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Transition-metal dichalcogenides with intrinsic spin-valley degree of freedom have enabled great potentials for valleytronic and optoelectronic applications. However, the degree of valley polarization is usually low under nonresonant excitation at room temperature due to the phonon-assisted intervalley scattering. Here, achiral and chiral Au arrays are designed to enhance the optical response and valley polarization in monolayer and bilayer WS2. A considerable band edge emission with 7 times increment is realized under the resonant coupling with Au dimer-prism arrays. Valley polarization enhancement is quantitatively predicted by the inherent mechanisms from elevated electromagnetic field intensity and radiation efficiency and further realized in polarized photoluminescence. A tunable valley polarization up to 30.0% is achieved in bilayer WS2 under a nonresonant excitation at room temperature. All of these results provide a promising route toward the development of room-temperature valley-dependent optoelectronic devices.

Memristors with non-volatile storage performance and simulated synaptic functions are regarded as one of the critical devices to overcome the bottleneck in traditional von Neumann computer architecture. van der Waals heterostructures integrating excellent properties of two-dimensional semiconductor materials, possessing controllable optoelectronic properties and high compatibility with conventional microelectronic technology, have paved a new way for the development of advanced memristors. Herein, we demonstrate a two-dimensional planar memristor with both electrical and optical controllability based on ReS2/WS2 van der Waals heterostructure. The device shows a typical unipolar non-volatile behavior with a high Ron/Roff ratio, multiple tunable resistance states, and desirable endurance and retention. It also successfully realizes biological synaptic functions and plasticity, including spike rate-dependent plasticity and paired-pulse facilitation. Furthermore, the developed device shows a significant gate controllability and a remarkable optical tunability. The superior performance is attributed to the unique optoelectronic property and the interlayer interaction in the heterostructure. The research presented here demonstrates the potential of two-dimensional van der Waals heterostructures for high-performance memristor applications and further developments in modelling biological synapses.

Monolayer two-dimensional transition-metal dichalcogenides, such as tungsten disulfide (WS2), are regarded as promising candidates for optoelectronic and electronic applications. Although theoretic...

Memristors are excellent candidates for non-volatile memory and neuromorphic computing. Controlling the resistive switching polarity in certain materials holds great promise for the design and integration of multifunctional memristors. Here, bipolar and unipolar nonvolatility were realized by modulating the crystal structures, which exhibit high switching ratios, desirable cyclability, and long retention. By investigating the defect level and conductivity difference in single-crystal and polycrystal ReS2, the competition between electric field and Joule heat effects were revealed as the essential mechanism governing the switching process and memristive polarities. These results promote the design of two-dimensional memristors with controllable polarity.

Transition-metal dichalcogenides with intrinsic spin-valley degree of freedom have enabled great potentials for valleytronic and optoelectronic applications. However, the degree of valley polarization is usually low under nonresonant excitation at room temperature due to the phonon-assisted intervalley scattering. Here, achiral and chiral Au arrays are designed to enhance the optical response and valley polarization in monolayer and bilayer WS

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 WSe

Two-dimensional Janus materials have great potential for the applications in spintronic devices due to their particular structures and novel characteristics. However, they are usually non-magnetic in nature. Here, different transition metals (TMs: Co, Fe, Mn, Cr, and V) adsorbed WSSe frameworks are constructed, and their structures and magnetic properties are comprehensively investigated by first-principles calculations. The results show that the top of W atom is the most stable absorption site for all the TM atoms, and all the systems exhibit magnetism. Moreover, their magnetic properties significantly depend on the adsorbed elements and the adsorbent chalcogens. A maximal total magnetic moment of 6 μB is obtained in the Cr-adsorbed system. The induced magnetism from S-surface-adsorption is always stronger than that for the Se-surface-adsorption due to its larger electrostatic potential. Interestingly, the easy magnetization axis in the Fe-adsorbed system switches from the in-plane to the out-of-plane when the adsorption surface changes from Se to S surface. The mechanism is analyzed in detail by Fe-3d orbital-decomposed density of states. This work provides a guidance for the modification of magnetism in low-dimensional systems.

Inspired by the profound physical connotations and potential applications of the spintronics and valleytronics, two-dimensional (2D) monolayer manganese chalcogenophosphates alloys are constructed, and the strain modulated spin-valley characteristics are investigated through the first principles calculations. For both the MnFePS3and MnFePSe3, the conductivity can be tuned reversibly between semiconductive and half-metallic, while and magnetic stability is controllable between ferromagnetism and antiferromagnetism. Large valley splitting of up to 1000 meV is achieved in MnFePS3under a -4% strain. Simultaneous spin splitting of 219 meV and valley splitting of 160 meV are acquired in MnFePS3under a 4% strain. Strain tunable magnetic moment and interaction between Mn, Fe and S/Se atoms are revealed as the internal mechanisms of controlling the magnetic stability, spin and valley polarizations in the two structures. All the findings in this work provide a strategy for the manipulation of spin and valley degrees of freedom in 2D magnetic materials.

Hydrothermally stable ZnAl2O4 nanocrystals with controlled surfaces including Zn-rich and Al-rich structures are synthesized by a facile method. The Zn-rich surfaces are utilized to design long-lasting PdZn/ZnAl2O4 catalysts evaluated in the aqueous phase hydrogenolysis of glycerol for 10 consecutive cycles showing high activity and selectivity.

Monolayer two-dimensional transition-metal dichalcogenides, such as tungsten disulfide (WS

Group-IIIA metal-monochalcogenides with the enticing properties have attracted tremendous attention across various scientific disciplines. With the aim to satisfy the multiple demands of device applications, here we report a design framework on GaSe monolayer in an effort to tune the electronic and magnetic properties through a dual modulation of vacancy doping and electric field. A half-metallicity with a 100% spin polarization is generated in a Ga vacancy doped GaSe monolayer due to the nonbonding 4p electronic orbital of the surrounding Se atoms. The stability of magnetic moment is found to be determined by the direction of applied electric field. A switchable magnetic configuration in Ga vacancy doped GaSe monolayer is achieved under a critical electric field of 0.6 V/A. Electric field induces redistribution of the electronic states. Finally, charge transfers are found to be responsible for the controllable magnetic structure in this system. The magnetic modulation on GaSe monolayer in this work offers some references for the design and fabrication of tunable two-dimensional spintronic device.

The design and manipulation of magnetism in low-dimensional systems are desirable for the development of spin electronic devices. Here, we design two kinds of Co-adsorbed monolayer WS

We report tunable optical performances of gallium selenide (GaSe) layers in phonon vibrations, band edge emission, circular polarization, and anisotropic response via strain manipulation. By applying a uniaxial tensile strain, frequency shift and peak broadening are observed in Raman spectra. A shrink in bandgap is demonstrated in photoluminescence (PL) spectra and confirmed by first-principles calculations. A continuously growing circular polarization from 3.8% to 37.9% is detected at room temperature when the tensile strain is increased from 0% to 0.35%, which is almost a ten-fold enhancement compared with that under the non-resonant excitation. Through the theoretical calculations, the decrease in exciton lifetime is revealed to be responsible for the overwhelming enhanced circular polarization. By deforing the lattices of GaSe layers, the Raman intensity was found to be suppressed in the strain direction. The intrinsic fourfold-symmetry of the E2g1 mode in angle-dependent Raman spectra is tuned to a two-fold symmetry. An anisotropic PL response is further regulated by changing the structural symmetry of GaSe lattices. A maximal polarization of 66.0% is achieved when the detection polarizations are perpendicular to the strain direction. All the findings in this study suggest a route for tuning the optical properties, particularly the polarized response in two-dimensional (2D) materials, and provide a strategy for developing flexible and anisotropic 2D optical devices.

Identically sized $\mathrm{Co}$ quantum dots (QDs) are constructed on monolayer tungsten disulfide (${\mathrm{WS}}_{2}$), forming coupled heterostructures. Topographical images investigated by means of in situ scanning tunneling microscopy (STM) show a bias-dependent feature. First-principles-calculated binding energies combined with the simulated STM images identify that the $\mathrm{Co}$ QDs possess a unique magic number, with a tetrahedral ${\mathrm{Co}}_{4}$ configuration. Numerical differential conductance measured by means of scanning tunneling spectroscopy indicates a p-type doping and an Ohmic contact property for the ${\mathrm{Co}}_{4}/{\mathrm{WS}}_{2}$ system. The mechanism of the transport and conduction properties of the coupled heterostructures is further revealed by analyzing the work functions and interfacial interaction. Our findings offer some references for the controlled fabrication of identically sized zero-dimensional and two-dimensional heterostructures, and we propose a feasible strategy for Ohmic interface contact in nanodevices.

Inspired by the profound physical connotations and potential application prospects of the valleytronics, we design a two-dimensional (2D) $\mathrm{W}{\mathrm{S}}_{2}/h\ensuremath{-}\mathrm{VN}$ magnetic van der Waals (vdW) heterostructure and study the control of valley degrees of freedom through the first-principles calculations. A considerable spin splitting of 627 meV is obtained at the $K$ valley, accompanied with a strong suppression of that at the ${K}^{\ensuremath{'}}$ valley. An intrinsic large valley splitting of 376 meV is generated in the valence band, which corresponds to an effective Zeeman magnetic field of 2703 T. Besides the valence band, the conduction band of $\mathrm{W}{\mathrm{S}}_{2}$ possesses a remarkable spin splitting also, leading to the possible formation of valley-labeled dark exciton states at the ${K}^{\ensuremath{'}}$ valley. The strengths of spin and valley splitting relying on the interfacial orbital hybridization are further tuned continually by the in-plane strain and interlayer spacing. Maximum spin and valley splitting of 654 and 412 meV are finally achieved, respectively, and the effective Zeeman magnetic field can be enhanced to 2989 T with a \ensuremath{-}3% strain. Time-reversal symmetry breaking and the sizable Berry curvature in the heterostructure lead to a prominent anomalous Hall conductivity at the $K$ and ${K}^{\ensuremath{'}}$ valleys. Based on these finding, a prototype filter device for both the valley and spin is proposed.

Group-IIIA metal-monochalcogenides have attracted increasing interests due to their optoelectronic and spin electronic properties. Achieving the modification of their magnetism is desirable for device applications. In our work, structural, electronic, and magnetic properties of the Fe adsorbed GaSe monolayer are studied systematically using density functional theory (DFT). It is found that there is a strong orbit coupling effect between Fe and the vicinal Ga and Se atoms, resulting in a half-metallicity with a 100% spin polarization. The magnetic moments of the nearest neighbour Ga and Se atoms are −0.004 μB and −0.178 μB, respectively, demonstrating an antiferromagnetic coupling with the adsorbed Fe atom. Meanwhile, the induced majority and minority spin states are inclined to distribute on the Se and Ga atoms separately. The hybridization of Fe-3d, Se-4p, and Ga-4p orbitals is responsible for the induced magnetic moments and spin polarization in the nonmagnetic GaSe monolayer. These investigations shed light on understanding the electronic and magnetic properties of the ferromagnetic/GaSe adsorption system, which offer some references for the design and construction of two-dimensional spin nanostructures.

The design and manipulation of magnetism in low-dimensional systems are desirable for the development of spin electronic devices. Here, we design two kinds of Co-adsorbed monolayer WS2 frameworks, i.e. Co1/WS2 and Co2/WS2, and comprehensively explore the dependences of their magnetic properties on injected charge by using first-principles calculations. The value of magnetic moment can be tuned almost linearly through injecting charge due to the modulated interaction and charge transferring between Co atom and monolayer WS2. A transition from ferromagnetism to non-ferromagnetism occurs in Co1/WS2 system when 1 e/unit cell charge is injected. Furthermore, the magnetic anisotropy can be manipulated by injecting charge as well. The magnetic anisotropy energy (MAE) in Co1/WS2 system sharply increases from -4.16 to 2.47 (0.99) meV when injected charge vary from 0.0 to 0.2 (-0.2) e/unit cell, meaning a transition of the magnetic easy axis from in-plane to out-of-plane direction. Similarly, in Co2/WS2 system, the magnetic easy axis also can be modified to out-of-plane direction through injecting 0.1 e/unit cell charge. It is found that the changes of Co-3d states are responsible for the tunable magnetic anisotropy. This work provides a theoretical understanding on effective manipulation of magnetism in low-dimensional system.

Fully electrical control of devices is keenly pursued for applications in optoelectronics and spintronics. Inspired by the continual discovery of interesting features in two-dimensional semiconductors, the authors investigate the electronic structure and properties of monolayer GaSe in a transverse electric field. Density functional theory predicts a giant Stark effect, switchable optical anisotropy, and controllable spin-orbit coupling and spin polarization in this group-IIIA monochalcogenide. This study provides welcome guidelines for exploiting GaSe in various realms of device physics.

Electrically controllable magnetic properties in 2D materials are promising for the development of the next-generation of magnetic and spintronic applications. In this paper, a theoretical design of magnetic property modulation in an Fe-doped GaSe monolayer is provided based on systematical density functional theory calculations. A ferromagnetic coupling between Fe and the vicinal Ga atom in the Fe-doped GaSe monolayer is found to be regulated to an antiferromagnetic coupling at a critical electric field of 0.2 V ??1, accompanied with the change of the magnetic moment and the spin polarization. By calculating the magnetization energy, total energy, charge redistribution, and orbital-decomposed densities of states, the significant effect of the electric field on the magnetic configuration and magnetic anisotropy in the Fe-doped GaSe monolayer is analyzed, and the related physics mechanism is revealed. The electric-field tunable magnetic properties offer a promising application in magnetic and spintronic devices.

In this work, the electronic and spintronic properties of GaSe/HfSe2 heterostructure under different strains are investigated through first-principles calculations. The results indicate that GaSe/HfSe2 heterostructure has an intrinsic type I band alignment, and the band structure is sensitive to the strain. A transition from type-I to type-II band alignment is found under a tensile stress. The evolution of the band structures is analyzed by the decomposed-projected band structures. Moreover, switchable spin textures of GaSe/HfSe2 heterostructure with different strains are also predicted. The controllable electronic spintronic properties of GaSe/HfSe2 heterostructure hold a great promise in applications of nanoelectronics and spintronics.

Development of two-dimensional (2D) nanoelectronics is appealing for electrically tunable properties in 2D materials. By means of density functional theory computations, we systematically study the modulation of vertical electric field on the electronic structure and spintronic properties of monolayer GaGeTe. A transition from indirect to direct bandgap is realized with an electric field of 0.11 V A−1. The direct bandgap monotonously diminishes due to the giant Stark effect with a Stark coefficient of 3.68 A. A critical electric field of 0.43 V A−1 is predicted, at which the semiconductor-to-metal transition occurs, and the effective mass of holes is correspondingly tuned to as low as 0.16 m 0. Electric-field-induced SOC plays an important role in the spintronic properties of monolayer GaGeTe so that a widely tunable SOC splitting from 0 to 78.3 meV is achieved, and switchable spin-polarization features are also predicted in the spin texture. The spin splitting is attributed to the Rashba effect with the Rashaba contribution coefficients tuned from 0 eVA to 0.15 eVA. As the all-electrical control method is demonstrated, the physical mechanism related to the redistribution of the electronic states is further revealed. The outstanding electronic and transport features as well as the widely controllable spintronic properties of monolayer GaGeTe hold a great promise for their applications in the field of 2D nanoelectronics and spintronics.

Perpendicular magnetic anisotropy is significantly important for realizing a long-term retention of information for spintronics devices. Inspired by 2D graphene with its high charge carrier mobility and long spin diffusion length, we report a first-principles design framework on perpendicular magnetic anisotropy engineering of a Fe atom adsorbed graphene by employing a O-terminated MgO (1 1 1) substrate. Determined by the adsorption sites of the Fe atom, a tunable magnetic anisotropy is realized in Fe/graphene/MgO (1 1 1) structure, with the magnetic anisotropy energy of −0.48 meV and 0.23 meV, respectively, corresponding to the in-plane and out of plane easy magnetizations. Total density of states suggest a half-metallicity with a 100% spin polarization in the system. Decomposed densities of Fe-3d states reveal the orbital contributions to the magnetic anisotropy for different Fe adsorption sites. Bonding interaction and charge redistribution regulated by MgO substrate are found responsible for the novel perpendicular magnetic anisotropy engineering in the system. The effective manipulation of perpendicular magnetic anisotropy in present work offers some references for the design and construction of 2D spintronics devices.