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Quantum Dot Thin Layers Templated on ZnO Inverse Opals

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 to be a powerful strategy to build up applications in photonics. In this way, photonic crystals (PCs) have turned out to be 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. This fact provokes the appearance of energy intervals, called photonic bandgaps (PBG), for which the propagation of light is prevented. Conveniently used, this specific property of PCs is the origin of an important number of fundamental phenomena that have been the subject of study for the last two decades. Present and future photonic technologies will make use of techniques such as lithography, holography, direct laser writing, and selfassembly in order to fabricate periodically ordered nanostructures able to shape and modify fundamental optical properties of the constituent materials. Among them, structures such as artificial opals, self-assembled face-centered cubic (fcc) structures of dielectric microspheres, have revealed to be a perfect playground in the field of PCs. Although as-grown, these systems do not present a complete PBG (a stopgap in all directions) and their symmetry and topology are fixed by the growth process, there is, up to now, no other method of fabrication that can provide the same large-scale fabrication, high optical quality, and versatility of processing in one package. Therefore, the combination of luminescent materials with PCs, in particular with opals, is one of the open challenges being addressed nowadays. 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 (NCs), matching the first (incomplete) pseudogap (pG). Among the materials for this purpose, ZnO appears to be a very interesting one because of its remarkable properties as a conductor and transparency in the visible range of the spectra. Recently, high-quality ZnO inverted opals grown by chemical vapor deposition (CVD) and atomic layer deposition (ALD) have been reported. Furthermore, the relatively high ZnO refractive index provides enough dielectric contrast in the inverted structure to open additional pGs in the high-energy regime, which have recently been used for enabling laser emission. Its ample range of transparency allows ZnO to host emitters in a broad spectral range from the IR to the UV with no hindrance derived from absorption. All these reasons make ZnO an appropriate template material where quantum dots (QD) can be conformally self-assembled. In this communication we present a large-area, high-quality, new composite material obtained by structuring, in two steps of hierarchical colloidal self-assembly, a ZnO inverted opal with colloidal CdTe nanocrystals. We also provide experimental results of the photonic effect that the original ZnO template produces on the spontaneous photoluminescence (PL) from CdTe QDs embedded in the structure. In particular, we chose the scarcely explored high-energy photonic bands of the ZnO inverted opal to show that the effect produced shows up as inhibition and enhancement of the QDs emission. Samples were prepared starting from large diameter (700 nm) polystyrene (PS) spheres in order to place the desired photonic features (high-energy gaps) matching the QD emission band (around 640 nm for a QD with diameter d = 4 nm). The CdTe QDs were synthesized in water following a previously described method by utilizing thioglycolic acid (TGA) as stabilizer. 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 were then infiltrated (Fig. 1b) with ZnO following a modified CVD method that allows a controlled conformal infiltration with precision within a few nanometers. The structure was then inverted (Fig. 1c) to obtain the ZnO template, eliminating the C O M M U N IC A TI O N S