Surface-Enhanced Raman Scattering on a Chemically Etched ZnSe Surface

We report the observation of surface-enhanced Raman scattering (SERS) from a chemically etched ZnSe surface using 4-mercaptopyridine (4-MPy) as probe molecules. A thin film of ZnSe is grown by molecular beam epitaxy (MBE) and then etched using a strong acid. Protrusions of herniellipsoidal nanoparticles are observed on the surface. Using the results of the Mie theory, we controlled the size of the nanoparticles to overlap significantly with maximum efficiency of near-field plasmon enhancement. In the Raman spectrum, we observe large enhancements of the a(1), b(1), and b(2) modes when 4-MPy molecules are adsorbed on the surface using a S514.5 nm laser for excitation, indicating strong charge-transfer contributions. An enhancement factor of (2 X 106) is observed comparable to that of silver nanoparticles. We believe this large enhancement factor is an indication of the coupled contribution of several resonances. We propose that some combination of surface plasmon, charge transfer, and band-gap resonances is most likely the contributing factor in the observed Raman signal enhancement because all three of these resonances lie close to the excitation wavelength.