Imaging Upconversion from NaYF 4 :Er:Yb Nanoparticles on Au and Ag Nanostructured Substrates
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Imaging Upconversion from NaYF4:Er:Yb Nanoparticles on Au and Ag Nanostructured Substrates Lanlan Zhong1, QuocAnh Luu2, Hari P. Paudel3, Khadijeh Bayat3, Mahdi Farrokh Baroughi3, P. Stanley May2 and Steve Smith1,* 1
Nanoscience and Nanoengineering, South Dakota School of Mines and Technology 2 Chemistry, University of South Dakota 3 Electrical Engineering and Computer Science, South Dakota State University *
Corresponding Author: [email protected]
ABSTRACT Near-infrared-to-visible upconversion materials have many promising applications, including use in luminescent solar concentrators, in next-generation displays, and as biological labels. NaYF4 nano-particles doped with Yb and Er exhibit efficient upconversion and are easily deployed in these applications. It is known that a rough metal surface may increase the yield of fluorescence of a nearby fluorophore, by local field enhancement due to plasmonic resonances, and by modification of the radiative rate(s) of the fluorophore. Thus, properly chosen metallic nanostructures can potentially increase the upconversion efficiency of lanthanide-doped nanoparticles, yet the optimal design of these nanostructures is still an active area of research. In our experiments, we use a spectroscopic imaging system to study the upconversion efficiency of NaYF4: Er3+/ Yb3+ through spatially-resolved upconversion spectra, using a custom-built scanning confocal microscope system with infra-red excitation, and wide-field fluorescence imaging. We present spectrally-resolved upconversion images of NaYF4:Yb3+/Er3+ nanoparticles on plasmonic substrates, including silver nanowires and patterned substrates of gold and silver, which show localized regions (a 1Pm) of relatively stronger intensity and modified upconversion spectra, and compare these to wide-field fluorescence images of samples with and without plasmonic substrates. 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].
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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 efficienci
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