Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe2 by Hydrohalic

Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe2by Hydrohalic Acid Treatment

Hau-Vei Han,?,?,?Ang-Yu Lu,?,?Li-Syuan Lu,§Jing-Kai Huang,?Henan Li,?Chang-Lung Hsu,∥Yung-Chang Lin,#Ming-Hui Chiu,?Kazu Suenaga,#Chih-Wei Chu,∥Hao-Chung Kuo,*,?

Wen-Hao Chang,§,⊥Lain-Jong Li,*,?and Yumeng Shi*,?

?Physical Sciences and Engineering Division,King Abdullah University of Science and Technology,Thuwal,23955-6900,Saudi Arabia ?Department of Photonics and Institute of Electro-Optical Engineering and§Department of Electrophysics,National Chiao Tung University,Hsinchu300,Taiwan

⊥Taiwan Consortium of Emergent Crystalline Materials(TCECM),Ministry of Science and Technology,Hsinchu300,Taiwan ∥Research Center for Applied Sciences,128Sec.2,Academia Road,Nankang,Taipei11529,Taiwan

#National Institute of Advanced Industrial Science and Technology(AIST),Tsukuba305-8560,Japan

*Supporting Information

defects(Se vacancies)and oxidized Se defects in CVD-grown

microscopy and X-ray photoelectron spectroscopy.These

excitons,leading to the smearing of free band-to-band exciton

treatment(such as HBr)is able to e?ciently suppress the trap-

emission in defective MoSe2monolayers through the p-doping

room temperature can be enhanced by a factor of30.We show

free exciton emissions even from highly defective MoSe2layers.

the n-doping in MoSe2but also reduces the structural defects.

tailoring the exciton emission from CVD-grown monolayer TMDCs.

diselenide,layered materials,photoluminescence,

tomically thin transition-metal dichalcogenides

(TMDCs)can be obtained by mechanical1,2or

chemical exfoliation3from their bulk crystals.Because of their unique and striking properties,monolayer TMDCs have attracted extensive attention.Semiconducting TMDC monolayers have been demonstrated to be feasible for various energy-related applications,where their electronic proper-ties1,4,5and high surface areas o?er opportunities for applications such as biosensors,6,7nanogenerators,8green electronics,9electrocatalytic hydrogen generation,10,11and energy storage.12,13Semiconducting TMDC monolayers,such as MoS2,14?16WS2,17?19MoSe2,20,21and WSe2,22?25exhibit direct band gaps and show attractive light emission properties in the visible and near-infrared spectral region,making them suitable for optoelectronic devices.26Applications based on TMDCs in light-emitting diodes27and photodetectors28,29have

been developed recently.Signi?cant e?orts have been devoted

to the control and tuning of photoluminescence(PL) properties in TMDCs.Several approaches including electro-

static gating,30chemical treatment,31?33surface plasmonic excitation,34photonic crystal cavities,35and strain engineer-

ing36,37have been adopted to modulate the trions and neutral

Received:November4,2015

Accepted:December30,2015

Article

excitons in monolayer TMDCs.CVD-grown TMDC layers could exhibit a unique PL property which is absent in their exfoliated TMDC counterparts.For example,extraordinary PL has been observed near the edge of CVD grown WS2 recently.17Edges and point defects in monolayer TMDCs formed during CVD could be engineered to tailor their electronic,optical and catalytic properties.38?42Interestingly, the structural defects in TMDCs have been reported to darken and blue shift the PL peak in MoS2and WS2.38Controversially, recent reports have also demonstrated that point defects could activate PL in TMDCs and increase the overall PL intensity.39 More experimental e?orts are needed to understand the point defects and excitonic characteristics in TMDC layers. Chemical doping is known as an e?ective way to modulate the optical and electrical properties of TMDCs.Matsuda et al. have reported that the PL intensity of mechanically exfoliated MoS2monolayer can be enhanced when it is interacted with a p-dopant2,3,5,6-tetra?uoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ),where the p-doing can switch the recombination of negative trions to the neutral exciton.32However,the PL intensity enhancement ratio is only around3with the F4TCNQ doping.32More recently,Amani et al.have reported that PL and minority carrier lifetime in exfoliated MoS2 monolayers can be uniformly enhanced by an organic super acid treatment.43However,the exact mechanism by which the passivation of surface defects is not fully understood. Furthermore,in the previous studies,the modulation of PL properties in TMDCs by surface chemistry and structural defect engineering have been mostly limited to mechanically exfoliated TMDCs.31,32,38,39Compared to the exfoliation method,CVD-grown TMDCs are more practical for large-scale device applications.44Since the CVD-grown TMDCs may exhibit a wide range of defect types and defect density levels, di?erent from the mechanically exfoliated TMDCs,the defect and optical emission relations are yet to be explored.

In this paper,we demonstrate that the PL intensity of monolayer MoSe2grown from CVD can be e?ectively enhanced after exposure to hydrohalic acid vapors,such as HCl,HBr,and HI.The overall PL intensity of MoSe2 dramatically increases more than30times by HBr treatment. Because of the structural defects that commonly exist in CVD grown TMDCs,neutral exciton and trion peaks are rarely observed in low-temperature PL measurement.We found the defect trapped exciton can be greatly suppressed by the HBr treatment which allow us to observe the neutral exciton and trion peaks from CVD grown MoSe2.Furthermore,our scanning transmission electron microscope(STEM)character-ization proves the presence of point defects in MoSe2 structures.Raman spectrum,PL,X-ray photoelectron spectro-scope(XPS)and STEM suggest that the HBr treatment imposes various e?ects which explain the PL enhancement such as removing impurities,p-doping to the MoSe2,and reducing the structure of MoSe2.

RESULTS AND DISCUSSION

Monolayer MoSe2was grown via the CVD process according to our previous reports,21where sulfur and MoO2powders were used as precursors.Figure1a illustrates the optical micrograph(OM)of the CVD MoSe2monolayers synthesized on sapphire substrates,where the monolayer MoSe2exhibits a triangular shape with the lateral size up to10μm.Here,we use hydrohalic acids as highly e?cient agents to tune the exciton PL in MoSe2monolayers grown by CVD.Figure1b schematically illustrates the experimental setup for the hydrohalic acid treatment.Aqueous solutions of HCl,HBr and HI were bubbled and brought to the sample by Ar gas,and the AgNO3was used to absorb the exhaust gas.Photo-luminescence from the as-grown and treated MoSe2mono-layers excited by a532nm laser are shown in Figure S1.Only one high intensity peak with photon energy around1.53eV was observed in both pristine and HBr treated MoSe2,which is close to the reported band gap of1.55eV for monolayer MoSe2.Figure1c compares the PL intensity spatial mappings for as-grown and treated MoSe2.As shown in Figure1d,e,PL signals are pronounced at the edge of the as-grown MoSe2and become very weak or absent toward the center of the as-grown monolayer MoSe2.A blue-shift was also observed from the center to the edge.Recently,a similar phenomenon was observed on CVD-grown WS2monolayer by Yu et al.,40where they suggest that the defects within the crystal act as nonradiative recombination sites and thus quench the intrinsic PL.Consistently,the edge enhanced PL emission has been observed in CVD grown WS2monolayer in previous studies, and the darkening of PL in the center of TMDCs island has been attributed to the charge defect-induced doping.17,45Our result shows that the overall PL intensity of MoSe2dramatically increases more than30times after HBr treatment,which suggests the PL tunability of defective monolayer MoSe2. Figures1d,e display the PL line scans for PL intensity and photon energy across the sample before and after HBr treatment,respectively.Remarkably,the PL intensity

increases Figure1.(a)Optical image of as-prepared CVD monolayer MoSe2 on sapphire substrates.(b)Schematic illustration of the experimental setup for hydrohalic treatments.(c)PL intensity mappings of an individual MoSe2?ake before and after HBr treatment.Pro?les in(d)and(e)show the PL intensity and photon energy modulation as a function of surface location along the solid line indicated in(c).

signi ?cantly and the photon energy blue shifts at the center of the triangle.For the spatial modulation of PL intensities and positions in MoSe 2,various factors are responsible,including external electrostatic doping,structural defects,and chemical composition change.The darkening of PL in the center of MoSe 2islands is attributable to the presence of structural defects such as point defects and dislocations within the metastable nuclei.38During the CVD growth procedure,the initial nucleation occurs at the beginning of the growth process followed by the incoming MoSe 2species coalescing into the nuclei leading to an enlarged MoSe 2grain.Therefore,the HBr molecules are more likely to adsorb at the nucleation center of MoSe 2and result in a more pronounced PL change.In addition to HBr,other hydrohalic acids such as HCl and HI also show similar PL enhancement e ?ects (see Figure S2).Among the three hydrohalic acids,HBr shows the best PL enhancement performance.In a previous report,46the light emission in mechanically exfoliated TMDC ?akes were shown to be sensitive to H 2O molecules.We have separately examined the e ?ect of moisture,and no signi ?cant PL enhancement is observed with only H 2O treatment (details in Figure S3).Therefore,the e ?ect of moisture can be excluded in this study.Furthermore,we have also excluded other possible gaseous adsorption e ?ects,such as O 2,by comparing the PL intensity change in vacuum for the sample before and after HBr treatment (details in Figure S4).All of the results consistently indicate HBr plays a major role in enhancing photo-luminescence in MoSe 2.In the following paragraphs,we discuss the HBr treatment e ?ect on the doping level,structural defects,and chemical composition and further reveal the mechanism of optical property modulation in MoSe 2.

Raman scattering is known to be sensitive to the doping level of 2D materials.We use Raman spectroscopy to investigate the charge ?phonon interaction in MoSe 2layers.For a monolayer MoSe 2only one Raman active mode (out-of-plane A 1g )appears.Figure 2a shows the typical Raman spectrum for the MoSe 2?akes before and after HBr treatment,where a

predominate A 1g mode at ～240.41/cm is identi ?ed for the as-grown MoSe 2.The out-of-plane vibrational A 1g mode becomes stronger and shifts to a higher frequency at 241.11/cm after HBr treatment.Figure 2b shows the statistical measurements for the Raman frequency,and Figure 2c shows the Raman intensity mappings for the as-grown and HBr treated MoSe 2monolayers.The A 1g mode results from the out-of-plane vibration of Se atoms in opposite directions,which couples more strongly with electrons than the in-plane vibrational mode does.Calculation based on ?rst-principles density functional theory shows that electron doping leads to occupation of the bottom of the conduction band at the K point states resulting in a signi ?cant change in the electron ?phonon coupling of A 1g .42It has also been demonstrated experimentally that n -doping typically results in the softening and intensity decrease of the A 1g phonon,while p -doping causes the hardening and intensity increase of A 1g phonon.31,47,48Therefore,the A 1g phonon renormalization can be used to estimate the doping level change in TMDCs.The intensity increase and frequency upshift in A 1g mode suggest that HBr treatment imposes p -doping to MoSe 2layers.49We provide quantitative analysis of the doping level (details in the Supporting Information ,Figure S5).The electron carrier densities are estimated to be 8.2×1011cm ?2for as-grown and 6.37×1010cm ?2for HBr-treated MoSe 2,respectively,which are within a comparable range with the previous report.30To further reveal the excitonic nature of MoSe 2monolayer and the e ?ect after HBr treatment,we performed temperature-dependence PL measurements down to 10K for the as-grown and HBr-treated MoSe 2layers.Figure 3a compares the PL emission from as-grown and HBr-treated MoSe 2at 10K.The neutral exciton of the MoSe 2monolayer has been reported to emit at around 1.66eV.30Figure 3a shows a strong emission at a lower energy (around 1.56eV)from our as-grown MoSe 2with a full width at half-maximum (fwhm)value of 80.6meV.The peak at 1.56eV indicates that the radiative excitons are bound to defects,which is in good agreement with the previous report 39since the defect-related,trapped excitons should lead to an emission energy lower than the band-to-band optical transition energy.After HBr treatment,the defect-related PL emission MoSe 2is dramatically suppressed as revealed in Figure 3a.In addition to the broad subband gap emission at around 1.59eV,we observed two additional peaks at 1.66and 1.63eV,assigned as the neutral exciton (X 0)and the trion peak (X ?),respectively.30Figure 3b shows the evolution of neutral exciton and trion peaks as a function of temperature from 10to 300K.The trion peak becomes negligible when the temperature is higher than 150K.Parts c and d of Figure 3show the temperature dependence of the X 0and X ?peak position and intensity extracted on the basis of the same ?tting method in Figure 3a.In addition,a binding energy of 30meV is extracted,which is in good agreement with previous reports.30The temperature dependence of the peak intensity ratio (X ?/X 0)for MoSe 2monolayers shown in Figure 3d suggests that our MoSe 2sample exhibits a higher trion to exciton ratio.The appearance of X 0emission suggests that the highly n-doping feature in pristine MoSe 2has been reduced considerably after HBr treatments.30The temperature-dependence measurement of MoSe 2PL suggests the defects within the as-grown MoSe 2crystals prohibit the intrinsic exciton emission and the dominate PL peak is mostly from trapped exciton states,while for the HBr-treated MoSe 2the trapped exciton state

is

Figure 2.(a)Raman spectra and (b)statistical analysis of Raman peak energy for the as-grown and HBr-treated MoSe 2.(c)Raman intensity maps of a monolayer MoSe 2?ake before and after HBr treatment.

greatly suppressed and both exiton and trion peaks are detectable at a low temperature.

Both theoretical 50,51and experimental 52,53works have demonstrated that the defects such as cation and anion vacancies in TMDCs could induce doping.In order to reveal the as-grown and treated MoSe 2microscopically,MoSe 2monolayers are transferred onto a transmission electron microscope (TEM)grid and visualized under TEM and STEM.The transfer process unavoidably induces the folding

of MoSe 2(top area in Figure 4a).Figure 4b displays the high-resolution TEM (HRTEM)image.The inset in Figure 4b corresponds to the fast Fourier transform (FFT)of the image,which clearly shows the hexagonal packing of MoSe 2crystals.In addition to the TEM characterization,a more precise lattice structure measurement was carried out using aberration-corrected STEM.We further visualized the ?ake via chemical analysis using atomic-resolution Z-contrast imaging with high-angle annular dark ?eld (HAADF)STEM.As the intensity of STEM images is directly related to the atomic number (Z-contrast),individual Se atoms can be easily recognized and di ?erentiated from the 3-fold coordinated two Se atoms.Parts c ancd d of Figure 4are the STEM images of MoSe 2before and after HBr treatment.By analyzing the column-to-column intensity ratio of metal and chalcogen sites in Figure 4c,d,the lattice structures have been reconstructed and are shown in Figure 4e,f.The STEM characterization identi ?es the presence of a large number of single Se vacancies,and we also observe Se point defects in the as-grown MoSe 2layer (as shown in Figure 4c,e,right corner).It was found that the individual Se vacancies are stable under TEM.Meanwhile,we note there are no Mo vacancies observed in the examined locations for both of as-grown and HBr treated samples.The presence of Se vacancies contributes to the n -doping and PL weakening in MoSe 2.We can draw the conclusion that HBr treatment can e ?ectively tailor the localized exciton in MoSe 2defect sites structured by Se vacancies and release the defect bound excitons.Qualitatively,we note that the Se vacancies in HBr-treated samples are relatively less than the as-grown MoSe 2monolayers.

To obtain more insights of HBr treatment on the lattice structure of MoSe 2,XPS was used to further reveal the chemical composition change of CVD grown MoSe 2.As shown in Figure 5a,the binding energies of Mo 3d 5/2and Mo 3d 3/2are around 229.4and 232.53eV,respectively,revealing the +IV oxidation chemical state of Mo.The small shoulder peaks at around 233.3and 236.43eV in Figure 5a can be assigned to Mo 3d 5/2and 3d 3/2core levels of the hexavalent state of Mo,54originated from not fully selenized MoO 3precursors used in CVD growth.We note the hexavalent state becomes barely detectable after HBr treatment,suggesting HBr treatment can remove these impurities.It is noteworthy that after HBr treatment there is

a

Figure 3.(a)Photoluminescence of the as-grown and HBr-treated monolayer MoSe 2at 10K.(b)Temperature dependence of PL for the MoSe 2after HBr treatment.(c)Trion and exciton peak energies.(d)Intensity of trion to exciton peak as a function of

temperature.

Figure 4.(a)Bright ?eld low magni ?cation TEM image of HBr treated monolayer MoSe 2.(b)High-resolution TEM image of HBr treated MoSe 2(inset:corresponding FFT pattern).(c,d)STEM images for pristine and HBr-treated MoSe 2.(e,f)Atomic model showing the surface structure of MoSe 2in (c)and (d).

red shift for the peak positions of Mo (IV)3d 5/2and 3d 3/2from 229.6to 229.4eV,which could suggest a reduction of Mo (IV)oxidation sate or a p -doping,55consistent with the conclusion drawn from Raman measurements.Additionally,the full width at half-maximum (fwhm)of Mo (IV)also decreases from 1.53to 1.06eV,indicating a more uniform chemical environment for Mo after the HBr treatment.In Figure 5b,the Se 3d 5/2and 3d 3/2doublets at (54.8eV;55.66eV)and (55.34eV;56.2eV)can be assigned to terminal Se 2?and bridging Se 22?,respectively,56and these are the two types of covalent bonds observed in both as-grown and HBr treated MoSe 2layers.The binding energy of 3d 5/2core levels at 55.9eV is ascribed to elementary Se (0)impurities originated from Se element or the oxidized MoSe 2(Mo ?Se ?O)state.The broad peak at around 59.2eV is attributed to the oxidized Se species such as the SeO 2impurity.57However,these peaks are observable in as-grown but not in HBr-treated MoSe 2layers.It is known that HBr is able to remove the oxygen in SeO 2through the formation of SeBr 4.58We suspect that HBr might be able to remove the oxygen species in oxidized MoSe 2,where the Mo ?Se ?O could

be transformed to Mo ?Se ?Br.This postulation agrees well with the observation of fwhm narrowing for Mo (IV)peak after HBr treatment.The binding energy evolution for Mo,Se,and Br is summarized in Table R3(Supporting Information ).Note that the Br ?ions are detected at around 68.6eV (Br 3d 5/2doublet)after HBr treatment as shown in Figure 5c,which con ?rms the reaction between MoSe 2and HBr.Note that in STEM we could not identify the Br atom since Br has an atomic number similar to that of Se (35and 34,respectively).However,we do observe the presence of Br in the XPS and energy-?ltered electron energy-loss spectrum (EELS)elemen-tary mapping (shown in Figure S6).This suggests that Br ions adsorb on MoSe 2as counterions.Since Se and Br are not distinguishable in STEM,we cannot exclude the possibility that Br ions ?ll in the Se vacancies as illustrated in Figure 5d.To further strengthen the arguments of structural repairing by HBr,we calculate the elemental ratio of Mo,Se,and Br based on their corresponding XPS peak areas.The atomic ratio of anions versus Mo (IV)is shown in Figure 5e.The ratio of as-grown MoSe 2is 1.686,suggesting the presence of the

high

Figure 5.(a ?c)XPS scans of the Mo,Se,and Br binding energies for pristine (upper)and HBr-treated MoSe 2(lower),respectively.(d)Proposed chemical structure change showing the e ?ects of HBr treatment.(e)Atomic ratio versus Mo (IV)for as-grown and HBr-treated MoSe 2.

density of Se vacancies and Se(0)and SeO2impurities. Remarkably,after HBr treatment,the anion to Mo atomic ratio increased to1.97;this increase is majorly caused by the incorporation of additional Br anions and the increase in Se2?, where the increase is likely from the conversion of the bridging Se22?to the Se2?.We also note the SeO2becomes undetectable,but with a dramatically increased percentage of Se2?.Revealed by our STEM characterization,the lattice structure defects are mainly the Se vacancies.In brief,all of the XPS results suggest that HBr not only p-dopes but also removes the impurities Se(0)and SeO2.Meanwhile,the HBr treatment most likely reduces the Se vacancies by?lling Se vacancies using the Se in the bridging form Se22?to Se2?.The passivation of Se vacancies with Br ions might also occur at the same time.

CONCLUSIONS

In conclusion,point defects formed by Se vacancies can greatly quench the PL of monolayer MoSe2due to the trapping of free charge carriers and nonradiative recombination.A low-temper-ature PL study shows that HBr can e?ectively suppress the trapped exciton states and populate both exciton and trion emission.Other hydrohalic acids such as HCl and HI also show similar PL enhancement e?ects.However,HBr is the most e?ective chemical with a suitable acidity and thus provides better controllability for tailoring the optical properties of MoSe2.The drastic modulation of optical properties by HBr can be attributed to various reasons,including removing impurities,p-doping to the MoSe2,and reducing the structure of MoSe2.Our method o?ers the insights for tuning the properties of2D TMDCs.

METHODS

Growth of MoSe2Layers.We have developed the CVD method to synthesize monolayer MoSe2.The precursor of Mo and Se are MoO2powder(Sigma-Aldrich,≥99.5%purity)and Se powder (Sigma-Aldrich,≥99.5%purity),respectively.MoO2powder(0.3g) was placed in a ceramic boat at the center of furnace with a1cm×5 cm sapphire substrate placed at downstream of ceramic boat.Selenium powder was heated by another furnace and carried by Ar/H2(Ar/H2= 65sccm/5sccm,50Torr).The center of the furnace was gradually heated from room temperature to800°C at the rate of25°C/min. The temperature of the Se powder at the upstream furnace was raised up to260°C when the growth furnace temperature reached800°C. In order to growing MoSe2triangle domains,the furnace was held at 800°C for15min.After growth,the furnace was naturally cooled down to room temperature.

Chemical Treatment of MoSe2Layers.After getting the MoSe2 triangle domains on sapphire substrate,we used hydrobromic acid (Alfa Aesar,49%liquid)or other hydrohalic acids as a chemical source. The bubble method was performed to spray the HBr droplets onto the sapphire substrate where the MoSe2domains were prepared.After the evaporation of chemical solvent,the MoSe2triangle domains were characterized by optical measurements.

Transfer Method for Pristine and HBr-Treated MoSe2.The TMDCs layers were transferred onto target substrates or TEM grids by a poly(methyl methacrylate)(PMMA)(950PMMA A4,Micro Chem)assisted method.First,PMMA thin?lm was spin-coated on top of the sample,and then the PMMA/MoSe2/sapphire was dipped in a6M HF solution to etch the MoSe2/sapphire interface.PMMA/ MoSe2was lifted from the etching solution,diluted in DI water,and then transferred onto target substrates.The PMMA layer was removed with acetone and2-propanol.

Characterizations.The aberration-corrected JEOL-2100F cold ?eld emission gun electron microscope equipped with a DELTA corrector was used to observe the annular dark?eld(ADF)images in this work.The ADF images were obtained at an accelerating voltage of 60kV at room temperature.The probe current we used to detect was about10?15pA.The scanning rate of the ADF images was set in the region of8to128μs/pixel.Photoluminescence spectra and Raman spectra were excited by a green laser with532nm wavelength in a Witec alpha300confocal Raman microscopic system.The spot size of laser beam is about0.5μm,and a0.9N.A.of100X objective from Carl Zeiss Microscopy GmbH was used to collected the emitted Raman (1800lines/mm grating)and PL signal(300lines/mm grating).A higher grating was used to obtain more details Raman line shapes.The Raman wavelength calibration used the Si Raman peak,which is at520 cm?1to be a reference.The sample was cooled to T=4K in a low-vibration cryogen-free cryostat for the low-temperature PL measure-ment.A0.42N.A.objective lens was used in the4K low-temperature measurement because it has a long working distance.Chemical con?gurations were determined by X-ray photoelectron spectroscopy (XPS,Phi V5000).XPS measurements were performed with an Al KαX-ray source on the samples.The energy calibrations were made against the C1s peak to eliminate the charging of the sample during analysis.

ASSOCIATED CONTENT

*Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acsnano.5b06960.

Figures S1?S6(PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail:hckuo@https://www.wendangku.net/doc/8711172479.html,.tw.

*E-mail:lance.li@https://www.wendangku.net/doc/8711172479.html,.sa.

*E-mail:yumeng.shi@https://www.wendangku.net/doc/8711172479.html,.sa.

Author Contributions

?These authors(H.-V.H.and A.-Y.L.)contributed equally. Notes

The authors declare no competing?nancial interest.

ACKNOWLEDGMENTS

L.-J.L.and Y.S.acknowledge support from King Abdullah University of Science and Technology(Saudi Arabia).L.-J.L. acknowledges the Ministry of Science and Technology (MOST)and Taiwan Consortium of Emergent Crystalline Materials(TCECM),Academia Sinica(Taiwan),and AOARD-134137(USA).H.-C.K.acknowledges support from MOST of Taiwan under Grant No.NSC104-3113-E-009-002-CC2.Y.-C.L.and K.S.acknowledge support from the Japan Science and Technology Agency research acceleration program.W.-H.C. acknowledges support from TCECM,MOST of Taiwan under Grant No.NSC102-2119-M-009-002-MY3,and the Center for Interdisciplinary Science of Nation Chiao Tung University. REFERENCES

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