| Using Surface Enhanced Raman Spectroscopy (SERS), I observed Raman enhancements (104-105) for pyridine, 4-tertbutylpyridine and other biological molecules adsorbed on a II-VI semiconductor CdSe/ZnBeSe sample of uncapped self-assembled quantum dots produced by molecular beam epitaxy. When a monolayer of Raman active molecule is adsorbed on these structures, excitation at 488 nm produces intense Raman spectra and a very large enhancement of the a1, b1 and b2 modes. This indicates the presence of charge-transfer as a contributor to the enhancement. Furthermore, the excitation wavelength is in the vicinity of several interband transitions located both in the quantum dots and the wetting layer, and it is likely that these resonances also contribute to the enhancement factor. The observed enhancement is among the largest yet seen on semiconductor quantum dot systems and is the first obseved on MBE grown quantum dots.;The observed Raman spectrum shows several differences from the normal Raman spectrum, and is closer in some ways to the SERS observed on Ag nanoparticles. Among other things, the prominence of non-totally symmetric lines indicates the importance of charge-transfer contributions to the overall enhancement. In Ag nanoparticle SERS, both the charge-transfer and surface plasmon resonances have been shown to be responsible for the effect. However, in semiconductors, the excitation wavelengths are far from the region of surface plasmon resonance for CdSe and so the magnitude of enhancement must be attributed to other possible resonances.;Bandgap dependence studies using 4-tertbutylpyridine on semiconductor quantum dots with varying bandgap energies, indicates that transitions to the bandgap are important contributors to the overall enhancement, and may possibly be regarded as a replacement for the surface plasmon resonance so important in metallic SERS. As in SERS, however, the observation of non-totally symmetric bands indicates additional contributions from charge-transfer resonances. It is most likely a combination of these two effects which is responsible for the rather large enhancements observed here. |
