Post-Print: J. Mater. Chem. , 2010, 20, 8240-8246

Post-print: J. Mater. Chem. , 2010, 20, 8240-8246

DOI: 10.1039/C0JM01508C

Versatility and multifunctionality of highly reflecting Bragg mirrors based on nanoparticle multilayers

Olalla Sánchez-Sobrado , Mauricio E. Calvo and Hernán Míguez *

Instituto de Ciencia de Materiales de Sevilla, (Consejo Superior de Investigaciones Científicas-Universidad de Sevilla), Américo Vespucio 49, 41092, Sevilla, Spain. E-mail:

The use of both supported and flexible self-standing nanoparticle -based one dimensional photonic crystal films as effective frequency selective filters in the UV-vis-NIR is herein evaluated. The requirements to achieve a flat spectral response at the desired frequency range are analyzed and a synthetic route to realize materials with such properties presented. Strict control over the structural parameters yields multilayers in which the opening or closing of higher order photonic band gaps can be devised, thus leading to films capable of blocking the UV and NIR ranges simultaneously. Furthermore, the physico-chemical properties of the mirror can be modified to yield either moisture-repelling or, on the contrary, environmentally responsive optical filters. These materials present a great potential to be used as versatile and multifunctional optical elements.

1 Introduction

Nanoparticle -based one-dimensional photonic crystals (NP 1DPCs) are multilayered materials in which films made of colloidal beads of different sorts alternate to built up a modulation of the refractive index in one spatial dimension. Optical interference occurring between the beams reflected and transmitted at each interface results in the opening of forbidden and allowed energy regions, which renders these materials useful as frequency selective mirrors to be used either in reflection or transmission modes. After the seminal works published in the late eighties by Thomas and co-workers,1 who focused on the optical characterization of such coatings as windows for high-power laser applications, no further studies were made on either the materials aspects or the technological potential of these slabs until very recently. A renewed interest in this type of materials arose when Rubner, Cohen and co-workers2 and our own group 3 independently reported on different deposition methods, as well as provided a detailed structural and optical characterization, of nanoparticle multilayers. This research was primarily boosted by the interesting technological possibilities offered by the combination of high optical quality and the presence of controllable and accessible porosity in the same film. Since then, numerous studies have deepened the understanding of their properties and widened their range of application. Ozin and co-workers have demonstrated that these materials can be uniformly infiltrated by a luminescent dye to lower the lasing power threshold and spectrally control the emission by means of photonic band gap edge effects.4 Environmentally-responsive optical resonators, in which the luminescent emission spectrum of embedded rare earth nanophosphors abruptly changes depending on the ambient conditions, have also been realized.5 In the field of photovoltaics, significant enhancement of the light harvesting efficiency of dye sensitised solar cells was also achieved by coupling such porous back reflectors to the nanocrystalline electrode .6 Potential applications as radiation shields or mirrors have also been studied. Druffel et al. reported flexible nanoparticle polymer nanocomposite films of high reflectivity in the NIR prepared by dip-coating .7 Interest in similar structures as near-UV mirrors to prevent bird-aircraft collision has also been demonstrated very recently.8 In this same context, we recently showed that infiltration of organic polymers allowed the attainment of flexible, self-standing, Bragg mirrors of high dielectric contrast.9 Initial studies were performed with polycarbonate and later extended to biocompatible silicones to show its applicability as UV protection shields.10 Within a fundamental scope, the complex sorption properties11 and capillary condensation2 processes that take place in these structures when exposed to different vapor pressures have been analyzed in detail.

In spite of the importance of the advances made up to date, a thorough analysis of all these relevant precedents reveals that there is still room for improvement affecting the optical quality of the NP 1DPCs. So far, to our knowledge, no nanoparticle multilayers presenting a flat spectral response at frequencies whose propagation is forbidden, indicative of a fully developed one dimensional photonic band gap, have been reported. Furthermore, this type of response should be inherited by any new composite developed by later infiltration of NP 1DPCs.

Herein, we present optimized preparation routes to attain both rigid and flexible nanoparticle multilayers capable of totally blocking perpendicularly incident radiation in a well-defined frequency range. Strict control over the structural parameters yields multilayers in which the opening or closing of higher order photonic band gaps can be devised. This allows tailoring to measure the optical response of the ensemble to create, for instance, films capable of simultaneously blocking the NIR and UV ranges while leaving the visible transparency unaltered. The prospective applications of these versatile materials as passive optical elements are analyzed. Results reported herein prove that nanoparticle -based one-dimensional photonic crystals constitute an excellent platform to develop novel multifunctional optical materials to be used in numerous fields.

Results and discussion

High reflectance multilayers in the UV-vis-NIR

Aiming at obtaining nanoparticle multilayers showing a flat spectral response in reflectance and transmittance at photonic band gap frequencies, we have optimized the spin-coating technique we previously proposed,3 and which has been successfully employed for a number of applications afterwards.4,6 In this method, precursor suspensions of nanometre sized colloids are deposited onto a substrate whose rotation speed is controlled. Full descriptions of the effect of the spin-coating protocol12 and the use of additives13 on the final structural quality have previously been reported. The intensity and width of the Bragg reflections observed in the final ensemble primarily depends on the contrast between the refractive index of the constituents layers and, especially for coatings made of just a few periods, on the number of unit cells . In this regard, although different types of nanoparticles can be employed to build multilayers, the TiO2–SiO2 pair is usually preferred for the large dielectric constant difference they present and the ease of preparing suspensions of them with controlled size distribution.

Experimentally, two main factors limit the maximum specular reflectance that can be achieved from a Bragg stack. First, as is well-known, mechanical stability is lost when films are too thick, due to tensile stress occurring between the substrate and the film. This determines that there will be a maximum number of unit cells that can be deposited before cracks arise. Second, the presence of defects in the individual layers, which may cause undesired diffuse scattering and loss of transparency in spectral pass-band regions. The first limiting factor makes the realization of highly reflecting mirrors operating in the red or near infrared more difficult, since thicker unit cells are required, while the second one has a larger impact on the UV or blue reflecting stacks, since they are made of thinner films and hence are more sensitive to imperfections.

We have overcome these obstacles and realize nanoparticle Bragg stacks showing close to a hundred percent reflectance at the desired spectral position within the UV-vis-NIR region. Since full details are given in the experimental section, it will suffice to say here that a strict control over the presence of aggregates in the original suspensions and, in some cases, intermediate thermal stabilization were the key factors leading to this improvement. Fig. 1(a) and (b) display two pictures, taken at low and high magnification respectively, of the cross section of one of the twelve-unit cell multilayers prepared in this way. Uniformity in the long range can be appreciated in Fig. 1(a), while the different morphology of the spherical SiO2 and the more irregular TiO2 particles is evident in Fig. 1(b).

Large areas can be covered with an acceptable degree of uniformity with the method proposed herein. In Fig. 2(a) we show a series of photographs, taken with a digital camera, of nanoparticle -based Bragg stacks devised to efficiently block different spectral ranges within the UV-vis-NIR region. All of them were deposited onto circular glass wafers of 5 cm diameter. Transmittance spectra are plotted in Fig. 2(b) and present a spectrally wide and deep drop of transmission at the forbidden propagation region. Secondary lobes are caused by the interference of beams partially reflected and transmitted at the top and bottom surfaces. The latter are thus the fingerprint of the finite size of the stacks.

A practical advantage of spin coating nanoparticles to create the multilayer is that mirrors can be prepared in a few minutes, since each layer is formed in seconds. As soon as the solvent evaporates, the structure is ready to be coated by a new layer. For those mirrors displaying Bragg reflections at λ > 500 nm, an intermediate calcination to mechanically stabilize the ensemble and allow further deposition of layers is required.

For NIR reflecting samples, transmittance dips corresponding to higher Bragg diffraction orders (labelled as m = 2, 3,…) of the multilayer can be detected in the visible and near infrared. This implies that NP 1DPCs can be engineered to attain mirrors capable of simultaneously blocking different spectral regions. Interestingly, judicious design of the unit cell allows suppression of some of the multiple reflections observed to yield mirrors that are fully transparent in the visible but behave as shields for NIR and UV radiation at the same time, as we demonstrate in the following section.

Engineering higher order diffraction

By careful design of the relative thickness of the layers, from the unit cell , it is possible to modify the intensity of the different diffraction orders, and even completely suppress one of them while keeping the rest almost unaltered. This effect can be put into practice to design wide transparency windows between the frequency regions whose blocking is sought after. In order to show the versatility of NP 1DPC in this respect, we have prepared a series of samples that display a similar fundamental band gap, but in which higher orders are selectively suppressed or allowed to arise. In a first approximation, the central wavelength of the Bragg reflection of order m is mλB = 2·(n1d1 + n2d2), where ni and di denote the refractive index and width of layer i. Hence the only condition to keep the different peaks at the same spectral position is that the sum of the optical thickness of the individual layers forming the unit cell remains constant. Then, relative peak intensity can be tailored through the ratio between the optical thickness of the layers, Z = n1d1/n2d2. Four mirrors presenting the same value for n1d1 + n2d2 but different Z were studied.

The reflectance of the different designs proposed was previously simulated using a code written in MATLAB. This program had been successfully employed several times to fit the optical response of multilayers.3,11 In this case, we have employed the values of refractive index previously attained for the SiO2 and TiO2 nanoparticle layers, namely n1 = 1.27 and n2 = 1.74, respectively. Details of the cross sections of the periodic nanoparticle multilayers prepared following this design, as seen in the FESEM , are shown in Fig. 3. The different relative thicknesses between the SiO2 and TiO2 layers can be readily appreciated. All samples used for this study present three unit cells and the parameter, Z, takes the values 1, 2, 3, and 4. The corresponding normal incidence specular reflectance spectra are plotted in Fig. 4, where it can be clearly seen that, as expected, the intensity of higher order peaks strongly depends on the value of Z. Thus, for Z = 1, the second order peak (m = 2) is absent, while the third diffraction order peak (m = 3) can be clearly observed. For Z = 2, the peak labeled as m = 2 is present and that corresponding to m = 3 has disappeared. The third diffraction order starts rising again for Z = 3, and, finally, all orders are clearly detectable when the optical thickness of the SiO2 layer is four times that of the TiO2 one. In all cases, the simulated spectra used as a guide to extract the structural parameters are included. Agreement between theory and experiment is fairly good. It should be noted that the possibility to eliminate the second diffraction order opens the route to the fabrication of films transparent to the eye and that can simultaneously protect against near infrared and UV radiation. This is experimentally demonstrated in the last section of the results and discussion, where we engineered the higher order diffraction peaks to create a flexible transparent film with such properties.

Combining optical and physico-chemical properties of porous Bragg mirrors

In principle, as multilayers get thicker, their response to environmental changes could be hindered by the non-uniform filling or closing of the pores at a certain depth, which might render part of the pore network inaccessible. We have performed vapor adsorption-desorption experiments to prove that the highly reflecting multilayers presented herein shows a similar response to those made of a much lower number of porous layers,11 as can be seen in Fig. 5. Thus, the flat spectral response at photonic band gap frequencies achieved by gradually stacking a large number of nanoparticle films red-shifts as the partial pressure of a selected solvent , in this case isopropanol, increases in the chamber. Consequently, as the pressure is lowered, the reflectance peak blue-shifts. The adsorption-desorption isotherm attained indicates the presence of mesopores of quite regular size, since the particles forming the ensemble are monodisperse, and display a certain hysteresis typical of mesostructured materials. Also, the effect of microporosity can be seen at lower pressures. As has been described before, the response to vapors seems to be controlled by the condensation within the smaller interstitial sites of the stack, i.e., those present in the void network of the TiO2 layers.11

Flexible Bragg mirrors with tailored responses

We have recently demonstrated that the porous mesostructure of nanoparticle multilayers can serve as a host for different types of guest compounds.9,13,14 This endows these Bragg stacks with added value that none of their dense counterparts possesses. In particular, a biocompatible flexible Bragg mirror, with potential interest for skin protection applications, has been developed by embedding polydimethylsiloxane (PDMS) within the void network of the periodic nanostructure .10

With respect to the advances discussed herein, flexible Bragg mirrors can also be devised to display flat spectral reflectance and controlled higher diffraction order modes. This is achieved by infiltrating a polymer , PDMS in our case, and then cooling down the hybrid structure below Tg. This is represented as route (i) in the scheme shown in Fig. 6. Contrarily, if the polymer overlayer is peeled off at room temperature, it is possible to attain a multilayer that is filled up right to the top layer of the nanoparticles , leaving part of their surface exposed to air. The steps to make this type of structure are summarized in route (ii) in Fig. 6. Such multilayers will show no porosity, thus becoming stable versus environmental changes.

Fig. 7 shows images of the outer surface of Bragg mirrors prepared following the steps described in route (ii) of the scheme drawn in Fig. 6, but leaving a different type of nanoparticle layer exposed. However, we can confirm that the physico-chemical properties of the surface are now determined by the polymer regardless of the type of nanoparticles that constitute the last layer.

In Fig. 8(a) and (b), we show images of water droplets deposited onto the surface of a NP 1DPC treated with polymer as shown in routes (ii) and (i) of Fig. 6, respectively. PDMS-protected NP 1DPC shows well-defined drops and no color changes are appreciated in the zones onto which they were deposited, demonstrating the absence of infiltration . In the case of the porous 1DPC (Fig. 8b) no drops are formed on the surface and the voids of the porous layers of the NP 1DPC are filled with water, thus changing the refractive index and the position of the Bragg peak. Contact angle, α, measurements further proved the water -repelling character of the infilled photonic crystal coatings (α = 105°) imposed by the polymer guest (α = 100°) versus the hydrophilic nature of the porous multilayer (α = 16°).

In Fig. 9 we show pictures of two flexible films in which the optical response has been tailored to provide (a) total blocking of visible radiation in the desired spectral range, and (b) almost total transparency in the visible while protecting against near infrared and ultraviolet radiation, respectively. The latter effect was devised by tuning the relative thickness of the SiO2 and TiO2 layers, as described in a previous section, to totally suppress the second diffraction order. The corresponding transmittance spectra (red lines) are also plotted in Fig. 9(c) and (d), along with the spectra of the multilayers prior to infiltration . Such structures present a wide potential in multiple applications, such as laser pointer blocking, UV and heat protection coatings for arbitrary surfaces (including skin), light weight adhesive mirrors, etc.‥.

Experimental

Preparation of particle suspensions

TiO2 nanoparticulated sols were synthesized using a procedure based on the hydrolysis of titanium tetraisopropoxide (Ti(OCH2CH2CH3)4 (97% Aldrich, abbrev. TTIP) as has been described before.3 Briefly, TTIP was added to Milli-Q water. The white precipitate was filtered and washed several times with distilled water. The resultant solid was peptized in an oven at 120 °C for 3 h with tetramethylammonium hydroxide (Fluka). Finally, the suspension obtained was centrifuged at 14[thin space (1/6-em)]000 rpm for 10 min. SiO2 nanocolloids were purchased from Dupont (LUDOX TMA, Aldrich). Both suspensions were diluted in methanol with concentrations between 2 wt% and 4 wt% and filtered later using syringe driven filters (Millipore) of 0.45 μm and 0.22 μm for SiO2 and TiO2 particles respectively.

Deposition of nanoparticle -based one-dimensional photonic crystals

Photonic crystals were built by an alternated deposition of TiO2 and SiO2 nanoparticulated suspensions, following a generic procedure previously reported by our group .3 These sols were deposited over glass slides using a spin coater (Laurell WS-400E-6NPP) in which both the acceleration ramp and the final rotation speed could be precisely determined. The first layer was deposited using 200 μL of SiO2 sol and the substrate was tilted and rotated to let the suspension cover the total glass surface. Then, the sample was accelerated up to different final speeds, in order to control the thickness of each layer. The final speed was chosen between a nominal value of 3000 and 6500 revolutions per minute (rpm) and the value of the acceleration was 11[thin space (1/6-em)]340 rpm s−1. The spin-coating process is completed in 60 s. Sequentially, another layer of a different type of nanoparticle is deposited following the procedure described above. The process is repeated until a final number of layers forming each sample has been deposited.