Ultrafast Dynamics Across Pressure‐Induced Electronic State Transitions, Fluorescence Quenching, and Bandgap Evolution in CsPbBr3 Quantum Dots

Abstract This work investigates the impact of pressure on the structural, optical properties, and electronic structure of CsPbBr3 quantum dots (QDs) using steady‐state photoluminescence, steady‐state absorption, and femtosecond transient absorption spectroscopy, reaching a maximum pressure of 3.38 GPa. The experimental results indicate that CsPbBr3 QDs undergo electronic state (ES) transitions from ES‐I to ES‐II and ES‐II to ES‐III at 0.38 and 1.08 GPa, respectively. Intriguingly, a mixed state of ES‐II and ES‐III is observed within the pressure range of 1.08–1.68 GPa. The pressure‐induced fluorescence quenching in ES‐II is attributed to enhanced defect trapping and reduced radiative recombination. Above 1.68 GPa, fluorescence vanishes entirely, attributed to the complete phase transformation from ES‐II to ES‐III in which radiative recombination becomes non‐existent. Notably, owing to stronger quantum confinement effects, CsPbBr3 QDs exhibit an impressive bandgap tuning range of 0.497 eV from 0 to 2.08 GPa, outperforming nanocrystals by 1.4 times and bulk counterparts by 11.3 times. Furthermore, this work analyzes various carrier dynamics processes in the pressure‐induced bandgap evolution and electron state transitions, and systematically studies the microphysical mechanisms of optical properties in CsPbBr3 QDs under pressure, offering insights for optimizing optical properties and designing novel materials.

][26][27][28][29][30][31][32] In recent years, researchers have conducted extensive studies on the crystal structure and optical properties of the CsPbBr 3 bulk counterparts and CsPbBr 3 NCs under high pressure.Zhang et al. discovered an isostructural phase transition (orthorhombic phase) in CsPbBr 3 bulk counterparts near 1.0 GPa, which was accompanied by an anomalous discontinuous evolution of the bandgap, with a tunable range of 0.044 eV. [33]In addition, the fluorescence gradually quenched under pressure and disappeared at 2.5 GPa.Xiao et al. also observed an isostructural phase transition (orthorhombic phase) in CsPbBr 3 NCs near 1.19 GPa, as well as significant pressure-modulated bandgap and pressureinduced fluorescence quenching (the fluorescence disappeared at 1.36 GPa). [34]Furthermore, Gao et al. revealed electronic state (ES) transitions in CsPbBr 3 NCs (average particle size of 10.4 nm) from ES-I to ES-II at 0.38 GPa and from ES-II to ES-III at 1.08 GPa, accompanied by anomalous discontinuous evolutions of the bandgap and the carrier lifetime. [35]They also observed the pressure-induced fluorescence quenching, with the fluorescence disappearing at 1.38 GPa.By comparison, the electronic state transition from ES-II to ES-III at 1.08 GPa corresponds to the isostructural phase transition mentioned before, while the transition from ES-I to ES-II at 0.38 GPa has not been discovered in CsPbBr 3 bulk counterparts.Our research team investigated the ultrafast carrier dynamics of the CsPbBr 3 bulk counterparts through in situ high-pressure femtosecond transient absorption (fs-TA) spectroscopy. [36]We discovered the electronic state transition from ES-II to ES-III but not the transition from ES-I to ES-II.Moreover, pressure effectively reduced the bandgap and prolonged the hot carrier relaxation and Auger recombination before the phase transition. [36]Based on the above observations, it can be concluded that CsPbBr 3 NCs exhibit different pressure-induced phase transition pathways, a larger tunable range of bandgap, and a lower pressure for fluorescence disappearing compared to CsPbBr 3 bulk counterparts.CsPbBr 3 QDs and NCs are similar in size, so they both have strong quantum confinement effects, high surface energy, and excellent chemical stability. [25]Therefore, CsPbBr 3 QDs and NCs have superior optical properties compared to their bulk counterparts.However, there hasn't been a systematic investigation on the ultrafast dynamics of the pressure-induced electronic state transitions, fluorescence quenching, and bandgap evolution in CsPbBr 3 QDs and NCs.
In this study, in situ high-pressure steady-state PL, Abs, and fs-TA spectroscopy experiments were carried out to explore the PL intensity, bandgap, and ultrafast dynamics of CsPbBr 3 QDs with an average particle size of 7.6 nm under high pressure.By analyzing the experimental results and comparing them with those of CsPbBr 3 NCs with an average particle size of 10.4 nm and CsPbBr 3 bulk counterparts, the connection between the microscale ultrafast dynamics of excited-state carriers and the macroscopic optical behavior was established, which revealed the physical mechanisms of the electronic state transitions, fluorescence quenching, and bandgap evolution in CsPbBr 3 QDs under pressure.

Characterization of CsPbBr 3 QDs at Ambient Pressure
The CsPbBr 3 QDs sample displayed a uniform morphology in the high-resolution transmission electron microscopy image, as shown in Figure S1a (Supporting Information).The corresponding size distribution was obtained via Gaussian fitting, revealing an average size of 7.6±0.1 nm, as depicted in Figure S1b (Supporting Information).In Figures S1c,d (Supporting Information), the fluorescence peak of CsPbBr 3 QDs located at ≈ 510 nm and the first excitonic absorption edge appeared at ≈ 498 nm under ambient conditions. [78]The bandgap value was calculated to be 2.40 eV using the Tauc plot extrapolation, [79] which was consistent with the results reported by Protesescu et al.. [24] 2.2.Pressure-Induced Fluorescence Quenching of CsPbBr 3 QDs Pressure-induced fluorescence quenching refers to the phenomenon where the fluorescence intensity of a material decreases or disappears under high pressure conditions. [60,80]To study the effect of pressure on the fluorescence radiative ability of CsPbBr 3 QDs, we carried out in situ high-pressure steady-state PL spectroscopy measurements on the CsPbBr 3 QDs.Figure 1a shows steady-state PL spectra of the CsPbBr 3 QDs during compression.At 0.18 GPa, CsPbBr 3 QDs exhibit green emission, and the fluorescence intensity progressively diminishes until it vanishes as the pressure increases.From 0.18 to 0.28 GPa, the fluorescence peak undergoes a slight redshift, and the fluorescence intensity is slightly enhanced.From 0.38 to 1.08 GPa, the fluorescence peak of CsPbBr 3 QDs continues to redshift, but the fluorescence intensity begins to diminish gradually, which may be attributed to the electron state transition from ES-I to ES-II. [35][35] Above 1.08 GPa, the fluorescence intensity decreases significantly with the increase of pressure, and the fluorescence disappears at 1.68 GPa.This is related to the electron state transition that occurs from ES-II to ES-III. [35]In Figure 1b, during decompression, the emission color gradually recovers to green.When the pressure decreases to 0.88 GPa, the fluorescence peaks start to appear.From 0.88 to 0.08 GPa, a blueshift in the fluorescence peak is observed, and the intensity continues to enhance.When the pressure reaches 0.08 GPa, the fluorescence peak is located around 509 nm, which reveals that the changes in PL properties of CsPbBr 3 QDs under pressure are reversible.However, there is a pressure relaxation of 0.8 GPa between the disappearance of the pressure-induced fluorescence peak and the reappearance of the pressure-relieved fluorescence peak.
By comparison, it can be seen that the overall trend of the fluorescence behavior under high pressure of CsPbBr 3 QDs, NCs, [35] and bulk counterparts [33] is consistent.However, due to the quantum confinement effect in QDs, two different phenomena are observed.First, the fluorescence intensity of QDs shows a slight enhancement at 0.28 GPa, while this is not observed in NCs and bulk counterparts.The slight enhancement of pressureinduced fluorescence intensity is attributed to the fact that QDs have a strong quantum confinement effect, and the enhanced passivation of surface states and defects in smaller QDs under high pressure, [62,63,81] increasing in the participating radiative recombination carriers.Second, the fluorescence disappearance is more sensitive to pressure in QDs and NCs compared to the bulk counterparts.As shown in Table 1, the pressure at which fluorescence vanishes for bulk counterparts is 2.5 GPa, while that for NCs, and QDs is 1.38 and 1.68 GPa respectively.Previous studies attributed the pressure-induced fluorescence disappearance in the CsPbBr 3 bulk counterparts and NCs to the increase in non-radiative recombination caused by the ES-III pressureinduced amorphization, [33][34][35]82] but require further investigation. Therfore, the physical mechanism of the pressure-induced fluorescence quenching of the CsPbBr 3 bulk counterparts, NCs, and QDs is still unclear.

Pressure-Induced Bandgap Evolution of CsPbBr 3 QDs
To investigate the changes of the electronic band structure during the pressure-induced electron state transition in CsPbBr 3 QDs and further reveal the physical mechanism of the pressureinduced fluorescence quenching, in situ high-pressure steadystate Abs spectroscopy measurements on CsPbBr 3 QDs were conducted.As shown in Figure 2a, there are two distinct discontinuous changes in the absorption edge of CsPbBr 3 QDs within the pressure range of 0.02 to 2.08 GPa.At ≈ 0.38 GPa, the absorption edge of CsPbBr 3 QDs starts to have a significant redshift as the pressure increases.Then, above 1.08 GPa, the absorption edge undergoes a continuous blueshift with increasing pressure.These changes correspond to the electronic state transitions of CsPbBr 3 QDs from ES-I to ES-II and from ES-II to ES-III respectively, [35] which are also consistent with our steadystate PL measurements.Additionally, above 1.08 GPa, the first excitonic absorption edge starts to weaken, and eventually disappears completely at 1.68 GPa.This agrees well with the substantial weakening and disappearance of fluorescence observed in Figure 1a. Figure 2b displays steady-state Abs spectra of CsPbBr 3 QDs during the decompression process.When the pressure is released from 2.08 to 0.88 GPa, the first exciton absorption edge reappears, [78] which corresponds to the re-emergence of the fluorescence peak at 0.88 GPa shown in Figure 1b.From 0.88 to 0.58 GPa, the absorption edge undergoes a redshift, indicating the electron state transition from ES-III back to ES-II.Finally, as the pressure is further decreased from 0.58 to 0.08 GPa, the absorption edge undergoes blueshift, returning to the initial ES-I.This confirms the reversibility of the electron state transition of CsPbBr 3 QDs under high pressure.
The bandgap of CsPbBr 3 QDs at different pressures was calculated by the Tauc plot extrapolation method. [79,83]As shown in Figure 2c, from 0.02 to 1.08 GPa, the bandgap value of CsPbBr 3 QDs decreases with increasing pressure.This is attributed to the shortening of Pb-Br bond lengths and the enhanced coupling between the Pb 6s and Br 4p orbitals under pressure, resulting in the narrowing of the bandgap between the valence band maximum (VBM) and the conduction band minimum  [33,35] The inset illustrates the pressure-dependent bandgap from 0 to 1.1 GPa, the green-shaded region corresponds to the electron state transition interval from ES-II to ES-III in CsPbBr 3 QDs.
6]84,85] The abrupt decrease of the bandgap value at 0.38 GPa (as shown in the inset of Figure 2c) corresponds to the electronic state transition from ES-I to ES-II. [35]6]84,85] The significant increase in the bandgap value at 1.18 GPa corresponds to the electronic state transition from ES-II to ES-III. [35]During decompression, the changes in bandgap with pressure are reversible.
Through comparison, the bandgaps of CsPbBr 3 bulk counterparts, NCs, and QDs have similar evolution under pressure, and they all undergo a structural phase transition around 1.08 GPa.However, pressure regulates many of their physical properties differently due to different sizes.First, from 0 to 2.08 GPa, QDs have a bandgap tuning range of 0.497 eV, which is 1.4 times that of NCs and 11.3 times that of bulk counterparts, as shown in Table 2.This is because the smaller the size of CsPbBr 3 , the stronger the quantum confinement effect, [24,86] and the more discrete the energy level structure, [87,88] which leads to more significant changes in the bandgap under pressure.Second, due to the strong quantum confinement effects, QDs and NCs exhibit discontinuous changes in the bandgap at 0.38 GPa, indicating an electron state transition from ES-I to ES-II, which is absent in the bulk counterparts. [24]Nevertheless, the electronic state transition interval from ES-II to ES-III exists in the CsPbBr 3 QDs, NCs, and bulk counterparts, and ES-III is considered to be radiate-free fluorescence.82] Based on the experimental results of in situ high-pressure steady-state Abs spectra, [33][34][35] the comparative analysis shows that after the disappearance of the pressure-induced fluorescence, obvious steady-state Abs spectra of CsPbBr 3 QDs, NCs, and bulk counterparts can be still obtained.It is confirmed that CsPbBr 3 QDs, NCs, and bulk counterparts still maintain a good lattice structure (orthogonal phase), and are not amorphous.These results indicate that the disappearance of the pressureinduced fluorescence of CsPbBr 3 QDs, NCs, and bulk counterparts is caused by the complete transformation from ES-II to non-fluorescent ES-III under pressure, rather than the pressureinduced amorphous in ES-III.The ES-III of CsPbBr 3 NCs at 1.08-1.38GPa reported by Gao et al. should be a mixed state of ES-II and ES-III. [35]This corresponds to the mixed state of CsPbBr 3 NCs in the 1.19-1.45GPa reported by Xiao et al., [34] and that of the CsPbBr 3 bulk counterparts in the 1.21-2.03GPa reported by Zhang et al. and Xiao et al.. [33,34] It is also consistent with the

Materials
Octahedral distortion pressure Bandgap tuning range CsPbBr 3 bulk counterparts [ 33] 1.0 GPa 0.044 eV CsPbBr 3 NCs (10.4±1.8 nm) [ 35] 1.08 GPa 0.355 eV CsPbBr 3 QDs (7.6±0.1 nm) 1.08 GPa 0.497 eV QDs measured in our work (as shown in the green-shaded area in Figure 2c).At present, the physical mechanism of the pressureinduced fluorescence quenching of CsPbBr 3 QDs, NCs, and bulk counterparts is still not elucidated clearly, which prompts us to investigate ultrafast dynamical behaviors of CsPbBr 3 QDs under pressure by using in situ high-pressure fs-TA spectroscopy.

In Situ High-Pressure Ultrafast Dynamics of CsPbBr 3 QDs
][91] To investigate the ultrafast dynamics of the pressure-induced electronic state transitions, fluorescence quenching, and bandgap evolution in CsPbBr 3 QDs, and reveal the physical mechanism of the fluorescence enhancement, quenching, and disappearance of different electronic states under high pressure, we conducted in situ high-pressure fs-TA spectroscopy measurements on CsPbBr 3 QDs.The experimental results were compared with our previous data of the in situ highpressure ultrafast dynamics of the CsPbBr 3 bulk counterparts. [36]-TA spectra of CsPbBr 3 QDs at ambient pressure (as shown in Figure 3) obtained in our measurement are similar to that of CsPbBr 3 NCs reported by Anunay Samanta et al.. [28] Figure 3a shows 3D image plots of fs-TA spectra, where two positive-signal peaks are observed near 450 and 520 nm, respectively, and one negative-signal peak near 500 nm. Figure 3b shows 2D image plots of fs-TA spectra with a delay time of 0-10 ps.The positive and negative signals are observed in the region of 425 to 575 nm.Figures 3c,d show the evolution of fs-TA spectra at different delay times (where ΔA is the change in absorption).The broad absorption band located from 425 to 475 nm (positive signal) is labeled PA1.PA1 is attributed to the absorption of the first excited state carrier and is formed by the transition of the lowest excited state carrier to the higher excited state by absorbing pump light energy. [28,57]The peaks near 498 nm and acromial peaks near 480 nm (negative signal) are labeled PB1 and PB2, respectively.Among them, PB1 is attributed to the ground-state bleach and is formed by the weakened absorption of ground-state carriers to the probe light. [28,57]PB2 is attributed to the hot carrier-induced bleach, and the rapid recovery of PB2 (>700 fs) caused the broadening of the spectral envelope of PA1. [95]The broad absorption band in the region of 510 to 575 nm (positive signal) is labeled as PA2, and PA2 is attributed to the absorption of hot carriers, which is formed by the transition of the hot carriers from the absorption of the pump light energy in the lower excited state to the higher excited state. [28,57]o further study the ultrafast dynamics of excited state carriers in CsPbBr 3 QDs under pressure, we carried out in situ highpressure fs-TA spectroscopy measurements of CsPbBr 3 QDs.Figure 4 shows normalized fs-TA spectra of CsPbBr 3 QDs for a 10 ps probe delay under different pressures.As shown in Figure 4a, from 0.16 to 1.08 GPa, PB1 continuously redshifts with increasing pressure.Above 0.38 GPa, the redshifted trend of PB1 becomes more obvious, which is related to the electronic state transition from ES-I to ES-II. [35]At 1.18 GPa, the energy value (about 2.3 eV) corresponding to the PB1 signal peak is consistent with the bandgap value of ES-II, while a new negative-signal peak appears near 440 nm, and the energy value corresponding to its peak position is ≈ 2.8 eV, which is consistent with the bandgap value of ES-III in Figure 2c.Therefore, this new negative-signal peak is regarded as the ground-state bleach (labeled as PB3) in ES-III.As shown in Figure 4b, above 1.08 GPa, both PB1 and PB3 continuously blueshift with increasing pressure.The change in PB1 trend and the appearance of PB3 are attributed to the transformation from ES-II to ES-III. [35]With the increase of pressure, the PB1 signal representing ES-II gradually weakens, while the PB3 signal representing ES-III appears and gradually increases.Until 1.68 GPa, the disappearance of PB1 signifies the disappearance of ES-II, and the mixed state of ES-II and ES-III completely transforms into the non-radiative ES-III, which is consistent with the disappearance of the fluorescence peak observed in Figure 1a for CsPbBr 3 QDs.
The global fitting of in situ high-pressure fs-TA spectra of CsPbBr 3 QDs at different pressures was performed for further analysis. [92][95] From Figure 5a,  1 increases with the increase of pressure from 0.16 to 0.98 GPa.One of the reasons is that the Pb-Br bond lengths of ES-I and ES-II shrink under pressure, and the resulting repulsive force causes the lattice hardening, [96] which makes it difficult for the hot carriers to cool down by interacting with the lattice.Another reason is that the decreasing bandgap value (as shown in Figure 2c) leads to an increase in the number of the hot carriers that transition to the highly excited state, and thus an increase in the number of the hot carriers participating in the hot carrier relaxation process. [36,91]The discontinuous increase of  1 at 0.38 GPa can be attributed to the electronic state transition from ES-I to ES-II. [35]When the pressure is above 1.58 GPa,  1 disappears, indicating that the mixed states of ES-II and ES-III are completely transformed into ES-III.As a result of the wide bandgap of ES-III, the carriers cannot transition to the high excited state, so there is no hot carrier relaxation process for ES-III.Then,  1 decreases with increasing pressure from 0.98 to 1.58 GPa, which is due to the decrease in the proportion of ES-II with hot carrier relaxation.The reduction of  1 at 1.08 GPa can be attributed to the electronic state transition from ES-II to ES-III. [35]he changing trends of  1 with the pressure of CsPbBr 3 QDs and bulk counterparts are similar. [36]However, it is worth noting that CsPbBr 3 QDs undergo an electronic state transition from ES-I to ES-II at 0.38 GPa, [35] resulting in a discontinuous increase in  1 , which is not observed in the bulk counterparts. [33,35]n Figure 5b,  2 decreases as the pressure increases from 0.16 to 0.28 GPa, which is because the density of the pressureinduced defect states in ES-I of CsPbBr 3 QDs decreases, resulting in a reduction in the number of the carriers trapped by the defect states. [91,94]From 0.28 to 0.98 GPa,  2 increases slightly, indicating that the defect state of ES-II is less affected by pressure and its density increases little.The slight increase of  2 at 0.38 GPa can be attributed to the electronic state transition from ES-I to ES-II. [19]From 0.98 to 1.58 GPa,  2 increases due to the increase in the density of pressure-induced defect states in ES-II, which increases the number of carriers trapped defect states. [51,91,97]The rapid increase of  2 at 1.08 GPa is caused by the electronic state transition from ES-II to ES-III. [35] 2 disappears above 1.58 GPa suggests CsPbBr 3 QDs are completely transformed into ES-III, and ES-III does not have the carriertrapping process of defect states.From the fitting results,  4 increases slightly as the pressure increases from 1.18 to 3.38 GPa, which demonstrates that the surface state of ES-III is less affected by pressure and its density increases little.
From 0.16 to 0.28 GPa,  3 increases with the increase of pressure, as shown in Figure 5c, which is due to the decrease in the density of pressure-induced defect states in ES-I of CsPbBr 3 QDs, increasing in the number of the carriers participating in radiative recombination. [91,94]This well explains the slight enhancement of the fluorescence at 0.28 GPa in Figure 1a.With further compression,  3 decreases from 0.28 to 0.98 GPa, which is caused by the increase of the trapped carriers in the ES-II defect state ( 2 increases slightly), resulting in the decrease of the carriers participating in the radiative recombination.Through the analysis of the pressure-dependent carrier lifetime changes with pressure, a schematic diagram for the ultrafast dynamics of different electronic states in CsPbBr 3 QDs under pressure is plotted, as shown in Figure 6.It can be concluded that the microphysical mechanism of the pressure-induced fluorescence enhancement in ES-I is the inhibition of defect trapping and the promotion of radiative recombination.However, the pressure-induced fluorescence quenching in ES-II is due to the promotion of defect trapping and the inhibition of radiative recombination.This also validates our previous views that the reason for the pressure-induced fluorescence disappearance of QDs is that ES-II completely transforms into non-radiative ES-III under pressure, rather than the pressure-induced amorphization in ES-III.

Conclusion
In this study, the ultrafast dynamics in the pressure-induced electronic state transition, fluorescence quenching, and bandgap evolution of CsPbBr 3 QDs were explored by in situ high-pressure steady-state PL, Abs, and fs-TA spectroscopy experiments.The experimental results indicate that the pressure-induced electronic state transition path in CsPbBr 3 QDs is different from that of the bulk counterparts.The electronic state transitions from ES-I to ES-II and from ES-II to ES-III occur in CsPbBr 3 QDs at 0.38 and 1.08 GPa, respectively.In comparison, only electronic state transitions from ES-II to ES-III occur in CsPbBr 3 bulk counterparts at 1.0 GPa.Notably, through in situ high-pressure fs-TA spectroscopy measurements, the ground-state bleach of ES-II and ES-III can be effectively distinguished from 1.08 to 1.68 GPa, which confirms the coexistence of these two electronic states.Moreover, the pressure-induced fluorescence quenching of CsPbBr 3 QDs above 0.38 GPa is due to the promotion of the pressure-induced defect state trapping process and the inhibition of the radiative recombination process in ES-II.However, the reason for the disappearance of the pressure-induced fluorescence is that the ES-II with radiative recombination completely transforms into the non-radiative recombination ES-III, instead of the pressureinduced amorphous in ES-III.The fluorescence disappearance phenomenon appeared at 2.5 GPa for CsPbBr 3 bulk counterparts, and 1.68 GPa for CsPbBr 3 QDs.This reflects that because of the quantum confinement effect, QDs are more prone to E-II to E-III electronic state transition than their bulk counterparts.In addition, CsPbBr 3 QDs have a larger bandgap tuning range (0.497 eV) from 0 to 2.08 GPa, which is 11.3 times that of the bulk counterparts.The hot carrier relaxation, defect state trapping, radiative recombination, surface state trapping, and non-radiative recombination processes of CsPbBr 3 QDs are all regulated by the pressure-induced electronic state transition and bandgap evolution.The development of this study is crucial for improving the photophysical properties of perovskite materials and provides important guidance for broadening their applications in areas such as optoelectronic detectors, light-emitting diodes, and solar cells under pressure conditions.

Experimental Section
Sample Preparation: CsPbBr 3 QDs (>99% purity) were purchased from Xiamen Lumansci Technology Co., Ltd.The solution concentration was 2 mg mL −1 , and the n-hexane was used as a dispersant.
Experimental Conditions for In Situ High-Pressure Optical Experiment: All in situ high-pressure optical experiments were conducted at room temperature.The high-pressure generating device used in the experiment was DAC, and the two diamond anvils were IIa ultra-low fluorescent diamonds with an anvil surface size of 600 μm.DAC was used to prepress a piece of T301 stainless steel with an initial thickness of 250 to 150 μm for the gasket.A 500 μm diameter hole was drilled in the center of the gasket by laser as the sample chamber.Pressure calibration was performed using rubies encapsulated in the sample chamber. [98,99]xperimental Conditions for Steady-State PL Spectroscopy Measurement: The 355 nm line of a UV DPSS laser with an output power of 10 mW was used as the excitation light source.To eliminate external radiation along the emission path, interference filters and color filters were employed.For steady-state PL spectroscopy measurements, a high-sensitivity fiber optic spectrometer (QE Pro, Ocean Insight) with a measurement range of 355 to 1000 nm was utilized.
Experimental Conditions for Steady-State Abs Spectroscopy Measurement: A deuterium-tungsten halogen lamp was utilized as the light source.For steady-state Abs spectroscopy measurements, a high-sensitivity fiber optic spectrometer (QE65 Pro, Ocean Insight) was employed with a measurement range of 250 to 1000 nm.
Experimental Conditions for fs-TA Spectroscopy Measurement: The used femtosecond laser (Newport, USA) had a laser pulse width of 100 fs, a center wavelength of 800 nm, and a repetition rate of 1 kHz.An optical parametric amplifier (OPA) generated a pump wavelength of 330 nm.The 330 nm pulses were split into two beams in a 9:1 ratio.One beam served as the pump light for sample excitation, while the other beam was directed through a computer-controlled delay line onto an optical non-linear transparent medium (CaF 2 ) to generate supercontinuum white light.This supercontinuum white light served as the probe light to detect changes in absorbance induced by the pump light in the sample.The probe and pump lights were kept overlapped in the sample chamber. [57,58,77]Detection was carried out using a charge-coupled device (CCD) detector.The acquired fs-TA spectroscopy was globally fitted using the CarpetView software.

Figure 1 .
Figure 1.In situ high-pressure steady-state PL spectra of CsPbBr 3 QDs.a) Steady-state PL spectra during compression, with insets showing the micrographs of CsPbBr 3 QDs in the sample chamber at 0.18, 0.78, 1.08, and 1.68 GPa.b) Steady-state PL spectra during decompression, with insets showing the micrographs of CsPbBr 3 QDs in the sample chamber at 0.98, 0.88, 0.68, and 0.08 GPa.

Figure 2 .
Figure 2. In situ high-pressure steady-state Abs spectra of CsPbBr 3 QDs.a) Steady-state Abs spectra during compression.b) Steady-state Abs spectra during decompression.c) Pressure-dependent bandgaps of CsPbBr 3 QDs during compression (red ball) and decompression (red ball), along with the pressure-dependent bandgaps of CsPbBr 3 bulk counterparts (black square) and CsPbBr 3 NCs (black pentagram) during compression.[33,35]The inset illustrates the pressure-dependent bandgap from 0 to 1.1 GPa, the green-shaded region corresponds to the electron state transition interval from ES-II to ES-III in CsPbBr 3 QDs.

Figure 3 .
Figure 3. Fs-TA spectra of CsPbBr 3 QDs at ambient pressure.a) 3D image plots.b) 2D image plots.c) Time evolution plots in the delay time range of 0 fs-4 ps.d) Time evolution plots in the delay time range of 4 ps-1.45ns.

Figure 6 .
Figure 6.The schematic diagram for ultrafast dynamics of different electronic states in CsPbBr 3 QDs under pressure.
From 0.98 to 1.58 GPa,  3 continues to decrease, which is caused by the increase of trapped carrier of the ES-II defect state ( 2 increases) and the increase in the number of the carriers participating in non-radiative recombination of ES-III ( 5 increases). 3 disappears above 1.58 GPa, indicating that the mixed states of ES-II and ES-III are completely transformed into ES-III, and there is no radiative recombination process in ES-III.From 1.18 to 3.38 GPa,  5 increases with increasing pressure, corresponding to the increase of ES-III carriers participating in non-radiative recombination.

Table 2 .
The bandgap tuning range and octahedral distortion pressure of CsPbBr 3 bulk counterparts, NCs, and QDs under high pressure.