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Abstract
Core-shell Bi2SiO5 nanosystem with uniform morphology and narrow size distribution has been successfully synthesized via a facile template-assisted route. With the introduction of Eu3+, detailed studies are performed to evaluate its promise as Eu3+-based phosphor host. The yielded Bi2SiO5:Eu3+ nanospheres are proven to be pure tetragonal phase via X-ray diffraction and Rietveld refinement. Moreover, the phosphor particles consist of monodisperse spheres with an average diameter of approximately 285 nm by high-resolution electron microscopy. When excited by near-ultraviolet (NUV) light, the abnormally high-intensity emission at 703 nm arising from the 5D0→7F4 transition of Eu3+ is observed. The temperature-dependent photoluminescence spectra show that the optimized Bi2SiO5:20%Eu3+ have satisfactory thermal stability with 63.7% of emission intensity at 423 K relative to 303 K. The deep-red light-emitting diode (LED) device fabricated by coating NUV chip with the Bi2SiO5:20%Eu3+ phosphors is demonstrated. The newly-developed Bi2SiO5:Eu3+ nanophosphors display commendable photoluminescence properties, demonstrating their promise as deep-red phosphor candidates for use in phosphor-converted LEDs.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Rare earth based luminescent materials have aroused considerable attention over the last decades because of the demands for a wide range of advanced applications including solid-state lighting [1,2], information storage [3,4], displays [5,6], solid-state laser [7], solar cells [8,9] and bioimaging [10,11]. These rare earth based phosphors are most commonly found in the form of solid-state solutions, which consist of rare earth based matrix doped with small concentrations of lanthanide ions. Based on the unique electronic [Xe]4fn configurations (n = 0–14), rare earth based phosphors are rich in optical characteristics [12]. The 4f-intraconfigurational transitions of lanthanide ions are usually independent of crystalline size, morphology as well as the chemical environment surrounding the luminescent lanthanide ions, due to the shielding of 4f orbitals by the filled 5s2 and 5p6 shells [13,14]. As a result, the inner-shell 4f-4f transitions give rise to sharp and recognizable light emissions. Furthermore, multicolour emissions covering from UV to near-infrared spectral region can be easily achieved by the proper selection of different lanthanide ion dopants for their rich variety of electronic levels [15]. Additionally, in contrast to other luminescent materials, rare earth based materials do not suffer from photobleaching happened in fluorescent dyes, or from surface recombination and carrier diffusion issues present in the semiconducting materials, resulting in high photochemical stability [16]. On top of these, rare earth based phosphors usually feature low toxicity because of the absence of heavy metal ions such as Cd3+ and Pb2+ [17,18]. Therefore, the excellent optical properties, high photochemical stability and low toxicity make these rare earth based phosphors ideal candidates for photoluminescence applications in optoelectronics and biomedicine.
Among the reported rare earth based phosphors, rare earth silicates have attracted great interest as promising host matrixes owing to their remarkable chemical and thermal stability, high luminescence quantum yields, and abundant resources, in which the cations of these hosts are composed of rare earth elements [19–21]. However, increased demand for rare earth elements has strained supply and elevated prices. In fact, it costs much more to obtain a large number of high purity rare earth elements than it does for the main group elements such as bismuth, which can be easily purified in large quantities by techniques like zone refining [22,23]. On the other hand, the electronic configuration of bismuth element is (Xe)4f145d106s26p3 [24]. Bi3+ can be formed after losing three valence electrons, which is the most common and most stable oxidation state [25]. When the Bi3+ ions act as cations of the host matrix, the yielded bismuth-based compounds feature easy doping with trivalent luminescent lanthanide (Ln3+) ions to obtain the optical functionality due to the similar ionic radius and valence with lanthanide ions [26,27]. Based on these reasons, bismuth silicates luminescent materials have been attracting much attention for the development and in-deep study of next generation phosphor materials in very recent years [28–31]. Bismuth silicates are usually composed of a series of crystalline compounds obtained by the reaction of Bi2O3 and SiO2 at different synthesis parameters, with the representative ones including Bi12SiO20, Bi4Si3O12 and Bi2SiO5 [32,33]. Among them, the sillenite Bi12SiO20 phase and the eulytite structured Bi4Si3O12 phase are emerging as promising phosphor hosts [31,34]. On the other hand, the metastable Bi2SiO5 phase is alternately stacked in a two-dimensional structure by [Bi2O2]2+ and [SiO3]2- layers [35,36], which is demonstrated to work as appealing lead-free ferroelectric material and promising photocatalyst recently [37–39]. Nevertheless, to date, there are few reports about the use of Bi2SiO5 as the host for lanthanide ion doping [30,40–43], even though it is characterized by low-cost, nontoxicity, favorable physicochemical stability and large lanthanide admittance.
Trivalent europium (Eu3+), as a line emitter, is the most extensively studied rare earth ion among the ions of luminescent lanthanide series [44–46]. Its luminescence stems from the internal 4f-4f electronic transitions, resulting in very narrow emission bands mainly located in the orange to the red part of the visible region [47]. The emission energies of Eu3+ vary little in different chemical environments. However, the relative intensities of different emission bands do depend on the local symmetry of Eu3+ site [48], allowing a saturated red emission color in hosts with low symmetry. Moreover, there are several excitation bands in the NUV spectral region of 360–405 nm, due to the transitions from 7F0 to the 5D4, 5G2, 5L6 and 5D3 excited states, which coincide with the emission of commercial NUV LED chips [49,50]. Currently, the Eu3+ doped luminescent materials are emerging as promising red phosphor candidates for high-quality white LEDs due to their unique properties including good color saturation, high spectral sensitivity, and high luminous efficacy [47,51,52]. As a result of intensive research, a series of Eu3+ doped phosphors, emitting in the orange part of visible spectrum due to the 5D0→7F1 magnetic-dipole transition or in the red part due to the 5D0→7F2 electric-dipole transition, are wildly studied and are being used as commercial red phosphors in solid-state lighting and displays [49,50]. In contrast, the intense deep-red emission at ∼700 nm from 5D0→7F4 transition in Eu3+ doped phosphors are rarely reported [53–58].
Here, we report a new deep-red emitting Eu3+-doped Bi2SiO5 phosphor with an intense 5D0→7F4 transition by means of a mild and controllable template-assisted route, in which the insitu reaction of Eu3+ doped bismuth precursor spheres with the silica shell is responsible for the formation of uniform Bi2SiO5:Eu3+ spheres. To the best of our knowledge, this is the first time to report the photoluminescence properties of Bi2SiO5:Eu3+ phosphors with uniform spherical morphology and narrow size distribution. The phase transition and morphology evolution of the as-prepared nanospheres were systematically investigated by X-ray powder diffraction, Fourier-transform infrared spectroscopy and high-resolution electron microscopy. The photoluminescence excitation and emission spectra, temperature-dependent emission spectra and decay curves, absolute luminescence quantum yields of the obtained nanospheres were collected in detail. The results evidence the potential of the as-synthesized Bi2SiO5:Eu3+ nanospheres for phosphor-converted LEDs.
2. Materials and methods
2.1 Materials
Analytical grade Bi(NO3)3·5H2O was provided by Alfa Aesar. Eu(NO3)3·6H2O (99.99%) was purchased from Aladdin Reagents. Polyvinyl pyrrolidone (PVP, average Mw ≈ 40,000) was obtained from Sigma-Aldrich. Nitric acid (HNO3, 68 wt%), ethylene glycol (EG), urea (CO(NH2)2), ethanol, ammonium hydroxide (NH3·H2O, 25–28 wt%) and Tetraethyl orthosilicate (TEOS, 98%) were obtained from Sinopharm Chemical Reagents. All of the chemical reagents were directly used as starting materials without further purification.
2.2 Synthesis of Eu3+-doped precursor spheres
Eu3+-doped bismuth-based precursor spheres were prepared by using the hydrothermal method [59]. In a typical synthetic process, Bi(NO3)·5H2O (0.32 mmol) and Eu(NO3)·6H2O (0.08 mmol) were dissolved into nitric acid solution (5 mL, 2M). Then, CO(NH2)2 (2.8 mmol) and PVP (0.3 g) dispersed in 25 mL of EG were slowly poured into the above solution. The mixture was stirred for 15 min and then transferred to a 50 ml Teflon-lined autoclave. The autoclave was sealed and heated at 150 °C for 5 h. After the system was gradually cooled to room temperature, the 20%Eu3+-doped precursors were obtained by centrifugation, washed several times with water and ethanol, and then dried at 60 °C for 12 h. Other samples were prepared by a similar process, except for different amounts of Eu3+ ions.
2.3 Synthesis of core-shell structured Bi2SiO5:Eu3+ nanospheres
Eu3+-doped precursor@SiO2 spheres (hereafter referred to precursor@SiO2) were synthesized by a modified Stöber process [40]. Firstly, 100 mg of precursor spheres were dispersed in 40 ml ethanol and 10 ml water under ultrasonic treatment for 30 minutes. Then, 1 ml NH3·H2O and 0.2 ml TEOS were dropped into the above solution with constant stirring at room temperature for 4 h. The resulting products were collected by centrifugation and washed with water and ethanol for several times, and the white products were dried at 75 °C for 12 h. At last, the resulting precursor@SiO2 spheres were calcinated at 750–1000 °C for 2 h in air to obtain the core-shell structured Bi2SiO5:Eu3+ nanospheres.
2.4 Characterization
Crystal structure and phase composition were carried out using Powder X-ray diffraction (XRD, DMAX-2500PC, Rigaku) with a scanning speed of 10° min−1. The crystal structure refinement was carried out using the General Structure Analysis System (GSAS) program. The morphology of the samples was measured by a field-emission scanning electron microscopy (SEM, JSM-7800F, JEOL) and a high-resolution transmission electron microscopy (HRTEM, 2100F, JEOL). The composition of the as-prepared samples was verified by inductively coupled plasma–mass spectrometry (ICP-MS, Xseries II, Thermo Scientific). Fourier transform infrared spectroscopy (FTIR) spectra were studied by an IS50 spectrometer with the KBr pellet technique. Diffuse reflectance spectra were obtained on a Specord 200 Plus UV-vis spectrophotometer. The photoluminescence (PL) excitation and emission spectra, temperature-dependent emission spectra and decay curves were recorded by Edinburgh FLS1000 spectrofluorometer equipped with a continuous 400 W xenon lamp and a µF900 flash lamp as the excitation sources, and a heating stage (TAP-02) as the temperature-controlled stage. The absolute quantum yield measurements were performed using an Edinburgh FS5 spectrofluorometer with an attached integrating sphere. Prototype deep-red LED devices were fabricated by coating NUV chips with the as-prepared nanospheres. The electroluminescence (EL) properties of the fabricated devices such as EL spectra, correlated color temperature (CCT), color rendering index (CRI) and chromaticity coordinates were measured by a HAAS 2000 photoelectric measuring system.
3. Results and discussion
3.1 Crystallinity, morphology and composition analysis of Bi2SiO5:Eu3+ nanospheres
Figure 1(a) shows The XRD patterns of the Bi2SiO5 doped with 1–30 mol% Eu3+ ions after calcination at 750 °C for 2 h. The tetragonal structured bismuth silicate (Bi2SiO5, JCPDS No. 75-1483) is identified from the XRD results. It is noted that no second phase is observed when the Eu3+ ions concentration increases from 1 to 20%. The evolution of the crystal structure of Bi2SiO5 system from the monoclinic to tetragonal is firstly introduced in (Bi1−xLax)2SiO5 (0≤x ≤0.1) crystal [60,61], and it is further confirmed by Back and co-workers when the system is doped with other lanthanide ions (Yb3+/Er3+, Yb3+/Tm3+, and Nd3+) [40–42]. The doping of Eu3+ ions can also stabilize the tetragonal structure of Bi2SiO5 sample, which is consistent with the previous reports. However, when the Eu3+ concentration reaches up to 30%, a detailed analysis of the XRD pattern shows very weak diffraction peaks at 31.3°, indicating the appearance of an unidentified impurity phase. When the Eu3+ doping concentration is higher than 30%, additional diffraction peaks become more obvious, as shown in Supplementary Fig. S1. In order to clearly study the effect of different Eu3+ doping concentrations on the phase composition, the zoom-in XRD patterns from 28.5° to 35° is depicted in Fig. 1(b). With increasing the concentration of Eu3+ ions, two main XRD peaks steadily shift toward the high angle side, indicating an apparent contraction of the crystal cell volume due to the difference between the cationic radius of Eu3+ (CN = 8, 1.066 Å) and Bi3+ (CN = 8, 1.170 Å).
Fig. 1. (a) XRD patterns of Bi2SiO5:x%Eu3+ (x = 1, 5, 10, 20, 30) nanospheres obtained at 750 °C for 2 h. (b) Zoom-in XRD patterns from 28.5 to 35 degree. (c) XRD patterns of the as-synthesized 20%Eu3+-doped precursor, precursor@SiO2 and Bi2SiO5:20%Eu3+ nanospheres obtained at different temperatures (750 °C, 800 °C, 850 °C, 900 °C and 1000 °C). (d) The Rietveld refinement results of Bi2SiO5:20%Eu3+ nanospheres obtained at 800 °C.
XRD patterns of 20%Eu3+-doped precursor, precursor@SiO2 and Bi2SiO5:20%Eu3+ obtained at different temperatures (750 °C, 800 °C, 850 °C, 900 °C and 1000 °C) are depicted in Fig. 1(c). Three broad bands are observed in the XRD result of the precursor, indicating the low crystallinity of the obtained products via the mild hydrothermal synthesis process. The XRD pattern of precursor@SiO2 sample shows an analogous profile with that of the precursor. When the precursor@SiO2 samples are calcined at 750 °C for 2 h, all diffraction peaks can be readily assigned to pure tetragonal phase of Bi2SiO5 (ICSD#245035). No additional peaks of other phases are observed, which indicates that the reaction of precursor core and SiO2 shell at high temperature successfully results in the formation of pure metastable Bi2SiO5 phase and the introduction of Eu3+ has no effect on the tetragonal structure of Bi2SiO5. With the increase of the calcination temperature to higher temperatures (800 °C, 850 °C and 900 °C), the diffraction peaks become stronger and narrower, which indicates that the calcination at higher temperature can increase the crystallization of the yielded Bi2SiO5:Eu3+ nanospheres. It should be noted that, when the precursor@SiO2 are calcinated at 1000 °C, the diffraction peaks of cubic Bi4Si3O12 (JCPDS No. 76-1726) begin to appear, indicating that there is a further chemical reaction between Bi2SiO5 and SiO2 shell. XRD evolution results in Fig. 1(c) clearly demonstrate that pure Bi2SiO5:Eu3+ nanospheres can be synthesized in the calcination temperature range of 750–900 °C.
In order to further understand the detailed crystal structure and extract lattice parameters of the Bi2SiO5:20%Eu3+ nanospheres synthesized at 750 °C, the Rietveld XRD refinement was performed using the General Structure Analysis System software, and the refinement results are summarized in Fig. 1(d). The tetragonal Bi1.9La0.1SiO5 crystal system was used as the initial model. It was assumed that a certain amount of Eu3+ substitute the sites of Bi3+. Figure 1(d) shows the raw data, calculated values, differences and Bragg positions for the refinement of Bi2SiO5:Eu3+. It is obvious that the measured and calculated XRD patterns coincide well with each other. The reliability factors are Rwp = 9.05%, Rp = 6.47% and χ2 = 3.971 respectively, indicating the validity of refinement results. The refinement results further demonstrate the feasibility that the Eu3+ ions can replace the Bi3+ sites in Bi2SiO5 host lattices. Meanwhile, as listed in Table S1, the lattice parameters of the Bi2SiO5:Eu3+ are slightly smaller than that of pure tetragonal Bi2SiO5. the volumetric constriction of the sample also suggests that the lanthanide ions have been successfully doped into the lattice and occupied the sites of Bi3+ in Bi2SiO5 host.
SEM and TEM were used to clearly examine the changes of morphological and structural features of the samples during the formation process of core-shell structure. The average diameter and statistic of particle size distribution of 20%Eu3+ doped precursor, precursor@SiO2 and the final Bi2SiO5:20%Eu3+ nanospheres obtained at different calcination temperatures (750 °C, 800 °C and 850 °C), were obtained by counting more than 200 individual particles of the corresponding SEM images, which were used to analyze the movement behaviors at the interface between precursor core and SiO2 shell. As shown in Fig. 2(a)–(c), 20%Eu3+ doped precursor synthesized via hydrothermal treatment is composed of monodisperse spheres with a mean diameter of about 210 nm. When the precursor core is coated by a SiO2 layer via a modified Stöber process, uniform core-shell structured precursor@SiO2 spheres with an average particle size of about 280 nm is obtained (Fig. 2(d)–(f)). It is found that the gray SiO2 layer is evenly coated on the surface of precursor core and the thickness of the SiO2 shell is about 35 nm. After the precursor@SiO2 particles is calcinated at 750 °C, the uniform core-shell structured spheres with an average diameter of about 285 nm are obtained, as shown in Fig. 2(g)–(i). The obtained samples after calcination at 750 °C are considered to be Bi2SiO5:Eu3+ nanospheres according to the XRD results in Fig. 1. Compared with the precursor@SiO2 samples, the particle size has increased slightly from 280 nm to 285 nm. These particles show the same spherical morphology but larger particle size when compared with other lanthanide ions doped Bi2SiO5 nanoparticles synthesized by Back et al [40–42]. Moreover, it is found that the thickness of the outer SiO2 layer has been decreased a lot and the cavity begins to appear due to the chemical reaction between the precursor core and SiO2 shell. This is because that the outward diffusion of Bi and Eu atoms has a faster rate than that of inward diffusion of Si atoms in the interface during the chemical reaction process between the precursor core and SiO2 shell at high temperature. Therefore, some Bi atoms most probably diffuse to the outermost edge of silica shell to form Bi2SiO5 particles, resulting in the growth of particle size and the formation of hollow core-shell structure. When the calcination temperature is further increased (e.g., 800 °C and 850 °C), the obtained products can still maintain the uniform core-shell structure. But the outer layer surrounded the core particles become thinner and the yielded nanospheres shows a slight increase in size (Fig. 2(j)–2(o)). Thanks to the outer SiO2 layer on the surface of precursor core, the Bi2SiO5:20%Eu3+ nanospheres obtained at different calcination temperatures from 750 °C to 850 °C can still keep their uniform spherical morphology and narrow size distribution. The outer silica layer plays an important role in the formation of uniform core-shell structure, which not only acts as the silicon source, but also provides physical barriers to avoid the direct contact between precursor cores when calcinated at high temperature [40–42,62]. The SEM images of bismuth-containing precursor doped with different concentrations of Eu3+ (x=1, 5, 10, 20 and 30), precursor@SiO2 and the final Bi2SiO5:x%Eu3+ nanospheres are shown in Supplementary Fig. S2-S4. Moreover, the particle size distributions of Bi2SiO5:x%Eu3+ nanospheres were determined by counting more than 200 individual particles of the corresponding SEM image for each Eu3+ concentration, and the results are given in Supplementary Fig. S5. It is found that all the samples have an average diameter of about 275 nm ± 50 nm, suggesting that the particle size distribution is largely independent of the Eu3+ doping concentration. The above results indicate that the Eu3+ doping concentration in the range of 1 to 30 have no obvious influence on the particle size and spherical morphology of the precursors, precursor@SiO2, and the final Bi2SiO5:Eu3+ nanospheres. It should also be noted that the yielded nanospheres can still keep uniform spherical morphology and narrow size distribution when the precursor@SiO2 are calcinated at 900 °C. However, when the calcination temperature is further increased to 1000 °C, the yielded particles show severe aggregation and irregular morphology, as shown in Fig. S6.
Fig. 2. SEM (a, d, g, j, m), TEM images (b, e, h, k, n) and size distributions (c, f, i, l, o) of 20%Eu3+ doped precursor, precursor@SiO2 and the final Bi2SiO5:20%Eu3+ nanospheres obtained at different calcination temperatures (750 °C, 800 °C and 850 °C).
3.2 Optical band gap
The band gap of the synthesized Bi2SiO5:Eu3+ samples can be determined by extrapolating the linear portion based on their diffuse reflection (DR) spectra. The diffuse reflectance spectrum and the Kubelka–Munk function of the Bi2SiO5:20%Eu3+ nanospheres are shown in Fig. 3(a). The strong absorption band in the shorter-wavelength UV spectral region is attributed to the inter-band absorption of Bi2SiO5 host. The characteristic absorption peaks of the Eu3+ ions due to the intra-configurational 4f-4f transitions are detectable in the NUV and visible regions. The UV-Vis results further confirm that the Eu3+ has been successfully doped into Bi2SiO5 lattice. The content of Bi and Eu in Bi2SiO5:20%Eu3+ were also quantified by using inductively coupled plasma–mass spectrometry. The Bi/Eu molar ratio is determined to be 92.2/7.8, as shown in Table S2. The result indicates that there is the loss of Eu content in the final samples. We speculate that the loss of Eu content is mainly ascribed to the use of urea precipitant agent, which produces a weaker alkaline environment and cannot precipitate the added Eu3+ ions completely in the precursor solution during the hydrothermal synthesis. As shown in Fig. 3(b), the bandgap energy value of the Bi2SiO5:20%Eu3+ phosphor is estimated by extrapolating the intercept of the fitted straight line at (F(R)·hν)2 = 0 in the plot of (F(R)·hν)2 versus energy (hν) according to the following equation [30]:
where F(R) represents the absorption coefficient, R represents the reflectance at the given wavelength, hν is the photon energy, the exponent n denotes the nature of the transition (in our case, a direct allowed transition is considered; thus, n = 1/2), A is the absorption constant and Eg represents the optical bandgap value. The value of optical band gap is determined to be 4.00 eV for Bi2SiO5:20%Eu3+ nanospheres, which is consistent with the reported results [30].Fig. 3. (a) Diffuse reflectance spectrum of Bi2SiO5:20%Eu3+ nanospheres with the Kubelka–Munk function (inset). (b) The bandgap estimation for the Bi2SiO5:20%Eu3+ nanospheres.
3.3 Photoluminescence properties of Bi2SiO5:Eu3+ nanospheres
In order to evaluate the potential application of the obtained core-shell Bi2SiO5:Eu3+ spheres as red-emitting phosphors for solid-state lighting application, the photoluminescence properties of Bi2SiO5:Eu3+ nanospheres were investigated. Figure 4(a) displays the photoluminescence excitation and emission spectra of Bi2SiO5:20%Eu3+ nanospheres synthesized at 800 °C. the excitation spectrum monitored at 703 nm emission covers the UV and blue spectral regions. The broad absorption band in the shorter-wavelength UV region is assigned to the absorption of Bi2SiO5 host. The sharp peaks in the NUV and blue spectral regions correspond to the intra-configurational f-f transitions of Eu3+ ions, i.e., 7F0→5D4 (362 nm), 7F0→5G3 (377 nm), 7F0→5G2 (383 nm), 7F0→5L6 (394 nm), 7F0→5D3 (414 nm) and 7F0→5D2 (465 nm). The strong absorption in the region of 360-400 nm coincides well with the emission of NUV LED chips, which means that this phosphor can be used as LED conversion phosphors. When exciting with 394 nm light, Bi2SiO5:20%Eu3+ nanospheres show several narrow emission bands mainly in the orange and red part of the visible spectrum due to the typical 5D0→7FJ (J=0, 1, 2, 3, and 4) transition of Eu3+. Among these emission bands, the intense emission line at 703 nm originating from 5D0→7F4 transition is dominated. The other weak emission peaks correspond to the 5D0→7F0 (579 nm), 5D0→7F1 (587, 595 nm), 5D0→7F2 (613, 621 nm) and 5D0→7F3 (652 nm), respectively. The presence of the 5D0→7F0 transition at 579 nm indicates that the Eu3+ occupies only one site in the host lattice, that is, the Bi3+ sites of the Bi2SiO5 host. In most cases, the dominant emission peaks of Eu3+-doped phosphors come from either magnetic-dipole transition (5D0→7F1) or electric-dipole transition (5D0→7F2). However, in Bi2SiO5:Eu3+ nanospheres, the emission intensity of 5D0→7F4 transition is enhanced greatly, nearly three times higher than that at 595 nm and 613 nm. This is because that the geometry of the Eu3+ doped Bi2SiO5 is distorted from monoclinic symmetry to higher tetragonal one [40–42,60,61], resulting in an intense 5D0→7F4 transition and a weak 5D0→7F2 transition in our sample.
Fig. 4. (a) Photoluminescence excitation and emission spectra of Bi2SiO5:20%Eu3+ phosphors. (b) Emission spectra and (c) decay curves of Bi2SiO5:x%Eu3+ (x=1, 5, 10, 20, 30) phosphors. (d) Emission spectra of the Bi2SiO5:20%Eu3+ phosphors obtained at 750 °C, 800 °C, 850 °C, 900 °C and 1000 °C for 2 h, respectively.
The emission spectra of Bi2SiO5:Eu3+ nanospheres with different Eu3+ doping concentration ranging from 1% to 30% under 394 nm excitation are presented in Fig. 4(b). It can be seen that with increasing the doping concentration of Eu3+ from 1% to 20%, the emission intensity increases rapidly until reaching its maximum value and then decreases sharply due to the concentration quenching effect and the existence of the impurity phase in the material. For most of the lanthanides doped luminescent materials, the doping concentrations can directly affect the emission intensity. When the doping concentration is low, the emission intensity increases with the increase of doping concentration, and the interaction between the activators can be neglected. With the further increase of the doping concentration to a certain value, the emission intensity begins to decrease because of the nonradiative energy transfer among activators, which brings the excitation energy to killer sites and results in the occurrence of concentration quenching. The nonradiative energy transfer usually occurs as a result of radiation reabsorption, exchange interaction or multipole-multipole interaction. Radiation reabsorption interaction can happen when there exists a broad overlap between the excitation spectrum of the activator and the emission spectra of the sensitizer. So the radiation reabsorption can be excluded in our sample because Eu3+ ion shows the forbidden 4f→4f transition and no overlap exists between the excitation and emission spectrum. In view of exchange interaction, according to the Blasse's theory [63], the critical transfer distance (Rc) can be estimated by the following equation:
where V is the volume of the unit cell, Xc represents the critical concentration and N is the number of Bi3+ ions in the unit cell. In the case of Bi2SiO5:Eu3+ phosphor, the value of V is 223.37 Å3, Xc = 0.2 and N = 4. Then Rc is calculated to be about 8.11 Å, which is much larger than 5 Å. Therefore, according to the Blasse's theory, exchange interaction becomes ineffective in the Bi2SiO5:Eu3+ nanospheres and multipole-multipole interaction is responsible for the concentration quenching effect.Figure 4(c) shows the decay curves of the Bi2SiO5:xEu3+ nanospheres as a function of Eu3+ ion concentration monitored at 703 nm with the excitation wavelength of 394 nm. A single-exponential equation is used to fit the lifetime curves [50]:
where I(t) and I0 represent the PL intensity at time t and the background intensity for dark current of the detector. A refers to constant; and τ is the decay time. It is found that the lifetime gradually decreases with the increase of Eu3+ content, which is attributed to the increase of nonradiative relaxation rates at higher Eu3+ doping concentration.To evaluate the influence of calcination temperature on the photoluminescence properties of the Bi2SiO5:Eu3+ nanospheres, photoluminescence emission spectra of the samples calcinated at different temperatures ranging from 750 °C to 1000 °C were recorded, as shown in new Fig. 4(d). The PL spectra of the samples obtained in the temperature range of 750–900 °C share an almost identical spectral profile with each other. Moreover, the emission intensity increases with the increase of calcination temperature, which can be attributed to the good dispersion of doping components inside the host lattice and the increased crystallization of Bi2SiO5 host. However, for the sample obtained at 1000 °C, the luminescence intensity of 5D0→7F4 transition (703 nm) decreases obviously because the existence the Bi4Si3O12 impurity phase. The calculated Commission Internationale de I’Eclairage (CIE) chromaticity coordinate for the Bi2SiO5:20%Eu3+ phosphor obtained at 800 °C is located at (0.627, 0.373), as shown in Supplementary Fig. S7.
3.4 Temperature-dependent decay curves and thermal stability of Bi2SiO5:20%Eu3+ nanospheres
In order to further investigate the effect of environment temperature on the photoluminescence properties of Bi2SiO5:20%Eu3+ phosphor obtained at 800 °C, the temperature-dependent decay curves by monitoring 703 nm emission under 394 nm excitation were recorded from 303 K to 573 K, as shown in Fig. 5(a). The calculated values of lifetime for Bi2SiO5:20%Eu3+ nanospheres decrease from 2.47 ms to 1.73 ms with the increase of environment temperature, mirroring the increase of probability of non-radiative transition at elevated temperature.
Fig. 5. (a) Temperature-dependent decay curves of Bi2SiO5:20%Eu3+ nanospheres in the ranges of 303–573 K. (b) Temperature-dependent emission spectra of the Bi2SiO5:20%Eu3+ nanospheres. (c) Dependence of emission intensity on the temperature. (d) Plot of ln(I0/I - 1) versus 1/kT.
The thermal stability is an indispensable parameter to evaluate a phosphor for its application in LEDs, because it has a great effect on chromaticity, light output and color rendering index. When excited at 394 nm, the temperature-dependent PL emission spectra of red-emitting Bi2SiO5:20%Eu3+ nanospheres in the temperature range of 303–513 K were measured, and the results are presented in Fig. 5(b). It is noted that, with the increase of temperature, the spectral shape and peak position in the emission spectra remain unchanged, which is beneficial to obtain stable emitting color at elevated temperature. However, the emission intensity decreases gradually owing to the thermal quenching effect. As shown in Fig. 5(c), the emission intensity at 423 K can still maintain approximately 63.7% of its initial value at 303 K, which suggests that the Bi2SiO5:Eu3+ nanospheres have relatively good thermal stability and have the potential for white LED application. In order to better understand the thermal quenching phenomenon, the activation energy of the Eu3+ ions in the studied Bi2SiO5 host was evaluated by the following expression [64,65]:
where I0 is the initial emission intensity, I is the emission intensity at the studied temperatures, the residual coefficients of A, ΔE and k are associated with constant, activation energy and Boltzmann constant, respectively. The value of k is fixed at 8.629 × 10−5 eV K-1. Figure 5(d) displays The plot of ln(I0/I - 1) versus 1/kT. Through the best fit using the Arrhenius equation, the activation energy (ΔE) is determined to be 0.22 eV for the Bi2SiO5:Eu3+ nanospheres. The temperature-dependent emission spectra in the temperature range from 93 K to 483 K is also given in the Supplementary Fig. S8. It is found that the thermal quenching begins at 93 K and the emission intensity decreases with the increase of temperature. The internal quantum efficiency (QE) was measured under the excitation of 394 nm light by using a barium sulfate coated integrating sphere attached to the spectrophotometer. Details of the QE measurements are described in Supplementary Fig. S9. The value of quantum efficiency in Bi2SiO5:20%Eu3+ phosphor is measured to be 29%. Table 1 shows the luminescence parameters of well-known Eu3+-doped phosphors, including CIE color coordinate, thermal stability, activation energy, and internal quantum efficiency. These Eu3+ doped phosphors, mainly emitting in the red part of visible spectrum due to the 5D0→7F2 electric-dipole transition, are wildly studied and are being used as red phosphors in solid-state lighting. In contrast, the abnormally high intensity deep-red emission at 703 nm from 5D0→7F4 transition in Bi2SiO5:Eu3+ phosphor is reported for the first time.
Table 1. Comparison of the luminescence performances of Eu3+ doped phosphors.
3.5 Electroluminescence (EL) performance of the fabricated deep-red LED device
To further confirm the suitability of the as-synthesized nanospheres for solid-state lighting application, a red-emitting LED device was constructed by coating Bi2SiO5:20%Eu3+ nanospheres on the surface of a 394 nm NUV LED chip. the EL emission spectrum of the fabricated LED device was recorded under the injection current of 60 mA, as shown in Fig. 6(a). The EL emission spectrum consists of several narrow emission bands in the wavelength range of 550–750 nm corresponding to the characteristic emissions of Eu3+ ions. The high intensity emission at 703 nm arising from the 5D0→7F4 transition of Eu3+ is also observed. Meanwhile, under the drive current of 60 mA, the developed LED device emits deep-red emission with color coordinate of (0.626, 0.374), as shown in Fig. 6(b). Additionally, the El emission spectra of the fabricated red LED device under diverse drive currents ranging from 20–80 mA are given in Supplementary Fig. S10. With increasing the drive current, the spectral shape and peak position in the EL emission spectra remain constant, except that the emission intensity increases. The color coordinates, CCT and CRI values of the packaged red LED device change little with the increase of injected current, as listed in Table S3, suggesting that the Bi2SiO5:Eu3+ nanospheres are promising deep-red phosphor candidates for use in phosphor-converted LEDs.
Fig. 6. (a) EL spectrum of the fabricated deep-red LED device under the drive current of 60 mA. (b) CIE chromaticity diagram of the fabricated deep-red LED device. Inset shows the digital image of the fabricate deep-red LED device without and with injection current.
4. Conclusion
In summary, core-shell structured Bi2SiO5:Eu3+ deep-red emitting nanospheres with uniform morphology and narrow size distribution have been successfully synthesized via a facile template-assisted route. The as-synthesized Bi2SiO5:Eu3+ nanospheres show spherical morphology with an average diameter of about 285 nm. Thanks to the incorporation of Eu3+ ions, the yielded Bi2SiO5:Eu3+ nanospheres can emit characteristic emissions of Eu3+ when exciting with NUV light. The 5D0→7F4 transition at 703 nm is the most intense emission band, a favorable attribute for deep-red emitting phosphors. The NUV based LED device fabricated by using the as-synthesized Bi2SiO5:20%Eu3+ sample as a red phosphor component, can produce deep-red emission with chromaticity coordinates (0.626, 0.374). Overall, not only can Bi2SiO5 host accommodate high concentrations of Eu3+, leading to an unusual and intense 5D0→7F4 transition at 703 nm, it is comparatively easy to synthesize, contains earth-abundant component elements, and has good chemical stability once formed.
Funding
National Natural Science Foundation of China (51902184); Natural Science Foundation of Shandong Province (ZR2019BEM028); Qi-Lu Young Scholar Fund (31370088963167); Major Basic Research Projects of Shandong Natural Science Foundation (ZR2018ZB0104).
Disclosures
The authors declare no conflicts of interest.
See Supplement 1 for supporting content.
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