Francesca Sarto

Heterojunction solar cell with 2% efficiency based on a Cu2O substrate

We report on the fabrication of heterojunction solar cells made by deposition of transparent conducting oxide (TCO) films on Cu2O substrates. The TCO films have been grown by ion beam sputtering on good quality Cu2O sheets prepared by oxidizing copper at a high temperature. The best solar cell has reached an open-circuit voltage of 0.595V, a short-circuit current density of 6.78mA∕cm2, a fill factor of 50%, and a conversion efficiency of 2% under simulated AM1.5G illumination, which is the highest efficiency value reported for this kind of heterojunction devices. These devices represent a good starting point for the development of very low cost solar cells.

Nanotechnology of transparent metals for radio frequency electromagnetic shielding

The aim of this paper is to present an innovative one-dimensional photonic bandgap structure on plastic substrate, for electromagnetic field shielding applications in the radio frequency range. A complete study is performed, from material characterization and design, to fabrication and experimental test of a prototype sample consisting of seven alternating zinc oxide and silver (Ag) layers, on Lexan polycarbonate. The transparent metal is designed to obtain high shielding effectiveness (SE), not just as a theoretical prediction, but as experimentally measured according to the standard ASTM-D 4935/89. Actually, the shielding performance of multilayered coatings having similar values of sheet resistance but different design are strongly affected by the bonding conditions. The following properties of the structure, which contains only 66 nm of Ag, are highlighted: 1) it remains transparent in the visible range; 2) it retains the conductivity of bulk metal; 3) it has SE in the 30-kHz to 1-GHz range over 40 dB; 4) the overall thickness of the multilayer shielding coating is 312 nm; and 5) it has been realized on both glass or plastic substrate.

Accessing the optical limiting properties of metallo-dielectric photonic band gap structures

The optical limiting properties of a one-dimensional, transparent metallodielectric photonic band gap structure are studied. Due to light confinement in the structure that enhances the nonlinear response of the layers, a nonlinear transmission dependence on the incident light intensity is found. Experimental results are reported for a four period sample where the single period consists of ZnO and Ag layers 109 and 17 nm thick, respectively. The structure was designed to exhibit a transmission resonance at 532 nm. Under the action of a Q-switched frequency doubled Nd:yttrium–aluminum–garnet laser, a decrease in transmission of approximately 50% is obtained for a maximum incident light intensity of 2 GW/cm2. These results are explained in terms of a dynamic change of the absorption coefficient due to the enhancement of the two-photon absorption process. These results suggest that the structure is suitable for optical limiting applications in the visible range.

Nanolayered lightweight flexible shields with multidirectional optical transparency

New nanolayered coatings are designed and deposited on flexible plastic substrate having the thickness of 100 /spl mu/m, in order to realize lightweight ultrathin transparent shielding foils. The structure of the coating is optimized considering three figures of merit: the average transmittance in the visible range for normal incidence, the normalized average transmittance for oblique incidence at 550 nm, and the transmittance quality factor. The nanotechnology exploited for the deposition of the transparent metals is the dual ion beam sputtering. Tests of durability, optical transmission, and shielding effectiveness demonstrate that the film has a high adhesion under mechanical solicitation, high resistance against aging, peak transmittance in the visible range higher than 70%, omnidirectional properties in the range 0/spl deg/-60/spl deg/, and shielding effectiveness of 40 dB up to 6 GHz.

Laser damage dependence on structural and optical properties of ion-assisted HfO2 thin films

Abstract Laser damage studies at 248 nm (KrF excimer laser) have been performed on HfO 2 films of 300 nm thickness deposited on silica substrates by the Xe ion-assisted electron beam evaporation technique. The assistance parameters (ion mass, energy and current density) have been adjusted to investigate the effect of the Xe-ion momentum transfer parameter P on the film optical and structural properties. Then, the dependence of laser damage fluence on film properties has been studied. Higher laser damage fluences have been found for the HfO 2 films deposited at lower P values and characterized by a fully crystalline structure with the grain of smaller size and randomly oriented.

Adhesion enhancement of optical coatings on plastic substrate via ion treatment

The main limitation in using plastics for optical components is the softness of their surface, which is responsible for low impact and abrasion resistance and for weak adhesion between the film and the substrate. Protecting the substrate with a hard film can increase the impact resistance of polymeric lenses. However, deposition of thin films on chemically cleaned plastic substrates has resulted in fast delamination of the coating. This paper discusses the problems associated with coated plastics, it presents the ion assistance as one of the solutions and it finally identifies the relevant parameters. A hard oxide layer with additional ion pre-treatment is proposed as protection for plastic optics. The adhesion between the film and the substrate has been evaluated by a scratch-test and the hardness has been calculated by measuring the radius of the stylus track.

Synthesis and characterization of ZnO nanorods with a narrow size distribution

The development of novel materials for energy harvesting applications or strain sensing has generated great interest towards zinc oxide (ZnO) nanostructures, and in particular towards the synthesis of ZnO nanowires or nanorods with well controlled morphology and properties. The high-yield mass production of such nanostructures by catalyst-free methods is a crucial aspect to enable a cost-effective large-scale development of new ZnO-based piezoelectric devices and materials. In the present work, we propose a method for the mass-production of high-purity ZnO-nanorods with a uniform size distribution, based on the combination of thermal decomposition of zinc acetate dihydrate and probe sonication in acetone. The quality of the produced ZnO nanorods is assessed through multi-technique characterization using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and photo-luminescence spectroscopy (PL). The adopted synthesis method is simple, cost effective and feasible for large-scale production. Various process parameters such as precursor amount and growth time have been found to play an important role in controlling the formation of the as grown nanostructures with high uniformity in size and morphology. Size distribution curves were employed to depict the effect of various process parameters for tailoring the morphology, homogeneity and aspect ratio of the nanorods. Our results reveal that the high crystallographic quality of ZnO nanorods grown by a long-time thermal decomposition method is not affected by probe sonication, which is proposed as a post-synthesis step necessary to produce ZnO nanorod powder with a uniform distribution of diameters and lengths.

Nitrogen-doped graphene films from chemical vapor deposition of pyridine: Influence of process parameters on the electrical and optical properties

Summary Graphene films were produced by chemical vapor deposition (CVD) of pyridine on copper substrates. Pyridine-CVD is expected to lead to doped graphene by the insertion of nitrogen atoms in the growing sp2 carbon lattice, possibly improving the properties of graphene as a transparent conductive film. We here report on the influence that the CVD parameters (i.e., temperature and gas flow) have on the morphology, transmittance, and electrical conductivity of the graphene films grown with pyridine. A temperature range between 930 and 1070 °C was explored and the results were compared to those of pristine graphene grown by ethanol-CVD under the same process conditions. The films were characterized by atomic force microscopy, Raman and X-ray photoemission spectroscopy. The optical transmittance and electrical conductivity of the films were measured to evaluate their performance as transparent conductive electrodes. Graphene films grown by pyridine reached an electrical conductivity of 14.3 × 105 S/m. Such a high conductivity seems to be associated with the electronic doping induced by substitutional nitrogen atoms. In particular, at 930 °C the nitrogen/carbon ratio of pyridine-grown graphene reaches 3%, and its electrical conductivity is 40% higher than that of pristine graphene grown from ethanol-CVD.

Radiation resistance of single and multilayer coatings against synchrotron radiation

Optical coatings for the use in free electron laser systems have to withstand high power laser radiation and the intense energetic background radiation of the synchrotron radiation source. In general, the bombardment with high energetic photons leads to irreversible changes and a discoloration of the specimen. For the development of appropriate optical coatings, the degradation mechanisms of available optical materials have to be characterized. In this contribution the degradation mechanisms of single layer coatings (fluoride and oxide materials) and multilayer systems will be presented. Fluoride and oxide single layers were produced by thermal evaporation and high energetic ion beam sputter deposition. The same methods were employed for the deposition of multilayer systems. High reflecting coatings for the wavelength region around 180 nm were chosen for the irradiation tests. All samples were characterized after production by spectrophotometry covering the VUV , VIS, and MIR spectral range. Mechanical coating stress was evaluated with interferometric methods. Synchrotron irradiation tests were performed at ELETTRA, using a standardized irradiation cycle for all tests. Ambient pressure and possible contamination in the vacuum environment were monitored by mass spectrometry. For comparison, the optical coatings were investigated again in the VUV, VIS, and MIR spectral range after irradiation. On selected samples XRD measurements were performed. The observed degradation mechanisms comprise severe damages like coating and substrate surface ablation. Color centre formation in the VIS spectral range and an increase of VUV absorption were found as a major origin for a severe degradation of VUV transmittance On the basis of the performed investigations, a selection of coating materials and coating systems is possible in respect to the damage effects caused by synchrotron radiation.

The influence of ion mass and energy on the composition of IBAD oxide films

Abstract Dual ion-beam sputtering deposition is a very promising technique for fabricating optical coatings thanks to its very good control of the deposition parameters. Unfortunately, the different sputtering yields of the elements composing the films modify the stoichiometry and, consequently, may cause an increase of the absorption in the UV range and an undesirable variation of the refractive index. In this work we investigate the influence of ion-beam assistance on some oxide materials: SiO 2 , ZrO 2 , and HfO 2 . The sputtering yield has been measured by varying the mass (Ar, Xe) and energy (100–1000 eV) of the bombarding ions. The yield measurements were compared with the calculated yields using Sigmunds model. Different screening functions for different characteristic energy ranges were necessary to fit the experimental results.