Elsevier

Current Applied Physics

Volume 16, Issue 10, October 2016, Pages 1374-1381
Current Applied Physics

Enhanced down and upconversion emission for Li+ co-doped Gd2O3:Er3+ nanostructures

https://doi.org/10.1016/j.cap.2016.08.004Get rights and content

Highlights

  • Uniform sized synthesis of Er3+/Li+ co-doped Gd2O3 Nanostructures showing up and downconversion emission.

  • Lithium incorporation drastically enhance upconversion emission intensity.

  • Effect of Li co-doping on crystal structure was studied in detail.

Abstract

A phosphor with higher emission efficiency in the visible region can be very helpful in various optoelectronics applications such as solar cells. Effective conversion of UV and NIR radiations to more useful visible and red emissions were carried out through Gd2O3:Er3+ nanostructures synthesized by conventional co-precipitation method. The spectroscopic study of Gd2O3:Er3+ samples were carried out as a function of dopant concentration under the excitation wavelengths of 378, 975 and 1540 nm, respectively. The doping of erbium show downconversion emission at 561 and 660 nm under 378 nm excitation while upconversion emission under 975 and 1540 nm excitations shows upconversion of NIR photons to 561 and 660 nm visible emissions. The enhanced red emission (660 nm) at higher erbium concentration was observed due to the dominant cross-relaxation processes. The efficiency of upconversion luminescence is highly dependent on the various relaxation and energy transfer processes involved in different energy levels of erbium. The intensity of downconversion and upconversion emission was enhanced by lithium co-doping in Gd2O3:Er3+ nanostructures. The incorporation of alkali metal ions in Gd2O3:Er3+ shows considerable enhancement in downconversion and upconversion emission intensity making the phosphor more suitable for optoelectronic applications.

Introduction

Luminescent materials plays an important role in various technologies, including display screen, optical communication amplifiers, lamps and solid state lasers [1], [2]. Rare earth ions possess large number of metastable energy levels. These energy levels are responsible for several optical transitions under suitable excitation wavelength. Rare earths doped into oxides shows an attractive property of frequency downconversion as well as upconversion via a multiphoton absorption or through energy transfer mechanism. For upconversion phenomenon, host materials doped with rare earth ions having lower phonon frequencies and non-hygroscopic nature can significantly helpful for enhancing emission efficiencies of the phosphors. Fluoride based hosts with lower phonon frequencies have limited applications due to their non-hygroscopic and toxic nature [3]. Oxide hosts are preferred due to their high chemical durability, thermal stability, non-toxic nature, non-hygroscopic behavior and lower phonon frequencies [4].

Gadolinium oxide (Gd2O3) is one of the promising host material for upconversion phosphors due to its high chemical and thermal stability and lower phonon energy (phonon cutoff ≈ 600 cm−1) which makes it suitable for various practical applications. The lower phonon energy of gadolinium oxide can increase the possibility of radiative transitions and in turn results in high quantum yield of upconversion process [5]. Gd2O3 nanocrystals can be easily doped with various rare earth ions for down and upconversion phenomenon [6], [7], [8]. Dosev et al. [9]. have suggested some biomedical applications of upconverting nanophosphors Gd2O3 doped with Er3+ or Sm3+. The biolables for immunoassays is more focused on their applications, since upconversion nanoparticles possess distinct advantages such as the absence of autofluorescence of biomolecules and no need for time-resolved detection as compared to commonly used down converting phosphors. They also suggested possible use of rare earth doped Gd2O3 as a fluorescence and MRI (magnetic resonance imaging) labels.

Erbium ion is one of the excellent candidate for upconversion phosphors, since its metastable levels 4I9/2 and 4I11/2 can be conveniently populated by commercial near-infrared laser diodes [10], [11], [12], [13], [14]. The upconversion mechanism of single Er3+-doped oxide nanocrystals under the excitation wavelength of 970 nm has been studied with various oxide hosts such as Y2O3 [14], [15], [16], Gd2O3 [17], ZrO2 [18], and ZnO [19], [20]. The oxide nanocrystals have high chemical and mechanical durability, thermal stability, excellent optical properties and relatively lower phonon cutoff energy for the choice of suitable host material. During recent years, the optical properties of nanocrystals have attracted considerable interest in both fundamental research and practical applications. The phosphors with higher efficiency and smaller size are in demand [21].

Cai's and Zhang's groups reported a strategy of Li+ ions doping in upconversion nanoparticles, which gave rise to significant enhancement in upconversion emission without changing phase and particle size [22], [23]. It was reported that Li+ ions can increase the upconversion luminescent intensity in Y2O3:Er3+ and Gd2O3:Eu3+ [24], [25]. Similar study has also been carried out by co-doping Li in Y2O3:Tb3+,Yb3+ by Pandey et al. [26]. It is well-known that upconversion emission intensity is dependent on their intra 4f transition probabilities of rare earth ions which is affected by their local crystal field symmetry. The improvement of upconversion luminescence can be originated from the breaking and distortion of local crystal field symmetry around rare earth ions by alkali metal [27], [28].

Considering the requirement of a phosphor material which can show excellent downconversion and upconversion properties with single ion doping and can be utilized as photon absorber in solar cell technologies, we have synthesized Gd2O3:Er3+ nanostructures by conventional co-precipitation method. The energy levels of erbium can absorb both UV as well as NIR photons and that makes erbium a unique doping material. The phosphor with only UV or NIR absorption have much limitations such as loss of energy due to heat or lattice mismatch in solar cell technology. Thus, if the phosphor can have an ability to absorb both UV and NIR photons and converting them to visible light then it will increase the photocurrent and ultimately solar cell efficiency will be enhanced. Focusing particularly on the spectroscopic studies, Gd2O3:Er3+ was later co-doped with Li+ ion to enhance emission efficiency. Due to smaller ionic radius, Li+ ion can be easily doped into the host lattice substitutionally or interstitially which will break the crystal field symmetry around Er3+ ions. The nano-sized Gd2O3:Er3+ phosphor show enhancement in excitation and emission intensity in UV, NIR and Visible regions, respectively after successful Li+ co-doping.

Section snippets

Materials

Gadolinium (III) chloride hydrate (GdCl3•xH2O, 99.99%, Aldrich Chemicals), erbium chloride hexahydrate (ErCl3·6H2O, 99.99%, Aldrich Chemicals), lithium carbonate (Li2CO3, 99.997%, Aldrich Chemicals), oleic acid (C18H34O2, Tech. grade, Aldrich Chemicals), sodium chloride (NaCl, ≥99%, Sigma Aldrich Chemicals), sodium carbonate anhydrous (Na2CO3, >99.5%, Tedia Co. Inc. Ohio, USA) were used without further purification.

Synthesis

The synthesis of Gd2O3:Er3+ nanocrystals have been carried out using

Structure and morphology

The cubic phase Gd2O3 belongs to the Ia-3 space group and can be considered as a fluorite–related arrangement containing one–quarter O atoms. Gadolinium has six–coordination to oxygen, forming corner and edge sharing distorted polyhedra as shown in Fig. 1 [8]. Similar to cubic phase Y2O3 crystal, Er doping in Gd2O3 takes place at two symmetry sites such as C2 and S6, where C2 is low symmetry and S6 is centrosymmetric site. The crystal structure of annealed nanocrystals of Gd2O3 doped with

Conclusion

Uniform sized nanostructures of cubic phase Gd2O3 were prepared with Er doping by simple co-precipitation method. The sample showed good spectroscopic response to UV and NIR excitation by converting them to more useful visible (green and red regions) radiations. To enhance emission intensity, the prepared sample were co-doped with Li+ ions. The effect of Li+ co-doping on crystal structure, unit cell parameters and optical properties were studied in detail. The upconversion emission excited with

Acknowledgement

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2015060315). This work was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2010-0029634). Er3+/Li+ co-doped Gd2O3 Nanostructures, supplied by the Display and Lighting Phosphor Bank at Pukyong

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