Manganese-Doped, Lead-Free Double Perovskite Nanocrystals for Bright Orange-Red Emission.

Lead-free halide perovskite nanocrystals (NCs) have recently attracted attention due to their nontoxicity and stability as alternatives to lead-based perovskite NCs. Here, we report undoped and manganese-doped all-inorganic, lead-free double perovskite (DP) NCs: Cs2NaIn x Bi1-x Cl6 (0 < x < 1) and Cs2NaIn x Bi1-x Cl6:Mn (0 ≤ x ≤ 1) NCs. Undoped NCs exhibit blue emission. Through doping Mn2+ ions into Cs2NaIn x Bi1-x Cl6 NCs, we can avoid self-trapped exciton emission and realize bright orange red emission of Mn2+ dopants with the highest photoluminescence quantum efficiency of 44.6%. The photoluminescence (PL) is tunable from yellow emission to orange-red emission corresponding to a red shift from 583 to 614 nm with increasing In content. Interestingly, the PL emission of Mn-doped NCs shows a red shift from the bulk size to the nanoscale. The Mn-doped NCs show good stability in air. In addition, we further prove the process of dark self-trapped state-assisted Mn2+ emission in DP NCs by ultrafast transient absorption techniques.

Introduction

All-inorganic, lead-free perovskite nanocrystals (NCs) have aroused great interest due to their simple synthesis process, nontoxicity, and high stability for optoelectronic applications,[1−3] especially for double perovskites (DPs) NCs, which are characterized by a three-dimensional (3D) network similar to lead-based perovskite NCs and have the potential to be excellent light-emitting materials.[4−10] Doping and alloying strategies can improve the photophysical properties and stability of DP NCs.[6−15] Recently, the broadband, large red-shifted emission is the main focus of attention attributed to bright self-trapped exciton (STE) in DP NCs.[8−13] However, other specific wave-band emission and dopants emission have been rarely studied in the visible region, in terms of dopants emission in DP NCs, which is often accompanied by bright STE emission,[16] and for lead-based NCs, which is often accompanied by free exciton (FE) emission or bright STE emission.[17−19] Thus, avoiding FE emission or bright STE emission can promote single, pure dopants emission. In addition, Mn-doped, Ag-based DP NCs have been reported, which facilitate dopants emission owing to the direct band gap structure,[7] but those NCs have parity-forbidden transition and inhibit further improvement of photoluminescence (PL) properties.[20] Moreover, there is a significant difference in the photophysical properties of DP NCs and DP bulk crystals. We have previously studied the size effect of alloying DPs for STE emission,[11] but the size effect of doping DPs has not been investigated for dopants emission. These problems limit the application of these semiconductor materials as light emitters in the illumination field. It is a great challenge and significance to further enhance the light emission and clarify the exciton dynamics mechanism. To the best of our knowledge, there has been no report of a Mn-doped, direct bandgap of the allowed transition, Na-based DP colloidal NCs.

Here, we successfully synthesize undoped and Mn-doped lead-free direct bandgap Na-based mixed In/Bi DP NCs: Cs2NaInBi1–Cl6 (0 < x < 1) and Cs2NaInBi1–Cl6:Mn (0 ≤ x ≤ 1) NCs. Undoped NCs show blue-light emission. Through introducing Mn2+ ions into Cs2NaInBi1–Cl6 NCs, the PL properties are changed and improved to achieve bright orange-red emission with the best photoluminescence quantum efficiency (PLQE) of 44.6%. The PL are tunable from yellow emission to orange-red emission corresponding to a red shift from 583 to 614 nm with increasing In content. The Mn-doped NCs have an ultralong PL lifetime around 3–9 ms. Interestingly, the PL emission of Cs2NaIn0.75Bi0.25Cl6:Mn NCs shows a red shift from the bulk size to the nanoscale. We further prove the process of the dark STE state assisted Mn2+ PL in DP NCs by time-resolved PL (TR-PL) measurements and ultrafast transient absorption (TA) techniques. In addition, the Mn-doped NCs show better stability in air.

Results and Discussion

Undoped Cs2NaInBi1–Cl6 DP NCs

The Cs2NaInBi1–Cl6 (x = 0, 0.25, 0.5, 0.75, and 0.9) NCs are synthesized by variable temperature, one-pot hot injection.[10] X-ray diffraction (XRD) patterns reveal that these NCs have face-centered cubic structures and Fm3m space group similar to Cs2NaBiCl6 (Figure a). Also, these NCs show high crystallinity and a single pure phase. These NCs exhibit a monotonical shift of diffraction peaks to higher 2θ values with an increasing In/Bi ratio, as shown in the case of the (220) peak in Figure a, attributed to the ionic radius of In3+ (94 pm), which is smaller than that of Bi3+ (117 pm). To obtain the actual In/Bi ratio, inductively coupled plasma optical emission spectrometry (ICP-OES) was carried out (Table S1). The transmission electron microscopy (TEM) image shows that Cs2NaIn0.75Bi0.25Cl6 NCs are mainly cubic shape with a mean edge length of 10.59 nm and exhibit relatively even distribution (Figure b,d). A high-resolution TEM (HRTEM) image of single NC confirms a single pure phase and high crystallinity (Figure c). The lattice fringes can be clearly observed and correspond to the (220) lattice plane. The TEM images of x = 0, 0.25, 0.5, and 1 undoped NCs also exhibit relatively even distribution, having mean sizes of 10.93 ± 0.9 nm (details in Figure S1).

Figure 1

(a) XRD pattern of undoped Cs2NaInBi1–Cl6 (x = 0, 0.25, 0.5, 0.75, and 0.9) NCs compared with the standard XRD patterns of Cs2NaBiCl6 single crystal (PDF#04-005-9059) (left) and magnified image of the (220) diffraction peaks (right). (b) TEM image of Cs2NaIn0.75Bi0.25Cl6 NCs. (c) HRTEM image of Cs2NaIn0.75Bi0.25Cl6 NCs. (d) Size distribution histogram of Cs2NaIn0.75Bi0.25Cl6 NCs.

In order to investigate the optical properties of Cs2NaInBi1–Cl6 (x = 0, 0.25, 0.5, 0.75, and 0.9) NCs, the steady-state absorption spectra are performed and presented in Figure a. The narrow excitonic absorption peak of Cs2NaBiCl6 NCs is observed at 326 nm. The absorption peaks of these Na-based mixed In/Bi DP NCs (0 < x < 1) are similar to Cs2NaBiCl6 NCs, and only exhibit a slight blue shift (∼2 nm) with increasing In content (Figure a). We then check the PL spectra of undoped Cs2NaInBi1–Cl6 NCs (0 ≤ x < 1). These undoped NCs show a blue PL emission peak, whose position blue shifts from 429 to 424 nm as the In content increases (Figure b), consistent with the change trend of the absorption result. The PL emission of Cs2NaInBi1–Cl6 NCs is different from the broad emission with a large Stokes Shift caused by bright STEs of reported DP NCs. Thus, The Cs2NaInBi1–Cl6 NCs have a dark STE state.[10] These will be discussed in detail below. The highest PLQE of Cs2NaIn0.9Bi0.1Cl6 NCs is up to 38%. The time-resolved PL (TR-PL) spectra using the time-correlated single-photon counting (TCSPC) technique are recorded and shown in Figure S2. The average PL lifetimes of these NCs all are on the order of nanoseconds, corresponding to the radiative recombination of FEs.

Figure 2

(a) Steady-state absorption spectra of undoped Cs2NaInBi1–Cl6 (x = 0, 0.25, 0.5, 0.75, and 0.9) NCs (left) and magnified portion of absorption spectra (310–350 nm) (right). (b) Normalized PL spectra of undoped Cs2NaInBi1–Cl6 NCs (left) and magnified portion of PL spectra (390–500 nm) (right).

Compared to reported lead-free DP NCs, we find an interesting rule. Na-based DP NCs have no broad, large red-shifted PL emission attributed to the dark STE state. However, as long as the DP NCs contains an Ag element (Ag-based,[4−7,11,12,14−16] Ag-alloyed,[8,9,13] or Ag-doped[10] DP NCs), it has a broad PL emission with large Stokes shift attributed to a bright STE state. This demonstrates that the Ag element is the key to the formation of bright STEs, which facilitates broad, large redshift PL emission in DP NCs. In addition, Na-based DP NCs have three advantages over reported Ag-based DP NCs.[14] First, their morphology is more regular. Second, FE emission does not require reprocessing (surfactant passivation). Third, the PLQE of Na-based DP NCs is slightly higher than that of Ag-based NCs in blue emission.

Mn-Doped Cs2NaInBi1–Cl6 DP NCs

Na-based DP NCs are not conducive to a broad, large redshift PL emission due to their dark STE state, which is beneficial to FE trapping. We speculate that the dark STE state may act as an intermediate state to avoid FE emission or bright STE emission and promote single, pure PL emission of Mn2+ dopants. To improve PL emission, we attempt to dope Mn2+ into Cs2NaInBi1–Cl6 (0 ≤ x ≤ 1) DP NCs. The synthesis of Mn-doped Cs2NaInBi1–Cl6 (x = 0, 0.25, 0.5, 0.75, 0.9, and 1) NCs are also performed using variable temperature hot injection. X-ray diffraction (XRD) patterns of these NCs confirm the absence of undesired secondary phases (Figure a). XRD patterns revealed that the diffraction peaks are monotonically shifted to high angles with increasing In content. The (220) peak, which shifts 0.8° from x = 0 to x = 1, coincides with the result of undoped NCs. We show that Mn2+ ions have been successfully doped into Cs2NaIn0.75Bi0.25Cl6 NCs by X-ray photoelectron spectroscopy (XPS) (Figure S3).

Figure 3

(a) XRD pattern of Mn-doped Cs2NaInBi1–Cl6 (x = 0, 0.25, 0.5, 0.75, 0.9, and 1) NCs compared with the standard XRD patterns of Cs2NaBiCl6 single crystal (PDF#04-005-9059) (left) and magnified image of the (220) diffraction peaks (right). (b) TEM image of Cs2NaIn0.75Bi0.25Cl6:Mn NCs. (c) HRTEM image of Cs2NaIn0.75Bi0.25Cl6:Mn NCs. (d) Size distribution histogram of Cs2NaIn0.75Bi0.25Cl6:Mn NCs.

A transmission electron microscopy (TEM) image of Cs2NaIn0.75Bi0.25Cl6:Mn NCs is shown in Figure b. The NCs are evenly distributed cubic-shaped having a mean size of 9.43 nm (Figure b,d). A high-resolution TEM (HRTEM) image of single NC shows a single pure phase and high crystallinity (Figure c). The high-angle annular dark-field (HAADF) and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) element mappings of Cs2NaIn0.75Bi0.25Cl6:Mn NCs confirm visually that without phase separation and the homogeneous distribution of Cs, Na, In, Bi, Mn, and Cl in the single pure phase (Figure S4 and Table S2). The TEM images of x = 0, 0.25, 0.5, and 1 Mn-doped NCs also exhibit relatively even distribution and are mainly a cubic shape with mean sizes of 10.45 ± 0.5 nm (details in Figure S5). These results also indicate that the doping of the Mn2+ cation does not induce the formation of crystal defects. According to previous studies,[21,22] doping Mn2+ ions can enhance the stability of perovskite NCs in air. The Na-based DP NCs are no exception, and Mn-doped NCs show better stability than undoped NCs in air (Figure S6).

Normalized steady-state absorption (dashed line) and PL (continuous lines) spectra of Mn-doped Cs2NaInBi1–Cl6 (x = 0, 0.25, 0.5, 0.75, and 0.9) NCs.

The optical absorption spectra of Mn-doped Cs2NaInBi1–Cl6 (x = 0, 0.25, 0.5, 0.75, and 0.9) NCs are similar to the undoped NCs (Figure S7). It indicates that doping Mn2+ has little effect on the absorption of undoped NCs, consistent with absorption results of lead-based perovskites.[19,23] As expected, Mn-doped NCs show bright pure Mn2+ PL, which are tunable from yellow emission to orange-red emission corresponding to from x = 0 to x = 0.9 (Figure S8a). However, Cs2NaInCl6:Mn NCs have very weak Mn2+ PL emission centered at 614 nm (Figure S8), due to the parity-forbidden transition of In-based DP.[20] The highest PLQE of Cs2NaIn0.9Bi0.1Cl6:Mn NCs is up to 44.6%, much higher than the PLQE of Mn-doped Cs2AgInCl6 DP NCs.[7] Excitingly, with increasing In content, the PL peak position of Mn-doped NCs exhibits a red shift from 583 to 614 nm (Figure S8b), contrary to the trend of absorption peak. TR-PL measurements are operated. The PL decay curves of Mn-doped NCs are shown in Figure S9, giving a long average lifetime of 3–9 ms, caused by the spin forbidden nature of the Mn2+ ions (4T1 → 6A1 transition).[24,25] The PL lifetime of Mn-doped Na-based DP NCs is longer than that of Mn-doped, Ag-based DP and Pb-based perovskites NCs.[7,21−23]

Size Effect on PL Property

As is known to all, semiconductor material decreases from the bulk size to the nanoscale, due to the quantum confinement effect,[26,27] and the optical properties will be significantly different, which is generally manifested as the XRD diffraction peak becomes wider, and the absorption and PL peak become narrower and blue shift. For the reported Cs2AgInCl6:Mn NCs and bulk crystals,[7,28] the optical properties are consistent with the above. To our surprise, the PL peak of Mn-doped Na-based DP NCs appears red shifted compared with the reported bulk crystals.[29]

In order to facilitate the following research, we synthesize Cs2NaIn0.75Bi0.25Cl6:Mn bulk crystals for comparison and discussion. The XRD pattern of Cs2NaIn0.75Bi0.25Cl6:Mn NCs corresponds to that of bulk crystals, and the XRD diffraction peak of NCs is wider than that of bulk crystals (Figure S10). Both Cs2NaIn0.75Bi0.25Cl6:Mn NCs and bulk crystals show obvious exciton absorption peaks in Figure a. The central position of the absorption peak has barely changed. The full width at half-maximum (fwhm) of the absorption peak of NCs (31.5 nm) is 17.4 nm narrower than that of bulk crystals (14.1 nm), so the absorption band edge is blue shifted. The PL spectra of Cs2NaIn0.75Bi0.25Cl6:Mn NCs and bulk crystals are also shown in Figure a. The fwhm of the PL peak of NCs (74.2 nm) is 10.5 nm narrower than that of bulk crystals (84.7 nm). Intriguingly, it is obvious that the position of the PL peak exhibits a red shift (∼17 nm), contrary to Mn-doped or undoped Ag-based DP.[5,7,11,28,30,31] This interesting phenomenon is rare in perovskite systems. The quantum confinement effect explains well the results of XRD and absorption, but it is obviously unreasonable to explain the PL properties of Mn-doped DP. This is because Mn2+ PL belongs to the sub-band radiation transition (d–d transition of Mn2+). This phenomenon can be explained by contraction of the 4T1 state of Mn2+ (details in Supporting Information (SI)). The scheme is illustrated in Figure S11. In addition, the average PL lifetime (∼8.7 ms) of NCs is longer than that of bulk crystals (∼0.6 ms) (Figure b).

Figure 5

(a) Steady-state absorption (dashed line) and PL (continuous lines) spectra of Cs2NaIn0.75Bi0.25Cl6:Mn NCs (orange) and bulk crystals (olive green). (b) TR-PL decay curves of Cs2NaIn0.75Bi0.25Cl6:Mn NCs (τavg = 8.7 ms) and bulk crystals (τavg = 0.6 ms).

Transient Absorption Measurement

To further reveal the process of the dark STE state assisted Mn2+ emission in DP NCs, the femtosecond TA technique was carried out. The pseudocolor TA plot of the Cs2NaIn0.75Bi0.25Cl6:Mn NCs is shown in Figure a. With 310 nm laser excitation, a broad positive photoinduced absorption (PIA) is observed across the probe region, which is direct evidence of the STEs.[10,32,33] The same decay curves probed at different wavelengths (470, 505, 540, 580, and 630 nm) confirm that the PIA signal reflects the property of the same excited state in the inset of Figure b. The rise process of the PIA signal is clearly observed in Figure S12, but the rise time is too fast within the instrument response resolution (∼100 fs). This result demonstrates that there is no potential barrier separating the FEs and STEs, and the dark STE state is beneficial to FE trapping. The PIA decay signal can be fitted by three components: an ultrafast lifetime of τ1: 1–3 ps, a middle lifetime of τ2: 100 ± 50 ps, and a slow lifetime of τ3: > 2 ns (Figure b).

Figure 6

(a) Pseudocolor TA plot of Cs2NaIn0.75Bi0.25Cl6:Mn NCs. (b) PIA decay dynamics of Cs2NaIn0.75Bi0.25Cl6:Mn NCs probed at 580 nm. Inset: Normalized PIA decay dynamics probed at different wavelengths.

For DP NCs, the smaller the size, the larger the specific surface area, the higher the surface activity, and a shorter crystallization time and larger temperature gradient of crystallization result in more permanent defects.[11] Bulk perovskite single crystals have a low trap-state density.[34−36] Meanwhile, a doping strategy can reduce the defect state.[10,21,37] To eliminate the influence of surface defects, the femtosecond TA of bulk Cs2NaIn0.75Bi0.25Cl6:Mn single crystal is characterized and analyzed (Figure S13a). As expected, the PIA signal of the bulk crystal is similar to that of NCs, but the middle-lifetime process disappears (Figure S13b). It is reasonable that the middle component of 100 ± 50 ps is attributed to surface defects trapping.[6] The surface defects are clearly observed in the HRTEM image of single Cs2NaIn0.75Bi0.25Cl6:Mn NC (Figure S14). Energy transfer (ET) is an extremely fast picosecond process in semiconductor nanomaterials.[38] The ultrafast component (1–3 ps) is assigned to the energy transfer from the dark STE state to the 4T1 excited state of Mn2+. The slow lifetime of >2 ns is assigned to the dark STEs, consistent with our previous study.[10] The femtosecond TA of undoped Cs2NaIn0.75Bi0.25Cl6 NCs also has a positive PIA signal and a slow component of >2 ns (Figure S15). It proves that dark STE state is independent of Mn doping or undoping in Na-based mixed In/Bi DP NCs. Combined with the above, the overall exciton dynamics of Cs2NaIn0.75Bi0.25Cl6:Mn NCs is illustrated in Scheme .

Scheme 1

Exciton Dynamics Model of Mn-Doped Cs2NaIn0.75Bi0.25Cl6 DP NCs

Conclusion

In conclusion, we have successfully synthesized a series of undoped and Mn-doped all-inorganic, lead-free, direct bandgap Na-based mixed In/Bi PD NCs using variable temperature hot injection. Undoped NCs exhibit band edge blue-light emission with the highest PLQE of 38%. Through doping Mn2+ ions into Cs2NaInBi1–Cl6 NCs, PL properties of Mn-doped NCs are changed and improved to achieve bright orange-red emission due to 4T1 → 6A1 transition of Mn2+ dopants and have an ultralong PL lifetime around 3–9 ms. The PL are tunable from yellow emission to orange-red emission corresponding to a red shift from 583 to 614 nm with increasing In content. The best PLQE of Mn-doped NCs is 44.6%. Mn-doped NCs show better stability than undoped NCs to air exposure for more than two months. Therefore, it is promising as a new highly efficient orange-red-light emitting material for applications of LEDs. In addition, we find an interesting phenomenon is that the PL emission of Cs2NaIn0.75Bi0.25Cl6:Mn NCs shows a red shift from the bulk size to the nanoscale. We further prove the process of dark STE state assisted Mn2+ PL in DP NCs by the TR-PL measurements and ultrafast TA techniques. This work highlights the rational use of the advantages of excited states and the design of new high-performance semiconductor nanomaterials by introducing appropriate dopants, in order to obtain desired photophysical properties for photovoltaic devices.

Experimental Section

Materials

Cesium acetate (Cs(OAc), 99.99%, Aladdin), sodium acetate (Na(OAc), 99.99%, anhydrous, Aladdin), indium acetate (In(OAc)3, 99.99%, Alfa Aesar), bismuth acetate (Bi(OAc)3, 99.99%, Sigma-Aldrich), manganese acetate (Mn(OAc)2, 98%, anhydrous, Alfa Aesar), chlorotrimethylsilane (TMSCl, 99%, Sigma-Aldrich), 1-octadecene (90%, Alfa Aesar), oleylamine (OLA, Aladdin, 80%), oleic acid (OA, 90%, Alfa Aesar), toluene (99.5%, Sinopharm Chemical Reagent Co., Ltd., China), n-hexane (97%, Aladdin), cesium chloride (CsCl, 99%, Aladdin), sodium chloride (NaCl, 99.99%, Alfa Aesar), indium chloride (InCl3, 98%, Sigma-Aldrich), bismuth chloride (BiCl3, anhydrous, 99%, Alfa Aesar), manganese chloride (MnCl2, 99.99%, Alfa Aesar), and hydrochloric acid (analytical pure, Sinopharm Chemical Reagent Co., Ltd., China). All chemicals were used as received without further purification.

Synthesis of Undoped and Mn-Doped Cs2NaInBi1–Cl6 (x = 0, 0.25, 0.5, 0.75, 0.9, and 1) NCs through Variable Temperature Hot Injection

The undoped Cs2NaInBi1–Cl6 NCs were prepared by variable temperature hot injection. A total of 0.65 mmol Cs(OAc), 0.45 mmol Na(OAc), x mmol In(OAc)3, and (0.5 – x) mmol Bi(OAc)3 were added in a mixture of oleic acid (2.8 mL), oleylamine (0.7 mL), and octadecene (10 mL), which was heated to 110 °C under a vacuum for 60 min. The reaction mixture was heated through a temperature gradient of 6 °C/min under a nitrogen atmosphere, and TMSCl (0.4 mL) was swiftly injected at 168 °C. The reaction mixture continued to 180 °C and was immediately cooled to room temperature in an ice–water bath. The reaction mixture was then decanted into a centrifugal tube and centrifuged at 9000 rpm for 20 min. The supernatant was removed. The precipitate was washed in 10 mL of toluene and centrifuged at 10 000 rpm for 15 min. The supernatant was discarded. The precipitate was redispersed in 10 mL of hexane with sonication and centrifuged at 6000 rpm for 15 min, and colloidal Cs2NaInBi1–Cl6 NCs were obtained by discarding the bottom precipitate. The synthesis of Mn-doped Cs2NaInBi1–Cl6 NCs was performed using the same methods, but adding another 0.14 mmol of Mn(OAc)2 to the starting reaction mixture for Cs2NaInBi1–Cl6:Mn NCs.

Synthesis of Mn-Doped Cs2NaIn0.75Bi0.25Cl6 Bulk Crystals by a Hydrothermal Method

For Cs2NaIn0.75Bi0.25Cl6:Mn bulk crystals, 2 mmol of CsCl, 1 mmol of NaCl, 0.75 mmol of InCl3, 0.25 mmol of BiCl3, and 0.2 mmol of MnCl2 are dissolved in 8 mL of 12 M hydrochloric acid in a 25 mL Teflon liner. Then it was heated at 180 °C for 18 h in a stainless steel Parr autoclave and was slowly cooled to room temperature (RT) at a speed of 5 °C/h. Finally, the crystals were filtered and dried in a vacuum at 80 °C for 12 h.

Structural Characterizations

Powder X-ray diffraction (PXRD) was performed on an PANalytical Empyrean diffractometer equipped with Cu Kα X-ray (λ = 1.54056 Å) tubes, and the acquisition was done for every 0.04° increment over the Bragg angle range of 10–60°. Inductively coupled plasma optical emission spectrometry (ICP-OES) was performed on PerkinElmer ICP-OES 7300DV. The transmission electron microscopy (TEM) measurements were performed by using the JEM-2100. The STEM-HAADF and STEM-EDS elemental mapping images were obtained using a FEI Tecnai G2 F20 at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a ThermoFisher ESCALAB 250Xi with the X-ray source of monochromatic Al Kα (hν = 1486.6 eV).

Optical Absorption Spectroscopy

For colloidal NCs, steady-state optical absorption measurement was performed by using a PerkinElmer Lambda 35 double-beam spectrometer equipped with an integrating sphere to exclude the signal due to light scattering.

For single crystal powder, steady-state absorption spectra were recorded using a UV–vis (SHIMADZU UV2600) spectrometer. Optical diffuse reflectance measurements were performed by equipping with an integrating sphere at room temperature and BaSO4 as the 100% reflectance reference. The reflectance data were converted to absorption according to the Kubelka–Munk equation:where R is the reflectance, α and S are the absorption and scattering coefficients, respectively.

PL Spectroscopy

Steady-state PL spectra were recorded on a Horiba JobinYvon FluoroMax-4P spectrofluorometer. Time-resolved PL (TR-PL) measurements were carried out on an OB920 luminescence lifetime spectrometer (Edinburgh Instruments Ltd., UK) using time-correlated single photon counting (TCSPC) technology. The PLQE measurement was performed using an absolute PL quantum yield spectrometer (Hamamatsu C11347).

Ultrafast Transient Absorption Spectroscopy

TA experiments are operated by using a homemade femtosecond pump–probe setup. Laser pulses (800 nm, 50 fs pulse length, 1 kHz repetition rate) were generated by a Ti: sapphire femtosecond laser source (Hurricane, Spectra-Physics). An optical parametric amplifier was used to change the laser wavelength. For the probe, we use the supercontinuum generation from a thin CaF2 plate. The mutual polarization between pump and probe beams was set to the magic angle (54.7°) by placing a Berek compensator in the pump beam. The kinetics of the different scans stayed the same showing no sign of degradation. Excitation power and spot size measurements were used to determine the excitation fluence (pump wavelength: 310 nm; pump fluence: 6.2 × 1014 photons/pulse/cm2). During the measurement, millimeter-scale bulk crystal was placed on the quartz substrate; colloidal NCs were contained in a 1 mm cuvette.