Low-Dimensional Excitons in Aromatic Nanoparticles

Abstract

Frenkel excitons confined in nanoparticles reveal novel characters in the density of states, the exciton bandwidth [1–3], the k-selection rule [3] and the exciton-phonon interaction [4,5]. All those characters depend very much on the crystallite size. Most of the crystallite specimens that have been applied to such studies are aromatic materials, such as anthracene, pyrene and perylene which were dispersed in PMMA (polymethylmethacrylate). The specimen includes a variety of crystallites with sizes with a Gaussian distribution [6,7]. The average crystallite size that we fabricated was 65 Å in the case of anthracene. The absorption band appears as a wide band. Smaller crystallites contribute to the high-energy side of the absorption band and larger ones to the low-energy side. When the specimen is shone, crystallites with various sizes are excited simultaneously. Nevertheless, if one monitors the luminescence at a certain photon energy, one obtains the excitation spectrum of microcrystallites with a specific size. From this excitation spectrum, one gets the exciton bandwidth [2]. The exciton bandwidth is an important parameter for discussing excitonic processes in organic molecular solids. The excitation spectrum has a sharp, intense peak at the lowest excitation energy but at a higher energy it is weak and broad [2,3]. In Sect. 13.3, a surplus exciton bandwidth which was observed in a specific size range of anthracene microcrystallites is interpreted with the use of a one-dimensional model [8], assuming the presence of electric charge located on the surface of the crystallites. In Sect. 13.4 an abrupt change in the exciton bandwidth at a critical size is interpreted in terms of the phonon scattering capability of the exciton.