The Temperature-Dependent Bandgap in 2D Perovskitesï¼Thermal

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Energy Conversion and Storage; Plasmonics and Optoelectronics

The Temperature-Dependent Bandgap in 2D Perovskites# Thermal Expansion Interaction and Electron-Phonon Interaction Shuai Wang, Jiaqi Ma, Wancai Li, Jun Wang, Haizhen Wang, Hongzhi Shen, Junze Li, Jiaqi Wang, Hongmei Luo, and Dehui Li J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01011 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 4, 2019

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The Journal of Physical Chemistry Letters

The Temperature-Dependent Bandgap in 2D Perovskites：Thermal Expansion Interaction and Electron-Phonon Interaction Shuai Wang,1 Jiaqi Ma,1 Wancai Li,1 Jun Wang,1 Haizhen Wang,2 Hongzhi Shen,1 Junze Li,1 Jiaqi Wang,1 Hongmei Luo 2 and Dehui Li1* 1School

of Optical and Electronic Information and Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, 430074, China; 2Department of Chemical and Materials Engineering, New Mexico State University, Las Cruces, NM 88003, United States;

*Correspondence to: Email: [email protected].

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The Journal of Physical Chemistry Letters 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

Abstract: Two dimensional (2D) organic-inorganic perovskites have attracted considerable interest recently. Here, we present systematic study of the temperature-dependent photoluminescence on pure phase (n-BA)2(MA)n−1PbnI3n+1 (n=1-5) and (isoBA)2(MA)n−1PbnI3n+1 (n=1-3) microplates obtained by mechanical exfoliation. The photoluminescence (PL) peak position gradually changes from a red-shift for n=1 to a blue-shift for n=5 with the increasing temperature in (n-BA)2(MA)n−1PbnI3n+1 (n=1-5) series while only a monotonous blue-shift has been observed for (isoBA)2(MA)n−1PbnI3n+1 (n=1-3) series, which can be attributed to the competition between the thermal expansion interaction and electron-phonon interaction. In (nBA)2(MA)n−1PbnI3n+1 (n=1-5) series, the thermal expansion interaction and electronphonon interaction are both gradually enhanced and the former progressively plays a dominant role over the latter from n=1 to n=5, resulting in the bandgap versus temperature changing from a red-shift to a blue-shift. In contrast, both those two factors show a weaker layer thickness dependence, leading to the monotonous blue-shift in (iso-BA)2(MA)n−1PbnI3n+1 (n=1-3) series.

Table of contents (TOC) Graphic


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Three dimensional (3D) hybrid organic-inorganic lead halide perovskites have recently been extensively studied due to the unprecedented progress in photovoltaics and optoelectronics beneficial from their large absorption coefficient,1 modest charge mobility2 and long carrier diffusion length.3 We have witnessed that the certified power conversion efficiency of perovskite-based solar cells rapidly soared up to 23% within a few years4,5 and perovskite-based photodetectors,6-9 lasers10,11 and light emitting devices12-15 have been consecutively demonstrated with decent performance. Nevertheless, 3D perovskites are inherent unstable in ambient conditions which limits their practical applications and thus 2D perovskites with better environmental stability are emerging to address the stability issue in 3D perovskites.16,17 In addition, 2D perovskites possess naturally formed quantum well structures, layered nature and unique optical properties and are recently attracting a growing attention.18-22 The general formula of 2D perovskites is (A)2(B)n−1MnX3n+1, where A is a spacer cation, B is monovalent organic cation, M is a divalent metal, X is a halide anion, and n is a positive integer from 1 to ∞, representing the number of [MX6]4− octahedral sheets sandwiched between two layers of A spacer cations.23,24 The organic chain A serves as dielectric layer with a smaller dielectric constant compared with that of inorganic layer, resulting in a large exciton binding energy due to the dielectric confinement,25 which would find promising applications in polaritonic devices.26 The weak Van der Waals coupling between the organic layers and inorganic layers allows us to exfoliate thin flakes from their respective bulk crystals and integrate with other 2D materials to extend their functionalities.19 By simply tuning the chemical compositions and layer number n, the band gap can be readily tuned from violet to near infrared wavelength range.21 Thus, 2D perovskites exhibit rich and tunable optoelectronic properties and might have promising potential applications in solar cells,27 light emitting devices,28 3 ACS Paragon Plus Environment


photodetectors29 and lasers.30 In this end, it is crucial to understand the fundamental optical properties of 2D perovskites with pure phase for further device designing and performance improvement.19 Temperature-dependent photoluminescence (PL) spectroscopy can provide rich information on defects, impurities, phase transition and temperature evolution of band gap.31-34 Previous temperature dependent PL studies in 3D perovskites reveal that the band gap shows a blueshift with the increasing temperature, which is strikingly different from traditional semiconductors probably due to the contraction of the lattice with temperature resulting from the anti-bonding feature in the valence band.31,35 In 2D perovskites, it has been demonstrated that the bandgap shows a quite different behavior with temperature depending on the species of the organic layer and the layer number n.20,36,37 Understanding how the organic species and layer number n affect the temperature evolution of the bandgap for 2D perovskites is of great importance not only for fundamental research but also for their optoelectronic applications; however, study on this aspect is still largely unexplored. Here, we report on a systematical investigation on the temperature evolution of the bandgap in pure phase 2D perovskite (n-BA)2(MA)n−1PbnI3n+1 (n=1-5) and (isoBA)2(MA)n−1PbnI3n+1 (n=1-3) (BA=C4H9NH3 and MA=CH3NH3) microplates by temperature-dependent photoluminescence (PL) spectroscopy. Pure phase 2D perovskite microplates with different layer number n are obtained by mechanical exfoliation from their respective bulk crystals.19 The trend of the temperature dependent emission peak exhibits a strong dependence on the layer number n and organic layer, which can be attributed to the synergetic effect of thermal expansion interaction and electron-phonon interaction.38-41 With the increase of the layer number n in (nBA)2(MA)n−1PbnI3n+1 series, both the thermal expansion interaction and the electron4 ACS Paragon Plus Environment

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phonon interaction are gradually enhanced while the thermal expansion interaction starts to play a dominant role over the electron-phonon interaction for the larger layer number n. This model has been successfully applied to the (iso-BA)2(MA)n−1PbnI3n+1 series although the temperature dependent emission peak is rather different from that of (n-BA)2(MA)n−1PbnI3n+1 series.

a) CH3NH3+

[PbI6]4-

C4H9NH3+

n

1

5 b) 

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




1.6

77 K 260 K

1.8

2.0

2.2

2.4

2.6

Energy (eV)

Figure 1. a) The schematic illustration of the crystal structures of (n-BA)2(MA)n1PbnI3n+1(n=1-5).

b) The PL spectral of the as-exfoliated 2D perovskite microplates at

both 77 K and 260 K excited by 473-nm (n=2-5) and 405-nm (n=1) lasers. Figure 1a displays schematic illustration of the crystal structures of (n-BA)2(MA)n1PbnI3n+1

(n=1–5), which are composed of n layers of [PbI6]4− octahedral sheets

sandwiched by two layers of insulating spacers. By tuning the molar ratio of the precursors, we successfully synthesized 2D perovskite crystals with different layer number n by solution method while the pure phase 2D perovskite microplates were mechanically exfoliated from their respective bulk crystals to further investigate the basic optical properties.21 It should be noted that the we cannot directly synthesize the pure phase bulk crystals for the layer number n larger than 2. Therefore, we adopted 5 ACS Paragon Plus Environment


the previously reported mechanical exfoliation method to obtain pure phase microplates with a single n number, which was further confirmed by PL measurement.19,42 Optical microscopy (OM) images and atomic force microscopy (AFM) images show that the as-foliated 2D perovskite microplates have rather smooth surface with a lateral size of the order of microns and thickness of tens of nanometers (Figure S1). To check the phase purity and crystalline of the as-foliated microplates which is critical for studying the temperature dependent bandgap behavior, PL measurement at both 77 K and 260 K has been carried out excited with either a continuous wave (CW) 473-nm or a CW 405nm laser as shown in Figure 1b. At 77 K while two emission peaks with a low-energy shoulder have been observed for n=1 due to the coexistence of the high-temperature phase and low-temperature phase and phonon replica and/or self-trapped states,43-45 only one emission peak and a low-energy shoulder are present for n=2-5 microplates arising from the free exciton emission and phonon replica and/or self-trapped states, suggesting that those microplates have pure phase and excellent crystalline quality. The phase purity of the exfoliated microplates is further supported by the PL spectra at 260 K and 290 K (Figure 1b and Figure S2). Since PL peaks are well separated among n=1-5, we can safely draw the conclusion that the samples are pure phase with single n-number when only one emission peak is observed for n=1-5 respectively at 290K under a weak excitation according to previous report.19 This method has been utilized to identify the phase purity in 2D perovskites before.42 For a much larger n, the separation between PL spectra for successive n number is too small to distinguish different phase by PL measurement. In addition, our PL spectra at room temperature exhibit the similar peak position and peak width with previous report,42 further suggesting that our as-prepared microplates are pure phase with single n-number for n=1-5 (Figure S2). Since a first-order solid-solid orthorhombic-to-orthorhombic phase 6 ACS Paragon Plus Environment

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transition takes place above 260 K for n=1,46 we displayed the PL spectra at 260 K to make the comparison easier among flakes with different layer number n. We have examined whether the measured samples have undergone degradation during the measurement for each sample. This is done by measuring the PL spectra at room temperature before and after low temperature PL measurements and checking whether there is any difference between those two spectra. Figure S3 displays PL spectra of a typical (BA)2PbI4 microplate before and after low temperature PL measurements. There is no noticeable difference for the emission peak position and peak shape, suggesting that the sample did not undergo severe degradation. This is understandable since all measurements were carried out in high vacuum chamber and under a low power excitation. With the increase of the layer number n from 1 to 5, the peak position of PL spectra at both 77K and 260K gradually redshifts due to the reduction of the quantum confinement effect and dielectric confinement according to previous reports.25 Close inspection of the PL spectra for each pure phase microplate reveal that the emission energy difference between 77 K and 260 K strongly relies on the layer number n, changing from 39 meV for n=1 to -21.6 meV for n=5 (Figure 1b). In order to understand the underlying mechanism, temperature-dependent PL spectra were further conducted on pure phase microplates with n=1-5 measured in a 20 K step (Figure 2b-f).



b)

n=1

150 100 2.4 Energy (eV)

2.6

150

e)

n=4

2.0 2.2 Energy (eV)

Temperature (K)

150

1.8 Energy (eV)

2.0

0.8

150

250

200 150

0.6 1.8

f)

n=5

2.0 Energy (eV)

2.2

0.4

n=

200

0.2

150

0 100

100

100

200

2.4

250

200

PL (norm.) 1

100

1.8

250

1.6

200

100 2.2

n=3 250

Temperature (K)

200

d)

c)

n=2 250

Temperature (K)

Temperature (K)

250

Temperature (K)

a)

Temperature (K)

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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1.6

1.8 Energy (eV)

2.0

1.4

1.6 1.8 Energy (eV)

2.0

Figure 2. The temperature dependent PL maps of (n-BA)2(MA)n−1PbnI3n+1 (a-f correspond to n=1-5 and ∞) microplates from 77 K to 290 K in 20 K steps. It is essential to first understand the origin for each emission peak in PL spectra, and then we carry out detailed analysis on how the layer number n influences the temperature evolution of the bandgap. We achieve this based on the temperature dependent and excitation power dependent PL studies together with previous report.25 By fitting the PL spectra at 77 K using the Voigt function, two emission peaks and a low-energy shoulder are present in n=1 while only one emission peak and one lowenergy shoulder are observed for other 2D perovskite investigated (Figure S4). Temperature dependent PL studies indicates that the emission peak with higher emission energy for n=1 disappears above 260 K, when the phase transition takes places. Therefore, the two emission peaks for n=1 can be ascribed to free exciton emission of low-temperature phase and high-temperature phase based on previous studies.43,45 For n=2-5, the temperature dependent PL peak indicates the emission peak is due to the free exciton emission.25 In terms of the low-energy shoulder, it is likely resulted from the 8 ACS Paragon Plus Environment

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self-trapped states according to previous reports.45,47 In order to precisely determine the origins of those emission peaks and exclude possible defect-related emission, we have further carried out the excitation power dependent PL studies at 77 K and extracted the emission intensity via multiple peak fitting (Figure S5). Compared our fitting results with previous reported ones,21,25,42,45,48 the nearly linear correlation between the intensity of PL spectra and excitation power further supports our above peak assignment as suggested in previous reports.49,50 In the rest of our article, we focused our studies on temperature evolution of the free-exciton emission peak (free exciton emission peak of low-temperature phase for n=1), which was extracted by multiple peak fitting using Voigt function. The temperature evolution of the bandgap shows a rather different behavior for different layer number n. It should be noted that the free exciton emission follows the similar trend as that of bandgap provided the exciton binding energy is independent on temperature. For n=1 microplates, the PL peak shows a gradually redshift as the temperature increases from 77 K to 260 K and a sudden shift between 260 K and 280 K due to an orthorhombic-to-orthorhombic phase transition (Figure 2a).46 With the increase of the temperature, the emission peak exhibits first a blueshift and then redshift for n=2 and n=3 samples (Figure 2b-c) while a monotonic blueshift was observed for n=4 and 5 samples, which is similar to the emission peak shift of 3D perovskite case within both low temperature and high temperature phase (Figure 2d-f).44 Those trends are more clear if we extracted the emission peak from Figure 2a-e and plotted against the temperature (Figure 3a), from which we can see a transition from the red-shift of n=1 to the blue-shift of n=5 with increasing temperature. We have measured a few samples for each pure phase perovskite and the same trend was observed, which indicates that those temperature evolution emission peak is intrinsic for those samples 9 ACS Paragon Plus Environment


b)

1.892 2.010

2.200

n=3 fit

2.003 2.164

n=2 fit

2.158

n=1 fit

2.540 2.520 50

n=4 fit

100

150

200

Temperature (K)

250

n=2 fit

-40

4.0

-80

3.0

-120

2.0

-160

1.0

-200

0.0

2.150

d) 800 2.100

2.050

Electron-phonon interaction 100

150

200

250

400

40

ΔEP ΔTE ΔEP+ΔTE

20

0

0 -20

-400 -800

Temperature (K)

Energy (meV)

1.903

n=5 fit

Thermal expansion interaction

AEP (meV)

1.815

c) 2.250

Energy (meV)

1.830

Energy (eV)

a)

ATE (meV/K)

with different layer number n.

Energy (eV)

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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-40 1

2

3

4

5

n

Figure 3. a) The emission peak versus temperature extracted from Figure 2a-e. The solid lines are the fittings according to Eq. (3) for n=1-5. b) The temperature dependent PL peak of n=2 with the calculated contribution from thermal expansion interaction and electron-phonon interaction. The solid line is the fit according to Eq. (3) and the dashed and the dashed-dotted lines show the individual contributions of thermal expansion (TE) interaction and electron-phonon (EP) interaction, respectively. c) The weight of both the TE and EP interaction of n=1-5. d) The contribution of the thermal expansion interaction ∆TE (defined as ETE (260 K) - ETE (77 K)) and the electron-phonon interaction ∆EP (EEP (260 K) - EEP (77 K)) extracted from b and Figure S7. According to the previous report, the temperature affects the emission peak (band gap) via thermal expansion interaction and electron-phonon interaction.38-41 While the thermal expansion interaction changes the lattice constant to alter the electronic band structure through temperature, the electron-phonon interaction would give rise to the change of the electronic band structure via lattice vibration caused by the temperature.38,39,41 Under constant pressure and a quasi-harmonic approximation, the temperature dependence of the bandgap can be generally expressed as40,41 10 ACS Paragon Plus Environment

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Eg T



Eg V E 1  r ( g )(n j ,qr  ) V T j ,q n j ,qr 2

(1)

where n j ,qr is the number of phonons at j branch with wave vector q , and follows the Bose-Einstein distribution

n j ,qr 

1 exp(h j ,qr / k BT )  1

(2)

where  j ,qr is the angular frequency of the phonon mode. The first term of Eq. (1) can be attributed to the thermal expansion of the lattice originated from the anharmonicity of the potentials between atoms while the second term of Eq. (1) corresponds to the contribution from the electron-phonon interaction. For a given semiconductor, the coefficient

Eg V

can be approximated as a temperature independent constant which

can be either positive or negative depending on the specifics of the bonding parameters as well as the detailed structure of the bandgap.41 For instance, in (n-BA)2PbI4, theoretical calculation suggests that the valence band maximum (VBM) is the hybridization between the p orbit of I and s orbit of Pb.51 As the temperature increases, the thermal expansion of the crystal lattice would reduce the interaction between those two orbits, resulting in a decrease in the valence bandwidth and an increase in the bandgap. Thus, the

Eg V

coefficient is determined to be positive for n=1 samples.40,52

In terms of the second term of Eq (1), the summation includes all possible phonon modes within the entire Brillouin zone, which makes it almost impossible to precisely calculate it.38 In practice, only a few phonon modes are considered and many models have been developed in order to explain the experimental results according to the number of the dominant phonon modes that contribute to the bandgap evolution.38-41 Among those models, the two-oscillator model is most frequently used to interpret the 11 ACS Paragon Plus Environment


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temperature dependent bandgap change, in which only one branch of acoustic and optical phonons are considered.38,39 Previous studies reveal that the one-oscillator model is more appropriate to describe the temperature evolution of bandgap within 3D perovskites.40 In our 2D perovskite case and temperature range we investigated (77K260K), the electron-phonon interaction is believed to be dominated by longitudinal optical (LO) phonon scattering via Fröhlich coupling and the acoustic phonon scattering can be negligible.47,53 Therefore, we adopted the one-oscillator model in our following analysis. It should be noted that the thickness of as-foliated 2D perovskite microplates we used in our investigations is on the order of tens of nanometers (Figure S1), which suggests that the microplates are multiple layer not monolayer, according to the reported thickness of monolayer for different n number.42 Under such case, the vertical direction coupling in 2D perovskites has been included in the theoretical model we used. In order to verify this, we have carried out the same measurement for a bulk n=1 crystal since the phase of n=1 crystal is always pure. The comparisons of bulk crystal and microplate with a thickness of 50 nm show the similar trend of temperature dependent free exciton emission peak (Figure S6), supporting that our results can be well described by the theoretical model we used. By assuming a linear relationship between the lattice constant and temperature together with the one-oscillator model we discussed above, Eq. (1) can be simplified as38,40,41

Eg (T )  E0  ATET  AEP (

2  1) exp(h / k BT )  1

(3)

where ATE and AEP are the weight of the thermal expansion interaction and electronphonon interaction, respectively, k B is Boltzmann constant and h is the average optical phonon energy contributing to the electron-phonon coupling. E0 is the 12 ACS Paragon Plus Environment

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unrenormalized bandgap and Eg (T  0)  E0  AEP after taking into account the quantum factor in the Bose-Einstein distribution. By applying this equation to fit our temperature dependent PL peak curves, an excellent agreement with the experimental data has been produced for 2D perovskite microplates with all different layer number n (solid lines in Figure 3a and the extracted fitting parameters are listed in Table S1), suggesting that the simplified model captures the dominant factors for the change of the bandgap with temperature in our samples. To separate the contribution from thermal expansion interaction and electronphonon interaction explicitly, we calculated their respective contribution to the bandgap change with temperature according to the fitting parameters and plotted in Figure 3b for n=2 and in Figure S7 for other pure phase perovskites. We can clearly see that the PL peak shift is determined by the competition between the thermal expansion interaction and the electron-phonon interaction (Figure 3b and Figure S7) and eventually they together produce the temperature evolution of the bandgap. Through fittings, we also can investigate how those two factors contribute to the temperature evolution of bandgap for samples with different layer number n. As shown in Figure 3c, the weight of both the thermal expansion interaction and electron-phonon interaction continuously increases with the increases of the layer number n. Similarly, the contribution of the thermal expansion interaction ∆TE (defined as ETE (260 K) - ETE (77 K)) and the electron-phonon interaction ∆EP (EEP (260 K) - EEP (77 K)) extracted from Figure 3b are both gradually enhanced but the thermal expansion interaction gradually plays a dominant role over electron-phonon interaction with the increase of the layer number from n=1 to 5 (Figure 3d). As a result, the synergetic effect of thermal expansion interaction and electron-phonon interaction leads to the change of the bandgap from red-shift to blue-shift as the layer number n increases. 13 ACS Paragon Plus Environment


This result is further supported by the PL lifetime studies in 2D perovskites.20 While the enhanced electron-phonon interaction at a higher temperature would facilitate the nonradiative recombination and thus a decreased PL lifetime, the more active thermal expansion interaction at a higher temperature would prevent the exciton recombination, resulting in an increase of PL lifetime.20,40 Previous experiments have demonstrated that the PL lifetime increases with the increasing layer number n,20 further verifying that the thermal expansion interaction plays a more important role in the 2D perovskites with a larger layer number n. b)

n=1

250

200 150

c)

n=2

250

Temperature (K)

Temperature (K)

250

Temperature (K)

a)

200 150

PL (norm.) 1

n=3

0.8 200

0.6 0.4

150

0.2 100 2.4 Energy (eV)

2.6

d) 1.998

2.145 2.132

n=2 fit

2.4

0

e) n=3 fit

2.2 Energy (eV)

4.0 3.0

-100

AEP (meV)

1.971

2.0

2.0

-200 1.0

2.384 2.368 50

n=1 fit 100 150 200 250 Temperature (K)

1.8

2.0 Energy (eV)

0

2.2

250

f)

0

40

ΔEP ΔTE ΔEP+ΔTE

30

Energy (meV)

2.2

100

ATE (meV/K) Energy (meV)

100

Energy (eV)

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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20

-300

1

2

3

0.0

-250

n

1

2

3

n

Figure 4. a)-c) The steady-state temperature-dependent PL maps of (isoBA)2(MA)n−1PbnI3n+1 (n=1-3) microplates from 77 K to 290 K in 20 K steps. d) The temperature dependent emission peak extracted from a)-c). The solid lines are the fittings according to Eq. (3) for n=1-3. e) The weight of both the TE and EP interaction of n=1-3. f) The contribution of the thermal expansion interaction ∆TE (defined as ETE (260 K) - ETE (77 K)) and the electron-phonon interaction ∆EP (EEP (260 K) - EEP (77 K)) extracted from d. This model has been successfully applied to (iso-BA)2(MA)n−1PbnI3n+1 series, which 14 ACS Paragon Plus Environment

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shows a strikingly different temperature dependent bandgap. Compared with (nBA)2(MA)n−1PbnI3n+1 series, the organic cations in (iso-BA)2(MA)n−1PbnI3n+1 series were replaced by the short branched chain butylamine (iso-BA) cations which are structural isomers to linear n-BA cations.54,55 Figure 4a-c display the steady-state temperature-dependent PL maps of (iso-BA)2(MA)n−1PbnI3n+1 (n=1-3) microplates excited by a 473-nm laser from 77 K to 280 K in a 20 K step. Similar to the (nBA)2(MA)n−1PbnI3n+1 series, we also focus on the temperature evolution of free exciton emission of the pure phase (iso-BA)2(MA)n−1PbnI3n+1 (n=1-3) microplates based on the Voigt fittings. Compared to the red-shift of PL peak for (n-BA)2PbI4 with increasing temperature, the PL peak of (iso-BA)2PbI4 shows a blue-shift behavior and no phase transition within the temperature range we investigated, which can be further supported by the Differential scanning calorimetry (DSC) scans (Figure S8b). The similar behavior was observed for (iso-BA)2(MA)n−1PbnI3n+1 samples with n=2 and n=3. With the increase of the layer number n, similar red-shifts of the peak position occurs at 77 K and 260 K due to the weakening of the quantum confinement effect (Figure S8a). Following the same procedure in (n-BA)2(MA)n−1PbnI3n+1 series, we extracted the emission peak and plotted against the temperature as shown in Figure 4d. In order to be consistent with that in (n-BA)2(MA)n−1PbnI3n+1, we selected the same temperature range from 77 K to 260 K as well to analyze the temperature evolution of bandgap in (isoBA)2(MA)n−1PbnI3n+1 series. In contrast to that in (n-BA)2(MA)n−1PbnI3n+1 series, the emission peak exhibits a monotonic blue-shift with the increase of the temperature, which agrees well with 3D perovskites (Figure 4d). Eq. (3) also gives excellent fittings of the temperature dependent bandgap for all n=1-3 samples, from which we can separate the contribution from thermal expansion interaction and electron-phonon interaction (solid lines in Figure 4d and Table S2). In (iso-BA)2(MA)n−1PbnI3n+1 series, 15 ACS Paragon Plus Environment


the weight of the thermal expansion interaction shows only a slightly decrease from n=1 to n=2 and no change from n=2 to n=3 while the weight of the electron-phonon coupling almost maintains the same from n=1 to n=2 and exhibits a sudden increase from n=2 to n=3 (Figure 4e), both of which are in striking contrast to that in (nBA)2(MA)n−1PbnI3n+1 series and might be due to the more effective packing of branched alkane chains. The unchanged thermal expansion interaction and the sudden change of electron-phonon interaction from n=2 to n=3 result in that the curve of the emission peak versus temperature changes from nearly linear in n=2 to significantly nonlinear in n=3. Finally, the contribution of the thermal expansion interaction ∆TE (defined as ETE (260 K) - ETE (77 K)) and the electron-phonon interaction ∆EP (EEP (260 K) - EEP (77 K)) were extracted from Figure 4d and plotted in Figure 4f. As the layer number n increases, both of them decreases, also in contrast to that in (n-BA)2(MA)n−1PbnI3n+1 series (Figure 3d). As a result, those two factors together give rise to the blue-shift of the bandgap with increasing temperature for all samples with different n number we investigated. Previous studies indicate that the organic species in 2D perovskites would not significantly alter the room-temperature emission spectra but be able to control the phonon modes that the excitons can interact with.47 Our findings suggest that the organic species could not only affect the thermal expansion interaction but also the electron-phonon interaction, resulting in the strikingly different temperature evolution of the bandgap. With the same organic species, the change of the layer number n in 2D perovskites would also alter those two factors, leading to the different temperature dependent bandgap behavior for 2D perovskites with different layer number n. Furthermore, the structural phase transition also strongly depends on the organic species and layer number n. Therefore, the optical and optoelectronic properties of the 16 ACS Paragon Plus Environment

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2D perovskites can be greatly tuned in a wider range via organic species and layer number n due to the change of the quantum confinement effect, phase transition, thermal expansion interaction and electron-phonon interaction. This significant difference in temperature dependent PL from the organic species is probably mainly due to the different degree of the lattice distortion driven by n-BA and iso-BA cations. This different degree of the lattice distortion would lead to the different thermal expansion and electron-phonon interaction strength, giving rise to the different temperature dependent PL behavior we observed. How the organic chains affect the electronic properties of 2D perovskites has also been theoretically investigated previously, agreeing with our experimental results.56 In summary, we have systematically investigated how the organic species and the layer number n affect the temperature evolution of bandgap in 2D perovskites by temperature dependent PL spectroscopy. While the temperature dependent emission peak changes from red-shift for n=1 to blue-shift for n=5 with increasing temperature for (n-BA)2(MA)n−1PbnI3n+1 series, only a monotonous blue-shift has been observed for n from 1 to 3 in (iso-BA)2(MA)n−1PbnI3n+1 series. Those strikingly different temperature dependent bandgap change can be ascribed to the synergistic effect of the thermal expansion interaction and electron-phonon interaction tuned by the organic species and the layer number n. Our studies are not only of great significance for the fundamental understanding the optical and optoelectronic properties of 2D perovskites, but also have important implications for the development of the optoelectronic devices based on 2D perovskites at low temperatures.

Methods Sample preparation. 2D perovskites were synthesized according to the previous reports21. Methylammonium iodine (MAI) was synthesized by neutralizing an aqueous 17 ACS Paragon Plus Environment


methylamine (MA) and an aqueous hydroiodic acid (HI) with a molar ratio of 1:1 with stirring for 2 h at 0°C. The solvent was then evaporated at 60 °C and washed by diethyl ether three times followed by dried at 70°C for 12h. For synthesis of n-butylammonium iodide (n-BAI) solution, the same procedure was used except that the methylamine was replaced by n-butylamine (n-BA) and the stirring time was extended to 4h. For synthesis of (n-BA)2(MA)n-1PbnI3n+1 (n=1–5), PbO powder (0.5 g) was dissolved in a mixture of 57% w/w aqueous hydriodic acid (HI) solution (3ml) and 50% w/w aqueous hypophosphorous acid (H3PO2) solution (0.5 ml) by heating to 140°C under constant magnetic stirring. Subsequently, 2.5 mmol BAI solution was added into the resultant solution for the synthesis of (n-BA)2PbI4 plates while a mixture solution of (1.25mmol; 1.67 mmol; 1.86 mmol; 2 mmol) MAI and (1.75 mmol; 0.83 mmol; 0.62 mmol; 0.5 mmol) BAI were successively injected to the resultant solution for the synthesis of (nBA)2(MA)n-1PbnI3n+1 (n = 2–5) plates. Afterwards, the stirring was stopped, and the solution was left to naturally cool to room temperature. The precipitation was left to stand overnight for the growth to be completed. For synthesis of (iso-BA)2(MA)n1PbnI3n+1 (n=1-3)

solution, the same procedure was used except that the linear n-BA was

replaced by its isomer-short branched iso-BA. Material characterizations. Optical images were acquired on the Olympus BX53M system microscope and the atomic force microscopy images (AFM) were captured by a Bruker Dimension EDGE. The PL measurement were carried out in a backscattering conﬁguration using a Horiba HR550 system equipped with a 600 g/mm excited by a 473-nm and a 405-nm solid-state lasers with a power of 0.01~20 μW. The objective is a 50X with a long work distance of 11 mm and a numerical aperture (NA) of 0.60. The diameter of the laser spot size on samples is around 2 µm while the detected area is around 3 µm2. A liquid nitrogen continuous flow cryostat (Cryo Industry of America, 18 ACS Paragon Plus Environment

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USA) was used to control temperature variation from 77K to 290 K. Differential scanning calorimetry (DSC) measurement was conducted on a PSTA449F3 ( Netzsch, Germany). Acknowledgements D. L. acknowledges the support from NSFC (61674060) and the Fundamental Research

Funds

for

the

Central

Universities,

HUST

(2017KFYXJJ030,

2017KFXKJC002, 2017KFXKJC003 and 2018KFYXKJC016). We thank Testing Center of Huazhong University of Science and Technology for the support in differential scanning calorimetry (DSC) measurement. Supporting Information Available: OM images and AFM images of the as-foliated microplates; the related PL spectra and linewidth; the normalized PL spectra for a typical (BA)2PbI4 microplate at 280 K before and after low temperature PL measurements; the PL spectra with fitting results; the power-dependent PL spectra; the temperature dependent emission peak; DSC scans. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Green, M.A.; Ho-Baillie, A.; Snaith, H.J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506-514. (2) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; Losovyj, Y.; Zhang, X.; Dowben, P.A.; Mohammed, O.F.; Sargent, E.H.; Bakr, O.M. Low Trap-State Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519522. (3) Johnston, M.B.; Herz, L.M. Hybrid Perovskites for Photovoltaics: Charge-Carrier Recombination, Diffusion, and Radiative Efficiencies. Acc. Chem. Res. 2016, 49, 146154. 19 ACS Paragon Plus Environment


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Phys. Chem. Lett. 2018, 9, 3416-3424.


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