Raman transverse phonons. The nonpolar E2 modes
Raman spectroscopy has been used to detect the disorder induced in the host lattice due to dopant incorporation. Figure 3(a) represents room temperature Raman spectra of undoped and cobalt doped ZnO nanorods. The wurtzite crystal symmetry of ZnO belongs to space group C46v and group theory analysis predicts that the zone-center optical modes have the symmetries A1+2B1+E1+2E2. 100 Out of these modes, B1 modes are forbidden, and A1 and E2 modes are doubly degenerate. E1 modes are polar with different energies for the longitudinal and transverse phonons. The nonpolar E2 modes have two frequencies represented as E2 (low) and E2 (high), and E2 (high), are Raman active only.101,102 The undoped ZnO nanostructures exhibits five Raman-active peaks at around 332 cm-1, 438 cm-1, 482 cm-1, 574 cm-1, and 657 cm-1, which can be ascribed to the A1 (TO), E2 (High), A1 (TO) +E2 (Low), E1 (Low) and E2 (Low) +B1 (High) symmetric phonon modes respectively.
103,104 The sharpest and strongest peak at around 438 cm-1 is found in the wurtzite crystal structure. For the cobalt doped ZnO, an additional peak at 527 cm-1 and 694 cm-1 appear and its intensity increases with increasing cobalt concentration. These peaks can be assigned to local vibration of substitutional cobalt or cobalt-oxygen vacancy created during doping.
104 Our observation shows that, as the cobalt concentration is increased, the intensity of 438 cm-1 peaks decreased and shifted towards lower frequencies, whereas peak intensity at 527 cm-1 and 694 cm-1 increases. To date, the studies on the evolution of additional Raman vibrational modes in the transition-metal-doped ZnO have been rather limited, and the explanation regarding the additional vibrational modes especially located in between 450 cm-1 – 500 cm-1 and between 600 cm-1 – 700 cm-1 in doped ZnO is still vague. Phan et al.105 explained the presence of cobalt in ZnO lattice leads to the slight shift of the 437 cm-1 band toward lower frequencies. Also, they observed two additional modes at 524 cm-1 and 546 cm-1 in cobalt-doped ZnO samples. We believe that the incorporation of Co++ into the ZnO host lattice induced defects and internal strain that triggered anomalous modes around 524 cm-1-546 cm-1. Similar studies performed by Wang et al.
106 on cobalt doped ZnO, where the authors have found a mode at 530 cm-1 and suggested it as shallow-donor defects bound on the tetrahedral cobalt sites. Chu et al.107 also observed the vibrational mode at 484 cm-1 in annealed cobalt-doped ZnO samples, but they did not specify its identity. Zhou et al.108 detected the vibrational modes of Co3O4 phase in the subtle region of 5 to12 at.% of cobalt-doped ZnO.
Samanta et al.109 observed the additional modes at around 470 cm-1 and 680 cm-1 in 15at.% of cobalt-doped ZnO and assigned them to the vibration of ZnCo2O4 phase. Sudakar et al.110 also observed the vibrational modes of ZnCo2O4 phase in 4.7at.% of cobalt-doped ZnO and they tentatively assigned the mode at ?690cm?1 to the Zn–O–Co local vibration due to cobalt doping.