Hongmei Deng

Structural phase transition, narrow band gap, and room-temperature ferromagnetism in [KNbO3]1−x[BaNi1/2Nb1/2O3−δ]x ferroelectrics

Structural phase transition, narrow band gap (Eg), and room-temperature ferromagnetism (RTFM) have been observed in the [KNbO3]1−x[BaNi1/2Nb1/2O3−δ]x (KBNNO) ceramics. All the samples have single phase perovskite structure, but exhibit a gradual transition behaviour from the orthorhombic to a cubic structure with the increase of x. Raman spectroscopy analysis not only corroborates this doping-induced change in normal structure but also shows the local crystal symmetry for x ≥ 0.1 compositions to deviate from the idealized cubic perovskite structure. A possible mechanism for the observed specific changes in lattice structure is discussed. Moreover, it is noted that KBNNO with compositions x = 0.1–0.3 have quite narrow Eg of below 1.5 eV, much smaller than the 3.2 eV band gap of parent KNbO3 (KNO), which is due to the increasing Ni 3d electronic states within the gap of KNO. Furthermore, the KBNNO materials present RTFM near a tetragonal to cubic phase boundary. With increasing x from 0 to 0.3, the magnetis...

Magnetism switching and band-gap narrowing in Ni-doped PbTiO3 thin films

Ions doping-driven structural phase transition accompanied by magnetism switching and band-gap narrowing effects has been observed in PbTi1−xNixO3−δ (xPTNO, x = 0.00, 0.06, and 0.33) thin films. With the increase of x, the xPTNO thin films exhibit not only a phase transition from the pseudotetragonal structure to a centrosymmetric cubic structure but also a drastic decrease of grain size. Moreover, the as-grown Ni-doped PbTiO3 (PTO) thin films show obvious room-temperature ferromagnetism and an increased saturation magnetization with increasing the Ni content, in contrast to undoped PTO, which shows diamagnetism. A bound magnetic polaron model was proposed to understand the observed ferromagnetic behavior of PTO-derived perovskite thin films. Furthermore, the 0.33PTNO thin film presents a narrowed band-gap, much smaller than that of PTO, which is attributed to new states of both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in an electronic structure with the presence ...

Synthesis and characterization of earth-abundant Cu2MnSnS4 thin films using a non-toxic solution-based technique

Earth-abundant Cu2MnSnS4 (CMTS) thin films were fabricated through a non-toxic spin-coating technique. The precursor solution is based on a 2-methoxyethanol solvated thiourea complex with acetyl-acetone used as an additive agent, and the spin-coated films were post-annealed at 570 °C under a N2 atmosphere. The influence of annealing time on the structure, composition, morphology, and optical properties of the processed precursor films has been studied in detail. We found that a longer annealing time during CMTS growth can improve the phase purity, promote the preferred orientation along the (112) direction, and enhance grain growth in the micrometer range. Film annealed for 10 min gives a pure CMTS phase, whereas other films annealed for lower and/or higher than 10 min (especially 13 min) can form secondary phases (i.e., SnS, MnS). The band gap energy is estimated as 1.63–1.18 eV for post-annealed films depending on the heat treatment, compared to 1.69 eV for as-prepared film. An efficiency of 0.49% for the device fabricated here has been achieved with an open-circuit voltage of 308.4 mV, a short-circuit current density of 4.7 mA cm−2, and a fill factor of 33.9%. It offers a new research direction for the application of a CMTS absorber layer in low-cost solar cells.

Strain-induced structural phase transition, ferromagnetic and optical properties of Bi1−x Tb x FeO3 thin films

In this paper, we report on the structural phase transition, surface morphologies, ferromagnetic and optical properties that have been observed in Bi1−x Tb x FeO3 (BTFO) thin films with (x = 0, 0.03, 0.06 and 0.09). X-ray diffraction (XRD) indicated that all the films have a single phase with rhombohedral structure and exhibit gradual transition behavior to pseudo-tetragonal structure with the increase of x, the strain values were estimated to be 0.1–0.8, depending on composition. A possible mechanism, strain-induced structural phase transition, is discussed. The micro-Raman spectroscopy analysis was in good agreement with structural phase transition verified by the XRD patterns. Moreover, with an increase in Tb3+ ions from x = 0–0.06, the saturation magnetization of the BTFO thin films shows a linear increase, which is mainly due to the Tb3+ ions. However, a sharp increase emerges with x from 0.06 to 0.09, the phase transition mainly dominates the higher ferromagnetism in BTFO films. Moreover, BTFO films with x > 0 exhibit a lower band-gap than that with x = 0, originating from local rhombohedral to pseudo-tetragonal structure transition. These results are helpful for a deeper understanding of structural transition, magnetic, and optical properties in perovskite oxides and show the potential roles, which such materials can play in multiferroic applications and solar energy devices.

Structural, optical and magnetic tunability in KBiFe2O5 multiferroics

KBiFe2O5, a highly promising multiferroic for perovskite solar cells, has been fabricated using a one-step thermal treatment method. The resulting products were characterized by X-ray diffraction, scanning electron microscopy and ultraviolet-visible-near-infrared spectroscopy. The effect of temperature on the formation of KBiFe2O5 polycrystallites was assessed, and we found that the reaction temperature is the key factor in determining the optical properties of the final products. Pure multiferroic KBiFe2O5 forms at a temperature of 850 °C with a narrow band gap of 1.65 eV, which is due to the stronger covalent character of Fe–O in FeO4 than that in FeO6 accompanying the inverted t2g/eg orbitals of tetrahedra. The magnetic transition from paramagnetism to ferromagnetism corresponds to the site of Fe3+, and the magnetic moment modification in the ferromagnetic phase in KBiFe2O5 could be correlated with the temperature and distortion of the unit cell. These results are helpful in the deeper understanding of the relationship between the crystal structure and the physical properties in perovskite-like oxides and show the potential role such materials can play in perovskite solar cells and multiferroic applications.

Band-gap narrowing and magnetic behavior of Ni-doped Ba(Ti0.875Ce0.125)O3 thin films

Band-gap narrowing and magnetic effects have been observed in a Ni-doped Ba(Ti0.875Ce0.125)O3 (BTC) thin film. Structural characterizations and microstructural analysis show that the as-prepared Ba(Ti0.75Ce0.125Ni0.125)O3−δ (BTCN) thin film exhibits a cubic perovskite structure with an average grain size of 25 nm. The Ce doping at the Ti-site results in an increasing perovskite volume to favour an O-vacancy-stabilized Ni2+ substitution. Raman spectroscopy, however, shows the cubic symmetry of crystalline structures is locally lowered by the presence of dopants, significantly deviating from the ideal Pm3m space group. Moreover, BTCN presents a narrowed band-gap, much smaller than that of BaTiO3 and BTC, due to new states of both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in an electronic structure with the presence of Ni. Also, magnetic enhancement driven by co-doping has been confirmed in the films, which mainly stems from the exchange interaction of Ni2+ ions via an electron trapped in a bridging oxygen vacancy. These findings may open an avenue to discover and design optimal perovskite compounds for solar-energy devices and information storage.

Structural, optical and electrical properties of Sb2Te3 films prepared by pulsed laser deposition

Revealing travel patterns and city structure with taxi trip data Xi Liu, Li Gong , Yongxi Gong , Yu Liu a,b* a Institute of Remote Sensing and Geographical Information Systems, Peking University, Beijing 100871, PR China b Beijing Key Lab of Spatial Information Integration and Its Applications, Peking University, Beijing 100871, PR China c Shenzhen Key Laboratory of Urban Planning and Decision Making, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, PR China Abstract: Delineating travel patterns and city structure has long been a core research topic in transport geography. Different from the physical structure, the city structure beneath the complex travel-flow system shows the inherent connection patterns within the city. On the basis of massive taxi trip data of Shanghai, we built spatially-embedded networks to model the intra-city spatial interactions and introduced network science methods into the issue. The community detection method is applied to reveal sub-regional structures, and several network measures are used to examine the properties of sub-regions. Considering the differences between longand short-distance trips, we reveal a two-level hierarchical polycentric city structure of Shanghai. Further explorations on sub-network structures demonstrate that urban sub-regions have broader internal spatial interactions, while suburban centers are more influential in local traffic. By incorporating the land use of centers from the travel pattern perspective, we investigate sub-region formation and center–local places interaction patterns. This study provides insights into using emerging data sources to reveal travel patterns and city structures, which could potentially aid in applying urban and transportation policies. The sub-regional structures revealed in this study are more easily interpreted for transportation-related issues than other structures, such as administrative divisions.

Microstructure and optical properties of Zn-doped BaTiO3 thin films

The high-quality BaTiO3 and Zn-doped BaTiO3 thin films were prepared on Si substrates by choosing the suitable raw materials and optimizing the preparation of thin films and the post process of annealing. Through the characterization technique of x-ray diffraction (XRD) and atomic force microscope (AFM), the influences of the different doping levels was discussed on the sizes, shapes and uniformities of BaTiO3 and Zn-doped BaTiO3 thin films respectively. Raman spectra measurements were carried out to study the lattice vibration modes of thin films. Meanwhile, the type of dopant and the occupied site in the BaTiO3 lattice were also discussed in detail. For investigating of the mechanism of the doping effects, the optical properties of thin films were studied by spectroscopic ellipsometry furthermore. And the optical constant refractive index and extinction coefficient have been obtained in the wavelength from 400nm to 700nm.

Optical properties of LuFeO3 thin films prepared by pulsed laser deposition

The LuFeO3 thin films were successfully prepared by pulsed laser deposition on silicon and LaNiO3-coated silicon substrates, respectively. The crystalline structure of the films has been studied by x-ray diffraction and indicated that all the films were well-crystallized. Atomic force microscope was used to characterize the surface morphology of the films and the surface was uniform and dense. Raman spectra measurements were carried out to study the lattice vibration modes of the films in the 200-1000 cm−1 wave number range. Polarization-electric field hysteresis loops of the LuFeO3 thin films were measured at applied electric field.

Structural and optical properties of LuFeO3 thin films prepared on silicon (100) substrate by pulsed laser deposition

The LuFeO3 (LFO) nanocrystalline films with the average grain size of 20 nm have been grown on silicon (100) substrates by pulsed laser deposition. X-ray diffraction (XRD) analysis shows that LFO films are polycrystalline. The microstructure and surface morphology of LFO film are analyzed by Atomic force microscope (AFM) and the surface shows the film is compact. Spectroscopic ellipsometry (SE) was used to extract the optical properties of LFO films in the 1.1-3.1 eV (400-1100 nm) photon energy range at room temperature. By fitting the measured ellipsometric parameter data with a multilayer model system for the LFO thin films, the optical constants and thicknesses of the thin films have been obtained. The refractive index n and extinction coefficient k of the LFO thin films both decreases with increasing thickness.