Photoluminescence Room-temperature photoluminescence spectra of all the samples are shown in Figure 5a.
All samples exhibited two dominant peaks. The first and sharpest peak is centered on 378 nm and was assigned to the near-band edge (NBE) emission or to the free exciton emission. The intensity of the NBE emission decreases with the increase of Cu concentration for both precursors Cu(CH3COO)2 and Cu(NO3)2. This may have resulted from the formation of the nonradiative centers in the Cu-doped Selleck Luminespib samples [28]. In comparison between the two precursors, the Citarinostat solubility dmso nanorods doped with Cu(NO3)2 (samples S4 and S5) showed a higher NBE emission compared to the nanorods doped with Cu(CH3COO)2 (samples S2 and S3). This observation could be due to the
higher anion concentration in samples S2 and S3 [35]. The UV emission peak of the Cu-doped samples showed a small redshift (approximately 6 nm) relative to the undoped ZnO, where the shift is clearer for the samples doped with Cu(NO3) (S4 and S5). This may be attributed to the rigid shift in the valence and the conduction bands due to the coupling of the band electrons and the localized Cu2+ impurity spin [16]. It can be observed that there is a small shoulder at around 390 nm, and it becomes pronounced for sample 3, which is doped with 2 at.% Cu from Cu(CH3COO)2, and this shoulder is ascribed to the free electron-shallow acceptor Microtubule Associated inhibitor transitions [25, 26]. Additionally, there is a luminescence peak at around 544 nm, which is called the deep-level emission (DLE) or blue-green emission band. When 1 at.% Cu is added from Cu(CH3COO)2, the intensity of this peak increased slightly (sample S2) and decreased again when 2 at.% Cu is added from the same precursor (sample S3),becoming nearly identical with the undoped ZnO nanorods (sample S1). This result suggests that the green emission is independent of Cu concentration. On the other hand, when we use Cu(NO3)2 as the Cu source (samples S4 and S5), the green emission enhanced significantly for sample S5 (doped with 2 at.%). Interestingly, the origin of the green
emission is questionable because it has been observed in both undoped and Cu-doped ZnO nanorod samples. Vanheusden et al. [36] attributed the green emission selleck compound to the transitions between the photoexcited holes and singly ionized oxygen vacancies. Based on these arguments, the high oxygen vacancy concentration may be responsible for the higher green emission intensity of sample S5. Additionally, the ratio (R) of the NBE emission intensity to the DLE intensity is shown in Figure 5b. The R decreases with the increase of Cu concentration. Figure 5 PL spectra and relative ratio. (a) Room-temperature PL spectra of undoped and Cu-doped ZnO nanorods; the inset shows the blue-green emission bands. (b) The relative ratio of PL intensity (R = I(UV)/I(DLE)).