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Experimentally, we observed a clear trend in the Raman peak intensity ratio of the signals at 210 and 203 cm–1 in the FTA mode: the intensity ratio ...
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Using Visible Laser-Based Raman Spectroscopy to Identify the Surface Polarity of Silicon Carbide Yi-Chuan Tseng,† Yu-Chia Cheng,† Yang-Chun Lee,† Dai-Liang Ma,‡ Bang-Ying Yu,‡ Bo-Cheng Lin,‡ and Hsuen-Li Chen*,† †

Department of Materials Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan Materials and Electro-Optics Research Division, National Chung-Shan Institute of Science and Technology, P.O. Box 90008-8-1, Lungtan, Taoyuan 32599, Taiwan

‡

S Supporting Information *

ABSTRACT: In this study we developed an approach to identify the surface polarity of silicon carbide (SiC) by using an excitation laser possessing a photon energy (2.33 eV) much lower than the band gap of 4H-SiC (3.30 eV). By gradually attenuating the intensity of the excitation laser, the effective depth that the laser could generate Raman signals could eventually be limited to within the ultrashallow region of the SiC wafer. Through three-dimensional finite-difference-timedomain (3D-FDTD) simulations, we found that the depth of the high electric field region could be limited from several micrometers below the surface to the near-surface region of 4H-SiC, merely by attenuating the power of the incident laser. Experimentally, we observed a clear trend in the Raman peak intensity ratio of the signals at 210 and 203 cm−1 in the FTA mode: the intensity ratio of the Si face was always higher than that of the C face regardless of the measurement position on the 4H-SiC wafer. Even through the carrier concentrations in the 4H-SiC wafer were nonuniform, the resulting variability in peak intensity did not influence the trend in the intensity ratio, which could, therefore, be used to identify the surface polarity. This approach might also allow characterization of different polytypes of SiC, for example, 6H-SiC and 3C-SiC, the optical band gaps of which are lower than that of 4H-SiC. Because this optical approach using low-photon-energy laser-based Raman spectroscopy is nondestructive, simple, and rapid and employs excitation light of low photon energy, it should be very applicable for characterizing the surface properties of various other crystalline materials.

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INTRODUCTION Silicon carbide (SiC)-based devices have displayed excellent stability in the applications requiring high-temperature, highpower, and high-radiation conditions.1,2 Because of its outstanding properties, SiC has been applied in a broad range of applications, including optoelectronics, micromechanical sensors, biomedical devices, and astronomical telescopes.3−6 In particular, 4H-SiC is the preferred polytype in most of the main applications of SiC-based devices due to its wide band gap and high electron mobility.7 Crystallographic polarity generally has a large influence on the physical and chemical properties of the surfaces and interfaces of heterostructures due to the different chemical nature of the terminating atoms. SiC is a typically polar semiconductor with different terminating atoms on each surface; that is, it presents “silicon (Si) faces” and “carbon (C) faces.” Because the surface polarity influences the rates of nucleation and surface migration, it would be advantageous if we could rapidly and nondestructively identify the surface polarity of a SiC wafer.8 For example, the different surface polarities of SiC can influence the growth rates and electrical properties of epitaxial graphene on SiC wafers. The preparation of single© 2016 American Chemical Society

layer graphene through thermal decomposition on SiC has been proposed as a feasible route for the growth of large-scale graphene.9 Nevertheless, the structural quality of the resulting graphene is highly dependent on the surface crystallographic polarity of the SiC wafer. The influence on the graphene quality is due to the effect of the different surface atoms during hightemperature annealing. On the Si face, vacuum annealing leads to the growth of homogeneous graphene, but with small domains; in contrast, large domains of multilayered, rotationally disordered graphene are produced on the C face.9−12 The crystallographic polarity can be determined using several methods, including low-energy electron diffraction (LEED),13 reflection high-energy electron diffraction (RHEED),14 X-ray diffraction (XRD), 15,16 X-ray photoelectron diffraction (XPD),17 convergent beam electron diffraction,18,19 Auger electron spectroscopy,20,21 and electron energy-loss spectroscopy.22 None of these methods, however, can be employed to determine the surface polarity of SiC; because they characterize Received: April 12, 2016 Revised: June 30, 2016 Published: August 9, 2016 18228

DOI: 10.1021/acs.jpcc.6b03713 J. Phys. Chem. C 2016, 120, 18228−18234

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The Journal of Physical Chemistry C

the excitation laser was approximately 0.4 μm2; the integration time was 5 s for a laser power intensity of 100% and 150 s for laser power intensities of 0.1 and 1%.

SiC through interactions between incident electrons and a longrange-ordered atomic arrangement of the substrate, they cannot be used to analyze only the surface layer of the SiC wafers. Previous studies have revealed that the surface polarity can be identified in terms of the etching, growth, and oxidation rates.12,23 For example, the surface polarity of SiC can be determined from the rate of formation of graphene or thermal oxide.12 These approaches are, however, destructive and slow. Therefore, the development of rapid, in situ, nondestructive methods for characterizing the surface polarity of SiC, or other crystalline materials, is not only desirable but also necessary. Raman scattering, a nondestructive method, provides abundant information relating to the bonding, symmetry, and vibration of molecules.24,25 Raman scattering is also a practical method for characterizing the properties of the surface layer of a sample, when the penetration depth of the excitation light is limited within the surface region. Nakashima et al. employed a Raman scattering approach to distinguish the Si and C faces of a SiC wafer.26 By using deep ultraviolet (DUV) laser-based Raman spectroscopy, they found that the relative intensities of the doublet of Raman signals were different for the Si and C faces of a SiC wafer. This approach can successfully identify the surface polarity because the penetration depth of DUV light is very shallow; thus, the Raman signals were contributed mostly by the surface region of the SiC wafer. Although this optical approach is rapid, DUV lasers are relatively expensive and provide photon energy so high that it might damage the surface to the SiC wafer. Moreover, DUV light would also excite fluorescence light in a SiC wafer, thereby increasing the difficulty of using DUV-based measurement systems in practical applications. In this study, we developed a nondestructive approach, using visible laser-based Raman spectroscopy, to identify the Si and C faces of SiC wafers. As the excitation laser for Raman spectroscopy, we selected a visible laser (532 nm) possessing a photon energy (2.33 eV) much lower than the band gap of 4H-SiC (3.3 eV).27 Unlike the DUV light (244 nm, 5.08 eV) reported in the previous study, the low photon energy of this excitation laser prevented the generation of photoluminescence and did not cause any damage to the 4H-SiC surface through overexposure. To ensure that the penetration depth of the excitation light was limited to the surface region, we gradually attenuated the laser power. As the effective penetration depth of the excitation light decreased, the Raman signal was contributed to a greater extent by the surface region of the SiC wafer. This method of varying attenuation proposed herein is very easy to perform and has great potential for use in identifying the surface polarity of various other crystalline materials.

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RESULTS AND DISCUSION Figure 1a displays a schematic representation of the microRaman system used to characterize the SiC wafers. The laser

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EXPERIMENTAL SECTION Sample Preparation and Characterization. The 4H-SiC wafers were purchased from SiCrystal AG. Prior to characterization, the wafers were washed with acetone and isopropanol, rinsed with deionized water (DI), and dried under a N2 flow. Micro-Raman spectra of the SiC wafers were measured using a commercial micro-Raman microscope (UniRAM, UniNanoTech) equipped with a monochromator having a focal length of 75 cm. The wavelength of the excitation laser line was fixed at 532 nm (diode laser). The laser beam was focused by a 100× objective having a numerical aperture of 0.95. The power intensity of the excitation laser was gradually attenuated from 30 mW/μm2 (100%) to 30 μW/μm2 (0.1%). The spot size of

Figure 1. (a) Schematic representation of a micro-Raman system with a variable attenuator used to characterize a SiC wafer. (b) Schematic representation of a 4H-SiC wafer; the C-terminated (000−1) and Siterminated (0001) polar faces appear at the top and bottom of the wafer, respectively. (c) Measured Raman spectrum of the 4H-SiC wafer; inset: photograph of the 4H-SiC wafer. 18229

DOI: 10.1021/acs.jpcc.6b03713 J. Phys. Chem. C 2016, 120, 18228−18234

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The Journal of Physical Chemistry C power intensity was attenuated gradually from 30 mW/μm2 to 30 μW/μm2 via the variable attenuator filter. The slit was fixed at 50 μm to guarantee a sufficiently large signal-to-noise ratio. Notably, we employed a monochromator having a focal length of 75 cm to obtain Raman signals of good spectral resolution, necessary to distinguish the close vicinal Raman peaks of 4HSiC wafers to identify their surface polarity. Figure 1b provides a schematic representation of a 4H-SiC wafer, with the Cterminated (0001̅) and Si-terminated (0001) polar faces on the top and bottom, respectively. In this study, we changed the intensity of the excitation laser from 100 to 0.1% in an attempt to limit the main Raman excitation to within the near-surface region of the SiC wafer. Figure 1c displays a photograph of a SiC wafer, corresponding to the 4H-SiC polytype, and its Raman spectrum. The Raman spectrum features two typical optical phonon peaks at 210 and 770 cm−1, corresponding to the FTA and FTO vibration modes, respectively.28 To investigate how excitation laser light of various intensities propagates into a SiC wafer, we used a commercial simulation software (R-Soft) based on the three-dimensional finitedifference-time-domain (3D-FDTD) approach to simulate the behavior of excitation light interacting with a SiC wafer. The 3D-FDTD simulations revealed the electromagnetic fields over the entire computational domain as they evolved over time, providing animated displays of the electric field’s movement and allowing analysis of the electric field distribution beneath the surface of the SiC wafer. To compare the penetration depths of DUV and visible light within a SiC wafer, we modeled the propagation of planar waves having wavelengths of 244 and 532 nm, respectively, upon the surface of a 4H-SiC wafer. The optical constants (refractive index n and extinction coefficient k) of SiC we used in the simulation are n = 3.392, k = 0.884 at a wavelength of 244 nm and n = 2.681, k = 0.058 at a wavelength of 532 nm, respectively.29 The simulation volumes (a unit cell) were 400 × 400 × 4000 nm3 at a wavelength of 244 nm, 400 × 400 × 7000 nm3 at a wavelength of 532 nm, and 400 × 400 × 2500 nm3 at a wavelength of 532 nm, respectively. The grid sizes were 5 nm in the x, y, z direction. The unit cell was repeated periodically in the x and y direction, and the perfect matched layer (PML) was adopted as the boundary of the z direction. We used several detectors below the SiC surface to monitor the behavior of light propagating into the SiC wafer. Figure 2a displays the electric field distributions of incident light having a wavelength of 244 nm propagating into a SiC wafer. Here, E02 and E2 represent the squares of the electric field amplitudes of the incident light and the light propagating into the SiC wafer, respectively. For incident light having a wavelength of 244 nm, the value of E2 decayed to e−1 times E02 at approximately 7 nm below the surface of SiC, as displayed by the white dotted line in Figure 2a; the value of E2 decayed to 10−3 E02 at approximately 70 nm beneath the SiC surface, as displayed by the red dotted line. The rapid decay and ultrashallow extinction depth originated from the high absorption and apparent reflection of SiC for light having a wavelength of 244 nm. Therefore, it is understandable that DUV lasers can be used as light sources to generate Raman scattering signals when obtaining the surface crystallographic information on wide-band-gap semiconductors (e.g., SiC).26,29,30 Figure 2b displays the electric field distributions of incident light having a wavelength of 532 nm, but intensity (100%) identical to that in Figure 2a, into a SiC wafer. Because of the low absorption of SiC at this wavelength, the value of E2

Figure 2. Electric field distributions of incident light having wavelengths of (a) 244 nm with an initial intensity of 100%, (b) 532 nm with an initial intensity of 100%, and (c) 532 nm with an initial intensity of 1%, propagated into a 4H-SiC wafer.

decayed to e−1 times E02 at approximately 500 nm below the surface of SiC, as displayed by the white dotted line in Figure 2b; the value of E2 decayed to 10−3 E02 at approximately 3800 nm beneath the surface, as displayed by the red dotted line in Figure 2b. Because of the relatively large region of high electric field distribution, a Raman excitation laser having a wavelength 532 nm would provide integrated information about the bulk SiC rather than the surface properties of the SiC wafer. To overcome this problem and characterize the surface polarity of 18230

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Table 1. Optical Constants (n, k) at a Wavelength of 532 nm, Band Gaps, and Propagating Depths at Which the Electric Field Intensities Decreased to e−1 and 10−3 Times the Initial Intensity for the Polytypes 3C-SiC, 4H-SiC, and 6H-SiC polytype

n/k@532 nm

Eg (eV)

depth of E2 = E02·e−1 @ 532 nm (nm)

depth of E2 = E02·10−3 @ 532 nm (nm)

ref.

3C-SiC 4H-SiC 6H-SiC

2.675/0.398 2.681/0.0577 2.673/0.1223

2.3 3.3 3

70 500 180

730 3800 2300

31 32 32

SiC using visible light, we attenuated the intensity of the 532 nm light from 100 to 1%. Figure 2c reveals that the large electric field distribution occurred within a relatively shallow region; the red dotted line indicates that an intensity of 10−3 E02 appeared 200 nm beneath the surface of the SiC wafer. We suspected that the shorter propagating depth of incident visible light for generating Raman signals might be applicable to characterizing the surface properties of SiC wafers. We also compared the penetration depths of incident light at a wavelength of 532 nm into three different polytypes of SiC: 3C-SiC, 4H-SiC, and 6H-SiC. Table 1 lists the optical properties of these three common polytypes of SiC in terms of their optical constants (n, k), band gaps, and propagating depths at which incident light (E02) decayed to e−1·E02 and 10−3E02. Because the three polytypes of SiC possess different band gaps, they had different propagating depths for incident light at a wavelength of 532 nm. The propagating depth for 4HSiC, which possesses the widest band gap and lowest absorption coefficient, was longer than those for 6H-SiC and 3C-SiC. Therefore, it would be most difficult to characterize the surface polarity of 4H-SiC when using a visible light-based Raman system. In this study, we attempted to experimentally characterize the surface polarity of 4H-SiC, the transparency of which in the visible regime is higher than those of 6H-SiC and 3C-SiC. Accordingly, any approach that can identify the surface polarity of 4H-SiC when using a visible laser must also be practical for characterizing the surface polarities of 6H-SiC and 3C-SiC wafers. We investigated the effect of different intensities of 532 nm laser light propagating into a SiC wafer. Nevertheless, the propagating depth of the incident light would not necessarily be equal to the depth at which Raman signals could be generated beneath the surface of a SiC wafer. To further estimate the depth at which Raman signals could be generated, we performed the following investigation. First, we experimentally determined the threshold intensity of the excitation laser (532 nm) to generate 4H-SiC Raman signals by gradually decreasing the intensity of the laser light from 30 mW/μm2 (100%) to 30 μW/μm2 (0.1%). We used an integration time of 5 s for the Raman signals at a laser power intensity of 100% and 150 s for intensities ranging from 0.1 to 1%. As displayed in Figure 3, the 4H-SiC wafer emitted Raman signals when the light from the excitation laser had intensities ranging from 30 mW/μm2 (100%) to 42 μW/μm2 (0.14%) in our Raman system. Nevertheless, the Raman spectra of the 4H-SiC wafer featured only the FTO mode when the light intensity decreased to 42 μW/μm2 (0.14%); the FTA mode, which is used to identify the surface polarity of SiC, could not be observed when the intensity was less than 1%. Upon further decreasing the incident power of the excitation laser to 36 μW/μm2 (0.12%), both the FTO and FTA modes disappeared. Therefore, it appears that an intensity of 1% (300 μW/μm2) in our system was the threshold intensity for generating the FTA mode and identifying the surface polarity of a 4H-SiC wafer.

Figure 3. Raman spectra of a 4H-SiC wafer generated using light from an excitation laser having a wavelength of 532 nm, recorded upon decreasing the intensity from 30 mW/μm2 (100%) to 36 μW/μm2 (0.12%).

Next, as displayed in Figure 4, we simulated the power distributions of 532 nm incident light at initial intensities of

Figure 4. Distributions of power at different depths within a 4H-SiC wafer irradiated with visible light at a wavelength of 532 nm with intensities of 0.12, 1, and 100% and with DUV light at a wavelength of 244 nm with an intensity of 100%.

0.12, 1, and 100% propagating into a 4H-SiC wafer. Based on our experimental results, the power of the incident light having an intensity of 0.12% was the threshold power of the excitation laser necessary to generate Raman signals, marked by the red horizontal line in Figure 4. The red horizontal line intercepts the curves for the light with intensities of 1 and 100% at points “a” and “b”, respectivelyrepresenting the depths (245 and 3830 nm, respectively) within which most of the Raman signals were contributed from within 4H-SiC. The simulation 18231

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Figure 5. (a, c) Measured FTA modes and (b, d) intensity ratios of the signals at 210 and 203 cm−1 on the Si and C faces in the Raman spectra of 4H-SiC under excitation with 532 nm laser light (a, b) without attenuation and (c, d) with attenuated power density of 1%.

suggested that the region of generated Raman signals could be limited to relatively near the surface merely by attenuating the intensity of the excitation laser. Moreover, the power of the excitation laser required to generate the signals of the FTA mode was higher than that of the FTO mode. Therefore, the region of generated FTA signals was presumably more limited to near the SiC surface. Therefore, the surface polarity of a SiC wafer could be distinguished more readily by considering its FTA mode in Raman spectra when attenuating the power of the excitation laser. Next, we verified that the surface polarity of a SiC wafer could be distinguished using visible laser-based Raman spectroscopy. Under visible laser (532 nm) excitation, as displayed in Figure 3, major peaks appeared for the Raman bands of the FTA mode near 210 cm−1 and the FTO mode at 770 cm−1. The FTA mode of 4H-SiC has been reported as having sensitivity to the atomic arrangement of the surface layer.26 Figure 5a displays the FTA mode of a 4H-SiC wafer excited by a 532 nm laser without attenuating (100%, ca. 30 mW/μm2). The signal of the FTA mode appeared of a doublet, with a sharp peak at 210 cm−1 and a weaker peak at 203 cm−1 (Figures 5a and 5c). To investigate any differences in the Raman spectra of the C and Si faces of a 4H-SiC wafer, we aligned the laser spot at the same position on both sides of a 4H-SiC wafer by using a horizontally moving two-axis stage. As displayed in Figure 5b, we then measured and calculated the Raman peak intensity ratios of the signals at 210 and 203 cm−1 obtained from the C and Si faces at the same position of the 4H-SiC wafer. We noted that the Raman spectra of two FTA doublet modes did not display flat backgrounds. The details about how we determined the intensity of the two FTA modes

were illustrated in the Supporting Information. Notably, the process of SiC crystal growth sometimes results in some inhomogeneous areas in a 4H-SiC wafer. As displayed in the photograph of 4H-SiC in Figure 1c, these inhomogeneous areas could appear as dark regions. In Figures 1c and 5b, we use the numbers 1−6 and the letters a−c to represent different positions in the bright and dark regions, respectively. In Figure 5b, the points “bright 1−6” and “dark a−c” represent the averages of the peak intensity ratios of points 1−6 and a−c, respectively. There is no evident trend in the intensity ratios of the C and Si faces because the Raman signals were contributed mostly by the inner region of the 4H-SiC wafer when the excitation laser power was high. Therefore, we performed the same measurement on the same wafer using a lower-intensity light from the excitation laser. Figure 5c displays the FTA mode of the 4H-SiC wafer excited under a 532 nm excitation laser with an attenuated intensity of 1% (ca. 300 μW/μm2). For this experiment we increased the integral time to acquire sufficient intensity to obtain more precise spectral information. The spectral shape and peak position in Figure 5c are very similar to those in the spectrum generated by the laser light without attenuation (Figure 5a). When the laser power was attenuated to 1%, we observed and calculated the peak intensity ratio between the signals at 210 and 203 cm−1 as displayed in Figure 5d. The peak intensity ratio of the signals at 210 and 203 cm−1 was always higher when measured on the Si face than on the C face, for all positions (1−6 in bright region, a−c in dark region), as displayed in Figures 1c and 5d. Therefore, this clear trend allows identification of the surface polarity of 4H-SiC when using merely visible laser-based Raman spectroscopy. We note that the more measured data are provided in the Supporting 18232

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The Journal of Physical Chemistry C Information. We attribute this performance to the fact that Raman signals were mostly collected from the surface region when the visible laser light was applied with attenuated power. Almost the same result was performed by using another visible laser (473 nm). The detailed discussion can be found in te Supporting Information. In Figures 5b and 5d, the peak intensity ratios of the signals at 210 and 203 cm−1 varied depending on their positions. Although the variation in peak intensity did not influence our ability to identify the surface polarity, the origin of this effect would still have a bearing on the characterization of a SiC wafer. We suggest that the variation in peak intensity might be due to different carrier concentrations at different points of the SiC wafer. In previous reports, the electrical properties (e.g., free carrier density) of SiC wafers were examined through measurement of the LO phonon plasmon coupled (LOPC) mode near 970 cm−1.28 The LOPC mode in a Raman spectrum would broaden asymmetrically, with the peak also blue-shifting, upon increasing the carrier density in the SiC wafer.28 Thus, we used micro Raman spectroscopy to investigate the carrier concentrations at different regions of the 4H-SiC wafer. Figure 6 displays the signals for the LOPC mode measured from a 4H-

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CONCLUSION

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ASSOCIATED CONTENT

We have demonstrated that visible laser-based Raman spectroscopy can be used to readily identify the surface polarity of a SiC wafer. By gradually attenuating the intensity of the excitation laser (532 nm), the effective depth that the visible laser light could generate Raman signals could be confined to the ultrashallow region near the surface of a 4H-SiC wafer. For this reason, the Raman signals were mostly collected from the near-surface regions of the SiC wafer. Through 3D-FDTD simulations we found that the depth of the high electric field region of 532 nm light could be reduced from several micrometers below the surface to the near-surface region of the 4H-SiC wafer upon attenuating the incident power. Experimentally, we found that the threshold intensity of visible laser light that could generate the Raman signals from 4H-SiC was 36 μW/μm2. We also observed that the signal of the 4HSiC wafer’s FTA mode, which could be used to identify the surface polarity, appeared only when the intensity of the excitation laser was greater than 300 μW/μm2. When we measured the peak intensity ratios of the signals at 210 and 203 cm−1 for the FTA mode of a SiC wafer, we observed a clear trend: the intensity ratio on the Si face was always greater than that on the C face, regardless of the position on the 4H-SiC wafer. We attribute this behavior to the fact that the FTA-mode Raman signals were collected mostly from the surface region after the intensity of incident visible laser light had been attenuated sufficiently. Moreover, even though the 4H-SiC wafer featured nonuniform carrier concentrations, the variations in peak intensity did not influence the trend in the intensity ratio, which could, therefore, be used to identify the surface polarity. This optical approach is nondestructive and simple and applies excitation light of low photon energy, much lower than the band gap of 4H-SiC. We suspect that this approach might also allow characterization of the different polytypes of SiC, including 6H-SiC and 3C-SiC, which have optical band gaps lower than that of 4H-SiC, because the high electric field region of the incident laser light would be even more confined within the shallow region. This characterization method, based on attenuated excitation laser power, appears very applicable for the development of techniques for characterizing the surface properties of many other crystalline materials in conjunction with low-photon-energy laser-based Raman spectroscopy.

Figure 6. Raman spectra of LOPC modes measured at the different positions in bright (1−3) and dark (a−c) regions of the 4H-SiC wafer.

SiC wafer identical to that used to characterize the surface polarity. Here, we use the numbers 1−3 and the letters a−c to represent different positions in the bright and dark regions, respectively; note that the positions of 1−3 and a−c are identical to those in the experimental setup for characterization of the surface polarity. Compared with the bright region (1−3), the LOPC mode provided asymmetrically broader peaks and a blue-shifted peak frequency in the dark region (a−c), suggesting that the carrier concentration in the dark region of the 4H-SiC wafer was higher than that in the bright region. In addition, the Raman LOPC mode varied slightly at the different positions, even in the same dark or bright region. Although these results suggest that variations in the Raman spectra over the whole 4H-SiC wafer occurred because of nonuniform carrier concentrations, the peak intensity ratio of the signals at 210 and 203 cm−1 could still identify the surface polarity of a SiC wafer when using visible laser-based Raman spectroscopy.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03713. Measurements of more positions on SiC wafers; determination of the intensity of two FTA doublet modes; and identification of the surface polarity of SiC by using a Raman system having 473 nm excitation laser (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +886-2-3366-3240. Notes

The authors declare no competing financial interest. 18233

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology, Taiwan, for supporting this study under contracts MOST-103-2221-E-002041-MY3 and MOST-103-2221-E-002-092-MY3. We also thank the National Chung-Shan institute of Science and Technology, Taiwan, for supporting this study under contracts CSIST-010-V401(103) and NCSIST-102-V209(105).

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DOI: 10.1021/acs.jpcc.6b03713 J. Phys. Chem. C 2016, 120, 18228−18234