Role of Surface States in Silver-Doped CdSe and CdSe/CdS Quantum Dots
journal contributionposted on 27.04.2018, 00:00 by David Morgan, David F. Kelley
The effects of Ag doping of CdSe and CdSe/CdS quantum dots (QDs) have been studied as a function of the surface composition. Throughout these studies, static and time-resolved luminescence and transient absorption measurements are used to determine the nature and rates of radiationless decay mechanisms and how these vary with the concentration of Ag dopants. In tributylphosphine-ligated CdSe, the photoluminescence quantum yield (PL QY) varies nonmonotonically with dopant concentration, increasing at low concentrations of Ag+ then decreasing with further addition. The initial increase is assigned to the passivation of preexisting surface hole traps by interaction of interstitial Ag+ with surface Se2– ions, whereas the subsequent decrease is due to the introduction of substitutional Ag+ dopants, which act as a new source of hole traps. Ligation of the particle surface with alkyl amines largely passivates the surface hole traps and thereby eliminates the initial increase in PL QY upon doping. CdSe/CdS QDs ligated with oleylamine (and no phosphines) exhibit extensive thermal population of empty surface orbitals by valence band electrons. Photoexcitation of these surface-charged particles results in what is essentially a positive trion, which can undergo rapid Auger decay. The presence of oleylamine reduces much of the Ag+ to Ag0, and the addition of Ag0 dopants passivates these empty surface states and eliminates the surface charging and hence positive trion formation.