The effects of surface and surface coatings on fluorescence properties of hollow NaYF 4 :Yb,Er upconversion nanoparticle
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Hollow nanoparticles of hexagonal close-packed (hcp)-NaYF4:Yb,Er were synthesized by thermal decomposition of trifluoroacetate precursors at 340 °C via vacancy diffusion, likely due to the Kirkendall effect and Ostwald ripening mechanism. The average outer diameter, inner diameter, and shell thickness of these hollow particles were 14 6 3 nm, 7 6 2 nm, and 4 6 1 nm, respectively. The surface effects on the fluorescence properties of these hollow particles were studied by comparing with that of solid NaYF4:Yb,Er (average size ;15 6 3 nm) and solid NaYF4 core/NaYF4:Yb,Er shell (NaYF4 core ;10 6 1 nm and NaYF4:Yb,Er shell ;3 6 2 nm) nanoparticles containing similar composition of Yb and Er ions. The green, red, and total emission intensities decreased with increasing upconversion active volume-normalized surface area. Surface coatings of undoped NaYF4 on both inner and outer surfaces of the hollow nanoparticles enhanced the total emission intensity by ;19 and ;5 times compared with those of the hollow and solid NaYF4:Yb,Er nanoparticles, respectively.
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.30
host.7 It was reported that the surface of particles may significantly differ from the interior environment because of compositional segregation of dopant ions at the surface, resulting in significant change of optical properties.4 A recent report demonstrated that the UC properties of the prism-, plate-, and rod-shaped NaYF4:Yb,Er nanoparticles were attributed to the surface-to-volume ratio and the particle shapes.8 The relationship between luminescence decay time and the ratio of surface area-to-volume of the nanoparticles was reported.8,9 To minimize the nonradiative loss, the UC active surface ions of the particle core were passivated by surface coating of low phonon energy materials that are commonly undoped host materials.10,11 The surface coating of undoped host on the UC core, referred to as undoped shell, provides a barrier to prevent undesired interactions between the UC active surface ions of the core and high phonon energy environment, thus leading to emission enhancement. However, our previous study showed that the emission intensity of UC core/undoped shell nanoparticles increased with increasing thickness of the undoped shell, which had no further enhancement when the thickness exceeded 3 nm.5 Recently, hollow structures have gained growing interest because of the potential applications in drug delivery, catalyst, and photonic devices.12–16 The hollow structures of UC nanoparticles may find combined use in bioimaging and drug-delivery applications. However, the hollow nanoparticles have even larger numbers of surface atoms due to both inner and outer surfaces, accentuating the surface-dependent properties. The hollow nanospheres are thermodynamically less stable compared with the solid
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Ó Materials Research Society 2011
I. INTRODUCTION
Nanostructured materials have been attracting great attention because of their size and surface structure dependent properties.1
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