van der Waals heterojunctions formed by stacking various two-dimensional (2D) materials have a series of attractive physical properties, thus offering an ideal platform for versatile electronic and optoelectronic applications. Here, we report few-layer SnSe/MoS2 van der Waals heterojunctions and study their electrical and optoelectronic characteristics. The new heterojunctions present excellent electrical transport characteristics with a distinct rectification effect and a high current on/off ratio (∼1 × 105). Such type-II heterostructures also generate a self-powered photocurrent with a fast response time (<10 ms) and exhibit high photoresponsivity of 100 A W–1, together with high external quantum efficiency of 23.3 × 103% under illumination by 532 nm light. Photoswitching characteristics of the heterojunctions can be modulated by bias voltage, light wavelength, and power density. The designed novel type-II van der Waals heterojunctions are formed from a combination of a transition-metal dichalcogenide and a group IV–VI layered 2D material, thereby expanding the library of ultrathin flexible 2D semiconducting devices.Keywords: dissimilar material systems; rectification; self-power photocurrent; type-II band alignment; van der Waals heterostructure;

The elastocaloric effect (eCE) is studied in a Ni57Mn18Ga21In4 alloy undergoing the magnetostructural transition from paramagnetic austenite (PMA) to ferromagnetic martensite (FMM). Superelasticity associated with the stress-induced transition from PMA to FMM is realized at room temperature. Simultaneously, a large eCE marked by a temperature change ΔTad up to 9.6 K is detected. It is revealed that both the lattice vibration and magnetization change positively contribute to the large eCE. This work may open the possibility of tuning the eCE by simultaneous application of magnetic field and bias stress.Compressive (a) stress-strain curves and temperature change as a function of time (b) simultaneously tested for the Ni57Mn18Ga21In4 single crystal measured at different strain rates ranging from 0.025 mm/s to 0.4 mm/s.Download high-res image (273KB)Download full-size image

•Special dual-phase microstructure containing < 100 > orientation columnar grains and particle precipitates are prepared in Fe-Ga-Tb alloys.•Considerable magnetostriction of 255 ppm is obtained in directional solidified (Fe0.83Ga0.17)99.9Tb0.1 alloy.•Giant tensile strain of ~ 6.6% is achieved in (Fe0.83Ga0.17)99.95Tb0.05 alloy induced by the special dual-phase microstructure.•A practical magnetostrictive alloys with integration of structural and functional properties are produced by trace Tb doping and directional solidification method.Decent magnetostriction of ~ 255 ppm and giant tensile strain of ~ 6.6% is obtained through trace Tb addition and directional solidification method in (Fe0.83Ga0.17)100 − xTbx (x = 0–0.5) alloys. The particle-shaped Tb-rich precipitates are formed and dispersively distributed in the matrix along the (sub-) grain boundaries; preferred 〈100〉 orientation is achieved in the alloys by appropriate directional solidification process, which makes contribution to improve the magnetostriction. The formation of Tb-rich precipitates induces the transition of the fracture mechanism from brittleness to ductility. The maximal room-temperature tensile strain is obtained in x = 0.05 specimen, which is superior to existing literatures about the mechanical properties of Fe-Ga based alloys. The regular columnar grain morphologies and certain distribution pattern of granular precipitates are considered to be responsible for the excellent ductility in Fe-Ga-Tb alloys. This work may provide a new thinking to design high ductility Fe-Ga magnetostrictive alloys and significantly broaden the application of these materials in industry field.Download high-res image (163KB)Download full-size image

•Short-range ordering exists in both melt-spun and single crystal Fe-Ga alloys.•Local lattice strain and long-range symmetry-breaking lead to giant magnetostriction.•Ga-Ga pairs can aggregate into nano-sized precipitates under fast cooling.•Short-range ordering Ga-Ga pairs cause peak broadening while their nano-scale aggregations induce peak splitting.Melt-spun ribbons and single crystal plates of Fe83Ga17 alloys were studied at both nano-scale and atomic-scale to investigate the structure origin of their giant magnetostriction. The presence of nano-precipitates in the melt-spun ribbons was verified using synchrotron x-ray diffraction and was validated by small angle scattering experiment. The nano-precipitates were found to be inhomogeneously distributed within the ribbons, as peak splitting clearly observed in the diffraction patterns disappeared into broad peaks when the synchrotron beam was focused onto different parts of the same sample. Further analysis using x-ray diffuse scattering and extended x-ray absorption fine structure indicates the existence of short-range ordering (SRO) Ga-Ga pairs along the (001) direction in both the ribbon and single-crystal alloys. All experimental evidence suggests that this modified D03-type SRO leads to both a short-range lattice strain and a long-range symmetry breaking tetragonal distortion of the A2 lattice. This gives rise to giant magnetostriction in the Fe-Ga alloys. In the melt-spun ribbons, the Ga-Ga pairs can aggregate into nano-sized precipitates under fast cooling and enhance the magnetostriction further.Download high-res image (533KB)Download full-size image

•Direct correlation between nanoinclusions and magnetostriction is established.•Nanoinclusions induce tetragonal distortion of the matrix.•A tetragonal model is proposed for the B2 or fcc-like nanoinclusions.Traditionally, magnetostriction of ferromagnetic materials was considered as an intrinsic homogeneous effect. More recently, the magnetostriction in certain binary systems such as Fe-Ga alloys, has been associated with nanoscale heterogeneities, but evidence of a direct relation between the heterogeneities and magnetostriction was lacking. Here heterogeneous magnetostriction controlled by nanoinclusions is reported in Fe-Co alloys, where the largest values are found in two-phase coexistence regions. Systematic investigation of the nanostructure-dependent magnetostriction in Fe-Co alloys using synchrotron X-ray diffraction and high-resolution TEM with geometric phase analysis has allowed us to identify the nature of the B2-like and fcc-like nanoinclusions that are responsible for the enhanced magnetostriction in this system. The relation between magnetostriction and the nanoheterogeneities is established by tailoring the nanoinclusion size with alloy composition and annealing temperature. This provides the direct experimental evidence of nanoinclusion-induced enhanced magnetostriction. Finally, a model is presented to explain how coherent inclusions with the ordered B2-like structure or fcc-like structure can induce tetragonal distortions of the matrix in the relevant two-phase regions and thus enhance the magnetostriction. The results clarify the magnetostriction mechanism and may stimulate a search for new magnetostrictive materials.Download high-res image (196KB)Download full-size image

Vertically stacked van der Waals (vdW) heterojunctions based on two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted a great deal of attention and have created a powerful new material platform for novel, high-performance electronic and optoelectronic devices. Here, we report the construction of multilayer p-MoTe2/n-MoS2 vdW heterostructures with remarkable rectification behavior, self-powered photoresponse and distinct photosensitivity at different laser wavelengths and power densities. Field effect transistors (FETs) fabricated by MoTe2/MoS2 heterojunctions exhibit excellent gate-tunable rectification behavior and p–n junction transport characteristics, with the n-type dominating. The MoTe2/MoS2 heterojunction devices generate a self-powered photocurrent at zero bias voltage with a considerable on–off ratio reaching ∼780 and achieve a stable and fast photoresponse, due to the type-II band alignment facilitating efficient electron–hole separation. Utilizing the advantages of a p–n junction with type-II band alignment, this MoTe2/MoS2 vdW heterostructure provides more opportunities for future electronic and optoelectronic applications.

Herein we present a systematic study on the effects of different gas molecules on the photoelectric response of few-layer GaSe phototransistors before and after introducing defects. After introducing defects by thermal annealing, the phototransistors become largely photo-responsive (18.75 A W−1) with high external quantum efficiency (EQE) (∼91.53%), high photocurrent on–off ratio, fast photo-response, and good stability in an O2 rich environment when illuminated by 254 nm ultraviolet light.

Heterostructure engineering of atomically thin two-dimensional materials offers an exciting opportunity to fabricate atomically sharp interfaces for highly tunable electronic and optoelectronic devices. Here, we demonstrate abrupt interface between two completely dissimilar material systems, i.e, GaTe-MoS2 p–n heterojunction transistors, where the resulting device possesses unique electronic properties and self-driven photoelectric characteristics. Fabricated heterostructure transistors exhibit forward biased rectifying behavior where the transport is ambipolar with both electron and hole carriers contributing to the overall transport. Under illumination, photoexcited electron–hole pairs are readily separated by large built-in potential formed at the GaTe–MoS2 interface efficiently generating self-driven photocurrent within <10 ms. Overall results suggest that abrupt interfaces between vastly different material systems with different crystal symmetries still allow efficient charge transfer mechanisms at the interface and are attractive for photoswitch, photodetector, and photovoltaic applications because of large built-in potential at the interface.Keywords: ambipolar behavior; dissimilar material systems; p−n heterojunction; rectification; self-driven photocurrent

•Coupled magneto-structural transition were observed in single-phase Ni50Mn29Ga20.9Tb0.1 ribbon.•Sizable magnetocaloric effect was monitored from the magneto-structural transition.•The magnetocaloric effect was enhanced by the solid solution of Tb.Ni50Mn29Ga21−xTbx (x=0-1) ribbons with the solid solution of Tb element were synthesized by the melt spinning method. The phase transformation, magnetic properties and magnetocaloric effect were investigated. With the increasing Tb content the martensitic transformation temperatures were gradually increased while the Curie temperature was monotonously decreased. According to the detected phase transition temperatures, a phase diagram was established to describe the dependence of the magneto-structural transition on the Tb content. Three types of magneto-structural transitions were observed. Especially, the martensitic transformation was coincided with the magnetic transition in the single phase alloy with x = 0.1, giving rise to the coupled magneto-structural transition from ferromagnetic martensite to paramagnetic austenite. Sizable magnetic entropy change of 4.31 J/Kg K was induced from the coupled magneto-structural transition by the application of magnetic field of 50 kOe at 349.5 K.

The correlation between growth twinning and crystalline reorientation of faceted growth materials during directional solidification was demonstrated through matrix calculation and Thompson regular tetrahedron model diagrams. The correlation is that the crystalline reorientation results from the mirror symmetry operation of initial orientation on different twin planes. In total, seven possibilities of crystalline reorientation caused by single-twinning from ⟨110⟩, ⟨111⟩, and ⟨112⟩ initial axial orientation were predicted, four of which were verified experimentally in the representative Tb0.3Dy0.7Fe2 alloys.

SmCo5/Co nanocomposites with exchange-coupling are synthesized by a reverse design where Co is decomposed from SmCo5 nanoparticles by hydrogen disproportionation process to fabricate two-phase nanocomposites. The novel design makes SmCo5 nanoparticles uniformly surrounded by Co nanoparticles, which avoids the alloying in traditional methods and achieves exchange-coupling.

The addition of rare earth elements (RE) is an effective way to improve the mechanical properties of Ni–Mn–Ga alloys. Unlike previous investigations which were completely focused on the N–Mn–(Ga,RE) alloys, in this work Ni50−xTbxMn30Ga20 (x=0–1) alloys in the form of (Ni,RE)–Mn–Ga were systematically studied with the microstructure, martensitic transformation, shape memory effect and mechanical properties. Dual-phase microstructure containing body-centered tetragonal martensite and Tb-rich hexagonal precipitates were formed. With the increasing Tb content, the volume fraction of the precipitates was also increased. Yet stable martensitic transformation occurring at about 80 °C was observed over the whole composition range. Both the transformation temperatures and hysteresis (∼11 °C) were quite insensitive to the composition variation. Sizable shape memory effect is obtained in all the polycrytalline alloys. Shape memory effect of 2.68% was detected for the alloy x=1 with large amount of precipitate. With the increasing volume fraction of precipitates both the compressive strength and strain were well increased for x≤0.2 and gradually decreased for 0.2≤x≤1. The transition of fracture mode from intergranular fracture to transgranular fracture and then to interphase fracture was revealed to contribute to the evolution of the mechanical properties.

Remarkable enhancements in tensile strength (∼625 MPa) and elongation ratio (∼1%), being three and five times, respectively, as large as those of the binary Fe81Ga19 alloy, were obtained in situ dual-reinforcement (TaCparticle+TaCdendrite)/Fe81Ga19 magnetostrictive composites due to grain refinement and secondary-phase reinforcement.

  (Fe0.83Ga0.17)100−xDyx (0 ⩽ x ⩽ 0.42) ribbons were made by the melt-spinning method. The increases in the lattice parameters Tc and Ms indicated the presence of Dy element solution in the A2 matrix. Dy-doped Fe83Ga17 ribbons showed a significant improvement in magnetostriction, which was the same effect of small additions of Tb in the Fe83Ga17 melt-spun ribbons. The maximum perpendicular magnetostriction λ⊥ in the direction of the ribbons length was −620 ppm for x = 0.25, which was three times larger than that of the binary Fe83Ga17 ribbon.

In order to study the texture evolution and magnetostriction behavior in the rolled Fe–Ga–B sheets during the heat treatments from low to high temperatures, (Fe81Ga19)98B2 sheets were prepared and investigated. The phase structure, recrystallization, grain size, texture evolution, and magnetostriction behavior during the annealing from 525 to 1200 °C for 1–5 h were investigated using X-ray diffraction, electron backscattering diffraction, and standard strain-gauge measurements. Results indicated that the primary recrystallization temperature for 1-h annealing was found as 525–575 °C in (Fe81Ga19)98B2 sheets. Annealing the sample below 575 °C for 1 h, the release of rolling stress and increase of 〈100〉 η-fiber texture during the primary recrystallization jointly resulted in a rapid improvement in magnetostriction. After annealed between 575 and 1100 °C for 1 h, the grains of the sheets underwent a normal growth, and the three (α-, γ- and η-fiber) types of textures kept an approximate balance, leading to a plateau of magnetostriction around 75 ppm. When the abnormal grain growth proceeded above 1100 °C for 1 h, the proportion of η-fiber texture markedly increased, and the magnetostriction was subsequently increased to 97 ppm. For longer annealing durations, the strong ideal cube texture (η-fiber) was firstly formed and then changed to undesired texture (γ-fiber), producing a corresponding magnetostriction peak of 136 ppm at 2 h for the annealing at 1200 °C. The clear correlation among heat treatments, recrystallization, texture, and magnetostriction provides an essential understanding for Fe–Ga–B alloy sheets.

Permanent magnetic materials capable of operating at high temperature up to 500 °C have wide potential applications in fields such as aeronautics, space, and electronic cars. SmCo alloys are candidates for high temperature applications, since they have large magnetocrystalline anisotropy field (6–30 T), high Curie temperature (720–920 °C), and large energy product (>200 kJ·m−3) at room temperature. However, the highest service temperature of commercial 2:17 type SmCo magnets is only 300 °C, and many efforts have been devoted to develop novel high temperature permanent magnets. This review focuses on the development of three kinds of SmCo based magnets: 2:17 type SmCo magnets, nanocrystalline SmCo magnets, and nanocomposite SmCo magnets. The oxidation protection, including alloying and surface modification, of high temperature permanent magnets is discussed as well.

Polycrystalline Fe81Ga19 rods were directionally solidified by an induction heating zone melting method at different growth velocities. The microstructure, preferred orientation, and magnetostriction were investigated. As the growth velocity increased, the solidification morphology transited from columnar to equiaxed. Simultaneously, the solid/liquid interface morphology evolved from planar to cellular, accompanied by an increase in the interface concave height. The 〈001〉 preferred orientation was detected along the axial direction of the grown rod at 10 mm/h. With the increased growth velocity, the 〈001〉 preferred orientation deviated from the axial direction, and the 〈001〉 orientation degree was weakened. The saturation magnetostriction of the grown rod was 305 ppm at 10 mm/h, but it deteriorated at higher velocities. The decreased magnetostriction was attributed to the microstructure transition and the weakened 〈001〉 preferred orientation.

Herein we present a systematic study on the effects of different gas molecules on the photoelectric response of few-layer GaSe phototransistors before and after introducing defects. After introducing defects by thermal annealing, the phototransistors become largely photo-responsive (18.75 A W−1) with high external quantum efficiency (EQE) (∼91.53%), high photocurrent on–off ratio, fast photo-response, and good stability in an O2 rich environment when illuminated by 254 nm ultraviolet light.