Quantum confinement and carbon nanodots

Context. The nature of the diffuse interstellar band (DIB) carriers is perhaps the most studied and longest-standing unresolved problem in astronomy. While four bands have been associated with the fullerene cation (C60+), the vast majority (>550) remain unidentified. Aims. This work is an attempt to provide a conceptual framework for the typical energy transitions that are central to explaining the origin of DIBs; however, it does not make an association between these transitions and any particular DIBs. Methods. The effect of quantum confinement on excitons, including charge transfer excitons, was used to construct a generic basis for the electronic transitions that could, in principle, be coherent with the energies associated with DIBs. In this model the carriers are carbon nanodots (CNDs) modelled as nanodiamonds and a-C(:H) nanoparticles. Results. These preliminary results seem to show that particle size dependent effects in nanodiamond and a-C(:H) CNDs could be consistent with the positions of, and intervals between, some of the DIBs. One particular strength of the model is that it predicts single bands from the majority of single-size particles, and at most two bands from some of these same carriers. In the latter case, the two bands come from different transitions and may or may not correlate, depending upon the local environment. Conclusions. This generic framework indicates that the size-dependent fundamental transitions in CNDs could provide a viable scenario for the origin of some DIB-type bands. While this work does not identify a single DIB, it furnishes a conceptual view of the DIB origin, and suggests that a more refined exploration of quantum confinement size effects and exciton physics within the astronomical domain might prove fruitful. This work also hints at the requirement for stable configurations for particular size domains in order to explain DIB wavelength stability.