Investigating the thermal stability of ultra-small Ag, Au and AuAg alloy nanoparticles embedded in a silica matrix.

Thermal growth kinetics of embedded bimetallic (AuAg/SiO2) nanoparticles are explored and compared with their monometallic (Au/SiO2 and Ag/SiO2) counterparts, as their practical applicability demands stability and uniformity. The plasmonic properties of these nanoparticles (NPs) significantly improve when their size falls in the ultra-small region (diameter < 10 nm), owing to their large active surface area. Interestingly, the bimetallic NPs exhibit better optical properties and structural stability as compared to their monometallic counterparts. This calls for a thorough understanding of the nucleation and temperature-dependent growth to ensure size stability against thermal coarsening that most bimetallic NPs completely lack. Herein, the atom beam sputtered AuAg NPs are systematically analysed over a wide range of annealing temperatures (ATs), and the results are compared with those of Au and Ag NPs. The X-ray photoelectron spectroscopy spectra and other experimental results confirm the formation of AuAg alloy NPs inside the silica matrix. Furthermore, techniques like transmission electron microscopy and grazing-incidence small-/wide-angle X-ray scattering were used to explore the temperature-dependent structural and morphological stability of the NPs. Our results show that the deposited AuAg NPs retain their spherical shape and remain as an alloy for the entire range of ATs. When the AT increases from 25 °C to 800 °C, the size of the NPs also increases from 3.5 to 4.8 nm; beyond that, their size grows substantially to 13.6 nm at 900 °C. We observed that the NPs remain in the ultra-small size range (∼5 nm) until an AT of 800 °C. Beyond that Ostwald ripening is ascribed to be the major cause of particle growth, resulting in an active surface area loss. Based on the outcomes, a three-step nucleation and growth mechanism is proposed.

A Combinatorial Study Investigating the Growth of Ultrasmall Embedded Silver Nanoparticles upon Thermal Annealing.

Ultrasmall nanoparticles (NPs) with a high active surface area are essential for optoelectronic and photovoltaic applications. However, the structural stability and sustainability of these ultrasmall NPs at higher temperatures remain a critical problem. Here, we have synthesized the nanocomposites (NCs) of Ag NPs inside the silica matrix using the atom beam co-sputtering technique. The post-deposition growth of the embedded Ag NPs is systematically investigated at a wide range of annealing temperatures (ATs). A novel, fast, and effective procedure, correlating the experimental (UV-vis absorption results) and theoretical (quantum mechanical modeling, QMM) results, is used to estimate the size of NPs. The QMM-based simulation, employed for this work, is found to be more accurate in reproducing the absorption spectra over the classical/modified Drude model, which fails to predict the expected shift in the LSPR for ultrasmall NPs. Unlike the classical Drude model, the QMM incorporates the intraband transition of the conduction band electrons to calculate the effective dielectric function of metallic NCs, which is the major contribution of LSPR shifts for ultrasmall NPs. In this framework, a direct comparison is made between experimentally and theoretically observed LSPR peak positions, and it is observed that the size of NPs grows from 3 to 18 nm as AT increases from room temperature to 900 °C. Further, in situ grazing-incidence small- & wide-angle X-ray scattering and transmission electron microscopy measurements are employed to comprehend the growth of Ag NPs and validate the UV + QMM results. We demonstrate that, unlike chemically grown NPs, the embedded Ag NPs ensure greater stability in size and remain in an ultrasmall regime up to 800 °C, and beyond this temperature, the size of NPs increases exponentially due to dominant Ostwald ripening. Finally, a three-stage mechanism is discussed to understand the process of nucleation and growth of the silica-embedded Ag NPs.

Localized thermal spike driven morphology and electronic structure transformation in swift heavy ion irradiated TiO2 nanorods.

Irradiation of materials by high energy (∼MeV) ions causes intense electronic excitations through inelastic transfer of energy that significantly modifies physicochemical properties. We report the effect of 100 MeV Ag ion irradiation and resultant localized (∼few nm) thermal spike on vertically oriented TiO2 nanorods (∼100 nm width) towards tailoring their structural and electronic properties. Rapid quenching of the thermal spike induced molten state within ∼0.5 picosecond results in a distortion in the crystalline structure that increases with increasing fluences (ions per cm2). Microstructural investigations reveal ion track formation along with a corrugated surface of the nanorods. The thermal spike simulation validates the experimental observation of the ion track dimension (∼10 nm diameter) and melting of the nanorods. The optical absorption study shows direct bandgap values of 3.11 eV (pristine) and 3.23 eV (5 × 1012 ions per cm2) and an indirect bandgap value of 3.10 eV for the highest fluence (5 × 1013 ions per cm2). First principles electronic structure calculations corroborate the direct-to-indirect transition that is attributed to the structural distortion at the highest fluence. This work presents a unique technique to selectively tune the properties of nanorods for versatile applications.