On Structural,Magnetic And Optical Properties Of Mn²⁺Doped ZnSe/ZnS Semiconductor Nanocrystals

Diluted magnetic semiconductors,based on traditional semiconductors,possess ferromagnetic properties by doping magnetic atoms.Hence the charge and the spin properties of the charge carriers can be simultaneously controlled in these systems.Mn atom with the unique 3d5 electronic structure,is one of the most important dopants in dilute magnetic semiconductors.This thesis aims to study the magnetic and optical properties of Mn2+doped ZnSe/ZnS(Zn1-xMnxSe/y ZnSe/z ZnS)nanocrystals with core/shell structure.From the perspective of energy levels,we study the mechanisms for the relationship between magnetic and optical properties and establish a systematic and quantitative luminescence model.We firstly synthesized Mn2+doped ZnSe/ZnS nanocrystals(Zn1-xMnxSe/y ZnSe/z ZnS)with mono-exponential luminescence decay dynamics.The single channel decay property indicates that the magnetic interactions between the doped Mn2+ ions are monodispersed.With the co-nucleation doping method,the dopant ions and the host cations of the nanocrystal core are mixed in the solution.Under the effect of the high activity of anion precursors,the dopant ions are uniformly distributed in the host crystal lattice by adjusting the molar ratio of anion precursors to cationic precursors.ZnSe and ZnS shells are fturther epitaxially grown onto the core nanocrystals to increase the stability.The as-synthesized doped nanocrystals have fine crystallinity,favorable stability,minimum surface defects and high luminescence quantum yield.As the coordination environment of the doped ions within the nanocrystal core is identical,the only difference among the samples with different Mn2+ concentrations is the magnetic exchange strength between the dopants.This provides a single variable for the study of magnetic and optical properties of the doped nanocrystal systems.The magnetic measurement and electron paramagnetic resonance spectroscopy(EPR)were applied to study the temperature dependent magnetic moment of nanocrystals with different Mn2+contents.Nanocrystals with the lowest Mn2+ content shod paramagnetism during the whole temperature range;while those with high dopant contents exhibit antiferromagnetism at low temperatures but paramagnetism at high temperatures.We indicate that the alternation of the distribution of electrons in energy levels leads to the change of magnetic moment and transformation of magnetic species.The spin quantum numbers at different temperatures are fitted with the Boltzmann distribution.In addition,hysteresis loops indicate that the as-synthesized samples have weak ferromagnetism at room temperature.We suspect that the weak ferromagnetism is caused by anisotropic superexchange interaction.The influence of magnetic properties on optical properties is mainly by alternating the energy levels.With the temperature dependent steady-state and transient fluorescence spectra,we found that the emission of samples with high Mn2+ contents showed the characteristics of multi-level emission.Due to the highly uniform properties of Mn2+ emissive centers,we can identify that the multi-level emission is the intrinsic properties of individual Mn2+ emissive centers.Given the characteristics of the energy levels after magnetic coupling,we established the rate equations of the excited states to quantitatively simulate the temperature dependent emission peak positions and full-width-of-half maxima.The long lifetime components of the transient emission spectra at low temperatures are not from different emissive centers,but from the non-radiative exciton transition centers.Thus the final emitters remain the Mn2+-Mn2+ aggregates.