WO3 Film for Indium Tin Oxide

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Reactive Sputter Deposition of WO3/Ag/WO3 Film for Indium Tin Oxide (ITO)-Free Electrochromic Devices Yi Yin, Changyong Lan, Huayang Guo, and Chun Li* State Key Laboratory of Electronic Thin Film and Integrated Device, and School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu, 610054, China S Supporting Information *

ABSTRACT: Functioning both as electrochromic (EC) and transparent-conductive (TC) coatings, WO 3 /Ag/WO 3 (WAW) trilayer film shows promising potential application for ITO-free electrochromic devices. Reports on thermalevaporated WAW films revealed that these bifunctional WAW films have distinct EC characteristics; however, their poor adhesive property leads to rapid degradation of coloringbleaching cycling. Here, we show that WAW film with improved EC durability can be prepared by reactive sputtering using metal targets. We find that, by introducing an ultrathin tungsten (W) sacrificial layer before the deposition of external WO3, the oxidation of silver, which leads to film insulation and apparent optical haze, can be effectively avoided. We also find that the luminous transmittance and sheet resistance were sensitive to the thicknesses of tungsten and silver layers. The optimized structure for TC coating was obtained to be WO3 (45 nm)/Ag (10 nm)/W (2 nm)/WO3 (45 nm) with a sheet resistance of 16.3 Ω/□ and a luminous transmittance of 73.7%. Such film exhibits compelling EC performance with decent luminous transmittance modulation ΔTlum of 29.5%, fast switching time (6.6 s for coloring and 15.9 s for bleaching time), and long-term cycling stability (2000 cycles) with an applied potential of ±1.2 V. Thicker external WO3 layer (45/10/2/100 nm) leads to larger modulation with maximum ΔTlum of 46.4%, but at the cost of significantly increasing the sheet resistance. The strategy of introducing ultrathin metal sacrificial layer to avoid silver oxidation could be extended to fabricating other oxide−Ag−oxide transparent electrodes via low-cost reactive sputtering. KEYWORDS: transparent conductor, WO3/Ag/WO3 film, reactive sputtering, sacrificial layer, electrochromic

1. INTRODUCTION Electrochromic (EC) material is defined by a characteristic of changing its optical properties reversibly if an external potential is applied, associated with ion insertion and extraction processes.1 By virtue of its wide optical transmittance tunability and simple device structure, EC glass is particularly promising for energy-saving smart windows. However, the economy and reliability of commercially available EC glass has been proven: the high cost hinders its rapid commercialization.2 Tungsten oxide (WO3) is the most widely studied EC material.3−15 Currently, industrial-scale mass production of EC glass mainly relies on the sputter-deposited WO3 film on indium tin oxide (ITO) transparent conductive (TC) glass. The continuous increasing of ITO price motivates people to exploit new TC materials16 such as aluminum-doped ZnO, silver nanowire films, graphene, and dielectric−metal−dielectric (DMD) trilayer films,17 etc., which may be potentially substituted for ITO. Among them, WO3/Ag/WO3 (WAW) trilayer film has been extensively investigated as transparent electrode for solar cells,18−23 thin-film transistors,24 and organic light emission diodes.25,26 Recently, WAW film was proposed for both TC and EC bifunctional coatings.27−29 This ITO-free bifunctional coating could considerably reduce the cost of © 2016 American Chemical Society

WO3-thin-film based smart windows (see the section entitled “Material Cost Analysis”, in the Supporting Information). The pioneer work on bifunctional WAW film that was reported by Leftheriotis et al.27 revealed that the WAW (47/15/155 nm) film with a “thin” external WO3 film was unresponsive after 200 continuous voltammetric coloring-bleaching cycles, while “thick” external layer WAW (59/15/389 nm) performed better with stabilizing to the final shape of the 500th cycle. Therefore, they claimed that the external WO3 layer should be thick enough to effectively protect the sensitive silver layer from the liquid electrolyte. Later, however, Li et al. reported28 that WAW (30/10/50 nm) film also made via electron beam evaporation can sustain 3000 cycles without significant EC performance degradation within the similar sweeping voltage range. Although the reason that causes such apparent disagreement has not been clarified, both of their results did demonstrate that WAW bifunctional films have the potential for ITO-free EC devices. Received: November 5, 2015 Accepted: January 4, 2016 Published: January 4, 2016 3861

DOI: 10.1021/acsami.5b10665 ACS Appl. Mater. Interfaces 2016, 8, 3861−3867

Research Article

ACS Applied Materials & Interfaces

voltammetry (CV) and chronoamperometry (CA) were carried out to study the long-term stability and EC response time, respectively. The applied voltage sweep rate was maintained at 0.1 V/s for all of the CV measurements. The dynamic optical transmittance of the films was measured by recording the photocurrent of a standard Si photodetector, which monitors the transmitted light of a 632.8 nm laser diode through the film. To measure the coloring-bleaching response time, the optical transmittance of the films were recorded while the voltage applied on the films was turned on and off periodically.

Sputter deposition, which is a well-established industrial-scale thin-film coating technique, possesses distinctive advantages, such as better substrate adhesion, large-scale uniformity, and unique capability to control the film composition, compared with thermal evaporation. To the best of our knowledge, reports on the sputtering deposition of WAW film are still very limited. Recently, Chiang et al.30 prepared WAW film with an average transmittance of 74.2% by reactive sputtering; however, all the films were insulating. In this work, we demonstrate that a transparent and conductive WAW film with a sheet resistance of 16.3 Ω/□ and a luminous transmittance of 73.7%, which can be prepared by a sophisticated reactive sputtering technique, i.e., inducing an ultrathin tungsten (W) sacrificial layer prior to the deposition of external WO3 layer to avoid the Ag oxidation. The effects of the thickness of the tungsten sacrificial layer on the structural, electrical, and optical properties of WAW thin films were investigated. The optimized WAW film made by sputtering shows improved EC cycling stability, compared with the thermally evaporated one. Our sputter-deposited bifunctional WAW film with compelling TC and EC performance offers a new material platform for large-scale and low-cost EC devices.

3. RESULTS AND DISCUSSION 3.1. Transparent Conductive (TC) Performance of Sputtered WAW Film. For a transparent conductor with a DMD structure, the electrical conductivity is mainly determined by the middle metal layer, which requires the metal layer to be thick enough to reach a structural continuity. On the other hand, to obtain high optical transparency, the metal layer should be as thin as possible to minimize absorption. In addition, the dielectric layer should have a proper thickness to suppress the surface plasmon coupling-related absorption.17 Therefore, the thickness of each layer is crucial to ensure both high conductivity and high transparency. Generally, the optimized film thickness for each layer can be determined by the calculation according to the characteristic matrix theory.17 Here, for simplicity, the thickness of each layer of WAW film in our experiment was adopted to be 45/12/45 nm as a starting parameter, according to the previous reports.16,17 We find that our tentative deposition of WAW (45/12/45 nm) film following the normal layer-by-layer sequential deposition method always leads to a hazy optical appearance and all the films were insulating. However, after predepositing an ultrathin tungsten film prior to the deposition of external WO3, the WAW film becomes optical clear, i.e., no observable optical haze by the naked eye. We also find that both optical transmittance and electrical conductivity of the WAW film were sensitive to the thickness configuration of the tungsten and silver layer. Careful tuning the thickness of the tungsten and silver layers allows us to achieve WAW film with optimized TC performance. Figure 1 shows the optical transmittance of the as-deposited WAW films. We can see that, as the total thickness of silver and tungsten layer increase, the transmittance gradually decreases. The variation of transmittance of the WAW films can be also seen from their optical images shown in the lower inset of Figure 1. The colors of the WAW films on glass substrate turn to brown as the metal layer thickness increases (see bottom inset), and an obvious hazy optical effect of the WAW film without the sacrificial tungsten layer can be clearly seen from an oblique angle (center inset). In contrast, for the films with a predeposited tungsten layer, they all exhibit a clear appearance (no observable optical haze) from any viewing angle. For a high-quality TC coating, the material should possess both high electrical conductivity and high optical transparency. The film without introducing a sacrificial tungsten layer has an extremely large sheet resistance (>1 MΩ/□). With a sacrificial tungsten layer, the sheet resistance is significantly reduced to as low as 8 Ω/□ (WAW (45/12/2/45 nm)) at the cost of slight transmittance degradation. In order to quantitatively evaluate the performance of a transparent conductive film, so-called figure-of-merit (FTC) is introduced, which can be defined as18

2. EXPERIMENTAL SECTION 2.1. Thin-Film Preparation. The commercial available soda−lime glass (BBL-001, produced by Zhuhai Kaivo Optoelectronic Technology Co., Ltd.) with a thickness of 1.1 mm was used as the substrate. The substrates were ultrasonically cleaned in acetone and ethanol and then rinsed in deionized water. The substrates then were dried with high-purity nitrogen gas. The WAW films were deposited using a direct current/radio frequency (DC/RF) combined magnetron sputtering system with a background pressure below 5 × 10−4 Pa. Both the top and bottom WO3 layers were prepared at a working pressure of 0.46 Pa, an RF power of 200 W, and a Ar/O2 gas-flow-rate ratio of 2:3. The silver interlayer was subsequently deposited on the bottom WO3 by DC sputtering with a power of 10 W and a gas pressure of 0.35 Pa. The tungsten sacrificial layer was deposited under the same deposition conditions as the WO3, except that the O2 flow was blocked. The deposition rates of WO3, silver, and tungsten layer were controlled to be 0.04, 0.3, and 0.25 nm/s, respectively. During the deposition, no intentional substrate heating was applied. The distance between the substrate and the target was kept at 7 cm. For comparison, the thermally evaporated WAW film was prepared according to our previous report.31 Briefly, the WAW films were deposited under the vacuum pressure of 4 × 10−4 Pa. Films with an optimized thickness of WO3 (45 nm), silver (12 nm), and WO3 (45 nm) were sequentially deposited on glass substrates without breaking the vacuum. The evaporation rates of WO3 and silver were 0.1 and 0.2 nm/s, respectively. The measured sheet resistance of the thermally evaporated WAW film was 9.0 Ω/□. 2.2. Structural, Electrical, Optical, and Electrochromic Measurements. The crystal structure, cross-sectional morphology, and surface roughness of the films were examined by a glancingincidence X-ray diffraction (XRD) system (Rigaku, Model D/max-rA, Cu Kα radiation), a field-emission scanning electron microscopy (FESEM) system (Karl Zeiss, Model ULTRA55), and an atomic force microscopy (AFM) system (Veeco Multimode), respectively. The film transmission was measured by a fiber spectrometer (Stellar Net, Inc., Model BLK-C-SR). The sheet resistance was examined by a four-point probe instrument (Suzhou Jingge Electronic Co., Ltd., Model ST2258A). The electrochemical measurements were carried out using an electrochemical workstation (Shanghai Chenhua, Inc., Model CHI660). The EC properties of the films were investigated using a standard three-electrode system, with the WAW film on glass as the working electrode, a platinum plate as the counter electrode, and a Ag/ AgCl (1 M KCl) as the reference electrode. The electrolyte was 1 M LiClO4 dissolved in propylene carbonate (PC) solution. Cyclic

FTC = 3862

Tlum10 Rs

(1) DOI: 10.1021/acsami.5b10665 ACS Appl. Mater. Interfaces 2016, 8, 3861−3867

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ACS Applied Materials & Interfaces

indeed the optimal one among all the films with varying bottom/top WO3 thickness, as shown in the Supporting Information (Figure S1 and Table S1). 3.2. Mechanism of TC Performance Improvement. In order to elucidate the function of the tungsten layer, the crystal structure, surface morphology, and cross-sectional microstructure of the typical WAW films were analyzed by XRD, atomic force microscopy (AFM), and scanning electron microscopy (SEM), respectively. Figure 2 shows the XRD

Figure 1. Transmittance spectra of the WAW film on glass substrates with varying silver and tungsten layer thickness. For comparison, the transmittance of a ITO glass substrate was also included. The samples were numbered from 1 to 7. The center inset shows photographs of the typical samples with (sample 4) and without (sample 1) a sacrificial tungsten layer, which clearly show an optically clear and hazy appearance, respectively. The lower inset shows top-view photographs of the deposited samples and the bare glass substrate. The sample size is ∼1 cm × ∼4 cm.

Figure 2. X-ray diffractometry (XRD) patterns of the WAW films without (black curve) and with (red curve (10 nm Ag layer) and blue curve (12 nm Ag layer)) introducing a sacrificial tungsten layer. The diffraction peaks can be assigned to Ag.

where Tlum is the luminous transmittance, which is defined as Tlum =

∫ T ( λ )f (λ ) d λ ∫ f (λ ) d λ

pattern of the sample 1 (WO3/Ag/WO3, 45/12/45 nm), sample 4 (WO3/Ag/W/WO3, 45/10/2/45 nm), and sample 6 (WO 3 /Ag/WO 3 , 45/12/2/45 nm). For the film with predeposited tungsten (samples 4 and 6), the sharp peaks attributed to the characteristic diffraction peaks from crystalline silver can be clearly seen. In contrast, for the WAW films without predepositing tungsten (sample 1), no detectable Ag diffraction peaks can be observed, indicating the crystalline silver layer could presumably be oxidized into amorphous state. Note that none of the patterns show any characteristic peaks of WO3, reflecting that the WO3 layer was amorphous under our deposition conditions, which is beneficial to the EC performance.1 The Ag degradation or oxidization can be also deduced from the comparison of the surface topographies and cross-sectional microstructures of the films with and without introducing a sacrificial tungsten layer. For the film with predeposited tungsten, a well-established sandwich trilayer structure can be seen from the cross-sectional SEM image (Figure 3a). In addition, the surface is very smooth, with a root-mean-square (RMS) roughness of ∼1.2 nm (Figure 3c). In contrast, for the

(2)

Rs is the sheet resistance, f(λ) the eye-sensitive luminous spectral efficiency, and T(λ) the measured film transmittance. Note that the superscript “10” in eq 1 means that Tlum is raised to the power of 10. The integration range of λ is from 400 nm to 780 nm. The sheet resistances, luminous transmittances, and figure-of-merits (FTC) of the films with varying silver and tungsten thickness are listed in Table 1. We can see that the films with the structure of WO3 (45 nm)/Ag (10 nm)/W (2 nm)/WO3 (45 nm) has the largest FTC of 2.88 × 10−3 Ω−1 with a sheet resistance of 16.4 Ω/□ and a luminous transmittance of 73.7%, indicating its optimal structure among all the prepared films in our experiments. We note that such a performance can be comparable to the ITO with moderate quality.16 In addition, we also find that the TC performance of the sputtered film does not show considerable improvement by post-thermal annealing treatment, as in the case of the thermally evaporated one.31 This may be attributed to the presence of an essential substrate heating effect during sputtering deposition. In addition, the film with WO3 (45 nm)/Ag (10 nm)/W (2 nm)/WO3 (45 nm) is

Table 1. Sheet Resistance, Luminous Transmittance, and Figure-of-Merit of the Sputtered WAW Films sample No.

thickness (nm) (WO3/Ag/(W)/WO3)

sheet resistance, R (Ω/□)

luminous transmittance, Tlum (%)

figure-of-merit, FTC (× 10−3 Ω−1)

1 2 3 4 5 6 7

45/12/0/45 45/8/2/45 45/8/3/45 45/10/2/45 45/10/3/45 45/12/2/45 45/12/3/45

>1 M 47.5 61.2 16.4 21.2 8.42 15.5

82.7 72.2 58.1 73.7 61.8 62.1 52.5

0 0.81 0.07 2.88 0.38 1.01 0.10

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of an ultrathin tungsten film, the silver oxidation can be effectively avoided, so that the WAW films always exhibit clear optical appearance. This means a 2 nm ultrathin tungsten film is sufficient to protect the silver layer from oxidation with minimum visible light absorption. Therefore, we focused on the optimized sample, i.e., sample 4 (WO3/Ag/W/WO3, 45/10/2/ 45 nm) for the following EC performance measurement. 3.3. Electrochromic (EC) Performance of Sputtered WAW Films. To evaluate the EC performance of the sputterdeposited WAW film, the cathodic and anodic behaviors of the film were monitored in a solution containing Li+. Coloration and bleach state can be clearly distinguished upon Li+ insertion and extraction. For EC window applications, a key requirement is the long-term durability. In order to investigate the EC stability, the CV measurement of sputtered WAW film was performed from 1 to 2000 cycles, as shown in Figure 4a. It reveals that the current response was almost identical for the initial 200 cycles. After 200 cycles, a slight decrease in the current was observed. In contrast, the thermally evaporated WAW film becomes unresponsive after only 50 cycles before the current density diminishes to almost zero (Figure 4b). Inspection of the EC coating revealed that the thermally evaporated film becomes nonconductive and the sheet resistance (>1 MΩ/□) exceeds the detection limit of our equipment, while the sheet resistance of the sputtered WAW film slightly increased from 16.4 to 22.7 Ω/□. To quantitatively investigate the EC properties, the transmittance of colored and bleached states for the sputtered WAW film after the 1st and the 2000th voltammetric cycles were measured as shown in Figure 4c. The film exhibits apparent optical modulation in the visible light range, accompanied by an average optical modulation (ΔT = Tb − Tc, where Tb and Tc represent the transmittance in the bleached and colored state, respectively) of ∼35.5% for transmittance at 650 nm and

Figure 3. Cross-sectional SEM and surface AFM images of the WAW films (a, c) with and (b, d) without introducing a tungsten layer. For better SEM observation, ultrathin platinum films were deposited on the cross-sectional samples to eliminate the charging effect.

film without a predeposited tungsten layer (Figure 3b), no image contrast for the silver interlayer can be found. Moreover, the film surface is very rough, with a RMS roughness of ∼34.6 nm (see Figure 3d). We believe that, without predepositing an ultrathin tungsten film, the silver thin film directly exposes the oxygen-ion-enriched environment in the subsequent reactive sputtering of the external WO3 with an Ar/O2 ratio of 2:3. Therefore, the silver thin film could be easily oxidized by the energetic oxygen-ion bombardment and further broken and aggregated into AgOx islands. The discontinuous silver film leads to a substantial increase in sheet resistance. Meanwhile, a large surface roughness significantly enhances the diffuse light scattering, forming a “foggy” appearance. Via in situ deposition

Figure 4. EC performance characterization of sputtered WAW film (WO3/Ag/W/WO3, 45/10/2/45 nm): (a) CV cycling stability of sputterdeposited and (b) thermal-evaporated WAW film, respectively. (c) Optical transmittance spectra of the colored and bleached states after the 1st and the 2000th CV cycling of the optimized sputter-deposited sample. Inset shows the corresponding optical images of bleached (top) and colored (bottom) states after the 2000th cycling. (d) Response time for the coloration-bleaching switch operation of the sputter-deposited WAW film. 3864

DOI: 10.1021/acsami.5b10665 ACS Appl. Mater. Interfaces 2016, 8, 3861−3867

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ACS Applied Materials & Interfaces ∼29.5% for luminous transmittance (see Table 2). After the 2000th scan cycles, ΔT under the same condition was slightly Table 2. Major Parameters of the EC Performance of the Sputtered WAW Film (WO3/Ag/W/WO3, 45/10/2/45 nm) EC state

Tlum (%)

T (%, 650 nm)

τ (s)

bleached colored

70.7a 41.2

70.2 34.7

15.9 6.6

ΔTlum = 29.5%

ΔT = 35.5%

a

Note that the luminous transmittance (Tlum) at the bleached state is slightly lower than the value of the original as-prepared WAW film (73.7%).

reduced to 31.4% for transmittance at 650 nm and 23.7% for luminous transmittance, which can be attributed to the variation of the film refraction index caused by the slight oxidation of the embedded silver layer. The coloration efficiency can be defined as the change in optical density (ΔOD) at a given wavelength (here, we use the transmittance at 650 nm) for the charge consumed per unit of electrode area, which can be calculated using the following formula: η=

log(Tb/Tc) ΔOD = Q Q

Figure 5. Luminous transmittance of the sputtered WAW films in bleached (circle) and colored states (triangle) with increasing external WO3 thickness. The red curve with square data points shows optical modulation ΔTlum between the bleached and colored states.

and bleached states (Figure S2 and Table S2)). Considering the case of large-scale EC window application, a special partitioning technique, which involves exposing the edge of the silver layer for electrode connection, is necessary to ensure delivery of a uniform electric field on the external WO3 EC film.

(4)

where Q is the injected charge density, corresponding to the ratio of the inserted charge over the device area. The calculated η values after the 1st and 2000th scan cycles are 28.3 and 32.8 cm2 C−1, respectively, which is comparable to the directsputtered WO3 film on an ITO substrate with a thickness of 500 nm.32,33 In addition, the cyclic CA and the corresponding transmittance measurements were employed to study the switching characteristics of the sputtered WAW film. The cycling of CA stepping from −1.2 to 1.2 V with a 40 s interval. The time required for the shift in coloration (τc) and bleaching (τb) is based on the optical modulation depth of 90% at a wavelength of 632.8 nm. As shown in Figure 4d, the typical values for τc and τb were 15.9 and 6.6 s, respectively, which is also a competitive value to most of the WO3 EC devices on ITO substrates with similar thicknesses.2 These results indicate that the sputtered WAW film exhibits much stronger durability than the thermally evaporated one. Such an improved stability can be intuitively attributed to a greatly compacted film structure, rather than the evaporated one, so that the silver film can be well-protected by the compact sputtered WO3 film. In contrast, the silver layer of the thermally evaporated WAW film with relatively loose external WO3 can readily react with liquid electrolyte during the Li+ injection process, which results in its poor cycling stability. Furthermore, we note that there might be trace amount of tungsten retained at the top Ag/WO3 interface, which may work as a diffusion barrier to avoid the reaction between silver and liquid electrolyte during Li+ injection. This is also another possible factor that contributes to the long-term stability. Finally, we find that increasing the thickness of external WO3 can slightly enlarge the EC optical modulation to the maximum ΔTlum value of ∼46.4% for the film of 45/10/2/100 nm, as shown in Figure 5, which is already comparable to some commercially available EC windows.2 However, this is achieved at the cost of an apparent reduction in optical transmittance and a substantial increase in sheet resistance (see the Supporting Information for the transmission data of colored

4. CONCLUSIONS In summary, we report that the indium tin oxide (ITO)-free bifunctional WO3/Ag/WO3 (WAW) film on glass substrate with compelling electrochromic (EC) performance can be fabricated by reactive sputtering. By introducing an ultrathin tungsten layer as a sacrificial layer, silver oxidation, which leads to the film insulation, can be effectively avoided. Careful tuning of the silver and tungsten layer thickness allows us to obtain a maximum figure-of-merit of 2.88 × 10−3 Ω−1 for WO3 (45 nm)/Ag (10 nm)/W (2 nm)/WO3 (45 nm) film with an average luminous transmittance (73.7%) and sheet resistance (16.3 Ω/□). These films can serve simultaneously as electrodes and EC materials. They exhibit decent performance, with an optical contrast of 35.5% at 650 nm and a coloration efficiency of 28.3 cm2 C−1, considering that the effective WO3 thickness was only 90 nm. The sputtered WAW film shows substantial improvement of long-term cycling stability (2000 cycles), compared to the thermally evaporated one. Furthermore, the strategy of introducing a sacrificial layer of ultrathin metal to avoid silver oxidation, as presented here, could be extended to prepare other types of oxide−Ag−oxide transparent electrodes via low-cost reactive sputtering.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10665. Material cost analysis, transparent conductive performance of sputtered WAW films with varying of bottom and top WO3 thickness, and electrochromic performance of sputtered WAW films with increasing thickness of external WO3 (PDF) 3865

DOI: 10.1021/acsami.5b10665 ACS Appl. Mater. Interfaces 2016, 8, 3861−3867

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ACS Applied Materials & Interfaces

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 61421002, 61475030, 61504020, and 61522403), the Program for New Century Excellent Talents in University (Grant No. NCET-13-0092), the State Key Laboratory of Electronic Thin Film and Integrated Device Program (No. KFJJ201408), and the Central University Basic Scientific Research Business Expenses (Nos. 2672013ZYGX2013J060 and 2672013ZYGX2013J061).

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DOI: 10.1021/acsami.5b10665 ACS Appl. Mater. Interfaces 2016, 8, 3861−3867

Research Article

ACS Applied Materials & Interfaces (33) Lee, K. D. Influence of Film Thickness on the Chemical Stability of Electrochromic Tungsten Oxide Film. J. Korean. Phys. Soc. 2001, 38 (1), 33−37.

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DOI: 10.1021/acsami.5b10665 ACS Appl. Mater. Interfaces 2016, 8, 3861−3867