Abstract
In the recent years, there is an extensive effort concentrated towards the development of nanoparticles with near-infrared emission within the so called second or third biological windows induced by excitation outside 800–1000 nm range corresponding to the traditional Nd (800 nm) and Yb (980 nm) sensitizers. Here, we present a first report on the near-infrared (900–1700 nm) emission of significant member of cubic sesquioxides, Er-Lu2O3 nanoparticles, measured under both near-infrared up-conversion and low energy X-ray excitations. The nanoparticle compositions are optimized by varying Er concentration and Li addition. It is found that, under ca. 1500 nm up-conversion excitation, the emission is almost monochromatic (>93%) and centered at 980 nm while over 80% of the X-ray induced emission is concentrated around 1500 nm. The mechanisms responsible for the up-conversion emission of Er - Lu2O3 are identified by help of the up-conversion emission and excitation spectra as well as emission decays considering multiple excitation/emission transitions across visible to near-infrared ranges. Comparison between the emission properties of Er-Lu2O3 and Er-Y2O3 induced by optical and X-ray excitation is also presented. Our results suggest that the further optimized Er-doped cubic sesquioxides represent promising candidates for bioimaging and photovoltaic applications.
Highlights
Among the rare earth oxides, cubic Lu2O3 is an attractive phosphor host for lanthanide (Ln) activators due to its high mass density (9.4 g cm−3), good phase stability, low thermal expansion, low phonon energy, and broad optical transparency from the visible to the near infrared (NIR) regions[1,2,3,4]
Upconversion emission and indirect/direct decay measurements were performed considering multiple absorption transitions in the visible and near-infrared with specific aim to assess the effect of Li on the emission properties and to elucidate the up-conversion mechanisms
To compare the effect of improved crystallization triggered by thermal annealing to that induced by Li addition, Li free samples were calcined at 1000 °C and denoted as reference samples
Summary
Among the rare earth oxides, cubic Lu2O3 is an attractive phosphor host for lanthanide (Ln) activators due to its high mass density (9.4 g cm−3), good phase stability, low thermal expansion, low phonon energy (phonon cutoff ~600 cm−1), and broad optical transparency from the visible to the near infrared (NIR) regions[1,2,3,4]. The same authors suggested that the much stronger (up to two orders of magnitude) up-conversion emission intensity measured with Er-Lu2O3 compared to Er-Y2O3 nanoparticles under 980 nm excitation was related to reduced adsorption of CO2 and H2O on the Lu2O3 surface induced by the greater particle size of Lu2O3 (50 nm) than Y2O3 (20 nm). It is well known that the efficiency of solar cells is increased by converting the photons with energies below the absorption threshold (λ > 1100 nm) into higher energy photons (λ < 1100 nm)[22] Such interesting applications explain the recent growing number of studies on Er based systems that present a relative intense up-conversion emission centered at 980 nm following excitation at 1500 nm, such as Er-CeO223,24, Er-Y2O325 and Er-NaYF426, Er(Yb)-Gd2O2S27–29, Er-BaTiO330, Er-BaY2F831 and Er-LiYF432,33. Extending the excitation range of upconversion nanocrystals outside 800–1000 nm corresponding to traditional Nd and Yb sensitizers represents an active and formidable challenge targeting applications such as anticounterfeiting, bioimaging, display, photovoltaics and information storage[35,36,37,38,39]
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