Unusual weak interparticle distance dependence in Raman enhancement from nanoparticle dimers

Here, we report surprisingly weak distance dependence in Raman enhancement from a raspberry-like gold nanoparticle termed raspberry-MM. A raspberry-MM is composed of closely packed gold nanobeads assembled on a polymer core. Due to the abundant built-in hot spots between adjacent gold nanobeads, bright and uniform Raman signals were observed from isolated single raspberry-MMs. Interestingly, dimers of raspberry-MMs also showed highly reproducible Raman signals, indicating that the dimer SERS signal is not strongly dependent on the nanoparticle separation. Finite-difference time-domain (FDTD) modeling shows that a strong hot spot is created at the dimer-gap, as expected. However, since there are many more built-in hot spots in each raspberry-MM, the contribution of the dimer-gap hot spot to the total Raman enhancement remains low even for 2 nm separation, which explains the observed weak distance dependence. This result is in stark contrast with many previous SERS studies on nanoparticle dimers and clusters, and provides an important guideline on how to design bright and highly reproducible Raman substrates.