Abstract
The synthesis of semiconductor nanocrystals with controlled doping is highly challenging, as often a significant part of the doping ions are found segregated at nanocrystals surface, even forming secondary phases, rather than incorporated in the core. We have investigated the dopant distribution dynamics under slight changes in the preparation procedure of nanocrystalline ZnO doped with manganese in low concentration by electron paramagnetic resonance spectroscopy, paying attention to the formation of transient secondary phases and their transformation into doped ZnO. The acidification of the starting solution in the co-precipitation synthesis from nitrate precursors lead to the decrease of the Mn2+ ions concentration in the core of the ZnO nanocrystals and their accumulation in minority phases, until ~79% of the Mn2+ ions were localized in a thin disordered shell of zinc hydroxynitrate (ZHN). A lower synthesis temperature resulted in polycrystalline Mn-doped ZHN. Under isochronal annealing up to 250 °C the bulk ZHN and the minority phases from the ZnO samples decomposed into ZnO. The Mn2+ ions distribution in the annealed nanocrystals was significantly altered, varying from a uniform volume distribution to a preferential localization in the outer layers of the nanocrystals. Our results provide a synthesis strategy for tailoring the dopant distribution in ZnO nanocrystals for applications ranging from surface based to ones involving core properties.
Highlights
One of the most studied semiconductors of the past decade, nanocrystalline zinc oxide doped with transition metal ions continues to hold a great interest for the researchers in materials science, as changing the nature and concentration of the dopant ion is a well-tested method to tailor the material properties for a wide range of applications in spintronics, nano- and/or optoelectronics[1,2,3,4]
Sometimes only a small fraction of dopant ions is present inside the nanocrystals, the largest part being localized in the intergrain region, often in secondary phases[8,10,16,17,18]
In order to assess the influence of the synthesis parameters on the dopant distribution and the formation of intermediate secondary phases, we have prepared several samples following a similar synthesis route, but varying one synthesis parameter from sample to sample
Summary
One of the most studied semiconductors of the past decade, nanocrystalline zinc oxide (nano-ZnO) doped with transition metal ions continues to hold a great interest for the researchers in materials science, as changing the nature and concentration of the dopant ion is a well-tested method to tailor the material properties for a wide range of applications in spintronics, nano- and/or optoelectronics[1,2,3,4]. Doping colloidal semiconductor nanocrystals is still an experimental challenge[5,6,7,8,9,10,11], the main issues being the control of the concentration level and distribution uniformity of the incorporated dopant. For larger dopant concentrations impurity-rich secondary phases are formed which affect the magnetic, electrical and optical properties of the nanostructured semiconductors[11,20,21,22,23,24,25,26]. Atom probe tomography (APT) boosted the knowledge in the field of the chemical elements distribution and segregation phenomena at atomic scale[33,37,38] Techniques have a strong local character, being less statistically relevant and not always suitable for dopant concentrations lower than a few percent
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