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Self-Assembled Photonic Crystals of Monodisperse Dendritic Fibrous Nanosilica for Lasing: Role of Fiber Density Ayan Maity, Sushil Mujumdar, and Vivek Polshettiwar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04732 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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ACS Applied Materials & Interfaces

Self-Assembled Photonic Crystals of Monodisperse Dendritic Fibrous Nanosilica for Lasing: Role of Fiber Density Ayan Maitya, Sushil Mujumdar,b* Vivek Polshettiwara* a

Department of Chemical Sciences, Tata Institute of Fundamental Research (TIFR), Mumbai, India. Email: [email protected] Nano-Optics and Mesoscopic Optics Laboratory, Tata Institute of Fundamental Research (TIFR), Mumbai, India. Email: [email protected]

b

ABSTRACT: Photonic crystals are essentially a periodic (‘crystalline’) arrangement of dielectric nanoparticles that respond in unison to incident light. They can be used to harvest light in various applications such as photocatalysis, solar cells and lasing. In this work, we prepared photonic crystals of dendritic fibrous nanosilica (DFNS), by their self-assembly. Due to the narrow particle size distribution of the assynthesized DFNS, they readily formed colored photonic crystals. The photonic band gap was found to be tunable by using DFNS of various sizes, and fiber density. Notably, even after having similar particle sizes (but with different fiber density), they showed different photonic band gaps, indicating that the fiber density plays a role in the band gap of photonic crystals. Such observations are not reported before. This can be arising from the difference in their refractive index due to difference in their fiber density and hence variation in silica content, leading to a different optical signature. Keywords: Self-assembly, photonic crystals, lasing, refractive index, dendritic fibrous nanosilica and fiber density.

Introduction: Photonic crystals are essentially a periodic (‘crystalline’) arrangement of dielectric nanoparticles that respond in unison to incident light, allows controlling and manipulating light flow.1-9 The partial reflection from the various Bragg planes formed by the nanoparticles forbids certain frequencies from existing in the crystal, thus realizing the bandgap. They are being used in range of interesting applications including optics, coatings, and even in photocatalysis and solar cells.1-9 In photonic crystals (PhC), the wave propagation gets affected when it enters into a material, by various properties of that material. The behavior of a photon with a certain frequency will depend on the propagation direction within the photonic crystals. The modulation of the refractive index will cause that certain energies and directions are forbidden for photons. This region of energies where the photonic crystal does not allow photons to propagate irrespective of their direction and polarization is called a photonic band gap. The band gap of the photonic crystals of self-assembled nanoparticles is mainly depend upon the diameter of the nanoparticles of specific refractive index. In turn, the range of lasing wavelengths is determined by the diameter and the refractive index of nanoparticles together. However, the tuning the refractive index of particle without changing its diameters has not been entirely realized in the field of self-assembled photonic crystals and their lasing. Such tunability can make more unconventional ranges of lasing wavelengths accessible. Photonic crystal lasers exploit the properties of the bandgap to achieve lasing.10-22 Essentially, spontaneous emission is inhibited in the bandgap region, realizing easier population inversion in an amplifying medium. When the emission spectrum of the amplifying medium matches the bandgap, efficient lasing is obtained. In general, PhC-based dye-lasers have been fabricated using a bottom-up self-assembly of nanoparticles, such as spherical polymeric or silica spheres. However, due to their low surface area, the loading of dye is low, which decreases the active surface concentration, and that can be overcome by using high-surface-area silica spheres.

In this work, we showed tuning of refractive index by tuning the fiber density (number of fibers in a sphere) of dendritic fibrous nanosilica (DFNS)23-26 without changing its particle size. This allowed tuning of PhC bandgap and then their lasing behavior. DFNS possesses a unique fibrous morphology with high surface area due to its fibers (thin sheets). We have successfully used DFNS in various applications, such as nanocatalysis, photocatalysis and CO2 capture.23-26 Since our discovery, DFNS has been used worldwide in many applications, such as catalysis, energy harvesting and storage, coatings, sensors, CO2 capture-conversion, and biomedical applications.27 There are no other silica spheres with such a fibrous morphology, and hence such a PhC bandgap tuning was not possible before. It was also not reported before for the photonic crystals of solid silica or polymer spheres. Another important point that can be foreseen for DFNS photonic crystals is that their fibrous structure will sustain a unique light intensity distribution within the particle, and therefore, within the crystal, enhancing their lasing efficiency. In addition, fibrous structure of high surface area DFNS provides an open morphology, unlike in mesoporous particles or solid spheres. This may allow better loading of the dye in DFNS, and hence enhanced lasing, whereas conventional materials suffer the poor to moderate loading of the dyes due to low surface area and/or pore blocking.

Results and Discussion: Synthesis of DFNS Photonic Crystals: First, we prepared photonic crystals of (DFNS) and study the effects of size and fiber density of DFNS on the photonic crystal properties, particularly the photonic band gap. DFNS with varying sizes and fiber density was prepared using reported protocol by varying reaction time.28 Reaction time of 6, 9 and 12 h yielded DFNS-6, DFNS-9 and DFNS-12 with particle size of 260 nm, 311 nm and 297 nm respectively. DFNS-9 and DFNS-12 has approximately same particle size, but difference in their fiber density. The

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fiber density (number of fibers/sheets within one sphere) was more in DFNS-12 as compare to DFNS-9.28 SEM images of the photonic crystals prepared using these DFNS via a vertical deposition technique demonstrated the formation of good quality photonic crystals (PhC) (Figure 3). The images indicate that the packing efficiency is dependent on the fiber density as well as the degree of monodispersity. DFNS-6 and DFNS-9 exhibit a narrow particle size distribution arranged in a perfect manner (Figure 1a, b). The DFNS-9 exhibits a mild imperfection in the packing of the internal layers, if seen from the side view DFNS-12 exhibits a moderate particle size distribution showing several surface defects due to poor surface packing (Figure 1c), although with good internal packing (side view).

Optical Studies of DFNS Photonic Crystals: The initial optical characterization of the PhC samples was carried out in air using standard UV-Vis spectrophotometer, in a transmission (T) mode. Silica has zero absorption in the visible and hence in absence of absorption, the reflectance (R) was taken as 1-T. This measurement merely endorsed the wavelength selectivity of the samples and for more detailed angle dependent measurements (in air and methanol), new setup was made separately (Figure S1). The glass slide with the PhC sample was held vertical on an anglevariable mount. Illumination was provided by a collimated white

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light source (Xenon lamp), and the detection was done by a mounted multimode (core diameter 200 um) optical fiber. The relative angular position between the sample and the optical fiber was kept constant. Thus, as the sample & optical fiber together was angularly moved with respect to the source beam, and the change in reflectance (wavelength) was measured. Notably, from the UV-Vis spectra (Figure 2), a correlation between the particle size of DFNS and the reflection maxima due to the photonic band gap (a frequency band for which light propagation is forbidden or suppressed) was observed. DFNS-6 with average particle size of 260 nm showed λmax at 416 nm, while DFNS-9 and DFNS-12 with average particle sizes of 311 and 297 nm showed λmax at 566 and 497 nm, respectively (Figure 2). Thus, the photonic band gaps can be tuned by tuning the particle size of the DFNS and can exist in the visible light range (Figure 2). Notably, DFNS-9 and DFNS-12 have very similar particle sizes and distributions, but their photonic band gaps are significantly different, indicating that the fiber density plays a role in the band gap of photonic crystals, which is observed for the first time. This effect can be intuitively understood as arising from the effective refractive index of the nanoparticles. With changes in the fiber density, the silica content in the nanoparticle changes, leading to a different optical signature and, hence, forming a different photonic band gap.

Figure 1. SEM images of photonic crystals of various DFNS, a) DFNS-6 b) DFNS-9 c) DFNS-12. 2


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Figure 2. UV-VIS spectra of photonic crystals of DFNS-6, DFNS9 and DFNS-12, exhibiting emission maxima at λmax = 416 nm, 566 nm and 497 nm, respectively.

Lasing by Self-Assembled DFNS Photonic Crystals: We then hypothesized that, due to the unique fibrous morphology and large surface area, photonic crystals of DFNS may exhibit an enhanced lasing action over conventional photonic crystals.10-22 An important signature of a photonic crystal bandgap is the angle dependent reflectance from the crystal. When broadband light is

incident onto the surface of the crystal, the bandgap wavelengths show reflectance depending on the angle of the reflected light with respect to the surface normal. To measure the reflectance, we employed a broadband source (Ocean Optics HL-2000 Tungsten Halogen light source) that illuminated the various PhC samples, and a fiber optic probe was used to collect the reflectance that was analyzed by a spectrometer (Ocean optics, 200 nm – 1100 nm). The input end of the probe was mounted on an angle-variable mount, which enabled us to measure the reflectance as a function of the angle. The results are exhibited in Figure 3 wherein subplots a, b and c depict measurements for the DFNS-6, DFNS-9 and DFNS-12 photonic crystals, respectively. Figure 3a1 illustrates various spectra measured at the angles listed in the legend. The spectrum varies strongly with the reflected angle, with a drop in the peak emission wavelength λmax, which is a signature of the photonic bandgap effect. The bandwidth also varies with angle. The systematic drop of λmax with angle Ɵ (Figure3a2) shows a variation of ~45 nm (380 nm to 425 nm) over an angle variation of ~25 degrees. The bandgap clearly lies in the blue region of the visible spectrum. Figure 3b1, b2 and c1, c2 depict the same for the samples DFNS-9 and DFNS12, respectively. For these two samples, their bandgaps exist over the green-yellow range (520 nm – 600 nm) and the blue-green range (435 nm – 510 nm), respectively.

Figure 3. Angle-dependent reflectance of a) DFNS-6, b) DFNS-9, and c) DFNS-12 photonic crystals in air (a1-a2, b1-b2, c1-c2) and in methanol (a3-a4, b3-b4, c3-c4). 3



Angle-dependent reflectance study was carried out using dry samples of DFNS photonic crystals. To achieve lasing, an appropriate gain medium needs to be doped into the photonic crystal. An effective lasing medium is a laser dye, which, when dissolved in an alcoholic solvent, exhibits a quantum efficiency of the order of 90% or more. We chose Rhodamine 6G dye dissolved in methanol as the lasing medium. The emission spectrum of Rhodamine 6G in methanol is centered at 565 nm, with a typical bandwidth of ~40 nm. To account for the effect of the alcoholic solvent, the photonic crystal first needs to be characterized in the presence of the solvent. To that end, the three crystals were doped with methanol first, and their bandgaps were subsequently characterized. Figure 3a3 and a4 depict the angle-dependent reflectance measurements for DFNS-6 in methanol, and a notable change in the bandgap was seen, where the bandgap redshifted to the blue-green range (λmax ~ 485 nm – 515 nm). At some angles, the reflectance approached 540 nm, which is in the vicinity of the Rhodamine emission wavelength but not overlapping. Hence, some Bragg feedback can exist, but only sub-optimally. Similarly, DFNS-9 in methanol exhibits a bandgap in the red region (600 nm – 680 nm), and here too, a minor overlap is seen with Rhodamine wavelengths at certain angles. Hence, weak feedback can also be expected in this crystal. Finally, DFNS12 shows a bandgap in the orange red wavelengths (540 nm-620 nm), and it is exactly centered over the desired range of wavelengths. Therefore, the last crystal has the most appropriate band structure for lasing. Importantly, the clean monotonic linear decay seen in figure 3c2 implies that the bandgap effect is still strong and overrides any deficiencies due to the visible defects seen in Figure 1, a fact that bodes well for lasing. The lasing studies were carried out on the photonic crystals doped with a 5 mM solution of Rhodamine 6G in methanol. A frequencydoubled NdYAG laser (pulse width ~5 ns, repetition rate 10 Hz) was made incident onto the sample as the excitation beam, and the fiber optic probe was used for the spectral studies, as shown earlier. Figure 4 summarizes the results. Figure 4a depicts the observed spectra at two pump powers. At an excitation of ~0.011 W/cm2, the

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spectrum has a low-amplitude and large bandwidth, typical of the dye fluorescence. At approximately 0.042 W/cm2, the spectral peak is strongly enhanced by about a factor of 10, while the bandwidth is narrowed significantly, thus showing lasing behavior. The inset compares the normalized spectra illustrating the bandwidth collapse. A systematic study of the peak intensity and the bandwidth as function of pump power is depicted in figure 4b, where a clear lasing threshold is observed at 0.014 W/cm2. At the threshold, the peak intensity diverges, and a maximum decrease in the bandwidth occurs at values near the pump power. Thus, the photonic crystal DFNS-12 provides lasing obtained from the efficient inversion, as can be seen from the spectral overlap of the emission peak and the bandgap region (Figure 4c). Figure 4d exhibits the behavior of the three crystals at a comparable pump power. At a pump power of 0.033 W/cm2, the DFNS-12 lases with a high output intensity. In comparison, the other two photonic crystals provide only a limited gain of less than half the peak output, at an input power of 0.034 W/cm2and 0.036 W/cm2 respectively. The diminished feedback due to spectral mismatch in the bandgap region and the emission band of Rhodamine results fail to take the laser above the threshold. Figure 5 illustrates the behavior of DFNS-6 and DFNS-9 in subplots (a) and (b), respectively. In the former, the peak intensity shows a gradual rise, in contrast to a sharp threshold. In the latter, a threshold is seen, but at an increased pump power of ~ 0.042 W/cm2. A comparative study is shown in figure 5c, which graphically illustrates the threshold power (blue bars, left Y axis) and the minimum bandwidth (red bars, right Y axis) of lasing. Clearly, DFNS-12 has the lowest threshold and smallest bandwidth. Both these characteristics point to the efficient feedback realized in DFNS-12 compared to that in the other two photonic crystals, on virtue of the maximal overlap of the bandgap with the Rhodamine gain profile. As described earlier, DFNS-9 and DFNS-12 have comparable sizes but different refractive indices due to different fiber density, thus leading to different lasing properties. This study, thus, underlines the unique way of the tunability of refractive index and in turn their lasing properties, by simply controlling their fiber density.

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Figure 4. Lasing properties of the DFNS-12 crystal: (a) the blue line shows the fluorescence spectrum of the dye-doped crystal at a pump power of 0.011 W/cm2 compared to the lasing emission (red line) at 0.044 W/cm2. The inset shows the normalized spectra to emphasize the spectral narrowing. (b) Lasing threshold identification by observing the peak intensity (blue triangles, left Y-axis) and FWHM (red triangles, right Y-axis) as a function of pump power, showing a threshold at 0.014 W/cm2 (power dependent spectrums are given in figure S5); (c) Overlap of the lasing peak (blue markers) and the bandgap region (black line) of the crystal. (d) Comparison of the emission at the same pump power (~0.035 W/cm2) for DFNS-12 (green line), DFNS-9 (black line) and DFNS-6 (red line). Inset shows the comparison of the bandwidth. Fabry-Perot resonator feedback mechanism was ruled out from the emission dynamics (Figure S2, S3).

Figure 5. Lasing behavior and threshold identification of (a) DFNS-6 and (b) DFNS-9, showing a higher threshold power due to diminished feedback. A comparative chart for the threshold and FWHM is shown in (c). 5



The FWHM obtained in our lasing studies is about 7-8 nm. The absolute narrowed bandwidth depend upon the efficiency of the feedback, in other words, the effective quality factor of an equivalent resonator. In our case, although the strength of DFNS lies in the tunability of the refractive index, its refractive index is lesser than solid silica spheres. Consequently, the quality factors of these crystals limits the final lasing bandwidth. If high narrowing is a desired goal of the experiments, then techniques such as coating of high-index materials on DFNS can be attempted. However, our aim here is not the best narrowed lasing, but more a proof-of-principle lasing that is tunable with DFNS fiber density. Figure S4 shows the behavior of lasing intensity with emission angle for the three DFNS PhC samples. As can be seen from figure S4, the emission intensity is maximum at 26 degrees for the DFNS12, and rapidly falls off that angle. In comparison, the other two samples provide lesser emission intensity, and the variation with angle is also weaker, due to the lack of the participation of the bandgap in the emission dynamics. Nonetheless, the sharp dependence of intensity on angle endorses the origin of lasing intensity from the photonic bandgap.

Size and Fiber Density Dependent Refractive Index Estimation of DFNS: Furthermore, an added advantage of the photonic crystal analysis is that it provides a means to estimate the effective refractive index of the crystal and, hence, even for the individual DFNS nanoparticles, which is otherwise not possible. The Bragg-Snell law that determines the wavelength of the reflectance peak is written as 4 ŋ Ɵ, where d is the separation in the Bragg planes, Ɵ is the angle between the reflected light and the normal to the Bragg plane, and ŋ is the effective refractive index of the photonic crystal medium. Thus, a plot of vs Ɵ will show a linear behavior, whose slope and intercept will provide both d and ŋ . Figure 6 (a1, b1, c1) shows the relevant vs Ɵ plots for the photonic crystals of DFNS-6, DFNS-9 and DFNS-12, respectively. The data clearly reveal the expected linear profile, to which the red lines show the best fit. Simultaneously, the effective refractive index of a bi-component medium is also derived from the individual volume fractions and their individual refractive indices, as ŋ ŋ ŋ , such that + = 1. Considering the packing fraction as 0.74 for an FCC crystal and the refractive index ŋ and fraction for air as 1 and 0.26, respectively, we obtain the refractive index of an individual nanospheres of DFNS. The refractive indices for the three nanospheres were found to be 1.30±0.03 (DFNS-6), 1.34±0.04 (DFNS-9) and 1.24±0.05 (DFNS-12). These measurements enable us to estimate the actual silica content in a single nanosphere from the equation ŋ ŋ ŋ , applied to a single sphere. Accordingly, DFNS-6. DFNS-9 and DFNS-12 are measured as having a silica content of 51±7%, 67±9% and 44.5±9.5%, respectively. This calculation allowed us to estimate the refractive index and silica contents of individual DFNS particles, which can be used to

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quantify fiber density of these spheres that was until now qualitative estimation from SEM/TEM images. From the SEM images of PhC of DFNS-9 (Fig. 3), it is observed that the internal packing of the crystal is moderate and hence value of 0.74 for DFNS-9 may not be exactly correct. Because of this, the silica contents for DFNS-9 is overestimated. We attempted to minimize the errors in silica content calculations, by normalizing them with their BJH adsorption pore volume value obtained from N2 sorption measurement. DFNS-12 has pore volume is 0.898 cm3/g while DFNS-9 has 0.791 cm3/g. Ratio of these pore volume suggests DFNS-12 has 1.14 times more empty space compare to DFNS-9. Since for DFNS-12, silica content was 44.5±9.5% with empty space of 55.5±9.5%, using pore volume ratio, DFNS-9 silica content can be 51.3±8.3% with empty space of 48.7±8.3%.

Figure 6. Plot of vs Ɵ of photonic crystals in air and TEM image of the DFNS: a) DFNS-6, b) DFNS-9, and c) DFNS-12.

Conclusions: Due to the narrow particle size distribution of the as-synthesized DFNS, they readily formed colored photonic crystals. The photonic band gap was found to be tunable by using DFNS of various sizes, and fiber density. Notably, even after having similar particle sizes (but different fiber density), DFNS-9 and DFNS-12 had different photonic band gaps, indicating that the fiber density plays a role in the band gap of photonics crystals. Such observations are not reported before. This can be arising from the difference in their refractive index due to difference in their fiber density and hence variation in silica content, leading to a different optical signature. Photonic Crystals of DFNS have shown unique lasing action. The photonic crystal DFNS-12 provides highest lasing efficiency compared to other two photonic crystals, due the maximal overlap of its bandgap with the Rhodamine 6G gain profile. Photonic crystals are generally prepared using solid spheres of polymer of silica, which 6


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limits the dye loading due to their moderate surface area, while due to high surface area of DFNS, one can load large amount of dye. Notably, DFNS-9 and DFNS-12 have comparable sizes but different refractive indices due to different fiber density, which lead to different lasing properties. Fibrous structure of high surface area DFNS provides an open morphology, allowing better loading of the dye in DFNS, whereas conventional materials (solid spheres) suffer from the poor to moderate loading of the dyes due to low surface area and/or pore blocking. This, this study thus underlines the unique way of the tunability of refractive index and in turn their lasing properties, by simply controlling their fiber density, for the first time.

Experimental: Synthesis of DFNS Photonic Crystals Photonic crystals of DFNS were prepared using a vertical deposition technique, which is based on the evaporation of the liquid forcing the particles to arrange in the meniscus formed between a vertical substrate, the suspension and air. In a typical procedure, DFNS (50 mg) was suspended in 10 mL of water in a small glass beaker and sonicated for 2 h. 1.5 mL of this suspension was taken in another 6 mL beaker and diluted with 3.5 mL water, in which a clean glass slide was kept vertically tilted and covered with a glass lid. The closed glass breaker was then heated in an oven at 80 °C for 3 days to grow the photonic crystals on the glass slide. Lasing Study The detailed experimental set up for the lasing study is given in the supporting information (Figure S6). 5 mM solution of Rhodamine 6G

in methanol was added into the PhC and then the lasing experiments were performed. A frequency-doubled NdYAG laser (pulse width ~5 ns, repetition rate 10 Hz) used as the excitation beam, and 100 µm multimode fiber optic probe was used for the spectral studies. Laser spot size of 3 mm was maintained throughout the study. Supporting Information. Supporting information mentioned in the MS is included. Experimental Set up for angle dependent reflectance and Lasing measurement, Fabry Perot computed spectrum, Intensity vs angle dependency study, Power dependent lasing spectrum.

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Acknowledgements

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We thank the Department of Atomic Energy (DAE), Government of India, for funding. We also acknowledge the use of EM facilities of TIFR, Mumbai. SM acknowledges support from the Swarnajayanti Fellowship of the DST, India.

22. Wang, J.; Hu, J.; Wen, Y.; Song Y. and Jiang, L. Hydrogen-BondingDriven Wettability Change of Colloidal Crystal Films: From Superhydrophobicity to Superhydrophilicity. Chem. Mater. 2006, 18, 4984-4986. 23. Polshettiwar, V.; Cha, D.; Zhang, X.; Basset, J. M. High Surface Area Silica Nanospheres (KCC-1) with Fibrous Morphology. Angew. Chem. Int. Ed. 2010, 49, 9652-9656.

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26. Maity, A.; Polshettiwar, V. Dendritic Fibrous Nanosilica for Catalysis, Energy Harvesting, Carbon Dioxide Mitigation, Drug Delivery, and Sensing. ChemSusChem, 2017, 10, 3866-3913. 27. Maity, A.; Das, A.; Sen, D.; Mazumder, S.; Polshettiwar, V. Unraveling the Formation Mechanism of Dendritic Fibrous Nanosilica. Langmuir, 2017, 33, 13774-13782. 28. Maity, A.; Polshettiwar, V. Scalable and Sustainable Synthesis of Size Controlled Monodisperse DFNS Quantified by E-Factor. ACS Appl. Nano Mater, 2018, accepted.

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