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Activities

The defining objective of this two-year PRF grant was to develop photonic crystals with a complete band gap in the visible and study their potential for efficient solar energy conversion. Year 1 focused on 1) identification of natural photonic structures, occurring in exoskeleton scales of colored beetles, 2) photonic band structure calculations to explore the potential of these structures for modifying the energy distribution of optical excitations, 3) development of synthetic strategies for converting these biological structures into high-dielectric, semiconducting materials. The activities of year 2 built upon the findings and results of year 1 and were directed towards 1) structural and optical characterization of high-dielectric bio-templated photonic crystals and 2) studying excited state dynamics of light harvesters (quantum dots) placed into these photonic crystals.

Findings

In year 1 we discovered that the weevil Lamprocyphus augustus obtains its green coloration from a diamond-based photonic crystal lattice, the champion of photonic structures. Interestingly, our calculations revealed that this structure with a reduced high-dielectric volume fraction (30-40%) would open a complete photonic band gap at visible frequencies—the first of its kind—when prepared out of titanium dioxide (titania) with a refractive index of at least 2.1. To transform the bio-polymeric structure into high-dielectric replica, we developed a double-imprint sol-gel bio-templating route via a sacrificial silica intermediary.

Figure 1. a and b) SEM cross-sectional view of the bio-templated diamond-based titania photonic crystals. c) Corresponding calculated band structure diagram. The complete PBG is indicated by a gray rectangle. Inset: Calculated dielectric function showing three orthogonal planes (air: gray; high-dielectric: green).

Initial year 2 activities focused on optimizing the bio-templating parameters. Particular emphasis was focused on 1) minimizing shrinkage of the overall structure, 2) significantly reducing the high-dielectric volume fraction from 50-60% (beetle structure) to 30-40%, and 3) optimizing the crystallinity/density of the titania framework to achieve the required refractive index (>2.1). In short, shrinkage was minimized by fabricating the silica intermediary out of hybrid SBA-type silica and removal of the bio-polymeric template by acid-etching and the titania volume fraction and refractive index were optimized by repeated infiltration-crystallization cycles. Figure 1a and b display SEM images of titania replicas and show that the original beetles diamond-based structure—a lattice of ABC stacked layers of hexagonally ordered air cylinders in a surrounding high-dielectric matrix—was excellently preserved. We found an average lattice constant of 366 ± 24 nm and volume fractions between 30 and 40%. The corresponding calculated band structure diagram is shown in Figure 1c and reveals a 5% wide complete photonic band gap!

To experimentally confirm the presence of a complete photonic band gap, we studied the angle-dependent optical reflectance behavior of our samples by collecting a series of reflectance spectra covering a 30° angle (Figure 2). The obtained series of intensity normalized spectra (inset Figure 2) displayed no significant dependence of the reflectance peak position on the recording-angle, confirming our calculations of opening of a complete band gap. The slightly off-center location of the calculated overlap of all directional stop gaps is grounded in the Γ-L direction. While this direction constitutes the high-energy limit of the complete photonic band gap, its orientation within the pixilated multi-domain organization is mostly parallel to the scale's top-surface, thus contributing to the measured overall reflectance intensity only at higher collection angles (red spectrum in Figure 2).

Figure 2. Angle-dependent reflectance measurements of a titania replicated beetle photonic structure recorded over a 30° angular range; normal incidence (black), –15° off-normal (blue), and +15° off-normal (red). The gray vertical bar shows the position and width of the calculated complete PBG. Inset: Intensity-normalized angle-dependent reflection spectra.

The experimental realization of a complete photonic band gap at visible frequencies opens new avenues in optical and energy research. One of the most interesting properties of photonic band gap crystals is their ability to strongly modify the photonic density of states. Importantly, the dynamics of fundamental radiative transitions, such as absorption and spontaneous emission, are directly proportional to the density of states. Given the pivotal role of these fundamental processes in solar energy conversion and photocatalysis, photonic band gap materials are prime candidates for enhancing solar conversion efficiency. We tested the ability of our photonic band gap materials to modify the photonic density of states by evaluating the dynamics of spontaneous emission from incorporated core-shell CdSe/ZnS nanocrystal quantum dot light sources. We performed excited state decay rate (lifetime) studies of quantum dot light sources emitting within the band gap, at the edge of the band gap and far outside (at the low energy side) of the band gap. First results of such studies are given in Figure 3 and clearly show the enormous impact of our photonic band gap crystals on the dynamics of spontaneous emission. While quantum dots emitting far outside the photonic band gap have an excited state lifetime of around 15 ns (a typical value for CdSe/ZnS nanocrystals on titania), we observed significant changes for quantum dots emitting within or close to the band gap. Emission dynamics of quantum dots at the edge of the band gap show enhancement behavior with lifetimes as short as 6 ns. On the other hand emission taking place within the band gap is strongly suppressed with excited state lifetimes approaching 100 ns.   

Figure 3. Excited state decay curves and evaluated lifetime numbers of spontaneous emission from CdSe/ZnS core shell quantum dots embedded into our photonic band gap crystals. Black: emission recorded far outside band gap; blue: emission recorded at the edge of the band gap; red: emission recorded within the band gap.

Research activities supported by this PRF grant resulted in two major findings: We succeeded in fabricating the first photonic crystal with a complete band gap at visible frequencies, as confirmed by theoretical, structural and optical characterization. Studies of spontaneous emission dynamics revealed that our photonic band gap crystals have the ability to strongly modify the photonic density of states with significant implications for enhanced photon management in solar energy conversion and photocatalysis.