Intrinsic Defect-Driven Synergistic Synaptic Heterostructures for Gate-Free Neuromorphic Phototransistors.

The optoelectronic synaptic devices based on two-dimensional (2D) materials offer great advances for future neuromorphic visual systems with dramatically improved integration density and power efficiency. The effective charge capture and retention are considered as one vital prerequisite to realizing the synaptic memory function. However, the current 2D synaptic devices are predominantly relied on materials with artificially-engineered defects or intricate gate-controlled architectures to realize the charge trapping process. These approaches, unfortunately, suffer from the degradation of pristine materials, rapid device failure, and unnecessary complication of device structures. To address these challenges, an innovative gate-free heterostructure paradigm is introduced herein. The heterostructure presents a distinctive dome-like morphology wherein a defect-rich Fe7S8 core is enveloped snugly by a curved MoS2 dome shell (Fe7S8@MoS2), allowing the realization of effective photocarrier trapping through the intrinsic defects in the adjacent Fe7S8 core. The resultant neuromorphic devices exhibit remarkable light-tunable synaptic behaviors with memory time up to ≈800 s under single optical pulse, thus demonstrating great advances in simulating visual recognition system with significantly improved image recognition efficiency. The emergence of such heterostructures foreshadows a promising trajectory for underpinning future synaptic devices, catalyzing the realization of high-efficiency and intricate visual processing applications.

Manipulation and Optical Detection of Artificial Topological Phenomena in 2D Van der Waals Fe5 GeTe2 /MnPS3 Heterostructures.

2D ferromagnet is a good platform to investigate topological effects and spintronic devices owing to its rich spin structures and excellent external-field tunability. The appearance of the topological Hall Effect (THE) is often regarded as an important sign of the generation of chiral spin textures, like magnetic vortexes or skyrmions. Here, interface engineering and an in-plane current are used to modulate the magnetic properties of the nearly room-temperature 2D ferromagnet Fe5 GeTe2 . An artificial topology phenomenon is observed in the Fe5 GeTe2 /MnPS3 heterostructure by using both anomalous Hall Effect and reflective magnetic circular dichroism (RMCD) measurements. Through tuning the applied current and the RMCD laser wavelength, the amplitude of the humps and dips observed in the hysteresis loops can be modulated accordingly. Magnetic field-dependent hysteresis loops demonstrate that the observed artificial topological phenomena are induced by the generation and annihilation of the magnetic domains. This work provides an optical method for investigating the topological-like effects in magnetic structures and proposes an effective way to modulate the magnetic properties of magnetic materials, which is important for developing magnetic and spintronic devices in van der Waals magnetic materials.

Manipulating exchange bias in 2D magnetic heterojunction for high-performance robust memory applications.

The exchange bias (EB) effect plays an undisputed role in the development of highly sensitive, robust, and high-density spintronic devices in magnetic data storage. However, the weak EB field, low blocking temperature, as well as the lack of modulation methods, seriously limit the application of EB in van der Waals (vdW) spintronic devices. Here, we utilized pressure engineering to tune the vdW spacing of the two-dimensional (2D) FePSe3/Fe3GeTe2 heterostructures. The EB field (HEB, from 29.2 mT to 111.2 mT) and blocking temperature (Tb, from 20 K to 110 K) are significantly enhanced, and a highly sensitive and robust spin valve is demonstrated. Interestingly, this enhancement of the EB effect was extended to exposed Fe3GeTe2, due to the single-domain nature of Fe3GeTe2. Our findings provide opportunities for the producing, exploring, and tuning of magnetic vdW heterostructures with strong interlayer coupling, thereby enabling customized 2D spintronic devices in the future.

Nonlocal Manipulation of Magnetism in an Itinerant Two-Dimensional Ferromagnet.

Two-dimensional (2D) magnets are crucial in the construction of 2D magnetic and spintronic devices. Many devices, including spin valves and multiple tunneling junctions, have been developed by vertically stacking 2D magnets with other functional blocks. However, owing to limited local interactions at the interfaces, the device structures are typically extremely complex. To solve this problem, the nonlocal manipulation of magnetism may be a good solution. In this study, we use the magneto-optical Kerr effect technique to demonstrate the nonlocal manipulation of magnetism in an itinerant 2D ferromagnet, Fe3GeTe2 (FGT), whose magnetism can be manipulated via an antiferromagnet/ferromagnet interface or a current-induced spin-orbital torque placed distant from the local site. It is discovered that the coupling of a small piece of MnPS3 (∼40 µm2) with FGT can significantly enhance the coercive field and emergence of exchange bias in the entire FGT flake (∼2000 µm2). Moreover, FGT flakes with different thicknesses have the same coercive field at low temperatures if they are coupled together. Our study provides an understanding of the basic magnetism of 2D itinerant ferromagnets as well as opportunities for engineering magnetism with an additional degree of freedom.

Spin mapping of intralayer antiferromagnetism and field-induced spin reorientation in monolayer CrTe2.

Intrinsic antiferromagnetism in van der Waals (vdW) monolayer (ML) crystals enriches our understanding of two-dimensional (2D) magnetic orders and presents several advantages over ferromagnetism in spintronic applications. However, studies of 2D intrinsic antiferromagnetism are sparse, owing to the lack of net magnetisation. Here, by combining spin-polarised scanning tunnelling microscopy and first-principles calculations, we investigate the magnetism of vdW ML CrTe2, which has been successfully grown through molecular-beam epitaxy. We observe a stable antiferromagnetic (AFM) order at the atomic scale in the ML crystal, whose bulk is ferromagnetic, and correlate its imaged zigzag spin texture with the atomic lattice structure. The AFM order exhibits an intriguing noncollinear spin reorientation under magnetic fields, consistent with its calculated moderate magnetic anisotropy. The findings of this study demonstrate the intricacy of 2D vdW magnetic materials and pave the way for their in-depth analysis.

Air-Stable 2D Cr5 Te8 Nanosheets with Thickness-Tunable Ferromagnetism.

2D magnetic materials have aroused widespread research interest owing to their promising application in spintronic devices. However, exploring new kinds of 2D magnetic materials with better stability and realizing their batch synthesis remain challenging. Herein, the synthesis of air-stable 2D Cr5 Te8 ultrathin crystals with tunable thickness via tube-in-tube chemical vapor deposition (CVD) growth technology is reported. The importance of tube-in-tube CVD growth, which can significantly suppress the equilibrium shift to the decomposition direction and facilitate that to the synthesis reaction direction, for the synthesis of high-quality Cr5 Te8 with accurate composition, is highlighted. By precisely adjusting the growth temperature, the thickness of Cr5 Te8 nanosheets is tuned from ≈1.2 nm to tens of nanometers, with the morphology changing from triangles to hexagons. Furthermore, magneto-optical Kerr effect measurements reveal that the Cr5 Te8 nanosheet is ferromagnetic with strong out-of-plane spin polarization. The Curie temperature exhibits a monotonic increase from 100 to 160 K as the Cr5 Te8 thickness increases from 10 to 30 nm and no apparent variation in surface roughness or magnetic properties after months of exposure to air. This study provides a robust method for the controllable synthesis of high-quality 2D ferromagnetic materials, which will facilitate research progress in spintronics.

Strong Second- and Third-Harmonic Generation in 1D Chiral Hybrid Bismuth Halides.

Breaking the symmetry of a crystal structure can enable even-order nonlinear activities, including second-harmonic generation (SHG). The emerging chiral hybrid organic-inorganic metal halides feature unique optical and electronic properties and flexible crystal structures, making them a class of promising nonlinear optical materials. However, their nonlinear response performances are currently inferior to traditional nonlinear crystals, because of the lack of research on resonant enhancement and third-harmonic generation (THG). Herein, we designed chiral hybrid bismuth halides with naturally nonsymmetrical structure to enable SHG. Simultaneously, these chiral compounds preserve 1D crystal structures to create strong free exciton, broad self-trapped exciton (STE), and discrete band energy levels, which facilitate the resonant enhancement of SHG and THG susceptibilities. These new chiral films showcase superior effective SHG susceptibility (χ(2) ∼ 130.5 pm V-1 at an interesting wavelength of 1550 nm), exceeding that of the reference, a commercial LiNbO3 (χ(2) ∼ 83.4 pm V-1) single-crystal film. Furthermore, their THG intensities are even higher than their SHG intensities, with effective THG susceptibility (χ(3)) being ∼9.0 × 106 pm2 V-2 at 1550 nm (37 times that of the reference monolayer WS2). Their high SHG and THG performances indicate the promising future of these 1D chiral hybrid bismuth halides toward nonlinear optical applications.

Room-Temperature Magnetic Field Effect on Excitonic Photoluminescence in Perovskite Nanocrystals.

Magnetic-field-enhanced spin-polarized electronic/optical properties in semiconductors are crucial for fabricating various spintronic devices. However, this spin polarization is governed by weak spin exchange interactions and easily randomized by thermal fluctuations; therefore, it is only produced at cryogenic temperatures, which severely limits the applications. Herein, a room-temperature intrinsic magnetic field effect (MFE) on excitonic photoluminescence is achieved in CsPbX3 :Mn (X = Cl, Br) perovskite nanocrystals. Through moderate Mn doping, the MFE is enhanced by exciton-Mn interactions, and through partial Br substitution, the MFE is stabilized at room temperature by exciton orbital ordering. The orbital ordering significantly enhances the g-factor difference between electrons and holes, which is evidenced by a parallel orbit-orbit interaction among excitons generated by circular polarized laser excitation. This study provides a clear avenue for engineering spintronic materials based on orbital interactions in perovskites.

Enhancement of the Coercive Field and Exchange Bias Effect in Fe3GeTe2/MnPX3 (X = S and Se) van der Waals Heterostructures.

Fe3GeTe2/MnPS3 and Fe3GeTe2/MnPSe3 van der Waals heterostructures were fabricated by mechanical exfoliation. Via the magneto-optical Kerr effect and reflected magnetic circular dichroism measurements, we have observed nearly three times enhancement of the coercive field, improvement of Curie temperature, and exchange bias effect in both heterostructures. These observations may provide new insights into the emergent heterostructure devices between itinerant ferromagnets and metal thio- and selenophosphates for both applied and fundamental research studies in magnetic correlations.

Enhanced Valley Splitting in Monolayer WSe2 by Phase Engineering.

Lifting the valley degeneracy in two-dimensional transition metal dichalcogenides could promote their applications in information processing. Various external regulations, including magnetic substrate, magnetic doping, electric field, and carrier doping, have been implemented to enhance the valley splitting under the magnetic field. Here, a phase engineering strategy, through modifying the intrinsic lattice structure, is proposed to enhance the valley splitting in monolayer WSe2. The valley splitting in hybrid H and T phase WSe2 is tunable by the concentration of the T phase. An obvious valley splitting of ∼4.1 meV is obtained with the T phase concentration of 31% under ±5 T magnetic fields, which corresponds to an effective Landé geff factor of -14, about 3.5-fold of that in pure H-WSe2. Comparing the temperature and magnetic field dependent polarized photoluminescence and also combining the theoretical simulations reveal the enhanced valley splitting is dominantly attributed to exchange interaction of H phase WSe2 with the local magnetic moments induced by the T phase. This finding provides a convenient solution for lifting the valley degeneracy of two-dimensional materials.

Nanoscale Ultrasensitive Temperature Sensing Based on Upconversion Nanoparticles with Lattice Self-Adaptation.

Upconversion nanoparticles have recently received increasing attention due to their outstanding performance in temperature sensing at the nanoscale. Although much effort has been devoted to improve their thermal sensitivity, there is no efficient way for achieving significant enhancement. Here, we show that lattice self-adaptation can unlock a new route for remarkably enhancing the thermal sensitivity of upconversion nanoparticles. The thermally sensitive fluorescence intensity ratio (FIR) of the dopant Er3+ is used for indicating the temperature variation, while a heterojunction of NaGdF4/NaYF4 is prepared as host material to produce a lattice distortion at the interface which is also sensitive to temperature. With the increase of temperature, the FIR of the transitions 2H11/2/4S3/2 → 4I15/2 increases, accompanied by the self-adapted decrease of interface lattice distortion that leads to the additional increase in FIR. Using core/shell upconversion nanoparticles with lattice self-adaptation, we achieve an enhanced thermal sensitivity three times higher than core-only nanoparticles.

Air-Stable 2D Intrinsic Ferromagnetic Ta3FeS6 with Four Months Durability.

2D ferromagnetic materials provide an important platform for the fundamental magnetic research at atomic-layer thickness which has great prospects for next-generation spintronic devices. However, the currently discovered 2D ferromagnetic materials (such as, CrI3, Cr2Ge2Te6, and Fe3GeTe2) suffer from poor air stability, which hinders their practical application. Herein, intrinsic long-range ferromagnetic order in 2D Ta3FeS6 is reported, which exhibits ultrahigh stability under the atmospheric environment. The intrinsic ferromagnetism of few-layer Ta3FeS6 is revealed by polar magneto-optical Kerr effect measurement, which exhibits giant MOKE response and has Curie temperature of ≈80 K. More importantly, few-layer Ta3FeS6 nanosheet exhibits excellent air stability and its ferromagnetism remains unchanged after 4 months of aging under the atmosphere. This work enriches the family of 2D ferromagnetic materials, which will facilitate the research progress of spintronics.

Proximity-Coupling-Induced Significant Enhancement of Coercive Field and Curie Temperature in 2D van der Waals Heterostructures.

Magnetism in 2D has long been the focus of condensed matter physics due to its important applications in spintronic devices. A particularly promising aspect of 2D magnetism is the ability to fabricate 2D heterostructures with engineered optical, electrical, and quantum properties. Recently, the discovery of intrinsic ferromagnetisms in atomic thick materials has provided a new platform for investigations of fundamental magnetic physics. In contrast to 2D CrI3 and Cr2 Ge2 Te6 insulators, itinerant ferromagnetic Fe3 GeTe2 (FGT), which has a larger intrinsic perpendicular anisotropy, higher Curie temperature (TC ), and relatively better stability, is a promising candidate for achieving permanent room-temperature ferromagnetism through interface or component engineering. Here, it is shown that the ferromagnetic properties of FGT thin flakes can be modulated through coupling with a FePS3 . The magneto-optical Kerr effect results show that the TC of FGT is improved by more than 30 K and that the coercive field is increased by ≈100% due to the proximity coupling effect, which changes the spin textures of FGT at the interface. This work reveals that antiferromagnet/ferromagnet coupling is a promising way to engineer the magnetic properties of itinerant 2D ferromagnets, which paves the way for applications in advanced magnetic spintronic and memory devices.

Two-dimensional ferromagnetism in CrTe flakes down to atomically thin layers.

Two-dimensional (2D) ferromagnetism has attracted intense attention as it provides a platform for the investigation of fundamental physics and the emerged devices. Recently, the discovery of intrinsic 2D ferromagnet has enabled researchers to fabricate ultrathin devices, which can be controlled by external fields. Nevertheless, 2D ferromagnetic materials are mostly obtained by mechanical exfoliation methods with uncontrollable size and thickness, which make the device fabrication processes time-consuming and difficult to expand in industries. Therefore, the development of a controllable fabrication process for the synthesis of 2D intrinsic magnetic materials is necessary. In this study, a new 2D ferromagnet, chromium tellurium (CrTe), was successfully synthesized by the chemical vapor deposition (CVD) method, and the magnetism was studied by the magneto-optical Kerr effect (MOKE) technique. The results demonstrated that CrTe flakes exhibit hard magnetism with strong perpendicular anisotropy. As the thickness varies from 45 nm to 11 nm, the hard magnetism sustains quite well, with the Curie temperature TC decreasing from 205 K to 140 K. Our study presents a new ultrathin hard magnetic material, which has the potential to be fabricated and applied in spintronic devices massively.

A stable dysprosium(iii) complex with a terminal fluoride ligand showing high resolution luminescence and slow magnetic relaxation.

An air-stable dysprosium(iii) complex, [C(NH2)3]4[DyF(piv)4](piv)2 (piv = pivalate), with a terminal fluoride ligand protected by forming extensive hydrogen bonds with peripheral guanidiniums has been isolated. Though the magnetic data failed to determine the energy barrier for the magnetic reversal (Ueff) accurately, the fine emission spectrum of the 4F9/2→6H15/2 transition at 4.2 K and ab initio calculations reveal that the Ueff is about 376(20) K, which is among the highest for high-coordinate (coordination numbers ≥ 9) lanthanide mononuclear magnets.

Experimental Determination of Magnetic Anisotropy in Exchange-Bias Dysprosium Metallocene Single-Molecule Magnets.

We investigate a family of dinuclear dysprosium metallocene single-molecule magnets (SMMs) bridged by methyl and halogen groups [Cp'2 Dy(µ-X)]2 (Cp'=cyclopentadienyltrimethylsilane anion; 1: X=CH3 - ; 2: X=Cl- ; 3: X=Br- ; 4: X=I- ). For the first time, the magnetic easy axes of dysprosium metallocene SMMs are experimentally determined, confirming that the orientation of them are perpendicular to the equatorial plane which is made up of dysprosium and bridging atoms. The orientation of the magnetic easy axis for 1 deviates from the normal direction (by 10.3°) due to the stronger equatorial interactions between DyIII and methyl groups. Moreover, its magnetic axes show a temperature-dependent shifting, which is caused by the competition between exchange interactions and Zeeman interactions. Studies of fluorescence and specific heat as well as ab initio calculations reveal the significant influences of the bridging ligands on their low-lying exchange-based energy levels and, consequently, low-temperature magnetic properties.

Halogenated-Methylammonium Based 3D Halide Perovskites.

3D perovskites with typical structure of ABX3 are emerging as key materials to achieve high-performance optoelectronic devices. The variation of A-site cation is promising to achieve enhanced properties; however, is limited to a few available choices of methylamine, formamidine, and cesium. In this work, halogenated-methylammoniums are developed as A cation to broaden the family of hybrid perovskites. Single crystals and colloidal nanocrystals of halogenated-methylammoniums based perovskites are successfully synthesized, showing bright future as alternatives for device exploration. In particular, the improved thermal stability and low exciton binding energy from single crystals measurements are demonstrated and bright tunable emission from blue to green for colloidal nanocrystals is achieved.

Gap-Induced Giant Third-Order Optical Nonlinearity and Long Electron Relaxation Time in Random-Distributed Gold Nanorod Arrays.

The third-order optical nonlinearities and the hot electron relaxation time (τ) of random-distributed gold nanorods arrays on glass (R-GNRA) have been investigated by using Z-scan and optical Kerr effect (OKE) techniques. Large third-order optical susceptibility (χ(3)) with the value of 2.5 × 10-6 esu has been obtained around the plamsonic resonance peak under the excitation power intensity of 0.1 GW/cm2. Further decrease of the excitation power intensity down to 0.3 MW/cm2 will lead to the significant increase of χ(3) up to 6.4 × 10-4 esu. The OKE results show that the relaxation time of R-GNRA around the plasmonic peak is 13.9 ± 0.4 ps, which is more than 4 times longer than those of the individual gold nanostructures distributed in water solutions. The Finite-difference time domain simulations demonstrate that this large enhancement of χ(3) and slow down of τ are caused by the gap-induced large local field enhancement of GNRs dimers in R-GNRA. These significant results offer great opportunities for plasmonic nanostructures in applications of photonic and photocatalytic devices.

Ultralow-Threshold and Color-Tunable Continuous-Wave Lasing at Room-Temperature from In Situ Fabricated Perovskite Quantum Dots.

Room-temperature-operated continuous-wave lasers have been intensively pursed in the field of on-chip photonics. The realization of a continuous-wave laser strongly relies on the development of gain materials. To date, there is still a huge gap between the current gain materials and commercial requirements. In this work, we demonstrate continuous-wave lasers at room temperature using rationally designed in situ fabricated perovskite quantum dots in polyacrylonitrile films on a distributed feedback cavity. The achieved threshold values are 15, 24, and 58 W/cm2 for green, red, and blue lasers, respectively, which are one order lower than the reported values for the conventional CdSe quantum dot-based continuous-wave laser. Except for the high photoluminescence quantum yields, smooth surface, and high thermal conductivity of the resulting films, the key success of an ultralow laser threshold can be explained by the interaction of polyacrylonitrile and perovskite induced "charge spatial separation" effects. This progress opens up a door to achieve on-chip continuous-wave lasers for photonic applications.

Optical spectroscopy study of Ca3(Ru0.91Mn0.09)2O7 single crystal in high magnetic fields.

The magneto-optical spectrum, with magnetic fields up to 42 T, of double layered ruthenates Ca3(Ru0.91Mn0.09)2O7 (CRMO) single crystal is studied. Both the temperature and magnetic field induced insulator-to-metal transitions (IMTs) are observed via magneto-optical properties of the crystal. The critical magnetic field (H // c) of IMT for CRMO is found to be as large as 35 T at 5 K. The fine structure of optical spectra identified the antiferromagnetic/ferro-orbital-ordering configurations of Ru 4d orbitals at low temperatures. Meanwhile, the configuration of orbital polarization of such double-layer CRMO single crystal is discussed. These results suggest that the orbital degree of freedom plays an important role in the field induced IMT of multi-orbital system.