Changing the Microstructural and Chemical Properties of Graphene Oxide Through a Chemical Route.

The aim of this work is to investigate the possibility of engineering desired molecular sp2 structures in graphene oxide, via controlled oxidation of graphite powder, in order to achieve tunable chemical and microstructural properties useful for optoelectronics or sensing applications. Specifically, GO powder is obtained by a modified Hummers method, by using different concentrations of potassium permanganate (KMnO4) in order to change the number of oxygen functionalities in the graphitic structure. Then, a successive alkaline treatment is performed by increasing the KOH concentration. The alkaline treatment induces a noticeable variation of the GO microstructural and chemical properties, which is accompanied by a strong enhancement of photoluminecence. PL and PLE measurements reveal that the configuration of electronic energy states changes as a function of the KMnO4 and KOH concentration, by introducing further electronic n levels available for n→π* transitions. In particular, the number of sp2 small domains embedded among oxygen-sp3 domains, increases under the KOH treatment, due to the addition of OH groups. Most of these sp2 domains are lifted-off from GO and thrown away in the surnatant giving it high blue photoluminescence excited at λexc ∼ 319 nm. The employ of combined spectroscopy techniques allows a deep investigation of the microstructural and chemical changes induced by chemical treatments, opening the way to the fine tuning of GO functional properties.

Simulation of optical properties of layered metallic nanoparticles embedded inside dielectric matrices: interference method or Maxwell Garnett effective-medium theory?

Optical characterization of composite films consisting of a ceramic matrix with embedded layered metal nanoparticles have recently received increasing interest. In particular, two methods have been mainly proposed in order to obtain optical performances of dielectric matrices containing layered nanoclusters (NCs): the first method is based on the simulation of the layered system as composed of alternated films of dielectric material and effective-medium material. Therefore, the optical response of the multilayer stack is calculated, assigning to the effective-medium layers the dielectric constant epsilon(f)(Yama), obtained by the Yamaguchi theory, and calculating the interference between the beams reflected and refracted at each interface inside the stack. The second method considers the multilayer stack as a single-layer effective-medium film whose dielectric constant is calculated by the Maxwell Garnett (MG) theory. In particular, this second method is recognized to be valid in the case of nanoparticles uniformly distributed inside a dielectric matrix. The present study shows that the interference method, as it has been applied up to now, does not allow reproducing reflectance and transmittance spectra calculated by the MG theory in the case of a uniform distribution of NCs.