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 CientíficasUniversidad de Sevilla), Américo Vespucio 49, 41092, Sevilla, Spain. E-mail:
hernan@icmse.csic.es
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
1
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
2
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
3
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 adsorption4
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°).
5
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.
Preparation of self-standing one-dimensional photonic crystals
Flexible self-standing films were made by infiltrating the multilayer with an elastomer (Sylgard
184®, Dow Corning). The curing mix was prepared in a polystyrene vessel and deposited onto
the multilayers using a roller blade or by spin coating (40 s, 700 rpm). After that, samples were
cured at 120 °C in a stove for 30 min. Next, we made straight incisions in the PDMS layer with a
blade, and we immersed the infiltrated multilayer supported on the glass in liquid nitrogen (77
6
K). Then we allowed the samples to return to room temperature and at the same time, the
films were lifted mechanically from the substrate.
Characterization
The reflectance spectra were performed using a Fourier Transform infrared
spectrophotometer (Bruker IFS-66 FTIR ) attached to a microscope and operating in reflection
mode with a 4X objective with 0.1 of numerical aperture (light cone angle ±5.7°). The
transmittance spectra were obtained using a ultraviolet-visible scanning spectrophotometer
(SHIMADZU UV-2101PC). FESEM cross section images of the multilayers films were taken with
a microscope Hitachi S-4800 operating at 2 kV. The optical images of the films were acquired
using a digital camera (Canon EOS 400D). Contact angle measurements were obtained using a
CAM 100 goniometer from KSV Instruments.
Conclusions
In this paper we have demonstrated that it is possible to achieve multifunctional Bragg mirrors
made of nanoparticle multilayers with a spatially uniform response and spectrally flat
reflectance/transmittance. Also, the opening or closing of higher order photonic band gaps can
be devised and have been experimentally proved. This has allowed tailoring to measure the
optical response of the ensemble to create, for instance, either rigid or flexible films capable of
simultaneously blocking the NIR and UV ranges while leaving the visible transparency
unaltered. Although porous Bragg mirrors based on nanoparticles constitute a new class of
optical materials whose potential has just started to be investigated, there are already several
examples of the added value offered by the combination of optical quality and high and
accessible porosity.
Acknowledgements
We thank the Spanish Ministry of Science and Innovation for funding provided under grants
MAT2008-02166 and CONSOLIDER HOPE CSD2007-00007, as well as Junta de Andalucía for
grant FQM3579 and FQM5247. The methods and materials herein presented are protected
under patent ES P200900275.
7
Notes and references
1. A. Urbas, R. Sharp, Y. Fink, E. L. Thomas, M. Xenidou and L. J. Fetters, Adv. Mater., 2000, 12,
812
2. D. Lee, M. F. Rubner and R. E. Cohen, Nano Lett., 2007, 7, 1444
3. S. Colodrero, M. Ocaña and H. Míguez, Langmuir, 2008, 24, 4430
4. F. Scotognella, D. P. Puzzo, A. Monguzzi, D. S. Wiersma, D. Maschke, R. Tubino and G. A.
Ozin, Small, 2009, 5, 2048
5. O. Sánchez-Sobrado, M. E. Calvo, N. Núñez, M. Ocaña, G. Lozano and H. Míguez, Nanoscale,
2010, 2, 936
6. S. Colodrero, A. Mihi, L. Haggman, M. Ocaña, G. Booscholoo, A. Hagfeldt and H. Míguez,
Adv. Mater., 2009, 21, 764
7. T. Druffel, N. Mandazy, M. Sunkara and E. Grulke, Small, 2008, 4, 459
8. P. Kurt, D. Banerjee, R. E. Cohen and M. F. Rubner, J. Mater. Chem., 2009, 19, 8920
9. M. E. Calvo, O. Sánchez-Sobrado, G. Lozano and H. Míguez, J. Mater. Chem., 2009, 19, 3144
10. M. E. Calvo and H. Míguez, Chem. Mater., 2010, 22, 3909
11. S. Colodrero, M. Ocaña, A. R. Gonzaález-Elipe and H. Míguez, Langmuir, 2008, 24, 9135
12. M. E. Calvo, O. Sánchez-Sobrado, S. Colodrero and H. Míguez, Langmuir, 2009, 25, 2443
13. D. P. Puzzo, F. Scotognella, M. Zavelani-Rossi, M. Sebastian, A. J. Lough, I. Manners, G.
Lanzani, R. Tubino and G. A. Ozin, Nano Lett., 2009, 9, 4273
14. O. Sánchez-Sobrado, K. Thomas, I. Povey, M. E. Pemble and H. Míguez, Small, 2010, 6,
1283
8
Figure captions
Figure 1. (a) Low and (b) high magnification SEM images of the cross section of a twelve-unit
cell nanoparticle -based Bragg mirror made of alternated SiO2 (spherical beads) and TiO2
(irregularly shaped nanocrystals ) particles.
Figure 2. (a) Pictures of NP 1DPC having a different spatial period of the refractive index
modulation and deposited onto circular 5 cm diameter borosilicate glass. (b) Transmittance
spectra of twelve unit cell NP 1DPC displaying a flat response at photonic band gap frequencies
within the UV-vis range.
Figure 3. SEM images showing the unit cell of different NP 1DPCs, whose ratios between the
optical thickness of the constituent layers are (a) Z = 1, (b) Z = 2, (c) Z = 3, (d) Z = 4. Layer
thickness is indicated using white (TiO2 layer) and yellow (SiO2 layer) lines. The thickness of a
complete unit cell is shown with a red arrow.
Figure 4. Reflectance spectra corresponding to three-unit cell multilayers built with a ratio
between the optical thickness of SiO2 and TiO2 of (a) Z = 1, (b) Z = 2, (c) Z = 3, and (d) Z = 4.
Red lines correspond to the spectra simulated and used as a guide for the design of the
multilayers. Black lines are the experimental reflectance of the actually realized 1DPC
Figure 5. (a) A series of reflectance spectra from a porous 1DPC attained at different partial
pressures of isopropanol. (b) Evolution of the position of the reflectance maximum for
different partial pressures of isopropanol
Figure 6. A scheme of the polymer infiltration process and the subsequent possible thermal
post-treatments to obtain either (i) a self-standing 1DPC or (ii) a self-cleaning supported 1DPC.
Figure 7. SEM images showing the outer surfaces of two nanoparticle -based multilayers
infilled with PDMS. The structure of the outer surface can be tailored by choosing the type of
nanoparticle layer that is left exposed, (a) SiO2 and (b) TiO2.
Figure 8. Pictures obtained depositing water droplets onto NP 1DPC (a) infiltrated with
polymer and (b) with an empty void network. Photographs taken to estimate the contact angle
between water droplets and the outer surface of these samples are shown in (c) and (d)
respectively. (e) Similar contact angle measurements were performed from pure PDMS films.
Figure 9. Pictures of two flexible NP 1DPCs, with (a) a flat optical response in the visible range
and (b) no reflection maximum in the visible region. (c) Transmittance spectra of samples
shown in Figures 9(a) and (b). (red lines). The black spectra correspond to the non-infiltrated
samples used as matrices for the polymer .
9
Figure 1
10
Figure 2
11
Figure 3
12
13
Figure 4
14
Figure 5
15
Figure 6
16
Figure 7
17
Figure 8
18
Figure 9
19