Physical Synthesis and Study of Ag@CaF₂ Core@Shell Nanoparticles: Morphology and Tuning of Optical Properties

Abstract

Pre‐formed Ag nanoparticles (NPs) and Ag@CaF2 core–shell NPs are physically synthesized using DC magnetron‐based NP source and deposited on Si‐SiOx wafers. The samples are prepared by co‐depositing Ag nanoparticles and CaF2 produced by an evaporation source, or by sequential deposition method, i.e., by depositing in a sequence a CaF2 buffer layer, the Ag NPs generated by the NP source and a capping CaF2 layer. The supported films are characterized by Scanning Electron Microscopy (SEM), X‐ray Photoelectron Spectroscopy (XPS), and Surface Differential Reflectivity (SDR). SEM shows that Ag NPs deposited directly on Si‐SiOx tend to diffuse and to agglomerate, affecting the size distribution of the nanostructures. The presence of a CaF2 buffer layer between Ag and Si‐SiOx limits this effect, while XPS reveals electrical charging, caused by the insulating nature of the CaF2 continuous film. The surface plasmon resonance behavior for different samples is analyzed using SDR with p‐polarized light. There is a clear evidence of a blue shift in the plasmon excitation due to the presence of CaF2 on Si, which can represent a potential advantage for the technological applications in photovoltaics and optoelectronics.

Morphology and Optical Properties of Gas-Phase-Synthesized Plasmonic Nanoparticles: Cu and Cu/MgO

“…Overall, the microscopy results obtained on the Cu/MgO NP samples, the XPS data, and the intensity and position of the LSPR in the absorbance spectrum strongly indicate that MgO constitutes a good protective and transparent material for the Cu NPs, which are otherwise subject to oxidation under exposure to air. This result was also found for Ag/MgO [37] and Ag/CaF 2 [38] NPs that were grown using the same method. The formation of shells with a predominance of Cu(II) oxide around metallic Cu core was also found to be an effective way to block the effect of progressive oxidation [21,36].…”

“…These shapes are clearly an effect of the formation of MgO shells/matrices embedding the Cu NPs, in a similar way to what was observed on the Ag/MgO [37] system, obtained by using the same apparatus and synthesis method. On the basis of previous results [37,38], as well as the XPS data found in this study (see the next subsection), it can be inferred that this peculiar arrangement is essentially due to two main reasons: (1) a higher reactivity of Mg to O species, resulting in the preferential oxidation of Mg compared with the metal clusters during co-deposition, and (2) a higher sticking coefficient of MgO to metal NPs than inert substrates, such as Si/SiOx and carbon films used in TEM experiments. This synthesis recipe results in either the formation of metal- The effect of co-deposition of Cu NPs and Mg in an oxygen atmosphere on the NP film morphology is clearly shown in Figure 4, where a SEM image obtained from Cu nanoclusters and MgO co-deposited on Si/SiO x is shown.…”

“…An alternative one-step procedure is the co-deposition of the pre-formed NPs with an inert and transparent material that can act either as a matrix or as a shell. This method has been successfully tested on Ag NPs co-deposited with MgO [37] and CaF 2 [38]. Ag/MgO NPs produced in this way were also deposited on the porous TiO 2 layer of perovskite solar cells, giving rise to an increase in PCE by 5% [15].…”