A full-color emitting phosphor Ca9Ce(PO4)7:Mn2+, Tb3+: Efficient energy transfer, stable thermal stability and high quantum efficiency
Graphical abstract
Herein, we report a series of phosphate phosphors Ca9Ce(PO4)7:xTb3+/yMn2+, exhibiting much efficient energy transfer, stable thermal stability and high quantum efficiency. To prove their promising application, the proof-of-concept LED lamps are fabricated by combining the phosphate phosphors Ca9Ce(PO4)7:Mn2+, Tb3+ with a UV LED chip.
Introduction
As new solid-state light sources, white light-emitting diodes (WLEDs) have attracting more and more attention because they can offer benefits such as long lifetime, high luminous efficiency, low-energy, environment-friendly, green lighting, etc [1], [2], [3], [4], [5], [6], [7]. There is no doubts that WLEDs will replace the traditional light sources (e.g., incandescent lamp, fluorescent lamp), and become one of the most influential light sources in the 21st century [8], [9]. Until now, phosphor-converted white light is considered to be the most economical method, therefore a fast growing interest in the development of phosphors has been aroused. For phosphor-converted WLEDs, there are several ways to achieve the white light emission. The common approach for white LEDs is based on the combination of an InGaN blue LED and a yellow phosphor, Y3Al5O16:Ce (YAG:Ce) [10], [11]. However, the lack of a red spectral component usually results in a highly correlated color temperature (CT = 7756 K) and a poor CRI (Ra = 75), which largely restricts the broader applications of white LEDs [10], [11]. An alternative way to achieving white light is through the combination of an ultraviolet (UV) LED with red, green and blue (RGB) multi-phased phosphors [12], [13], [14]. This approach provides white light with high CRI and suitable CT. Unfortunately, the multi-phase phosphors produce an inevitable problem of fluorescence re-absorption, resulting in a loss of blue emission efficiency and a time-dependent shift of the color point. Compared with the above two methods, the single-phase phosphor with tunable emission containing white emission has the advantages of a higher CRI, tunable CT, pure CIE chromaticity coordinates [10]. Meanwhile, developing single-phase white-light-emission materials might effectively solve the re-absorption problem existing in RGB phosphors and improve the quality of white light. Compared to the n-UV and blue excited white LEDs, deep UV LED chips have more opportunities to stimulate the single-phase white phosphors directly and achieve the white light with high luminous efficacy, high chromatic stability and brilliant color-rendering properties [10]. What’s more, the single-phase white phosphor for a UV LED-pumped white LEDs would make device packaging easier. It is possible that deep UV excited white LEDs might provide more competitiveness in price against the traditional fluorescent lamps since their price competitiveness critically rely on the optical properties of phosphors [10].
A large number of single-phase white-light-emitting phosphors that are excited effectively for UV-pumped WLEDs were reported, and most of them are based on Eu2+→Mn2+ [15], [16], [17], Ce3+→Mn2+ [18], [19], Eu2+→Tb3+→Mn2+ [20], [21], [22], [23] and Ce3+→Tb3+→Mn2+ [24], [25], [26] systems because Eu2+ and Ce3+ serve as the sensitizers by transferring a part of their energy to activator ions (Tb3+ or Mn2+). Generally, their energy transfer efficiency is much low, resulting in the emission of lower quantum efficiency. Consequently, it is impossible that the fabricated WLED devices give an ideal luminous efficiency by coating the single-component white-light-emitting phosphors onto a UV LED chip. Apart from energy transfer efficiency and quantum yields, thermal stability of the obtained phosphor need to be improved. As is known to us, a high input current or high power inevitably results in high local heat flux in InGaN-based chips. The junction temperature of chips will be rising as high as 150-200 °C [27], [28], [29]. Obviously, a high junction temperature leads to the thermal quenching effect including a decrease of phosphor emission intensity and wavelength shift of their main emission peaks. Therefore, it is critical to evaluate the thermal stability of the as-synthesized phosphor particular at the high temperature. Unfortunately, the commercial phosphors, BaMgAl10O17:Eu2+ and (Sr, Ba, Ca)3MgSi2O8:Eu2+, are known to have a high efficiency but suffer from poor thermal stability [30], [31]. The commercially available Y3Al5O12:Ce3+ yellow phosphor also faces the same problem in WLEDs due to reduced thermal stability [32], [33]. As a great challenge, thermal stability has been an urgent and hot issue for the fabrication of high-power WLEDs. Up to now, a lot of effort has been devoted to developing single-phase white-light-emitting materials including phosphors, glasses and ceramics [34], [35], [36]. However, most single-phased white-light-emitting materials present poorly in energy transfer, thermal stable and quantum efficiency [37], [38], [39], [40], [41]. Thus, it is highly desirable to find a novel phosphor with excellent thermal stability and high efficiency.
Recently, phosphate phosphors have emerged as an important family of luminescent materials due to high thermal stability. Being isostructural with β-Ca3(PO4)2, Ca9Ce(PO4)7 host has a favorable chemical and photophysical stabilities and its structure can provide a wide range of possible cationic substitutions since there are different inequivalent sites of calcium presenting a large-scale of sizes and coordination spheres. This special crystal structure allows rich heterovalent substitution of Ca2+ by R+ (monovalent), R3+ (trivalent) and R4+ (quadrivalent) cations [42], [43]. Considering these merits, Ca9Ce(PO4)7 host attracts us to concentrate on the structure and composition dependent energy transfer process in Ca9Ce(PO4)7: Tb3+, Mn2+ phosphor. Ca9Ce(PO4)7 presents a outstanding feature that the concentration of sensitizers (Ce3+) is up to 100%, providing more energy for activators such as (Tb3+ and Mn2+). The emission of the Ca9Ce(PO4)7: Tb3+, Mn2+ phosphor consists of a full spectral range of the visible region, leading to a high white emission. Ca9Ce(PO4)7:xTb3+/yMn2+ exhibit excellent luminescent properties of low thermal quenching (ΔE > 0.2 eV) and high quantum efficiency (QE > 60%).Ca9Ce(PO4)7: Tb3+/Mn2+ phosphors do not lose their application in the phosphor-converted WLED in spite that the maximum excitation wavelength of the phosphors is around 300 nm and there is almost no absorption in the >330 nm range. Because a proof-of-concept white LEDs is fabricated successfully by employing the single-phase white phosphor Ca9Ce(PO4)7: Tb3+, Mn2+ with a deep UV LED chip (the main emission wavelength of deep UV LEDs is located at 280 nm), which proves Ca9Ce(PO4)7: Tb3+, Mn2+ to be a promising single-phase white-light-emitting candidate for deep UV-pumped WLEDs.
Section snippets
Synthesis
A series of Ca9-xCe(PO4)7:xMn, Ca9Ce1-y(PO4)7:yTb and Ca9-xCe1-y(PO4)7: yTb3+, xMn2+ samples were prepared using the simple solid state method. The starting raw materials are listed as follows: CaCO3 (AR), (NH4)HPO4 (AR), MnCO3 (AR), CeO2 (99.99%), Tb4O7 (99.99%). Ca9Ce(PO4)7:0.15Tb3+, 0.05Mn2+ sample was taken as an example. The synthesis procedure is as follows: 1.760 g CaCO3, 0.344 g CeO2, 1.849 g (NH4)2HPO4, 12 mg MnCO3 and 56 mg Tb4O7 were thoroughly mixed in an agate mortar for 40 min. They
Conclusions
In brief, a single-component phosphor Ca9Ce(PO4)7: Tb3+/Mn2+ is developed with the white light and tunable color emission by the energy transfer from Ce3+ to Tb3+ or Mn2+. The energy transfer efficiency from Ce3+ to Mn2+ and Tb3+ reaches 72% and 91% in Ca9Ce(PO4)7 host. The energy transfer mechanism from Ce3+ to Mn2+/Tb3+ is proved to be an electronic dipole-dipole (d-d) interaction. The as-synthesized phosphate phosphors exhibit an excellent thermal stability (ΔE > 0.200 eV) and present a high
Acknowledgements
This work is supported by the National Natural Science Foundation of China (No. 21471038) and Research Fund for the Doctoral Program of Higher Education of China (20134407110008).
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