We report on the observation of surface enhancement of the Raman signal by semi-conductor quantum dots. Until recently surface enhanced Raman spectroscopy has been observed only on metals, especially Ag and Au. One requirement for the effect is that the metal be in the form of nanoparticles. Enhancement factors of up to six orders of magnitude and even larger have been reported. Two sometimes-competing explanations for the effect have been suggested: either plasmon resonances or charge-transfer resonances between the molecule and metal.

We have recently observed surface enhancement in molecules adsorbed on semiconductor quantum dot surfaces. We have observed these in MBE grown InAs and CdSe quantum dots, as well as colloidal quantum dots such as ZnS, ZnO and CeTe. Enhancement factors of up to four orders of magnitude are observed. It is impossible to explain these observations using plasmon resonance, which for semiconductors are in the infrared. We explore the possible application of the charge-transfer theory to semiconductor dots. Application of the Herzberg-Teller theory to such systems implicates polaron (electron-phonon) coupling in the suggested mechanism.