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Fig. 4 shows the Raman spectra (LABRAMHR Confocal Laser MicroRaman spectrometer, 514.5 nm, 250 mW) of the as-synthesized NiO nanorods. It can be seen that there are four Raman peaks at 420, 546, 712, and 1092 cm1, respectively. Compared to the predicted data of cubic NiO single crystal [26], we can assigned the peaks at 420 and 546 cm1 to the first-order transverse optical (TO) and longitudinal optical (LO) phonon modes of NiO, respectively. The remaining peaks at 712 and 1092 cm1 can be assigned as combination of 2TO and 2LO, respectively, and are all shifted down in frequency about 68 cm1. Since k ¼ 0 phonons in the paramagnetic (cubic) phase transform as C4 , no firstorder Raman scattering is expected. Therefore the observed first-order Raman peaks near the 420 (TO) and 546 cm1 (LO) must derive from paritybreaking imperfections [26]. This conclusion is confirmed by the enhancement of the first-order Raman scattering in the black NiO, where the nickel vacancy concentration is high. The combination Raman scattering trends to vanish at the C point but is usually strong elsewhere in the Brillouin zone. This explains why the combination peaks are all shifted down in frequency, since both LO and TO branches are expected to be depressed in frequency away from the zone center. |
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benmao_mogu
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lastzealot
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