Controlled Synthesis And Optoelectronic Properties Research Of Ⅱ-Ⅵ Semiconductor Doped Nanocrystals

In order to exploit transition-metal（TM） ions doped II-VI semiconductor nanocrystals,trace tributylphosphine（TBP） was employed to initiate cation exchange and generate single crystalline II-VI NCs with trace TM ions residue inside. Their unusual optical, electronic,magnetic properties, such as the doped luminescence and electronic impurities（p-, n-type doping） were studied.Here, the substitutional Cu（I） dopants with controllable concentrations were confirmed by local atom-specific fine structure from X-ray absorption near edge structure（XANES）, extended X-ray absorption fine structure（EXAFS） spectroscopy, elemental analysis characterizations from X-ray photoelectron spectroscopy（XPS） and the electron spin resonance（ESR） measurement. The dominant and strong Cu（I） dopant fluorescence were verified by their absorption and photoluminescence（PL） spectra, and PL lifetime. The620 nm kinetic curve could be well fitted with a two-exponential model, resulting in two lifetimes of 0.2 μs and 1.3 μs, respectively. This long-lived fluorescence further supported the dominance of Cu dopant related emission. The self-quenching problem was solved here due to the large Stokes shift of 0.88 e V. Finally, the band positions and the p-type conductivities of as-prepared Cu（I） doped Cd S NCs were identified by ultraviolet photoelectron spectroscopy（UPS） measurements. The high monodispersity of NCs enabled their strong film-scale self-assembly and would hasten their subsequent applications in device.Besides, the substitutional Ag（I） dopants with controllable concentrations were synthesized through cation exchange reaction between Cd ions and amorphous Ag₂ X nanoparticles. Steady-state ultraviolet-visible-near-infrared absorption/photoluminescence spectra and time-resolved carrier kinetics characterizations have verified the stable,dominant, and strong dopant fluorescence. The 640 nm kinetic curve could be well fitted with a two-exponential model, resulting in two lifetimes of 0.2 μs and 1.3 μs, respectively.The self-quenching problem has also been solved here due to the large Stokes shift of 0.71 e V. The absolute quantum yields of 1% Ag doped Cd S NCs reached to 42%. Femtosecond transient absorption（TA） spectroscopy was used to further probe the photophysics of Ag doped Cd S NCs. Based on carrier dynamics characterizations from TA analysis, the efficient energy transfer from the excited state to the Ag trap state quenched excitonicemission and inspired the dopant-related emission from the Ag dopant level to the ground state.Also, this new cation exchange method was conducted on the preparation of Ag and magnetic ions co-doped Cd S NCs, which was verified to be effective. Typically,superconducting quantum interference device（SQUID） and the electron spin resonance（EPR） characterizations were carried out on the Ag & Mn ions co-doped Cd S NCs. Mn existed as Mn²⁺ ions located on the surface of Cd S NCs, which was confirmed by the clear cleavage from the EPR analysis. The coercivities of Ag & Mn ions co-doped Cd S NCs were 100 Oe and 50 Oe from the hysteresis loop at 10 K and 300 K, respectively. The saturation magnetizations were 0.04 and 0.01 emu/g accordingly. Finally, the as-prapared NCs were identified to have room-temperature ferromagnetism.