Properties of One-Dimensional Photonic Crystal Coatings

Enhancement of One-Dimensional Photonic Crystal Coating Properties Through Rare Earth Element Doping

1. Introduction

Photonic crystals are next-generation materials capable of controlling light, consisting of regularly arranged nanoscale materials with high and low dielectric constants [1]. Their potential to control light enables photonic crystals to be used in numerous photonic and optoelectronic fields such as photonic crystal lasers, optical switches, sensors, solar energy, and light-emitting diodes (LEDs) [2, 3]. Photonic crystals are divided into three basic groups according to their arrangement geometries: one-dimensional, two-dimensional, and three-dimensional [4, 5]. The simplest type of photonic crystal is the one-dimensional photonic crystal (1-D PC), in which dielectric contrast is created in only one direction [6]. The popularity of multilayer 1-D photonic crystals is increasing daily due to their advanced optical properties and they find applications in structural colors [7], Bragg mirrors [8], thermal collectors [9], thermal sensors [10], energy-saving spectrally selective coatings [11], transparent heat reflectors [12], optical filters [13], and anti-reflection side-view car mirrors, among many others [14]. Particularly in structural color applications, special high/low/high refractive index thin film layers are used to modulate reflectivity at specific wavelengths [15].

A specific bright color called structural color can only be obtained from multilayer films where the optical path length is close to the wavelength range of visible light.

In general, the working principle of paints or pigments is based on light-matter interaction that can absorb light only at specific wavelengths and reflect the remaining frequency bands of the spectrum, leading to transmission or reflection resonance peaks [16]. Structural colors were developed inspired by natural materials such as butterfly wings, seashells, feathers, and insect exoskeletons [3]. Humans have long benefited from nature's color-formation method. Some examples derived from nature are summarized in Armstrong and colleagues' work as shown in Figure 1 [5]. The famous Morpho butterfly, which naturally displays a dazzling blue color, has become one of the examples attracting the most attention from researchers. Another well-known example is the chameleon, which has the ability to rapidly change colors depending on changes in background or environment [16]. In their study, Khalil and Abbas demonstrated that one-dimensional CaF₂/TiO₂ photonic crystals can achieve desired wavelength ranges in optical resonators and micro-reflectors, which have high reflectance from the photonic bandgap (PBG) structure by selecting appropriate values of control parameters [17]. Yasuda and colleagues produced the TiO₂/SiO₂ multilayer stacks shown in Figure 2 [7]. As can be seen, different colors were obtained under reflection and transmission conditions [7]. Figure 1. Natural photonic crystals [5]; (a) Brilliant blue Morpho butterfly and scanning electron microscopy image [18], (b) multicolored peacock feather with scanning electron microscopy image of the blue region in the wing [19], (c) natural opal stone together with SEM images of its contained silica spheres [20], (d) male Sasakia Charonda butterfly wing and SEM image [21], (e) schematic representation of one-, two-, and three-dimensional photonic crystal structures [22].

Figure 2. Optical photographs of (a) reflection and (b) transmission obtained from multilayer SiO₂/TiO₂ surfaces [7]

Doping has become a notable research topic for next-generation materials due to its ability to modify material properties, particularly electronic and photonic characteristics [23]. Rare earth element doping is preferred over metal or transition metal doping to improve optical properties [24]. Currently, many studies on photonic crystal doping have generally focused on three-dimensional photonic crystals [25]. This study on one-dimensional photonic crystals doped with rare earth elements is significant from a literature perspective. In this work, in order to investigate the effect of rare earth element type and concentration doped into a one-dimensional SiO₂-TiO₂ photonic crystal system, samarium (Sm) and cerium (Ce) elements were added to the SiO₂ layers and characterization studies of the resulting coatings were performed.

2. Experimental

SiO₂-TiO₂ multilayer photonic crystals were coated onto borosilicate glass substrates using the sol-gel method and spin-coating technique with silicon- and titanium-based starting chemicals. For the production of pure TiO₂ and SiO₂ layers using the sol-gel method, titanium tetraisopropoxide (TTIP, Acros Organics) and tetraethyl orthosilicate (TEOS, Merck) starting chemicals were used. For Sm and Ce doping into the SiO₂ layer, samarium (III) oxide (Merck) and cerium(III) chloride hexahydrate (Merck) starting chemicals were used. Doping concentrations were examined at 0.50% and 1% molar relative to the Si amount. The SiO₂-TiO₂ one-dimensional photonic crystal was produced as a 7-layer structure and doping was performed only on the SiO₂ layers. The layered photonic crystal structure produced is shown schematically in Figure 3. Each of the SiO₂ and TiO₂ layers was subjected to drying at 110°C after the spin-coating process, and the final 7-layer structure was subjected to heat treatment at 500°C.

Figure 3. Schematic representation of one-dimensional photonic crystal production

Table 1. Nomenclature of produced samples according to doping element type and quantity

The phase structures of coatings produced by the sol-gel method were examined using an X-ray diffractometer (XRD, Rigaku D-max-2200 PC) at 40 kV and 36 mA using CuKα radiation. The chemical bonding structure of the coatings was analyzed using Fourier Transform Infrared Spectroscopy (FTIR, Perkin Elmer Spectrum BX) in the 650–4000 cm^-1 scanning range. Surface morphologies of the multilayer coatings were examined using scanning electron microscopy (SEM, JEOL-JSM 6060). The reflectance percentages of the coatings were measured using a Thermo Scientific Evolution 600 UV–VIS model device in the wavelength range of 350–800 nm. Structural colors of the coatings were imaged using an optical microscope (Nikon Eclipse ME600 model).

3. Results and Discussion 3.1. Structural Characterization

The production of doped and undoped multilayer photonic crystal coatings began with SiO₂ coating on glass substrate, and after production was completed at the determined number of layers, the produced sample was terminated with a SiO₂ layer. For this reason, phase analysis performed with XRD analysis was carried out on the SiO₂ layer and the obtained diffraction pattern is given in Figure 4. Based on the obtained diffraction pattern, it is observed that the doped and undoped SiO₂ layers have an amorphous structure. Obtaining the SiO₂ phase in crystalline form is possible with higher sintering temperature and duration; therefore, amorphous SiO₂ layers were obtained following the sintering process carried out at 550°C [26]. In XRD analysis performed on the surface of 6-layer production, the structure obtained was determined to be in the anatase (TiO₂) phase. As a result, the multilayer photonic crystal structures produced are observed to consist of amorphous SiO₂ and anatase-form TiO₂ phase.

Figure 4. XRD diffraction patterns of one-dimensional photonic crystal samples

The FT-IR spectra of the produced doped and undoped coatings are given in Figure 5. The peak observed at 2350 cm^-1 originates from CO₂ in the environment. The peak observed at 1073 cm^-1 belongs to the Si-O-Si asymmetric stretching vibration originating from the silica surface. Additionally, the peak found at 803 cm^-1 corresponds to symmetric stretching vibrations of O-Si-O [27]. The peak observed at 925 cm^-1 wavelength is thought to originate from the stretching vibration band of Ti–O–Si bonds. SiO₂ and TiO₂ layers can be bonded to each other in two different ways: physically and chemically. Chemical bonding occurs through this Ti-O-Si bond [28]. Upon examination of the results, it is observed that increasing doping amount causes a decrease in the peak intensity thought to correspond to the Ti-O-Si bond.

Figure 5. FT-IR spectra of produced coatings

3.2 Morphological Characterization

The surface morphologies of undoped, Sm-doped, and Ce-doped one-dimensional photonic crystal coatings are given in Figure 6. It is observed that the surface morphologies of the coatings show changes with the doping of Sm and Ce. Compared with undoped coatings, increasing Ce doping results in crack islands becoming smaller and distributed homogeneously. Additionally, it is observed that Sm doping reduces surface cracks.

Figure 6. Surface morphology images of (a) undoped, (b) 0.50% Ce, (c) 1% Ce, (d) 0.50% Sm, and (e) 1% Sm doped coatings

As a result, it is observed that Ce and Sm dopant elements generally contribute positively to the surface morphology of the coatings and that with increasing doping ratio for both dopant elements, the crack formation appearing in the coating structure is reduced. Upon examination of the literature, it is thought that the change appearing on the surfaces of high-doping-ratio coatings results from an increase in nucleation centers, which is a consequence of a reduction in nucleation energy barrier due to the presence of Sm and Ce dopant elements [29].

3.3 Optical Characterization

The optical images of undoped and doped one-dimensional photonic crystals are given in Figure 7. According to the images, the doped coatings show differences compared with the undoped coatings. No distinct coloration was observed on the undoped coatings. On the other hand, distinct coloration is observed in Ce- and Sm-doped coatings. The color vibrancy and diversity on the coating surfaces of Sm-doped samples increase with increasing Sm amount (Figure 7. b–7.e). However, yellow, green, and blue colors appear very vividly on the surfaces of 1% Sm-doped coatings. Furthermore, compared with Sm doping, Ce doping can be stated to be more effective in terms of color vibrancy. Particularly, 0.50% and 1% Ce-doped one-dimensional photonic crystal coatings are observed to have vibrant colors over larger areas. While many colors with yellow, pink, and red predominance are seen together in the 0.50% Ce-doped coating, blue, purple, and green colors are dominant in the 1% Ce-doped C100 coating. The obtaining of different colors from a single angle of measurement is a property of photonic crystal materials. Furthermore, it is understood that both Sm and Ce dopant elements improve the photonic crystal properties of the coatings.

Figure 7. Optical microscope images of coatings (a) undoped, (b) 0.50% Sm, (c) 1% Sm, (d) 0.50% Ce, (e) 1% Ce doped

Conclusion

The production of photonic crystal coatings with layers in SiO₂ and TiO₂ phases was successfully accomplished using the sol-gel method. According to the XRD analysis results, the obtained one-dimensional photonic crystal structure consists of amorphous silica and anatase layers. Based on FT-IR analysis, the Si–O–Si asymmetric stretching vibration and O-Si-O symmetric stretching vibrations are in good agreement with the silica surface. The Ti-O-Si stretching vibration band proved the existence of a chemical bond between layers in addition to physical bonding. Among the coating layers, Sm and Ce elements were doped into the SiO₂ layer. The obtained results show that increasing the doping ratios of Sm and Ce elements reduced crack formation and improved surface morphologies. Both dopant elements enhanced the photonic crystal property of the obtained structure by increasing color diversity and vibrancy.

Acknowledgments

We thank Dokuz Eylül University Department of Metallurgical and Materials Engineering and the Electronic Materials Production and Application Center for their valuable contributions. We also thank Sami Orçun Kortunay, Mehmet Furkan Çakıcı, and Dilay Tümen for their contributions to the work. Ramazan Dalmış Dokuz Eylül University Institute of Science Faculty of Engineering Metallurgical and Materials Engineering     N. Funda Ak Azem Dokuz Eylül University Institute of Science Faculty of Engineering Metallurgical and Materials Engineering Nanoscience and Nanoengineering     Işıl Birlik Dokuz Eylül University Institute of Science Faculty of Engineering Metallurgical and Materials Engineering Nanoscience and Nanoengineering     Erdal Çelik Council of Higher Education Bilkent      

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