退火處理致相變化之超薄酞菁鋅超快光譜研究

Abstract

Phthalocyanines (Pcs) are two-dimensional aromatic molecules with an inner ring. Due to their strong absorption in the wide spectral region, excellent thermal and chemical stability, and flexible processing, the thin layers of metallophthalocyanine (MPc) are accepted as one of the most promising materials for optoelectronic devices, photovoltaic devices, and organic field-effect transistors. The optical and chemical properties of MPcs can be modified by central metal atoms and among them, zinc phthalocyanine (ZnPc) is of particular interest for various applications due to its high absorption coefficient in a wide spectral range of solar radiation and high energy conversion efficiency. It has been known that the morphology of ZnPc films can be tuned by adjusting post-annealing temperature. Above certain temperatures, spherical grains at room temperature transform to elongated rod shaped grains at high temperatures above a critical temperature. While Pc molecules grown at room temperature form vertically aligned nanorods, annealing-induced elongated nanorods are aligned parallel to the substrate. In-plane orientation of nanorods will allow the carriers to diffuse along the stacking direction. In this work, we investigated the influence of annealing temperature on the structural properties and optical characteristics of ultrathin (20 and 50 nm) ZnPc films. In the studies of UV–VIS spectroscopy and grazing incident X-ray diffraction (GIXRD), a transition from α-phase to β-phase was observed when a ZnPc thin film was annealed at a temperature greater than 200 °C. The images of field emission scanning electron microscopy (FESEM) for β-ZnPc exhibit the horizontally aligned nanorods with a large length to diameter ratio. In the study of ultrafast pump-probe spectroscopy, the elongated nanorods in the ultrathin films showed distinctively different exciton dynamics for s- and p-polarized probe beams and even more significant difference was observed in the 20-nm-thick film. From the study of thickness-dependent exciton relaxation, we found that in the 20-nm-thick film the three-dimensional packing of molecules is suppressed to resemble nearly two-dimensional structure. The structural transition in the ultrathin ZnPc films was identified as the reorientation (inverting) of molecular columns along a-axis while preserving the packing of the molecules along the c-axis.

Phthalocyanines (Pcs) are two-dimensional aromatic molecules with an inner ring. Due to their strong absorption in the wide spectral region, excellent thermal and chemical stability, and flexible processing, the thin layers of metallophthalocyanine (MPc) are accepted as one of the most promising materials for optoelectronic devices, photovoltaic devices, and organic field-effect transistors. The optical and chemical properties of MPcs can be modified by central metal atoms and among them, zinc phthalocyanine (ZnPc) is of particular interest for various applications due to its high absorption coefficient in a wide spectral range of solar radiation and high energy conversion efficiency. It has been known that the morphology of ZnPc films can be tuned by adjusting post-annealing temperature. Above certain temperatures, spherical grains at room temperature transform to elongated rod shaped grains at high temperatures above a critical temperature. While Pc molecules grown at room temperature form vertically aligned nanorods, annealing-induced elongated nanorods are aligned parallel to the substrate. In-plane orientation of nanorods will allow the carriers to diffuse along the stacking direction. In this work, we investigated the influence of annealing temperature on the structural properties and optical characteristics of ultrathin (20 and 50 nm) ZnPc films. In the studies of UV–VIS spectroscopy and grazing incident X-ray diffraction (GIXRD), a transition from α-phase to β-phase was observed when a ZnPc thin film was annealed at a temperature greater than 200 °C. The images of field emission scanning electron microscopy (FESEM) for β-ZnPc exhibit the horizontally aligned nanorods with a large length to diameter ratio. In the study of ultrafast pump-probe spectroscopy, the elongated nanorods in the ultrathin films showed distinctively different exciton dynamics for s- and p-polarized probe beams and even more significant difference was observed in the 20-nm-thick film. From the study of thickness-dependent exciton relaxation, we found that in the 20-nm-thick film the three-dimensional packing of molecules is suppressed to resemble nearly two-dimensional structure. The structural transition in the ultrathin ZnPc films was identified as the reorientation (inverting) of molecular columns along a-axis while preserving the packing of the molecules along the c-axis.