A bright yellow light from a Yb³⁺,Er³⁺-co-doped Y₂SiO₅ upconversion luminescence material

Abstract

A bright yellow light from Yb³⁺ and Er³⁺ doped Y₂SiO₅ upconversion materials in particle and fiber form were prepared by a co-precipitation, and electrospinning method. The morphologies of the prepared samples were investigated through FT-IR, FE-SEM, XRD and Raman measurements. The upconversion properties of the samples are carefully studied based on the absorption, fluorescent and decay time measurements. Under 980 nm excitation, the prepared material shows bright yellow emission. By controlling the concentration of Yb³⁺ or Er³⁺ and the excitation laser power, the ratio of red to green emission intensity can be varied to adjust the color tuning properties of the phosphor. The upconversion mechanism, and transition probabilities were elucidated owing to Judd–Ofelt theory. The radiative quantum efficiencies for the red (⁴F_9/2) and green (⁴S_3/2) bands were estimated to be 87% and 55%, respectively. The color coordinates of the system were evaluated as a function of the dopant concentrations and plotted on a standard CIE index diagram. The change of band intensity ratio with dopant concentrations gives a promising potential of the current phosphor for lighting application.

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Introduction

Upconversion (UC) is an anti-Stokes process in which a longer wavelength radiation is converted to a shorter wavelength via a multiphoton mechanism. In recent years, upconversion materials have attracted considerable interest owing to their unique optical properties.¹ The rare-earth (RE)-doped UC materials have been studied extensively, but most reports were focused on fluoride materials, such as NaYF₄,^2,3 NaGdF₄,^4,5 NaYbF₄,⁶ BaYF₅,⁷ and LaF₃.⁸ However, the upconversion materials based on other hosts including Y₂O₃,⁹ Bi₂Ti₂O₇,¹⁰ BiOBr,¹¹ CaTiO₃,¹² Y₂SiO₅ (ref. 13) have been rarely reported.

It is well known that Y₂SiO₅ (YSO) is an ideal host material for both photoluminescent and cathodoluminescent phosphors due to the combined chemical–physical characteristics of the matrix, such as high chemical and thermal stability, and elevated mass absorption coefficient with high stopping power.^14–17 For example, Ce³⁺-doped YSO was considered as a candidate to substitute ZnS.¹⁸ Tb³⁺-doped YSO exhibited a high efficient green phosphor excited by UV light-emitting diode.¹⁷ To date, a few studies on the upconversion of RE-doped YSO have been reported, where YSO was doped with Pr³⁺, Tb³⁺, Tm³⁺, Li⁺.^13,19–21 There are studies reporting the UC effect of YSO for converting visible to ultraviolet,¹⁹ violet, cyan and yellow to UVC (under polychromatic excitation)¹³ and IR to blue/red, based on Pr³⁺-doped YSO.²² However, UC from IR to green in Yb³⁺,Er³⁺-co-doped Y₂SiO₅ (YSO:YbEr) has been rarely investigated.

It is worthwhile to investigate the efficiency of rare-earth ions in host material because the interactions between dopants and host materials play a key role in controlling the excitation dynamics. From a fundamental point of view, the physical understanding of the luminescence properties of rare-earth ions in host material is very important. Furthermore, it is difficult to make direct measurements of radiative rate constants for the emitting states of Er³⁺ because the emitting states include numerous non-radiative paths (multiphonon relaxation, Er–Er relaxation, and Yb–Er energy transfer, etc.).²³ Judd–Ofelt (JO) theory describes the origin of electric-dipole intensity for the nominally parity forbidden f–f transitions. According to that a well-established means to estimate the radiative rates was provided.^24,25 It has been extensively used to determine the basic spectroscopy properties of RE-doped materials. To quantify the three JO intensity parameters, Ω_λ (λ = 2, 4, 6), an absorption spectrum is often used. However, the Judd–Ofelt theory have been often applied to transparent bulky amorphous or crystals. In order to apply Judd–Ofelt theory to powder samples, the powdered samples were often pressed to a very thin semi-transparent pellet at high pressure and the absorption spectra were obtained.²⁶ Luo et al.²⁷ used the excitation spectra and the proportional relationship between the excitation and absorption spectra for determining JO parameters.

In materials science, controlled synthesis of different morphological forms has been a focus of worldwide research work, since like flower, fiber, hollow porous sphere structure hold higher specific surface area, and better permeation.^28–31 Here, we present the synthesis and UC spectroscopic studies of YSO:YbEr material in two different morphologies, namely powder (YSO:YbEr) and fiber form (FYSO:YbEr), prepared by different methods. The UC mechanism of the products has been proposed and discussed in detail. Judd–Ofelt theory has been first time adapted to estimate the transition probabilities, fluorescence branching ratios, and quantum efficiencies of the various emission bands quantitatively for elucidating the energy transfer UC process. The color coordinates of the system were evaluated and plotted on a standard CIE index diagram. To the best of our knowledge, this is the first report on the quantitative analysis of the fluorescent properties and UC process in YSO:YbEr material.

Results and discussion

Phase and morphology

The XRD patterns of the 1000 °C heat treated YSO and YSO:YbEr (18 mol% Yb³⁺, 2 mol% Er³⁺) (YSO:YbEr-1000) are shown in Fig. 1a. There is no difference in the peak positions in XRD patterns of the two samples. This confirms that Yb³⁺ and Er³⁺ ions didn't create other phases and entered in the crystal phase of YSO. All diffraction peaks could be indexed to the monoclinic phase of Y₂SiO₅ (JCPDS no. 52-1810); the crystal belongs to the P2₁/c space group. Rare earth oxyorthosilicates form two polymorphs, both of which are monoclinic. From La to Gd, they have the P2₁/c space group called the X₁ phases.³² Whereas the elements from Dy to Lu are found in the C2/c space group called the X₂ phases, Tb and Y exist in both the X₁ and X₂ phases.³³ The phase transition from low temperature crystalline phase (X₁) to high temperature crystalline phase (X₂) occurred around 1200–1300 °C.^32,33 In this study, the X₁ phases always existed when the samples were annealed below 1300 °C (data not shown here). The X₁–X₂ phase transition appeared above 1300 °C agreed with previous reports. Both phases belong to the same monoclinic crystal system with different space group. For the samples heat treated at 1400 °C (YSO:YbEr-1400) there is an elongation along c axis. The crystal belongs to the I2/a space group, as shown in Fig. 1b (JCPDS no. 74-2011). The c value was changed from 6.64 Å to 12.49 Å resulted the increasing of cell volume from 397.38 ų to 852.66 ų. Liu's³² group proposed that the change in the coordination of two heavy cation sites to oxygen, corresponding to a silicon-bonded oxygen from the isolated tetrahedral and a non-silicon-bound oxygen may be reason for different structures. In X₁ phase, Y³⁺ ions occupied two sites, where they are surrounded by nine and seven oxygen ions, whereas only six oxygen ions are involved in X₂ phase to result an elongation c axis.³⁴ The appearance of X₂-phase could be originated from the replacement of Y³⁺ ions with Yb³⁺ and/or Er³⁺ ions after the samples were calcined at high temperature. The ionic radii of Y³⁺, Yb³⁺, and Er³⁺ are 1.02, 0.99, and 1.00 Å, respectively. It should be noted that there were no noticeable differences in the XRD data of YSO and YSO:Yb,Er. No second phase is detected at this doping level, indicating that the Yb³⁺ and Er³⁺ ions can be efficiently incorporated into the YSO host lattice by substitution for the Y³⁺ ion or occupying interstitial sites of the crystal lattice. Additional phases or impurity peaks did not appear in both cases. The characteristic peaks were consistent with JCPDS no. 74-2011. The XRD pattern of the fiber is similar.

Fig. 1 XRD patterns of heat treated samples (a) at 1000 °C with the reference JCPDS no. 52-1810, and (b) at 1400 °C with the reference JCPDS no. 74-2011.

Fig. 2a and b show the SEM images of YSO:YbEr-1400 powder samples with low and high magnification, respectively. As can be seen in Fig. 2b, the obtained particles were uniform and like “boomerang form” with average size of 400 nm. It is well known that the morphology and diameter of the fibers can be influenced by many eletrospinning parameters. These parameters include the polymer concentration, the distance between the spinneret and collector, the value of high voltage, and the spinning rate. The results were shown that the diameter of FYSO:YbEr was clearly increased when the voltage was increased from 15 kV to 20 kV. At higher voltage (25 kV), the diameter of fibers change was not significant, as shown in Fig. S1a–c.† The length of the fibers also depends on the distance between the spinneret and collector. It was shortened with increasing the distance at voltage of 20 kV, as shown in Fig. S1d–f.† The shortening might be a result of the complete fiber drying due to evaporation of solvent before reaching the collector. Fig. 2c presents the SEM image of post calcined FYSO:YbEr fiber. As can be observed, the fibers inhibited relative uniform structure without any beads-on-string morphology. Fig. 2d also depicted the average diameter of 150 nm for calcined-FYSO:YbEr fiber. The results from EDS data confirmed the existence of Yb and Er in all prepared samples, as shown in inset of Fig. 2a.

Fig. 2 SEM images of YSO:YbEr with low magnification (a), with high magnification (b); FYSO:YbEr fiber samples before (c), and after calcination at 1400 °C (d).

FT-IR spectrum of the 1400 °C heat treated sample is shown in Fig. S2.† The peak at 1010 cm⁻¹ originates from the absorption of Si–O–Si asymmetric stretching vibrations. Inorganic silicates have a characteristic, strong band cantered around 1100 cm⁻¹ which in some cases appears as multiple bands.³⁵ In our case, the band at 1010 cm⁻¹ of the samples transformed into three well differentiated absorption peaks of SiO₄ at 1010, 915, and 870 cm⁻¹ in the sample suggesting the formation of well-crystallized silicate. The band at 590 cm⁻¹ is due to bonding vibrations of the Y–O bonds.³⁵ The obvious bands at 1020–1080, and 846, 930–950 cm⁻¹ may arise from the asymmetric and symmetric stretching vibration of Si–O–Si, respectively.³⁰

Raman spectra of YSO:YbEr samples were investigated, as shown in Fig. 3. The low wavenumber region (up to 300 cm⁻¹) is related to the motion of the cations, the bands between 300 and 800 cm⁻¹ relates to the vibration of RE-O, and higher energy vibrations are due to the motion of Si–O atoms.^17,36 Ricci group¹⁷ reported that the vibrational frequencies in the 880–930 cm⁻¹ range are a fingerprint that can be used to assign the effective crystal phase of the samples. Besides that, the two vibration modes at 891 and 918 cm⁻¹ can be assigned to the symmetric stretching motion of the O atom with respect to the central cation (ν₁ mode).³⁶ The temperature-induced phase transformation from P2₁/c to C2/c to structure has been observed through a splitting of the stretching mode in that band, as shown in Fig. 3.³⁶ There are slight shifts (∼3 cm⁻¹) and change of relative intensities of these peaks that can be related to the polarized nature of the Raman bands in oxyorthosilicate in the YSO:YbEr samples. In addition, the quite high intensity of bands in range of 632–796 cm⁻¹ in the spectra of both samples can relates to the absence of a cation-deficient type.¹⁷

Fig. 3 Raman spectra at 300 K of YSO:YbEr-1000 and YSO:YbEr-1400 samples.

The absorption spectrum of the YSO:YbEr sample was shown in Fig. 4 along with standard notations of the observed spectral transitions of both Er³⁺ and Yb³⁺. It can be seen that a strong band below 280 corresponds to the intrinsic absorption band edge of the YSO crystal.³⁷

Fig. 4 Absorption spectrum of YSO:YbEr-1400 sample.

Upconversion study

Fig. 5 displays the upconversion emission spectra of YSO:YbEr sample under the excitation with a 980 nm laser. Spectra shows two strong bands: one band located at the green spectral range is due to 4f–4f radiative relaxation from the thermally coupled states ²H_11/2, ⁴S_3/2 to the ground state ⁴I_15/2 of Er³⁺, and another intense band located at the red spectral range due to 4f–4f radiative relaxation from the excited state ⁴F_9/2 to the ground state ⁴I_15/2 of Er³⁺ ion.³⁸ Results show that upconversion fluorescent spectra depend on calcination temperature of the samples, as shown in Fig. 5a. It can be seen that the upconversion intensity linearly increases with the annealing temperature. The increase in intensity is due to the increase in crystallinity due to the change in the coordination of two heavy cations sites to oxygen, corresponding to a silicon-bonded oxygen from the isolated tetrahedral and a non-silicon-bounded oxygen.³² These changes result in different alterations of the crystal-field splitting in the energy levels of the erbium due to the change of coordination number around the Yb³⁺, Er³⁺. Besides, the transition ⁴F_9/2 → ⁴I_15/2 is more sensitive to the annealing temperature than the transitions ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2. The yellow emission (⁴F_9/2 → ⁴I_15/2) arises from two main possible mechanisms in YSO. First one, the non-radiative relaxations originating from the ⁴S_3/2 state of Er³⁺ result in the increasing population of the ⁴F_9/2. Other possible mechanism is the energy transfer from Yb³⁺ (²F_5/2) to Er³⁺ (⁴I_11/2). The ⁴I_13/2 level is populated owing to the non-radiative relaxation from the upper ⁴I_11/2, excited-state absorption ⁴I_13/2 + a photon → ⁴F_9/2 and cross relaxation between Er³⁺ ions ⁴I_13/2 + ⁴I_11/2 → ⁴I_15/2 + ⁴F_9/2.³⁹

Fig. 5 Upconversion spectra as a function of (a) heat treatment temperature, (b) Yb³⁺ concentration, and (c) Er³⁺ concentration.

As shown in Fig. 5b and c the concentration of Yb³⁺ and Er³⁺ has a strong influence on the relative intensities of the green (G) and red (R) UC bands. Upconversion processes in all Yb/Er doped samples are due the multiphoton processes happening from the upper excited sates which involve mainly the ⁴I_13/2 and ⁴F_11/2 states of Er. Under different concentrations these levels are populated in different ways and the emission intensity is proportional to the population of these excited levels. Since green and red are the major upconversion emission bands in all Yb/Er based upconversion phosphors the relative emission intensities of these emission band determine the G/R ratio. It can be seen that the R/G ratio increases with increasing Yb³⁺ concentration. The (R/G) ratio obtained for 5, 10, 18, 23 and 27 mol% of Yb³⁺ are 2.4, 4.5, 8.0, 5.8, and 5.8 respectively. In case, of fixed-Yb³⁺ concentration of 18 mol%, the R/G values are 5.1, 5.9, 8.0, and 3.7 corresponding to 0.5, 1, 2, and 3 mol% of Er³⁺ (Fig. 5c). Both these results indicate that a strong energy transfer is occurring between Yb³⁺ and Er³⁺ ions. The UC intensity increased with increasing Yb³⁺ concentration and got the maximum value at 18 mol% and decreased with further increase in Yb³⁺ concentration. The decrease of UC intensity can be related to the excess Yb³⁺ ions in the lattice (concentration quenching effect). Too much Yb³⁺ ion will end up in forming Yb–Yb pairs or more complex clusters competing with the Yb–Er interactions.

The UC spectra of FYSO:YbEr fiber and YSO:YbEr powder are the same except that the intensity of fiber was lower than that of the powder form. The reason relates to the surface quenching effect⁴⁰ because the fiber consists of the small particles (some ten nanometers) whereas the size of YSO:YbEr powder was big (some hundreds nanometer).

In order to verify the multi-photon excitation process, the dependence of the UC intensity of YSO:YbEr samples on pump power were examined. The relationship between UC intensity I_UC and pump power I_p can be described as:

I_UC ∼ I_p^m

where, m- is the number of photons participating in the UC process. The plot of log I_UC vs. log I_p yields a straight line with slope n. The relationship between pump power of laser and UC intensity is shown in Fig. 6. The results shows that the UC intensity of 1400 °C heat treated YSO:YbEr samples are more sensitive to laser pump power than that of 1000 °C heat treated samples. The slope n = 2 indicate that the two photon process is primary mechanism of up-conversion luminescence in YSO:YbEr material.

Fig. 6 log–log plots of UC fluorescence emission as a function of the 980 nm pump power.

Judd–Ofelt analyses

Many researchers have applied the Judd–Ofelt theoretical analyses to determine important spectroscopic parameters. The absorption spectrum of YSO:YbEr is shown in Fig. 4. Most of the bands can be attributed to the 4f–4f transition of Er³⁺ from the ⁴I_15/2 ground state to different excited states and only one band corresponding to the transition from the ²F_7/2 level to ²F_5/2 of Yb³⁺. Six bands can be assigned to the ground state of Er³⁺ (⁴I_15/2) to the excited state ⁴I_9/2, ⁴F_9/2, ⁴S_3/2, ²H_11/2, ⁴F_7/2 and ⁴F_5/2 transitions.

The oscillator strength, f_exp, of these absorption bands were determined experimentally using the following formula:^41,42

where ε is the molar extinction coefficient [L mol⁻¹ cm⁻¹] at energy ν [cm⁻¹]. Judd–Ofelt theory provides the calculated oscillator strength f_cal of a transition from the ground state to an excited state. By Judd–Ofelt model, the calculated oscillator strength for an electric dipole transition from the ground state to an excited state is given by^24,25where J is total angular momentum quantum number of the ground state, ν is the energy of the transition, Ω_λ is Judd–Ofelt intensity parameter and ‖U_λ‖² is the squared double reduced matrix element of the unit tensor operator of rank λ = 2, 4, 6 calculated from the intermediate coupling approximation.

By equating the measured and calculated values of the oscillator strength (f_exp = f_cal) and solving the system of equations by the method of least squares, the Judd–Ofelt intensity parameters Ω_λ have been calculated numerically. The resulting Ω_λ (×10⁻²⁰ cm²) parameters for this YSO sample are found to be Ω₂ = 1.29, Ω₄ = 0.29, and Ω₆ = 2.78 × 10⁻²⁰ cm². These results are represented in Table 1 together with the Judd–Oflet parameters of other samples for comparison. Jørgensen and Reisfeld⁴³ noted that the Ω₂ parameter is indicative of the amount of asymmetry and covalent bonding while the Ω₆ parameter is related to the rigidity of the host. Generally, it is found that the magnitude of Ω₂ is much greater than Ω₄ and Ω₆ in most of the Er³⁺ doped glasses due to the high asymmetry of the amorphous state. A comparative study of the intensity parameters for complexes containing Er³⁺ allowed Jorgensen and Reisfeld to conclude that the Ω₆ parameter is related to the rigidity of the medium in which the Er³⁺ ions are embedded and the Ω₆ parameter increases with increasing vibrational amplitude of the RE-X distance (X = O, F…), which in turn is related to the vibrational phonon energy. Phonon energy of the host increases in the order fluorite < aluminate < silicate.

Table 1 Judd–Ofelt parameters (Ω_λ) in comparison with other hosts. All parameters are in units of 10⁻²⁰ cm²

Host	Ω2	Ω4	Ω6
a This work.
Y2SiO5^a	1.29	0.29	2.78
Germo-tellurite^44	4.99	1.47	0.76
Zinc tellurite^50	0.85	0.01	0.02
Ag-doped tellurite^41	6.63	2.47	2.92
NaYF4 (ref. 51)	4.65	0.82	1.11
Fluorindate^52	2.91	1.27	1.11
Lu2SiO5 (ref. 53)	4.57	3.11	1.42

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It was observed that Ω₂ is related with the symmetry of host, while Ω₆ decreases with the increasing of the covalence of the Er–O bond.⁴⁴ The value of Ω₂ is smaller in magnitude as compared to some other hosts, as shown in Table 1. The smaller values of Ω₂ in this system indicate that the Er³⁺ ions occupy ionic sites with small distortion. In comparison with the Lu₂SiO₅ glass sample, the value of Ω₂ is smaller than that of Lu₂SiO₅ because the distortion of the glass system is higher. The value of Ω₆ is larger than those of germo-tellurite, fluoride. This suggests that the covalency of Er–O bond in YSO:YbEr is lower than that of germo-tellurite, fluoride.

The phenomenological parameters Ω_λ were used in the calculation of radiative decay rate A(J → J′) according to the formula

The reciprocal of the sum of the radiative decay rate yields the radiative decay times of the excited states. The calculated values of the emission spectroscopic parameters of Er³⁺ doped YSO are presented in Table S1.† The ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions are especially interesting in this study. The radiative transition rate A_R (s⁻¹), branching ratio β (%) and the radiative lifetime of the excited level τ (ms) of these transitions are compared with those of the same material reported by Li et al.⁴⁵

Fluorescence decay curves of the red and green emission bands in YSO:YbEr-1400 sample are shown in Fig. 7. In order to estimate the accuracy of the JO analysis in this case, the calculated life time is compared with the measured life time obtained from the decay curves. The calculated radiative decay times of the green and red bands are respectively 0.187 ms and 0.405 ms. The experimental values of the life times of these transitions are 0.164 ms and 0.226 ms, respectively. From the calculated and measured life times of the ⁴F_9/2 and ⁴S_3/2 states, the radiative quantum efficiency for the red (⁴F_9/2) and green (⁴S_3/2) band are 87% and 55%, respectively.^46,47

Fig. 7 Decay curves of green (a) and red (b) emissions in YSO:YbEr-1400 sample.

It should be noted that the emission bands around 525 and 545 nm correspond to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, respectively, which are originated from levels ⁴H_11/2 and ⁴S_3/2. At room temperature, thermalization of these two levels occurs and therefore, a simple three-levels system comprised of ⁴I_15/2 (level 0), ⁴S_3/2 (level 1) and ²H_11/2 (level 2) could be used to describe the thermalization of the ²H_11/2 state by the following relation:⁴⁸

where A(⁴S_3/2) = 5360 s⁻¹, A(²H_11/2) = 3940 s⁻¹ are the calculated total spontaneous emission rate, hν₁ = 18 351 cm⁻¹ and hν₂ = 18 692 cm⁻¹ are the highest experimental Stark energy level of the luminescence band ⁴S_3/2 → ⁴I_11/2 and the lowest experimental Stark energy level of the luminescence band ²H_11/2 → ⁴I_15/2 respectively. g₁ and g₂ are degeneracies (2J + 1) = 4 and 12 of ⁴S_3/2 and ²H_11/2 levels, respectively. The I(⁴S_3/2) and I(²H_11/2) represent the integrated measured emission intensities of the observed transitions at room temperature, k is Boltzmann constant, and ΔE is the energy gap between the ²H_11/2 and ⁴S_3/2 levels.

Using the experimental values of emission spectra and the predicted results of the Judd–Ofelt analysis and using eqn (4), the energy gap ΔE is predicted to be 465 cm⁻¹. This predicted value is in good agreement with the energy separation of 341 cm⁻¹ between two nearest Stark levels of the ⁴S_3/2 → ⁴I_15/2 and ²H_11/2 → ⁴I_15/2 bands. The difference of 120 cm⁻¹ could be explained by the inherent error of about 15–20% in the Judd–Ofelt calculations.⁴⁹ From these remarks, we conclude that the Judd–Ofelt analysis performed in this study could be used to explain satisfactorily the obtained experimental results.

Since Yb³⁺ has a much larger absorption cross-section than Er³⁺, the GSA of Er³⁺ from ⁴I_15/2 → ⁴I_11/2 is not the main reason for the population of the ⁴I_11/2 level of Er³⁺. The energy transfer process contributes to the population accumulation on the ⁴I_11/2. This level serves as the intermediate state for the upconversion process. The life time of the ⁴I_11/2 level was high enough to confirm its role. The ⁴S_3/2 state lie close to ²H_11/2 (ΔE = 465 cm⁻¹) and resulted in the existence of thermal population process. In this process, the electrons can migrate between ⁴S_3/2 and ²H_11/2, so that both level states have a common transition probability.^54,55 This transition probability was expressed by

with ΔE = 465 cm⁻¹, A_R(²H_11/2) = 3940 s⁻¹, and A_R(⁴S_3/2) = 5360 s⁻¹ Boltzmann constant k_B = 0.695 cm⁻¹ K⁻¹, T = 300 K. Using above results, the calculated common transition probability A_t is 5025 s⁻¹ and the life time of ⁴S_3/2 state was corrected to be 0.199 ms.

Evaluation of color coordinate

The luminous color was estimated by studying color co-ordinates and color ratios of the 1400 °C heat treated samples under 980 nm excitation. From the emission spectra of the samples, the value of chromaticity co-ordinate of the powder was estimated from CIE-1931 system (as shown in Table S2†). The values were found to locate in the yellow or near white region, as shown in Fig. 8. The values shifted from the bright yellow region to the yellow region with increasing Er³⁺ ions (fixed Yb³⁺ ions), as shown in Fig. 8a. The values slightly changed with a change in Yb³⁺ ions (at 2 mol% Er³⁺ ions) and located on near white region, as shown in Fig. 8b. Overall the effect of Er³⁺ concentration is found to be more prominent that Yb³⁺ concentrations. This is clear from the emission data of Fig. 3b and c where emission intensity shows sudden decrease after 2 mol% of Er³⁺. The pump power influenced the value of the chromaticity co-ordinate, as show in Fig. 8c. The change of R/G ratios by pump power is a reason for the change of yellow color intensity. Therefore, by adjusting to a suitable Er³⁺ concentration, the chromaticity coordinates of the sample can be adjusted to near white light zone. This clearly shows that the prepared YSO:YbEr-1400 samples will be more suitable candidates for light emitting applications.

Fig. 8 CIE chromaticity coordinates diagrams for YSO:Yb,Er as a function of (a) Er³⁺ concentration, (b) Yb³⁺ concentration, and (c) pump power.

Experiment

Yttrium oxide (Y₂O₃), ytterbium oxide (Yb₂O₃), erbium oxide (Er₂O₃), tetraethylorthosilicate (TEOS), and anhydrous ethanol were obtained from Aldrich Chemical Co. Ammonia solution, nitric acid (HNO₃), were obtained from Alfa Co. All the reagents were Analar grade and used without further purification.

Synthesis

Synthesis of powder sample. Yb³⁺,Er³⁺-co-doped Y₂SiO₅ powder samples were synthesized according to the following procedure. First, stoichiometric amount of Y₂O₃, Yb₂O₃ and Er₂O₃ (1 mmol) were dissolved in 15 mL of HNO₃ 0.1 N at 70 °C and the solution was allowed to cool to room temperature. Second, 1 mmol TEOS in 12 mL ethanol were added to the cooled rare-earth solution with mechanical stirring for 1 h resulting in the formation of a transparent solution. Droplet of ammonia solution was subsequently added to form a white precipitate immediately. This process was continued until no further precipitation occurred. The mixture was centrifuged at 4000 rpm for 15 min and washed 5 times thoroughly with water, followed by subsequent wash with 2-isopropanol in order to remove excess reagents. The product was dried in a vacuum oven at 60 °C for 12 h. The powder samples were annealed at different temperatures in range 1000–1400 °C with heating rate of 5 °C min⁻¹ for 3 h.

Synthesis of fiber sample. 1.1 g of Y₂O₃, Yb₂O₃ and Er₂O₃ (Y : Yb : Er = 80 : 18 : 2 mol%) were dissolved in 10 mL of solution H₂O : HNO₃ (1 : 1 v/v), then evaporated to powder form. The RE (NO₃)₃ powder was dissolved in 3.5 g of DMF. TEOS (360 mg) was added to the mixture under stirring for 6 h, and then 2.1 g of PVP (MW: 360 000) was added under vigorous stirring for 6 h to form electrospun sol. In the electrospinning process, the mixture was injected through a stainless steel needle (26G in orifice diameter of 450 nm) that was connected to a high voltage DC supply. The parameters for preparing FYSO:Yb,Er, namely the distance between the spinneret and collector, the value of high voltage, and the spinning rate, were set at 15 cm, 20 kV, 0.7 mL h⁻¹, respectively. The fibers were heated up to 1400 °C with heating rate 2 °C min⁻¹ and kept at this temperature for 3 h in order to remove the polymer and maintain the fiber form.

Characterization

X-ray powder diffraction data was recorded by using an X'pert PRO MPD X-ray diffractometer (Panalytical, Netherlands). The morphology of the samples was investigated by using field emission scanning electron microscopy (FE-SEM), MIRA3 (Tescan, Czech). FT-IR spectra were observed by using a Nicolet iS10 FT-IR spectrometer (Thermo Fisher Scientific, Madison, Wisconsin, USA). The absorption spectra were recorded using a UV-VIS/NIR spectrometer (V-670, Jasco, Japan). The prepared samples were compressed into near transparent pellets of 0.4 mm thickness using a hydraulic press for the absorption measurements.

The UC luminescence emission spectra were measured with a fluorescence spectrophotometer (Acton SpectraPro 750-Triplet Grating Monochromator) from a CCD detector (Princeton EEV 1024 × 1024 and PI-Max 133 Controller), using a 980 nm semiconductor CW laser diode as an excitation source, which was placed at an angle of 45° in front of the sample holder. All of the measurements were performed at room temperature. Photoluminescence decay curves were measured on a Quanta Master 40 system (Photon Technology International Inc., US) using a pulsed 980 nm laser as the excitation source. The collected decay curve was analyzed using built in software provided by PTI.

Raman scattering measurements were performed using a Raman spectrophotometer (NRS-3300 Jasco, Japan) at room temperature in a single monochromator. For excitation, the 532 nm line of a solid-state laser system was focused on the sample by mean of a 100× objective, while the laser power was kept below 2 mW in order to avoid laser-heating effects. The accumulation time was typically two collections of 2 s.

Radiative properties from Judd–Ofelt theory

To quantitatively estimate the radiative properties of Er³⁺, Judd–Ofelt theory was adapted based on the oscillator strengths of the absorption band (f_exp) from absorption spectra. The experimental oscillator strengths were fitted with theoretical oscillator strengths (f_cal) following the Judd–Ofelt procedure.^41,56 By fitting the measured oscillator strengths to the theoretical oscillator strengths using a least-squares method, the three Judd–Ofelt parameters (Ω₂, Ω₄, and Ω₆) were obtained. The radiative transition probability (A_rad), radiative lifetime (τ_rad), and branch ratios of a transition were estimated from these Ω₂, Ω₄, and Ω₆ parameters.

Conclusions

In summary, a bright yellow emitting Yb³⁺,Er³⁺-co-doped Y₂SiO₅ up-conversion material was prepared through a co-precipitation, and electrospinning procedure. The X₁–X₂ phases were determined through XRD and Raman measurements. The upconversion emission originates from two photon process. Results from the Judd–Ofelt theory are in good agreement with the experimental results. The color coordinates of the system were evaluated and plotted on a standard CIE index diagram. By varying the concentrations of Yb³⁺ or Er³⁺ and pump power, the red to green band ratio of YSO:YbEr can be adjusted so as to obtain a color tunable phosphor for various lighting applications. Using co-precipitation method, large amount of YSO:YbEr powder sample can be obtained. The electrospinning technique has been used to make YSO:YbEr hollow fiber that will be studied for some applications, such as drug delivery, cell imaging in future.

Acknowledgements

This work was supported by the NRF-2015K2A1A2070664 project through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. Authors GAK would like to acknowledge the financial support from the National Science Foundation Partnerships for Research and Education in Materials (NSF-PREM) grant No. DMR-0934218.

Notes and references

1. G. Chen, H. Qiu, P. N. Prasad and X. Chen, Upconversion Nanoparticles: Design, Nanochemistry, and Applications in Theranostics, Chem. Rev., 2014, 114(10), 5161–5214.
2. X. Huang, Giant enhancement of upconversion emission in (NaYF₄:Nd³⁺/Yb³⁺/Ho³⁺)/(NaYF₄:Nd³⁺/Yb³⁺) core/shell nanoparticles excited at 808 nm, Opt. Lett., 2015, 40(15), 3599–3602.
3. X. Huang and J. Lin, Active-core/active-shell nanostructured design: an effective strategy to enhance Nd³⁺/Yb³⁺ cascade sensitized upconversion luminescence in lanthanide-doped nanoparticles, J. Mater. Chem. C, 2015, 3(29), 7652–7657.
4. X. Huang, Dual-model upconversion luminescence from NaGdF₄:Nd/Yb/Tm@NaGdF₄:Eu/Tb core–shell nanoparticles, J. Alloys Compd., 2015, 628, 240–244.
5. S. B. Wang, Q. Li, L. Z. Pei and Q.-F. Zhang, Branch-shaped NaGdF₄:Eu³⁺ nanocrystals: Selective synthesis, and photoluminescence properties, Mater. Charact., 2010, 61(8), 824–830.
6. J. A. Damasco, G. Chen, W. Shao, H. Ågren, H. Huang, W. Song, J. F. Lovell and P. N. Prasad, Size-Tunable and Monodisperse Tm³⁺/Gd³⁺-Doped Hexagonal NaYbF₄ Nanoparticles with Engineered Efficient Near Infrared-to-Near Infrared Upconversion for In Vivo Imaging, ACS Appl. Mater. Interfaces, 2014, 6(16), 13884–13893.
7. T. Grzyb, S. Balabhadra, D. Przybylska and M. Węcławiak, Upconversion luminescence in BaYF₅, BaGdF₅ and BaLuF₅ nanocrystals doped with Yb³⁺/Ho³⁺, Yb³⁺/Er³⁺ or Yb³⁺/Tm³⁺ ions, J. Alloys Compd., 2015, 649, 606–616.
8. X. Huang, Enhancement of near-infrared to near-infrared upconversion luminescence in sub-10-nm ultra-small LaF₃:Yb³⁺/Tm³⁺; nanoparticles through lanthanide doping, Opt. Lett., 2015, 40(22), 5231–5234.
9. A. N. Meza-Rocha, E. F. Huerta, U. Caldiño, S. Carmona-Téllez, M. Bettinelli, A. Speghini, S. Pelli, G. C. Righini and C. Falcony, Dependence of the up-conversion emission of Li⁺ co-doped Y₂O₃:Er³⁺ films with dopant concentration, J. Lumin., 2015, 167, 352–359.
10. Y. Cun, Z. Yang, J. Li, B. Shao, J. Yang, Y. Wang, J. Qiu and Z. Song, Enhanced upconversion emission of three dimensionally ordered macroporous films Bi₂Ti₂O₇:Er³⁺, Yb³⁺ with silica shell, Ceram. Int., 2015, 41(9), 11770–11775.
11. Y. Li, Z. Song, Z. Yin, Q. Kuang, R. Wan, Y. Zhou, Q. Liu, J. Qiu and Z. Yang, Investigation on the upconversion emission in 2D BiOBr:Yb³⁺/Ho³⁺ nanosheets, Spectrochim. Acta, Part A, 2015, 150, 135–141.
12. S. P. Tiwari, M. K. Mahata, K. Kumar and V. K. Rai, Enhanced temperature sensing response of upconversion luminescence in ZnO–CaTiO₃:Er³⁺/Yb³⁺ nano-composite phosphor, Spectrochim. Acta, Part A, 2015, 150, 623–630.
13. E. L. Cates and J.-H. Kim, Upconversion under polychromatic excitation: Y₂SiO₅:Pr³⁺,Li⁺ converts violet, cyan, green, and yellow light into UVC, Opt. Mater., 2013, 35(12), 2347–2351.
14. T. Hirai and Y. Kondo, Preparation of Y₂SiO₅:Ln³⁺ (Ln = Eu, Tb, Sm) and Gd_9.33(SiO₄)₆O₂:Ln³⁺ (Ln = Eu, Tb) Phosphor Fine Particles Using an Emulsion Liquid Membrane System, J. Phys. Chem. C, 2006, 111(1), 168–174.
15. F. Zhang, Photon Upconversion Nanomaterials, Springer-Verlag, Berlin Heidelberg, 2015.
16. Z. Sun, M. Li and Y. Zhou, Thermal properties of single-phase Y₂SiO₅, J. Eur. Ceram. Soc., 2009, 29(4), 551–557.
17. P. C. Ricci, C. M. Carbonaro, R. Corpino, C. Cannas and M. Salis, Optical and Structural Characterization of Terbium-Doped Y₂SiO₅ Phosphor Particles, J. Phys. Chem. C, 2011, 115(33), 16630–16636.
18. P. J. Marsh, J. Silver, A. Vecht and A. Newport, Cathodoluminescence studies of yttrium silicate:cerium phosphors synthesised by a sol–gel process, J. Lumin., 2002, 97(3–4), 229–236.
19. C. Hu, C. Sun, J. Li, Z. Li, H. Zhang and Z. Jiang, Visible-to-ultraviolet upconversion in Pr³⁺:Y₂SiO₅ crystals, Chem. Phys., 2006, 325(2–3), 563–566.
20. N. Rakov, S. A. Vieira, R. B. Guimarães and G. S. Maciel, Cooperative upconversion luminescence in Tb³⁺:Yb³⁺ co-doped Y₂SiO₅ powders prepared by combustion synthesis, J. Solid State Chem., 2014, 211, 32–36.
21. N. Rakov and G. S. Maciel, Three-photon upconversion and optical thermometry characterization of Er³⁺:Yb³⁺ co-doped yttrium silicate powders, Sens. Actuators, B, 2012, 164(1), 96–100.
22. J. Li, C. Sun, X. Wang, Y. Yu, H. Yang, C. Hu and Z. Jiang, IR-to-blue and red wavelength upconversion in Pr³⁺:Y₂SiO₅ crystals, Phys. B, 2007, 398(1), 144–147.
23. G. Yao, C. Lin, Q. Meng, P. Stanley May and M. T. Berry, Calculation of Judd–Ofelt parameters for Er³⁺ in β-NaYF₄:Yb³⁺,Er³⁺ from emission intensity ratios and diffuse reflectance spectra, J. Lumin., 2015, 160, 276–281.
24. B. R. Judd, Optical Absorption Intensities of Rare-Earth Ions, Phys. Rev., 1962, 127(3), 750–761.
25. G. S. Ofelt, Intensities of Crystal Spectra of Rare-Earth Ions, J. Chem. Phys., 1962, 37(3), 511–520.
26. M. C. Tan, G. A. Kumar, R. E. Riman, M. G. Brik, E. Brown and U. Hommerich, Synthesis and optical properties of infrared-emitting YF₃:Nd nanoparticles, J. Appl. Phys., 2009, 106(6), 063118.
27. W. Luo, J. Liao, R. Li and X. Chen, Determination of Judd–Ofelt intensity parameters from the excitation spectra for rare-earth doped luminescent materials, Phys. Chem. Chem. Phys., 2010, 12(13), 3276–3282.
28. Y. Han, S. Gai, P. A. Ma, L. Wang, M. Zhang, S. Huang and P. Yang, Highly Uniform α-NaYF₄:Yb/Er Hollow Microspheres and Their Application as Drug Carrier, Inorg. Chem., 2013, 52(16), 9184–9191.
29. M. Wang, Y. Shi, Y. Tang and G. Jiang, Hierarchical nanowires-assembled YVO₄ microspheres: Synthesis, characterization and photocatalytic properties, Mater. Lett., 2014, 132, 236–239.
30. L. Wang, Z. Hou, Z. Quan, C. Li, J. Yang, H. Lian, P. Yang and J. Lin, One-Dimensional Ce³⁺- and/or Tb³⁺-Doped X₁-Y₂SiO₅ Nanofibers and Microbelts: Electrospinning Preparation and Luminescent Properties, Inorg. Chem., 2009, 48(14), 6731–6739.
31. H. Yu, Y. Jia, C. Yao and Y. Lu, PCL/PEG core/sheath fibers with controlled drug release rate fabricated on the basis of a novel combined technique, Int. J. Pharm., 2014, 469(1), 17–22.
32. Y. Liu, C. N. Xu, H. Chen and H. Tateyama, Influence of calcining temperature on photoluminescence and thermal quenching in europium-doped Y₂SiO₅ using the MOD process, J. Lumin., 2002, 97(2), 135–140.
33. M. D. Dramićanin, B. Viana, Ž. Andrić, V. Djoković and A. S. Luyt, Synthesis of Y₂SiO₅:Eu³⁺ nanoparticles from a hydrothermally prepared silica sol, J. Alloys Compd., 2008, 464(1–2), 357–360.
34. M. Yin, C. Duan, W. Zhang, L. Lou, S. Xia and J.-C. Krupa, Site selectively excited luminescence and energy transfer of X₁-Y₂SiO₅:Eu at nanometric scale, J. Appl. Phys., 1999, 86(7), 3751–3757.
35. R. Krsmanović, Ž. Andrić, M. Marinović-Cincović, I. Zeković and M. D. Dramićanin, Optical and Thermal Investigation of Sol–Gel Derived Eu³⁺:Y₂SiO₅ Nanoparticles, Acta Phys. Pol., A, 2007, 112(5), 975–980.
36. P. C. Ricci, D. Chiriu, C. M. Carbonaro, S. Desgreniers, E. Fortin and A. Anedda, Pressure effects in lutetium yttrium oxyorthosilicate single crystals, J. Raman Spectrosc., 2008, 39(9), 1268–1275.
37. X. Wang, J. Qiu, J. Song, J. Xu, Y. Liao, H. Sun, Y. Cheng and Z. Xu, Simultaneous three-photon absorption induced ultraviolet upconversion in Pr³⁺:Y₂SiO₅ crystal by femtosecond laser irradiation, Opt. Commun., 2008, 281(2), 299–302.
38. J. D. Suter, N. J. Pekas, M. T. Berry and P. S. May, Real-Time-Monitoring of the Synthesis of β-NaYF₄:17% Yb,3% Er Nanocrystals Using NIR-to-Visible Upconversion Luminescence, J. Phys. Chem. C, 2014, 118(24), 13238–13247.
39. Y. Wu, X. Shen, S. Dai, Y. Xu, F. Chen, C. Lin, T. Xu and Q. Nie, Silver Nanoparticles Enhanced Upconversion Luminescence in Er³⁺/Yb³⁺ Codoped Bismuth–Germanate Glasses, J. Phys. Chem. C, 2011, 115(50), 25040–25045.
40. F. Wang, J. Wang and X. Liu, Direct Evidence of a Surface Quenching Effect on Size-Dependent Luminescence of Upconversion Nanoparticles, Angew. Chem., Int. Ed., 2010, 49(41), 7456–7460.
41. R. J. Amjad, M. R. Dousti and M. R. Sahar, Spectroscopic investigation and Judd–Ofelt analysis of silver nanoparticles embedded Er³⁺-doped tellurite glass, Curr. Appl. Phys., 2015, 15(1), 1–7.
42. B. T. Huy, M.-H. Seo, J.-M. Lim, Y.-I. Lee, N. T. Thanh, V. X. Quang, T. T. Hoai and N. A. Hong, Application of the Judd–Ofelt Theory to Dy³⁺-Doped Fluoroborate/Sulphate Glasses, J. Korean Phys. Soc., 2011, 59(5), 3300–3307.
43. C. K. Jørgensen and R. Reisfeld, Judd–Ofelt parameters and chemical bonding, J. Less-Common Met., 1983, 93(1), 107–112.
44. Y. Gao, Q.-H. Nie, T.-F. Xu and X. Shen, Thermal stability, Judd–Ofelt theory analysis and spectroscopic properties of a new Er³⁺/Yb³⁺-codoped germano-tellurite glass, Spectrochim. Acta, Part A, 2005, 61(13–14), 2822–2826.
45. C. Li, C. Wyon and R. Moncorge, Spectroscopic properties and fluorescence dynamics of Er³⁺ and Yb³⁺ in Y₂SiO₅, IEEE J. Quantum Electron., 1992, 28(4), 1209–1221.
46. H. Desirena, E. De la Rosa, V. H. Romero, J. F. Castillo, L. A. Díaz-Torres and J. R. Oliva, Comparative study of the spectroscopic properties of Yb³⁺/Er³⁺ codoped tellurite glasses modified with R₂O (R = Li, Na and K), J. Lumin., 2012, 132(2), 391–397.
47. M. Pokhrel, G. A. Kumar, S. Balaji, R. Debnath and D. K. Sardar, Optical characterization of Er³⁺and Yb³⁺ co-doped barium fluorotellurite glass, J. Lumin., 2012, 132(8), 1910–1916.
48. D. C. Yeh, W. A. Sibley, M. Suscavage and M. G. Drexhage, Multiphonon relaxation and infrared to visible conversion of Er³⁺ and Yb³⁺ ions in barium thorium fluoride glass, J. Appl. Phys., 1987, 62(1), 266–275.
49. C. G. Walrand and K. Binnemans, Handbook on Physics and Chemistry of Rare Earths, ed. K. A. Gschneidner and J. L. Eyring, Elsevier, 1998.
50. A. Awang, S. K. Ghoshal, M. R. Sahar, M. Reza Dousti, R. J. Amjad and F. Nawaz, Enhanced spectroscopic properties and Judd–Ofelt parameters of Er-doped tellurite glass: Effect of gold nanoparticles, Curr. Appl. Phys., 2013, 13(8), 1813–1818.
51. H. Ping, D. Chen, Y. Yu and Y. Wang, Judd–Ofelt analyses and luminescence of Er³⁺/Yb³⁺ co-doped transparent glass ceramics containing NaYF₄ nanocrystals, J. Alloys Compd., 2010, 490(1–2), 74–77.
52. A. Florez, Y. Messaddeq, O. L. Malta and M. A. Aegerter, Optical transition probabilities and compositional dependence of Judd–Ofelt parameters of Er³⁺ ions in fluoroindate glass, J. Alloys Compd., 1995, 227(2), 135–140.
53. Q. Dong, G. Zhao, J. Chen, Y. Ding, B. Yao and Z. Yu, Spectroscopic properties of Ho³⁺ ions in biaxial Lu₂SiO₅ crystal, Opt. Mater., 2010, 32(9), 873–877.
54. B. Klimesz, G. Dominiak-Dzik, M. Żelechower and W. Ryba-Romanowski, Optical study of GeO₂–PbO–PbF₂ oxyfluoride glass single doped with lanthanide ions, Opt. Mater., 2008, 30(10), 1587–1594.
55. L. Riseberg and H. Moos, Multiphonon Orbit-Lattice Relaxation of Excited States of Rare-Earth Ions in Crystals, Phys. Rev., 1968, 174(2), 429–438.
56. N. T. Thanh, V. X. Quang, V. P. Tuyen, N. V. Tam, T. Hayakawa and B. T. Huy, Role of charge transfer state and host matrix in Eu³⁺-doped alkali and earth alkali fluoro-aluminoborate glasses, Opt. Mater., 2012, 34(8), 1477–1481.

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Footnote

† Electronic supplementary information (ESI) available: SEM images of different parameters prepared fiber, FT-IR spectra, table: calculated radiative properties; CIE calculate. See DOI: 10.1039/c6ra21768k

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