Extinction coefficient per CdE (E = Se or S) unit for zinc-blende CdE nanocrystals

The extinction coefficient of semiconductor nanocrystals is a key parameter for understanding both the quantum confinement and applications of the nanocrystals. The existing extinction coefficients of CdE (E = Se, S) nanocrystals were found to have an unacceptable deviation for the zinc-blende CdE quantum dots (QDs). The analysis reveals that, in addition to the interference of impurities, the commonly applied extinction coefficient per CdE nanocrystal is sensitive to the size, shape, and density of the surface ligands of nanocrystals. The extinction coefficient per CdE unit does not depend on accurate information of the size, shape, and number of surface ligands of the nanocrystals. A new three-step purification scheme was developed to investigate three classes of possible impurities for accurate determination of the extinction coefficient per CdE unit, including CdE clusters not considered previously. Given that the sole ligands of zinc-blende CdE nanocrystals are cadmium fatty acid salts (CdFa₂), a universal formula for the nanocrystals can be written as (CdE)_n(CdFa₂)_m. The n: m ratio was accurately determined for purified nanocrystals. The resulting extinction coefficients per unit for both CdSe and CdS QDs were found to decrease exponentially as the size of the QDs increases, with the corresponding bulk value as the large-size limit.