On the Absorption Cross Section of CdSe Nanocrystal Quantum Dots

The linear absorption cross section is a crucial parameter to the design of nanocrystal quantum dot devices and to the interpretation of spectroscopic data. We measure and report the size-dependent absorption cross section of CdSe nanocrystal quantum dots. We compare the results for absorption far above the band edge, where the quantum dot density of states may be approximated as a continuum, to simple theoretical models of light scattering from light-absorbing small particles. Excellent agreement with theory is found for dilute dispersions in hexane. We find that for absorption at 350 nm the per particle absorption cross section Cabs (in cm2) for CdSe is Cabs = (5.501 × 105)a3 cm-1, where a is the particle radius in cm. The absorption cross section is observed to be largely insensitive to the solvent refractive index. Detailed modeling of the effect of the ligand shell may be necessary to understand the lack of sensitivity of the absorption properties of nanocrystal quantum dots to the refractive index of the medium.