Currently, there are various technologies for controlling the electric field using the micro-nano structure of noble metal films, including electron beam lithography (EBL) and focused ion beam (FIB) etching. Most of these processing methods are complicated, with limited processing areas and high costs. These limitations significantly restrict the development of photoelectric functional films with electric field enhancement characteristics. According to current theories, the enhancement mechanism of surface-enhanced Raman scattering (SERS) technology can be divided into electromagnetic enhancement mechanism (EM) and chemical enhancement mechanism (CM). Compared to CM, EM generally exhibits stronger increases in the Raman scattering signals. Therefore, it is important to develop EM-based SERS substrates. The enhancement of the electromagnetic field mainly originates from the local surface plasmon resonance (LSPR) of the metal nanoparticles, which is independent of the adsorbed molecules and is an inherent property of metal nanoparticles. Therefore, classical electronic dynamics methods such as the finite difference time domain (FDTD) method can be used to describe the electromagnetic field. The purpose of this study is to develop a large-area, low-cost, and reproducible SERS substrate using FDTD simulations combined with mechanical grating ruling and electron beam evaporation deposition.
In this study, Silver (Ag) films with different blazed grating structures are simulated using FDTD method. The LSPR of the Ag films with blazed grating structures is significantly enhanced by adjusting the period. Under excitation at 633 nm, we find that Ag films with a grating period of 1/1200 mm generate a significant LSPR effect. Large-area electric-field-enhanced Ag grating films that can be produced in batches are realized using a mechanical grating ruling process and electron beam evaporation deposition. We successfully apply this electric field-enhanced Ag film to SERS to detect methylene blue dye. The Ag grating films significantly enhance the SERS intensity, and the test results show good uniformity and reproducibility of the substrate, consistent with the FDTD simulation results.
As shown in Fig. 1, FDTD software is used to design and simulate two types of blazed grating film structures, and the electric field intensity and absorption spectra of the Ag films are compared. The results of the FDTD simulation (Fig. 2) show that compared to the Ag film with a grating structure, the pure Ag film exhibits lower electric field intensity and light absorption. Compared with other two types of structures, the Ag films on Grating 1 exhibit the strongest electric field intensity and absorbance in the local area, indicating that the Ag films with a grating structure exhibit a stronger LSPR effect at 633 nm. Blazed Grating 1 and Grating 2 structures are prepared using a mechanical scribing process and the required diffraction gratings are produced in batches through grating replication. Finally, the processed gratings are coated with Ag films through electron beam evaporation deposition to obtain Ag grating films with the required electric field enhancement effect. The prepared Ag films with grating structures are characterized using X-ray diffraction (XRD) (Fig. 4), atomic force microscopy, and optical microscopy (Fig. 5). The microscopic images and XRD results illustrate that the large-area Ag films with a grating structure are successfully prepared. The surface uniformity of the prepared gratings is good with equal spacing between the lines. A Raman spectrometer is used to verify the electric-field enhancement ability of films on Grating 1. As shown in Fig. 6(a), the Ag thin films with a grating structure produce more 'hot spots,' which significantly enhances the local electromagnetic field on the surfaces of the grating samples, thus substantially improving the Raman scattering signal of methylene blue. Figures. 6(b)‒(d) show that the corresponding relative standard deviation (RSD) values of the characteristic peaks at 1623 cm-1 and 1394 cm-1 for methylene blue in different sites of the Grating 1 sample are less than 17%, confirming the good reproducibility of the SERS substrate.
In this study, a simple and low-cost technology for fabricating large-area patterned electric-field-enhanced Ag films is proposed. First, Ag films with a blazed grating structure is simulated using the FDTD method, and a film structure with strong electric field enhancement at 633 nm wavelength is obtained by adjusting the period, which effectively tunes the LSPR effect on the film surface. Ag thin films with corresponding structures are prepared using mechanical grating ruling and electron-beam evaporation deposition. Through verification by a Raman spectrometer, the Ag films with a blazed grating structure improve the SERS intensity, consistent with the simulation results. This electric-field-enhanced film provides an efficient substrate for the enhancement of Raman scattering signals and an idea for the actual detection of trace molecules.
