Synthetic approaches toward tungsten photonic crystals for thermal emission

The efficiency of standard incandescent light sources is limited by strong thermal emission in the infrared regime. It is possible that emission of light may be more efficient when the conventional tungsten filament is replaced by metallic photonic crystals that have large photonic band gaps in the infrared and can suppress the thermal emission of blackbody emitters. One approach toward fabricating photonic crystal structures with highly ordered periodic features on an optical length scale involves colloidal crystal templating to produce inverse opals. Metallic inverse opals were synthesized using chemical vapor deposition (CVD) and wet chemical methods capable of producing granules, thin films and monolithic pieces. Thin films were prepared by infiltrating silica opal films with tungsten hexacarbonyl in a CVD process, reducing tungsten in hydrogen and removing the silica template by HF etching. A range of soluble metal precursors, including tungsten(VI) chloride, tungsten(V) ethoxide and acetylated peroxotungstic acid, were infiltrated into self-assembled, colloidal crystal arrays comprised of monodisperse poly(methy) methacrylate) (PMMA) spheres. The infiltrated composites were processed under reducing conditions to produce metallic inverse replicas of the template. The influence of processing conditions on structural properties, including thickness of skeletal walls, window openings and solid filling fraction, was studied. A monolithic tungsten inverse opal with dimensions of 0.5 × 0.5 × 0.2 cm was resistively heated in an inert atmosphere and thermal emission was observed. The wet chemical methods provide a low cost alternative to expensive nanolithographic methods for the fabrication of three-dimensional periodic metallic structures.