Simultaneously improved SERS sensitivity and thermal stability on Ag dendrites via surface protection by atomic layer deposition

Silver has been considered as one of the most promising materials for surface-enhanced Raman scattering (SERS) thanks to its ultra-high sensitivity. Nevertheless, Ag based nanostructures suffer from both thermal and chemical instability, inhibiting their practical applications. Surface coating is effective for improving their stability, nevertheless, which sacrifices the SERS sensitivity. Here we proposed a novel strategy to fabricate highly sensitive and stable SERS substrates using atomic layer deposition (ALD) protected Ag nanogaps. The purpose of this work is to provide an ultrathin and conformal protective layer without destroying the hot spots in nanogaps. This idea was implemented in ALD protected Ag dendrites. Ultrathin ALD layer with precisely controlled thickness can be conformally coated on the Ag surface without blocking the nanogaps between adjacent branches. Experimental results revealed Ag dendrites@10-cycle ZnO exhibit the improved SERS sensitivity compared to the pristine Ag dendrites. COMSOL theoretical simulations also demonstrated that ultrathin ZnO coating can promote the plasmonic coupling in Ag nanogaps. More importantly, this SERS substrate also shows excellent thermal stability at high temperature of 200 °C. Therefore, both SERS sensitivity and thermal stability of Ag dendrites can be enhanced via ALD surface protection.