Photoionization of individual CdSe/CdS core/shell nanocrystals on silicon with 2-nm oxide depends on surface band bending

CdSe/CdS nanocrystals are studied by electrostatic force microscopy (EFM) on n- and p-type silicon with 2-nm surface oxide and graphite for comparison. In the absence of above-band-gap excitation, the nanocrystals are not in electrical equilibrium with the substrates. Upon continuous photoexcitation, the particles behave differently on each substrate. On n-type silicon, the particles exhibit predominantly positive charge. However, on p-type silicon, fewer charged particles overall are seen, blinking is more common, and the ratio of negative to positive particles is higher. The low ionization yield on p-Si is apparently due to the fast return of photoexcited electrons that are confined near the silicon/oxide interface by band bending. Nanocrystal photoionization dynamics are sensitive to the state of the silicon across the 2-nm oxide. On graphite, the particles show a greater propensity to oscillate between positive, negative, and neutral states than on the other substrates.