Hybrid materials themed issue

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This article was published as part of the

Hybrid materials themed issue

Guest editors Clément Sanchez, Kenneth J. Shea and Susumu Kitagawa

Please take a look at the issue 2 2011 table of contents to access other reviews in this themed issue

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ISSN 0306-0012

0306-0012(2011)40:2;1-Z

www.rsc.org/chemsocrev Volume 40 | Number 2 | February 2011 | Pages 453–1152

Themed issue: Hybrid materials

Chemical Society Reviews

2 | 2011 Chem Soc Rev Pages 453–1152Themed issue: Hybrid materials

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CRITICAL REVIEW

Clément Sanchez, Philippe Belleville, Michael Popall and Lionel Nicole Applications of advanced hybrid organic–inorganic nanomaterials:

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Cite this: Chem. Soc. Rev ., 2011, 40, 696–753

Applications of advanced hybrid organic–inorganic nanomaterials: from laboratory to market w

Cle´ment Sanchez,*

^ab

Philippe Belleville,

^c

Michael Popall

^d

and Lionel Nicole

^ab

Received 7th October 2010 DOI: 10.1039/c0cs00136h

Today cross-cutting approaches, where molecular engineering and clever processing are synergistically coupled, allow the chemist to tailor complex hybrid systems of various shapes with perfect mastery at different size scales, composition, functionality, and morphology. Hybrid materials with organic–inorganic or bio–inorganic character represent not only a new field of basic research but also,viatheir remarkable new properties and multifunctional nature, hybrids offer prospects for many new applications in extremely diverse fields. The description and discussion of the major applications of hybrid

inorganic–organic (or biologic) materials are the major topic of thiscritical review. Indeed, today the very large set of accessible hybrid materials span a wide spectrum of properties which yield the emergence of innovative industrial applications in various domains such as optics, micro-electronics, transportation, health, energy, housing, and the environment among others (526 references).

1. Introduction

One remarkable feature of natural materials is that they have found optimal trade-oﬀs between durability, mechanical properties and other functions such as density, permeability, colour, hydrophobicity, etc. Indeed, mother nature is an example of this, in the use of soft and sustainable synthetic conditions for organic and inorganic matter for the generation of organic–inorganic (O–I) nanocomposites such as those forming crustacean carapaces, mollusc shells and bone or

aUPMC Univ Paris 06, UMR 7574, Laboratoire Chimie de la Matie`re Condense´e de Paris, Colle`ge de France, 11 place Marcelin Berthelot F-75231 cedex 05, Paris, France

bCNRS, UMR 7574, Laboratoire Chimie de la Matie`re Condense´e de Paris, F-75005, Paris, France. E-mail: [email protected], [email protected]; Fax: +33-1-44271504; Tel: +33-1-44271529

cCEA, DAM, LE RIPAULT, F-37260 Monts, France.

E-mail: [email protected]; Fax: +33-2-47345676;

Tel: +33-2-47344982

dFraunhofer ISC, Neunerplatz 2, 97082 Wuerzburg, Germany.

E-mail: [email protected]; Fax: +49 931 4100 559;

Tel: +49 931 4100 522

wPart of the themed issue on hybrid materials.

Cle´ment Sanchez

Cle´ment Sanchez, Director of Research CNRS, leads the Laboratory Chimie de la Matie`re Condensee of Paris at the University of Paris VI.

After a PhD at the University of Paris VI in 1981, he did post-doctoral work at the University of Berkeley.

Today, he currently leads a research group working on sol–gel chemistry and physical properties of nanostructured porous and non-porous inorganic and hybrid organic–inorganic materials shaped as monoliths, microspheres and ﬁlms. He has been awarded many scientiﬁc French and international prizes.

Philippe Belleville

Philippe Belleville received his PhD degree in Chemistry from the Pierre & Marie Curie Paris University in 1991 (Prof. J. Livage Laboratory).

He is currently working for the French Commission for Atomic Energy (CEA) as a senior scientist. His research topics include sol–gel chemistry for optical coatings, solar and fuel cell materials, ferroelectric and high ‘‘k’’ thin ﬁlms, and smart layers for gas sensors.

He is a member of the Sol–Gel Optics Program Committee of the SPIE, Optical Society of America (OSA), Technical Committee (TC-16) of the International Commission on Glass (ICG). He was the recipient of the 2003 Ulrich Award for Excellence in Sol–Gel Technology. Since 2007, he is one of the directors of the board of the International Sol–Gel Society (ISGS).

www.rsc.org/csr CRITICAL REVIEW

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teeth.¹Moreover, it is said that minerals likely act as catalysts for natural processes. For example, polypeptides can be formed from the catalytic activation of organic matter (condensation of amino acids) conﬁned within the interstitial gap or porosity of inorganic catalysts such as clays or zeolites, thus emphasizing the possible role of hybrid O–I interfaces in the construction of living systems.^2,3

Because of their swelling and soft-matter-related properties, clays have always been commonly employed as raw materials that humankind could process, modify or even ‘‘alloy’’ with organic components. In doing so, we have been continuously challenging ourselves towards such a complex level of integration to produce, likely through serendipity, artistic or functional hybrid composites such as those found in green bodies of china ceramics, blue Maya pigments, some prehistoric frescos and more. For example, Maya blue is a nice example of a man-made hybrid O–I material that combines the color of the organic pigment (natural blue indigo) and the resistance of the inorganic clay palygorskite.⁴

However, in an industrial context, O–I composites were developed as late as in the middle of the last century. This concept of mixing organic and inorganic components, mainly polymers, has been part of manufacturing technologies since the 1940’s.⁵Some mixed O–I materials were already patented by companies such as Dupont, Dow Corning, 3M,etc. . .and also manufactured in various domains within them: paints (inorganic nano-pigments suspended in organic mixtures), paper(cellulosic polymers cross-linked by metal oxo-species), coupling agents such as silanes, silicones or other metallo- organic molecules allowing the modiﬁcation of glass, ceramics or metallic surfaces. . .

At the end of the 1950’s, several scientiﬁc communities were already giving important contributions to the ﬁeld of mixed O–I compounds. A few of the highlights from an exhaustive list of research domains concern the intercalation of organic components inside clays and lamellar compounds,^2,3,6–9 the

organically templated growth of zeolites, the development of silicones and organosilanes.^10–12The period of 1980–1995 was particularly fruitful thanks to the scientiﬁc melting pot resulting from the constitution of the sol–gel community.

The meeting between material scientists mainly working on glass and ceramics with molecular and polymer chemists allowed an intensive growth in the domain with the creation of many mixed O–I nanocomposites. It is worth mentioning here some of the new material achievements based on hybrid compositions.

The modification or even functionalisation of polymers and macromonomers with sol–gel derived inorganic components, mainly silica or siloxane based species, was performed to improve mechanical, barrier or permeation properties.^13–18 Because of the mild conditions provided by sol–gel chemistry, sol–gel derived silica glasses were doped or hybridized with numerous organic dyes^19–21 or biomolecules,²² giving birth to two important domains known as ‘‘sol–gel optics’’ and biohybrids.^23,24The synthesis of materials resulting from the polycondensation of organosilanes and metal alkoxides,²⁵and the design of transition-metal oxide based hybrids,^26–29 particularly with nanobuilding block (NBB) approaches, pioneered by the Paris and Vienna groups,^30,31also represent major contributions in the field. The name ‘‘hybrid’’ O–I materials was evoked around the 1990s³² when the input of molecular chemistry was obviously creating a ‘‘scientific tsunami’’ in the domain of nanomaterials science.

In that time period (1990–1995), the possibilities of textural, structural and compositional tailoring of hybrids were ampliﬁed with the development of the chemistry of bridged and cubic polysilsesquioxane.^33–47 The 1990’s were very productive with the birth of two other important research domains based on hybrid chemistry strategies. The ﬁrst one concerned the synthesis of periodically organized mesoporous materials obtained via sol–gel condensation templated via the formation of micellar lyotropic assemblies generated by

Michael Popall

Dr Michael Popall, born 1955 in southern Germany, studied chemistry at the Technical University Munich. He received his PhD in 1986 in metal–organic chemistry, at the same university. In 1987 he went to Fraunhofer Institut fu¨r Silicatforschung (ISC) in Wu¨rzburg, Germany and changed from fundamental to applied contractual research with industry and established research on inorganic–organic sol–gel class-II hybrids for use in microelectronic, photonic as well as electrochemical applications. He is now head of ISC-International at Fraunhofer ISC. His materials research is on organic–inorganic hybrids, ORMOCER^ss, for optical, dielectric and electrochemical applications. He received the German Science Prize in 2002 (Stifter foundation) and the NanoTech Future Award 2005 (Tokyo, Japan).

Lionel Nicole

Lionel Nicole is Associate Professor at the UPMC-Univ Paris 06. After a PhD in 2002, he held successively two postdoctoral positions in Dr Cle´ment Sanchez’s group (UPMC-Univ Paris 06) and in Professor MacCraith’s laboratory (OSL-DCU, Dublin).

His main interests are the conception and production of original hybrid materials for optical applications with a particular concern in the environmental domain and in response to social needs.

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amphiphilic molecules or polymers.^48–52The second corresponds to the very interesting family of hybrids known as MOFs (Metal Organic Frameworks) that can be categorized as nanoporous and crystalline hybrid coordination polymers.^53–70

The evolution of this dynamic and mushrooming domain has been paved with numerous international symposia, conferences and workshops totally or partly devoted to the scientiﬁc achievements made with new hybrid materials;

among them MRS and E-MRS symposia, the 1st European workshop on hybrid materials, Euromat, International sol–gel conferences, MOFs conferences, IMMS, Hybrid Materials International Conference. . .⁷¹

Nowadays, hybrid materials chemistry is a unifying scientiﬁc domain for chemists and is obviously impacting numerous societal and industrial demands. Hybrid materials chemistry represents an inherent interdisciplinary ﬁeld of research and development bridging together a variety of communities such as organometallics, colloids and nano- objects, soft matter and polymers, coordination polymers including MOFs, sol–gel, catalysis and surfaces, clays and lamellar compounds, nanocomposites, nanoporous and mesoporous materials, biomaterials, biochemistry and engineering.

Hybrid materials represent an inexhaustible source of inspiration for this large scientiﬁc community. The so-called hybrid O–I materials can be broadly deﬁned as nanocomposites with intimately mixed organic and inorganic components. The control of the physico-chemical nature and

the extension of hybrid O–I interfaces is paramount since it regulates a material’s transparency, chemical homogeneity, and stability. Moreover, its tuning can obviously generate hybrid materials with properties that are not only the sum of the individual contributions of both phases.

Due to the central role played by the hybrid interface, its nature has been used to categorize these materials into two distinct classes.^26,72InClass Imaterials, organic and inorganic components additively exchange weak bonds (hydrogen, Van der Waals or ionic bonds). In Class II materials, the two phases are totally or partly linked together through strong chemical bonds characterized by a strong orbital overlap (covalent or iono-covalent bonds). Independent of the types or applications, as well as the nature of the interface between organic and inorganic components, a second important feature in the tailoring of hybrid networks concerns the chemical pathways that are used to design a given hybrid material. General strategies for the synthesis of sol–gel derived hybrid materials have been extensively discussed in several reviews.15,26,27,40,72–79

However, to facilitate the reading of this critical review, the general chemical background required in order to understand all synthetic approaches explored to date for the designed construction of hybrid materials will be brieﬂy described in part 2 of this article.

Today, the potential of hybrid materials is reﬂected by the fact that many of them are entering a variety of markets.

New materials and systems produced by man in the future

Fig. 1 Arborescence representation of hybrid materials on both the academic and industrial scenes.

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must aim toward higher levels of sophistication and miniaturisation, be recyclable and respect the environment, be reliable and consume less energy or help save energy. The increasing impact of hybrid materials science, both on the academic and industrial scene, can be pictured following the arborescent representation shown in Fig. 1. The design of hybrid materials clearly follows a bottom-up approach.

Starting from molecular precursors or well-defined nanobuilding blocks, the materials are processed directly as particles, fibres, coatings, foams or monoliths. They can additionally be obtained with peculiar micropatterning or with hierarchical structures by coupling colloidal fluids with the physical chemistry of complex fluids, soft matter and top down processing strategies.

These cross-cutting approaches, where molecular engineering and ingenious processing are synergistically coupled allow the chemist to develop complex systems of varying shapes with perfect mastery at different size scales, composition, functionality, and morphology. Achieving hierarchical hybrid architectures involves cross-cutting synthetic strategies opening a land of opportunities to discover new materials which will find applications in numerous fields such as environment, energy, housing, health, automotive, micro-optics, micro-electronics. . . Today a first blossoming of hybrid material applications has already occurred as illustrated by the few examples pictured in the tree branches of Fig. 1.

Smart coatings and membranes, photovoltaics/fuel cells, therapeutic (bio)vectors, (bio)sensors, catalysts (including bio- and photocatalysis), O–I textiles, and automotive parts give a good but incomplete picture of hybrid material products, components or devices that already exist. These developments only correspond to ‘‘the tip of the iceberg’’

and should therefore expand in the future as new and stricter requirements are being set up to achieve greater harmony between the environment and human activities. Hybrid materials are highly promising advanced materials for which the development has just reached its adolescence. Without a doubt, hybrid materials synthesized through green chemistry, will generate increasing numbers of smart membranes, new catalysts and sensors, novel generations of photovoltaics and fuel cells, smart microelectronics, micro-optical and photonic components and systems, or intelligent therapeutic vectors that combine targeting, imaging, therapy and controlled release properties.

The present review summarizes the general chemical pathways for processing hybrid materials and presents the most striking examples of applications of functional hybrids, selecting them from among the existing prototypes or commercially available hybrid materials.

2. Chemistry background and general synthesis strategies for hybrid materials

2.1 Chemistry background

Hybrid materials are constituted by organic components (molecules) or networks (organic polymers) intimately mixed at the molecular or nanoscopic level with inorganic components

mainly metal oxides and metal–oxo polymers but also phosphates, carbonates, chalcogenides and allied derivatives.

Today the main hybrid materials that ﬁnd applications in industry are based mostly on the association between metal oxides or metal–oxo polymers and organic molecules or macromonomers of all kinds including bio-components. An exhaustive description of all the chemical reactions involved in the construction of organic and inorganic components is beyond of the scope of this article, and the reader is referred to some excellent reviews and books.7,15,26,27,40,72–81

Therefore the short discussion on the chemical background will be restricted to the chemical reactions commonly used to produce these two main components (metal–oxo polymers and organic polymers) and their coupling via a covalent or ionocovalent bonding to yield class II hybrids. In order to simplify, the chemical description of the formation of the organic and inorganic components and their interface will be presented separately. To summarize:

(a) Inorganic part.Inorganic metal oxides and metal–oxo polymers are usually produced as amorphous networks, nano- crystalline networks or metal–oxo clustersviacondensation of metal organic precursors (metal alkoxides, modiﬁed metal alkoxides) or metallic salts. These inorganic polymerisation reactions belong to the family of hydrolytic and non-hydrolytic sol–gel chemistry. Sol–gel polymerisation can be driven through hydrolysis reactions (addition of water to reactive precursors such as alkoxides,or chemical or thermal modiﬁcation of the pH of aqueous solutions containing metallic salts) to form reactive M–OH species that condense through oxolation and/

or olation reactions yielding metal–oxo oligomers and polymers assembledviaM–O–M and/or M–OH–M bridges. Metal–oxo species can also be generated through thermal elimination of organic moieties wherein the respective departure creates M–O–M bridges. Ester elimination, ether elimination, and alkyl chloride elimination are well known examples of leaving molecule that have been used to produce metal-oxides through thermally induced non hydrolytic sol–gel chemistry.^82–84

(b) Organic part.Organic components can be introduced into an inorganic network in two different ways, as network modifiers (molecules) or network formers (macromolecules).^25,26,85 The most commonly used network modifiers or network formers are coupled to inorganic moieties through organo silicon alkoxides or chlorides. Some examples of functional organosilanes are gathered in Table 1.

The introduction of organic network formers into an inorganic network to form hybrid materials can be performed following two main strategies: (i) by using already pre-synthesised functional macromonomers that are compatibilized with the inorganic component eitherviachemical grafting (class II) or through embedding with a growing inorganic network in a common solvent to form class I hybrid materials; (ii) they can be generated in situ through photo- or thermally induced polyadditions in the presence of a radical initiator, Atomic Transfer Radical Polymerization, polycondensation (polyesters, formophenolic, polyimine, polyamides. . .), chemically or electro- chemically promoted oxidative polymerisation (polypyrrole, polyaniline, polythiopheneetc. . .).

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(c) Hybrid interface. This third part will concern the chemical nature of the interfaces commonly developed for the processing of hybrids constructed from sol–gel derived metal oxo species (including metal–oxo polymers, clusters, oxide nanoparticles) and organics.

For class I hybrids, homogeneity can be tuned by controlling the extension of a fuzzy interface that optimizes hydrogen

bonding between both organic and hydrophilic inorganic components.

For class II hybrids, the extended but weak class I interface can be reinforced through strong chemical bonding. The hydrolytic stability of the resulting chemical bond depends on the nature of the metallic center. Its knowledge is paramount because, this stability controls the ﬁnal homogeneity and Table 1 Some examples of network modiﬁers, network formers and functionalised macromonomers

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durability of these hybrids. In the basis of the abundant sol–gel literature, lets summarize the observed general trends.

Fig. 2 schematizes the traditional strategies that are employed to bond organic components labeled R (R = molecule, oligomer, polymer. . .).

M–OR bonds are not stable in a hydrolytic medium, they are cleaved yielding reactive M–OH groups that are the starting point of sol–gel polymerisation. However, even if these bonds are not hydrolytically stable, upon drying and thermal treatment they can back condense with the hydroxylated metal–oxo species located in their close vicinity.

In most common sol–gel synthesis conditions Si–R and Sn–R are hydrolytically stable while M–R are not (M = all transition metals and lanthanides, Ti, Zr, Nb, V, W, Eu, Y, Ce. . ., and Al).

However two main conditions must be fulfilled: (i) The chemical bonding should be made through Si–C_sp3or Sn–C_sp3 links typically as in Si–CH2–CH2– or Sn–CH2–CH2– (Fig. 2A), (ii) However such a bond becomes hydrolytically unstable if hydrogen atoms located in a and b positions of the metal centre are substituted by highly electronegative atoms such as fluorine. This is the reason for which commercially available fluorinated silanes are fluoroalkyl- silanes such as (RO)3–Si–CH2–CH2–(CF2)n–CF3 but not (RO)3–Si–(CF2)n–CF3.

Si–C_sp, Sn–C_sp and Sn–C_sp2 links are not stable in the presence of water and other nucleophilic hydroxylated species as methanol and their stabilities range as follows:

Sn–Csp2> Si–Csp> Sn–Csp.

Si–Csp2as in (RO)3–Si–phenyl are thermally (up to 4001C) and hydrolytically stable. However, care should be taken,

because as soon the aromatic ring is substituted by electro- attracting groups such as –F or –SO3H, the organic part can be easily cleaved by water and the organic functionality is lost during materials processing.

For transition metal oxo-based hybrids the grafting of organic moieties can be performed via two strategies (Fig. 2B). The indirect one involves a stable organosilane ((RO)3–Si–CH2–CH2–R) linked to a transition metal alkoxide viaan oxo M–O–Si bridge. The stability of the hybrid interface will depend on the stability of the M–O–Si bridge. Such a strategy has been used extensively to synthesize hybrids made of PDMS chains crosslinked by metal–oxo particles (with M = TiÎV, ZrÎV, AlÎII, Nb^V, Ta^V. . .)^86–88 or made from organically modified poly-oxo tungstates for instance.^89–91

A direct bonding of the organic component to the transition metals or lanthanides can be performed through the use of complexing ligands (Fig. 2C). The stability of the linkage depends directly on the couple metal-complexing ligand.

Considering hard metal alkoxides such as M(OR)n(M = Ti, Zr, Nb, Ta. . .), a common precursor in sol–gel chemistry, the metal-complexing ligand stability ranges as follows:

phosphonates > b-diketones and derivatives > carboxylic acids. For a given ligand, the complexing power can be obviously increased by the polydentate modiﬁcations. Polyacids, such asa-orb- hydroxy acids are better complexing ligands of titanium and zirconium than monocarboxylic acids.

This short tutorial part on the chemistry and hydrolytic stability of the hybrid interfaces will help the reader to enter more deeply in the ﬁeld of hybrid materials. Hybrid materials are not only limited to sol–gel process based approaches. Most of the general strategies for the synthesis of a wide range hybrid materials are schematically represented in Fig. 3.

Fig. 2 Traditional strategies employed to bond organic components labeled R.

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2.2 General synthesis strategies for hybrid materials

This part is inspired from our previous article on applications of hybrids.⁷⁷However the authors are convinced of the need to present a summarized global view of the diﬀerent approaches that allow the generation of hybrid materials to guide the reader along the wide range of possibilities oﬀered by bottom up strategies to design advanced materials.

Following our previous review on applications of hybrids, these general routes can be split into three main chemical paths (A, B, C).

Route Acorresponds to soft chemistry-based routes including conventional sol–gel chemistry, the use of speciﬁc bridged and polyfunctional precursors, hydrothermal synthesis including the synthesis of coordination polymers such as MOFs.

Conventional sol–gel pathways to hybrid networks are obtained through hydrolysis of organically modified metal alkoxides or metal halides condensed with or without simple metallic alkoxides. These strategies are simple, low cost and yield amorphous nanocomposite hybrid materials. These materials, exhibiting an infinity of microstructures, can be transparent and easily shaped as films or bulks. They are cheap, very versatile, present many interesting properties and consequently they give rise to many commercial products

shaped as films, powders or monoliths. However, they are generally polydisperse in size and locally heterogeneous in chemical composition. Better academic understanding providing an accurate control of the local and semi-local structure of the hybrid materials and their degree of organization is an important issue, especially if in the future tailored properties are sought. Two main approaches are used to achieve such control (Fig. 3). The first one is on the use of bridged precursors such as silsesquioxanes X₃Si–R⁰–SiX₃ (R⁰ is an organic spacer, X = Cl, Br, –OR)^74,75,92where the chemical tailoring of the organic bridge allow the improvement of supramolecular interactions yielding materials with a better degree of organisation. Indeed, in recent work, the organic spacer has been complemented by using two terminal functional groups (urea type).^92–98 The combination within the organic bridging component of aromatic or alkyl groups and urea groups allows better self-assembly through the capability of the organic moieties to establish both strong hydrogen bond networks and efficient packing via p–p or hydrophobic interactions.^92–98 The second one includes all aspects of hydrothermal synthesis performed at moderates temperatures (20–2001C) in polar solvents (water, formamide, toluene, alcohol, DMF,. . .). The presence of organic templates has given rise to numerous microporous hybrid materials Fig. 3 Schematic representation of the chemical routes that can be used for the synthesis of O–I hybrids.

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such as organically templated zeolites that already deserve an extensive number of applications in the domain of adsorbents or catalysts. More recently, a new generation of crystalline microporous hybrid solids (Metal Organic Frameworks = MOFs) have been heavily developed by several groups (Yaghi,^64–70 Ferey,^53–60 Kitagawa,^61–63 and Cheetham and Rao).⁹⁹ These hybrid materials are coordination polymers built from telechelic or polyfunctional spacers that coordinate or linkin situgenerated metal-containing small oligomers such as metallic oxo-clusters (see part 6).

Route Bcorresponds to the hybridization of well-deﬁned nanobuilding blocks (NBBs) (viaassembling or intercalation or intercalation and dispersion) which are suitable methods to reach a better deﬁnition of the inorganic component. The use of highly pre-condensed species presents several advantages:

(i) they exhibit a lower reactivity towards hydrolysis or attack of nucleophilic moieties than metal alkoxides, and (ii) the nanobuilding components are nanometric, monodispersed, and with better deﬁned structures, which facilitates the characterization of the ﬁnal materials. These NBB are perfectly calibrated preformed objects such as clusters, organically pre- or post-functionalized nanoparticles (metallic oxides, metals, chalcogenides, etc. . .), nano-core–shells¹⁰⁰ or layered compounds (clays, layered double hydroxide, lamellar phosphates, oxides or chalcogenides) that can host organic components.3,6,8,9,101–106

These NBBs are selected to be able to keep their integrity in the final material.²⁷They can be capped with polymerizable ligands or connected through organic spacers, like telechelic molecules or polymers, or functional dendrimers (Fig. 3). The variety found in the nanobuilding blocks (nature, structure, and functionality) and links allows one to build an amazing range of different architectures and O–I interfaces, associated to different assembling strategies. More- over, the step-by-step preparation of these materials usually allows for a high control over their semi-local structure.

One important set of the NBB-based hybrid materials that are already on the market are those resulting from the intercalation, swelling, and exfoliation of nanoclays by organic polymers. Their applications have been extensively described in several excellent reviews.^6,8,9,77 These properties and applications will be summarized in the tables presented in part 5 of the present article.

Route C corresponds to procedures based on the self- assembly of amphiphilic molecules or polymers coupled with sol–gel polymerisation. During the last ten years, a new ﬁeld has been explored, which corresponds to the organization or the texturation of growing inorganic or hybrid networks, its growth has been templated by organic surfactants.50,51,107–111

In this ﬁeld, hybrid O–I phases are very interesting due to the versatility they demonstrate in the building of a whole continuous range of nanocomposites, from ordered dispersions of inorganic bricks in a hybrid matrix to highly controlled nanosegregation of organic polymers within inorganic matrices. In the latter case, one of the most striking examples is the synthesis of mesostructured hybrid networks.¹⁰⁸ A recent strategy developed by several groups consists of a templated growth (with surfactants) of mesoporous hybrids by using bridged silsesquioxanes as precursors. This approach yields a new class of periodically organised mesoporous hybrid

silicas with organic functionality within the walls. These nanoporous materials present a high degree of order and their mesoporosity is available for further organic functionalisation through surface grafting reactions.¹¹² Another possibility corresponds to the combination of self-assembly and NBB approaches.²⁷ These strategies combining the nanobuilding blocks approach with the use of organic templates that self- assemble and allow one to control the assembling step are also appearing. These NBB, with tunable functionalities allowing the generation of a large variety of hybrid O–I interfacesvia covalent bonding, complexation and electrostatic interactions, can, through molecular recognition processes, permit the development of a kind vectorial chemistry.^113,114

Finally the construction of hierarchically structured hybrid materials that are integrated in real devices follows integrative synthesis pathways where chemistry, physics and processing are strongly coupled. Indeed, the synthesis strategies reported above mainly oﬀer the controlled design and assembling of hybrid materials in the nanoscopic to mesoscopic range. The micron to centimetre ranges can be reached by including either other templating strategies and/or processing methods.

Recently, micro-molding methods have been developed, in which the use of controlled phase separation phenomena, emulsion droplets, latex beads, bacterial threads, colloidal templates or organogelators lead to the control of the shapes of complex objects in the micron scale.^51,109 Moreover, the combination between these strategies and those above described along paths A, B, and C allow to construct hierarchically organized materials in terms of structure and functions.^51,109 These synthesis procedures coupled with top-down approaches such as two photon adsorption (TPA), electrospinning, ink-jet and aerosol printing, dip-pen lithography, X-ray lithography, electron beam writing, nano-imprinting and plasma processing could enable us to design and build ever more challenging and sophisticated novel hybrid materials.1,51,109,115–119

These coupled synthesis procedures will probably help to construct hybrid materials and composites as smart as those observed in natural systems.

These developments will impact important domains of applications such as those associated with energy, the environment and sustainable development, biology and medical sciences, sciences and techniques of information (transfer, writing, reading), housing, automotive and personal comfort.

Some commercial products corresponding to developments in these areas are gathered in the bottom of Fig. 3 and part 5.

3. Processing of hybrid materials

The successful development of hybrid O–I materials is intrinsically linked to the versatility of their processing in addition to the cost and availability of both precursor species and processing devices. Indeed because the rheology of the colloidal hybrid dispersions can be easily adjusted, hybrid materials can be processed through many methodologies used for organic polymers among them: film deposition methods,¹²⁰ fiber extrusion,¹²⁰fiber pulling,¹²¹electrospinning,^122–124electro- chemical deposition,¹²⁰ (soft)lithography based techniques (dip-pen, X-Ray. . .),^125–127TPA,^128,129aerosol or spray,^130,131 ink-jet printing. . . (Fig. 4). One difficulty, but also one Downloaded by UNIVERSITE DU MAINE-SCD on 04 October 2012 Published on 12 January 2011 on http://pubs.rsc.org | doi:10.1039/C0CS00136H

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advantage of hybrid sols can be their reactivity associated to the evolution of the polycondensation and aggregation reactions.

These reactions present a partial reversibility and they can be controlled (quenched or speed up) by tuning chemical and physical parameters such as water content, nature of solvent, pH, dilution, temperature, chemical additives. . . During processing the hybrid oligomers or polymers present in the sols are triggered to form extended solids through evaporation.

The consolidation of the hybrid network can be thermally or photochemically assisted. An eﬃcient coupling between the mastered chemistry and processing methods is paramount for any industrial development. The coupling of hybrid materials chemistry with these processing methods and templating strategies allow the construction of hierarchically structured hybrid materials with controllable functionality and multiscale texturation.

The basic understanding of this coupling is paramount, it dictates a more systematic use of in situ characterization techniques that follow, in real time, formation and structuration from the molecular precursor solutions to the ﬁnal stabilized hybrid materials.¹³²Today, amongst the various hybrid materials processing methods compatible with sol–gel chemistry (Fig. 4), ﬁlm depositions are the most commonly developed in industry.

Basically, the sol state prior to gelation is ideal for preparing thin ﬁlms or macro/micro/nano-patterns by chemical solution deposition techniques. Some of these deposition techniques were developed for large-scale substrates and continuous

coating while others are well-suited for small substrates and batch processing. The aim of this chapter is not to detail each processing technique, but rather to provide a brief description of the main employed or promising processing techniques.

Dip-coating120,133–135

Among the available wet chemical deposition techniques, dip coating is the oldest and the most widely used in industry (production and R&D laboratories).¹²⁰This could be explained by its ease of use, its cost-efficiency, the high coating quality and its flexibility (several modified techniques based on dip coating have been developed for specific substrate shapes and applications such as drain coating, angle-dependent dip coating, thickness gradients, single-sided coating, withdrawal under rotation, and inside coating). Usually dedicated to coat small flat substrates (mainly in academic and R&D laboratories), this deposition technique allows also coatings on large area and complex shaped substrates. This process results from the immersion of a substrate into the coating solution. The film is formed by removing the substrate from the solution at a precise withdrawal speed under controlled environmental conditions (temperature and atmosphere). The formation of thin films occurs through the evaporation of solvents (mainly alcohol and water) which concentrate the system in non- volatile species leading then to aggregation and gelation.

During the drying step, environmental conditions, such as Fig. 4 Processing routes to materials using sol–gel methods.

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relative humidity, temperature and air flow have to be carefully controlled since sol–gel chemistry is highly sensitive to the water content and temperature. The film thickness is mainly governed by the withdrawal speed, the content of non-volatile species and the viscosity of the solution. Usually performed in batch conditions, dip coating has been adapted to a semi-continuous process for endless flexible substrates like webs or filaments.¹³⁴Film defects could be observed such as voids, pinholes, thickness variations, wavy surfaces and incomplete substrate coverage. Voids and pinholes are usually caused by air bubble formation during film deposition or substrate surface contamination. Thickness and uniformity are sensitive to flow conditions in the liquid bath and gas overhead. Moreover chemical stability of large dip-coating baths under atmospheric conditions could also be altered.

Spin-coating120,135–138

This coating technique has been mainly developed for the spin-on glasses in microelectronics and also substrates presenting a rotational symmetrye.g.optical lenses or eyeglass lenses. This also is the main application technology of dielectrics (e.g.hybrids like ORMOCER^ss) and photoresists in processing of the dielectrical interconnection on the backend of chips: i.e. its multilayers, established by spinning of dielectrics and its photolithographic patterning, sputtering of copper, spinning of photoresist and its photolithographic patterning, etching of copper, and spinning of the dielectrics again and so on. In this technique, the desired liquid is deposited onto a substrate held in place using a rotatable fixture (often a motor-driven vacuum chuck). The substrate is then accelerated to very high angular velocities during which the excess liquid (most of the applied liquid) is spread out over the substrate leaving a thin uniform film. This coating technique involves the equilibrium between the centrifugal forces created by rotational accelerations and the viscous forces determined by the viscosity of the liquid. Both thickness and uniformity depend on the rotational speed, the content of non-volatile species in the sol, the volatility and the viscosity of the liquid. The major advantages of spin coating are low cost, reproducibility, uniformity, simplicity, ease of integration, and ability to use different substrate materials. The main disadvantage of the method is that a smooth, flat substrate should be used and a surplus of material is lost, depending on spinning speed.

Moreover, depending on the sol quality and substrate characteristics, some defects could be observed such as striations, gradual radial thickness variations, chuck marks, or ‘‘comets’’.

Spray coatings120,139–143

Spray coating techniques are widely used in industry for organic lacquers and are suitable for coating substrates presenting irregular/complex shapes. In this process, very ﬁne droplets are formed from the solution using atomisers or nebulisers.

These ﬁne droplets are then carried into the coating chamber with a carrier gas and deposited on the substrate by gravity or with an electrostatic ﬁeld. The quality of the coating is determined by the size of the droplet, which can be reduced by decreasing the viscosity of the solution, increasing the

atomising pressure or using a Venturi nozzle. Spray coating techniques oﬀer several advantages compared to the spin and dip coating techniques: it is a faster coating process, waste of coating sols can be much smaller, coating sols with rather short pot lives can be used and the coating step is compatible with continuous coating processes and commercial equipment (Venkajob).¹²⁰Recently reported by Brinker’s group,¹⁴⁴spray coating combining an aerosol-droplet generator system with a robot-controlled deposition apparatus controlled by a computer-aided program allowed the printing of complex shapes on various (curved and ﬂat) substrates (silicon wafers, glass slides, plastics). Further, with INKtelligent printing^sof silver nanoparticles (Fraunhofer IFAM) and ORMOCER^s dielectrics (Fraunhofer ISC) by an aerosol based ink-deposition technique software controlled 3D-printed circuit boards and modules to be produced.

Doctor blading120,145,146

Doctor blading is one of the widely used techniques for producing thin films on large substrates with a well-defined thickness. The technique works by placing a sharp blade at a fixed distance from the substrate surface that is to be coated.

The coating solution is then placed in front of the blade that is then moved linearly across the substrate leaving a thin wet film after the blade. The final wet thickness of the film is ideally half the gap width but may vary due to the surface energy of the substrate, the surface tension of the coating solution and the viscosity of the coating solution. Compared to spin-coating or dip-coating, the loss of coating solution could be greatly minimized. This technique could also work in batch or in continuous process.

Capillary or meniscus coating^120,147

This technique which has a limited use in industry was developed conjointly in the 1990’s by the CEA/Limeil- Valenton and Convac-group Company. Capillary coating is dedicated to coating large ﬂat substrates at room temperature with a very high thickness uniformity and weak edge eﬀect.

The process could be basically described as following: a chuck holds the substrate in an upside-down state using a vacuum, a tubular dispense unit can gently be moved under the surface of the substrate using a driving unit on guide rails. The dispense unit consists of a circulating solution reservoir collecting the excess ﬂuid and a pumping system to ensure a continuous solution feed during the deposition travel. No physical contact exists between the tube applicator and the substrate, but a narrow gap permits the creation of a spontaneous meniscus.

Flow coating

In the ﬂow coating process, the liquid coating is poured over the substrate. The coating thickness depends on the angle of inclination of the substrate, the coating liquid viscosity, and the solvent evaporation rate. As a variation of this process, the spinning of the substrate after coating may be helpful in order to obtain more homogeneous coatings. If no spinning process is employed, the coating thickness increases from the top to the bottom of the substrate.

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Roll coating

Basically, roll coating is a process whereby liquid flows into a narrow gap between two rotating cylinders, the surfaces of which move either in the same or opposite directions. Some of the liquid passes through the gap and splits downstream into two thin films, each coating one of the rolls. This technique is extensively used in the painting, photographic and tape- recording industries for covering a large surface area with one or several uniform layers. Especially the reverse curvature coating allows the application of hybrids like ORMOCER^ss onto foil to obtain a barrier, even ultra barrier coatings/foils for application in flexible solar cells and flexible OLED devices (Fraunhofer POLO).

4. Rational design of hybrid materials

In advanced technologies, the optimisation of numerous applications need a set of requirements sustained in many cases by divergent properties that are very diﬃcult to satisfy by using single component (either organic or inorganic) based materials. Multifunctional materials constitute a golden gate giving access to a large variety of tuneable modern systems and devices. Hybridization allows one to combine, in a single material, properties of organic or biological molecules with those of inorganic compounds. However, these ‘‘simultaneous’’

assimilations of the two diﬀerent organic and inorganic networks from molecular precursors retaining advantageously both organic and inorganic functionalities is not always a trivial goal. In the course of the synthesis of simultaneous hybrid networks, a preferred segregation between ionic and hydrophilic oxide species from covalent and lipophilic organic moieties can occur. Nevertheless, the size of the phase separation or the degree of interpenetration of the respective organic and inorganic components can be kept at the nanometre scale by using several strategies that eventually can be coupled:

Modulation of the reactions rate for the formation of the inorganic and organic components,

Tuning of supramolecular forcesviaweak chemical bonds (Van der Waals, hydrogen bond, hydrophobic interactions) between both components,

Judicious adjustment of the hydrophilic/hydrophobic natures,

Construction of strong bonds (covalent or iono-covalent) between the organic and inorganic components.

The chemistry of hybrid O–I compounds has been strongly developed during the last 30 years for several metal–organic precursors such as M = Si, Sn, Ti, Zr, Al. . .. Currently, the most common way to introduce an organic group into an inorganic silica network is through the use of organoalkoxy- silane precursors, R⁰nSi(X)_4n (wherein R⁰n is an Si–Csp3

bond). In most sol–gel conditions, the particular Si–Csp3bond remains stable towards hydrolysis and the R⁰ group brings possibilities of new properties to the inorganic network (flexibility, hydrophobicity, refractive index modification, optical response,. . .). Organic groups R⁰ can be introduced into an inorganic network in two different ways: as network modifiers (if R⁰is a non-hydrolyzable group bonded to silicon such as Si–CH₃, Si–phenyl Si–dye, isocyanate, amine,etc. . .)

or network formers (if R⁰ can react with itself because it contains a vinyl, a methacryl, pyrrole, thiophene or an epoxy group or any additional polymerisable monomers or macromonomers). Both functions have been achieved in the so-called CERAMERS and ORMOCER^ss. Indeed, these strategies allow a bypass of micro or sub-micro phase separation that may occur otherwise due to the thermo- dynamic incompatibility between the inorganic and organic constituents of the network. As a result new multifunctional hybrid nanocomposite materials can be obtained. Moreover, to this day, the improved knowledge of sol–gel chemistry widens the path in real rational designs of hybrid materials.

At this time, a performance and property driven approach is pervasive in chemistry today. Today’s outstanding individuals display the capability to tailor hybrid materials utilizing this kind of performance–property driven methodology.

Importantly, this approach stems from the requirement of an extensive and integrative scientiﬁc background that over- comes current divisions between respective ﬁelds of Chemistry, Physics, and Engineering and, thus, provides a basic platform to be truly multidisciplinary. This particular way of thinking developed by such individuals can be delineated into two main steps. First, they list and understand the targeted properties and the requirements, at hand. Then, they open their ‘‘toolbox of knowledge’’ containing precursors (eventually if needed they design and synthesize new ones), and they expertly select the appropriate chemistries and coupled-processing routes.

Second they proceed with the synthesis and characterisation of the targeted hybrids without ever forgetting two important rules: (i) the coupling between chemistry, structure, processing;

and (ii) the growth of sol–gel derived polymers is often controlled by kinetics. The second rule strongly influencing the first one, wherein recent development ofin situcharacter- isation methods allows one to follow the structural and compositional modifications of the processed hybrid materials in real time. This has been paramount for knowledge improvement, control and reproducibility of sol–gel derived materials.¹⁴⁸

An illustration of the rational construction of O–I hybrid nanomaterials for the development of hybrid polymers is described in the following.

4.1 Developments in micro-electronics and photonics

As shown in Fig. 5, it is of utmost importance to know in detail what application is targeted. Most materials nowadays have to fulfil numerous test conditions and norms, and need to be adaptable to processing requirements etc. The classical thinking of tuning dielectric properties towards high-k or low-k as well as considering optics for certain refractive indices is not enough to establish a new materiale.g., for optical and electrical applications. Very often, more than a hundred so-called specifications have to be fulfilled, mostly related to certain production processes and their conditions as well as to the later utilisation and environment. Furthermore, solvents and chemicals to be used are limited according to laws like ‘‘REACH’’, safety rules or just because of price, availability and green practices. O–I sol–gel hybrids fit perfectly for the described industrial needs (an easy choice of Downloaded by UNIVERSITE DU MAINE-SCD on 04 October 2012 Published on 12 January 2011 on http://pubs.rsc.org | doi:10.1039/C0CS00136H

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(multi)functionality and playing with organic and inorganic chemistry give easy access to materials that are well adapted to product and manufacturing conditions).

Over a period of 30 years, a database was established at Fraunhofer ISC on diﬀerent compositions and resultant materials with their functions and processing properties. In combination with fundamental knowledge of chemistry, mostly regarding organic cross-linking reactions or the driving of the kinetics of the co-condensation reaction, as described above, inorganic–organic structures can be described and manufactured in a reproducible way.

Fundamental scientists like to have a clear and well-defined structure in the final material, which in principle is possible by use of well-defined nano-building blocks (NBBs), like POSS, silsesquioxanes, or clusters. However, very often their price is a drawback. In addition, one clearly needs to keep in mind, that in most applications, sudden temperature changes are possible. This implies that the coefficient of thermal expansion (CTE) of the inorganic and organic units/phases/multi-layers etc. have to be taken into account and adapted. Failure to do so will lead to resultant stresses causing nano-cracks which will develop into larger ones; giving rise to a reduced lifetime of the final product. A certain mix of NBBs and sol–gel oligomers or the addition of an organic monomer could help to overcome the issue of inner stress.

The in situ building of sol–gel oligomers, such as ORMOCER^sresins, starts from with a mix of cubes, rings and more or less open silsesquioxanes. Depending on the catalyst driving the co-condensation or alkoxolation reactions of the different starting silanes in certain amounts (mostly mono- (R⁰SiOR₃) or difunctionalised alkoxysilanes/silanoles (R^{0 0}₂SiOR₂) have been chosen) a quantitatively well defined mix of D, T, Q structures will be obtained (D = difunctionalised alkoxysilanes R⁰⁰2Si(OR)2, T = monofunctionalised alkoxysilanes R⁰Si(OR)3and Q = Si(OR)4). As the resins consist of several structures, as well as the finally organically crosslinked siloxanes, isotropic material cannot be structurally characterised by classical X-ray methods. It has been shown¹⁴⁸that a combination of spectroscopic techniques with simple visualisation and modelling tools like compass force field^149–151render it possible to obtain information about the oligomer structure and the

statistical functionality. Having allocated the ﬁnal base structure/composition in terms of D, T, Q units of the siloxane, the quantitative spread of D, T, Q in the

29Si-NMR measurement of the co-condensates proves to be a good base for batch control in industrial scale-up production.

After documentation of the kinetics of each involved starting silane by NMR spectroscopy, the classiﬁcation of the spectra of the co-condensates becomes possible and in subsequent measurements for batch control in production, one has only to compare the quantitative spread of diﬀerent Si–O–Si signals.

ORMOCER^ss, basic sol–gel class II hybrids that display specific properties, can be the starting point for a step by step optimisation leading to final materials well-suited for a targeted application. As shown in Fig. 6, one will start with a first modification, e.g., a slight change in the starting composition or concentration of catalyst or change in reaction temperature etc. In the next step, the condensate will be characterised by near infrared (NIR) and infrared (IR) spectroscopy, small angle X-ray scattering (SAXS) and ²⁹Si-NMR.

The data will be the input for simple modelling and one can obtain a visualisation of a possible siloxane structure.

It is not necessary to know the ﬁnal structure, just a hint about match or mismatch of the sterical needs of for instance the reactive groups for later organic cross-linking of silsesquioxanes provides input for the next modiﬁcation/

optimisation, the cross-linking capability of the oligomers as well as whether there are too many buried inside the silsesquioxanes. In parallel, one can also get an input for estimations on density of later material which is directly linked to the refractive index and the permittivity. Repeating this approach several times allows a step by step tuning of the mentioned properties. In further cycles, also a closer look towards other desired properties, like transparency, distribution of certain functions, porosity, polarity,etc., can be done to find the final match of specificationsversusproperties.

Based on the methods described in Fig. 5 and 6, a large number of ORMOCER^ss were developed and are on the market as different products.⁷⁷To update the earlier review, two new examples are given. The difference in comparison to the older published systems is that, here, an additional benefit of sol–gel class-II hybrids is used: ORMOCER^ss can easily be applied to various curing methods and patterning technologies Fig. 5 Specification (here from the electronic industry) driving the

choice of precursors and their functional groups for cost-eﬀective in situbuild-up of ORMOCER^ss.

Fig. 6 Modification/optimisation of ORMOCER^s properties towards desired specifications of the final application. (* and other required properties).

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like UV-nano-imprinting and multi-photon absorption based on a femto-laser technology in high resolution, allowing complex applications in micro-optics and integrated optics. Moreover, they enable a drastic cost reduction in the processing line. They can be passed through: (a) a wafer-scale production of micro- optical arrays for LED lighting and sensing systems with no need of further alignment and packaging and (b) a two-photon absorption-based processing of electro-optical high density printed circuit boards in 3 steps instead of 17.

Fig. 7 shows the reproducible fabrication of refractive micro- lenses and arrays with parameters derived from system design calculations. The resolution/lens gap is 1mm. It has a high ﬁll factor and lateral precision. The homogeneity of the focal length is1% across the wafer. The patterning of the ORMOCER^s results from a UV-replication/nano-imprint technology with ORMOCER^s resins (oligomers) initiated by certain photo- initiators. Commonly utilised resins are Irgacure 369 and Lucirin.

Applications are laser/fiber collimation, beam-forming elements, homogenisers, fill factor enhancement on detector arrays, field-of-view matching for displays and miniaturised imaging systems as well as sensors.^152–162

Fig. 8 gives the scheme of a two-photon absorption (TPA) waveguide writing process for the preparation of optoelectrical boards. Due to the TPA laser writing process, the refractive

index of the ORMOCER^s is increased and 3-D waveguide cores are produced in the material. In contrast to standard technologies, this new ORMOCER^s and TPA technique allows,via higher densiﬁcation (diﬀerent crosslinking of the core, based on femto-laser pulses), the fabrication of wave- guides with one ORMOCER^slayer. The subsequent steps to manufacture a board are: (i) placement of the opto-electrical (o/e) components onto the inner layer of a printed circuit board (standard precision assembly), (ii) application of the ORMOCER^s on the surface (by print, spin-on, curtain coating etc.) (iii) waveguide pattering (core, here waveguide bundle) between the o/e components using TPA technology, (iv) lamination of the surface plane and parallel thermal curing of the residual non-TPA-cured ORMOCER^s resin, (v) connection of the embedded o/e components usingm-vias and (vi) surface mount assembly of the components.

Fig. 9 displays the cross section through the directly patterned ORMOCER^s layer. The TPA process forms the waveguide bundles by focused cross-linking of the cores, whereas the other ORMOCER^s oligomer was thermally polymerised during the lamination process (200 1C and 20 bar) resulting in a lower refractive index than that of the TPA-processed cores, forming the cladding.

Finally, Fig. 10 illustrates the completely processed opto-electrical circuit board which consists of integrated opto-electronic components (VCSEL- and photodiode-chips) and driving components (VCSEL driver and trans impedance ampliﬁer chips (TIA)). The board presents an optical line Fig. 7 ADouble-sided beam homogeniser with 3 diﬀerent zones NA

0.1, 0.2, 0.3 and a buried aperture array.B–CSi CMOS wafer with lens arrays on top of the detector area by selective UV curing.

UV-replication in ORMOCER^s.Courtesy of Fraunhofer–IOF.

Fig. 8 Two-photon absorption (TPA) waveguide writing process.

Courtesy of Langer et al. at AT&S.

Fig. 9 ORMOCER^s matrix with waveguide array, (4 waveguide bundles arranged in an array).Courtesy of Langer et al. at AT&S.

Fig. 10 Opto-electronic circuit board demonstrator. Courtesy of Langer et al. at AT&S.