Optimally Functionalized Adhesion for Contact Transfer Printing of

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Optimally Functionalized Adhesion for Contact Transfer Printing of Plasmonic Nanostructures on Flexible Substrate Jihye Lee,†,‡ Jun-Young Lee,†,‡ and Jong-Souk Yeo*,†,‡ †

School of Integrated Technology and ‡Yonsei Institute of Convergence Technology, Yonsei University, Incheon, 406-840, Republic of Korea S Supporting Information *

ABSTRACT: This paper demonstrates a facile method to achieve high yield and uniform fabrication for the transfer printing of nanoplasmonic structures on a flexible substrate by providing novel understanding on adhesion layers. The mercapto alkyl carboxylic acids and the alkyl dithiols are used as functionalized adhesion layers and further optimized by controlling the terminal group as well as the length and composition of the functionalization on flat and nanostructured gold surfaces. Our approach of optimized adhesion has been successfully implemented to the transfer printing of functionalized gold nanostructure arrays, thus producing much higher yield of 97.6% and uniform fabrication of nanostructures on a flexible substrate and enabling applications such as flexible nanoplasmonic devices and biosensing platforms. KEYWORDS: self-assembled monolayer, contact transfer printing, adhesion energy, nanoplasmonic structure, polar surface energy, dispersive surface energy merging flexible technology as a new growth engine is currently expanding research fields into integrated photonics/electronics and finding new applications in personalized medicine and point-of-care (POC) diagnostics.1 To realize these applications, several attempts are underway in fabricating functional nanomaterials or nanostructures on a flexible and nonplanar substrate with unconventional lithographic methods. Regarding the unconventional methods, there are noncontact printing and contact printing processes.2 In noncontact printing methods, nanostencil lithography,3 screen printing,2 and laser-assisted inkjet printing4 have been used for fabricating the flexible sensors and electronics. These noncontact printing methods allow the fabrication of structures by dispensing the solution or target materials through open nozzle and mask. In contact printing methods, there are micromolding in capillaries,5 transfer printing,6 nanoimprinting lithography,7 and gravure offset printing.8 The gravure offset printing needs an engraved cylinder for rolling on a moving substrate. The micromolding, imprinting, and transfer printing methods require elastometric or rigid stamps to fabricate structures. Especially for the applications enabled by nanoplasmonics with metallic nanostructures, the contact transfer printing provides unique advantage of transferring functional nanostructures on various substrates.6,9 The process of transfer printing demands a careful control of surface energies. Increasing or decreasing the surface energies require either physically roughened or chemically functionalized surfaces.10 For the physically meaningful roughening of surfaces, corona discharge treatment or plasma treatment with active oxygen

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© XXXX American Chemical Society

and nitrogen have been extensively studied to enhance the adhesion on plastic polymers.11 These treatments render a surface that not only induces a mechanical interlocking but also provides a receptive chemical bonding, thus exhibiting hybrid characteristics according to the adhesion theory. The approaches are generally effective in promoting the adhesion of cells, target biomolecules, and inorganic materials on flexible polymer substrates. However, these processes lead to randomly functionalized surfaces in terms of composition and these surface composition for functionalization degrade over time.12 Thus, chemical functionalization is more desirable as it provides stable control of the surface energies. The chemical control on the surface leads to two major factors determining the proper transfer of the purposed nanostructures: adhesion13 and antiadhesion.14 The properties of adhesion layers are depending on their mechanisms such as physical adsorption, chemical bonding, diffusion, electrostatic, mechanical interlocking, or weak boundary layer.11 Physical adsorption is mainly due to the interaction between a permanent dipole and an induced dipole, thus generating a van der Waals force. In this case, quantum-induced polarization contributes to the physically induced dispersion force between the interface layers. One such example is the interface adhesion between polydimethylsiloxane (PDMS) and silicon substrate, characterized mainly as van der Waals interaction by Hsia et Received: October 7, 2016 Accepted: January 13, 2017 Published: January 13, 2017 A

DOI: 10.1021/acsami.6b12739 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Schematics for the Transfer Printing of Functionalized Nanostructures on the Flexible Substrate by Optimizing the Adhesion Energya

(a) Functionalized gold nanostructure on the silicon mold-FOTS layer is placed on a hot plate and then a flexible film is placed on the top with pressure. The transfer printing of functionalized nanostructure is enabled by the attachment layer for the adhesion of the nanostructured metal to the flexible substrate and the detachment layer for the release of the metal from the rigid silicon mold. (b) The functionalized nanostructure is detached from the mold and attached to the flexible substrate with optimized chemical functionalization. (c) The nanoplasmonic structure with chemical functionalization is printed on the target flexible substrate. a

al.15 In the soft lithography process, optimal control of interface adhesion is required to prevent a collapse of roof top structures on the PDMS mold. The interfacial adhesion in chemical bonding is defined by the formation of covalent, ionic, or hydrogen bonds throughout the interface.11 Lee et al. demonstrated the thin film solar cell on various substrates with “peel and stick” process by controlling the chemical bonding and debonding between the metal and silicon oxide interface by using a moisture.16 Another way to control the adhesion at the interface is to utilize an adhesion promoter. An adhesion promoter and a surfactant for adhesion are used for promoting the adhesion of resins,17 coating materials,18 and metals to a target substrate.19 They are frequently exploited in nanotechnology research. Ahn et al. have shown the high-speed roll-to-roll nanoimprint lithography on a flexible substrate by using an adhesion promoter which can improve the adhesion of the resist pattern to the PET substrate.17 They not only used the adhesion promoter, but also added a fluorosurfactant to reduce the adhesion between printed pattern and mold surfaces. Using the combination, denser nanostructures and high throughput nanopatterning can be achieved. Despite the elaborate studies on adhesion, there has been little consideration regarding a chemical functionalization at the interface between a metal and a polymer substrate. Such control of the interface is quite important in the transfer printing process of a metal to a flexible substrate. The process requires the analysis of interface using surface free energies for unknown surfaces.20 One of the methods to verify the characteristics of the interface is to analyze a type of interfacial interaction by measuring a contact angle for the wettability of the surface. In this paper, we have systematically controlled the surface functionalization to optimize the properties of the interfacial

adhesion. The adhesion at the interface can be changed not only with a functionality of a terminal group but also with a different body chain in the functionalization. The body chain of different length or composition can lead to a change in an inductive effect. The inductive effect is an electronic effect by the transmission of electrons through a chain of atoms in a molecule, thus affecting the polarization in a bond. The electrons can be transmitted through the sigma (σ) bonds due to the difference of the electronegativity in molecules.21 The inductive effect from different body chain can contribute to the adhesion of the functional terminal group to a substrate. The functionalized adhesion can be optimized for the stable transfer of nanoplasmonic structures to a flexible substrate. The nanoplasmonic structures are based on the resonant oscillations of conduction electrons in the metallic nanostructures so that their optical properties are dependent on their sizes, shapes, and materials.22 These properties as well as the plasmonic coupling between the adjacent nanostructures can lead to characteristic radiative colors. Optically meaningful nanoplasmonic structures are used for biosensing applications with an appropriate functionalization using biochemically active molecules. The representative example is thiolate groups that have been conventionally used to modify the surface functionality of noble metals such as gold, silver, copper, platinum, palladium, and nickel with a strong molecule−metal linkage.23,24 Terminal groups of thiol chains provide a surface with the functionality of recognizing specifically targeted bio or chemical substances. In a transfer printing process, the terminal functionality should also provide a proper adhesion with a desired polymer substrate. Examples of terminal groups are carboxylic acid (COOH), hydroxy (OH), amine (NH3), sulfhydryl (SH), and methyl (CH3) B


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Figure 1. (a) Chemical structures of 3-MPA (C3), 11-MUA (C11), and 16-MHA (C16) for carboxylic acid terminal (left) and 1,2-EDT (C2), 1,8ODT (C8), and 1,10-DDT (C10) for thiol terminal (middle) with increasing number of carbon (C) atoms. Chemical structures of 1,8-ODT (C8 w/ o O) and 3,6-DODT (C6 O2) for thiol terminal with different compositions (right). Graphs showing the change of dispersive and polar surface energies with respect to the number of C atoms on (b) flat surface (FS) and (c) nanostructured surface (NS) with functionalization of carboxylic acid terminal group and (d) FS and (e) NS with functionalization of thiol terminal group. The effect of different compositions in body chain with only carbon atoms (C8 w/o O) and with carbon and oxygen atoms (C6 O2) on the (f) FS and (g) NS.

group. Each terminal group provides a distinctive surface energy to determine a work of adhesion at the interface. The oxygen and nitrogen atoms provide a polar surface compared to the carbon and sulfur atoms. So selecting a terminal group with a body chain of atoms is important for determining the surface energy. To understand and enhance the adhesion between the chemically functionalized nanostructure and flexible polymer substrate, we have controlled the length and the composition of the functional group with a same terminal and changed the type of the terminal group, all of which are chemisorbed molecules on plasmonic nanostructures. The surface energies from all the chemical functionalizations for enhancing the adhesion were compared with the surface energy of releasing layer, trichloro(1H, 1H, 2H, 2H-perfluorooctyl)silane (FOTS) as a reference to evaluate the degree of adhesion. The change in surface energy characterizes the electron state of the terminal group, thus allowing us to evaluate the adhesion. Scheme 1 shows the contact transfer printing of functionalized nanostructure to the flexible substrate with the control of the adhesion energy using a chemical functionalization. There are two layers: attachment layer (adhesion layer) and detachment layer (releasing layer) (Scheme 1a). The attachment layer plays a role of adhesion of the metal to the target

substrate. The detachment layer helps to release the metal from a nanostructured silicon mold. The releasing layer and gold deposited layer are applied to a nanostructure on the silicon mold. The releasing layer of FOTS underneath the gold layer enables the release of the gold with its low adhesion energy of 49.2 mJ/m2 (Figure S1). This value was calculated using the surface energies from the FOTS and the gold on the nanostructures. The releasing layer is made on the nanostructured silicon mold by the vaporization of FOTS for 2 h in a desiccator. Then, the gold layer is deposited by an evaporation on the FOTS layer. Following the gold deposition, an attachment layer is formed by immersing the gold nanostructure for 12 h in the prepared 1 mM mercapto alkyl carboxylic acid and alkyl dithiol solution diluted by an ethanol. By this solution-based functionalization, the thiolate group is chemisorbed as a selfassembled monolayer onto the metal with a strong metal− sulfur bonding and then serve as an adhesion layer. This preparation of the silicon mold with a release layer and an attachment layer enables the transfer printing (Scheme 1b). The terminal group of the functionalization on the silicon mold is placed in direct contact with the targeted flexible substrate at the condition of 80 °C for 30 min and then 100 °C for 2 h on the hot plate under a 0.56 psi pressure. (The detailed methods C


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increasing length of the alkyl chain as shown in black dotted line of Figure 1b. Previous reports have only mentioned that the surface energy depends on the terminal group.26 In contrast, our new finding not only considers the role of terminal groups but also points out the importance of the length of the functionalization determined by the number of carbons for controlling the surface energy. For the carboxylic acid terminal, the carbon is slightly charged positive relative to the slightly negative oxygen, thus providing high polarity. As carbon atoms are added with the increasing length of alkyl chain, more electrons from the carbon atoms can be transmitted to the terminal, thus reducing the polarity in the carboxylic acid terminal. This leads to the decrease in the polar component of surface energy for a long chain of carbon atoms. This change of polarization due to the transmission of electrons through a chain of atoms in a molecule is “inductive effect”.21 The polarity is decreased more with the increasing distance between the terminal and headgroup. Hence, the long alkyl chain in the functionalization has a reduced polar component of the surface energy compared to the short alkyl chain as shown in red dotted line of Figure 1b. This effect from carboxylic acid terminal group was also investigated on the nanostructured surface (NS) (Figure 1c). The trends of polar and dispersive surface energies on the NS are similar to those of FS but with much larger value. The larger increase in polar surface energy on the nanostructured surface can be explained by the roughness factor that affects the measurement of contact angle and wettability.27 When the surface layer provides strong hydrophilic properties (Figure 1b), the presence of nanostructure on the surface can dramatically enhance the hydrophilicity (Figure 1c). In order to see the effect of different terminal group, surface energies were measured with increasing alkyl chain length for thiol functionalization using a 1,2-EDT (C2), 1,8-ODT (C8), and 1,10-DDT (C10). As shown in Figure 1d, e, the thiol terminal group also provides similar trends in the changes of polar (γps ) and dispersive (γds ) surface energies with increasing number of carbon atoms on both FS and NS. In contrast to the carboxylic acid terminal group, polar surface energy on the NS shows only slight increase from FS for thiol terminal group since hydrophilicity is smaller in thiol compared to carboxylic acid so the effect of nanostructure is not as distinct. To compare the effect of different body chain in the functionalization, we have prepared the 1,8-ODT (C8 w/o O) and 3,6-DODT (C6 O2) functionalization. These chemical groups can show the effect on the surface energies by replacing two carbon atoms with oxygen atoms within a body chain in same thiol terminal group. As shown in Figure 1f, g, the polar components of surface energies dramatically increase while the dispersive components decrease with the presence of oxygen atoms in the body chain both on flat and nanostructured surfaces. These changes can be explained by the inductive effect of the body chain. The electron-rich oxygen atoms influence the permanent dipole of the terminated thiol, thus enhancing the terminal polarity and the corresponding polar surface energy. This enhanced polarity of the body chain leads to a lessdispersive component of the surface energy. Therefore, the compositional change to the oxygen atoms in the body chain influences the terminal surface properties. The effect of terminal functionality on the surface energy also has been verified with carboxylic acid and thiol by comparing the results from Figure 1c, e. The polar component of the surface energy for carboxylic acid ranges in 56−62 mJ/m2

are described in the Supporting Information for the fabrication of nanoscale silicon mold, condition of the FOTS coating on silicon mold, deposition of gold on the FOTS layer-silicon mold, preparation of the solutions for making a functionalized gold nanostructure, and transfer printing of functionalized gold nanostructure.) Following these processes, the printed nanoplasmonic structure with chemical functionalization is prepared (Scheme 1c). On the basis of the results, we aim to identify the optimal condition for the transfer printing by measuring the surface energies, thus finding the work of adhesion between the functionalized nanostructures and the flexible substrate. Figure 1 shows the types of chemical functionalization on gold surface with its properties depending on the length, the composition, and the terminal group. To correlate the surface properties with chemical functionalization, we have measured a contact angle and calculated the surface energy using different surface tension liquids: water (surface tension (SFT) of 72.8 mN/m; polar surface energy (γpl ), 50.2 mN/m; dispersive surface energy (γdl ), 22.6 mN/m) and ethylene glycol (SFT of 48.0 mN/m; γpl , 19.0 mN/m; γld, 29.0 mN/m). All these measurements for chemical functionalization and flexible substrate are summarized in Table S1. The effect of various chain lengths has been studied using the solutions of 3-mercaptopropionic acid (3-MPA, C3), 11mercaptodecanoic acid (11-MUA, C11), and 16-mercaptohexadecanoic acid (16-MHA, C16) for carboxylic acid terminal group, while using the solutions of 1,2- ethanedithiol (1,2-EDT, C2), 1,8-octanedithiol (1,8-ODT, C8), and 1,10-decanedithiol (1,10-DDT, C10) for thiol terminal group. To change the composition of the chain, 1,8-ODT (C8 without (w/o) O), and 3,6-dioxa-1,8-octanedithiol (3,6-DODT, C6 O2) are used. Carboxylic acid and thiol terminal groups have been used to evaluate the effect of different terminal groups. All the chemical structures of functionalization are shown in Figure 1a. Now, we need to find out the work of adhesion between the two solid surfaces to optimize the transfer process. The work of adhesion is defined by the surface energies of both surfaces. When a solid surface is in contact with a liquid, the total surface energy of solid (γs) and liquid (γl) is the sum of the γps , γds , and γpl , γdl , respectively, where γp represents polar interaction due to ion-induced, hydrogen bonding, or acid−base interactions, and γd represents dispersion due to van der Waals interaction.20 With the known values of γpl and γdl for a liquid, the dispersive and polar components (γds , γps ) of solid surface can be calculated with the Wu methods by using the following eq 1 and measuring the contact angles θ on the solid for two different types of liquids.25 γl d(1 + cos θ ) = γs + γl −

4(γs dγl d) γs d + γl d

−

4(γs pγl p) γs p + γl p

(1)

The first type of functionalization was applied on the flat gold surface by using mercapto alkyl acid solutions: the 3-MPA has the contact angle of 49.6° ± 2.2 for a water droplet, the 11MUA has 67.6° ± 2.5, and then the 16-MHA has 68.3° ± 4.0. Also, ethylene glycol was used for another liquid and all the results are summarized in Table S1. According to these measurements and corresponding calculations, the polar component (γps ) of surface energy decreases when the alkyl chain length of functionalization increases for a same terminal group on flat gold surface (FS) as shown in Figure 1b with red dotted line. On the other hand, the dispersive component (γds ) of the surface energy increases proportionally with the D


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Figure 2. XPS data of (a) S 2p and (b) C 1s spectrum corresponding to the self-assembled monolayer (SAM) monolayer. Plots of adhesion energies at various interfaces between the flexible substrates (PET, PS, and Kapton) and (c) flat surface (FS) and (d) nanostructured surface (NS) functionalized by carboxylic acid terminal group (color coded by different shades of violet): 3-MPA (C3-carboxyl), 11-MUA (C11-carboxyl), and 16MHA (C16-carboxyl) and by thiol terminal group (color coded by different shades of green): 1,2-EDT (C2-thiol), 1,8-ODT (C8-thiol), and 1,10DDT (C10-thiol), and without functionalization (color coded gray). Plots of adhesion energies between the flexible substrates (color coded by different shades of blue) and (e) FS and (f) NS functionalized by 1,8-ODT (C8 w/o O) and 3,6-DODT (C6 O2).

(Figure 1c) compared to the range of 20−23 mJ/m2 for thiol (Figure 1e). The carboxylic acid terminal provides more polar surface free energy than thiol terminal group because the polarity on the oxygen group of the carboxylic acid is stronger than the polarity of the thiol. While the terminal functionality plays the key role for determining the surface energy, it is still important to tune the factors such as the length and composition of the functionalization for more comprehensive optimization of surface characteristics. To evaluate the surface energies accurately, we have prepared the high quality monolayer film only to account for the terminal group, length and constituent of the body chain of the molecules. Atomic force microscopy (AFM) is used to prove the high quality surface of the gold and to measure the thickness of self-assembled monolayer (SAM) as shown with the topological data in Figure S2. The roughness of the SAM is presented by the root-mean-square (RMS) value (Rq) of the surface. The roughness values of the SAMs match each length of the molecule layer provided by the Sigma-Aldrich, respectively. Thus, these surfaces satisfy the reasonable requirements for the analysis of the surface energy and work of adhesion with high reliability. In Figure 2, XPS data was plotted for S 2p and C 1s spectrum to evaluate the chemical bonding. All chemical functionalizations have sulfur bound to the gold nanoparticles (S−Au bonding at 162.1 eV) and unbound thiol groups (S−H bonding at 163.2 eV). The peak at 162.1 eV from the plot in Figure 2a indicates that chemical bonding with sulfur and gold has been formed stably by chemisorption process. The slightly higher intensity of the S−H bond compared to the intensity of the S− Au bond indicates the presence of thiol terminated chemical group such as 1,2-EDT, 3,6-DODT, 1,8-ODT, and 1,10-DDT. In Figure 2b, the peaks at 284.6 eV from C 1s spectrum verify the presence of C−C bonding common to all the chemical

groups. In case of carboxylic acid terminated chemical groups such as 3-MPA, 11-MUA, and 16-MHA, the peaks at 288.7 and 286.6 eV correspond to O−CO bonding and C−O bonding, respectively. The 3,6-DODT is a thiol-terminated chemical group similar to 1,8-ODT but the two carbon atoms are replaced with two oxygen atoms as indicated by the peak at 286.6 eV for C−O bonding. Detailed analyses of the individual peaks from the XPS data are provided in Supporting Information S3. These XPS measurements confirm the chemical bonding characteristics of the functional layers on the flat Au surface. Now that the polar and dispersive components of the surface free energy on these stably functionalized surfaces are calculated, the adhesion at the interface can be given by using the following harmonic mean eq 2. ⎛ γ dγ d γ pγ p ⎞ W12 = 4⎜⎜ d1 2 d + p1 2 p ⎟⎟ γ1 + γ2 ⎠ ⎝ γ1 + γ2

(2)

where W12 is a work of adhesion between the surface 1 and the surface 2, γnd is the surface energy from the dispersive component and γpn is the surface energy from the polar component of the surface n. Using the harmonic mean equation above, the work of adhesion between the functionalized nanostructure array and flexible substrate can be obtained. In Figure 2c−f, overall results on the adhesion energies are compiled to identify the optimal condition for transfer printing with (1) flexible substrates of PET, PS, and Kapton, (2) different terminal groups of carboxylic acid and thiol, (3) different lengths of body chain with increasing number of carbon atoms, (4) different compositions of body chain with only carbon atoms and with carbon and oxygen atoms, and (5) flat and nanostructured surfaces. To compare the adhesion energy clearly, we have plotted the data by histograms grouped E


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Figure 3. Atomic force microscopy (AFM) images and profiles showing (a) the uniform nanostructure arrays in silicon mold (inset) along with their profile and (b) the uniformly transferred gold nanostructure arrays on PET (inset) and their profile. (c, d) Dark-field microscopy images and (e, f) scanning electron microscopy (SEM) images of the printed nanostructures on PET using thiol and carboxylic acid functionalization: (c, e) respective images from 1,2-EDT functionalized nanostructure with the adhesion energy of 74.46 mJ/m2, (d, f) respective images from 3-MPA functionalized nanostructure with 112.45 mJ/m2. (g) Yield plot of the transfer printed nanostructure indicates that the improved adhesion energy of 112.45 mJ/m2 provides higher yield of 97.6% compared to 78.0% with the adhesion energy of 74.46 mJ/m2. (Scale bar in dark-field images, 10 μm; scale bar in SEM images, 5 μm.)

in PET, PS, and Kapton (Figure 2c, d) for different terminal groups with increasing number of carbon atoms: 3-MPA (C3carboxyl), 11-MUA (C11-carboxyl), 16-MHA (C16-carboxyl), 1,2-EDT (C2-thiol), 1,8-ODT (C8-thiol), and 1,10-DDT (C10-thiol). We have also plotted the data by histograms for the flexible substrates of PET, PS, and Kapton (Figure 2e, f) grouped in the body chain compositions of thiol terminal group without and with oxygen atoms: 1,8-ODT (C8 w/o O) and 3,6-DODT (C6 O2). In Figure 2c, the functionalization on FS enhances the adhesion on the flexible substrate with about 1.5 times larger adhesion energy compared to the FS without chemical functionalization. The adhesion energies of 3-MPA (C3carboxyl) to the substrates such as polyethylene terephthalate (PET), kapton, and polystyrene (PS) were larger than those of 11-MUA (C11-Carboxyl) and 16-MHA (C16-Carboxyl) with longer chain. For same functionalization, the PET substrate has shown higher adhesion than the other two flexible substrates. This is because high polar components of surface free energies from the PET and 3-MPA lead to a stronger physical adsorption, thus providing a higher adhesion between the flexible substrate and the functionalized nanostructure. As previously explained for Figure 1c, the adhesion energy tends to decrease slightly with the increasing number of carbon atoms shown in 3-MPA (C3-carboxyl), 11-MUA (C11-carboxyl), and 16-MHA (C16-carboxyl). This effect also was applied to the thiol terminated 1,2-EDT (C2-thiol), 1,8-ODT (C8-thiol) and 1,10-DDT (C10-thiol). On the basis of this data, the functionalization with 3-MPA and the flexible substrate of PET were selected for the efficient transfer of gold nanostructure arrays benefiting from the higher adhesive energy. In order to compare the effect of functionalization on nanostructure, flat surface was used as a reference as shown in Figure 2d. The adhesion energy on the FS clearly decreases with the increasing number of carbon atoms in the n-mercapto alkyl acid as well as thiol functionalization. This trend for NS is similar to FS showing the decrease of adhesion energy with the increasing number of carbon atoms but at higher adhesion

energy. This increased adhesion for the NS can be described by roughness factor27 as discussed previously. The difference in water contact angle between flat and nanostructured surface was as large as 20° resulting in higher adhesion energy for NS. Therefore, the terminal group, length of the chemical chain, and the nanoroughness are the important factors to consider in optimizing the adhesion at the interface. The effect of composition is evaluated for the adhesion to the flexible substrates using the dithiols of 1, 8-ODT (C8 w/o O) and 3,6-DODT (C6 O2) functionalized on FS (Figure 2e). As previously explained in Figure 1 (e), the adhesion energy increases for the dithiol due to the presence of oxygen atoms. The same trend was shown in Figure 2f on NS. For the interface between dithiol functionalization and PET substrate, the adhesion energy for the body chain with two oxygen was 17 mJ/m2 which is larger than the one without oxygen as shown in Figure 2f. Therefore, the composition of the functionalization at the interface also needs to be considered for further optimization of a transfer process. Using the optimized adhesion energy of 112.45 mJ/m2, we have transferred the gold nanostructure on PET substrate and performed the AFM measurement of the nanostructure arrays on the silicon mold with evaporated gold before transfer printing and the printed gold nanostructure arrays after the transfer to PET. By AFM data, the height and the width of the silicon mold are measured as 600 and 545 nm in average, respectively (Figure 3a). The thickness and the width of the printed gold nanostructure on PET are measured as 90 and 530 nm, respectively (Figure 3b). The size of the nanostructure is measured with reference to the top surface. AFM images (inset) show the uniform arrays of gold nanostructures in silicon mold before transfer and also show the effectively transferred uniform nanostructure arrays on PET. To evaluate the yield of transfer printing, we compared the dark-field images and SEM images of selected thiol terminal group (1,2-EDT) (Figure 3c, e) and carboxylic acid terminal group (3-MPA) (Figure 3d, f). The SEM images are able to show the details of printed nanostructures on PET substrate in higher magnification, especially for the printed shape and defect F


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Figure 4. Uniform transfer printing of the plasmonic nanostructure arrays in thickness and size due to the optimally controlled interfacial energy. (a) Photographic image of scattering response at normal incident illumination from optical microscopy (upper left with an inset showing an enlarged image with a scale bar of 5 mm) and schematic of scattering response at the normal incident illumination at 0° (lower left) indicating uniform scattering response in green due to the uniform transfer printing. (b) Simulation of electric field distributions showing the resonances at different wavelengths depending on the illumination angle from surface normal; 0, 30, 45, and 60° (middle). (c) Photographic image of scattering response due to illumination at various angles using condenser lens from dark-field microscopy (upper right with an inset showing an enlarged image with a scale bar of 5 mm) and schematic of scattering response at angled lighting conditions (lower right) with their colors corresponding to the simulation results in b.

of nanostructures, whereas the dark-field images at relatively lower magnification can show the larger regions of printed images with scattering signals that allow yield comparisons for the transfer printing with different conditions. Dark-field images in these figures represent the forward scattered plasmonic response from the printed nanostructures, which is captured by integrated CCD camera. The selected flexible substrate was PET. The total yield of transfer printing was 78.0% using the condition with 1,2-EDT having the lowest adhesion energy of 74.46 mJ/m2, whereas 3-MPA condition with the highest adhesion energy of 112.45 mJ/m2 provided the transfer yield of 97.6% (Figure 3g). The yield of 97.6% was calculated by counting the number of nanostructures scattering light among the array of 20 × 20 nanostructures in the area of 3600 μm2. The results indicate that the 3-MPA offers an optimal adhesion energy so the condition has been applied for the transfer printing of plasmonic nanobio sensing platform with high yield and reproducibility.9 In our experiment, several shapes and sizes of nanostructures have also been fabricated by careful control of etching process for silicon mold. This allowed the transfer of various shapes such as circle, half-moon, and square as shown in Supporting Information S4. The results can support the flexibility of our fabrication process for various applications.

In this paper, the capability for uniform transfer printing is demonstrated by printing the functionalized nanostructure arrays on flexible PET substrate using the maximum adhesion energy of 112.45 mJ/m2. The uniformity in thickness and size for the plasmonic nanostructure arrays can be examined by the optical response for normal and angled illuminations. Figure 4 shows the photographic images of the printed nanostructure arrays, schematics of scattering response from different lighting conditions, and electric field distributions at various illumination angles. The images in Figure 4a, c represent the backward scattered plasmonic responses from the printed nanostructures with normal illumination and angled illumination, respectively. The size of the printed nanostructures is 550 ± 50 nm with 100 nm thickness having 3 μm spacing. Overall fabrication area as shown in Figure 4a, c was 49 mm2 with 5 × 5 array of a printed unit, each containing 100 × 100 array of 550 nm wide nanostructures. For normal incident illumination under regular lighting condition of optical microscope, the printed nanostructures show uniform green color which indicates uniformity in printed thickness as shown in Figure 4a. For angled illumination under condensing optical condition of dark-field microscope, the printed nanostructures now show multicolour scattering with respect to the incident angle of light as shown in Figure 4c. Continuous change of colors from the plasmonic G


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ACS Applied Materials & Interfaces scattering of the nanostructures again indicates the uniformity in printed size and thickness. These experimental results were verified by simulating the electric field distribution by Lumerical software to find resonant wavelengths at different angles of illumination on the nanostructure arrays (the condition of the simulation is described in the Supporting Information). The top nanostructure array showing a green color should be at the condition with normal illumination based on the observation from Figure 4a. Continuous shifts of resonant wavelength in the subsequent arrays correspond to the simulation results at different angles of incidence. In Figure 4b, the simulations of electric field distributions show the resonances at different wavelengths depending on the illumination angle from surface normal: 0, 30, 45, and 60°. Depending on the angle of incident light, scattering wavelengths resonant with the plasmon lengths along the nanostructures demonstrate continuously changing colors for the nanostructures that are uniform in size, shape, and period.28 The uniform color of green at normal incidence as well as the multicolour at varying angle of incidence from the plasmonic nanostructures demonstrate the capability of uniform transfer printing on a transparent and flexible PET substrate. The results indicate that the printed nanostructure arrays have uniform thickness and size with the help of the optimally controlled interfacial adhesion. Taking advantage of the uniformly transfer printed nanostructure arrays, further applications such as refractive index sensing and plasmon coupling based sensing have been demonstrated with the wavelength shift of plasmonic nanostructures as shown in Figures S5 and S6. In summary, this work highlights the enhanced transfer printing methods through controlling the flexible polymer− metal adhesion. The flexible polymer−metal adhesion can be tuned by chemical functionalization by changing their terminal group, length, and composition of the body chain. Based on the theory of physical adsorption and the principle of inductive effect, an optimal condition for transfer printing has been determined by functionalizing with 3-MPA and using PET as a flexible substrate. This optimally functionalized adhesion on the flexible substrate provides the higher yield of 97.6% in transfer printed nanoplasmonic arrays. The uniform color at normal illumination as well as the multicolour at angled illumination suggest that optimally controlled adhesion provides a high yield transfer of the purposed nanostructures with uniform thickness and size. Leveraging this optimally transferred nanostructure arrays, the highly sensitive refractive index sensing platform as well as the plasmon coupling based sensing platform have been demonstrated. Therefore, it is important to optimize the functionalized adhesion in order to transfer nanoplasmonic arrays effectively, thus enabling multifunctional applications such as multispectral display, imaging,29 high-density optical storage, and anticounterfeiting technologies.30

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sensing and plasmon coupling effect application using a printed nanoplasmonic structure (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jihye Lee: 0000-0003-4970-9881 Author Contributions

J.L. designed and performed the experiments and analyzed the data; J.-Y.L. analyzed the wetting data and discussed the results; J.-S.Y. supervised the project. All authors contributed to the writing of the manuscript and approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Seungmuk Ji for helpful discussion. This research was supported by the MSIP(Ministry of Science, ICT and Future Planning), Korea, under the “ICT Consilience Creative Program” (IITP-R0346-16-1008) supervised by the IITP (Institute for Information & communications Technology Promotion) and also under the “Mid-career Researcher Program” (NRF-2016R1A2B2014612) supervised by the NRF(National Research Foundation).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12739. Experimental methods; contact angle of all solutions; atomic force microscopy (AFM) image of SAMs; XPS fitting data; AFM profile of printed nanostructure on PET; fabrication of various shape and size of nanostructure on flexible substrate; reflective index H


Letter

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Optimally Functionalized Adhesion for Contact Transfer Printing of

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