Synthesis, electronic structures, and photoluminescence properties of an efficient and thermally stable red-emitting phosphor Ca₃ZrSi₂O₉:Eu³⁺,Bi³⁺ for deep UV-LEDs

Abstract

A series of red-emitting Ca₃ZrSi₂O₉:Eu³⁺,xBi³⁺ phosphors was synthesized using a conventional high temperature solid-state reaction method, for the purpose of promoting the emission efficiency of Eu³⁺ in a Ca₃ZrSi₂O₉ host. The site preference of Bi³⁺ and Eu³⁺ in the Ca₃ZrSi₂O₉ host was evaluated by formation energy. The effects of Bi³⁺ on electronic structure, luminescent properties, and related mechanisms were investigated. The inner quantum yield of the optimized sample increased to 72.9% (x = 0.08) from 34.6% (x = 0) at 300 nm ultraviolet light excitation. The optimized sample (x = 0.08) also showed excellent thermal stability, and typically, 84.2% of the initial emission intensity was maintained when the temperature increased to 150 °C from 25 °C, which is much higher than that without Bi³⁺ doping (70.1%). The mechanisms of emission properties and thermal stability enhancement, as well as the redshift of the charge transfer band (CTB) induced by Bi³⁺ doping in the Ca₃ZrSi₂O₉:Eu³⁺ phosphor, were discussed. This study elucidates the photoluminescence properties of Bi³⁺-doped Ca₃ZrSi₂O₉:Eu³⁺ phosphor, and indicates that it is a promising luminescent material that can be used in ultraviolet light-emitting diodes.

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1. Introduction

Phosphor-converted white light-emitting diodes (pc-WLEDs) have the advantages of long lifetime, high luminous efficacy, and environmental friendliness, and are becoming an indispensable solid-state light source in our daily life.^1–5 However, conventional WLEDs using blue InGaN chips combined with phosphors are very likely to cause health problems because excessive exposure to monochromatic blue light often results from their use.^6,7 Therefore, ultraviolet (UV) LEDs combined with phosphors are an important and promising alternative for the fabrication of full spectrum lighting sources that are healthy and comfortable to human beings.^8,9 With their rapid development, UV LEDs are mainly divided into near-UV LEDs (300–400 nm) and deep UV LEDs (200–300 nm).¹⁰ Correspondingly, new highly efficient and thermally stable phosphors should be rapidly developed so as to match the emission wavelength of UV LED chips.¹¹ Generally, Y₂O₃:Eu³⁺ is considered a red-emitting phosphor for UV LEDs that are solely composed of rare earth elements. Because rare earth elements are non-renewable resources, it is necessary to develop efficient and thermally stable red-emitting phosphors with inexpensive raw materials and comparative properties to replace the use of Y₂O₃:Eu³⁺ in UV LEDs.

Recently, Kim et al. reported a cuspidine-type Ca₃ZrSi₂O₉:Eu³⁺ red-emitting phosphor possessing a relative emission intensity that reached 84% of commercial red-emitting Y_1.94Eu_0.06O₃ phosphors, and it seemed promising as a replacement for Y₂O₃:Eu³⁺, but the inner quantum yield was only 41% at 268 nm excitation.¹² In order to further enhance the emission efficiency, Zuo et al. tried to partially substitute Zr⁴⁺ ions with Al³⁺ ions in the host Ca₃ZrSi₂O₉, and finally the inner quantum yield was increased to 46%.¹³ This enhancement was mainly ascribed to the charge compensation effect, which is helpful for improving the crystallinity of the phosphors. However, the relatively low quantum yield still limits its practical use.

Here, we report another route for enhancing the quantum yield of this phosphor by introducing the sensitizer Bi³⁺ into the lattice. It is known that Bi³⁺ has an ns²-type ground state electric configuration (¹S₀ state) and ¹S₀ → ³P₁ and ¹S₀ → ¹P₁ spin-allowed transitions, which are expected to have a reasonable absorption strength.¹⁴ The aborted energy can be efficiently transferred to luminescence center ions, such as Eu³⁺ and Mn⁴⁺, which act as a sensitizer, or directly emitting photons, acting as a luminescence center due to the transitions of ³P₁ → ¹S₀ and ¹P₁ → ¹S₀.^15–19 For this reason, many commercially available Eu³⁺-activated phosphors, such as Y₂O₃:Eu³⁺ and Y(P,V)O₄:Eu³⁺, adopt Bi³⁺ as sensitizer.^20–22 In this approach, the inner quantum yield of Ca₃ZrSi₂O₉:Eu³⁺,Bi³⁺ was greatly promoted, and the thermal stability was also enhanced. To further understand this enhancement, first principles calculations were performed to investigate the site preference of dopants and electronic structure variations induced by Bi³⁺ doping. The synthesis and photoluminescence properties were also experimentally characterized in detail. The photoluminescence investigation demonstrates that the Ca₃ZrSi₂O₉:Eu³⁺,Bi³⁺ phosphor is a promising red-emitting phosphor that can be incorporated into UV LEDs.

2. Methodology

2.1 Experimental details

The samples with formula (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0–0.16) were synthesized by the high-temperature solid-state reaction method. The starting materials consisting of CaCO₃ (Aldrich, 99.95%), Bi₂O₃ (Aldrich, 99.9%), ZrO₂ (Aldrich, 99.5%), SiO₂ (Aldrich, 99.99%) and Eu₂O₃ (Aldrich, 99.95%) were weighed according to the stoichiometric ratio. The mixed powder was evenly ground in an agate mortar, and then the homogeneous mixtures were placed in an alumina crucible and continually heated at 1400 °C in an air atmosphere for 6 h. The samples were gradually cooled to room temperature and then ground once more until a fine powder was obtained.

The powder X-ray diffraction (XRD) patterns were measured using an X-ray powder diffractometer (Rigaku, Japan) with Cu-Kα radiation (λ = 1.5406 Å). The photoluminescence spectra and the decay curves of Eu³⁺ lifetime values were measured using a FLS-980 fluorescence spectrophotometer (Edinburgh Instruments) equipped with a xenon lamp (450 W, Osram) as the excitation source. The quantum yield and temperature-dependent emission spectra were measured using the QE-2100 quantum yield measurement system (Otsuka Electronics Co., Ltd., Japan), composed of an integrating sphere, a heating apparatus, and a Xe lamp used as an excitation source and white BaSO₄ powder as a reference.

2.2 Computational details

Structural optimization of Ca₃ZrSi₂O₉, Ca₃ZrSi₂O₉:Bi³⁺, and Ca₃ZrSi₂O₉:Eu³⁺ was firstly performed using the density functional theory (DFT) method with the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional,²³ as implemented in the VASP software package.^24,25 The electronic properties of Ca₃ZrSi₂O₉:Bi³⁺ were then calculated based on the optimized geometries using the hybrid PBE functional. The Ca₃ZrSi₂O₉ host crystal containing 60 atoms was used as the computational model. The Ca 3s²3p⁶4s², Zr 4s²4p⁶5s²4d², Si 3s²3p², O 2s²2p⁴, Bi 5d¹⁰6s²6p³, and Eu 5s²5p⁶4f⁶5d¹6s² electrons were treated as the valence electrons, whose interactions with the ion cores were treated with the projected augmented wave (PAW) method.²⁶ The geometric structures were fully relaxed with the convergence criteria of 10⁻⁶ eV used for the change in the total energy and 0.01 eV Å⁻¹ used for Hellmann–Feynman forces on atoms. The cut-off energy of 550 eV was used for the basis set of the plane waves. The Brillouin zone integrations were sampled using a 4 × 2 × 2 Monkhorst–Pack k-point mesh.

3. Results and discussion

3.1 Phase purity, crystal structure, and site preference of dopants

Fig. 1 presents the powder XRD patterns for the synthesized (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0–0.16) samples compared with the standard pattern (PDF no. 54-0710) for Ca₃ZrSi₂O₉. All the diffraction peaks of these samples can be indexed to the P2₁/c space group (no. 14) of the monoclinic system,²⁷ indicating that a single phase with high purity has been synthesized, and the doped Bi³⁺ and Eu³⁺ did not generate any impurity or induce significant changes in the host structure. The Ca₃ZrSi₂O₉ compound can be regarded as being derived from the well-known cuspidine Ca₄Si₂O₇F₂ by substituting Zr-O₂ for Ca-F₂.²⁸ Thus, the crystal structure (shown in Fig. 2) of Ca₃ZrSi₂O₉ is similar to Ca₄Si₂O₇F₂, composed of [SiO₄] tetrahedrons, [ZrO₆] octahedrons, [CaO₆] octahedrons, and [CaO₇] decahedrons, forming a three dimensional network. There are three types of Ca²⁺ sites and one type of Zr⁴⁺ site, and they are available for Eu³⁺ and Bi³⁺ to occupy.

Fig. 1 Powder XRD patterns of the synthesized (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0–0.16) samples, and the standard card (PDF no. 54-0710) of Ca₃ZrSi₂O₉ is given for comparison.

Fig. 2 Crystal structure of Ca₃ZrSi₂O₉ views along the (100) direction; and three types of Ca sites, namely, Ca1, Ca2, and Ca3 with seven-, six-, and seven-coordinations, respectively.

To identify the site preference, the formation energy E_form was calculated according to the following equation:^29,30

E_form = E_M:CZSO − E_CZSO + μ_{Ca or Zr} − μ_M (1)

where CZSO refers to Ca₃ZrSi₂O₉, M represents metal Eu or Bi, E_M:CZSO and E_CZSO is the total energy of the metal-doped CZSO system and undoped CZSO, respectively, and μ is the total energy per atom of the bulk metal. According to this equation, the formation energies of Eu³⁺ and Bi³⁺ at possible cation sites were calculated and are shown in Fig. 3. As presented, both Eu³⁺ and Bi³⁺ exhibit much lower formation energy in the Ca²⁺ sites than in the Zr⁴⁺ site. The formation energy for Eu³⁺ and Bi³⁺ in three types of Ca²⁺ sites is very similar, but Eu³⁺ entering Ca₍₃₎ and Bi³⁺ entering Ca₍₁₎ provide slightly lower formation energy. These results indicate that Eu³⁺ and Bi³⁺ only occupy Ca²⁺ sites, but nearly have equal possibility to enter any type of Ca²⁺ site during the high temperature sintering process. The formation energy results also suggest that it is much more difficult to dope Bi³⁺ into the lattice than Eu³⁺, mainly due to its larger ionic radius and size mismatch effect.

Fig. 3 Formation energy of Eu³⁺ and Bi³⁺ doping into Zr⁴⁺ and three types of Ca²⁺ sites in the Ca₃ZrSi₂O₉ host.

3.2 Electronic structure of Bi³⁺-doped and undoped Ca₃ZrSi₂O₉

Fig. 4a presents the total and partial density of states (DOSs) of the Ca₃ZrSi₂O₉ host. As shown, the conduction band is mainly composed of Ca 3d and Zr 4d states, which show a hybridization character with O 2p and Si 3p states. The bottom of the conduction band is mainly determined by Zr 4d states. The top of the valence band reflects the p electronic character, deriving from O 2p states. The valence bands at the lower energy are mainly attributed to Si 3s/3p and O 2s/2p states. The band gap is calculated to be 4.59 eV, corresponding to the optical absorption at 270 nm of Zr⁴⁺–O²⁻. This energy value is slightly underestimated compared to the experimentally observed value of 255 nm,³¹ due to the drawback of the DFT method.³²

Fig. 4 Total and partial density of states of the (a) Ca₃ZrSi₂O₉ host; (b) Bi³⁺ doping into the Ca₍₁₎ site; and (c) Bi³⁺ doping into the Zr⁴⁺ site.

Fig. 4b shows the total and partial density of states of Bi doped in the most possible Ca₍₁₎ site. In this case, the bottom of the conduction band is mainly composed of Zr 4d and Bi 6p states, and a strong hybridization between them can be observed. The p electronic character of the O 2p states remains at the top of the valence band. Additionally, the Bi 6s states emerge at the band gap, slightly above the top of the valence band (by approximately 0.18 eV), presenting a strong hybridization with O 2p states. The band gap is reduced to 4.41 eV.

The total and partial density of states of Bi doped in the Zr site were also investigated and are presented in Fig. 4c. In this situation, the Bi 6p states are higher than the Zr 4d states, and thus, the bottom of the conduction band is determined by the Zr 4d states. The top of the valence band still presents the p electronic character of O 2p states, while the location of Bi 6s is significantly higher than that with Bi doping at the Ca₍₁₎ site. Also, a strong hybridization between the Bi 6s and O 2p states can be detected. The band gap is not influenced by the doped Bi and remained at 4.59 eV.

3.3 Photoluminescence properties of Ca₃ZrSi₂O₉:Eu³⁺,xBi³⁺

The photoluminescence emission (PL) spectra of samples (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0–0.16) excited by 300 nm UV light are presented in Fig. 5. As shown, the spectrum exhibits typical characteristics of Eu³⁺, with several sharp emission peaks located at 579 nm, 586/595 nm, 610/622/626/630 nm, 648/654 nm, and 704/707/709/712 nm corresponding to the transitions of ⁵D₀ → ⁷F_J (J = 0–4),^33,34 respectively. The emission intensity gradually increases and then decreases with the increasing concentration of Bi³⁺ doping. The optimized Bi³⁺ concentration is x = 0.08, with an inner quantum yield of 72.9% and external quantum yield of 59.7% under 300 nm excitation, while the sample without Bi³⁺ doping has an inner quantum yield of 34.6% and external quantum yield of 27.3% under the same measured conditions. This emission enhancement is mainly ascribed to the sensitization provided by Bi³⁺ because Bi³⁺ can absorb energy through ¹S₀ → ³P₁ and ¹S₀ → ¹P₁ transitions and transfer its energy to Eu³⁺, improving the absorbance of this phosphor. However, when the Bi³⁺ doping concentration exceeds x = 0.08, more defects may form due to nonequivalence substitution, resulting in a slight decrease in emission intensity.³⁵

Fig. 5 The photoluminescence emission spectra of samples (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0–0.16) excited by 300 nm UV light. The emission intensity of the dominant peak (610 nm) as a function of Bi³⁺ concentration was inserted.

The normalized photoluminescence excitation spectra of (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0 and 0.08) monitored at 610 nm are compared and shown in Fig. 6. It can be observed that the excitation peaks ascribing to f–f transitions of Eu³⁺ are in good agreement at each wavelength, while the charge transfer band (CTB) presents a redshift of approximately 10 nm (approximately 0.14 eV) when Bi³⁺ was introduced in this phosphor, which indicates that the 4f energy levels are not influenced by Bi³⁺ doping, but the band gap and Eu³⁺–O²⁻ interactions may have been affected.^16,36 This redshift is mainly ascribed to the existence of a hybrid level constituted by O 2p and Bi 6s states above the valence band, which decrease the energy between the O 2p and Eu 4f states. The energy difference (approximately 0.14 eV) corresponding to the redshift in CTB is very close to the energy separation (approximately 0.18 eV) between the above mentioned hybrid level and the top of the valence band obtained by DFT calculation (shown in Fig. 4b), which further confirms that Bi³⁺ prefers to enter Ca²⁺ sites, especially the Ca₍₁₎ site. Additionally, we can also find the redshift of the Zr⁴⁺–O²⁻ absorption band at a high energy, and this shift should be ascribed to the lowering of the conduction band induced by Bi³⁺ doping. The involved energy level variations are shown in the sketch inserted in Fig. 6.

Fig. 6 The normalized photoluminescence excitation spectra of (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0 and 0.08) monitored at 610 nm. A sketch of involved energy level variations was inserted.

Fig. 7 illustrates the normalized fluorescence decay curves of (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0–0.16) excited by 300 nm UV light and monitored at 610 nm. These decay curves are unable to be fitted by a single exponential function, but can be well fitted by a two-exponential function. Generally, Eu³⁺ occupies three types of Ca²⁺ sites, and it is more reasonable to fit the decay curves by a three-exponential function than a two-exponential function. However, due to the similarity between the coordination numbers and symmetry of the Ca₍₁₎ site and the Ca₍₃₎ site, Eu³⁺ enters either the Ca₍₁₎ site or Ca₍₃₎ site, and the luminescent properties are quite similar. Thus, it is reasonable to fit the decay curves by a two-exponential function. In this method, the decay times were determined to be 1.36 ms, 1.36 ms, 1.46 ms, 1.49 ms, 1.47 ms, and 1.47 ms for (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0–0.16) phosphors with increasing Bi³⁺ concentration. In terms of the values of decay times, it is difficult to find obvious regularity, but a trend is exhibited of first an increase and then a decrease. The reason for the increased decay time is mainly due to the energy transfer from Bi³⁺ to Eu³⁺, while a further increase in Bi³⁺ concentration will create more defects that trap the excited electrons, reducing the decay times.^37,38

Fig. 7 The normalized fluorescence decay curves of (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0–0.16) excited by 300 nm UV light and monitored at 610 nm, and lifetimes that were calculated by fitting two exponential functions.

The thermal quenching property, namely emission loss with increasing temperature, is one of the key application criteria for practical phosphors.³⁹ The temperature-dependent normalized inner quantum yield of (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0 and 0.08) phosphors under 300 nm UV excitation are presented in Fig. 8a. It can be detected that the Bi³⁺-doped phosphor has better thermal stability than that without Bi³⁺ doping at any point in the measured temperature range (25–250 °C). We observed that the quantum yield as well as emission intensity (shown as Fig. 8b) first increases up to a temperature of 50 °C and then decreases in the (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0.08) phosphor. The photoluminescence (PL) intensity increase is generally ascribed to the formation of defect levels in the phosphor, which can capture electrons and then release them with increasing temperature, causing the increase of the PL intensity.^40,41 Because of the effect of defects in increasing the thermal stability, this Bi³⁺-doped phosphor can remain at 84.2% of the initial quantum yield (at room temperature) when the temperature was increased up to 150 °C. The yield is better than that of Y₂O₃:Eu³⁺ (approximately 81%) and comparable to commercial Sr₂Si₅N₈:Eu²⁺ (approximately 85%),^42–44 and is much higher than that without Bi³⁺ doping (70.1%). This thermal stability enhancement is different from the traditional reduction of cross relaxation or thermal ionization,^45,46 mainly ascribing to the appropriate amount of defects created by Bi³⁺ doping.

Fig. 8 (a) The temperature-dependent normalized inner quantum yield of (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0 and 0.08) phosphors under 300 nm excitation and (b) the temperature-dependent emission spectrum of the (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0.08) phosphor.

Furthermore, the CIE chromaticity coordinates for (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0–0.16) excited at 300 nm were calculated, but the values were the same, which reflects the fact that Bi³⁺ doping did not change the occupancy of Eu³⁺ at any type of Ca²⁺ site. The CIE chromaticity coordinates were determined to be (0.65, 0.35) (depicted in Fig. 9), which are close to those of standard red (0.67, 0.33) given by the NTSC (National Television Standards Committee).⁴⁷ Additionally, a digital photo of the (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0.08) phosphor under daylight and a 254 nm UV lamp is shown in the inset of Fig. 9, demonstrating an intense red light emission from this phosphor under UV light excitation. With further synthetic process optimization, the luminescent properties of this phosphor are expected to be further promoted.

Fig. 9 CIE coordinates of (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0–0.16) phosphors under excitation at 300 nm. A digital photo of the (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0.08) phosphor under daylight and a 254 nm UV lamp is shown at the upper right.

4. Conclusions

We have synthesized (Ca_2.83−xEu_0.17Bi_x)ZrSi₂O₉ (x = 0–0.16) phosphors by a conventional high-temperature solid-state reaction method using inexpensive raw materials. The DFT calculation demonstrates that the doped Bi³⁺ and Eu³⁺ prefer to occupy Ca²⁺ sites. The redshift of the charge transfer band (CTB) induced by Bi³⁺ doping of the Ca₃ZrSi₂O₉:Eu³⁺ phosphor should be ascribed to the existence of a hybrid level constituted by the O 2p and Bi 6s states located above the valence band. The inner quantum yield of the optimized sample with x = 0.08 was promoted to 72.9%, due to the efficient sensitizing of Eu³⁺ with Bi³⁺. The formation of the appropriate amount of defects created by nonequivalence substitution, namely Bi³⁺ substituting Ca²⁺, is beneficial for increasing the thermal stability of Eu³⁺ in this phosphor. The optimized sample can remain at 84.2% of the initial quantum yield (at room temperature) when the temperature was raised to 150 °C. These parameters indicate that the Bi³⁺-doped Ca₃ZrSi₂O₉:Eu³⁺ phosphor exhibits a particularly high quantum yield and excellent thermal stability, and can serve as red-emitting phosphors for future deep UV LEDs.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This present work was financially supported by the National Basic Research Program of China (2014CB643801); the National Natural Science Foundation of China (No. 51702057); China Postdoctoral Science Foundation (2017M622627); Major Program for Cooperative Innovation of Production, Education & Research of Guangzhou City (201704030106; 201604016029); and Science and Technology Planning Projects of Guangdong Province of China (2014B050505020; 2016201604030035).

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