Aluminum-doped zinc oxide (AZO) is one of the most promising transparent conductive oxides, valued for its low cost, high transparency and low electrical resistivity. Its tunable plasmonic response in the near-infrared region and the low optical losses make it a promising material for photonic applications. Here, we investigate the role of nanostructuration in the ultrafast optical response of engineered AZO metasurfaces. We design periodic arrays of nanocylinders, nanorods, and L-shaped nanostructures as a function of their relevant structural parameters. Selected metasurfaces are fabricated after an electron beam lithography and focused ion beam process. They are characterized from a dynamic point of view via ultra-fast mid-infrared pump probe technique, demonstrating marked differences of the optical behavior in the nanostructures. Simulations predict broad resonances in the 1500-3000 nm range, with peak absorption from 20% to 40%. In the AZO film and larger nanostructures, hot carriers decay rapidly (similar to 100 fs) in a thermalized population and in similar to 300 fs to the ground state, while for nanorods only one population decaying rapidly is evident. Our results establish that it is possible to modify and control the ultrafast response of AZO metasurfaces through nanostructuration making them promising building blocks for infrared plasmonics and nanophotonic applications.

The modification of the electronic properties of CeO2 thin films through Cu doping is a promising approach to enhancing their performance in photoinduced water splitting (WS). In this study, undoped and Cu-doped CeO2 films of 5 nm thickness with 5 and 11% Cu atomic concentrations were characterized under ambient pressure conditions by near-edge X-ray absorption fine structure (NEXAFS) to provide insights into the film modification and the water splitting process during exposure to water and to laser light at different temperatures. The analysis of NEXAFS data, acquired at the Ce M-5 and Cu L-3 edges, reveals temperature-dependent changes in the oxidation states of Ce and Cu. Notably, a temperature-dependent evolution of the Ce3+ concentration is observed during water exposure, accompanied by spectral changes consistent with a partial reduction of Cu2+ to Cu1+ in the film with the highest dopant concentration. The ambient-pressure NEXAFS measurements highlight the role of Cu dopant ions in modifying the electronic structure of CeO2 and the catalytic response in the presence of water and illumination, enabling more efficient hydrogen production. In parallel, micro gas chromatography analysis revealed a marked increase in hydrogen production from the 11% Cu-doped CeO2 film compared to the undoped CeO2 and the 5% Cu-doped films. These results provide crucial insights into the structure-function relationship in Cu-doped CeO2, offering pathways to optimize the design of materials for hydrogen production and related applications.

Gold nanoclusters are known to slide easily on a graphite surface. In this study, we confirm the slipperiness of the gold-carbon interface by studying the sliding behavior of fullerene adsorbates on gold by using a quartz crystal microbalance (QCM). More precisely, we transfer high-quality gold electrodes deposited on an atomically flat mica substrate to the QCM. By means of an effusion cell, we deposit C60 molecules on the QCM gold electrode kept in ultrahigh vacuum. We observe the pinning of the fullerene adsorbates at room temperature. As the temperature increases above 320 K, the fullerene adsorbates begin to slide. This thermally activated sliding is explained in terms of a simple diffusive model.

Niobium oxide can be stabilized in three distinct stoichiometries, each exhibiting unique physicochemical properties relevant to various technological applications. This study presents a novel procedure for fabricating niobium oxide films and tuning their stoichiometry among the three most stable oxide phases. Starting with a magnetron-sputtered film predominantly composed of Nb2O5, its structure and stoichiometry are optimized through thermal treatment in an O2/N2 flux. A vacuum reduction treatment transforms the as-grown film into the NbO phase, which can then be reoxidized under controlled oxygen partial pressure to achieve the NbO2 phase. The films are characterized in terms of surface composition using X-ray photoemission spectroscopy, structure through X-ray diffraction, optical properties via UV-vis spectrophotometry, and morphology using scanning electron microscopy. Additionally, we show that X-ray absorption near-edge spectroscopy at the Nb K-edge, performed with X-ray free-electron laser radiation, can provide insights into the electronic structure and subsurface stoichiometry of the films. The ultrafast mechanisms underlying photoinduced processes in NbO2 are also discussed.

Pump-probe spectroscopies utilizing X-ray free-electron lasers offer element-specific insights into the processes occurring in photocatalysts following photoexcitation, which are essential for the rational optimization of the efficiency of these materials. This study examines the dynamic evolution of the electronic and atomic structure in stoichiometric cerium oxide films following photoexcitation, employing ultrafast pump-probe X-ray absorption spectroscopy (XAS) at the Ce L3 edge in both the near-edge and extended energy ranges using an X-ray free electron laser. The results reveal a rapid relaxation pathway occurring within the first few hundred femtoseconds, followed by the formation of an excited state with a lifetime on the order of hundreds of picoseconds. The analysis of pump-probe XAS in the extended energy range identifies a structural distortion consistent with the formation of a photoinduced small polaron state. The observed time correlation between the photoinduced electronic and structural changes further reinforces the hypothesis of photoinduced polaron formation. Constrained density functional theory simulations offer insights into the electronic modifications and structural distortions in the photoexcited material. The consequences of the observed processes on material functionality are discussed.

We report the room temperature, low-voltage-enabled modulation of the optical response of hybrid transparent-conductive-oxide/ferroelectric multilayers. We have fabricated an optical multilayer consisting of Al-doped ZnO (AZO) and BaTiO3 films deposited on a Nb-doped SrTiO3(110) substrate. Applying a low voltage between the AZO film and the substrate, a significant variation of the system's optical response has been detected by means of in operando spectroscopic ellipsometry. The voltage-induced variations have been ascribed to a combination of charge accumulation/depletion at the insulator/semiconductor interface and the field-induced Pockels effect in the BaTiO3 layer and successfully reproduced by an optical model including these effects. We have deduced a variation of the refractive index in AZO in the infrared range by more than 0.1 at an applied bias of 0.2 V and by more than 2 at an applied bias of just 3 V at room temperature, which can be strongly appealing for voltage-modulated active optical systems.

The doping of metal oxides is an interesting route to increase catalyst activity and lower activation temperatures in H-2 dissociation to replace Pt in catalysts for electrochemical devices. In this process, the roles of both the matrix and dopant cations are fundamental to understanding and designing more efficient catalysts. In this work, we have investigated the reduction process in pure and doped CeO2 films. We followed the oxidation states of Ce and dopants (Cu and Fe) during H-2 exposure at ambient pressure by combining X-ray absorption spectroscopy and gas chromatography on 5 nm films in the temperature range of 300-620 K. We have observed that Cu doping (at concentrations of 5 and 14 at. %) promotes the ceria reduction, while the addition of Fe seems to have a limited impact on the oxide chemical reactivity only at low temperatures. Moreover, thanks to the chemical sensitivity of operando X-ray absorption spectroscopy, we were able to follow simultaneously the evolution of Ce and Cu oxidation states during the reaction, which has permitted to identify two distinct reduction processes taking place above and below 500 K. These measurements show that at low temperatures, the H-2 dissociation takes place at the Cu1+ sites, thus explaining the higher reactivity of the Cu-doped samples. The described mechanism can help in the design of Pt-free catalysts with enhanced performances.

Three Positron Annihilation Spectroscopy (PAS) techniques have been employed to investigate the point defects of Al-doped Zinc Oxide (AZO) thin films grown by Radio Frequency (RF) magnetron sputtering with different substrates and deposition parameters. The films were grown with thickness varying from 100 to 300 nm, and their crystalline quality ranged from single crystalline epitaxial to partially amorphous. We found that the main defect in the crystalline samples is the 3VZn−VO four vacancy complex, with a concentration around 1018−1019 cm−3. In polycrystalline films larger vacancy clusters, within 10%−20% of the total concentration, were detected. These vacancy clusters are inferred to be most likely located at the grain boundaries. In partially amorphous films the concentration of these larger vacancy clusters, located either at grain boundaries or in the amorphous regions of the film, approached even the 40%, and also some sub-nano voids have been observed.

Realizing very thin transparent conductive (TCO) films is a key aspect of many applications, but often the film quality tends to worsen at very small thicknesses. In this work we demonstrate that Al-doped ZnO (AZO) thin films grown epitaxially on SrTiO3 retain their optimal properties even at thicknesses as low as 30 nm. We deposit by radio frequency magnetron sputtering and investigate the film morphology, structure, crystallinity, electrical and optical properties. We prove that the Hall mobility of epitaxial films is limited mostly by ionized impurity scattering, with a negligible contribution of grain boundary scattering, leading to comparably high mobility in thin epitaxial films. On the contrary, in polycrystalline films the properties of AZO films strongly deteriorate at reduced film thickness, due to grain boundary contributions. The optical carrier density and mobility do not change significantly between epitaxial and polycrystalline films, suggesting that the different electrical properties are indeed mainly due to grain boundary scattering and not to a reduction of carriers and intrinsic mobility. In this way we obtain an epitaxial thin TCO film with the optimal properties of a bulk by a scalable technique, paving the way to applications in plasmonic devices and in thin films solar cells.

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.

Nanomaterials can be game-changers in the arena of sustainable energy production because they may enable highly efficient thermoelectric energy conversion and harvesting. For this purpose, doped thin film oxides have been proven to be promising systems for achieving high thermoelectric performances. In this work, the design, realization, and experimental investigation of the thermoelectric properties exhibited by a set of five Al:ZnO thin films with thicknesses of 300 nm and Al doping levels ranging from 2 to 8 at.% are described. Using a multi-technique approach, the main structural and morphological features of the grown thin films are addressed, as well as the electrical and thermoelectrical transport properties. The results show that the samples exhibited a Seebeck coefficient absolute value in the range of 22–33 μV/K, assuming their maximum doping level was 8 at.%, while the samples’ resistivity was decreased below 2 × 10−3 Ohm·cm with a doping level of 3 at.%. The findings shine light on the perspectives of the applications of the metal ZnO thin film technology for thermoelectrics.

Fe2+-doped ZnSe nanoparticles, with varying concentrations of Fe2+ dopants, were prepared by the hydrothermal method and investigated using a multi-technique approach exploiting scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy, as well as measurement of the electrical transport properties and Seebeck coefficient (S). The doped nanoparticles appeared as variable-sized agglomerates on nanocrystallites upon SEM investigation for any doping level. Combined XRD and Raman analyses revealed the occurrence of a cubic structure in the investigated samples. Electric and thermoelectric (TE) transport investigations showed an increase in TE performance with an increase in Fe atom concentrations, which resulted in an enhancement of the power factors from 13 µWm−1K−2 to 120 µWm−1K−2 at room temperature. The results were also dependent on the operating temperature. The maximum power factor of 9 × 10−3 Wm−1K−2 was achieved at 150 °C for the highest explored doping value. The possible applications of these findings were discussed.

Space and mirror charge effects in time-resolved photoemission spectroscopy can be modeled to obtain relevant information on the recombination dynamics of charge carriers. We successfully extracted from these phenomena the reneutralization characteristic time of positive charges generated by photoexcitation in CeO2-based films. For the above-band-gap excitation, a large fraction of positive carriers with a lifetime that exceeds 100 ps are generated. Otherwise, the sub-band-gap excitation induces the formation of a significantly smaller fraction of charges with lifetimes of tens of picoseconds, ascribed to the excitation of defect sites or to multiphoton absorption. When the oxide is combined with Ag nanoparticles, the sub-band-gap excitation of localized surface plasmon resonances leads to reneutralization times longer than 300 ps. This was interpreted by considering the electronic unbalance at the surface of the nanoparticles generated by the injection of electrons, via localized surface plasmon resonance (LSPR) decay, into CeO2. This study represents an example of how to exploit the space charge effect in gaining access to the surface carrier dynamics in CeO2 within the picosecond range of time, which is fundamental to describe the photocatalytic processes.

We investigate the interaction between molecular hydrogen and ultrathin epitaxial CeO2 films modified with a 2% concentration of Cu atoms using X-ray photoemission spectroscopy (XPS) during thermal reduction cycles in H2. The XPS measurements are combined with density functional theory calculations to obtain further insight into the observed modifications of the film surface. Our results show that the presence of Cu atoms decreases the barrier for H2 dissociation in comparison to that on pure ceria surfaces, leading to the formation of surface OH groups after exposure to H2. Moreover, surface oxygen vacancies are generated already at mild temperatures (470 K), most likely via water formation and desorption. The presence of surface oxygen vacancies and hydroxyls contributes to the observed large increase in surface Ce3+ concentration with increasing reduction temperature. In spite of these atomic scale modifications, the surface morphology observed by scanning tunneling microscopy remains substantially unchanged on the length scale of tens of nm.

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.

Expanding the activity of wide bandgap semiconductors from the UV into the visible range has become a central goal for their application in green solar photocatalysis. The hybrid plasmonic/semiconductor system, based on silver nanoparticles (Ag NPs) embedded in a film of CeO2, is an example of a functional material developed with this aim. In this work, we take advantage of the chemical sensitivity of free electron laser (FEL) time-resolved soft X-ray absorption spectroscopy (TRXAS) to investigate the electron transfer process from the Ag NPs to the CeO2 film generated by the NPs plasmonic resonance photoexcitation. Ultrafast changes (<200 fs) of the Ce N4,5 absorption edge allowed us to conclude that the excited Ag NPs transfer electrons to the Ce atoms of the CeO2 film through a highly efficient electron-based mechanism. These results demonstrate the potential of FEL-based TRXAS measurements for the characterization of energy transfer in novel hybrid plasmonic/semiconductor materials.

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.

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.

In the framework of piezoelectric/ferromagnetic patterned heterostructures, the purpose of this work is to electrically control the magnetic properties by tuning the morphology, especially by modifying the magnetic shape anisotropy through patterned strain. We have thus designed and studied a heterostructure with bottom nano-striped and top full film electrodes. ZnO piezoelectric and CoFeB magnetic materials were chosen to respond at critical criteria of its geometry. In addition, numerical simulations and magnetostatic calculations were performed to understand the reproduction of the pattern across the multiferroic heterostructure. Calculations have shown that the geometry of the heterostructure presents strict constraints, as for instance the distance between stripes versus the piezoelectric thickness. This study is a preliminary step towards reversible patterning of magnetic properties.