These studies shed light on the dependence of PAH aggregate fluorescence on their monomers' optical properties and their intermolecular configuration. However, the above-mentioned studies have primarily focused on aggregates of very simple PAH molecules and homogeneous aggregates. In contrast, results from the molecular imaging of asphaltene molecules with scanning tunneling microscopy (STM) and atomic force microscopy (AFM) revealed a formidable structural diversity of asphaltene monomers. Although generally consistent with the "island" structure proposed by the modified-Yen model, the molecular weight, shape, and elemental composition of asphaltene monomers vary greatly. The heterogeneous aggregates composed of these complex molecules will be the majority of asphaltene nanoaggregates so conclusions derived from calculations on a few PAH dimers may lack generalizability. It is still unclear whether the fluorescence of asphaltene aggregates adheres to some universal rules or can only be discussed case by case.