Doping-Dependent Optical Response of a Hybrid Transparent Conductive Oxide/Plasmonic Medium

Understanding the interaction between plasmonic nanoparticles and transparent conductive oxides is instrumental to the development of next-generation photovoltaic, optoelectronic, and energy-efficient solid-state lighting devices. We investigated the optical response of hybrid media composed of gold nanoparticles deposited on aluminum-doped zinc oxide thin films with varying doping concentration by spectroscopic ellipsometry. The dielectric functions of bare AZO were addressed first, revealing doping-induced effects such as the band gap shift and the appearance of free carriers. In the hybrid media, a blue-shift of the localized surface plasmon resonance of Au NPs as a function of increasing Al doping of the substrate was observed, ascribed to the occurrence of a charge transfer between the two materials and the doping-dependent variation of the polarizability of the substrate

Optical and electronic properties of transparent conducting Ta:TiO2 thin and ultra-thin films: effect of doping and thickness

The development of low-dimensional transparent conducting systems is nowadays gaining interest in view of novel optoelectronic applications. In this paper, we investigate the evolution of optical and electronic properties of Ta-doped TiO2 films as thickness is decreased down to 5 nm and as a function of Ta doping (5-10% at.), and we correlate the observed behavior to the structural properties, showing a high degree of tunability. Ta:TiO2 polycrystalline anatase films are synthetized via pulsed laser deposition, followed by vacuum annealing. For films thick 50-200 nm, the electrical resistivity is ~8×10-4-1×10-3 Ωcm and charge carrier density increases with doping content while mobility decreases. Below a thickness of 20 nm the electrical properties partially deteriorate, but still conductive ultra-thin films can be obtained down to 5 nm. The optical response changes with Ta addition, i.e. the absorption band in the UV range blue-shifts, according to the Moss-Burstein effect, while absorption in the IR increases because of free carriers. Finally, we provide estimates of the effective mass and the plasma energy range in the IR. The fine tunability of the optoelectrical properties of Ta:TiO2 films makes them suitable for devices as transparent conductive components and for photonic or plasmonic applications in the visible and IR range

Tunable optical and plasmonic response of Au nanoparticles embedded in Ta-doped TiO2 transparent conducting films

Localised Surface Plasmon Resonances (LSPR) are fascinating optical phenomena occurring in metal nanostructures, like gold nanoparticles (Au NPs). Plasmonic excitation can be tailored efficiently in the visible range by acting on size, shape and NP surrounding, whereas carrier density is fixed, thus restricting the LSPR modulation. Transparent Conductive Oxides (TCOs), on the other hand, are gaining increasing interest for their transparency, charge carrier tunability and plasmonic features in the infrared. The combination of these two materials into a metal-TCO nanocomposite can give access to unique electrical and optical characteristics, to be tailored in view of the desired optoelectronic application. In this study Au NPs and Ta-doped TiO2 TCO films have been merged with the aim to master the Au plasmon resonance by acting on the dielectric properties of the surrounding TCO. Morphology, structure and electrical properties have been investigated as well, in order to understand the optical response of the nano-systems. The role of the embedding geometry has been explored, revealing that the largest LSPR shift (550-760 nm) occurs when the nanoparticles are sandwiched in the middle of the film, and not at the “bottom” of the film (substrate/film interface). Ta doping in the TCO has been varied (5-10% at. and bare TiO2) to induce a permittivity change of the matrix. As a result, Au LSPR is clearly blue-shifted when decreasing the dielectric permittivity at higher Ta content in the sandwich configuration. Despite the non-optimal electrical performance caused by defectivity of the films, Au-Ta:TiO2 multifunctional nanocomposites are promising candidates for their optical behavior as highly tunable plasmonic conductive metamaterials for advanced light management

Original design of a patterned multiferroic heterostructure for electricalcontrol of the magnetic shape anisotropy

In the framework of piezoelectric/ferromagnetic patterned heterostructures, the purpose of this work is toelectrically control the magnetic properties by tuning the morphology, especially by modifying the magneticshape anisotropy through patterned strain. We have thus designed and studied a heterostructure with bottomnano-striped and top fullfilm electrodes. ZnO piezoelectric and CoFeB magnetic materials were chosen to re-spond at critical criteria of its geometry. In addition, numerical simulations and magnetostatic calculations wereperformed to understand the reproduction of the pattern across the multiferroic heterostructure. Calculationshave shown that the geometry of the heterostructure presents strict constraints, as for instance the distancebetween stripes versus the piezoelectric thickness. This study is a preliminary step towards reversible patterningof magnetic properties

Quantitative Ultrafast Electron-Temperature Dynamics in Photo-Excited Au Nanoparticles

The femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photoemission spectra around the Fermi edge of gold nanoparticles providing a direct, unambiguous picture of the ultrafast electron-gas dynamics. These results will be instrumental to the refinement of existing models of femtosecond processes in laterally-confined and bulk condensed-matter systems, and for understanding more deeply the role of hot electrons in technological applications