Study on MA(Pb,Cu)Br₃ Provskite Nanocrystal with Both Controlled Color Emission and Improved Stability

the toxicity of perovskite quantum dots, and the Cu²⁺ is selected as alternative metal cations to develop a new luminescent material in this paper.^[14,15]

With the doping concentration of Cu²⁺ increases significantly, the tolerance factor value increases in the octahedral structure of quantum dots, indicating that this organic-inorganic hybrid perovskite possesses stabilized internal structures. Furthermore, the radiation recombination of electrons in the conduction band and the holes have overlapped with T₂ level of Cu²⁺,^[16,17] and thus the color of Cu²⁺ doped quantum dots can be controlled with changing the proportion of Cu^{2 +} and Pb^{2 +}. However, it is difficult for the divalent transition metal cation to be doped in the perovskite quantum dots. The luminous intensity is sharply reduced due to the lattice distortion of [PbX₆]^4- octahedral with increasing the doping concentration of metal ions.^[18,19] Therefore, it is particularly important to study the synthesis process of MA(Pb,Cu)Br₃ quantum dots.

To address the above mentioned challenges, the MA(Pb,Cu)Br₃ quantum dots have been synthetized via the auxiliary ligand precipitation in this paper. In order to improve the stability and water solubility of quantum dots, the SiO₂ coating method was adopted to change the poor water solubility of MA(Pb,Cu)Br₃.^[20,21] Finally, the phase evolution, controlled-color and fluorescent properties of MA(Pb,Cu)Br₃ and MA(Pb,Cu)Br₃@SiO₂ quantum dots were systematically studied by various instruments including X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL), UV-vis and fluorescence decay analysis.

2. Experiment procedure

2.1 Materials

In a study, hydrobromic acid (HBr, 40%, McLean), methylamol solution (CH₃NH₂, 99.7%, traditional Pharmaceutical Group Chemical Reagent Co., Ltd.), anhydrous methanol (CH₃OH, 99.7%, McLean), lead bromide (PbBr₂, 99.999%, McLean), copper bromide (CuBr₂, 99.9%, McLean), N, N-dimethyl formamide (DMF, 99%, Tianjin Fuyu Fine Chemical Co., Ltd.), oleic acid (C₁₈H₃₄0₂, 90%, McLean), octylamine (C₈H₁₉N, 99%, McLean), tetramethoxysilane (C₄H₁₂O₄Si, 95%, McLean), toluene (C₇H₈, 99.5%, Yantai far East Fine Chemical Co., Ltd.), and acetone (CH₃COCH₃, AR, McLean), etc were used without any further purification.

2.2 Preparation procedure

2.2.1 Synthesis of MABr precursor

Methylamine alcohol solution (17.4 mL) was added to methanol (30 mL) for dilution in 100 mL three-mouthed flask. The mixture was cooled to 0~5 ^oC while the hydrobromic acid (13.6 mL) was mixed at the rate of 0.5 mL/min. The mixture solution continually reacted for 2 h in the same environment. After the reaction was over, the target products were removed from the mixture solutions by rotary evaporation. Lastly, the depositions were washed repeatedly three times using methanol and acetone, and dried at 80 ^oC for 24 h to obtain white products.

2.2.2 Synthesis of MA(Pb,Cu)Br₃ quantum dots

The methylamine salt, PbBr₂ and CuBr₂ were mixed as the mother salt according to the chemical formula of MA (Pb_1-xCu_x)Br₃, and dissolved in DMF with a total volume of 5 mL. Then, 20 μL octylamine and 0.5 mL oleic acid were added and stirred fully to form the precursor solution. After that, 1 mL of the precursor solution was rapidly injected into 10 mL of toluene solution and vigorously stirred to obtain quantum dots solution. Finally, the colloidal solution was centrifuged at the speed of 8000 rpm/s for 5 min, and washed repeatedly with toluene to remove the by-products, and finally the solution of MA(Pb, Cu)Br₃ quantum dots was obtained.

2.2.3 Synthesis of MA(Pb,Cu)Br₃@SiO₂ quantum dots

Tetramethyl orthosilicate (TMOS, 120 μL) was added into 25 mL bottle which contained 15 mL of the MA(Pb, Cu)Br₃ perovskite quantum dots toluene solution. The mixture solution was stirred for 5 min at room temperature. After that, the bottle was put on a light-shielded environment at room temperature, and stirred for 48 h. After the reaction was over, the target products were obtained from the mixture solutions by centrifugation. Lastly, the depositions were washed thrice with acetone, and dried at 80 ^oC for 24 h to obtain the yellow products.

2.3. Characterization

The phase composition analyses were performed by XRD. And the patterns were recorded at room temperature using nickel-filtered Cu K_α radiation in the 2θ range 10-50° at a scanning speed of 4.0° 2θ/min (Model D8 ADVANCE, BRUKER Co., Germany). The morphology of resultant products was collected via the HR-transmission electron microscope (HR-TEM, JEM-2100F, JEOL, Japan). The UV-vis absorption spectra were performed by UV-vis spectrophotometer (UV-3600, Shimadzu Corporation). The photoluminescence (PL) spectra were obtained using a Fluorescence Spectrophotometer (FP-6500, JASCO Co., Japan) at room temperature equipped with a Φ60-mm integrating sphere (ISF-513, JASCO, Tokyo, Japan) and a picosecond laser/diode was used as the excitation source. The optical performances for all samples were conducted under identical conditions with the slit breadth of 5 nm. The sample was excited with a selected wavelength and the intensity of the intended emission was recorded as a function of elapsed time after the excitation light was automatically cut-off.

3. Results and discussion

Fig. 1(a-f) shows the TEM images of MA(Pb, Cu)Br₃ perovskite quantum dots with different concentrations of Cu²⁺. It can be seen that the MA(Pb, Cu)Br₃ perovskite quantum dots possess nanosphere morphologies with a good dispersion compared with the MAPbBr₃ perovskite quantum dots from Fig. 1(g), and the average size is ~6 nm. As shown in the inset of Fig. 1(a), the HR-TEM micrograph of nanospheres MA(Pb, Cu)Br₃ perovskite quantum dots indicates that the interplanar spacing of MA(Pb, Cu)Br₃ perovskite quantum dots is ~0.253 nm, which corresponds to the (210) lattice plane.

The XRD patterns of MA(Pb, Cu)Br₃ quantum dots with different doping concentrations of Cu²⁺ are shown in Fig. 1(h). The results show the similar diffraction peaks of (100), (110), (200), (210) and (300) to the XRD diffraction behavior of MAPbBr₃ quantum dots in publish reports^[22-24] indicated that the doping of Cu²⁺ has no effect on the crystal phase structure of the quantum dots. Furthermore, the strong diffraction peaks of (100) and (200) drift towards high angles with gradually increasing the doping concentrations of Cu²⁺. The lattice shrinkage of quantum dots increases with increasing the concentrations of Cu²⁺, owing to the smaller radius of Cu²⁺ (0.073 Å) than that of Pb²⁺ (0.119 Å).

Fig. 1 (a-f) the TEM morphologies of MAPbBr₃: xcu²⁺ (x=0.1-0.6) quantum dots, and Fig. 1(g)  TEM morphologies of MAPbBr₃ quantum dots. The inset of Fig. 1(a) shows the HR-TEM micrograph of MA(Pb,Cu)Br₃ perovskite quantum dots; and Fig. 1(h) the XRD patterns of MA(Pb,Cu)Br₃ perovskite quantum dots with different concentrations of Cu²⁺ (x=0-0.6).

The PL spectra and UV-vis spectra of MA(Pb, Cu)Br₃ quantum dots with different concentrations of Cu²⁺ are depicted in Fig. 2(a) and 2(b), respectively. As marked in the figure, the MA(Pb, Cu)Br₃ perovskite quantum dots exhibit green emission (~525 nm) monitoring at 400 nm excitation. The fluorescence intensity decreases with changing the doping concentration of Cu²⁺ from 0.1 to 0.6. Moreover, the luminescence peak exhibited a slight blue shift from 528 to 515 nm with increasing the concentration of Cu²⁺. The lattice shrinkage of MA(Pb, Cu)Br₃ perovskite quantum dots can be ascribed to the smaller radius of Cu²⁺ than that of Pb²⁺. Furthermore, the lattice shrinkage of MA(Pb, Cu)Br₃ perovskite quantum dots is attributed to the shortened Pb-halide bond and a stronger ligand field within the [PbX₆]^4- octahedral. Meanwhile, this phenomenon of blue shift can also be seen in Fig. 2(b), and is similar to the result of PL spectra. In addition, the wide absorption peaks at 650-800 nm are vest in the absorption of Cu²⁺ ions, which may be from the Cu²⁺ ions in a quantum dot solution that is not completely purified. ^[25]

Fig. 2. (a) The PL spectra of MA(Pb,Cu)Br₃ quantum dots with different concentrations of Cu²⁺ monitoring at 400 nm, and (b) the UV-vis spectra of MA(Pb,Cu)Br₃ quantum dots with different concentrations of Cu²⁺ (x=0.1-0.6).

In order to further study its fluorescence properties, the Commission International de L'Eclairage (CIE) chromaticity coordinates for the emission of the MA(Pb,Cu)Br₃ perovskite quantum dots under 400 nm excitation are shown in Fig. 3(a). The MA(Pb_1-xCu_x)Br₃ perovskite quantum dots are analyzed to have color coordinates (x,y) of (~0.15, ~ 0.75), (~0.15, ~0.74), (~0.13, ~0.73), (~0.14, ~0.69), (~0.16, ~0.65), and (~0.14, ~0.61) with the temperature of ~8117 K, ~8171 K, ~8579 K, ~8673 K, ~8573 K and ~9397 K for x=0.1, 0.2, 0.3, 0.4, 0.5 and 0.6, respectively, which prove the results of emission peaks in Fig. 3(a). Fig. 3(b) shows the fluorescence decay curve of MA(Pb,Cu)Br₃ perovskite quantum dots. It was found that the decay data can be well fitted with the following double exponential equation in each case:

         (1)

                                             (2)

where I is the luminescence intensity at a certain moment, t is the time, A₁, A₂ and B are constant, τ₁ and τ₂ show the time with the luminescence intensity rapidly and slowly weaken, respectively. Among them, the A₁, A₂, B, τ₁ and τ₂ are obtained from the double exponential fitting curve, and the effective decay time (τ) is calculated by

Fig. 3. (a) the CIE coordinates of MA(Pb,Cu)Br₃ quantum dots with different concentrations (x=0.1-0.6); (b) the fluorescence decay curves of MA(Pb,Cu)Br₃ quantum dots monitored at emission wavelength of 400 nm with the concentrations of Cu²⁺ increase from 0.1 to 0.6, and the embedded curve shows the decay times curve changing of MA(Pb,Cu)Br₃ quantum dots.

formula (2).^[26-28] From Fig. 3(b), it can be seen that the fluorescence lifetime of MA(Pb, Cu)Br₃ quantum dots is 19.91-2.71 ns when the doping concentration of Cu²⁺ change from 10 to 60 at.%, which is similar to the published results.^[29,30] Fig. 3(b) shows the decay time curve changing of MA(Pb, Cu)Br₃ quantum dots. From which can be seen that the fluorescence lifetime gradually decreases with the increase of the doping concentration of Cu²⁺. The reason can be explained that the surface defects of MAPbBr₃ quantum dots increase with gradually increasing the doping concentration of Cu²⁺. Moreover, the stability of the [PbX₆]^4- octahedral in MA(Pb, Cu)Br₃ perovskite quantum dots becomes worse when the Cu²⁺ gradually replaces Pb²⁺. The above two reasons lead to the decrease of quantum dot lifetime.

The poor stability of perovskite quantum dots seriously limits the application as luminescent materials, when they are dispersed into strong polar solvent (such as water and alcoholic solution).^[31-34] SiO₂ is a more important shell structure in the coating material at present. The material is coated by the principle of hydrolysis of tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS) with water to form SiO₂ shell. Based on this, the MA(Pb,Cu)Br₃@SiO₂ perovskite quantum dots were successfully obtained (depicted in the inset of Fig. 4a) via the slow hydrolysis of TMOS in toluene for 48 h and then dried in vacuum for 24 h. Fig. 4(a) shows the TEM morphologies of MA(Pb_0.8Cu_0.2)Br₃@SiO₂ perovskite quantum dots. The average size of MA(Pb_0.8Cu_0.2)Br₃@SiO₂ perovskite quantum dots phosphor exhibiting the spherical morphologies is ~200 nm, and larger than the size of MA(Pb,Cu)Br₃ quantum dots (~6 nm). To further analyze the coating of quantum dots, the HR-TEM morphology analysis of MA(Pb_0.8Cu_0.2)Br₃@SiO₂ quantum dots with nanospheres is shown in Fig. 4(b), from which it can be seen that the interplanar spacing of MA(Pb_0.8Cu_0.2)Br₃@SiO₂ quantum dots is ~0.285 nm (the inset of Fig. 4b) corresponding to the (200) lattice plane, and the MA(Pb,Cu)Br₃ quantum dots are embedded in SiO₂ shell. Therefore, the quantum dots wrapped by SiO₂ shell will improve water-stability.

Fig. 4. (a) TEM images of MA(Pb_0.8cu_0.2) Br₃@SiO₂ perovskite quantum dots, and the inset of Fig. 4(a) is the digital image of the MA(Pb_0.8Cu_0.2)Br₃@SiO₂ perovskite quantum dots phosphor under 365 nm UV excitation from a hand-held UV lamp; the HR-TEM micrograph of MA(Pb_0.8Cu_0.2)Br₃@SiO₂ perovskite quantum dots is shown in Fig. 4(b), and the inset of Fig. 4(b) is the partial enlarged drawing of Fig. 4(b).

In order to further investigate the fluorescence attenuation behavior of the quantum dots in the mixed solution of toluene and water (4:1), the PL behaviors of the MA(Pb_0.8Cu_0.2)Br₃@SiO₂ and MA(Pb_0.8Cu_0.2)Br₃ quantum dots are shown in Fig. 5(a) and 5(b), respectively. It can be seen that the green emissions of MA(Pb_0.8Cu_0.2)Br₃@SiO₂ and MA(Pb_0.8Cu_0.2)Br₃ quantum dots are depicted in PL spectra with the emission bands located at ~525 and ~523 nm, respectively, and the PL intensity decreases over the exposure period. However, the phenomenon has been discovered that the quenching rate of MA(Pb_0.8Cu_0.2) Br₃@SiO₂ quantum dots is slower than that of the MA(Pb_0.8Cu_0.2)Br₃ quantum dots by contrasting MA(Pb_0.8Cu_0.2)Br₃@SiO₂ and MA(Pb_0.8Cu_0.2)Br₃ quantum dots. From Fig. 5(c), the fluorescence quenching is observed in the MA(Pb_0.8Cu_0.2)Br₃ quantum dots after 16 hours, while MA(Pb_0.8Cu_0.2)Br₃@SiO₂ quantum dots maintain 40% of the original fluorescence intensity after 36 hours. This is further proved that the SiO₂ shell coated on the surface of the MA(Pb_0.8Cu_0.2)Br₃ quantum dot can protect the quantum dots better, and then improves the water stability and water solubility of the quantum dots effectively. In addition, the full-width half-maximum of MA(Pb_0.8Cu_0.2)Br₃@SiO₂ quantum dots (~28 nm) increases compared with that of MA(Pb_0.8 Cu_0.2)Br₃ quantum dots (~23 nm) from Fig. 2(a), which is mainly due to the aggregation of quantum dots as the coating proceeds. Fig. 5(d) shows the XRD contrast patterns of MA(Pb_0.8Cu_0.2)Br₃@SiO₂ and MA(Pb_0.8cu_0.2)Br₃ quantum dots. From this, it can be seen that the crystal diffraction of MA(Pb_0.8Cu_0.2)Br₃@SiO₂ quantum dots appears at (100), (110), (200), (210) and (300) comparing to MA(Pb_0.8Cu_0.2)Br₃ quantum dots, which match the crystal phase of MAPbBr₃ quantum dots in Fig. 1(h).^[22-24] The wide peaks at range of 15^o-35^o are attributed to the unformed silica. Therefore, the result certifies the success of SiO₂ coating.

Fig. 5. the PL spectra of the MA(Pb_0.8Cu_0.2)Br₃@SiO₂ (a) and MA(Pb_0.8Cu_0.2)Br₃ quantum dots (b) which are dispersed in the mixed solution of water and toluene, and the proportion of water and toluene is 1:4; (c) is the luminescence intensity comparison diagram of MA(Pb_0.8Cu_0.2)Br₃@SiO₂ and MA(Pb_0.8Cu_0.2)Br₃ quantum dots, and (d) is the XRD comparison diagram of MA(Pb_0.8Cu_0.2)Br₃@SiO₂ and MA(Pb_0.8Cu_0.2)Br₃ quantum dots.

The temperature-dependent PL spectra for MA(Pb, Cu)Br₃ perovskite quantum dots are shown in Fig. 6(a). It can be seen that the emission intensity gradually decreases with the increase of temperature from 60 to 300 K, and the half-maximum of the sample increases gradually. The lattice shrinkage of sample caused by thermal expansion is severe when the temperature is set to 60 K, and the lattices of sample expand with increasing the temperature which led to the increased half-maximum. Furthermore, as the temperature rises from 60 to 100 K, the reduction of the emission intensity is not obvious. As the temperature increases from 100 to 300 K, the fluorescence emission peaks of MA(Pb, Cu)Br₃ perovskite quantum dots show an obvious blue shift from about 560 to 540 nm which is due to the conversion from bound exciton to free exciton, and the emission of bound exciton was inhibited. Then, the blue shift of the emission peaks occurred. These phenomena are consistent with the temperature- dependent of CdSe and PbS quantum dots. ^[35,36]

In order to further discuss the thermal quenching, the results can be explained by Arrhenius equation: ^[37]

              (3)

where E_a, T, A and k refer to activation energy, temperature (K), constant and Boltzmann constant, respectively. The I₀ and I represent the emission intensity at room temperature and operating temperature, respectively. The relationship between ln(I₀/I-1) and 1/kT for the thermal quenching of MA(Pb, Cu)Br3 perovskite quantum dots is

Fig. 6. (a) the PL spectra of MA(Pb,Cu)Br₃ perovskite quantum dots as a function of temperature; (b) the relationship between ln(I₀/I-1) and 1/kT for the thermal quenching of MA(Pb,Cu)Br₃ perovskite quantum dots.