편광 분해 분광법을 통한 이차원 PTCDA 결정에서의 층내 및 층간 엑시톤 상호작용 연구

Abstract

Molecular crystals are appealing alternatives to two-dimensional (2D) materials including graphene and transition metal dichalcogenides (TMDCs) due to their high absorption coefficients, ease of growth on large scales, and flexibility. Because of their high surface-to-volume ratio, 2D materials have been observed to be vulnerable to their environment. For examples, Raman active modes of graphene and MoS2, one of the TMDCs, have distinct forms or wavenumbers at various thickness due to interlayer couplings. Furthermore, the changes of temperature and humidity promote chemical doping of 2D materials on silicon oxides via a redox reaction between water and oxygen at 2D interface. Similarly, 2D molecular solids are expected to have analogous susceptibility to their surroundings and their environments such as crystalline structure, dielectric conditions can be easily manipulated by controlling the interface interaction by controlling the type of substrates and the number of thickness. Despite their importance in fundamental physics and applicable potential in electrical devices, the behavior of molecular excitons has not been captured in single-atom limits. In this dissertation, we exhibit in-plane and out-of-plane interaction in atomically thin layers of perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) solids assembled on hexagonal BN crystals. 2D PTCDA assembly grown on hBN crystals by physical vapor deposition (PVD) method exhibited similar crystallographic orientations to PTCDA bulk crystals with layered structure via atomic force microscope (AFM) and electron beam diffraction patterns. And, polarization-resolved spectroscopies revealed their molecular orientations of two herringbone bases, resulting in the Davydov splitting of excitonic transition energy dependent on excitation polarization. Following Kasha’s description, 0-0 emission was energetically split in monolayer PTCDA and exhibited a remarkable energy inversion with decreasing surrounding temperature, favoring in-plane delocalization of the Frenkel (FE) excitonic pair. As the increase of the layer number, charge transfer excitons were formed in the out-of-plane direction along the nearby manner of π-π stack axis. Notably, the in-plane reorientation of the transition dipoles of CT excitons occurred with increasing thickness. This phenomenon was successfully addressed by a FE-CT mixing model, which is described as a linear combination of FE and CT states. In the following work, few-layered PTCDA solids were grown on few-layered graphene and the screening of excitonic Coulomb potentials between dipole-dipole (Davydov splitting energy) and electron-hole (binding energy) in 2D PTCDA by semi-metallic graphene were observed by polarization-resolved reflectance spectroscopy. Because of ultrafast nonradiative relaxation of PTCDA exciton by enormous charge quenching of the graphene. It also increased the linewidths and decay rates of Davydov pair compared to PTCDA assemblies on hBN. This works will contribute to a better understanding of molecular excitonic behaviors bound in low-dimensional organic materials, allowing for the construction of molecular structures with novel functionalities.