Modifying electronic and optical properties of violet phosphorus through variable fluorine coverage.

We present a comprehensive investigation of the electronic properties of fluorinated monolayer violet phosphorus using first-principles calculations. Our results reveal a strong dependence of the electronic properties on the different fluorine coverages of fluorination. As the fluorine coverage increases, monolayer violet phosphorus undergoes a significant transition from a wide direct bandgap semiconductor to a narrow indirect bandgap semiconductor. Moreover, both semi-fluorinated and fully fluorinated monolayer violet phosphorus exhibit advantageous semiconducting characteristics, with a tunable bandgap of 0.50 ~ 1.04 eV under biaxial strain ranging from -6% to 6%. Notably, the fully fluorinated monolayer violet phosphorus demonstrates a higher coefficient of light absorption within the visible range. Therefore, our findings highlight the tunability of monolayer violet phosphorus properties through the absorption of various fluorine coverages, providing valuable insights for the design and development of novel semiconductor devices based on this material.

Improved resistive switching performance and realized electric control of exchange bias in a NiO/HfO2 bilayer structure.

The fluctuation of switching parameters is unavoidable in conductive filaments (CFs)-type resistive switching (RS) devices, which restricts the application in resistive random-access memory. Here, we employed an uninsulated antiferromagnetic (AFM) NiO layer adhered to a well-insulating HfO2 layer to effectively suppress the RS fluctuation by achieving forming-free, narrower set voltage distribution, a more stable on/off ratio, and better endurance in comparison with single-HfO2-layer based RS devices. The conduction scaling behavior indicates that the NiO/HfO2 bilayer has a smaller scale parameter S0 (lateral dimension of the bottleneck for the CFs). Besides this, considering some preexisting conductive paths in the NiO layer, the electric fields and the formation/rupture of CFs can be highly localized, leading to reduced switching fluctuation and improved RS performance in the NiO/HfO2-based RS devices. Moreover, asymmetric I-V curves measured in a high resistance state (HRS) in positively and negatively biased regions and the electric modulation of exchange bias (EB) arising from the Co-NiO interfacial coupling are favorable for revealing the inherent mechanism for RS. The coexistence of RS and EB is also useful to the design of novel multifunctional memory devices.

Exchange Bias Modulated by Antiferromagnetic Spin-Flop Transition in 2D Van der Waals Heterostructures.

Exchange bias is extensively studied and widely utilized in spintronic devices, such as spin valves and magnetic tunnel junctions. 2D van der Waals (vdW) magnets, with high-quality interfaces in heterostructures, provide an excellent platform for investigating the exchange bias effect. To date, intrinsic modulation of exchange bias, for instance, via precise manipulation of the magnetic phases of the antiferromagnetic layer, is yet to be fully reached, owing partly to the large exchange fields of traditional bulk antiferromagnets. Herein, motivated by the low-field spin-flop transition of a 2D antiferromagnet, CrPS4, exchange bias is explored by modulating the antiferromagnetic spin-flop phase transition in all-vdW magnetic heterostructures. The results demonstrate that undergoing the spin-flop transition during the field cooling process, the A-type antiferromagnetic ground state of CrPS4 turns into a canted antiferromagnetic one, therefore, it reduces the interfacial magnetic coupling and suppresses the exchange bias. Via conducting different cooling fields, one can select the exchange bias effect switching among the "ON", "depressed", and "OFF" states determined by the spin flop of CrPS4. This work provides an approach to intrinsically modulate the exchange bias in all-vdW heterostructures and paves new avenues to design and manipulate 2D spintronic devices.

Facile Synthesis of Two-Dimensional Natural Vermiculite Films for High-Performance Solid-State Electrolytes.

Ceramic electrolytes hold application prospects in all-solid-state lithium batteries (ASSLB). However, the ionic conductivity of ceramic electrolytes is limited by their large thickness and intrinsic resistance. To cope with this challenge, a two-dimensional (2D) vermiculite film has been successfully prepared by self-assembling expanded vermiculite nanosheets. The raw vermiculite mineral is first exfoliated to thin sheets of several atomic layers with about 1.2 nm interlayer channels by a thermal expansion and ionic exchanging treatment. Then, through vacuum filtration, the ion-exchanged expanded vermiculite (IEVMT) sheets can be assembled into thin films with a controllable thickness. Benefiting from the thin thickness and naturally lamellar framework, the as-prepared IEVMT thin film exhibits excellent ionic conductivity of 0.310 S·cm-1 at 600 °C with low excitation energy. In addition, the IEVMT thin film demonstrates good mechanical and thermal stability with a low coefficient of friction of 0.51 and a low thermal conductivity of 3.9 × 10-3 W·m-1·K-1. This reveals that reducing the thickness and utilizing the framework is effective in increasing the ionic conductivity and provides a promising stable and low-cost candidate for high-performance solid electrolytes.

Metal-Free Carbon Nitride Nanosheet Supported the Pentacoordinated Silicon Intermediates for Photocatalytic Overall Water Splitting.

Photocatalytic overall water splitting is a promising approach to overcome the environmental and energy crisis. However, developing effective photocatalysts with well activity, light absorption, and photogenerated carrier lifetime is still a challenge. Herein, combining extensive first-principles and nonadiabatic molecular dynamics calculations, we find that microporous carbon-nitride nanosheets with a pyridinic nitrogen, such as C2N and C6N6, possess the pentacoordinated silicon intermediates' bonding environment. The pentacoordinated silicon as intermediates exhibits good photocatalytic performance for the difficult four-electronic oxygen evolution reaction. The overpotential is only 0.55 V for C2N, which is significantly lower than that of the tetracoordinated silicon intermediates (2.00 V). Simultaneously, the decoration of the silicon group not only widens the absorption range of visible light but also maintains the lifetime of photogenerated carriers on the nanosecond scale, which enhances the application efficiency of solar energy. Our work paves a new route for advancing photocatalytic overall water splitting.

Field-Free Spin-Orbit Torque-Induced Magnetization Switching in the IrMn/CoTb Bilayers with a Spontaneous In-Plane Exchange Bias.

Spin-orbit torque (SOT)-induced magnetization switching in ferrimagnetic materials is promising for application in a new generation of information storage devices. Here, we demonstrate SOT-induced field-free magnetization switching of the perpendicularly magnetized CoTb ferrimagnet layer in the IrMn/CoTb bilayer, in which the in-plane magnetic inversion symmetry is broken by a spontaneous in-plane exchange bias (IEB) established by isothermal crystallization of the IrMn layer. We obtain a significant SOT effective field acting on the CoTb layer and a large effective spin Hall angle in this system, derived by the second harmonic voltage measurement method. Moreover, the IrMn/CoTb bilayer demonstrates multistate magnetic switching behavior with different amplitudes of current pulses at zero field, which can mimic the synaptic weight updates in the neuromorphic network. These findings make the IrMn/CoTb bilayer with spontaneous IEB a promising candidate for potential applications in multilevel storage and neuromorphic computing.

Electric Control of Exchange Bias at Room Temperature by Resistive Switching via Electrochemical Metallization.

Electric field control of exchange bias (EB) plays an important role in spintronics due to its attractive merit of lower energy consumption. Here, we propose a novel method for electrically tunable EB at room temperature in a device with the stack of Si/SiO2/Ta/Pt/Ag/Mn-doped ZnO (MZO)/Pt/FeMn/Co/ITO by resistive switching (RS) via electrochemical metallization (ECM). The device shows enhanced and weakened EB when set at high-resistance state (HRS) and low-resistance state (LRS), respectively. For the device at LRS, the aberration-corrected scanning transmission electron microscopy (STEM) characterizations unambiguously reveal that the Ag filaments grow initially from the Ag anode and then elongate toward the ITO cathode. It is inferred that at LRS, a small portion of Ag filaments have passed through the MZO and the intervening thin Pt layer and extended into the FeMn layer. After applying reverse voltage, these Ag filaments are electrochemically dissolved and ruptured near the MZO/Pt interface. This is considered to be the main mechanism responsible for RS and switchable EB as well. This work presents a new strategy for designing low-power, nonvolatile magnetoelectric random access memory devices.

Cooling Field Dependence of Exchange Bias in Mn-Doped Metal- Organic Framework [NH2(CH3)2][FeIIIFeII(HCOO)6].

Herein, an intriguing exchange bias (EB) effect manifesting itself from positive to negative with an increase in the cooling field (HFC) is reported in the single crystal of Mn-doped metal-organic framework (MOF) [NH2(CH3)2][FeIIIFeII(HCOO)6] (1) by finely tuning the exchange interactions between the magnetic ions. Note that the doping ratio of Mn relative to the total metal ions is about 15%. Negative magnetization and EB below the compensation temperature were both observed in 1, and the EB field (HE) changes its sign from positive to negative when HFC is larger than ∼10 kOe. The abnormal HFC dependence of EB can be interpreted explicitly by a combination of negative magnetization and couplings among the ions of Fe3+, Fe2+, and Mn2+ with varying the HFC. This work demonstrates a tunable EB in MOFs, in favor of designing novel magnetic devices.

Full Electric Control of Exchange Bias at Room Temperature by Resistive Switching.

Electric control of exchange bias (EB) is of vital importance in energy-efficient spintronics. Although many attempts have been made during the past decade, each has its own limitations for operation and thus falls short of full direct and reversible electrical control of EB at room temperature. Here, a novel approach is proposed by virtue of unipolar resistive switching to accomplish this task in a Si/SiO2 /Pt/Co/NiO/Pt device. By applying certain voltages, the device displays obvious EB in the high-resistance-state while negligible EB in the low-resistance state. Conductive filaments forming in the NiO layer and rupturing near the Co-NiO interface are considered to play dominant roles in determining the combined resistive switching and EB phenomena. This work paves a new way for designing multifunctional and nonvolatile magnetoelectric random access memory devices.

Correction: Enhanced resistive switching effect upon illumination in self-assembled NiWO4 nano-nests.

Correction for 'Enhanced resistive switching effect upon illumination in self-assembled NiWO4 nano-nests' by Bai Sun et al., Chem. Commun., 2014, 50, 13142-13145.

Enhanced resistive switching effect upon illumination in self-assembled NiWO4 nano-nests.

The resistive switching effect of devices with metal-oxide-metal structure is a fascinating candidate for next generation nonvolatile memory devices. Here, self-assembled NiWO4 nano-nests on a Ti substrate were synthesized by a hydrothermal process. Moreover, a resistive switching memory device with Ag/NiWO4/Ti structure is demonstrated. The device shows an enhanced bipolar resistive switching effect under white-light illumination. This study is useful for exploring multifunctional materials and their applications in light-controlled nonvolatile memory devices.