(La_0.97RE_0.01Yb_0.02)₂O₂S Nanophosphors Converted from Layered Hydroxyl Sulfate and Investigation of Upconversion Photoluminescence (RE=Ho, Er)

Abstract

Phase-pure (La_0.97RE_0.01Yb_0.02)₂O₂S upconversion (UC) nanophosphors (average crystallite size ~ 45 nm; RE=Ho, Er) were annealed from their hydrothermally crystallized layered hydroxyl sulfate precursors in flowing hydrogen at 1200 °C for 1 h, with water vapor as the only exhaust. Under 978-nm laser excitation (up to 2.0 W), the Ho³⁺-doped phosphor exhibited green (medium), red (weak), and near-infrared (strong) emissions at ~ 546 (⁵F₄ → ⁵I₈), 658 (⁵F₇ → ⁵I₈), and 763 nm (⁵F₄ → ⁵I₇), respectively, and has the stable chromaticity coordinates of about (0.30, 0.66) in the visible-light region (400–700 nm). The Er³⁺-doped UC phosphor, on the other hand, showed weak green (~ 527/549 nm, ²H_11/2,⁴S_3/2 → ⁴I_15/2), weak red (~668/672 nm, ⁴F_9/2 → ⁴I_15/2), and strong near-infrared (~ 807/58 nm, ⁴I_9/2 → ⁴I_15/2) luminescence, whose emission color in the visible region drifted from yellowish-green [(0.36, 0.61)] to green [(0.32, 0.64)] with increasing excitation power. Analysis of the power-dependent UC luminescence found three- and two-photon processes for RE=Ho and Er, respectively, and the possible UC mechanisms were proposed.

Background

Upconversion (UC) phosphor is drawing considerable attention due to its unique ability to convert longer wavelength radiation into shorter wavelength fluorescence [1, 2] and is finding wide applications in the fields of solid-state lasers [3], multi-color displays [4], drug delivery [5], fluorescent biological labels [6], wavelength converters for solar cells [7], and so forth. A UC phosphor is commonly formed by doping a host lattice with a sensitizer/activator pair, where the sensitizer is usually Yb³⁺ and the activator is frequently being Ho³⁺, Er³⁺, or Tm³⁺. This is because Yb³⁺ can efficiently absorb 980-nm near-infrared laser excitation and the three types of activators have the ladder-like energy levels that are beneficial to sequential photon absorption and energy transfer [8]. The fundamentals of UC luminescence and the energy transfer in lanthanide upconversion can be found in the review articles by Auzel [9] and Dong et al. [10], respectively. Gai et al. [8] recently compiled the recent progress achieved in rare-earth micro/nanocrystals for downconversion (DC) and UC purposes, including soft chemical synthesis, luminescent properties, and biomedical applications. Wang et al. [11], on the other hand, extensively summarized in their review article the application of rare-earth ion-doped UC and DC phosphors in optical thermometry. The property of a UC phosphor is significantly affected by the type of host lattice, sensitizer/activator combination, dopant concentration, particle/crystallite morphology, crystallinity, excitation power, and the actual lattice site where the dopant ion resides [8,9,10,11,12,13]. For example, two non-equivalent Gd-activated crystallographic sites were identified in an Er³⁺-doped hexagonal Na_1.5Gd_1.5F₆ phosphor, and through time-resolved spectroscopy, it was proved that the two green emissions from the ⁴S_3/2 level of Er³⁺ separately originate from the Gd1 (540 nm) and Na2/Gd2 (550–555 nm) crystallographic sites while the 657-nm red emission from the ⁴F_9/2 level only originates from the Na2/Gd2 site [12]. A recent study of novel Er³⁺-doped transparent Sr_0.69La_0.31F_2.31 glass ceramics, on the other hand, illustrated that the spectrum split, thermal quenching ratio, population stability, and temperature sensitivity from the three thermally coupled energy levels (TCL) of ²H_11/2/⁴S_3/2, ⁴F_9/2(1)/⁴F_9/2(2), and ⁴I_9/2(1)/⁴I_9/2(2) are dependent on the pump power of 980 nm laser, and a new fitting method was developed to establish the relation between fluorescence intensity ratios and temperature [13]. Rare-earth (RE) halides (such as NaYF₄:Yb/Er) are currently the most efficient UC phosphors owing to their low phonon energies (ℏω < 400 cm⁻¹) [8, 10, 11, 14], though the toxic raw materials involved in the synthesis and the air sensitivity of many halides restrain their application and production. Another type of widely investigated UC phosphors is RE₂O₃ (such as Y₂O₃:Yb/Er), whose relatively high phonon energy (ℏω ~ 600 cm⁻¹; ~ 591 cm⁻¹ for Y₂O₃ and 612 cm⁻¹ for Lu₂O₃) [15], however, lowers the efficiency of UC luminescence due to photon-phonon coupling. From the view point of biocompatibility, Li et al. [16] synthesized Yb³⁺- and Ho³⁺-codoped fluorapatite crystals (nanorods of 16 by 286 nm) via hydrothermal reaction, and UC luminescence of Ho³⁺ at 543 and 654 nm was attained through a two-photon process under 980-nm laser excitation. The crystals also exhibited clear fluorescent cell imaging after the surfaces were grafted with hydrophilic dextran [16].

RE₂O₂S oxysulfide is an important family of compounds in the phosphor field and can be advantageous over oxide for luminescent applications. For example, the occurrence of S²⁻ → Eu³⁺ charge transfer transition in Eu³⁺-activated RE₂O₂S significantly extends the effective excitation wavelength to ~ 400 nm [17,18,19], which makes the phosphor useful as the red component in near-UV (365–410 nm)-excited white LEDs as reviewed by Ye et al. [20]. The most matured technique to synthesize RE₂O₂S is solid-state reaction, which has the advantages of high yield and convenience, but high reaction temperature, uncontrollable product morphology, and especially the employment of environmentally harmful sulfur sources are apparent shortcomings [21,22,23]. Sulfurization of RE₂O₃ by H₂S or CS₂ gas at an elevated temperature [24,25,26] is another frequently used strategy to produce RE₂O₂S. Since the methodology for controlled synthesis of RE₂O₃ is rich and well developed, RE₂O₂S with various particle morphologies has thus been produced through the sulfurization route, though the complicated procedures are less feasible for industrial production. Other techniques for RE₂O₂S synthesis may include precipitation [27], hydrothermal reaction [28], two-step solution gel polymer thermolysis [29], gelatin-templated synthesis [30], gel thermolysis [31], solvothermal pressure-relief synthesis [32], and combustion [33]. The involvement of harmful sulfur sources or by-products (such as C₂S, H₂S, and thiourea) is, however, still hard to avoid. The appearance of sulfate-type layered rare-earth hydroxide (RE₂(OH)₄SO₄∙2H₂O, SO₄ ²⁻-LREH) in 2010 [34] provided a unique chance to solve the aforementioned issues, since this group of compounds has exactly the same RE/S molar ratio of RE₂O₂S. Homogenous hydrolysis of RE₂(SO₄)₃·8H₂O in the presence of Na₂SO₄ and hexamethylenetetramine (C₆H₁₂N₄) is the classic technique to produce SO₄ ²⁻-LREH but is limited to RE=Pr–Tb in the lanthanide family [34]. We extended the group of compounds to RE=La–Dy via reacting aqueous solutions of RE(NO₃)₃·nH₂O and (NH₄)₂SO₄ under hydrothermal conditions [17,18,19] and subsequently manifested that RE₂O₂S can be facilely produced through thermolysis of SO₄ ²⁻-LREH in a reducing atmosphere [17,18,19]. RE₂O₂S was recently identified to have relatively low phonon energy (ℏω ~ 500 cm⁻¹) [1], good chemical stability, and particularly high UC efficiency comparable to halides [35, 36], but the study on this type of promising UC phosphors is yet far from sufficiency [8, 10, 11, 37, 38]. La³⁺ does not have unoccupied 4f sub-orbital and is optically inert, and thus, its compounds are suitable host lattices for luminescence. We thus synthesized in this work La₂O₂S:Yb/RE UC phosphors (RE=Ho, Er) via annealing the hydrothermally crystallized SO₄ ²⁻-LREH in flowing H₂, and the luminescent properties and UC processes were elaborated in detail.

Methods

The starting materials of RE(NO₃)₃·6H₂O (RE=La, Ho, Er, and Yb; > 99.99% pure), (NH₄)₂SO₄ (> 99.5% pure), and NH₃·H₂O solution (28%, ultrahigh purity) were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan), and were used as received. Yb³⁺/Ho³⁺- and Yb³⁺/Er³⁺-doped La₂(OH)₄SO₄·2H₂O was separately synthesized via hydrothermal reaction. The dopant contents are 2 at.% for Yb³⁺ and 1 at.% for both Ho³⁺ and Er³⁺ according to the literature [39]. In a typical synthesis [17], 6 mmol of (NH₄)₂SO₄ was dissolved in 60 ml of an aqueous solution of the rare earths (0.1 mol/L for total RE³⁺), followed by dropwise addition of NH₃·H₂O until pH = 9. After continuous stirring for 15 min, the resultant suspension was transferred into a Teflon-lined autoclave of 100-ml capacity for 24 h of hydrothermal crystallization in an electric oven preheated to 100 °C. The resultant product was collected via centrifugation, washed with filtered water three times and ethanol once, and finally dried in air at 70 °C for 24 h. The La₂O₂S:Yb/RE UC phosphors were then annealed from their SO₄ ²⁻-LREH precursors in flowing H₂ (200 mL/min) at 1200 °C for 1 h, with a heating rate of 5 °C/min in the ramp stage.

Phase identification was performed via X-ray diffractometry (XRD; Model RINT2200, Rigaku, Tokyo, Japan) under 40 kV/40 mA, using nickel-filtered Cu-Kα radiation (λ = 0.15406 nm) and a scanning speed of 1°/min. Structure parameters of the products were derived from the XRD data using the TOPAS software [40]. Particle morphology was observed by field emission scanning electron microscopy (FE-SEM; Model S-5000, Hitachi, Tokyo) under an acceleration voltage of 10 kV. UC luminescence spectra were obtained at room temperature using an FP-6500 fluorospectrophotometer (JASCO, Tokyo) under 978-nm near-infrared laser excitation of the phosphors with a continuous wavelength (CW) laser diode (Model KS3–12322-105, BWT Beijing Ltd., Beijing, China). The signal/noise ratio (S/N) of the spectrometer is of ≥ 200, and the sensitivity was set low due to the strong UC luminescence of the phosphors. The experimental setup can be found in Additional file 1: Figure S1.

Results and Discussion

Figure 1 shows XRD patterns of the hydrothermal products, where it is seen that in each case, all the diffraction peaks can be well indexed with the layered compound of La₂(OH)₄SO₄·2H₂O [17, 18]. In an aqueous solution containing SO₄ ²⁻, the rare-earth (RE) cations would undergo hydration and partial hydrolysis to form the complex ions of [RE(OH) _x (H₂O) _y (SO₄) _z ]^3-x-2z [17,18,19]. Either a higher temperature or solution pH will promote RE³⁺ hydrolysis, leading to more OH⁻ while less SO₄ ²⁻ (smaller SO₄ ²⁻/OH⁻ molar ratio) in the complex ion. Under the optimized hydrothermal conditions of 100 °C and pH = 9 [17,18,19], the complex ion may have a proper SO₄ ²⁻/OH⁻ molar ratio, and thus, the aimed SO₄ ²⁻-LREH compound can be crystallized via condensation reactions. The structure parameters of the hydrothermal products are summarized in Table 1. It is clear that (La_0.97Ho_0.01Yb_0.02)₂(OH)₄SO₄·2H₂O has larger lattice constants (a, b, c) and cell volume (V) than (La_0.97Er_0.01Yb_0.02)₂(OH)₄SO₄·2H₂O. This is understandable in view that Ho³⁺ (1.072 Å for CN = 9) is larger than Er³⁺ (1.062 Å for CN = 9). Both the products have smaller cell constants and cell volume than the un-doped La₂(OH)₄SO₄·2H₂O (SO₄ ²⁻-LLaH), in accordance with the fact that La³⁺ is the largest (1.216 Å for CN = 9) among the four types of RE ions. The differing cell parameters provided direct evidence of solid-solution formation.

Fig. 1

XRD patterns of the (La_0.97RE_0.01Yb_0.02)₂(OH)₄SO₄·2H₂O layered compounds obtained via hydrothermal reaction at 100 °C and pH = 9 for 24 h

Table 1 Structure parameters of the (La_0.97RE_0.01Yb_0.02)₂(OH)₄SO₄·2H₂O layered compounds obtained in this work and La₂(OH)₄SO₄∙2H₂O·(SO₄ ²⁻-LLaH) [17]

Figure 2 shows XRD patterns of the products annealed from their SO₄ ²⁻-LREH precursors at 1200 °C for 1 h in flowing H₂. The diffraction peaks can be fully indexed with the hexagonal structured La₂O₂S in each case (space group: P-3m1; JCPDS card no. 00-075-1930). SO₄ ²⁻-LLaH would decompose to La₂O₂SO₄ up to 1200 °C in air through the reactions of La₂(OH)₄SO₄·2H₂O → La₂(OH)₄SO₄ + 2H₂O (dehydration) and La₂(OH)₄SO₄ → La₂O₂SO₄ + 2H₂O (dehydroxylation) [17]. In a H₂ atmosphere, the S⁶⁺ in SO₄ ²⁻ would be reduced to S²⁻ following the reaction of La₂O₂SO₄ + 4H₂ → La₂O₂S + 4H₂O, and thus, La₂O₂S can be resulted with water vapor as the only by-product [17]. The lattice parameters and cell volume of (La_0.97RE_0.01Yb_0.02)₂O₂S are shown in Table 2 together with those of La₂O₂S [17]. The decreasing cell dimension towards a smaller RE³⁺ indicates the successful formation of solid solution.

Fig. 2

XRD patterns of the (La_0.97RE_0.01Yb_0.02)₂O₂S upconversion phosphors calcined from their layered precursors in flowing H₂ (200 ml/min) at 1200 °C for 1 h. The standard diffractions of La₂O₂S are included as bars for comparison

Table 2 Structure parameters of the (La_0.97RE_0.01Yb_0.02)₂O₂S phosphors obtained in this work and those of La₂O₂S [17]

Figure 3 shows the particle morphology of the layered precursors and the resultant UC phosphors. It is seen that the SO₄ ²⁻-LREH crystallized as nanoplates of ~ 150–550 nm in lateral size and ~ 20–30 nm in thickness. The nanoplates underwent significant disintegration upon calcination at 1200 °C to yield rounded particles. The average crystallite size was assayed with the Scherrer equation to be ~ 45 nm for the UC phosphors.

Fig. 3

FE-SEM particle morphologies of the (La_0.97Ho_0.01Yb_0.02)₂(OH)₄SO₄·2H₂O (a) and (La_0.97Er_0.01Yb_0.02)₂(OH)₄SO₄·2H₂O (b) layered precursors and the (La_0.97Ho_0.01Yb_0.02)₂O₂S (c) and (La_0.97Er_0.01Yb_0.02)₂O₂S (d) upconversion phosphors

Figure 4a shows UC luminescence spectra of the (La_0.97Ho_0.01Yb_0.02)₂O₂S phosphor under 978-nm laser excitation. The emissions at ~ 546, 658, and 763 nm are attributed to the ⁵F₄ → ⁵I₈, ⁵F₇ → ⁵I₈, and ⁵F₄ → ⁵I₇ transitions of Ho³⁺, respectively [36], with the 763-nm NIR emission being predominant. The sensitizing effect of Yb³⁺ is significant, and ~ 15 and 20 times stronger green (546 nm) and NIR (763 nm, not sensitive to human eyes) emissions, respectively, were produced by codoping of 2 at.% Yb³⁺ (Additional file 1: Figure S2a). Under 50-mW laser pumping, vivid and strong green emission was observed for the (La_0.97Ho_0.01Yb_0.02)₂O₂S phosphor with naked eyes, as shown in the insert in Fig. 4a. In spite of the excitation power, the CIE color coordinates calculated from the emission spectra in the visible light region (400–700 nm) are stable at about (0.30, 0.66), typical of a vivid green color (Additional file 1: Table S1 and Figure S3).

Fig. 4

Upconversion luminescence spectra (a) and the relationship between log(I _em) and log(P) (b) for the (La_0.97Ho_0.01Yb_0.02)₂O₂S phosphor, where I _em and P are the emission intensity and excitation power (in watt), respectively. The inset in a is a photograph showing the appearance of strong UC emission under 50 mW of 978-nm laser excitation

In general, the number of photons required to populate the upper emitting state under unsaturated condition can be obtained from the relation I _em∝P ⁿ [41], where I is the luminescence intensity, P the pumping power, and n the number of laser photons. Figure 4b shows the log(I _em)-log(P) plot of the above relation, from which the n value was determined from the slope of the linear fitting to be ~3.02, 3.14, and 2.92 (approximately 3) for the UC emissions peaked at ~546, 658, and 763 nm, respectively. The results thus suggest that a three-photon process was involved to generate the observed UC luminescence.

In principle, three basic population mechanisms, namely excited state absorption (ESA), energy transfer (ET), and photon avalanche, may be involved in an UC process [8,9,10]. Since no power threshold was observed in the range of this study, the photon avalanche mechanism can be neglected. The energy diagram of Yb³⁺/Ho³⁺ in La₂O₂S has rarely been reported and is not available to us for comparison. Nonetheless, UC luminescence involving three phonons was seen from the previous work on Yb³⁺/Ho³⁺-codoped other material systems [42, 43]. Therefore, the energy diagram and UC process of (La_0.97Ho_0.01Yb_0.02)₂O₂S were constructed in Fig. 5 by referring to these previous studies and are detailed below: (1) excitation of Yb³⁺ by laser photons [ESA; ²F_7/2(Yb³⁺) + hν (978 nm) → ²F_5/2(Yb³⁺)]; (2) population of the ⁵I₆ energy level of Ho³⁺ after Yb³⁺ absorbing the first laser photon and transferring energy to Ho³⁺ [ET1; ²F_5/2(Yb³⁺) + ⁵I₈(Ho³⁺) → ²F_7/2(Yb³⁺) + ⁵I₆(Ho³⁺)]; (3) non-radiative (NR) relaxation to the ⁵I₇ level of Ho³⁺ [NR; ⁵I₆(Ho³⁺) ~ ⁵I₇(Ho³⁺)]; (4) excitation of Ho³⁺ from the ⁵I₇ to ⁵F₅ level after Yb³⁺ absorbing the second laser photon and transferring energy to Ho³⁺ [ET2; ²F_5/2(Yb³⁺) + ⁵I₇(Ho³⁺) → ²F_7/2(Yb³⁺) + ⁵F₅(Ho³⁺)]; (5) NR relaxation to the ⁵I₅ level of Ho³⁺ [⁵F₅(Ho³⁺) ~ ⁵I₅(Ho³⁺)]; (6) excitation of Ho³⁺ from the ⁵I₅ to ⁵F₄/⁵S₂ level after Yb³⁺ absorbing the third laser photon and transferring energy to Ho³⁺ [ET3; ²F_5/2(Yb³⁺) + ⁵I₅(Ho³⁺) → ²F_7/2(Yb³⁺) + ⁵F₄/⁵S₂(Ho³⁺)]; and (7) back-jumping of the excited electrons from the populated ⁵F₄/⁵S₂ level to the ⁵I₈ ground state to produce the green emission (~546 nm; ⁵F₄,⁵S₂ → ⁵I₈). The electrons can also relax to the ⁵F₅ and ⁵I₄ levels via NR processes, from which the red (~658 nm; ⁵F₅ → ⁵I₈) and near-infrared (~763 nm; ⁵I₄ → ⁵I₈) emissions were resulted. The strong near-infrared UC emission at ~763 nm may imply that NR relaxation to the ⁵I₈ energy level is significant.

Fig. 5

A schematic illustration of the energy levels and UC processes for the (La_0.97Ho_0.01Yb_0.02)₂O₂S phosphor

The ²F_5/2 → ²F_7/2 emission transition of Yb³⁺ and the ⁴I_15/2 → ⁴I_11/2 excitation transition of Er³⁺ have well-matching energies, which makes Yb³⁺/Er³⁺ the most widely investigated activator/sensitizer pair for UC luminescence in various types of host lattices [8,9,10,11,12,13]. Similar to Ho³⁺, the UC emission of Er³⁺ was also dramatically enhanced by Yb³⁺ codoping (Additional file 1: Figure S2b). Taking the 527-nm green emission for example, 2 at.% of Yb³⁺ improved the Er³⁺ luminescence by a factor of ~ 14. Under 978-nm laser excitation, the (La_0.97Er_0.01Yb_0.02)₂O₂S UC phosphor exhibits emission bands in the green (~ 527 and 549 nm), red (~ 668 and 672 nm), and near-infrared (~ 807 and 858 nm) regions (Fig. 6a), which are arising from the ²H_11/2/⁴S_3/2 → ⁴I_15/2, ⁴F_9/2 → ⁴I_15/2, and ⁴I_9/2 → ⁴I_15/2 transitions of Er³⁺, respectively [32]. The color coordinates determined for the UC luminescence in the visible light region (400–700 nm) drifted from the yellowish-green [(0.36, 0.61)] to green [(0.32, 0.64)] region in the CIE chromaticity diagram along with increasing excitation power from 0.7 to 2.0 W (Additional file 1: Figure S3b and Table S2). The color change also agrees well with the gradually larger intensity ratio of the green to red emissions (I ₅₄₉/I ₆₆₈ and I ₅₂₇/I ₆₆₈, Additional file 1: Table S3) under a higher excitation power. Excitation-power-dependent emission-color tuning of the Yb³⁺/Er³⁺ pair was previously observed in Y₂O₂S [44]. The number of pumping photons required to populate the emitting states was derived from the slope of the log(I _em)-log(P) plot (Fig. 6b), and the three groups of emissions were found to have the similar n values of ~ 2. This indicates that a two-phonon process is largely responsible for the observed UC luminescence.

Fig. 6

Upconversion luminescence spectra (a) and the relationship between log(I _em) and log(P) (b) for the (La_0.97Er_0.01Yb_0.02)₂O₂S phosphor, where I _em and P are the emission intensity and excitation power (in watt), respectively. The inset in a is a photograph showing the strong UC emission of (La_0.97Er_0.01Yb_0.02)₂O₂S under 50 mW of 978-nm laser excitation

The energy diagram and photon process leading to the UC luminescence of (La_0.97Er_0.01Yb_0.02)₂O₂S are schematically shown in Fig. 7. Excited-state absorption and Yb³⁺ → Er³⁺ energy transfer excitation are mainly involved in the UC mechanism, with the latter being dominant [8,9,10,11,12,13,14, 39, 43]. Upon excitation with 978-nm laser, the ²F_7/2 ground state electrons of Yb³⁺ are pumped to the ²F_5/2 excited state (ESA). Since the ²F_5/2 level of Yb³⁺ and the ⁴I_11/2 level of Er³⁺ are matching well with each other, energy transfer from Yb³⁺ to Er³⁺ readily takes place. The Er³⁺ electrons can thus be excited from the ⁴I_15/2 ground state to the ⁴I_11/2 level with the energy transferred from Yb³⁺ (one photon, ET1). The absorption cross section of Er³⁺ is smaller than that of Yb³⁺ at ~ 980 nm [42, 45], so energy transfer (ET) dominates the real excitation of Er³⁺. The excitation energy at the ⁴I_11/2 level may non-radiatively (NR) relax to the ⁴I_13/2 level, from which the electrons can be excited to the ⁴F_7/2 state by ET of a second laser photon (ET2). After NR processes, the three groups of emissions (Fig. 6a) can then be produced via the electronic transitions shown in Fig. 7. The photon reactions of the whole UC process can be presented as follows: (1) ²F_7/2(Yb³⁺) + hν (978 nm) → ²F_5/2(Yb³⁺) and ⁴I_15/2(Er³⁺) + hν (978 nm) → ⁴I_11/2(Er³⁺); (2) ²F_5/2(Yb³⁺) + ⁴I_15/2(Er³⁺) → ²F_7/2(Yb³⁺) + ⁴I_11/2(Er³⁺); (3) ⁴I_11/2(Er³⁺) ~ ⁴I_13/2(Er³⁺); (4) ²F_5/2(Yb³⁺) + ⁴I_13/2(Er³⁺) → ²F_7/2(Yb³⁺) + ⁴F_7/2(Er³⁺); (5) ⁴F_7/2(Er³⁺) ~ ²H_11/2/⁴S_3/2(Er³⁺), ⁴F_9/2(Er³⁺), and ⁴I_9/2(Er³⁺); and (6) ²H_11/2/⁴S_3/2(Er³⁺) → ⁴I_15/2(Er³⁺) + hν (~527 and 549 nm), ⁴F_9/2(Er³⁺) → ⁴I_15/2(Er³⁺) + hν (~668 and 672 nm), and ⁴I_9/2(Er³⁺) → ⁴I_15/2(Er³⁺) + hν (~807 and 858 nm). The aforementioned emission color change may suggest a more efficient population of the ⁴F_7/2(Er³⁺) energy level under a higher excitation power, and the ⁴F_7/2(Er³⁺) ~ ²H_11/2/⁴S_3/2(Er³⁺) NR process becomes successively stronger than ⁴F_7/2(Er³⁺) ~ ⁴F_9/2(Er³⁺).

Fig. 7

A schematic illustration of the energy levels and UC processes for (La_0.97Er_0.01Yb_0.02)₂O₂S phosphor

Conclusions

(La_0.97RE_0.01Yb_0.02)₂O₂S upconversion (UC) nanophosphors (RE=Ho, Er) were successfully produced via thermal decomposition of their layered hydroxyl sulfate precursors in flowing H₂ at 1200 °C, with water vapor as the only exhaust. The precursors crystallized as nanoplates with the lateral sizes of ~ 150–550 nm and thicknesses of ~ 20–30 nm, which disintegrated into rounded nanoparticles (average crystallite size: ~ 45 nm) upon thermal decomposition. The oxysulfide phosphors exhibit strong UC luminescence under 978-nm laser excitation, through a three-photon process for Ho³⁺ and a two-photon process for Er³⁺. For the UC luminescence in the visible region (400–700 nm), the chromaticity coordinates of (La_0.97Ho_0.01Yb_0.02)₂O₂S are stable at around (0.30, 0.66), while those of (La_0.97Er_0.01Yb_0.02)₂O₂S changed from about (0.36, 0.61) to (0.32, 0.64) along with the excitation power increasing from 0.7 to 2 W.