Resonance Raman and polarized Raman scattering of single-crystal hematite

Researchers have demonstrated that the Raman spectra collected on hematite nanoparticles undergo an intensity enhancement, due to a dependence on excitation energy. Especially, the bands at approximately 660 and approximately 1320 cm(-1), which are assigned as the 1LO and 2LO modes, respectively, experience the strongest intensity enhancement, which has been attributed to resonance Raman scattering when excited with excitation energy at or close to the band gap. However, the resonance Raman scattering mechanism is not clear and further has not been experimentally determined. To this end, we have undertaken measurements to examine the excitation energy dependence and polarized Raman scattering on a pure single-crystal of hematite to gain a better understanding of the resonance enhancement mechanism for the 1LO and 2LO modes. Resonance enhancement mechanisms giving rise to LO phonons in semiconductors can be attributed to either the deformation potential (DP) or Frohlich interactions (F) scattering, or a combination of the two. The DP and F contributions that give rise to LO phonon scattering in semiconductors are distinguishable due to their contrasting selection rules. Here, employing these selection rules, we conducted the following polarization Raman scattering measurements: ZoXXTHORNZ, ZoXYTHORNZ, ZoYYTHORNZ, ZoYXTHORNZ, XoYYTHORNX, XoZZTHORNX, and YoXXTHORNY on single-crystal hematite to distinguish between DP (dipole-allowed) and F (dipole-forbidden) scattering. Our polarization experiments strongly suggest that the Frohlich LO phonon interaction dominates the Raman scattering over the weaker DP contribution. Furthermore, polarization experiments collected using the following geometries Z X 0Xo 0THORNZ and Z Y 0 Yo 0THORNZ demonstrate that the interference between dipoleallowed and dipole-forbidden Raman scattering is constructive for LO phonons. Therefore, we attribute the mechanism of resonance Raman scattering of the 1LO and 2LO modes in the Raman spectrum of hematite due to contributions from F and DP processes.

Automated summary

Researchers have found that the Raman spectra collected on hematite nanoparticles undergo intensity enhancement when excited with energy close to the band gap. However, the resonance Raman scattering mechanism for this enhancement is not clear. This study examined the polarization Raman scattering on single-crystal hematite to better understand the resonance enhancement mechanism. The results suggest that the Frohlich LO phonon interaction dominates the Raman scattering, while the deformation potential contribution is weaker.