雜化和無機鹵化物鈣鈦礦的光學研究

Abstract

NoneThe growing interest in hybrid and inorganic halide perovskites is driven by significant advancements in their properties, which show great promise for optoelectronic applications. A thorough understanding of their optical and electronic excitations, as well as phase transitions across a wide range of temperatures and energies, is essential for realizing the full potential of hybrid and inorganic halide perovskites-based optoelectronic devices. This thesis examines the vibrational and electronic properties of select hybrid and inorganic halide perovskite compounds: 3D-CH3NH3PbBr3, 2D-(C10H16N)2CuBr4, and 2D-Cs4MnBi2Cl12 single crystals, employing Raman spectroscopy and spectroscopic ellipsometry supported by theoretical calculations. In addition, phase stability and transitions are analyzed using X-ray diffraction (XRD) and thermal gravimetric analysis (TGA).Room temperature XRD analysis confirms that the CH3NH3PbBr3 exhibits cubic symmetry, with no secondary phases above the background level. Temperature-dependent optical spectra indicate semiconductor behavior, with thermo-optic coefficients (dn/dT) of -4.18 × 10^(-4) (600 nm) and -3.29 × 10^(-4) K^(-1) (1200 nm). Thermal hysteresis observed during cooling-heating cycles, as reflected in the extinction coefficient, suggests a first-order phase transition. Furthermore, optical absorption spectrum at room temperature reveals the band gap of 2.35 eV and the exciton binding energy of 37.2 meV. Additionally, the band gap is found to decrease with decreasing temperature.Single-crystal XRD (C10H16N)2CuBr4 reveals a single-phase orthorhombic structure, with no structural transition observed between 100 and 300 K. The Raman spectrum identifies 10 phonon modes, primarily associated with the in-plane and out-of-plane vibrations of the C10H16N and CuBr4 octahedra layers. Room temperature optical absorption spectrum indicates a band gap of 2.47 eV and the binding energy of exciton of 107 meV. As the temperature decreases, both the band gap and the exciton peak position shift to higher energies, while the exciton binding energy remains temperature-independent.Single-crystal XRD analysis reveals that Cs4MnBi2Cl12 crystallizes in a trigonal structure with R3 ̅ symmetry within the temperature range of 130 – 300 K. The Raman spectrum identifies five phonon modes, primarily associated with Mn – Cl and Bi – Cl vibrations. The room temperature optical absorption spectrum exhibits the band gap of 3.28 eV. Additionally, the band gap is observed to decrease with increasing temperature.The reduction in the band gap with increasing temperature observed in the (C10H16N)2CuBr4 and Cs4MnBi2Cl12 is primarily due to electron-phonon coupling and lattice expansion as well as the weakening of interatomic bonding. This results in a reduction in the energy needed to excite electrons into the conduction band, a behavior commonly observed in semiconductor materials. In contrast, the CH3NH3PbBr3 system exhibits opposite behavior, which can be caused by the reverse ordering of band-edge states. As temperature increases, orbital splitting decreases, causing the valence band maximum (VBM), which is dominantly influenced by anti-bonding orbitals, to shift downward. This is accompanied by spin-orbit coupling (SOC) in the degenerate states of the conduction band. Furthermore, exciton features are observable in the CH3NH3PbBr3 and (C10H16N)2CuBr4 systems, but are less prominent in the Cs4MnBi2Cl12 system. Notably, the exciton binding energy in (C10H16N)2CuBr4 is approximately three times higher than that in CH3NH3PbBr3, which is related to the dimensionality. The reduction in dimensionality enhances exciton binding energy, which is influenced by screening effects that modify the electron-hole Coulomb interaction.We have presented a detailed analysis of the optical response, electronic excitations, and phase stability and transitions of these materials. These findings provide valuable insights into the vibrational and electronic properties of hybrid and inorganic halide perovskites, which are crucial for the development and fabrication of hybrid and inorganic halide perovskites-based devices for optoelectronic applications across a range of temperatures.