Iridescence and Luminescence from Opal Matrices for Show Business

Abstract

The paper reports on obtaining visually appealing images from opal matrices to artificial samples comprising regular packing of monodisperse silica globules. We show the images of iridescence, photoluminescence, and both of them simultaneously, exciting upconversion luminescence of Er³⁺ ions from BaTiO₃ xerogel/opal matrix. Opal matrix with BaTiO₃ xerogel doped with Er³⁺ and Yb³⁺ ions demonstrates upconversion luminescence under excitation with the wavelength 980 nm of the laser with the main bands ranging from 500 to 570 nm and 640–700 nm, corresponding to the transitions from the excited states ²H_11/2, ⁴S_3/2, ⁴F_9/2, ⁴I_9/2 to the ground state ⁴I_15/2 of trivalent Er ions. In our view, the synthesis of opal matrices along with the generation of luminescent xerogels doped, for example, with trivalent lanthanides, is a promising approach for obtaining colorful images, always very individual and often very attractive, bringing joy and pleasure at concerts and other show business events.

1. Introduction

As was reported in 1968 by Sanders, “in gem quality opal the particles are remarkably uniform in size, and they were therefore able to pack together regularly to form a pseudo-crystal” [1]. The effect of iridescence of artificial opals is explained by the photonic band gap effect of 3D photonic crystals [2,3]. Photonic band gap is shifted for the opals impregnated with liquids or after synthesis of solids like xerogels in space between the globules [3,4,5]. Photonic band gap materials like opals or inverted opals influence spontaneous emission from the excited states of the embedded species [6,7].

Opal matrices are an intermediate product of opal synthesis; they are more porous and more fragile and about ten times cheaper compared to natural jewelry opals. Unlike natural or synthetic gem opals, opal matrices may not exhibit iridescence. However, opal matrices can exhibit iridescence in liquids, and the color of this type of photonic crystal can change. Recently we proposed to use the images of opal matrices for show and to create the color images using simultaneously the effects of iridescence and luminescence using opal matrices containing light-emitting species inside [8]. In this paper, we begin to describe several experiments that we conducted to create color images using the iridescence and luminescence effects of the opal matrix that contains luminescent xerogels inside. We also demonstrate how the user possessing several opal matrices can manipulate the creation of virtual color images using almost any available improvised experimental tools.

2. Materials and Methods

In the experiment 4 opal matrices produced in accordance with the patent [9] were used. These matrices were chosen in order to demonstrate iridescence and change in the color upon liquid impregnation as well as to observe room-temperature up-conversion luminescence after synthesis of Er- and Yb-doped BaTiO₃ xerogel. Ethanol was used for immersion of the opal matrices in order to reduce diffuse scattering of the media of silica globules and interglobular space of the opal matrices [3]. With the aim of obtaining color images, 4 samples were investigated, designated as samples #1, #2, #3, #4:

• Opal matrices without xerogel inside (as-grown) immersed in ethanol (samples #1 and #2).
• Opal matrix immersed once in sol corresponding to Er- and Yb-doped BaTiO₃ xerogel (BAT-sol) and annealed at 450 °C for 30 min (sample #3) and immersed in ethanol.
• Opal matrix with BaTiO₃ xerogel doped with Er and Yb (BaTiO₃:(Er,Yb) xerogel) inside and annealed at 600 °C (sample #4). This matrix was immersed in BAT-sol several times, followed by drying and annealing at 450 °C for 30 min after each immersion, then the matrix was finally annealed at 600 °C for 30 min.

To observe upconversion luminescence under excitation by a 980 nm laser line, a sol with an erbium to ytterbium ratio of 1 to 7 was prepared to obtain a BaTiO₃:(Er,Yb) xerogel, namely Ba_0.76Er_0.03Yb_0.21TiO₃, using the following procedure. Two solutions were prepared. Titanium isopropoxide (Ti(OC₃H₇)₄) (97%, Sigma-Aldrich, Steinheim, Germany) was dissolved in acetylacetone (CH₃COCH₂COCH₃) (analytical grade, AO Vekton, Saint Petersburg, Russia) and stirred until the solution cooled. Barium acetate (Ba(CH₃COO)₂) (ACS reagent 99%, Sigma-Aldrich, Steinheim, Germany) was separately dissolved in distilled water and stirred until completely dissolved. Erbium acetate hydrate (Er(CH₃COO)₃·xH₂O) (99.9% trace metal basis, Sigma-Aldrich, Milwaukee, WI, USA) was added to the barium acetate solution and stirred until completely dissolved. Then, ytterbium acetate hydrate (Yb(CH₃COO)₃·xH₂O) (99.95% trace metal basis, Sigma-Aldrich, Milwaukee, WI, USA) was added to the barium and erbium acetate solution and stirred until completely dissolved. Acetic acid (CH₃COOH) was added to the Ba, Er, and Yb acetate solution and stirred for 5 min. Then, solutions of titanium isopropoxide and Ba, Er, and Yb acetates were mixed and stirred for 5 min. Then, ethanol (C₂H₅OH) was added to this solution and stirred for 1.5 h. The amount of titanium isopropoxide, barium acetate, and erbium and ytterbium acetate hydrates was selected so that the Ti/(Ba + Er + Yb) ratio corresponded to the stoichiometric composition of barium titanate in the films (i.e., Ti:(Ba + Er + Yb) = 1:1).

The reflection spectra were measured on an MS122 spectrophotometer (PROSCAN Special Instruments, Minsk, Belarus).

Experiments with upconversion photoluminescence (PL spectra and images) were carried out under continuous wave (CW) optical excitation. A focused 980 nm laser beam of a 200 mW diode module was used for the excitation of the upconversion PL (power density J ~10 W/cm²) in the CW mode. The emission in the visible range was focused on the entrance slit of a 0.6 m grating spectrometer equipped with 1200 gr/mm gratings, and the PL intensity was measured using a R 9110 Hamamatsu photomultiplier tube (Hamamatsu photonics K.K., Hamamatsu City, Japan) that was sensitive in the spectral range of 200–850 nm. The spectral resolution of the PL measurement system was ~0.6 nm. A lock-in amplifier with mechanical chopping at a frequency of approximately 20 Hz was used for the signal recovery.

Images of opal matrices were obtained with the use of a microscope camera High Cloud (Shenzhen, China) with a 48-megapixel matrix, 4k (3840 × 2160) frame resolution, and a 130× lens.

3. Results and Discussion

Figure 1, Figure 2 and Figure 3 show images of opal matrices (samples #1, #2, and #3) obtained using only one white light source—a white LED flashlight from a mobile phone. As the opal matrix is impregnated with ethanol, diffuse scattering decreases and the sample becomes more transparent [3]. The white spot in the center of the sample outlines an area that is not yet impregnated and therefore less transparent. The shape of this unimpregnated area is an individuality of the opal matrix, as is the matrix itself. An example of impregnation with ethanol, illustrating the time scale information, is given for sample #1 by the Supplementary Materials S1, Video S1 ; the duration of the video is 11 s, and it is in real time. The images from (a) to (e) in Figure 1 were captured within two minutes.

Figure 2 demonstrates sample #2 impregnated with ethanol. This sample is impregnated completely, as is the sample in Figure 1e, unlike the sample shown in Figure 1a–d. The images from (a) to (e) differ from each other only because of the change in the angle of illumination and the distance from the light source to the sample manually. This sample displays a rich range of colors, and again we used only one light source–a white LED flashlight.

Figure 3 also illustrates the stages of impregnation in ethanol for sample #3. As impregnation occurs, the iridescence is coming. After impregnation in ethanol, the sample also demonstrates bright iridescence.

Application of other solvents that reduce diffuse scattering of opals or opal/xerogel structures is also possible; however, it should be noted that the photonic band gap position and color of the opal matrix depend on the refractive index of the solvent [3,5].

This sample #3, which was immersed in BAT-sol and annealed at 450 °C for 30 min, does not demonstrate upconversion luminescence. Noteworthy, both of the effects—upconversion luminescence and iridescence—were observed for sample #4 also immersed in BAT-sol followed by annealing at 600 °C for 30 min (Figure 4). At higher annealing temperatures or prolonged annealing for several hours, sintering of the xerogel/opal matrix structure can lead to the disappearance of the photonic stop band and iridescence, respectively [Supplementary Material S2]. The samples annealed at 450 and 600 °C were found to be X-ray-amorphous.

Yb³⁺ is known as an effective sensitizer to perform upconversion luminescence for Er³⁺ [10,11]. Upconversion in materials doped with Er³⁺ and Yb³⁺ ions occurs due to energy transfer between the lanthanide ions and is effective for the excitation wavelength of 980 nm due to the large absorption cross-section of ytterbium ions [10,11,12]. We may assume that the dominant mechanism of upconversion luminescence under 980 nm excitation for the BaTiO₃:(Er,Yb) xerogel in the opal matrix and onto the surface of the opal matrix is the absorption by Yb ions and the energy transfer from Yb³⁺ to Er³⁺ [12]. As a result, the luminescence bands corresponding to the transitions from the excited states ²H_11/2, ⁴S_3/2, ⁴F_9/2, ⁴I_9/2 to the ground state ⁴I_15/2 of trivalent Er ions are observed from PL spectra (Figure 5a), and upconversion luminescence is recorded by the camera (Figure 4). Schematic partial energy-level diagrams for the Er³⁺–Yb³⁺ couple demonstrating energy transfer between the lanthanide ions is given in Figure 5b.

It is worth noting that the camera also records scattered light of the 980 nm laser, adding orchid color to the image. Thus, the penetration path of the scattered light in the xerogel/opal structure also contributes to the individuality of the color image, and this depends on the defects in the opal matrix and the distribution of the xerogel in mesoscopic pores of the opal matrix. In more detail, the scattering of laser radiation in the opal/xerogel structure will also depend on the type of xerogel and its refractive index, i.e., by changing the composition of the sol and the heat treatment temperature, it is possible to obtain new color images with iridescence effects and registration of scattered laser radiation. Examples of these images are given in Figure 6.

Unlike samples #1 and #2, samples #3 and #4 have lower transmission. The decrease in transmission is due to both an increase in diffuse light scattering after the introduction of xerogel into the pores and partial sintering of the opal matrices during heat treatment. Nevertheless, opal matrices such as samples #3 and #4 retain their mesoporous structure, clusters with regular packing of the globules, photonic stop-band, and iridescence. For example, impregnation of sample #4 with ethanol leads to a red shift in the photonic band gap by 33 nm and an increase in its iridescence (Figure 7).

Significantly, controlled, short-term heat treatment at 600 °C allows for the observation of both iridescence and upconversion luminescence simultaneously. Thus, from our point of view, application of sol–gel synthesis in opal matrices is an intriguing method of producing color images for show effects. Without a doubt, inverted opal structures made of titania, vanadium oxide, barium titanate, and other materials [13,14,15,16], as well as opal matrices with liquid crystals [17] and photonic crystals with polarization-dependent reflectivity anisotropy [18], can also be used to create show effects using iridescence and luminescence simultaneously.

4. Conclusions

In conclusion, we report that inexpensive opal matrices allow beautiful multicolor images to be obtained using a very simple experimental setup. The iridescence of the initially opaque opal matrices is enhanced by impregnating them in ethanol, which makes the matrices more transparent by reducing diffuse scattering. The simultaneous recording of iridescence and scattered laser radiation enhances the visual appeal of opal matrix images when they are illuminated by a laser. The synthesis of xerogel in the mesoporous space between the globules of the opal matrix, in turn, makes the image of scattered laser radiation unpredictable and attractive. Finally, upconversion luminescence of trivalent erbium ions from the xerogel/opal structure adds additional color to the palette of the iridescent sample. Heat treatment at 600 °C for 30 min of BaTiO₃:(Er,Yb) xerogel/opal matrix allows for observation of iridescence and upconversion luminescence simultaneously. Opal matrix images as a video or slide show can be accompanied by music or vice versa. An example of a video is provided in Supplementary Materials S3, Video S2. Images of opal matrices that change during impregnation with liquids, when lighting changes, and when luminescence is excited can be used at concerts as footage to denote graphic content and color palette for the screen during the artists’ performance.