Electron Dynamics in Films Made of Transition Metal Nanograins Embedded in SiO2: Infrared Reflectivity and Nanoplasma Infrared Resonance

N. E. MASSA

Rosario, Sta. Fe

Seminario; Tercer Taller-Escuela Latinoamericano sobre Materia Condensada “Interacciones en Nanosistemas”; 2011

Ad-hoc

One of the current issues at the basis of the understanding of novel materials is the degree of the role played by spatial homogeneities in their intrinsic properties. Transition metal oxides from manganites to high Tc superconductors present a plethora of properties that might relate to the presence several competing states due to subtle phase separations. These may appear as ranging from nanoscale granularity to oxygen vacancies distributed at random in lattice inhomogeneous patterns. To clarify this picture here we will compare the plain glass network disorder response of transition metal granular films with different metal fractions against what it is known for conducting oxides.Our ~550 nm thick granular films, prepared by cosputtering SiO2 and the different transition metal (TM) volume fractions on SiO2 glass substrates, are made of nanoparticles and nanoclustering aggregates. Films for TMx(SiO2)1-x (x= 1.0, 0.84, 0.61, 0.54, 0.28) were studied by temperature dependent far infrared measurements with techniques that allow a quantitative and reliable evaluation of their intrinsic properties. While for pure TM the spectra show a flat high reflectivity response, the ones for x ~0.84 has a Drude component, vibrational modes mostly carrier screened at the lowest frequencies, and a long tail that extents toward near infrared associated with hopping electron conductivity and the presence of strong electron-phonon interactions. The sample for Ni0.61(SiO2)0.39 has a near normal reflectivity in which the relative reduction in the number of carriers allows less screened phonon bands on the top of a continuum and the emergence of a wide and overdamped oscillator at mid-infrared frequencies. Co0.51(SiO2)0.49 and Ni0.28(SiO2)0.72 have infrared spectra with well defined vibrational bands and a sharp threshold at ~1450 cm-1. It is most remarkable a distinctive resonant sharp peak found for the p-polarized angle dependent specular reflectivity. It originates in a conduction electron cloud bounded in the nanoparticle that, beating against the positive background, generates the electric dipole detected in the infrared.  Overall we conclude that the spectra thus far obtained are analogous to those regularly found in conducting oxides in which we verified, as for transport measurements, that with a suitable percolating network, polarons are formed. (Massa et al, J. Apply. Phys. 105, 114306 (2009).