Anomalous Temperature-Dependent ExcitonâPhonon Coupling in

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C: Physical Processes in Nanomaterials and Nanostructures

Anomalous Temperature-Dependent Exciton-Phonon Coupling in Cesium Lead Bromide Perovskite Nanosheets Xiangzhou Lao, Zhi Yang, Zhicheng Su, Yitian Bao, Jun Zhang, Xiong Wang, X.D. Cui, Minqiang Wang, Xi Yao, and Shijie Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00091 • Publication Date (Web): 03 Feb 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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The Journal of Physical Chemistry

Anomalous Temperature-Dependent Exciton-Phonon Coupling in Cesium Lead Bromide Perovskite Nanosheets Xiangzhou Lao†, Zhi Yang‡, Zhicheng Su§, Yitian Bao†, Jun Zhang†,#, Xiong Wang†, Xiaodong Cui†, Minqiang Wang‡, Xi Yao‡, and Shijie Xu*† †Department

of Physics, and Shenzhen Institute of Research and Innovation (HKU-SIRI), The University of Hong Kong, Pokfulam Road, Hong Kong, China ‡Electronic Materials Research Laboratory (EMRL), Key Laboratory of Education Ministry; International Center for Dielectric Research (ICDR), Shaanxi Engineering Research Center of Advanced Energy Materials and Devices, Xi’an Jiaotong University, Xi’an 710049, China §Laboratory of Solid State Physics and Magnetism, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium #Faculty of Mathematics and Physics, Huaiyin Institute of Technology, Huaian 223003, China *Corresponding

author. E-mail: [email protected] (S.J.X.)

Abstract: The role of exciton-phonon coupling in light emission in cesium lead bromide (CsPbBr3) nanosheets is investigated with a combined photoluminescence spectroscopy and the multimode Brownian oscillator (MBO) model. Good agreement between theory and experiment in a low temperature range of 5 to 40 K enables us determine several key parameters, including the dimensionless Huang-Rhys factor characterizing the exciton-longitudinal-optical (LO) phonon coupling strength and the damping constant accounting for the phonon bath (quasicontinuous acoustic phonons) dissipation. It is found that the Huang-Rhys factor of the free excitons peculiarly tends to diminish upon increasing the temperature in the interested low temperature range. However, the damping constant shows a linear increase with temperature in the interested temperature range. These new findings may deepen the understanding of the exciton-phonon coupling in CsPbBr3 nanosheets and relevant solids.

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1. Introduction

In recent years, all inorganic CsPbX3 (X=Cl, Br, I) perovskites in various forms of nanostructures, such as nanocrystals, nanorods and nanosheets, have attracted an increasing interest due to their extraordinary optical properties, defect tolerance, good thermal stability and potential functionalities,1-10 especially light emission functions.11 Among the investigated CsPbX3 perovskites, CsPbBr3 nanocrystals show the greatest thermal stability.11 In addition, slow cooling rate of hot carriers and the associated “hot-phonon bottleneck” effect has been firmly verified to be the strongest in the all inorganic CsPbBr3 crystal, compared with organicinorganic hybrid analogues.12-19 Despite the key mechanisms of slow cooling of hot carriers in perovskites are still under debate, all available results indicate the occurrence and the vital role of unusual electron (exciton)-phonon coupling in lead-halide perovskites. For example, Evans et al. experimentally investigated the initial electron cooling and polaron formation dynamics in single-crystal CsPbBr3 perovskite, and suggested that temperature-dependent formation of large polarons due to the electron-longitudinal-optical (LO) phonon interaction could also slow down hot carrier cooling.17 Hopper et al. utilized “pump-push-probe” ultrafast spectroscopy to show that the cooling rate is limited by the “hot-phonon bottleneck” imposed by the lattice vibrations.18 Ramade et al.’s polarized micro-photoluminescence (PL) measurements on single CsPbBr3 nanocrystals revealed that acoustic and LO phonons play dominant roles at low and high temperature, respectively. In particular, the line broadening of excitonic luminescence is mainly ruled by the Fröhlich-type LO phonon coupling because of the polar nature of CsPbBr3 crystal.19 In fact, phonon coupling, especially LO phonon coupling in polar semiconductors such as GaN and ZnO may result in phonon sidebands or replicas and even more fascinating manybody Fano structures in the PL spectra of excitons.20-28 These studies not only verify the dominant role of LO phonons in the generation of phonon sidebands (PSBs) and the broadening 2 ACS Paragon Plus Environment

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of the zero-phonon lines (ZPL) in polar semiconductors, but also unveil the size dependence of exciton-LO-phonon coupling in polar semiconductors. For example, the exciton-LO-phonon interaction was observed by Hsu et al.26 to decrease with reducing ZnO quantum dot size due to the quantum confinement effect that makes the exciton less polar. Yan et al. observed local suppression of exciton-LO-phonon coupling in bent ZnO nanowires with PL spectroscopy.27 We observed the similar effects of strong quantum confinement and reduced Fröhlich exciton– phonon coupling in ZnO quantum dots from PL and optical reflectance spectra.28 Pelekanos et al. reported an effective reduction of the exciton-LO-phonon coupling in quasi-twodimensional (Zn,Cd)Se/ZnSe quantum wells with absorption spectroscopy, and attributed it to the quantum confinement effect.29 Utilizing resonant Raman spectroscopy, Zhang et al. investigated the exciton-phonon coupling in high-quality cubic phase ZnTe nanorods, and found that the Huang-Rhys factor of individual nanorods, and thus the exciton-LO-phonon coupling strength increases with the diameter of nanorods.30 However, an unexpectedly enhanced polar exciton-LO-phonon interaction was found by Heltz et al.31 in strained lowsymmetry InAs/GaAs quantum dots. Their calculations in the adiabatic approximation indicate that the quantum confinement and piezoelectric effect together account for the enhanced coupling. These results have provided new insights into the long-standing problem of the exciton-phonon interactions in low-dimensional semiconductor systems, but also suggest more in-depth studies to be done for deepening and even solving the issue of exciton-phonon coupling in low-dimensional lead-halide perovskites and other polar semiconductors. Of course, such studies are also of technological significance in light of improvement of optoelectronic devices’ performance of perovskites. Recently CsPbBr3 nanosheets (NSs) with lateral dimensions as large as several hundred nanometers have demonstrated exceptional photophysical properties.6,8,32 For example, high carrier mobility and large diffusion length make CsPbBr3 NSs appealing for optoelectronic device applications. As a quasi-two-dimensional system, meanwhile, it may have different 3 ACS Paragon Plus Environment


properties from those of 3D, 1D and 0D systems. Therefore, it is of great interest and significance importance to enhance the understanding of optoelectronic processes in CsPbBr3 NSs, especially the roles of phonons in optoelectronic processes. The perovskite sample studied here was CsPbBr3 NSs prepared with a typical hot-injection method. The sample preparation and characterization including the morphology and size of nanosheets have been previously described elsewhere in some details.32, 33 Under the extremely weak excitation conditions such as ~ 7 μW 405 nm light dispersed with a grating monochromator from a Xe lamp, the sample emits the two groups of emissions of free and trapped excitons at cryogenic temperature.33 Meanwhile, we have shown that the free excitonic transition becomes dominant either at higher temperatures under the extremely weak excitation levels or at cryogenic temperature but under the relatively higher excitation level. A study of the excitation power dependence of the sample PL spectra can refer to a previous publication.33 As stated earlier, an in-depth investigation on the exciton-phonon coupling, in particular, on the coupling between free excitons and LO phonons in CsPbBr3 NSs is the major subject of the present study. We thus utilized a relatively stronger excitation power (e.g. ~ 3 mW) to make the principal emission line of free excitons always be the main PL band in the interested low temperature range of 5-40 K. Under such circumstance, the free-exciton principal ZPL line can be observed and investigated in detail. To gain more insight into the exciton-phonon coupling in the quasi-two-dimensional CsPbBr3 NSs, we adopted the multimode Brownian oscillator (MBO) model34 taking into account both the exciton-LO-phonon coupling and the dissipative effect of the phonon bath modes to model the observed PL spectra at different temperatures.35,36 Good agreement between theory and experiment was achieved when only two adjustable parameters, namely, the dimensionless Huang-Rhys factor characterizing the exciton-LO phonon coupling strength and the damping coefficient accounting for the phonon bath dissipation, were taken. Variable-temperature PL measurements of the sample were carried out on a home-assembled high-resolution PL setup previously described elsewhere.37 The excitation light source was a 405 nm laser diode. Room4 ACS Paragon Plus Environment

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temperature Raman scattering measurements of the sample were performed under the backscattering geometric configuration using a 633 nm He-Ne laser as the excitation light.

2. Experimental Section

The CsPbBr3 NSs were synthesized with the hot-injection method and detailed description on the synthesis procedures can be referred to a previous study.32 The steady-state PL spectral measurements were carried out on a home-assembled system consisting of a Spex 750M monochromator and a Hamamatsu R928 photomultiplier detector. A standard lock-in amplification technique was employed with a chopper to achieve a high signalto-noise ratio. The sample was mounted with silver paint on the cold finger of a Janis closed cycled cryostat providing a varying temperature range from 5 K to 300 K. The excitation light source was a continuous-wave laser diode with central wavelength of 405 nm. The laser spot here was around 3mm in diameter. And the excitation power was controlled by a variable neutral density filter. Room-temperature Raman scattering measurements of the sample were performed under a back-scattering geometric configuration using a 633 nm He-Ne laser as the excitation light source. The laser beam was focused on the sample with a ×100 objective. The Raman light was collected by the same objective and analyzed with a grating spectrometer equipped with the Andor Newton CCD. 3. Results and Discussion

Figure 1(a) and (b) show the measured PL spectra and the color plot of the sample at various temperatures in the range of 5-40 K, respectively. The PL spectra exhibit asymmetric line shape with a low energy tail, especially at low temperatures. The low energy tail comprises actually the first- and even second-order LO phonon sidebands (PSBs) whose energy separations from the free-exciton principal ZPL line are one and double characteristic energy (~18 meV) of LO phonons, respectively. To confirm such judgement, a Raman scattering spectrum of the 5 ACS Paragon Plus Environment


studided CsPbBr3 NSs is illustrated in Figure 2. The characteristic energy (ℏωLO) of LO phonons is determined to be about 18 meV (~147 cm−1) from the Raman spectroscopy, while the tranverse optical (TO) phonon to ~14 meV (~112 cm-1). These phonon energies are in good agreement with the reported data in literature for CsPbBr3.38, 39 The small LO and TO phonon energies of CsPbBr3 are largely due to the hevay mass of lead element in the alloy.40 Also due to the small LO phonon energy as well as the inhomogeneous broadening, the ZPL line and its LO PSB significantly overlap so that only asmmetric lineshape as whole but not separated peaks are observed in the PL spectra even at 5 K. As shown and argued later, the experimental PL spectra can be well modelled as the supersitioning result of a ZPL line and its first- and secondorder LO PSBs with the MBO model.


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(b) 40 35

1

Temperature (K)

30

0.8 25

0.6 20

0.4 15

0.2 10

0 5 530

535

540

545

550

555

Wavelength (nm)

Figure 1. (a) PL spectra of the CsPbBr3 NSs measured at different temperatures under the excitation of ~3 mW 405 nm laser diode. The downward arrows from left to right indicate the ZPL, the first-order LO and the second-order LO PSBs at 5 K. (b) Color plot of the temperature-dependent PL spectra shown in (a). Blue shit in peak position, increase in intensity, and narrowing in line width is apparently observed for the main PL peak in the interested temperature range.

Experimental Fit Peak 1 (112 cm-1) Fit Peak 2 (147 cm-1) Cumulative Fit Peak

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

100

150

200 -1

Raman shift (cm )


250


Figure 2. Raman scattering spectrum (noisy curve) collected from the sample under the back-scattering geometric configuration at room temperature using a red 633 nm He-Ne laser as the excitation light source. Decoupled peak 1 and peak 2 corresponding to transverse optical (TO) and LO phonon mode, respectively, and the cumulative fit curve are shown.

Before discussing the MBO theoretical curves, we consider temperature dependence of both full-width at half maximum height (FWHM) and asymmetry factor (AS) of the experimental PL spectra. Fascinatingly, both key spectral parameters tend to linearly decline as the temperature increases, as shown in Figure 3. For instance, the FWHM reduces from ~24 meV at 5 K to ~16.5 meV at 40 K, while the AS parameter shrinks from 1.42 at 5 K to the unit at 40 K. The FWHM data for above 40 K and the definition of asymmetry factor can be referred to the supplementary document. These results strongly indicate that the LO PSBs fade gradually out with the rise of temperature, and eventually become unobservable. Thermally induced diminishing of the LO PSBs of free excitonic luminescence sounds unusual in polar semiconductors,41 and reminds us again that the exciton-phonon coupling in low-dimensional polar semiconductors is indeed a long-standing challenging subject.31 Regarding the unusual blue shift of the ZPL line peak with temperature, it can be well interpreted as a result of thermal lattice-constant expansion.33,42,43 In the interested low temperature range of 5-40 K, the peak position of the ZPL line shows an almost perfect linear blue shift with temperature, as theoretically predicted.42,43 Nevertheless, we still need a sophisticated theoretical calculation to understand the temperature induced decline of the LO PSBs of free exciton luminescence in the studied CsPbBr3 NSs. The MBO model, in which a complicated solid-state luminescence system is simplified as a composite primary oscillator (e.g. coupled exciton and LO phonon) dissipated by a quasi-continuous acoustic phonon bath, may provide a good theoretical platform for the purpose. Although the theory was developed by Mukamel and his co-workers for calculating the nonlinear transient response of a solvent-solute system (e.g., a Brownian-like 8 ACS Paragon Plus Environment

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molecule dissovlved in a liquid soultion),34 it has been successfully applied by Shi and her coworkers to calculate the variable-temperature PL spectra of color centers and quantum dots in polar semiconductors.35,36 (a)

(b) 24

Exp. Linear fit

23

1.5 Exp. Linear fit

1.4

Asymmetry factor

R2=0.971

22

FWHM (meV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21 20 19 18

R2=0.944 1.3

1.2

1.1

17 1.0

16 0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

Temperature (K)

Temperature (K)

Figure 3. Both FWHM (a) and asymmetry factor (b) of the PL spectra exhibits linear decline with the rise of temperature in the interested low-temperature range. Solid squares and lines represent the experimental data and linear fit with the least-squares method, respectively. Errors in the experimental data were estimated and given. The squares of the vertical deviations R2 are also presented in the figures.

Usually, the three key parameters, namely the frequency or energy of primary oscillator, the Huang-Rhys factor S characterizing the electron (exciton)-primary oscillator (phonon) coupling strength, and the damping constant γ accounting for the dissipation of acoustic phonon bath, shall be taken into account in the MBO calculations.34-36 Among the three parameters, the dimensionless Huang-Rhys factor is essentially important because it physically represents the lattice relaxation amount and hence mean phonon numbers involved within absorption and luminescence process.44 For a given material like the CsPbBr3 studied here, the frequency of LO phonons shall be taken as a constant. We thus adopted the Raman peak shift of the LO phonons, e.g. ~147 cm−1, as the characteristic frequency of the primary oscillator in the MBO calculations. To calculate the ensemble PL spectrum of many CsPbBr3 NSs with the MBO 9 ACS Paragon Plus Environment


model, the inhomogeneous broadening induced by size distribution etc. may be represented by a Gaussian function36

f (eg )  exp[2W 2 (eg  eg0 ) 2 ]

.

(1)

The PL line shape of an exciton−phonon coupling system with inhomogeneous broadening may be hence expressed as36

I PL ( ) 

1







0



f (eg )d eg  Re  exp[i (  eg  S hLO )t  g * (t )]dt 0

,

(2)

where g*(t) is the complex conjugate of a spectral response function g(t)34

g (t )  

1 2







d

3  2 S hLO  [1  coth(  h / 2)](e  it  it  1) 3 2 2 2 2     (LO   ) .

(3)

Here, heg0 represents the pure electronic transition energy (e.g., the peak position of the ZPL line) and hLO the LO phonon energy. Energy (eV) 1.0

2.35

2.3

2.25

Energy (eV) 2.2

2.302 eV

2.15

2.4

Normalized intensity (a.u.)

2.4


5K

0.8 0.6 0.4

~18 meV

0.2 0.0 520

530

540

550

560

570

2.35

2.3

2.25

2.2

2.307 eV

1.0

2.15

20 K

0.8 0.6 0.4

~18 meV

0.2 0.0

580

520

530

Wavelength (nm)

540

550

560

570

580

Wavelength (nm) Energy (eV)

2.4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0

2.35

2.3

2.25

2.2

2.314 eV

2.15

35 K

0.8 0.6 0.4

~18 meV 0.2 0.0 520

530

540

550

560

570

580

Wavelength (nm)

Figure 4. Measured (open circles) and calculated (solid lines) emission spectra of the sample at three


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different temperatures. Three parallel vertical lines are drawn to mark the energetic locations of the ZPL and its first-, second-order LO PSBs.

Figure 4 shows theoretical (solid lines) at three different temperatures. For direct comparison, the corresponding experimental PL spectra (open circles) are also depcted in the figure. At 5 K, the parameter values S=0.55, γ =100 cm-1, eg0 =2.302 eV were adopted in the calculation. The Huang-Rhys factor adopted in the present study is in good agreement with the value S=0.38±0.05 obtained by Iaru et al.45 using the intensity ratio between the first phonon sideband and the ZPL at 8 K as the Huang-Rhys factor. It comes actually no surprise because that the Huang-Rhys factor is approximated as the intensity ratio between the first-order phonon sideband and the ZPL, especially at cryogenic temperature.41 As seen in Figure 4, good agreement between theory and experiment may provide an unprecedented understanding of the exciton-phonon interactions in low-dimensional CsPbBr3 nanostructures.

Damping constant  (cm)

0.6

Linear fit 0.5

R2=0.987

0.4

0.3

0.2

150

Linear fit R2=0.991

140 130 120 110 100

0.1

0

5

10

15

20

25

30

35

40

45

0

5

10

15

20

2.318

Linear fit

2.316

R2=0.986

2.314 2.312 2.310 2.308 2.306 2.304 2.302 2.300

0

25

30

35

Temperature (K)

Temperature (K)

ħeg (eV)

Huang-Rhys factor S

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5

10

15

20

25

30

35

40

45

Temperature (K)


40

45


0 Figure 5. Huang−Rhys factor S, damping constant γ, energetic position heg of the ZPL of the CsPbBr3

NSs vs. temperature. The solid lines represent the best linear fit curves. Errors in the experimental data were estimated and given. The squares of the vertical deviations R2 are also presented in the figures.

Figure 5 shows the Huang-Rhys factor S, the damping constant γ, the ZPL heg0 adopted in the MBO calculations vs. Temperature. The solid lines represent the best linear fit curves. Cleraly, the Huang-Rhys factor decreases almost linearly with increasing the temperature, suggesting that the exciton-LO-phonon coupling thermally diminishes in the interested temperature range from 5 K to 40 K. However, the damping constant γ increases linearly with the rise of temperature in the range. The thermal enhancement of the damping constant can be well understood as the increased thermal dissipation by thermally activated acoustic phonons.35,36 But the thermal diminishing tendency of the Huang-Rhys factor in the CsPbBr3 NSs seems difficult to be understood. In polar semiconductor of GaN, the coupling strength between free exciton and LO phonon has been firmly observed to increase with increasing the temperature.21,22,48 Such increase in the intensity ratio of 𝐼1 𝐼𝑍𝑃𝐿 (𝐼1 is the luminescence intensity of the first-order LO PSB, while 𝐼𝑍𝑃𝐿 represents the luminescence intensity of the ZPL line) can be well interpreted within the Segall-Mahan theory.22 In this theory, the LO phononassisted excitonic transition probability depends peculiarly on the kinetic energy of free exciton. As the temperature increases, the kinetic energy of free exciton increases, resulting in an increase of the LO PSBs. Desipte the studied CsPbBr3 is also one kind of polar semiconductor, the observation of thermally induced decline of the coupling strength between the free exciton and the LO phonon in the CsPbBr3 NSs seems contradictory with the theoretical prediction and the observed result in GaN. To seek for the possible physical mechanism, we need to examine the preculiar optolectronic properties of CsPbBr3 semiconductor.


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It is well established that in polar semiconductors Fröhlich-type exciton-LO-phonon coupling may play a dominant role in both luminescence and Raman scattering processes.46-48 A general concluding remark of available theoretical studies on exciton-phonon coupling in semiconductors is that Huang-Rhys factor depends critically on the effective Bohr radius of excitons.46,49,50 For example, the Huang-Rhys factor is inversely proportional to the effective Bohr radius of free excitons.49 In other words, the smaller the effective Bohr radius, the larger the Huang-Rhys factor. Such theoretical prediction has been demonstrated for various excitons in GaN.41 Beside the Bohr radius of exciton, the Huang-Rhys factor S is also theoretically predicted to be peculiarly on the dielectric constants of semiconductor:48-50 S

1 aB

 1 1  ,    0 

(4)

where aB   0 ex is the Bohr radius of exciton, ex is the reduced mass of exciton, while   and  0 are the dielectric constants of semiconductor at high and low frequency, respectively. Justifying from the electronic structure of CsPbBr3 crystal, such as the valence band consisting of Br 4p and Pb 6s functions, respectively, and the lowest conduction bands of Pb 6p, and the lowest gap occuring at the R point of the Brillouin zone,51 it could be hard for one to conclude that the effective mass of electron and hole (hence exciton) in the studied CsPbBr3 NSs significantly changes with the rise of temperature in the low temperature range. However, the static dielectric constant of the lead halide perovskite could be a strong function of temperature.52 According to the study by Bosman and Havinga,52 for low-ɛ compounds the dielectric constant increases with increasing temperature, whereas for high-ɛ compounds the dielectric constant decreases with increasing temperature. As a low-ɛ (~7.3) compound,53 the dielectric constant of CsPbBr3 shall increase with the rise of temperature. If so, the Bohr radius of free exciton shall become larger with temperature for CsPbBr3. Then by Eq. (4), we may



conclude that the Huang-Rhys factor shall diminish with increasing the temperature. This is just observed tendency in the present study.

Figure 6. LO-phonon vibration along with the Pb-Br atomic chain.

It could be now useful for us to turn to discuss the LO mode lattice vibration, as schematically in Figure 6. It is well established that the LO phonon with the characteristic energy of ~18 meV or ~147 cm−1 in CsPbBr3 crystal is due to the pure Pb-Br stretching mode.54 Large polarons have been argued by Miyata et al.54 to form predominantly from the deformation of the PbBr3− frameworks. When the lattice temperature is increased, the lattice volume naturally expands. As a result, several effects may be observed. (A) The band gap of perovskite semiconductors exhibits a linear blue shift with increasing the temperature. In the interested low temperature range of 5-40 K, almost perfect linear blue shift is indeed verified, as shown in Figure 5. (B) The decrease in the number of polarizable particles per unit volume as the temperature increases, which is a direct result of the volume expansion.52 (C) The increase of the macroscopic polarizability due to the volume expansion may appear. It is found that the volume-dependent contribution (B+C) is always positive for the dielectric constant. In other words, the dielectric constant of CsPbBr3 perovskite may increase with the rise of temperature, especially in the interested low temperature range.53 Consequnetly, the exciton-LO-phonon coupling strength, 14 ACS Paragon Plus Environment

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and hence the Huang-Rhys factor exhibits an unusual declining behavior as the temperature is increased, which is seemingly consitent with the large polaron idea in the CsPbBr3 perovskite.

4. Conclusions

In conclusion, the free excitonic emission band of CsPbBr3 NSs has been investigated in the low temperature range of 5-40 K. Besides the principal ZPL band, the LO PSBs are observed as a low energy tail at low temperatures. As the temperature is increased, the LO PSBs tend to diminish so that unusual temperature dependence of the dominant exciton-LO-phonon coupling strength (hence the Huang-Rhys factor) is observed in the CsPbBr3 NSs for the first time. The MBO model with three adjustable parameters is employed to reconstruct the experimental PL spectra. Virtuous agreement between theory and experiment further verifies that the HuangRhys factor reduces upon increasing temperature in the range of 5-40K. This study thus provides a valuable insight into the complicated many-body nature of exciton-phonon interactions in low-dimensional CsPbBr3 perovskite and other polar semiconductors.

Supporting Information PL spectra, FWHM data and fitting for temperatures above 40 K, and the definition of asymmetry factor.

Acknowledgments

The study was supported by the SRT on New Materials of HKU. Z.Y. and M.Q.W. wish to acknowledge the financial support received from the National Natural Science Foundation of China (Grant No. 61604122, 51572216 and 61774124).

References



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