Wenbo Mi

Titanium-Defected Undoped Anatase TiO2 with p-Type Conductivity, Room-Temperature Ferromagnetism, and Remarkable Photocatalytic Performance

Defects are critically important for metal oxides in chemical and physical applications. Compared with the often studied oxygen vacancies, engineering metal vacancies in n-type undoped metal oxides is still a great challenge, and the effect of metal vacancies on the physiochemical properties is seldom reported. Here, using anatase TiO2, the most important and widely studied semiconductor, we demonstrate that metal vacancies (VTi) can be introduced in undoped oxides easily, and the presence of VTi results in many novel physiochemical properties. Anatase Ti0.905O2 was synthesized using solvothermal treatment of tetrabutyl titanate in an ethanol-glycerol mixture and then thermal calcination. Experimental measurements and DFT calculations on cell lattice parameters show the unstoichiometry is caused by the presence of VTi rather than oxygen interstitials. The presence of VTi changes the charge density and valence band edge of TiO2, and an unreported strong EPR signal at g = 1.998 presents under room temperature. Contrary to normal n-type and nonferromagnetic TiO2, Ti-defected TiO2 shows inherent p-type conductivity with high charge mobility, and room-temperature ferromagnetism stronger than Co-doped TiO2 nanocrystalline. Moreover, Ti-defected TiO2 shows much better photocatalytic performance than normal TiO2 in H2 generation (4.4-fold) and organics degradation (7.0-fold for phenol), owing to the more efficient charge separation and transfer in bulk and at semiconductor/electrolyte interface. Metal-defected undoped oxides represent a unique material; this work demonstrates the possibility to fabricate such material in easy and reliable way and thus provides new opportunities for multifunctional materials in chemical and physical devices.

Microstructure, magnetic, and optical properties of sputtered Mn-doped ZnO films with high-temperature ferromagnetism

The microstructure, magnetic, and optical properties of Mn-doped ZnO films have been examined. It has been found that Mn doping could improve the growth of ZnO (002) orientation without Mn oxide formation. All the films are ferromagnetic with a Curie temperature of above 350K. The ferromagnetism comes from the ferromagnetic interaction activated by oxygen vacancies between the Mn ions that replace Zn ions, but not from Mn oxide impurities. At an atomic fraction of 2.2% Mn, the average moment per Mn ion reaches a maximum of 0.55μB. With the further increase of Mn atomic fraction, the average moment per Mn ion decreases because the antiferromagnetic energy is lower than the ferromagnetic one due to the reduced distance between the adjacent Mn ions. Meanwhile, the optical band gap value increases from 3.120to3.162eV with the increase of Mn atomic fraction from 0% to 7.5%.The microstructure, magnetic, and optical properties of Mn-doped ZnO films have been examined. It has been found that Mn doping could improve the growth of ZnO (002) orientation without Mn oxide formation. All the films are ferromagnetic with a Curie temperature of above 350K. The ferromagnetism comes from the ferromagnetic interaction activated by oxygen vacancies between the Mn ions that replace Zn ions, but not from Mn oxide impurities. At an atomic fraction of 2.2% Mn, the average moment per Mn ion reaches a maximum of 0.55μB. With the further increase of Mn atomic fraction, the average moment per Mn ion decreases because the antiferromagnetic energy is lower than the ferromagnetic one due to the reduced distance between the adjacent Mn ions. Meanwhile, the optical band gap value increases from 3.120to3.162eV with the increase of Mn atomic fraction from 0% to 7.5%.

First Principles Prediction of the Magnetic Properties of Fe-X6 (X = S, C, N, O, F) Doped Monolayer MoS2

Using first-principles calculations, we have investigated the electronic structure and magnetic properties of Fe-X6 clusters (X = S, C, N, O, and F) incorporated in 4 × 4 monolayer MoS2, where a Mo atom is substituted by Fe and its nearest S atoms are substituted by C, N, O, and F. Single Fe and Fe-F6 substituions make the system display half-metallic properties, Fe-C6 and Fe-N6 substitutions lead to a spin gapless semiconducting behavior, and Fe-O6 doped monolayer MoS2 is semiconducting. Magnetic moments of 1.93, 1.45, 3.18, 2.08, and 2.21 μB are obtained for X = S, C, N, O, and F, respectively. The different electronic and magnetic characters originate from hybridization between the X and Fe/Mo atoms. Our results suggest that cluster doping can be an efficient strategy for exploring two-dimensional diluted magnetic semiconductors.

Large Spin-Valley Polarization in Monolayer MoTe2 on Top of EuO(111).

The electronic properties of monolayer MoTe2 on top of EuO(111) are studied by first-principles calculations. Strong spin polarization is induced in MoTe2 , which results in a large valley polarization. In a longitudinal electric field this will result in a valley and spin-polarized charge Hall effect. The direction of the Hall current as well as the valley and spin polarizations can be tuned by an external magnetic field.

Magnetism by Interfacial Hybridization and p-type Doping of MoS2 in Fe4N/MoS2 Superlattices: A First-Principles Study

Magnetic and electronic properties of Fe4N(111)/MoS2(√3 × √3) superlattices are investigated by first-principles calculations, considering two models: (I) Fe(I)Fe(II)-S and (II) N-S interfaces, each with six stacking configurations. In model I, strong interfacial hybridization between Fe(I)/Fe(II) and S results in magnetism of monolayer MoS2, with a magnetic moment of 0.33 μB for Mo located on top of Fe(I). For model II, no magnetism is induced due to weak N-S interfacial bonding, and the semiconducting nature of monolayer MoS2 is preserved. Charge transfer between MoS2 and N results in p-type MoS2 with Schottky barrier heights of 0.5-0.6 eV. Our results demonstrate that the interfacial geometry and hybridization can be used to tune the magnetism and doping in Fe4N(111)/MoS2(√3 × √3) superlattices.

Origin of the butterfly-shaped magnetoresistance in reactive sputtered epitaxial Fe3O4 films

Epitaxial Fe3O4 thin films were synthesized by facing-target reactive sputtering Fe targets. The epitaxy of the Fe3O4 film on MgO (100) was examined macroscopically using x-ray diffraction, including conventional θ-2θ scan, tilting 2θ scan, φ scan, and pole figure. The observed low-field butterfly-shaped magnetoresistance (MR) are explained by the primary fast rotation of the spins far away from antiphase boundaries and the high-field MR changing linearly with magnetic field can be understood by the gradual rotation of the spins near the antiphase boundaries. It is magnetocrystalline anisotropy that causes an increase in MR below Verwey transition temperature.

Structure and magnetic properties of facing-target sputtered Co–C granular films

We studied the structure and magnetic properties of as-deposited and subsequently annealed CoxC100−x granular films fabricated by a DC facing-target magnetron sputtering system at room temperature using atomic force microscopy, x-ray diffraction (XRD), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy and a vibrating sample magnetometer. The average roughness, Rα, of the as-deposited CoxC100−x granular films is smaller than that of the Si(100) substrates. XRD and TEM analyses indicate that the as-deposited CoxC100 − x granular films are composed of ~2 nm amorphous cobalt grains dispersed in an amorphous carbon matrix, and their morphology is composition independent. The high resolution TEM image of the as-deposited Co30C70 film shows that cobalt and carbon have already separated during the deposition, even if the aggregation of cobalt is not complete. Annealing at 300–450°C causes the crystallization of amorphous cobalt followed by an increase in grain size and the graphitization of the amorphous carbon matrix. The constriction arising from the structural environment results in the coexistence of the hcp and fcc Co phases at temperatures higher than the phase transformation point of 425°C. Magnetic measurements reveal that the coercivity of the as-deposited CoxC100−x granular films decreases with the increase in cobalt concentration, and increases with decrease in film thickness. The enhanced coercivity can be attributed to the weakened intergrain interaction because of the increased percolation threshold and/or the destruction of long-range domain structures caused by the reduction in film thickness.

Spin-polarized transport of electrons from polycrystalline Fe3O4 to amorphous Si

Polycrystalline Fe3O4∕amorphous Si heterostructure was prepared by facing-target sputtering and its microstructure and electrical transport properties were studied. The polycrystalline Fe3O4 layer was grown in column structure. The electrical transport mechanism across the disordered interface between polycrystalline Fe3O4 and amorphous Si layers is tunneling above the Verwey temperature [Nature (London) 144, 327 (1939)] of 120K. Nonlinear I‐V characteristics of the Schottky diode reveal thermionic emission∕diffusion mechanism below the Verwey temperature, and Schottky barrier height is 0.27eV, calculated by a standard theory of thermionic emission∕diffusion. Based on a simplified band structure, the spin polarization of the polycrystalline Fe3O4 layer was determined to be ∼45%.

Evolution of structure, magnetic and transport properties of sputtered films from Fe to Fe3O4

Polycrystalline iron oxide films were fabricated using the reactive sputtering method without substrate heating. Structure characterization indicates that the dominant phases in the films evolve from α-Fe to pure Fe3O4 with the increasing O2 flow rate. In polycrystalline Fe3O4 films, disordered atoms exist at the grain boundaries. Magnetic properties analyses reveal that the room-temperature magnetization first decreases and later increases due to the variation of the volume fraction of the paramagnetic FeO phase with a Neel temperature of 198 K. The magnetoresistance MR (= [R(H) − R(0)]/R(0)) of the films increases from 0.1% for pure Fe film to 10.6% for the Fe3O4 film at 80 K under a 90 kOe field. The transport mechanism of FeO–Fe3O4 and Fe3O4 films is suggested to be the tunnelling process, which satisfies the log ρ ~ T−1/2 relation. The Hall resistivity of the Fe3O4 film decreases with increasing temperature. The ordinary and extraordinary Hall coefficients of the Fe3O4 film at 300 K are about 100 and 420 times larger than those of bulk Fe.

Prediction of spin-dependent electronic structure in 3d-transition-metal doped antimonene

We investigate the geometric structure and electronic and magnetic properties of 3d-transition-metal atom doped antimonene using spin-polarized first-principles calculations. Strong orbital hybridization exhibits between 3d-transition-metal and Sb atoms, where covalent bonds form in antimonene. A spin-polarized semiconducting state appears in Cr-doped antimonene, while half-metallic states appear by doping Ti, V, and Mn. These findings indicate that once combined with doping states, the bands of antimonene systems offer a variety of features. Specific dopants lead to half-metallic characters with high spin polarization that has potential application in spintronics.