Post-print of: Energy Environ. Sci., 2011, 4, 4800-4812

DOI: 10.1039/C1EE02081A

Porous one dimensional photonic crystals: novel multifunctional materials for environmental and energy applications

Mauricio E. Calvo, Silvia Colodrero, Nuria Hidalgo, Gabriel Lozano, Carmen López-López, Olalla Sánchez-Sobrado and Hernán Míguez *

Multifunctional Optical Materials Group, Instituto de Ciencia de Materiales de Sevilla, Consejo Superior de Investigaciones Científicas Universidad de Sevilla(US-CSIC), Américo Vespucio 49, 41092, Sevilla, Spain. E-mail: hernan@icmse.csic.es

In recent times, several synthetic pathways have been developed to create multilayered materials of diverse composition that combine accessible porosity and optical properties of structural origin, i.e., not related to absorption. These materials possess a refractive index that varies periodically along one direction, which gives rise to optical diffraction effects characteristic of Bragg stacks or one-dimensional photonic crystals (1DPCs). The technological potential of such porous optical materials has been demonstrated in various fields related to energy and environmental sciences, such as detection and recognition of targeted biological or chemical species, photovoltaics, or radiation shielding. In all cases, improved performance is achieved as a result of the added functionality porosity brings. In this review, a unified picture of this emerging field is provided.

Broader context

Biological and chemical sensors often use optical signals either to detect a specific compound or to warn of its presence. Also, the efficiency of photovoltaic cells is determined by their ability to capture incoming photons and convert them into charge carriers. These two examples illustrate well the relevance of achieving control over light propagation in order to optimize the performance of environmental or energy devices. This becomes particularly relevant when it comes to new photo-electrochemical technologies whose viability depends on how light is managed. With this aim, novel optical materials capable of providing such control and that, at the same time, are compatible with these emerging tools are required. In our review, we summarize a series of recent developments that have led to a new class of multilayered structures that present a unique combination of interconnected porosity and well-defined photonic crystal properties. Such features, for instance, endow them with color responsive properties to ambient changes, of interest for sensing, or can be used to enhance the light harvesting in heterojunction energy devices in which highly efficient optical elements had not been integrated before.

1. Introduction

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The development of new materials that can be used in fields related to the generation of renewable energy and protection of the environment is experiencing a boost due to the high social demand for fast and effective improvement of such disciplines. Photonic materials are no exception. Although their application have been traditionally restricted to the realization of elements of use in optoelectronics and optical communications, the last two decades have seen the emergence of novel structures that combine photonic properties with other functionalities whose synergic effects can be put into practice in fields such as photovoltaics or chemical and biological sensing. Indeed, both periodic and disordered photonic and plasmonic structures have been successfully integrated in solar cells to improve their light harvesting efficiency.1–4 Similar materials have been applied to build sensing devices with higher performance in the detection and recognition of targeted species by means of the change of an optical signal.5,6

The objects of this review are porous one-dimensional photonic crystals (1DPCs) or Bragg stacks, which belong to the group of multilayered structures in which the refractive index varies periodically along one dimension.7,8 These are commonly built by sequential deposition of layers of two materials of high refractive index contrast. Refractive index modulation gives rise to optical interference effects that cause the blocking of well-defined wavelength ranges that can be determined by the structural and optical parameters of the multilayer. Substrates typically coated with this sort of multilayers are rigid ones made of quartz , sapphire, borosilicate glass, etc. A wide number of technological fields make use of these periodic dielectrics both as passive and active optical elements, in which strict control over the density of photon modes at selected spectral regions can be achieved through the rational devise of the refractive index.

Interest in porous Bragg stacks boosted in the mid-nineties due to the advent of porous silicon photonic crystals.9,10 Until then, dense materials, usually grown by physical vapor deposition or sputtering techniques, had been employed in order to ensure performance stability versus ambient changes. Porous silicon photonic crystals constituted a first and versatile research platform to investigate, precisely, the potential of the interplay between controlled mass transport , given by the presence of interconnected and accessible porosity, and structural color. In fact, relevant advances in the field of optical detection of different species, among them some potentially harmful for the environment, have been made based on porous silicon one-dimensional photonic crystals with functionalized inner walls.6 In the last few years, this concept has been extended to many other materials, which has greatly enlarged the potential of application of this sort of structures. These can be as diverse as nanoparticles of all sorts, organically templated ordered mesostructured films, anodically oxidized metals, or thin oxide coatings attained by different types of vapor or liquid phase precursor deposition techniques. Potentially relevant applications of such porous optical materials have been identified in various fields such as biological and chemical sensing,11–15 detection and recognition,16 photovoltaics,1 conducting17 and photoconducting coatings,18 radiation shielding,19,20 or light emission.21

In this review, we will provide an overview of the application of porous 1DPCs to the development of better energy conversion or biological or chemical sensing devices of environmental interest. We will focus on recent advances that take advantage of the

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combined effects of the functionality brought about by the presence of porosity and those coming from the high optical quality of the material. We will show that these versatile optical materials allow improving the performance of a wide range of photoelectrochemical devices by their integration in matrices, which is possible only because of their porous nature. We cover all aspects ranging from their synthesis and microstructural analysis (Section 2), the study of their optical properties and their applications as novel materials for gas and liquid optical sensing (Section 3), sun radiation shielding (Section 4) and improved luminous-to-electric energy conversion devices (Section 5).

2. Methods of preparation and microstructure of porous Bragg stacks

As happens for other nanostructures , the methods of preparation of multilayers with an interconnected network of pores can be readily classified as top-down or bottom-up strategies. Both the built up porosity and the physico-chemical behavior will present completely different features depending on the type of synthetic route chosen. Top-down approaches are typically those in which the periodic spatial modulation of the refractive index is achieved by etching an active substrate using a technique that allows a strict control of the porosity at the tens of nanometres length scale. On the other hand, bottom-up ones are based on the sequential and alternated deposition of layers of materials with different refractive index. The different strategies that have been employed so far are reviewed in what follows.

2.1 Electrochemical etching

This method relies on the selective elimination of well-defined regions of a metal wafer through the combination of oxidation and latter dissolution in extreme pH media. This process generates a pore network whose size distribution and overall void volume are determined by the modulation of the current density applied to the substrate. The resulting pores are interconnected and present typically a tubular shape. The refractive index of a specific layer of the structure is determined by the thickness of the pore walls, which is in turn controlled by the current density. Since the difference in the porosity of the neighboring layers attained by this technique is usually low, it is necessary to build up many periods to obtain a well-defined Bragg peak in the optical reflectance spectrum . The advantage of this technique is the great reproducibility and accuracy of the pore modulation achieved, whereas the main drawback is that it can only be applied to materials that can be obtained in the shape of conducting wafers.

2.1.1. Porous silicon. Silicon was the first material shaped as a porous 1DPC. Since the late fifties,22,23 it has been known that a crystalline silicon wafer can be made porous by passing an anodic current in a fluorhydric acid solution. The mechanism involves a nucleophilic attack of the fluoride ions to silicon atoms of the lattice, releasing silicon hexafluoride. The size of the pores can be precisely tuned from 2 nanometres to a few microns depending on the charge density, the concentration of the ion fluoride, and the doping level of the silicon wafer.24,25 One direct path to attain porous silicon 1DPCs is to modulate the anodic current in time by means of an external source, which leads to a multilayer structure of well-defined slabs of different porosity and thus refractive index.9,10 In Fig. 1a we show an image of the cross-section of a porous silicon 1DPC, where the periodic modulation of porosity can be readily observed. The preferential pore growth direction is the one perpendicular to the substrate, which determines their tubular shape. The group of Sailor has pioneered the research with this

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type of porous lattices and has made significant advances in the control of the photonic properties of these stacks as well as of related structures, such as optical resonators.26 They have also explored in depth different ways to functionalize the silicon walls in order to endow these materials with a selective response to a wide spectrum of molecules (vide infra).

2.1.2 Anodized alumina. The development of the hard anodization technique that generates long pores in alumina27 from an aluminium foil, used as the working electrode , was the key that led to the first porous 1DPC made of this material. Voltages applied are high, typically in the range of tens of volts, which allows diminishing the length of the treatments down to a few hours rather than days, as demanded when low voltages are applied. In a first electrochemical step, a mirror-polished aluminium substrate is anodized in an acidic media to generate a surface array of regularly arranged stem pores . In a second anodic step, current is sinusoidally modulated to generate a pore structure from the seminal pores obtained previously. In Fig. 1b we show a cross-sectional image of a porous alumina 1DPC, extracted from ref. 14, where the connected tubular-shape porous structure can be appreciated. The voltage applied and the nature of the acid determines the pore size. The most commonly used acids are oxalic, phosphoric or sulfuric. Porosities are typically controlled on the order of a few tens per cent.

As in the case of silicon, 1DPCs made in this way present a homogeneous composition and the one-dimensional modulation of the refractive index is obtained through the controlled variation of the anodic current.14,28,29 The contrast between refractive index of the layers achievable by means of varying the porosity of alumina is low, so a large number of layers must be created to detect clear Bragg reflector properties.

2.2 Physical vapor deposition at glancing (oblique) angle

The glancing angle deposition (GLAD) is a bottom-up technique that combines the traditional thin film vacuum deposition with a particular geometry in which the substrate is tilted with respect to the line connecting the target and the substrate.30 In this way, the vapor-phase compound arrives obliquely to the substrate onto which it will be deposited. Due to this particular geometry, the microstructure of the deposited material is that of tilted pillars. The ability to control the column orientation throughout the thickness leads to interesting applications due to the highly accessible and tunable interconnected pore network of the entire structure. The versatility of GLAD to create photonic crystal structures in different dimensions and the precision it offers to control different types of columnar structures have been thoroughly studied by the group of M. Brett, who pioneered both the technique and the development of the first photonic structures based on them.11 One of the main advantages of GLAD is that it allows obtaining uniform porous layers with homogeneous optical properties over large areas, which makes it compatible with industrial processes.

Since GLAD allows controlling the porosity of the layers and thus refractive index by means of the angle of deposition, multilayers made of a single material, i.e., from a single target, can be prepared by alternating periodically the tilt angle between target and substrate.11 The finality of using only one material is to preserve its intensive bulk properties, bringing photonic functionalities. Examples can be found in the literature of deposited multilayers of titanium dioxide11 or indium tin oxide ,17 which lead in both cases to photoconducting or conducting porous Bragg mirrors, respectively. In Fig. 1c a cross-sectional image of a TiO2 photonic crystal

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grown with this technique is shown. Different refractive index areas can be readily identified as brighter and darker slabs, corresponding to higher and lower refractive index layers, respectively.

On the other hand, the use of GLAD to deposit alternated layers of different oxides was recently carried out and the photonic properties of the final ensemble demonstrated.31 In this work, the sequential deposition of SiO2 and TiO2 led to a modulation of the refractive index in a structure in which the shape and size of the pores remained unaltered throughout the whole multilayer. This technique permits controlling the size and the density of the interstitial sites very precisely for each layer individually.

2.3 Wet deposition methods

Under this title, we include a series of bottom-up approaches that have been developed in the last few years to attain porous photonic multilayers and that are based on the sequential deposition of solution-derived precursors to build porous stacks of thin films of precisely controlled thickness and porosity. The materials that have been used for this purpose are supramolecularly templated metal oxides , clays and nanoparticles of different sort. In all cases, multilayers in which structural color is combined with the properties of the constituent base materials are attained. Such multi-functionality has been put into practice in a variety of applications and prototype devices.

In general, the experimental conditions of the deposition and the composition of the liquid precursor dispersions yield control over the pore size distribution and overall porosity of the mesoporous networks. A porous 1DPC by the sequential deposition of two types of porous layers has been prepared mainly by spin-coating 33,34 and dip-coating ,19,35 both methods providing good results in terms of optical quality and pore connectivity between the resulting layers.

Wet deposition methods are inexpensive and give rise to uniform coating of substrates of areas about a few tens of square centimetres. Control over the thickness of the layers is achieved through the variation of the parameters that determine the deposition dynamics (acceleration ramp and substrate rotation speed, in the case of spin-coating ; and speed of retrieval of the substrate from the liquid precursor, in the case of dip-coating ) or by the concentration of the inorganic phase in the precursor suspensions or solutions. Structural inhomogeneities, which may be produced by border effects, Marangoni convection, the presence of aggregates, or differential solvent evaporation , are the main disadvantages of these methods.33 Building units of a variety of compositions and shapes have been used to obtain porous 1DPCs by wet deposition methods, as described in the following subsections.

2.3.1 Nanoparticle based 1DPC. Seminal studies on the use of nanoparticles as building blocks for Bragg stacks were performed in the late eighties and early nineties by Thomas et al.36,37 Their main interest was the development of laser filters capable of resisting high temperatures rather than taking advantage of their porous nature. However, a full description of the preparation procedure and the structural aspects of the resulting materials were only recently presented.33,38 In order to build a stack in which layers of nanoparticles of different sort are alternated, both spin-coating and dip-coating of precursor suspensions have been successfully

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employed.39 In a combined alternative approach, Rubner and Cohen used a layer-by-layer self-assembly dip coating method to deposit layers from liquid media containing nanoparticles and polymers .19,34 Next, polymers are removed by thermal treatment and a porous nanoparticle 1DPC results.

The principal challenge to overcome when using nanoparticles is to find the right compromise between optical quality of the final layer and the particle size distribution, which will determine the characteristics of the pore network. For narrow size distributions, pore size will increase with the average particle size. The presence of large pores favors interconnectivity and accessibility of the porous networks, which is one of the main added values of these lattices. On the other side, particle size cannot be too big or it will hinder the formation of smooth layers with optical quality. In general, nanoparticle suspensions employed present an average size between 5 nm and 50 nm, with the absence of aggregates to prevent imperfections that will give rise to diffuse light scattering in the deposited layer. A wide variety of particles can be employed, one of the preferred choices made being metal oxides since they form very stable colloids and present diverse optical and electronic properties.40 Also, they offer the possibility of tailoring their surface properties to attain functionalized materials.41

Sol–gel methods are used to synthesize metal oxide colloidal suspensions with features that fulfill the requirements described above.42 The size and shape of the particle are controlled through the reactant concentration, added catalyst and peptizer or complexing agents. As in previously discussed 1DPCs, SiO2 and TiO2 are the preferred particulated materials to build multilayers due to their high refractive index contrast.43 However, many other types of nanoparticles have been employed, such as SnO2, Sb:SnO2, SiO2@Au, Eu:YF3, etc.44 (vide infra). Also, our group demonstrated that the alternated deposition of layers of TiO2 particles with different aggregation state yields the required modulation of refractive index as to observed strong Bragg reflections.18 A cross-sectional FESEM image of a SiO2–TiO2 nanoparticle stack reveals a well-defined layer thickness with smooth interfaces between them (Fig. 1d). Spheres are SiO2 particles whereas smaller particles are nanocrystalline TiO2 particles. Fig. 2a and d display pictures of nanoparticle multilayers attained by different methods, strong colored reflections being observed in all cases.

2.3.2. Supramolecularly templated metal oxide 1DPC. This approach, simultaneously developed by the Ozin group 45 and our group ,35,46 relies on the alternated stacking of meso-ordered metal oxide thin films. In these systems, the pore size and the network geometry of each layer can be precisely designed by an adequate choice of the metal oxide precursor, solvent and organic templating agent. The number of materials that can be shaped as mesostructured thin films, hence used to build 1DPCs, is limited practically to SiO2 and TiO2.35,45 Alcoholic solutions of titanium(IV) or silicon(IV) compounds were used as precursors whereas cationic surfactants such as CTAB (cetyltrimethyl ammonium bromide) or block copolymers such as Pluronics F127 were used as templates. The resulting liquid solutions are sequentially deposited by dip-coating or spin coating followed by a thermal treatment to consolidate the inorganic walls and to remove the polymer leading to a highly specific surface area film. Fig. 1e shows a FESEM image of the cross-section of a photonic crystal made using a TiO2–SiO2 mesostructured templated multilayer. A regular pore structure can be easily distinguished in the image. It should be mentioned that, very recently, the use of non-standard

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organic templates has allowed achieving the first Bragg stack made exclusively of mesostructured TiO2, which implies that it keeps its photoconducting properties intact and opens the route to applications in photoelectrochemical devices.47

The strict control over the ordered mesophase is mandatory to ensure its reproducibility. The resulting narrow pore size distribution brings the possibility of using them as base materials for optical sensing in which the probe will induce a change in the optical response depending on its size.48 In addition, different types of radicals can be anchored to the metal oxide walls of these porous 1DPCs, which provides added functionality to these versatile materials.35

2.3.3 Clay 1DPC. Clays have also been used to build Bragg stack.15,49 Clays belong to the family of layered silicates, which are well known for their versatility as adsorbents, ion exchange materials or catalysts . Reported Clay Bragg stacks (CBS) were made by alternating layers of laponite (a synthetic philosilicate clay of empirical formula Na0.7[(Si8Mg5.5Li0.3)O20(OH)4]) and metal oxide porous layers. A cross-section image of a CBS is shown in Fig. 1f. Laponite layers were successfully deposited from water based precursor dispersions and templated with polystyrene spheres to improve the flow of liquid and gas species through them. Denser slabs in the image correspond to regions of TiO2 nanoparticles . The importance of the use of clay layers as building blocks is the capability of these compounds to intercalate, adsorb, and exchange ions into their structure. All these processes modify the optical thickness of clay layers altering the size of the unit cell promoting a change in the optical properties of the whole ensemble.

2.4 Other 1DPC

Very recently, the group of Yang developed a variation of porous 1DPCs combining organic polymer layers (derivatives of poly methylacrylates) with porous TiO2 layers.50,51 Since these materials are partially porous, its structure and optical response are closely related to the materials herein described. In fact, they make use of the porosity of the TiO2 layers to give different solvents access to the polymer layers, that swell yielding a change of the optical properties (vide infra). Refractive indexes of the organic polymer and TiO2 layers are near 1.50 and 1.83 respectively, which provides enough refractive index contrast as to achieve strong reflections with 10 periods.

3. Optical properties of porous Bragg stacks

The periodic modulation of the refractive index in one spatial dimension, achieved through the alternated deposition of materials of different nature or porosity, leads to the opening of a photonic band gap. Light beams reflected at the interfaces of layers with different refractive index can interfere constructive or destructively. As a result, the propagation of electromagnetic waves through the photonic structure is forbidden for photons of certain energies. In fact, the phenomenon of constructive interference in the backwards direction leads to the appearance of the maximum in the reflectance spectrum characteristic of these structures. When this peak is located in the visible region of the electromagnetic spectrum , bright colors are seen, as it is illustrated in the different pictures selected in Fig. 2.

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The reflectance spectrum of a one-dimensional photonic crystal depends on the number of layers stacked, the refractive index of the constituting layers, and their thickness. As the difference between the refractive index of neighboring layers increases, the reflectance peak gets more intense and broader. A similar effect occurs when the number of layers in the stack rises. On the other hand, as the optical thickness of the layers (which results from multiplying the actual thickness by its refractive index) increases, the Bragg reflection shifts to lower energies. The evolution of the optical properties with the number of layers deposited is illustrated in Fig. 3a for a 1DPC prepared using the GLAD technique.31 The reflectance primary maximum becomes more intense and narrower as the number of layers in the stack increases, in good agreement with theoretical predictions. Secondary maxima are the result of Fabry–Perot oscillations resulting from the interference between beams reflected at the upper and bottom face of the multilayer. Fig. 3b shows the transmittance spectra of a series of nanoparticle based 1DPCs whose stop bands cover different wavelength ranges within the visible spectrum (cf.Fig. 2a).52 In this case, the Bragg reflection is detected as a minimum in the transmittance spectra .

It is also possible to disrupt the periodicity of these ordered structures to create allowed states in the photonic stop band. In Fig. 3d we show the cross section image of a porous silicon 1DPC in which a controlled optical defect has been embedded in the middle of the multilayer.53 The consequence of the presence of these optical cavities is the opening of a sharp transmission dip within the forbidden frequency range, as illustrated in Fig. 3c. The position of this minimum will depend on the optical thickness of the middle layer. Interestingly, these relatively sharp resonances can be used to monitor very small changes of refractive index in the pores of the cavity, allowing us to detect the presence of adsorbed vapors onto the pore walls of the structure with great sensitivity.54

4. Environmental response

4.1 Fundamentals

One of the environmental applications envisioned for these porous 1DPCs is based on the dependence of their optical properties on changes in the surrounding media. The basic principle behind this phenomenon can be explained by means of the following approximate expressions. The position of the reflectance maximum (or transmittance minimum), λBragg, at normal incidence can be calculated using the following equation:

mλBragg = 2neffΔ

where m is the diffraction order, Δ is the period or unit cell thickness, and neff is the effective refractive index that can be estimated using the expression:

where n1 and n2 are the refractive indexes of the layers and d1 and d2 their thickness (Δ = d1 + d2). For an individual porous layer, the refractive index is in turn a function of both the refractive index and the filling factor of each one of the components of the layer. Since within

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the layer variations of the dielectric constant occur in distances much smaller than the wavelength, its average refractive index must be estimated using the Bruggeman equation:

where ns and np (ffs and ffp) are the refractive index (filling factor) of the solid phase and the pores , respectively. This expression can be extended to as many components as necessary for more complex mesostructures. By combining eqn (1), (2) and (3), it can be readily concluded that the modification of the refractive index of the pores by the presence of a guest compound will cause a shift of the Bragg peak to longer wavelengths. A similar effect will occur if the thickness of the layers is increased by swelling, as a result of the incorporation of a solvent to the structure. These are the main mechanisms explored so far to attain a change in the optical response of the ensemble versus the presence of a guest compound. Also, some other characteristic parameters of the Bragg peak such as its intensity and spectral width will be strongly affected. Their analysis requires the fitting of the full reflectance or transmittance spectrum , which can be easily done by employing a transfer matrix approach.55

As the porosity of these systems is on the order of 30–40% of the total volume and the pore structure is well connected, the diffusion of vapors, liquids or any fluid guest compound takes place easily. Typically, in the narrowest pore multilayers, the thermodynamic equilibrium is reached after a couple of minutes after exposure to solvent vapors that adsorb onto the pore walls.13 In the case of liquids, equilibrium occurs in a few seconds. In addition, the response of these materials is reversible, rendering them recyclable and low cost sensing platforms with simple optical read-out.

4.2 Sensing by pore filling

As mentioned before, porous silicon 1DPCs were the first of their kind developed and tested as base material for optical sensing.12,26 Simultaneously, many groups in the world developed diverse techniques to functionalize silicon pore walls to make the optical structure sensitive to a specific compound.12,26,56,57 Complementarily, drug release from porous structures is another technological approach that has been afforded with porous silicon stacks.58

Although hitherto porous silicon was the only material explored to build 1DPC based sensors, today metal oxides are being considered and analyzed for similar purposes. They might open the range of applications since they present improved stability in aqueous solutions, the option to treat the pore walls with a wider range of functional groups , and the possibility of tailoring the physico-chemical properties of the ensemble from the synthesis.

The optical behavior of these porous photonic systems under different vapour pressures has so far been analyzed placing the multilayer film into a closed chamber previously air-evacuated, and in which vapors can be gradually introduced. Reflectance or transmittance is measured in situ as the partial pressure increases and decreases. Above a certain pressure, they condensate onto the pore walls by a process that can be understood in terms of the capillary forces acting in confined spaces. This phenomenon is described by the Kelvin equation, which establishes

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that, at a fixed temperature, condensation occurs at a partial pressure that only depends on the pore size and the specific gas phase compound.

In general, it is observed that vapors diffuse easily through the void network of most porous multilayers. For those stacks in which each layer presents a different and well-defined pore size distribution, changes in the optical response indicate a strong interplay between the two condensation processes.59 In an interesting approach, capillary condensation has been proven to provide in turn a unique way to selectively functionalize surfaces at the nanoscale.60

Examples of the type of changes observed in the optical response of porous 1DPCs when exposed to increasing vapor pressures are depicted in Fig. 4. Fig. 4a shows the reflectance spectra obtained from a 24-layer porous 1DPC, built using nanoparticles of TiO2 and SiO2, as isopropanol pressure increases from 10−3 mtorr to 40 torr (that is, the saturation vapor pressure of isopropanol at 25 °C). In Fig. 4b, the featured optical properties are those of a porous optical cavity made using GLAD. In this case, a disruption of the periodicity leads to an optical resonant mode whose fingerprint is a maximum in transmittance.

In both cases, as pressure increases in the chamber, the spectral position of the Bragg peak shifts to higher wavelengths indicating the condensation of vapors within the pore structure. Moreover, the plot of the spectral position of the Bragg peak or the resonant mode against the pressure for a closed pressure loop (from PVap 0 to PVap = 1 and then to PVap 0 again) resembles a typical adsorption–desorption isotherm obtained when classical physisorption techniques are applied to a bulk mesoporous material, including the hysteresis loop (Fig. 4c and d). The gradual increase of the peak spectral position gives valuable information related to the changes of vapor concentration in the surrounding environment. Such response is determined by the pore size distribution in the multilayer.61 Alcohols , other organic volatile compounds and water concentration in air are easily detected using this technique. Other porous 1DPCs have been tested as base materials to build optical sensors, such as mesoporous ordered materials35 where the optical response is conditioned by the different surface chemistry of the pore walls. This methodology allows detecting the presence of vapors at concentrations of few nanomoles per cubic centimetre.54

Infiltrated liquids produce the same optical shift described with vapors but in this case, the changes in the optical response are much more abrupt, as it is typically the process of infiltration . As porous multilayers are soaked in a solvent , pores are filled producing a shift in the position of the maximum of the Bragg peak proportional to the refractive index of the liquid introduced. This information constitutes the basis for the identification of the solvent that is infiltrating the structure. Porous alumina, nanoparticle multilayers and glancing angle deposited photonic crystals have been satisfactorily infiltrated with series of liquids.

Finally, it should be remarked that porous 1DPCs have allowed the development of the concept of the photonic nose,16 coined and developed by the Ozin group . Porous Bragg stacks were functionalized with different alkoxysilanes to create an array of photonic crystal “pixels” with different surface energy characteristics. As a proof of concept, they exposed the functionalized Bragg stack array to different bacteria cultures, which yielded different responses in each single functionalized photonic detector . In this case, the response is due to the different chemical volatile metabolites that bacteria produce and that can infiltrate the

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photonic crystals. Software color analysis of each pixel in the sensor array allows the identification of the different colonies.

4.3 Sensing by swelling

Another approach to sense different compounds with porous 1DPCs is the swelling of some of the constituent layers in the presence of external compounds, which causes the change of the optical properties of the dielectric mirror. Clay Bragg stacks constituted the first example of a material with this kind of responsive behavior. The intercalation of compounds such as bulky polyalkyl cationic surfactants like CTAB (cetyltrimethylammonium bromide) or PDDA (poly(diallyldimethylammonium chloride)) into the clay layer are in this case responsible for the swelling. In general, surface adsorption, ion exchange and intercalation are the proposed mechanisms that promote the optical change observed in clay multilayers.49 In addition these systems are totally reversible by thermal treatment or by soaking the system in a sodium chloride solution that replaces the surfactant , contracting the structure to recover the original spectrum (see Fig. 5a).

The second option to change the lattice parameter of the photonic crystal with an external agent is the alternating deposition of particulated TiO2 layers and a polymer designed “a la carte” that swells when put in contact with different kinds of liquids or ions. The presence of porous TiO2 is necessary to permit the flow of compounds throughout the structure and grant accessibility to targeted species. It also provides the high refractive index contrast necessary to observe intense Bragg reflections. Experiments in the group of Yang show significant shifts in the position of the Bragg peak when these multilayers are soaked into different solvents such as water (please see the series of spectra plots in Fig. 5b), ethanol, DMSO, etc. Again, the color of the multilayer indicates the nature of the solvent that fills the structure (Fig. 5c).

4.4 Sensing with embedded optically active nanomaterials or dyes

Another potentially relevant approach to sensing based on porous multilayers is based on the use of these 1DPCs to embed nanomaterials that present optical activity or resonant absorption. In fact, luminescent62 and metallic particles, the latter displaying localized surface plasmon resonances ,63 have been successfully integrated into nanoparticle based 1DPCs. The key to the successful achievement of a high optical quality structure is sequential deposition, which allows intercalating different types of particles into the structure, and the great adaptability of the nanoparticle layers, which helps to smoothen the roughness of the underlying layer.

Integration of luminescent nanophosphors (Eu-doped YF3) into an optical porous resonator has recently been realized. Both the reflectance (thin black curve) and the luminescence (thick red curve) spectra of this ensemble are plotted in Fig. 6a. The porous photonic environment is designed so that the resonance (identified as a dip in the reflectance spectrum ) matches the energy of one of the emission lines of europium.64 When this porous structure is exposed to ethanol, the liquid fills the voids modifying the spectral position of both the resonant mode and the photonic band gap edges. Hence, the optical cavity and the emission mode no longer coincide, thus new luminescence peaks, previously blocked by the stop band, are now allowed to exit in the direction perpendicular to the layers. The abrupt change of the emission

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observed reveals the potential of these materials for detection. This concept could be extended to dye molecules included in porous photonic structures, which is also a matter of research. Ozin and collaborators have integrated a rhodamine dye into a nanoparticulated 1DPC and have observed improved lasing at emission wavelengths matching the spectral position of the edge of the Bragg peak,21 for which the density of photon modes increases. Upon infiltration of the structure, such edge would shift and variations of the laser emission wavelength could be used to detect the presence of a guest compound.

It should be mentioned that the same kind of abrupt response versus environmental changes can be attained using gold nanoparticles (caped with a SiO2 shell) embedded in a porous multilayer.63 In that case, the interplay between the localized surface plasmon resonant absorption of the gold particles and the photonic environment determines the response of the whole stack. Again, as liquids or vapors gradually fill the void network, clear changes are detected in the optical absorptance spectrum of the resonator.

5. Radiation shielding

From the point of view of radiation shielding, the use of Bragg filter coatings are advantageous with respect to absorption filters since the mechanism of protection is based on the reflection of the incident photons . This implies no generation of chemical reactive species and no heat accumulated or dissipated, preventing any undesirable chemical reactions.65

Porous 1DPCs offer the possibility to create flexible and highly adaptable mirrors since their void space can be infiltrated with polymers , thus inheriting their mechanical properties. The first example was made with a polymer solution (4% poly(bisphenol-A-carbonate in methylene chloride)).20 This new hybrid organic–inorganic photonic material can be lifted off the substrate to obtain a flexible interference filter that can be transferred and attached onto a substrate with arbitrary shape. This technique is useful in depositing a dielectric mirror in surfaces not compatible with the described deposition methods or of irregular shape. Another synthetic route to attain these hybrid materials is by infiltration with a mixture of polymer precursors (monomers, dimmers or oligomers) with a curing agent. Next, the polymerization is activated in situ by heat or light.66

Fig. 7a and b display images of self-standing hybrid polymer –nanoparticle photonic crystals made of polycarbonate (Fig. 7a) and polydimethylsiloxane (PDMS, Fig. 7b). In both cases, as the polymer infiltrates completely the structure, the optical properties of the ensemble remain unaltered under environmental variations, hence equaling the performance of dense filters against the environmental variations. In the case of PDMS, its biocompatibility makes it a suitable candidate to build ultraviolet (UV) radiation shields for therapeutical applications. Besides, it has recently been shown that it is a suitable matrix to carry and deliver drugs topically.67

Another alternative method to obtain flexible one dimensional photonic crystals is to perform a sequential deposition of a mixed dispersion of nanoparticles in a polymeric medium.68,69 In this case the polymer –nanoparticle interactions must be designed to avoid flocculation or aggregation of the inorganic phase.70

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Applications of these flexible materials are foreseen in the prevention of UV degradation of food and beverage, storage of cosmetics, or, generically, as adaptable coatings for a variety of UV sensitive environments. The introduction of spectral selectivity in that range might also lead to the use of these films to protect selected areas of the human skin from sunburns.71,72 This new approach is an alternative to the current technology for shielding that is based on the inclusion of UV-absorbers into a liquid or solid matrix.73

It should be reminded that flexible Bragg stacks can be made by sequential deposition of polymer layers74 or the self-organization of diblock copolymers .75 The advantage of the hybrid materials herein reviewed over purely polymeric 1DPCs is that they preserve the high refractive index contrast between the inorganic parts, which gives rise to almost total blocking of the desired radiation with a few unit cells (normally 10–12). In contrast, polymer multilayers must be built with many periods to achieve high reflectance values, since the refractive index contrast is much lower. Also, this affects the blocked spectral bandwidth, which also decreases as the refractive index contrast does.

6. Applications in energy

There is a considerable interest in finding ways to manage light propagation and absorption to improve the efficiencies of solar energy conversion devices.76,77 Nowadays, the integration of photonic (including plasmonic) materials in solar cells constitutes one of the most exciting challenges in the field of materials for photovoltaic applications.78 The role of photonic crystal structures is the localization or the re-injection of non-absorbed photons back into the working electrode , where spatial charge separation takes place.79 In this way the photonic crystal acts as a sort of low loss dielectric mirror that increases the electromagnetic field intensity in the absorbing layer. A compromise must be found between the adequate optical design to achieve the highest photon harvesting and the smoothest integration of such structure in the device. Furthermore, other features of the photovoltaic cell such as charge diffusion and low recombination rate must be optimized after the incorporation of the photonic materials.

There are precedents of inclusion of dense one dimensional photonic crystal made of porous silicon80,81 and porous alumina82 integrated in silicon photovoltaic devices with satisfactory results. Porous photonic crystals have also attracted a great deal of interest in the field of dye solar cells (DSCs ).83 In this kind of photovoltaic device, the light is absorbed by a dye attached to the surface of a wide band-gap nanocrystalline semiconductor (commonly TiO2 or ZnO).84 When light shines, the dye is excited and transfers an electron to the conduction band of the semiconductor. The electrical circuit is closed with a redox pair that diffuses from the counter electrode to regenerate the dye . In this way, electrolyte must embed the sensitized semiconductors nanoparticles to allow rapid regeneration of the dye . Since the electrodes are mounted on transparent conductive glass (fluorine doped tin oxide , FTO) and the semiconductor nanoparticles are in the tens of nanometres particle size range, the cell remains transparent. Porosity plays a major role in this architecture, so any optical element added into the cell must preserve the original diffusion of the solution species. A successful design to collect non-absorbed photons is based on the use of polydisperse packings of sub-micron size spheres as highly diffusive reflecting layers.85 Although this approach is used in almost all record efficiency DSCs developed today, it implies the loss of the semi-transparency of the

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cell , which is one of its main added values, and a property that originally boosted research aiming at their integration in buildings as window modules.

Porous photonic crystals open the possibility to boost photon harvesting and hence conversion efficiency while preserving the transparency of the cell , which makes them excellent candidates as photon harvesting enhancers for applications in which transparency is required. Instead of reflecting non-absorbed photons diffusely, as a random packing of large (100–400 nm) scattering particles or cavities would do, a periodic structure allows reflecting light back into the cell specularly. It should be noted that the maximum efficiency one can reach is slightly lower than that achievable when diffuse scattering is used, since the wider angular distribution of scattered light into the absorbing electrode obtained in the latter case yields longer optical paths and thus higher probabilities of absorption. However, periodic structures also allow to precisely define the spectral range at which the absorption enhancement will take place, thus preventing the scattering of light at the longest visible wavelengths (λ > 650 nm, the range of vanishingly small absorption coefficient for the most commonly employed dyes ), and consequently maintaining the semitransparency of the cell .

Complete theoretical analysis of the different phenomena causing light absorption enhancement when photonic crystals are included in dye solar cells can be found elsewhere.79,86 It suffices to say here that the main source of increment of the photocurrent is the longer photon path length of photons reflected back into the absorbing electrode . It should be noted that the effect of optical cavity-like resonances arising from partial photon confinement due to the interference of beams reflected by the highly reflecting materials (i.e., the transparent conducting oxide and the photonic crystal) surrounding the absorbing electrode must also be accounted for in the case of very thin electrodes .87

The first experimental and theoretical attempts with photonic crystals in DSC were made in the field of 3D ordered structures.83,88 In such cases, the thickness of the scattering layer was as large as that of the working electrode it was coupled to. Also, due to the high density of intrinsic defects they present, the enhancement effects observed were due to a combination of specular and diffuse scattering, and the semi-transparency of the cell was also strongly affected.

It has recently been demonstrated that porous 1DPCs accomplish all requirements to behave as light harvesting enhancers without affecting negatively the performance of other parts of the cell .1 In Fig. 8a we show an image of the cross section of a working electrode of a DSC in which a standard 4 μm nanocrystalline TiO2 film is coupled to a 1DPC made of nanoparticles . Porosity in the photonic crystals allows not only performing a post-treatment with a dye solution to sensitize the TiO2 layer, but also to infiltrate the structure with the electrolyte containing the redox pair. The multilayer can be devised to present a Bragg peak that precisely matches the absorption band of the dye , but that has no effect on the longer visible wavelengths, keeping the reddish transparency of the cell unaltered. Photons with energies matching the photonic band gap that have not been absorbed by the dye after passing through the working electrode are reflected back into the cell by the photonic crystal.87

In Fig. 8b, we show three DSC coupled to different photonic crystals with the Bragg peak tailored to reflect blue, green or red light (a standard reference is included for comparison, see

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the top left picture). This response of the photonic crystal can be tuned to match the absorption band of different dyes used in the cell . It also opens the opportunity to achieve panchromatic cell absorbers89 by, for instance, stacking photonic crystals of different unit cells to cover fully the desired absorption range.

In Fig. 9a a current intensity/voltage graph is plotted for a reference cell and a cell coupled to a photonic crystal. It can be seen that the 1DPC causes a significant raise of the photocurrent while preserving, or even enlarging, the photovoltage.90Fig. 9b displays the transmittance spectra of a standard cell , a cell coupled to a diffuse scattering layer and another one in which a porous 1DPC has been integrated. It can be readily seen that the semi-transparency of the cell is preserved when the Bragg mirror is employed.

A related approach has also been studied in DSC by the introduction of TiO2/SiO2 1DPC flakes behind the working electrode .91 Flakes were made by spin coating deposition of a 1DPC on a glass substrate treated with methyl cellulose, which is later removed by immersing the whole structure in water. The polymer dissolves leading to 1DPC flakes. After that, the little 1DPC pieces were integrated on the back of the TiO2 nanocrystalline electrode to act as a highly efficient mirror. In this example, electrolyte flows through the voids betweens flakes.

Finally, the use of mirrors based on polymer –nanoparticle based hybrid photonic crystals,66 as described in the previous section, has also been demonstrated to increase the photon absorption in solar cells . In this case, they are attached to the back side (counter electrode ) of the cell and act as an external low weight, highly adaptable mirror, which reflects the red part of the spectrum where the dye has less probability of absorbing light.

7. Conclusion

We have reviewed the current state of the art of a novel type of photonic structure, namely porous one-dimensional photonic crystals, with great potential relevance in different areas of application linked to energy and environmental sciences. We have illustrated how the introduction of this sort of mirrors in various devices, in which high quality optical materials could not be introduced a few years ago, provides new opportunities for sensing, radiation shielding and photovoltaics. Such multilayers are extremely versatile in terms of the materials they can be made of and the type of porosity they can present. Future research should aim at developing scale-up techniques that allow integration of these materials on large surfaces, of relevance for building integrated photovoltaics, and initiate further studies on the functionalization of inner pore walls with radical groups to anchor specific chemical and biological species to be detected. All these features would provide a unique platform to devise novel multifunctional optical devices for energy and environmental applications.

Acknowledgements

We thank the Spanish Ministry of Science and Innovation for funding provided under grants MAT2008-02166, MAT2011-23593 and CONSOLIDER HOPE CSD2007-00007 to Junta de Andalucía for grants FQM3579 and FQM5247; MEC and MCLL thank the Junta de Andalucía for funding of their contract.

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Figure captions

Figure 1. 1Cross-section scanning electron microscopy images of porous one dimensional photonic crystals made of (a) silicon, (b) alumina, (c) titanium dioxide, (d) TiO2 and SiO2 nanoparticles , (e) supramolecularly templated layers of SiO2 and TiO2 and (f) Laponite–TiO2 system. Images reprinted with permission from ref. 32 (a), 14 (b), 11 (c), 38 (d), 35 (e) and 49 (f). (a), (b), (d) and (f), Copyright 2007, 2008 and 2010 American Chemical Society. (c) Copyright 2006 Elsevier. (e) Copyright 2007 John Wiley and Sons.

Figure 2. Optical Images of porous Bragg stacks obtained through different preparation methods . (a) Spin coating alternated deposition of TiO2 and SiO2 nanoparticles . (b) Anodized alumina. (c) Glancing angle deposition of SnO:F. (d) TiO2 SiO2 layer by layer deposition. Images reprinted with permission from ref. 52 (a), 14 (b), 17 (c) and 19 (d). (b) Copyright American Chemical Society. (c) Copyright 2007 American Institute of Physics.

Figure 3. (a) Experimental and theoretical specular reflectance spectra obtained for 1DPC with different number of TiO2/SiO2 layers deposited by GLAD (solid lines) and compared with theoretical data (dashed lines) (b) Transmittance spectra of a series of nanoparticle based 1DPC that covers the entire visible region, spectra were obtained from the samples showed in Fig. 2a. (c) Reflectance spectrum of a porous optical resonant cavity made of silicon. Transmittance allowed mode is located near 1300 nm (d) Cross section image of a porous silicon optical resonator. Reprinted with permission from ref. 31 (a),52 (b),53 (c and d). (c) and (d) Copyright 2008 American Institute of Physics.

Figure 4. (a) Reflectance spectra of porous 1DPC at different isopropanol vapor pressures. (b) Transmittance spectra of a porous 1DPC with an optical resonant mode exposed to increased humidity content. (c) Plot of the position of the Bragg peak obtained from reflectance spectra of (a) against the increment of isopropanol reduced pressure. Open circles show gradual pressure increase and closed circles gradual pressure decrease. (d) Plot of the spectral position of the optical resonant mode shown in (b) against relative humidity. Reprinted with permission from ref. 52, (a and c), and,11 (b and d). (c) and (d) Copyright 2006 Elsevier.

Figure 5. 5(a) Wavelength shift versus different consecutive cycles of intercalation and removal of CTAB into clay Bragg stacks. (b) Reflectance spectra obtained when using different water concentration (inside graph legend) from a periodic stack of TiO2 and swellable polymer (c) Optical image of photonic multilayers that show the colors obtained from clay Bragg stacks soaked in different solvents . Image reproduced with permission from ref. 50, Reprinted with permission from ref. 49 (a) and51 (b and c). (a) Copyright 2008 American Chemical Society. (b) and (c) Copyright 2010 John Wiley and Sons.

Figure 6. Luminescence of nanophosphors sandwiched between two nanoparticle based photonic crystals to form an optical resonator (a) before (red curve) and (b) after (orange curve) being infiltrated with ethanol. The corresponding optical reflectance spectra are also plotted. Reprinted with permission from ref. 62

Figure 7. Images of (a) a flexible 1DPC prepared from the combination of porous 1DPC and poly(bis-phenol)-A-carbonate when being lifted off, and (b) a flexible 1DPC made with PDMS.

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Figure 8. (a) Cross section FESEM image of a porous 1DPC deposited on top of a nanocrystalline TiO2 electrode supported onto conductive glass (FTO) (b) Optical images of three different 1DPCs integrated in a DSC (the upper left picture has no photonic crystal). Pictures were taken in reflection mode illumination from the counter electrode .

Figure 9. (a) IV curves of a 4.0 micrometres thick electrode coupled to different 1DPCs under 1 sun illumination. The corresponding IV curve for a reference cell is also plotted (black line). (b) Transmittance spectra measured under frontal illumination conditions of two PC-based solar cells together (red and green lines), a reference (black solid) line with the transmission spectrum of a cell that includes a scattering layer. Image 9b reproduced with permission from ref. 1. Copyright 2009 John Wiley and Sons.

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