Photoluminescence Of Phase-doped Rhenium Disulfide And Room-Temperature Ferromagnetism Of S-doped Titanium Dioxide

The scaling down of traditional transistor is approaching the limit of miniaturization due to quantum tunneling and gate leakage.In order to further enhance the performance of transistors,it is necessary to develop new materials and design devices based on new principles.In this aspect,two-dimensional materials and spin electronic devices have become the new darlings of scientific research.It is well known that graphene is in the gapless state and thus not suitable for optoelectronic application.Despite of the sizable bandgaps of transition metal dichalcogenides?TMDCs?represented by MoS₂,the bandgap is strongly related to the layer thickness because of the crossover from the direct band gap of the monolayer to the indirect band gap of the bilayer as well as the multilayer,which also hinders their application in optoelectronic devices.Sulfide rhenium disulfide?ReS₂?,a new member of the TMDC family,exhibits much weaker interlayer coupling.Bulk behaves just like vibrationally and electronically decoupled monolayers stacked together,which has great potential in the field of optoelectronic devices without special preparation process.Meanwhile,the reduced symmetry leads to strong in-plane anisotropy.Although great progress is achieved in the study of photoluminescence?PL?properties,the anisotropy of electronic structure has not yet been fully utilized to realize visible polarization-resolved PL.The realization of spintronic devices is based on room-temperature ferromagnetic semiconductors.Since the first report on the observation of room-temperature ferromagnetism in cobalt-doped TiO₂ thin films,dilute magnetic oxides?DMOs?have attracted considerable attention.However,despite the many investigations,the origin of observed high-temperature ferromagnetism is still in controversy.Due to the existence of surface oxygen vacancies?OVs?,it is still not sufficiently clear how doped host and OVs contribute to the observed magnetic hysteresis persisting above room temperature.The exploitation of magnetic mechanism is an important research topic in the field of spintronics.Although TiO₂ has a great number of promising applications in photocatalysis,the revelation of the origin of the ferromagnetism will widen the range of applications much further.In this paper,the polarized visible PL of ReS₂ is realized by phase doping and the origin of room-temperature ferromagnetism of S-doped TiO₂ is unveiled by regulating the concentration of sulfur doping and OVs.The main results are described as follows:1.The theoretical research shows that if metallic T phase ReS₂?T-ReS₂?is incorporated to semiconducting Td-ReS₂ matrix,the increased overlap between the wave function and screened Coulomb interactions will make emission peak blue-shift from infrared to visible region.The initial Td-ReS₂ single crystals were fabricated by the Br2-assisted chemical vapor deposition method.A standard mechanical exfoliation method was adopted to isolate the monolayer and few-layer ReS₂ films.The photoluminescence of the pristine nanosheets peaks at 1.51 eV and scarcely depends on the layer number.The T phase was introduced to Td-ReS₂ matrix?T@Td-ReS₂?by ultrasonic chemical exfoliation so as to achieve the visible PL.The T-ReS₂ concentration can be adjusted by changing ultrasonic time.High-resolution transmission electron microscopy?TEM?and spectroscopic characterizations reveal the coexistence of the T and Td phases in a nanosheet.The PL peaks of the T@Td-ReS₂ nanosheets blue-shift continuously with T phase concentration increasing,resulting in the visible light emission in the range from 1.5 to 2.25 eV.In addition,the PL of the T@Td-ReS₂ nanosheets still possesses a weak dependence on the layer number..2.The polarized PL spectra of the T@Td-ReS₂ nanosheets are investigated as well.The largest PL intensity is observed when both excitation and detection polarization are aligned with the direction of the Re-Re atomic chain formed.Besides,regardless of the excitation polarization,the emitted light is strongly angle-dependent,indicating an anisotropic excitonic nature.The exciton binding energy was determined through the PL excitation?PLE?measurements and the density-functional theory.The experimentally measured energy?0.43eV?matches the theoretical result?0.45eV?reasonably well.The mechanism of highly anisotropic and robust excitons is also explored.Under laser irradiation,electrons in T-ReS₂ can easily transfer to Td-ReS₂ because of the photothermal effect,which constructs a built-in electric field at the phase interface.The small difference in the work function between the two phases?0.12 eV?as well as the metal-semiconductor contact increases the effective transfer range of "hot" electrons to reach 25 nm.Because the average distance between the T-ReS₂ phase regions is about 9 nm,electron transfer occurs easily.The"hot" electrons are transferred to Td-ReS₂ to adjust the electron-hole interactions,leading to anisotropic visible light emission.The phase engineering can be generalized to other 2D materials to tune anisotropic PL properties.3.The sulfur-doped TiO₂ were prepared by hydrothermal synthesis method.To regulate sulfur doping amount and OV concentrations,the samples were annealed at different temperatures under Ar and O₂,respectively.Spectroscopic characterizations indicate that sulfur is adsorbed on the surface of TiO₂ particles in the form of S-O bonds before annealing,and the amount of doped S decreases at nearly the same rate in Ar and O₂ as annealing temperature increases.The adsorbed S atoms began to find interstitial or substitutional sites to form more stable structures at high temperature,leading to the further decrease of adsorbed S atoms.The concentration of OVs drops in O₂ and rises in Ar as annealing temperature increases.4.The room temperature magnetic properties of S-doped TiO₂ annealed under different conditions were investigated as well.The results demonstrate that the as-prepared sample before annealing has the largest saturation magnetization?0.03 emu/g?,and the magnetism of all samples attenuates after annealing treatment.The saturation magnetization of the samples annealed in O₂ is smaller than that of samples annealed in Ar.In addition,their saturated magnetization decreases sharply when the annealing temperature was further increased.These results imply that the magnetism is related to the synergistic effect of OVs and adsorbed S.Density functional theory?DFT?calculations reveal that magnetic moments originate from unpaired Ti atoms close to OVs.Electronic coupling between adsorbed sulfer and OV makes the superfluous electron of an adsorbed S atom transfer to an unpaired Ti atom via S-0 bonding bridge,inducing the rearrangement of the spin polarization of p orbit of the Ti atom.If the S atoms are introduced to the lattice,the electron of S atom will be localized at the doped site and no free electron can be used to affect the spin arrangement.Without S adsorption,the existence of a certain OVs cannot contribute to ferromagnetism,too.Thus,the room-temperature ferromagnetism originates from the electronic interaction between sulfur adsorption and OV.