Inverted opals are of great interest as photonic materials. Here, an inexpensive chemical vapor deposition method of synthesis of ZnO and ZnS inverse opals (see Figure) is described, based on infiltration and calcination of ZnO in synthetic polystyrene opals. Excellent control of the degree of infiltration, i.e., the layer thickness of ZnO, and thus the properties of the material, is reported.
An extended investigation has been made of the electrical and photovoltaic properties of heterojunctions prepared by spray-pyrolysis deposition of thin ZnO films on single-crystal p-type CdTe. The principal experimental variables were the substrate temperature and the postdeposition temperature for annealing in H2. Under actual sunlight the optimum cell showed an open-circuit voltage of 0.54 V, a short-circuit current of 19.5 mA/cm2, and a solar efficiency (referred to the active area) of 8.8%, the highest value obtained to date for an authentic heterojunction on CdTe. The nature of the forward transport mechanism has been investigated, and a tunneling model in which bulk and interface deep traps control the forward characteristics is shown to provide good correlation with the experimental data.
Physisorbed and structurally bound (surface and internal) water in silica opals are distinguished and quantified by thermogravimetry. By controlled dehydroxylation with thermal annealing, we correlate these forms of water with the silica chemistry. In particular, we find that the silica capability to physically adsorb water from ambient moisture exhibits three regimes, associated with the distinct condensation behavior of bonded and unbonded surface silanols. Features in both opal IR absorbance and photonic band gap reproduce the physisorbed water regimes. This allows direct assessment of the water content and its evolution just by routine optical spectroscopy, being a useful tool for local and nondestructive analysis of colloidal silica. Besides, this provides a simple recipe for accurate tuning of the opal photonic band gap (about 10% in position and width) by just selecting the annealing temperature.
Photonic crystals can be viewed just as a subclass of a larger family of material systems called metamaterials in which the properties largely derive from the structure rather than from the material itself. Opals have only a relatively recent history as photonic bandgap materials and have received a strong thrust from their adequacy as scaffoldings for further templating other materials with photonic applications for instance. The tortuous route from materials to devices might perhaps find reward in the ease and low cost of fabrication of these materials. In this paper we present a review of recent work and work under way in our laboratory tending towards synthesis based on self-assembly to realize metamaterials in the optical range. This comprises the formation of the templates (opals) and subsequent synthesis of guest materials such as semiconductors, metals and insulators. The possibility of further processing allows additional two-dimensional and quasi-two-dimensional patterning for the design of new structures. In this paper we show how the raw matter can be checked for quality and learn how to use its optical properties to evaluate application potential. Issues relating to the optical properties (such as crystalline quality, finite size effects and infiltration with other materials) are examined. We show some examples where opals are used to pattern the growth of other materials with photonic applications (such as metals and semiconductors) and developments leading to both vertical and lateral engineering are shown.
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