Molding the flow of light
It is well known that nanostructuring materials in a complex or ordered fashion can modify their fundamental properties. This powerful route can also combine different materials and be used to explore new properties of the resultant heterostructures, enhancing or hindering the individual properties of the constituents. In particular, by nanostructuring light emitting materials one can modify their emission properties and tailor their response in what has revealed as a powerful strategy to build up applications in photonics. In this way, photonic crystals (PCs) have turned out a promising tool to engineer at will, or even inhibit, the emission of light. In these systems the refractive index is modulated periodically in one, two or three dimensions.
Light emitters such as organic dyes and semiconductors have been incorporated within the structure of artificial opals during the last few years. Very recently, partial inhibition and enhancement of the spontaneous emission has been shown in inverse titania opals doped with CdSe nanocrystals, matching the first (incomplete) pseudogap (pG)]
Figure 1 schematically depicts the method used to fabricate the composite material. Thin film opals (fig. 1a) were grown using the vertical deposition method and then were infiltrated (fig. 1b) with ZnO following a modified CVD method, which allows a controlled conformal infiltration with few nanometres precision. The structure was then inverted (fig. 1c) to obtain the ZnO template, eliminating the polymer backbone by calcination. The CdTe QD infiltration (fig. 1d) of the templates
(fig. 1d) was performed as follows: ZnO inverse opals were immersed in a dilute solution containing CdTe nanocrystals and were vertically pulled with the help of a stepper motor. This leads to an accurate deposition of QDs on the surface of the ZnO shells as figure 1d schematically reproduces. This process can be repeated as many times as needed. We should remark the fact that QDs are deposited both on the inner and the outer ZnO surfaces. The inner growth is possible because the spherical air cavities are connected by windows produced in the contact points between the original spheres, which remain open after calcination. The size of these windows depends on the degree of sintering and the grown material, in this case ZnO, but typically they are around 10% of the diameter (70 nm in our case). This interior growth is possible while these windows remain open. Once the desired amount of QDs are assembled, a thin layer of ZnO can be re-grown to bury (fig. 1e) them. Additional ZnO infiltrations can be subsequently performed to obtain the desired photonic effects in the final composite (fig. 1f).