Fabrication And Optical Magnetic Properties Of Rare Earth Doped In₂S₃ Quantum Dots

Fluorescent nanomaterials with magnetic properties have been applied in many fields of chemistry,such as nanoelectronics,information storage,photocatalysis,fluorescent probes,biotechnologies,and biosensors.?-In₂S₃ is a typical defect spinel structure with a large amount of vacancies that is particularly favorable for incorporation of guest ions.Rare earth?RE?ions are quite suitable dopants because they have the special 4f and 4f5 d energy level and the structure of charge transition state.The doped rare-earth ions enter into the crystal lattices by randomly substituting the In atoms and introduce deep levels.These levels control the type and mobility of the conducting carrier acting as activators or killers in luminescent process and also give rise to the magnetization of the samples.Furthermore,the doped ion concentration also can influence the intensity of the luminescence and the magnetization.In this paper,a series of In₂S₃-based fluorescent quantum dots with magnetic properties were prepared by doping different concentrations of rare earth ions Tb³⁺,Ho³⁺,Dy³⁺ and codoping of Dy³⁺,Tb³⁺.The influence of different RE doping on the optical and magnetic properties of these quantum dots was discussed.The theoretical calculation on the band structure and the spinpolarized total and partial density of states was performed using VASP to further explore the intrinsic physical mechanism.This paper summarizes the magnetic operating mechanism and bandgap variation of In₂S₃-based rare earth doping quantum dots,which provides experimental and theoretical guidance for the application of solar converters,projection television screens,biomedical probes and photocatalysis,and the development of spintronic applications.The main research findings are as follows:1.In₂S₃:Tb³⁺ nanoparticles of about 4-6 nm in size with different doping concentrations were fabricated and the bandgap value decreases as the doping concentration increases.The emission peak at about 467 nm is assigned to the intrinsic emission or excitonic recombination and the lower energy peaks?at 481 and 491 nm?can be ascribed to near-impurity/near-defect excitonic luminescence.And doping leads to an increase in PL intensity.The synthesized quantum dots exhibit strong room temperature ferromagnetism and their saturation magnetizations are relevant to Tb³⁺ concentration whose maximum are at 1.24 at.%.The luminescence decay curves can be well fitted by the double exponential function.The fitted lifetime values are only about 1 nanosecond,and gradually decrease as the Tb³⁺ ion content increases.The band structure calculation shows that the bandgap variation tendency is consistent with the measured absorption spectrum.The peaks of Tb³⁺-doped In₂S₃ become sharper between the both sides of the Fermi level,indicating that the overlap between the valence band and conduction band decreases.Meanwhile,doping strengthens the bonding interaction of In₂S₃ and thus leads to the quantum confinement effect stronger.2.The In₂S₃:Ho³⁺ quantum dots?3–5 nm?with different doping concentrations were synthesized.The UV-vis presents a characteristic step like absorption band and the band gap energy can be tuned by doping concentration varying from 3.19 to 3.62 e V.The PL emission spectra exhibit a significant Stokes shift compared to the corresponding absorption peak.In addition,the decrease in PL intensity with Ho³⁺ concentration increasing further is a consequence of the concentration quenching.The synthesized quantum dots exhibit room temperature ferromagnetism and their saturation magnetizations?Ms?appear to increase at first and then decreases gradually.The Ms reache a maximum 0.00463 emu/g at 0.95 at%,which is explained by the percolation of bound magnetic polaron?BMP?theory.The first-principle calculations show that the band gap of In₂S₃:Ho³⁺ is increasing in comparison with that of pure In₂S₃,which is in accordance with the UV-vis analysis.The introduced Ho-4d orbital mainly contributes to the bottom of the conduction band.3.The high purity In₂S₃:Dy³⁺ quantum dots?around 3-5 nm?with different doping concentrations were synthesized.The PL emission spectra exhibit obvious blue-shift compared with the In₂S₃ quantum dots reported previously and the emission peak at 563 nm is derived from 4F9/2?6H13/2 transition of Dy³⁺.The synthesized quantum dots exhibit strong room temperature ferromagnetism and their saturation magnetizations are relevant to Dy³⁺ concentration whose maximum are at 1.19 at.%,which is also explained and discussed based on BMP theory.The ZFC-FC curves further confirm the coexistence of ferromagnetic?FM?and antiferromagnetic?AFM?phases and AFM interation plays a dominant position until a certain doping concentration value.The first-principles calculations indicate that the ferromagnetism originates not only from the Dy atoms but also from the In vacancies.Doping Dy³⁺ can strongly decrease the defect formation energy and favour producing more In vacancies.4.The high purity In₂S₃:6%Dy³⁺,y Tb³⁺?y = 2%,4%,6%,8%?quantum dots?around 3-6 nm?with different doping concentrations were synthesized,the band gap energy can be tuned by doping concentration varying from 3.17 to 3.51 e V.The PL emission spectra exhibit the transition emission peaks of Dy³⁺ and Tb³⁺,and the fluorescence intensity decreases with the increase of Tb³⁺ doping concentration.The synthesized quantum dots have room temperature ferromagnetism,and their saturation magnetization is related to the dopant concentration of Tb³⁺ in the double dopant.As the Tb³⁺ concentration increases,the value of the Ms increases initially and then reaches a maximum value 0.0147 emu/g at In₂S₃:6%Dy³⁺,4%Tb³⁺,becomes a decrease afterwards.This phenomenon is related to the antiferromagnetic interaction occurring between adjacent dopants.The theoretical calculations show that the magnetic properties of the system mainly originates the hybridization of In-5p,In-5s and S-3p in the In vacancy defects.