Tuning the Refractive Index Sensitivity of LSPR Transducers Based on Nanocomposite Thin Films Composed of Noble Metal Nanoparticles Dispersed in TiO2.

This work reports on the development of nanoplasmonic thin films consisting of Au, Ag, or Au-Ag nanoparticles dispersed in a TiO2 matrix and the optimization of the deposition parameters to tune their optical response. The thin films were produced by reactive DC magnetron sputtering of a Ti target with Au and/or Ag pellets placed on the erosion zone. The thicknesses (50 and 100 nm) of the films, the current density (75 and 100 A/m2) applied to the target (titanium), and the number of pellets placed on its surface were the deposition conditions that were used to tailor the optical (LSPR) response. The total noble metal content varied between 13 and 28 at.% for Au/TiO2 films, between 22 and 30 at.% for Ag/TiO2 films, and 8 to 29 at% for the Au-Ag/TiO2 systems with 1:1, 1:1.5, and 1:2 Au:Ag atomic ratios. After thermal annealing at 400 and 600 °C, LSPR bands were found for all films concerning the Au-TiO2 and Au-Ag/TiO2, while for Ag/TiO2, only for thin films with 28 and 30 at.% of Ag concentration. Refractive index sensitivity (RIS) was evaluated for Au and Au-Ag/TiO2 thin films. It was found that for bimetallic nanoparticles, the sensitivity can increase up to five times when compared to a monometallic nanoplasmonic system. Using Au-Ag/TiO2 thin films can decrease the cost of fabrication of LSPR transducers while improving their sensitivity.

The Role of Hydrogen Incorporation into Amorphous Carbon Films in the Change of the Secondary Electron Yield.

Over the last few years, there has been increasing interest in the use of amorphous carbon thin films with low secondary electron yield (SEY) to mitigate electron multipacting in particle accelerators and RF devices. Previous works found that the SEY increases with the amount of incorporated hydrogen and correlates with the Tauc gap. In this work, we analyse films produced by magnetron sputtering with different contents of hydrogen and deuterium incorporated via the target poisoning and sputtering of CxDy molecules. XPS was implemented to estimate the phase composition of the films. The maximal SEY was found to decrease linearly with the fraction of the graphitic phase in the films. These results are supported by Raman scattering and UPS measurements. The graphitic phase decreases almost linearly for hydrogen and deuterium concentrations between 12% and 46% (at.), but abruptly decreases when the concentration reaches 53%. This vanishing of the graphitic phase is accompanied by a strong increase of SEY and the Tauc gap. These results suggest that the SEY is not dictated directly by the concentration of H/D, but by the fraction of the graphitic phase in the film. The results are supported by an original model used to calculate the SEY of films consisting of a mixture of graphitic and polymeric phases.

Optimization of Au:CuO Thin Films by Plasma Surface Modification for High-Resolution LSPR Gas Sensing at Room Temperature.

In this study, thin films composed of gold nanoparticles embedded in a copper oxide matrix (Au:CuO), manifesting Localized Surface Plasmon Resonance (LSPR) behavior, were produced by reactive DC magnetron sputtering and post-deposition in-air annealing. The effect of low-power Ar plasma etching on the surface properties of the plasmonic thin films was studied, envisaging its optimization as gas sensors. Thus, this work pretends to attain the maximum sensing response of the thin film system and to demonstrate its potential as a gas sensor. The results show that as Ar plasma treatment time increases, the host CuO matrix is etched while Au nanoparticles are uncovered, which leads to an enhancement of the sensitivity until a certain limit. Above such a time limit for plasma treatment, the CuO bonds are broken, and oxygen is removed from the film's surface, resulting in a decrease in the gas sensing capabilities. Hence, the importance of the host matrix for the design of the LSPR sensor is also demonstrated. CuO not only provides stability and protection to the Au NPs but also promotes interactions between the thin film's surface and the tested gases, thereby improving the nanocomposite film's sensitivity. The optimized sensor sensitivity was estimated at 849 nm/RIU, which demonstrates that the Au-CuO thin films have the potential to be used as an LSPR platform for gas sensors.

Metallic filamentary conduction in valence change-based resistive switching devices: the case of TaOx thin film with x∼ 1.

The resistive switching in metal-oxide thin films typically occurs via modulation of the oxygen content in nano-sized conductive filaments. For Ta2O5-based resistive switching devices, the two current models consider filaments composed of oxygen vacancies and those containing metallic Ta clusters. The present work tries to resolve this dispute. The filaments in Ta2O5 were formerly shown to exhibit the same electrical transport mechanisms as TaOx thin films with x∼ 1.0. In this paper, sputtered thin films of pure ß-Ta and of TaOx with different oxygen concentrations are studied and compared in terms of their structure and electrical transport. The structural analysis reveals the presence of Ta clusters in the TaOx films. Identical electrical transport characteristics were observed in the TaOx films with x∼ 1.0 and in the ß-Ta film. Both show the same transport mechanism, a carrier concentration on the order of 1022 cm-3 and a positive magnetoresistance associated with weak antilocalization at T < 30 K. It is concluded that the electrical transport in the TaOx films with x∼ 1.0 is dominated by percolation through Ta clusters. This means that the transport in the filaments is also determined by percolation through Ta clusters, strongly supporting the metallic Ta filament model.

Low-temperature fabrication of layered self-organized Ge clusters by RF-sputtering.

In this article, we present an investigation of (Ge + SiO2)/SiO2 multilayers deposited by magnetron sputtering and subsequently annealed at different temperatures. The structural properties were investigated by transmission electron microscopy, grazing incidence small angles X-ray scattering, Rutherford backscattering spectrometry, Raman, and X-ray photoelectron spectroscopies. We show a formation of self-assembled Ge clusters during the deposition at 250°C. The clusters are ordered in a three-dimensional lattice, and they have very small sizes (about 3 nm) and narrow size distribution. The crystallization of the clusters was achieved at annealing temperature of 700°C.