Colloidal Organometal Halide Perovskite (MAPbBrxI3−x, 0≤x≤3) Quantum Dots: Controllable Synthesis and Tunable Photoluminescence

Organic-inorganic perovskite materials, typically methylammonium lead trihalide (MAPbX3: MA = methylammonium; X = Br, I), are recently attract enormous attention for their distinguished photo-electronic properties. The control of morphology, composition and dispersability of MAPbX3 perovskite nanocrystals is crucial for the property tailoring and still a major challenge. Here we report the synthesis of colloidal MAPbBrxI3−x(0 ≤ x ≤ 3) nanocrystals at room temperature by using alkyl carboxylate as capping ligands. These nanocrystals exhibit continuously tunable UV-vis absorption and photoluminescence (PL) across the visible spectrum, which is attributed to the quantum confinement effect with certain stoichiometry. Their unique exciton recombination dynamics was investigated and discussed.

Lead halide based perovskites have become famous semiconductor materials for their industrial prospect in solar cells. Up to date, the efficiency of perovskite solar cells has unceasingly boosted up to 22.1% [1][2][3][4][5][6][7][8][9] . The application potentials of perovskites in light emitting devices [10][11][12][13][14][15][16][17] and lasers 18,19 were also demonstrated. In nanometer scale, the huge specific surface area and thus the abundant interface or surface states of the perovskites exhibit prominent effects on the electronic and photoelectronic properties. Therefore, the challenge emerges for chemists of nano-science and -technology to develop synthesis methods to achieve size-controllable perovskite nanocrystals. In the most recent year, colloidal cesium lead halide perovskites nanocrystals and nanowires were successfully synthesized. Kovalenko et al. reported the synthesis of highly luminescent perovskite CsPbX 3 (X = Cl, Br, I) of 4-15 nm, with the photoluminescence tuned within 410− 700 nm 20,21 . Yang et al. developed a solution synthesis of single-crystalline CsPbX 3 (X = Cl, Br, I) nanowires 22 . Prato et al. further found that the composition of perovskite nanocrystals can be feasibly tuned by post-synthesis halide anion exchange 23 . Despite the success for the all-inorganic cesium lead halide perovskite colloidal nanocrystals, the synthesis methods of colloidal organometal halide perovskite nanocrystals are comparably less developed. Although there are numerous literatures on preparing organic-inorganic perovskite micro/nanocrystals, most of them are synthesized or grow on substrates. For examples, the synthesis of MAPbI 3 nanoplates and nanowires on substrates were demonstrated by Jin 24 , Horvath 25 , Grätzel and Park et al. 26 . We reported the synthesis of MAPbI 3 crystals on porous TiO 2 substrate with the size controllable within 40-700 nm 27 . The substrates served as a scaffold for the precursors were used to control the perovskite growth kinetics [27][28][29][30][31] . However, the substrates prevented from scale up the products and the generality for different substrates is limited.
Recently, advances have been made in synthesis of MAPbX 3 (X = Br, I, Cl) nanocrystals in solution without a substrate [32][33][34][35][36] . All these reported methods use organic ammonium cation with a long alkyl chain, such as the octylammonium bromide (CH 3 (CH 2 ) 7 NH 3 Br) or octadecylammonium bromide (CH 3 (CH 2 ) 17 NH 3 Br). The ammonium cation serves as the surface capping ligands of the nanocrystals, limiting the crystal growth in one, two or three dimensions. In this article, we use a different surface modulation strategy by applying alkyl Scientific RepoRts | 6:35931 | DOI: 10.1038/srep35931 carboxylate, the lead oleate (Pb(CH 3 (CH 2 ) 7 CH = CH(CH 2 ) 7 COO) 2 , abbreviated as Pb(OA) 2 ), as both the lead resource and capping ligands. The as synthesized MAPbBr x I 3−x (0 ≤ x ≤ 3) nanocrystals exhibit continuously tunable UV-vis absorption and photoluminescence (PL) spectra across the visible realm, which is attributed to the size related quantum confinement effect with a fixed stoichiometry of the halide composition.

Results and Discussion
The reaction solution contains two solvents, the cyclohexane and the isopropanol. The Pb(OA) 2 is soluble in the former, and the MAI or MABr (or the mixture) is soluble in the later. When these two solutions are mixed at room temperature, MAPbBr x I 3−x (0 ≤ x ≤ 3) forms immediately which can be easily identified by the color change and the photoluminescence under a portable 365 nm ultraviolet lamp (Fig. 1).
The XRD spectra of the typical products are shown in Fig. 2 (and Figures S1-S4 in Supporting Information). For the samples P-I-2/3/4/5, strong split peaks at 14.0° and 14.1° that respectively corresponding to (002) and (110) crystal plane and the split peaks of the (004) and (220) peaks can be clearly identified. This split feature verified that the as prepared MAPbI 3 are tetragonal perovskite phase (space group I4/mcm) [37][38][39][40][41] . While for the rest of the samples containing Br − , no split can be identified at (001) or (002) peaks, indicating they are cubic phased perovskite (space group Pm-3m). The perovskite phases via the regulation of I − and Br − are generally observed by various researchers [38][39][40][41] . For all the samples no impurity of MAI or Pb(OA) 2 is found. This may due to the fast reaction rate and excess amount of MAI to consume the Pb(OA) 2 . Also none of the other known and related MA n PbI m (n, m = 2,4, 3,5, 4,6) phases are observed 41 .
For all the samples of MAPbBr x I 3−x (0 ≤ x ≤ 3) nanocrystals, the typical FTIR vibration modes of organolead halide perovskites are distinctly presented ( Figure S5). The 3300-3000 cm −1 broad strong peak is assigned to N-H stretching; the 2950-2820 cm −1 peaks are assigned to symmetric and asymmetric stretching vibrations of CH 2 and CH 3 36,42-45 ; the peak at 1730-1630 cm −1 corresponds to the COO − modes and the peak at 1440-1360 cm −1 is assigned to the C-H bending 44 . The symmetric O-H stretch and antisymmetric O-H stretch of H 2 O were not found in 3600-3800 cm −1 from Figure S5. Thus, no adsorbed H 2 O was detected, suggesting its good temporary stability in ambient air condition. The good dispersability of the perovskites in cyclohexane also suggests that the surface of MAPbBr x I 3−x (0 ≤ x ≤ 3) nanocrystals is coordinated by oleate ligand, like typical oleate or alkylamines modified colloidal nanocrystals. The schematic illustration of a perovskite nanocrystal stabilized by oleate as surface ligands is shown in Fig. 3(a). Figure 3(b,c) are typical TEM and HRTEM images of perovskite nanocrystals  of sample P-Br-5, respectively. It indicates that these nanocrystals are well dispersed and the measured crystal lattice matches well with the that of the MAPbBr 3 32,36 . Else TEM images of colloidal perovskite nanocrystals of The SEM measurement indicates that with the decrement of MA:Pb from 5:1 to 2:1, the size of all the as synthesized MAPbBr x I 3−x (0 ≤ x ≤ 3) nanoparticles increases to ~300 nm (Fig. 4). This trend may find its origin in the nucleation and growth mechanism. Take the nucleation and growth of MAPbI 3 as an example, the reaction in the solution is as follows: The alkyl carboxylate, lead oleate Pb(OA) 2 , acts as not only as the lead resource to react with MAI, but also as the capping ligands. In addition, the reaction rate is higher with the higher MAI concentration. The faster the nucleation bursts, the more completely Pb(OA) 2 consumed in solution in a short time, leading to smaller MAPbI 3 nanocrystals. While in relatively lower MAI concentration, the nucleation rate is slower and the perovskite growth is dominated following the nucleation, resulting bigger MAPbI 3 nanocrystals. Obviously, it shows significance for all these perovskite materials.
The UV-vis absorption spectra and corresponding PL spectra of four sets of samples (P-I-2/3/4/5, P-I3Br2-2/3/4/5, P-I1Br1-2/3/4/5, P-Br-2/3/4/5) are shown in Fig. 5. The band edge absorption peak locates at 764 nm for big MAPbI 3 nanocrystals (P-I-2, ~300 nm by SEM). It undergoes a blue shift to 735 nm for smaller MAPbI 3 nanocrystals (P-I-5, ~5 nm by TEM). Accordingly, the photoluminescence (PL) peak shifts from 767 nm to 747 nm with the size decreasing. The synthesized MAPbBr 3 and MAPbBr x I 3−x (0 ≤ x ≤ 3) nanocrystals also show similar UV-vis absorption and PL spectra dependent on the size. This size related monotonic blue shift of both spectra can be induced by the intrinsic quantum confinement effect, for the small crystal size is comparable to the Bohr diameter of MAPbBr x I 3−x (0 ≤ x ≤ 3) [46][47][48][49][50] . There remains only one concern, that since the spectra of P-I3Br2-2/3/4/5 and P-I1Br1-2/3/4/5 are over lapped, it is doubtful if the spectrum shift is caused by the component deviation. By comparing the XRD peaks of P-I3Br2-2/3/4/5 and P-I1Br1-2/3/4/5, this possibility is excluded. As shown in Fig. 6, the (002) and (201) peaks of each set are separated from each other, while almost fixed for a same set. It suggests that the perovskites of the same sets have the same crystal parameter, thus have the same component. Also, the widening of the XRD peaks indicates the size of nanocrystals decreases (Figures S1-S3). These features further support the intrinsic quantum confinement effect for the observed spectra shift. The PL lifetimes of MAPbBr x I 3−x (0 ≤ x ≤ 3) were measured ( Figures S9-11) to get insight into the exciton recombination dynamics. These PL decays can be well fitted by the bi-exponential function ( Figure S12): where τ 1 and τ 2 are the fitted decay lifetimes; A 1 and A 2 are the weighting parameters. The average lifetime τ ave is calculated by equation (3): The results are shown in Table 1. It suggests two dynamics, a fast decay (τ 1 ) and a longer-lived component (τ 2 ). With the decreasing size of the nanocrystal or the blue shift of the PL peak, the τ ave increases for samples of P-I-2/3/4/5, P-I1Br1-2/3/4/5 and P-Br-2/3/4/5. For sample P-I-2, it has a very short lifetime τ ave of 3.0 ns. The lifetime of the sample P-I-5 is incredibly prolonged to 166.2 ns. This lifetime range covers the ever reported lifetimes of tetragonal MAPbI 3 in the form of nanowire, nanorod, film or bulk ( Table 2). A very short lifetime of P-I-2 indicates that there exist high effective nonradiative recombination channels in the material [50][51][52] . It is proposed to be chemical or structural defects in the MAPbI 3 nanocrystals. The long lifetime of the sample P-I-5 is unexpected, because it has a big specific surface area which may induce vast surface states. Possibly it suggests that the surface of MAPbI 3 nanocrystals is well passivated by the oleate ligands [53][54][55] . It was noticed that from P-I-2 to P-I-5, both τ 1 and τ 2 increase drastically. The fast decay τ 1 of P-I-5 is 25.5 ns, a value much bigger than the slow decay τ 2 of P-I-2 4.8 ns. The mechanisms how the τ 1 and τ 2 are separately or cohesively tuned are presently unclear. However, it is not likely to originate from quantum confinement effect, but more possible related to the surface or defect states and charge carrier delocalization in MAPbI 3 20 . For the samples from P-I1Br1-2 to P-I1Br1-5, the lifetime τ ave boosts from 1.1 to 42.1 ns. While for the samples from P-Br-2 to P-Br-5, the lifetime τ ave only increases from 13.0 to 17.1 ns, with the small PL peak shift from 532 to 525 nm. Compared with MAPbBr 3 single crystal or film (Table 2), these as prepared MAPbBr 3 nanocrystals exhibit shorter lifetime, suggesting more nonradiative recombination effect involved 13,56,57 . Unfortunately, there was no lifetime data of MAPbBr x I 3−x (0 ≤ x ≤ 3) quantum dots synthesized by using ammonium cation with a long alkyl chain as surface capping ligands [35][36][37][38][39] . Further investigation on how surface ligands type would affect the fluorescence lifetime may give us a deeper understanding of the exciton recombination dynamics, and thus better control of the PL for future use in construction of devices.

Conclusions
Organometal halide perovskite MAPbBr x I 3−x (0 ≤ x ≤ 3) colloidal nanocrystals were synthesized by mixing Pb(OA) 2 cyclohexane solution and MAX(X = I or/and Br) isopropanol solution. The size of the perovskites was  successfully controlled by the oleate as the ligands, and also by the Pb:MA ratio. The UV-vis absorption and PL spectra show a blue shift as the size decreasing monotonically, which is ascribed for quantum confinement effect. Significantly, these colloidal organometal halide perovskite nanocrystals were dispersed well in nonpolar organic solvent, e.g. toluene or cyclohexane. This would be useful for fabricating perovskite thin films in various substrates such as silicon, polymer or glass. In addition to the method could be generally applied to synthesize other organometal halide perovskite materials, this study would bring chances to the design and fabrication of new photovoltaic and electronic devices.

Materials and Methods
Materials. Lead acetate trihydrate (AR), sodium oleate (> 99.5%) and cyclohexane (> 99.5%) were bought from Sinopharm. Isopropanol (anhydrous, 99.5%) was purchased from J&K. Hydroiodic acid (HI, 55-58%), hydrobromic acid (HBr, 47.0%) were purchased from Sigma-Aldrich. Methylamine (40% in methanol) was bought from TCI. All chemicals were used as received unless specified otherwise.   MAPbI 3 nanocrystals can be isolated by centrifugation (3 min at 14000 rpm) and redispersed in solvent of cyclohexane or toluene. The MAPbBr x I 3−x (0 ≤ x ≤ 3) nanocrystals was synthesized similarly, by introducing MABr to the reaction. The samples are numbered in the form of P-I-y, P-IaBrb-y, or P-Br-y, where P = perovskite; a and b are the I:Br ratio of a:b such as I1Br1 or I2Br3; y = 2, 3, 4 or 5 means the MA:Pb ratio of y:1 (Table S1). "P-I-2/3/4/5" means four samples of different y values: P-I-2, P-I-3, P-I-4 and P-I-5. To eliminate the effects from the solvent, the volume ratio of cyclohexane to isopropanol is controlled as a constant of 1:1 for all the reactions in the context. The Pb(OA) 2 is kept as a constant. The concentration of MA in isopropanol is investigated as a variable.

Synthesis of Pb
Instruments. The powder X-ray Diffraction (XRD) spectra were performed on a Bruker D8 Advance instrument with a Cu Kα radiation (λ = 1.5418 Å). The scanning electron microscopy (SEM) images were obtained on the S-4800 SEM at 20 kV. The transmission electron microscopy (TEM) images were measured on a HT7700 (Hitachi) TEM at an acceleration voltage of 100 kV. The FTIR spectra were recorded on a Vector 22 spectrometer with a resolution of 2 cm −1 by using KBr pellets. The absorption spectra were obtained by a Shimadzu UV-2700 UV/Vis spectrophotometer. The PL spectra were collected with a Hitachi F-4600 fluorescence spectrophotometer. PL lifetimes were measured with a Zolix Omini-λ 300 fluorescence spectrophotometer and a picosecond pulsed diode laser (Edinburgh Instruments Ltd.) operating at 379.2 nm.