Initially, we fully characterized each individual layer of the proposed capacitor with various microscopic and spectroscopic techniques. We fabricated reference materials and performed spectroscpic ellipsometry on each layer prior to the the fabrication of the capacitor in order to extract their dielectric properties.
After the optimization of the experimental parameters for the fabrication of each layer and the study of their optical properties, we then fabricated the capacitors. We performed ellipsometric measurements under bias but we didn’t detect any significant changes on the dielectric properties of AZO upon voltage, in the photon energy range of the ellipsometer, 245-1700 nm. For that reason, it was scientifically unwise to proceed with the Au NPs growth on top of the TCO electrode of the capacitor before the active tuning of the AZO properties was achieved. In fact, the results from the above measurements, forced us to rethink the experiment overall and perform optical simulations in order to exploit the expected response of these systems upon bias that can be a guide for their future fabrication. The study of the optical properties of each layer led to the modeling of a capacitor with BTO as the insulating layer, NSTO as the conductive bottom gate, and AZO film as the top gate. The successful fabrication of the AZO/ BTO/NSTO hybrid system with tunable properties upon gating is a first step toward the fabrication of a prototypedevice for plasmoelectronics. These simulations represent a useful benchmark for the determination of the optimum configuration for monitoring any variation in the optical properties of the TCO upon gating.
Moreover, we studied the optical response of hybrid media composed of Au NPs deposited on AZO thin films with varying doping concentration, by means of Spectroscopic Ellipsometry. Understanding the interaction between plasmonic NPs and transparent conductive oxides is instrumental to the development of next-generation photovoltaic, opto-electronic and energy-effcient solid-state lighting devices. We addressed the dependence of the LSPR frequency of metallic NPs upon the doping of a TCO substrate (here, AZO films). We focused on achieving high-precision values of the dielectric response of the system as a mean to understand the NP-TCO interaction in depth. We noticed a systematic blueshift of the LSPR of Au NPs as a function of increasing Al-doping, that cannot be simply due to the dielectric environment of the plasmonic nanoparticles, suggesting that a doping-dependent charge transfer between the substrate and the NPs is responsible for the effect.
We investigated the hot-electron injection process in the NS-TCO system by performing pump-probe photoemission experiments. Thanks to the strong collaborative effort of different research groups, we were able to report for the first time a direct measurement of the ultrafast electron-temperature dynamics in plasmonic Au NPs in the first femtoseconds after the excitation. By performing ultrafast state-of-the-art photoemission measurements with high temporal and unprecedented spectral resolution, we traced the electron-temperature evolution of photo-excited Au NPs on the femtosecond time scale in a totally model-independent fashion. Our results are both significant and timely because current understanding of the energy dissipation phenomena mentioned above relies on indirect experimental information, rather than the direct assessment of quantities such as electron or ion-lattice temperatures, that unambiguously define the thermal state of the system.
Parts of the work of the Action have been presented in Conferences and Scientific Journals.
