The fabrication of photonic crystal heterostructures is important for the applications in the fields of integrated photonic crystal chips and multi-frequency optical Bragg filters or mirrors; here we report on the fabrication and characterization of opal-based photonic crystal heterostructures. These heterostructures are created by using multilayer deposition of silica and polystyrene spheres. In the specific the fabricated structures involved both different lattice constant and dielectric function. Scanning electron microscopy (SEM) and VIS-NIR transmittance and reflectance spectroscopy are used to characterize the systems. The SEM images show good ordering of the two-layer colloidal crystals constituting the heterostructures. The transmittance and reflectance spectra measured from the (111) plane of the heterostructure show that the composite colloidal photonic crystals have double photonic stop bands that matches the stop bands of the individual photonic crystals. This behaviour can be seen as a superposition of the properties of each individual layer.
Photonic crystals have attracted great interests since the pioneering works of Yablonovitch and John [1,2]. These materials have the feature of the photonic bandgap (PBG) that forbids the propagation of the light in a particular range of frequencies through the structure. This property is due to the periodic variation of the refractive index of the crystals in one, two or three dimensions [3-5]. They can be used in several applications, such as optical switches [
Although top-down methods such as nanolithography, holography and direct laser writing can successfully design 3D PC, the high cost and the complex process have led to the development of efficient bottom-up techniques based on the colloidal crystals consisting of highly organized monodispersed colloidal particles [13-16]. It has been demonstrated from a theoretical point of view [
From a technological point of view several deposition techniques have been used for the fabrication of two-layer opal films structure such as the Langmuir Blodgett (LB) technique [
In this paper we report on the synthesis of monosize silica and polystyrene particles of different dimensions and we describe a suitable deposition protocol based on convective assembly that permits to grow on a silica opal different polystyrene (PS) colloidal crystals. Morphological and optical assessment of the systems realized is discussed.
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 [
The position in term of wavelength of the pseudo band gap, at normal incidence, can be expressed by a modified Bragg’s Law [
where d111 = 0.816·D is the interplanar spacing of the (111) planes and D corresponds to the diameter of the nanoparticles; the neff is the effective refractive index that can be considered as follows [
where nspheres corresponds to the refractive index of the NPs used, nmedium 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
Moreover from
In
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 [
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
Analysing
We have demonstrated a suitable protocol for the realization of opal photonic crystal heterostructures based on multilayer deposition of silica and polystyrene colloidal films. This method permits to deposit on a silica opal, polymeric colloidal crystals with different lattice constant. In fact SEM images performed on the two HTs realized (HT_350-PS/320-SiO2 and HT_230-PS/320- SiO2) have shown a good ordering of the two-layer colloidal crystals and large domains for the top layer. The double photonic stop bands observed in the reflectance and transmission measurements confirm the formation of the HT system. Moreover the position of the stop bands of heterostructures matches the position of the stop band of single layers; this behaviour can be seen as just a superposition of the properties of each individual layer. Further investigations are in progress in order to minimize the effect of the roughness at interface that is responsible of the reduction of the optical quality of the heterostructures compared with each single opal system.
This research was performed in the framework of the CARITRO project.