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 photo emission 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.

ZnO Thin Films Growth Optimization for Piezoelectric Application.

The piezoelectric response of ZnO thin films in heterostructure-based devices is strictly related to their structure and morphology. We optimize the fabrication of piezoelectric ZnO to reduce its surface roughness, improving the crystalline quality, taking into consideration the role of the metal electrode underneath. The role of thermal treatments, as well as sputtering gas composition, is investigated by means of atomic force microscopy and x-ray diffraction. The results show an optimal reduction in surface roughness and at the same time a good crystalline quality when 75% O2 is introduced in the sputtering gas and deposition is performed between room temperature and 573 K. Subsequent annealing at 773 K further improves the film quality. The introduction of Ti or Pt as bottom electrode maintains a good surface and crystalline quality. By means of piezoelectric force microscope, we prove a piezoelectric response of the film in accordance with the literature, in spite of the low ZnO thickness and the reduced grain size, with a unipolar orientation and homogenous displacement when deposited on Ti electrode.

In-Operando Optical Spectroscopy of Field-Effect-Gated Al-Doped ZnO.

Transparent conductive oxides (TCO) have the unique characteristics of combining optical transparency with high electrical conductivity; such a property makes them uniquely alluring for applications in visible and infrared photonics. One of their most interesting features is the large sensitivity of their optical response to the doping level. We performed the active electrical manipulation of the dielectric properties of aluminum-doped ZnO (AZO), a TCO-based on Earth-abundant elements. We actively tuned the optical and electric performances of AZO films by means of an applied voltage in a parallel-plate capacitor configuration, with SrTiO3 as the dielectric, and monitored the effect of charge injection/depletion by means of in-operando spectroscopic ellipsometry. Calculations of the optical response of the gated system allowed us to extract the spatially resolved variations in the dielectric function of the TCO and infer the injected/depleted charge profile at the interface.

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.

Highly efficient plasmon-mediated electron injection into cerium oxide from embedded silver nanoparticles.

The coupling with plasmonic metal nanoparticles (NPs) represents a promising opportunity to sensitize wide band gap oxides to visible light. The processes which come into play after the excitation of localized surface plasmon resonances (LSPRs) in the NPs largely determine the efficiency of the charge/energy transfer from the metal NP to the oxide. We report a study of plasmon-mediated energy transfer from mass-selected silver NPs into the cerium oxide matrix in which they are embedded. Femtosecond transient absorption spectroscopy is used to probe the dynamics of charge carrier relaxation after the excitation of the LSPR of the silver nanoparticles and to evaluate the plasmon-mediated electron transfer efficiency from the silver nanoparticles to the cerium oxide. High injection efficiencies in the 6-16% range have been identified for excitation between 400 and 600 nm. These high values have been explained in terms of plasmon-mediated direct electron injection as well as indirect hot electron injection from the NPs to the oxide. The information obtained provides an important contribution towards a knowledge-driven design of efficient cerium oxide based nanostructured materials for solar to chemical energy conversion.

Frictional transition from superlubric islands to pinned monolayers.

The inertial sliding of physisorbed submonolayer islands on crystal surfaces contains unexpected information on the exceptionally smooth sliding state associated with incommensurate superlubricity and on the mechanisms of its disappearance. Here, in a joint quartz crystal microbalance and molecular dynamics simulation case study of Xe on Cu(111), we show how superlubricity emerges in the large size limit of naturally incommensurate Xe islands. As coverage approaches a full monolayer, theory also predicts an abrupt adhesion-driven two-dimensional density compression on the order of several per cent, implying a hysteretic jump from superlubric free islands to a pressurized commensurate immobile monolayer. This scenario is fully supported by the quartz crystal microbalance data, which show remarkably large slip times with increasing submonolayer coverage, signalling superlubricity, followed by a dramatic drop to zero for the dense commensurate monolayer. Careful analysis of this variety of island sliding phenomena will be essential in future applications of friction at crystal/adsorbate interfaces.