Elsevier

Optical Materials

Volume 33, Issue 10, August 2011, Pages 1500-1505
Optical Materials

NIR-to-visible and NIR-to-NIR upconversion in lanthanide doped nanocrystalline GdOF with trigonal structure

https://doi.org/10.1016/j.optmat.2011.02.029Get rights and content

Abstract

Codoped Er3+/Yb3+, Tm3+/Yb3+, Ho3+/Yb3+ and triply doped Er3+/Tm3+/Yb3+ gadolinium oxyfluoride nanoparticles were prepared in aqueous solution by a simple coprecipitation method and a suitable heat treatment at 500 °C. From the experimental X-Ray powder diffraction patterns, a Rietveld analysis was carried out and it was determined that the nanoparticles are single phase trigonal GdOF. Electron microscopy images show that the average particle size is approximately 25 nm, even though a certain degree of agglomeration is evidenced. The spectroscopic properties of the lanthanide doped nanoparticles are investigated in terms of emission spectra. For proper lanthanide concentrations, the nanoparticles show visible upconversion upon excitation at 980 nm, making them useful as luminescent nanomaterials for photonic applications.

Highlights

► The color of the upconversion can be tuned with a proper choice and concentration of lanthanide ions. ► A trigonal GdOF structure has been found by Rietveld refinement of the XRPD patterns. ► Bright upconversion in the visible and near infrared range is observed upon excitation at 980 nm. ► The color of the upconversion emission can be tuned in the visible and near infrared with a proper choice of lanthanide ions and their concentration.

Introduction

Significant efforts have been undertaken in the recent past to develop new nanostructured lanthanide doped luminescent materials for their possible use in cutting-edge photonic applications [1], [2]. In particular, these materials have been demonstrated to be of paramount importance in light emitting diodes (LED and organic LED) [3], [4], solar energy converters [5], [6] and as materials for biomedical applications, in particular in the field of optical imaging [7], [8]. Recent research activities have been devoted to investigations on new lanthanide doped nanocrystalline hosts showing strong luminescence [9]. The preparation method of the materials is also of great importance as it should be easy, fast and environmentally friendly.

Lanthanide doped luminescent fluoride nanocrystalline materials are currently being intensively investigated, due to their low phonon energy cut off and in turn to their low probability of the multiphonon deexcitation pathways [10], [11]. Oxyfluorides have been also studied as hosts that can easily accommodate luminescent lanthanide ions as dopants. Few papers have appeared in the literature reporting the synthesis and spectroscopic properties of lanthanide doped based oxyfluorides. For instance, bulk samples of Eu3+ or Tb3+ doped lanthanum, gadolinium and yttrium oxyfluorides [12], [13], thin films of Eu3+ doped LaOF oxyfluoride [14] and Eu3+ doped gadolinium fluoride nanoparticles embedded in a silica film [15] have been recently studied. Moreover, cubic gadolinium oxyfluoride nanoparticles doped with lanthanide ions have also been prepared starting from trifluoroacetate precursors in high-boiling solvents and their luminescence properties have been investigated [16]. Very recently, a paper on the upconversion (UC) properties of lanthanide doped Er3+/Tm3+/Yb3+ Gd4O3F6 nanoparticles has been published [17]. Aside from these few cases, to the best of our knowledge, no other papers have been appeared in the literature concerning nanocrystalline upconverting GdOF samples.

For these reasons, we found it interesting to prepare Er3+/Yb3+, Ho3+/Yb3+, Tm3+/Yb3+ and Er3+/Tm3+/Yb3+ doped GdOF nanocrystalline samples with a trigonal structure and to investigate the structural and spectroscopic properties, in particular the upconversion emission. The codoped nanocrystalline samples have been prepared via a facile coprecipitation technique using water as the solvent and inexpensive metal and fluorine precursors. The structural features, such as lattice parameters and average particle size, have been investigated using a Rietveld refinement while the morphological analysis has been carried out using Transmission Electron Microscopy (TEM). Laser excited Stokes and UC luminescence of the lanthanide doped materials have been measured and analyzed.

Section snippets

Synthesis

Lanthanide doped gadolinium oxyfluoride nanocrystalline samples of composition Gd0.89Ln0.01Yb0.1OF (Ln = Er, Ho and Tm) and Gd(0.98−x)Er0.01Tm0.01YbxOF (x = 0.01, 0.10) were prepared by a simple coprecipitation procedure as follows. Reagent grade metal nitrates (Gd(NO3)3 · 6H2O, Aldrich, 99.999%, Er(NO3)3 · 6H2O, Aldrich, 99.9%, Tm(NO3)3 · 6H2O, Aldrich, 99.999%, Ho(NO3)3 · 6H2O, Aldrich, 99.999%, Yb(NO3)3 · 6H2O, Aldrich, 99.999%) and NH4F (Aldrich, 99.9%) were used as starting reagents. 0.886 mmol of gadolinium nitrate

Structural and morphological characterization

The XRPD powder patterns of all the synthesized compounds clearly show the presence of a single trigonal crystal phase with space group R-3m (space group no. 166) according with the reference card no. 00-050-0569 of the PDF-4+2008 database. A Rietveld refinement [18], [19] exploiting the trigonal structural model proposed by Hölsa et al. [20] for SmOF, was carried out on the powder pattern for the Tm3+/Yb3+ codoped oxyfluoride sample (see Fig. 1). In this compound, of GdOF composition, all the

Conclusions

Nanocrystalline gadolinium fluoride samples codoped with Er3+/Yb3+, Tm3+/Yb3+, Ho3+/Yb3+ and triply doped with Er3+/Tm3+/Yb3+ ions in several molar ratios have been prepared by a simple coprecipitation technique. A Rietveld refinement of the XRPD patterns revealed that all the samples are single phase with trigonal structure and GdOF stoichiometry, pointing out that only one regular site for the lanthanide ions is present in the lattice structure. It must be underlined that the Tm3+/Yb3+ doped

Acknowledgement

Fondazione Cariverona (Verona, Italy) is gratefully acknowledged for financial support.

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