Nanocrystalline Phosphors

Encyclopedia of Nanoscience and Nanotechnology

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Nanocrystalline Phosphors Guangshun Yi, Baoquan Sun, Depu Chen Tsinghua University, Beijing, China

CONTENTS 1. Introduction 2. Semiconductor Nanocrystalline Phosphors 3. Doped Nanocrystalline Phosphors 4. Conclusion Glossary References

1. INTRODUCTION Phosphors are defined as solid, inorganic, crystalline materials that show luminescence upon excitation [1]. According to the excitation source, phosphors can be divided into photoluminescence phosphors, cathode luminescence phosphors, X-ray luminescence phosphors, electroluminescence phosphors, etc. The excitation sources are, respectively, photons, electrons with a high kinetic energy, X-ray, and electrons with a low kinetic energy [2]. Phosphors have been studied for a long time, and have been widely used in such areas as fluorescent lamps (FLs), X-ray photography, cathode-ray tube (CRTs), electroluminescence display (ELDs), and so on. With the development of nanoscience and nanotechnology, many investigations have focused on nanocrystalline phosphors, including semiconductor nanocrystals and doped nanocrystals. It is interesting to know what the differences are between the bulk and nanoscale phosphors in terms of their properties. In addition, there is currently a great deal of interest in the production of novel types of bright, high-resolution, and high-contrast emissive displays, such as high-definition TVs, field-emission displays, plasma displays, and electroluminescent devices. In these applications, the required properties are high purity, compositional uniformity, high luminous efficiency, low-energy excitation source, and small and uniform particle-sized powders [3, 4]. Nanocrystalline (1600  C) without an external heat source; at the same time, a large amount of gaseous product such as N2 , CO2 , H2 O is produced, which generates nanostructure phosphors with higher surface areas and high luminescence emission. Tao et al. have prepared nanoscale Y2 O3 :Eu phosphors by glycine–nitrate solution combustion synthesis [134]. An average particle size of 8, 40, 70, and 160 nm was obtained by adjusting the glycine-to-nitrate ratio. Y3 Al5 O12 :Eu (YAG:Eu) nanocrystalline phosphors are prepared by employing a combustion synthesis with urea as a fuel [135]. The size of the phosphor particles is in the range of 60–90 nm. The disadvantage of this method is that it can only be used to prepare oxide phosphors.

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Colloidal Chemical Methods Colloidal chemical methods have often been utilized to prepare colloidal solutions of highly crystalline and well-separated nanoparticles. These methods have been widely used for the preparation of semiconductor nanocrystals, such as CdSe, as mentioned in Section 2. They have also been applied to the preparation of doped nanocrystals. Colloidal nanocrystals of LaPO4 :Eu, CePO4 :Tb [136], LaPO4 :Ce, and LaPO4 :Ce,Tb [137] have been prepared by this method. Hydrothermal Method Hydrothermal synthesis is a lowtemperature and high-pressure decomposition technique that produces fine, well-crystallized powders. YVO4 :Ln (Ln = Eu, Sm, Dy) nanocrystals have been prepared via a hydrothermal method at 200  C. Highly crystalline particles ranging in size from about 10 to 30 nm were obtained [138]. The authors have prepared La2 (MoO4 )3 :Yb,Er upconversion nanocrystalline phosphors through the hydrothermal method [139]. For some time, a number of papers have reported on the preparation and luminescence of nanocrystalline rareearth doped II–VI semiconductors, such as ZnS:Tb3+ [140], ZnS:Eu3+ [141], and ZnS:Er3+ [142]. Recently, Bol et al. repeated some of the experiments, and investigated the results systematically [143]. They concluded that it was not possible to incorporate rare-earth ions in the nanocrystalline semiconductors due to the large size, chemical differences, and the need for charge compensation for rare-earth ions. The observed rare-earth emission is really from the ions absorbed on the surface.

3.1.2. Optical Properties Emission Wavelength Li et al. found that, when the size of nanocrystalline Y2 O3 :Eu [127] and Y3 Al6 O12 :Ce [144] became smaller, a blue shift was observed for the emission spectrum. For example, nanocrystalline Y2 O3 :Eu of different sizes with an average diameter of 43, 55, 68, 71 nm were prepared; the central emission wavelengths were, respectively, 610, 612, 614 and 614 nm. Luminous Efficiency Bhargava reported [145] that the photoluminescence efficiency of nanocrystalline Y2 O3 :Tb3+ with a size between 2–4 nm was at least five times higher than that of bulk samples. He also proved that smaller nanocrystalline Y2 O3 :Tb3+ particles were far more efficient than larger ones or bulk-like samples. For the phosphor of Y2 O3 :Eu, Lee et al. [131] found that Y2 O3 :Eu nanoparticles produced by a reverse microemulsion method with an average size of 30 nm displayed a stronger photoluminescence intensity than the bulk samples. They attributed this fact to higher crystallinity and more densely packed crystal with few void spaces. Sharma et al. reported that the peak emission intensity of Y2 O3 :Eu3+ increased approximately fivefold as the average particle size decreased from 6 m to 10 nm [126]. Ihara and co-workers synthesized glass-coated ZnS:Tb and ZnS:Eu nanocrystals. They found that the photoluminescence intensities were about three times higher than those of bulk [146]. However, Li et al. [127] obtained a different result. Nanocrystalline Y2 O3 :Eu3+ has been prepared with a homogenous precipitation method. The relative luminescent

intensity changed from 100 to 64% as the size decreased from 68 to 43 nm. Quenching Concentration Nanosized phosphors doped with rare-earth elements have increased the quenching concentration. Tao et al. reported that the quenching concentration of Y2 O3 :Eu prepared by conventional synthesis is 6% mol europium, but for their nanoscale samples prepared by combustion synthesis, the quenching concentration was apparently 14% mol [134]. Li et al. found that the quenching concentration was 8% for nanocrystal Y2 O3 :Eu [147], as compared with 6% in bulk samples. Luminescent Lifetime As for the lifetime of Y2 O3 :Eu nanocrystals, Tissue and co-workers have done much research. They observed that the fluorescent lifetime was obviously longer in the monoclinic Y2 O3 :Eu3+ nanocrystals than in the bulk material [5]. Li et al. also obtained the same result [148].

3.2. Transition Metal-Doped Nanocrystals 3.2.1. Synthesis Colliodal Chemical Method For the synthesis of transition metal-doped nanocrystals, colloidal chemical methods have been widely used, and the most thoroughly investigated systems are manganese-doped zinc sulfide and cadmium sulfide. Liu et al. have prepared well-dispersed CdS:Mn nanocrystals in an aqueous solution by using mercapto acetate as a capping reagent [149], while our group has synthesized ZnS:Mn nanocrystals in a similar way by using histidine as a capping reagent [150]. In both methods, transparent colloidal solutions of CdS:Mn and ZnS:Mn nanocrystals were acquired. Bawandi and coworkers [151] synthesized TOPO-capped CdSe:Mn using two different manganese precursors. They found that almost all of the manganese resides near the surface in the doped sample obtained by using manganese salts as the manganese source, whereas by use of an organometallic complex [Mn2 (-SEMe)2 (CO)8 ], manganese was incorporated in the lattice. Recently, TOPO-capped, nearly monodispersed ZnS:Mn and CdS:Mn nanocrystals were prepared by Malik et al. [152] Coprecipitation Method Through this process, nanocrystalline ZnS:Mn was prepared by coprecipitation of zinc acetate and manganese acetate with sodium sulfide in methanolic media [153]. ZnS:Cu+ , ZnS:Cu2+ [154], and ZnS:Cu,In [155] nanocrystalline phosphors have been obtained by the chemical homogeneous precipitation of cation solutions, with S2− as the precipitating anion, which was formed by the decomposition of thioacetamide. Reverse Microemulsion Method By using this method, ZnS:Mn [156, 157], ZnS:Cu [157, 158], and CdS:Mn [159] nanoparticles with a narrow-size distribution have been prepared successfully.

3.2.2. Optical Properties Luminous Efficiency In 1994, Bhargava et al. [160] first observed that nanocrystalline ZnS:Mn has a high luminous efficiency. The efficiency in these nanocrystals was measured to be 18%, as compared to 16% for the bulk.

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Table 3. Nanocrystalline phosphors reported in the literature.

Semiconductor nanocrystalline phosphors

Semiconductor

Size (nm)

CdSe

1.1–11.5 Nanorods Nanorods and fractal 3 3–8 Nanorods, teprapods 6–20 × 100–500 (nanorods)

Optical properties

Quantum efficiency 20–85%

ZnSe

2–9 4–8 1–6 3–7 Less than or equal to 6 2–3 2–5 4.3–6 4–8

Quantum efficiency 20–50%

ZnS CdTe

2–5 2.5–7

Quantum efficiency 40% Quantum efficiency 30–65% Quantum efficiency 18%

ZnTe InAs

4.2–5.4 2.5–6

InP

7.4 11.3–20

CdS

Quantum efficiency 20–50%

Bandgap: 1.05–1.55

Bandgap: 2.1–2.5 eV 2–6.5 GaP

GaAs PbS

PbSe Si Y2 O3 :Eu Doped nanocrystalline phosphors

Y2 O3 :Tb Zn2 SiO4 :Eu Zn2 SiO4 :Tb Y2 SiO5 :Eu Y3 Al5 O12 :Eu LaPO4 :Eu, CePO4 :Tb YVO4 :Ln (Ln = Eu, Sm, Dy) La2 (MoO4 )3 :Yb,Er Y3 Al6 O12 :Ce ZnS:Tb and ZnS:Eu ZnS:Mn

20–40 × 200–500 nanorod 11–21 2–6.5 6–36 8–30 Nanofilm

Film Porous silicon 3.8–2.7 23 23 60 6 m, 10 nm 43, 55, 68, 71 10–30 8, 40, 70, and 160

2–4 40–100 50 60–90 5 10–30 50 2, 3 8.3 3.5–7.5

Spectral blue shift Bandgap blue shift from IR region to near-UV region Enhanced photoluminescence Quantum-size effect Enhanced photoluminescence Enhanced photoluminescence Longer fluorescent lifetime

Enhanced photoluminescence (fivefold) Emission blue shift decreased emission intensity Enhanced photoluminescence Quenching concentration increased to 14% Quenching concentration increased to 8% Long luminescent lifetime Enhanced photoluminescence (five times greater)

Enhanced up-conversion fluorescence Emission blue shift Enhanced photoluminescence (three times higher) High luminous efficiency, short lifetime (4 ns)

Ref. [40] [41] [42] [43] [44] [45] [46] [83] [47] [48] [49] [50] [51] [52] [53] [82] [54] [55] [56] [57] [58] [59] [60–61] [86] [62] [64–65] [88] [66] [67] [68] [69] [70] [71] [89] [93] [94] [91] [95] [101] [5] [118] [119] [126] [127] [131] [134] [147] [148] [145] [122] [123] [135] [136] [138] [139] [144] [146] [150] [160] continued

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Nanocrystalline Phosphors Table 3. Continued Semiconductor

Size (nm)

CdS:Mn CdSe:Mn ZnS:Cu ZnS:Cu,In

3–5 100 4 5 2–2.5 2–3

Luminescent Lifetime Bhargava et al. also reported an ultrafast decay time for nanocrystal ZnS:Mn. The lifetime of the Mn emission was shortened from 1.8 ms in the bulk powder to 4 ns in the doped nanocrystalline phosphor, which is five orders of magnitude shorter than that of bulk ZnS:Mn [160]. However, it was later shown that lifetime shortening in nanocrystalline ZnS:Mn did not occur. The Mn2+ emission of nanocrystalline ZnS:Mn has a normal millisecond lifetime. To test the result, a systematic investigation was carried out by Bol and Meijerink [161]. From lifetime measurements and time-resolved spectroscopy, they concluded that the 4 T1 –6 A1 transition of the Mn2+ had a normal decay time of about 1.9 ms. The short decay time reported by Bhargava was ascribed to a defect-related emission of ZnS, and was not from the decay of the 4 T1 –6 A1 transition of the Mn2+ impurity. More recent work on ZnS:Mn [156, 162] and CdS:Mn nanocrystals [159, 163] confirmed this conclusion [156].

3.3. Application of Doped Nanocrystals As we reviewed earlier, doped nanocrystalline phosphors have many new optical characteristics. Such a system offers numerous possibilities for the next-generation devices in the field of lighting, displays, sensors, bio tags, and lasers. In the following, we will discuss the application of display and bio tags in detail.

3.3.1. Display Applications The development of new types of flat-panel and projection displays has created a need for optical phosphors with new or enhanced properties [164]. For application in these areas, thermally stable, high-luminous-efficiency, radiation-resistant, fine particle size powders are required [3]. Nanophase and nanocrystalline materials, typically particles with diameters of 100 nm or less, offer new possibilities for advanced phosphor applications [165–167]. On the other hand, for display applications, multiple particle layers are required to achieve optimal light output [5]. Large particles require thicker layers, increasing the phosphor cost, and also producing more light scattering [168]. Small and uniform particles with high luminous efficiency are preferred for new flat-panel displays [3]. Dinsmore et al. [169] found that ZnS:Mn nanoparticles exhibit less current saturation than bulk phosphors, which is an important feature for use in field-emission displays. Furthermore, the nanoparticles were annealed at a temperature far below the processing temperatures of standard phosphors.

Optical properties Normal lifetime of 1.9 ms Low-voltage excitation

Ref. [169] [149] [151] [154] [155]

By using nanometer-sized rare-earth doped phosphors as layers, Chinese scientists have successfully manufactured a field-emission display recently. This display has the advantages of high definition, bright emission, sharp dynamic color image, large angle of view (nearly 180 ), and so on [170].

3.3.2. Biological Application Nowadays, most of the biological luminescent tags are organic dyes, such as rodamine, FITC, Cy3, and Cy5. In comparison, the luminescence of doped phosphors is stronger, nonfading, not significantly influenced by pH or temperature, and has a longer lifetime [171]. Inorganic phosphors with a size of 100–300 nm, such as Zn2 SiO4 :Mn, As, ZnS:Ag, and Y2 O2 S:Eu, have already been used for the detection of proteins and nucleic acids [171–173]. By using these kinds of labels, 10 fg protein and 300 fg nucleic acids have been detected. Nanosized phosphors with a narrow size distribution and high fluorescent efficiency are theoretically advantageous and favorable [172]. The uses of up-conversion phosphors as fluorescent labels for the sensitive detection of biomolecules have attracted even more interest recently [174, 175]. Phosphors that emit lower energy photons when excited by higher energy photons are down-conversion phosphors. For example, ZnS:Mn and Y2 O3 :Eu are well-known down-conversion phosphors. On the other hand, phosphors that emit higher energy photons after absorbing lower energy excitation photons are upconversion phosphors. At least two low-energy photons are required to generate a higher energy photon. In comparison with organic dye labels, up-conversion phosphor fluorescent labels show very low background noise without photobleaching. By using up-conversion phosphor as labels, 1 ng/L DNA could be detected, which is four times more sensitive than that labeled with cy5 [116].

4. CONCLUSION Here, we have presented an overview of the synthesis and optical properties of nanocrystalline phosphors (see Table 3). The potential applications of these novel materials are also highlighted. For semiconductor nanocrystalline phosphors, a wide range of synthetic methods are now available. Particles with diameters in the range of 1–20 nm have been prepared, and quantum-size effects have been observed experimentally for many nanocrystalline semiconductors. However, most of the studies up to now have focused on II–IV and III–V compounds; there is still a major problem associated with the reproducible preparation of this kind of material that will be needed for technological applications, and the applications of semiconductor nanocrystals remain scarce, except for CdSe.

474 Studies of doped nanocrystalline phosphors, including synthesis, optical properties, and their applications, are discussed. Some new nano-related optical properties, such as high efficiency, high quenching concentration, and so on, have been reported. However, research in this area is preliminary; some results are quite confusing, and even contradicting. Most of the studies concentrated on a few of this kind of material, such as ZnS:Mn and Y2 O3 :Eu. There is still a lack of a guiding theory for the study of doped nanocrystalline phosphors. Systematic research needs to done.

Nanocrystalline Phosphors 15. 16. 17. 18.

19. 20. 21. 22.

GLOSSARY Down-conversion phosphor Phosphor that emits lower energy photons when excited by higher energy photons. Emission spectrum Wavelength distribution of the emission, measured at a single constant excitation wavelength. Excitation spectrum Dependence of emission intensity, measured at a single emission wavelength, upon the excitation wavelength. Fluorescence Emission of light by a substance immediately after the absorption of energy from light of usually shorter wavelength. Luminescent lifetime Average fluorescence time between its excitation and its return to the ground state. Luminous efficiency Also called quantum yields, is the number of emitted photons relative to the number of absorbed photons. Phosphors Solid, inorganic, crystalline materials that show luminescence upon excitation. Up-conversion phosphor Phosphor that emits higher energy photons after absorbing lower energy excitation photons. At least two low-energy photons are required to generate a higher energy photon.

23. 24. 25. 26. 27. 28. 29. 30. 31.

32. 33. 34. 35. 36.

37.

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