Transferrable Plasmonic Au Thin Film Containing Sub-20 nm

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Transferrable Plasmonic Au Thin Film Containing Sub-20 nm Nanohole Array Constructed via High-resolution Polymer Self-assembly and Nanotransfer Printing Soonmin Yim, Suwan Jeon, Jong Min Kim, Kwang Min Baek, Gun Ho Lee, Hyowook Kim, Jonghwa Shin, and Yeon Sik Jung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16401 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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ACS Applied Materials & Interfaces

Transferrable Plasmonic Au Thin Film Containing Sub-20 nm Nanohole Array Constructed via Highresolution Polymer Self-assembly and Nanotransfer Printing Soonmin Yim, Suwan Jeon, Jong Min Kim, Kwang Min Baek, Gun Ho Lee, Hyowook Kim, Jonghwa Shin, and Yeon Sik Jung* Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Republic of Korea

*Corresponding author’s e-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The fabrication and characterization of nanoscale hole arrays (NHA) have been extensively performed for a variety of unique characteristics including extraordinary optical transmission phenomenon observed for plasmonic NHAs. Although the size miniaturization and hole densification are strongly required for enhancement of high-frequency optical responses, from a practical point-of-view, it is still not straightforward to manufacture NHA using conventional lithography techniques. Herein, a facile, cost-effective, and transferrable fabrication route for high-resolution and high-density NHA with sub-50 nm periodicity is demonstrated. Solventassisted nanotransfer printing with ultrahigh-resolution combined with block copolymer selfassembly is used to fabricate well-defined Si nanomesh master template with four-fold symmetry. An Au NHA film on quartz substrate is then obtained by thermal-evaporation on the Si master and subsequent transfer of the sample, resulting in NHA structure having a hole with a diameter of 18 nm and a density over 400 holes/µm2. A resonance peak at the wavelength of 650 nm, which is not present in the transmittance spectrum of a flat Au film, is observed for the Au NHA film. Finite-difference time-domain (FDTD) simulation results propose that the unexpected peak appears due to plasmonic surface guiding mode. The position of the resonance peak shows the sensitivity toward the change of the refractive index of surrounding medium, suggesting it as a promising label-free sensor application. In addition, other types of Au nanostructure arrays such as geometry-controlled NHA and nanoparticle arrays (NPAs) shows the outstanding versatility of our approach.

KEYWORDS: Block copolymer, Directed self-assembly, Solvent-assisted nano-transfer printing, Plasmonic film, Nanohole array


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Nanoscopic hole arrays have been applied as practical templates for various applications such as templated nanofabrication, color filters, perfect absorbers, cell trapping membranes, X-ray grating array, lithography masks, and leak detection.1, 2 In particular, plasmonic nanohole arrays (NHA) formed in metallic films have received considerable attention due to their extraordinary optical transmission (EOT) phenomena and extensive applications for label-free biosensors, plasmonic photovoltaics, and surface-enhanced spectroscopy. Among them, label-free biosensors based on NHA have been suggested as a promising potential application due to their excellent sensitivity,3-5 which originates from the surface plasmons (SP) concentrated near the interface between the metal surface and the surrounding environment. Enhancement of optical transmission through a metal NHA at a specific wavelength range is caused by interaction between the incident light and the generated SP at the metal surface, and the wavelength of spectral resonance is dependent on the geometry and arrangement of nanoholes.4 Thus, the transmission characteristic of NHAs can be finely controlled through the rational design of NHAs, which can be advantageous for label-free sensor applications.4 Moreover, NHA-based biosensors demonstrate advantages such as a small device footprint, simple measurement setup, and easy integration of sensors with an imaging system.6 While pursuing the various applications of NHA, it can be miniaturized to be integrated into a single sensing chip to improve the performance and reduce manufacturing cost. For example, the incorporation of an NHA into a microfluidic system can provide the advantages of further extension of its application range and improved sensor performance.7 In addition, adjusting the flowing direction of the solution containing target elements through a NHA could enhance the sensitivity of nanohole-array sensors.7 On the other hands, further densification of nanoholes would be desirable because, by integration of high-density NHAs into a single chip, each array in


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a multi-nanohole array system can function as an independent sensor, and thus the sensor system provides high-speed detection capability.8, 9 However, despite the advantages and potential of high-density NHA, conventional highresolution fabrication methods based on serial patterning processes such as e-beam lithography (EBL) and focused ion beam (FIB) milling suffer from low throughput and high manufacturing expense.3, 5 Although other nanofabrication tools such as colloidal lithography and nanoimprint lithography have also been applied,9,

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developing a more practical processing strategy with

sufficiently high resolution, high throughput, cost-effectiveness, and scalability remains an important issue. In this sense, nanotransfer printing (nTP) can be considered for practical manufacturing of sensors containing NHA due to its confirmed scalability and cost-effectiveness. However, miniaturization of the pattern size fabricated by nTP is generally limited to the submicron range.11 Consequently, the operation wavelength of optical nanostructures fabricated through sub-micron nTP11, 12 remains in the near-infrared (NIR) range. For example, an NIRoperating metamaterial structure composed of multi-stacked meshes with a submicron unit cell size was successfully formed by the repetitive implementation of nTP.11, 12 By further reducing the size of the building blocks that make up optical metamaterials, the operating wavelength would reach the visible wavelength range, significantly improving the practicality of optical metamaterials. Meanwhile, to overcome the resolution limit of photolithography, bottom-up based nanofabrication methods such as block copolymer (BCP) self-assembly can be an alternative route due to beneficial characteristics such as high-resolution down to sub-10 nm, costeffectiveness, high-throughput, and scalability.13,

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The extremely high resolution and high

throughput of BCP nanopatterning is highly suitable for practical NHA fabrication. However, the


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self-assembled polymeric nanopatterns without optical functionality in the visible range need to be converted into metallic nanostructures using dry etching or lift-off processes. For the direct formation of functional metallic nanostructures while maintaining ultra-high resolution and high throughput, solvent-assisted nanotransfer printing (S-nTP) was recently developed by our group.15 S-nTP consists of replication of reusable templates, metal deposition, and solventassisted adhesion control, allowing rapid and high-yield transfer-printing of nanostructures without any surface treatment.15 S-nTP provides direct and simple patterning of various metallic nanostructures in a wide size range from several nanometers to microns. Herein, we demonstrate a facile and cost-effective fabrication method of preparing highdensity Au NHA with a sub-50 nm periodicity based on the synergic combination of BCP selfassembly and nTP. The high resolution of the resultant NHA structures benefits from the nanoscale self-assembly of Si-containing BCPs, which can be converted into silica nanopatterns and used as reusable templates. Moreover, we show that the high-throughput and large-area formation of nanomesh structures is realized by S-nTP, and thus Au NHA can be simply formed by evaporative deposition of an Au film on the nanomesh template. As a result, Au NHA with a diameter of 18 nm and with an extremely high hole density (over 400 holes µm-2) was fabricated without using any high-cost lithographic processes. We show that the incorporation of highdensity nanoholes affect the optical transmission characteristics of the Au film. A resonance peak at around 650 nm, which is not present in the transmittance spectrum of a flat Au film, is observed for the Au NHA film, suggesting that the new transmission peak stems from a plasmonic effect induced by the densely packed nanohole structures. A finite-difference timedomain (FDTD) simulation is also performed to elucidate the mechanism of the modulated optical properties. The sensitive dependence of the resonance peak position on the refractive


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index of the surrounding medium also implies its promising potential for high-performance sensor applications.

RESULTS AND DISCUSSION Figure 1 shows the entire procedure of nanohole array fabrication in Au films. A linear master template with a line width of 20 nm was obtained from the self-assembly and plasma oxidation of a cylinder-forming Poly(styrene-b-dimethylsiloxane) (PS-b-PDMS) BCP.16, 17 Cr nanowires (NWs) were then transfer-printed onto a Si wafer via the replication of the BCP patterns, formation of discrete Cr NWs on the replica using oblique-angle deposition of Cr, and S-nTP of the NWs onto the Si substrate.15 After forming the first layer of aligned Cr NWs, second Cr NWs were transfer-printed on the first Cr NWs with an 90°alignment angle. The exposed area of the Si wafer was then etched by repeated SF6 and C4F8 plasma dry etching (the Bosch process). During the plasma etching process, the crossed Cr NWs serve as an etch mask, resulting in Si nanowell structures after completing the plasma etching of Si and removal of Cr NWs, as schematically shown in Figure 1j. The well-defined structure of the Si nanowell was confirmed by scanning electron microscopy (SEM), as presented in Figure 2a. For the formation of Au nanohole structures, Au was deposited by thermal evaporation on the Si nanotemplate. Controlled deposition of Au provided the separate formation of (1) a perforated Au film at the mesa region of the Si nanowell master template and (2) Au dot structures at the bottom of the trench region, as illustrated in Figure 1l. The control of the sidewall profile of the Si nanowell template was found to be an important factor for the formation of well-defined Au NHA. The reliable separation between the perforated Au film on the mesa of the Si nanowell and Au dots formed on the trench region required a reverse taper structure of the sidewall. Thus, the


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dry etching parameters were finely tuned to fabricate a well-defined Si nanowell template with the desired sidewall profiles, as shown in Figure S1a in the Supporting Information and the inset of Figure 2a. As shown in Figure 2b, the Au film structure precisely replicates the structure of the Si master template. A slight shrinkage of the nanohole size from that of the Si nanowell was observed, which is caused by lateral growth of Au during the deposition process. The statistic histogram shown in Figure 2c indicates a decrease of the hole size from 15.02 nm (Si template) to 14.22 nm (Au film on the master). The Au film containing high-density NHA was then transferred onto a quartz substrate using a polymer carrier film. (Figure 1m-o) KOH solution was used to detach the polymer/Au mesh film from the Si master template. After transferring the Au structure, the polymer thin film was removed by rinsing with toluene. It was observed that the transferred Au structure with a high-density NHA did not accompany any Au dots (Figure S1b and Figure 2d), which was achieved by the precise control of the side-wall profile of the Si NWs and the deposition conditions of Au. The excellent area scalability is one of the advantages of the fabrication method. As shown in Figure 2d, the entire size of the transferred sample was 1.5 × 1.5 cm2, which corresponds to the cross-sectional dimension of the pristine BCP master template (1.5 × 2.0 cm2); it can easily be scaled up by extending the size of the template substrate for BCP self-assembly, which is highly viable because wafer-scale directed self-assembly (DSA) pattern formation was already demonstrated and its uniformity was confirmed.16 In addition, the Au NHA film can be transferred onto diverse unconventional substrates such as vials, cuvettes, and pipettes, and thus its integration with other devices will be straightforward. (Figure S2) We then characterized the optical properties of the fabricated Au mesh sample in comparison with a continuous Au thin film with the same thickness (27 nm) on quartz. As shown in Figure


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3a, the Au film having a nanohole array exhibited an additional transmission peak at around 650 nm, which is not observed for a flat Au film. The additional transmission peak may originate from plasmonic resonance induced by the structured pattern composed of a hole array and DSA guide patterns.3, 18, 19 On the other hand, our proposed approach has the advantage of applicability to fabricate a nanohole array with different alignment angles, which may provide additional polarizationdependent optical responses (Figure S3). Recently, a template stripping (TS) method using an optical adhesive has been widely applied to the fabrication of plasmon nanostructures for manipulation of surface plasmons.5,

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Using the TS method, we fabricated NHA on a poly

urethane acrylate (PUA) film. (Figure S4) Although the TS method can provide the benefit of reusability of the master template, the reliability of the optical property is poorer than with our polymer-assisted transfer method because Au NPs are also randomly detached by PUA infiltrated in nanoholes. Therefore, coupling of NHA with Au NPs (disks) may have resulted in a red shift and a broad peak width in the additional peak, as shown in Figure S4d.21 On the other hand, an NHA film can be used as a shadow mask for fabricating nanoparticle arrays (NPAs), which provide a useful structure for various applications such as nanoscale plasmon lasing, sensors, and electroluminescence.22 The yield of Au NP formation in the Si template is considerably enhanced by blocking the infiltration of PUA into the holes via increasing the depth of holes in the Si template and depositing thicker Au. As shown in Figure S5, thick Au deposition (> 40 nm) on the Si master effectively protected the underlying Au NPAs from contacting PUA, and thus well-ordered NPAs can be obtained after selective delamination of the covered Au film.


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For a more in-depth analysis of the phenomenon, the optical property of the gold nanohole structure was analyzed via finite-difference time-domain (FDTD) simulations. The morphology of the actual nanohole array sample was approximated by an array of identical, toroidally rounded holes, with the following structural parameters: the period of unit cell (a), the height of Au film (d), the radius of the toroidal cross section (r), and the inner radius of circular nanoholes (Rin) were chosen to be 50 nm, 27 nm, 16.5 nm, and 14.25 nm, respectively, based on the statistical measurement data in Figure 2c and Figure S6. The pattern from the DSA guide template was also incorporated in the simulation. The complete simulation domain contains a uniform hole array region (950 nm width), a slot array region (130 nm), and a uniform film region (170 nm). The total period of the pattern was 1250 nm and the structural details are described in Figure S7a. The transmission spectrum from the simulation in Figure 3b reproduces important features from the experimentally measured spectrum in Figure 3a, especially the two predominant peaks in the visible range. The shorter-wavelength peak around 565 nm is due to decreased reflectance resulting from the diminished absolute permittivity at frequencies slightly lower than the interband transition of gold. As it is due to the intrinsic property of gold, this peak appears for many other gold thin film samples as well, whether they are structured or non-structured. On the other hand, the second peak at 690 nm possibly originated from an interaction between a light and structured pattern, which does not appear in the Au film. To investigate the origin of the side peak, we analyzed the electric field of surface plasmonic resonance (SPR) and surface guiding modes in the FDTD simulation results (Figure 3c and Figure S7). The excited mode on NHA appears to be confined by the DSA guide pattern and oscillates like a mode in the optical cavity. Actually, the DSA guide pattern not only works as a mirror of Fabry-Perot resonance, but also as


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a SPR coupler (Figure S8). Without the guide pattern, the SPR is not excited in the periodic hole array, which means the hole structure itself is too small to excite the SPR, but the scale of the guide pattern is adequate to couple and maintain the confined mode. Thus, we conclude that the additional peak originated from Fabry-Perot resonance of surface guided waves along the nanohole array structure and the guide pattern period corresponds to the Fabry-Perot condition.23 Meanwhile, the peak positions in the transmittance simulation is slightly deviated compared to the experimental results, and this may be due to the fact that the actual permittivity of gold is influenced by increased electron scattering if the feature size is less than 20 nanometers. The small disagreement may also be caused by deformation24 and inhomogeneity25 of hole geometry in the actual sample. The plasmonic phenomena can be utilized as a refractive index sensor if the plasmonic mode is effectively exposed to the surrounding medium. Thus, the change of the refractive index can lead the change of the plasmonic mode condition.5, 26, 27 In our experiment, we systematically controlled the refractive indices of the surrounding medium by mixing ethanol (n = 1.33) and toluene (n = 1.48) with different ratios.28 Transmittance spectra were obtained using a quartz cuvette to minimize unintentional absorption from the experimental setup, as shown in the inset of Figure 4a. The relationship between the resonance peak position and the refractive indices of the surrounding media was investigated. In order to eliminate the effect of light absorption by solvents, base lines were set by the transmittance of a blank solvent prior to the measurement of the NHA samples. As a result, the shift of the transmittance spectra was clearly observed, as shown in Figure 4a. Figure 4a presents that as the refractive index increases, the resonance peak is red-shifted. The shift of the peak can be more clearly confirmed by deconvolution of the measured transmission spectra, as shown in Figures 4b, c. The peak shift phenomenon was also


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simulated by the FDTD method, as shown in Figure 4d, indicating the trend of the calculation result is well matched with the experimental data. The small inconsistency in the peak position and shape between the measured data (Figure 3a and Figures 4a – c) and the simulated data (Figure 3b, Figure 4d) may originate from the size and geometrical distribution of the nanoholes. Also, the polycrystalline nature of the deposited Au film would cause surface roughness (rootmean-square (RMS) roughness = 0.64 ± 0.11 nm) of the samples, which can affect the LSPR characteristics of the NHA samples. In addition, we assumed the nanoholes are perfect circles for the theoretical analysis of structure, whereas the actual shape of the nanoholes varied between square and circular, as shown in Figure S6a. However, overall, the simulation data are consistent with the entire tendency of the experimental data. The sensitivity of the high density nanohole array is 115 nm RIU-1 (refractive index unit) measured from the slope of the fitted line, as shown in the inset of Figure 4c. The FDTD simulation shows an almost consistent but relatively higher sensitivity value (177 nm RIU-1), as shown in the inset of Figure 4d. The enhanced sensitivity in the simulation may come from structural perfectness of the unit cell geometry. Although the sensitivity of NHA is not as high as that reported in previous works,5 the NHA may be more surface-sensitive to environment change. As presented in Figure S9, using a FDTD simulation, we compared the NHA and a 25-nm-thick gold film with variation of the proximity refractive index within 30 nm distance from the top surface. When the index is changed from 1 to 1.5, the NHA shows a four times larger peak shift than the film. To analyze the surface sensitivity, we also measured the decay length of the mode and field intensity distribution (Figure S10 and Table S1). The decay length of NHA was half of that of the thin film, and the intensity was significantly concentrated in the hole. Because the electric field of the SPR mode is more focused around the high-density nanoholes, the NHA becomes more surface-sensitive than


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the SPR in the flat film. In this regard, the NHA may be promising for highly sensitive biosensing and other proximity index sensing.

CONCLUSIONS In summary, we demonstrated a facile, cost-effective, and scalable fabrication method to prepare a high-resolution and high-density plasmonic nanohole array based on the combination of BCP self-assembly and a nanoscale transfer-printing process. An Au periodic nanohole array with a four-fold symmetry was prepared from a Si nanowell master template, which was fabricated by the replication of a self-assembled BCP template with sub-20 nm feature size and transfer-printing of well-aligned Cr nanowires, and subsequent reactive ion etching of the Si substrate. Even without using a laborious or high-cost lithographic process, the dimension of the nanoholes is comparable with previously reported resolution obtained using high-cost and lowthroughput patterning tools such as EBL and FIB. Moreover, the perforated Au film with a macroscopic area was successfully transferred to a quartz substrate for optical characterization. The plasmonic nanohole sample exhibited additional transmittance peaks at about 650 nm, which are not observed for flat Au films. The unusual transmission behavior was attributed to the surface plasmon mode, which is guided along the uniform hole array and confined by the guide structure. In addition, the location of the resonance peak was strongly influenced by the changes of the refractive indices of the surrounding medium, suggesting promising application as a plasmonic index sensor. These experimental data were consistent with FDTD simulation results. Au NHA also was used as a master template for the fabrication of well-ordered and high-density NPA, and thus demonstrates the possibility of wide usage of the NHA film. The successful fabrication of high-density and high-resolution nanohole array structures on a macroscopic area


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suggests that this approach can be further extended to the volume-manufacturing of highperformance optical, chemical, or biological sensors and even metamaterials working in the visible range.

EXPERIMENTAL SECTION Preparation of sub-20 nm Si master template Poly(styrene-b-dimethylsiloxane) (PS-b-PDMS) BCPs were purchased from Polymer Source Inc. (Canada) and used without further purification. A total molecular weight, volume fraction of PS, and polydispersity index (PDI) of BCP are 48 kg mol-1, 66.3%, and 1.18, respectively. A PS-b-PDMS BCP with a molecular weight of 48 kg mol-1 dissolved in toluene (0.8 wt%) coated on grapho-epitaxy templates with a depth of 40 nm, width of 1 µm, and a period of 1.25 µm. PDMS-OH brush (5 kg mol-1, Polymer Source Inc.) was grafted on the template substrate at 150°C in a vacuum chamber for 90 min, which was followed by washing away the unreacted polymer. The BCP film was solvent-annealed for 12 hours at room temperature (RT) using toluene vapor. For pattern development, reactive ion etching (RIE) with CF4 plasma (source power = 50 W, 21 sec) and O2 plasma (source power = 60 W, 30 sec) was used to etch the top PDMS layer and PS matrix while converting PDMS to oxidized PDMS. The working pressure and gas flow rate were kept at 15 mTorr and 30 sccm, respectively. Fabrication of Si nanowell master template Aligned Cr nanowires (NW) were fabricated and printed by a previously reported solventassisted nano-transfer printing (S-nTP) method.15 PDMS-OH brush was treated on the master template to reduce surface energy and to promote the release of polymer replica. PMMA (M.W. = 100 kg mol-1, Sigma-Aldrich Inc.) dissolved (4 wt%) in a mixed solvent (toluene, acetone,


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and heptane with 4.5:4.5:1 by volume) was then spin-cast on the master template. A polyimide adhesive tape was used to detach the PMMA polymer replica from the master template. Replicated PMMA thus had an inverted geometry of the BCP master. Metallic nanowires (NWs) are then fabricated through glancing-angle deposition of a metal source. In this study, we used a chromium (Cr) source due to its mechanical and chemical robustness, which is desirable as an etch mask. Cr was deposited by an e-beam evaporation system with an optimized incidence angle. A Cr NWs/polymer replica/PI film was then exposed to saturated solvent vapor in a preheated (55oC) chamber system. After 25 - 35 sec, the sample was taken out from the chamber and transfer-printed to target substrate. After the first Cr NWs were printed on a flat Si substrate, a mild washing process was applied using toluene. The same washing process was applied after the second Cr NW printing process, forming cross-stacked NWs. The thermal washing process was employed to remove the residual organic components. Crossed Cr NWs were then used as mask structures for the fabrication of a high-density nanowell array on the Si substrate. We used the Bosch process (repeated SF6 and C4F8 plasma treatments) for the anisotropic etching of Si. SF6 plasma (source power = 80 W, 10 sec) followed by C4F8 plasma (source power = 80 W, 7 sec) treatment was repeated six times. Cr NWs were then removed by Cr-5 wet etchant (Cyantek, USA) for 3 min. Formation of Au mesh structure An Au film was formed by thermal evaporation of an Au source with a deposition rate of 1.0 Å s-1. The deposited film thickness was confirmed by a cross-sectional SEM image (27 nm). An Au mesh, which was located on top of the surface, was then transferred using a polymer carrier film. We used polystyrene (PS, molecular weight = 280 kg mol-1) dissolved in methylene chloride (MC, 10 wt%). After the PS film was spin-cast onto the Au mesh, the entire sample


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was dipped in KOH solution (10 % in weight) for delamination of the PS/Au mesh from the Si master. With a short treatment of 1 – 2 minutes, the delaminated film floated on the surface of the solution, and then was fished out using a quartz substrate. Finally, the carrier polymer was washed away using MC. Optical and surface characterization Transmittance of the quartz plate containing the Au nanohole array was measured using a UV–vis spectrophotometer (Mecasys, Optizen POP, Korea) with reference to air. In order to measure the transmittance of the perforated Au film in the solvent, a quartz cuvette was used. Top- and cross-sectional structures of the Au mesh were obtained using a field emission scanning electron microscope (FE-SEM, Hitachi, S-4800). For measurement of the crosssectional structure of the transferred Au mesh (as shown in Figure S1b), a Si wafer was employed as a substrate to avoid a charging effect during FE-SEM observation. Finite-Difference Time-Domain (FDTD) Simulation The nanohole array was simulated with a 3D finite-difference time-domain (FDTD) method provided by Lumerical Inc. (Vancouver, Canada) The FDTD numerically solves Maxwell’s equation with a discretized unit cell (period = 1250 nm) in the designed structure.29 The gold material data provided by Johnson and Christy is used and the permittivity of gold is fitted by manipulating a weight of an imaginary part.30 For simplicity, the nanohole structure is designed as a unit cell, which contains a single nanohole with 50 nm periodicity. The entire structure (1250 nm) is composed of a uniform hole structure (950 nm), slit (130 nm), and film (170 nm). The boundary condition of the x and y direction is periodic and a perfect match layer (PML) is set in the z direction boundary with a 0.3 nm unit mesh. The optical property of the nanohole


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structure was simulated for the normal-incident plane wave until most of the field was absorbed by boundaries.

ASSOCIATED CONTENT Supporting Information. Supporting Information. Supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Yeon Sik Jung E-mail: [email protected] Author Contributions This manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS This work was supported by the Center for Advanced Meta-Materials (CAMM- No. N01160654) and National Research Foundation of Korea (NRF- 2017R1A2B2009948) grant funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project. And this work was also supported by Creative Materials Discovery Program (NRF-2016M3D1A1900035) through the NRF funded by the Ministry of Science, ICT and Future Planning.


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(12) Chanda, D.; Shigeta, K.; Gupta, S.; Cain, T.; Carlson, A.; Mihi, A.; Baca, A. J.; Bogart, G. R.; Braun, P.; Rogers, J. A. Large-area flexible 3D optical negative index metamaterial formed by nanotransfer printing. Nat. Nanotech. 2011, 6, 402-407. (13) Kim, J. M.; Kim, Y.; Park, W. I.; Hur, Y. H.; Jeong, J. W.; Sim, D. M.; Baek, K. M.; Lee, J. H.; Kim, M. J.; Jung, Y. S. Eliminating the Trade‐off between the Throughput and Pattern Quality of Sub‐15 nm Directed Self‐assembly via Warm Solvent Annealing. Adv. Func. Mater. 2015, 25, 306-315. (14) Jung, Y. S.; Chang, J.; Verploegen, E.; Berggren, K. K.; Ross, C. A path to ultranarrow patterns using self-assembled lithography. Nano Lett. 2010, 10, 1000-1005. (15) Jeong, J. W.; Yang, S. R.; Hur, Y. H.; Kim, S. W.; Baek, K. M.; Yim, S.; Jang, H. I.; Park, J. H.; Lee, S. Y.; Park, C. O.; Jung, Y. S. High-resolution nanotransfer printing applicable to diverse surfaces via interface-targeted adhesion switching. Nat. Commun. 2014, 5387. (16) Jeong, J. W.; Hur, Y. H.; Kim, H.-j.; Kim, J. M.; Park, W. I.; Kim, M. J.; Kim, B. J.; Jung, Y. S. Proximity injection of plasticizing molecules to self-assembling polymers for large-area, ultrafast nanopatterning in the sub-10-nm regime. ACS Nano 2013, 7, 6747-6757. (17) Baek, K. M.; Kim, J. M.; Jeong, J. W.; Lee, S. Y.; Jung, Y. S. Sequentially Selfassembled Rings-in-Mesh Nanoplasmonic Arrays for Surface-enhanced Raman Spectroscopy. Chem. Mater. 2015, 27, 5007-5013. (18) Masson, J.-F.; Murray-Méthot, M.-P.; Live, L. S. Nanohole arrays in chemical analysis: manufacturing methods and applications. Analyst 2010, 135, 1483-1489. (19) Gordon, R.; Sinton, D.; Kavanagh, K. L.; Brolo, A. G. A new generation of sensors based on extraordinary optical transmission. Acc. Chem. Res. 2008, 41, 1049-1057. (20) Yoo, D.; Johnson, T. W.; Cherukulappurath, S.; Norris, D. J.; Oh, S.-H. Templatestripped tunable plasmonic devices on stretchable and rollable substrates. ACS Nano 2015, 9, 10647-10654. (21) Li, W.-D.; Hu, J.; Chou, S. Y. Extraordinary light transmission through opaque thin metal film with subwavelength holes blocked by metal disks. Opt. Express 2011, 19, 21098-21108. (22) Wang, W.; Ramezani, M.; Väkeväinen, A. I.; Törmä, P.; Rivas, J. G.; Odom, T. W. The rich photonic world of plasmonic nanoparticle arrays. Mater. Today 2017, DOI: 10.1016/j.mattod.2017.09.002


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FIGURES

Figure 1. Fabrication procedure of Au nanohole array. (a-e) Fabrication of reusable block copolymer (BCP) master template. PS-b-PDMS BCP with a molecular weight of 48 kg mol-1 was used to fabricate the cylindrical morphology of the master template. (f-j) Si nanowell template fabrication via solvent-assisted nanotransfer-printing (S-nTP) process. Cross-point geometry of Cr NWs blocked induced plasma. (k-o) Perforated Au thin film fabrication using thermal evaporation of Au. The fabricated Au nanohole array film is finally transferred to a quartz substrate using a carrier polymer and KOH solution.


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Figure 2. Structure of fabricated Au nanohole array. SEM images of (a) Si master template (bird’s-eye-view) (inset: tilt SEM image of Si master template) and (b) 27-nm-thick Au deposited on the Si master (c) Size distribution of nanoholes before and after Au deposition (d) Long range uniformity of Au nanohole array (inset: photograph and high-magnification image of the sample) after transfer onto a quartz substrate. The observed cross-lines at the low-magnifiedSEM image are guide patterns (period: 1250 nm) that were originally used for DSA of BCP.


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Figure 3. Spectral property of plasmonic nanohole array (a) Transmittance of Au film with and without nanohole structures, (b) Transmission data obtained by FDTD simulation (inset shows design of unit cell for FDTD simulation). (c) Electric field distribution at absorption peak wavelength (670 nm peak in Figure S7). The top and bottom images show the field at the airNHA-substrate interface and the intensity at a specific y position in the hole array region, respectively.


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Figure 4. Transmittance characteristics of Au nanohole array under various surrounding media (a) Transmittance of Au film in various refractive index (n) condition (inset: Schematic of measurement setup) Au nanohole array sample formed on a quartz substrate is located in a quartz cuvette filled with mixture solvents. (b) Deconvolution of transmittance data to two spectra


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(black line: transmittance spectra centered at 510 nm, colored lines: additional resonance peak originated from nanohole structures) (c) Shifts of resonance peaks as a function of the change of n of the surrounding solvent media (inset: sensitivity of Au nanohole array calculated by fitting the location of resonance peak with respect to n) (d) Result of FDTD simulation of Au nanohole array under the same refractive indices with measurement system.


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TABLE OF CONTENTS GRAPHIC


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Transferrable Plasmonic Au Thin Film Containing Sub-20 nm

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