Y₂BaZnO₅:Er³⁺ microcapsules with enhanced upconversion by vanadium ion codoping

Abstract

Microcapsules of Y₂BaZnO₅:Er³⁺ nanocrystals were successfully prepared by a simple hydrothermal method using citric acid as chelating agent. The sizes of the spherical microcapsules varied between 1–5 μm with a wall thickness of about 50–100 nm. The underlying microcapsule formation mechanism was discussed using the gas-bubble-assisted Ostwald ripening process. The Y₂BaZnO₅:Er³⁺ microcapsules exhibited characteristic upconversion (UC) emissions of Er³⁺ with intense yellowish orange color under dual laser pumping at 808 nm as well as 980 nm. Concentration dependent studies revealed that the optimum Er³⁺ concentration for intense UC at 550 nm and 660 nm was 5 mol% and 7 mol%, respectively. In addition, the optimum Er concentration differed from the excitation wavelength indicating a different UC mechanism. Importantly, the optimum amount of the V⁵⁺ co-doping into the Y₂BaZnO₅:Er³⁺ phosphors intensified the UC intensity by more than 3 fold. The Y₂BaZnO₅:Er³⁺,V⁵⁺ phosphor exhibited intense yellowish UC pumped by an 808 nm laser and will have potential applications for in vivo biological labelling, and biomedical imaging and therapies.

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1. Introduction

Upconversion is a process in which two or more low-energy photons are converted to a single high-energy photon. Various processes can lead to upconversion, including two/three/multiple-photon absorptions, ground state followed by excited state absorption, energy transfer upconversion, second harmonic generation, and photon avalanche.^1,2 Since its discovery in the 1960s, upconversion (UC) has been the focus of many researchers.^2–5 These UC materials with strong luminescence in the NIR to visible to near UV region have been extensively investigated in recent years due to their potential applications in solar cells,³ novel 3D display technologies,^2,6,7 inks for secure printing,⁸ optical communication and amplifiers,^9,10 high density optical storage,² infrared detection and sensors,¹¹ and more recently biomedical labelling, imaging and therapies.^4,12,13

Lanthanide ions are often employed as UC luminescence centres due to the abundant energy states of 4f configurations in Ln³⁺ ions and hence the UC emission proceed via various excited state absorption (ESA) and energy transfer (ET) pathways. Among the various lanthanide ions, Er³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Pr³⁺ were studied for UC luminescence.^9,13–17 However, there are very limited f–f transitions from ground state of these RE³⁺ ions whose energy matches the energy of the excited photons, thus the ground state absorption (GSA) of these ions is extremely poor. The Yb³⁺ is the most commonly used sensitizer in UC luminescence to harvest pump photons and subsequently promotes neighbouring activator ions (like Er³⁺) to the excited states because of its wider absorption cross-section around 980 nm, simple two-level structure ensuring its very high quenching concentration and large spectral overlap (Yb³⁺; ²F_5/2 to ²F_7/2) with Er³⁺ absorption (⁵I₈ to ⁵I₆) which ensures efficient energy transfer from Yb³⁺ to Er³⁺ and other RE³⁺ ions.^4,6,9 Thus, UC luminescence of RE³⁺ in RE³⁺/Yb³⁺ co-doped systems have been extensively studied, and predominant green to red UC emissions have been obtained, especially in low phonon energy hosts, such as fluoride,^18,19 garnets,^9,16 sulfide and oxy-sulfide^16,20 and glasses.^9,19

For bio-imaging and therapies, nanoparticles with high brightness with continuous emission are desired. In addition, they should be resistant to photo-bleaching. Lots of effort has been made to utilize NaYF₄:Er³⁺ for bioimaging and therapies.^2,4,12,21,22 However, they suffer severely due to high toxicity, low sensitivity, and low penetration depth of 980 nm laser in the body tissue, since the excellent optical window of the body tissue is 700–900 nm.²³ Importantly, present upconversion nanoparticles (UCNP) suffer badly due to their low efficiency.²⁰ In UC based techniques, detection sensitivity depends on the luminescence intensity of the phosphor. Recently, several attempts in case of UCNPs have been made to increase the luminescence intensity, such as codoping with sensitizer ions,^9,14,24 crystal surface coating,^2,25 core/shell synthesis,²⁶ etc., but most of them are not enough to improve the UC intensity to desire level (should be pumped by as low as 1 mW laser power to protect human tissue from burning by laser heat). Hence, the luminescence intensity enhancement of UCNPs is still a challenging topic.

Compared to the huge number of studies on the NaYF₄:Yb,Er nanocrystals, very small effort has been paid on the work of oxide based UC phosphors.^{9–11,13–15,17,19,20,24,27} Moreover, the study of 808 nm excited visible UC luminescence has not been in focus despite most suitable spectral range for the bioimaging and therapies due to best match of optical window of body tissue (700–900 nm). We have previously reported efficient oxide based NIR to NIR upconversion phosphor (CaMoO₄:Tm,Yb,K) pumped by 980 nm laser.²⁸ However, the 980 nm laser has low penetration depth compared to the 808 nm laser. Thus, in this paper we present efficient dual NIR laser (808 nm and 980 nm) pumped VIS UC emission in an oxide based sub micrometer phosphor, Y₂BaZnO₅:Er³⁺. Further, V⁵⁺ co-dopant has been identified first time for the enhancement of the UC intensity of Er³⁺ ions. Luminescence and absorbance measurements were reported for a series of Er³⁺-doped and Er³⁺, V⁵⁺ co-doped nanocrystalline microcapsules to achieve the highest UC intensity and to provide insight in the mechanism responsible for the UC luminescence.

2. Experimental

2.1 Sample preparation

Y₂BaZnO₅ phosphor doped with Er³⁺ and/or V⁵⁺/Yb³⁺ ions were prepared by a simple hydrothermal (HT) process. In a typical hydrothermal reaction, 10 mM mixture of metal ions viz. Y, Ba, Zn, RE and/or Yb, V (Y(NO₃)₃·6H₂O, Ba(NO₃)₂, Zn(NO₃)₃·6H₂O, Er(NO₃)₃·5H₂O, Yb(NO₃)₃·5H₂O, NH₄VO₃) were prepared in a beaker with distilled water by constant stirring with magnetic stirrer. Citric acid was used as chelating agent (total metal ions to citric acid ratio was fixed to 4 : 1) and urea as precipitating agent (total metal ions to urea was fixed to 1 : 40). The pH of the resulting solution was adjusted by aqueous ammonia/nitric acid. 30 ml of the mixture was poured into a 50 ml Teflon lined stainless steel autoclave and kept at 120 °C for 12 hours. The white solid obtained was centrifuged, washed multiple times with distilled water followed by ethanol and freeze dried for 12 h. Each sample was further calcined at 700–1200 °C for 3 h to get the final highly crystalline Y₂BaZnO₅:Er³⁺ phosphor.

2.2 Characterizations

Phase identification was carried out using a Shimadzu XRD-6300 instrument with Cu Kα radiation. The morphology of the phosphor particles were characterized by scanning electron microscopy (Hitachi-S3000N). Elemental analysis and mass percentage of the constituent ions were estimated using EDX coupled with SEM. Before SEM measurements, each sample was coated roughly 5 nm in thickness with platinum–palladium using Hitachi E-1030 ion sputter. Precise elemental analysis for the powder sample was carried out using Rayny EDX-800HS energy dispersive X-ray fluorescence spectrometer. The upconversion (UC) emission spectra, pumped by 808 nm and/or 980 nm cw laser (≤1–200 mW output power) were recorded using USB 4000 UV-VIS-NIR miniature fiber optic spectrometer (Ocean optics). Absorbance spectra were recorded using Shimadzu 6545 UV-VIS-IR spectrometer using integration sphere mounting the sample in quartz slides. All measurements were carried out at room temperature unless specified.

3. Results and discussion

Fig. 1(a) presents the phase analysis of Y₂BaZnO₅ doped with different moles of Er³⁺ ions. The XRD patterns of all the samples agree very well with the Y₂BaZnO₅ standard JCPDS pattern (52-1688) and no distinguished secondary phases were identified. The Y₂BaZnO₅ consists of orthorhombic crystal structure with Pnmb space group as depicted in Fig. 1(b). Further, isotropic substitution by Er³⁺ (and Yb³⁺) in to the Y³⁺ sites did not altered the Y₂BaZnO₅ phase. Cell parameters obtained from the XRD data of the typical sample (3 mol% Er doped Y₂BaZnO₅) were; a = 7.068 Å, b = 5.705 Å and c = 12.334 Å, slightly smaller than the reported data of pure host [JCPDS: 52-1688], which is attributed to the slightly smaller ionic radii of the Er³⁺ (0.100 nm) compared to the host ion Y³⁺ (0.102 nm).²⁹ Very similar XRD patterns without distinguished impurity was observed by co-doping with various Yb³⁺ moles as shown in ESI.†

Fig. 1 (a) XRD patterns of various moles of Er³⁺ substituted Y₂BaZnO₅:Er³⁺ phosphors and (b) crystal structure created by VESTA program.

Fig. 2 reveals the morphology of various Y₂BaZnO₅:0.03Er phosphor particles as prepared hydrothermally at 120 °C for 12 h as well as heat treated at 1000 °C. The SEM images revealed that almost mono-dispersed microcapsules of size between 1 to 5 μm were obtained. A closer look of the SEM images as shown in the Fig. 2(b) and (c) suggested that the microcapsules are really hollow with wall thickness ranging between 50–200 nm (see ESI Fig. S1 for detail†). The wall of the micro-capsules consisted of nanoparticles of size between 30–100 nm. Further, some non-oriented nanograins were observed whose particles dimensions varied between some tens of nanometers up to 200–300 nm as shown in Fig. 2(d). It is important to state that the microcapsules were stable up to relatively high temperatures, ∼1000 °C as shown in Fig. 2(e) and the ESI Fig. S2.† During the thermal treatment process, the size of the microcapsules slightly reduced, but the capsular shape remained intact. However, further increase of sintering temperatures above 1000 °C destroyed the microcapsules and formed agglomerated nanoparticles as shown in Fig. S2.† The EDX spectrum of the selected surface of microcapsules (Fig. 2(f)) proved that the microcapsules really consisted of Y₂BaZnO₅ phase. Further, precise compositional analyses were carried out using X-ray fluorescence spectroscopy (see ESI S4† for the mass composition). The elemental compositions confirmed that the doped ions, especially Er, Yb and V were really incorporated in to the Y₂BaZnO₅ matrix.

Fig. 2 SEM images of Y₂BaZnO₅:0.03Er hollow spheres as prepared hydrothermally at 120 °C for 12 h under different magnifications (a–d), sample (a) heat treated at 1000 °C for 3 h (e), and EDX spectra of selected area (f).

Under, these experimental results, the formation mechanism of those microcapsules can be outlined by gas-bubble-assisted Ostwald ripening through self templated, self-assembly process as shown in the scheme of Fig. 3.³⁰ First, citric acid dissolved in the solution and citrate anions were formed. Then the metal ions get complexes with the citrate molecules and controlled precipitation through hydrolysis forms crystal nuclei or nanograins. As the reaction progress, the nanograins have tendency to aggregate to minimize the total surface energy. Thus, at high temperature hydrothermal treatment process, nanograins begin to aggregate via an Ostwald ripening process and form the aggregated structures.³¹ During the hydrothermal treatment at high pressure, these aggregates roll over smoothly and form spherical balls. During the progress of the HT treatment time, more nanograins self-assemble on to the surface of the sphere to minimize the total surface energy and bigger spheres are expected. Here, each sphere acts as a template for nanograins formed from the same sphere itself. This Ostwald ripening and gas-bubble-assisted-assembly process going on, more and more nanograins attached on the surface of the sphere and bigger balls are formed. The spherical, somehow solid or semi solid spheres as depicted in the ESI S3(b) and (b-1)† can be well explained by above mention mechanism. However, if excess urea (metal ions/urea = 1/40) was used, microcapsules with hollow architectures were formed as depicted in Fig. 2 and ESI S3(a) and (a-1).† As mentioned by Liu et al.,³⁰ when excess urea is used during the HT treatment process, ammonia/N₂ gas-bubbles were continuously generated. These gas-bubbles provide aggregation centres for nanograins and gas-bubble-nanograins aggregates are formed as shown in scheme in Fig. 3. Here, the citrate ions acts as surfactants and inhibits the hydrolysis of the metal cations used in the reactions (here, Zn²⁺, Ba²⁺, Y³⁺, RE³⁺) and facilitates the slow growth of the nanoparticles, otherwise nanograins agglomerates rapidly and forms irregular solids. Further, citrate ions increased the viscosity of the HT solution. At viscous solution, stabilized gas-bubbles form regular hollow cores. Further, slow precipitation and movement rate of nanograins in viscous solution allow enough time to form regular spheres. This Ostwald ripening and gas-bubble-assisted-assembly process going on, more and more nanograins attached on the surface of the sphere and bigger balls with thicker wall are formed as shown in the ESI Fig. S3(a) and (a-1).† A similar Ostwald self-assembled, self-templated process of CeO₂, ZnO and Al₂O₃ spheres formation has been reported in gas-bubble free conditions.^31–34 The size of the microcapsules, void size are expected to change according to the air-bubble size whereas the wall thickness are expected to control according to the HT reaction time and concentration of the citrate anions (rate of metal ions hydrolysis). The core size of the microcapsules varied remarkably in our results due to formation of different sizes air-bubbles. The air-bubble size could be control according to the citrate concentration (viscosity of the solution) and rate of bubbles formation (concentration of urea) and underlined phenomena are being carried out and expected to publish elsewhere in near future.

Fig. 3 Schematic illustration of the formation mechanism of Y₂BaZnO₅:Er microcapsules by gas-bubble-assisted Ostwald ripening.

The UC emission phenomena of the Er³⁺ and Er³⁺ codoped with Yb³⁺ were studied by 808 nm as well as 980 nm laser excitation at room temperature and results are presented in Fig. 4. Y₂BaZnO₅:Er phosphors exhibited green emission centered around 520–560 nm corresponding to the (²H_11/2, ⁴S_3/2 → ⁴I_15/2) electronic transition and comparatively stronger orange emission band centered around 650 nm corresponding to the (⁴I_9/2 → ⁴I_15/2) electronic transition of Er³⁺ ion. With increasing Er concentration, both the 808 nm pumped as well as 980 nm pumped UC intensity increased up to 7 mol% Er. However, the increased trend differ remarkably for the green and orange emission bands as well as under pumping energy. Under both 808 nm and 980 nm pumping, the orange emission peak monotonically increased with increasing Er concentration, even though the increase was much pronounced at 808 nm pumping (>40 fold increase, see inset in Fig. 4(a) and (b)). However, the increase of green peak was less pronounced under both 808 nm (∼10 fold increase, see inset in Fig. 4(a)) and 980 nm pumping (∼5 fold increase, see inset in Fig. 4(b)). Further, the green emission peak saturated as early as ∼3 mol% Er ions concentration but the orange emission peak was not saturated until 7 mol% Er. It is worth mentioning that the concentration quenching was started at lower concentration under 980 nm pumping compared to the 808 nm pumping. Obviously, 808 nm photons were absorbed by Er ions itself corresponding to (⁴I_15/2 → ⁴I_9/2) electronic transition, but 980 nm was absorbed by Yb (²F_7/2 → ²F_5/2) and energy transfer from Yb → Er generates the Er UC. It is assumed that cross relaxation from higher energy states (²H_11/2, ⁴S_3/2) to lower energy states (⁴I_9/2) of Er ions and Er (⁴I_11/2) to Er (⁴I_11/2) energy migrations were pronounced earlier under 980 nm excitation. When 808 nm laser was used, direct two step absorption followed by lattice relaxation easily populates higher energy states of Er (²H_11/2, ⁴S_3/2) and concentration quenching was observed at relatively higher Er concentrations (>7 mol% Er). However, under low energy excitation (980 nm), the phonon assisted relaxation from ⁴I_11/2 to the lower lying ⁴I_13/2 states is more pronounced, thus it is difficult to populate higher energy states (²H_11/2, ⁴S_3/2) to generate green UC. The detail UC mechanism will be discussed later.

Fig. 4 UC emission profiles of Y₂BaZnO₅:x mol% Er, 7 mol% Yb phosphors: (a) excitation at 808 nm and (b) excitation at 980 nm. Insets are the integrated intensities of green (520–570 nm) and red (640–690 nm) emissions as a function of Er ions concentration.

Fig. 5 represents the effect of Yb³⁺ ions concentration at fixed 3 mol% Er ions concentration. Under 980 nm excitation, the intensities of the green emissions band gradually increased with increasing Yb³⁺ ions concentration up to 15 mol% above that it started decreasing (Fig. 5(b)). However, the orange emission band monotonically increased even up to 20 mol%. Completely different trend was observed when 808 nm laser was used; green band decreased with increasing Yb concentration from the beginning but orange band increased first up to 10 mol% Yb and then decreased further increasing its concentration. When direct Er is excited by 808 nm laser, with increasing Yb concentration back transfer of excited photons from ⁴I_11/2 (Er) to ²F_5/2 (Yb) increases which decreases the photons population at ⁴I_11/2 (Er) states and consequently the excitation from ⁴I_11/2 (Er) to higher states ⁴F_7/2 (Er) is suppressed and decreased the green UC. On the contrary, at low Yb ions concentrations, the competition between energy transfer from ⁴I_11/2 (Er) → ²F_5/2 (Yb) to internal relaxation to lower lying states (⁴I_11/2 (Er) → ⁴I_13/2 (Er)) increases followed by upper states population (⁴I_13/2 → ⁴F_9/2) through ESU process increases the orange upconversion. But as the Yb concentration further increased beyond 10 mol%, the energy transfer from ⁴I_11/2 (Er) → ²F_5/2 (Yb) exceeds the internal relaxation leading to quenching of the Er upconversion. This concentration quenching effect could be explained by the energy transfer between the nearest activator to sensitizers and sensitizer to sensitizer internal relaxation. As the dopants concentration increased, the distance between dopants ions shrinks allowing non-radiative energy migration such as exchange, dipole or multi-pole interactions. Further, at higher rare ions concentrations, the solid solubility limits reached the maximum and a secondary phase appeared and hampered the UC emission.

Fig. 5 UC emission profiles of Y₂BaZnO₅:3 mol% Er, x mol% Yb phosphors: (a) excitation at 808 nm and (b) excitation at 980 nm laser. Insets are the integrated intensities of green (520–570 nm) and red (640–690 nm) emissions as a function of Yb ions concentration.

According to Judd–Ofelt theory, the partially forbidden intra 4f–4f transitions are enabled by crystal field coupling with the unoccupied electronic configurations of the rare earth ions in higher energies or with ligand to metal charge transfer configuration.³⁵ At low symmetry, not only the degeneracy of the free ions states removed, the transition probabilities also increased due to the overall increase in the crystal field strength. Thus, to develop highly efficient UC phosphors that utilizes the specific energy level schemes of the rare earth ions, symmetry breaking is one of the most promising way. Use of monovalent charge compensator ions (such as Li⁺, Na⁺, K⁺, Ag⁺ etc.) showed promising effect on the enhancement of emission intensity of the alkali or alkaline earth based materials.^36–39 S. Das et al. explained that the K⁺ ions addition on the CaSO₄:Dy³⁺ phosphor and enhanced the PL intensity significantly.⁴⁰ J. H. Chung et al. significantly increased the UC intensity of CaMoO₄ blue phosphor by excess Li⁺ ions addition.⁴¹ We have previously reported many fold improvement of UC intensity of blue, green and orange emissions in CaMoO₄:RE³⁺ (Tm/Er/Ho) by alkali metal ions substitution at the divalent Ca²⁺ ions sites and explained the effect by distortion of the crystal field of RE³⁺ (Tm/Er/Ho) ions due to size and charge difference.^13,39 Recently, Y. Hu et al. remarkably improved the red UC of NaYF₄:Yb,Er by Fe³⁺ and Mn²⁺ doping and explained the underlined mechanism due to crystal field and symmetry lowering around the Er ions enhancing the f–f transition probabilities.²² However, they have used very high concentration of Fe and Mn ions that significantly altered the crystal phase. Herein, we have reported for the first time, the significant improvement of the both green and orange upconversion in oxide based phosphor using the very similar crystal field breaking and symmetry lowering technique by doping very small amount of the V⁵⁺ ions into the Y₂BaZnO₅:Er phosphor.

Fig. 6(a) shows the effect of V⁵⁺ ions substitution on the green and orange emission of the Y₂BaZnO₅:3 mol% Er phosphors excited by 808 nm. Substitution of V⁵⁺ ions significantly intensified the UC emission intensities of both the green and orange emission bands upto 3 mol% V⁵⁺ ions substitution, which was more than 3 times compared to the V⁵⁺ ions unsubstituted sample as shown in the inset of Fig. 6(a). Further increase of V⁵⁺ ions above 3 mol% decreased the UC intensity. We have recorded the absorption spectra of selected samples and expressed in Fig. 6(b). There was no significant absorption band related to the V⁵⁺ ions. However, the Er³⁺ related absorption band were improved significantly compared to the V⁵⁺ ions undoped sample. It confirmed that the improvement of the UC intensity was not related to the sensitization effect of the V⁵⁺ ions like Yb³⁺ rather it is related to the change of crystal field and its symmetry around the Er³⁺ ions due to the V⁵⁺ ions substitution. As seen from the XRD in Fig. 7, the doping of V⁵⁺ ions showed significant change in the XRD peak position to higher 2θ value indicating shrinkage of lattice due to substitution of bigger Zn²⁺ (0.090 nm) sites by smaller V⁵⁺ (0.070 nm) sites. It suggests that the V⁵⁺ ions occupied the Zn sites in the host lattice. The small V⁵⁺ ions with higher charge density significantly distort the crystal field around the Er³⁺ ions in the Y₂BaZnO₅ matrix that cause severe crystal field break and disorder around Er³⁺ ions. This causes the redistribution of electron cloud around the Er³⁺ ions, as a consequence, the 4f–4f transition probabilities of Er³⁺ ions increased and significantly increases the UC emission intensities.^{7,22,35,38,42,43} However, at higher V concentrations, secondary phase was clearly observed marked by stars in the Fig. 7 and hampered the UC.

Fig. 6 UC emission profiles of Y₂BaZnO₅:3 mol% Er, x mol% V phosphors (excitation at 808 nm) and (b) absorption spectra of selected samples. Inset in (a) is the integrated intensities of green (520–570 nm) and red (640–690 nm) emissions as a function of V ions concentration.

Fig. 7 (a) XRD profiles of the Y₂BaZnO₅:3 mol% Er, x mol% V phosphors and (b) portion of XRD patterns showing peak shift with V doping concentration.

Fig. 8 shows the 442 nm excited stokes emissions of Er recorded from 500 nm to 1050 nm range. Due to sufficiently strong excitation and hence population of upper excited states various fⁿ → fⁿ transitions were observed besides certain bands observed at 808 nm and 980 nm excitation. The inset of Fig. 8 shows the normalised emission intensity (integrated over whole range viz. 540–1030 nm) as a function of V ions concentration. The same trend was found for the variation of emission intensities with V substitution as observed while pumping at 808 nm. It further implies that all the electronic transitions of Er were intensified significantly by V-doping.

Fig. 8 PL emission profiles of Y₂BaZnO₅:3 mol% Er, x mol% V phosphors excited at 442 nm. Inset is the normalised emission intensity (integrated over 540–1030 nm) as a function of V ions concentration.

Fig. 9 shows the excitation power dependence of green and orange UC emissions in the Y₂BaZnO₅:Er phosphors. It is well known that for unsaturated UC processes, the number of photons required to populate the upper lying states can be obtained by the following general equation:

I ∝ Pⁿ

where n is a number of pumping photons required to excite the emitting state, I is the UC intensity and P is the laser pumping power. The calculated result (Fig. 9) indicates that the slope is ∼1.96 for orange UC, 1.97 for green UC emissions. This result indicates that the green (²H_11/2/⁴S_3/2 → ⁴I_15/2) and orange (⁵F₅/⁵F₄/⁵S₂ → ⁵I₈) upconversion processes involves two photons upconversion processes.^4,5,15,20,44

Fig. 9 (a) Pump power dependence upconversion intensity of Y₂BaZnO₅:3 mol% Er, 7 mol% Yb phosphors by 808 nm laser excitation. Inset is the n dependent UC relationship from pump power verses UC intensity and (b) simplified energy level diagrams of Er³⁺(Yb³⁺):Y₂BaZnO₅ phosphors.

According to above results the upconversion mechanism of Y₂BaZnO₅:Er phosphor can be generalized as in Fig. 9(b). The UC mechanism under 980 nm lasers pumping was well studied and can be seen in detail at ref. 11 and 20. Under 808 nm lasers pumping, the ⁴I_9/2 state gets populated (⁴I_15/2 → ⁴I_9/2). Successive excitation (excited state absorption, ESA) of the ⁴I_9/2 state populates the ²H_9/2 state. From the ²H_9/2 state the excited photons relax nonradiatively to the ²H_11/2 and then to the ⁴S_3/2 level, from where green emission is observed (²H_11/2/⁴S_3/2 → ⁴I_15/2; green emission). At the same time, photons from the ⁴I_9/2 state relax nonradiatively to the ⁴I_11/2 state. The lifetime of ⁴I_11/2 state is comparatively long,¹⁵ the energy transfer upconversion (ETU) from the nearby Er³⁺ ions populates the ²H_11/2 and then to the ⁴S_3/2 levels through the nonradiative relaxation from higher ⁴F_3/2 and ⁴F_7/2 states and radiative transition from the ²H_11/2/⁴S_3/2 levels gives the prominent green emission (²H_11/2/⁴S_3/2 → ⁴I_15/2). The non radiative relaxation from the ⁴S_3/2 state to the low lying ⁴F_9/2 state or radiative relaxation from the (⁴F_3/2/⁴F_7/2) to the ⁴F_9/2, populates the ⁴F_9/2 state, from where transition to the ground state (⁴I_15/2) generates the red UC (⁴F_9/2 → ⁴I_15/2; red emission). Here, important point is that when Er concentration was high enough, ETU dominates, which populates only ⁴F_9/2 states as a consequence, the red UC will be prominent. It means, the quenching concentration of the red UC is comparatively larger than the green UC due to competition between ETU and ESA at higher Er concentration. It can be further supported by the indirect 980 nm pumping through Yb³⁺ ions. When 980 nm laser is used, the ²F_5/2 states of Yb³⁺ gets populated. Then non-resonant energy transfer from the ²F_5/2 (of Yb³⁺) level to the ⁴I_11/2 level of Er³⁺ populates the ⁴I_11/2 level. Nonradiative relaxation to the ⁴I_13/2 level followed by ETU populates the ⁴F_9/2 states to generate red UC. At the same time, due to long life time of the ⁴I_11/2 state, the ²H_11/2 and ⁴S_3/2 states get populated partially through nonradiative relaxation from ⁴F_7/2. Thus, excited state population of the ²H_11/2 and ⁴S_3/2 states are comparatively low than the population of the ⁴F_9/2 states while pumping at 980 nm, which is seen as strong red and weak green UC at 980 nm pumping but strong green and weak red UC at stronger (808 & 442) nm pumping.

4. Conclusion

A facile hydrothermal method was used to synthesize microcapsules of Y₂BaZnO₅:Er³⁺ upconversion phosphor. Spherical balls of 1–5 μm size and hollow architectures of about 50 nm shell thickness were fabricated by HT treatment and these microcapsules were stable even at high temperature of 1000 °C. The underlined microcapsule formation mechanism was discussed using gas-bubble-assisted Ostwald ripening process. Yellowish green UC emissions were achieved by pumping with 808 as well as 980 nm lasers. The optimum Er³⁺ doped concentration for 550 nm and 660 nm UC luminescence intensities were 5 mol% and 7 mol%, respectively. In addition, the optimum Er concentration differed from the excitation wavelength indicating different mechanism under 808 and 980 nm pumping. Importantly, optimum amount of the V⁵⁺ co-doping into the Y₂BaZnO₅:Er³⁺ phosphors intensified the UC intensity by more than 3 fold. The redistribution of electron cloud around the Er³⁺ ions by crystal field distortion and symmetry lowering of the crystal after introducing V⁵⁺ was suggested to be the origin of the UC improvement. The Y₂BaZnO₅:Er³⁺,V⁵⁺ phosphor exhibited very intense yellowish UC pumped by 808 nm laser and will have potential applications for the in vivo biological labelling, biomedical imaging and therapies.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26042f

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