Nature of Self-Trapped Exciton Emission in Zero-Dimensional Cs2ZrCl6 Perovskite Nanocrystals

Low dimensional perovskite-inspired materials with self-tapped exciton (STE) emission have stimulated a surge of cutting-edge research in optoelectronics. Despite numerous efforts on developing versatile low-dimensional perovskite-inspired materials with efficient STE emissions, there is little emphasis on the intrinsic dynamics of STE-based broad emission in these materials. Here, we investigated the excited state dynamics in zero-dimensional (0D) Cs2ZrCl6 nanocrystals (NCs) with efficient blue STE emission. By using femtosecond transient absorption (fs-TA) spectroscopy, the ultrafast STE formation process within 400 fs is directly observed. Then, the formed STEs relax to an intermediate STE state with a lifetime of ∼180 ps before reaching the emissive STE state with a lifetime of ∼15 μs. Our work offers a comprehensive and precise dynamic picture of STE emission in low-dimensional metal halides and sheds light on extending their potential applications.


Sample preparation
Firstly, 300 mg Zr(CO3)2(zirconium decarbonate, HWRK Chem, 99.90%), 10 ml ODE (1-octadecene, Alfa Aesar, 90% ) and 325 μL CH3COOH (acetic acid, Sigma, ≥ 99%) were added into a 3-neck round flask and heated at 105 o C to get a transparent solution.After around 30 minutes, 136 mg CsOAc (cesium acetate, Acros Organics, ≥ 98.5%), 2.8 mL oleic acid and 615 μL OLA (oleylamine, Acros Organics, ≥ 96%) were sequentially added.The mixture was degassed at low pressure condition to remove water and oxygen for 1 h at 105 o C. Later, the temperature was raised up to 200 o C under N2 atmosphere.Once reaching 200 o C, 400 μL TMSCl (trimethylchlorosilane, > 98.0%, TCI) was swiftly injected within 20 seconds.Afterwards, the ice-water bath was used to cool it down.To get rid of the left reagents in solution, the raw mixture was isolated by centrifugating at 6500 rpm for 10 min.The white sediment was dispersed in 10 mL chloroform-d (CHCl3) or heptane, and then centrifuged at 6500 rpm for 10 min again.Finally, the colloidal NCs could be obtained from the supernatant.For femtosecond transient absorption (fs-TA) and streak camera measurements, the prepared Cs2ZrCl6 NCs were transfer to heptane by evaporating CHCl3 and dispersed in heptane with the same volume to exclude the potential solvent effect under high excitation photon energy.
The morphology and shape of NCs was obtained from transmission electron microscopy (Tecnai G2 T20 TEM).The NCs' concentration was measured by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES, Perkin Elmer Optima 8300).Powder X-ray diffraction (XRD) measurements were performed on a Rigaku D-MAX 2500/PC diffractometer equipped with Cu Kα radiation.Time-resolved PL spectra within microseconds was measured using a FLS1000 Edinburgh Instruments spectrofluorometer.The Streak camera measurements were performed with the excitation pulses generated by a Ti:Sapphire laser (Spectra-Physics, Tsunami) with 60 fs pulse at 800 nm and repetition rate of 80 MHz.Time-resolved PL spectra at nanosecond scale were imaged onto the input slit of the streak camera (Hamamatsu C6860) set at 150 μm with the excitation of 266 nm.

Femtosecond transient absorption (fs-TA) spectroscopy
TA measurements were conducted by using a femtosecond pump-probe homemade setup.Laser pulses (8 W, 796 nm, 60 fs, 4 kHz) come out of Solstice (Spectra Physics) amplifier seeded by a femtosecond oscillator (Mai Tai SP, Spectra Physics).The pulsed laser light was divided into two parts to use for pump and probe beams.For the probe, the super-continuum white light is generated from a thin CaF2 plate, covering the whole visible light region (430 ~ 775 nm).For pump pulse (30 mW, 266 nm, 130 fs) was produced by collinear optical parametric amplifier (Tripler 100M (X), Spectra Physics).Inside, the laser pulse light goes though BBO crystal to achieve double frequency, and then mixes with Mai Tai laser fundamental to get sum frequency.The output pump lights consist of 266, 400 and 800 nm, then the pure pump light of 266 nm is obtained by using two higly selective (for 266 nm) reflective mirrors.The global analysis (GLA) was performed by Glotaran software package (http://glotaran.org).The simple sequential or parallel decay model with various components was used to do SVD global fitting. 1

Figure S1 .
Figure S1.UV-vis absorption and PLE spectra of Cs2ZrCl6 NCs and organic reagents for comparison.

Figure S2 .
Figure S2.The plot of PL intensity versus excitation fluence pumped on Cs2ZrCl6 NCs. Green scatter and pink solid line stand for experimental data and linear fitting results, respectively.The blue star stands for the excitation fluence used in fs-TA measurements.