Lattice Dynamics and Thermal Stability of Cubic-Phase CsPbI3

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials 3

Lattice Dynamics and Thermal Stability of Cubic Phase CsPbI Quantum Dots Wei Zhou, Fan Sui, Guo-Hua Zhong, Guanming Cheng, Mingyue Pan, Chunlei Yang, and Shuangchen Ruan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02036 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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The Journal of Physical Chemistry Letters

Lattice Dynamics and Thermal Stability of Cubic Phase CsPbI3 Quantum Dots Wei Zhou,1, † Fan Sui,2, † Guohua Zhong,2 Guanming Cheng,2 Mingyue Pan,2 Chunlei Yang2, * & Shuangchen Ruan1, *

1

Shenzhen Key Laboratory of Laser Engineering, College of Optoelectronic

Engineering, Shenzhen University, Shenzhen, 518060, China. 2

Center for Photovoltaics and Solar Energy, Shenzhen Institutes of Advanced

Technology, Chinese Academy of Sciences, Shenzhen, 518055, China.

†

These authors have contributed equally to this work.

*

Correspondence and requests for materials should be addressed to S. R.

([email protected]) and C. Y. (email: [email protected]).

1

ACS Paragon Plus Environment

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ABSTRACT: Cubic phase CsPbI3 quantum dots (QDs) have been synthesized recently with merits of excellent optoelectronic performance. However, vital properties of cubic CsPbI3 including lattice dynamics and stability at high temperature remain poorly explored. We fabricate cubic CsPbI3 QDs, and study its lattice dynamic and thermal stability to 700 K. We obtain Raman modes of cubic CsPbI3 QDs from 300 to 500 K at ultra-low frequency range down to 15 cm-1, consistent with first principles calculations. Above 550 K, the modification of Raman features suggests sample degradation. Consistently, temperature dependent PL measurements indicate the absence of other luminescent phases up to 700 K. With increasing temperature, CsPbI3 QDs photoluminescence peak has a blue-shift with exponentially decreasing intensity, showing faster electronic degradation than structural degradation. Our work provides detailed investigation of CsPbI3 QDs lattice dynamics, band gap and their high temperature behavior, potentially useful for their emerging optoelectronic applications.

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Inorganic perovskites CsPbX3 (X=Cl, Br, I) have emerged as excellent optoelectronic materials, owing to their merits of ultrahigh luminescence quantum yield (~100%) and tunable broad spectrum emissions from the ultraviolet to the visible (UV-VIS) spectral range.1,2 Recently, cubic phase CsPbI3 QDs were fabricated at room temperature,3 whereas the bulk CsPbI3 of cubic phase can only exist above 320 oC.3,4 Besides non-volatile inorganic structure, the merits of cubic CsPbI3 QDs include the most suitable bandgap of 1.73 eV, low cost and high light absorption efficiency for solar cell applications,5 high crystallinity with low intrinsic defects, as well as self-surface passivation without the core-shell structure of general chalcogenides QDs.6 Therefore, CsPbI3 QD is among outstanding candidates for optoelectronic

applications

including

light-emitting

diodes,6,7

solar

cells,5,8

photodetectors9 and lasers.10,11 Structure stability and device performance at room and high temperatures are main obstacles for optoelectronic applications of cubic CsPbI3 QDs. With increasing temperature, CsPbI3 has several phases including two orthorhombic (black γ and non-perovskite yellow δ), tetragonal (β), and cubic (α) phases.12 Their different crystal and electronic structures will result in different lattice vibrations and band gaps, key factors to determine material property and performance. For example, the anomalous band gap blue-shift with increasing temperature of cubic CsPbI3 is due to the strong carrier-phonon coupling and giant band gap renormalization effect,13,14 which are also present in lead halide perovskite nanocrystals15 and cubic phase hybrid perovskite CH3NH3PbI3.16 The highly correlated crystal and electronic structures of perovskites 3



are also shown through the long range carrier transport distance and lifetime.17,18 However, to the best of our knowledge, the experimental observation of cubic CsPbI3 lattice vibrations still remains less explored, the main difficulty is the metastable bulk CsPbI3 of cubic phase at room temperature. Recently, nanocrystals of cubic CsPbI3 like QDs can be fabricated at room temperature.3 It provides a good opportunity to investigate vital vibrational properties of cubic CsPbI3 through experiments: whether CsPbI3 QDs’ lattice dynamics is consistent with numerical simulations, whether CsPbI3 QDs’ can remain stable at high temperature, and whether CsPbI3 QDs’ temperature dependent band gap is related to lattice vibrations. In this work, we first fabricate 20 nm size high quality colloidal CsPbI3 QDs, then we study its lattice dynamics and luminescent properties by Raman and photoluminescence (PL) measurements from 300 to 700 K. The observed Raman features are consistent with our first principles calculations and remain stable up to 500 K, above which the disappearance of both characteristic Raman and PL features suggests sample degradation. Meanwhile, PL measurements show blueshift of PL peak and exponential luminescence quenching with increasing temperature, indicating a faster electronic degradation than structural degradation. Our work provides useful information of cubic CsPbI3 QDs lattice dynamics, phase stability, band gap and their high temperature behavior for further theoretical and experimental exploration. CsPbI3 QDs were synthesized via the hot injection route.1 The light absorption spectrum has an absorption edge at 700 nm, as shown in Fig. 1(a). Under 450 nm excitation, QDs exhibited bright red luminescence emission centered at 690 nm with 4


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narrow width of 34 nm, and no photobleaching was observed. In Fig. 1(b) of powder x-ray diffraction (XRD) result (Bruker D8 diffractometer with Cu Kα radiation), the standard Bragg positions of cubic perovskite crystal structure with space group of Pm3m were confirmed. In the scanning transmission electron microscopy (STEM) image (Fig. 1(c)), cubic shape CsPbI3 QDs have size distribution of 20~25 nm. In the high resolution transmission electron microscopy (HRTEM) image (Fig. 1(d)), the lattice fringes spacing was 6.28 Å, corresponding to the planar spacing of cubic perovskite structure,19 also match our XRD results. CsPbI3 nanocubes crystallized into the cubic structure at room temperature, rather than the orthorhombic phase of bulk CsPbI3. The existence of cubic phase CsPbI3 QDs at room temperature is due to the effect of both synthesis temperature and surface energy.1 As a comparison, CsPbI3 nanowires synthesized using large oleylamine surfactant amount and long reaction time were reported to be the orthorhombic phase, whose crystal lattice might be distorted during the anisotropic growth of certain crystal facet.3 We first spin coated colloidal QDs on a cleaned Si wafer, then deposited 50 nm Al2O3 layer above the film by atomic layer deposition (ALD). Quality and thickness of the Al2O3 layer were confirmed in the scanning electron microscope (SEM) cross section image (insert of Fig. 2). Raman measurements were performed using a Horiba-JY T64000 system in the backscattering geometry, the incident laser wavelength is 514.5 nm with spot size about 2 µm in diameter. A Linkam stage was used to increase temperature from 300 to 700 K in the step of 50 K, and the dwelling time was 15 minutes to ensure the thermal equilibrium before measurements. For 5



ultra-low frequency Raman measurements, Horiba low frequency Raman suite was used to obtain signals down to 15 cm-1. Laser power was kept as 0.5 mW and 90s was used for every spectrum. For PL measurements, the continuous wave 514.5 nm laser power was as low as 0.01 mW to minimize the possible heating effect, and 0.1s was used for every PL spectrum. Raman shifts and PL peaks were analyzed by the Lorentzian fitting procedure. We show CsPbI3 QDs Raman spectra and shifts at selected temperatures in Fig. 2 and Fig. 3, separately. Using the non-resonant 514.5 nm excitation laser, we obtained symmetric Stokes and anti-Stokes Raman modes consistently. At 300 K, there are four dominant Raman modes at 24.5, 35.3, 55.1 and 108.8 cm-1, and three weak ones at 20.5, 47.8 and 85.1 cm-1. To analyze the observed Raman modes, we calculated lattice dynamic of CsPbI3 by using the density functional perturbation theory as implemented in a plane-wave pseudopotential method in the QUANTUM ESPRESSO code.20 In the calculation, the Hartwigsen-Goedecker-Hutter-Teter (PZ-HGH) pseudopotentials were adopt and the lattice parameter was set as the experimental value of 6.28 Å. Fig. 4(a) shows the crystal configurations viewed from different directions. The calculated Raman shifts are consistent with our experimental observation and previous calculations.21 In Fig. 4(b), the calculated modes of 17.8, 26.0, 33.3, 55.9 and 114.6 cm-1 fit well with those measured ones at 20.5, 24.5, 35.3, 55.1 and 108.8 cm-1. The Raman shift difference can be induced by the quantum confinement effect, which typically leads to a redshift of Raman frequency with increased full width at half maximum (FWHM) for nanostructured samples with 6


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decreasing size. Meanwhile, selection rules of bulk samples can be relaxed with some forbidden modes observed. For several important modes, we investigated the vibration character to identify the origination. As shown in Fig. 4(c), the T1u mode at -27.5 cm-1 completely originates from the Cs atomic vibrations opposite to I ions. The relative large and inverse vibrations of Cs and I atoms result in the imaginary frequency, which supports the structure instability and non-existence of bulk cubic phase CsPbI3 single crystals at ambient conditions, consistent with first principles calculations of structural instability of cubic CsPbI3 and CsSnI3.22,23,24 The low frequency mode at 26.0 cm-1 mainly results from the Pb atom coupling with the vibrations of I atoms. The moderate frequency mode at 55.9 cm-1 mainly results from the bending vibrations of Pb-I bonds. The highest A1g mode at 114.6 cm-1 was driven by the stretching vibrations of I atoms in octahedron. Additionally, the phonon density of states (PhDOS) shown in Fig. 4(d) determines the phonon vibrations feature from the full momentum space. As shown in Fig. 2, during sample degradation process at high temperature, dominant and weak mode intensities have slight difference, weak peaks at low wavenumbers shift to lower energies and disappear first. The difference meets theoretical expectation since the weak bonds of Cs cations in the lattice are affected by the elevated temperature while the stronger bonds in Pb-I octahedron can preserve. Similar to the behavior of normal materials, Raman mode peak positions redshift with increasing temperature are plotted in Fig. 3. From 300 to 500 K, 7 modes around 20.5, 24.5, 35.3, 47.8, 55.1, 85.1, and 108.8 cm-1 only shift 0.6, 2.3, 2.9, 0.5, 3.4, 2.2 and 7



2.3 cm-1, corresponding to temperature coefficients from -0.0025 to -0.017 cm-1/K. The similar thermal behavior of CsSnCl3 single crystal has also been observed, such as temperature coefficient of -0.004 cm-1/K for the sharp 24.1 cm-1 Ag mode.25 Clear and similar Raman features of CsPbI3 QDs from 300 to 500 K indicate the absence of phase transition and samples degradation. At 550 K, the weak mode at 82.9 cm-1 has a sudden intensity increase and other 6 modes become much weaker. Meanwhile, a new mode at 137.4 cm-1 is present with strong intensity. These two modes become main Raman features up to 700 K, with several weak modes around 20 and 50 cm-1. After cooling down to 300 K, the Raman spectrum contains one strong 137 cm-1 mode, and several weaker ones different from those of CsPbI3 QDs. There is no reversible recrystallization process observed, we believe CsPbI3 QDs have degraded or decomposed to non-luminescent materials at such high temperatures, consistent with the following PL measurements. PL spectra of CsPbI3 QDs at different temperatures are shown in Fig. 5(a). From 300 to 500 K, although PL spectra have similar shapes and peak positions, PL intensity is strongly decreased and eventually quenched. In Fig. 5(b), the solid line is the exponential fitting of PL peak intensity using the formula of I=I1exp (-T/T1)+C, where I1 and C are constant and background, T and T1 are temperature and temperature constant. T1 is fitted as a small value of 7.2 K, indicating the sensitive temperature response of CsPbI3 QDs luminescence. The similar PL intensity quenching above 300 K has also been observed in cubic CsPbBr3 nanocrystals.26 In Fig. 5(c), PL peak position is plotted with respect to temperature and the solid line is 8


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the linear fitting. From 300 to 500 K, PL peak position increases from 1.792 to 1.821 eV, with the temperature coefficient approximately 0.15 meV/K. The positive temperature dependence of band gap has also been observed in cubic CsPbBr3 and CsSnI3,26,27 different from general semiconductors such as Si and GaAs. The underlying mechanism is attributed to the thermal contribution effect, where the band gap increases linearly with lattice parameter under heating from first principles calculations.27 Whereas for general semiconductors, the electron-phonon interaction dominates the thermal effect due to their much smaller effective electron mass, and induces decreased band gaps with increasing temperature. Meanwhile, FWHM of CsPbI3 QDs has increased from 0.1 to 0.18 eV, 80% increasing as shown in Fig. 5(d), also similar to the high temperature behavior of cubic CsPbBr3 nanocrystals.26 As shown in Fig. 5(a) for spectra above 550 K and back to 300 K, no clear PL peaks are observed, showing the irreversible sample degradation to non-luminescent materials, consistent with Raman results. Besides the absence of photobleaching effect, we observe no orthorhombic PL features, which contain strong PL peaks around 540 and 430 nm (2.3 and 2.9 eV), and PL peak position should redshift rather than blueshift with increasing temperature.3 Since the photo-excitation and light scattering effects can mark different phase perovskites clearly, PL and Raman spectroscopy provide convenient and sensitive approaches to probe sample phase and quality, such as to determine the quality and multiple stage structure transformation of perovskite CH3NH3PbI3 under light illumination by Raman spectroscopy.28,29 Compared with the stable crystal structure up to 500 K revealed by Raman 9



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measurements, PL measurements show a fast degradation of luminescence, implying cubic CsPbI3 electronic transitions are more sensitive to temperature than lattice vibrations. The decoupling of structural and electronic degradation is similar to the recent photoexcited insulator-to-metal transition in V2O3, where ultrafast x-ray scattering and nanoscopy techniques indicate the decoupling of structural and electronic responses in time domain with a transient photoinduced precursor phase.30 Perovskites

structural

and

electronic

transitions

under

temperature

and

photoexcitation are highly correlated, their strong electron-phonon interaction,17 giant band gap renormalization due to lattice vibrations,13 long carrier lifetime and diffusion distance18 deserve future work especially ultrafast measurements. In summary, we have synthesized high quality 20 nm size cubic phase perovskite CsPbI3 QDs, and investigated its high temperature lattice vibrations and luminescence behavior by ultra-low frequency Raman and PL experiments. At room temperature, characteristic Raman modes of cubic CsPbI3 QDs are observed, consistent with first principles calculations. With increasing temperature from 300 to 500 K, Raman modes of CsPbI3 QDs have small peak position and intensity variations, showing its lattice dynamics stability to thermal perturbations. Meanwhile, the PL peak position blueshifts with a rate of 0.15 meV/K and the PL intensity exponentially decreases. Around 550 K, Raman and PL signals both indicate the disappearance of cubic phase. Our results provides useful information of CsPbI3 QDs lattice vibrations, luminescence and their high temperature behavior. Furthermore, our work indicates that optical spectroscopy is a convenient and reliable method to monitor perovskites 10


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quality and phase stability due to their strong light-matter interaction and sensitive response to environments.

ACKNOWLEDGMENTS The authors declare no competing financial interest. This work was supported by the National Natural Science Foundation of China (61575129, 21701185, 61574157, 61774164, 51474132 and 61604166), Natural Science Foundation of SZU (2017014).

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Optical Mater. 2017, 5, 1700231. (14) Yang, R. X.; Skelton, J. M.; da Silva, E. L.; Frost, J. M.; Walsh, A. Spontaneous Octahedral Tilting in the Cubic Inorganic Cesium Halide Perovskites CsSnX3 and CsPbX3 (X= F, Cl, Br, I). J. Phys. Chem. Lett. 2017, 8, 4720−4726. (15) Iaru, C. M.; Geuchies, J. J.; Koenraad, P. M.; Vanmaekelbergh, D.; Silov, A. Y. Strong Carrier–Phonon Coupling in Lead Halide Perovskite Nanocrystals. ACS Nano 2017, 11, 11024−11030. (16) Yang, J. Y.; Hu, M. Temperature-Induced Large Broadening and Blue Shift in the Electronic Band Structure and Optical Absorption of Methylammonium Lead Iodide Perovskite. J. Phys. Chem. Lett. 2017, 8, 3720−3725. (17) Kawai, H.; Giorgi, G.; Marini, A.; Yamashita, K. The Mechanism of Slow Hot-Hole Cooling in Lead-Iodide Perovskite: First-Principles Calculation on Carrier Lifetime from Electron-Phonon Interaction. Nano Lett. 2015, 15, 3103−3108 (18) Guo, Z.; Wan, Y.; Yang, M.; Snaider, J.; Zhu, K.; Huang, L. Long-Range Hot-Carrier Transport in Hybrid Perovskites Visualized by Ultrafast Microscopy. Science, 2017, 356, 59-62. (19) Marronnier, A.; Roma, G.; Boyer-Richard, S.; Pedesseau, L.; Jancu, J. M.; Bonnassieux, Y.; Katan, C.; Stoumpos, C. C.; Kanatzidis, G. M.; Even, J. Anharmonicity and Disorder in the Black Phases of Cesium Lead Iodide Used for Stable Inorganic Perovskite Solar Cells. ACS Nano 2018, 12, 3477−3486.

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Captions: Figure 1. (Color online) Optical properties, phase and morphology characterizations of colloidal perovskite CsPbI3 QDs: (a) UV-VIS absorption (black) and PL spectra (red) of CsPbI3 QDs. (b) collected powder XRD pattern fits the Bragg peaks of cubic phase. (c) STEM image of CsPbI3 QDs nanocube morphology; (d) TEM image of lattice fringe of the plane of CsPbI3 QDs to confirm the cubic crystal structure. Figure 2. Raman spectra of CsPbI3 QDs at selected temperatures from 300 to 700 K. Spectra are shifted vertically for clarity. The arrow indicates temperature changing order and spectra are shifted vertically for clarity. The inset is the cross section SEM image, the grey color is the silicon substrate, covered by 50 nm Al2O3 layer. The scale bar is 500 nm. Figure 3. (Color online) Raman shifts of CsPbI3 QDs for selected temperatures from 300 to 700 K. Figure 4. (Color online) The structure and dynamic properties of CsPbI3 bulk: (a) crystal configurations viewed from different directions, (b) calculated Raman shift comparing with experimental values, (c) mode patterns for selected modes in CsPbI3 and (d) phonon density of states projected on each atom. Figure 5. (Color online) (a) PL spectra of CsPbI3 QDs at selected temperatures. The black arrow indicates temperature changing order. (b) PL intensity, (c) peak energy and (d) FWHM with respect to temperature. The solid lines are the fittings of data.

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Figure 1. (Color online) Optical properties, phase and morphology characterizations of colloidal perovskite CsPbI3 QDs: (a) UV-VIS absorption (black) and PL spectra (red) of CsPbI3 QDs. (b) collected powder XRD pattern fits the Bragg peaks of cubic phase. (c) STEM image of CsPbI3 QDs nanocube morphology; (d) TEM image of lattice fringe of the plane of CsPbI3 QDs to confirm the cubic crystal structure.

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Figure 2. Raman spectra of CsPbI3 QDs at selected temperatures from 300 to 700 K. Spectra are shifted vertically for clarity. The arrow indicates temperature changing order and spectra are shifted vertically for clarity. The inset is the cross section SEM image, the grey color is the silicon substrate, covered by 50 nm Al2O3 layer. The scale bar is 500 nm.

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Figure 3. (Color online) Raman shifts of CsPbI3 QDs for selected temperatures from 300 to 700 K.

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Raman shift (cm )

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Figure 4. (Color online) The structure and dynamic properties of CsPbI3 bulk: (a) crystal configurations viewed from different directions, (b) calculated Raman shift comparing with experimental values, (c) mode patterns for selected modes in CsPbI3 and (d) phonon density of states projected on each atom.

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Figure 5. (Color online) (a) PL spectra of CsPbI3 QDs at selected temperatures. The black arrow indicates temperature changing order. (b) PL intensity, (c) peak energy

1E2

1E3 1E2 1E1 1E0

(c)

1.84 1.82

1E1

1.8 0.4

(d)

0.3

1E0

0.2 0.1 1E-1

1.6

1.8

2.0

2.2

Energy (eV)

300

Peak energy (eV)

1E3

(b)

350

400

450

500

Temperature (K)

22


550

FWHM (eV)

300 K 350 K 400 K 450 K 500 K 550 K 300 K

(a)

PL intensity (a.u.)

and (d) FWHM with respect to temperature. The solid lines are the fittings of data.

PL 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

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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


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140

120 -1

Raman shift (cm )

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


100

80

60

40

20 300

400

500

Temperature (K)


600

700

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


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1E3 1E2 1E1 1E0

(c)

1E2

1.84 1.82

1E1

1.8 0.4

(d)

0.3

1E0

0.2 0.1 1E-1

1.6

1.8

2.0

Energy (eV)

2.2

300

PL intensity (a.u.)

1E3

(b)

Peak energy (eV)

300 K 350 K 400 K 450 K 500 K 550 K 300 K

(a)

PL 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


350

400

450

500

Temperature (K)


550

FWHM (eV)

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Lattice Dynamics and Thermal Stability of Cubic-Phase CsPbI3

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