Up-conversion luminescence of Er(Yb)-CeO2: Status and new results
Graphical abstract
Introduction
Cerium oxide (CeO2) represents an attractive host for the luminescent lanthanide (Ln) dopants due to its low phonon energy (∼470 cm−1), high chemical and thermal stability, high optical transparency in the visible region (band gap of ∼3.2 ÷ 3.5 eV), high values of refractive index (between 2 and 2.2) and ease of doping with trivalent Ln ions due to the small differences between ionic radius of Ln3+ and Ce4+ [1], [2], [3], [4]. Down- and up-conversion (UPC) emission studies on Ln doped CeO2 nanoparticles (NPs) represents a topic of great interest due to their potential applications in theranostics, photonics and bioimaging [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. To date, of the UPC studies regarding Ln doped CeO2, those on Er (Yb)-CeO2 are the most numerous [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. The first report on luminescence properties of Er doped CeO2 NPs described the effect of the nanoparticle size (18, 38 and 70 nm) on the down-conversion emission intensity under 488 nm excitation [22]. It was found that the red to green emission ratio value increased with decrease of the particle size. The down-conversion luminescence properties of Er doped CeO2 were also studied by Mu et al. [6] who analysed the emission properties of Er-CeO2 thin film under 340 nm excitation and observed that the red to green emission ratio increases monotonously for up to 5 at.% Er. Also, the maximum intensity of the green emission corresponds to 1% Er concentration while for red and near infrared (∼1500 nm) emission corresponds for 5% Er concentration.
Guo et al. [7] analysed the UPC emission properties of Er doped CeO2 with 0.5, 1, 3 and 6% Er concentration under 785 nm cw laser excitation. They observed that the maximum of the UPC emission intensity was attained for 3%Er concentration. The potential for therapy applications of CeO2 (co-)doped with Yb and Ln (Ln = Er, Ho or Tm) was first investigated by Seal et al. [8]. They reported that Er,Yb-CeO2 nanoparticles, excited with near-infrared radiation (975 nm) can be used as contrast agents for diagnostic imaging and as therapeutic agents for treatment of cancer. Shehata et al. [9] analysed the UPC emission of Er-CeO2 via both theoretically and experimentally approaches. The Er-CeO2 with 1, 3, 5, 10% Er concentration were excited at 780 nm and the maximum of UPC emission intensity was obtained for 5% Er. The same group also analysed the effect of calcination temperature on the UPC emission intensity and showed that the no-calcinated sample did not emit light under infrared excitation [9]. Following co-doping Er-CeO2 with the Yb sensitizer, Cho et al. [10] showed that the red to green emission ratio increases linearly with Yb co-doping concentration. The dependence of the red to green emission ratio value on Er and Yb concentrations was also observed by Cheng et al. [11]. They demonstrated a green to red UPC color tunability for three dimensional ordered macroporous Er,Yb-CeO2 excited at 980 nm and reported pure red up-conversion emission for 0,5%Er and 10%Yb. Han et al. [23] showed that the calcination temperature promoted crystallization of Er,Yb-CeO2 nanoparticles and also enhanced the UPC emission intensity. For Er,Yb-CeO2, generally a two-photon process was found responsible for both the red and green up-conversion emissions [9], [10], [11], [12], [13], [14], [23], [24], [25].
In our previous reports on the UPC emission of Ln doped CeO2 [5], [16], [28] we have shown that Ln generate significant charge-compensation defects, mostly oxygen vacancies, which modify the phonon spectrum by introducing characteristic phonon modes at higher energy than the characteristic Raman active F2g mode of pure CeO2 (∼464 cm−1). We also observed that the amount of defects in the Er-CeO2 lattice induced by heterovalent co-dopants such as La and Li ions, changed the nanoscale environment around Er ions and strongly modulated both the intensity and shape of the Er emission [16]. The Er-CeO2 co-doped with concentration up to 20% La or 15% Li were analysed under X-ray and optical down-conversion excitation into CeO2 charge transfer band (CT) or f-f absorption of Er as well as UPC excitation at ∼1500 nm. We found that the low-lying charge-transfer band of CeO2 acted as a selective antenna sensitizer of Ln emission, that is, the red emission was sensitized more via CT band whilst the green emission was excited mainly via the f-f absorptions of Er [16], [28]. Co-doping with La induced a strong reduction of Er emission intensity compared to Er single doped CeO2 which was explained by a simultaneous decrease of crystallite size, increase of the oxygen vacancy defects and OH surface defects. On the other hand, the co-doping with Li improved the crystallization and reduced the OH surface defects which leaded to a remarkable enhancement of the UPC emission intensity by a factor of ∼17 for excitation at ∼1500 nm. Moreover, under X-ray and up-conversion excitation at ∼1500 nm, the measured contribution of the near-infrared emission at 980 nm to the total emission intensity approached 90% respectively 98% [16].
Here, we report on the down- and up-conversion processes in Er doped and Er,Yb co-doped CeO2 NPs by use of time-gated luminescence spectroscopy under optical down/up-conversion and X-ray excitation. Optical excitation was pursued by use of ns pulsed laser in the range of 300–1500 nm and the emission was measured in the range of 500–1100 nm. The mechanisms involved in the up-conversion emission of Er (excitation at 650, 790, 977 and 1470 nm) and Er,Yb-CeO2 (excitation at 971 nm) were investigated in detail in terms of UPC excitation spectra, UPC emission decays and evolution of red to green emission ratio with Er concentration and delay after the laser pulse. The structural properties of Er and Er,Yb-CeO2 nanoparticles were analysed by X-ray diffraction (XRD), Raman, Diffuse Reflectance optical (DR-UV-Vis) and Diffuse Reflectance Infrared Fourier Transform (DRIFTS) spectroscopies. Our results are also discussed in the frame of comprehensive review of current literature which highlights the potential of the Er, (Yb) –CeO2 nanoparticles for bio-imaging, theranostics and solar cell applications (see Table 1).
Section snippets
Materials and characterization
Samples of 0.3, 1, 3% Er and 1%Er20%Yb doped CeO2 were synthesized by a citrate complexation method as described elsewhere [29]. All samples were calcined in air at 1000 °C (heating rate of 10 °C/min) for 5 h. The samples containing 0.3, 1, 3% Er and 1%Er 20% Yb, are denoted as: 0.3Er-CeO2, 1Er-CeO2, 3Er-CeO2 and 1Er20Yb-CeO2.
Powder X-ray diffraction (XRD) patterns were recorded on a Schimadzu XRD-7000 diffractometer using Cu Kα radiation (λ = 1.5418 Å, 40 kV, 40 mA) at a step of 0.02° and a
Structural characterization
Fig. 1 gathers the structural characterization of 1Er-CeO2 and 1Er20Yb-CeO2 pursued by X-ray diffraction, Raman, DRIFT and DR UV/Vis spectroscopies.
The XRD patterns (Fig. 1a) show that both 1Er-CeO2 and 1Er20Yb-CeO2 samples exhibit the cubic phase of ceria (JCPDS No. 34-0394) with no additional lines for erbium or ytterbium oxides (Fig. 1a). By use of Scherrer equation, the crystallite sizes for 1Er-CeO2 and 1Er20Yb-CeO2 were calculated as ∼48 and ∼44 nm, respectively. For 1Er20Yb-CeO2 samples,
Conclusions
Herein, we present a comprehensive study on the down- and up-conversion properties of Er (0.3, 1 and 3%)-CeO2 and Er(1%)Yb(20%)-CeO2 nanoparticles. The samples were synthetized by citrate complexation method and the structural properties were characterized by X-ray diffraction, Raman, Diffuse Reflectance UV/Vis and Diffuse Reflectance Fourier Transform Infrared spectroscopies. Down-conversion emission properties were analysed under excitation into f-f absorption of Er, charge transfer band of
Acknowledgements
DA and CT acknowledge the Romanian National Authority for Scientific Research and Innovation, the program NUCLEU, contract 4N/2016 for the financial support.
IP acknowledge the Romanian National Authority for Scientific Research (CNCS-UEFISCDI) (project number PN-II-ID-PCE-2011-3-0534) for the financial support.
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