Red luminescence of Zn /ZnO core–shell nanorods in a mixture of LTZA/Zinc acetate matrix: Study of the effects of Nitrogen bubbling, Cobalt doping and thioglycolic acid

Abstract

Zn/ZnO core–shell nanorods were synthesized using a very simple low temperature and low pressure Solvothermal method. Effect of N2 gas bubbling, insertion of thioglycolic acid (TGA) and cobalt doping with Co/Zn=x: 0%, 0.1%, 0.3% and 0.5% was studied by means of scanning electron microscopy, transmission electron microscopy, x-ray diffraction, Furrier transformed infrared(FTIR) spectroscopy, UV–Vis spectroscopy and photoluminescence (PL) measurement. XRD analysis showed that hexagonal metallic zinc and wurtzite ZnO phases are available in all the samples. Zinc acetate crystallizes together with Zn and ZnO phases as well, but it transforms to l-thereonine zinc acetate(LTZA) after the insertion of TGA. Moreover, gas bubbling and cobalt doping do not change the crystal phases. FTIR spectroscopy also confirms the formation of zincacetate and LTZA phases. Williamson–Hall analysis indicates that with the insertion of TGA, mean crystallite size decreases and the value of microstrain is the least for x=0.1%. SEM images depict that with the insertion of TGA, the morphology of the compound changes from rounded particles to nanorods. TEM images revealed that the microstructure of the compounds is of the Zn/ZnO core–shell type. UV–vis spectroscopy demonstrates the blue shift of absorption edge and the size reduction due to insertion of TGA. PL spectroscopy shows that the samples have strong blue emission with peaks range from 316nm up to 480nm. Bubbling simply causes that the samples emit a narrow and sharp band peaked at 605nm. Insertion of TGA prevents the blue emission and enhances the pure red emission band. At the presence of TGA, the optimum value of pure red emission for Co-doping was found to be 0.1% which was compatible with XRD analysis. With x>0.1%, the blue and red emission of samples started quenching and light emission for the value of x=0.5% vanished completely. The results show that the other crystal phases such as zincacetate, LTZA do not affect the luminescence of the samples. It is suggested that N2 gas bubbling accompanied by the growth of Zn/ZnO core–shells gives rise to a dramatic increase of the number of deep levels at the interface of metal/semiconductor. This would cause the appearance of a narrow red emission at room temperature. TGA insertion successfully incorporated as new capping agent for the enhancement of ZnO emission.