Core-defect reduction in ZnO nanorods by cobalt incorporation

Abstract

Zinc oxide (ZnO) nanorods grown by the low-temperature (90 °C) aqueous chemical method with different cobalt concentration within the synthesis solution (from 0% to 15%), are studied by electron paramagnetic resonance (EPR), just above the liquid helium temperature. The anisotropic spectra of substitutional Co2+ reveal a high crystalline quality and orientation of the NRs, as well as the probable presence of a secondary disordered phase of ZnO:Co. The analysis of the EPR spectra indicates that the disappearance of the paramagnetic native core-defect (CD) at is correlated with the apparition of the Co2+ ions lines, suggesting a gradual neutralization of the former by the latter. We show that only a little amount of cobalt in the synthesis solution (about 0.2%) is necessary to suppress almost all these paramagnetic CDs. This gives insight in the experimentally observed improvement of the crystal quality of diluted ZnO:Co nanorods, as well as into the control of paramagnetic defects in ZnO nanostructures.

Toward the Optimization of Low-temperature Solution-based Synthesis of ZnO Nanostructures for Device Applications

“…[56][57][58] Moreover, high crystal quality well-oriented Co-doped ZnO NRs were also synthesized by manipulating the way of mixing the synthesizer solution. [34,52,59] All these parameters were precisely selected considering that the final properties of the ZnO NRs are susceptible to the synthesis conditions. A typical LT-ACS of ZnO NSs consists of two main steps: first, a clean substrate is subjected to spin coating with a ZnO seed layer and then ZnO NSs are subsequently, synthesized from a zinc ion containing solution.…”

“…[20][21][22][23][30][31][32][33][34][35] In particular, in all luminescence processes, these defects in ZnO are involved as steps in the excitation and recombination paths (e.g., radiative and non-radiative recombinations), and thus influencing the absorbed/emitted light properties. The room-temperature luminescence of ZnO NSs is usually distinguished by a sharp UV emission peak centered at ~ 380 nm and attributed to a near-band-edge emission (NBE), and a broad emission peak covers the entire visible region extending between 400 -750 nm as shown in Figure 2.3.…”

“…[34,52,59] The influence of these two processes in the incorporation of Co 2+ inside the ZnO NRs crystal lattice and the subsequent effect of them on the crystal quality, orientation, and defects properties were systematically investigated. [34,52,59] Figure 3.3 shows the synthesized Co-doped ZnO NRs by using the first and second approach for preparing the synthesis solution.…”

“…[34,52,77] In a typical X-band EPR experiment, a paramagnetic material is placed within a resonant cavity and in a quasistatic magnetic field (B) which split a degenerated 1/2 spin level into its two-spin projection -½ and + ½. Then magnetic field (B) is slowly varied from 0 to 1.4 T, and at the same time, a continuous stationary microwave of constant energy (hν) is sent into the cavity as illustrated in Figure 4.5.…”

“…[20][21][22][23][30][31][32][33] However, identifying the nature of such defects is a complex issue, particularly in NSs, where the information on anisotropy is usually lost due to the lack of coherent orientation. [34] To investigate the origin of these defects in more details and to fulfill all the advantages of ZnO NRs in device applications several synthesis techniques have been utilized.…”