Objective Plasma nanostructures composed of multiple metals have been widely applied in various fields such as photocatalysis, medical imaging, solar cells, surface-enhanced Raman scattering (SERS), biosensors, and information technology, due to their localized optical near-field properties and surface plasmon resonance effects. Compared with single-metal nanostructures, multi-metal plasma nanostructures exhibit significant enhanced resonance effects in the UV-VIS wavelength range. At present, there are few studies on multi-metal plasmonic nanostructures, and the fabrication methods are complicated, such as tedious processing, poor controllability, and long preparation period. Therefore, in this study, a scheme based on multi-metal thin film plasma nanostructures was designed, and simulation methods were used to demonstrate that the designed multi-metal plasma nanostructures have the characteristic of enhanced electric field. Furthermore, multi-metal plasma nanostructures were fabricated and evaluated using Rhodamine 6G (R6G) with a femtosecond laser direct writing system, demonstrating the enhanced SERS signal of the structure.
Methods This article describes the construction of a femtosecond laser direct writing system. A titanium-sapphire oscillator laser (with an output power of 3.5 W, a central wavelength of 800 nm, and a repetition frequency of 85 MHz) is used as the femtosecond laser source (Fig.1). Magnetron sputtering technology was used to deposit a dual-layered gold-silver metal film on a silicon dioxide substrate. Rhodamine (R6G) solution was used as the test molecule for evaluating the SERS performance of multi-metal plasmonic nanocavity structures. Confocal Raman spectroscopy imaging was used to analyze the SERS performance of the multi-metal plasmonic nanocavity structures.
Results and Discussions A multi-metal plasmonic nano-cavity structure was fabricated using a femtosecond laser direct writing system. Different sizes of nanoparticles were produced by adjusting the laser power and pulse irradiation time. The three-dimensional morphology of the experimental results was characterized using AFM and SEM, verifying the size variation law of multi-metal plasmonic nanostructures fabricated by femtosecond laser processing (Tab.1, Tab.2). The FDTD simulation software was used to simulate and analyze the changes in the electric field intensity. The electric field distribution of the planar metal was clearly reorganized, mainly concentrated at the edge of the metal plasmonic nanostructure, and the electric field intensity of the multi-metal plasmonic nanostructure was significantly enhanced compared to that of the single metal, usually manifested as an increase in the localized surface plasmon resonance effect (Fig.2, Fig.3). Evaluation using Rhodamine (R6G) solution showed that the gold-silver bilayer metal plasmonic nanostructure exhibited a stronger Raman signal, while the single-layer planar metal film still did not show any peak (Fig.5, Fig.6).
Conclusions Based on the high-precision, high-flexibility, simple and convenient femtosecond laser processing technology, the metal plasmonic nanostructures were directly fabricated on the surface of metal thin films in this study. Through continuous optimization of processing parameters, uniform and regular nanostructures were obtained, and the structure was characterized to demonstrate the significant enhancement of localized surface plasmon resonance in multi-metal plasmonic nanostructures. Surface-enhanced Raman scattering (SERS) signal enhancement was verified using Rhodamine (R6G). The Raman test results showed that the structure had excellent SERS signal enhancement performance. Experimental simulations were performed using FDTD software, and the results showed that the electric field intensity between multi-metal plasmonic nanostructures was significantly enhanced. Femtosecond lasers can be used to process any material, such as semiconductors, polymers, alloys, and others, with various processing methods. In the future, spatiotemporally shaped femtosecond laser direct writing technology will be used to expand the size processing range of femtosecond lasers and control more material properties.
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