A Review on High Refractive Index Nanocomposites for Optical Applications
Yuanrong Cheng1, Changli Lü2 and Bai Yang ,1
1State Key Laboratory of Supramolecular Structure & Materials, College of Chemistry, Jilin University, Changchun
130012, P. R. China, 2College of Chemistry, Northeast Normal University, Changchun 130024, P. R. China
Received: June 24, 2010; Accepted: July 25, 2010; Revised: September 25, 2010
Abstract: Nanocomposite optical materials with high refractive index (RI) have been one of the most attractive issues due to their good optical property that can be used in many fields. The method for preparing high RI nanocomposites is to incorporate high RI inorganic nano-building blocks, for example, metal oxides and metal chalcogenide semiconductor nanoparticles into polymer matrices. The general design requirements and synthetic methods of these high RI nanocomposites are discussed in this review. We classify the synthetic methods into two parts: in situ particle generation method and ex situ particle generation method, and all the methods are reviewed. Some optical applications on antireflection coatings, volume holographic recording materials, high RI LED encapsulant materials, photonic band gap materials and other applications are also reviewed. The relevant patents are discussed in this article.
Keywords: High refractive index, nanocomposites, nano-building blocks, optical applications.
1. INTRODUCTION
Over the decades, high refractive index (RI) optical materials have attracted considerable attention from both fundamental and applied research due to their potential optical applications in lenses, optical waveguides, antiref-lection films, reflectors, holographic recording materials, photonic crystals and light emitting diode (LED) encapsulant materials, etc. [1-8]. Many inorganic optical materials usually show good mechanical properties (such as high strength, high hardness and high rigidity) and high RI larger than 2.0 [9], but they have many disadvantages such as high densities (>2.5g/cm3) and low flexibility for process of optical devices, such as lenses, reflectors, optical waveguides, Fresnel lenses, and hologram materials.
Compared to inorganic materials, organic polymer materials have the advantages of light weight, excellent impact resistance, good flexibility and processability which can benefit the manufacture of optical devices. However, the conventional organic polymer materials have a low RI of 1.4~1.6 [10]. Polymers with high RI (usually higher than 1.6) can be prepared by incorporating such structures or organic groups as aromatic groups, halogen atoms, phosphorus and sulfur atoms with high molar refractions into the backbone or side chain of polymers according to classical Lorentz-Lorenz equation [11]. Many new designed high RI polymer materials such as poly[S-alkylcarbamate] [12], polythiourethane [13], epoxy and episulfide-type polymers [14], poly(thioether sulfone) [15], polyphospha-zenes with a –P=N– backbone [16] and polyimide derived from sulfur-containing aromatic diamines and aromatic
*Address correspondence to this author at the State Key Laboratory of Supramolecular Structure & Materials, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China; Tel: 86-431-85168478; Fax: 86-431-85193423; E-mail: [email protected]
dianhydrides [17] exhibit a high RI from 1.60 to 1.76. Some monomers containing metal element for high polymers were also synthesized and their structures are listed below (Formula 1-4) [18-21].
The optical resins produced by polymerizing above monomers exhibited a high RI exceeding (nd) 1.7, high transparency, good heat resistance and mechanical strength. We should note that it is very difficult to prepare these kinds of monomers for polymer materials with a RI higher than 1.8.
In recent years, nanocomposites consisting of inorganic and organic components with controllable length scales of inorganic nano-building blocks ranging from a few angstroms to a few tens of nanometers have attracted broad attention from both fundamental and applied research [22-24]. The incorporation of inorganic nano-building blocks with polymer can improve the mechanical and thermal properties of the polymer materials. The nanocomposite optical materials may possess both the advantages of organic polymers (lightweight, flexibility, good impact resistance, and excellent processability) and that of inorganic materials (high RI, high thermal stability and other physical properties i.e. optical, magnetic, etc.). So the nanocomposite optical
O S Sn
4
OS Sn
4
OS
O
Sn
4
S S Sn
4
Formula 1 Formula 2
Formula 3 Formula 4
16 Recent Patents on Materials Science 2011, Vol. 4, No. 1 Cheng et al.
materials can replace the traditional polymer and inorganic optical materials due to their distinguished physical, che-mical properties and well processable properties for optical applications [22, 23, 25, 26]. High RI organic-inorganic nanocomposite is to incorporate nano-sized inorganic building blocks (such as TiO2, ZrO2, ZnS and PbS) with high RI into polymer or organic matrices. There have been more and more papers and patents dealing with high RI organic-inorganic nanocomposites, focused on the synthetic method and properties as well as on the potential optical application [27-29]. Our aim of this article is to review the present patents on high RI organic-inorganic nanocomposites for optical applications. The general design principle and syn-thetic methods of these hybrid nanocomposites are discussed first. Finally, some optical applications of these nanocom-posites will be reviewed.
2. DESIGN REQUIREMENTS AND FABRICATION METHODS OF HIGH RI ORGANIC-INORGANIC
NANOCOMPOSITES
2.1. Design Requirements
Optical transparency is one of the most key issues to be solved for preparation of high RI organic-inorganic nanocomposites, especially for the nanocomposites with high inorganic loadings [29]. According to Rayleigh’s law, the transparency decline or opaque of the nanocomposites is mainly due to the light scatter caused by the large inorganic domain size and the RI mismatch between the inorganic phase and the organic matrices [29]. There are two routes to minimize the scattering losses and improve the transparency of the nanocomposites at a given wavelength. The first way is so-called RI matching method (RI of the inorganic building blocks is very close to that of the matrix). Many transparent organic-inorganic hybrids composites have been prepared by this strategy [30-35]. However, it’s difficult to prepare high transparency of materials in most cases where the RIs of the inorganic building blocks and the organic matrices differ greatly. The other effective and necessary way to improve the transparency of nanocomposites is by decreasing the particle size much below the visible light wavelength. The pronounced optical scattering loss can be avoided when the inorganic domain size (typically < 40nm) is less than one-tenth of the visible light wavelength (400-800nm) [29]. That the inorganic particles have a diameter below 20nm is better to avoid the particle aggregates having a diameter larger than 100nm which result in the strong scattering in the visible region, and cause haze or even opacity [36, 37]. So, the size of the inorganic domain utilized for fabricating transparent nanocomposites is generally below 20nm.
In order to improve the RI of the nanocomposites, inor-ganic nano-building blocks with high RI can be introduced into organic matrix. Many inorganic materials with high RI and low absorption coefficients can be good candidates as nano-building blocks. The most frequently employed inor-ganic materials for high RI organic-inorganic nanocom-posites are some metal oxide, i.e. TiO2 (n=2.7, 500nm, rutile type), ZrO2 (n=2.2, 589nm), ZnO (n=2.0, 550nm), CeO2 (n= 2.18, 500nm) and some sulfides i.e. ZnS (n=2.41, 500nm) and PbS (n=4.35, 500nm).
The RI of nanocomposites can be adjusted by the amount of definite type of nano-building blocks introduced into polymer matrices [27, 38]. The RI of the nanocomposites can be approximately estimated by the equation [38]:
ncomp = pnp + orgnorg
Where ncomp, np, and norg are the RI of the nanocomposite,
nanoparticles (NPs), and organic matrix, respectively, p
and org are the volume fractions of the nano-building
blocks and organic matrix, respectively. Then, a definite RI of the nanocomposite can be achieved with a definite type of nano-building blocks and the volume fraction of the nano-building blocks. This can ensure that the nanocomposites can meet the different need of RI for various applications.
The RI of the inorganic materials generally is also remarkably affected by the quantum size effect when their characteristic sizes are less than the corresponding exciton Bohr diameters [37, 39]. The influence of the quantum size effect on the RI of inorganic building blocks should be also considered in the design of the high RI nanocomposites when one attempts to control the size of the inorganic particles, especially the narrow band-gap inorganics with large Bohr diameter.
2.2. Preparation Methods
There have been many methods for the preparation of high RI polymer nanocomposites. According to the gene-ration process of nano-building blocks in the nanocom-posites, we classify all the methods into two strategies (shown in Fig. (1)). One way is to prepare NPs in situ in a system consisting of NPs precursors and polymer or mono-mers, we call this procedure as in situ particle generation method. Another way is to prepare NPs firstly, then intro-duce the NPs into monomers, oligomers, polymer or their solutions, we call this procedure as ex situ particle generation method.
2.2.1. In Situ Particle Generation Route
In situ particle generation route is a facile and effective strategy for one-step fabrication of nanocomposites with NPs generated from corresponding precursors in the presence of polymer networks, polymer solutions or polymer precursor (monomer). In this route, the NPs are formed inside the polymer matrix which can effectively control the particle size, whilst maintaining a good spatial distribution in polymer matrices without the irreversible agglomeration. Some typical patents about in situ particle generation method are shown in Table 1. There are two major methods for preparation of high RI nanocomposite materials. One is re-treatment of polymer with metal ion by H2S, etc. for metal sulfide nanocomposites; the other is in situ sol-gel proces-sing for metal oxide nanocomposite.
The general scheme for the preparation of metal sulfide (such as ZnS, PbS) NPs/polymer nanocomposites by in situ method is shown in Fig. (2) [40, 41]. The polymer and metal ions are first mixed in solution, and then exposed to the counter ion source such as S2- in the form of S2- ions
High Refractive Index Nanocomposites Recent Patents on Materials Science 2011, Vol. 4 No. 1 17
Fig. (1). General classification schematic for the preparation methods of high RI nanocomposites.
Table 1. Components, Descriptions and RI of Nanocomposites by in Situ Particle Generation Method.
Entry Nano-Building Block Descriptions RI Refs.
1 PbS In situ generation of nanoparticles 2.2 [42]
2 ZnS Poly carbonate — [43]
1.60-1.76 [49] 3 TiO2, ZrO2(amorphous) In situ sol-gel
6 TiO2(Bi2O3) Monomers with metal for polymerization and sol-gel 1.60,1.69 [52]
Fig. (2). Schematic of in situ synthesis methods of NPs in polymer networks [40]. Copyright 1997 American Chemical Society.
18 Recent Patents on Materials Science 2011, Vol. 4, No. 1 Cheng et al.
dissolved in solution or H2S gas. The composite can be cast as a film before or after exposure to the counter ion [42, 43]. For example, in US20070293611 [42], high RI nanocom-posite film was prepared by the in-situ synthesis of PbS NPs in poly(oxy-3-methyl-l,4-phenyleneoxy-1,4- phenylenecar-bonyl-1,4-phenylene) (methyl PEEK). Methyl-PEEK was dissolved in dimethylacetamide with Lead dodecyl thiolate (a lead precursor). The solution was heated under 80oC and H2S gas was passed through the mixture for in situ synthesis of PbS NPs. Then the solution was spin-coated on a silicon wafer and dried. The prepared nanocomposite film showed high RI of 2.2. Metal (Zn, Pb) precursors can be integrated into polymer chains by preparing metal precursor for polymerization and H2S treatment. For examples, we have prepared metal sulfide NPs in polymer networks by copolymerization of lead methacrylate or zinc methacrylate with styrene before treatment using H2S gas [44, 45]. We also synthesized high RI PbS/polymer nanocomposites from reactive lead-containing precursors (Pb(SCH2CH2OH)2) and polythiourethane (PTU) oligomers terminated with isocyanate groups, followed by in situ gas/solid reaction [46]. The highest RI of the resulting nanocomposite films was 2.06 at the precursor content of 67 wt%.
In situ sol-gel process of inorganic oxide NPs either in the presence of a preformed polymer or in parallel with the formation of the organic polymer has been a broad field of investigation [47]. This method is an efficient method to introduce metal oxide such as TiO2, ZrO2, TaO2 and CeO2. Titanium alkoxides and zirconium alkoxides are most frequently used as precursors in the synthesis of high RI organic-inorganic hybrid materials because of the high RI of their gels. Besides, the derivant of titanium alkoxides such as poly(dibutyltitanate) can also be the precursors for sol-gel processing [48].
In order to interconnect the polymer matrix and inorganic building blocks, one way is modification of polymer by end group modification of polymer with reactive functional group or preparation of copolymer with side group such as triethoxysilane for further sol-gel reaction. For example, poly(arylene ether phosphine oxide) (PEPO), poly(arylene ether) ketone or poly(arylene ether) sulfone having amine end groups was modified by reaction with 3-isocyana-topropyltriethoxysilane to prepare triethoxysilane-capped polymer. This functionalized polymer can react with titania sol by sol-gel method for hybrid materials [49, 50]. Rao et al. prepared nanocomposites by utilizing copolymer with side group of triethoxysilane for further sol-gel reaction with metal oxide precursors [51]. The copolymer was obtained by the copolymerization of styrene and (acry1oxypropy)l trime-thoxysilane (APTMS) monomers. In the copolymer solution of toluene, titanium isopropoxide as the precursor of TiO2 NPs was added for sol-gel reaction to prepare transparent high RI nanocomposite films. The maximum RI of the resultant film can reach 1.86 [51].
Another way is to prepare monomer with metal for both polymerization and sol-gel reaction. Su et al. have prepared film sample and monolith sample of titanium methacryl ethoxide, titanium bismuth methacryl ethoxide with high RI and low birefringence [52]. The metal acryl alkoxides were synthesized from exchanging acryl alcohol with metal
alkoxides. Then they were polymerized to prepare organic/ inorganic hybrid materials containing metal oxide in polyacrylate. Film samples were prepared by gelation in the presence of exact amount of water, following with polymeri-zation irradiated with an UV light in the presence of UV photoinitiator. Besides, monolith samples were also prepared with 2,2'-azobis(isobutyronitrile) free-radical initiator for bulk polymerization in mold.
2.2.2. Ex Situ Particle Generation Method
Ex situ particle generation method is another important route for the preparation of the organic-inorganic nanocom-posites in which the inorganic nano-building blocks such as inorganic NPs are prepared separately, isolated, and purified or modified by surface capping agents before immobilization into polymer matrix [29, 53]. The pre-prepared inorganic nano-building blocks such as inorganic NPs can be integrated into the polymer or monomers for subsequently polymerization to form nanocomposites. This method has advantages of full synthetic control over the particle size, shape, crystal form and size distribution as well as the surface properties of NPs. Therefore the ex situ particle generation method is more suitable for introducing inorganic nano-building blocks with excellent physical properties into polymer matrix compared with the methods of in situ generation in system. The NPs tend to aggregate unless they are modified at their surface to decrease the interparticle attraction, and it’s usually difficult to disperse NPs into polymer matrix for their incompatible physical properties for example, high interface energies, polarity, the surface charge of NPs, etc. So, the key issue is to appropriately design and tailor the surface of NPs and the polymer or the monomer structure simultaneously to make sure that the NPs can be dispersed in polymer or monomers for polymerization. That is to develop a method to prepare NPs in large mass production, and these NPs should possess good dispersibility and long-term stability in the polymer or its monomer. In order to improve the compatibility or miscibility between inorganic NPs and polymer matrix, several surface func-tionalization and stabilizing techniques have been developed: surface engineering of inorganic NPs, engineering of polymer matrix including modification of polymer, or synthesis of suitable polymer matrix, etc. In conclusion, the rational design and tailoring for the NPs and the structures of polymer matrix, selecting proper co-solvents is necessary to improve their compatibility whilst avoiding phase separation.
Generally, surface engineering of inorganic NPs are achieved by the surface modification of inorganic NPs by chemi- or physisorption of surfactant molecules onto the particle surface in the synthesis process or post-treatment process of the particles. The surface modified particles can be readily redispersed after isolation in the organic medium which is compatible with the surfactant layer grafted or deposited to the particle surface. As to surface engineering of metal oxide NPs, silane coupling agent with group such as RSi(OMe)3 are frequently used as capping agent. These capping agents can react with the hydroxyl group of the surface of metal oxide NPs (TiO2, ZrO2, etc.) by sol-gel reaction [54]. Thiol, thiophenol can be good capping agents for metal sulfide (ZnS, PbS, etc.). Some other capping agents
High Refractive Index Nanocomposites Recent Patents on Materials Science 2011, Vol. 4 No. 1 19
with group of –PO3H2, –COOH, –SO2OH, or –NH2 are also effective as surface capping agents [55-57]. Trioctylphos-phine oxide (TOPO), trioctylphosphine (TOP), and quater-nary ammonium cation surfactant can be capping agents for NPs of metal oxide, metal sulfide etc. For the design of high RI nanocomposites, the surface capping agents with higher RI than the conventional ones are desirable because the large amount of surface capping agents with low RI will reduce the average RI of particles, which finally results in the RI decrease of the resultant nanocomposites although the high RI inorganic particle cores are used [57].
Surface engineering of inorganic NPs can be realized directly when synthesizing the NPs. Besides, it also can be realized by ligand exchange for prepared NPs when necessary. For instance, NPs prepared in organophilic phase can be transported to hydrophilic phase by exchange of surface capping agents [58].
The methods for ex situ particle generation in system can be classified to two series: one is blending with polymer, and the other is blending with monomers including oligomers for in situ polymerziation.
2.2.2.1. Blending with Polymer
This route is a general route for preparing nanocom-posites via blending NP solution with solution of polymer or via directly melts compounding [55, 56, 59-67]. Some typical patents by this route are concluded in Table 2. This method includes two independent steps of the preparation of surface modified NPs and the synthesis of polymers, in which the compatibility of NPs and polymer matrices is considered to be a crucial issue for improving the trans-parency of nanocomposites. Besides, the selection of proper co-solvents is also necessary to improve their compatibility whilst avoiding phase separation. Generally, the uniform transparent dispersion of NPs in a solution of polymers with reactive groups is cast into a container or deposited on a glass or plastic substrate by spin-coating or dip-coating process, and then the transparent nanocomposite films or sheets can be obtained after heating treatment.
Engineering of polymer including modification of polymer, or synthesis of suitable polymer, is another way to
improve the compatibility with inorganic NPs. The func-tional group of polymer chain should have similar polarity to that of the surface of inorganic NPs, or it should form a chemical bond with the inorganic fine particles [55, 56, 61]. For example, Suzuki et al. have synthesized a series of polymers with a terminal of phosphoric acid (Formula 5-7). These polymers have strong interaction with inorganic NPs including TiO2 and ZrO2 [55].
In JP2007238929 [56], they also synthesized a series of copolymers by using ordinary monomers like St, MMA with monomers (Formula 8-13) with functional groups. These copolymers also have good compatibility with metal oxide NPs including TiO2 and ZrO2 [56].
NPs can also be introduced into other polymer matrices such as epoxy resin, polyester, PU, PTU, and PI etc. Nakayama et al. reported the preparation of transparent free-standing nanocomposite films with high RI of 1.58-1.81 by dispersing propionic acid modified TiO2 NPs into
Table 2. Components, Descriptions and RI of Nanocomposites by Blending with Polymer Method.
Entry Nano-Building Blocks Descriptions RI Refs.
1 TiO2 (rutile) particle by laser pyrolysis, Surface treatment, and blends with PAA 1.5-1.8 [59]
2 TiO2 Copolymer of methyl methacrylate and methacrylic acid 1.49-1.80 [60]
3 ZnS, TiO2 Copolymer of styrene, Poly(bisphenol-A and epichlorohydrin) 1.63,1.69 [63]
4 TiO2(Sn, Zr) Poly(bisphenol-A and epichlorohydrin) 1.61,1.81 [62]
20 Recent Patents on Materials Science 2011, Vol. 4, No. 1 Cheng et al.
poly(bisphenol-A and epichlorohydrin) using n-butanol and toluene as the co-solvents [62, 63]. Other metal oxide such as ZrO2, CeO2 and SnO2 were also introduced into siloxane polymers to prepare high RI nanocomposites [64].
Bulk nanocomposites can also be made by using this blending route [55, 56, 65-67]. Suzuki and co-workers reali-zed this by hot press molding of nanocomposite blending concentration residue after concentrated distilling off of the solvent from the blending solution under heating and reduced pressure. The 1-mm-thick transparent plastic solid with high RI (up to 1.71) was obtained [55, 65]. Taima prepared bulk nanocomposites by adding a powder mixture of inorganic NPs to a molten cyclic olefin polymer for kneading under the setting temperature and rotating rate to disperse the inorganic particles, thus transparent resin composition were obtained and the RI of some composites can reach 1.86 [67].
2.2.2.2. In Situ Polymerization
Transparent nanocomposites can also be obtained by blending inorganic NPs with organic monomers for in situ polymerization. In this route, surface modified NPs (or their solution) can be blended with monomers (or their solution) for further polymerization by UV-curing, thermal initiation, and others. This approach has been recently developed to prepare high RI organic-inorganic nanocomposites see Table 3.
2.2.2.2.1. In Situ Polymerization with Co-Solvent
Blending nano-building blocks as NPs with monomers in co-solvent is another advantageous strategy for preparing transparent nanocomposites [54, 68-85]. The nano-building blocks in solvent can keep stable without aggregation, which facilitates the nano-building blocks blending with monomers without precipitations. Especially, many nano-building blocks such as TiO2, ZrO2 and ZnS have been utilized to prepare high RI nanocomposites by this method [68-82]. Some surface modified metal oxide prepared by sol-gel method has been used to prepare high RI nanocomposites [54, 72-75, 78, 79]. For example, in order to prepare surface modified ZrO2 NPs, dialyzed ZrO2 sol was treated by methacyloxypropyl trimethoxysilane in a mixture solution of water and methoxypropanol, then concentrated and successively treated by NH3·H2O solution [74].
High RI monomers dissolved in co-solvent can also be utilized to improve the RI of the nanocomposites [69, 72-74]. In order to improve the RI of the compositions, the inventors from 3M Corporation prepared high RI monomers for polymerization. For example, a series of new aromatic sulfur acrylate monomers (Formula 14-17) with high RI are shown below [73]:
The monomers with bromine substituted fluorene structure with high RI were also prepared in US7297810 [74]. The combination of functionalized ZrO2 NPs, multi-functional acrylate crosslinkers and high RI monomers, about 1.58 or greater, can produce the abrasion resistant hard coats that have relatively high RI above 1.6.
The NPs (TiO2, ZrO2, Ta2O5, ZnS, PbS, etc.) can also be introduced into polythiourethane (PTU), epoxy resins, and others to prepare high RI optical materials [81-85]. Recently, we developed a blending reaction route to fabricate ZnS-polythiourethane (PTU) nanocomposite optical thin films with tunable RI [84]. The thiophenol (PhSH)/ mercapto-ethanol (ME) capped ZnS NPs were covalently integrated into the PTU matrix via polyaddition of hydroxyl groups in the ME molecule on the surface of particles with isocyanate groups of the PTU oligomers to prepare high RI nanocom-posite films. Another strategy is blending thiophenol (PhSH)/4-thiomethyl styrene (TMSt)-capped ZnS NPs with urethanemethacrylate macromer (UMM) via combining UV initiated free radical polymerization process. The capping agents (PhSH and TMSt) also have high RI, avoiding the RI decrease of the resultant nanomaterials caused by the incorporation of the low RI capping agents of particles.
O
OO
PO(OH)2
O
OCOOH
n=1, 4, 5
n
SO3H
HN
O CH2SO3H
OO
O
P
O
OH
OH
P
O
OH
OH
Formula 8 Formula 9
Formula 10 Formula 11
Formula 12 Formula 13
S
O
O
S
O
O
SO
O
SO
O
S
S S
O
O
O
O
S S
O
O
O
O
Formula 14
Formula 15
Formula 16
Formula 17
High Refractive Index Nanocomposites Recent Patents on Materials Science 2011, Vol. 4 No. 1 21
Besides, the polymerizable capping agent can react with macromer or oligomers to form integrated polymeric materials, which can effectively prevent phase separation of the nanocomposites and improve the stability of ZnS NPs in the polymers.
2.2.2.2.2 “Monomer as Solvent” Route for In Situ Polymerization
“Monomer as solvent” route for in situ polymerization is to disperse NPs directly into monomers for polymerization to attain transparent nanocomposites. However, the key challenge is to obtain stable and transparent dispersions of NPs before subsequent polymerization because the low viscosity of monomers with low molecular mass may result in the precipitation of inorganic particles. Phase separation may occur during the polymerization process. Generally, for a given monomer used for preparing nanocomposites, the corresponding design and tailoring for surface characteristics of the capping inorganic particles is very important in order to make the particles possess good compatibility with the monomers by the strong interfacial interaction between the monomer molecules and the inorganic particles or the organic capping shell protected inorganic particle core.
Wegner et al. prepared tert-butylphosphonic acid modified ZnO-PMMA transparent nanocomposites by this method [36]. The nanocomposites exhibit light transmittance in visible region, strong UV absorption and moderately high RI as the volume fraction of ZnO particles with 22nm is below 7.76%. However, the NP content in the polymer still needs to be improved in order to further increase the RI of polymer matrix.
Especially, this strategy is a very effective method for preparing thicker bulk nanocomposites as compared with above mentioned methods [57, 86]. In patent US20060216508 [86], ZnS NPs capped with high RI carbon acid for example 2-phenoxybenzoic acid were dispersed into 2-carboxyethyl acrylate (CEA) using an ultrasonic disperser. The formed transparent and curable viscous liquid was used for further UV-curing to prepare a film having a thickness of 100μm between two polyester films.
Recently, we also developed the above strategy to prepare thick bulk transparent nanocomposites with high nanophase contents by incorporating pre-made ZnS NPs into the selected co-monomers, followed by bulk polymerization [37, 87]. A key of this strategy is that we employed polar N, N-dimethylacrylamide (DMAA) as a co-monomer, which
Table 3. Components, Descriptions and RI of Nanocomposites by Blending with Monomer for in-Situ Polymerization Method.
Entry Nano-building Blocks Description RI Refs.
1 TiO2 Sol, UV-curing 1.533-1.675 [68]
2 ZrO2 Sol, High RI monomers, UV-curing 1.49-1.87 [69]
3 Ta2O5 Sol, UV-curing 1.488-1.82 [70]
4 Nb2O5 Sol, UV-curing 1.492-1.843 [71]
5 HIT-32M(TiO2·ZnO2·SnO2,
commercial)
High RI monomers, UV-curing
High RI surface capping agents
1.736,1,796,1.711 [72]
6 ZrO2 Modification of sol, high RI monomers and UV-curing 1.59-1.66 [72]
7 ZrO2 Modification of sol, high RI monomers and UV-curing 1.551-1.639 [74]
8 ZrO2 Modification of sol , UV curing 1.562-1.652 [75]
9 TiO2, BaTiO3 Sol prepared by hydrothermal methods, UV-curing 1.67-2.01 [76]
10 TiO2, ZrO2 UV-curing, sol-gel 1.65,1.94 [77]
11 TiO2 sol, Sb2O3 Modification of sol 1.530-1.653 [78]
12 ZrO2, Al2O3 Modification of sol — [79]
13 TiO2(Sn, Zr) Sol, UV-curing 1.61,1.81 [62]
14 TiO2, ZrO2/TiO2 Sol, UV-curing, desiccation of mixture for bulk materials 1.61-1.83 [111]
15 TiO2,ZrO2,TaO2(sol) Hybrid with polythiourethane (PTU) 1.633-1.88 [81]
16 TiO2(sol) Hybrid with polythiourethane (PTU) 1.53-1.66 [82]
17 ZnS Surface ligand exchange PTU — [83]
18 ZnS PU 1.574-1.848 [84]
19 ZnS In situ polymerization bulk materials 1.55-1.64 [87]
20 ZnS carboxylic acid as capping agent, UV-curing 1.63,1.64 [86]
22 Recent Patents on Materials Science 2011, Vol. 4, No. 1 Cheng et al.
can be used as both solvent and ligand for the ZnS NPs to effectively disperse the NPs. In addition, the -ray irradiation polymerization also offers the quick polymerization of monomers in a mild condition, which is favorable for the fabrication of nanocomposites with NPs dispersed homo-geneously within polymer matrix.
3. APPLICATIONS OF HIGH RI ORGANIC-INORGANIC NANOCOMPOSITES
Another advantage of organic-inorganic nanocomposite materials is to provide a kind of materials with reduced temperature sensitivity compared to that of polymer materials [67, 88-90]. These nanocomposite materials can show high optical stability with respect to temperature. High RI organic-inorganic nanomaterials can be utilized for various optical applications, including antireflection coa-tings, ophthalmic lenses, prisms, optical waveguides, etc. Some promising optical applications of the organic-inorganic nanocomposites with controlled high RI will be discussed here.
3.1. Antireflection (AR) Coatings
Antireflection (AR) coatings are a type of optical coating applied to the surface of lenses and other optical devices to diminish the reflection loss at the surface of the optical component and increase light transmittance [91, 92]. They have provided benefits to a wide variety of technological applications from ophthalmic lenses, optical filters and photovoltaics (solar cells, photodetectors) to windows, display screens, optical data storage, and other optoelectronic devices in which reflections hamper device performance. Multilayer AR coatings comprising alternatively layers of high RI and layers of low RI are more effective over the entire visible and near infrared spectrum, and lower reflection compared to a single layer anti-reflection coating, which can be made non-reflective only at one wavelength, usually at the middle of the visible region [93, 94]. However, conventional AR coatings are usually applied expensively via vacuum processes [95]. Nanocomposite materials with high RI is very good candidate for the design and fabrication of multilayer AR coatings because nanocomposite materials with continuous adjustable RI by controlling loading of high RI NPs in nanocomposite can satisfy the need of special layer with special RI [54, 75, 77, 96-102]. The properties (reflectance spectrum, abrasion resistance, haze and trans-mission value) of the resultant nanocomposite AR coatings are well comparable to those produced using state-of-the-art vacuum.
Typically, Kato developed a method for preparing multilayer antireflection film which structure is shown in Fig. (3) [77]. The hard coating layer was prepared by UV curing of the mixture of dipentaerythritol pentaacrylate (DPPA) and dipentaerythritol hexaacrylate (DPHA) with silica particle dispersed in methyl-ethyl-ketone dispersion liquid, methyl ethyl ketone, cyclohexanone, and photo-polymerization initiators. On the above-mentioned hard coating layer, the medium RI layer was prepared by UV curing of coating liquid consisting of TiO2 NPs, DPHA, photosensitizer, etc. Then, the high RI layer was prepared by heat-treatment of the coating liquid consisting of high
Fig. (3). Schematic structure of the multilayer anti-reflection coatings [77].
concentration of TiO2(Co). The low RI layer was formed on the high RI layer by UV curing and then heat treatment of the coating liquid consist of heat cross-linking fluorine-containing polymer and silica NPs. The multi-antireflection film has no crack, and shows good transparency, low haze, and low reflectance (the average reflectance was 0.8% in the wavelength range of 450-650nm).
3.2. Volume Holographic Recording Materials
Another application of the high RI organic-inorganic nanocomposites is used as the holographic recording mate-rials for volume hologram recording. Holographic recording is generally achieved through photo-induced RI modulation arising from periodic variation of composition and density induced by photopolymerization of the monomers and subsequent two-directional diffusion of the components during the exposure to the interference pattern. A large RI modulation is desired to maximize the dynamic range or the data storage capacity of the materials [3]. However, to achieve large RI modulation has proven to be rather difficult for pure polymer materials because of relatively limited RI differences between the photopolymerizable monomers and binders [103].
Organic-inorganic hybrid materials made mainly of inorganic particles (TiO2, ZrO2, etc.) and photopoly-merizable monomers have been investigated in order to achieve large RI modulation, improve environment resis-tance and durability. Different models with different mecha-nisms of phase separations were designed for holographic recording materials [104-108]. Figure 4 presents a formation mechanism of a permanent modulation of RI. For the composites of reactive monomers and photo-insensitive NPs, photo-insensitive NPs are not consumed and undergo counter-diffusion from the bright to the dark regions of the interference pattern. Their chemical potential increases in the bright regions due to consumption of the monomer. Thus the mutual diffusion process continues until photopolymeri-zation is complete. Redistribution of NPs under holographic exposure is thus accomplished resulting in compositional and density difference between the bright and the dark regions. Thus a RI grating is created.
High Refractive Index Nanocomposites Recent Patents on Materials Science 2011, Vol. 4 No. 1 23
In order to obtain good hologram image characteristics, it is necessary that the medium has a light transmittance of 50% or more. So the NPs should be colorlessness (absence of the absorption in visible and near-IR region), small size (< 50nm) and narrow size distribution. Hayashida et al. have developed a hologram recording material which attains high RI change flexibility, high sensitivity, low scattering, environment resistance, durability, low shrinkage, and high multiplicity, and is suitable for volume hologram recording [109]. The hologram recording material solution was made by blending diphenyldimethoxysilane and titania sol solution, photomonomer polyethylene glycol diacrylate, and photopolymerization initiator. Hologram recording material solution was further coated on a glass substrate for volume hologram recording medium. The resultant hologram recor-ding medium sample shows high value of the diffraction efficiency. The transmittance at 405nm (the recording wave-length) is 83% and does not decrease after the recording. The development of an optimum recording material is still one of the challenges in the field of holographic data storage.
3.3. High RI LED Encapsulant Materials
High RI nanocomposite materials also have good application as semiconductor (light-emitting diodes) LEDs encapsulant or package materials. LEDs are semiconductors that convert electricity into light [83, 110-116]. Once used just as indicator lights for electronics, LEDs have evolved into a major lighting technology that may change the future of general illumination for their long life and high energy efficiency. Yet as a fairly new and rapidly changing lighting technology, much research is still needed in order to fully realize the energy and cost savings potential of LEDs. One of the fundamental limitations in the light extraction efficiency of LEDs is the occurrence of trapped light within high RI (usually larger than 2.5) semiconductors such as III-V phosphides, arsenides, and nitrides. Based on Snell’s law, the angle of the light escape cone for the semiconductor-air interface is very small due to the high RI contrast which is
easy to cause the total internal reflection to limit the light escaping. For example, the escape-cone angle for unencap-sulated AlGaInP (nAlGaInP=3.35) is only 17°, resulting in light-extraction efficiencies of only a few percent. The light outside the escape cone is trapped inside the semiconductor and thus likely lost by eventual absorption. Although major efforts have been made to alleviate the light-escape problem, for example, by optimizing the geometrical shape of the LED die and by surface roughening, the fundamental problem of the high RI contrast between semiconductor and air remains unsolved. Encapsulants such as epoxy and silicone reduce the RI contrast resulting in a larger light escape cone, as shown in Fig. (5a). However, the RI of typical encapsulants was only 1.5, far below the refractive indices of III-V nitrides and phosphides (n =2.5-3.5).
High RI nanocomposites with good heat resistance and high transparence were designed as high RI encapsulants to improve the extraction efficiency. As the RI of the encapsulant increases, the calculated light-extraction efficiency increases rapidly as shown in Fig. (5b). Mont et al. have prepared high RI encapsulants by incorporating TiO2 NPs into epoxy to obtain surfactant-coated TiO2-NP-loaded epoxy having RI of 1.67 at 500nm, significantly higher than that of conventional epoxy (n = 1.53) [110]. 4% Mg Treated Coated TiO2 NPs were also incorporated into polymer matrix separately to prepare high RI epoxy encapsulant, high RI epoxy-terminated reactive-silicone encapsulant, and vinyl-terminated reactive-silicone encapsu-lant respectively. High RI nanocomposites as encapsulants are still in development to improve RI, colorlessness and stability to temperature, etc.
3.4. Application in Photonic Band Gap Materials
Photonic band gap (PBG) materials are attractive optical dielectric materials for controlling and manipulating the flow of light. Essentially, PBG materials contain regularly repea-ting internal regions of high RI (the dielectric) interspersed
Fig. (4). Schematic formation of a volume grating in a two-monomer mixture with NPs. M1, P1-multifunctional monomer (polymer), M2, P2-monofunctional monomer (polymer) [108]. Copyright 2009 IOP Publishing.
24 Recent Patents on Materials Science 2011, Vol. 4, No. 1 Cheng et al.
with regions of low RI (the air holes). If there is large contrast in RI between the two regions, then most of the light will be confined either within the dielectric material or the air holes. This confinement results in the formation of allowed energy regions separated by a forbidden region - the so-called PBG. Since the wavelength of the photons is inversely proportional to their energy, the patterned dielec-tric material will block light with wavelengths in the PBG, while allowing other wavelengths to pass freely.
Generation of PBG with high RI materials is a necessary condition toward a complete band gap in order to achieve large RI contrast [117-120]. There have been many reports on preparing PBG with high RI materials for example, bulk Si, GeAs, TiO2, etc. However, the fabrication methods are very difficult, complicated and strict, for example, etching (drilling) holes through the article normal to the layered structure, two-photon lithography [117].
Fink and Thomas made a series of PBG materials by incorporating inorganic NPs into copolymers to enhance the RI contrast [120]. The structure includes periodic, phase separated microdomains alternating in RI, the domains sized to provide a PBG in the UV-visible spectrum. A block copolymeric species of methyltetracyclododecene (MTD) and [2,3-trans-bis((tert-butyl-amido) methyl)norborn-5-ene] were prepared by ring-opening metathesis polymerization technique with ZnPh2 as catalyst in benzene. Periodic structures of alternating separate copolymeric domains are created by casting from solution while, near the point of complete removal of benzene. The polymeric structure then is exposed to H2S vapors and following other treatments. The result is a phase-separated article including ZnS clusters in the nanometer size range in one domain selectively, the overall structure having a RI ratio between domains of approximately 1.5. CdSe, CdTe etc. also were introduced
into copolymers to improve the RI contrast (RI ratio). One, two, and three dimensional periodic band gap structure materials were fabricated.
3.5. Other Applications
High RI materials can also be made into many other articles for applications. For example, brightness enhance-ment films with microstrusture-bearing surface made by high RI materials can be used for backlit liquid crystal display (LCD) [121-123]. High RI materials can also be used in a reflective image display for total internal reflection (TIR) materials having a plurality of transparent hemi-beads, each having a reflective region surrounding a non-reflective region. Images can be displayed by controllably frustrating total internal reflection (TIR) to switch selected pixels of a multi-pixel display between a reflective state where light incident on those pixels undergoes TIR, and a non-reflective state in which TIR is frustrated at those pixels [124]. High RI nanocomposite materials can also be used in other fields, for example, nonlinear optical (NLO) materials [125], transparent dielectric materials [64], light guiding plate, high RI resist for deep UV immersion lithography [126, 127] and so on. The application of high RI nanocomposite materials will facilitate more and more other complicate optical device designed and manufactured.
CURRENT & FUTURE DEVELOPMENTS
Nanocomposites with high RI have attracted a great deal of attention for their potential application in optical field. The structure and properties can be manipulated by the precise design and tailoring of the nano-building blocks and organic matrix at molecular level. The RI of the nanocom-posite materials generally is expected to be approximately a
Fig. (5). (a) Escape cone of an LED without and with encapsulation. (b) Light-extraction efficiency ratio for GaN and GaP as a function of the encapsulant refractive index [110]. Copy right 2008 American Institute of Physics.
High Refractive Index Nanocomposites Recent Patents on Materials Science 2011, Vol. 4 No. 1 25
linear combination by the volume ratios of the inorganic nano-building blocks, which means that we can design nanocomposites with different RI for the demand we need. However, the high RI (e.g., n > 1.8) organic-inorganic nanocomposites with high inorganic nano-building block loading are still difficult to prepare. Especially, for the ex situ method, how to carry out the large-scale preparation of the high RI NPs which has good compatibility with polymer matrix or monomers is the key challenge. Besides, it’s necessary to optimize the structure of the nanocomposites for their macroscopic performance, including optical properties, thermal and mechanical properties, light stability, and processability, which is very important for the precise design of these nanomaterials for the need in application. In addition, the attention should also be paid to the high RI multifunctional nanocomposites with other function for example, high hardness, UV-absorption, photochromic and self-cleaning properties, dielectric etc., which will make the high RI nanocomposites more extensive applications in optical fields. The development of high RI organic-inorganic nanocomposites will still be challenging and potential subjects in the future.
ACKNOWLEDGEMENTS
The authors appreciate the financial support of the National Natural Science Foundation of China (Nos. 20704004 and 20534040) and the Training Fund of NENU’s Scientific Innovation Project (NENU-STC07003).
CONFLICT OF INTEREST
No.
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