Hierarchical Materials Design by Pattern Transfer ... - ACS Publications

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Hierarchical Materials Design by Pattern Transfer Printing of SelfAssembled Binary Nanocrystal Superlattices Taejong Paik,†,‡,⊥ Hongseok Yun,‡ Blaise Fleury,‡ Sung-Hoon Hong,†,# Pil Sung Jo,§,∥ Yaoting Wu,‡ Soong-Ju Oh,§,∇ Matteo Cargnello,‡,○ Haoran Yang,‡ Christopher B. Murray,*,‡,§ and Cherie R. Kagan*,†,‡,§ †

Department of Electrical and Systems Engineering, ‡Department of Chemistry, and §Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ∥ Complex Assemblies of Soft Matter, CNRS-SOLVAY-PENN UMI 3254, Bristol, Pennsylvania 19007, United States ⊥ School of Integrative Engineering, Chung-Ang University, Seoul, 06974, South Korea # Electronics and Telecommunications Research Institute, Daejeon, 34129, South Korea ∇ Department of Materials Science and Engineering, Korea University, Seoul 02841, South Korea ○ Department of Chemical Engineering and SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: We demonstrate the fabrication of hierarchical materials by controlling the structure of highly ordered binary nanocrystal superlattices (BNSLs) on multiple length scales. Combinations of magnetic, plasmonic, semiconducting, and insulating colloidal nanocrystal (NC) building blocks are self-assembled into BNSL membranes via the liquid−interfacial assembly technique. Free-standing BNSL membranes are transferred onto topographically structured poly(dimethylsiloxane) molds via the Langmuir−Schaefer technique and then deposited in patterns onto substrates via transfer printing. BNSLs with different structural motifs are successfully patterned into various meso- and microstructures such as lines, circles, and even threedimensional grids across large-area substrates. A combination of electron microscopy and grazing incidence small-angle X-ray scattering (GISAXS) measurements confirm the ordering of NC building blocks in meso- and micropatterned BNSLs. This technique demonstrates structural diversity in the design of hierarchical materials by assembling BNSLs from NC building blocks of different composition and size by patterning BNSLs into various size and shape superstructures of interest for a broad range of applications. KEYWORDS: Transfer patterning, nanocrystals, self-assembly, binary superlattices, liquid interfacial assembly

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driven self-assembly.15−17 By controlling the size and shape of the NCs, the concentrations of the different NC building blocks, and the conditions (e.g., solvent, temperature, and so forth) used in assembly, diverse BNSL structures with tight control over the positions and arrangements of the NC building blocks are realized.18−20 These BNSLs show novel collective properties, distinct from the simple sum of the NC constituents, that emerge from interparticle interactions and are precisely tuned by the choice and number of NC building blocks,21,22 the ligand length and functionality separating the NCs,23 and the NC symmetry that directs the coupling between multicomponent building blocks in the super-

ierarchical architectures created by structuring dissimilar materials at the nano-, meso-, and microscales promise the design of complex materials with emergent behaviors and multiple functionalities not achievable in conventional monolithic materials.1−3 Colloidal nanocrystals (NCs) are an attractive materials class from which to construct hierarchical materials. Today synthetic methods allow the preparation of a library of NCs that are tailored in size, shape, and composition to tune their electronic, optical, and magnetic properties. Selfassembly techniques enable the codeposition of combinations of two different types of NCs to form highly ordered materials structured at the nanoscale, known as binary NC superlattices (BNSLs). BNSLs are assembled by techniques including drying-mediated self-assembly,4,5 liquid-interfacial assembly,6−8 Langmuir−Blodgett assembly,9 dip-coating,10 doctor-blade casting,11 DNA-assisted assembly,12−14 and electrostatically © 2017 American Chemical Society

Received: October 12, 2016 Revised: January 20, 2017 Published: February 1, 2017 1387

DOI: 10.1021/acs.nanolett.6b04279 Nano Lett. 2017, 17, 1387−1394

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Figure 1. Schematic of the fabrication of meso- and micropatterned BNSL superstructures via liquid interfacial assembly of BNSLs on a DEG subphase and pattern transfer printing using PDMS molds.

lattice.6,24,25 This allows for a variety of BNSLs to be explored for applications in electronics,23 catalysis,21,26 plasmonics,25,27 and magnetics.24,28 In addition to engineering the nanoscale structure and function of self-assembled NC superlattices, patterning superlattices on meso- or microscales promises to allow coupling of physical phenomena characteristic of different length scales. While unexplored in BNSLs, meso- and microscale structuring of assemblies of a single type of NC are yielding materials with unique properties.29−34 For example, by controlling the interparticle interactions in colloidal gold (Au) NC assemblies and patterning these NC assemblies into subwavelength superstructures, metamaterials are realized with plasmonic responses characteristic of the dielectric function of the NC assemblies and the size and shape of the patterned superstructures.29,30 Meso- or microscale photonic structures such as photonic crystals,35,36 waveguides,37,38 and optical resonators39,40 constructed from NC assemblies allow for synergistic light-matter interactions that enhance the size-dependent absorption, scattering, and emission of light from NC building blocks. In addition, the micro- or mesoscale patterning of semiconducting NCs to form the active area of devices is also essential to exploiting the nanoscale properties of colloidal NC assemblies in electronic41,42 and optoelectronic applications.43,44 Various techniques have been reported to pattern meso- and microstructures of colloidal NC assemblies including nanoimprinting,29,30 template-assembly,45,46 electrophoretic deposition,47,48 e-beam lithography,41,49 inkjet printing,50,51 and transfer printing.52−55 Among them, the transfer printing technique is particularly interesting as it is a cheap, scalable fabrication process with high reproducibility.56 In addition, it is a low-temperature technique compatible with many different types of substrates, including flexible plastics. Most importantly, it enables the patterning of complex and well-defined, mesoand micropatterned NC superstructures that can be readily changed by modifying the patterns on the transfer printing molds. Although transfer printing of NC membranes has been reported in the literature,44,57−59 such research has focused only on printing single component NC assemblies and not BNSLs. Development of fabrication methods to create micro- and

mesoscale structures with the well-defined nanoscale order and materials combinations possible in BNSLs would expand the degrees of freedom in the design of hierarchical architectures with structural and functional diversity. Herein, we report the design of hierarchical materials by pattern transfer printing of self-assembled BNSLs to fabricate large-area, meso- and micropatterned superstructures composed of long-range ordered superlattices of different combinations of NCs. First, colloidal magnetic, plasmonic, semiconducting, and insulating NCs with different size and composition are self-assembled into BNSL membranes using the liquid interfacial assembly technique.6 The nanoscale structure of the BNSL allows the size and compositiondependent properties of NC building blocks and their collective interactions in assemblies to be exploited. Then, the BNSLs are patterned into meso- and microscale structures to achieve hierarchical architectures by the transfer printing technique using topographically patterned poly(dimethylsiloxane) (PDMS) molds. Mesoscales structures are printed in the shapes of lines, pillars, and even grid-type three-dimensional (3D) structures on various substrates. Electron microscopy and grazing incidence small-angle X-ray scattering (GISAXS) reveal that the long-range nanoscale ordering of multicomponent NC building blocks is well preserved after patterning the BNSLs at the meso- and microscales, allowing creation of these hierarchical architectures. Figure 1 describes the fabrication of meso- and micropatterned BNSLs via liquid interfacial assembly and pattern transfer printing. BNSL formation requires precise control of the size ratio and relative concentration of the two NC building blocks. For example, 12.2 ± 0.6 nm magnetic FeOx and 5.4 ± 0.4 nm plasmonic Au NCs are chosen as the building blocks of BNSLs (see Supporting Information Figure S1 for statistical analysis of NC size from transmission electron microscopy (TEM) images). The relative size ratio including the length of organic ligands between the two NCs is approximately 0.55, which falls into the range where the AlB2-type superlattice is predicted to be a stable structure.18,19,60 A drop of a codispersion of FeOx (3 mg mL−1) and Au NCs (1 mg mL−1) in hexane is deposited on top of a diethylene glycol (DEG) subphase and slowly dried in a closed system.6 The 1388


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Figure 2. (a-c) SEM images and (d) low-magnification (inset selected-area electron diffraction) and (e) high-magnification TEM images of a transfer printed array of lines composed of AlB2-type BNSLs assembled from FeOx and Au NCs.

the BNSL membranes are transferred onto carbon-coated TEM grids. Figure 2d,e shows representative TEM images of patterned FeOx and Au BNSLs. Continuous and smooth patterns are observed with the mosaic texture of multiple BNSL domains separated by grain boundaries. TEM images and smallangle, selected-area electron diffraction (SAED) patterns are characteristic of the (010) lattice projection of AlB2-type BNSLs, corroborating the square symmetry of the NCs seen in SEM images (Figure 2c, inset, and Supporting Information Figure S3).20,64 In one region, the SAED patterns between adjacent BNSL lines show identical orientation of crystallographic direction (Supporting Information Figure S3), indicating the continuity of the crystallographic orientation and symmetry of the BNSLs across lines in the patterned array. This result demonstrates that the membrane sitting on the raised regions of the PDMS mold are transferred without significant damage to the BNSL ordering. Atomic force microscopy (AFM) images indicate that the BNSL pattern reproduces the dimensions of the original PDMS mold used in transfer printing and the thickness of the BNSL formed during liquid phase assembly. For example, the BNSL line structures are found to be on average 2.42 ± 0.03 μm in width with a 0.77 ± 0.12 μm spacing (Supporting Information Figure S4), consistent with the 2.47 ± 0.07 μm line width and 0.77 ± 0.06 μm trench width design of the PDMS mold (Supporting Information Figure S5). The thickness of the BNSL patterns measured by AFM is approximately 34.3 ± 1.9 nm, which corresponds to 2−3 layers of the assembly structure. The thickness is controlled by changing the concentration of the NC dispersion used in liquid interfacial assembly. AFM height profile images reveal the patterned binary membranes are smooth with a root-mean-square surface roughness (Rq) of 4.5 ± 1.7 nm (Supporting Information Figure S4). The pattern edges are well-defined, with a line edge roughness of 33.2 ± 7.8 nm as characterized from TEM images (Supporting Information Figure S6). Considering the sizes of the constituent NCs,

assembly process is completed within 30 min resulting in the formation of densely packed BNSL membranes floating on top of the DEG subphase. The BNSL membranes are transferred onto the surface of prepatterned PDMS molds via the Langmuir−Schaefer process61 and dried under a vacuum to remove residual DEG. The BNSL membranes are mechanically robust;6,62,63 that is, most membranes remain intact after transfer to the PDMS molds and vacuum drying. The quick drying of the membranes avoids crack formation in the BNSLs due to shrinkage of the film under vacuum. After drying, the BNSLs membranes on the PDMS molds are transferred on to a substrate via pattern transfer printing using a home-built apparatus. (Supporting Information Figure S2) The substrate and PDMS mold with the BNSL membrane are placed in between a metal plate and an elastic silicone sheet. Vacuum (typically up to 0.8−1.0 mbar) is applied to evacuate the air between the metal plate and silicone sheet, mildly pressing together the substrate and PDMS stamp. The BNSL thin film on the raised regions of the patterned PDMS mold is brought into conformal contact with the substrate, selectively transferring the BNSLs. The low surface energy of the molds and the high adhesive affinity of NC membranes allows the patterned BNSL membranes to be readily transferred to a variety of substrates including Si wafers, glass, metal foils, and organic layers, highlighting the versatility of this pattern transfer technique. Figure 2a−c shows scanning electron microscopy (SEM) images of AlB2-type FeOx and Au BNSLs patterned to form large-area arrays of micron-scale lines on a Si substrate using the process described in Figure 1. SEM images reveal that the densely packed BNSL membrane are transferred intact onto the Si substrate (Figure 2a) and the patterned assembly maintains the square lattice of the NC building blocks characteristic of the AlB2-type BNSL (Figure 2b,c). More detailed TEM analysis is performed to visualize the individual NCs in the BNSL and their symmetry in the assembled structure. For TEM analysis, 1389


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Figure 3. (a) SEM and (b) TEM images of patterned FeOx and Au BNSLs. (c) GISAXS data of the patterned BNSLs with (hkl) indexing of the calculated diffraction peaks from the transmission channel (stars). Peak (002) with circular markers indicating the anticipated postions of intensity from the reflection channel.

Figure 4. (a) Low-magnification and (b) high-magnification SEM images of a line grating patterned from AlB2-type BNSLs of semiconducting CdSe@CdS core/shell quantum dots and dielectric β-NaGdF4 NCs. (c) TEM image of the patterned BNSLs and (inset) the corresponding SAED pattern. (d) GISAXS data of the corresponding patterned BNSLs. Spots labeled in blue (respectively black) corresponds to [120] (respectively [001]) crystal orientation. Circle and star markers correspond to reflection and transmission channels.

These values are in agreement with TEM images where a and c are estimated to be 14.4 nm. Compared to a perfect AlB2 structure, this sample presents a vertical compression of 6.7% (see Supporting Information for the calculation).66,67 The shape of the GISAXS features gives us insight into the size and the thickness of the BNSL as it separates in-plane and out-ofplane scattering. For thin films, peaks are expected to be narrow in the qy direction and broader in the qz direction. The in-plane 2π coherence length S is defined as S = fwhm and is ∼900 nm for

the line edge roughness value corresponds to only 2−3 unit cells of the binary pattern. The finely defined structures of the BNSL pattern reproduce the 54.9 ± 8.5 nm line edge roughness of the PDMS mold (characterized by AFM Supporting Information Figure S5), which is less than 3% of the line width. To confirm the long-range order of the patterned structures, GISAXS measurements are performed on the patterned BNSLs. Figure 3 shows SEM, TEM, and corresponding GISAXS measurements of BNSLs of FeOx and Au NCs patterned on Si substrates. In agreement with SEM and TEM images (Figure 3a, b), the GISAXS reflections can be identified as scattering from an AlB2-type BNSL oriented in the [120] direction (Figure 3c), perpendicular to the substrate (i.e., facet (010) parallel to the substrate). Using GIXSGUI software,65 a model of the sample hexagonal structure is fit to the data. Under the space group 191 (P6/mmm), the lattice parameters obtained are a = c = 14.3 nm, b = 13.6 nm and γ = 121.8°.

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the (100) peak. The coherence length is more than one-third the width of the patterned lines. The out-of-plane coherence 2π length T, defined as T = fwhm is ∼24 nm for the (111) peak, z

which is consistent with the average thickness of the BNSL membranes formed via liquid interfacial assembly. The (111) peak is used as it is better resolved (see details in Supporting Information and Figure S7). In both cases, the fwhm are 1390


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Figure 5. TEM images of (a) circular and (b,c) square arrays of AlB2-type-BNSLs of FeOx and Au NCs. (d−f) TEM images of a grid pattern of AlB2type BNSLs of FeOx and Au NCs fabricated by sequential transfer printing of self-assembled BNSL membranes.

corrected from the experimental resolution Res = 4 × 10−4 Å−1. Even though these values are only rough estimates of the inplane size and thickness of an average BNSL domain, these results confirm large-area, long-range ordering in the patterned BNSL film, which is often limited by the pattern size (Supporting Information Figure S3). Thus, SEM, TEM, and GISAXS data confirm that the transfer printing technique can achieve wide-area meso- and microscale patterns successfully without damaging the nanoscale ordering of the BNSLs. The NC compositions in the patterned BNSLs are readily tunable by changing the type of building blocks used for selfassembly. Figure 4 shows a patterned line grating of BNSLs self-assembled from 4.2 ± 0.3 nm semiconducting CdSe@CdS core@shell quantum dots and 13.2 ± 0.6 nm dielectric βNaGdF4 NCs (Supporting Information Figure S1). SEM and TEM images and small-angle SAED (Figure 4a−c) show largearea, grating patterns composed of a square lattice of NC building blocks characteristic of densely packed, AlB2-type BNSLs viewed along the (010) projection. GISAXS measurements (Figure 4d and Supporting Information Figure S8) confirm long-rage ordering of the quantum dots and dielectric NCs. This GISAXS image shows a more complex scattering distribution compared to that for the FeOx/Au BNSLs (Figure 3c). The more complex pattern arises as (i) the sample contains two crystal orientations [120] and [001] perpendicular to the substrate, and (ii) for each superlattice the features are split in two contributions called the “reflection channel” and “transmission channel”, which doubles the number of spots on the diagram. Detailed analysis of the GISAXS data for patterned CdSe@CdS and β-NaGdF4 BNSLs is given in Supporting Information and Figure S8. Similar data analysis is performed sequentially for the two orientations. For the [120] orientation, the data is fit to the SG191 P6/mmm crystal structure with lattice parameters a = c = 16.2 nm, b = 15.5 nm and γ = 121°. These values indicate a slight compression (5.3%) of the lattice perpendicular to the substrate. The in-plane and out-of-plane coherence lengths obtained from (111) peaks are S = 1 μm and

T = 220 nm. For the [001] orientation, the BNSL lattice parameters are found to be a = b = c = 15.9 nm and γ = 120°. This BNSL does not exhibit detectable compression in the cdirection, perpendicular to the substrate. Values of 8 to 12% of compression were previously described in AB2-type FeOx/Au BNSLs self-assembled by slow drying on substrates.64,66,67 The smaller compression measured in our films may be explained by the reduced stress during BNSL assembly on the liquid interface compared to a solid substrate. From the (202) peaks, the calculated in-plane coherence length is limited by instrumental resolution with S > 1.3 μm and the out-of-plane coherence length is T = 250 nm. The presence of features over 100 nm in thickness in this sample, as can be seen by AFM (Supporting Information Figure S9), explains the higher Tvalue, even though the average film thickness is 41.1 ± 16.5 nm. The in-plane coherence measurements show that the pattern transfer printing technique can be applied to BNSLs constructed from various types of NCs and assembled in various orientations without destroying the long-range order. The shape of patterned BNSLs is also tunable simply by changing the topographical structures on the PDMS molds. Figure 5a−c shows pillar-patterned arrays of AlB2-type Au/ FeOx BNSLs. The shape of the BNSLs after pattern transfer printing reproduces the 5 μm (Figure 5a) and the 10 μm size pillar (Figure 5b) of the molds. Figure 5c shows grain boundaries highlighting the polycrystalline structures in each molded BNSL. Each pixel is successfully transferred over large area while maintaining the binary symmetry originally formed during the liquid interfacial assembly, which gives us confidence that the pattern transfer printing technique can be implemented with a wide array of patterned molds. More complex 3D architectures of patterned BNSL arrays are fabricated via sequential pattern transfer printing of BNSLs, as presented in Figure 5d−f. For these 3D assemblies, a BNSL membrane is assembled on the liquid interface and transferred to a PDMS mold with a line array. A first grating patterned BNSL is deposited on the substrate via pattern transfer 1391


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followed by a centrifugation (8000 rpm, 2 min). The sediment is redispersed in hexane and washed further for three times by isopropyl alcohol to remove extra amount of surfactants and solvent. The final product is dispersible in a nonpolar solvent such as hexane or toluene. Synthesis of 5.4 nm Au NCs. Au NCs are synthesized following a published method with slight modifications.68 TBAB (0.5 mmol) is dissolved in the mixture of oleylamine (1 mL) and tetralin (1 mL). In the meanwhile, 100 mg HAuCl4· 3H2O is dissolved in the mixture of 10 mL of oleylamine and 10 mL of tetralin in a 50 mL flask by brief sonication. At room temperature in air, the TBAB solution is swiftly injected into the flask. Upon injection, the solution changes color instantly from orange to dark red but the reaction is left stirring in air for 1 h. Particles are collected by centrifugation (8000 rpm, 3 min) with 60 mL of isopropyl alcohol and then purified by two rounds of precipitation in hexane/isopropyl alcohol (5 mL/15 mL). For each washing cycle, 20 μL of oleylamine is added to the hexane solution of particles before the addition of isopropyl alcohol to prevent the particles from fusing. Synthesis of CdSe@CdS Core/Shell NCs. Oleic acidprotected CdSe QDs were prepared as described by Yang et al.69 by using cadmium oleate (0.5 M in ODE, 1 mmol) and elemental selenium (0.5 mmol) as precursors. After reaction at 240 °C for 30 min, ZnEt2 and (TMS)2S (1 mmol each) dissolved in 6 mL of TOP were injected using a syringe pump at 0.1 mL min−1. In the case of further CdS coating, cadmium oleate (0.5 M in ODE) and (TMS)2S (2 mmol each) were dissolved into 12 mL of TOP and injected at 0.1 mL min−1. The mixture was cooled to room temperature and NCs were isolated and purified by centrifugation using isopropyl alcohol and methanol as antisolvents. Synthesis of β-NaGdF4 NCs. β-NaGdF4 spherical NCs are synthesized according to the method previously reported with slight modification.70 Gadolinium trifluoroacetate (0.98 g) and 0.44 g of sodium fluoride are added into the reaction mixture of oleic acid and 1-octadecene (30 mL/30 mL, respectively) in 125 mL of three neck flask. The reaction mixture is heated at 125 °C under vacuum for 2 h to remove water. Then, the solution is heated to 320 °C under N2 environment at a rate of 15 °C/min and maintained at this temperature for 30 min. Purification is performed by adding excess of ethanol and centrifugation at 6000 rpm for 2 min. Purification processes are conducted twice. Residual sodium fluoride is removed by precipitation after dispersing NCs in hexane and centrifugation at 3000 rpm for 2 min. Liquid Interfacial Assembly and Transfer Printing. Liquid interfacial assembly is performed according to the procedure previously reported.6 A Teflon well is filled with polar subphases such as ethylene glycol or diethylene glycol. Then, 30 μL of NCs dispersed in hexane is added on top of the surface of subphase. The well is then covered with a glass slide to reduce the hexane evaporation rate. Once the BNSL membrane is formed, it is transferred onto a prepatterned PDMS stamp via Langmuir−Schaefer technique. BNSL films on the PDMS mold are dried under vacuum for 30 min to remove residual ethylene glycol subphase. Pattern transfer printing of the BNSL membranes are performed on a substrate such as a silicon wafer or carbon-coated TEM grid using homebuilt apparatus. Preparation of 3D Grid-Type Patterned BNSLs. First, a layer of line-patterned BNSL film is fabricated on a substrate such as a silicon wafer and carbon-coated TEM grid as described above.

printing. Another PDMS mold, replicated from the same master template, is used to transfer a BNSL membrane assembled on another area of the liquid interface and subsequently used to print another layer on top of the first but with a 90° rotation. The first and second orthogonally printed line arrays form a grid-type pattern consisting of highly ordered BNSLs. TEM images in Figure 5d and Supporting Information Figure S10 shows that this grid-type BNSL pattern is achieved over a large area (∼20 μm × 20 μm). The grid width and spacing can be easily tuned by adjusting the line width of the mold pattern. If the composition or assembly structure of the second layer is different from that of the first layer, the collective properties of the superimposed area may be further differentiated. Therefore, the property of each microsized domain is tunable depending on the choice of NC size, shape, and composition and the nanoscale assembled and meso- to microscale pattern structure of the BNSLs. This gridtype BNSL assembly suggests that sequential pattern transfer possesses great potential for the fabrication of complex threedimensional matrices with multiple functionalities. In summary, we have demonstrated the fabrication of patterned BNSLs using a combination of liquid interfacial assembly and pattern transfer printing techniques. Selfassembled BNSL membranes are transferred into various structures in the shapes of lines and pillars on various types of substrates, including glass, Si wafers, and carbon-coated TEM grids. The structure and order of the BNSL membranes self-assembled on the liquid interface are preserved during the transfer printing process resulting in well-defined, highly ordered BNSL patterns over large areas, as corroborated by GISAXS analysis. Finally, we demonstrate that sequential deposition of patterned BNSLs enables the formation of more complex structures such as grid-type 3D BNSLs. The fabrication of patterned BNSLs by combining liquid interfacial assembly and sequential pattern transfer printing offers a unique opportunity to tailor materials properties not only by accessing new types of binary phases self-assembled from different types of NC building blocks but also by manipulating the size and shape of the superstructures and sequentially stacking the structures in 3D. Methods. Chemicals. All the chemicals were used as received. Iron(III) acetylacetonate (99+ %), 1-octadecene (technical grade, 90%), hydrogen tetrachloroaurate (III) hydrate (HAuCl 4 ·3H 2 O), oleylamine (C18 content: 80∼90%), 1,2,3,4-tetrahydronaphthalene (tetralin, 98+ %), and sodium fluoride (99+ %) were purchased from Acros Organics. Oleic acid (technical grade, 90%), oleylamine (technical grade, 70%), tert-butylamine-borane complex (TBAB, 97%), trioctylphosphine (TOP, technical grade, 90%), diethylzinc (ZnEt2), hexamethyldisilathiane ((TMS)2S), and gadolinium(III) oxide (99.99%) were purchased from Sigma-Aldrich. Gadolinium trifluoroacetate precursors are synthesized by refluxing gadolinium oxide powder in water/ trifluoroacetic acid mixture (50:50 vol %). Synthesis of 12.2 nm Iron Oxide NCs. Iron(III) acetylacetonate (6.36 g), 31.8 mL of oleic acid, 38.4 mL of oleylamine, and 72.0 mL of 1-octadecene are mixed in a 250 mL flask. The reaction mixture is heated up to 110 °C and put under a vacuum. After 2 h, the reaction vessel is refilled with nitrogen gas and the temperature of the system is increased to 300 °C at a rate of 10 °C/min. After 1 h at 300 °C, the reaction mixture is cooled down to room temperature and iron oxide NCs are precipitated by adding ethanol and isopropyl alcohol 1392


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electron microscopy was performed in facilities supported by the NSF MRSEC Program under Award No. DMR-1120901. C.B.M. is grateful for the support of Richard Perry University Professorship. C.R.K. thanks for the support of Stephen J. Angello Professorship.

Then, after rotating the silicon wafer by 90°, another layer of line-patterned BNSL film is printed on top of the first layer by gently pressing the PDMS stamp, resulting the 3D grid type patterned BNSLs as displayed in Figure 5. Characterization. TEM and electron diffraction analysis are performed using a JEM-2100 microscope operating at 200 kV. Scanning electron microscopy images are collected on a JEOL 7500F HRSEM. AFM measurements are conducted in tapping mode with Bruker Icon AFM. Grazing incidence small-angle Xray scattering was measured at Advanced Photon Source Sector 8-ID-E (Argonne National Lab). The beam size is 50 μm tall and 100 μm wide. Beam energy is 7.35 keV and incidence angle α ∼ 0.2°. The detector is a Pilatus 1 M at a distance of ∼130.4 cm. The q resolution in both qy and qz direction is 4 × 10−4 Å−1. Line edge roughness information is extracted using ImageJ and a plugin file, “Analyze_Stripes_v.2.4.4.ijm”, in the ImageJ Web site.

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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/acs.nanolett.6b04279. Additional TEM and SEM images, AFM analysis result. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: (C.R.K.) [email protected]. *E-mail: (C.B.M.) [email protected]. ORCID

Taejong Paik: 0000-0003-0111-8513 Hongseok Yun: 0000-0003-0497-6185 Yaoting Wu: 0000-0002-4363-9870 Author Contributions

T.P and H.Y contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We want to acknowledge Advanced Photon Source and the beam scientists of 8-ID-E, Joseph Strzalka and Zhang Jiang, for their help with GISAXS experiments. The authors are grateful for primary support of this work from the Office of Naval Research Multidisciplinary University Research Initiative Award No. ONR-N00014-10-1-0942 for the synthesis NaGdF4 NCs, NC self-assembly, transfer printing, and microscopic analysis. GISAXS measurement and analysis was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering under Award No. DE-SC0002158. Synthesis of FeOx NCs was supported by the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001004. Synthesis of Au NCs was supported by the NatureNet Science Fellowship offered by the Nature Conservancy. CdSe/CdS NCs was supported by the National Science Foundation (NSF) under Award No. NSF CBET1335821. AFM characterization was supported by and scanning 1393


Letter

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