Tetrahedral framework of inverse opal photonic crystals defines the optical response and photonic band gap

Abstract

By forming anatase TiO2 inverse opals by infiltration of an opal photonic crystal, we demonstrate that the optical response and angle-resolved blue-shift of the band-gap of the inverse opal structure is defined by a particular three-dimensional structure of the infilled voids. The optical structure of TiO2 inverse opals usually displays significant deviation from its physical structure and from theoretically predicted position of the photonic band-gap. Following rigorous structural characterization of the parent opal template and TiO2 inverse opals, alternative explanations for the signature of optical transmission through inverse opals is proposed. These approaches posit that, for light-matter interaction, an inverse opal is not precisely the inverse of an opal. Accurate parameters for the structure and material properties can be obtained by invoking a Bragg FCC selection rule-forbidden (-211) plane, which is not a realistic model for diffraction in the IO. Alternatively, by assuming optical interactions with just the periodic arrangement of tetrahedral filled interstitial sites in the structure of inverse opal, a complete reconciliation with spectral blue-shift with angle, photonic band gap and material parameters are obtained when a reduced unit cell is defined based on interstitial void filling. The analysis suggests a reduced interplanar spacing (d = 1/√3 D, for pore diameter D), based on the actual structure of an inverse opal in general, rather than a definition based on the inverse of an FCC packed opal. This approach provides an accurate and general description for predicting the spectral response and material parameters of ordered inverse opal photonic crystal materials.

Semiconducting Metal Oxide Photonic Crystal Plasmonic Photocatalysts

“…[ 30 ] In comparison, the morphology of TiO 2 IOs is comprised of interconnected TiO 2 NPs, as shown in Figure 2c,d with internal pore diameters that are ≈460 nm in diameter. [ 33 ] Similar pore dimensions that are also found on the V 2 O 5 IO structures (Figure S4, Supporting Information). A colloidal solution of monodisperse Au NPs with a mean diameter of 4.5 nm, shown in Figure 2e, was immobilized onto the IO supports giving well‐dispersed Au NPs across the IO as shown in Figure 2f.…”

Filling in the gaps: The nature of light transmission through solvent-filled inverse opal photonic crystals

“…IOs [52], applying a reduced interplanar spacing of 𝑑 = √ Figure 1 (j) shows the characteristic optical transmission data for a typical TiO2 IO as detailed in Eq.…”

“…Currently, there are several methods used in a wide variety of literature to approximate the effective refractive index of composite material [69,70]. Two common models applied to inverse opal materials to determine 𝑛 eff include the Drude [16,48,52,[71][72][73] and Parallel [58][59][60][61][62] models. The Drude model for inverse opal systems can be formalized as:…”

“…In other cases [50,51], such as those with inverse opals prepared via annealing an opal template, there is a clear difference between the estimation of material properties from the spectral data and the independently measured microscopy data. It has been suggested that the optical behavior of inverse opal materials is inherently different from the reciprocal of an opal, with the possibility of an axial compression [51], reduced unit cell parameter [52] and, most commonly, reduced crystalline material volume fractions [53][54][55][56][57] suggested as possible explanations. Currently, there is an absence of consensus on the explanation underpinning the differences observed in the PBG for inverse opal structures.…”

Simplifying the synthesis of carbon inverse opals

“…5 Photonic crystals in the form of inverse opals (IOs) of various materials have been identied as a promising structure for gas sensors, waveguide applications and as electrode materials for Li-ion batteries, due to their unique physical properties such as photonic band gap, high surface area and high level of porosity. [6][7][8][9][10][11][12] IOs are typically prepared via inltration of a sacricial sphere template with a precursor solution, a chemical treatment step to remove the spheres, followed by thermal treatment. 13 Various inltration methods have previously been reported, including dip coating, spin coating, drop casting, chemical vapor deposition and electrodeposition.…”