Synthesis and Spectroscopic Properties of Sm³⁺-Activated Li₆Y(BO₃)₃ Phosphor for Light-Emitting Diode Applications

Abstract

A series of orange-red emitting Li₆Y(BO₃)₃: Sm³⁺ (LYBO: Sm³⁺) phosphors were produced via the high temperature solid-state method. The structure, morphology, element distribution and photoluminescent behavior of these phosphors were thoroughly examined. XRD analysis confirmed that all samples exhibited a pure phase. Under 404 nm excitation, the emission spectra included four distinct transitions of Sm³⁺, attributed to ⁴G_5/2→⁶H_5/2 (565 nm), ⁴G_5/2→⁶H_7/2 (613 nm), ⁴G_5/2→⁶H_9/2 (647 nm) and ⁴G_5/2→⁶H_11/2 (708 nm). The ideal doping level for LYBO: xSm³⁺ is x = 0.05, and the concentration quenching primarily stems from electric dipole–dipole interactions among the ions. As the amount of Sm³⁺ dopant was increased, the fluorescence lifetime decreased. The CIE indicates that LYBO: 0.05Sm³⁺ is located in the orange-red region, exhibiting a high color purity (99%) and low color temperature (1711 K). The phosphor demonstrated excellent thermal stability and its activation energy was 0.3238 eV. In summary, LYBO: Sm³⁺ is a potential orange-red phosphor that can be coated onto near-ultraviolet chips suitable for W-LEDs.

1. Introduction

The light-emitting diode (LED) as the revolutionary fourth generation lighting innovation has been widely used in lighting, display and other fields [1]. Compared with traditional solid-state lighting technology such as incandescent lamps and fluorescent lamps, LEDs have advantages such as a reduced power draw, an increased longevity, a high light conversion efficiency, simple operation, and environmental friendliness [2,3]. At present, the common production methods for white LED (WLED) are divided into multi-chip combined type and single-chip fluorescent conversion type [4]. Among the aforementioned methods, the multi-chip combined approach stands out for its simplicity and straightforward implementation. Notably, the control circuitry required can be intricate, contributing to an overall higher cost. Furthermore, the variability in light decay among individual chips leads to suboptimal output light stability, which can be a significant drawback in certain applications [5]. Phosphor particles mixed with silica gel are coated on the corresponding chip surface as a coating material in the process of packaging fluorescent conversion LEDs. There are three ways to produce white light in WLEDs: (1) Blue LED chip excited yellow phosphor: the typical process is to use blue LED chips (440–470 nm) coated with Y₃Al₅O₁₂: Ce³⁺ (YAG: Ce³⁺) yellow phosphor to achieve commercial production. However, LED lighting also presents challenges, notably the issue of a high color temperature and the problem of poor color rendering stemming from an insufficient amount of red-light components [6,7]. (2) The blue LED chip stimulates the red and green phosphor [8,9]. (3) The near-ultraviolet chip excites the red, green and blue trichromatic phosphor [10,11].

Currently, fluorescent conversion WLEDs dominate the lighting market due to the low cost of trichromatic phosphors, their amenability to mass production, and their inherent attributes of high light efficiency and remarkable color uniformity [12,13,14]. Regardless of the technology solution used, the efficient use of red fluorescent powder is essential to create high-performance white light LEDs [15,16]. Due to its rich energy level, the high-purity orange-red emissions produced by Sm³⁺ are attributed to 4f–4f transitions, which can effectively compensate for red light emissions from WLEDs and adjust color temperatures [17,18]. Borate compounds offer advantages that encompass ease of preparation and stability across a wide range of physical and chemical conditions, making them potentially valuable in luminescent materials and attracting the interest of many researchers [19]. Li₆R(BO₃)₃ (R = rare earth) is a good optical material for use in borate materials and has a large band gap, which provides favorable conditions for the energy level to accommodate rare earth ions [20].

In this paper, Li₆Y(BO₃)₃: Sm³⁺ (LYBO: Sm³⁺) phosphor was made using the high temperature solid phase method at a low temperature of 700 °C. The phase composition, elementary composition, luminescence properties, decay lifetimes, CIE chromaticity, thermal stability and energy transfer mechanism were thoroughly studied. These analyses show that LYBO: Sm³⁺ is an orange-red phosphor with potential application value in WLEDs.

2. Materials and Methods

A series of Li₆Y(BO₃)₃: xSm³⁺ (x = 0.01, 0.05, 0.10, 0.15, 0.20) samples were produced using the high temperature solid phase method. Lithium carbonate (Li₂CO₃, A.R), boric acid (H₃BO₃, 99.99%, excess 3% to compensate for evaporation), yttrium oxide (Y₂O₃, 5N) and samarium oxide (Sm₂O₃, 3N) serves as the primary raw materials. The masses of these elements calculated for the LYBO: 0.05 Sm³⁺ sample were Li₂CO₃ (0.5542 g), H₃BO₃ (0.4776 g), Y₂O₃ (0.2681 g) and Sm₂O₃ (0.0218 g). The required amount of each component was weighed according to the stoichiometric ratio and ground in an agate mortar for 30 min to blend well. The well-mixed raw material powder was put into the alumina crucible, pre-burned at 450 °C for 2 h in the muffle furnace, ground again for 30min after removal, and then calcined at 700 °C in the muffle furnace for 5 h. The phosphor was meticulously ground after it had cooled down slowly to normal room temperature.

The X-ray Diffraction (XRD) of the samples was tested utilizing a Bruker D8 X-ray diffractometer (Bruker, Karlsruhe, Germany), equipped with a Cu-Kα radiation source (λ = 0.1541 Å), operating at 40 kV and 40 mA. The surface morphology of the samples was meticulously examined by a Carl Zeiss Gemini SEM500 (Carl Zeiss, Jena, Germany) field emission scanning electron microscope, manufactured in Germany. PHI VersaProbe 4 X-ray photoelectron spectrometer (ULVAC-PHI, Tokyo, Japan) was utilized to investigate the elemental makeup of the sample. C1s (284.8 eV) was used to calibrate the binding energy of other chemical elements. Additionally, the photoluminescence spectra, fluorescence lifetime and temperature-dependent spectra were recorded by an Edinburgh-FLS 920 fluorescence spectrophotometer (Edinburgh Instruments, Scotland, UK) along with relevant accessories, using a 150 W xenon lamp as the excitation light source.

3. Results and Discussion

Figure 1a shows the XRD pattern of LYBO: xSm³⁺ (x = 0.05, 0.10, 0.15, 0.20). All the diffraction peaks are consistent with PDF# 80-0843, which indicates that the obtained sample is in the pure phase and no other phases have been produced. When Sm³⁺ (r_Sm³⁺ = 1.079 Å, CN = 8) replaces the position of Y³⁺ (r_Y³⁺ = 1.019 Å, CN = 8), the peak position is shifted by a smaller angle and the crystal structure remains unchanged [21,22]. LYBO exhibits a monoclinic crystal structure belonging to the P21/c space group, with crystal parameters of a = 7.157 Å, b = 16.378 Å, c = 6.623 Å, α = γ = 90°, β = 105.32° and Z = 4 [23]. Figure 1b shows the crystal structure of LYBO. The LYBO sample is composed of a three-dimensional composite structure formed by B atoms and O atoms in a triangular coordination, as well as polyhedrons of Li and Y [24]. The Li forms LiO₄ and LiO₅ polyhedrons, and all oxygen ions are part of the BO₃ plane triangle [25]. The Y in LYBO forms irregular YO₈ dodecahedra that share a chain to form a zigzag chain.

Figure 2a presents an image of the LYBO: 0.05 Sm³⁺ phosphor sample at 1000 magnification obtained through Scanning Electron Microscopy (SEM). It can be observed that the synthesized particles have irregular shapes and rough surfaces, and the particle size is between 5 and 15 μm. Due to the effect of surface energy on the process of high-temperature calcination, the grain boundary of the large grain expanded towards the center of the small grain, resulting in a further reduction in the grain size, and then the appearance of the clustering phenomenon [26]. The Li, Y, B, O and Sm components can be seen to be uniformly distributed in the selected particles in Figure 2b–f. Figure 2g is an element distribution diagram of LYBO: 0.05 Sm³⁺ obtained with an Energy Dispersive Spectrometer (EDS), and there are no other impurities in the produced sample. Li could not be detected in the element distribution map because Li is a super optical element which limits the measurement of X-rays [27]. The above tests further show that Sm³⁺ was effectively incorporated into the matrix.

X-ray Photoelectron Spectroscopy (XPS) analysis of LYBO: Sm³⁺ was performed in order to substantiate the composition and elucidate the chemical interaction of the phosphors more thoroughly. Figure 3a is the full XPS spectrum of LYBO: Sm³⁺, and the corresponding binding energy confirms the presence of Li, Y, B, C and O elements without other impurities. In addition, Sm elements were also observed, indicating that lanthanide activator ions had been incorporated into the LYBO. Figure 3b–f depict the fine XPS spectra of various elements in the phosphors. Figure 3b shows the peaks corresponding to Li 1s at 55 eV and 56.6 eV, and Y 3d consists of two consecutive double peaks (158.2 eV, 159.8 eV) in Figure 3c. In Figure 3d, B1s contains two peaks located at 192.25 eV and 199 eV, which are due to the oxidation state of B³⁺ [28]. O1s can be well fitted as a single peak at 531.875 eV which indicates that only lattice oxygen exists, as shown in Figure 3e [29]. In Figure 3f, the two peaks at 1072.3 eV and 1097.2 eV come from Sm³⁺ (³d_5/2) and Sm³⁺ (³d_3/2), indicating the +3 oxidation state of Sm in the host [30].

In Figure 4a, photoluminescence excitation (PLE) and the corresponding photoluminescence emission (PL) spectra of the LYBO: 0.05 Sm³⁺ sample are presented. Under the monitoring of 647 nm, the wide spectrum of 225–275 nm is the charge transfer band formed by the migration of electrons from the 2p orbital within the O²⁻ to the 4f orbital of Sm³⁺, and the intense excitation peak between 300 and 500 nm is due to the 4f–4f energy transition of Sm³⁺ ion [31]. A series of excitation peaks are located at 306 nm, 317 nm, 334 nm, 346 nm, 364 nm, 377 nm, 404 nm, 419 nm and 468 nm, corresponding to ⁶H_5/2→⁴P_5/2, ⁶H_5/2→⁴P_3/2, ⁶H_5/2→⁴G_7/2, ⁶H_5/2→⁴H_9/2, ⁶H_5/2→⁴D_3/2, ⁶H_5/2→⁴D_1/2, ⁶H_5/2→⁴F_7/2, ⁶H_5/2→⁴M_19/2 and ⁶H_5/2→⁴I_11/2 transitions, respectively [32]. Under 404 nm excitation, the emission peaks of Sm³⁺ can be found at 565 nm, 613 nm, 647 nm and 708 nm, which can be attributed to ⁴G_5/2→⁶H_5/2, ⁴G_5/2→⁶H_7/2, ⁴G_5/2→⁶H_9/2 and ⁴G_5/2→⁶H_11/2 [33]. In order to better illustrate the luminescence process, we drew a schematic of the energy levels of Sm³⁺ in LYBO: Sm³⁺ in Figure 4b. When excited by ultraviolet light, the electrons of Sm³⁺ transition from the ground state ⁶H_5/2 to the excited state, then undergo a non-radiative transition to the ⁴G_5/2 state, and finally these electrons return to the various ⁶H_J (J = 5/2, 7/2, 9/2, 11/2) ground states of Sm³⁺.

At the luminescence center, the variation in Sm³⁺ ions will significantly affect the intensity of the luminescence [34]. Figure 5a depicts the emission spectra (λ_ex = 404 nm) of LYBO: xSm³⁺ (x = 0.01, 0.05, 0.10, 0.15, 0.20) with different Sm³⁺ concentrations. When excited by 404 nm, the strongest emission peak is at 647 nm (⁴G_5/2→⁶H_9/2). The shape and peaks of the PL spectra remain unchanged with the increase in Sm³⁺, except the luminescence intensity. The emission intensity of phosphor is continuously increased with an increase in dopants, and is strongest at x = 0.05 as can be seen in Figure 5b, before decreasing because of concentration quenching [35].

In an effort to explain the mechanism of quenching effect, we calculated the critical distance R_c of Sm³⁺ using the Blasse theory [36]:

$$R_c\approx 2{\left[{\displaystyle \frac{3V}{4\pi x_{c}N}}\right]}^{{\displaystyle \frac{1}{3}}}$$

(1)

Here, V indicates the unit cell volume, N represents the number of cation sites occupied by Sm³⁺ in the unit cell, and x_c is the critical concentration. In this phosphor, V = 748.6 ų, x_c = 0.05 and N = 4, and the critical distance calculation is R_c ≈ 19.265 Å [36]. Generally, when R_c < 5 Å, the concentration quenching mechanism is an exchange between ions; otherwise, it is an electric multipole interaction [37]. The interaction between ions can be describe by Van Uitert’s theory [38]:

$$\lg \left({\displaystyle \frac{I}{x}}\right)=C-\left({\displaystyle \frac{Q}{3}}\right)\lg x$$

(2)

In which C is a constant, I is the luminescence intensity corresponding to the doping concentration when concentration quenching occurs, and x represents different doping concentrations. When Q = 6, 8, 10, the interactions correspond to electric dipole–electric dipole (d-d), electric dipole–electric quadrupole (d-q) and electric quadrupole–electric quadrupole (q-q) interactions [39]. The relationship between the lg(I/x) and lgx of LYBO: xSm³⁺ (x = 0.05–0.20) samples is depicted in Figure 5c. The slope is −2.03096 and the Q value is approximately 6. The concentration quenching mechanism of the LYBO: Sm³⁺ sample is considered to be a d-d interaction, accordingly. Figure 5d shows the emission peak integral areas at 613 nm and 647 nm which vary with Sm³⁺ concentration; the orange emissions and the red emissions correspond to ⁴G_5/2→⁶H_7/2 and ⁴G_5/2→⁶H_9/2. The relative contributions of ⁴G_5/2→⁶H_7/2 and ⁴G_5/2→⁶H_9/2 is in parallel with the change in the doping concentration, suggesting that a rise in Sm³⁺ does not affect the result [40].

In order to further analyze the luminescence processes of the obtained samples, the fluorescence decay curve of LYBO: xSm³⁺ (x = 0.01–0.20) was measured with 404 nm as the excitation wavelength and 647 nm as the monitoring wavelength. The fluorescence life curve is clearly decaying from single-index to multi-index; this may be due to cross relaxation or an energy transfer process between two adjacent ions that creates additional attenuation channels [41]. The deviated decay curves can be fitted by the following formula [41,42]:

$$\tau ={\displaystyle {\int }_0^{\infty }t{I}_{(t)}}dt/{\displaystyle {\int }_0^{\infty }{I}_{(t)}}dt$$

(3)

where I_(t) corresponds to the fluorescence intensity at time t. The average fluorescence lifetime of samples with various Sm³⁺concentrations (x = 0.01, 0.05, 0.10, 0.15, 0.20) was calculated as 2.64 ms, 1.59 ms, 0.83 ms, 0.42 ms and 0.38 ms, respectively. The decay lifetime of phosphors reduces with the increase in Sm³⁺ doping in Figure 6b, and the lifetimes of the samples are all less than 2.7 ms. The reason for the lifetime decrease is that increasing the amount of dopant leads to a reduction in the distance between ions and an enhancement of their interaction, and the probability of non-radiative transition between ions is increased [43].

CIE chromaticity coordinates of phosphors with different Sm³⁺ concentrations are shown in Figure 7a. The CIE coordinates of the optimal LYBO: 0.05 Sm³⁺ are (0.5977, 0.4012), and the phosphor shows a very small change in its chrominance coordinates with a change in the doping concentration. Figure 7b shows that the chromaticity coordinates of the fluorescence powder fluctuate slightly up and down with concentration changes. The standard deviation of the chromaticity coordinates Δx = 0.000006644 and Δy = 0.000002484 can be calculated, indicating that the chromaticity coordinates are relatively stable.

In addition, color purity can be calculated using Formula (4) [44,45]:

$$P={\displaystyle \frac{\sqrt{{\left(x-x_i\right)}^2+{\left(y-y_i\right)}^2}}{\sqrt{{\left(x_d-x_i\right)}^2+{\left(y_d-y_i\right)}^2}}}\times 100\%$$

(4)

In which (x_i, y_i) is the light source coordinate (0.333, 0.333), (x_d, y_d) represents CIE chromaticity coordinate of the main wavelength in the sample and (x, y) shows the CIE of LYBO samples. The calculated color purities of the LYBO: xSm³⁺ (x = 0.01–0.20) phosphor samples were 99.99%, 99.57%, 99.35%, 99.36% and 99.12%, which indicate that the prepared phosphor has a high color purity.

The correlation of color temperatures of LYBO: Sm³⁺ samples with different doping concentrations is calculated using Equation (5) [46,47]:

$$T=-437n^2+3601n^2-6861n+5514.32$$

(5)

Here, n = (x − 0.3320)/(y − 0.1858). The correlated color temperature (CCT) calculation results for LYBO: xSm³⁺ (x = 0.01–0.20) were 1711 K, 1710 K, 1710 K, 1711 K and 1715 K, which are relatively low and suitable for use as the red component in warm indoor lighting sources.

To evaluate the application of LYBO: Sm³⁺ in WLEDs, it is necessary to investigate its thermal quenching characteristics [48]. Figure 8a demonstrates the variable temperature fluorescence PL spectra (298–423 K) of LYBO: 0.05Sm³⁺ (λ_ex = 404 nm). The shape and peaks have not changed with the increase in temperature, but the PL intensity diminishes gradually due to thermal quenching [49]. A detailed visual representation of the corresponding contour plot for LYBO: 0.05 Sm³⁺ over the temperature range of 298 K to 423 K is shown in Figure 8b. Figure 8c depicts a histogram of LYBO: Sm³⁺’s normalized light intensity at various temperatures. Compared with the luminescence intensity at room temperature, the luminescence intensity at 423 K is 68.4% of that at 298 K.

The thermal quenching performances of the materials were evaluated according to the modified Arrhenius equation [50]:

$$ln\left({\displaystyle \frac{I_0}{I}}-1\right)=lnA-{\displaystyle \frac{\Delta E}{kT}}$$

(6)

Here, I₀ and I represent the luminous intensity at room and certain other temperatures, ΔE is the activation energy and A and k represent the constants. Figure 8d shows ln(I₀/I − 1) and 1/(kT) of the sample at 298–423 K, and the activation energy of LYBO: 0.05 Sm³⁺ is ΔE = 0.3238 eV, which is larger than some reported phosphors, such as NaSrLa(MoO₄)₃: Sm³⁺ (0.3100 eV), La₆Ba₄Si₆O₂₄F₂: Sm³⁺ (0.1600 eV) and Ca₂LaSbO₆: Sm³⁺ (0.2689 eV) [48,51,52]. The result shows that LYBO: 0.05 Sm³⁺ has a good thermal stability.

4. Conclusions

A series of LYBO: xSm³⁺ (x = 0.01–0.20) samples were successfully prepared via a high-temperature solid-phase reaction. XRD confirmed that the phosphors were pure phase. SEM and EDS showed that the elements in the powders were evenly distributed and the particle sizes of the powders were all less than 15 μm. Under excitation at 404 nm, the sample emitted orange-red light with a maximum peak at 647 nm. The optimal doping amount for LYBO: Sm³⁺ (x = 0.01–0.20) was x = 0.05 within the experimental range, and the quenching mechanism of fluorescence concentration was attributed to d-d interactions. In addition, the fluorescence lifetime became shorter as the amounts of dopants increased. The CIE coordinates of LYBO: 0.05 Sm³⁺ are (0.5977, 0.4012) in the orange-red region with a high color purity above 99%. The sample had a good thermal stability and an ΔE of 0.3238 eV. In conclusion, LYBO: Sm³⁺ could serve as a promising orange-red phosphor for LED applications.