Influence of Ag film on upconversion luminescence enhancement of a single β-NaYF4:Yb3+/Er3+ microcrystal particle

Abstract

Rare-earth-doped upconversion (UC) is a nonlinear optical phenomenon in which the successive absorption of two or more low-energy photons via intermediate energy states leads to high-energy light emission (anti-Stokes emission). It endows their unique optical properties including low auto-fluorescence, high signal to noise ratio, a large anti-Stokes shift, and low toxicity. Therefore, rare-earth-doped UC materials have been widely used in photovoltaics, color displays, optical storages, biological imaging, and solar cells. However, the UC luminescence efficiency is much lower and largely limited the practical applications. The noble metal nanostructures as promised enhanced substrates could efficiently improve the intensity of UC luminescence and expand its applications. In this work, Yb3+and Er3+ co-doped β-NaYF4 microcrystals are synthesized via a simple and facile hydrothermal method with assistance of sodium citrate to control the crystal growth. Then, a Ag films/β-NaYF4:Yb3+/Er3+ hybrid structure that the β-NaYF4:Yb3+/Er3+ microcrystals were deposited on silver films with different thickness is constructed. The crystal phase and the morphology of the β-NaYF4:Yb3+/Er3+ microcrystals are analyzed with X-ray power diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM). According to the results of XRD, SEM, and TEM, the pure hexagonal phased NaYF4:Yb3+/Er3+ are arc-shaped hexagonal plate microcrystals with smooth surface. The average overall dimension of each microcrystal is about 5.0 µm in diameter. The UC luminescence property of Ag film/β-NaYF4:Yb3+/Er3+ hybrid structure is investigated carefully by a confocal setup equipped with an optical microscope. When 980 nm continuous wave (CW) laser is focused on the central of a single microcrystal particle deposited on silver films with different thickness, the intensity of blue (2H9/2→4I15/2), green (4S3/2→4I15/2 and 2H11/2→4I15/2), and red (4F9/2→4I15/2) emission bands increases with increasing the Ag film thickness. The maximum enhancement factor (EF) for each emission band occurs at the Ag film thickness of 100 nm. But we realize that the intensity EF of each emission band is different, the EF for the green emission is greater than the blue and red one. A comprehensive analysis, based on the rate equations and the numerical calculation | E |2, reveals that the optical reflection and local electric field enhancement effect of the Ag film play a role in the UC luminescence enhancement for the single β-NaYF4:Yb3+/Er3+ microcrystal. This finding breaks the limitation of conventional core-shell system, and it also provides a unique platform for micro-optoelectronic devices, solar energy conversion, optical sensors, and biological applications.