Interfacial Manganese‐Doping in CsPbBr3 Nanoplatelets by Employing a Molecular Shuttle

Abstract Mn‐doping in cesium lead halide perovskite nanoplatelets (NPls) is of particular importance where strong quantum confinement plays a significant role towards the exciton–dopant coupling. In this work, we report an immiscible bi‐phasic strategy for post‐synthetic Mn‐doping of CsPbX3 (X=Br, Cl) NPls. A systematic study shows that electron‐donating oleylamine acts as a shuttle ligand to transport MnX2 through the water–hexane interface and deliver it to the NPls. The halide anion also plays an essential role in maintaining an appropriate radius of Mn2+ and thus fulfilling the octahedral factor required for the formation of perovskite crystals. By varying the thickness of parent NPls, we can tune the dopant incorporation and, consequently, the exciton‐to‐dopant energy transfer process in doped NPls. Time‐resolved optical measurements offer a detailed insight into the exciton‐to‐dopant energy transfer process. This new approach for post‐synthetic cation doping paves a way towards exploring the cation exchange process in several other halide perovskites at the polar–nonpolar interface.


Preparation of Cs-Oleate (Cs-OA) solution.
To prepare Cs-OA solution, 0.16 g of Cs2CO3, 0.5 mL of dried Oleic acid (OA) and 8 mL of ODE were added into a 50 ml three-neck flask. The mixture was degassed under vacuum at 120 o C for 30 min, and then the temperature was increased to 150 o C under N2 atmosphere. After the solid phase was completely dissolved, the temperature was set back to 120 o C for further use.

Synthesis of CsPbBr3
NPls. The CsPbBr3 NPls were prepared following our previously reported literature. [1] In a typical synthesis of CsPbBr3 NPls, 0.069 mg (0.188 mmol) of PbBr2 was added into a 50 mL three-neck flask containing 0.5 mL of OA, 0.5 mL of OLA and 5 mL of ODE. The mixture was degassed under vacuum at 120 o C for 20 min and then changed to N2 atmosphere. When the PbBr2 was completely dissolved, the solution was cooled to room temperature. Subsequently, 0.4 mL of hot Cs-OA was injected into the solution. The mixture was heated up under an N2 atmosphere. Once the temperature reached 100 o C (150 o C or 180 o C), the reaction was quenched by an ice-water bath. The product was separated by centrifugation at the speed of 8000 rpm for 5 min. The precipitate was redispersed into 10 mL of hexane. To get the final product, the hexane solution was centrifuged at 3000 rpm for 5 min, and then the precipitate was discarded.

Mn doping process in One-step two-phase post-synthetic (OPS) method.
In a typical experiment, several concentrations of manganese salt in 50 L of water was mixed with 1 mL of as-prepared CsPbBr3 NPls and 1 mL of hexane. Subsequently, the two-phase mixed solution was subjected to stirring at the speed of 400 rpm for 1 h at ambient conditions. Then after waiting for a minute before there is a phase separation, the hexane layer was pipetted out from the upper phase. Generally, this step is not time-sensitive, however, prolonged mixing (over days) between the two phases can destroy the NPls to some extent (see Fig. S12). Furthermore, if the stirring is continued beyond 1 hr, it takes several days (~17 days) for complete degradation of the perovskite, with negligible PL emission. Therefore, it is better to separate the phases soon after the reaction. The hexane phase was then transferred to a centrifuge tube and treated by centrifugation at the speed of 9000 rpm for 5.
The precipitate was discarded, and the final product will be in hexane.

Mn doping process in two-step two-phase post-synthetic (TPS) method.
In this method, the Mn doping process needs two steps. Firstly, 50 L saturated aqueous solution of a manganese salt was added into 2 mL of hexane which contains 10 L of OLA. The mixed solution was subjected to oscillation by a vortex for 30 seconds at vigorous conditions. Then, the hexane solution on the upper phase was transferred to a centrifuge tube, similar to section 1.4, and treated by centrifugation at the speed of 9000 rpm for 5 min. The hexane solution with MnX2 on the upper phase was retained for further use, and the precipitate was discarded. Next, 1 mL of hexane with MnX2 prepared in the first step was mixed with 1 ml as-prepared CsPbBr3 NPls. The Mn-doped CsPbBr3 NPls will generate immediately. The mixture could be stirred for a certain time to make sure the reaction is completed.

Transmission electron microscopy (TEM).
Scanning transmission electron microscopy in high-angle annular dark-field mode was performed with a probe corrected FEI Titan Themis operating at 300 kV (Department of Chemistry, LMU Munich). TEM specimen was prepared on a carbon-coated Cu grid by dropping 10 L of dilute colloidal dispersions on each sample.

Electron Paramagnetic Resonance (EPR) spectroscopy.
The presence of Mn in the doped NPls is determined by an EPR spectroscopy from ADANI Spinscan X (2020 model). The conditions are used as following: 20mW, 9.4506 GHz. To prepare samples for the EPR measurement, first the prepared Mn-doped NPls (via OPS method) was precipitated with an anti-solvent (ethyl acetate) and collected after centrifugation, which would remove the undoped Mn ions in the solution. To get a sufficient amount of samples for measurements, 10 typical batches (NPls in 10 ml hexane) of the two doping concentrations were made and collected together. The solid sample was then filled into the quartz tube for EPR measurement. The EPR signals of the empty tube were first taken as the baseline.

Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES).
The amount of Mn-doping in the NPls is determined by ICP-OES measurement from Varian Vista RL simultaneous spectrometer with autosampler. To prepare samples for the ICP-OES measurement, first the prepared Mn-doped NPls (4M and 7M) (via OPS method) were precipitated with an anti-solvent ethyl acetate and collected after centrifugation, which would remove the undoped Mn ions in the solution. To get a sufficient amount of samples for measurements, 5 typical batches (NPls in 10 ml hexane) of the two doping concentrations were made and collected together. Then the solid doped NPls were dissolved in 400 μL aqua regia (freshly made and stirred for over 30 min) under strong sonication, which were further diluted to 20 ml aqueous solutions. The samples were filtrated before measured in the ICP-OES equipment.

UV-Vis absorption and Photoluminescence (PL) spectroscopy.
The absorption spectra were measured by a Cary 5000 UV-Vis-NIR spectrophotometer. A Horiba Jobin Yvon Fluorolog-3 FL3-22 spectrometer with a 450W Xe lamp was used to acquire the photoluminescence spectra.

Time-resolved fluorescence spectroscopy.
A homemade device with a white light laser (NKT SuperK Extreme EXR-20), extended with an EXTEND-UV box, was used to perform the time-resolved fluorescence measurements. The samples were excited with 350 nm laser. The signal was detected with a Princeton Instruments monochromator fiber connected to a low-noise Avalanche Photodiode from Excelitas.

Femtosecond transient absorption spectroscopy. A custom-built transient
absorption setup from Newport was used for the ultrafast transient absorption measurement, similar to our earlier work. [2] A femtosecond multi-pass Ti: Sapphire regenerative amplifier laser system (Libra HE+, Coherent Inc.) produces ~100 fs laser pulse having 1 kHz repetition rate at 800 nm. A fraction of this laser pulse is frequency doubled using a β-barium borate (BBO) crystal to produce the pump beam at 400 nm for the sample excitation. Another fraction of the laser pulse (~1 J) is used to generate the white-light probe pulse by focusing part of the initial 800-nm beam through a 2-mm-thick motorized CaF2 crystal. The pump and the probe pulses are spatially and temporally overlapped at the sample, with the pump pulse passing through at 0.5 kHz chopper to read out the changes in probe transmission for every pump-probe cycle. A motorized optical delay line is placed in the probe arm to offer a time delay between the two pulses. The NPls dispersed in hexane were kept in a 2-mm cuvette for the pump-probe experiment. The average number of excitons were kept low enough (<<1) to avoid the multi-exciton phenomenon.

Possible Mechanism of Mn-incorporation into the NPls as a function of thickness.
To understand the underlying mechanism of Mn incorporation into the NPls as a function of the thickness of parent NPls, we performed a detailed investigation of the stability of the undoped and doped NPls prepared at 100 and 180 o C, by probing the PL spectra. As shown in Fig. S10-12, the undoped NPls prepared at 100 o C is only stable for a few days (<10 days), while the one prepared at 180 o C is stable for more than 10 days, as the PL spectra remain unchanged on the 10 th day. The same phenomenon has also been observed for the corresponding Mn-doped NPls. Here the intensity of the Mn PL for the doped NPls obtained from the Un-100 NPls (named Mn-100) drops significantly at the 10 th day while that for the Mn-180 NPls remains quite similar to day 1, albeit a slight blue shift in the Mn PL is observed for both Mn-100 and Mn-180 NPls (see section 3.2). One possible reason for the better stability of the NPls prepared at a higher reaction temperature (180 o C) corresponds to thicker NPls (5 ML) due to better surface passivation. [3] This directs that most of the ligands (both OA and OLA) are engaged towards the surface passivation of the NPls prepared at a higher reaction temperature. As revealed in the main text, the post-synthetic method developed in this work is based on the shuttling effect of the OLA, which is liable for the MnX2-transport at the water-hexane interface. Due to better surface passivation, the number of available shuttling ligands is limited in the hexane solution, leading to low incorporation of Mnion into the 5 ML NPls prepared at 180 o C.
Conversely, the NPls prepared at 100 o C showed lower stability due to poorer surface passivation. This not only leads to additional availability of the shuttling ligands in the solution but also opens more room for Mn-ion incorporation through poorer surface passivation, eventually leads to more Mn-ion incorporation into the NPls (i.e. Mn-100). The additional available OLA ligand plays the shuttling role, which can be verified from the fact that if the amount of OLA is increased in the hexane solution (as described in Fig. 2a), the incorporation of Mn-ion into the NPls increases (see Fig. S6).
As a result, one can conclude that the amount of the Mn-ion incorporation into the NPls in the post-synthetic method described in this work depends on the number of available shuttling ligands (here OLA) in the nonpolar solvent. Moreover, as Mn-100 (7M) NPls are less passivated compared to Mn-180 (7M) NPls, this indicates the organic ligand may plays an essential role in the energy transfer process in the two system, as described later.
One should also note that, a small side band along with the main exciton PL peak is observed in some of the samples (Fig. S5, S8, S9). Most probably, these are due to formation of different size/thickness NPls, which forms possibly due to different solvent mixture used in the post-synthetic doping.

Mn PL tunability as a function of NPls thickness.
The atomic like Mn PL in the doped NCs or NPls is supposed to appear ~600 nm (2.1 eV), independent of the host NCs/NPls. However, it is also reported the Mn PL peak position tunability upto ~55 nm, starting from ~585 nm to ~640 nm, in perovskite nanocrystals [4] . The atomic like 4 T1-6 A1 Mn PL peak position tunability depends on a few factors [5] : 1) ligand field strength,

2) Mn -Mn interaction,
3) doping induced lattice contraction effect in the crystal field strength, etc.
In the present investigation, the Mn PL peak position appears ~618 nm for the Mn-180 (7M) NPls. On a closer inspection suggests the Mn PL peak position blue-shifted upon reduction of the parent NPls thickness; for example, the Mn PL appears at ~608 nm and ~605 nm for Mn-150 (7M) and Mn-100 (7M) NPls, respectively (see Fig. 3b). On the contrary, the Mn PL peak position is independent of Mn amount for a particular thickness, e.g. it always appears ~618 nm for all Mn-180 NPls (see Fig. 1g), indicating the Mn -Mn interaction and doping induced lattice contraction may not be the underlying reason for Mn PL position tunability in the present case. We believe the Mn PL peak position tunability in the present case may be related to the ligand field strength.
Here the Mn-dopant in the NPls with a lower thickness (e.g. Mn-100 NPls) faces a higher ligand field interaction from the surrounding shuttle ligands due to additional available shuttle ligands. This eventually leads to more splitting of the 4 T1-6 A1 energy, causing the blue shift from Mn-180 to Mn-100 NPls.
Another thing also needs to be considered: upon Mn-halide saturated aqueous solution addition, a certain amount of Mn-halide shuttled to hexane via OLA, all of which (i.e. Mn-OLA complex) indeed does not lead to doping in the NPls. As a result, the doping process still may continue after several days, even after separating the two phases. This observation apparently can be seen from the blue-shift in the Mn PL spectra after 10 days, presented in Fig. S10-12. For example, although at day 0, the Mn PL peak appears at ~620 nm for Mn-180 (7M) NPls, after 10 days, the Mn PL peak shifted to ~600 nm. Presumably, over time surface doping increases in the NPls from the surrounding Mn-OLA complex, leading to a more ligand field interaction (i.e. higher splitting of the 4 T1-6 A1 energy), causing the blue shift in the Mn PL. However, a detailed investigation is necessary to have a clear understanding of such doping mechanism and the resultant Mn PL tunability.

Estimation of the energy transfer efficiency and time.
To estimate the energy transfer efficiency, we model the bleaching dynamics (in Moreover, by recalling the PL lifetime measurement (Fig. 4, Fig. S13 and Table S2, S3), we found the undoped NPls prepared at a lower temperature (Un-100) has higher exciton lifetime than the one prepared at a higher temperature, Un-180 (4 ns vs. 3.1 ns).
This indicates the non-radiative exciton relaxation decreases in the Un-100 NPls due to reduced interaction with ligands (less surface passivated). Therefore, the exciton can transfer the energy more efficiently to the Mn-dopant in the Mn-100 NPls than the Mn-180 NPls. In short, the organic ligands act as a non-radiative center for exciton trapping and thereby reduced the energy transfer efficiency.