| Author: | Gao, Wei |
| Title: | Upconversion lasing from metal halide perovskites with different dimensionalities |
| Advisors: | Yu, Siu Fung (AP) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Semiconductor lasers Quantum dots Perovskite materials Metal halides Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xvii, 143 pages : color illustrations |
| Language: | English |
| Abstract: | Metal halide perovskites (MHPs) as a large family of crystalline direct semiconductors possess excellent optoelectronic properties, such as tunable energy bandgap, high carrier mobility, low trap state density, high absorption coefficient, and low-cost fabrication. Since they were applicated in solar cells in 2009, they have attracted intensive attention due to their diverse optoelectrical properties with a variety of structures. They were first used as laser gain medium with a facile fabrication process in 1998, suggesting that they are potential candidates for developing flexible and low-cost laser to replace the traditional III-V semiconductors. In past years, they have been well studied in the field of solar cells, light-emitting diodes, photodetectors and lasers, and they are considered as a new generation of outstanding materials for optoelectronics. The optical gain medium is one of the most important parts of a laser system, which is used for amplifying the stimulated emission to generate a strong coherent light. However, the traditional gain medium of a laser diode is limited by its rigid architecture and complicated fabrication process with high cost. To address this problem, an organic dye is appearing as new laser gain media with flexibility. But the liquid gain media is not able to integrate them with other external cavities. Compared with organic dye and traditional III-V semiconductors, MHPs with low-cost fabrication methods have balanced charge carrier mobility, tunable emission wavelengths coving the whole visible region, higher carrier recombination rate, and so forth. Herein, MHPs with different dimensionalities as laser gain media were systematically investigated. The solution-processed fabrication of MHPs distinguishes them from traditional III-V semiconductors, which enable MHPs to be easily coated or grow on a flexible substrate to realize flexible lasers. Our study started by using zero-dimensional MHPs quantum dots (QDs) to explore the performance of flexible MHPs lasers. The QDs with an average size of 8 nm were synthesized by using the modified methods published previously in 2015, and their photoluminescence quantum yield is as high as 97%. Then they were coated on flexible Ni foam by spin-coating as a thin film with a thickness of 200 nm. In the previous studies, flexible substrates were organic polymers or plastics. The advantage of using flexible metallic structures as the substrate of flexible lasers over plastic materials is their strong mechanical strength and high thermal conductivity. Another advantage is Ni foam has mesh structures, which can provide random feedback among these voids. Under 800 nm excitation at room temperature, we observed incoherent random lasing emission at ~537 nm. This incoherent random lasing can be tuned to amplified spontaneous emission by changing the voids size of the Ni porous foam. Their corresponding lasing threshold can also be controlled by the external deformation of Ni foam. More importantly, this flexible laser is stable enough that the laser is robust to intensive bending (>1000 bending cycles) with minimum effect on the lasing intensity. Due to the stability and low spatial coherent characteristics of the flexible random lasers, it is shown to be an ideal light source to produce high-quality micro images. Hence, these results indicate that MHP QDs are a potential laser gain media to develop high-performance flexible lasers Compared to QDs, the two-dimensional Ruddlesden−Popper perovskites (RPPs) have higher stability, higher binding energy, and stronger exciton confinement due to the quantum well (QW)-like structures. Lasing emission has been demonstrated from RPP films which can be fabricated by the spin-coating technique or solution-crystallization method. These quasi-2D perovskites compose of mixed dimensionalities containing various n values, and they are difficult to control the uniformity of n values, leading to the shift in absorption and emission energy. Lasing emissions from homologous RPP bulk or the mechanically exfoliated flakes are rarely reported because the single crystal of RPPs with pure phase is hard to obtain. We used the modified method to grow the RPPs with high quality of single n value. These were confirmed by the XRD, absorption spectra, and PL emission spectra. The PL spectra excited by 800 nm pulse laser showed their nonlinear property of two-photon absorption. Then their nonlinear properties were further studied by using the micro Z-scan technique. A giant two-photon absorption (TPA) coefficient was calculated by fitting the experimental data. Their TPA coefficient is as high as 3.6 × 103 cm GW−1, which is at least 3 orders of magnitude higher than their 3D perovskites counterparts. Then the upconversion lasing from RPPs with FP cavities by two-photon excitation was achieved at low temperature. Calculation results and microscopic transient absorption measurements reveal the reasons for lasing from the RPP flakes, which are 1) high-efficiency upconversion transitions under TPA, 2) high differential gain to overcome the biexciton Auger recombination losses inside the laser microcavities, and 3) small electron-phonon coupling strength reduces non-radiative recombination. Although the MHPs nanoparticles have been widely used as laser gain medium, the unstable crystal structures in moisture environment limited their further applications, especially for the red emission lasers. The red emission can only be achieved from the metal iodide perovskites, which are unstable even at room temperature due to the lattice structure decomposing or transferred from cubic to orthorhombic phases. There are some methods to solve this issue, 1) doping Br− to improve the stability of the lattice structure, 2) protecting nanocrystals by external matrix. Herein, we use the glass matrix to separate MHPs from the moisture environment and oxygen, which is expected to improve their stability. Then the stability of MHPs nanocrystals with different ratios of I− and Br− were investigated to confirm the optimized ratio (1:1). Before using them as laser gain media, their optical gain was investigated by variable-stripe-length measurements. Then the red emission lasing is realized from our samples at low temperature, and their stability in water is significantly enhanced. These findings distinguish RPPs as promising gain media for developing efficient upconversion lasers. |
| Rights: | All rights reserved |
| Access: | open access |
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