Elsevier

Journal of Luminescence

Volume 170, Part 2, February 2016, Pages 707-717
Journal of Luminescence

Nanostructured rare earth doped Nb2O5: Structural, optical properties and their correlation with photonic applications

https://doi.org/10.1016/j.jlumin.2015.08.068Get rights and content

Highlights

  • Vis and NIR emission from nanostructured lanthanide doped Nb2O5.

  • Eu3+-doped Nb2O5 as Red luminophor.

  • Multicolor tunability of intense upconversion emission from lanthanide doped Nb2O5.

  • Potential application as biological markers.

  • Broad band NIR emission.

Abstract

In the present work, we report on a systematic study on structural and spectroscopic properties Eu3+ and Er3+-doped Nb2O5 prepared by sol–gel method. The Eu3+ ions were used as structural probe to determine the symmetry sites occupied by lanthanide ions. The Eu3+-doped Nb2O5 nanocrystalline powders were annealed at different temperatures to verify how the different Nb2O5 crystalline phases affect the structure and the luminescence properties. Er3+-doped Nb2O5 was prepared showing an intense NIR luminescence, and, visible luminescence on the green and red, deriving from upconversion process. The synthetized materials can find widespread applicability in photonics as red luminophor for white LED (with tricolor), optical amplifiers and upconverter materials.

Introduction

Luminescent rare earth doped micro- and nanostructured materials have been extensively explored for high tech applications like optical sensor (chemical sensor), temperature sensor [1], [2], luminescent labeling of (bio)molecules [3], [4], light emitting in fluorescence lamps and white light emitting diodes [5], field emission displays, solar concentrator for solar cells [6], lasers, optical amplifiers for telecom [7] and others as photonic or biophotonic devices. Rare earth doped materials can be obtained as single crystals, or in the form of micron or submicron size, and most recently, a lot of papers are devoted to nanoparticles, which can be prepared in a vast range of morphological properties (sizes and shapes, core shell structures and functionalized surface). In this sense, the most appropriate material will be dependent of specific application, for instance single crystals for laser and micrometric particles for lamps, which can be efficiently excited and has spatially homogeneous emission. The dimension of particles can provide different matter properties, and in some cases, influence the luminescence, due to its correlation with structural features of the host. Accordingly, the structure (crystalline or amorphous), vibrational and electronic features, and refractive index, rare earth distribution (symmetry sites and ion–ion distance) have to be taken into account to select the host.

Oxide material has a huge significance as rare earth host for photonics and biophotonics, and in this work the niobium oxide has been used as rare earth host for visible and near infrared (NIR) emitter.

Niobium oxide (Nb2O5) has been recognized in literature in a series of applications, e.g., on catalysis [8], [9], [10], eletrochromism [11], [12], sensors [13], [14] and biocompatible coatings [15], [16]. Regarding on optical applications niobium-based compounds doped with trivalent lanthanide ions (Ln3+) have been reported in the literature for application in the field of optoelectronics and light-emittingdevices [17], [18], photorefractive memories [19] and solid-state laser materials [20], [21].

Nb2O5 shows a high refractive index (n=2.4), a wide band gap (3.6 eV) [22], high dielectric constant (29–200, depending on crystalline phase) [23] and relatively low phonon energy (~700 cm−1) [24]. Nevertheless, different Nb2O5 crystalline phases [25] can be obtained depending on preparation method and annealing temperature. There are a lot of discussion about this matter on literature, but two crystalline phases are mainly recognized, the T-phase and the H-phase. The orthorhombic T-phase or γ-phase is the lowest temperature form, crystallizing at 500 °C and the monoclinic H-phase is the highest temperature stable form crystallizing at 1100 °C [25]. The M-phase that crystallizes at around 800 °C has been described in literature as an intermediate or a poorly crystallized H-phase and many authors have stated that both polymorphs are essentially the same [25], [26], [27], [28]. However, recently, we could observe distinct spectroscopic features related to M- and H-phases in Er3+-doped SiO2–Nb2O5 containing Nb2O5 nanocrystals embedded in silica based host [29]. There are still other polymorphs like N, P and TT that have been described, by some authors, using specific preparation methods [25], [30].

Recent structural studies and their correlation to the spectroscopic properties of Er3+-doped SiO2-Nb2O5 nanocomposites [29] revealed that the Er3+ ions were mainly located in Nb2O5-rich regions and the NIR luminescence intensity and the FWHM (full-width half-maximum) values changed with the Nb5+ content and the different Nb2O5 crystalline phase formation. An unusual broadening in the near infrared band, even with the lanthanide ions distributed in a crystalline environment made them interesting for WDM and telecom applications.

In the present work, we intend to go further on the study of luminescent properties in the visible and near infrared region, not to mention the evaluation of the symmetry sites occupied by rare earth ions on Nb2O5 host.

One of the well-known strategies on the structural characterization is the Eu3+-doping as structural probe of symmetry sites in many materials like glasses, glass-ceramics and crystals. This is possible because the 5D07F1 transition has pure magnetic-dipole character, which not depends on the local ligand field of the Eu3+ ion and consequently may be used as reference for the entire spectrum. The number of the Stark components added to the relative intensities of the 5D07FJ (J=0–6) may be used to estimate the symmetry site of Eu3+ ions and also the intensities parameters, radiative lifetimes and emission quantum efficiency [31]. Additionally to the structural probe application, our results indicate that Eu3+ may be employed as red luminophor for white LED (with tricolor) [32], since a high excitation efficiency in the blue and green has been observed. Today, one of the strategies to obtain white LEDs is combining phosphors with blue or near-ultraviolet (NUV) LEDs [33], and the most used is the combination of a blue InGaN with the yellow-emitting Y3Al5O12:Ce3+ phosphors [34], [35], which exhibits low color rendering index (CRI). In this sense, the fabrication of white LEDs with tricolor (red, green, and blue) phosphors excited by a NUV chip [36] may solve this problem.

Most of the works on Eu3+-doped Nb2O5-based materials on literature are about niobates (XNbO3, X=alkaline metal ions) and their application as orange–red emitter using UV, blue excitation [37] and infrared (IR) excitations [38], and as structural probe [39].

The sol–gel process has been successfully adopted as a versatile method on the preparation of a wide range of materials like nanoparticles, oxides, thin films, hybrid materials, xerogels, aerogels, monoliths, glasses, glass-ceramics, fibers, etc [40]. There are some works on Nb2O5-based materials prepared by sol–gel process on literature like powders, thin films, catalysts, glasses, etc. To the best of our knowledge there are no other works reporting on the preparation of Eu3+ and Er3+-doped Nb2O5 materials by sol–gel process. Regarding Eu3+-doped Nb2O5 we could only find reports on preparation and characterization of Eu3+-doped Nb2O5 prepared by the Pechini method [41], [42]. The authors have obtained a nanocrystalline powder as confirmed by XRD analysis and observed a strong inhomogeneous broadening on their luminescence spectra, which in their first study [41] was attributed to disorder environment around the Eu3+ ions. In a further study [42], the authors used the EXAFS technique and nanocrystalline powders doped with Eu3+ and Er3+ ions and have concluded that both lanthanide ions enter the Nb2O5 structure as substitutional defects at Nb nine-fold coordination site.

In the present work, we report on a systematic study on structural and spectroscopic properties Eu3+-doped Nb2O5 prepared by sol–gel method. The Eu3+ ions will be used as structural probe in order to evaluate the symmetry sites occupied by lanthanide ions. The Eu3+-doped Nb2O5 nanocrystalline powders were annealed at different temperatures to verify how the different Nb2O5 crystalline phases affect the structure and the luminescence properties. Finally, Er3+-doped Nb2O5 were prepared showing an intense NIR luminescence, and, visible luminescence on the green and red, deriving from upconversion process. Therefore, the synthetized materials are potential candidates to photonic applications.

Section snippets

Experimental procedure

Eu3+-doped Nb2O5 nanostructured particles were prepared by the sol–gel method. Optical grade niobium oxide (Nb2O5 – 99.9%) from CBMM (Companhia Brasileira de Metalurgia e Mineração, Brazil) was dissolved in HCl under stirring at 70 °C for 5 h, followed by careful drying at 80 °C with addition of anhydrous ethanol, which afforded the stock solution. The Eu3+ ions (0.1, 0.5 and 1.0 mol%, in relation to Nb ions) and Er3+ ions (0.75 mol% in relation to Nb ions) were added from its chloride solution,

Structural properties

Fig. 1A shows the X-ray diffractograms of 0.5 mol% Eu3+-doped Nb2O5 annealed at 600 °C, 900 °C and 1100 °C, which is representative of all Eu3+-doping concentrations in the present work. Analyzing the different annealing temperatures diffractograms, it can be observed the formation of three distinct Nb2O5 crystalline phases for different annealing conditions. The assignments of the main planes are also shown in Fig. 1A. The reflection patterns for the sample annealed at 600 °C can be attributed to

Conclusions

Eu3+ and Er3+-doped Nb2O5 materials were successfully prepared by the sol–gel method. A colloidal precursor prepared from optical grade Nb2O5 was used as an alternative to alkoxide that are generally adopted on literature reports. A systematic study on spectroscopic and structural properties was performed. Nb2O5 polymorphism was clearly observed with T, M and H crystalline phases formation, for the annealing temperatures at 600, 900 and 1100 °C, respectively. The Eu3+-doped Nb2O5 materials were

Acknowledgments

The authors acknowledge FAPESP (Brazil) and CNPq (Brazil) for financial support, CAPES for the scholarship, and CBMM (Companhia Brasileira de Metalurgia e Mineração) for donating the Nb2O5 used in this study. The authors also acknowledge Prof. Sidney Jose Lima Ribeiro for the opportunity to obtain the luminescence spectra, and Patrick Aschehoug for technical support to obtain the emission decay curves

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