Pressure-induced emission and remarkable piezochromism of two-dimensional cesium antimony bromide perovskites

Abstract

There exist some perovskites materials with no photoluminescence (PL), which will greatly limit the practical applications in photodetection, display and lighting. Here, we achieve an exotic pressure-induced emission (PIE) at a mild pressure of 0.8 GPa from initially non-emissive 2D all-inorganic perovskite Cs₃Sb₂Br₉ quantum dots, when the sample was subjected to external pressure. With the increase of pressure, the PL intensity gradually increases and the emission color transforms from red to green. Combined with subsequent experiments and computations, thus PIE behavior and piezochromism result from the SbBr₆ octahedral distortion, accompanied by a structural phase transition from trigonal to monoclinic under pressure. Our work provides a robust strategy to boost the emission efficiency and to construct multi-functional PIE materials with piezochromism in environmentally friendly perovskites, thus facilitating the diverse applications in futural practices.

GRAPHICAL ABSTRACT

KEYWORDS:

• Halide perovskite
• pressure-induced emission
• piezochromism
• high pressure

Introduction

Recently, the enormous optical applications of halide perovskites have attracted wide attention of scientists [1–5]. CsPbBr₃, a prime example known for its high monochromaticity and quantum yield, has been extensively studied and reported [6–8]. However, the toxicity associated with the lead element and the sensitivity to heat and oxygen significantly limits its practical applications. Therefore, exploring new materials with high stability and low toxicity is currently a research focus [9–16]. During this process, scientists have shown interest in studying all-inorganic A₃M₂X₉ structures with excellent stability [17,18]. The distinguishing feature of these structures compared to traditional ABX₃ structures is that one-third of the B sites in the conventional ABX₃ structure are replaced by vacancies. In addition, the problem of high toxicity is effectively solved by lead-free ions such as Bi³⁺ or Sb³⁺ acting as M position [19–22].

Nonetheless, A₃M₂X₉ structures often demonstrate low photoluminescence (PL) efficiency, which largely limits their practical applications. To optimize their structure and electronic states, it is imperative to precisely regulate external conditions such as pressure or temperature. These external factors possess the inherent capability to modulate optical and electrical properties through lattice compression or expansion [23–27]. Therefore, extensive prior investigations have been conducted to exploit this approach for tailoring the characteristics of the material, thus augmenting their practical utility [28–39]. Recently, the pressure-induced emission (PIE) has been reported in compressed low-dimensional perovskites [40–43]. It is believed that high-pressure treatment enables the possibility of harvesting emergent properties under ambient conditions. In addition, the undesirable toxic lead element or unstable organic cation in well-established PIE systems still restrict the functional applications. Besides, the previously reported PIE was confined into the monochromatic emission enhancement with nearly fitted wavelength, which leads to the lack of application diversity. Accordingly, designing multi-functional PIE materials is the current research hotspot.

In this work, we synthesized monodisperse Cs₃Sb₂Br₉ quantum dots (QDs) which exhibit no PL emission under ambient conditions. Note that an exotic PIE from ‘0’ to ‘1’ was achieved at 0.8 GPa when the sample was subjected to external pressure. Furthermore, the PL intensity increased with pressure and two distinct peaks appeared, accompanied by a remarkable change in emission color from red to green. Subsequent analyses including UV/vis absorption, PL spectra, Raman spectroscopy, and angle-dispersive synchrotron X-ray diffraction (ADXRD) were conducted to elucidate the mechanism, thus underlying PIE and piezochromism. First-principles calculations further revealed that distortion of the halide SbBr₆ octahedra, associated with structural phase transition from trigonal to monoclinic under pressure, played a pivotal role in PIE and piezochromism. This study offers a new class of multi-functional PIE materials with piezochromism in environmentally friendly perovskites, facilitating diverse applications in futural practices.

Results and discussion

The colloidal Cs₃Sb₂Br₉ QDs were synthesized by using the ligand-assisted reprecipitation method [44]. The Cs₃Sb₂Br₉ QDs possess a spherical morphology with an average size of 6.4 nm, as shown in Figure 1(a,c). The high-resolution transmission electron microscope image shows a lattice constant of 0.2 nm, which matches with the (204) planes of trigonal Cs₃Sb₂Br₉ (Figure 1(b)). It is observed that the unit cell of Cs₃Sb₂Br₉ QDs consists of two SbBr₆ octahedra (Figure 1(d)). The Cs₃Sb₂Br₉ exhibits two-dimensional (2D) perovskite structure, as depicted in Figure 1(e), where one Sb atom every three halide octahedra layers along the 〈111〉 crystalline direction of 3D structured CsSbBr₃ is removed.

Figure 1. (a) Transmission electron microscopy (TEM) image of as-prepared Cs₃Sb₂Br₉ QDs. (b) High-resolution TEM (HRTEM) image of Cs₃Sb₂Br₉ QDs before compression, and (c) the corresponding size distribution. (d) Cs₃Sb₂Br₉ unit cell structure. (e) Removal of every third Sb layer along the 〈111〉 direction in the perovskite structure.

The initially Cs₃Sb₂Br₉ QDs show no emission under atmospheric pressure. To investigate the emission behavior of Cs₃Sb₂Br₉ QDs upon compression, in situ high-pressure PL experiments were conducted under 355 nm laser irradiation. As shown in Figure S1, upon compression to 0.8 GPa, an abnormal broadened PL peak at around 662 nm was detected, indicating the occurrence of PIE. With the increase of pressure, the PL peak of Cs₃Sb₂Br₉ QDs exhibited a continuous blue-shift. Moreover, the PL intensity rapidly enhanced and reached the maximum as the pressure increased up to 4.8 GPa (Figure 2(a)). Further increase in pressure resulted in decreased PL intensity and the appearance of another PL peak (Figure 2(b)), followed by gradual red-shift for both peaks. Subsequently, we proceeded to increase the applied pressure and observed a discernible alteration in the relative intensity of the two PL peaks, as depicted in Figure S2. At higher pressure (>18.2 GPa), only the higher-energy PL peak remained while the lower energy counterpart (∼600 nm) almost disappeared. High-pressure PL photographs under UV irradiation are depicted in Figure 2(c). It is evident that within the pressure range of 0–16.0 GPa, a notable transition of emission color from initial red to vibrant lime green can be observed. The variations of wavelength and PL intensity as function of pressure can be obviously distinguished in Figure 2(d,e). The chromaticity diagram data under different pressures indicated a transition from (0.35, 0.50) at 1.1 GPa to (0.53, 0.41) at 16.0 GPa (Figure 2(f)). During the pressure release process, we observed a distinct phenomenon different from that observed during pressurization, as depicted in Figure S3a. The ‘R0’ and ‘R1.1’ meant the pressure point of one atmosphere and 1.1 GPa during the decompression process. Upon releasing pressure to 4.1 GPa, the high-energy PL peak continued to increase while the low-energy PL peak remained absent. Nevertheless, when the pressure was released from 4.1 to 2.1 GPa, the high-energy fluorescence peak abruptly vanished and was replaced by the previously absent low-energy fluorescence peak, leading to a rapid change in emission color (Figure S3c).

Figure 2. (a–b) Pressure-dependent PL spectra of Cs₃Sb₂Br₉ QDs. (c) Optical micrographs of the pressure-induced changes in Cs₃Sb₂Br₉ QDs. (d–e) Wavelength and intensity variations plotted against pressure. (Peak 1): high-energy PL peak, (Peak 2): low-energy PL peak. (f) Chromaticity coordinates of emission peaks as a function of pressure for Cs₃Sb₂Br₉ QDs.

In situ high-pressure absorption experiments were further conducted on Cs₃Sb₂Br₉ QDs, as shown in Figure 3(a). Under ambient conditions, the absorption spectrum of Cs₃Sb₂Br₉ QDs shows an absorption onset at ∼420 nm, defined as E_e. Additionally, there is an absorption band edge at slightly shorter wavelengths results from the transition of free carriers from valance band to conduction band, which correspond to the bandgap E_g. It is noted that the exciton absorption peak is apparent in the spectra up to 10 GPa. However, during decompression process, it remains absent until 2.0 GPa (Figure S3b), accompanied by the emergence of the high-energy PL peak. The pressure hysteresis effect of results in a delayed recovery during decompression process. The relationship between the E_e and E_g under pressure is illustrated in Figure S4. The free-exciton binding energy under ambient conditions is calculated to be 585 meV, which matches well with previous reports [45]. As depicted in Figure 3(b), the exciton binding energy maintains decreasing upon compression progress. The turning point at ∼6 GPa indicates the existence of internal conversion in Cs₃Sb₂Br₉ QDs. The low exciton binding energy under high pressure implies a gradual decrease in the involvement of excitons in the transition process. Consequently, as the exciton absorption peak diminishes, there is an absence of excitons participating in the radiation.

Figure 3. (a) Optical absorption measurements on Cs₃Sb₂Br₉ QDs under pressure. (b) Exciton binding energy evolution of Cs₃Sb₂Br₉ QDs with pressure. (c) Representative synchrotron ADXRD patterns obtained from Cs₃Sb₂Br₉ QDs under different pressures. (d) Raman spectra recorded under selected pressures for Cs₃Sb₂Br₉ QDs.

To gain a deeper understanding of the PIE phenomenon, in situ high-pressure angle-dispersive synchrotron X-ray diffraction (ADXRD) patterns of the Cs₃Sb₂Br₉ QDs were collected, as illustrated in Figure 3(c). As the pressure increased, all Bragg diffraction peaks consistently shifted toward high diffraction angles, indicating a continuous lattice contraction of the Cs₃Sb₂Br₉ QDs. When the pressure reached 0.9 GPa, some Bragg peaks of Cs₃Sb₂Br₉ exhibited asymmetry and eventually split into two distinct peaks under higher pressure (Figure 3(c)). However, the sudden appearance of peaks were not the new diffraction peaks that induced by a phase transition under pressure. That is because all three diffraction peaks are actually composed of two or three individual diffraction peaks respectively, and the initial diffraction peaks were separated from each other under high pressure. Upon compression to about 7.0 GPa, the diffraction peak at ∼13.5° started to get asymmetric, which was attributed to the emergence of new Bragg diffraction peak, associated with structural phase transition. This is consistent with the pressure point where the intensity of the two fluorescence peaks began to relatively change to each other and the turning point of energy gaps (Figures 2(b) and S4). The refined ADXRD patterns demonstrated that the samples underwent a reversible structural phase transition from trigonal (space group P-3m1) to monoclinic (space group C2/C) phase, as depicted in Figure S5. Therefore, it indicated that the observed high-energy emission of Cs₃Sb₂Br₉ QDs was associated with the monoclinic phase.

As depicted in Figure 3(d), we further conducted Raman experiments under high-pressure conditions. The lower vibrations in the Raman spectrum came from the Cs⁺ cation loosely bound with the SbBr₆ octahedron, while three high-frequency vibrations originated from strongly bound SbBr₆ octahedron. The Raman peak at 88 cm⁻¹ belonged to the F_2g(v₅) Sb–Br bending mode, and two sharp and symmetric bands at 182 and 210 cm⁻¹ could be attributed to a symmetrical Sb–Br stretch A_1g(v₁) and asymmetrical Sb–Br planar contraction E_g(v₂), respectively [46]. As the pressure increased, all Raman peaks continuously shifted to higher wavenumbers, because of the shrinkage of the SbBr₆ octahedra. Note that when the external pressure increased up to 7.5 GPa, a new sharp and symmetric mode peak at 247 cm⁻¹ appeared, which was consistent with the phase transition deduced from the ADXRD evolution. After the complete release of the external pressure, the Raman spectrum recovered to its initial state under ambient conditions.

First-principles calculations indicated that Cs₃Sb₂Br₉ exhibited a direct bandgap of 1.97 eV under ambient conditions (Figure S6). The projected density of states revealed that the valence band maximum is dominated by Sb-5s and Br-4p orbitals, while the conduction band minimum was mainly composed of Sb-5p, Br-4s and Br-4p orbitals. It meant that the overlap of wave functions was affected by the Sb–Br motif under high pressure, thus determining the eventual optical properties of Cs₃Sb₂Br₉ QDs. In addition, the third-order Birch–Murnaghan equation was adopted to fit the experimental pressure–volume (P–V) data, and details could be found in the Supporting Information. The P–V curve indicated the occurrence of phase transition under 7.0 GPa, as depicted in Figure S7. Calculated bulk modulus B₀ for the trigonal and monoclinic phase of the Cs₃Sb₂Br₉ were 13.0 and 47.0 GPa, respectively. We further accessed the relationship between the distortion of Sb-Br octahedra and optical properties. The deviation of the octahedron can be judged using the following parameters:

Distortion of bond length: (1) $\begin{array}{c}\mathrm{\Delta }d=\frac{1}{6}\sum _{i=1}^6{\left[\left(d_i-d_0\right)/d_0\right]}^2\end{array}$(1)

Variance in octahedral angle: (2) $\begin{array}{c}{\delta }^2=\frac{1}{11}\sum _{i=1}^{12}{\left({\theta }_i-90\right)}^2\end{array}$(2) where $d_0$ is the mean of bond distance and $d_i$ is individual bond distance, the average $\mathrm{\Delta }d$ is the factor to describe the distortion degree of the structure. The ${\theta }_i$ is the Br–Sb–Br bond angle of the octahedron. By using $\mathrm{\Delta }d$ and $\delta$, we can describe the distortion levels of the individual octahedron and the degree of the lattice distortion. As illustrated in Figure S8, ${\delta }^2$ kept increasing while $\mathrm{\Delta }d$ decreased upon increasing pressure, indicating the contraction of SbBr₆ octahedron and the increased degree of bond angle distortion. This would subsequently strengthen the electron–phonon coupling, leading to an increase in the concentration of STEs. This can reasonably explain the observed enhancement in PL intensity. In addition, to explore the mechanism behind the luminous phenomenon, we used the time-dependent density functional theory (TD-DFT) method to calculate the absorption of excited states for two phases, as depicted in Figure S9. It was evidenced that, under high pressure, the FE state disappeared, while only FC state existed.

The mechanism of PIE and piezochromism for Cs₃Sb₂Br₉ QDs is illustrated in Figure 4. Under ambient conditions, as depicted in Figure 4(a), the large exciton binding energy facilitates the exciton formation process (indicated by black arrows), so the transition is mainly accomplished by free excitons on the FE. The relatively symmetrical octahedral structures possess low activation energy for detrapping (E_detrap) due to the weak electron–phonon coupling strength. The photoexcited carriers are readily detrapped from the self-trapped states and dissipated through thermal relaxation. Therefore, no emission can be observed at ambient pressure. With the increase of pressure, the electron–phonon coupling strength is enhanced, resulting in difficult detrapping of STEs via thermal activation because of the increased E_detrap. As shown in Figure 4(b), when the pressure reaches 0.8 GPa, some excitons located in ST₁ begin to transit to the valence band rather than detrapping due to relatively higher E_detrap, the PIE phenomenon can thus be found. Subsequently, with much higher pressure, the SbBr₆ octahedra are further contracted and distorted, which further enhances the E_detrap. Therefore, a persistent increase in low-energy fluorescence peak can be observed with increasing pressure, associated with the improved possibility of radiative recombination. When the pressure reaches approximately 7.0 GPa, the occurrence of phase transition leads to the emergence of ST₂, which generates the emergence of the high-energy PL peak (Figure 4(b)). After the occurrence of phase transition at 7.0 GPa, two phases mixed, resulting in the co-existence of ST₁ and ST₂. Meanwhile, the smaller exciton binding energy deduces the exciton transformation, thus some free carriers starts to direct relaxation to the ST₁ and ST₂. Upon complete phase transition from trigonal to monoclinic, only ST₂ exists. Therefore, there is no low-energy PL peak left.

Figure 4. Configuration coordinate models for Cs₃Sb₂Br₉ QDs at (a) 1 atm, (b) 0.8 GPa, (c) 9 GPa, and (d) 18.2 GPa. (FC): free carrier state, (FE): free exciton state, (ST): self-trapped state, (E_detrap): the activation energy for detrapping.

Conclusion

In summary, the multi-functional PIE with considerable piezochromism was achieved in the synthesized 2D all-inorganic Cs₃Sb₂Br₉ QDs through pressure engineering. The intriguing PIE was realized under high pressure, accompanied by a remarkable change in emission color from red to green. First-principles calculations and comprehensive characterizations including UV/vis absorption, PL spectra, Raman spectroscopy, and synchrotron ADXRD were performed on Cs₃Sb₂Br₉ QDs. These results indicated that Cs₃Sb₂Br₉ QDs experience a structural phase transition from trigonal to monoclinic under pressure, which was responsible for the piezochromism phenomenon. Our work provides a robust strategy to boot the emission efficiency and to construct multi-functional PIE materials with piezochromism in environmentally friendly perovskites, thus facilitating the diverse applications in futural practices.

Acknowledgements

This work was mainly performed at BL15U1 at the Shanghai Synchrotron Radiation Facility (SSRF).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work is supported by the National Key R&D Program of China [grant number 2023YFA1406200], the National Natural Science Foundation of China [grant numbers 12174144 and 12304014], the Zhejiang Provincial Natural Science Foundation of China [grant number LR22B010001], the Jilin Provincial Science and Technology Development Program [grant number 20220101002JC], the Beijing Institute of Graphic Communication [grant numbers Ea202412 and 27170124039], the Graduate Innovation Fund of Jilin University [grant number 2024CX201], and the Fundamental Research Funds for the Central Universities.