In a recent paper published in Nanomaterials, researchers conducted a comprehensive investigation into the synthesis and nonlinear optical (NLO) properties of silver (Ag) and gold (Au) alloy nanoparticles (NPs) using a two-step pulsed laser ablation in liquid (PLAL) method. The study aimed to explore the impact of laser settings on the alloying process and to characterize the NPs using various techniques.
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Background
NLO materials have gained significant attention, particularly with advancements in ultrashort laser pulses. These materials are crucial for developing electro-optical and all-optical systems used in light-driven chemical processes, optical communications, and optical computing. Nanostructured materials, in particular, stand out due to their large surface-to-volume ratios and unique structures, often resulting in superior optical and electrical properties compared to bulk forms.
Metal NPs are a key area of research because of their outstanding optical properties and wide range of applications, including drug delivery, biosensing, optoelectronics, and catalysis. Ag and Au NPs, in particular, have gained interest for their unique features and broad applications. Quantum dots (QDs), tiny metal particles with diameters typically between 2 and 10 nm, are also significant due to their size-dependent electrical, optical, and catalytic properties.
About the Research
The study aimed to synthesize Ag/Au alloy NPs by re-irradiating a mixture of separately generated Ag and Au NPs using the PLAL technique. The researchers examined how laser settings, such as exposure time and intensity, influenced the alloying process. The alloy NPs were analyzed using transmission electron microscopy (TEM), ultraviolet-visible (UV-Vis) spectrophotometry, and inductively coupled plasma (ICP) analysis.
Additionally, the researchers used both closed-aperture (CA) and open-aperture (OA) Z-scan with femtosecond laser pulses to examine the nonlinear absorption coefficient (γ) and nonlinear refractive index (n2) of Ag, Au, and Ag/Au alloy NPs. The NLO properties of the Ag/Au alloy were examined at different wavelengths (750 and 850 nm) and laser powers (0.6 and 1.2 W). The study also explored the optical limiting (OL) effects of solutions containing Ag, Au, and Ag/Au alloy NPs.
Research Findings
The results showed the formation of Ag/Au alloy NPs, indicated by the appearance of a single peak between the Ag and Au NP peaks in the UV-Vis absorption spectra. This occurred at a laser intensity of 11.4 MW/cm² and an exposure time of 15 minutes. The average size of the combination decreased to 8.5 nm at 11.4 MW/cm² and further to 8 nm with 30 minutes of exposure, reaching the quantum dot size range (<10 nm).
The OA Z-scan measurements revealed that both monometallic (Ag and Au) and bimetallic (Ag/Au) solutions exhibited reverse saturable absorption (RSA) behavior, indicating a positive nonlinear γ. The CA Z-scan measurements showed that nonlinear n2 of the Ag, Au, and Ag/Au NP samples were linked to the self-defocusing effect, indicating a negative nonlinearity for the NPs. The NLA and NLR properties of the Ag/Au NPs colloids were influenced by the incident power and excitation wavelength.
The study also found that the OL effect of the Ag/Au alloy NPs was greater than that of the monometallic NPs, suggesting that the Ag/Au bimetallic NPs are more suitable for optical-limiting applications.
Applications
This research has several potential applications. The unique optical properties of Ag/Au alloy NPs, which can be tuned by adjusting laser parameters, make them useful in various fields. In photonics and optoelectronics, their NLO properties, such as the nonlinear n2 and γ, make them suitable for all-optical and electro-optical systems, optical communications, and optical computing. The superior OL effect of Ag/Au alloy NPs compared to monometallic NPs suggests their use as protective materials against unwanted laser exposure.
In biosensing and bioimaging, the distinctive optical characteristics of Ag/Au alloy NPs make them ideal for nanoscale optical biosensors, bioimaging, and biolabeling. Additionally, their size-dependent optical, electrical, and catalytic properties in the quantum size regime can be leveraged for advancements in catalysis and energy applications.
Conclusion
In summary, the researchers successfully synthesized Ag/Au alloy NPs using the PLAL method and investigated their NLO properties. They demonstrated that laser intensity and exposure time significantly influenced NP formation, with average sizes reaching the quantum dot range.
Future research should optimize the synthesis process to control the size, shape, and composition of Ag/Au alloy NPs, further enhancing their NLO properties and exploring their potential in different fields.
Journal Reference
Abd El-Salam, Y., et al. (2024). Using Femtosecond Laser Pulses to Explore the Nonlinear Optical Properties of Ag/Au Alloy Nanoparticles Synthesized by Pulsed Laser Ablation in a Liquid. Nanomaterials. DOI: 10.3390/nano14151290, https://www.mdpi.com/2079-4991/14/15/1290
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