Jiannian Yao and colleagues from the Chinese Academy of Sciences, in Beijing and collaborators from the Royal Institute of Technology, Stockholm and the University of Science and Technology of China, Heifei, have combined both experimental and theoretical chemistry to study single molecular behaviour.

Probing single molecular behaviour using surface-enhanced Raman scattering (SM-SERS) has attracted considerable attention recently, although there are a few drawbacks. One major problem is the strong spectral-fluctuation (also known as 'signal-blinking') that is associated with single molecules, which prevents detailed information being extrapolated from the molecule and its surroundings. Yao says 'up to now SM-SERS studies have been limited to a few dye molecules that form strong chemical bonds with the metal substrate.' This means that the formation of the chemical bond between the molecule and the substrate leads to stable spectra with a strong signal. However, this provides little consensus on how to determine the structure of the single molecule.

Yao conducted SM-SERS measurements for a non-bonding molecule, perylene, which was physically adsorbed on (uniformly assembled) colloidal silver nanoparticles. As expected, at room temperature, the SM-SERS spectra of a perylene molecule displayed strong spectral-fluctuation, however, this was effectively eliminated by lowering the temperature of the substrate. 

'It sounds straightforward' explains Yao, 'but this needs special consideration of the experimental set up and a good understanding of SM-SERS mechanism. With this improvement, we can get a stable spectrum and in combination with first principle calculations we can find out where the single molecule prefers to stay after cooling and under Plasmon excitation.'

This technique should have a strong impact on the field and will help widen the scope of the SM-SERS for the future.  

Emma Shiells