Layer-Number Dependent Excitons Recombination Behaviors of

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C: Physical Processes in Nanomaterials and Nanostructures

Layer-Number Dependent Excitons Recombination Behaviors of MoS2 Determined by FLIM Ting Wang, Yirui Zhang, Yuanshuang Liu, Junyi Li, Dameng Liu, Jianbin Luo, and Kai Ge J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02393 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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The Journal of Physical Chemistry

Layer-Number Dependent Excitons Recombination Behaviors of MoS2 Determined by FLIM Ting Wang1, Yirui Zhang1,2, Yuanshuang Liu1, Junyi Li1, Dameng Liu1* and Jianbin Luo1*, Kai Ge3 1

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, People’s

Republic of China 2

Department of Mechanical Engineering, Massachusetts Institute of Technology,

Massachusetts 02139, USA 3

China Petroleum Technology And Development Corporation, Beijing 100028,

People’s Republic of China

E-mail: [email protected] and [email protected]

1

ACS Paragon Plus Environment

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Abstract: The fluorescence lifetime imaging microscopy (FLIM) technique is utilized to probe the photoluminescence properties of individual flakes MoS2. This measurement allows identification of the layer number of the flakes: two fluorescence decay lifetimes (τ1 and τ2) exhibit linear relationships with layer-number. Our investigation of the fluorescence lifetime reveals exciton dynamics in monolayer and multilayers MoS2. We find the distinct difference on the decay rates between A exciton (fast) and B exciton (slow). K′/ Г emission has different decay behaviors with respect to the layer number (N) because of its variable energy in monolayer and multilayer samples. The interplay of these transition channels also plays an important impact on the overall decay. Our results demonstrate that FLIM is an effective measurement for studying the luminescence properties of TMDs.

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Introduction Transition metal dichalcogenides (TMDs) has attracted increasing attention as an exciting class of two-dimensional materials for electronics, optics and optoelectronics， as well as for dry lubrication1-4. TMDs consists of stacked structure with hexagonal layers of metal atoms sandwiched between two hexagonal layers of chalcogen atoms. By stoichiometry, these TMDs can be recorded as MX2, where M is metal atom and X is chalcogen atom. In 1923, the crystal structure of MoS2 was first determined by Dickinson and Pauling5, which has recently emerged as a promising candidate for the semiconducting analogue of graphene6. Based on the known study, its properties may be changed remarkably as the thickness is reduced down to single layer or only a few layers

7-16

. The emergence of photoluminescence at room temperature in ultrathin

layers of MoS2 has been discussed by many reports.9-11,

17-21

Unlike the III-V 17, 19, 22

semiconductors, the optical properties are dominated by excitons

with strong

bonding energies, which are resulted from the strong Coulomb interaction between electrons and holes. Several recent time-resolved photoluminescence (TRPL) studies for MoS2 point out the excitons recombination dynamics8,

22-24

. In this work, we

explored another effective measurement--florescence lifetime imaging microscopy (FLIM)--to examine the luminescence properties of single- and few-layer MoS2. We find two representative excitons recombination lifetimes (the short component τ1 and the long component τ2) through analyzing the FLIM results, which shows linear relationship with layer number (N). Based on fitting the PL spectrum by bi-Gaussian method, the origination of τ1 and τ2 can be determined under separating the monitored 3



particular wavelength ranges. The short component τ1 and the long component τ2 control the short-lived and long-lived part of the overall configuration, respectively. These results also suggest different channels for excitons recombination in the monolayer and multi-layers MoS2. In addition to the main bright A-exciton, the intravalley (B-exciton) and the dark intervalley transition (K′/ Г exciton) also have important impacts on the emission decay. Our findings also reveal that FLIM is an alternative tool to identify the layer number of the flakes.

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Experimental Section Ultrathin MoS2 layers were prepared by mechanical cleavage technique. Scotch tape was used to exfoliate mono- and few-layer MoS2 flakes from the bulk crystals (SPI Supplies) and then the thin MoS2 were deposited onto the gel film and mica substrates. The layer number (N) of MoS2 was firstly identified by the frequency difference of two high-frequency Raman modes. Here, a Raman Jobin-Yvon HR800 system was applied for the Raman scattering measurement. The Raman are performed using a solid-state laser operating at a wavelength of 532 nm and a spectral resolution of ∼0.6 cm−1, while the PL measurements are at a excitation of 488 nm. To avoid laser-induced sample heating, we used a low laser power of ∼0.15 mW. Scanning photoluminescence lifetime measurements were performed using a laser

scanning

confocal

microscope

(IX

83,

Olympus),

equipped

with

photoluminescence lifetime measurement mode (Picoquant). The samples were excited using a 485 nm pulsed laser diode through a 40× objective (N.A. = 0.95, UPLSAPO). Long-passed filter (Semrock, 561nm) was employed to choose the wavelength range from 561 nm to 700 nm for fluorescence detection. Band-passed filters (Semrock) were employed to choose two different passing ranges from 585 nm to 645 nm and from 662.5 nm to 737.5 nm for fluorescence detection. The detector was connected to a counting board for time correlated single photon counting (Picoquant, PicoHarp 300). During laser scanning fluorescence lifetime imaging, the sample was scanned by the piezo stage, recording a fluorescence lifetime trace for 5



each pixel. All measurements were performed at room temperature and in an air ambient environment. The software named SymPhoTime 64 was employed to do the PL decay curves fitting. All the calculation were performed by using spin-polarized density functional theory (DFT) in VASP code25. The generalized gradient approximation with the exchange and correlation interaction of Perdew-Burke-Ernzerh (GGA-PBE)26 is implemented. When calculate the band structure, spin-orbit coupling is included27-28. During geometry relaxation and the total energy calculation, a 4*4*1 supercell is adopted and a cutoff energy of 400 eV converges the total energy to less than 1*10-6 eV. The atomic structure is fully relaxed with a residual force of less than 0.01 eV/Å. During the calculations, the high symmetry points are along “Γ-M-K-Γ” direction. A vacuum slab with 15Å is inserted between layers in the Z-direction.

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Result and discussions The three-dimensional MoS2 is composed of a vertically stacked layers crystallized in a 2H prototype structure with space group P63/mmc( ). The

multi-layers are held together by the weakly van der Waals interactions, which allow for micromechanical exfoliation of single layers from bulk crystalline samples similar to the fabrication of graphene from graphite. Single layer MoS2 has a sanwiches-like crystal structures, composed of three atomic sheets S-Mo-S. In the alternating sheets Mo stoms and S atoms are hexagonally arranged due to strong covalent bonds between Mo and S, giving rise to a trigonal prismatic coordination (Fig. 1(a)) . The quasi-two-dimensional nature of MoS2 makes the trigonal prismatic arrangement keep stable, whereas its symmetry is reduced to be a noncentrosymmetry because of a loss of inversion symmetry.

Figure 1 (a)Lattice structures of monolayer and bilayer of MoS2. (b)~(e) The band 7



structure of monolayer, bilayer, trilayer and bulk of MoS2 calculated by the DFT. The blue line indicates the Femi energy level. The orange line shows the CB minimum level. ∆E is the energy difference between the CB minimum and the VB maximum, which is observed to be decreased with the thickness. The red arrows display the indirect transition in the layered MoS2. The purple arrows reflect the direct photoluminescence transition in MoS2 regardless of the thickness.

An illusration of electronic band structure calculated for monolayer, bilayer, trilayer and bulk MoS2 within density functional theory (DFT)26-29 is given in Fig.1(b)~(e). The band gap is direct for single layer in contrast to the indirect gap of bilayer and thicker mutilayers due to the missing interlayer interaction, which have been confirmed by many studies including several photoluminescence experiments11. The DFT calculations show that the electronic distributions are strongly correlated to the atomic spatial structure. Also it can be considered that the unusual band structure of monolayer and mutilayers MoS2 originate from characteristics of the d-electron orbitals which comprise the valence bands (VB) and the conduction bands (CB) of MoS2. In details, the CB minimum at the K-point is mainly attributed to localized d-electron orbitals of the Mo atoms. Since Mo atoms are located in the middle of the trigonal prismatic coordination, they are weakly effected by the interlayer coupling, as a result the CB minimum remains at the K-point as the thickness changed. However, the VB maximum near the Γ-point of the bulk MoS2 originating from the combinations of the antibonding pz-orbitals of the S atoms and the d-electron orbitals of the Mo atoms is strongly aﬀected by the interlayer coupling. The VB maximun shifts from the Γ-point to the K-point as the layer number decreases to a monolayer. 8


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Therefore, the band gap shifts from an indirect one at the Γ-point (Fig. 1(c)~(e)) to a direct one at the K-point (Fig. 1(b)) and the energy difference (∆E) between VB maximum and the CB minimum increased significantly in the thinner layers. Noted that the direct excitonic states near the K-point are relatively unchanged.

Figure 2 (a)~(e)Raman spectra of 1~3L MoS2 measured by 532 nm laser. (a)~(c) choose gel film as the substrate and (d)~(e) are on the mica subtrate. (f) indicates the photoluminescence spectrum of the regions displayed on (d) excited by 488 nm laser.

Table 1 Frequency of high frequency Raman modes E and A of 1~3L MoS2 measured by 532 nm laser. The frequency differences ∆ω are also compiled in the 5th column. Layer number N in the 6th column are analyzed from ∆ω following the equation of ∆ω(cm-1)=25.8-8.4/N.

Subst rate Gel film

Region No. a-1L a-3L b-1L

Frequency of Frequency cm-1)

of (cm-1)

385.0 383.2 385.0

405.0 407.0 405.1

Frequency difference ∆ω (cm-1) 20 23.8 20.1

9


Layer number N 1 3 1


mica

b-2L c-1L c-2L c-3L d-1L d-2L d-3L e-1L e-3L

383.6 385.0 383.7 383.0 385.6 385.1 383.7 386.9 384.5

406.2 405.1 406.3 406.9 405.1 406.7 406.7 405.1 407.9

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22.6 20.1 22.6 23.9 19.5 21.6 23.0 18.2 23.4

2 1 2 3 1 2 3 1 3

Figure 2 (a)~(e) show the high-frequency Raman spectroscopy, accompanied with optical micrograph in the inset figures. Figure 2(f) performed the emission spectra of the investigated regions in Fig. 2(d). Well-defined steps with the apparent color in the optical micrograph indicate the areas with different layer numbers, which allows for fast identification of sample thickness due to optical interference. The Raman spectroscopy was examined as an effective and nondestructive tool for accurate indentification of the thickness of 2D MoS2 flakes. The layer number was confirms by the Raman spectroscopy measurement. The high-frequency Raman spectroscopy consists mainly of two characteristic Raman modes, the E mode and the A mode. And as reported previously 30-32, The frequency difference ∆ω of the E mode and the A mode, following the formula of ∆ω(cm-1)=25.8-8.4/N33, in which N is labeled as the layer number. Figure 2(a)~(e) shows that the E mode and the A mode are located at ~385 cm-1 and ~405 cm-1, respectively. The layer number of the individual regions can be found out in the figures and in Table 1. Photo luminescence (PL) measurements at room temperature are depicted in Fig. 2(f). The PL spectra exhibit two prominent peaks centered at 1.86 eV (668 nm) and 10


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2.01 eV (618 nm) at the excitation wavelength of 488 nm, corresponding to the direct excitonic transition at Brillouin zone K point. The energy difference between the two peaks is resulted from the spin-orbital splitting of the VB, which are known as the transitions of the A and B excitons34-35. According to earlier conventions, A is labeled as the resonance peak centered at 1.86 eV and B is corresponding to the direct transition at 2.01 eV. Sometimes the B-exciton emission peak is not expected to present even in the monolayer PL spectrum. Steinhoff et al. discussed the situation where the B-exciton emission came to be presence or absence36. They point out that for the excitation wavelength of 488 nm regarded as quasi-resonant excitation, the presence of B-exciton emission is explained in terms of the coherent excitation at the B-exciton resonance that results in holes occupying the lower-lying conduction band at K point, from where B-exciton emission is possible. The positions of the two resonances barely change with the increasing layer number due to the unchanged energy difference at K point, as shown in Fig.1. For thin layers MoS2, the intensity of the photoluminescence peaks become weaker with the layer thickness as a result of the transition from a direct band gap to an indirect one. Remarkably, this luminescence is absent in the indirect bandgap bulk MoS2 sample. The sharp peak located at approximately 1.88 eV is originated from the background as depicted with BG in Fig. 2(f).

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Figure 3 (a1)~(e1) exhibited the optical image of the flakes with different thickness. Fluorescence-lifetime imaging microscopy (FLIM) (a2)~(e2) and the PL lifetime decay curves (a3)~(e3) of layered MoS2 are also displayed corresponding to the optical image located at variety 12


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of regions.

To obtain more time-resolved photoluminescence information of layered MoS2, we examined the fluorescence-lifetime imaging microscopy (FLIM) for a variety of flakes and identified separate regions ranging from monolayer to few-layer thickness, as shown in Fig. 3 (a2)~(e2). The fluorescence lifetime decay curves are recorded for each pixel in the microscopy image. Figure 3 also displays the corresponding optical microscopy in (a1)~(e1) and the extracted decay curves of the selected regions in (a3)~(e3). The three pictures in the same row origin from the same sample flakes. Comparisons between the optical microscopy and the lifetime imaging indicate that FLIM results exhibit more distinct contrast between regions with different thickness. For all samples, it is found that the fluorescence intensity decay rate shows strong layer dependence: the decay curve drops the most dramatically in the monolayer facet and it decays more smoothly with the increasing thickness. This PL decay behavior indicates that the photoluminescence time evolution is much faster in monolayer than in the multilayer and the bulk MoS2. Following a double-exponential lifetime equation of = + ⁄ + ⁄ , we fitted all the decay curves as shown in Fig. 3 (a3) ~ (c3). The obtained lifetime (τ1 and τ2), the deviation (∆τ1 and ∆τ2) and corresponding percentum (τ1% and τ2%) are compiled in Table 2. Variation of each lifetime and its percentage with respect to layer number (N) are also visible in Fig. 4 (a) and 4 (b), suggesting different decay behaviors of τ1 and τ2 in monolayer and multi-layer structures. As plotted in Fig. 4(a), the photoluminescence decay rate τ1 and τ2 increased 13



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linearly against the layer number N, and the solid lines are least-squares linear fits through the data. The two linear relationships are respectively expressed as τ1(ns)=0.037N+0.053 and τ2(ns)=1.64N+4.42. The two equations suggest that FLIM is another potential tool to identify the flakes thickness of 2DMs. The respective percentage (τ1% and τ2%) exhibit opposite trends as the layer number (N) increases as shown in Fig. 4(b): τ1% is the highest in the monolayer MoS2, while τ2% increases linearly in spite of the added layers. This behavior of the photoluminescence lifetime indicates that the slower luminescence decay curve observed in the multilayers is originated from two effects. First, both of the decay lifetime (τ1 and τ2) increased linearly with the thickness. Remarkably, τ2 shows more dramatic growth than τ1. Second, the rising τ2% suggests that the longer lifetime part (τ2) gives more contribution to the whole decay rate in the thicker layer samples. Table 2 Photoluminescence lifetime (τ1 and τ2), deviation (∆τ1 and ∆τ2) and corresponding percentum (τ1% and τ2%) on different regions.

Region a1-1L b1-1L c1-1L b1-2L c1-2L a1-3L c1-3L

N 1 1 1 2 2 3 3

τ1/ns 0.095 0.090 0.083 0.134 0.122 0.176 0.149

∆τ1 0.0032 0.0024 0.0014 0.0085 0.0065 0.0160 0.0086

τ1% 0.988 0.993 0.993 0.972 0.972 0.880 0.932

τ2/ns 7.65 5.62 5.12 8.10 6.86 10.57 8.31

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∆τ2 0.22 0.13 0.12 0.21 0.21 0.36 0.33

τ2% 0.0116 0.0068 0.0073 0.0280 0.0284 0.1199 0.0684

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Figure 4 (a) Photoluminescence lifetime (τ1 and τ2) plotted with deviation (∆τ1 and ∆τ2) with respect to number of layers. (b) Lifetime percentum (τ1% and τ2%) as a function of layer number N.

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Figure 5 (a) Fitting curves by Bi-Gaussian method of PL measurements for monolayer MoS2. (b)~(d) TRPL measurements of different spectral ranges for layered MoS2. (e)~(f) TRPL measured with different layer samples of D(A) and D(B+A).

To determine the respective source of τ1 and τ2, it is necessary to separate A and B excitons by using appropriate bandpass filters. Thus, the emission spectrum of A and B excitons was firstly fitted by bi-Gaussian method as shown in Fig.5(a) and two peaks of the fitting curves were fixed at 1.86 eV and 2.01 eV, respectively. Accordingly, two band-passed filters, labeled as 615-60 and 700-75, were employed 16


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to separate A and B emission. Here, filter 615-60 means it allows the light with a wavelength range of 60 nm and centered at 615 nm to pass, i.e. it has a passing range of 585 nm ~ 645 nm (1.92~2.12 eV) (red range in Fig.5(a)). The same goes for 700-75 filter, which is ranging from 662.5 nm to 737.5 nm (1.68~1.87 eV) (blue range in Fig.5 (a)). It can be considered that the TRPL using filter 700-75 only contains the contribution of the A excitons, while the one using filter 615-60 contains both of A and B contributions and mainly with the B contributions. Hereafter we called filter 615-60 and filter 700-75 as F615 and F700 for short. And D(A) is meaning of decay curves measured with F700 and D(B+A) is behalf of decay curves measured with F615. For contrast, D(L) plotted in Fig.5 shows the decay curves measured with long-passed filter, ranging from 561 nm to 700 nm. Figure 5(b)~(d) display the fluorescence lifetime curves with different pass filters. (b) is for monolayer sample, (c) is corresponding to 2L sample and (d) is 3L. Of the same tendency in the three figures is faster D(A) and slower D(B+A) regardless of the layer numbers. From this point of view, the faster decay lifetime τ1 seems to corresponding to D(A), which comes from the only A excitons as mentioned above. Indeed, all of D(A) curves are not experimentally observed to be single-exponential function (Fig.5(e)). Compared D(A) curves of 1L, 2L and 3L in Fig. 5(e), it is obvious that the decay rate increased with the layer numbers. This goes opposite to the assumption that D(A) only originates from the A excitons. In contrast to monolayer MoS2, for 2L and 3L MoS2, the dark indirect transition K′/ Г, coming from the exciton formed by the hole located in the Γ valley and the electron in the K hill, lies energetically slightly 17



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below the bright KK exciton. Excitons are efficiently scattered by phonons from the bright KK to the dark K′/ Г exciton state because of the lower energy as sketched in Fig. 6. This opens a new scattering channel for excitons in MoS2, which can explain the observed non-single-exponential decay curves of D(A). With increasing layer number, the dark K′/ Г exciton energy decreased and leads to enhanced exciton-phonon scattering into the K′/ Г state, which has a slower decay rates than the bright KK exciton. As a consequence, the decay rate of D(A) increased with the layer numbers. Regarding D(B+A), it contains at least two contributions as observed from the PL measurements. It is believed that the presence of the higher-lying B peak results from the presence of holes in the lower-valence-band maximum at K point

36

. Since the

holes in the lower-valence-band maximum and electrons in the conduction-band form an exciton with total angular momentum is not necessarily equal to 1, they generate bright or dark exciton, of which the optical transition is spin-allowed or spin-forbidden. The radiative lifetime is strongly affected by the present of the spin-forbidden intravalley dark states. Due to its slower relaxation rate with the increasing layer, it is reasonably expected that the dark K′/ Г-excitons also have a strong impact on the lifetime of D(B+A). In all the investigated MoS2 materials (Fig.(b)~(d)), we observe distinct decay rates between D(A) and D(B+A), which are resulted from different radiative recombination coupling with non-radiative and phonon-induced K′/Г emission. And it can be observed that the configuration of D(L) is similar with D(A) in the time scale 18


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of 0~3 ns, but with D(B+A) in the time scale after 3 ns. At room temperature, τA originating from the bright A-exciton relaxation is calculated to be on the order of several tens of ps and to exceed a ns37, which is on the same order of magnitude as τ1. The longest-observed lifetimes of effective radiative exciton have been measured in super-acid treated samples to be about 10.8 ns21, which may be related to B-exciton and on the same order of magnitude as our measurements (τ2). In this regards, τ1, which is a fast decay process in the transition, is related to interplay of the bright A-exciton and the dark K′/ Г-exciton transition. τ2, which shows much slower relaxation process, is governed by the B-exciton transition coupling with the influence of the K′/ Г-exciton emission. Thus, the short component τ1 and the long component τ2 are suggested to control the short-lived and long-lived part of the overall configuration, respectively. Notably, τ1% keeps much higher than 80%, indicating the A-exciton transitions always play a dominate role in the recombination channels. In addition, it should be also noted that the overall decay of the exciton is effected by the presence of defects24 and disorder exciton densities, as well as the formation of exciton complexes such as biexcitons and trions38.

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Figure 6 Possible exciton recombination channels for MoS2. Figure 6 summarize the possible exciton recombination channels for MoS2. The K′/K state is the lowest-energy transition, where the electrons and the holes located at the highest hill of the K and the lowest valley of the K′ point, respectively. It is a direct energy dissipation pathway for the radiative decay and is expected to be the bright exciton, which is consistent with A-exciton emission of the PL measurements. Based on the above result, the A-exciton transition dominates the optical properties and exhibits the shortest radiative lifetime, which is calculated to be on the order of several tens of ps to exceed a nanosecond depending on the temperature37. Besides the optically accessible bright excitons, the MoS2 system also shows a variety of dark excitons. Some of the B-exciton emission is an effective dark and intravalley transition due to its spin-forbidden (inactive) excitons as illustrated in Fig.6. Under qusi-resonance incident light, the B-exciton transitions have longer lifetime than the A-excitons due to its partially optical darkness, which is estimated as 5~10 ns in this work. The K′/Г exciton state represents the electrons occupy the K′ valley and the

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holes stay at the Г hill, which is an indirect transition as plotted in Fig.6 and plays a more important role in multilayers MoS2. They are not visible in the optical spectra, but can strongly influence the lifetime and linewidth of the emission from the bright exciton states.22

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Conclusion In summary, we used MoS2 as a template to study the N-dependent photoluminescence properties determined by FLIM. The double exponential fitted lifetime of τ1 and τ2 can be observed to increase linearly with respect to layer number, leading to slower decay rate in thicker samples. The linear relationships of the variable lifetimes of τ1 and τ2 with respective to N offer some suggestions for identifying the layer number with FLIM. Though the analysis of lifetime component, it reveals that the A exciton, B exciton and K′/Г exciton play crucial roles in the luminescence energy dissipation. With the increasing thickness, the τ1 proportion (τ1%) holding more than 80% suggests that the short-time component decay is always the dominate relaxation pathway.

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ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (Grant No. 51575298, No.51527901and No. 51705284).

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