The effect of dielectric constants on noble metal/semiconductor SERS enhancement: FDTD simulation and experiment validation of Ag/Ge and Ag/Si substrates

Abstract

The finite-difference time-domain (FDTD) method was employed to simulate the electric field distribution for noble metal (Au or Ag)/semiconductor (Ge or Si) substrates. The simulation showed that noble metal/Ge had stronger SERS enhancement than noble metal/Si, which was mainly attributed to the different dielectric constants of semiconductors. In order to verify the simulation, Ag nanoparticles with the diameter of ca. 40 nm were grown on Ge or Si wafer (Ag/Ge or Ag/Si) and employed as surface-enhanced Raman scattering substrates to detect analytes in solution. The experiment demonstrated that both the two substrates exhibited excellent performance in the low concentration detection of Rhodamine 6G. Besides, the enhancement factor (1.3 × 10(9)) and relative standard deviation values (less than 11%) of Ag/Ge substrate were both better than those of Ag/Si (2.9 × 10(7) and less than 15%, respectively), which was consistent with the FDTD simulation. Moreover, Ag nanoparticles were grown in-situ on Ge substrate, which kept the nanoparticles from aggregation in the detection. To data, Ag/Ge substrates showed the best performance for their sensitivity and uniformity among the noble metal/semiconductor ones.

Figure 1. Intensity () distributions obtained from 3D FDTD calculations at wavelength 633 nm of silver nanoparticles, with 40 nm diameter and interparticle separation of 5 nm, on (a) Ge wafer and (b) Si wafer.

Figure 2. The SEM images of Ag NPs growing on Ge wafer: (a) in a low magnification, and (b) in a high magnification.

Figure 3. The high-resolution XPS spectra of (a) Ag 3 d, and (b) Ge 3 d.

Figure 4. Schematic of Ag NPs grown on the Ge wafer.

Figure 5. The Raman spectrum of R6G aqueous solution (1 × 10⁻¹⁰ M) on the as-prepared Ag/Ge substrate (upper part), and the SERS contour (lower part).

Figure 6. The intensities of the main Raman vibrations of R6G aqueous solution (1 × 10⁻¹⁰ M) in 200 spots SERS line-scan spectra collected on the as-prepared Ag/Ge substrate.

Figure 7. The Raman spectrum of MBA aqueous solution (at 1 × 10⁻⁹ M) on the as-prepared Ag/Ge substrate (upper part), and the SERS contour (lower part).

Figure 8. The intensities of the main Raman vibrations of MBA aqueous solution (at 1 × 10⁻⁹ M) in the 200 spots SERS line-scan spectra collected on the as-prepared Ag/Ge substrate.