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 (<50 nm) luminescent materials are potentially well suited for the applications [5]. The demand of these new technologies has stimulated a search for new materials and synthesis techniques to improve the performance of phosphors.

ISBN: 1-58883-062-4/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

In the early stage, most of the studies in nanocrystalline phosphors focused on semiconductor phosphors. This is because, when semiconductor particles become smaller than the Bohr radius, the so-called quantum-size effect occurs. As a result, these semiconductor nanocrystals not only provide many unique opportunities for studying physics in low dimensions, but also exhibit novel optical properties which are potentially useful for technological applications. It is important in both basic and applied research. Recently, the number of scientific publications on the optical properties of transition or rare-earth metal-doped nanocrystals has increased. Some new nano-related phenomena, such as luminous efficiency, lifetime, quenching concentration, and so on, have been reported. Many new methods for the preparation of nanocrystalline phosphors have also been developed. This chapter reviews recent work on the synthesis, luminescent properties, and potential applications of nanocrystalline phosphors. In Section 2, we discuss semiconductor nanocrystalline phosphors, and in Section 3, we discuss doped nanocrystalline phosphors.

2. SEMICONDUCTOR NANOCRYSTALLINE PHOSPHORS Semiconductor nanocrystals (also known as quantum dots or QDs) have attracted a great deal of interest from all disciplines, including chemistry, physics, materials, and even biology, due to their attractive properties such as sizetunable optical properties [6, 7]. Furthermore, many promising technical applications in the field of biomolecular labeling [8, 9], solar cell [10, 11], laser [12], and light-emitting diodes [13, 14] have attracted commercial interest. Over the last decade, rapid progress has been made in the preparation of nanocrystal semiconductors. They mainly include the II–IV compounds (ZnS, CdSe, CdS) [15], III–V compounds (InP, GaAs) [16], IV–VI compounds (which mostly refers to some kinds of chalcogenides [17]), and Si [18]. In the following, we discuss them respectively. Because the II–VI and III–IV semiconductor nanocrystals share almost the same synthesis procedure and optical properties as CdSe, these two parts are discussed together.

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 6: Pages (465–476)

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2.1. II–IV and III–V Compounds 2.1.1. Synthesis Synthesis of CdSe Nanocrystals In particular, II– IV and III–V compounds have attracted much attention because the preparation of nanocrystalline CdSe leads to an unprecedented degree of monodispersity and crystalline order [19]. Shape-controlled methods have been developed to prepare nanocrystals of different shapes [20, 21]. The investigation of the size-dependent optical absorption has been carried in detail [22]. In the following section, CdSe is used as an example for semiconductor nanocrystals. Its preparation, optical properties, and technical application are discussed. The synthesis of other II–IV and III–V semiconductor nanocrystalline phosphors is shown in Table 1. There are two main synthetic methods for the preparation of nanosized CdSe. However, both need ligands to derivate the surface to keep the nanocrystals stable in a solution. Stabilizing ligands must be present during growth to prevent aggregation and precipitation of the nanocrystals. The stabilizing molecules attach onto the nanocrystal surface as a monolayer through covalent, dative, or ionic bonds. Most of them use organic surface derivation in organic solution, and

others use organic or inorganic ones (such as cetyltrimethylammonium bromide, CTAB) in an aqueous solution, as shown in Figure 1. The synthesis of colloid semiconductor nanocrystal CdSe is based on the pyrolysis of organometallic reagent (cadmium dimethyl, Cd(CH3 )2 ) with selenium (Se) powder by injection into a vigorously stirred flask containing a hot coordinating solvent (200–360  C) [23]. The coordinating solvent can be amine, phosphonic acid, or phosphine oxide, which serves as a ligand/solvent for the synthesis of high-quality CdSe nanocrystals, as shown in Eq. (1): Cd(CH3 ) + Se–TOP (or TBP) → CdSe [technical grade TOPO (90%) as solvent]

(1)

Furthermore, fatty acids have also been developed as useful coordinating solvents. This method provides temporally discrete nucleation, and permits controlled growth of macroscopic quantities of nanocrystallites. Size-selective precipitation of crystallites from portions of the growth solution isolates samples with narrow size distributions

Table 1. Synthesis of other II–IV and III–V nanocrystals. Semiconductor

Synthetic method

Size (nm)

CdSe

Cd(CH3 )2 + Se-TOP (in hot TOPO) CdCl2 + Na2 Se (CTAB as surfactant) Cd2+ + SeO2− 3 + N2 H4 (hydrothermal method) [Cd(SePh)2 ] thermolysis Cd[Se2 CNRR
]2 thermolysis CdO[or Cd(C2 O4 )] + HPA (or TDPA, ODPA) + Se TOPO CdC2 O4 + Se Li4 [Cd10 Se(Sph)16 ], pyrolysis S(in THF) + CdCl2 + KBH4 CdO + S + oleic acid + octadecene Cd[S2 CNRR
]2 thermolysis Cd(NO3 )2 + P2 S5 in ethanol Cd(CH)3 + S(TMS)2 in hot TOPO

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

[40] [41] [42] [43] [44] [45] [46]

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

[47] [48] [49] [50] [51] [52]

Li4 [Zn10 Se(Sph)16 ], pyrolysis Zn(CH3 )2 + Se-TOP (hexadecylamine/trioctylphosphine)

2–5 4.3–6

S(in THF) + ZnCl2 + KBH4 Zn[S2 CN-Me(C6 H13 )]2 in hot TOPO

4–8

H2 Te + Cd(ClO4 )2 + RSH Cd(CH3 )2 + Te (TOP + dodecylamine as solvents) Cd2+ + NaHTe + mercaptoacetic acid

2–5 2.5–7

CdS

ZnSe ZnS CdTe

Quantum efficiency

20–50%

Ref.

[53] [54] [55]

40% 30–65% 18%

[56] [57] [58]

ZnTe

[Zn(TePh)2 ][TMEDA] with TOP in dodcylamine

4.2–5.4

[59]

InAs

InCl3 + As[Si(CH3 )3 ]3 (TOP) In(Ac)3 + As(TMS)3 + octadene

2.5–6

[60] [61]

InP

In(PBut2 )3 + 4-ethylpyridine InCl3 + KBH4 + P In(Ac)3 + P(TMS)3 + octadene [R2 InP(TMS)2 ]2 in hot TOPO and TOP InX3 (X = Cl, Br, I) + P(SiMe3 )3

7.4 11.3–20

[62] [64] [65] [66] [67]

2–6.5

GaP

Na, P, and GaCl3 in benzene (Na/K)3 P + GaCl3 in 1,4–dioxane [R2 GaP(TMS)2 ]2 in hot TOPO and TOP

20–40 × 200–500 nanorod 11–21 2–6.5

[68] [69] [70]

GaAs

(Na/K)3 As + GaCl3 in 1,4–dioxane GaCl3 + As(SiMe3 )3 in quinoline

6–36 4.5

[71] [72]

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Nanocrystalline Phosphors CdCl2

+ Na2Se

Cd(CH3)2 + Se-TOPO

CTAB +

N

Br-

CdSe Aqueous solution

HO P O

OH

CdSe Hot TOPO

Figure 1. Two major surface modification schemes for the CdSe nanocrystals.

(<5% rms in diameter). Although numerous reports have been published in which CdSe was prepared using inorganic cadmium salts in an aqueous solution [24], the luminescence is much lower than organometallic paralysis. One possible reason is that the paralysis process operates at a much higher temperature, and may crystallize better than in an aqueous solution. It would be very helpful for the colloid nanocrystals to have a good optical property [25]. It is well known that organometallic compounds are very unstable, apart from being toxic, expensive, and explosive. They are usually stored in the refrigerator inside a glove box. Fortunately, some alternative cadmium compounds have been developed via a different route. Simple inorganic and organic cadmium such as CdO [26] and Cd(Ac)2 [27] can be candidates for the cadmium precursor. They are lowcost, safe, and easily controlled precursors. The resulting nanocrystal is nearly monodispersed without any exceptional size separation, and is reproducible. The reaction is simple and mild. Furthermore, the formation of nanocrystals with the concept of “user-friendly chemistry” was achieved by growing the nanocrystals in a mixture of octadecen (ODE) and oleic acid with CdO as the cadmium precursor [28]. Apart from spherical nanocrystals, the formation of nanocrystals of various shapes, including nanorods, -arrows, -teardrop, -tetrapod, and branched tetrapod-shaped nanocrystals, has also been achieved by adjusting the components of the coordinating solvent [21, 29]. The coordinating solvent is a mixture of pure trioctylphosphine oxide (TOPO) and hexylphonsphonic acid (HPA). In general, experimental results show that the coordinating solvent TOPO must be technical grade if the semiconductor nanocrystal is synthesized using an organometallic precursor. In fact, TOPO and HPA are very similar. HPA can be called a ligand, which can be substituted by tetradecylphosphonic acid or octadecylphosphonic acid. The initial cadmium and selenium precursor ratio (Cd:Se ratio), the injection and growth temperature, and the concentration of the precursor all play key roles in determining the shape of the resulting nanocrystals and luminescent quantum efficiency. Usually, low temperature and a high precursor concentration lead to the formation of perfect nonorods. In comparison with bulk semiconductors, there are more surface atoms for nanocrystals. Thus, the semiconductor nanocrystals are more likely to undergo epitaxial overgrowth than other inorganic compounds. Furthermore, there are many defective surface atoms unless they are passivated. To remove these defects, core/shell-type composite CdSe/ZnS

has been studied by several groups [30–32]. Overcoating the core of lower bandgap nanocrystals with a higher bandgap shell has improved the photoluminescence quantum yield by passivating surface nonradiative recombination sites. It has also been reported that the epitaxial growth graded CdS/ZnS shells on the colloidal CdSe nanorods [33]. The core/shell nanorods have increased the photoluminescence quantum yield and improved the photostability. Modification of CdSe As mentioned earlier, nanocrystal CdSe is prepared in a coordinating solvent such as an amine, or a phosphine oxide with long alkyl tails. These organic reagents are usually called surfactants, a capping group, or ligands. They improve the stability of nanocrystals by adsorbing a monolayer on the surface. If nanocrystals lose the surface ligand, they will precipitate out, and the luminescence is totally lost. The ligands are usually hydrophobic. So the nanocrytal CdSe can easily be dispersed in many nonpolar solvents such as hexane, toluene, and pyridine. Usually, it is necessary to modify the nanocrystals with a hydrophilic reagent if they are going to be dispersed in an aqueous solution. Some strategies have been pursued to develop hydrophilic nanocrystals [8, 34, 35]. The trick depends on supplying the nanocrystal surface with thiolated molecules having a free carboxyl group facing the solution that warrants water solubility. In general, the longer the alkyl chain is, the longer the modified nanocrystals remain dispersed in the aqueous solution. To improve the stability of thiol-coated nanocrystals in an aqueous solution, chelating agents were introduced for surface modification, instead of the commonly used monothiols [36]. In fact, all monothiolcoated CdSe nanocrystals have shown very profound photochemical instability in an aqueous solution [37]. The main reason is that the mercapto ligands on the surface can be oxidized photochemically to disulfides, which then leave the surface and expose the nanocrystal in air. The naked nanocrystals could also be photooxidized, and eventually precipitate out. So free thiols in a solution help to improve the stability of nanocrystals modified with hydrophilic thiols. According the above mechanism of the photochemical instability of the thiol-coated nanocrytals, it is believed that a thick and densely packed ligand layer would be helpful in inhibiting the photooxidation. Organic dendric ligands have been developed to stabilize nanocrystals [38]. They are hyperbranched organic molecules with thiol groups as the central focal point of the binding site for the cationic elements, and hyperbranched groups to inhibit oxygen diffusion to the surface of nanocrystals. The closely packed and tangled ligand shell provides sufficient stability to withstand the rigor of the coupling chemistry. An alternative was reported in which CdSe nanocrystals were embedded in a polymerized silica shell [9, 39]. Mercapto silane was attached onto the surface, and a siloxane shell of 1–5 nm thickness was developed through pyrolysis of the silane reagent. The siloxane shell can be functionalized with thiols and/or amines on the surface. The silanized nanocrystal can be dispersed in buffer solutions in a wide pH range, even at a high salt concentration. The photoluminescence properties depend weakly on the pH of the solution due to the protection of the silicon shell. The silanized nanocrystals have quite a good quantum yield, and

468

Nanocrystalline Phosphors

are stable for months. They also have optical properties similar to that of thios-coated nanocrystals. However, the thiol modification is much easier to carry out. The silanization process, although rather difficult to develop, results in better stability. Synthesis of Other II–IV and III–V Nanocrystals The popular pyrolysis method with TOPO as the solvent medium has also been used by Alivisatos [8, 9] in the synthesis of InP (2–5 nm) or InAs nanocrystals (2.5–6 nm). The method was based on the reaction of InCl3 and X[Si(CH3 )3 ]3 (X = P or As) in TOPO at elevated temperatures, as shown in Eq. 2: 100  C
12 h

InCl3 + TOPO −−−−−−→ In–TOPO XSiCH3 3 3
heated
days

−−−−−−−−−−−−−−−→ InX (X = P, As)

(2)

The nanocrystals are highly crystalline, monodisperse, and soluble in various organic solvents. Improved size distributions have been obtained by size-selectively reprecipitating the nanocrystals. X-ray photoelectron spectroscopy (XPS) shows that the nanocrystals have a nearly stoichiometric ratio of indium to phosphorus, with the TOPO surface coverage ranging from 30 to 100%. The synthesis of other II–IV and III–V nanocrystals is summarized in Table 1.

2.1.2. Optical Properties Optical Properties of CdSe Due to the quantum-size effect, semiconductor nanocrystals exhibit many unique properties when their size is smaller than their Bohr radius (about 1–5 nm) [7, 73]. Semiconductor nanocrystals of different sizes show diverse properties. The optical absorption spectrum of semiconductor nanocrystals supplies accessible and straightforward information to evaluate the quantum-size confinement effect. An electron was excited from the valence band to the conduction band via absorbing a photon, which is associated with the bandgap energy (Eg ). The absorption of photons with energy similar to that of the bandgap, hv ≥ Eg , leads to an optical transition producing an electron in the conduction band and a hole in the valence band. CdSe semiconductor nanocrystals have a broad excitation range and a narrow emission range compared with temporal organic dyes. Their spectral width (full width at half maximum) usually spans 27–40 nm at room temperature, compared with 20 nm for a CdSe single crystal because of the inhomogeneous emission properties of the nanocrystals [74, 75]. Different from organic dyes, there is a

substantial Stokes shift in the luminescence of nanocrystals. The optimized excitation and emission filters for semiconductor nanocrystals can allow light passage for nearly the entire excitation and emission peaks, which permits detection of more fluorescence output than that of organic dyes. Semiconductor nanocrystals have a rather high cross section (∼105 mol/cm), which means that they can efficiently absorb the light that is irradiated. And the photoluminescence quantum yield for the as-prepared nanocrystals increases by about 20% for bare dots, and by 30–50% for dots passivated with ZnS at wavelengths of 520 and 600 nm. Photoefficiency is lower at wavelengths below 520 nm or longer than 600 nm. [76]. It can be concluded that the semiconductor nanocrystals have a strong emission due to their large cross section and high quantum yield. The CdSe nanocrystals are rather stable against photobleaching. The emission wavelength of CdSe can be tuned from 450 (center) to 650 nm (center) by changing the particle size [77, 78]. It has been reported that light emission from single CdSe under continuous excitation turns on and off intermittently with a characteristic time scale of about 0.5 s [79–81]. The on/off time can change when a passivating, high-bandgap shell of ZnS encapsulates the nanocrystal. Optical Properties of Other II–IV and III–V Nanocrystals The optical properties of other II–IV and III–V semiconductor nanocrystals are shown in Table 2.

2.2. IV–VI Compounds (Lead Chalcogenides) Just as we reviewed earlier, II–IV and III–V semiconductor nanocrystals have been widely investigated. As for the IV–VI semiconductor nanocrystals, reports in this area mainly focus on lead chalcogenides, that is, PbS, PbSe, and PbTe. The growth of these semiconductor nanocrystals of uniform size distribution has been reported [89–92]. These nanocrystals are of interest because of their large Bohr radius (10 nm for PbS and 15 nm for PbSe), which permits quantumsize effects to be clearly visible, even for large particles or crystallites. These large Bohr exciton radii result in a strong confinement of the electron–hole pair. Nasu et al. [93] observed a significant bandgap shift from the IR region to the near-UV region, depending on the small size of the PbS crystallites. An enhanced photoluminescence was also reported [94] for PbS nanocrystals by Thielsch and co-workers. But reports on their optical properties remain scarce; studies in this area are preliminary; more work needs to be done.

Table 2. Optical properties of other II–IV and III–V semiconductor nanocrystals.

Semiconductor ZnSe CdSe CdTe CdS InAs InP

Bandgap (bulk, eV)

Electron affinity (eV)

Bandgap (nanocrystals, eV)

258 174 15 253 036 128

409 495 428 479 49 44

1.88–3.62 1.9–2.2 3.0–4.06 [85] 1.05–1.55 [86] 2.1–2.5 [88]

Quantum efficiency (nanocrystals, %) 20–50 [82] 20–85 [83] 35–65 [84] 8–20 [87]

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Nanocrystalline Phosphors

2.3. Si Since the strong room-temperature visible luminescence in porous silicon was discovered by Canham [95] in 1990, the study of the fabrication and light-emitting properties of various Si-based nanomaterials has received worldwide attention over the past few years. Si-based nanomaterials are some new photoelectronic and informational materials developed rapidly in recent years. These kinds of materials include nanoscale porous silicon, Si nanocrystallineembedded SiO2 matrices, Si nanoquantum dot, Si/SiO2 superlattice, etc. The synthesis of Si-based nanomaterials with high quality is very important in order to obtain efficient and stable luminescence. Thus far, various growth technologies have been developed, including chemical-vapor deposition [96, 97], Si-ion implantation [98], laser-ablation deposition [99], self-assembling growth [100], and so on. When the Si nanocrystallite size is reduced, the bandgap will become large, and will create the quantum energy level in the conductor band and valence band. As a result, not only is the luminescent intensity increased, but also the peak energy is blue shifted [101]. Kanzawa et al. [102] measured the photoluminescence spectra of Si nanocrystals embedded in SiO2 films as a function of size. They found that, as the average particle size decreased from 3.8 to 2.7 nm, the photoluminescence peak energy exhibited a blue shift from 1.42 to 1.54 eV, and its peak intensity increased progressively. This phenomenon was also observed by Takagi et al. [103]. They found that the luminescent intensity was inversely proportional to the square of the crystallite size from 2.8 to 5.6 nm.

Consequently, semiconductor nanocrystals might be promising biological tags. There have been reports on immunoassay [104], immunohistochemical assay [105], live-cell detection [106], and multicolor detection [107].

2.4.2. Photovoltaic Device and Solar Cell The semiconductor nanocrystals possess a relative large electron affinity and spectrally narrow luminescence compared to many traditional electroluminescent organic polymers. They show some desirable properties in that they can be spin coated, can be used on a large area. They are flexible, with potential thermal and electronic stability [108–109]. Consequently, the semiconductor nanocrystals have been made into bilayer light-emitting diodes with organic electroluminescent semiconducting polymers such as poly(p-phenylenevinylene). The device made from different sized semiconductor nanocrystals can emit from red to green with a relatively high conversion efficiency, a low operating voltage, and hundreds of hours of lifetime under constant current flow. As a class of electroluminescent materials, semiconductor nanocrystals offer many of the processibility advantages of polymers, along with potentially stable, long-lived operation. If the semiconductor nanorods are mixed with some conjugated polymers, they can be manipulated readily [110]. Tuning the length and diameter of the nanorods, the distance of electron transfer and the overlap between the absorption spectrum and the solar emission can be optimized. The power conversion efficiency of the reported device can be 1.7%.

2.4.3. Laser 2.4. Applications of Semiconductor Nanocrystals Due to the attractive characteristics of colloid semiconductors, that is, variable emission colors, electron affinities, and ionization potentials, they have showed significant potential as luminescent chromophores ranging from biological luminescent tags, lasers, as well as light-emitting diodes to solar cells.

2.4.1. Biological Application The fluorescent tag is a popular tool in biological detection. Organic dyes are most widely used in this area. Semiconductor nanocrystals have been applied as promising luminescent tags. Compared with the dyes, they have several potential advantages: a rather large Stokes shift, wide excitation and a relatively narrow emission peak (full width at half maximum, 20–30 nm), various emission wavelengths (400–2000 nm), a large extinction coefficient in the visible and ultraviolet range (∼105 M−1 · cm−1 ), high quantum yield (core– shell nanocrystals, >50%), a rather strong antiphotobleaching capability, and a relatively long luminescence lifetime (∼30–100 ns) at room temperature. Furthermore, semiconductor nanocrystals of different sizes may be excited with a single wavelength, resulting in many emission colors that may be detected simultaneously. The environment of semiconductor nanocrystals contributes little to their emission.

Theoretically, semiconductor nanocrystals have some advantages as a gain medium, including temperature insensitivity, lower lasing threshold, and a gain profile concentrated into a much narrower luminescence spectrum. Such advantages have motivated the use of semiconductor nanocrystals as the medium of the laser. Semiconductor nanocrystal-based lasers with optical pumping have been successfully developed using nanocrystal–titania chemistry [111]. The output color of the laser can be selected by choosing appropriately sized nanocrystals, operating at room temperature or below. The experience with semiconductor nanocrystal thinfilm systems has opened the possibility of UV and IR lasers using corresponding nanocrystals.

3. DOPED NANOCRYSTALLINE PHOSPHORS In addition to semiconductor phosphors, there is another very important class of phosphors, called transition or rareearth metal-doped phosphors. For the doped phosphors, transition or rare-earth metals act as luminescent centers. Since the wavelength of the characteristic luminescence of the doped metals hardly changes with size confinement, doped nanocrystals do not require the stringent control of size needed in semiconductor nanocrystalline phosphors. In fact, most of the phosphors employed in technological applications belong to doped phosphors. Such phosphors find

470 applications ranging from conventional fluorescent lighting to color TV picture tubes, X-ray photography, and so on. For example, the three components in trichromatic fluorescent lamps are BaMgAl10 O7 :Eu2+ (blue), CeMgAl10 O19 :Tb3+ (green), and Y2 O3 :Eu3+ (red) [112]. Nanoscale doped-phosphors are currently attracting a great deal of interest for both fundamental photophysics study, and important applications including display and sensor technologies [5, 113–115]. New synthetic techniques and effects of nanoscale confinement have motivated significant new interest in these materials [116, 117]. It is important and interesting to study the doped nanocrystalline phosphors. So far, reports on doped nanocrystalline phosphors mainly focus on the following areas: (1) rare-earth metal-doped nanocrystals, such as Y2 O3 :Eu, Y2 O3 :Tb, ZnS:Eu, ZnS:Tb, ZnS:Sm, YVO4 :Ln, LaPO4 :Ln, ZnSiO4 :Ln(Ln = Eu3+ , Tb3+ ), Y2 SiO5 :Eu, and (2) transition metal-doped nanocrystals, such as ZnS:Mn, ZnS:Cu(I), ZnS:Cu(II), ZnS:Cu,In, and CdS:Mn. In the following part, the preparation method, nanosize-related optical properties, and technical applications of doped nanocrystalline phosphor will be reviewed.

3.1. Rare-Earth Metal-Doped Nanocrystals 3.1.1. Synthesis The reported preparation methods include gas-phase condensation, flame-spray pyrolysis (FSP), coprecipitation, sol– gel, block copolymer, reverse microemulsion, combustion synthesis, colloidal chemistry, and hydrothermal method. Gas-Phase Condensation Method Tissue and coworkers have prepared monoclinic nanocrystals by a gasphase condensation method using CO2 -laser vaporization of Y2 O3 :Eu ceramic pellets, and condensing the nanocrystals from the gas phase [5, 118]. Y2 O3 :Eu with an average diameter of 23 nm was obtained. The average production rate was 11 mg/h. The main problem of the method is, when the doped concentration of Eu3+ is above 0.7%, Y2 O3 :Eu and a secondary Eu2 O3 phase will occur due to a kinetic effect of the gas-phase condensation method. Flame-Spray Pyrolysis Method There is also a well-established technology called flame-spray pyrolysis (FSP), which is used by the industry to produce nanocrystalline phosphors [119–121]. This process involves combusting aerosols of single-metal and mixed-metal metalloorganic alcohol solutions with oxygen or air in a reaction chamber at temperatures of 1200–2000  C. By this method, rare-earth doped Y2 O3 nanocrystalline phosphors with a size of 60 nm have been prepared. Sol–Gel Method For the sol–gel method, dilute solutions of metallorganics or metal salts are reacted to form an irreversible gel, then dried and shrunk to an amorphous or weakly crystalline mass. This kind of method has been widely used to prepare different kinds of doped nanosized phosphors, including Tb3+ and Eu3+ doped Zn2 SiO4 [122], Y2 SiO5 :Eu [123], Y2 O3 :Eu [124], SrAl2 O4 :Dy,Eu [125], etc. Sharma et al. have synthesized nanocrystalline Y2 O3 :Eu by an improved sol–gel method in the presence of surface modifiers. The particle size gradually decreased from 6 m

Nanocrystalline Phosphors

to 10 nm with an increase of the modifier from 0 to 10wt% with respect to Eu2 O3 /Y2 O3 [126]. The advantage of this method is that atomically mixed powders are obtained in the as-synthesized condition. However, these as-synthesized materials must also be heat treated to high temperatures to crystallize the desired phase. Also, the processing steps to prepare the precursor powders are complicated and time consuming. Homogeneous Coprecipitation Method In this kind of method, urea or thioacetamide is used to produce OH− or S2− . Due to the characteristics of urea and thioacetamide, OH− or S2− may be increased very slowly and homogeneously throughout the whole solution. The gradual and uniform rise in OH− or S2− can result in the nucleation and growth of uniformly nanosized hydroxide or metal sulfide particles. Nanocrystalline Y2 O3 :Eu has been prepared by the homogeneous precipitation of Y3+ and Eu3+ with urea [127]. Block Copolymer Method This method uses copolymers as steric stabilizers. The copolymers form micelles, and serve as nanoreactors for the preparation of nanoparticles. For example, by utilizing polyacrylamide gel, nanosized Y2 O3 :Eu3+ [128] and Y3 Al5 O12 :Ce3+ (YAG) [129] have been prepared. ZnS:Tb nanocrystals [130] have also been synthesized in a similar way. This kind of synthesis scheme could be used with various combinations of metal salts to yield many different doped nanocrystalline phosphors. Also, the size of the nanoparticle can be controlled by adjusting the size of the micelle. Reverse Microemulsion Method Reverse microemulsion is also called the reverse micelle method. In reverse microemulsion, the aqueous phase is dispersed as microdroplets, which are usually less than 100 nm. So the dispersed water droplets behave as nanoreactors for the synthesis of nanoparticles. By using this method, 10 nm Y2 O3 :Eu [131] nanoparticles with a narrow-size distribution have been prepared successfully. Combustion Synthesis Method (or Propellant Synthesis) The combustion synthesis process involves the exothermic reaction of an oxidizer such as metal nitrate, ammonium nitrate, and ammonium perchlorate [132], and an organic fuel, typically urea, carbohydrazide, or glycine [3, 133]. During the process, the chemical energy released from the exothermic reaction between the metal nitrate and fuel can rapidly heat the system to high temperatures (>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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Nanocrystalline Phosphors

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.

472

Nanocrystalline Phosphors

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

473

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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