Topological insulators (TIs) have a protected surface state that exhibits interesting plasmonic properties. [P D Pietro et al, Nat Nano. 2013] In particular, interfaces between TIs and vacuum show coexistant 2D electron gas (2DEG) and topological surface states (TSS) that are expected to interact and produce excitations in the THz and even optical regime that may be useful for integrating plasmonics and spintronics. [M Bianchi et al, Nat Comm. 2010.] To achieve this, however, the surface states must be engineered and controlled in order to modify scattering, chemical potential and Rashba coupling. Typically, this has been done through the use of adatoms, especially Rb and In, but these adatoms cannot be modified once implanted, leading to system which has been conditioned, rather than controlled. [M Bianchi et al, ACS Nano, 2012] Here, we present the results of doping a TI surface with C60 molecules. Organic molecules and nanocarbon have both been observed to alter the surface properties of TIs, but plasmonic excitations in these systems have not yet been studied. [T Kitazawa et al, Adv. Mater. Int. 2020] In addition, though single mono-layers have been studied in detail via ARPES, the effect of molecular films have not been observed. We determine that C60 introduces a strongly confining 2D potential at the TI interface which can tune the energy of the surface pi-plasmon. We can directly measure, via EELS, the localisation of different plasmonic excitations in the film and quantify their dispersion. DFT simulations reveal that this interface state is expected to exhibit strong Rashba coupling as a result of interfacial strain. Furthermore, we have observed that highly crystalline fullerite layers naturally form on TI surface during evaporation and propose that these layers could be used to actively control the topological character of the TI surface.