Silica nanoparticles (NPs) are prepared following the Stober method [13,29]. In particular tetraethylorthosilicate (TEOS), ethanol, concentrated ammonia and distilled water are used as reagents. Two mother solutions are prepared, containing TEOS-ethanol and ammoniawater-ethanol respectively, and then they are quickly combined in a reaction vessel. The mixture is stirred for 24 h with a magnetic stirrer. Subsequently, the silica suspensions are centrifuged at 3000 rpm for 30 min and washed with water. The centrifuging/washing procedure is repeated six times and finally the particles are dried at 50˚C overnight. Latex spheres are synthesized according to a single-stage polymerization process based on formation and growth of polymeric nuclei dispersed in an emulsion constituted by water, styrene, potassium persulfate (KPS) and sodium dodecyl sulfate (SdS) [30,31].

Briefly, the polymerization is carried out in a 500 ml glass reactor equipped with a stirrer, a reflux condenser and a heating jacket. All chemicals are used as received but styrene monomer, which is washed with NaOH and water to remove the polymerization inhibitor. The standard procedure is as follows: 245 ml of water, a suitable amount of sodium dodecyl sulphate dissolved in 13.6 ml of water and 27.2 ml of styrene monomer are premixed in the reactor at the temperature of 80˚C for 2 min and at a stirring speed of 300 rpm. To start the polymerization, an amount equal to 0.952 g of KPS dissolved in 13.6 mL of water is injected in the solution. After 4 h the polymerization is completed and after cooling down the colloidal solution is purified by repeated centrifugation/redispersion cycles.

The formation of direct colloidal crystals is obtained by vertical deposition (convective deposition method), which is based on the evaporation at 45˚C of the liquid (water) forcing the nanospheres to arrange in the meniscus formed between a vertical substrate, the suspension and air [13, 32]. In the specific the formation of the colloidal crystal films is achieved using silica nanoparticles about 320 nm in diameter and PS spheres of about 230 nm and 350 nm.

The morphology and structural properties of the heterostructures are characterized by scanning electronic microscopy (SEM) (JEOL6700). All the samples studied are not metalized. Optical properties are obtained by measuring reflection spectra using miniature fiber optic spectrometer with a 2 nm resolution, the incident light is perpendicular to the (111) plane of the colloidal crystal. Transmittance measurements are performed using a double beam spectrophotometer in the VIS-NIR range. An UV-Ozone cleaner is used in order to clean glass substrate for the deposition of the colloidal crystals and for the hydrophylization of the silica opal.

As a first step silica and polystyrene colloidal crystal films are deposited onto a glass slide by the vertical deposition method. In Figures 1(a) and (b) are reported the transmittance spectra of single colloidal crystal systems of PS and SiO₂ and their photonic band structures obtained using the plane wave expansion method (MIT Photonic-Bands package) developed by Steven G. John-

son [33]. From Figure 1(a) it is possible to observe a good agreement between experimental and theoretical results about the position of the pseudo band gap. In Figure 1(a) is also possible to notice the presence of a minimum in the transmittance spectrum at about 550 nm that arises due to the Bragg diffraction from the (111) planes of ordered structures. This minimum corresponds to the opening of a pseudo gap at about a/λ ~ 0.6 in the calculated Γ-L band diagram, where “a” is the lattice constant. Similarly for the case of the opal constituted by SiO₂ NPs, reported in Figure 1(b), it is possible to observe the presence of an intense diffraction peak at about 700 nm in agreement with the position of the calculated pseudo gap along the Γ-L direction.

The position in term of wavelength of the pseudo band gap, at normal incidence, can be expressed by a modified Bragg’s Law [16]:

where d₁₁₁ = 0.816·D is the interplanar spacing of the (111) planes and D corresponds to the diameter of the nanoparticles; the n_eff is the effective refractive index that can be considered as follows [13]:

where n_spheres corresponds to the refractive index of the NPs used, n_medium is assumed equal to 1 and f is the filling factor that for a face cubic centred structure is 74%.

The formation of the heterostructures occurs depositing on the SiO₂ NPs colloidal film (bottom layer), opal structures of PS (top layer) that present different dimension in size; here two different type of HT systems will be discussed:

(i)1) 350 nm PS NPs on 320 nm SiO₂ NPs (HT_350-PS/ 320-SiO₂);

(ii)2) 230 nm PS NPs on 320 nm SiO₂ NPs (HT_230-PS/ 320-SiO₂)

For both the structures an ozone treatment has been carried out in order to increase the wettability of the bottom layer and assure the adhesion of the second one. In Figure 2 is reported a typical SEM image relative to the cross section of a heterostructure (HT_350-PS/320-SiO₂). Analyzing Figure 2 we can notice that the growth of the two photonic layers occurs and a good ordering of the structures realized is evident. This is confirmed from Figure 2(a) where the presence of the hexagonal arrangement for both the structures (bottom and top layer) can be clearly seen.

Moreover from Figure 2(b) we can observe a large ordered area of the top layer of the PS NPs, indicating the good quality of the structure obtained.

In Figure 3 are reported transmission and reflectance

measurements performed on HT_350-PS/320-SiO₂ system where we can clearly observe the presence of a double peak centered at about 800 and 700 nm, indicating the realization of the heterostructure. Furthermore the position of the stop bands of the double layers HT matches the position of the stop band of each single layer.

This behaviour can be seen as just a superposition of the properties of each individual layer [19].

Moreover, we have verified that the approach used permits to obtain heterostructure systems based on the deposition of smaller NPs on bigger ones (HT_230-PS/320- SiO₂). In fact in Figure 4 we report the transmittance and reflectance spectra obtained on these heterostructure.

Analysing Figure 4 we can notice the presence of a double peak centered at about 550 and 700 nm. These peaks match the position of the stop band of each single layer (Figure 1) that compose the heterostructure; confirming that the method used permits to deposit colloidal crystals constituted by smaller NPs on bigger ones. Moreover we can affirm that the protocol developed permit to obtain an improvement in term of optical properties of the heterostructures realized respect to some of the recent results published by Piret and Khokhar [27,34].