Enhanced green up-conversion luminescence in In2O3:Yb3+/Er3+ by tri-doping Zn2+

https://doi.org/10.1016/j.jlumin.2020.117029Get rights and content

Highlights

  • In2O3:Yb3+/Er3+ with or without Zn2+ were prepared by solid-state reaction method.

  • After doping Zn2+, luminescence intensity increased and crystallinity improved.

  • The tri-doping Zn2+ makes emitting light change from faint red to bright green.

  • The increase of pump power is good to level population for green light emission.

  • Effects of Zn2+ on mechanism of up-conversion luminescence are discussed.

Abstract

By co-doping Yb3+/Er3+ into ordinary material In2O3, the up-conversion glow behavior will be triggered up. On this basis, the luminescence intensity can be significantly improved by tri-doping Zn2+ ions. Compared with In2O3:Yb3+/Er3+, the luminescence intensity of In2O3:Yb3+/Er3+/Zn2+ is significantly enhanced and the emitting light changes from faint red to bright green. This can be attributed to the reduction of site symmetry of the luminescence center and the promotion of crystallinity because of lattice distortion caused by Zn2+ ions. The up-conversion luminescence of the In2O3:Yb3+/Er3+/Zn2+ in the red and green light regions are two-photon excitation process, and the quantum yield (QY) is calculated to be 0.1%. The luminescence at the 524 nm emission peak corresponds to the transition Er3+:2H11/2 → 4I15/2. Other two emission peaks in green area (550 nm, 564 nm) correspond to the transition Er3+:4S3/2 → 4I15/2. And the emission peaks in red area (659 nm, 683 nm) root in the transition Er3+:4F9/2 → 4I15/2.

Introduction

Stokes' law states that materials can only receive the excitation of high energy and emit low-energy light. Whereas, some other materials can absorb near-infrared light and emit visible light, which is called up-conversion luminescence or anti-stocks luminescence [1]. Up-conversion luminescence materials have great application potential in fields of biological imaging, medical testing, gene knockout, targeted drug delivery, infrared detection, and alarming signs, due to their advantages of deep detection depth, low tissue damage, low background noise, high contrast and long luminescence lifetime [[2], [3], [4], [5]]. Besides, the up-conversion materials can transfer the infrared light, which accounts for 43% of the total energy of sunlight, into visible light for battery absorption and utilization. And this is of great significance to the improvement of photoelectric conversion efficiency [6]. Nevertheless, there are still some urgent issues that need to be addressed before put into practice. On account of the up-conversion luminescence process, the quantum yield of up-conversion materials is very low. Therefore, improving up-conversion quantum efficiency and luminescence intensity will always be the research focus [7]. Also, to less negative impacts on biological tissue caused by unabsorbed high-energy laser, it is significant to obtain up-conversion luminescence materials with higher laser absorption and utilization capacity [8,9].

In order to realize efficient up-conversion luminescence, activator ions should have suitable long-lifetime metastable intermediate energy levels, of which electrons have more chances to gain enough energy from either sensitizers or the excitation source to complete a further transition to higher energy levels, and therefore realize the up-conversion luminescence [10]. Lanthanide ions (except La3+, Ce3+ and Lu3+) undergo up-conversion luminescence under appropriate conditions. Particularly, Er3+, Tm3+, and Ho3+ ions are ideal activators due to their step-like energy level structure, long-lived metastable energy levels and appropriate energy level spacing [11]. The commonly used sensitizer is ytterbium (Yb3+), because of its relatively high molar extinction coefficient, simple energy level structure and large absorption cross section at 980 nm [[12], [13], [14]]. Besides, excited state of Yb3+ ion is exactly above or equal to metastable intermediate states of Er3+, Tm3+, and Ho3+ ions, leading to a prominent energy transfer process [11].

Up to now, host materials for up-conversion luminescence are mainly fluoride, oxides, sulfur compounds, fluorine oxides, halides, and so on. Oxide up-conversion phosphors have advantages of simpler preparation process, better stability, lower thermal expansion coefficient and less demanding of the environment [[15], [16], [17], [18], [19], [20]]. Different symmetries of crystal fields have various influences on transitions of trivalent rare earth ions. Low symmetry can increase the transition probability of f-f, decrease the influence of the non-radiation transition, and therefore promote the up-conversion mechanism. By introducing mono- or di-valent cations, such as Li+, Ca2+, Mg2+, Zn2+, Cu2+, Bi3+, Fe3+, Mo3+, etc., to the host lattice, luminescence intensity can be significantly enhanced [[21], [22], [23], [24], [25], [26], [27], [28], [29]]. The reason is that introduced ions cause interstitial ions and vacancies because of charge compensation, also lead to lattice distortion owing to the mismatch of ion radius [30].

Indium oxide (In2O3) is a kind of semiconductors with a wide band gap, and has a cubic manganese-iron ore with good chemical stability. The space group of the In2O3 is Ia-3 (No.266). There are 16 In2O3 molecules in a cell, and the coordinator number of oxygen and indium are 4 and 6, respectively. In one cubic lattice, the top corner contains 6 oxygen atoms and leaves 2 vacancies. Thus, there are two crystallographically unbalanced In3+ positions [[31], [32]]. This paper first reports the introduction of Zn ions into up-conversion luminescent materials In2O3:Yb3+/Er3+, and the absorption spectra, X-ray diffraction patterns, up-conversion emission spectra are executed. It is manifest that Zn2+ ions will largely improve the up-conversion luminescence intensity and efficiency. Meanwhile, the mechanism of the enhanced up-conversion luminescence by Zn2+ tri-doping is discussed in detail.

Section snippets

Experiment

Samples of In2O3:5 at% Yb3+/0.5 at% Er3+ (In2O3:Yb3+/Er3+) with or without Zn2+ tri-doping were fabricated by conventional solid-state reaction method. In2O3(99.99%,Meryer)、Er2O3 (99.99%,Aladin)、Yb2O3 (99.99%,Merda)、ZnO (99.99%, Macklin) and anhydrous ethanol (AR.) were used as raw materials. Oxide raw materials were accurately weighed according to chemical stoichiometric ratio, and then filled into Nylon ball grinding tanks with appropriate amount of anhydrous ethanol and zirconia grinding

Characterization

The powder X-ray diffractive (XRD) patterns were carried out on a D8 Advance (Bruker, Germany) X-ray power diffraction system using nickel filtered Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 10–80°, operating at 40 kV and 40 mA. The SEM imaging was carried out by a field-emission scanning electron microscope (JSM-6700F,JEOL Inc. Japan). The X-ray photoelectron spectroscopy (XPS) was performed using an X-ray photoelectron spectrometer (Thermo Scientific ESCALAB 250X). The absorption

Phase and morphology

Fig. 1(a) shows XRD patterns of powder In2O3:Yb3+/Er3+ doped with various concentration of Zn2+. All the peaks can be exactly indexed with crystalline In2O3 phase (JCPDS, card No.06–0416) and no other impurity peaks were detected, indicating that pure phase of In2O3 was maintained by doping with Yb3+/Er3+ and Zn2+ ions. While, the doping of Zn2+ had somewhat effect on diffraction peaks, as shown in Fig. 1(b), where the enlarged region exhibits that the main diffraction peak of the (222)

Conclusions

The up-conversion phosphors In2O3:5 at% Yb3+/0.5 at% Er3+ with x wt% Zn2+ ions were fabricated by conventional solid-state reaction method. XPS and XRD results showed that Zn2+ ions were introduced into the host lattice through occupying the interstitial sites and there was no other heterophases produced. SEM results exhibited that crystallinity of In2O3:Yb3+/Er3+ was improved by Zn2+ ions. The introduction of Zn2+ ions can decrease the site symmetry of the position for luminescence center and

CRediT authorship contribution statement

Yuzhen Wang: Investigation, Writing - original draft. Zicheng Wen: Methodology, Validation. Wanggui Ye: Methodology, Validation. Chong Zhao: Methodology, Validation. Chuandong Zuo: Resources. Yanbin Li: Resources. Zhiquan Cao: Visualization, Data curation. Zhijun Cao: Visualization, Data curation. Chaoyang Ma: Formal analysis, Supervision, Writing - review & editing. Yongge Cao: Conceptualization, Funding acquisition.

Declaration of competing interest

None.

Acknowledgements

This work is supported by the National Key R&D Program of China (2017YFB0403200).

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