Electrodeposited 3D Tungsten Photonic Crystals with Enhanced Thermal Stability

Photonic band gaps can emerge when materials are periodically organized with characteristic dimensions on the order of the wavelength of light. Such materials are commonly known as photonic crystals. Proposed applications for photonic crystals include a wide range of functional optical devices including lowloss optical fibers, three-dimensional waveguides, zero-threshold lasers, sensors, and enhanced efficiency solar energy converters. Periodic micro/nanoarchitectures with metallic components are especially interesting because they interact very strongly with light, resulting in unique optical, plasmonic, and thermal emission properties. Solar thermophotovoltaic (sTPV) energy harvesting offers an attractive opportunity to achieve ultrahigh efficiency solar energy harvesting by effectively compressing the broad solar spectrum into a narrow band of energies, which can then be efficiently absorbed by a solar cell. 16 For maximum efficiency, electromagnetic radiation arriving at the solar cell should be concentrated at an energy just above the band gap energy of the solar cell. In this configuration, sub-band gap photons are not wasted and excess energy due to thermalization of high-energy photons is minimized, resulting in the high overall efficiency. Several materials/designs have been proposed to achieve narrow band emission, including rare earth doped materials, plasmas, optical filtration systems, and metallic photonic crystals. Metallic photonic crystals are particularly strong candidates to generate narrow band emission for sTPV energy harvesting systems. Previous studies demonstrated that metallic photonic crystals could exhibitmodified thermal emission as a direct result of the modulation of the optical density of states by their multidimensional periodic structure. For a blackbody in thermal equilibrium, Kirchoff’s law tells us that emittance ≈ absorptance = 1 reflectance. Thus, we can expect thermal emission to be greatly suppressed within the photonic band gap of a metallic photonic crystal and emission (absorption) to be enhanced at the photonic band gap edges. Fleming et al. reported modified thermal emission due to the photonic bad gap of a lithographically prepared tungsten woodpile photonic crystal. The woodpile fabrication is, however, complex. Thus, it is desirable to use a large area, low cost selfassembly process to fabricate metallic photonic crystals. On the basis of experimental studies with Ni inverse opals and simulations of tungsten inverse opals, we expect that a properly designed tungsten inverse opal photonic crystal could modulate thermal emission. Both control of surface termination and 3D filling fraction are important to ensure that light penetrates the 3D architecture and probes the photonic band structure. The concept of using a self-assembled opal template to fabricate tungsten inverse opals via CVD or solution-based inversion processes has been explored by a number of groups. 22 A significant problem with these inverse opal structures, however, is that they suffer from severe sintering and grain growth at elevated temperatures (beginning at 800 C) resulting in the loss of the 3D periodicity and thus, the photonic band structure. Stein et al. published a notable exception, demonstrating thermal stability of a thin tungsten coating on a carbon scaffold up to 1000 C for 5 h. Thermal stability at temperatures of 1000 C or, preferably, greater, for extended times, is a key requirement for a sTPV emitter. In accordance with the Stefan Boltzmann equation, the radiated power from a blackbody source will increase proportionally to the fourth power of the temperature of the emitter. Various models suggest the overall energy conversion efficiency of a sTPV device will increase with increasing emitter temperatures up to possibly