First order raman scattering in bulk Bi₂S₃ and quantum dots: Reconsidering controversial interpretations

In the present work, the nonresonance and resonance Raman scattering by Bi₂S₃ quantum dots, with an average size of 2.3 nm and standard deviation of 0.4 nm, is studied. The Raman spectra of a bulk sample, used as reference, also helped to identify recurrent misleading interpretations present in the literature. A blue-shift of 0.58 eV was observed in the band gap of the quantum dots with respect to the bulk value. In nonresonance conditions, the Raman bands of the Bi₂S₃ quantum dots (those at 186, 236, and 259 cm^-1) are asymmetrically broadened, and the intensity of the peak at 259 cm^-1 is enhanced, when compared to the bulk reference. In resonance condition, the Raman spectrum intensity of the Bi₂S₃ nanostructures is enhanced by a factor of 26 at 237 cm^-1 with respect to the nonresonance case and 18 times with respect to that corresponding to its bulk counterpart. The observed results are qualitatively explained in the framework of the microscopic Raman scattering theory of spherical quantum dots.