Resonance Raman and polarized Raman scattering of single‐crystal hematite

Abstract

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 Fröhlich 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: Z()XXtrueZ0.25em¯, Z()XYtrueZ¯, Z()YYtrueZ¯, Z()YXtrueZ¯, X()YYtrueX¯, X()ZZtrueX¯, and Y()XXtrueY¯ on single‐crystal hematite to distinguish between DP (dipole‐allowed) and F (dipole‐forbidden) scattering. Our polarization experiments strongly suggest that the Fröhlich LO phonon interaction dominates the Raman scattering over the weaker DP contribution. Furthermore, polarization experiments collected using the following geometries Z()X′X′trueZ¯ and 0.25emZ()Y′Y′trueZ¯ demonstrate that the interference between dipole‐allowed 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.

Chromaticity coordinate vector principle for charge-transfer-type thermochromic material design: case of Fe/Cr-(co)doped α-Al2O3host

“…59 The bands at approximately 1341–1407 cm −1 (v 11 ) can be assigned to a two-phonon scattering process, which arises from the overtone of the LO mode at approximately 660 cm −1 (superimposed with that of A 1g mode (v 9 )) and is referred to as the 2LO mode. 56,60 α-Al 2 O 3 shows fluorescence bands at 678 and 693 nm, which have been verified as characteristic of highly crystalline alumina. 61 In the spectrum of the Al 1.9 Fe 0.1 O 3 sample, two obvious peaks can be found at 1364.4 cm −1 and 1394.6 cm −1 (Fig.…”

Molecular-Enhanced Raman Spectroscopy Driven by Phosphoester Electron-Transfer Bridge

“…As shown in Figure b, the I 1574 / I 1605 value increased significantly within 30 min along with the peak splitting and red-shift phenomenon at 1574 cm –1 (Figure S16a). This can be explained by the fast charge injection (Figure S16b) into the ring backbone via H-bonds, which increases the skeleton conjugation length, breaks the symmetry structure, and makes the Raman-forbidden mode become the permissive mode . The subsequent re-emergence of the peak suggests that molecular symmetry can be restored by further charge doping.…”

“…This can be explained by the fast charge injection (Figure S16b) into the ring backbone via H-bonds, which increases the skeleton conjugation length, breaks the symmetry structure, and makes the Raman-forbidden mode become the permissive mode. 38 The subsequent re-emergence of the peak suggests that molecular symmetry can be restored by further charge doping. After 30 min, the H-bond formation becomes slow due to the gradually saturated adsorption of APM on the POE-Ag@Cu surface.…”