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Naqvi Furqan Ul Hassan,Naqvi Furqan Ul Hassan,Kim Tae Heon,Ahn Chang Won,Hwang Younghun 한국물리학회 2022 Current Applied Physics Vol.44 No.-
Structural and vibrational characteristics of methylammonium(MA) halide perovskite MAPbBr3-xClx single crystals (x = 0–3) were investigated by using powder X-ray diffraction and Raman scattering experiments. The lattice constant in the cubic phase obtained from powder X-ray diffraction peaks at room temperature showed nearly a linear dependence as a function of composition. On the other hand, some of the Raman mode frequencies obtained from single crystals exhibited substantial changes in both frequency shifts and half widths over the investigated composition range. Especially, the MA torsional mode showed a significant change from 325 to 485 cm 1 and mode splitting as Cl was replaced by Br. This mode splitting was more clearly seen at low temperatures reflecting the symmetry lowering of the local structure. The contrast between the linear change in the lattice constant and the substantial change in the vibrational frequencies and half widths of the MA torsional mode in the intermediate composition range indicates the local heterogeneous environment for the MA cations caused by the substitutional disorder.
Naqvi Furqan Ul Hassan,Ko Jae-Hyeon,Lim Ki-Soo 한국물리학회 2020 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.76 No.10
The vibrational properties of tetragonal tungsten-bronze lead barium niobate, Pb(1-x)}BaxNb2O6, with x = 0.5 were investigated by using Raman spectroscopy over a wide temperature range from room temperature to 550 °C. The Raman modes in the measured spectrum were divided into three spectral ranges, internal symmetric and antisymmetric vibrational modes of the oxygen octahedra and low-frequency external lattice modes. The temperature dependences of the Raman shifts showed anomalous changes at the ferroelectric phase transition temperature of ~350 °C. Especially, low-frequency modes below ~300 cm-1 commonly exhibited multiple softening upon heating toward the transition temperature, indicating that the lattice becomes softer on approaching the ferroelectric phase transition temperature.
Furqan Ul Hassan Naqvi,Jae-Hyeon Ko,Ki-Soo Lim 한국물리학회 2020 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.76 No.4
The elastic constants of tetragonal tungsten-bronze lead barium niobate, Pb(1-x)BaxNb2O6 with x = 0.5, were determined under poled and unpoled conditions by using Brillouin spectroscopy. The elastic constant C11 was larger than C33 similar to other ferroelectric tungsten-bronze systems. The C11 did not show any appreciable change between the poled and unpoled conditions while the C33 increased from 117 to 173 GPa upon poling the sample. This substantial change was attributed to the higher density of domain walls and the polar nanoregions in the unpoled condition and the resulting strong coupling between the longitudinal acoustic waves propagating along the polar c-axis and the one-dimensional order parameter aligned along the same direction. This elastic anisotropy associated with the dimensionality affected the acoustic damping and the central peak behaviors.
Naqvi Furqan Ul Hassan,고재현,Kim Tae Heon,Ahn Chang Won,Hwang Younghun,Sheraz Mahshab,Kim Seungdo 한국물리학회 2022 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.81 No.3
Lead bromide perovskites have proven to exhibit high open-circuit voltage, which is a signifcant factor in enhancing the overall performance of a solar cell. Single crystals of FAPbBr3, MAPbBr3 and CsPbBr3 are synthesized by solvent evaporation method. This study investigates the thermal stability, photo physical properties, and optical phonon modes of lead bromide perovskites using various spectroscopic techniques. The thermal stability measurements prove CsPbBr3 to be more stable than two hybrid perovskites. Raman spectroscopic studies for all three perovskites are performed in a wide frequency range probing both low and high-frequency modes. Fitting analysis is done to know the exact comparison of the Raman shifts. The torsional mode shows large diference in frequency depending upon the cation. Correlation times are estimated to get more insights into the dynamics of cations and inorganic octahedral frameworks. The diference in vibrational modes upon substitution of cation, resulting in complex chemical interplay is investigated in detail