The identification of non-bandgap electronic transitions of colloidal semiconductor magic-size clusters (MSCs) via optical absorption has been challenging, due to possible presence of other-bandgap ensembles in synthetic batches. Herein, we report one example, CdSe MSC-415, referred to as the optical absorption peak position in nanometers of wavelength. For the CdSe MSC-415 ensembles reported, their syntheses were performed in aqueous or organic solutions, with phosphine/phosphine oxide, phosphinate, carboxylate, or amines as surface ligands. In addition to the bandgap absorption peak at 414 to 422 nm, another two absorption peaks in the range from 382 to 398 nm and from 352 to 365 nm have been often reported. For the two shorter-wavelength peaks, their absorbance is much weaker and they have been generally taken respectively as due to the secondary and third electronic transitions of MSC-415. For the origin of two commonly documented absorption peaks at ~381 and ~351 nm, we used our two-step approach to synthesize the MSCs. We show that the evolution of the two peaks does not synchronize with that of the ~415 nm peak and seems to be related to the disappearance of MSC-391 and MSC-361, respectively. Accordingly, these two peaks detected are probably not due to higher order electronic transitions in MSC-415. The present study suggests that it is necessary to re-evaluate previous experimental results and calls for efforts to develop advanced theoretical models to better comprehend the quantized energy levels of semiconductor MSCs.

Colloidal semiconductor CdSe magic-size clusters with 415 nm bandgap

2019 Spring National Meeting

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