Abstract
Insulating spacer layers of MgO were used to identify the enhancement mechanisms of the ZnO band-edge and visible luminescence in ZnO-MgO-Ag and ZnO-MgO-Au multilayers. Purcell enhancement of the ZnO band-edge emission by both Ag and Au surface plasmon polaritons is confirmed by demonstrating that the exponential decay of this emission as a function of increasing MgO thickness is consistent with the Ag and Au SPP evanescent decay lengths. Local surface plasmons excited in Ag and Au nanoparticles and rough films are also shown to enhance the ZnO visible donor-acceptor-pair photoluminescence by dipole-dipole scattering, again with an appropriate dependence on the thickness of the MgO spacer layer. We also confirm that both Ag and Au nanoparticles enhance the ZnO band-edge emission by charge transfer when the MgO spacer layer is absent.
Highlights
Zinc oxide (ZnO) has a band gap of 3.37 eV and exciton binding energy of 60 meV, the latter providing excellent thermal stability compared, for example, with GaN
Purcell enhancement of the ZnO band-edge emission by both Ag and Au surface plasmon polaritons is confirmed by demonstrating that the exponential decay of this emission as a function of increasing MgO thickness is consistent with the Ag and Au surface-plasmon polaritons (SPPs) evanescent decay lengths
By depositing variable thicknesses of MgO between metal films and ZnO films, we have demonstrated the scattering of Au and Ag SPPs to photons when the MgO thickness is much greater than the calculated SPP fringing field thickness
Summary
Zinc oxide (ZnO) has a band gap of 3.37 eV and exciton binding energy of 60 meV, the latter providing excellent thermal stability compared, for example, with GaN (binding energy 25 meV). The band-edge PL enhancement in Au-ZnO nanostructures has been attributed to excitation of LSPs by ZnO impurity emission, followed by transfer of hot electrons to the ZnO conduction band [6,8] Both the band-edge enhancement and the Purcell factor have a similar dependence on temperature, lending further credence to the idea that enhanced band-edge emission results from coupling to SPPs [11]. We show that the band-edge enhancement in nanoparticulate metal-ZnO film bilayers disappears upon introduction of an insulating MgO spacer film This implies that the enhancement is due to charge transfer, as proposed in earlier studies of Au and Ag nanoparticles in contact with ZnO nanoparticles and nanorods [6,8].
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