Yongli Song

Bio: Yongli Song is an academic researcher from Harbin Institute of Technology. The author has contributed to research in topics: Dielectric & Grain boundary. The author has an hindex of 7, co-authored 10 publications receiving 269 citations.

Origin of colossal dielectric permittivity of rutile Ti0.9In0.05Nb0.05O2: single crystal and polycrystalline

Abstract: In this paper, we investigated the dielectric properties of (In + Nb) co-doped rutile TiO2 single crystal and polycrystalline ceramics. Both of them showed colossal, up to 10(4), dielectric permittivity at room temperature. The single crystal sample showed one dielectric relaxation process with a large dielectric loss. The voltage-dependence of dielectric permittivity and the impedance spectrum suggest that the high dielectric permittivity of single crystal originated from the surface barrier layer capacitor (SBLC). The impedance spectroscopy at different temperature confirmed that the (In + Nb) co-doped rutile TiO2 polycrystalline ceramic had semiconductor grains and insulating grain boundaries, and that the activation energies were calculated to be 0.052 eV and 0.35 eV for grain and grain boundary, respectively. The dielectric behavior and impedance spectrum of the polycrystalline ceramic sample indicated that the internal barrier layer capacitor (IBLC) mode made a major contribution to the high ceramic dielectric permittivity, instead of the electron-pinned defect-dipoles.

The contribution of doped-Al to the colossal permittivity properties of AlxNb0.03Ti0.97−xO2 rutile ceramics

Abstract: The search for colossal permittivity (CP) materials continues to attract considerable interest motivated by not only academic research but also potential applications. Very recently, CP with low dielectric loss was reported in In + Nb co-doped rutile TiO2 polycrystalline ceramics. However, the mechanism of CP in this material system and the effect of doping ions are still unclear. Here, we investigated the dielectric properties of AlxNb0.03Ti0.97−xO2 (x = 0, 0.01, 0.03 and 0.05) ceramics. CP with low dielectric loss was found in AlxNb0.03Ti0.97−xO2 samples (x ≤ 0.03). Once the amount of Al-doping exceeds Nb, the CP disappears. The change in dielectric response with varying Al-doping concentration and Ti3+ concentration together with the conductive activation energy in AlxNb0.03Ti0.97−xO2 ceramics clearly suggest that the CP mechanism in co-doped TiO2 ceramics could be attributed to the internal barrier layer capacitor effect. We believe that our research provides comprehensive guidance for the development of CP materials.

Origin of ferromagnetism in aluminum-doped TiO2 thin films: Theory and experiments

Abstract: In this paper, we combine first-principles calculations and experiments to investigate the magnetic properties of aluminum-doped TiO2 films of rutile structure. Density-functional theory with generalized gradient approximation based calculations were carried out for three cases, where the TiO2 lattice contains oxygen vacancies VO only, an oxygen is substituted by a fluorine atom, or a Ti is substituted by an aluminum. Magnetic moments associated with the formation of Ti3+ ions are found in all cases but they couple differently resulting in different magnetic states. Al-doped samples prepared in our labs exhibit ferromagnetism at room temperature with a TC near 340 K. The experimental results are consistent with the first principles calculations, and the magnetism is associated with the VO defect electrons induced by the Al doping. The defect electron occupies nearby Ti sites giving rise to the Ti3+ moments and, at the same time, has spatially extended wavefunctions assuring overlapping between neighbors.

Colossal dielectric permittivity in (Al + Nb) co-doped rutile SnO2 ceramics with low loss at room temperature

Abstract: The exploration of colossal dielectric permittivity (CP) materials with low dielectric loss in a wide range of frequencies/temperatures continues to attract considerable interest. In this paper, we report CP in (Al + Nb) co-doped rutile SnO2 ceramics with a low dielectric loss at room temperature. Al0.02Nb0.05Sn0.93O2 and Al0.03Nb0.05Sn0.92O2 ceramics exhibit high relative dielectric permittivities (above 103) and low dielectric losses (0.015 < tan δ < 0.1) in a wide range of frequencies and at temperatures from 140 to 400 K. Al doping can effectively modulate the dielectric behavior by increasing the grain and grain boundary resistances. The large differences in the resistance and conductive activation energy of the grains and grain boundaries suggest that the CP in co-doped SnO2 ceramics can be attributed to the internal barrier layer capacitor effect.

Experimental and theoretical evidence for the ferromagnetic edge in WSe2 nanosheets

Abstract: Bulk TMDCs are diamagnetic materials; however, two-dimensional TMDCs exhibit spin polarized edge states, which results in a coupling between the unsaturated transition metal and chalcogenide atoms at the edges. The magnetism in two-dimensional TMDCs broadens their applications in spintronic and multi-functional devices. Herein, by combining macro/micro-magnetic experimental measurements and density functional theory (DFT) calculations, we demonstrate that among five possible edge-terminated WSe2 nanosheets only two types have a magnetic ground state, corresponding to the 100% Se edge terminated and 50% Se edge terminated nanosheets, respectively. The calculation results on WSe2 clusters and WSe2 zig-zag nanoribbons with different terminations and Se coverage rate confirmed that the unpaired electrons of the edge atoms play a crucial role in the appearance of ferromagnetism in WSe2 nanosheets. Furthermore, due to the possible quantum confinement effect and surface effect, there exist thickness-dependent magnetic properties, and the magnitude of magnetism at the edge increases as the number of layers decreases.

Emerging chemical strategies for imprinting magnetism in graphene and related 2D materials for spintronic and biomedical applications

Abstract: Graphene, a single two-dimensional sheet of carbon atoms with an arrangement mimicking the honeycomb hexagonal architecture, has captured immense interest of the scientific community since its isolation in 2004. Besides its extraordinarily high electrical conductivity and surface area, graphene shows a long spin lifetime and limited hyperfine interactions, which favors its potential exploitation in spintronic and biomedical applications, provided it can be made magnetic. However, pristine graphene is diamagnetic in nature due to solely sp2 hybridization. Thus, various attempts have been proposed to imprint magnetic features into graphene. The present review focuses on a systematic classification and physicochemical description of approaches leading to equip graphene with magnetic properties. These include introduction of point and line defects into graphene lattices, spatial confinement and edge engineering, doping of graphene lattice with foreign atoms, and sp3 functionalization. Each magnetism-imprinting strategy is discussed in detail including identification of roles of various internal and external parameters in the induced magnetic regimes, with assessment of their robustness. Moreover, emergence of magnetism in graphene analogues and related 2D materials such as transition metal dichalcogenides, metal halides, metal dinitrides, MXenes, hexagonal boron nitride, and other organic compounds is also reviewed. Since the magnetic features of graphene can be readily masked by the presence of magnetic residues from synthesis itself or sample handling, the issue of magnetic impurities and correct data interpretations is also addressed. Finally, current problems and challenges in magnetism of graphene and related 2D materials and future potential applications are also highlighted.

Effects of Secondary Phases on the High-Performance Colossal Permittivity in Titanium Dioxide Ceramics.

Abstract: The intensive demands of microelectronics and energy-storage applications are driving the increasing investigations on the colossal permittivity (CP) materials. In this study, we designed a new system of Dy and Nb co-doped TiO2 ceramics [(Dy0.5Nb0.5)xTi1–xO2] with the formation of secondary phases, and then the enhancement of overall dielectric properties (er ∼ 5.0–6.5 × 104 and tan δ < 8%) was realized in the broad composition range of 0.5 ≤ x ≤ 5%. More importantly, effects of secondary phases on microstructure, dielectric properties, and stability were explored from the views of defect-dipoles and internal barrier layer capacitance (IBLC) effect. According to the defect-dipoles theory, the CP should mainly originate from Nb5+, and the Dy3+ largely contributes to the decreased dielectric loss. Both CP and low dielectric loss were obtained for co-doping with Dy3+ and Nb5+. Besides, the Dy enrichment induced the formation of secondary phases, which were regarded as the low loss unit dispersed into the cer...

Colossal permittivity of (Mg + Nb) co-doped TiO2 ceramics with low dielectric loss

Abstract: A high dielectric loss is one of the difficulties that hinder the application of colossal permittivity (CP) materials. Here we report the CP behaviors in ceramics of rutile (Mg + Nb) co-doped TiO2, i.e., (Mg1/3Nb2/3)xTi1−xO2. The room-temperature dielectric properties of the pure homogenous ceramics include a relatively high CP (>104) and an acceptable dielectric loss (<0.1) at frequencies from 102 to 105 Hz in the doping concentration range of 0.5% to 7%. In particular, an excellent low dielectric loss of 0.0083 and a high dielectric permittivity of 3.87 × 104 at 1 kHz were obtained for the 1% doped sample. Moreover, the temperature stability (room temperature ∼ 180 °C) and frequency stability (102–105 Hz) of the CP properties were studied. X-ray photoelectron spectroscopy suggests that the superior CP properties could be explained by the electron-pinned defect-dipole mechanism.

Crystalline Structure, Defect Chemistry and Room Temperature Colossal Permittivity of Nd-doped Barium Titanate.

Abstract: Dielectric materials with high permittivity are strongly demanded for various technological applications. While polarization inherently exists in ferroelectric barium titanate (BaTiO3), its high permittivity can only be achieved by chemical and/or structural modification. Here, we report the room-temperature colossal permittivity (~760,000) obtained in xNd: BaTiO3 (x = 0.5 mol%) ceramics derived from the counterpart nanoparticles followed by conventional pressureless sintering process. Through the systematic analysis of chemical composition, crystalline structure and defect chemistry, the substitution mechanism involving the occupation of Nd3+ in Ba2+ -site associated with the generation of Ba vacancies and oxygen vacancies for charge compensation has been firstly demonstrated. The present study serves as a precedent and fundamental step toward further improvement of the permittivity of BaTiO3-based ceramics.