative Research Initiative Center for Multi-Dimensional Directed Nanoscale Assembly KAIST Daejeon 34141, Republic of Korea sangouk.kim@kaist.ac.kr Y. C. Kim, Prof. S. Y. Kim School of Energy and Chemical Engineering Ulsan National Institute of Science and Technology (UNIST) Ulsan 44919, Republic of Korea soyounkim@unist.ac.kr DOI: /smll as a straightforward route. [3] Organic materials, such as block copolymer (BCP) micelles [4] and biological molecules [5] can also be used as templates. Nevertheless, spatial arrangement of the synthesized nanostructures still remains challenging despite the significance in the advanced applications in electronics and photonics. [6 8] BCP lithography exploits self-assembled nanostructures in BCP thin films for lithographic template application. [9 12] This approach holds great promise for ultrafine nanoscale patterning with enormous advantages, such as large-area scalability and cost effectiveness. For the effective pattern transfer of organic BCP self-assembled morphology into functional nanopatterns, various methods have been introduced, including vapor deposition, [13] sequential infiltration synthesis, [14 16] particle decoration, [17] etching, [18] and ion loading. [19,20] Unfortunately, large-area processing without area-selectivity commonly leads to nanopatterns with homogeneous chemical composition over the entire pattern area. It is hard to expect chemical and functional diversity in the final patterned nanostructure, which greatly diminishes the potential application fields. In this work, we present area-selective chemical segmentation of metallic nanowire arrays prepared from BCP 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 7)

2 full papers lithography. To induce desired local chemical segmentation, we exploited reversible metal ion loading, where metal ion complexation in BCP nanodomains can be unloaded and replaced by another type of metal ion complex. The spatial location of reversible metal ion complexation can be exactly controlled by means of pattern masks. Our approach can be employed for the highly aligned BCP nanopatterns prepared by in-plane shear alignment and successfully creates metal nanowire arrays with desired local segmentation. 2. Results and Discussion Figure 1 illustrates schematic procedure for the controlled segmentation of metal nanowire arrays. Asymmetric BCP polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) thin film was spin-cast on a substrate and self-assembled into aligned surface parallel nanocylinder arrays via thermal annealing with external shear stress (Figure S1, Supporting Information). [21 23] The resultant nanoscale morphology in the BCP thin film was observed with scanning electron microscope (SEM). The samples for SEM characterization were immersed in ethanol for 10 min to selectively swell P2VP cylinder nanodomains for imaging contrast. The top layer of BCP thin film originally covered with PS block becomes perforated during the swelling of P2VP block; [19] thus, the aligned trace of underlying nanocylinder array can be directly observed. As shown in Figure S2 (Supporting Information), well-aligned nanowire arrays could be obtained over large area (>4 cm 2 ) for various molecular weights of PS-b-P2VP: periodicity from 25 to 75 nm. Prepared BCP thin film was then immersed into acidic aqueous precursor solution for the selective metallic anion complexation at P2VP nanocylinder domains, which was followed by thorough rinsing with deionized (DI) water to remove unloaded precursors. Afterward, polyacrylic acid (PAA) was spun-cast onto the BCP thin film as a sacrificial layer for the lift-off of the photoresist (PR) overlay. [24] PR line pattern was prepared by conventional photolithography and transferred onto the PAA-coated BCP thin film. Following oxygen plasma etching selectively removed the bare BCP parts without PR cover and generated metal nanowires replicating the BCP nanocylinder morphology. For the reliable protection of BCP layer under PR mask, highly etch-resistant negative tone PR was used. The remaining PR mask could be easily removed by immersing the sample in water while PAA sacrificial layer is dissolved in water. The metal ions in the remained P2VP domains in BCP thin film (PR covered region) were unloaded by immersing in concentrated acidic aqueous solution without any other additive. A different kind of metallic anion was newly loaded into the P2VP nanocylinders. Following oxygen plasma treatment etched out remaining BCP part; therefore, chemically segmented nanowire arrays could be obtained over a large area. The fabricated nanowire array could be either in oxidized or pure metal states. Figure 2 shows the reversible ion loading process for P2VP block at each processing step. Figure 2A schematically illustrates reversible platinum (Pt) and gold (Au) complex anion loading and unloading. Figure 2B shows a SEM image of cylindrical PS-b-P2VP BCP thin film before ion loading. Due to the almost similar level of electron scattering from PS and P2VP block, self-assembled nanoscale morphology was not clearly shown. We also performed X-ray photoelectron spectroscopy (XPS) measurement during the reversible ion loading. Before metal ion loading, no observable peak was found in pure BCP films. After Pt ion loading, platinum complexed P2VP nanocylinders clearly look bright under SEM (Figure 2C) and XPS detected Pt 4f peaks. Figure 2D shows SEM image of the BCP thin film after Pt ion unloading in acidic aqueous solution. Pt anions Figure 1. Schematic illustration of heterogeneously segmented nanowire array using PS-b-P2VP thin film. PAA layer acts as a sacrificial layer to remove PR pattern and each uncovered and covered region contains different metallic components through reversible ion loading process (2 of 7) Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

3 Figure 2. A) Schematic procedure of reversible ion loading to P2VP block. In the experiment, chloroplatinate anion was loaded first and unloaded, then chloroaurate anion was reloaded. SEM images of substrate parallel oriented cylindrical PS-b-P2VP thin film B) before ion loading, C) after platinum ion loading, D) after platinum ion unloading, and E) after gold ion reloading. Figures right to each SEM image show XPS spectra at each step. complexed at protonated pyridinic site compete with chloride anion which is abundant in the solution. As a result, chloride anion substitutes for the pre-existing metallic complex anion at the pyridinic site. In the SEM image, P2VP block seems no longer brighter without metal ions. However, there seem some popped traces along the P2VP cylinders, resulting from the swelling of P2VP block in strong acid. Consistent with the SEM image, Pt 4f peaks disappeared in the XPS measurement. Figure S3 (Supporting Information) shows the SEM image of the sample in Figure 2D after the polymer removal by oxygen plasma etching. We could not found any metallic nanowire array remained on the substrate, therefore, we could confirm no residual metal ion left in the template BCP film. After successful unloading another metallic anion could be complexed at the P2VP cylinders through the same process as done with the first ion loading. As shown in Figure 2E, Au ion could be successfully reloaded and the resultant welldefined nanowire structure was observable by SEM again. XPS detected Au 4f peak concurrently. To fabricate segmented nanowire array, negative tone PR pattern was prepared by conventional photolithography. The PR pattern was prepared on a silica substrate, which plays the role of sacrificial layer for transfer. We used polydimethylsiloxane (PDMS) pad as a supporting material for the effective transfer onto the desired location at BCP films. [25] Figure 3D shows the SEM image after PR pattern transfer and first oxygen plasma treatment of randomly aligned BCP thin film. After the plasma treatment, exposed platinum nanowire structure is observed and negative tone PR pattern still exists covering the remaining BCP thin film. Figure 3E shows the SEM image after dissolving PAA sacrificial layer in DI water bath to remove upper PR pattern. Platinum ion loaded BCP thin film underlying the PR layer is well-maintained on the substrate. Noteworthy that we introduced graphene oxide (GO) layer to avoid the direct contact between the BCP thin film and PAA sacrificial layer (Figure 3A). As shown in Figure 3B, more than half area of BCP film (darker horizontal lines in the SEM image) was detached out from the substrate without GO layer, which is presumably due to the strong associative interaction between interpenetrating PAA sacrificial layer and BCP thin film. By contrast, Figure 3C confirms that GO layer successfully prevented the damage of BCP layer, and the stripe pattern morphology was well-maintained over a large area of BCP thin film after the removal of PAA and PR layers. Figure 4A shows the SEM image of well-aligned underlying cylinders using external shear stress after PAA and PR removal and Figure 4B shows the SEM image of heterogeneously segmented nanowire arrays composed of platinum (darker) and gold (brighter) segments after the final processing step. Periodicity of nanowire array was 43 nm and the areal width of PR covered and uncovered region were 600 and 400 nm, respectively. The edge of remaining BCP thin film became rough presumably after plasma etching process. For the imaging contrast between chemically different segments, a less amount of platinum was loaded than gold. As shown in Figure 4B, heterocomponents of platinum and gold nanowire are continuously connected with well-defined sharp interface morphology. Although platinum and gold segments were created from the different parts of BCP thin film and loaded separately, the continuity between them is well-maintained while the metal agglomeration under oxygen plasma treatment. [26] Combinations of heterocomponents could be varied with other metallic components by simply changing the metal 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (3 of 7)

4 full papers Figure 3. A) Schematic illustration of GO layer insertion between BCP thin film and PAA layer. Broad field SEM images of B) partially detached remaining BCP thin film (darker horizontal lines) during PAA sacrificial layer removal and C) well-preserved remaining BCP thin film due to graphene oxide preventing layer. SEM images of platinum nanowire arrays D) before removing PAA sacrificial layer with transferred PR pattern and E) after removing PAA sacrificial layer and PR lift-off. Randomly aligned PS-b-P2VP structure which was under the PR still remained after the process. precursor in the loading solution. Gold, platinum, palladium, iron, cobalt, and alloying mixture of these can be prepared by the reversible ion loading method. [27] The inset image in Figure 4B shows the successful fabrication of heterogeneous segmented nanowire array over a large area. Noteworthy that the fabrication method for segmented nanowire consists of all parallel processing over large area. Thus, our demonstration can be readily scaled up. Moreover, the morphology of heterogeneously segmented nanowire array could be varied by simply modifying the shape of PR pattern or transfer angle. As shown in Figure 4C, platinum gold nanowire array with 45 tilted segmentation was obtained by tilting the angle of transferred PR pattern and BCP alignment direction. Besides the tilted segmentation, the position and shape of segmentation could be modified in a variety of ways. As shown in Figure 4D, we obtained ellipsoidal shaped platinum segmented gold nanowire array by transferring PR pattern and stacking with different angles. As seen from the examples, reversible metal ion loading process with PR can readily yield chemical segmentation in BCP metal nanostructure with great versatility of diverse pattern shapes and scalability. To date, many successful approaches for directed selfassembly have been introduced to control the orientation and lateral ordering of BCP nanodomains. However, crucial limitations still remain: the direction of self-assembled BCP nanostructure is subordinate to that of an external guidance such as physical geometry, [28 31] chemical contrast, [32] external field, [21 23,33] and so on. We note a small modification of the presented study can circumvent the limitation. Since we could selectively leave BCP thin film with desired spatial position and orientation, we created patterned nanowire arrays with orthogonal nanodomain alignment to the direction of remaining BCP thin film in the PR covered region. Figure 5A schematically illustrates the procedure of patterned nanowire array. The experimental procedure is similar with that of fabrication of heterogeneously segmented nanowire array. Transferred PR plays the role of etch mask to protect the bare BCP thin films underneath during the oxygen plasma treatment. After removing PAA sacrificial layer and PR in the DI water bath, metallic anion is loaded to remaining BCP thin film. Upon the subsequent oxygen plasma treatment, metallic nanostructure was created. Consequently, we could obtain nanowire array with orthogonal orientation to prepattern direction over large area, as shown in Figure 5B and its inset. Similar patterned nanowire array fabrication is also applicable to the tilted PR pattern transfer angles and various shapes of PR patterns, as shown in Figure 5C. This method could offer various pattern (4 of 7) Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

5 Figure 4. A) SEM images of platinum nanowire array during heterogeneously segmented nanowire array fabrication procedure from Mn = 62.5 kg mol 1 PS-b-P2VP. PAA sacrificial layer and upper PR pattern were removed. B) SEM image of platinum (darker) gold (brighter) heterogeneously segmented nanowire array. The inset shows the large area image. SEM images of C) platinum gold segmented nanowire array from 45 tilted transfer of PR pattern and D) ellipsoidal shaped segmentation from stacking PR pattern with different directions. Figure 5. A) Schematic illustrations of patterned nanowire array. The procedure is similar to heterogeneous segmented nanowire array; however, loading metallic anion is done after PAA and PR removal and reloading metallic anion is skipped. SEM images of B) patterned nanowire array which is oriented perpendicularly to prepattern and large area image C) patterned nanowire array with ellipsoidal-shaped hole by stacking PR pattern with different directions from Mn = 62.5 kg mol 1 PS-b-P2VP Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (5 of 7)

6 full papers shapes required in electronic or other applications as the alignment direction of nanowire arrays can be independently controlled from prepattern masks. The future work will deal with the detailed study of hierarchical variation of nano- and micromorphology. [34] 3. Conclusion We have demonstrated a facile fabrication of chemically heterogeneous segmented nanowire arrays based on scalable self-assembly principle. This is the first time observation of reversible ion loading in BCP thin film and its application in the segmented metallic nanostructures based on BCP self-assembled nanopatterning. Directed self-assembly with external shear stress can create well-aligned nanowire arrays while reversible ion loading process with transferable PR pattern mask can afford the chemical diversity of nanowire arrays at desired location. Taken together, our method overcomes the limitations of current nanowire assembly and BCP nanopatterning by introducing positional ordering and chemical diversity in the resultant nanostructures. We anticipate that more complicated functional nanostructures can be attained with our approach, while keeping with the inherent advantages of BCP nanopatterning, such as large-area scalability and process simplicity. layer, 2 wt% of PAA aqueous solution was spun-cast (2000 rpm for 60 s) and thermally treated at 100 C for 2 min to remove humidity. A photopatterned SU-8 photoresist layer was transferred onto the PAA layer surface with a PDMS transfer master. Subsequent oxygen plasma treatment (O Torr, 100 W, 4 min) over the entire sample area led to the area-selective formation of metallic nanowire array at the BCP film area without photoresist layer. After the removal of remaining photoresist, PAA, and graphene oxide layers by spin-washing with deionized water, metal ion unloading/reloading and sequential 2 min of oxygen plasma treatment (O Torr, 50 W) resulted in highly aligned segmented nanowire array. Fabrication of Photoresist Pattern: A negative-tone photo resist layer of SU-8 (MicroChem Corp.) was spun-cast onto a piranha cleaned Si substrate (500 nm silica deposited silicon). Soft baking was performed at 90 C for 1 min. The SU-8 layer was photopatterned with an I-line photolithography equipment (Midas/ MDA-6000 DUV, wavelength: 365 nm; 9.5 mw cm 2 ). Post baking was performed at 110 C for 80 s to induce chemical cross-link in the radiation exposed area. The pattern development was performed by subsequent immersion in a commercial SU-8 developer (MicroChem Corp.) and isopropyl alcohol (IPA). Characterization: Nanoscale morphology of the BCP thin films and metallic nanostructures were characterized using S-4800 FE-SEM (Hitachi). XPS measurements were performed using ESCA 2000 system (Thermo VG Scientific). 4. Experimental Section BCP Thin Film Preparation: Asymmetric BCPs, PS-b-P2VPs ( k, k, k, k, and kg mol 1 for PS, and P2VP number-average molecular weights, respectively, purchased from Polymer Source Inc.) were used as self-assembling materials. Thin films of the BCPs were prepared by spin-casting of proper thicknesses for monolayer P2VP cylinders. (Spin-casting condition: 33.5 kg mol 1, 1 wt% at toluene, 2500 rpm; 42.5 kg mol 1, 1.1 wt% at toluene, 2500 rpm; 62.5 kg mol 1, 1.3 wt% at toluene, 3300 rpm; 94.5 kg mol 1, 1.5 wt% at toluene, 3000 rpm; 184 kg mol 1, 1.6 wt% at toluene, 2500 rpm.) External shear stress was applied to the BCP thin films over large area by means of PDMS pads conformally contacted at the thin film surfaces. For all different molecular weights of BCPs, 20 kpa of shear stress was applied at C for h. Reversible Metal Ion Loading: Metal complex anion loading was performed by the immersion of self-assembled BCP films in the 1 wt% HCl acidic aqueous solution containing metal ion precursors and the subsequent rinsing with deionized water for 30 s to 3 min. (The loading amount increases with time and becomes saturated.) [17] Unloading of metal anions was done by immersing 3 wt% HCl acidic aqueous solution without any metal ion precursor ( 30 min) and subsequent thorough rinsing with deionized water. Reloading of different metal ions was done by the same procedure with initial metal loading process. The metallic complex anion precursors used in this work were Na 2 PtCl 4 H 2 O and HAuCl 4 (purchased from Strem Chemicals, Inc.). Fabrication of Segmented Nanowire Array: Graphene oxide layer was spun-cast on the surface of metal ion loaded shear aligned BCP thin film with N 2 gas blowing. Over the graphene oxide Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This work was supported by the National Creative Research Initiative (CRI) Center for Multi-Dimensional Directed Nanoscale Assembly (2015R1A3A ) and the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B ). Y.C.K. and S.Y.K. were supported through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016M3A7B ). [1] J. Sharma, T. Imae, J. Nanosci. Nanotechnol. 2009, 9, 19. [2] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 2003, 15, 353. [3] C. R. Martin, Nanomaterials A Membrane-Based Synthetic Approach, DTIC Document, [4] H. Wang, A. J. Patil, K. Liu, S. Petrov, S. Mann, M. A. Winnik, I. Manners, Adv. 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