Composite Nanospheres Comprising Luminescent Carbon Dots Incorporated into a Polyhedral Oligomeric Silsesquioxane Matrix

We produced composite nanospheres, which consist of nanometer-sized luminescent carbon dots (CDs) incorporated in a dielectric matrix of polyhedral oligomeric silsesquioxane (POSS). The POSS-CD composite structure is built from N-doped sp(2)-hybridized carbon domains formed by solvothermal treatment in inverse micelles of citric acid surrounded by POSS molecules. The size of the nanospheres can be tuned by varying the ratio between the precursors (citric acid and POSS) over the range of 20-60 nm; they emit in the blue region at 460-480 nm with a high photoluminescence quantum yield (PL QY) of up to 50%. Furthermore, the POSS-CD nanospheres which are initially soluble in nonpolar solvents can be easily transferred into a broad range of polar and/or aprotic solvents (such as water, methanol, ethanol, dimethylsulfoxide, dimethylformamide, ethylene glycol, etc.) by partial chemical etching of the POSS matrix with tetramethylammonium hydroxide. At the final stage of the etching procedure, the initial large (several tens of nanometers) composite nanospheres transform into small (similar to 5 nm) carbon nanoparticles resembling "classical" chemically synthesized CDs, which further confirms their "raisin bun"-like architecture. Little alteration of the emission peak position and shape occurs upon this transformation, while the PL QY remains high (15-40%), which also confirms that the carbon domains inside the POSS matrix dominate the optical transitions and the luminescence of the POSS-CD nanospheres.