Photoluminescent particle surfaces in mesoporous hosts

Alkaline earth oxide (AEO) nanocrystals show a so far unnoticed potential for applications as inorganic phosphors with adsorption-dependent optical properties. To connect these nanoscopic objects to our macroscopic world under preservation of their interface-related optical properties, new approaches for nanoparticle immobilization inside mesoporous host matrices need to be developed. The goal of the present project is to create nanoscale AEO particles (MgO, CaO, BaO, SrO) in a controlled way inside inert as well as chemically reactive mesoporous coatings (SiO2, TiO2, mixed TiO2/SiO2 or MgO) using novel metal vapour infiltration techniques. The influence of the coating properties thickness, porosity, pore size distribution and pore connectivity on the deposition of AEO particle and consequently their optical properties will be investigated. The generation of multiple AEO particle concentration gradients within one mesoporous coating will be achieved by spatially controlled functionalization of sites for AE metal nucleation, e.g. by utilizing surface groups with variable thermal stabilities, or by patterning of the coating with lithographic or focused ion beam techniques. In addition, multiple coating steps will allow for structures with gradients in porosity and chemical composition, which will also be employed for the generation of AEO concentration gradients. On the basis of defined coating thicknesses of the host and spatially adjustable concentrations of various AEO particle types we envision composites where the deliberate photoexitation of one selected area inside the coating can produce photoluminescence emission in the second region based on energy transfer processes. As an alternative synthetic approach, we will employ the controlled adsorption of silanes onto AEO particle surfaces in combination with subsequent oxidative conversion steps to generate dense silica layers for their chemical protection. Respective products can then be directly incorporated into the evaporation-induced self- assembly process for the formation of the mesoporous coatings.

A critical issue for the utilization and integration of nanoparticle systems with desired properties in optical or electronic devices is their immobilization. Aiming at the linkage of alkaline earth oxide nanoparticles to the macroscopic world under preservation of their surface dependent chemical and optical properties their immobilization in porous host materials was investigated. We designed a new approach for the gas phase infiltration of SiO2 aerogels and also employed optical spectroscopies to explore the optical properties of the host materials with different pore size distributions and pore arrangements. Electron microscopy and powder X-ray diffraction were used to characterize the influence of metal infiltration on the structural properties of the host materials. Upon infiltration and subsequent thermal processing, composite materials contain either nanocrystalline alkaline earth metal oxides or depending on the hierarchy of the pore system - silicate phases. In order to achieve pinhole free surface coatings of MgO nanocubes we explored in a parallel activity vapour phase based surface functionalization approaches involving different silicon sources (Silane or SiCl4) and subsequent oxidation steps. Although we did not succeed in the formation of water resistant surface coatings, we discovered through serendipity the room temperature transformation of MgO nanocubes into magnesium oxychloride nanofibers in air and investigated the underlying formation mechanism using electron microscopy, X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy. Upon contact with water vapor the magnesium hydroxide needles grow out of agglomerates of highly dispersed MgO nanocubes with preadsorbed chlorine, a process which does not occur in case of low surface area materials. The presented growth approach is potentially extendable to other hydrolysable metal oxides at ultrafine dispersion. Corresponding spontaneous transformation processes are key to their chemical synthesis and application for two reasons: first, the underlying mechanisms may provide guiding principles for the synthesis and controlled spatial arrangement of anisotropic nanostructures. Second, knowledge about the transformation behaviour of nanomaterials in the environment is needed in order to reliably assess the potential risk to biological systems.