Multi-Technique Approach for Work Function Exploration of Sc2O3 Thin Films.

Thin films based on scandium oxide (Sc2O3) were deposited on silicon substrates to investigate the thickness effect on the reduction of work function. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), energy dispersive X-ray reflectivity (EDXR), atomic force microscopy (AFM), and ultraviolet photoelectron spectroscopy (UPS) measurements were performed on the films deposited by electron-beam evaporation with different nominal thicknesses (in the range of 2-50 nm) and in multi-layered mixed structures with barium fluoride (BaF2) films. The obtained results indicate that non-continuous films are required to minimize the work function (down to 2.7 eV at room temperature), thanks to the formation of surface dipole effects between crystalline islands and substrates, even if the stoichiometry is far from the ideal one (Sc/O = 0.38). Finally, the presence of BaF2 in multi-layered films is not beneficial for a further reduction in the work function.

Surface Nanotexturing of Boron-Doped Diamond Films by Ultrashort Laser Pulses.

Polycrystalline boron-doped diamond (BDD) films were surface nanotextured by femtosecond pulsed laser irradiation (100 fs duration, 800 nm wavelength, 1.44 J cm-2 single pulse fluence) to analyse the evolution of induced alterations on the surface morphology and structural properties. The aim was to identify the occurrence of laser-induced periodic surface structures (LIPSS) as a function of the number of pulses released on the unit area. Micro-Raman spectroscopy pointed out an increase in the graphite surface content of the films following the laser irradiation due to the formation of ordered carbon sites with respect to the pristine sample. SEM and AFM surface morphology studies allowed the determination of two different types of surface patterning: narrow but highly irregular ripples without a definite spatial periodicity or long-range order for irradiations with relatively low accumulated fluences (<14.4 J cm-2) and coarse but highly regular LIPSS with a spatial periodicity of approximately 630 nm ± 30 nm for higher fluences up to 230.4 J cm-2.

Frenkel-Poole Mechanism Unveils Black Diamond as Quasi-Epsilon-Near-Zero Surface.

A recent innovation in diamond technology has been the development of the "black diamond" (BD), a material with very high optical absorption generated by processing the diamond surface with a femtosecond laser. In this work, we investigate the optical behavior of the BD samples to prove a near to zero dielectric permittivity in the high electric field condition, where the Frenkel-Poole (FP) effect takes place. Zero-epsilon materials (ENZ), which represent a singularity in optical materials, are expected to lead to remarkable developments in the fields of integrated photonic devices and optical interconnections. Such a result opens the route to the development of BD-based, novel, functional photonic devices.

Enhanced and Selective Absorption of Molybdenum Nanostructured Surfaces for Concentrated Solar Energy Applications.

Surfaces of commercial molybdenum (Mo) plates have been textured by fs-laser treatments with the aim to form low-cost and efficient solar absorbers and substrates for thermionic cathodes in Concentrated Solar Power conversion devices. Morphological (SEM and AFM), optical (spectrophotometry), and structural (Raman spectroscopy) properties of the samples treated at different laser fluences (from 1.8 to 14 J/cm2) have been characterized after the laser treatments and also following long thermal annealing for simulating the operating conditions of thermionic converters. A significant improvement of the solar absorptance and selectivity, with a maximum value of about four times higher than the pristine sample at a temperature of 800 K, has been detected for sample surfaces treated at intermediate fluences. The effects observed have been related to the light trapping capability of the laser-induced nanotexturing, whereas a low selectivity, together with a high absorptance, could be revealed when the highest laser fluence was employed due to a significant presence of oxide species. The ageing process confirms the performance improvement shown when treated samples are used as solar absorbers, even though, due to chemical modification occurring at the surface, a decrease of the solar absorptance takes place. Interestingly, the sample showing the highest quantity of oxides preserves more efficiently the laser texturing. The observation of this behaviour allows to extend the applicability of the laser treatments since, by further nanostructuring of the Mo oxides, it could be beneficial also for sensing applications.

Charge Transport Mechanisms of Black Diamond at Cryogenic Temperatures.

Black diamond is an emerging material for solar applications. The femtosecond laser surface treatment of pristine transparent diamond allows the solar absorptance to be increased to values greater than 90% from semi-transparency conditions. In addition, the defects introduced by fs-laser treatment strongly increase the diamond surface electrical conductivity and a very-low activation energy is observed at room temperature. In this work, the investigation of electronic transport mechanisms of a fs-laser nanotextured diamond surface is reported. The charge transport was studied down to cryogenic temperatures, in the 30−300 K range. The samples show an activation energy of a few tens of meV in the highest temperature interval and for T < 50 K, the activation energy diminishes to a few meV. Moreover, thanks to fast cycles of measurement, we noticed that the black-diamond samples also seem to show a behavior close to ferromagnetic materials, suggesting electron spin influence over the transport properties. The mentioned properties open a new perspective in designing novel diamond-based biosensors and a deep knowledge of the charge-carrier transport in black diamond becomes fundamental.

LIPSS Applied to Wide Bandgap Semiconductors and Dielectrics: Assessment and Future Perspectives.

With the aim of presenting the processes governing the Laser-Induced Periodic Surface Structures (LIPSS), its main theoretical models have been reported. More emphasis is given to those suitable for clarifying the experimental structures observed on the surface of wide bandgap semiconductors (WBS) and dielectric materials. The role played by radiation surface electromagnetic waves as well as Surface Plasmon Polaritons in determining both Low and High Spatial Frequency LIPSS is briefly discussed, together with some experimental evidence. Non-conventional techniques for LIPSS formation are concisely introduced to point out the high technical possibility of enhancing the homogeneity of surface structures as well as tuning the electronic properties driven by point defects induced in WBS. Among these, double- or multiple-fs-pulse irradiations are shown to be suitable for providing further insight into the LIPSS process together with fine control on the formed surface structures. Modifications occurring by LIPSS on surfaces of WBS and dielectrics display high potentialities for their cross-cutting technological features and wide applications in which the main surface and electronic properties can be engineered. By these assessments, the employment of such nanostructured materials in innovative devices could be envisaged.