Exciton Recombination Dynamics And Spin Polarization Mediated By Polymorph Transformation In Cadmium Sulfide Nanocrystals

The electronic structures mediated by polymorphic transformation could change the exciton recombination dynamics and spin polarization mechanism. Exciton dynamics can probe the lifetime of the excitons and their associated relaxation or recombination pathways to gain fundamental insights into the relative electronic structure. Seeking high spin-polarized semiconductor nanocrystals at room temperature lay the foundation of the development of semiconductor spintronics. Spin polarization depends on spin-polarized density of states（DOS） at Fermi level. The variation in sizes could alter the bound state of exciton, energy band and magnetic domain. Doping could change the kind of carriers and introduce the impurity band in semiconductor, which could change the recombination path and the spin-polarized mechanism. High pressure causes the phase transformation, which changes the overlap degree of the neighbor atomic orbits, and alters the electronic structure,which provides a effective syntheis method for the study of the structure dependent excitonic recombination dynamics and spin polarization.As a typical and important class of semiconductor, the elaborate series of polymorphic phases of Cd S semiconductor that are fascinating in their own right offer a unique opportunity to study the mechanism of exciton recombination dynamics and spin polarization mediated by polymorph transformation. The main contents in this paper are as follows.Pure and Y³⁺ doped wurtzite Cd S semiconductor nanocrystals（8-10 nm） have been fabricated via an effective doping atomization method. The zinc-blende and rock-salt Cd S samples（8-10 nm） have been fabricated by the wurtzite Cd S nanocrystals as starting material under a fixed pressure of 5.2 GPa and 150 °C and 300 °C and then recovered to the ambient conditions, respectively. The 0.20 atom% rock-salt Cd S:Y samples（8-10 nm） have been fabricated by the 0.18 atom% wurtzite Cd S:Y nanocrystals as starting material under a fixed pressure of 5.2 GPa and 300 °C.As studied in the photoluminescence experiment, the emissions of the zinc-blende and rock-salt Cd S samples are both stronger in comparison to the wurtzite sample. The band-edge emission of the recovered zinc-blende and rock-salt sample show a blue-shift and red-shift in comparison to the wurtzite sample, respectively, indicating the larger and smaller tunable band gap after the structure transformation. The radiative decay lifetimes of wurtzite, zinc-blende and rock-salt Cd S samples at band edge emission states are 6.167 ns, 8.027 ns and 19.325 ns, respectively.The analyses of the band structures and exciton recombination paths of three Cd S polymorphs indicate that the additional requirement for the phonon assistance results in the longest radiative decay lifetime at band edge state of the indirect rock-salt Cd S semiconductor in comparison to the direct zinc-blende and wurtzite structures. The electron-hole wave function overlap of the direct zinc-blende and wurtzite Cd S semiconductors are 0.552 and 0.063 leading to the larger exciton recombination rate in comparison to the wurtzite Cd S structure, which is in agreement with the above obtained radiative decay lifetimes at band edge emission states.As suggested in the magnetic hysteresis loops and magnetic phase transition, the saturation magnetizations of wurtzite Cd S:Y nanocrystals increase at first and decrease afterwards with the inrreasing concentration of Y dopant, and the ferromagnetic transition temperatures of these samples are above 380 K, and the Y dopant can improve the ferromagnetic transition temperature. The saturation magnetizations（MS） at 10 K and 380 K are 0.031 emu/g and 0.023 emu/g for the wurtzite sample, and 0.210 emu/g and 0.107 emu/g for the rock-salt sample, respectively. Moreover, the ferromagnetic transition temperatures of the above two samples are above 380 K.The magnetic origin analyses manifest that the Cd vacancy defect is the mainly responsible for the ferromagnetic properties of the wurtzite and rock-salt Cd S:Y nanocrystals. The dopant Y can lower the defect formation energy and the spin configuration, which comprehensively leads to the saturation magnetizations of wurtzite Cd S:Y samples increasing firstly and then decreasing with the increase of Y dopant. Cd vacancy in the rock-salt is in favor of the larger spin polarization and spin density in comparison to the wurtzite sample which results in the more robust saturation magnetizations of rock-salt sample. The total energy differences of between the parallel and antiparallel spin orientations in wurtzite and rock-salt Cd S systems with two Cd vacancy defects are 116.220 me V and 103.083 me V, which indicates that the both two systems are room-temperature ferromagnetism and the wurtzite Cd S system has a higher ferromagnetic transition temperature.Therefore, the mechanism studies of exciton recombination dynamics in polymorphic Cd S nanocrystals and spin polarization in polymorphic Cd S:Y nanocrystals lay the foundation of the spin optoelectronic devices.