Spontaneous Registration of Sub-10 nm Features Based on Subzero

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Spontaneous Registration of Sub-10 Nanometer Features Based on Sub-Zero-Celsius Spin-Casting of Self-Assembling Building Blocks Directed by Chemically-Encoded Surfaces Jung Hye Lee, Hak-Jong Choi, ChulHee Lee, Seung Won Song, Joong Bum Lee, Daihong Huh, Yoon Sung Nam, Duk Young Jeon, Heon Lee, and Yeon Sik Jung ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03378 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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Spontaneous

Registration

of

Sub-10

Nanometer

Features Based on Sub-Zero-Celsius Spin-Casting of Self-Assembling

Building

Blocks

Directed

by

Chemically-Encoded Surfaces Jung Hye Lee,1,† Hak-Jong Choi,2,†,⊥ ChulHee Lee,1 Seung Won Song,1 Joong Bum Lee,1 Daihong Huh,2 Yoon Sung Nam,1 Duk Young Jeon,1 Heon Lee,2,* and Yeon Sik Jung1,* 1

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea 2

Department of Materials Science and Engineering, Korea University, 145 Anam-ro,

Seongbuk-gu, Seoul, 02841, Republic of Korea

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ABSTRACT For low-cost and facile fabrication of innovative nanoscale devices with outstanding functionality and performance, it is critical to develop more practical patterning solutions that are applicable to a wide range of materials and feature sizes while minimizing detrimental effects by processing conditions. In this study, we report that area-selective sub-10 nm pattern formation can be realized by temperature-controlled spin-casting of block copolymers (BCPs) combined with sub-micron-scale-patterned chemical surfaces. Compared to conventional room-temperature spin-casting, the low temperature (e.g., -5℃) casting of the BCP solution on the patterned SAM achieved substantially improved area-selectivity and uniformity, which can be explained by optimized solvent-evaporation kinetics during the last stage of film formation. Moreover, the application of cold spin casting can also provide highyield in situ patterning of light-emitting CdSe/ZnS quantum dot thin films, indicating that this temperature-optimized spin-casting strategy would be highly effective for tailored patterning of diverse organic and hybrid materials in solution phase.

Keywords: spontaneous registration, cold spin-casting, self-assembled monolayer, block copolymers, quantum dots


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High-precision patterning of various types of functional materials is requisite for highperformance micro/nanoscale integrated devices.1-4 More specifically, device performance and reliability can be improved by employing more advanced patterning techniques with high resolution, accuracy, controllability, and reproducibility.5,

6

Developing more practical

patterning solutions that are applicable to a wide range of functional materials and an extensive feature size range thus can have a powerful impact in this field.7-9 However, the processing conditions of high-resolution patterning techniques often detrimentally influence the physical and chemical properties of functional materials, especially for vulnerable organic and organic/inorganic hybrid materials.10 For example, photolithography, e-beam lithography, and nanoimprint lithography inevitably require processing steps based on UV exposure, solvent or thermal treatment, reactive plasma, and so on, and these steps often induce unfavorable conditions to the less robust materials.7,

9, 11

Direct micro/nano-molding and

transfer printing based on elastomeric stamps can circumvent such issues, but these methods accompany a resolution limitation or difficulty of precise spatial registration on desired locations.12-14 The use of a pre-patterned self-assembled monolayer (SAM) combined with various thin film casting techniques is one of the alternative approaches.15-19 Once a properly designed SAM pattern composed of two or more SAMs as a chemical guide is formed on a substrate, many different types of thin film materials can be controllably positioned according to the chemical information encoded in the guiding surface template.20, 21 For example, a fluorinecontaining SAM (F-SAM) can induce dewetting of almost all thin films due to its extremely low surface energy.22 Patterned thin films can thus be selectively formed on the other areas where the F-SAM is not present.23 A variety of inorganic thin films such as ZnO,24 TiO2,25 and ZrO226 have been selectively deposited on a SAM-patterned substrate via atomic layer deposition.27, 28 Also, solution-based casting techniques such as spin-coating can be employed 3 ACS Paragon Plus Environment

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for selective deposition of various functional materials such as metal particles,23 DNA,29 perovskite materials,30 and inorganic nanoparticles29 on SAM-patterned substrates. In general, however, the resolution of SAM-directed spin-coating is typically limited to the micron to sub-micron range, preventing the fabrication of high-density, high-performance nano-devices. Also, poor uniformity and insufficient area-selectivity of spun-cast films at desired locations remain critical challenges, as we will experimentally demonstrate. Moreover, from the viewpoint of advanced directed self-assembly (DSA) of block copolymers (BCPs), a more facile pattern customization methodology should be developed to directly generate deviceoriented geometries at selective locations.31-34 Herein, we report that on-demand sub-10 nm patterning technology can be realized by cold spin casting (CSC) combined with sub-micron SAM patterns (i.e., SAM-directed cold spin casting; S-CSC). The poor resolution of conventional SAM-directed nanopatterning is overcome by the use of self-assembling BCPs containing internal ultrafine domain structures, which provide 9-nm-wide line-and-space patterns after simple plasma oxidation. Moreover, we show that excellent area-selectivity of the desired pattern formation during spin casting can be achieved by low-temperature spin-casting of BCP solutions. Furthermore, the application of S-CSC is not limited to the patterning of polymers but also S-CSC can stabilize the patterned formation of light-emitting CdSe quantum dot thin films. This suggests that this reliable in situ nanopatterning strategy based on a patterned dewetting mechanism would be effective for a variety of organic and hybrid materials.

RESULTS AND DISCUSSION Figure 1 demonstrates the water contact angle values measured at room temperature (RT) on various substrates with different functionalized layers of hepadefluoro-1,1,2,2-tetrahydrodecyltrichlorosilane (HDFS), polydimethylsiloxane (PDMS), dodecyltrichlorosilane 4 ACS Paragon Plus Environment

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(DDTS), and polystyrene (PS). The relative contact angle changes suggest that HDFS and PS provide the lowest and highest surface free energy, respectively. For comparison of spincoating coverage and uniformity, a PS homopolymer with a molecular weight of 35 kg/mol in a toluene and heptane mixture (volume mixing ratio 5:5) was spun-cast at RT on different substrates. A uniform 100% coverage was obtained for the substrates functionalized with PS, DDTS, and PDMS, whereas almost 0% coverage was observed for the HDFS surface (lowest surface energy) due to serious dewetting of the PS solution. Interestingly, as the temperature of the solution (Tsol) decreases, the coverage of the PS films on DDTS and PDMS rapidly decreased. Also, for the low-temperature spin casting on the DDTS and PDMS SAMs, a solution concentration of PS lower than 1.0 wt% resulted in film coverage of less than 1. In contrast, regardless of Tsol and the solution concentration, 100% PS film coverage and 0% coverage were maintained for the PS-functionalized and HDFS-functionalizes substrates, respectively. These initial observations motivated us to systematically control Tsol and to adopt PS and HDFS as appropriate surface modifiers for reliable and uniform in situ patterning of organic materials via S-CSC. Figure 2a illustrates the overall process flow of S-CSC. For the patterning of chemically encoded surfaces, we employed soft lithography (transfer-printing) using a PDMS mold to achieve sharp line edge roughness of the features with sub-100 nm resolution on a large-area surface (see Figure S1 for more details).35, 36 First, PS was grafted on Si or glass substrates by spin-coating hydroxyl-terminated PS (PS-OH) followed by a hydrolysis reaction on the oxide surface through thermal-annealing in a vacuum oven. Monolayer PS with a thickness of 8 nm was then formed after rinsing unattached PS chains. Poly-benzylmethacrylate (PBMA) in chlorobenzene solution with a concentration of 1.5 wt% was spin-casted on a PDMS mold, which was brought in contact with the substrate surface and thermal-annealed at 120℃ under a pressure of 5 atm. The exposed PS-functionalization layer was then removed using reactive 5 ACS Paragon Plus Environment

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ion etching (RIE). The exposed substrate surfaces were coated with F-SAM by dipping the sample in a solution of HDFS in hexane for 10 min to induce a silanization reaction.37 Finally, SAM patterns composed of PS (surface energy = 40.7 mN/m) 38 and F-SAM (surface energy = 17.2 mN/m)23 lines were obtained and used for S-CSC. A scanning electron microscopy (SEM) image of the chemical guide templates with alternating PS and HDFS lines with a period of 0.8 µm and a 50% duty cycle is presented in Figure 2b. The contrast resulting from the difference of secondary electron yield between PS and HDFS clearly differentiates the two regions, confirming the well-defined structure of the patterns. Also, in order to investigate the long-range alignment of the SAM patterns, the substrate was analyzed via grazing-incidence small-angle X-ray scattering (GISAXS). When the grating direction of the SAM patterns is perfectly parallel with the direction of the incident X-ray beam, due to the large period (0.8 um) of the SAM patterns, the diffraction patterns gathered at the center and were screened by the beam blocker, as presented in Figure 2c. However, ring-like diffraction spots clearly appeared, as presented in Figure 2d, when the SAM grating structures are slightly misaligned with the X-ray beam direction. The observation of ring-shaped patterns resulted from the intersection of the Ewald sphere with the reciprocal lattice of the grating,39 and indicates the formation of well-defined SAM patterns over a macroscopic area. Directed self-assembly (DSA) of BCPs has been suggested for scaling down the lithography feature size to 5 - 15 nm.31-34,

40-45

Spontaneous phase separation of BCPs

combined with the guiding template forms patterns with pre-determined shape, size, and orientation, which also enables the generation of complex patterns.46-51 DSA pattern customization using hybrid organic/inorganic chemical patterns was also reported.52 On the other hand, direct writing of a BCP solution using electrohydrodynamic jet printing formed hierarchical structures.53 If BCP self-assembly can be combined with SAM-directed in situ 6 ACS Paragon Plus Environment

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nanopatterning, more convenient pattern customization can be realized and the resultant outstanding pattern resolution can be exploited for a variety of applications. We employed poly(styrene-b-dimethylsiloxane) (PS-b-PDMS) BCP (16 kg/mol, SD16), which is a BCP with a large Flory-Huggins interaction parameter (χ),54, 55 to generate sub-10 nm patterns by overcoming the order-disorder transition limit imposed by a short BCP chain. The BCP solution was spun-cast on SAM patterns with a periodicity of 0.8 um that were fabricated as mentioned above. The patterned PS brush was intended to attract the PS block in the PS-b-PDMS BCP thin films, while the HDFS (F-SAM) regions repel the BCP solution, leaving an empty region where the BCP is not coated. Although the spin-casting of the BCP thin films at RT showed partial selectivity on the PS region, a substantial portion of the PS region was not coated by the BCP, and some areas of the F-SAM patterns were covered with the BCP, thus showing poor selectivity. Inspired by the results shown in Figure 1, the solution casting temperature (Tsol) was controlled as depicted in Figure 3, and a noticeably different pattern selectivity was observed depending on Tsol. For a quantitative and statistic evaluation of the SAM-directed pattern quality, the BCP coating coverage on the PS region (Sn, 0 ≤ Sn ≤ 1, n = 1, 2, 3…, n) and on the F-SAM region (Fm, 0 ≤ Fm ≤ 1, m = 1, 2, 3…, m) was respectively measured. This quantitative analysis was conducted using randomly collected SEM images from more than 160 lines from each sample. As presented in Figure 3b, Sn increased from 0.61 to 0.94 as the solution temperature was changed from 25℃ to -5℃, and decreased slightly at -15℃. For the same temperature change, Fm was minimized at the temperature of -5℃. The total coverage, which is defined as 0.5 · (Sn + Fm), remained almost constant at 0.45 ± 0.02 as a function of Tsol, as presented in Figure 3d. Also, we defined the selectivity (S) as S = (∑ Sn)/n – (∑ Fm)/m (where S = 1 and S = 0 indicate maximum and zero selectivity values, respectively) (see Figure S2). From the definitions of S, Sn, and Fm, the highest selectivity can be achieved when Sn is maximized (1) 7 ACS Paragon Plus Environment

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and Fm is minimized (0). We discovered that the S values increased from 0.29 to 0.88 upon decreasing the solution temperature from 25℃ to -5℃. Further cooling of the solution (Tsol = 15℃) however, resulted in a decrease of S (0.65), as shown in Figure 3c. Successfully positioned BCP stripes with good uniformity are depicted in Figure S3. The linewidth, duty cycle (=linewidth/period), and line fluctuation of the coated polymer patterns on the PS region were also verified through an image analysis. While the average line width and duty cycle were almost constantly maintained regardless of Tsol (Figures 3g and 3f), the linewidth fluctuation of the patterned films decreased by 44 % by reducing Tsol from 25℃ to -5℃. It should be noted that these thin film patterns were characterized before the BCP development via O2 plasma etching to expose the sub-10 nm microdomain structures. The measured pattern quality parameters thus are employed to evaluate the S-CSC pattern quality. The selfassembled BCP pattern quality will be discussed in the latter part of this paper. In general, the spin coating phenomenon can be broken down into steps of dispensing of a solution, spreading, spin-up, spin-off, and solvent-evaporation.56 Area-selective dewetting of the solution and migration of the solution to the desired SAM location is thought to occur at the last stage – evaporation. This suggests that the evaporation kinetics of the solvent from the film surface is a key factor for the quality of SAM-directed patterning. Table S1 demonstrates the temperature dependence of the evaporation rate of the solvent calculated by the Antoine equation.57, 58 Because of the exponentially decreasing evaporation rate of the solvent with the reduction of Tsol, CSC provides longer time for the migration of the droplets to the targeted area (PS-coated region) before the BCP solution is completely dried. Given that the viscosity of a polymer solution rapidly increases with a decrease of solvent in the solution,59 the effective time for the migration of the BCP solution would be longer for lower Tsol, as illustrated in Figure S4. However, the degraded selectivity at a too low Tsol (e.g., -15℃) can be attributed to excessively depressed surface diffusion kinetics of the solution.59 These 8 ACS Paragon Plus Environment

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mutually competing effects can explain why the best pattern quality of S-CSC was obtained at Tsol = -5℃. The S-CSC-patterned SD16 BCP thin films were thermally annealed at 250℃ in a vacuum oven for self-assembly and were subsequently treated with O2 plasma to selectively remove the PS block and reveal the linear patterns of oxidized PDMS microdomains with an average line width of 9.4 nm. During the thermal annealing, the BCP chains were confined within the initially patterned dimension (PS region) due to the extremely low surface energy of the FSAM. The oxidized PDMS line patterns with an average width of 9.4 nm were aligned along the edges of the BCP films, as shown in Figures 3h, and 3j. The GISAXS patterns of the samples prepared with different Tsol showed clearly resolved diffraction patterns (Figures 3k and S5), suggesting the successful alignment of BCP microdomains in parallel with the longaxis direction of the SAM patterns. However, this surface confinement effect, and therefore the quality of the S-CSC patterns, strongly affected the straightness of the self-assembled patterns. The relatively smaller roughness of the S-CSC patterns resulted in the best pattern quality of BCP patterns, as shown in Figures 3j and S6. We investigated surface potential changes of the substrates after each step of S-CSC at Tsol = -5℃ using Kelvin probe force microscopy (KFM), which is useful for identifying the structures of organic molecular monolayers including F-SAM.60-63 Figure 4 schematically illustrates the cross-sections of each step (Figures 4a, 4e, 4i, and 4m), 3D topographic images (Figures 4b, 4f, 4j, and 4n), 3D surface potential mapping images (Figures 4c, 4g, 4k, and 4o), and corresponding 2D line profiles of the topography and surface potential (Figures 4d, 4h, 4l, and 4p). The KFM measurement on the patterned SAM substrate demonstrated a large difference (~ 45 mV) in the surface potential between the PS and F-SAM despite a very small height difference (~ 5 nm) in the topography, as shown in Figures 4a, 4b, 4c, and 4d. The negative surface potential of F-SAM is consistent with other previous results on fluorinated 9 ACS Paragon Plus Environment

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organosilanes grafted on silicon oxide.62, 63 After in situ nanopatterning of SD16 via S-CSC (Figures 4e, 4f, 4g, and 4h), the height difference (~ 31 nm) was substantially increased by the selective coating of the BCP on the PS region. In contrast, the surface potential difference decreased from 45 mV to 30 mV after BCP coating, which can be explained by the incorporation of PDMS with an extremely low surface energy in the BCP. The surface potential difference further decreased to 20 mV due to the generation of a continuous top PDMS layer at the air/BCP interface. The height difference slightly decreased while the film pattern width increased due to the formation of equilibrium thickness (~20 nm) of the BCP thin film during thermal annealing, as shown in Figure 4i, 4j, 4k, and 4l. Finally, as a result of removing the PS block and oxidizing the F-SAM, a small surface potential (~7 mV) between the native oxide of Si and the oxidized PDMS patterns was measured. We now demonstrate the application of S-CSC to in situ patterning of other materials such as organic-ligand-capped inorganic QD thin films. Because S-CSC does not require any posttreatment and the as-spun sample can be immediately used for applications, it can minimize the degradation of QDs in comparison with photolithography or transfer printing, which typically require removal of a photoresist or thermal or solvent treatments after patterning.6467

We synthesized red-emissive core-shell CdSe/ZnS QDs (average diameter = 6.33 ± 0.11

nm) capped with trioctylphosphine (TOP) ligands via the hot-injection method (see the Experimental section). The photoluminescence spectra of the solution-state QDs showed a peak wavelength of 640 nm, as shown in Figure 5a. As shown in Figures 5b and S7, the PL peak wavelength was not changed by either the RT- or low-temperature casting of the solution on the glass substrate, although the RT-cast sample showed a stronger PL emission intensity due to 151% larger film thickness compared to the CSC (Tsol = -5℃) sample. Overall, the spin-casting did not cause a change of emission characteristics. However, the florescence microscopy images presented in Figures 5c and 5d suggest that the CSC (Tsol = 10 ACS Paragon Plus Environment

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5℃) can provide better uniformity. The image of the RT-cast sample showed voids and roughness in the film, suggesting pores caused by rapid evaporation of the volatile solvent (toluene). Figure S8 presents the line-scan florescence intensity profiles of the samples, confirming superior smoothness of the CSC QD film surface compared to the RT-cast sample. The QDs in the toluene solution were also spin-casted on the SAM patterns with a periodicity of 9 um. As presented by the fluorescence and confocal microscopy (inset) images in Figures 5e and 5f, the SAM patterns successfully directed the area-selective formation of the QD films on the PS regions. However, the uniformity of the patterned QD films was improved significantly by the use of S-CSC compared to the RT casting, and the RT casting resulted in unintended QD residue spots on the F-SAM regions. This is supported by the linescan height profiles depicted in Figures 5g and 5h. The results of casting of QD solutions with different concentration also confirm the uniformity and selectivity improvement by SCSC in Figure S9. Similar to the case of BCP coating on the SAM, the S-CSC of QDs also caused a decrease of the surface potential difference (by 28.5%), as shown in Figures 5i, 5j, 5k. and 5l, and Figures 5m, 5n, 5o, and 5p.

CONCLUSIONS In summary, we introduced a facile in situ patterning strategy that provides sub-10 nm resolution using temperature-controlled spin-casting of a PS-b-PDMS BCP solution directed by SAM patterns composed of polystyrene and fluorine-containing molecules, providing a high contrast of surface energy. Compared to RT spin casting, low temperature (Tsol = -5℃) spin-casting of the BCP solution on the patterned SAM achieved markedly improved coating selectivity and uniformity. This is attributed to optimized solvent-evaporation kinetics during the last stage of spin coating. The subsequent thermal treatment and plasma oxidation reveals 0.8-µm-wide self-assembled patterns from the S-CSC patterned BCP thin films, achieving 11 ACS Paragon Plus Environment

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ultrahigh-resolution patterning on desired areas. Moreover, we showed that this approach can be extended to other solution-phase materials such as organic-ligand-capped colloidal quantum dots, demonstrating well-aligned core-shell QD patterns with perfectly preserved photoluminescence characteristics. We anticipate that our strategy can provide multiple advantages of high resolution, simplicity, cost-effectiveness, scalability, and versatility in patterning diverse functional materials in solution-phase without degrading their intrinsic properties.

METHODS Fabrication of lithographically patterned chemical template: Micron and sub-micron scale line patterned chemically guided templates (period = 9 µm and 0.8 µm, respectively) were fabricated using soft lithography and reactive ion etching (RIE). For the preparation of a chemically encoded surface, hydroxyl-terminated polystyrene (PS-OH, Mn = 38 kg/mol, purchased

from

Polymer

Source

Inc.)

and

heptadecaflouro-1,1,2,2-tetra-

hydrodecyltrichlorosilane (HDFS, purchased from Gelest) were used as a surface modification layer without further purification. First, PS-OH was dissolved in toluene and then the solution was spin-coated on Si or glass substrates. Subsequently, the PS-OH-coated substrate was thermally annealed in a vacuum oven at 150 °C for 4 h to form a covalent linkage between the substrate and PS, followed by rinsing with toluene and N2 blowing to eliminate residual unreacted PS-OH and to dry the substrate. Poly-benzylmethacrylate (PBMA, Mw = 75 kg/mol, purchased from Aldrich) in chlorobenzene solution with a concentration of 1.5 wt% was then spin-casted on a PDMS mold, which

was brought in

contact with the substrate under a pressure of 5 atm. The sample was held at 120℃ for 20 min under a pressure of 5 atm. The temperature of the sample was subsequently lowered to room temperature and the applied pressure was reduced to 1 atm. By peeling off the PDMS mold 12 ACS Paragon Plus Environment

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from the substrate, a PBMA pattern was successfully obtained on the PS-coated substrate as a removable etch mask. Thereafter, an oxygen RIE process was conducted to form the PS line patterned substrate and to expose Si or glass surfaces, which were coated with a fluorinefunctionalized self-assembled monolayer (F-SAM) by dipping in a 0.1 % of hepadefluoro1,1,2,2-tetra-hydrodecyltrichlorosilane (HDFS) in hexane for 10 min. Finally, a substrate where PS and F-SAM lines are alternately formed was obtained after completely removing the PBMA line pattern using N,N-dimethylformamide (DMF). Directed self-assembly of block copolymer (BCP) and plasma oxidation: Poly(styrene-bdimethylsiloxane) (PS-b-PDMS) BCPs were purchased from Polymer Source Inc. (Canada). All solvents were purchased from Sigma-Aldrich Co. (USA). The BCPs were dissolved in toluene + heptane + PGMEA with a specified mixing ratio (1:1:1, volume fraction, 0.5-0.7 w%) and cooled in a refrigerator or heated by using a hot plate. The BCP solution was spincasted on the patterned SAM Si template immediately and then annealed at 250℃ for 2 h in a vacuum oven. The spin-coated BCP films were treated with CF4 plasma (50 W, 19 sec) followed by O2 plasma (60 W, 20 sec) at 15 mTorr in order to remove the PDMS surface layer and PS matrix, resulting in SiOx line patterns with lateral dimensions of 9.4 nm. Because formation of uniform BCP film with precise thickness is crucial in generating selfassembled monolayer BCP patterns over the entire wafer, BCP solution concentration and spin-casting speed were precisely controlled and optimized. BCP characterization: The surface morphologies of the plasma-treated BCP patterns were observed using a field emission scanning electron microscope (FE-SEM, Hitachi S-4800) operated with an acceleration voltage of 7 kV and a working distance of 4.5 mm. For quantitative analyses of the linewidth, pitch, line width roughness (LWR), and line edge roughness (LER) using the SEM images, commercial image analysis software (SuMMIT) 13 ACS Paragon Plus Environment

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was used. The surface topographic and surface potential signals were obtained with a potential voltage of 5V (AC) and a trace height of 30 nm in a non-contact mode using by Kelvin probe force microscopy (KFM, Hitachi, AFM5000℃). Synthesis of CdSe/ZnS QDs: Cadmium oxide, oleic acid, 1-octadecene, trioctylphosphine, zinc diethyldithiocarbamate, and selenium were purchased from Sigma Aldrich. 5.3 mmol of cadmium oxide (CdO, 99.5%), 6 mL of oleic acid (OA, 90%), and 10 mL of 1-octadecene (ODE, 90%) were heated to 230°C with N2 gas. When the temperature at 230°C, 0.3 mmol of selenium dissolved in 1ml of trioctylphosphine (TOP, 97%) was quickly injected into the reaction mixture and

heated. Subsequently, a mixture

of 0.3 mmol of zinc

diethyldithiocarbamate (Zn DDTC, 97%) dissolved in 2 ml of TOP was injected into the reaction mixture at 225°C and heated for 10min. The temperature was then adjusted to 280°C and the solution was heated for 20 min. The final product was purified by adding chloroform and ethanol, followed by centrifuging at 6000 rpm. Positioning of CdSe QDs solution and characterization: CdSe/ZnS QDs were dissolved in toluene (0.3~0.5 w%) and spin-casted on the patterned SAM glass substrate via CSC. PL spectra vs. wavelength were acquired on a F-7000 fluorescence spectrometer operated with an emission wavelength of 450 nm. Fluorescence image maps were obtained using fluorescence microscopy (Leica, DMI 3000 B) and confocal microscopy (Carl Zeiss, LSM 780). The surface morphologies of the QDs were observed using a field emission scanning electron microscope (FE-SEM, Hitachi S-4800) operated with an acceleration voltage of 7 kV and a working distance of 4.5 mm. The topographic and surface potential signals were obtained with a potential voltage of 13 V (AC) and a trace height 30 nm in a non-contact mode using a KFM (Hitachi, AFM5000℃).


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Figure 1.

Surface wetting characteristics of various SAM-patterned substrates

depending on solution temperature (Tsol). Water contact angle on (a) HDFS, (b) PDMS, (c) DDTS, and (d) PS. Film coverage of PS on (e) HDFS, (f) PDMS, (g) DDTS, and (h) PS depending on Tsol and solution concentration. (i), (j), (k), and (l) Corresponding optical microscopy images for (e), (f), (g), and (h).


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Figure 2. Process flow of SAM-directed cold spin casting (S-CSC). (a) Schematic illustration of the S-CSC process vs. room-temperature spin-casting (RT-SC). (b) SEM image of the patterned SAM/Si template. (c), and (d) Grazing-incidence small-angle X-ray scattering (GISAXS) data of the SAM patterns for (c) aligned and (b) misaligned configurations between the long-axis direction of the SAM patterns and that of incident Xray.


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Figure 3. Directed self-assembly (DSA) of block copolymers (BCPs) using S-CSC. (a) PS-b-PDMS BCP thin films obtained with S-CSC. (b) Individual coverage on F-SAM and PS, respectively, (c) selectivity, (d) total coverage, (e) line width, (f) duty cycle, and (g) linewidth fluctuation of BCP thin films depending on Tsol. (h) and (i) Schematic illustration of BCP microdomains revealed by plasma oxidation. (h) S-CSC and (i) RT-SC. (j) SEM images of BCP microdomains and (k) Corresponding GISAXS images as a function of Tsol.


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Figure 4. Step-by-step characterization of surface topography and surface potential using Kelvin probe force microscopy (KFM). (a) , (b), (c), and (d) SAM patterns, (e), (f), (g), and (h) S-CSC of BCP thin film at -5℃, (i) , (j), (k), and (l), thermal annealing at 250℃ for 2 hours,

(m), (n), (o), and (p) O2 plasma etching. (a), (e), (i), (m) Schematic of each

step. (b), (f), (j), and (n) 3D representation of surface topography and (c), (g), (k), and (o) surface potential measured with KFM. (d), (h), (l), and (p) Corresponding line-scan profiles of height and potential.


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Figure 5. Application of S-CSC for the patterning of quantum dot thin films. (a) PL spectrum of red-emissive CdSe/ZnS QD solution. (b) PL spectra of CdSe/ZnS QD thin films formed on the bare glass or SAM-patterned glass (P-SAM glass) via S-CSC. Confocal microscopy images of (c) RT-SC and (d) -5℃ CSC QD thin films formed on glass substrate. Confocal and fluorescence (insert) microscopy images of (e) RT-SC and (f) -5℃ S-CSC QD thin films formed on SAM patterns. Line-scan intensity profile of PL spectra for (g) and (h). Surface topography and surface potential investigated with KFM. (i), (j), (k), and (l) Before and (m), (n), (o), and (p) after S-CSC of QD thin films on the patterned SAM with a periodicity of 9 um. (i) and (m) Illustration of each sample. (j), and (n) 3D representation of surface topography and (k), and (o) surface potential measured with KFM. (l), and (p) Corresponding line-scan profiles of height and potential.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website. Details of vapor pressure data of solvents depending on temperature, process flow of SAM patterning, definition of selectivity, SEM images, GISAXS data, pattern quality analysis of BCP thin films patterned with S-CSC, and illustration of effective migration time of solution depending on spin-casting temperature.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Author Contributions †J.H.L and H.-J.C contributed equally to this work. Present Address ⊥

University of Pennsylvania, Department of Electrical and Systems Engineering, 200 South

33rd street, Philadelphia, PA, USA, 19104

ACKNOWLEDGMENTS This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning. (NRF-2016M3D1A1900035) This research was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2013M3C1A3063046). This work was also 20 ACS Paragon Plus Environment

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supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT). (NRF-2017R1A2B2009948) The authors also acknowledge Pohang Accelerator Laboratory for grazing-incidence small-angle X-ray scattering (GISAXS) measurements.


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