High refractive index organic inorganic nanocomposites: design, synthesis and application

Transcription

1 FEATURE ARTICLE Journal of Materials Chemistry High refractive index organic inorganic nanocomposites: design, synthesis and application Changli L u ab and Bai Yang* a Received 16th September 2008, Accepted 12th November 2008 First published as an Advance Article on the web 4th February 2009 DOI: /b816254a Organic inorganic nanocomposites with high refractive index (RI) are typically constructed by integrating high RI inorganic nanoscale building blocks into a processable, transparent organic matrix. These nanocomposites combine the numerous advantages of organic and inorganic components, and have many promising applications in optical design and advanced optoelectronic fabrication. A substantial amount of work has been performed in this area. This Feature article summarizes the general design principles and different fabrication approaches of high RI nanocomposites, and reviews recent research advances and some important optical applications of these nanocomposites. 1. Introduction and background The refractive index (RI) is one of the most important and fundamental characteristics of materials used in optical design and application. In recent years, the need for optical materials with high RI in the areas of ophthalmic lenses, filters, optical adhesives, highly reflective and antireflection coatings as well as advanced optoelectronic fabrications is increasing. 1 8 Inorganic materials usually have a high RI (in the range of ), 9,10 but they have many disadvantages such as higher densities (>2.5 g cm 3 ) and lower flexibility. In particular, for the fabrication of antireflective coatings, the RI of inorganic materials cannot be tuned continuously, and the fabrication procedures are becoming more complex and expensive. Organic materials have the advantages of low weight, excellent impact resistance, and a State Key Laboratory of Supramolecular Structure & Materials, College of Chemistry, Jilin University, Changchun, , P. R. China. byangchem@jlu.edu.cn; Fax: ; Tel: b College of Chemistry, Northeast Normal University, Changchun, , P. R. China. lucl055@nenu.edu.cn good processability compared with inorganic materials. 1,2 However, most organic materials have a small adjustable RI range ( ) due to their chemical structure limitation. 11,12 For example, certain widely employed conventional polymer materials, such as poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and CR-39 have RI values of 1.49, 1.59, 1.58 and 1.50, respectively. Some special polymer materials, such as wholly aromatic polyamides and aromatic heterocyclic ring backbone polymers (n ¼ ), poly(thiophene) (n ¼ 2.12), 16 and aromatic conjugated polymers (n z 2.7), can exhibit an RI greater than 1.7, but these organic polymers usually consist of highly conjugated, aromatic-type, p-electron structures, and possess high optical dispersion and large birefringence. In particular, these highly conjugated polymers also tend to be insoluble and absorb strongly in the visible region, which restricts their practical application in the field of optics. Thus, the development of high RI organic optical materials with excellent optical properties is a challenging topic. The RI of a material is related to the molar volume (V M ) and polarizability (a) of the material. The classical Lorentz Lorenz Changli L u received his Ph.D. in polymer physics and chemistry in 2003 under the supervision of Prof. Jiacong Shen and Prof. Bai Yang at Jilin University. He is presently a professor of chemistry at Northeast Normal University. His main research interest is in the development of high refractive index polymer nanocomposites and multifunctional organic inorganic hybrid materials for optical applications. Bai Yang currently is a professor of chemistry and the director of the State Key Lab of Supramolecular Structure and Materials in the College of Chemistry at Jilin University. He received his Ph.D. in polymer physics and chemistry in 1991 under the supervision of Prof. Jiacong Shen at Jilin University. His research interests relate to the composite assembly of nanoparticles in polymers, nano micro-scale fabrication of ordered array structures, and high performance and functional polymer nanocomposite optical materials J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009

2 equation (1) can give reliable predictions of the RI of polymers from the group-contribution calculations based on the summation of the molar refractions (R LL ) of functional groups and the backbone repeating unit of the polymer. 17,22 n 2 1 V n 2 M ¼ 4 þ 1 3 pn Aa ¼ X ðr LL Þ i (1) i Therefore, according to this molecular design principle, high RI polymers can be prepared by incorporating such structures or organic groups as aromatic groups, halogen atoms, and sulfur atoms with high molar refractions into the backbone or side chain of polymers. In view of the molar refraction and molar dispersion, the introduction of the sulfide moiety into a polymer structure is considered an especially effective way to fabricate high RI polymers with lower dispersion (higher Abbe number). 23 Recently, there have been many reports on optical sulfurcontaining polymers including poly(s-alkylcarbamates), 24 polythiourethanes, 25,26 epoxy- and episulfide-type polymers, 1,27 poly(thioether sulfones) 28 and polyimides derived from sulfurcontaining aromatic diamines and aromatic dianhydrides These new transparent polymer materials exhibit a high RI ( ). Introducing inorganic elements with high molar refractions into polymer structures is also an effective strategy for improving their RI while retaining a high Abbe number. For example, transparent optical resins prepared by ternary copolymerization of lead dimethacrylate, styrene and methacrylic acid exhibit an increasing RI (up to 1.63) and high Abbe number with increasing lead ion content. 32 High RI polymetallocenes consisting of an alternating ferrocene unit 33 and polyphosphazenes with a P]N backbone 34,35 were also obtained by molecular tailoring, and both of them show a maximum RI of However, we should note that it is very difficult to obtain a polymer material with an RI higher than 1.8 using these approaches. Organic inorganic hybrid materials, which have benefited from the development of soft inorganic chemistry processes, can be defined as nanocomposites consisting of inorganic and organic components with controllable length scales ranging from a few angstroms to a few tens of nanometres These nanocomposites have received broad attention for optical applications in both fundamental and applied research in recent years, due to their outstanding physical and chemical properties resulting from their hybrid nature These hybrid nanocomposites combine the advantages of organic polymers (low weight, flexibility, good impact resistance, and excellent processability) and inorganic materials (high RI, good chemical resistance, and high thermal stability). 36,37,41,56 This research field, which bridges many different types of chemistry (organic, inorganic, organometallic and polymer) to material science, has provided a wide range of possibilities for RI engineering by adjusting the ratio of inorganic phase to organic phase. In recent years an increasing number of papers and patents have appeared dealing with high RI organic inorganic nanocomposites, focusing on the synthetic method and properties as well as on their potential optical applications. 10,52,55 The most prominent strategy for fabricating these hybrid nanocomposites is the incorporation of high RI inorganic building blocks (such as TiO 2, ZrO 2, PbS or ZnS) into organic matrices on the nanoscale. The aim of this Feature article is to give an overview of the present studies of high RI organic inorganic nanocomposites for optical applications. The general design principle and synthetic methods of these hybrid nanocomposites are discussed first. The subsequent sections will highlight the recent developments in transparent organic inorganic hybrid thin films and bulk nanocomposites with high RI. Finally, some optical applications of these nanocomposites will be discussed. 2. Design and synthesis of high RI organic inorganic nanocomposites An effective way of increasing the RI of materials is to introduce high RI inorganic building blocks into organic matrices. So, the selection and synthesis of high RI inorganic building blocks should be considered first. After that, it is also important to consider how the integration of these inorganic domains into an organic matrix could be carried out by an appropriate approach, based on the above inorganic building blocks. For the design of nanocomposites for optical applications, one of the technical challenges is the requirement to retain transparency whilst avoiding phase separation between organic and inorganic moieties. 2.1 High RI inorganic building blocks RI is an inherent physical parameter of a substance and it also varies as a function of wavelength for most real materials. A material that is used as an inorganic building block for a high RI organic inorganic nanocomposite must possess high RI (>2.0) and good transparency in the range of optical wavelengths used for practical applications. Table 1 lists the RIs and absorption coefficients at three different wavelengths in the visible range for Table 1 Refractive index and absorption coefficients at three different wavelengths in the visible range for some inorganic materials Refractive index (n) Absorption coefficient (k) Material 400 nm 500 nm 620 nm 400 nm 500 nm 620 nm Crystalline Si Amorphous Si Ge GaP InP PbS ZnS (sphalerite) TiO 2 (rutile) This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19,

3 some inorganic materials. 9 These inorganics can be integrated into organic matrices to increase the RI of materials by a suitable method. It should be noted that although the inorganics Si, Ge, GaP, InP and PbS have high RI (>4.0 at 500 nm), they exhibit higher absorption coefficients in the visible region than others, indicating that they are more suitable for optical application in the IR and near-ir regions. At present in fact, the most frequently employed inorganic materials in the literature concerning high RI organic inorganic nanocomposites are TiO 2 (n ¼ 2.7 at 500 nm in its rutile form), 9 ZrO 2 (n z 2.2 at 589 nm), 57 ZnO (n ¼ 2.0 at 550 nm), 58,59 CeO 2 (n ¼ 2.18 at 500 nm) 60 and ZnS (n ¼ 2.4 at 500 nm). 9 The main reasons these high RI inorganic materials are used as nanoscale building blocks are that they can be obtained easily both in the laboratory and commercially, and they have good transparency in the visible and near-ir regions. In addition, some inorganic materials such as ZnO, TiO 2 and CeO 2 are widely used as UV-shielding pigments in the field of optical research and in the polymer industry. 58,59,61 Besides the above-mentioned inorganic materials, other high RI inorganics such as Ta 2 O 5 (n ¼ 2.1 at 550 nm), 62 indium-doped tin oxide (ITO, n ¼ 2.0 at 550 nm), 63 Nb 2 O 5 (n ¼ 2.3 at 550 nm), 64 Bi 4 Ti 3 O 12 (n ¼ 2.3 at 520 nm), 65 and iron sulfides (n >3.5 at 633 nm) 66 are also promising materials as building blocks of high RI nanocomposites. The density of inorganic building blocks should also be considered in the design of high RI nanocomposites because of the linear dependency of nanocomposite RI on the volume fraction of inorganic phases, which has been found in many systems In order to achieve a high RI nanocomposite, it is necessary to incorporate a high content of inorganic material with high RI into an organic matrix. As a result, hybrid nanocomposites with balanced physical properties (specific gravity, optical transparency and mechanical properties) cannot be obtained. In view of this fact, high RI inorganic materials with low densities and low absorption coefficients appear to be the optimal candidates for inorganic building blocks in the design of high RI nanocomposites. 2.2 Optical transparency The greatest stumbling block to overcome when preparing high RI nanocomposites, especially those with higher inorganic loading for optical applications, is improving their transparency. The intense light scattering of inorganic domains, due to large inorganic domain size and RI mismatch between the inorganic phase and the matrix, is believed to be responsible for the decline in transparency or opaque appearance of these nanocomposites. The transparency loss (T) of the nanocomposites as a result of light scattering by randomly dispersed spherical particles with radius (r) and volume fraction (F p ) can be described by the Rayleigh scattering formula: 52,58 8 " T ¼ I ¼ exp 32p4 f p xr 3 n 4 2 # 9 < 2 m np =n m 1 = I 0 l 4 2 (2) : np =n m þ2 ; where I and I 0 are the intensities of the transmitted and incident light respectively, l is the wavelength of light, x is the optical path length, and n p and n m are the RIs of the particles and the matrix, respectively. It can be seen from the Rayleigh law that there are two routes to minimize the scattering losses and improve the transparency of nanocomposites at a given wavelength. The first way is the so-called RI matching principle (where the RI of the particles is very close to that of the matrix, n p z n m ). Many inorganic filled transparent organic inorganic hybrid composites such as glass-fiber reinforced PMMA, 70 organic chromophores or dye-loaded zeolite/polymer hybrid materials 71,72 and dental composite resins 73 have been prepared by this strategy. Li et al. have successfully prepared highly transparent epoxy nanocomposites using silica titania core shell nanoparticles with controlled RI. 74 However, the RI is an intrinsic feature of a material. The RIs of inorganic building blocks and the organic matrices differ greatly in most cases. In particular, it is very difficult to produce highly transparent materials for the design and synthesis of high RI nanocomposites by the RI matching strategy. So, another method for improving the transparency of nanocomposites is to decrease the particle size significantly below the visible light wavelength. It has been shown that the pronounced optical scattering loss can be avoided when the inorganic domain size (typically <25 nm) is less than one-tenth of the visible light wavelength ( nm). For the design of high RI nanocomposites with high particle-loading level, the large RI mismatch between the particles and the matrix should therefore be compensated by very small particle size in order to suppress losses by light scattering. Ideally, the inorganic particles should have a diameter below 10 nm because the presence of even a small percentage of particles or aggregates larger than 100 nm results in strong light scattering in the visible region, causing haze or even turbidity. 10,75 So the size of inorganic nanoparticles utilized for fabricating high RI nanocomposites is frequently below 10 nm. It is well known that the optical, electronic and physicochemical properties of nanoparticles appear to be essentially different from the corresponding properties of bulk materials due to the so-called quantum size effect, 76,77 the blue shift in the optical bandgap or exciton energy of semiconductor nanocrystalline materials being an archetypal example. The RI of inorganic materials generally is also significantly affected by the quantum size effect when their characteristic sizes are less than the corresponding exciton Bohr diameters. 68,78 It has been found that the RI of PbS nanoparticles shows a dependence on the particle size (the Bohr diameter is ca. 29 nm, 68 but the value reported by Wang is about 18 nm 78 ). For example, according to the results of the linear extrapolation of PbS volume fraction from the nanocomposites, the RI of pure PbS particles with an average diameter above ca. 25 nm at 633 nm is in accordance with that for bulk PbS (4.3), while this value drops to 2.3 for smaller size of PbS (ca. 4 nm). 68 It should be noted that PbS is a narrow-bandgap semiconductor, and its quantum size effect is expected to occur over a rather large size range. Our recent study on bulk nanocomposites with ZnS particles (2 5 nm), however, has shown that the extrapolated RI value (2.3) of ZnS particles is close to the value for bulk ZnS (2.36). 79 This result indicates that when compared to PbS nanoparticles, the RI dependence of ZnS nanoparticles on the quantum size effect is not obvious, probably because of the small Bohr diameter for the wide-bandgap ZnS semiconductor particles (about 5.5 nm). From the above discussion, we can conclude that the influence of the quantum size effect on the RI of inorganic building blocks should also be 2886 J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009

4 considered during the design of high RI nanocomposites when one attempts to control the size of inorganic particles, especially narrow bandgap inorganics with large Bohr diameters. 2.3 Synthetic method As discussed above, control over particle size and size distribution as well as uniform dispersion of the building blocks at the nanometre scale within the organic matrix is a critical issue for improving the transparency of nanocomposites. This is also a technological challenge for the design and synthesis of high RI organic inorganic nanocomposites because nanoscale building blocks, such as nanoparticles with high specific surface energies and inherent hydrophilicity, are prone to aggregation, even before incorporation into an organic matrix. Hence, the prerequisites for synthesizing high RI transparent nanocomposites, especially those with high nanophase content, are: the appropriate design and tailoring for the nanoscale building blocks and organic matrices, such as surface engineering of nanoparticles; the fabrication approaches of nanocomposites and the improvement of the compatibility between the inorganic domains and the organic matrix. Here we introduce several important methods developed for preparing high RI organic inorganic nanocomposites based on recent reports in the literature. The advantages and challenges of these methods for controlling phase morphology and improving compatibility between different components will be discussed Sol gel route. The sol gel process is based on the hydrolysis and condensation of molecular precursors such as metal alkoxides to prepare three-dimensional network-like inorganic or organic inorganic hybrid materials. This route is truly a bottom-up method, which is versatile and can offer the potential of low-cost mass design and fabrication of organic inorganic hybrid nanocomposites with domain size approaching the molecular level under mild conditions. Recently, a large number of reviews in this field have been published ,44,45,48 51,54 Here, we will discuss the one-step preparation method for the high RI nanocomposites based on the in situ sol gel route. In addition, the so-called ex situ sol gel method for nanocomposites will also be included in this section, in which the nanocomposites are prepared by dispersing pre-made nanoscale building blocks in sol gel-derived hybrid systems. Organic matrix materials generally can be broadly divided into two major classes in the design and synthesis of sol gel-derived organic inorganic nanocomposites with high RI. The first is organo(alkoxy)silanes with polymerizable (or reactive) organic substituents (Fig. 1). The commercially available organic inorganic hybrid materials termed ORMOSILs or ORMOCERs are usually prepared by hydrolysis condensation reaction from these organo(alkoxy)silanes, followed by an organic cross-linking of inorganic-oxidic units through the organic polymerizable groups. 45,51 Titanium alkoxides or zirconium alkoxides are most often used as precursors for the synthesis of inorganic phase in hybrid materials because of the high RI of their gels. High RI hybrid materials can be prepared directly by introducing these precursors into the ORMOSILs system via an acid-catalyzed co-condensation reaction However, the reactivity of titanium or zirconium alkoxides towards water is much higher than Fig. 1 Structural formulae of some organo(alkoxy)silanes used in sol gel processes. that of silicon alkoxides, and their hydrolysis leads immediately to the precipitation of the oxo-polymers. 38,39 This is because that the transition metals have a lower electronegativity and a higher coordination number than their valency, which results in what is called coordination expansion. 83 The reaction rate of metal alkoxides in the sol gel process also depends strongly upon the catalysts (acidic, basic or nucleophilic activation). So the reactivity of these transition-metal alkoxides must be controlled using chemical additives, otherwise phase separation and an opaque appearance may occur in the resultant nanocomposites. As usual, the organic acids or b-diketones and allied derivatives are used as chelating agent for controlling the reactivity of these transition-metal alkoxides. 80,84 Although the chemical additives are much more efficient for the preparation of transparent titania-based nanocomposites in the sol gel process, the additional chelating agent cannot be fully removed from the metallic centers in the hybrids, even for high hydrolysis ratios, due to their strong affinity for the transition metals. This result might affect the thermal, mechanical and optical properties of the hybrid materials, and might limit the further increase of RIs of materials. The incorporation of nanoscale building blocks such as clusters 85 and pre-made inorganic nanoparticles 86 into the hybrid sol systems is an alternative route to suppressing the above shortcoming (this method is defined as ex situ sol gel by us). The use of these nanoscale building blocks with better defined structures combines the advantages of their monodisperse and nanometric scale features, which makes the sol gel route more controllable at the nanoscale, while preventing macroscopic phase separation of the resultant nanocomposites. Functional group modified oligomers (prepolymers) or polymers such as triethoxysilane end- or side-chain-capped polymers are the second type of organic matrix generally used for the design of high RI nanocomposites In this case, the nanoscale building blocks such as titania domains are mainly generated in the organic matrices by a so-called in situ sol gel process. The interfacial force between organic and inorganic phases can play a major role in controlling the microstructure and properties of composite materials. 94 These functional groups consisting of polymers or oligomers offer good control over inorganic phase morphology on the nanoscale and a strong covalent linkage between nanoscale building block and organic matrix, which can prevent phase separation in the hybrid materials. Hence, highly transparent nanocomposites, especially with high inorganic loading level, can be obtained by this strategy. Fig. 2 shows a typical preparation route for high RI titania polymer This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19,

5 Fig. 2 Synthetic scheme for high RI titania polymer nanocomposites via the in situ sol gel route. nanocomposites with covalent linkages by the in situ sol gel process. These polymers with reactive functional groups may result in dual organic inorganic hybrid nanocomposites. 40 When the polymer matrix has a high content of functional groups such as triethoxysilane, a gelation effect is usually observed in the reaction system due to the fast hydrolysis and condensation reaction of titanium alkoxides. To solve this problem, acid-free polymerization instead of conventional acid catalysis was used to obtain the transparent polymer hybrid films with high titania content. 90,94 It is should be pointed out that the titania or zirconia domains formed in the hybrid materials by sol gel methods are generally amorphous and the RI is relatively low (<2.0). This is a disadvantage for preparing high RI nanocomposites. Furthermore, the large shrinkage during the drying process may result in high inner stress inside the sol gel-derived hybrid materials, with the high inorganic content leading to poor mechanical properties In situ formation of nanoparticles in polymer matrices. In situ synthesis of inorganic nanoparticles in a polymer matrix is a facile and effective route to preparing nanocomposites. This method allows one-step fabrication of nanocomposites with in situ-generated nanoparticles from corresponding precursors in the presence of polymer networks. In this case, the nanoparticles are nucleated and grown inside the polymer matrix, suppressing the undesirable irreversible particle aggregation caused by their isolation and handling process. The advantage of this route is the passivating or stabilizing effect of the polymer chain functional groups on the metal ions or formed nanoparticles, which can control particle size effectively, and thus prevent particle agglomeration whilst maintaining a good spatial distribution in polymer matrices. This is due to kinetic control because the polymers cannot provide a sufficiently fluid environment to allow individual particles to meet each other by diffusion. The obtained nanocomposites are more stable than colloidal solutions and have many possible applications in the design of optical and optoelectronic devices. However, the drawback of this method is that the unreacted educts or byproducts of the in situ reaction might influence the properties of the final material. The strong interaction between the inorganic precursor and the polymer matrix is very important for the control of particle size and polydispersity. So, careful design and tailoring of structure and composition of the polymers is necessary in this case. Polymers with hydroxyl, mercapto and sulfonic groups can provide the strong interactions with metal elements of inorganic precursors The general scheme for the preparation of semiconductor nanoparticle polymer nanocomposites by an in situ method is shown in Fig. 3. The polymer and metal ions are first mixed in solution, and then exposed to a counterion such as S 2 in the form of a gas or as ions dissolved in solution. The composite can be cast as a film before or after exposure to the counterion. Early work in high RI nanocomposites with PbS nanoparticles was performed by this in situ synthesis method The maximum RI of PbS-gelatin nanocomposite films has reached about 2.5 as the PbS loading level is 70 wt%. For high RI PbS polymer nanocomposites, other water-soluble polymers such as poly(vinyl alcohol) (PVA), poly(ethylene oxide) (PEO), poly(acrylic acid) (PAA) and poly(acrylamide) have also been chosen as polymer matrices. 67 Stable semiconductor polymer nanocomposites can be prepared by copolymerizing the polymerizable metal salts and styrene to form a soluble metal-containing microgel, followed by reaction with H 2 S gas. This method offers a well-controlled particle content, size and surface state. For instance, Yang et al. prepared metal sulfide nanoparticles in polymer networks by copolymerization of lead methacrylate or zinc methacrylate with styrene before treatment using H 2 S gas Later, they developed a novel route for fabricating cross-linked polymer nanocomposites with a high density of PbS nanoparticles by Fig. 3 Schematic of in situ synthesis of metal sulfide nanoparticles in a polymer matrix J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009

6 combining surface-initiated atom transfer radical polymerization (ATRP) of lead dimethacrylate and gas solid reaction. 104 The composite films are grown directly from the substrates by covalent bonds, which offers a basis for enhancing the stability of nanocomposite films, controlling film thickness and structure. Recently, we designed and synthesized high RI PbS polymer nanocomposites from reactive lead-containing precursors (Pb(SCH 2 CH 2 OH) 2 ) and polythiourethane (PTU) oligomers terminated with isocyanate groups, followed by in situ gas solid reaction Ex situ synthesis method. Another important route for the preparation of organic inorganic nanocomposites is to integrate pre-made nanoscale building blocks such as inorganic nanoparticles (NPs) into the polymer (blending route) or monomer which is subsequently polymerized to form composites (in situ polymerization). This route is defined as the ex situ synthesis of nanocomposites (Fig. 4). Here, the particles are prepared separately, isolated, and purified or modified by surface capping agents before immobilization in the polymer matrix. This method provides full synthetic control over the particle size and size distribution as well as the surface properties of nanoparticles. Therefore the ex situ synthesis method is more suitable for large-scale industrial applications than the above-mentioned methods. In particular, the in situ polymerization route is a quite promising approach for practical applications; however, it is known that the nanoparticles tend to aggregate unless they are modified at their surface to decrease the inter-particle attraction. So, a key challenge for this method is to be able to prepare nanoparticles which possess good dispersibility in the polymer or its monomer, have long-term stability against aggregation, and do not negatively interfere with the in situ polymerization process in large enough amounts. In fact to solve this problem, the surface engineering of nanoparticles is very important for minimizing the interface energies between nanoparticles and polymer matrix. Generally, the nanoparticles are modified by chemi- or physisorption of surfactant molecules onto the particle surface in both the synthesis and post-treatment processes, which terminates the growth of particles and modifies their surface properties. The surface-modified particles can be readily redispersed after isolation in an organic medium that is compatible with the surfactant layer grafted to, or deposited on, the particle surface. For the design of high RI nanocomposites, the surface capping agents or surfactants should have a higher RI than the conventional ones because a large amount of surface capping agent on the particles will reduce the average RI of particles, which ultimately results in the decrease of RI of the resultant nanocomposites even though high RI inorganic particle cores are used Blending route. The blending method is the simplest route for preparing nanocomposites. Usually, the uniform transparent dispersion of nanoparticles (NPs) in a solution of polymer or oligomers with reactive groups is cast into a container or deposited on a glass or plastic substrate by a spin-coating or dip-coating process, then the transparent nanocomposite films or sheets can be obtained after heating (Fig. 4). This method includes two independent steps for the preparation of surfacemodified nanoparticles and the synthesis of polymers, in which the compatibility of nanoparticles and polymer matrices is considered to be a crucial issue for improving the transparency of nanocomposites. For example, Lee et al. prepared high RI transparent nanocomposites by incorporating surface-modified ZrO 2 nanoparticles into a polydimethylsiloxane (PDMS) matrix via ligand molecule engineering. 106 The designed ligand molecule with PDMS-like siloxane tail structure has good chemical compatibility with a PDMS matrix, strong adsorption on ZrO 2 nanoparticles surface by its diamine head group, and a strong steric barrier due to its double-tailed structure. This ligand molecule plays a crucial role in efficiently dispersing ZrO 2 particles in a PDMS matrix. Besides the rational design and tailoring of the nanoparticles and the structures of the polymer matrix, selecting proper cosolvents is also necessary to improve their compatibility whilst avoiding phase separation. Nakayama et al. reported the preparation of transparent free-standing nanocomposite films with high RI of by dispersing propionic acid modified TiO 2 nanoparticles into poly(bisphenol-a and epichlorohydrin) (PHE) using n-butanol and toluene as the cosolvents In situ polymerization. There are a large number of research reports about transparent nanocomposites obtained by an in situ polymerization route. 59, This approach has been recently developed to prepare high RI organic inorganic nanocomposites. In particular, this strategy is a very effective route for preparing thicker bulk nanocomposites as compared with Fig. 4 Ex situ synthesis schemes for the preparation of nanocomposites from blending route and in situ polymerization process. This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19,

7 above-mentioned methods (Fig. 4). However, the key challenge is to obtain stable and transparent dispersions of nanoparticles before subsequent polymerization because the low viscosity of particle dispersions in monomers may result in the precipitation of inorganic particles during the long polymerization process. Generally, for a given monomer used for preparing nanocomposites, the design and tailoring of the surface characteristics of the inorganic particles is very important in order to make the particles compatible with the monomers through the strong interfacial interaction between the monomer molecules and the inorganic particles or the inorganic particle cores protected with organic capping shells. Our group has used this method to prepare novel high RI ZnS polymer nanocomposite films from thiophenol (PhSH)/ 4-(thiomethyl)styrene (TMSt)-capped ZnS nanoparticles and urethane methacrylate macromer (UMM) using a UV-initiated free radical polymerization process. 114 Our strategy has three prominent advantages: firstly, besides the polymer matrix, the capping agents also have high RI, avoiding the RI decrease of the resultant nanomaterials caused by the incorporation of low RI capping agents. Secondly, the polymerizable capping agent (TMSt) can copolymerize with the methacrylate-capped macromer to form integrated polymeric materials, which can effectively prevent phase separation of the nanocomposites and improve the stability of ZnS nanoparticles in the polymers. Another advantage is that the UMM macromers in solution have a viscosity which favors the formation of films during spin-coating. In addition, the UV curing procedure can provide rapid polymerization of the dispersion system, minimizing the density gradient of nanoparticles formed in the nanocomposite films during the long curing process, which is required because of the higher density of inorganic nanoparticles compared to organic medium. By this strategy, highly transparent nanocomposite films with nanoparticles dispersed homogeneously in both lateral and vertical dimensions within the polymer matrix can be easily prepared. Later, by using a similar strategy, high RI nanocomposite coatings were prepared from UV-curable monomers or hyperbranched polyesters and various organic-modified Table 2 Components, synthesis method and RI of some metal oxide polymer nanocomposites with high RI Entry Nanoscale building blocks Organic components Synthesis method RI Refs. 1 TiO 2 (amorphous) Triethoxysilane-capped oligomers of Sol gel ,88 poly(arylene ether ketone) (PEK) and poly (arylene ether sulfone) (PSF) 2 TiO 2 or ZrO 2 (amorphous) Triethoxysilane-capped diamino Sol gel diphenyl sulfone (DDS) and bis(3- aminophenoxy-4-phenyl)phosphine oxide (BAPPO) 3 TiO 2 (amorphous) Triethoxysilane-capped poly(arylene Sol gel ,123 ether phosphline oxide) (PEPO) 4 TiO 2 SiO 2 Organically modified silane Sol gel (ORMOSIL) 5 TiO 2 (amorphous) Triethoxysilane-capped poly(methyl Sol gel methacrylate) (PMMA) 6 TiO 2 /Bi 2 O 3 Poly(2-hydroxyethyl methacrylate) Sol gel (PHEMA) 7 TiO 2 (amorphous) Aminoalkoxysilane-capped Sol gel pyromellitic dianhydride (PMDA) 8 TiO 2 (partial rutile) Poly(vinyl alcohol) (PVA) Blending route TiO 2 (amorphous) Oligomeric phenylsilsesquioxane Sol gel TiO 2 (amorphous) Triethoxysilane-capped Sol gel polythiourethane (PTU) 11 TiO 2 (anatase) Triethoxysilane-capped PMMA Modified sol gel TiO 2 (amorphous) Combinations of Sol gel trimercaptothioethylamine and bisphenol-a epoxy resin 13 TiO 2 (amorphous) Triethoxysilane-capped Sol gel soluble polyimide (BTDA/DMMDA) 14 TiO 2 (anatase) Poly(bisphenol-A and Blending route epichlorohydrin) 15 TiO 2 (anatase) ZrO 2 UV-curable acrylate monomers In situ polymerization , TiO 2 Copolymers of styrene (St) and Sol gel MPTMS 17 TiO 2 (amorphous) Carboxylic acid capped soluble Sol gel polyimide (6FDA-6F p DA COOH) 18 TiO 2 (anatase) Sulfur-containing polyimide Blending route ZrO 2 Polydimethylsiloxane Blending route TiO 2 (anatase) Poly(acrylic acid)-graft-poly(ethylene Modified sol gel glycol methacrylate) 21 CeO 2 UV-curable monomer combinations In situ polymerization Bi 4 Ti 3 O 12 PMMA Blending route ZnO PMMA In situ polymerization J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009

8 inorganic nanoparticles including ZrO 2, 115,116 ZnS 117 and TiO 2 ZrO 2 particles. 118,119 The bead mills technology was also developed by Inkyo et al. to obtain well-dispersed APTMOScapped TiO 2 nanoparticle suspensions in MMA, and subsequent in situ polymerization was used to prepare transparent TiO 2 PMMA nanocomposites with higher RI than pristine PMMA. 120 However, the TiO 2 content in the polymer still needs to be improved in order to further increase the RI of polymer matrix. Wegner et al. prepared tert-butylphosphonic acid modified ZnO PMMA transparent nanocomposites by this method. 121 The nanocomposites exhibit light transmittance in the visible region, strong UV absorption and moderately high RI, as the volume fraction of ZnO particles with 22 nm is below 7.76%. In addition, the effect of different nanoparticles (ZnO, TiO 2, ZrO 2, etc.) on the free radical polymerization in the in situ polymerization process was also studied State-of-the-art developments of high RI nanocomposites High RI transparent organic inorganic nanocomposites have attracted significant interest because of their important optical applications. In recent years, many high RI nanocomposites with excellent properties have been prepared by the above-mentioned methods. In this section, we will briefly summarize the current research advances in high RI organic inorganic nanocomposites based on recent literature reported in this field. 3.1 High RI nanocomposite films The majority of research reports are focused on nanocomposite thin film materials with high RI due to their extensive applications and relative easy preparation process with respect to the thick bulk nanocomposites. High RI nanocomposites can be divided into two main groups, according to the high RI nanoscale building blocks used, metal oxide polymer systems and metal sulfides or semiconductor polymer systems High RI metal oxide polymer nanocomposites. Metal oxides, including TiO 2, ZrO 2, CeO 2, ZnO, Bi 4 Ti 3 O 12,Ta 2 O 5 / SiO 2, etc. have been used as nanoscale building blocks to develop high RI organic inorganic nanocomposites. Table 2 summarizes some representative examples reported in recent literature. Titania or zirconia are most often used as nanoscale building blocks (or nanoparticles) due to their easy preparation in both the laboratory and industry. The sol gel route is the dominant approach for the preparation of high RI metal oxide polymer nanocomposites. Early work with sol gel-derived high RI nanocomposites was performed by Wilkes and coworkers ,122,123 High-performance polymers (PEK, PSF, PEPO, etc.) functionalized with triethoxysilane were used as organic matrices. The RIs and Abbe numbers of the resultant polymer TiO 2 hybrid nanocomposites are in the range of and respectively, as the titania contents change from 0 to 85 wt%. Later, Chen et al. developed triethoxysilane-capped acrylate polymers as an organic matrix to prepare high RI nanocomposites because of their excellent optical properties, especially their low optical dispersion. 90 These transparent nanocomposites with a broad Abbe number range of exhibit an RI of up to at 633 nm. They have also used poly(silsesquioxanes) with excellent thermal, electronic and optical properties as matrix materials to fabricate high RI titania-based nanocomposites with an RI of up to with a titania loading level of 54.8 wt%. 93 A high density of bismuth oxide (Bi 2 O 3 ) was covalently incorporated into polyacrylate by co-hydrolysis/condensation reaction of the polymerizable alkoxide precursors of bismuth and titanium. 124 The highest RI achieved was at 633 nm for a nanocomposite consisting of a mixture of TiO 2 (1 wt%) and Bi 2 O 3 (10 wt%). It is known that crystalline forms of titania such as rutile have an RI of , but these values are generally obtained under high temperature (>800 C). However, as shown in the abovementioned titania-based nanocomposites, the titania nanophase obtained by the conventional sol gel route under low temperature is commonly amorphous, with an RI in the range of depending on the preparation conditions. 90,125 So, it is highly desirable, for the fabrication of high RI organic inorganic nanocomposites, that a crystalline titania phase is generated in the organic matrix by controlling the synthesis conditions. Hydrothermal (or water vapor) treatment of the hybrid materials has been developed to promote a condensation reaction and induce the crystallization of the titania phase in polymer matrices. 126,127 For example, Chen et al. have used this strategy to improve the RI of titania-based PMMA nanohybrids with different titania content from to (Fig. 5). 127 Another route for the preparation of crystalline titania-based nanocomposites at low temperature is to directly incorporate pre-made anatase titania obtained under strong acid conditions into organic matrices. 107,118,119,128 In view of their excellent mechanical, thermal resistance and dielectric properties as well as high RI, high-performance polyimides (PIs) have recently been employed as the matrix materials of high RI nanocomposites. Chen et al. first prepared titania PI hybrid optical films with RI range of from aminoalkoxysilane-capped PMDA and titanium isopropoxide by an in situ sol gel route. 91 However, the highly electronegative OR groups of titanium alkoxides are very susceptible to nucleophilic attack by poly(amic acid) (PAA), which results in the Fig. 5 Variation in the refractive index of acrylic polymer titania hybrid films with titania content, before and after hydrothermal treatment. Reprinted with permission from ref Copyright 2008 Wiley-VCH. This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19,

9 phenomenon of uncontrolled gelation, or a relatively large titania domain in the hybrid nanocomposites. Later, soluble PIs with trimethoxysilane and carboxylic acid end groups instead of PAA were used to prepare high RI nanocomposites. 92,129 For the carboxylic acid-capped PIs, the titania content in the crack-free hybrid films was as high as 90 wt%, the RI reaching Here, they employed an esterification reaction of the end carboxyl group of the PI with the titanium precursors to form covalent bonds while inhibiting phase separation. The conventional PIs in general exhibit poor optical transparency in the visible region because of the formation of charge-transfer complexes (CTC) between the electron-donating diamine moiety and the electron-accepting dianhydride moiety, which may prevent the PIs from being used for extensive optical applications. Recently, Liu and coworkers designed and synthesized a series of highly transparent sulfurcontaining PIs with RI in the range at 633 nm ,130 Optically transparent TiO 2 PI nanocomposites with RI of 1.81 at 633 nm were obtained from semi-alicyclic sulfur-containing PAA and silica-modified anatase TiO 2 particles with a diameter of 10 nm (n ¼ 2.0 at 589 nm) by the direct blending route. 130 It is desirable to develop high RI organic matrices (n > 1.6) for the preparation of higher RI nanocomposites. The advantage of this design is that the high RI nanocomposites can be fabricated with low inorganic filler content while maintaining their excellent mechanical and optical properties as well as processability. Although some hybrid nanocomposites with high inorganic loading have high RI, the poor mechanical properties make them useless in practical optical applications. Our group prepared high RI triethoxysilane-capped polythiourethane (n ¼ 1.632) to integrate with titania by the sol gel route. 94 The RI at 633 nm Fig. 6 AFM images of hybrid nanocomposite films with different titania content: (a) TCPTUTi10; (b) TCPTUTi30; (c) TCPTUTi50; and (d) TCPTUTi70 (R q : mean square roughness of hybrid films; R a : average roughness of hybrid films). The domain size decreases on average from nm to 5 20 nm with increasing content of titania and the variation of surface roughness for the hybrid films has the same trend. Reprinted with permission from ref. 94. Copyright 2003 Wiley-VCH. increases from for the polymer matrix to for the crack-free polymer nanocomposite film with 80 wt% titania loading. AFM studies showed that the titania phase is uniformly dispersed in the polymer matrix on a nanoscale and the nanocomposite films had an excellent surface planarity (Fig. 6). The interactions (hydrogen bonds or covalent bonds) between organic and inorganic components can prevent phase separation, and are the primary factor affecting phase size of the hybrid films. By a similar strategy, high RI TiO 2 /epoxy resin nanocomposite films were also fabricated. 131 Here, the design and synthesis of high RI triethoxysilane-capped trimercaptothioethylamine as coupling agents between the titania domain and the epoxy matrix is the key to successfully preparing these transparent nanocomposites. The high RI nanocomposite coatings were prepared from UV-curable multifunctional sulfur-containing methacrylate and TiO 2 ZrO 2 nanoparticles by in situ polymerization. 118,119 These hybrid coatings can be used as hard coatings with high abrasion resistance and good adhesion for high RI plastic lenses. Other metal oxides including CeO 2, 60 Bi 4 Ti 3 O 12, 65 ZrO 2, 106 ZnO, Ta 2 O 5 /SiO 2 and indium-doped tin oxide (ITO) 63 have been reported in the preparation of acrylate-based polymer nanocomposites with high RI by different methods High RI nanocomposites containing metal sulfides or semiconductor nanoparticles. Some semiconductors or metal sulfides have ultrahigh RI (>2.5) (Table 1). So, incorporating these inorganics as nanofillers can obviously increase the RI of polymer materials (Table 3). The pioneering studies on high RI metal sulfide polymer nanocomposites were performed by Suter and coworkers ,132 The highest RI (3.9) was achieved for PbS- PEO nanocomposites obtained by in situ formation of PbS nanoparticles in the presence of PEO. 68 Later, colloidal Si nanoparticles with an average diameter of nm were prepared by high-energy milling and were integrated into a gelatin matrix to fabricate polymer nanocomposites with RI up to Although the above-mentioned nanocomposites have higher RI, the polymer matrices used in these studies are watersoluble and there is no chemical bonding between nanoparticles and polymer matrices. As a consequence, these disadvantages may block the extensive application of such composite materials in many optical fields. Recently, we presented a novel method for in situ incorporation of PbS nanoparticles into water-insoluble polymer matrices to fabricate high RI nanocomposites. 105 Leadcontaining precursors for use as building blocks for the polymer network were designed to include functional hydroxyl groups and were introduced into a PTU matrix via a polyaddition reaction of the precursor hydroxyl groups with the isocyanate groups of the PTU oligomers. Then, the transparent PbS PTU nanocomposite films were created through exposing the precursors/ptu composites to H 2 S gas. The highest RI for the resulting nanocomposite films was 2.06 at a precursor content of 67 wt%. For optical applications, the transparency of materials in the wavelength regions of interest should be considered. Many optical designs and applications are mainly focused on the visible regions or a broader wavelength span. However, the aforementioned PbS or Si-based polymer nanocomposites have low optical transparency in this region due to the high absorption coefficients of these nanoparticles in the visible region. With 2892 J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009

10 Table 3 Some high RI nanocomposites with metal sulfides or semiconductor nanoparticles System Nature of bonds Synthesis method RI Refs. Poly(ethylene oxide)(peo) PbS Non-covalent In situ formation ,132 Gelatin PbS Non-covalent In situ formation PEO iron sulfides Non-covalent In situ formation Gelatin Si (50/50 w/w) Non-covalent Blending route Polythiourethane(PTU) ZnS Covalent Blending reaction Poly(urethane-methacrylate) ZnS Covalent In situ polymerization PTU PbS Covalent In situ formation Hyperbranched polyester ZnS Covalent In situ polymerization Fig. 7 Refractive index variation of TCZnS PTU nanocomposite films with weight content of the TCZnS particles. Reprinted with permission from ref Copyright 2003 Royal Society of Chemistry. these drawbacks in mind, our group recently developed high RI ZnS polymer nanocomposites with covalent bonds between particles and polymer matrix by an ex situ route. 114,133 The selection of ZnS as nanoscale building block is because of its high RI (n 2.36 at 620 nm) and low absorption coefficient over a broad range of wavelengths, from 400 to nm. The ZnS nanoparticles with small size of 2 6 nm were surface-modified by thiol capping agents with reactive functional groups such as hydroxyl groups or double bonds. Then, these functionalized ZnS particles were immobilized into a polymer matrix by the blending reaction or by in situ polymerization with isocyanate group terminated PTU oligomers or urethane methacrylate macromer (UMM), respectively. These transparent nanocomposites exhibit excellent thermal stability and optical properties. Here, the use of a high RI organic matrix (n ¼ 1.57 and for PTU and PUMM) is advantageous for improving the integrated RIs while maintaining the performance balance of the resultant nanocomposites. Fig. 7 shows the RI change of ZnS PTU nanocomposite films with the weight content of capped ZnS nanoparticles. By a similar strategy, Shi et al. prepared ZnShyperbranched polyester nanocomposites with an RI range of by using UV curing technology High RI bulk nanocomposites As mentioned above, high RI organic inorganic nanocomposites are mostly films with a thickness less than 10 mm. However, transparent bulk nanocomposites with more than 1 mm in thickness are desired in many optical applications such as ophthalmic lenses, filters, etc. In order to obtain high RI bulk nanocomposites, the inorganic nanofillers must achieve a high loading level in the bulk organic matrix. However, the nanoparticles are prone to aggregation in organic matrices because of their high specific surface energies and inherently hydrophilic character. In particular, the transmittance of nanocomposites will generally drop with the increasing sample thickness (x) according to the Rayleigh law (eqn (2)). Therefore, this makes it more challenging to prepare transparent bulk nanocomposites with high nanophase content. At present, very few studies have involved the fabrication of transparent organic inorganic nanocomposite blocks with respect to the nanocomposite film materials. Although some transparent nanoparticle polymer blocks have been previously reported, the content of inorganic particles in polymers is generally low (<5 wt%). 59,101,109,110,134,135 Recently, we developed a facile route to prepare thick transparent bulk nanocomposites with high nanophase content by incorporating pre-made ZnS nanoparticles into the selected co-monomers, followed by g-ray-initiated bulk polymerization. 79 The key to successful use of this strategy is the design and optimization of the nanoparticle surface and polymeric monomer, as well as the selection of a suitable polymerization technology. The polymer nanocomposites with a thickness of Fig. 8 UV vis transmittance (T) spectra and photograph (inset) of polymer bulk nanocomposites with 20 wt% ME-capped ZnS nanoparticles (sample thickness: 4 mm). Reprinted with permission from ref. 79. Copyright 2006 Wiley-VCH. This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19,

11 4 mm can be easily prepared by this method. As the content of capped ZnS particles in the polymer is below 40 wt%, the bulk nanocomposites have good transparency (Fig. 8). The RI of the bulk nanocomposites varies from 1.54 to 1.63 depending on the loading level of ZnS particles. Nakayama et al. prepared transparent bulk nanocomposite blocks with more than 1 mm thickness from acrylic acid modified TiO 2 ZrO 2 nanoparticles and thiol-acrylate by UV-induced crosslinking polymerization. 118 The nanocomposites obtained have a controllable RI in the range of with varying particle content from 0 to 30 wt%. 3.3 RI dependence on the inorganic content of nanocomposites The studies on the relationship between RI and inorganic content of nanocomposites can help with the precise prediction of optical properties, which is important for the design of optical coatings and devices. There are various reports on the linear dependence of RI on the weight fraction 59,87 92,107 or volume fraction 66 69,79,94,106,128,129 of inorganic fillers for various research systems. It is well known that the theories based on the Maxwell equation are very successful for predicting the RI of composites Experimentally, the RIs of different composites are generally found to fall between the upper and lower bound of the different models. 137,138 However, Rao et al. have recently found that the effective medium theories cannot adequately describe their so-called titania-based polymer molecular composite system. 139 They thought that the optical properties of these highly hybrid composites, with strong chemical bonds between neighboring constituents formed at the molecular level, could be better fitted to a molecular RI theory, such as the Lorentz Lorenz equation, (1). An empirical equation, (3), based on their system was proposed to describe the RI of nanocomposites. n comp ¼ n filler DnV 2 p (3) Where n comp is the composite RI, n filler is the RI of inorganic components, V p is the volume fraction of polymer, and Dn is the RI difference between the high and low RI materials. However, it should be pointed out that the theoretical analyses are in general difficult to perform with solutions, and possible only in certain special cases, although the linear or nonlinear relations between composite RI and inorganic filler may be determined either theoretically or empirically. 4.1 Antireflective coatings Antireflective (AR) coatings belong to the most important coating design, the purpose of which is to diminish the reflection loss at the surface of an optical component and obtain a clear view through the glazing by increasing light transmittance and avoiding negative effects on the visual observation like double image, reflection of light sources, etc Following the discovery of AR coatings on glass substrates in 1817 by Fraunhofer, 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. 140,141 Multilayer AR coatings in the visible and near-infrared regions are of high importance in the development of optical and electrooptical systems, 143 in which low-cost and high-performance AR coatings are desirable. However, application of conventional AR coatings usually requires expensive vacuum processes, and recently developed sol gel-derived AR coatings still require multiple-step processes, including a thermal or chemical curing stage For the design and fabrication of multilayer AR coatings, the choice of materials for the high RI layer is very important. Krogman et al. first developed a facile procedure for applying antireflective films by spin-coating polymer substrates with metal oxide nanoparticles. 60 Commercially available aqueous colloidal ceria nanoparticles with RI of 2.18 at 500 nm were introduced into a UV-curable monomer for making high RI layers of AR coatings. The RI of polymer layers can be tuned from 1.54 to 1.95 by using ceria nanoparticle loadings from 0 to 90 wt%. Low RI layers ( ) were fabricated by incorporating SiO 2 into polymer films. The properties (reflectance spectrum, abrasion resistance, haze and transmission value) of the resultant nanocomposite AR coatings are comparable to those produced using state-of-the-art vacuum and sol gel-based techniques. For example, a transmission of >96.3% has been achieved for twolayer AR films compared to 90% transmission for an uncoated 4. Optical applications of high RI nanocomposites Optical materials are expected to play a key role in future optical, optoelectric and information fields. The research on high RI organic inorganic nanomaterials with excellent mechanical properties and processability is driven by various optical applications, including antireflection coatings, ophthalmic lenses, prisms, optical waveguides, non-linear optical materials, and adhesives for optical components, etc. Here, we will mainly discuss some promising optical applications of organic inorganic nanocomposites with controlled high RI based on recent publications. Fig. 9 Variation of the reflectance with wavelength: (a) BK7 optical glass and (b) the three-layer anti-reflection coating. The inset figure shows the structure of the three-layer anti-reflection coating. Reprinted with permission from ref. 92. Copyright 2008 Royal Society of Chemistry J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009

12 acrylic substrate. In addition, the wavelength of minimum reflection of the cured two-layer films can also be controlled by varying the thickness and RI of the high and low index layers. Recently, Chen and coworkers used polyimide titania nanocrystalline nanocomposites as high RI layers to fabricate multilayer AR coatings on BK7 optical glass with an RI of A three-layer AR coating was created using polymethyl silsesquioxane (PMSSQ, n ¼ 1.379) and nanocomposites with a titania content of 90 wt% (n ¼ 1.943) and 30 wt% (n ¼ 1.625) as alternate depositing layers (Fig. 9). An average reflectance of less than 0.5% in the visible region was obtained compared with that (5%) of BK7 glass. The high RI nanocomposites mentioned previously also can be utilized to fabricate AR coatings. However, it should be noted that the solvent or monomers in nanocomposite systems must not destroy the substrate, especially plastic substrates, in the fabrication procedure for AR coatings. In addition, the high RI nanocomposites may play a more important role in the design of gradient-index AR coatings for higher RI substrates or device surfaces due to their tunable RI. 4.2 Optical waveguide materials Optical waveguide devices are expected to play a key role in highspeed broadband communications in the fields of very large scale integration photonics and optoelectronics. 147 Organic inorganic hybrid materials are useful for flexible optical waveguide materials because of their outstanding advantages, including improved thermal and mechanical properties over equivalent pure polymers, low processing temperature, increasing coating thickness without cracking, and because they are rendered micropatternable with photolithographic techniques by the incorporation of cross-linkable organic groups. 84, In particular, the optical properties (RI, optical loss, birefringence and thermooptic coefficient) of hybrid materials can be precisely tailored by manipulating their chemical composition. The tunable RI allows us to fabricate step-index or graded-index optical waveguide structures with well-defined and reproducible RI differences. Incorporation of high RI nanoscale building blocks into the hybrid is an effective strategy to allow precise tailoring of RI over a very broad range. Recently, there have been many studies on titania-based polymer nanocomposites for waveguide materials in view of the high RI of the titania phase. Prasad et al. prepared sol gel-derived SiO 2 TiO 2 poly(vinylpyrrolidone) nanocomposites for optical waveguide applications. 84 The RI of nanocomposites can be controlled in the range of at 633 nm by changing the ratio of SiO 2 to TiO 2. The author also used acetic acid as a co-solvent to control the hydrolysis rate of the titanium precursors, avoiding the precipitation of TiO 2 particles. A low optical propagation loss of 0.62 db cm 1 or lower at 633 nm in the slab waveguide configuration could be obtained from the resultant nanocomposites (Fig. 10). A transparent titania polyimide composite waveguide was successfully fabricated by an ex situ method from fluorinated PI and premade TiO 2 nanoparticles in a reverse micellar micro-reactor. 151 When 4 wt% TiO 2 nanoparticles were dispersed into a PI matrix, the RI of the nanocomposites increased from 1.55 for pure PI to 1.56, and the resultant slab composite waveguide possessed a optical propagation loss of 1.4 db cm 1 at 633 nm. Chen et al. prepared high RI oligomeric phenylsilsesquioxane (OPSQ) Fig. 10 Optical intensity decay of a streak line with respect to the displacement in a 20% TiO 2 waveguide. The slope of the line, 0.43, is the optical propagation loss in units of db cm 1. Reprinted with permission from ref. 84. Copyright 1996 American Chemical Society. titania hybrid materials with an absorption edge and RI range of nm and , respectively. 93 The planar waveguides were produced using OPSQ titania as the core layer and their optical loss decreased from to db cm 1 with increasing titania content from 0 to 15.9 wt% due to the reduction of C H bonding density in the nanocomposites. ORMOSIL systems have been developed for waveguide application High RI titania nanoscale building blocks have been commonly integrated into an ORMOSIL matrix to fabricate optical waveguides. 81,85,155,156 The results have shown that it is possible to make planar waveguides with optical losses typically below 1 db cm 1 in various communication windows by this method. 4.3 Non-linear optical materials New materials that exhibit a large and stable non-linear optical (NLO) response continue to be at the forefront of research, owing to their potential applications in various fields such as telecommunications, optical data storage and information processing. 157,158 Device-oriented stringent requirements of such materials include high laser damage threshold, high mechanical and dimensional stability, high thermal stability, and transparency in the IR vis region for low optical loss. Recently, the application of organic inorganic nanocomposites containing metal and semiconductor nanoparticles has emerged as one of the most interesting fields due to their large optical non-linearities. 43,48,49,159 It is known that the optical non-linearities of nanocomposite materials can be improved by increasing the nanoparticle concentration and dielectric confinement effect, the latter depending on the dielectric constant ratio of nanoparticles and their surroundings. So, there are two routes to enhance the dielectric confinement effect: by using high RI nanoparticles such as CdS, PbS to dope a low RI matrix such as PMMA, PVA, or by using high RI nanoparticles coated with a low RI layer. 160 In addition, the control of particle size and size distribution in the nanocomposites is still a critical issue to be solved for their NLO applications. 43,161 Some high RI transparent nanocomposites with high inorganic nanophase loading may well satisfy the above-mentioned conditions and find applications in NLO fields. Recently, Wang and coworkers studied the unique NLO This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19,

13 4.4 Volume holographic recording materials Fig. 11 Results of pump probe experiments on transparent nanohybrid thin films of TiO 2 in PMMA. The time-resolved probe differential transmittance DT/T signal was measured at the pump beam intensity, I, of 2.2 GW cm 2. Reprinted with permission from ref Copyright 2003 Royal Society of Chemistry. behavior of titania PMMA nanocomposites with high RI. 126,162 These nanohybrid films with up to 80% titania nanocrystalline content were prepared on quartz substrates by an in situ sol gel route, followed by spin-coating. The resultant nanohybrid films demonstrated large NLO optical behavior with an ultrafast response of <1.5 ps. 162 However, no signals were observed for pure PMMA and TiO 2 thin films prepared using the same conditions. Fig. 11 shows the temporal behavior of the photoinduced absorption change of different nanohybrids. The NLO properties of nanohybrid films exhibited a strong dependence on the titania loading level. This observed non-linear behavior can be accounted for by the nature of the TiO 2 nanocrystalline phase, which has a much higher RI than that of the surrounding polymer matrix. This can be attributed to the dielectric confinement effect of surface polarization, which in turn accelerates the separation of excited charges and enhances the electric field inside the nanoparticles. By controlling the crystallinity of titania in PMMA, the author also found that both linear or non-linear optical responses increased with the enhancement of titania crystallinity in nanocomposites. 126 A two-photon absorption coefficient (b) as high as 2260 cm GW 1 and a non-linear RI (n 2 ) as high as cm 2 GW 1 were observed for the nanocomposites with highest titania crystallinity. Oleic acid capped TiO 2 nanorods were also incorporated into a PMMA-co-MA copolymer by the direct blending route to prepare transparent highly homogeneous linear and non-linear optical nanocomposites. 163 The negative value for n 2 ( cm 2 W 1 ) and an almost negligible non-linear absorption were observed in this system. The negative n 2 is expected, principally due to the quadratic Stark effect, which overcomes both the positive nonlinearity of solvent and the positive non-linearity associated with indirect transition. Many methods for the tailoring or controlling of the structure and dispersion state of the nanophase in an organic matrix in the design of high RI nanocomposites are very effective and important for improving the NLO behavior of organic inorganic nanocomposites. A special application of high RI organic inorganic nanocomposites is used as volume holographic recording materials in optical data storage. The development of an optimum recording material is still one of the principal challenges in the field of holographic data storage. 164 Holographic recording is generally achieved through photo-induced RI modulation arising from periodic variation of composition and density induced by photopolymerization of 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 materials. A wide variety of photopolymers and photopolymerizable composites have been proposed so far to achieve high RI change and/or higher dimensional stability. However, achieving large RI modulation (Dn > ) has proven to be rather difficult for pure polymer materials, in part because of the limited diffusion of monomer molecules in binders and also because of relatively limited RI differences between the available monomers and binders. 165 Attempts to improve the RI modulation have been made by incorporating high RI species (HRIS), such as Zr-based HRIS 166 and different nanoparticles into photopolymerizable monomers, yielding volume holographic gratings with RI modulation up to Fig. 12 presents the formation mechanism of a permanent modulation of RI within the volume of different photopolymerizable materials. 166 It has been established that the spatial redistribution of monomers and nanoparticles during holographic exposure is responsible for the volume grating formation in such nanocomposites. For nanoparticle polymer composites, the nanoparticles are mainly concentrated in the fringes corresponding to the dark regions of the interference pattern. For the nanoparticle polymer-derived volume holographic recording materials, the inorganic nanoparticles should generally satisfy the following requirements: an RI much higher than that of typical polymers (n ¼ 1.5), colorlessness (absence of absorption in visible and near-ir regions), small size (<20 nm) and narrow size distribution. In addition, a substantive reduction in the scattering loss caused by aggregation of the inorganic nanoparticles is vital for holographic data storage applications Fig. 12 Formation of a permanent spatial modulation of refractive index within the volume of a photopolymerizable material by singlemonomer (left) and monomer-and-hris (right) diffusion following the diffraction pattern generated by interference of two coherent laser beams. Lighter colors represent low refractive index fringes, and darker colors represent high refractive index fringes. Reprinted with permission from ref Copyright 2006 Wiley-VCH J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009

14 where a photosensitive layer thicker than 500 mm is required to record a large number of holograms in the same area of a recording medium. Therefore, the surface modification of nanoparticles by an optimal organic shell is necessary for improving their compatibility with polymer matrix in order to suppress the above problems, especially, when the weight fraction of nanoparticles in a monomer mixture is required to be sufficiently high (not less than 15 wt%). Recently, Garnweitner et al. developed a large-scale, one-step nonaqueous synthesis pathway to monodisperse, highly crystalline ZrO 2 nanoparticles. 172 It has been shown that the low amounts of fatty-acid stabilizers, resulting in the zirconia nanoparticles containing less than 25 wt% of organics, render completely transparent dispersions in organic media, without any agglomerates remaining. The author successfully employed the obtained nanoparticles to generate effective volume-phase holographic gratings with high RI modulation amplitude n 1 up to (along with a reasonably low value of optical loss in the films), which is much higher than the values reported so far. Here, it should be noted that another advantage for the use of ZrO 2 nanoparticles is its low photocatalytic activity with respect to TiO 2 nanoparticles, thus avoiding the degradation of organic matrix. 4.5 High RI hard coatings for plastic lenses Although the optical plastics for ophthalmic lenses have the advantages of low weight, excellent impact resistance, good processability, and dyeability compared with glasses, the low surface hardness and poor abrasion resistance cause a faster decrease of the optical quality of uncoated plastics. In particular, the resins having a higher RI like polythiourethane (PTU, n ¼ 1.61) and polycarbonate (PC, n ¼ 1.59) exhibit a lower abrasion resistance. Therefore, hard coatings with a high abrasion resistance and a good adhesion to the substrate are in high demand. Furthermore, no interference fringe (color) must be generated between the lens surface and the coating layer, that is, the RIs of lens and coatings should be matched. Early work in this area was focused on the synthesis of organo(alkoxy)silane coatings by a crosslinking reaction between the vinyl and thiol groups. To increase the RI of the resulting coatings, the coatings can be modified by co-condensation with transition metal alkoxides such zirconium alkoxides by a sol gel route. However, the RI only increases to 1.53 by addition of about 12 mol% of complexed zirconium alkoxide. Thus it is desirable to develop higher RI (n > 1.6) hard coatings applicable on the high RI lenses. 173 Hwang et al. prepared scratch-resistant, UV-protective and transparent organic inorganic nanocomposite coatings on PC by incorporating commercially available colloidal TiO 2 particles into an ORMOSIL system by a sol gel method. 174 The TiO 2 particles were modified with 3-glycidoxypropyltrimethoxysilane (GPTMS). The ORMOSIL system was derived from methoxytrimethylsilane (MTMS), dimetyldimethoxysilane (DMDMS) and acetic acid. The resultant nanocomposite coatings with an RI of 1.60 showed excellent abrasion resistance and a maximum pencil hardness of 6H. Recently, Nakayama et al. developed organic inorganic nanocomposite hard coatings for high RI plastic lenses from UV-curable monomers and TiO 2 ZrO 2 nanoparticles by in situ polymerization. 118,119 The outer layer of the TiO 2 crystalline core was coated by ZrO 2 in order to depress the photocatalytic activity of TiO 2. The TiO 2 ZrO 2 nanoparticles with diameter 3 6 nm were modified with acrylic acid in order to improve their dispersibility in acrylate monomers and form covalent bonds between the organic and inorganic moieties. The UV-curable monomers include high RI thiol-acrylate and novel difunctional thiourethane methacrylate. The crack-free nanoparticle thiol acrylate resin nanocomposites exhibit an RI of and a pencil hardness of 3 5H depending on particle loading. 118 Although the TiO 2 ZrO 2 difunctional thiourethane methacrylate nanocomposites have relatively low RI in the range of , these coatings exhibit high abrasion resistance and excellent adhesion onto PC or PTU substrates due to the incorporation of difunctional thiourethane methacrylate Fabrication of photonic crystals The advanced functionalities of three-dimensional (3D) photonic crystal structures with a complete photonic band gap are vital to the next generation all-optical telecommunications and information systems. 175,176 Generation of 3D photonic crystals in high RI materials is a necessary condition toward a complete band gap in order to achieve large RI contrast. Bae et al. developed a cost-effective UV nanoimprint technique to fabricate polymeric photonic crystal nanostructures by using high RI organic inorganic hybrid materials (Fig. 13). 177 The high RI organic inorganic hybrid materials (hybrimers) with an RI of at 633 nm were synthesized from diphenylsilanediol (DPSD), MPTMS and Ti(OC 2 H 5 ) 4 in a 1:1:1 molar ratio by a non-hydrolytic sol gel process. The UV-curable hybrimer has low absorption loss and is very well polycondensed without significant shrinkage for soft lithographic applications. The sol gel-derived high RI organic inorganic hybrid composites were also used to fabricate multi-layer spherical voids in them using a femtosecond laserdriven micro-explosion method. 178 The hybrid materials with interpenetrated networks were synthesized from titanium tetrapropoxide (a precursor), MPTMS (a coupling agent) and multifunctional acrylate or methacrylate, respectively. An RI of approximately 1.70 was obtained for the hybrid material with 75 wt% TiO 2. However, the successful 3D fabrication can only be carried out for the hybrid materials with TiO 2 loading below 9 wt% because micro-domains were formed in the high loading hybrids, which resulted in differently sized void dots. 4.7 High RI photosensitive materials High RI photosensitive organic inorganic nanocomposites with patternability were also reported. The photosensitive (3-trimethoxysilyl propyl methacrylate)-modified titania poly(acrylic acid)-graft-poly(ethylene glycol methacrylate) hybrid nanocomposites were prepared by a sol gel route, followed by hydrothermal treatment. 127 The resultant nanocomposites have an RI in the range of when the titania nanocrystalline loading increases from 5.6 to 61.9 wt%. Micrometre-scale fine patterns with a line width of 20 mm were fabricated by a direct lithographic process. The photosensitive system consisting of sulfur-containing poly(amic acid)(paa-iia) with 45 wt% TiO 2 and a photobase generator (DNCDP) was also successfully used to generate a fine negative pattern with a resolution of This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19,

15 Fig. 13 Schematic diagram of the processes of UV-based nanoimprint technique and AFM and SEM images of each step: (A) silicon master of an hexagonal array of cylindrical rods with 100 nm diameter and 130 nm height, (B) embossed hole nanostructures with 105 nm diameter and 127 nm depth by hot embossing technique, (C) imprinted hole nanostructures with 105 nm diameter and 127 nm depth by UV-based nanoimprint technique. Reprinted with permission from ref Copyright 2005 Royal Society of Chemistry. nanocomposites with excellent properties also may find more potential applications in different fields, such as some special applications in new optical sensor devices for solvent detection 181 or in the fabrication of infrared mirrors 182 and smart biomimetic response structures Conclusion and perspectives Fig. 14 SEM image of negative PSPI pattern (film thickness: 1.0 mm). Reprinted with permission from ref Copyright 2008 American Chemical Society. approximately 4 mm by the i-line photolithography technique (Fig. 14). 130 This high RI (n ¼ 1.81) photoresist hybrid system combined with good thermal stability and high optical transparency is a good candidate in designing optical coatings for high-performance complementary metal oxide semiconductor (CMOS) image sensors. 179,180 With the higher demand for materials in the design of novel optical structure and devices, the high RI organic inorganic High RI organic inorganic nanocomposites have attracted considerable attention since their structure and properties can be easily manipulated, even at the molecular level, by the precise design and tailoring of the nanoscale building blocks and organic matrix. Although the promising applications of these nanocomposites have facilitated the rapid development of this area in both fundamental and applied research, there are still lots of questions that need to be addressed. The greatest obstacle to the industrial-scale production and commercialization of these nanocomposites is the dearth of cost-effective strategies for controlling the homogeneous dispersion of the nanoscale building blocks in polymer hosts, especially when high inorganic loading is used. The ex situ method is still considered to be a facile and feasible route for the generation of high RI nanocomposites, nevertheless the key challenge is to carry out the large-scale preparation of high RI nanoparticles with good compatibility with polymer matrices or monomers. Another 2898 J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009