Can “Hot Spots” Be Stable Enough for Surface-Enhanced Raman Scattering?
Mei Zhu, Moxia Li, Mengke Su, Jianfang Liu, Bingwu Liu, Yongjie Ge, Honglin Liu, Jiawen Hu
Abstract
Because of the random distribution and nonuniform enhancement of hot spots, surface-enhanced Raman scattering (SERS) usually shows poor spectral reproducibility, which greatly limits its practical applications, especially quantitative SERS detection. Here, we report our finding that the instability of hot spots is also a key limiting factor for the poor spectral reproducibility of SERS. We show that hot spots show a stability threshold to the excitation power, duration, and wavelength. During SERS measurement, large spectral fluctuation is observed at or above a threshold power because the laser used reshapes (here, weld) the hot spots, causing a continuous decrease in SERS intensity. The threshold power also differs for different excitation wavelengths because of their different resonance extents with the plasmon of the hot spots. Finally, we show that a random SERS substrate can deliver much improved stable, reproducible SERS signals when lowering the excitation power below the threshold or deliberately removing the more unstable hot spots. These results offer a new understanding to the spectral reproducibility of SERS, greatly facilitating reliable SERS detection and the design of robust hot spots.