Zacarias Eduardo Fabrim

Tuning the optoelectronic properties of amorphous MoOx films by reactive sputtering

In this letter, we report on the effect of oxygen partial pressure and sputtering power on amorphous DC-sputtered MoOx films. We observe abrupt changes in the optoelectronic properties of the reported films by increasing the oxygen partial pressure from 1.00 × 10−3 mbar to 1.37 × 10−3 mbar during the sputtering process. A strong impact on the electrical conductivity, varying from 1.6 × 10−5 S/cm to 3.22 S/cm, and on the absorption coefficient in the range of 0.6–3.0 eV is observed for the nearly stoichiometric MoO3.00 and for the sub-stoichiometric MoO2.57 films, respectively, without modifying significantly the microstructure of the studied films. The presence of states within the band gap due to the lack of oxygen is the most probable mechanism for generating a change in electrical conductivity as well as optical absorption in DC-sputtered MoOx. The large tuning range of the optoelectronic properties in these films holds strong promise for their implementation in optoelectronic devices.

Aging effects on the nucleation of Pb nanoparticles in silica

The ion beam synthesis of Pb nanoparticles (NPs) in silica is studied in terms of a two step thermal annealing process consisting of a low temperature long time aging treatment followed by a high temperature short time one. The samples are investigated by Rutherford backscattering spectrometry and transmission electron microscopy. The results obtained show that highly stable Pb trapping structures are formed during the aging treatment. These structures only dissociate at high temperatures, inhibiting the nucleation of NPs in the metallic phase and causing an atomic redistribution that renders the exclusive formation of a two dimensional, uniform and dense array of Pb NPs at the silica–silicon interface. The results are discussed on the basis of classic thermodynamic concepts.

Stabilization of perpendicular magnetic anisotropy in CeO2 films deposited on Co/Pt multilayers

Materials with perpendicular magnetic anisotropy (PMA) are of great interest as they have potential applications in high-density non-volatile memories, spin logic devices, and other spintronics applications. To attain perpendicular anisotropy, a number of material systems have been explored as ferromagnetic electrodes. Here, we use (Co/Pt)-multilayered films with PMA covered by a gold spacer-layer to induce the perpendicular magnetization in a ferromagnetic layer of cerium oxide and to control the reversible switching of its magnetization. The origin of the room-temperature ferromagnetism observed in nanocrystalline cerium oxide films remains controversial, but their wide energy band-gap and their transparency to visible light attracts attention for possible applications in magneto-optical devices. A weak magnetic stray field of 40 Oe emanates from the (Co/Pt)-multilayered film and permeates the gold spacer layer. Using a simple micromagnetic model based on the Stoner–Wohlfarth magnetization mechanism, the strength of the magnetic coupling between the ferromagnetic layers is estimated to be 18 μJ m−2. This magnetic coupling, which is almost independent of temperature, is sufficient to promote the reversible switching of perpendicular magnetization states in the field range of a only few hundred Oersteds at room temperature.

Tailoring the blueviolet photoluminescence from Sn-implanted SiO 2 using a two-step annealing process

The blue?violet photoluminescence (PL) response of ion beam synthesized Sn nanoparticles (NPs) in silica is studied in samples submitted to a two-step thermal annealing process. This process consists of a low temperature and long time ageing treatment (523?K, 100?h) followed by a high temperature and short time one (T?>?773?K, 0.5?h). The comparison between aged and non-aged control samples by means of transmission electron microscopy observations and Rutherford backscattering spectrometry measurements demonstrates that the ageing treatment causes the formation of a more thermally stable and dense array of rather small Sn NPs. This phenomenon is correlated with the enhancement in the blue?violet PL intensity caused by the formation of luminescent defects at the NP/matrix interface region, persisting even at annealing temperatures as high as 1373?K.

Electron irradiation effects on the nucleation and growth of Au nanoparticles in silicon nitride membranes

The formation of Au nanoparticles (NPs) in Au+ ion-implanted silicon nitride thin films and membranes was investigated as a function of post-implantation thermal treatments or room temperature electron irradiation at energies of 80, 120, 160, and 200 keV. The samples were characterized by Rutherford Backscattering Spectrometry and Transmission Electron Microscopy. High-temperature thermal annealing (1100 °C, 1 h) resulted in the formation of Au particles with a mean diameter of  ≈1.3 nm. In comparison, room-temperature electron irradiation at energies from 80 to 200 keV caused the formation of larger Au particles according to two growth regimes. The first regime is characterized by a slow growth rate and occurs inside the silicon nitride membrane. The second regime presents a fast growth rate and starts when Au atoms become exposed to the back free surface of the membrane. Realistic binary electron-atom elastic collision cross-sections were used to analyze the observed nanoparticle growth and membrane spu...

Titanium Nitride as a Strain Gauge Material

The performance of titanium nitride (Ti-N) thin films deposited by reactive magnetron sputtering for use as a strain gauge material was investigated. Films of different composition ranging from pure titanium to over-stoichiometric Ti-N were prepared by varying the nitrogen flow to total flow ratio (α). The atomic composition was measured using Rutherford backscattering spectrometry and energy-dispersive X-ray spectroscopy. The structure of the films was studied using X-ray diffraction and scanning electron microscopy, revealing the formation of a columnar film morphology consisting of cubicTi-N grains with preferential crystal orientations depending on the flow ratio α. The stability of the film resistivity was studied at 200 °C for 360 h. Films deposited at low (α ≤ 0.08) and high (α = 0.60) nitrogen flows exhibited a stable behavior relative to films deposited with intermediate nitrogen flows. The longitudinal piezoresistive gauge factor was found to increase with increasing nitrogen flow before settling at values of 6-6.35. Finally, the temperature coefficient of resistivity was found to be less than 300 ppm/°C for the films with the highest gauge factors. The results demonstrate that the Ti-N films have a good potential as a strain gauge material, especially due to the relatively high gauge factor.