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Vacuum-Deposited Organometallic Halide Perovskite Light-Emission Devices Kai-Ming Chiang, Bo-Wei Hsu, Yi-An Chang, Lin Yang, Wei-Lun Tsai, and Hao-Wu Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12805 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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ACS Applied Materials & Interfaces

Vacuum-Deposited Organometallic Halide Perovskite Light-Emission Devices Kai-Ming Chiang, Bo-Wei Hsu, Yi-An Chang, Lin Yang, Wei-Lun Tsai, and Hao-Wu Lin* Department of Materials Science and Engineering, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 300, Taiwan. *E-mail: [email protected] (Hao-Wu Lin) KEYWORDS: organometallic halide perovskite; vacuum deposition; light-emission device; vacuum sublimation.

ABSTRACT

In this work, sequential vacuum deposition process of bright, highly crystalline, and smooth methylammonium lead bromide and phenethylammonium lead bromide perovskite thin films are investigated and the first vacuum-deposited organometallic halide perovskite light-emission devices (PeLEDs) are demonstrated. Exceptionally low refractive indices and extinction coefficients in the emission wavelength range are obtained in these films, which contributed to high light out-coupling efficiency of the PeLEDs. By utilizing these perovskite thin films as emission layers, the vacuum-deposited PeLEDs exhibit a very narrow saturated green electroluminescence at 531 nm, with a spectral full width at half-maximum bandwidth of 18.6

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nm, a promising brightness of up to 6200 cd/m2, a current efficiency of 1.3 cd/A, and an external quantum efficiency of 0.36%.

1. INTRODUCTION Recently, organometallic halide perovskites have gained much attention owing to their unique optoelectronic properties, low-temperature fabrication processes, and in particular high powerconversion efficiencies in solar and dim light energy harvesting.1–4 In additional to photovoltaic devices, organometallic halide perovskites show a promising bright-light emission with a quantum dot format or a thin-film layer configuration. Light-emission devices (LEDs) utilizing these perovskite quantum dots or thin films as the emission layer (EML) have demonstrated some distinctive characteristics such as very narrow emission spectral widths and wavelength tunability across the ultraviolet (UV) to near-infrared spectrum ranges, with high external quantum efficiencies (EQEs) of up to 11.7%.5–9 These exciting developments make the perovskite LEDs (PeLEDs) an intriguing next-generation light-emission approach after the successful introduction of organic light-emission devices (OLEDs). Looking back at the history of OLEDs, both wet solution processes and dry vacuum deposition have been heavily investigated, with vacuum sublimation being the first wave of the mass-production techniques.10 The high batch-to-batch reproducibility and mature fabrication facilities owing to the well-developed semiconductor industry all contribute to the fast development of vacuum-processed OLEDs. However, after the first demonstration of efficient room-temperature-emissive PeLEDs in 2014,11 most of the PeLEDs have been solutionprocessed so far.5,7,8,12–19 The delay in the demonstration of vacuum-processed PeLEDs may be related to the difficulties in handling small-molecular-weight organic halides in the vacuum


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process. Recently, our group developed a method of “sequential vacuum deposition” to fabricate smooth perovskite thin films and efficient perovskite solar cells.20 This method can largely avoid the issue of contamination by organic halides in the vacuum chambers and in the same time gain a great control of the perovskite formation in a high vacuum. In this paper, we report, for the first time, the fabrication of efficient vacuum-deposited organometallic halide PeLEDs. Sequential vacuum deposition was utilized to fabricate the perovskite EMLs. Smooth and bright EMLs were formed by incorporating both methylammonium bromide (MABr) and phenethylammonium bromide (C6H5CH2CH2NH3Br, PEABr) in the process. With the mirror-like, reflective smooth morphology of the perovskite thin films, the optical constants were extracted by utilizing variable-angle spectroscopic ellipsometry (VASE). Exceptional low refractive indices and extinction coefficients were obtained. These properties are highly desired for maximizing the out-coupling efficiency of the devices. The PeLEDs showed a narrow saturated green electroluminescence (EL) band at 531 nm with a spectral full-width at half-maximum (FWHM) bandwidth of 18.6 nm; they also exhibited a promising brightness of up to 6200 cd/m2 and a current efficiency of 1.3 cd/A. Before utilizing sequential vacuum deposition to form the EMLs in the PeLEDs, the vacuum co-evaporation of cesium-based halides and lead-based halides was evaluated. However, even though efficient inorganic CsPbI3 and CsPbI2Br perovskite solar cells can be obtained with this vacuum co-sublimation method, bright photoluminescence (PL) was not observed in the coevaporated CsPbBr3 thin films.21 We then turned to the sequential vacuum deposition of organic halides instead of cesium halides, and the formation of a perovskite film was realized by the reaction of the first vacuum-deposited lead halide thin film with the organic halide in the vacuum. Compared with the very recent-reported PeLEDs fabricated by the chemical vapor


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deposition (CVD) method,22 which is also a solvent-free dry vacuum process, the LEDs made with vacuum deposition in this report exhibiting 23-fold higher EQE and 11-times higher brightness, showing a promising potential of our work.

2. EXPERIMENTAL SECTION 2.1 Materials Lead bromide (PbBr2) was purchased from Alfa Aesar. The organic ammonium bromides, MABr and PEABr were purchased from Dyesol. 2.2 Device and Thin-Film Preparation The glass substrates with a 145 nm coating of indium–tin oxide (ITO) were cleaned as follows. First, the substrates were brushed with detergent using cotton swabs to roughly remove the contaminant; they were then immersed in a sequence of ultrasonic baths containing detergent, deionized water, acetone, and methanol for 20 min each. The substrates were then transferred to beakers containing boiled acetone and methanol sequentially for 20 min each. Before the coating process, the substrates were treated by an UV–ozone cleaner to further remove the organic residues and improve the hydrophilicity. For PeLED fabrication, poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Clevios AI4083) was spin-coated onto substrates at a spinning speed of 5000 rpm for 30 s and then baked at 135 °C for 30 min in air to remove any residual solvent. After the spin-coating process, the samples were transferred to a vacuum chamber with a base pressure of ~2 × 10−6 Torr to deposit PbBr2 for the sequential vacuum deposition of the perovskites.20 The thickness of PbBr2 was precisely monitored by ellipsometry-calibrated quartz-crystal microbalance sensors in the vacuum chamber. Then, the samples were transferred to a separate chamber with a base pressure of ~4 × 10−6 Torr for the


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perovskite formation. MABr and PEABr were carefully weighed to obtain various MABr/PEABr weight ratios (1:0, 1:0.25, and 1:0.5) and pre-mixed in one crucible and loaded into the chamber. The MABr and PEABr evaporation process was controlled to remain at 130 °C for both sources and substrates, and the evaporation duration was ~1.5 h. The partial pressure of the organic halide vapors were maintained at ~4 × 10−5 Torr with a controllable gate valve23. After perovskite formation, the samples were then transferred back to the previous vacuum chamber for the deposition of the organic material, salt, and metal, 2,2,2"-(1,3,5-benzinetriyl)-tris(1phenyl-1-H-benzimidazole) (TPBi), 8-hydroxyquinolatolithium (Liq), and aluminum (Al) were used as the electron-transporting layer, electron-injecting layer, and cathode, respectively. The device structure was configured as follows: glass/ITO (145 nm)/PEDOT:PSS/perovskite/TPBi (60 nm)/Liq (2 nm)/Al (150 nm). 2.3 Characterization The absorption spectra were obtained using an UV–visible spectrophotometer (UV-2600, Shimadzu Corp.) with an integrating sphere. The Fourier-transform infrared spectroscopy (FTIR) spectra were taken by a Bruker FTIR Spectrometer. The surface morphology of the perovskite films was analyzed from images obtained with a field-emission scanning electron microscope (FE-SEM, SU8010, Hitachi). X-ray diffraction patterns were obtained using a desktop diffractometer (D2 phaser, Bruker) with CuKα radiation. The PL of perovskite films was detected in a front-facing configuration by using a charge-coupled device (CCD) camera (PIXIS 256BR, Princeton Instruments) and a diode laser with wavelength of 375 nm (LDH-P-C-375M, PicoQuant) as a pumping source. The transient PL of the perovskite films was measured by the time-correlated single photon counting (TCSPC) method with a 375 nm diode laser (LDH-P-C375M, PicoQuant) as the pumping source. The PL spectra were recorded at a room temperature


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of 24 °C and a relative humidity of 60%. PL images of the perovskite films were taken under the illumination of a UV flashlight (λ = 365 nm). Current density–voltage–luminance (J–V–L) characterization of the PeLEDs was performed using a sourcemeter (2636A, Keithley Instruments, Inc.) and a silicon photodetector calibrated using a colorimeter (PR-650, Photo Research, Inc.). The EL spectra of the devices were obtained using a spectrometer (Flame, Ocean Optics, Inc.). For the ellipsometry measurements of the vacuum-deposited perovskite thin films, the films were fabricated on fused silica substrates. Using a variable-angle spectroscopic ellipsometer (VVASE, J. A. Woollam Co.), ellipsometry measurements over the wavelength range of 300–1100 nm were performed in air in steps of 10 nm. The angle of light incidence was varied between 55° and 75° relative to the surface normal in steps of 10° for reflection ellipsometry and between 40° and 60° relative to the surface normal in steps of 10° for transmission ellipsometry. The analysis of the ellipsometric data was performed using commercial software (WVASE32, J. A. Woollam Co.). We first determined the thickness of a perovskite thin film by assuming n obeys the Cauchy equation and k = 0 in long-wavelength region (800–1100 nm). With the obtained perovskite thickness, the wavelength-dependent n and k could be obtained by fitting the ellipsometric data (Ψ and ∆) across the entire spectral range. The wavelength-dependent n and k were then treated as initial values to construct a Kramers–Kronig consistent oscillator model. Four Gaussian oscillators were included in the model to describe the absorption of the perovskite thin film. To confirm the Kramers–Kronig consistency, the oscillator model was fitted to measured Ψ and ∆ data. The optical simulation was performed by commercial OLED software (Setfos, FLUXiM) with ellipsometer-measured wavelength-dependent optical constants of all the layers as the input parameters. The mode distribution calculations were carried out at 531 nm.


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3. RESULTS AND DISCUSSION The fabrication procedure is illustrated in Figure 1, perovskite films fabricated with different proportions of precursors were investigated. Figure 2a and b show the absorption spectra and PL spectra, respectively, of the perovskite films fabricated with various MABr/PEABr source ratios. The bright-field and UV-light excited PL appearance are also shown in the insets of Figure 2a and b, respectively. Interestingly, the films all exhibited an absorption peak at ~520 nm, which is the characteristic absorption peak of the MAPbBr3 perovskite.24–27 An additional absorption peak at 400 nm was observed, and it can be attributed to absorption by the (PEA)2PbBr4 perovskite.12,17,28 Only one green emissive peak appears around 530–533 nm in the PL spectra of samples with MABr/PEABr = 1:0 and 1:0.25, with a spectral FWHM bandwidth as low as 18.6 nm. On the other hand, two emission peaks at 408 nm and 530 nm are clearly visible in the spectrum of the film with MABr/PEABr = 1:0.5. Hence, as shown in the inset of Figure 2b, dim emission and very bright green emission were observed from the thin films with MABr/PEABr = 1:0 and 1:0.25, respectively, and the film with MABr/PEABr = 1:0.5 exhibited bluish green emission. Figure 2c shows the corresponding time-resolved PL evolution of these emissions. The green emission of the thin film with MABr/PEABr = 1:0.25 displayed the longest average lifetime of 9.68 ns, followed by the green emission of the thin film with MABr/PEABr = 1:0.5 (4.95 ns). In contrast, the perovskite film fabricated without the PEABr precursor (MABr/PEABr = 1:0) showed highly quenched PL and the shortest PL lifetime of 0.68 ns. The blue emission of (PEA)2PbBr4 perovskite also exhibited a short PL lifetime of 1.33 ns. The time-resolved PL time constants are listed in Table S1 in the Supporting Information. It is worth noting that the PL of the thin film with MABr/PEABr = 1:0.25 was very stable in an ambient atmosphere with relative


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humidity of 60%, as shown in Figure S1. Using an integrated sphere measurement system, the absolute PL quantum yield (PLQY) of the thin film with MABr/PEABr = 1:0.25 was measured to be ~2%. Figure 3a and b show the scanning electron microscope (SEM) images and X-ray diffraction (XRD) patterns, respectively, of the vacuum-deposited perovskite films. Patterned features and pin holes were found in the thin films with MABr/PEABr = 1:0 and 1:0.5, while the thin film with MABr/PEABr = 1:0.25 showed a smooth morphology. The corresponding AFM images are shown in Figure S2. The triangles and stars in Figure 3b mark the XRD diffraction peaks of MAPbBr3 and (PEA)2PbBr4 perovskites, respectively.12,17,29,30 These results indicate that only MAPbBr3 perovskite diffraction characteristics were exhibited by the thin films with MABr/PEABr = 1:0 and 1:0.25, which echo the observation of only green MAPbBr3 emission from these two samples. The absence of PbBr2 characteristic peaks in all the films implies a fully transformation of the perovskite films under all of the fabrication conditions employed in this study.31 The diffraction intensity from the MAPbBr3(100) plane was apparently higher in the thin film with MABr/PEABr = 1:0.25, indicating that the crystallinity of the sample was higher than that of thin films prepared with different source mixing ratios. Based on the XRD, PL, and absorption results of the sample with MABr/PEABr = 1:0.25, one may deduce that even though PEABr was utilized in the fabrication process, a negligible (PEA)2PbBr4 perovskite existed in the final products. However, since higher crystallinity was found in the thin film with MABr/PEABr = 1:0.25, any PEABr produced in the vacuum deposition process acted as a morphology modifier to promote the crystallinity of the MAPbBr3, and most of the PEA eventually left the sample after the process. Nevertheless, when an excess amount of the PEABr precursor (MABr/PEABr = 1:0.5) was used, (PEA)2PbBr4 perovskites co-existed with MAPbBr3 in the resulting thin film


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and clear XRD, PL and, absorption characteristics of both perovskites were simultaneously detected. The FTIR of the samples are shown in Figure S3, showing that only MABr/PEABr = 1:0.5 sample exhibited aromatic C=C bond features, which further proves the above-mentioned hypothesis. Hence, we propose possible reactions as following: PbBr2 + MABr → MAPbBr3

(1)

PbBr2 + 2 PEABr → (PEA)2PbBr4

(2)

(PEA)2PbBr4 + MABr →(←) MAPbBr3 + 2 PEABr

(3)

The final reaction was separately confirmed by presenting a (PEA)2PbBr4 film in the MABr gas atmosphere and the film gradually changed the color from transparency to light yellow and showed the MAPbBr3 absorption characteristics (see Figure S4). VASE, a precise and non-destructive technique, was used to further investigate the optical properties of the perovskite films. In order to acquire more optical information from the thin films, both reflection and transmission ellipsometric data were obtained.32 Figure 4a and b show the measured and fitted ellipsometric data of a vacuum-deposited perovskite (MABr/PEABr = 1:0.25) thin film on a fused silica substrate, and good agreement was found between the experimental and the fitted data. The extracted wavelength-dependent complex optical constants—refractive indices (n) and extinction coefficients (k)—are shown in Figure 4c. The perovskite thin film possessed exceptionally low n values of approximately 1.6–1.8 in the entire visible-wavelength range. The k spectrum of the film exhibits a shoulder at 520 nm with a low value of 0.13 and increases monotonously with the decreasing wavelength. The zero values of k in the long-wavelength region (550–1100 nm) consist of the measured absorption data (Figure 2a). A comparison of the n and k values of perovskites fabricated in a solution process is shown in Figure 4d. Obviously, the perovskite thin films fabricated by sequential vacuum deposition


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exhibited a much lower n value of ~1.8 at the emission wavelength than the solution-processed thin film (n = 2.2) and single-crystalline perovskite (n = 2.5).33,34 This relatively low n value of the perovskite thin film offers a great advantage for the device light extracting. The effects of n value on a PeLED were studied by calculating the mode distributions of the PeLED. The device structure for the optical simulation was configured as follows (Figure S5a): air/glass (0.7 mm)/ITO (145 nm)/PEDOT:PSS (40 nm)/perovskite (50 nm)/TPBi (60 nm)/Liq (2 nm)/Al (150 nm). As shown in Figure S5b, the n values (ranging from 1.6 to 2.5) of the perovskite thin film were set as simulation parameters. As the n value was increased from 1.6 to 2.5, the simulated out-coupling efficiency (air modes) of the PeLEDs correspondingly decreased from 25% to 7%. The low out-coupling efficiency of the device with a high-n perovskite EML was due to the high trapped waveguiding modes that originated from the photons remaining in the high-n media. These results indicate that the relative high n of the solution-processed perovskite EML compared with the typical value of the organic EML (n ~1.8) can be an intrinsic disadvantage of the PeLEDs over OLEDs, and this can be avoided by using lower-n vacuumdeposited perovskite EMLs. The star symbol in Figure S5b indicates the values of n and outcoupling efficiency of the PeLEDs in this study. The plot of power dissipation as a function of normalized in-plane wave vector for different n values is also shown in Figure S5c, and the area under the curve in the air mode region indicates the amount of light that could be extracted to air. As shown in Figure S5c, the decreasing dissipated power in the air mode region reflects the decline in out-coupling efficiency with increasing n values of the EML. An out-coupling efficiency of 20% is expected, and with the 2% value of the thin film PLQY, one can estimate that the device would have an EQE of 20% × 2% = 0.4%, which is in a good agreement with the experimental EQE of 0.36% (vide infra).


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PeLEDs were then fabricated from vacuum-deposited perovskite films, and the results are shown in Figure 5. The device structure is shown in Figure 5a. ITO/PEDOT:PSS was used as the anode, and vacuum-deposited TPBi, Liq, and Al were utilized as the electron-transporting layer, electron-injecting layer, and cathode, respectively. The device with MABr/PEABr = 1:0.25 clear showed superior performance compared with that of the devices with MABr/PEABr = 1:0 and 1:0.5 PeLEDs. The PeLED with MABr/PEABr = 1:0.25 exhibited a high maximum brightness of 6200 cd/m2 with low turn-on voltage of 3.6 V (Figure 5b). A photo of an operating PeLED with MABr/PEABr = 1:0.25 is shown in the inset of Figure 5b. The EL spectra of the PeLEDs are shown in Figure 5c. The devices emitted narrow emission spectra with FWHM = 18.6 nm at the peak wavelength of 531 nm, and the EL color corresponds to the coordinate (0.20, 0.75) in the Commission Internationale d'Eclairage (CIE) 1931 color space, as indicated in the inset of Figure 5c. This saturated green color is desired for displays with high color gamut. The standard red, green, blue (sRGB) and Digital Cinema Initiatives-P3 (DCI-P3) color spaces are displayed in the figure. The color of the green PeLED was much more saturated than the sRGB and DCI-P3 green standards because of its narrow emission spectrum, and it even approaches the ITU-R Recommendation BT.2020 (Rec.2020) standard, which currently can only be achieved by a monochromatic green laser. A comparison of the PL, EL and optical simulated EL spectra is shown in Figure S6. The spectra all show a similar narrow FWHM bandwidth indicating that almost no microcavity effect was observed in the devices, and the narrow bandwidth of the EL originated from the already very narrow PL bandwidth. The FWHM = 18.6 nm of the EL and PL emission is apparently narrower than that of previously reported solution-processed green emission perovskite films and quantum dots (~20 nm).35,36 The EQE–J characteristics are shown in Figure 5d. The device with MABr/PEABr = 1:0.25 showed a much higher maximum EQE of


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0.36% and current efficiency of 1.3 cd/A than those of the devices with source ratios of 1:0 and 1:0.5 (~0.00061%, ~0.002 cd/A and ~0.0059%, ~0.02 cd/A, respectively). Details of the device characteristics are shown in Figure S7. The device efficiency was largely limited by the low PLQY of 2%. This is only the first demonstration of vacuum-deposited PeLEDs. We believe that by fine tuning the vacuum processes and the selection of the morphology modifier (beyond PEABr), the performance of PeLEDs can be further improved, so that it may eventually catch up with their efficiency of the solution-processed counterparts.

4. CONCLUSIONS Proof-of-concept sequential vacuum-deposition of perovskite light emission thin films were demonstrated. By incorporating of the PEABr precursor in the perovskite formation process, bright and saturated green-light-emitting thin films with smooth surface morphology were obtained. The PeLEDs utilizing these thin films showed promising highly saturated green EL with EQE of 0.36% and maximum brightness up to 6200 cd/m2. This is just the first step in the development of vacuum-deposited PeLEDs. By demonstrating the vacuum processability of PeLEDs, together with the unique emission prosperities of the perovskites, such as very narrow EL spectrum and wide color tunability, we have shown that vacuum-deposited PeLEDs can be one of the most promising future light-emission technologies after the current success of vacuum-deposited OLEDs.


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FIGURES

Figure 1. Fabrication of perovskite films from ammonium salt precursors in different proportions.


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Figure 2. (a) Absorption spectra of perovskite films; the inset shows a bright-field image of perovskite films fabricated with different precursor ratios. (b) Photoluminescence (PL) spectra of the perovskite films; a photographs of the UV-light-excited films is shown in inset. (c) Timeresolved PL the perovskite films.


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Figure 3. (a) SEM images of perovskite films fabricated with different precursor ratios, the scale bars are 1 µm. (b) XRD patterns of perovskite films; the triangles and stars mark the peaks of MAPbBr3 and (PEA)2PbBr4 perovskites, respectively.


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Figure 4. Experimental (polygonal symbols) and fitted (solid lines) (a) Ψ and (b) ∆ of the fused silica/perovskite sample. The measured parameters and fitted data are in the transmission mode for incident angles of 40° (□), 50° (○), and 60° (∆) and in the reflection mode for incident angles of 55° (■), 65° (●), and 75° (▲). Refractive index (n) and extinction coefficient (k) of (c) vacuum-deposited perovskite thin films and (d) referenced crystalline and solution-processed thin films.


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Figure 5. (a) Device structure of perovskite light-emitting devices (PeLEDs) examined in this study. (b) Luminance–voltage (L–V) characteristics of PeLEDs with emission layers fabricated from precursors in different proportions; the inset shows a photograph of an operating PeLED (MABr/PEABr = 1:0.25). (c) Electroluminescence (EL) spectra of the PeLEDs; the inset presents the corresponding CIE coordinate of the PeLED (MABr/PEABr = 1:0.25) EL. (d) External quantum efficiency vs. Current density (EQE–J) characteristics of the PeLEDs.


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ASSOCIATED CONTENT Figure S1—S7 and Table S1 are shown in supporting information (Word).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Hao-Wu Lin) ACKNOWLEDGMENT The authors would like to acknowledge the financial support received from the Ministry of Science and Technology of Taiwan.


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