# Synthesis, Structure and Photoluminescence Properties of 2D Organic-Inorganic Hybrid Perovskites
## 3 Results and Discussion
### The Laser Power Effect on the PL Intensity  
Figure 7a,c,e shows the bright field and dark field optical microscope images of the perovskites (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\), (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbI\({}_{4}\) and (C\({}_{4}\)H\({}_{9}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\), respectively. All of the perovskites present transparent crystals with regular geometric shapes. (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) and (C\({}_{4}\)H\({}_{9}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) emit blue photoluminescence under the femtosecond laser, while (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbI\({}_{4}\) emit green photoluminescence. Figure 7b,d,f shows the PL spectrum of the perovskites (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\), (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbI\({}_{4}\) and (C\({}_{4}\)H\({}_{9}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\). PL peaks are located at 415 nm (2.98 eV), 540 nm (2.30 eV) and 427 nm (2.81 eV), respectively. When the laser power increases from 10 mW to 100 mW, the PL intensity of the perovskite (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) increases from 117 to 2195 photon counts; the PL intensity of the perovskite (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbI\({}_{4}\) increases from 158 to 2771 photon counts; and the PL intensity of the perovskite (C\({}_{4}\)H\({}_{9}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) increases from 2320 to 13,224 photon counts. Figure 7: **(a)** The optical microscope images and **(b)** the two-photon PL spectrum of the perovskite (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\); **(c)** the optical microscope images and **(d)** the two-photon PL spectrum of the perovskite (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbI\({}_{4}\); **(e)** The optical microscope images and **(f)** the two-photon PL spectrum of the perovskite (C\({}_{4}\)H\({}_{9}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\). It can be seen that the photoluminescence intensity increases with the increase of laser power. Because the interaction between photons and perovskite molecules is strengthened with the increase of light intensity, a large number of electrons and holes will be generated inside the perovskite molecules. The recombination of electrons and holes will release the excess energy in the form of photoluminescence, so the photoluminescence intensity increases with the increase of laser power [38]. It was found that (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) and (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbI\({}_{4}\) had a significant difference in the strongest PL peak, and a longer PL wavelength with a higher halogen atomic number. The band gap of PbBr\({}_{2}\) is larger than that of PbI\({}_{2}\) in the inorganic part, resulting in the increase of exciton binding energy of (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\). Therefore, the PL spectrum of (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) and (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbI\({}_{4}\) show a redshift. We can see that the PL spectrum of (C\({}_{6}\)H\({}_{5}\)CH\({}_{2}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) and (C\({}_{4}\)H\({}_{9}\)NH\({}_{3}\))\({}_{2}\)PbBr\({}_{4}\) have two peaks. The lower peak is the photonic exciton generated by the inorganic layer. Binding exciton produces binding energy and emits photons with Coulomb forces after electron hole pair transition of excitons in perovskite. Therefore, the low peak is thought to be generated by the radiation excitation of excitons. At the higher peak, it can be found that compared with the front exciton emission peak, the peak width of the peak is very wide, and the energy of the luminescent peak is smaller than that of the front exciton emission peak.