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How to produce white light in a single-phase host? Cite this: DOI: 10.1039/c3cs60314h

Mengmeng Shang, Chunxia Li* and Jun Lin* White light-emitting diodes (WLEDs) as new solid-state light sources have a greatly promising application in the field of lighting and display. So far much effort has been devoted to exploring novel luminescent materials for WLEDs. Currently the major challenges in WLEDs are to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. In recent years, numerous efforts have been made to develop single-phase white-light-emitting phosphors for near-ultraviolet or ultraviolet excitation to solve the above challenges with certain achievements. This review article highlights the current methods to realize the white light emission in a single-phase host, including: (1) doping a single rare earth ion (Eu3+, Eu2+ or Dy3+)

Received 30th August 2013

into appropriate single-phase hosts; (2) co-doping various luminescent ions with different emissions into a

DOI: 10.1039/c3cs60314h

single matrix simultaneously, such as Tm3+/Tb3+/Eu3+, Tm3+/Dy3+, Yb3+/Er3+/Tm3+ etc.; (3) codoping different ions in one host to control emission color via energy transfer processes; and (4) controlling the

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concentration of the defect and reaction conditions of defect-related luminescent materials.

Key learning points (1) Summarize the current approaches to realize white emission in a single-phase host. (2) Explain the white luminescence mechanisms for all approaches. (3) Compare the advantages and disadvantages of each approach.

1. Introduction 1.1 Basic concepts for luminescence and luminescent material Luminescence is a common phenomenon in our daily lives, for example, the fluorescent lamp for lighting and architectural decoration, the screens of televisions and computers, and lasers in some medical devices. On the basis of the difference between the absorption and emission radiant energy, people have divided luminescence into up-conversion emission and down-shifting emission. Up-conversion (UC) is characterized by the conversion of long-wavelength radiation, for instance infrared or near infrared (NIR) radiation, to short-wavelength radiation, usually in the visible range. It refers to nonlinear optical processes characterized by the successive absorption of two or more pump photons via intermediate long-lived energy states followed by the emission of the output radiation at a shorter wavelength than the pump wavelength. Conversely, converting short-wavelength radiation, for instance ultraviolet

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, Jilin, China. E-mail: [email protected], [email protected]; Fax: +86-431-85698041; Tel: +86-431-85262031

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or near ultraviolet radiation to long-wavelength radiation is defined as down-shifting emission. What we call the luminescence usually refers to the type of down-shifting emission.1 The carrier of luminescence is a luminescent material (or phosphor), which is the critical part of the luminescent apparatus other than processing, large scale production and engineering and technical questions. Then, what are these luminescent materials? Generally speaking, luminescence can occur as a result of many different kinds of excitation source, such as X-rays, cathode rays, and ultraviolet light. The role of luminescent materials is to convert the radiation energy into visible or infrared light. According to their composition, luminescent materials can be classified into three groups: (1) some host materials themselves contain luminescent centers that can absorb the exciting radiation and subsequently give the radiative emission, such as vanadate and tungstate. We define them as host luminophores; (2) most luminescent materials consist of inactive host and luminescent ions (i.e. activators as luminescent centers). For example, Y2O3 as a host material can not luminesce directly. When Eu3+ ions are doped into Y2O3, it can emit red light. So we define these luminescent materials as ‘‘host + activator’’ type; (3) sensitizers also play an important role in the luminescent materials, and can absorb the excitation

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energy and transfer it to the activators. Consequently, the luminescent properties are improved or enhanced greatly.2 We define these luminescent materials as ‘‘host + sensitizer + activator’’ type. For example, the Ce3+ ion is a good sensitizer for the Tb3+ ion in the LaPO4:Ce3+, Tb3+ phosphor, and can transfer its excitation energy to Tb3+ and make the latter display green emission. A good luminescent material should fulfil some certain requirements. Obviously, the host material has to be a thermally and chemically stable compound to resist the elevated temperatures and to realize a long lifetime. Unless energy transfer between host and dopant occurs, the material should also be optically transparent for the emission light. Dopants have to be incorporated in the host material and able to efficiently absorb the ultraviolet or near-ultraviolet radiation and have tunable emission colors with high color rendering.

Mengmeng Shang was born in Shangdong, China, in 1987. She received her BS (2008) in chemistry from Liaocheng University, and a PhD degree (2013) from the Changchun Institute of Applied Chemistry. After graduation, she became an Assistant Professor in Prof. Jun Lin’s group. Her research focuses on the rare earth ions activated inorganic luminescent materials used for white light-emitting Mengmeng Shang diodes and field emission displays, including the development of novel inorganic luminescent materials and study of the physical chemical properties and the luminescence mechanism.

Chunxia Li

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Chunxia Li was born in Shandong, China, in 1977. She received her BS (2002) and MS (2005) in chemistry from Northeast Normal University, and a PhD degree (2008) from the Changchun Institute of Applied Chemistry. After graduation, she became an Assistant Professor in Prof. Jun Lin’s group and was promoted to Associate Professor in 2012. Her current research interests include the controllable synthesis of rare earth luminescence nanomaterials and their bioapplication.

By modeling the influence of the host’s band gap, phonon energy, and the local environment for the rare earth dopants (symmetry, coordination surroundings, bond distance, etc.), it is possible to predict to a certain extent the emission properties of some certain host–dopant materials. Finally, the phosphor production process should also be cheap and environmentally friendly. Although many host–dopant luminescent materials fail on one or more aspects, most of them are promising and deserve further in depth research. 1.2 Potential applications for single-phase white-light-emitting luminescent material Nowadays, luminescent materials are applied for a wide variety of applications, which include the fields of display and lighting, such as television tubes, mobile telephone screens, fluorescent lamps and the promising white light-emitting diodes (WLEDs).3,4 For these purposes, some tens of thousands of phosphors have been synthesized and characterized. White light generation is required on a daily basis, e.g., for room lighting, monitors, displays, enhanced solar cells, and other optical devices. Incandescent lamps, fluorescent lamps, light emitting diodes (LEDs), frequency downshifting layers in solar cells, and femtosecond laser pulses generating spectral continuum in glass fibers are the most effective realizable technology for the above applications. The recently developed WLEDs are the most promising white-light source, and the emergence of WLEDs is known as a great change in the field of lighting. WLEDs involves a electroluminescence process, which is a non-thermal generation of light resulting from the application of a voltage to a substrate. In the electroluminescence process, excitation is accomplished by recombination of charge carriers of contrary sign (electron and hole) injected into an inorganic or organic semiconductor in the presence of an external circuit.5 Compared with the traditional incandescent or

Jun Lin was born in Changchun, China, in 1966. He received BS and MS degrees in inorganic chemistry from Jilin University, China in 1989 and 1992, respectively, and a PhD degree from the Changchun Institute of Applied Chemistry in 1995. Then he went to City University of Hong Kong (1996), Institute of New Materials (Germany, 1997), Virginia Commonwealth University (U.S.A., 1998) and University of Jun Lin New Orleans (U.S.A., 1999) working as a postdoctoral researcher. He came back to China in 2000, and since then has been working as a professor in CIAC. His research interests include bulk and nanostructrued luminescent materials and multifunctional composite materials together with their applications in display, lightening and biomedical fields.

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fluorescent lamps, WLED-based lighting can provide significant power saving, longer lifetime, higher luminous efficiency and brightness, and environmental friendliness.6 In the following text, some figures of merit, such as correlated color temperature (CCT), the Commission Internationale de I’Eclairage (CIE) and color rendering index (CRI or Ra), that characterize WLEDs are referred to. Therefore, we list some basic concepts of fundamental aspects of phosphor converted WLEDs.6,7 (1) Correlated color temperature (CCT) is the temperature of a black body whose chromaticity most nearly resembles that of a light. Low CCT implies warmer (more yellow-red) light, while high color temperature appears to be a colder (more blue) light. It is important to install an electrical lighting system that emits warm or cold light as needed. (2) Commission International de I’Eclairage (CIE) is the most widely used method to describe the composition of any color in terms of three primaries (red–green–blue). Artificial ‘‘colors’’, denoted by X, Y, Z, also called tristimulus values, can be added to produce real spectral colors. By a piece of mathematic legerdemain, it is necessary only to quote the quantity of two of the reference stimuli to define a color since the three quantities (x, y, z) are made always to sum to 1. The x, y, z, i.e. the ratios of X, Y, Z of the light to the sum of the three tristimulus values, are the so-called chromaticity coordinates. (x, y) is usually used to represent the color. (3) The color rendering index (CRI or Ra) definition is based on comparing the color of test objects when illuminated by the light source under test, to the colors of the objects illuminated by a reference source. The introduction of this parameter is based on the fact that objects may look quite different in color under lamps that look quite alike in succession but are different in spectral distribution. When CRI is calculated, it can be rated on a scale from 0 to 100. A CRI of 100 would represent that all color samples illuminated by a light source in question would appear to have the same color as those same samples illuminated by a reference source. (4) Luminous efficacy is a figure of merit for light source. The luminous efficacy of a light source is defined as the ratio of the total luminous flux (lumens) to the power (watts or equivalent). The luminous efficacy is always in contradiction with CRI, because a high CRI value requires proper spectral dispersion over all the visible range, which would make the luminous efficacy far below 683 lm per W (the theoretically attainable maximum value). (5) Quantum yield involved in most references and reports is referred to the absolute quantum yield, i.e. the ratio between the number of emitted photons and the number of absorbed photons, which is an intrinsic property of the luminescence conversion process. The first WLED was fabricated by the combination of a blue LED chip and YAG:Ce3+ yellow-emitting phosphor, which is the most common method in lighting and display areas. However, YAG:Ce3+ yellow phosphor suffers some weaknesses, such as a poor color rendering index and low stability of color temperature. In addition, the lighting color of this device changes with the drive voltage and the phosphor coating thickness, and therefore,

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it is difficult to fabricate stable WLEDs in industrial production. To optimize the color rendering properties of white and colored light phosphors, the current focus of their fabrication shifts gradually from the YAG:Ce3+-based to the red–green–blue (RGB) emitting color phosphors excited by ultraviolet-LEDs. As human eyes are not sensitive to (near-) ultraviolet, the color obtained by white ultraviolet-LEDs only depends on the phosphors. Chiu et al. demonstrated that a near-ultraviolet LED chip coating with three different phases, red- (CaAlSiN3:Eu2+), green((Ba,Sr)2SiO4:Eu2+), and blue-emitting (Ca2PO4Cl:Eu2+) phosphors, gave a relatively high Ra of 93.4.8 With development of ultraviolet LED-driven inorganic phosphors, their use for general illumination is proposed to become price-competitive with current luminescent lamps by the year 2020. However, there still exist some problems that cannot be overcome so far for WLEDs fabricated with ultraviolet-LED chips and tri-color phosphors. Generally, the luminescent efficiency is relatively low in this system owing to the strong reabsorption of the blue light by the red and green phosphors.9 WLEDs with multiple emitting components can be problematic as the device is very complicated, and the color balance is difficult to control. In particular, most of the above problems can be avoided by using single-phase white-emitting phosphors with higher luminous efficiency and excellent color rendering index. Compared with blue chips coated with YAG:Ce3+ phosphor, the singlecomposition phosphors with tunable emission containing white emission have the advantages of a higher CRI, tunable CCT, pure CIE chromaticity coordinates and so on.10 Meanwhile, the development of the single-phase white light emission materials may effectively solve the reabsorption problem existing in RGB phosphors. Based on these applications, researchers have made numerous efforts to develop single-phase white-emitting phosphors excited with (near-) ultraviolet light, blue light, infrared (IR) light or low voltage electron beam, and have had some certain achievements in recent years. So, the present review paper is focused on four main methods for obtaining white light in single-phase host: (1) White emission can be generated by doping a single rare earth ion (Eu3+, Eu2+ or Dy3+) into appropriate single-phase hosts. (2) White light can be generated by the combination of multiple rare ions with red, green and blue or yellow and blue emission (such as Tb3+/Sm3+; Tm3+/Dy3+; Tm3+/Tb3+/Eu3+; Yb3+/Er3+/Tm3+). (3) White light emission can be generated by codoping ion pairs based on the energy transfer mechanism (Ce3+ - Eu2+; Ce3+ - Tb3+; Eu2+ - Mn2+; Ce3+ - Mn2+ etc.). (4) Defect-related luminescent materials can also emit white light by controlling the concentration of the defect and reaction conditions. Meanwhile, the existing problems and the future development are also discussed and expected in this review. Moreover, we hope to inspire research into the origins of the single-phase luminescent materials with white light emission and to encourage people to develop more single-phase white-light-emitting phosphors

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with appropriate CCT (3200–6000 K for warm white light; Z6500 K for cold white light), higher CRI (Z85%) and quantum yield (Z80%), and excellent quality of light.

2. Approaches to realize white-light emission in a single-phase host material 2.1

Single activator ion doped systems

Luminescent materials usually consist of a host and an activator, and the host composition determines the luminescence properties and performance of the activator. Fig. 1 shows the structure diagram of a functional unit (AXn, n represents the coordination number) for luminescence. Any changes of the host composition (X, P, M) can modify the energy transfer, crystal field strength and covalency, and ultimately influence the luminescent efficiency, color, and intensity of the materials. For example, the red/orange emission intensity ratio of Eu3+ and yellow/blue intensity ratio of Dy3+, as well as the d–f transition of Eu2+ depend strongly on the radius, charge and electronegativity of X (ligand, anion in acid radical), P (cation in acid radical) and M (cation in host) in the host compound. In this part, we will introduce briefly Eu2+, Eu3+ and Dy3+ ion solely doped single-phase materials with white emission, in which the luminescence properties are controllable by adjusting the host composition and the doping activator’s concentration (Table 1 gives some typical Eu2+, Eu3+ and Dy3+ ion

Fig. 1

Table 1

singly activated single-phase luminescence materials with white-light emission). A. Eu2+ doped systems. The Eu2+ ion has a 4f7 electron configuration, and its partial energy level diagram is shown in Fig. 2. As shown in Fig. 2, whether the emission spectra of Eu2+ ion can present broad band emission depends on the crystal field strength. Generally, the energy of the excited state 4f65d1 of the Eu2+ ion is lower than that of the lowest excited state 6 P in the 4f7 electron configuration, so the Eu2+ ion in most of the compounds presents broad band emission due to the 4f65d1 - 4f7 transition. Furthermore, the 5d states of Eu2+ are outer orbitals, and the coordination surroundings in the host have a prominent influence on its energies. The spectral position of the d–f transition lies closely with the nephelauxetic effect.11 The higher the charge and the smaller the radius of the ligand ion, the stronger the nephelauxetic effect and the lower the position of 5d energy level. So, the 4f - 5d transitions of the Eu2+ ion appear in a large wavelength range that depends greatly on the host lattice, and the emission color from the 4f65d1 - 4f7 transition can also vary from ultraviolet to yellow/ red light. For example, Chen et al.12 recently reported the tunable emission colors of (Ca,Mg,Sr)9Y(PO4)7:Eu2+ phosphors through

Fig. 2

Structure diagram of a luminescence functional unit.

Partial energy level diagram of Eu2+ ion.

Summary of single activator ion doped systems for single-phase white-emitting phosphors

Excitation source Phosphor 2+

BaSrMg(PO4)2:Eu Sr3MgSi2O8:Eu2+ LaOF:Eu3+ NaYF4:Eu3+ CaIn2O4:Eu3+ BaY2ZnO5:Dy3+ a

UV (nm)

CLa

Emission

CIE (x, y)

CCT

CRI

Ref.

| | | | | |

— — | — | –

460 nm, 550 nm 470 nm, 570 nm All the emissions from Eu3+: 5 DJ –7FJ 0 ( J, J 0 = 0, 1, 2, 3, 4)

(0.29, (0.32, (0.29, (0.29, (0.32, (0.32,

— 5892 K — — — —

>85 84 — — — —

16 17 21 22 23 24

385 375 274 397 397 355/351

489 nm (4F9/2–6H15/2) 579 nm (4F9/2–6H13/2)

0.35) 0.33) 0.34) 0.33) 0.32) 0.39)

CL: cathodoluminescence.

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Fig. 3 Dependence of emission spectra of (Ca0.993xyMgxSry)9Y(PO4)7: 0.007Eu2+ phosphors on concentrations of Mg2+ and Sr2+ (reproduced with the permission from ref. 12, copyright 2011, Royal Society of Chemistry).

crystal field splitting. Fig. 3 illustrates the emission spectra of (Ca0.993xMgx)9Y(PO4)7:0.007Eu2+ and (Ca0.993ySry)9Y(PO4)7: 0.007Eu2+, with x and y corresponding to various molar concentrations (0–0.5 mol%) of Mg2+ and Sr2+, respectively, under 380 nm UV excitation. As indicated in Fig. 3, there is a continuous blue-shift in the emission wavelength from 486 to 435 nm with increasing Mg2+ content (x r 0.5) and a redshift from 486 to 508 nm with increasing Sr2+ concentration ( y r 0.5). These results indicate that the emission color is tunable from blue to blue-green and to green in the visible region of the spectrum by varying the divalent metal ions due to the change of the crystal field splitting. According to reports by Robertson et al.13 and Jang et al.,14 crystal field splitting (Dq) can be determined by the following equation: 1 r4 Dq ¼ Ze2 5 6 R where Dq is a measure of the energy level separation, Z is the anion charge, e is the electron charge, r is the radius of the d wavefunction, and R is the bond length. When Ca2+ is substituted by a smaller Mg2+ ion, the distance between Eu2+ and O2 becomes longer and the magnitude of the crystal field decreases, resulting smaller crystal field splitting, so that there is a continuous increase in the blue-shift with the doped Mg2+ concentration. In contrast, when Ca2+ is substituted with a larger Sr2+ ion, the distance between Eu2+ and O2 becomes shorter, which in turn increases the magnitude or strength of the crystal field. As a result, the 5d band of Eu2+ is lowered, and the emission wavelength is redshifted from 486 to 508 nm as the doped Sr2+ content increases. Therefore, the Eu2+ ion with broad band emission is an important activator for luminescent materials and has been studied extensively.10,15 It is possible that an appropriate host material for Eu2+ ion doping can generate white light emission. Depending on the host material, high quantum efficiencies (generally above 0.8) in combination with good thermal quenching behavior (which means to maintain its quantum efficiency at the elevated temperatures) can be obtained.

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Fig. 4 Excitation spectra (a: lem = 447, b: lem = 536 nm) and emission spectra (c) (lex = 350 nm) of Ba0.97Sr0.99Mg(PO4)2:0.04Eu2+ powder (reproduced with the permission from ref. 16, copyright 2010, Elsevier).

BaSrMg(PO4)2:Eu2+ (BSMP:Eu2+) phosphor is a typical example for Eu2+ solely doped white-light-emitting phosphors,16 and shows two emission bands at around 447 nm and 536 nm under the excitation of 350 nm, as shown in Fig. 4. The crystal structure of the BSMP host is similar to the phosphates Ba2Mg(PO4)2 and SrMg2(PO4)2. Eu2+ ions occupying Ba2+ sites in the Ba2Mg(PO4)2 host show a yellow-emission, while Eu2+ ions substituting Sr in SrMg2(PO4)2 show a broad blue emission. Unlike Ba2Mg(PO4)2 and SrMg2(PO4)2, the BSMP host can provide two cation sites for Eu2+ ions substituting simultaneously. As shown in Fig. 4, the two emission bands peaking at 447 nm and 536 nm are attributed to the 4f7 - 4f65d1 transition of Eu2+ ion substituting for Sr2+ site and Ba2+ site in the host lattice, respectively. Consequently, the luminescence color of the material can be adjusted through tuning the proportion of Eu2+ entering in the two kinds of lattice sites in the BSMP host. The emission spectrum (curve c) of the sample with composition B0.97S0.99MP:0.04Eu2+ almost extends through the whole visible light region from 400 to 700 nm and its CIE chromaticity coordinates are calculated as x = 0.291, y = 0.349, which locate in white light region. For some silicate systems, such as M2SiO4, M3SiO5, MMgSi2O7 (M = Mg, Ca, Sr, Ba), and Sr3MgSi2O8, there usually exist two different cationic sites in the host lattices,17,18 which are suitable for Eu2+ ion doping because of the similar ion radii of the alkaline earth ions and Eu2+ ion. When Eu2+ ions simultaneously occupy the two cationic sites in the host lattice, the broadband emissions at different bands will be produced, and the position of the emission peak can be tuned by changing the composition of the host. Generally speaking, white light emission can be realized in Eu2+ ion singly activated phosphate or silicate systems that have two or more than two sites for Eu2+ replacing and the emission color can be tuned. Among the numerous hosts suitable for Eu2+ luminescence, silicate and phosphate are the most promising hosts since they can supply Eu2+ ions with multiple cationic sites and exhibit the tunable luminescent properties. However, the emission peaks of Eu2+ are mainly located in the blue and yellow(-green) region, which makes the obtained white light lack a red component,

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resulting in the lower CRI, similar to YAG:Ce3+ yellow phosphor with InGaN chip excitation. So, to improve the CRI of the material, the emission intensity of red light should be enhanced. B. Eu3+-activated systems. The trivalent europium ion (Eu3+) is well-known as a red-emitting activator due to its 5 D0–7FJ transitions ( J = 0, 1, 2, 3, 4). In addition to the above emission lines, whether those emissions from higher 5D levels, such as 5D1 (green), 5D2 (green, blue), and 5D3 (blue), can be observed depends upon the host lattice (phonon frequency as well as the crystal structure) and the doping concentration of Eu3+. In general, the efficiency of these emissions is low, less than 10%.19 If the doping concentration of Eu3+ is high (thus short Eu3+–Eu3+ distance), the higher 5D1,2,3 emissions might be quenched by cross relaxation occurring between two neighboring Eu3+ ions, such as Eu3+(5D1) + Eu3+(7F0) Eu3+(5D0) + Eu3+(7F3). Moreover, the higher-level 5D1,2,3 emissions of Eu3+ can also be quenched by multiphonon relaxation if the phonon energy (highest vibration frequency) of the host lattice is high enough. In this case, both the phonon frequencies of the host lattices and the doping concentration of Eu3+ should be low enough to avoid the multiphonon relaxation and cross-relaxation occurring among the energy levels of Eu3+, respectively. An appropriate selection of the host lattice and doping concentration of Eu3+ is possible to yield simultaneously red emission from the 5D0 level and blue and green emissions from the higher 5D levels (5D1, 5D2 and 5D3) of Eu3+ with comparable intensity, thus giving rise to white light emission from a Eu3+ singly-doped material.20 Generally, fluoride and oxyfluoride have low phonon frequencies and are appropriate hosts for Eu3+ to realize white emission.21,22 Fig. 5 shows the excitation and emission spectra of b-NaYF4:0.5 mol% Eu3+. Excitation into the strongest 7 F0 - 5L6 transition of Eu3+ at 397 nm yields the emission spectrum of b-NaYF4:0.5 mol% Eu3+, which consists of all the emission lines associated with the Eu3+ transitions from the excited 5D0,1,2,3 levels to the 7FJ level. The strongest one is 511 nm. These emission lines of Eu3+ cover the whole visible spectral region with comparable intensity, resulting in a white

Fig. 5 Excitation and emission spectra of b-NaYF4:0.5 mol% Eu3+ (reproduced with the permission from ref. 22, copyright 2009, American Chemical Society).

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bluish light emission. Its CIE coordinates are determined to be x = 0.2896, y = 0.3317. In addition, the white emission from Eu3+ can also be realized in some oxide hosts. For example, our group first reported the luminescence properties of CaIn2O4:Eu3+ phosphors as whitelight emission luminescence materials for WLEDs and FEDs.23 Under ultraviolet and low-voltage electron beam excitation, the CaIn2O4:Eu3+ phosphors exhibited a tunable color emission from white to orange and red by adjusting the doping concentration of Eu3+ appropriately. Fig. 6 shows the emission spectra (excited at 398 nm) of Ca1xIn2O4:xEu3+ phosphors and the corresponding CIE chromaticity diagram. From Fig. 6a, it can be seen that the higher energy level 5D3, 5D2, 5D1, and 5D0 emissions have comparable intensity for appropriate doping concentration (x = 0.008, 0.015), resulting in a white light emission. When the doping concentration is high, the higher energy level emissions (5D3, 5D2, and 5D1) are quenched by crossrelaxation in favor of the 5D0 emission, so the samples give red luminescence. In addition, the low vibration frequency available in the CaIn2O4 host lattice is also a critical factor for the emission of higher excited states (5D1, 5D2, and 5D3). The nmax of CaIn2O4 is determined to be 475 cm1, which is lower than that of Y2O3 (600 cm1), so it is reasonable that there is a larger probability of radiative transitions from the 5D1,2,3 states of Eu3+ in CaIn2O4 than in Y2O3 at above doping level, as shown in Fig. 7. As mentioned above, whether the higher levels of Eu3+ ion can emit or not depends on its doping concentration and the

Fig. 6 Typical photoluminescence spectra of CaIn2O4:xEu3+ with different Eu3+ ions concentrations (a) and the corresponding CIE chromaticity diagram (b) (reproduced with the permission from ref. 23, copyright 2007, American Chemical Society).

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Fig. 7 Excitation (a, lem = 512 nm) and emission (b, lex = 398 nm) spectra of CaIn2O4:0.01Eu3+, and the emission spectrum of Y2O3:1.0% Eu3+ (c, lex = 250 nm) (reproduced with the permission from ref. 23, copyright 2007, American Chemical Society).

Fig. 8 The diagram showing the basic conditions and process for Eu3+ to emit white light in single host lattices. DE is the energy difference between the involved levels; nmax is the highest available vibrational frequencies of the surroundings of the Eu3+ ion.

phonon frequency of the host lattice. As illustrated in Fig. 8, the doping concentration of Eu3+ is usually lower than 1 mol%, and the highest vibrational frequencies of the hosts (nmax) and the energy difference (DE) between the levels involved generally satisfy the following relationship: DE > 4–5 nmax. Only when these two conditions are satisfied simultaneously can we observe white emission from Eu3+ in the single-phase host materials. C. Dy3+-activated systems. Dy3+ ion has a 4f9 electron configuration, and usually exhibits two main emissions in the visible region: one in the blue region (470–500 nm) and one in the yellow region (570–600 nm), which originate from 4F9/2 6 H13/2 and 4F9/2 - 6H15/2 transitions of Dy3+ ions, respectively. Fig. 9 shows the energy level diagram of the Dy3+ ion. The yellow emission of Dy3+ is especially hypersensitive (DL = 2, DJ = 2) to the local environment, whereas the blue emission is not. The ratio of yellow/blue (Y/B) is mainly influenced by the nephelauxetic effect between Dy3+ and O2 in the composite oxides. The greater the nephelauxetic effect (viz. covalency), the stronger the yellow emission of the Dy3+ ion. Therefore, by suitably adjusting the yellow-to-blue intensity ratio, it is possible to

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Fig. 9

Partial energy level diagram of Dy3+.

obtain a near-white-light emission phosphor. Generally, the Y/B ratio of Dy3+ increases with the increasing of the ion radius of its surrounding elements. When Dy3+ substitutes for the equivalent ion in the host, the Y/B ratio can not vary with the concentration of Dy3+; however, if Dy3+ substitutes for the inequivalent ion in the host, the change in the Dy3+ concentration would affect the local symmetry of the crystal structure and consequently, the ratio of Y/B would vary with the Dy3+ concentration. Moreover, the temperature has no effect on the ratio of Y/B. Thus Dy3+-doped luminescent materials can potentially show white light emission by tuning the ratio of Y/B. For example, Dy3+-doped BaY2ZnO5 phosphor shows a nearwhite-light emission when excited by near UV light.24 The basic structure of the orthorhombic BaY2ZnO5 consists of YO7, BaO11, and ZnO5 polyhedra. Y is 7-fold coordinated inside a monocapped trigonal prism, and two such units join to form the basic structure of Y2O11. In the BaY2ZnO5 host, Dy3+ would occupy Y3+ sites and exhibit its characteristic emission. As shown in Fig. 10a, there are a series of strong and sharp excitation peaks between 300 and 500 nm, which are associated with the typical intra-4f transitions of the Dy3+ ions. Fig. 10b shows the emission spectrum (lex = 355 nm) of BaY2ZnO5:Dy3+ phosphor. It can be observed that there are two groups of strong peaks at 489 nm (blue) and 579 nm (yellow) due to the 4F9/2 - 6H15/2 and 4F9/2 6 H13/2 transitions of the Dy3+ ions, respectively. In addition, the concentration of Dy3+ ions did not affect the shape of emission curves and the asymmetry ratio (the emission intensity ratio of the 4F9/2 - 6H15/2 and 4F9/2 - 6H13/2). Therefore, the color coordinates of BaY2ZnO5:Dy3+ phosphor are in the near whitelight region, with CIE coordinates of x = 0.320 and y = 0.389. It is worth noting that Dy3+ can not be effectively excited by ultraviolet because its charge transfer band and 5d energy level both locate above 5000 cm1. In addition, the f–f transitions in the near ultraviolet region are forbidden transitions and the corresponding luminescence efficiency is low. In the above discussion, the tunable emission of the Eu2+ ion results from its 4f65d1 - 4f7 transition, while the characteristic emissions of Eu3+ and Dy3+ ions are from the transitions of their

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Sm3+, Er3+, Ho3+, Yb3+ and Pr3+, are suitable candidates for white light generation due to their abundant emitting colors. White light generation can be realized with different tints, depending on the dopants’ ratio in a single-phase host material. For example, our group has reported the multicolor-tunable emission properties of LaOCl:RE3+ (RE = Eu/Tb, Sm, Tm, Dy) phosphors.26–28 Under ultraviolet radiation and low-voltage electron beam excitation, the LaOCl:Eu3+, LaOCl:Sm3+, LaOCl:Tb3+, LaOCl:Tm3+ and LaOCl:Dy3+ samples give their characteristic transitions of Eu3+ (5D0,1,2 - 7F0,1,2,3,4, red), Sm3+ (4G5/2 6 H5/2,7/2,9/2, orange-red), Tb3+ (5D3,4 - 7F2,3,4,5,6, blue-green), Tm3+ (1D2, 1G4 - 3F4, 3H6, blue), and Dy3+ (4F9/2 - 6H15/2, 6H13/2, yellow), respectively. Moreover, there exists simultaneous luminescence of Tb3+, Tm3+, Eu3+, or Sm3+ individually when codoping them into the single-phase LaOCl host (for example, LaOCl:Tb3+, Eu3+/Sm3+; LaOCl:Tm3+, Eu3+/Sm3+; LaOCl:Tb3+, Tm3+, Eu3+/Sm3+ systems), which is beneficial to tune the emission colors (Fig. 11c).26 White emission was also realized by co-doping Tm3+ and Dy3+ (Fig. 11a),28 Tb3+ and Sm3+ (Fig. 11b),26 or Tb3+, Eu3+, and Tm3+ (Fig. 11d)26 with appropriate ratios into the LaOCl single host lattice. Chen et al.29 recently investigated Tm3+, Er3+, Yb3+ codoped b-YF3 UC nanocrystals, which can exhibit a perfect and bright white light with CIE-x = 0.310 and CIE-y = 0.358 under 976 nm laser excitation by adjusting the concentrations of the rare earth ions in the material, as shown in Fig. 12. Intense blue emissions at 451 and 478 nm of the Yb3+/Tm3+ codoped b-YF3 nanocrystals were observed, corresponding to Tm3+ 1D2 - 3F4 and 1G4 - 3H6 transitions, respectively; for Yb3+/Er3+ codoped b-YF3 nanocrystals, they exhibited intense green emissions at 530 and 550 nm, assigned to 2H11/2 - 4I15/2 and 4S3/2 - 4I15/2 Fig. 10 (a) Absorption spectra and PL excitation spectra of BaY2ZnO5:Dy3+ powders calcined at 1250 1C for 12 h. (b) Photoluminescence emission spectra of BaY2ZnO5:Dy3+ under an excitation wavelength of 355 nm (reproduced with the permission from ref. 24, copyright 2012, Elsevier).

intra-4f electrons. The different luminescence properties are just derived from the two different types of transition. The 5d states of Eu2+ are outer orbitals, and the coordination surroundings have a prominent influence on its energies; however, the 4f orbital lies inside the ion and is shielded from the surroundings by the filled 5s and 5p orbitals. Therefore, the influence of the host lattice on the optical transitions within the 4fn configuration is small. However, the host matrix influences the relative strength of the emission lines via selection rules associated with local symmetry (as the Y/B emission ratio of Dy3+ ion mentioned in the above discussion) and the host phonon energy influences the intensity of the f–f emission lines (like the emission of Eu3+ 5D1,2,3 energy level). 2.2

Multiple ions co-doping in a single host

It is well known that white light can be generated by using phosphors to convert UV, blue, or infrared (IR) light into combination of red, green and blue or yellow and blue.25 Trivalent rare earth (RE) ions, such as Eu3+, Tb3+, Dy3+, Tm3+,

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Fig. 11 (a, b, d) Typical cathodoluminescence spectra of LaOCl:RE3+ samples. The insets are their corresponding CL digital photographs. (c) The cathodoluminescence digital photographs of LaOCl:xEu3+, yTb3+, zTm3+ samples. Arrows A, B, C and D represent the co-doping of Eu3+/Tb3+ (high Tb3+ concentration), Eu3+/Tb3+ (moderate Tb3+ concentration), Eu3+/Tm3+ and Tm3+/Sm3+ in the LaOCl host, respectively (reproduced with the permission from ref. 26, 27 and 28; copyright 2012, 2009, and 2009; Wiley, Optical Society of America, and Royal Society of Chemistry).

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Fig. 12 (a) Upconversion emission spectra from (1) 0.5Yb3+/0.1Tm3+/ 0.02Er3+, (2) 0.5Yb3+/0.1Tm3+/0.05Er3+, (3) 0.5Yb3+/0.1Tm3+/0.1Er3+, and (4) 0.5Yb3+/0.1Tm3+/0.2Er3+ (mol%) triply doped glass ceramics (denoted as GC5102, GC5105, GC5110, and GC5120, respectively) under 976 nm excitation; (b) CIE (x, y) coordinate diagram showing the chromaticity points of the upconversion luminescence in the glass ceramics; the inset is a photograph of the white light luminescence in the GC5105 sample (reproduced with the permission from ref. 29, copyright 2007, American Institute of Physics).

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the ground and excited states of sensitizer and activator are equal (resonance condition) and if a suitable interaction between both systems exists. The interaction may be either an exchange interaction or an electric or magnetic multipolar interaction. In practice the resonance condition can be tested by considering the spectral overlap between the emission spectrum of the sensitizer and the absorption (excitation) spectrum of the activator. In some oxide-based luminescent materials, rare earth ions Ce3+ and Eu2+ could efficiently absorb near ultraviolet (300–400 nm) and give blue to red broadband emissions.30,31 Therefore, Ce3+ or Eu2+ ion can act as a good sensitizer, transferring a part of its energy to activator ions. In recent years, numerous efforts have been made to develop the single-phase white-emitting phosphors based on the mechanism of the energy transfer from sensitizer to activator, such as Ce3+/Mn2+,30 Eu2+/Mn2+,10 Ce3+/Tb3+,32 Ce3+/Eu2+,33 and Ce3+/Dy3+.34 Ca9Gd(PO4)7:Eu2+, Mn2+ (CGP: Eu2+, Mn2+) is a typical example of a single-phased white-emitting phosphor based on energy transfer from Eu2+ to Mn2+.10 The significant spectral overlap in Fig. 13a indicates that there may exist an energy transfer between sensitizer Eu2+ and activator Mn2+ in CGP host. This is because the PL spectrum of CGP:Eu2+ (solid line) with a broad band between 400 and 750 nm (4f65d1 - 4f7 electronic dipole transitions of Eu2+) has spectral overlap with the excitation peaks of CGP:Mn2+ centered at 251, 340, 357, 407, and 502 nm (dashed line, corresponding to the transitions from the 6 A1/6S ground state to the excited states 4T1(4P), 4E(4D), 4T2(4D), 4 [ A1(4G), 4E(4G)], and 4T1(4G) levels). Fig. 13b shows the emission spectra of CGP:0.007Eu2+, xMn2+ phosphors under near-UV 380 nm excitation. Doping 0.01 to 0.1 mol of Mn2+ ion in CGP:0.007Eu2+, xMn2+ phosphors generated blue-greenish and red emission bands centered at 494 nm (4f65d1 - 4f7 transition of Eu2+) and 652 nm [4T1(4G) - 6A1(6S) transition of Mn2+]. By tuning the Eu2+/Mn2+ ratio, the intensities of blue-green emission for Eu2+ and the red emission for Mn2+ can be

transitions of Er3+, respectively; red emissions were obtained in the Tm3+/Er3+ codoped b-YF3 nanocrystals. Based on the generation of red, green, and blue emissions, white light was produced by appropriate doping of Yb3+, Tm3+, and Er3+ in the present glass ceramic. In summary, white light generation can be realized by combining the emissions of trivalent rare earth ions (Tm3+, Tb3+, Eu3+, Er3+, etc.) due to their abundant emitting colors when they are simultaneously co-doped in a single-phase host material excited with ultraviolet, IR or low-voltage electron beam. This observation promises applications for RGB combination white light generation in luminescent lamps, flexible monitors, and displays. 2.3

Energy transfer approach

Energy transfer refers to the process that a certain excitation center transfers all or part of the excitation energy to another luminescent center. Generally, the donor ion is defined as the ‘‘sensitizer’’ and the acceptor ion is called the ‘‘activator’’. Energy transfer can only occur if the energy differences between

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Fig. 13 (a) Spectral overlap between the Eu2+ PL spectrum (solid line) and the Mn2+ PLE spectrum (dashed line); (b) the emission spectra of (Ca0.993x)9Gd(PO4)7:0.007Eu2+, xMn2+ phosphors under near-UV 380 nm excitation (reproduced with the permission from ref. 10, copyright 2010, American Chemical Society).

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2+

2+

Fig. 14 CIE chromaticity diagram of CGP:0.007Eu , xMn phosphors under 380 nm excitation: (1) x = 0; (2) x = 0.01; (3) x = 0.02; (4) x = 0.03; (5) x = 0.05; (6) x = 0.07; and (7) x = 0.1. The insets show CGP:0.007Eu2+, xMn2+ phosphors irradiated under 365 nm UV lamp box (reproduced with the permission from ref. 10, copyright 2010, American Chemical Society).

controlled and the emission color can be tunable. At an appropriate Eu2+/Mn2+ ratio, the blue-green emission intensity is comparable with the red emission intensity of Mn2+ and white light is generated. Fig. 14 shows the emission hue of Ca9Gd(PO4)7: 0.007Eu2+, xMn2+ from blue-greenish (0.219, 0.371) to whitelight (0.326, 0.328) and eventually to red (0.625, 0.307). Fig. 15 shows the electroluminescent spectrum of white LED lamps fabricated with a near-UV 380 nm chip combined with a singlephased white-emitting phosphor CGP:0.007Eu2+, 0.02Mn2+ driven by a 350 mA current. The electroluminescence spectrum clearly shows three emission bands at 380, 490, and 652 nm due to the near-UV chip, Eu2+ emission, and Mn2+ emission, respectively. The correlated color temperature of the white light LED is 6569 K with CIE coordinates of (0.312, 0.327). The inset of Fig. 15

Fig. 16 (a) PL and PLE spectra for Sr3B2O6:1% Ce3+, n% Eu2+ (n = 0.15). Inset: the conversion efficiency under UV excitation. (b) PL spectra for Sr3B2O6:1% Ce3+, n% Eu2+ phosphors excited at 351 nm. Inset: dependence of the energy transfer efficiency ZT on Eu2+ content n (reproduced with the permission from ref. 33, copyright 2007, American Institute of Physics).

shows a photograph of the white-light LED lamp under a forward bias of 350 mA. These results indicate that the CGP:0.007Eu2+, 0.02Mn2+ white phosphor through energy transfer between Eu2+ and Mn2+ may have promising applications for white-light near-UV LEDs. The Sr3B2O6:Ce3+, Eu2+ phosphor is presented as an example of Ce3+–Eu2+ codoped white light emission phosphor.33 The emission spectrum gives two emission bands with peaks at 434 and 574 nm, as shown in Fig. 16a, which are ascribed to the emissions of Ce3+ and Eu2+, respectively. With increasing Eu2+ content (n), the PL intensity of Eu2+ (Fig. 16b) increases systematically from n = 0.10 to 0.15, and reach saturation as n equal to or larger than 0.2, whereas that of Ce3+ was simultaneously found to decrease gradually from n = 0.10 to 0.70. So a tunable color from blue through white to red is obtained by adjusting the ratio of Ce3+/Eu2+. The energy transfer mechanism is dominated by an electric dipole–dipole interaction. So the energy transfer between sensitizer and activator plays an important role in tuning the luminescence color of the materials. The corresponding energy transfer mechanism has been frequently investigated. Generally speaking, the energy transfer mechanism can be determined by the following relation according to Dexter’s energy transfer formula:35   Z (1) ln 0 / C for exchange interaction Z Z0 / C n=3 for multipolar interaction Z

Fig. 15 The electroluminescent spectra of white LED lamps fabricated using a near-UV 380 nm chip combined with a white-emitting phosphor CGP:0.007Eu2+, 0.02Mn2+ driven by a 350 mA current (reproduced with the permission from ref. 10, copyright 2010, American Chemical Society).

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(2)

where Z0 and Z are the luminescence quantum efficiency of the sensitizer in the absence and presence of activator, respectively; C is the total content of sensitizer and activator; n = 6, 8, and 10, corresponding to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. Experimentally, I0 and I, which are luminescence intensity of the sensitizer in the

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absence and presence of activator, can be used to represent Z0 and Z according to Reisfeld’s approximation.36 That is to say, when the relationship between ln(I0/I) and C meets a linear relation, the energy transfer from the sensitizer to the activator could be considered to be through the exchange interaction; however, if the relationship between I0/I and Cn/3 satisfies a linear relation, the energy mechanism would be multipolar interaction. Therefore, n = 6 means that the energy transfer occurred through dipole– dipole interaction, while n = 8 or 10 corresponds to dipole– quadrupole or quadrupole–quadrupole interactions, respectively. For example, evidence shows that energy transfer from Eu2+ to Mn2+ is through the dipole–quadrupole interaction in most of the host materials.10 The energy transfer probability PSA (in s1) from Eu2+ to Mn2+ for dipole–quadrupole interaction is given by the following formula:35 2 ð 12 lS fq FS ðEÞFA ðEÞdE PDQ ¼ 3:024  10 (3) EuMn E4 R8 tS where fq is the oscillator strength of the involved absorption transition of the acceptor (Mn2+), lS (in Å) is the wavelength position of the sensitizer’s (Eu2+) emission, tS is the radiative decay time of the sensitizer (in seconds), R is the sensitizer–acceptor average distance (in Å), E is the energy involved in the transfer (in eV), and ð FS ðEÞFA ðEÞdE represents the spectral overlap between the E4 normalized shapes of the Eu2+ emission FS(E) and the Mn2+ excitation FA(E). The critical distance (RC) of energy transfer from the Eu2+ to Mn2+ is defined as the distance for which the probability of transfer equals the probability of radiative emission of donor, the distance for which PDQtS = 1. Hence, RC can be obtained from eqn (1) as ð FS ðEÞFA ðEÞdE (4) RC8 ¼ 3:024  1012 lS2 fq E4

Table 2

The oscillator strength of the Mn2+ electric quadrupole transition ( fq) is about 1010. So the critical distance RC of energy transfer can be calculated. Generally, the radiative emission from sensitizer (Eu2+) prevails when REu–Mn > RC, and energy transfer from Eu2+ to Mn2+ dominates when REu–Mn o RC. According the above formula, the energy transfer efficiency is correlated with RC and the spectral overlap. The energy transfer efficiency ZT can also be calculated by using the following formula, suggested by Paulose et al.:37 ZT ¼ 1 

tS tS0

(5)

where tS0 is the lifetime of the Eu2+ ions in the absence of the Mn2+ ions and tS is the lifetime of the Eu2+ ions in the presence of the Mn2+ ions. Additionally, there are some other co-doping ion pairs which could generate white light, such as Ce3+–Tb3+ in Ca4Y6(SiO4)6O,38 Ce3+–Dy3+ in Sr2Al2SiO7,39 and Bi3+–Eu3+ in Y2O3.40 A summary description of general properties and energy transfer mechanisms for some typical single-phase white-light-emission phosphors based on the energy transfer approach are listed in Table 2. Note that the above methods based on energy transfer to obtain white light in a single-phase host are realized by codoping luminescent ions into one host. However, some compounds, containing cations such as Zr4+, Ce4+, Ti4+, V5+, and W6+, themselves can generate broad ultraviolet-visible emissions and transfer their energy to an activator ion, in which the Mn+–O2 (n = 4, 5, or 6) charge transfer (M–O CT) transition is responsible for the emissions.41 In the case of charge transfer, the optical transition takes place between different kinds of orbitals or between electronic states of different ions. Such an excitation very strongly changes the charge distribution on the optical center, and consequently the chemical bonding also

Summary of representative single-phased white-emitting phosphors based on the energy transfer occurring between the doping ions

Representative examples Excitation Ca9Gd(PO4)7:Eu2+, Mn2+ 2+

2+

CaAl2Si2O8:Eu , Mn

MgY4Si3O13:Ce3+, Mn2+ 3+

Ca3Sc2Si3O12:Ce , Mn2+, Y3+ Sr2SiO4:Ce3+, Eu2+ 3+

2+

Sr3B2O6:Ce , Eu

Ca4Y6(SiO4)6O:Ce3+, Tb3+ Ca2Al2SiO7:Ce3+, Tb3+ 3+

3+

Sr2Al2SiO7:Ce , Dy

12CaO7Al2O3:Ce3+, Dy3+

Emission

Eu2+: blue-greenish emission band (peaking at 494 nm) + Mn2+: red emission band (peaking at 652 nm) 354 nm Eu2+: a broad band centered at 425 nm + Mn2+: a broad band centered at 568 nm 328 nm Ce3+: an asymmetric broad band peaking at 455 nm + Mn2+: orange-red emission band at 587 nm 450 nm Ce3+: a green emission band peaked at 505 nm + Mn2+: a yellow band at around 574 nm and a red band at around 680 nm 354 nm Ce3+: an asymmetric blue emission + Eu2+: a broad band covering the blue-green to yellow region 351 nm Ce3+: a broad asymmetric blue emission band centering at 434 nm + Eu2+: a broad yellow-orange emission band centering at 574 nm 352 nm A blue band (421 nm) of Ce3+ and the characteristic emission lines of Tb3+ (5D4–7F3,4,5,6) ranging from 470 to 650 nm with yellow-greenish emission 352 nm A blue band (419 nm) of Ce3+ and the characteristic emission lines of Tb3+ (5D4–7F3,4,5,6) ranging from 470 to 650 nm 335 nm Ce3+: a blue emission band at 408 nm + Dy3+: the emission bands at 491 and 573 nm 362 nm/ Ce3+: a broad band centered at 430 nm + Dy3+: two narrow electron beam bands centered at 476 and 576 nm 380 nm

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CIE (x, y)

Energy transfer mechanism Ref.

(0.326, 0.328) Dipole– quadrupole (0.33, 0.31) Dipole– quadrupole (0.36, 0.26) Dipole– quadrupole (0.30, 0.33) —

10

30





31

(0.31, 0.24)

Dipole–dipole

33

(0.278, 0.353) Dipole–dipole

38

(0.316, 0.336) —

32





39

(0.324, 0.323) —

34

15 43

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Fig. 17 Excitation and emission spectra of the K2YZr(PO4)3 (a) and K2EuZr(PO4)3 (b) phosphors at room temperature. (c) Emission spectra of the K2Y1xEuxZr(PO4)3 (x = 0–1) phosphors for various amounts of Eu3+ under 320 nm excitation at room temperature (reproduced with the permission from ref. 42, copyright 2011, Optical Society of America).

changes considerably. In these cases, therefore, very broad emission spectra are expected. In these materials, no intentional dopant is introduced, and for this reason they are also called self-activated luminescent materials. Therefore, these hosts can transfer their energy to the activator ions. The single-phase K2Y1xEuxZr(PO4)3 phosphors are presented as an example to illustrate white-emission based on energy transfer from host to activator ion.42 The single-phase K2Y1xEuxZr(PO4)3 (x = 0–1) phosphors were prepared by solid-state reaction. As shown in Fig. 17a, the greenish-blue Zr4+–O2 emission with peak at 475 nm is observed in the K2YZr(PO4)3 (x = 0) phosphor due to the Zr4+–O2 charge transfer transition under UV excitation. Together with the Zr4+–O2 emission, the red emission (Fig. 17b) of Eu3+ is progressively developed by replacement of Y3+ to Eu3+ over the full Eu-content (x = 0–1) in K2Y1xEuxZr(PO4)3. The emission color varies from greenish-blue to whitish with increasing Eu3+-content and the white-light emission is realized in single-phase phosphor of K2EuZr(PO4)3 (x = 1) by combining the Zr4+–O2 emission and the Eu3+-emission. As shown in Fig. 17b, the emission band of the Zr4+–O2 CT state overlaps the excitation lines of Eu3+. This spectral overlap gives rise to energy transfer from the Zr4+–O2 CT state to the Eu3+ ions resulting in changes in the intensity ratio of Eu3+/Zr4+–O2 CT for different Eu contents (Fig. 17c). 2.4

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due to the abundant emission colors based on their 4f - 4f or 5d - 4f transitions. A different type of phosphor relies for its photoluminescence on the creation of electron–hole pairs within specific sites or clusters in a lattice without the presence of doped activators.44 Here electronic states created between the valence and conduction bands are defect-induced and broad emission bands are the consequence of different chemical bonding environments in the ground and excited states.45 It has been reported that a kind of white-light-emitting silica spheres with high emissive broadband spectra (Fig. 18a) (emission maximum between 450 and 600 nm) can be synthesized from a sol–gel precursor and a variety of organic carboxylic acids with a low heat treatment.46 The PL quantum yield of these materials ranged from 20 to 45% under ultraviolet (365 nm) excitation with PL lifetimes of less than 10 ns. In addition, it is worth pointing out that the bright white photoluminescence was first attributed to carbon defect centers and the detailed scheme of the carbon substitutional defect for Si as the luminescent species in the lattice. Nitrides, such as hexagonal-boron nitride (h-BN)-based materials, have emerged as promising environmentally friendly phosphor candidates that are rare earth ion free. Most recently, Okuyama and co-workers reported a simple and useful synthetic route for a BCNO phosphor (a new carbon-based boron oxynitride phosphor).47 The color of the BCNO phosphor can be tuned by manipulating both the composition ratios of the raw materials as well as the reaction conditions. As shown in Fig. 19, the color can be tuned from violet to near red by changing the PEG/B ratio, operating temperature, and heating time. The measured PL peaks for the BCNO samples shifted

Defect related systems

As mentioned above, luminescent materials usually consist of a host and an activator. This kind of phosphor is based on an optically inactive host lattice into which activator ions are doped in small and optimized concentrations, typically a few mole percent or less. These activator ions have excitation energy levels that can be populated by direct excitation or indirectly by energy transfer, and emissions from these states are subsequently observed. As discussed above, rare earth ions have been playing an important role in this type of phosphor

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Fig. 18 Emission spectra of monodisperse SiO2 spheres excited at 300 nm (a) and the corresponding photographs (b) under a 365 nm lamp (reproduced with the permission from ref. 46, copyright 2006, American Chemical Society).

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Fig. 19 Photographs and corresponding emission spectra excited at 365 nm of BCNO samples prepared under various conditions by altering PRG/B ratio, temperature, and heating time [(a) 2.0  103, 900 1C, 30 min; (b) 2.0  103, 800 1C, 30 min; (c) 4.0  103, 700 1C, 60 min; (d) 4.0  103, 700 1C, 45 min; (e) 4.4  103, 700 1C, 30 min] (reproduced with the permission from ref. 47, copyright 2008, Wiley).

Fig. 20 Emission spectra of ZrO2 samples annealed at different temperatures: (a) 300 1C, (b) 350 1C, (c) 400 1C, (d) 450 1C, and (e) 500 1C under 365 nm excitation. The digital photos show luminescence photographs of the corresponding annealed samples under the excitation of a 365 nm UV lamp; the inset is an SEM image of ZrO2 spheres (reproduced with the permission from ref. 48, copyright 2009, American Chemical Society).

Table 3

from 387 (violet) to 571 nm (near-red) under excitation of 365 nm, as evidenced by the corresponding images (see insets in Fig. 19). The peak positions tended to shift to longer wavelengths as the PEG fraction was increased. One explanation for this result is that more carbon atom impurities, which are able to substitute for boron or nitrogen atoms in the t-BN crystallites, were incorporated into the BCNO particles. Consequently, the allowed electronic transitions in the t-BN type matrix were modified, and the states generated by carbon were changed. Namely, variations in the BCNO band gap, which result from changes in the chemical composition of the BCNO particles, caused shifts in the PL peaks. Our group has also made efforts to study defect-related phosphors with white light emission in recent years. For example, we firstly reported the monodisperse, spherical, luminescent ZrO2 particles with tunable emission by the simple hydrolysis of zirconium butoxide in alcohol solution, followed by an annealing process at different temperatures.48 As shown in Fig. 20, the emission intensity and emission color (peak position) vary obviously as a function of the annealing temperature. The peak center positions for the ZrO2 samples shift from 407 (blue emission) to 550 nm (dark-orange emission) when annealed from 300 to 500 1C. In particular, ZrO2 at 400 1C shows nearly white emission ranging from 370 to 620 nm centered at 462 nm. In our study, the luminescent centers might be attributed to the carbon impurities in the ZrO2 samples. The control of the carbon content by optimizing the annealing temperature is important for the production of ZrO2 phosphors with emission wavelengths tunable over the visible light emission range. Furthermore, there are some other defect-related luminescent materials which could generate white light, such as Y2O3,44 BPO4,49Al2O3.50 Table 3 gives a summary description of the general properties and luminescence mechanisms for defectrelated materials with white light emission. We can conclude from Table 3 that the BCNO phosphor may be the most promising system for application in WLEDs51 because the color emission of this novel BCNO phosphor has been easily tuned from the violet to the near-red region of the photoluminescence spectrum by varying the carbon content. In addition, high external quantum efficiency (79%) and a broad range of excitation wavelengths have been obtained. Furthermore, the preparation conditions, including synthesis method and raw materials, are readily available and realizable. Moreover, Okuyama’s group has made a

A summary description of the general properties and luminescence mechanisms for defect-related materials with white light emission

Representative examples

Fabrication route

Excitation

Emission

Possible mechanism

Ref.

SiO2 spheres

¨ber–Fink–Bohn Sto (SFB) process PSG process

300 nm

Maximum red-shifted with increasing APTES concentration Bluish-white emission (416–451 nm) Tunable emission (blue–white– dark orange) by varying the annealing temperature Bluish-white emission (350–600 nm) Tunable broad emission (400–650 nm)

Carbon/oxygen-related defect center Carbon impurities and oxygen-related defects Carbon impurities

46

2+

BPO4/Ba

ZrO2 spheres Al2O3

Hydrolysis and followed by a heat treatment PSG process

BCNO

One-step liquid process

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Broad band (220–450) 365 nm Broad band (235–425) Broad band (200–450 nm)

Carbon-related defect (radical carbonyl defect) Carbon impurities

49 48 50 47

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Fig. 21 An example application of BCNO phosphors in white LEDs. (a) A 5 mm reference GaN/SiC LED chip illuminating with blue light (commercially available from SloanLED, USA), (b) the same LED chip coated with a thin layer of the sample prepared at 700 1C after treatment with a polymer solution (before illumination), and (c) upon illumination, the same LED shows a bright white light (reproduced with the permission from ref. 51, copyright 2011, Royal Society of Chemistry).

WLED using a yellow-emitting BCNO phosphor coated on the surface of a commercial blue GaN/SiC LED chip and their research indicates that a bright white light was achieved, as shown in Fig. 21.

3. Summary and outlook With the development of modern technology, white LED technology gradually trends towards maturation, and white LED lighting becomes more and more popular under the environment of an energy shortage. In addition, white LEDs are being exploited for other applications, for example, white light communication technology and white light cameras. Moreover, single-phase white-emitting host materials can avoid color reabsorption and ratio control problems among phosphors, and improve the color rendering and luminous efficiency. Therefore, white light generation in a single-phase host is a hot research topic. In this tutorial review, we have given several methods for obtaining white light in a single-phase host material; nevertheless, research is still active, and the need for more efficient methods to generate whiteemitting material is still present. There are still many areas that need additional work, including but not limited to: (1) White emission can be generated by doping a single rare earth ion (Eu3+, Eu2+ or Dy3+) into appropriate single-phase hosts. The trivalent europium ion (Eu3+) is well-known as a redemitting activator due to its 5D0–7FJ transitions ( J = 0, 1, 2, 3, 4). However, if the doping concentration of Eu3+ and the phonon energy (highest vibration frequency) of the host lattice are low enough, white emission could be obtained, which comes from the simultaneous emission of red emission from the 5D0 level and blue and green emissions from the higher 5D levels (5D1, 5D2 and 5D3) of Eu3+ with comparable intensity. For the divalent europium ion (Eu2+), its 4f–5d transitions appear in a large wavelength range that deeply depends on the host lattice and hence, tunable emission colors including white emission can be obtained by appropriately modifying the composition of the host. As the Dy3+ ion has two dominant emission bands, one in the blue region (470–500 nm) and one in the yellow region (560–600 nm), it is possible to obtain a phosphor with near-white-light emission by suitably adjusting the yellow-to-blue intensity ratio. (2) White light can be generated by the combination of red, green and blue or yellow and blue. Therefore, white emission

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can be obtained by co-doping various luminescence ions with different emissions into a single matrix simultaneously, such as Tb3+/Sm3+, Tm3+/Tb3+/Eu3+, Tm3+/Dy3+, Yb3+/Er3+/Tm3+ etc. (3) White light emission can be generated by codoping ions based on the energy transfer mechanism, which has been a hot area of research for white LEDs in recent years. So, numerous efforts have been made to develop single-phase white-emitting phosphors that are based on the mechanism of the energy transfer from sensitizer to activator, such as Eu2+/Mn2+, Ce3+/Eu2+, Ce3+/Mn2+, Ce3+/Tb3+, Ce3+/Dy3+. In addition, white emission can also be obtained through transferring the energy of the host to activator ions. However, research on co-doping systems based on energy transfer is still needed to improve their performance. (4) Defect-related luminescent materials can also emit white light by controlling the concentration of the defect and reaction conditions. This kind of luminescent material has been greatly accelerated due to the emphasis on non-toxic, environment friendly and cheap materials. Generally, the methods of solely doping or multi-doping to obtain white light in a single-phase host material have good controllability of emission colors, while the luminous efficiency and intensity are lower. Conversely, the luminous efficiency and intensity of defect-related luminescent materials are higher, but the controllability of emission colors is poor. Therefore, numerous attempts have been made to develop single-phase white-emitting luminescence materials based on the energy transfer from sensitizer to activator, which not only has better controllability of emission colors but also has higher luminous efficiency and intensity. Although the single-phase white-emitting luminescence materials have unique advantages (avoiding the color reabsorption and ratio control problems among phosphors, and improving the color rendering and luminous efficiency compared with multiplecompositional phosphors), other important aspects of the phosphor interaction with WLEDs, like the cost, light scattering due to particle size and packing, also deserve attention and research. There is a long way to go to achieve the goal of efficient phosphors for WLEDs. In conclusion, a great number of newer methods and novel white-emitting materials are still in the research stage. Some of them may lead to much higher luminous efficiency and lower cost in the coming decades.

Acknowledgements This project is financially supported by the National Natural Science Foundation of China (NSFC 51332008, 51372243, 21221061), the Joint Funds of the National Natural Science Foundation of China and Guangdong Province (U1301242), and the National Basic Research Program of China (2010CB327704, 2014CB643803).

Notes and references 1 Y. Liu, D. Tu, H. Zhu and X. Chen, Chem. Soc. Rev., 2013, 42, 6924–6958. 2 P. Ghosh, A. Kar and A. Patra, Nanoscale, 2010, 2, 1196–1202.

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How to produce white light in a single-phase host?

White light-emitting diodes (WLEDs) as new solid-state light sources have a greatly promising application in the field of lighting and display. So far...
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