Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Silicon Oxycarbide Accelerated Chemical Vapor Deposition of Graphitic Networks on Ceramic Substrates for Thermal Management Enhancement Paul D. Garman,† Jared M. Johnson,‡ Vishank Talesara,§ Hao Yang,§ Dan Zhang,∥ Jose Castro,∥ Wu Lu,§ Jinwoo Hwang,‡ and L. James Lee*,† †
Department of Chemical and Biomolecular Engineering, ‡Department of Materials Science and Engineering, §Department of Electrical and Computer Engineering, and ∥Department of Integrated Systems Engineering, The Ohio State University, Columbus, Ohio 43210, United States
ACS Appl. Nano Mater. Downloaded from pubs.acs.org by 94.158.22.172 on 01/19/19. For personal use only.
S Supporting Information *
ABSTRACT: Ceramic materials such as aluminum oxide (Al2O3) and aluminum nitride (AlN) are commonly implemented as heat sinks for a variety of applications. However, the thermal conductivity of these ceramics is too low to act as effective thermal management materials in power electronics applications. With high lateral thermal conductivity, graphitic films would be ideal materials to enhance the thermal management abilities of ceramics. Current direct chemical vapor deposition (CVD) methods can only grow several layers of graphene on ceramic substrates with poor adhesion to the substrate. We demonstrate a simple atmospheric pressure chemical vapor deposition (APCVD) method to deposit micrometerscale graphitic networks directly on the surface of ceramics making use of dual silicon oxycarbide (SiOC) sources. The graphitic networks have good adhesion to aluminum oxide and can provide sufficient thermal mass combined with superior hear transfer properties for effective thermal management enhancement of ceramic substrates. KEYWORDS: chemical vapor deposition, graphitic networks, silicon oxycarbide, thermal management, electronic ceramics
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INTRODUCTION Ceramics are used for heat dissipation and thermal management applications which require materials with electric insulation, good thermal conductivity, high thermal stability, and strong mechanical properties. Examples include heat shields for automotive and aerospace applications as well as substrates and heat sinks for LEDs, computer processors, and power electronic devices. Compared to metals, ceramics display relatively poor heat transfer performance due to their low thermal conductivity. Because of their high planar and low transverse thermal conductivities (>2000 and 500 °C in air) and good mechanical strength, graphene/graphitebased materials would be ideal to be incorporated with ceramics to enhance their heat transfer performance.1−6 Graphene has been either integrated into the electronic device architecture or used as graphene/ceramic composites for heat sinks.7−10 Chemical vapor deposition (CVD) has proven to be an effective method for deposition of large-area graphene films.12−14 It has demonstrated the ability to deposit on a variety of substrates including those with curved and complex structures.10,16 This makes such a process ideal for ceramic heat sinks which have a three-dimensional large surface area. However, current “direct” graphene CVD processes can only grow nanoscale graphene films on a ceramic surface after several hours of growth time with poor adhesion to the substrate.10,16−18 Although these thin films have demonstrated © XXXX American Chemical Society
high surface thermal conductivity, a larger thermal mass (micrometer-scale films) and good bonding to the substrate are essential for successfully applying CVD graphene films to demanding thermal management applications such as wide band gap (WBG) semiconductor devices.20 In this paper, we describe an atmospheric pressure CVD (APCVD) method for synthesizing high-quality, micrometerscale thickness graphitic networks directly on porous ceramic substrates. The addition of a high-temperature silicone rubber and continuous feeding of tetraethyl orthosilicate (TEOS) to the typical reaction mixture used for graphene CVD allow very fast growth of thick and ordered graphite layers on ceramics with good intergraphene layer bonding and strong adhesion of the graphite layers on the substrate. This is due to the decomposition of silicone rubber and TEOS yielding silicon oxycarbide (SiOC) species which can accelerate graphene growth and cause intergraphene bonding.21,22 When grown on a low-cost aluminum oxide (Al2O3) direct-bonded copper (DBC) substrate, the coating was able to outperform a more expensive aluminum nitride (AlN) DBC substrate in terms of resistor lifetime.23 Here, we present the formation mechanism and characterization of this graphitic network on an aluminum oxide surface. Received: November 5, 2018 Accepted: January 2, 2019 Published: January 2, 2019 A
DOI: 10.1021/acsanm.8b01998 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Structural and Elemental Characterization Methods. Surface roughness and profiles of ceramic substrates were generated using a Veeko/WYCO optical profilometer. The coating thickness was measured by creating a scratch in the coating without damaging the substrate and imaging the step height using a Veeko/WYCO optical profilometer. A Renishaw inVia Raman microscope with an excitation wavelength of 514.5 nm was used to generate Raman spectra of graphitic networks. SEM images were taken using a FEI Nova NanoSEM 400 scanning electron microscope with low- and highresolution modes. All transmission electron microscopy (TEM) measurements were collected with a FEI Titan 60-300 image corrected S/TEM operated in scanning mode at 60 keV. Samples were prepared using the focused ion beam (FIB) method where cross sections are extracted using a gallium ion beam and milled until uniformly thin and electron transparent. From each sample, electron energy loss spectroscopy (EELS) spectra were collected using a Gatan Quantum spectrometer. For each scanning region, low and core loss spectra were obtained, and a Fourier-ratio deconvolution was applied. The resulting energy resolution was ∼0.3 eV. To calculate the sp2 fraction of graphitic networks using the deconvoluted EELS core loss spectrum, a ratio of the area under the curve within a 1 eV window centered at the π* peak and the area under the curve within a 20 eV window beginning at the onset of the π* peak was calculated. Normalization of this ratio with the ratio obtained from the EELS core loss spectrum of a reference material representing 100% sp2 carbon (HOPG) yielded the sp2 content for a given scan location.24 Annular dark field (ADF) images and EDS maps were acquired using a converged probe. Nanodiffraction patterns were obtained by scanning a converged probe of size ∼1−2 nm across the graphitic network. Each set of nanodiffraction patterns contains 100 patterns scanned 10 × 10 across and down the thickness of the graphitic networks. Energy-dispersive X-ray spectroscopy (EDS) maps were also obtained using a converged probe.
EXPERIMENTAL SECTION
SiOC-Accelerated CVD of Graphitic Networks on Ceramics. A ceramic substrate was placed in the center of a 5.08 cm o.d. quartz tube furnace, with 0.3 g of high-temperature silicone rubber being placed in a ceramic boat at the edge of the inlet side of the heating zone. Initially, the substrate was heated under argon flow at a rate of 10 °C min−1 where the argon flow rate was 50 standard cubic centimeters per minute (sccm). Once the temperature reached 400 °C, the argon flow rate was reduced to 15 sccm to build up SiOC species inside the furnace as well as to allow for the deposition of pyrolyzed silicone rubber on the surface of the substrate. At the desired growth temperature (1000−1100 °C), the argon flow rate was split into two streams. One stream of argon at a rate of 20 sccm was fed directly into the furnace, while the other bubbled through the TEOS at 30 sccm to a total argon flow of 50 sccm. The bubbling stream carried TEOS into the furnace at an approximate rate of 50 μg h−1. Argon streams were combined before entering the furnace. Methane flow was also turned on at a rate of 15 sccm. The furnace was held at 1000−1100 °C under these flow conditions for a desired growth time, after which both the methane and argon stream through TEOS were turned off, and the furnace was allowed to cool to room temperature under 50 sccm argon flow before the coated substrate was removed. Heat Transfer Experimental Setups. To demonstrate the heat transfer performance of ceramics coated with graphitic networks, 1.27 cm × 7.62 cm × 600 μm substrates of Al2O3 or AlN were coated with ∼20 μm graphitic networks using the following growth conditions: 15 sccm methane, 50 sccm argon (30 sccm through TEOS), 0.3 g of silicone rubber, 1100 °C for 2 h of growth time. Next, a 1.27 cm × 2.54 cm area located at one end of the substrate was attached to a heat source held at 180 °C by double-sided acrylic foam thermal tape. All other surfaces of the substrate were exposed to air. The temperature profile was monitored using an IR camera (FLIR). Prior to each experiment, the camera was calibrated to an experimentally determined emissivity of each coated sample surface to provide an accurate temperature profile. This was done by bringing each sample to a known temperature and then adjusting the emissivity setting in the camera until the camera temperature matched the actual temperature. Emissivity for the coated AlN substrate was found to be 0.47, while the emissivity for coated Al2O3 was 0.35. For the resistor failure experiments, a 2.54 cm × 7.62 cm × 600 μm DBC substrate with 200 μm thick copper bonded to both sides was cut using a water jet. The top surface copper was completely removed except for a 1.6 cm × 2.1 cm area at one end of the substrate which acted as a conductive surface for easy resistor attachment. This was achieved by immersing the sample in a bath of copper etchant solution (42% FeCl3, 1.1% HCl) held at 55 °C for 45 min. The backside copper and region for resistor attachment were protected from etching by the application of Kapton tape. Al2O3 substrates prepared by this method then underwent the SiOC-accelerated CVD process with the following growth conditions: 15 sccm methane, 50 sccm argon (30 sccm through TEOS), 0.3 g of silicone rubber, 1000 °C for 2 h of growth time. Graphitic networks grown on the copper regions of the substrate were removed prior to resistor attachment by sonication. A thick film resistor (Ohmite TEH100M10R0FE) was attached to the copper region on the end of the substrate using silver epoxy (Dupont 5504N) by curing at 150 °C for 3.5 h. A dc power supply was connected to the resistor, and the substrate was suspended in air. Thermocouples were attached to the front edge of the substrate opposite the resistor as well as directly under the resistor on the backside of the substrate using copper tape to monitor the temperature profile with a series data acquisition system from Omega. Once the setup was complete, the power supply was turned on, and the output was set to 21.5 W (15 V, 1.43 A). Once the resistor failed, the current would automatically return to 0 A, and the system would cool to room temperature. The point of failure was detected when temperature began to sharply drop to room temperature at both thermocouple locations. This was repeated with three resistors on the same substrate to check repeatability.
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RESULTS AND DISCUSSION The process setup is illustrated by Figure 1A. To grow 3D graphitic networks on ceramics, the silicone rubber provided an initial concentration spike of SiOC species in the furnace atmosphere to form a SiOC transition layer on the substrate surface. Upon reaching the growth temperature (1000−1100 °C), methane flow (15 sccm) was introduced to the tube furnace as a carbon source for graphene formation. Concurrently, argon flow was directed through a bubbler carrying a low concentration (∼50 μg h−1) of tetraethyl orthosilicate (TEOS) into the furnace. At high temperatures, the methane and TEOS decomposed into reactive carbon and SiOC species, respectively, which self-assembled into graphitic networks on the transition layer on the surface of the substrate (Figure 1B). The continuous feed of methane and TEOS resulted in an increased rate of the deposition of graphene layers (5−10 μm h−1) until it reached a threshold thickness where the graphitic network started to delaminate from the substrate to form flakes. Both Al2O3 and AlN substrates were implemented in this study for graphitic network growth. These ceramics, along with beryllium oxide (BeO), are commonly used as substrates for power electronic devices. However, BeO is carcinogenic and not considered here. The surface profile of each ceramic substrate was measured using optical profilometry and can be found in Figure S1. A substrate of each type was used to grow graphitic networks under the same conditions (0.3 g of silicone rubber, 30 sccm Ar through TEOS, 1100 °C for 2 h). These conditions resulted in ∼20 μm thick graphitic networks on the ceramic surface. Images of each substrate before and after graphitic network growth are shown in Figures 1C−F. The coating on Al2O3 revealed good adhesion to the substrate, B
DOI: 10.1021/acsanm.8b01998 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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adhesion coupled with the high heat transfer properties of the micrometer-scale graphitic networks (Figure 1G). On the other hand, upon removal from the furnace, coating on AlN revealed delamination from the substrate (Figures 1E,F). Because of poor coating adhesion, the heat transfer performance of the coated AlN substrate (Figure 1H) was inferior to that of the coated Al2O3 substrate, despite the higher thermal conductivity of AlN (∼150 W m−1 K−1) compared to that of Al2O3 (∼32 W m−1 K−1). Here, the initial temperature rise in the vicinity of the heat source of the coated AlN substrate was much slower compared to that of the coated Al2O3 substrate. After steady state was reached at 2 min, the coated AlN sample showed an overall lower saturated surface temperature profile compared to the coated Al2O3 sample. This again demonstrates the importance of coating adhesion to heat release. We believe a SiOC transition layer, formed during the pyrolysis of the SiOC source during heating, was less compatible with the AlN substrate because it contained no oxygen. Because adhesion between graphitic networks and AlN was poor using the current SiOC-accelerated CVD process setup, only graphitic networks grown on Al2O3 were investigated further using a variety of characterization techniques. The Raman spectra of Al2O3 before and after growth of thin (5 nm in 1 min) graphitic networks are shown in Figure 2A, D, G, and 2D bands located at 1353, 1588, and 2700 cm−1, respectively, were indicative of a graphitic structure, and could be clearly observed after a short growth time (1 min).25 Because of the transparent nature of the thin graphitic networks, the effect of the Al2O3 substrate could be observed in the Raman spectrum as the rise in signal intensity at high wavenumbers. As the growth time was increased to produce thick (8−10 μm) graphitic networks, we observed broadening of each peak as well as a high G:2D peak ratio, which could be attributed to the thickness of the graphitic networks.26−28 At this thickness, the coating was no longer transparent, so there was no rise in signal intensity at high wavenumbers. Scanning electron microscopy (SEM) was performed on a cross section of graphitic networks on Al2O3 (Figure 2B). The results show a well-defined layered structure for graphitic networks on
Figure 1. SiOC-accelerated graphene CVD on ceramics. (A) Process setup for graphitic network synthesis. (B) Schematic of graphitic networks with SiOC transition layer on ceramic substrate. Al2O3 substrate before (C) and after (D) SiOC-accelerated CVD of graphitic networks (15 sccm methane, 0.3 g of silicone rubber, 30 sccm Ar through TEOS, 50 sccm Ar total, 1100 °C for 2 h). AlN substrate before (E) and after (F) SiOC-accelerated CVD of graphitic networks using identical growth conditions. Temperature distribution during heating of coated Al2O3 (G) and AlN (H).
similar to that observed on nonceramic substrates in previous studies (Figures 1C,D).22 As a result, when using a simple heat transfer setup (Figure S2), there was a sharp temperature rise in the vicinity of the heat source due to the coating/substrate
Figure 2. Characterization of SiOC-accelerated graphitic networks on Al2O3. (A) Raman spectra of bare substrate (red), 1 min (blue) and 3 h (green) growth time (15 sccm methane, 15 sccm Ar through TEOS, 50 sccm Ar total, 1000 °C). (B) SEM images of graphitic networks on Al2O3 (15 sccm methane, 30 sccm Ar through TEOS, 50 sccm Ar total, 1000 °C for 1 h). EDS analysis of graphitic network/DBC Al2O3 interface. (C) TEM image. (D) Carbon, (E) silicon, (F) oxygen, (G) aluminum, and (H) copper EDS maps of the TEM image. C
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Figure 3. TEM characterization of SiOC-accelerated graphitic networks on Al2O3. (A) TEM cross section of graphitic networks on DBC Al2O3 with labeled EELS measurement locations (15 sccm methane, 30 sccm Ar through TEOS, 50 sccm Ar total, 1000 °C for 40 min). (B) EELS analysis of graphitic networks. (C) Cross section of amorphous region within graphitic networks on Al2O3. (D) EELS analysis of amorphous region. (E) TEM image of grain orientation impingement with nanodiffraction patterns at either side of interface. Arrows indicate growth direction from localized substrate surface. (F) SAED average pattern of blue square region from (E). (G) Schematic of grain boundary formation at low orientation mismatch. (H) Schematic of amorphous carbon formation at severe orientation mismatch.
ceramics. Because of the high surface roughness of the substrate (Ra ∼ 500 nm), graphene layers conformed to the porous surface and, although continuous, did not lie perfectly flat. In contrast, graphitic networks grown on smoother substrates such as quartz (Ra ∼ 10 nm) resulted in wellordered flat layers under SEM (Figure S3). Clearly, substrate surface morphology had a profound effect on the structure of SiOC-accelerated graphitic networks. A cross section of a coating grown on an Al2O3 substrate using 0.3 g of silicone rubber and 30 sccm of Ar through TEOS for 40 min at 1000 °C was prepared for transmission electron microscopy (TEM) using the method described in the Experimental Section. Here, a direct-bonded copper (DBC) Al2O3 substrate was used for graphitic network growth as it is commonly used in thermal management applications in the power electronics industry.29 The growth temperature when using DBC substrates was decreased to 1000 °C to prevent copper melting. This resulted in a slower growth rate (∼5 μm h−1) compared to growth at 1100 °C (∼10 μm h−1). However, both of these rates are significantly faster than other “direct” CVD methods reported in the literature. Copper was removed from the Al2O3 surface prior to the CVD process by the method described in the Experimental Section. The interface between the graphitic networks and Al2O3 was studied using energy dispersive X-ray spectroscopy (EDS). Elemental mapping for each present species is shown in Figures 2C−H. A SiOC layer, formed from the pyrolysis of silicone rubber during furnace heating, was observed between the graphitic networks and the porous ceramic surface (Figure 2E), which contained a number of voids. The origin of the voids was from the copper bonding procedure and did not affect coating adhesion.30 No silicon or oxygen was found within the graphitic networks down to the detection limit of EDS (∼1 at. %); therefore, the graphitic networks are primarily a carbon construct. To further investigate the effect of substrate surface morphology on the structure of graphitic networks, Electron energy loss spectroscopy (EELS) spectra were generated at six different locations in a graphitic network cross section (Figures
3A,B). This allowed for direct observation of the carbon structure and its distribution. Spectra generated near the surface of the film, in the intermediate regions of the film, and at the film/substrate interface all had sharp σ* and π* peaks located at 284 and 292 eV, respectively, which is a characteristic of layered, highly ordered sp2 structures.24,31,32 Comparison of the peak integral ratio with that of a 100% sp2 structure allowed for the estimate of the sp2 content of carbon materials. Using highly oriented pyrolytic graphite (HOPG) as a reference material, the sp2 content of graphitic networks on Al2O3 was found to vary between 80.8% and 100% (Table S1). This was lower than that found for graphitic networks grown on quartz using similar growth conditions. After further analysis, we discovered locations in the graphitic networks which were amorphous in structure (Figures 3C,D). This was indicated by the broadening of the σ* peak.33 The region was ∼400 nm in diameter with well-defined borders. Selected area electron diffraction (SAED) was also performed to investigate graphene layer stacking (Figures 3E,F). Nanodiffraction revealed graphene stacking parallel to the local substrate surface. Orientations of the nanodiffraction patterns rotated when different locations within the graphitic network on ceramic were scanned (Figure 3E). To the left of the labeled interface, nanodiffraction yielded patterns where the spots were rotated slightly clockwise from the vertical axis. Conversely, patterns recorded at locations right of the interface were rotated slightly counterclockwise from the vertical axis. Averaging the nanodiffraction patterns gathered from a 100 nm2 region spanning the interface (Figure 3F) showed two separate orientations of (001) spots, which were a result of graphene layer stacking.34 The individual nanodiffraction patterns could be viewed in succession to show localized stacking orientation change across the interface in Movie S1. We believe the relatively high sp3 content, amorphous carbon formation, and rotation of the stacking orientation within the graphitic networks were a consequence of the high surface roughness of the Al2O3 substrate. Graphene growth initiated from nucleation sites on the substrate surface formed graphene D
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Figure 4. Thermal management of resistor on DBC substrates. (A) Resistor failure experimental setup. Backside hot spot temperature profiles of (B) coated Al2O3 and (C) bare AlN DBC substrates.
“islands” oriented according to the substrate surface texture. On smooth quartz, nucleation sites had nearly identical orientations; therefore, layers appeared to be continuous and lie parallel to the surface.35−37 Because the ceramic surface is porous with relatively high surface roughness, the orientation angles of graphene “islands” were large with respect to each other that regions of different orientations could be detected by nanodiffraction. When two regions of different orientations impinged after sufficient graphitic network growth, they might form a continuous graphene layer, even though vastly different orientations along the layer could be observed by SEM. This is illustrated by the schematic in Figure 3G. When the angle between the regions was sufficiently large, the impinged graphitic networks might not form continuous graphene. Instead, a mixture of sp2 and sp3 (amorphous) carbon was formed (Figure 3H). This would result in a higher concentration of sp3 and amorphous carbon within graphitic networks grown on substrates with relatively higher surface roughness. To demonstrate the enhanced thermal management performance of ceramics provided by graphitic networks, a simple Joule heating setup implementing a thick film resistor attached to a DBC substrate was used (Figure 4A). Applying a sufficient power (21.5 W) continuously would result in elevated temperatures within the resistor well above its recommended operating temperature (175 °C). The temperature profiles were recorded at a location directly on the backside of the substrate beneath the resistor location (Figures 4B,C) of either a bare AlN or Al2O3 DBC substrate coated with 8−10 μm of graphitic networks as well as at the front edge (Figure S4). The backside location was a hot spot which approximated the junction temperature of the resistor and substrate. Because of prolonged operation at elevated temperatures, the resistor would eventually fail, and the time to failure was detected on the temperature profile at the point where the system began to cool to room temperature. A bare AlN substrate resulted in an average time to failure of 520 s with a maximum operation time of 655 s. Despite the difference in thermal conductivity between the two substrates, the Al2O3
substrate coated with graphitic networks significantly outperformed the AlN substrate in terms of time to resistor failure. Here, the resistor failed after an average of 28410 s with a maximum operation time of 33365 s. Within the first minute of heating, the backside temperature of the bare AlN sample rose quickly to over 220 °C compared to the coated Al2O3 sample, which only rose to a maximum of 205 °C. The 15 °C temperature difference between the two samples was observed within 20 s of heating. The delay in temperature rise that the graphitic networks provided to the resistor helped prevent thermal shock associated with the sharp temperature rise observed on the bare AlN substrate. The enhanced thermal management provided by micrometer-sized graphitic networks prevented failure within the first 6−10 min of operation. SiOCaccelerated graphitic networks on ceramics were also applied successfully to the operation of wide band gap (WBG) SiC MOSFETs on DBC substrates.23
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CONCLUSION
Through the use of SiOC-accelerated CVD, we could grow micrometer-sized graphitic networks onto Al2O3 substrates with strong coating adhesion. Deposition on AlN substrates resulted in poor coating adhesion which led to inferior heat transfer performance compared to the less conductive Al2O3 substrate. Graphitic networks on Al2O3 were characterized to have ordered layer stacking despite the surface roughness of the porous ceramic. However, the surface roughness resulted in a higher sp3 and amorphous carbon content within graphitic networks grown on ceramics compared to those grown on smoother substrates such as quartz. When deposited on a DBC substrate, the thermal management enhancement provided by 8−10 μm of well-bonded graphitic networks allowed for a lowcost Al2O3 substrate to outperform a more expensive AlN substrate in terms of prevention of device failure due to thermal shock. E
DOI: 10.1021/acsanm.8b01998 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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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/acsanm.8b01998. Optical profilometry of ceramics (Figure S1); heat transfer setup (Figure S2); graphitic networks on quartz (Figure S3); additional temperature profiles (Figure S4) (PDF) SAED analysis (Movie S1) (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Paul D. Garman: 0000-0002-1738-2442 Funding
This work was supported by Ohio Third Frontier IPP Program as well as Ohio Federal Research Network (WSARC-1077-20). Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acsanm.8b01998 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
