Optical Properties of a CsMnBr3 Single Crystal.

Lead-free perovskite materials with good stability are promising for various applications. In order to explore their application in optoelectronic devices, it is essential to investigate their fundamental optical properties. In this work, we have synthesized a CsMnBr3 single crystal (SC) with red emission at ∼621 nm and studied their optical properties. Through the measurement of temperature-dependent photoluminescence (PL) spectra, it is found that a phase transition occurs at approximately 100 K in the SC, which is absent in the CsMnBr3 nanocrystals (NCs). Furthermore, the SC exhibits stronger electron and longitudinal optical phonon coupling strength than that of the NCs at low temperatures. In addition, under the resonant excitation at 600 nm, the SC possesses strong saturable absorption property, with a modulation depth of ∼27%. Interestingly, the SC also exhibits a large two-photon absorption coefficient of ∼0.035 cm GW-1 at 800 nm and an excellent optical limiting behavior. The experimental results indicate that the CsMnBr3 SC is a class of excellent environmentally friendly optoelectronic materials.

Introduction

All inorganic perovskite materials (CsPbX3, X = Cl, Br, and I) have been widely used in various applications, such as light detection, photovoltaic cell, and laser, because of their excellent optoelectronic properties, including high photoluminescence quantum yield (PLQY), widely tunable PL emission, and large absorption cross section.[1] However, the intrinsic toxicity of these materials greatly hinders their practical applications. Therefore, to reduce the toxicity of the materials, researchers have synthesized various lead-free perovskite materials, with Cu2+, Sn2+, and Bi3+ as substitutes for Pb2+,[2−4] and developed their application in various optoelectronic devices.

In recent years, Mn2+ has been continuously studied as a dopant and an alternative element for Pb2+ due to its unique luminescence,[5,6] which has green emission in the form of tetrahedral coordination and red emission in the form of octahedral coordination. As a newly emerging lead-free perovskite material, CsMnBr3 has attracted intensive research interest.[7,8] However, although the research on CsMnBr3 nanocrystals (NCs) is relatively adequate, CsMnBr3 single crystals (SCs) have not been extensively studied. Compared with the NC counterparts, SCs are more suitable for preparing large-area optoelectronic devices.[9] The study on the optical properties of CsMnBr3 SCs is a prerequisite for realizing their relevant application. For example, the electron–phonon coupling effect in CsMnBr3 SCs can be investigated through the measurement of the temperature-dependent PL spectrum, which is crucial for developing high-efficiency photovoltaic and laser devices.[10,11] Investigating the nonlinear optical (NLO) behavior of CsMnBr3 SCs is important to develop their applications in Q-switching and mode-locking techniques[12,13] and frequency upconversion laser.[14,15] Nevertheless, there is still a lack of in-depth investigation on these fundamental optical properties of CsMnBr3 SCs, which is unfavorable to expand their various applications.

In this work, we synthesized a CsMnBr3 SC and studied its optical properties. Phase transition was observed at a temperature of approximately 100 K. In addition, it was found that the CsMnBr3 SC exhibited strong saturable absorption and two-photon absorption (TPA).

Methods

Sample Preparation

The CsMnBr3 SC was obtained through a slow evaporation method described in the previous literature.[7] Briefly, 0.55 mmol CsBr and 1 mmol MnBr2 were added to the mixed solution of water and hydrobromic acid (19:1 in weight) and then heated to 40 °C in ambient conditions. As a result, the CsMnBr3 SC with a size in millimeter level was formed after storage for 5 days. CsMnBr3 NCs were prepared through a hot-injection method according to a previous procedure with a slight modification.[8] During the synthesis process, 1-octadecene was used as a solvent, while oleic acid and oleylamine were used as ligands. The trimethylbromosilane precursor was rapidly injected into the solution of cesium acetate and manganese acetate precursor at 170 °C. After reaction for 20 s, an ice–water bath was used to put out the reaction. CsMnBr3 NCs were then obtained by a centrifugation method.

Characterization

The crystal structure of the CsMnBr3 SC was determined by using X-ray diffraction (XRD, Escalab 250Xi), while the morphology of NCs was analyzed by using transmission electron microscopy (TEM, JEM-2100F). Excitation and PL spectra and lifetimes were recorded by an Edinburgh FS5 spectrometer. PLQYs were obtained using an FS5 fluorescence spectrometer with an integrating sphere. The temperature-dependent PL spectra were collected from 10 to 300 K, which was excited by an He–Cd laser at 442 nm.

NLO Measurements

Our home-built microregion NLO system was used to investigate the NLO properties of CsMnBr3 SC. The fs pulses at 600 and 800 nm output from TOPAS (Spectra-Physics, 1000 Hz, 100 fs) were used as resonant and nonresonant excitation sources, respectively. The PL spectra under 800 nm excitation were collected by a 20× objective lens (NA = 0.45) and then detected by the charged coupling device (SpectraPro HRS-300). The TPA coefficient of CsMnBr3 SC was determined using the nonlinear transmittance method.[16]

Results and Discussion

The XRD pattern of CsMnBr3 SC is depicted in Figure a, which reveals its hexagonal crystal structure.[17] The TEM image of CsMnBr3 NCs confirms their hexagonal morphology (Figure b). Under the excitation of 278 nm, both CsMnBr3 SC and NCs exhibit red emission, with their PL spectra peaking at ∼621 and ∼653 nm, respectively (Figure a,b). The PLQYs of CsMnBr3 SC and NCs were determined as 26 and 53%, respectively. Although both the samples possess multiple peaks in the PL excitation spectra, only a single PL peak was observed. In addition, we measured excitation-wavelength-dependent PL spectra of the SC and NCs (Figure S1). It was found that the PL peaks of the two samples did not change at different excitation wavelengths, indicating that the same emissive states were involved for both of them. The PL emission for both the CsMnBr3 SC and NCs was ascribed to the transition from 4T1 to the ground state 6A1.[7] It was found that the CsMnBr3 SC still exhibited bright red emission even when it was kept at about 35% humidity for over 6 months. The time-resolved PL decay curves of CsMnBr3 SC and NCs were measured at 300 K under the excitation of 422 nm (Figure c,d), determining their lifetimes as ∼68.64 and ∼240.70 μs through the monoexponential fitting. Because more defects were formed in the SC and the surface defects on the CsMnBr3 NCs can be effectively passivated by the ligands, the PL lifetime of the former is much shorter than that of the latter. It should be noted that the PL lifetime in our CsMnBr3 NCs was much different from those in previous literature, which should be caused by the different local structures around Mn ions.[7,8]

Figure 1

(a) XRD pattern of CsMnBr3 SC. (b) TEM image of CsMnBr3 NCs.

Figure 2

(a,b) Excitation and emission spectra of CsMnBr3 SC and NCs. Insets: their photographs under UV light irradiation. (c,d) Time-resolved PL decay curves of CsMnBr3 SC and NCs.

To analyze the low-temperature optical properties of CsMnBr3 SC, its PL spectra were measured as a function of temperature ranging from 10 to 300 K by using a 442 nm He–Cd laser as the excitation source (Figure a,b). As depicted in Figure c, as the temperature increases from 10 to 100 K, the PL peak of CsMnBr3 SC exhibits a bathochromic shift. However, a blue shift of the PL peak was observed from 100 to 300 K. The sudden change of the PL peak energy at approximately 100 K should be associated with the occurrence of phase transition. Although the phase transition was reported in other relatively soft materials, such as the MAPbI3 bulk film,[18] this is the first time that the phase transition was observed in the CsMnBr3 SC. More experimental investigation was needed to further confirm the mechanism for the occurrence of phase transition in the SC.

Figure 3

Pseudo-color maps of the temperature-dependent PL spectra of (a) CsMnBr3 SC and (b) CsMnBr3 NCs. PL peak energy and fwhm as a function of temperature for (c,e) CsMnBr3 SC and (d,f) CsMnBr3 NCs.

In contrast, the PL peak of CsMnBr3 NCs exhibited a monotonous blue shift with increasing temperature (Figure d), indicating the absence of phase transition. Actually, previous literature implied that colloidal NCs generally exhibit better thermodynamic phase stability than bulk materials, which was caused by the reduced crystal-field strength and smaller splitting energy of the excited state during thermal expansion.[8,19] Compared with the SC, the better phase stability of CsMnBr3 NCs should be caused by the ligand-induced change in surface energy. Actually, such kind of stabilization phenomenon was also reported in CH3NH3PbBr3 and FAPbI3 that were modified by appropriate ligands.[20,21] Therefore, the phase transition observed in the CsMnBr3 SC may be ascribed to the absence of the ligand-induced phase stabilization effect.

The change of full width at half-maximum (fwhm) of the PL spectrum is correlated with electron-phonon coupling.[22] Such an effect has been widely investigated through the measurement of temperature-dependent PL fwhm in Mn-doped perovskite NCs,[23] but it has never been reported for the CsMnBr3 SC. Therefore, the PL fwhm of the CsMnBr3 SC and NCs at different temperatures was analyzed. As shown in Figure e, the PL fwhm of the CsMnBr3 SC broadens at temperatures ranging from 10 to 140 and 200 to 300 K. In contrast, the PL fwhm decreases between 140 and 200 K, probably caused by the occurrence of phase transition. As expected, the PL fwhm of CsMnBr3 NCs broadens monotonously as the temperature increases from 10 to 300 K (Figure f), which is similar to those of CsPbX3 (X = Cl, Br, and I) NCs.[24] The PL fwhm as a function of temperature can be expressed as[25,26]where Γinh is the inhomogeneous broadening term, σ is the coupling coefficient of electron and acoustic phonon, ΓLO is the coupling strength between the electron and longitudinal optical (LO) phonon, and ELO is the LO phonon energy. Through theoretical fitting, for the SC at 10–100 K, σ is ∼37.7 μeV K–1, while ΓLO and ELO are ∼132.7 and ∼8.4 meV, respectively. At 200–300 K, σ is ∼2.6 μeV K–1, while ΓLO and ELO are ∼40.1 and ∼20.3 meV, respectively. For the CsMnBr3 NCs, during the whole measured temperature range, σ is ∼13.7 μeV K–1, and ΓLO and ELO are ∼90.8 and ∼29.1 meV, respectively. Compared with CsMnBr3 NCs, the SC has stronger electron and LO phonon coupling strength at low temperatures but weaker coupling strength at high temperatures.

Next, the NLO properties of the CsMnBr3 SC and NCs were comparatively investigated under resonant and nonresonant excitation (600 and 800 nm). However, no obvious NLO signals were detected in the NCs under the excitation of above two wavelengths, probably caused by the low concentration of CsMnBr3 NCs (1 × 10–5 M). Considering that the SC has more advantages in the practical application of optoelectronic devices, only the NLO properties of CsMnBr3 SC were studied in the following discussions. Under the resonant excitation at 600 nm, it was found that the transmittance of the CsMnBr3 SC increased with increasing excitation intensity (Figure ), confirming the occurrence of saturable absorption. The excitation intensity-dependent transmittance of the sample can be fitted using the following formula[27]where As is the modulation depth, I0 is the incident intensity, Isat is the saturable optical intensity that is required in a steady state to reduce the absorption to half of its unbleached value, and ns is the non-saturable loss. Through the theoretical fitting using eq , the As value of the SC is determined as ∼27%, which is comparable to that of MoS2 dispersion (∼34%, 800 nm),[28] but much larger compared with the CsPbBr3 NC film (13.1%, 1060 nm).[29] Notably, the CsMnBr3 SC exhibits a high damage threshold (>4000 GW cm–2), implying that it is an excellent saturable absorber that may be used for Q-switching and mode-locking techniques.

Figure 4

Excitation intensity-dependent transmittance of the CsMnBr3 SC and the corresponding theoretical fitted curve under the excitation at 600 nm (100 fs, 1000 Hz).

Subsequently, the NLO behavior of the CsMnBr3 SC under the non-resonant excitation was investigated. Bright PL emission peaking at ∼621 nm was easily observed under 800 nm excitation (Figure a). In addition, it was found that the PL intensity was proportional to the square of excitation power (inset of Figure a), confirming the TPA mechanism at 800 nm.[30] The TPA coefficient of the SC was then measured using the nonlinear transmittance method.[16] The corresponding experimental data are shown in Figure b, which can be fitted using the following equationwhere L is the thickness of the sample and β is the TPA coefficient.[16] Through the theoretical fitting, the β value of the CsMnBr3 SC was determined as ∼0.035 cm GW–1, which is 1 order of magnitude larger than that of Cs3Cu2I5 SC (5.1 × 10–3 cm GW–1, 600 nm),[31] possibly due to the larger density of states in the former. Moreover, under the excitation intensity of 1.5 × 104 GW cm–2, the transmittance can decrease to 34% and no obvious saturation effect was observed. It indicates that the CsMnBr3 SC can be used as an excellent optical limiter, with a limiting threshold of ∼0.9 × 104 GW cm–2. It will be interesting to determine the wavelength-dependent 2PA coefficients of the CsMnBr3 SC. However, we cannot obtain the 2PA coefficients at other wavelengths. Even so, the 2PA coefficient of this material was determined at the most common wavelength, that is, 800 nm, which can provide important information for the potential application in NLO devices.

Figure 5

(a) PL spectra of the CsMnBr3 SC under different excitation intensities at 800 nm (100 fs, 1000 Hz). Insets: The relationship between the PL intensity vs excitation power and the microscopic image under 800 nm excitation. (b) Transmittance under different excitation intensities and the relevant theoretical fitting curve.

Conclusions

In conclusion, we investigated the low-temperature PL spectra and NLO properties of the CsMnBr3 SC. Unlike the colloidal NC counterpart, the SC underwent a phase transition at approximately 100 K, probably due to the absence of the ligand-induced phase stabilization effect. At low temperatures, the SC exhibited stronger electron and LO phonon coupling strength than that of the NCs. Notably, the CsMnBr3 SC possesses excellent saturable absorption and TPA properties, indicating that it is promising for the application in Q-switching and mode-locking techniques and as an optical limiter.