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Recrystallized NaCl from thin film to nano-micro sized sacrificial crystal for metal nanostructures Dong Kyu Lee, Tae Soo Kim, Jae-Young Choi, and Hak Ki Yu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00748 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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Crystal Growth & Design

Recrystallized NaCl from thin film to nano-micro sized sacrificial crystal for metal nanostructures Dong Kyu Lee1,2, Tae Soo Kim1, Jae-Young Choi3,*, and Hak Ki Yu1,2,* 1

Department of Materials Science and Engineering, Ajou University, Suwon, 16499, Korea

2

Department of Energy Systems Research, Ajou University, Suwon, 16499, Korea

3

School of Advanced Materials Science & Engineering & School of Advanced Institute of

Nanotechnology (SAINT), Sungkyunkwan University, Suwon, 16419, Korea Keywords: NaCl thin film, water-soluble, metallic nanostructures, Si nanorods

Abstract: We developed a breakthrough method for preparing metallic nanostructures using self-assembled NaCl nano sized sacrificial crystal and applied it to i) transparent flexible electrodes and ii) as catalysts for Si nanorod fabrication. Au nanostructures fabricated using the NaCl sacrificial crystal on a polyethylene terephthalate substrate showed low resistance (40 Ω/sq), relatively high transmittance (73%), and excellent mechanical stability. Wet chemical etching of Si with Ag nanostructures formed on a Si (100) substrate using the NaCl sacrificial crystal produced uniform vertical Si microrods with a thickness of 1–2 µm and length of about 3–4 µm. These Si microrods showed high light absorption (over 90%) in the wavelength range 350–800 nm, and various applications in optoelectronic devices are expected in future.

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1. Introduction Recently, metal nanostructures that have a various applications such as in transparent flexible electrodes, solar cells, optoelectronic devices, and catalysts have attracted considerable interest.1– 5

Many studies have used photolithography to fabricate metal nanostructures because of this

method’s high resolution, process repeatability, and large area patterning.6–9 However, despite the sophisticated nature of lithography, it has many disadvantages such as toxicity and high production costs.10,11 To overcome these limitations, alternative processes such as block copolymer lithography (BCP), colloidal lithography, and nanopatterning using anodic aluminum oxide (AAO) have been developed.12–14 These alternative lithography techniques to overcome the high cost, low throughput could fabricate nanopattern arrays with a 10 nm scale and relatively simple process compared to conventional lithography. However, colloidal lithography and block copolymer lithography could not free from the environment and safety issues because of byproducts and toxicity.15,16 Also, when fabricating nanostructures using the AAO mask, it is difficult to fabricate a large-area AAO mask and to clean the mask after fabrication using metal deposition.17,18 Moreover, these alternative lithography techniques could not free from material selection problems because only certain materials could be applied. Accordingly, there is a growing need for a nanostructures fabrication technique that is simple, consumes less material, and can be used universally for different substrates. Herein, we suggest a new method for the fabrication of metal nanostructures using sodium chloride (NaCl) as a water-soluble sacrificial crystal. After growing NaCl thin films on the desired substrate, they are placed in a moderate humidity atmosphere, resulting in the dissolution and recrystallization of NaCl (nano-/microsized cubic crystals).19,20 Due to the excellent water solubility of NaCl (360 g NaCl/1 kg water solvent),21,22 we can achieve a nanosized metal


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nanostructures very quickly and simply and apply it for the production of transparent flexible electrodes and Si nanorods (using Ag-metal-assisted chemical etching), as shown in Figure 1.

Figure 1. Schematic of fabrication of metal nanostructures for transparent flexible electrodes and metal-assisted chemical etching using NaCl sacrificial crystal. The method using self-assembled NaCl nano sized sacrificial crystal for the metal nanostructures has several advantages. Firstly, this method obtains a large area very easily and quickly compared to existing methods such as colloidal lithography, BCP lithography, and AAO masking. Lithography technique has various process steps such as exposure, baking, and cleaning after photoresist coating. Secondly, since a high-temperature process is not necessary


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and all processes are performed at room temperature, it can be easily applied to different substrates, such as polymers.23 Thirdly, it is possible to make various size of NaCl particles from 10 nm to several micro-meter by controlling the thickness, humidity, temperature, and substrate type. Finally, it is eco-friendly because it only requires salt and water and does not emit any toxic substances. In conventional lithography, various organic solvents such as acetone, benzene, and etc. are used to remove photoresist. Inhalation of such organic solvent vapors could cause various diseases and those solvents are very flammable. However, when NaCl is used as sacrificial crystal, there is no safety problem because water is solvent. 2. Experimental Section 2.1. NaCl film growth and recrystallization. NaCl thin films were deposited on polyethylene terephthalate (PET) and Si (100) substrates by thermal evaporation. The NaCl source material (99.5% purity; Sigma-Aldrich; product number S7653) was heated on a tungsten boat at 0.15 V and 9.1 A. The PET and Si (100) substrates were cleaned using acetone, ethyl alcohol, and deionized (DI) water. Additionally, the native oxide on the Si (100) substrate was removed by hydrofluoric acid. The chamber pressure was 8 × 10−6 Torr during deposition, and the substrates were maintained at room temperature. The deposition rate was 0.3 nm/s, and the film thicknesses were 10, 30, 50, and 120 nm. After NaCl thin film deposition, the humidity conditions (~80%, 10 s) were adjusted to dissolve the NaCl film. The dissolved NaCl particles were exposed to ~40% humidity for 7 days for recrystallization. 2.2. Fabrication of transparent flexible electrodes. After recrystallization of the NaCl film (thickness: 120 nm), Au film was grown on the recrystallized NaCl/PET substrate by electronbeam evaporation using high-purity Au pellets. The Au source material was evaporated at 7 kV


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and 86 mA. The chamber pressure was 5 × 10−6 Torr during the deposition, and the substrates were maintained at room temperature. The deposition rate was 0.05 nm/s, and the film thickness was 15 nm. After the deposition of the Au film, the sample was sonicated in DI water to dissolve the recrystallized NaCl particles and remove the Au film formed on the NaCl particles. 2.3. Metal-assisted chemical etching of Si substrate. After the recrystallization of the NaCl film (thickness: 120 nm), a Ag film was grown on the recrystallized NaCl/Si (100) substrate by electron-beam evaporation using Ag pellets. The Ag source was evaporated at 7 kV and 28 mA. The chamber pressure was 5 × 10−6 Torr during the deposition, and the substrates were maintained at room temperature. The deposition rate was 0.1 nm/s, and film thickness was 25 nm. After deposition of the Ag film, the sample was sonicated in DI water to dissolve the recrystallized NaCl particles and remove the Ag film formed on the NaCl particles. After the DI sonication process, the sample was dipped in an etching solution (H2O: 80 ml, HF: 20 ml, H2O2: 1 ml) for anisotropic etching of the Si substrate in the (100) direction. The etching time was 30 min. 2.4. Analysis. Optical microscopy (OM) images were recorded on a U-MSSP49 (Olympus, Tokyo, Japan) microscope. Scanning electron microscopy (SEM) images were obtained using a JSM-6700 (JEOL, Tokyo, Japan) device at 5.0 kV. The transmittance was measured using a UV– visible fiber-optic spectrometer (AvaSpec-ULS2048) with a light source (AvaLight-DH-S-BAL). The reflectance was measured using an ultraviolet–visible–near-infrared spectrophotometer (Cary 4000, Agilent). 3. Results and Discussion


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Figure 2a shows the OM images of each step of the NaCl samples of different thicknesses: deposited on the Si (100) substrate (as-dep stage), exposed to 80% humidity for 10 s (dissolving stage), and maintained at 40% humidity for 7 days (recrystallization stage). By adjusting the appropriate humidity, temperature and vacuum condition, the recrystallization time could be reduced to several seconds. It could be observed that in the recrystallization stage, the density of the NaCl particles increased with increasing thickness of the NaCl thin film. The thickness of NaCl particles (length: 1 um) is about 700 nm. The smaller the size in top view, the smaller the thickness of NaCl particles. The surface roughness in the as-dep stage was higher because the NaCl film reacted very quickly with moisture outside the chamber. With sufficient growth time, the larger NaCl particles could be obtained through Ostwald ripening.24 Without sufficient growth time, the NaCl particles are connected to each other (see Supporting Information, Fig. S1). The OM images for the recrystallized NaCl particles on different substrates is shown in Figure 2b. Due to the small lattice mismatch between NaCl (0.564 nm) and Si (0.543 nm), the NaCl film was recrystallized in the (100) orientation on the Si (100) substrate, resulting in rectangular particles. On the other hand, the NaCl film showed a (111) preferred orientation on the c-sapphire (0006) substrate because of 2/3 domain matching epitaxy. Therefore, the recrystallized NaCl particles on the c-sapphire substrate exhibited a triangular crystal morphology for the (111) facet of the rock-salt lattice.21 To understand the recrystallization of the NaCl nanocrystals in terms of surface energy, the recrystallization tendency was compared by depositing NaCl on a mechanically scratched Si substrate (based on the dashed line, the upper part was bare Si and the lower part was scratched Si). In the presence of the mechanical scratches, the number of recrystallized NaCl particles increased, and they aligned with the


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scratching direction. Thus, the size, spacing, and shape of the recrystallized NaCl particles could be varied by changing the substrate.


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Figure 2. (a) OM images of each step of NaCl samples of different thicknesses on Si (100) substrate and (b) OM images of recrystallized NaCl particles on different substrates (film thickness of NaCl is 10 nm). Prior to forming metal nanostructures using the recrystallized NaCl particles, Au or Ag noble metal (50 nm) was deposited on the 400-nm-thick NaCl thin film (without recrystallization) to confirm the interface characteristics between NaCl and the specific noble metal; the changes in morphology of metal surface over time were observed by SEM (shown in Figure 3a). The Au films showed very few cracks and holes even after 3 weeks, showing a stable surface morphology. On the other hand, the Ag films showed many nanosized cracks with increasing time, forming islands after 3 weeks. Intercalated water molecules (from the atmosphere) could dissolve the NaCl molecules at the interfaces and form ionic states of Na+ and Cl−, promoting AgCl formation (the change in Gibbs formation energy at 300 K and 1 atm was −155.75 kJ/mol for AgCl and −62.02 kJ/mol for AuCl). The Ag atoms could migrate due to the driving force of this interfacial reaction, resulting in the formation of gaps in the film.25,26 Therefore, when fabricating metal nanostructures using Ag, it is necessary to minimize the exposure time to the atmosphere. The SEM image of Figure 3b shows that the metallic nanostructures were produced by dissolving the NaCl crystals in DI water after depositing the 50-nm-thick Au film, which was relatively stable on the recrystallized NaCl particles (120-nm-thick NaCl film as the starting material).


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Figure 3. (a) SEM images of morphology of metal surface on NaCl thin film over time and (b) SEM images of metallic nanostructures fabricated using DI water sonication for dissolving NaCl particles.


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A schematic of the metallic nanostructures fabrication process on a flexible substrate for transparent flexible electrodes is shown in Figure 4a. A 15-nm-thick Au film was deposited on the recrystallized NaCl particle/PET substrate (see Supporting Information, Fig. S2). After the DI sonication process, the NaCl particles dissolved in water, and the Au film on top of them was removed from the substrate (OM images of NaCl recrystallized particles and the process of metallic nanostructures fabrication are shown in Figure 4b). The resulting Au metal nanostructures showed a higher transparency than the Au metal film due to the pores formed by the NaCl particles, as shown in Figure 4c. The 15 nm thick Au film has a 55% reduction in transmittance compared to the bare-PET substrate. However, it could be seen that the Au film with nanostructures increased the light transmittance from 55% to 73% compared with the Au film. This is insufficient transmittance in comparison with graphene (over 90 %) and ITO (over 80 %). However, if larger and more holes could be regularly ordered, the transmittance could be improved.27,28 Figure 4d shows the OM and SEM images of the Au nanostructures before and after the bending test (curvature: 5 cm−1, number of cycles: 10,000). The size, shape, and density of the pores were retained after the bending test, and no cracks or defects were observed in the metallic nanostructures. A comparison of the mechanical flexibility of the Ag and Au nanostructures obtained using the NaCl sacrificial crystal is shown in Figure 4e and 4f. The resistance of the Ag and Au metallic nanostructures was almost constant in the curvature range 0.1–10 cm−1, as shown in Figure 4e. Also, their resistance remained almost constant over 10,000 bending cycles at a constant severe bending curvature of about 5 cm−1, as shown in Figure 4f. The resistance of Ag nanostructures increased by 16 times and Au nanostructures increased by 4 times compared to bulk state of each metal.29,30 There are three reasons for the increase in resistance when making a metal film into nanostructures. i) Thickness: the thin metal film (15


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nm) has larger resistance compared to the bulk. ii) Reactivity with water: when the highly reactive metal nanostructures are made by dissolving NaCl particles, the higher resistance is obtained by metal agglomeration. iii) Sonication: in the case of metal with low adhesion to the substrate, the metal around the NaCl sacrificial crystal could also lift off when the metal deposited on the NaCl is removed by sonication. Therefore, the metal nanostructures made of Ag film have much higher resistance because of its high reactivity with water and low adhesion with substrate. As a result, the resistance ratio (R/R0, where R is the resistance with time and R0 is the initial resistance) of the Ag nanostructures abruptly increased after 2 h under 80% humidity conditions, whereas that of the Au nanostructures was stable even after 100 h, as shown in Figure 4g. The Ag nanostructures retained their bending stress flexibility but reacted with moisture and easily agglomerated into particles. The SEM images (inset of Figure 4g) show that the Ag nanostructures agglomerated into nanoparticles after 24 h.


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Figure 4. (a) Schematic of fabrication of transparent flexible electrodes using NaCl sacrificial crystal; (b) OM images of NaCl recrystallization and fabrication of Au nanostructures; (c) transmittance of Au metallic nanostructures and Au metal film; (d) comparison of SEM images of Au metallic nanostructures surface before and after bending test and comparison of stability of Au and Ag metal nanostructures; (e) change in resistance with respect to curvature; (f) change in resistance with respect to number of cycles; and (g) resistance ratio at 80% humidity and 25°C with respect to time. Figure 5a is a schematic showing the fabrication of Si nanorods by metal-assisted chemical etching using Ag nanostructures. A 25-nm-thick Ag film was deposited on the recrystallized NaCl particle/Si (100) substrate. After the DI sonication process, the Ag metallic nanostructures remained on the Si (100) substrate. The Si substrate was then etched using an etching solution (H2O: 80 ml, HF: 20 ml, H2O2: 1 ml) for 30 min.31,32 This etching solution caused anisotropic


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wet etching of the Si substrate in the (100) direction to form vertical Si rods.33,34 This method has the advantage of fabricating embossed Si nanorods as opposed to negatively shaped Si nanopores, which are formed when a Ag thin film is deposited and thermally processed to form nanoparticles and then etched (see OM and SEM images in Figure 5b and 5c). The thickness of the Si microrods was 1–2 µm and their length was about 3–4 µm, with the same shape as the rectangular NaCl particles. Since the Si substrate is etched along the shape of the NaCl particles, merged Si microrods could be obtained if the NaCl particles are merged through rapid evaporation. Deposition of 10 – 50 nm thin NaCl film (making smaller NaCl particles after recrystallization) could fabricate smaller size Si nanorods. Therefore, the size and density of the Si rods could be controlled by varying the humidity conditions and experiment duration and the shape of the Si rods could be controlled by changing the shape of the NaCl particles through regulation of the recrystallization mechanism. Figure 5d shows the light absorption spectra of the Si microrods and bare Si (100) substrate in a wavelength range of 350–800 nm. Assuming that light of all wavelengths was absorbed or reflected without transmission, absorption was calculated as (Absorption = 100 − Reflectance).35 The absorption of bare Si (100) at a wavelength of about 360 nm was the lowest because the reflectivity peak of Si could be observed at 280 nm and 370 nm because of the real and imaginary refractive index of Si.36 Therefore, the bare Si (100) substrate had a lowest absorption of about 30% at wavelength 360 nm, and the absorption increased with increasing wavelength. On the other hand, the etched Si (100) substrate (Si nanorods) showed a high absorption of over 90% at all wavelengths. This was because of the multiple scattering and trapping of incident light between the Si microrods.37,38 If the size and density of the recrystallized NaCl particles were controlled, the diameter and spacing of the Si rods could be tuned to control light absorption.


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Figure 5. (a) Schematic of Si nanorod fabrication using NaCl sacrificial crystal; (b) OM images of fabrication of Ag nanostructures and Si nanorods; (c) top-view (left), cross-sectional (middle), and 45° tilted (right) SEM images of Si microrods; and (d) light absorption spectra of Si microrods and bare Si (100) substrate. 4. Conclusion We studied self-assembled NaCl nano sized sacrificial crystal for fabricating metallic nanostructures and applied it to i) transparent flexible electrodes and ii) a catalyst for Si nanorod fabrication. Transparent flexible electrodes based on Au nanostructures fabricated using NaCl as sacrificial crystal on a PET substrate exhibited low resistance (40 Ω/sq) and relatively high transmittance (73%) during DI sonication. Also, they maintained almost constant resistance under severe bending and different humidity conditions without cracks or defects on the surface. Wet chemical etching of Si based on Ag nanostructures fabricated using recrystallized NaCl on


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the Si (100) substrate led to the formation of vertical Si microrods. The thickness of the Si microrods was 1–2 µm and their length was about 3–4 µm. These rods showed high absorption (over 90%) in a wavelength range of 350–800 nm. In this study, we fabricated metal nanostructures using recrystallized NaCl and applied them to various fields. The results of this study can be widely applied to various ceramic and polymer nanostructures beyond metals and are expected to be applicable to various fields in the future.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images of recrystallized NaCl particles on Si (100) substrate. Transmittance and resistance of Au on PET substrate as film thickness and OM images of recrystallized NaCl particles on PET and Si (100) substrate.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected] Author Contributions H. K. Y. designed the experiments, and D. K. L. and T. S. K. performed the experiments and analyzed the data. D. K. L., H. K. Y., and J. -Y. C. discussed the results and commented on the


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manuscript. H. K. Y. conceived and supervised this study and provided intellectual and technical guidance. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1C1B1009030). ABBREVIATIONS AAO, anodic aluminum oxide; UV, ultraviolet; PET, polyethylene terephthalate; DI, deionized; OM, optical microscopy; SEM, scanning electron microscopy REFERENCES (1) Brongersma, M. L.; Cui, Y.; Fan, S. Light Management for Photovoltaics using High-index Nanostructures. Nat. Mater. 2014, 13, 451-460. (2) Kim, B. H.; Onses, M. S.; Lim, J. B.; Nam, S.; Oh, N.; Kim, H.; Lee, C. H. High-resolution Patterns of Quantum dots formed by Electrohydrodynamic Jet Printing for Light-emitting Diodes. Nano Lett. 2015, 15, 969-973. (3) Cheng, T.; Zhang, Y.; Lai, W. Y.; Huang, W. Stretchable Thin-film Electrodes for Flexible Electronics with High Deformability and Stretchability. Adv. Mater. 2015, 27, 3349-3376. (4) Khan, A.; Lee, S.; Jang, T.; Xiong, Z.; Zhang, C.; Tang, J.; Li, W. D. High-performance Flexible Transparent Electrode with an Embedded Metal Mesh Fabricated by Cost-effective Solution Process. Small 2016, 12, 3021.


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“For Table of Contents Use Only” Recrystallized NaCl from thin film to nano-micro sized sacrificial crystal for metal nanostructures Dong Kyu Lee1,2, Tae Soo Kim1, Jae-Young Choi3,*, and Hak Ki Yu1,2,* 1

Department of Materials Science and Engineering, Ajou University, Suwon, 16499, Korea

2

Department of Energy Systems Research, Ajou University, Suwon, 16499, Korea

3

School of Advanced Materials Science & Engineering & School of Advanced Institute of

Nanotechnology (SAINT), Sungkyunkwan University, Suwon, 16419, Korea

Synopsis: Lithography-free method of making metal nanostructures using self-assembled NaCl nano sized sacrificial crystal.


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Recrystallized NaCl from thin film to nano-micro ... - ACS Publications

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