ZnO nanorods alloyed with Mo/Er. The effect of post-deposition treatment on defect states and luminescence

Abstract

Free-standing ZnO nanorods alloyed with Er/Mo were synthesized by the hydrothermal growth method. To characterize them, the number of experimental techniques was applied including X-ray diffraction (XRD), scanning emission microscopy (SEM), electron paramagnetic resonance (EPR), photo- and radioluminescence (PL, RL). EPR confirmed the existence of F+ centres common for ZnO-based structures in the ZnO:Er(30%) nanorods whereas in the ZnO:Mo(30%) this kind of defect was absent. Air annealing at elevated temperatures results in the reduction of F+ centres in all the materials studied. Moreover, Er3+ EPR signal also undergoes changes including broadening in the ZnO:Er. This allowed suggesting oxidation of Er ions on the ZnO nanorods surface. Red luminescence (~680 nm) appears in all studied samples regardless the dopant origin and doping level after the annealing in air. The exciton-related band at 380 nm never observed in the samples before the annealing appears upon the annealing at 350 °C in ZnO:Mo(10%) and ZnO:Er(30%). No such band was observed in the ZnO:Mo(30%) sample under the same conditions. According to SEM there are nanorods no more but microrods upon the content of Mo/Er as compared to the as-grown untreated ZnO as reported in a recent work.

Transformation of ZnO-based structures under heavy Mo doping: defect states and luminescence

“…In the ZnO:Mo(20, 25%) samples the flakes are getting mixed with the larger slabs of the averaged area of about 5 μm 2 . This is consistent with the previous study of the ZnO:Mo(30%) [7].…”

“…The red emission bands (1.8-2 eV) related to VZn [5,6] were recently also observed in the hydrothermally grown ZnO nano-and microrods [7,8]. It is known that low-level Mo doping (below 1%) can positively affect the luminescence properties of ZnO (both defect-related and exciton luminescence) [7,8], whereas doping with 10 and 30% of Mo deteriorates them. Moreover, Mo has the tendency to couple with ZnO creating some new phase other than ZnO itself.…”

“…For example, the experimentally observed broad red bands ranging from 1 to 2 eV were related to zinc (VZn) or oxygen (VO) vacancies [5]. The red emission bands (1.8-2 eV) related to VZn [5,6] were recently also observed in the hydrothermally grown ZnO nano-and microrods [7,8]. It is known that low-level Mo doping (below 1%) can positively affect the luminescence properties of ZnO (both defect-related and exciton luminescence) [7,8], whereas doping with 10 and 30% of Mo deteriorates them.…”

Free-Standing ZnO:Mo Nanorods Exposed to Hydrogen or Oxygen Plasma: Influence on the Intrinsic and Extrinsic Defect States

“…Molybdenum with the 3+–6+ row of charge states is an eligible candidate in this case since its improvement of the conductivity of ZnO:Mo nanorods and thin films has previously been observed [ 20 , 27 , 28 ]. In the past decade post-growth treatment, for example, annealing in air or exposure to hydrogen or oxygen plasma, has appeared the most efficient tool for the moderation of ZnO properties [ 13 , 14 , 19 , 20 , 21 , 29 , 30 , 31 , 32 ].…”

“…[ 29 ], ZnO:Mo(10 and 30%) were studied, whereas presently, the Mo doping level is not higher than 1%. The ZnO:Mo(10%) sample [ 29 ] was mostly composed of large microrods—1 μm thick and about 4–5 μm long, while the ZnO:Mo samples in the present case are nanorods about 400–500 nm thick and 1–2 μm long. The ZnO:Mo(30%) sample [ 29 ] is not ZnO at all.…”