Up- and Downconversion Luminescence in Ho3+,Yb3+-Co-Doped Y2O3 Transparent Ceramics Prepared by Spark Plasma Sintering

Park, Cheol Woo; In, Jun Hyeong; Park, Jae Hwa; Shim, Kwang Bo; Ryu, Jeong Ho

doi:2019.57.9.609

Korean Journal of Metals and Materials > Volume 57(9); 2019 > Article

Park, In, Park, Shim, and Ryu: Up- and Downconversion Luminescence in Ho3+,Yb3+-Co-Doped Y2O3 Transparent Ceramics Prepared by Spark Plasma Sintering

Research Paper

Korean Journal of Metals and Materials 2019; 57(9): 609-616.

Published online: August 21, 2019

DOI: http://dx.doi.org/10.3365/KJMM.2019.57.9.609

Up- and Downconversion Luminescence in Ho³⁺,Yb³⁺-Co-Doped Y₂O₃ Transparent Ceramics Prepared by Spark Plasma Sintering

Cheol Woo Park¹, Jun Hyeong In¹, Jae Hwa Park¹, Kwang Bo Shim¹, Jeong Ho Ryu^2,^*

¹Division of Advanced Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of Korea

²Department of Materials Science and Engineering, Korea National University of Transportation, Chungju, Chungbuk 27469, Republic of Korea

^*Corresponding Author: Jeong Ho Ryu [
Tel: +82-43-841-5384, E-mail: jhryu@ut.ac.kr]

- C. W. Park and J. H. Park: Researcher, J. H. In: Ph.D. Student, K. B. Shim and J. H. Ryu: Professor

Received May 3, 2019 Accepted July 10, 2019

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Cited By

Abstract

We employed spark plasma sintering (SPS) in the presence of sintering aids to fabricate high-quality Ho³⁺,Yb³⁺-co-doped Y₂O₃ transparent ceramics suitable for use as a laser gain medium, and analyzed their microstructure and optical properties. Ho³⁺,Yb³⁺-co-doped Y₂O₃ transparent ceramics with different contents of Yb³⁺ were fabricated by SPS and exhibited high transparency in both near-infrared and visible regions. All specimens showed high transparency in both the near-infrared and visible-light regions, and sample microstructure was almost unaffected by the amount of doped Yb³⁺. The transmission spectra of 1 mol% Yb³⁺ doped specimens exhibited transmissivities of 73.4 and 81.0% at wavelengths of 700 and 1500 nm, respectively. Relatively high transmissivity was quickly achieved through the addition of a sintering aid (1 mol% La₂O₃) which accelerated mass transport during sintering. The effect of Yb³⁺ doping on the upconversion photoluminescence of Y₂O₃:Ho³⁺,Yb³⁺ was examined. Irradiation at 980 nm resulted in strong green photoluminescence (552 nm) and weak red photoluminescence (669 nm). Excitation at 447 and 980 nm resulted in strong green emission (⁵F₄, ⁵S₂ → ⁵I₈) from Ho³⁺. The effects of Yb³⁺ content and laser input power on emission intensity and luminescence decay were investigated in detail. Under excitation at 980 nm, the presence of Yb³⁺ increased emission intensity over the entire range, while the opposite behavior was observed under excitation at 447 nm.

Key words: Y₂O₃, transparent ceramics, spark plasma sintering, upconversion, downconversion

1. INTRODUCTION

Transparent polycrystalline ceramics have recently emerged as advanced structural and functional materials of high importance [1], because such ceramics can often be excited by laser diode pumping systems, making them suitable for numerous applications in diverse areas of industry [2]. Non-linear optics–based high-power lasers emitting in the infrared range of 2–5 μm usually contain Ho³⁺-doped transparent ceramics as a gain medium. A great deal of research is currently focused on fabricating lasers based on Ho³⁺-doped ceramics for applications such as atmospheric pollutant monitoring, remote sensing, lidar, and medical surgery [3-5].

The luminescence efficiency of solid-state lasers is closely related to the fluorescence characteristics of their gain media. Based on changes in the incident and released photon wavelengths, that fluorescence is classified as either downconversion or upconversion. In the former case, two or more low-energy photons are produced from one high-energy photon [6], while in the latter case, low-energy photons are converted into a high-energy photon in an anti-Stokes process [7,8]. Among the various ions, Ho³⁺ show the highest upconversion photoluminescence efficiency due to their long fluorescence lifetime and the large region of stimulated luminescence [9]. Yb³⁺ ions are very easily excited by 980 nm radiation, and the energy of this radiation can be easily transferred to other ions [10]. This implies that the upconversion efficiency of Ho³⁺ can be significantly increased in the presence of co-doped Yb³⁺ [11].

The luminescence efficiency of fluorescent substances is greatly affected by the phonon energy of the host matrix, since the extent of non-radiative energy loss, and hence, that of quenching, decreases with decreasing phonon energy. Consequently, Y₂O₃ is viewed as a promising material for upconversion lasers in view of its high thermal conductivity, ideal physical/chemical stability, suitability for doping with trivalent rare-earth-element ions, high transparency in a broad spectral range (infrared to visible-light regions), and relatively low phonon energy (430–550 cm⁻¹) [12].

In general, most gain media used in solid-state lasers are of the single-crystal type [13]. However, the high melting point (~2430 °C, close to the melting point of Ir crucibles (2450 °C)) of Y₂O₃ and the occurrence of a cubic-to-hexagonal Y₂O₃ phase transition at 2350 °C complicate the fabrication of single crystals with high optical quality by conventional techniques such as the Czochralski method [14]. On the other hand, research has recently been directed at using Y₂O₃ transparent ceramics as laser media, as these materials exhibit the advantages of simple fabrication, low cost, uniform dopant concentration, and excellent mechanical properties [15].

Y₂O₃ transparent ceramics are fabricated using sintering methods such as hot pressing [16], hot isostatic pressing [17,18], and pressureless sintering [19-21]. In particular, spark plasma sintering (SPS) has the benefits of rapid temperature increase up to 2000 °C, accelerating the formation of necks between powder grains, and crystal grain surface purification, thermal diffusion, and electrotransport effects. This allows the process to achieve complete sintering in a very short time and at a low temperature [22-27].

Transparent ceramics are commonly manufactured using sintering aids, which govern the microstructure of these ceramics and can be used to increase their optical quality and promote densification. For example, the use of La₂O₃ in the sintering of Y₂O₃ ceramics results in improved transmissivity, as the former oxide induces grain growth and promotes densification by increasing grain boundary mobility [28] and thereby facilitates the annihilation of oxygen vacancies between grain boundaries [29].

In this study, we employed SPS in the presence of sintering aids to fabricate high-quality Ho³⁺,Yb³⁺-co-doped Y₂O₃ transparent ceramics suitable for use as a laser gain medium and analyzed their microstructure and optical properties. In particular, we confirmed the effect of Yb³⁺ content on photoluminescence intensity, excitation spectrum intensity, and fluorescence lifetime under 447 nm excitation. We further studied the photoluminescence intensity and the related emission mechanism at 545 and 655 nm under 980 nm excitation, and evaluated the effects of incident light intensity on output fluorescence intensity.

2. EXPERIMENTAL PROCEDURE

2.1. Sample preparation

High-purity Y₂O₃ (99.99%; Cenotec, Korea), La₂O₃ (99.9%; Kojundo, Japan), Ho2O3 (99.9%; Rare Metallic Co., Ltd., Japan), and Yb₂O₃ (99.9%; Shin-Etsu Chemical, Japan) powders were used. A mixture of La₂O₃ (1 mol%), Ho2O3 (1 mol%), Yb₂O₃ (0–3 mol%), and Y₂O₃ (balance) powders was ball-milled for 24 h with 1.5-mm-diameter zirconia balls at a 1:10 weight ratio and subsequently dried at 60 °C in an electric oven for 24 h. The dried powder was placed into a graphite foil–wrapped graphite die (internal diameter = 15 mm) and sintered at a temperature of 1625 °C, a vacuum of 10 Pa, and a uniaxial pressure of 30 MPa (Fig 1) using an S-515S SPS system (Sumitomo Coal Mining, Japan).

Three sintering processes were employed. Under process (I), the sample was heated from room temperature to the final sintering temperature (1625 °C) at a heating rate of 100 °C min⁻¹ and then held at this temperature for 15 min; under process (II), pressure was maintained at 30 MPa from the beginning. Under process (III), after sintering was complete, pressure was released at 1100 °C, and the electric current was blocked.

The sintered specimens (diameter = 15 mm, thickness = 1 mm) were annealed for 4 h under an oxygen atmosphere in a box furnace at 1000 °C using a heating rate of 10 °C min⁻¹. For microstructure and optical characteristics evaluation, both specimen sides were subjected to surface-grinding and then polished with 1 μm diamond paste.

2.2. Characterization

Crystal phase composition was determined using X-ray diffraction (XRD; Rigaku-denki, D/MAX-2500, Japan; Cu K_α, 40 kV, 30 mA) analysis in a 2θ range of 20–80°. The average grain size was determined by field-emission scanning electron microscopy (FE-SEM; NOVA Nano SEM 450, FEI, Czech Republic) at acceleration voltages of 5–15 kV. In-line transmittance and absorption were measured in a wavelength range of 300–2000 nm using a UV-Vis-NIR spectrophotometer (Lambda 950, Perkin-Elmer, USA). Irradiation at 980 nm was performed at room temperature employing an MDE981H8 laser (Latech Co., Korea), and the resulting upconversion emission was recorded in a wavelength range of 300–800 nm using a fluorescence spectrometer (LS55, Perkin-Elmer, USA). For fluorescence lifetime measurement, samples were irradiated at 445 nm and room temperature using a time-correlated single-photon counting spectrofluorometer (iHR320, Horiba, Japan) and a Xe lamp, and the resulting downconversion emission was measured.

3. RESULTS AND DISCUSSION

The XRD patterns of vacuum-sintered (1625 °C, 30 MPa, 15 min) Y₂O₃:Ho³⁺,Yb³⁺ specimens with different Yb³⁺ contents (Fig 2(a)) showed the presence of standard cubic Y₂O₃ (JCPDS Card No 41-1105) in all cases, with no other impurities were detected. La₂O₃ was found to be completely dissolved in Y₂O₃, and Ho³⁺ (1 mol%) and Yb³⁺ (0–3 mol%) matched perfectly with the space group Ia3¯. Examination of the (222) peak region (Fig 2(b)) confirmed that this peak shifted to higher angles with increasing content of Yb³⁺, which was ascribed to the concomitant shrinkage of the Y₂O₃ unit cell. In turn, this shrinkage was attributed to the fact that the radius of Yb³⁺ (0.868 Å [30]) is smaller than those of Ho³⁺ (0.901 Å [30]) and Y³⁺ (0.900 Å [30]).

FE-SEM imaging of the etched surface of the Y₂O₃:Ho³⁺,Yb³⁺ ceramics (Fig 3) confirmed that all specimens exhibited a relatively fine and uniform microstructure without abnormal growth. The average grain size (~13 μm) was found to be independent of Yb³⁺ content. This finding confirmed that the content of Yb³⁺ does not have a large effect on the sinterability of Y₂O₃, since Yb₂O₃ and Y₂O₃ have similar crystal structures, and the corresponding selfdiffusion coefficients are of the same order [31].

Figure 4 shows a photograph of the 1-mm-thick Y₂O₃:Ho³⁺,Yb³⁺ specimens, both sides of which were mirrorfinished after vacuum sintering (1625 °C, 30 MPa, 15 min) and annealing (1000 °C, 4 h, air). All of the specimens were transparent enough to allow one to read text placed behind them.

Figure 5(a) shows the transmission spectra of all samples, while Fig 5(b) shows the absorption spectrum of the 1 mol% Yb³⁺ specimen, revealing that this specimen exhibited transmissivities of 73.4 and 81.0% at wavelengths of 700 and 1500 nm, respectively. Thus, relatively high transmissivity was quickly achieved through the addition of a sintering aid (1 mol% La₂O₃) which accelerated mass transport during sintering. The absorption spectrum of the 1 mol% Yb³⁺ specimen featured peaks at 450, 540, 650, and 980 nm, which were ascribed to the ⁵I₈ → ⁵G₆, ⁵S₂, F₅ transitions of Ho³⁺ and the ²F_7/2 → ²F_5/2 transition of Yb³⁺, respectively [32]. Importantly, transmissivity at 980 nm decreased with increasing Yb³⁺ content of, which indicates that this ion absorbed radiation at 980 nm and transferred its energy to a different ion.

Figure 6(a) shows the effect of Yb³⁺ content on down-conversion photoluminescence intensity. Irradiation at 447 nm resulted in strong green photoluminescence (540 and 555 nm) and weak red photoluminescence (615 nm), with the former corresponding to the ⁵F₄, ⁵S₂ → ⁵I₈ transitions of Ho³⁺, respectively [33]. With the introduction of Yb³⁺, photoluminescence intensity decreased in all regions. This behavior was attributed to the fact that the ²F_5/2 level of Yb³⁺ is positioned between the ⁵I₅ and ⁵I₆ levels of Ho³⁺, which precludes the immediate relaxation of the latter ions to the ground state and results in cross-relaxation at the level of Yb³⁺, and, hence, in a decrease of Ho³⁺ emission energy [34].

Figure 6(b) shows the effect of Yb³⁺ concentration on the excitation spectrum for the 550 nm emission and reveals the presence of numerous transitions between 330 and 500 nm, e.g., ⁵I₈ → ³H₅, ⁵G₂; ⁵I₈ → ⁵G₄, ³K₇; ⁵I₈ → ⁵G₅; ⁵I₈ → ⁵G₆; ⁵I₈ → ⁵F₂, ³K₈; ⁵I₈ → ⁵F₃. Notably, the intensity of all signals decreased with increasing Yb³⁺ content, which was attributed to a concomitant increase in cross-relaxation occurrence frequency. In turn, this frequency increase resulted in the depopulation of ⁵F₄/⁵S₂ and ⁵F₅ levels and hence hindered downconversion photoluminescence at 550 nm [35].

Figure 7 shows the effect of Yb³⁺ content on fluorescence decay curves. These curves were fitted using a linear exponential function, and decay times of ~140, 125, 110, and 100 μs were obtained for Yb³⁺ contents of 0, 1, 2, and 3 mol%, respectively. As a result, the decay time decreased with increasing Yb³⁺ content, an effect ascribed to the provision of a new decay pathway by the Yb³⁺ ions, i.e., these ions facilitated the relaxation of Ho³⁺ [36].

Figure 8 shows the effect of Yb³⁺ content on the upconversion photoluminescence of Y₂O₃:Ho³⁺,Yb³⁺. Irradiation of the above samples at 980 nm resulted in strong green photoluminescence (552 nm) and weak red photoluminescence (669 nm). The former photoluminescence corresponds to the ⁵F₄, ⁵S₂ → ⁵I₈ transitions of Ho³⁺, while the latter is ascribed to the ⁵F₅ → ⁵I₈ transition of Ho³⁺ [37]. This behavior reflected the occurrence of an upconversion process, in which Ho³⁺ absorbs low-energy photons at 980 nm and emits higher-energy photons at 552 and 669 nm [32]. Doping with Yb³⁺ significantly increased photoluminescence intensity in all regions, with the largest increase observed for 2 mol% Yb³⁺. Notably, photoluminescence intensity decreased when going from 2 mol% to 3 mol% Yb³⁺. This is attributed to the enhancement of non-radiative transitions caused by interactions between Yb³⁺ ions at high Yb content [38].

Figure 9 shows that, like the upconversion photoluminescence intensity, green and red emission intensities increased with increasing near-infrared excitation power. Previously, upconversion photoluminescence intensity was found to be proportional to the n-th power of pump power. Here, the dependence of photoluminescence intensity (I_Intensity) on excitation intensity (I_pump) was modeled as I_Intensity ∝ I_pumpⁿ, where n is a constant [39]. The slopes (i.e., values of n) of each trend line in Fig 9 are summarized in Table 1.

Figure 10 shows the mechanism of fluorescence upon the excitation of Ho³⁺ and Yb³⁺ ions at 447 or 980 nm. In the case of downconversion (excitation at 447 nm), ground-state (⁵I₈) electrons are excited to the ⁵G₆ level via the ground state absorption (GSA) process [11], which is followed by slow transitions to lower-lying levels (e.g., ⁵F₂/³K₈, ⁵F₃, ⁵S₂, and ⁵F₅) through several stages of non-radiative transfer. Eventually, the ground state is re-populated, and the initially absorbed energy is released [33]. The presence of Yb³⁺ ions facilitates cross-relaxation which is characterized by the ⁵F₃ → ⁵I₅ and ⁵F₅ → ⁵I₇ transitions of Ho³⁺ [34].

With increasing Yb³⁺ content, the fluorescence induced by 447-nm excitation disperses to various wavelengths, which results in a decrease in fluorescence intensity in the visiblelight region as shown in Fig 6. Under 447 nm excitation, the interception of energy from the Yb³⁺ ions by the Ho³⁺ ions is more dominant than transition from the Ho³⁺ ions. This makes the Ho³⁺ ions energy levels less populated, resulting in decreased emission intensity [33,34].

In the case of upconversion (excitation at 980 nm), Ho³⁺ ions cannot directly absorb the incident radiation [39]. However, since these ions are embedded into the Y₂O₃ host matrix with a phonon frequency of 500 cm⁻¹, the energy difference between incident radiation and the ⁵I₆ level of Ho³⁺ can be bridged by one or two phonons, which allows the ground-state electrons of Ho³⁺ to be excited to the ⁵I₆ level and then to higher-lying ⁵S₂ and ⁵F₄ levels (via energy reabsorption through the excited state absorption (ESA) process) [37].

On the other hand, Yb³⁺ ions can directly absorb 980-nm radiation by undergoing a ²F^7/2 (ground state) → ²F_5/2 transition. Subsequent relaxation to the ground state results in a release in energy and induces transitions to the ⁵I₅ and ⁵I₆ levels (via ESA) as well as to the ⁵S₂ and ⁵F₄ levels (via GSA) of Ho³⁺ [36]. As shown in Fig 2(b), an XRD peak shift can be observed with increased Yb³⁺ doping concentration. The lattice distortion caused by the difference in the ionic radii of Y³⁺ ad Yb³⁺ decreases lattice constants and cell volumes, leading to slight structure distortion and decreasing distances between the Yb³⁺ and Ho³⁺ ions. As a consequence, this makes it easier to transfer energy among ions, increasing the intensity of emission under 980 nm excitation. Yb³⁺ ions can also receive energy via inverse energy transfer, which can be explained with the same theory [37-39]. Consequently, the intensity of green photoluminescence increases with increasing Yb³⁺ content as shown in Fig 8.

4. CONCLUSION

Highly transparent Y₂O₃:Ho³⁺,Yb³⁺ ceramics were prepared by SPS (30 MPa, 15 min) using La₂O₃ as a sintering aid. All specimens showed high transparency in both the near-infrared and visible-light regions, and sample microstructure was almost unaffected by the amount of doped Yb³⁺. In the downconversion, strong green photoluminescence (540 and 555 nm) and weak red photoluminescence (615 nm) were observed, and photoluminescence intensity decreased in all regions with increasing Yb³⁺ content. In the case of upconversion, strong green photoluminescence (545 nm) and weak red photoluminescence (655 nm) were observed. Upon the introduction of Yb³⁺, photoluminescence intensity greatly increased in every region, with the highest increase observed for 2 mol% Yb³⁺.

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2016R1D1A3B03933765).

Fig. 1.

Time dependence of the temperature and applied pressure during SPS process for Y₂O₃:Ho³⁺, Yb³⁺ with La 1 mol% powders.

Fig. 2.

X-ray diffraction patterns of Y₂O₃:Ho³⁺, Yb³⁺ transparent ceramics doped with various Yb³⁺ concentration of 0 mol%, 1 mol%, 2 mol%, and 3 mol in (a) 20 - 60° and (b) 29.0 - 29.5° range.

Fig. 3.

SEM micrographs of the Y₂O₃:Ho³⁺, Yb³⁺ transparent ceramics doped with various Yb³⁺ concentration of (a) 0 mol%, (b) 1 mol%, (c) 2 mol%, and (d) 3 mol%

Fig. 4.

Photographs of transparent Y₂O₃:Ho³⁺, Yb³⁺ ceramics specimen doped with various Yb³⁺, concentration of (a) 0 mol%, (b) 1 mol%, (c) 2 mol%, and (d) 3 mol% (1 mm thick).

Fig. 5.

(a) Transmission spectra of Y₂O₃:Ho³⁺, Yb³⁺ transparent ceramics doped with various Yb³⁺ concentration and (b) absorption spectrum of Y₂O₃:Ho³⁺, Yb³⁺ transparent ceramics with 1 mol% Yb³⁺ doping concentration.

Fig. 6.

(a) Downconversion photoluminescence spectra of Y₂O₃:Ho³⁺, Yb³⁺ transparent ceramics excited under 447 nm at room temperature, doped with various Yb³⁺ concentration of 0 mol%, 1 mol%, 2 mol%, and 3 mol%. (b) Photoluminescence excitation spectra of Y₂O₃:Ho³⁺, Yb³⁺ transparent ceramics about 550 nm emission at room temperature.

Fig. 7.

Emission decay curves of the Ho³⁺ 550 nm emission under 447 nm excitation.

Fig. 8.

Upconversion photoluminescence spectra of Y₂O₃:Ho³⁺, Yb³⁺ transparent ceramics excited under 980 nm at room temperature, doped with various Yb³⁺ concentration of 0 mol%, 1 mol%, 2 mol%, and 3 mol% in (a) 500 - 600 nm and (b) 600 - 700 nm range.

Fig. 9.

Plots of logarithm of I_Intensity versus I_pump in (a) 550 nm and (b) 650 nm of Y₂O₃:Ho³⁺, Yb³⁺ transparent ceramics.

Fig. 10.

Schematic energy level diagrams of Yb³⁺, Ho³⁺ in Y₂O₃ host matrix and the fluorescence emission mechanism of Y₂O₃:Ho³⁺, Yb³⁺ transparent ceramics under 447 and 980 nm excitation.

Table 1.

Slope of lines fitted to plots of 1 mol% Ho³⁺/x mol% Yb³⁺:Y₂O₃ (x = 0, 1, 2, 3) transparent ceramics.

Yb³⁺ concentration	0 mol%	1 mol%	2 mol%	3 mol%
550 nm	1.631	1.634	1.563	1.724
650 nm	0.714	1.803	1.961	1.866

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