Spectroscopic Imaging of Metal-Enhanced Upconversion on Plasmonic Substrates
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Spectroscopic Imaging of Metal-Enhanced Upconversion on Plasmonic Substrates R.B. Anderson1, J. Fisher1, A. Hor1, A. Lu2, H. Paudel3, K. Bayat3, M. Baroughi3, T-S. Luk4, P.S. May2 and S. Smith1 1
Nanoscience and Nanoengineering South Dakota School of Mines and Technology, Rapid City, SD 57701 2
Chemistry Department University of South Dakota, Vermillion, SD 57069 3
Electrical Engineering and Computer Science Department South Dakota State University, Brookings, SD 57007 4
Center for Integrated Nanotechnologies (CINT) Sandia National Laboratories, Albuquerque, NM 87185 Email: [email protected] ABSTRACT We use spectroscopic imaging to investigate the enhancement of infra-red to visible upconversion in rare-earth doped nano-particles (NaYF4:Yb:Er) supported on nano-fabricated plasmonic substrates consisting of square lattices of Au nano-pillars fabricated by electron beam lithography and designed to support a surface plasmon polariton at frequencies which are nearresonant with the rare-earth ion (Yb3+) absorption. We observe a systematic enhancement in the efficiency of upconversion associated with the interaction of the co-doped nano-particles with the plasmonic substrate. Spectrally-resolved imaging provides a massively parallel means of assessing the range of achievable enhancement and its relation to the specific configuration of the substrate / upconverting nano-particle system. Spectrally-resolved reflectivity of the plasmonic substrates confirms the role of the surface plasmon polariton in the upconversion enhancement. Experimental results are compared to Finite Difference Time Domain simulations of the frequency-dependent reflectivity of these metallic nanostructures. INTRODUCTION Two photon processes could be very important in implementing third generation solar cell technologies, and can play an important role in characterizing the materials and devices necessary to implement them. Complementary approaches are spectral upconversion [1], intermediate band absorption [2,3], and multi-exciton generation [4,5]. These methods all increase the fraction of the solar spectrum which is converted to electricity, and / or increase conversion efficiency by utilizing / recapturing carrier excess energy normally lost to heat in the photovoltaic process. The present study concentrates on spectral upconversion, using NaYF4 nanocrystals activated with Yb3+ and Er3+. NIR-to-visible upconverters such as Yb,Er co-doped in NaYF4 nanocrystals have potential as spectral converters and luminescent concentrators for solar cells [6], as well as other uses, such as nanolabels for biosensing or 3D displays [7]. However, a primary obstacle to the incorporation of upconversion phosphors into real devices has been the inability to obtain high upconversion efficiencies under modest excitation flux. It is now well-known that luminescence efficiencies of both organic and inorganic phosphors can be
enhanced d by their su uitable proxim mity to a meetal surface, due partly tto an increasse in the intrrinsic radiative decay ratess of
