Interpreting the Ultraviolet Absorption in the Spectrum of 415 nm-Bandgap CdSe Magic-Size Clusters

Colloidal semiconductor magic-size clusters (MSCs), a crucial link between molecular and bulk materials, have attracted attention in the past three decades. However, the identification of their nonbandgap electronic transitions via optical absorption has been challenging due to the possible presence of other-bandgap ensembles in synthetic batches. For CdSe MSC-415, referred to as the optical absorption (1S(e)-1S_3/2(h)) in nanometers of wavelength, we report our exploration on the origin of two commonly documented absorption peaks at 381 and 351 nm. We show that the evolution of the two peaks does not synchronize with that of the ∼415 nm peak and seems to be respectively related to the disappearance of MSC-391 and MSC-361. Accordingly, these two peaks detected are probably not due to higher order electronic transitions in MSC-415. The present study shows the necessity of re-evaluating previous experimental results and of developing advanced theoretical models to better understand the quantized energy levels of MSCs.