ZnO nanoflowers were synthesized by the hydrothermal process at an optimized growth temperature of 200 • C and a growth/reaction time of 3 h. As-prepared ZnO nanoflowers were characterized by x-ray diffraction, scanning electron microscopy, UV-visible and Raman spectroscopy. X-ray diffraction and Raman studies reveal that the as-synthesized flower-like ZnO nanostructures are highly crystalline with a hexagonal wurtzite phase preferentially oriented along the (1 0 1 1) plane. The average length (234-347 nm) and diameter (77-106 nm) of the nanorods constituting the flower-like structure are estimated using scanning electron microscopy studies. The band gap of ZnO nanoflowers is estimated as 3.23 eV, the lowering of band gap is attributed to the flower-like surface morphology and microstructure of ZnO. Room temperature photoluminescence spectrum shows a strong UV emission peak at 392 nm, with a suppressed visible emission related to the defect states, indicating the defect free formation of ZnO nanoflowers that can be potentially used for UV light-emitting devices. The suppressed Raman bands at 541 and 583 cm −1 related to defect states in ZnO confirms that the ZnO nanoflowers here obtained have a reduced presence of defects.
The magnetic properties of Mn‐doped ZnO (ZnO:Mn) nanorods grown by hydrothermal process at a temperature of 200 °C and a growth time of 3 h have been studied. The samples were characterized by using powder X‐ray diffraction with Rietveld refinement, scanning electron microscopy, energy‐dispersive X‐ray analysis and SQUID magnetometry. Mn (3 wt%) and (5 wt%)‐doped ZnO samples exhibit paramagnetic and ferromagnetic behavior, respectively, at room temperature. The spin‐glass behavior is observed from the samples with respect to the decrease of temperature. At 10 K, both samples exhibit a hysteresis loop with relatively low coercivity. The room‐temperature ferromagnetism in 5 wt% Mn‐doped ZnO nanorods is attributed to the increase in the specific area of grain boundaries, interaction between dopant Mn2+ ions substituted at Zn2+ site and the interaction between Mn2+ ions and Zn2+ ions from the ZnO host lattice.
M–H curve of hydrothermally grown ZnO:Mn (3 wt%) and ZnO:Mn (5 wt%) nanorods at 10 K.
This paper reports the pressure-dependent photoluminescence and Raman spectral investigation of hydrothermally synthesized ZnO nanoflowers at room temperature. Intrinsic near-band-edge UV emission from ZnO nanoflowers is monotonously blue-shifted under pressures up to 13.8 GPa with a pressure coefficient of 26 meV GPa(-1), and this pressure value is nearly 5 GPa above the transition pressure from the wurtzite to the rock salt phase for bulk ZnO. The Raman band corresponds to the wurtzite phase, the [Formula: see text] and [Formula: see text] modes were observed up to about 11 GPa from the spectra. The apparent discrepancy in the transition pressures as determined from photoluminescence and Raman studies suggests that it is a consequence of the gradual phase transition, in which the smallest nanoparticles are expected to remain in the wurtzite phase up to 13-15 GPa.
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