The electronic properties of monolayer MoTe2 on top of EuO(111) are studied by first‐principles calculations. Strong spin polarization is induced in MoTe2, which results in a large valley ...polarization. In a longitudinal electric field this will result in a valley and spin‐polarized charge Hall effect. The direction of the Hall current as well as the valley and spin polarizations can be tuned by an external magnetic field.
"Two-dimensional (2D) materials as electrodes" is believed to be the trend for future Li-ion and Na-ion battery technologies. Here, by using first-principles methods, we predict that the recently ...reported borophene (2D boron sheets) can serve as an ideal electrode material with high electrochemical performance for both Li-ion and Na-ion batteries. The calculations are performed on two experimentally stable borophene structures, namely β12 and χ3 structures. The optimized Li and Na adsorption sites are identified, and the host materials are found to maintain good electric conductivity before and after adsorption. Besides advantages including small diffusion barriers and low average open-circuit voltages, most remarkably, the storage capacity can be as high as 1984 mA h g(-1) in β12 borophene and 1240 mA h g(-1) in χ3 borophene for both Li and Na, which are several times higher than the commercial graphite electrode and are the highest among all the 2D materials discovered to date. Our results highly support that borophenes can be appealing anode materials for both Li-ion and Na-ion batteries with extremely high power density.
Enriching the functionality of ferroelectric materials with visible-light sensitivity and multiaxial switching capability would open up new opportunities for their applications in advanced ...information storage with diverse signal manipulation functions. We report experimental observations of robust intralayer ferroelectricity in two-dimensional (2D) van der Waals layered α-In2Se3 ultrathin flakes at room temperature. Distinct from other 2D and conventional ferroelectrics, In2Se3 exhibits intrinsically intercorrelated out-of-plane and in-plane polarization, where the reversal of the out-of-plane polarization by a vertical electric field also induces the rotation of the in-plane polarization. On the basis of the in-plane switchable diode effect and the narrow bandgap (∼1.3 eV) of ferroelectric In2Se3, a prototypical nonvolatile memory device, which can be manipulated both by electric field and visible light illumination, is demonstrated for advancing data storage technologies.
Manganese phosphorus trichalcogenides are widely used in the field of photocatalysis and magnetic studies due to their broadband gaps. Herein, an alloy engineering method for the few‐layer manganese ...phosphorus trichalcogenides (MnPS3–xSex, 0 ≤ x ≤ 3) in surface‐enhanced Raman scattering (SERS) is reported. A new strategy, with the coupling of exciton resonance (µex) and photoinduced charge transfer (PICT), is applied to screen out materials for SERS enhancement. According to the calculation of density functional theory, the bandgap of manganese phosphorus trichalcogenides (MnPS3) can be adjusted to match the band energy of Rhodamine 6G molecules by alloy engineering. Furthermore, a series of few‐layer MnPS3–xSex (0 ≤ x ≤ 3) are fabricated to study the PICT‐induced SERS behavior under resonance excitation. The good performance with a detection limit down to 10−9 m indicates that the synergistic resonances between µex and PICT are crucial to the enhancement.
Alloy engineering fabricating few‐layer MnPS3, MnPS2.4Se0.6, MnPS1.8Se1.2, MnPS1.2Se1.8, MnPS0.6Se2.4 and MnPSe3 for SERS measurement is predicted by a µex process. All samples could be used to probe the signal of R6G at a detection limit of 10−9 m on few‐layer MnPS2.4Se0.6. More SERS‐active materials are expected to be discovered by the µex process.
Alteration of the topological order by band inversion is a key ingredient of a topologically nontrivial material. Using first-principles calculations for HgTe, PtScBi, and B sub(2) Se sub(3), we ...argue that it is not accurate to ascribe the band inversion to the spin-orbit coupling. Instead, scalar relativistic effects and/or lattice distortions are found to be essential. Therefore, the search for topologically nontrivial materials should focus on band shifts due to these mechanisms rather than spin-orbit coupling. We propose an effective scheme to search for new topological insulators.
Exploiting the valley degree of freedom to store and manipulate information provides a novel paradigm for future electronics. A monolayer transition-metal dichalcogenide (TMDC) with a broken ...inversion symmetry possesses two degenerate yet inequivalent valleys, which offers unique opportunities for valley control through the helicity of light. Lifting the valley degeneracy by Zeeman splitting has been demonstrated recently, which may enable valley control by a magnetic field. However, the realized valley splitting is modest (∼0.2 meV T-1 ). Here we show greatly enhanced valley spitting in monolayer WSe2 , utilizing the interfacial magnetic exchange field (MEF) from a ferromagnetic EuS substrate. A valley splitting of 2.5 meV is demonstrated at 1 T by magnetoreflectance measurements and corresponds to an effective exchange field of ∼12 T. Moreover, the splitting follows the magnetization of EuS, a hallmark of the MEF. Utilizing the MEF of a magnetic insulator can induce magnetic order and valley and spin polarization in TMDCs, which may enable valleytronic and quantum-computing applications.
Functionalized graphene has been extensively studied with the aim of tailoring properties for gas sensors, superconductors, supercapacitors, nanoelectronics, and spintronics. A bottleneck is the ...capability to control the carrier type and density by doping. We demonstrate that a two-step process is an efficient way to dope graphene: create vacancies by high-energy atom/ion bombardment and fill these vacancies with desired dopants. Different elements (Pt, Co, and In) have been successfully doped in the single-atom form. The high binding energy of the metal-vacancy complex ensures its stability and is consistent with in situ observation by an aberration-corrected and monochromated transmission electron microscope.
2D organic–inorganic hybrid Ruddlesden–Popper perovskites (RPPs) have recently attracted increasing attention due to their excellent environmental stability, high degree of electronic tunability, and ...natural multiquantum‐well structures. Although there is a rapid development of photoelectronic applications in solar cells, photodetectors, light emitting diodes (LEDs), and lasers based on 2D RPPs, the state‐of‐the‐art performance is far inferior to that of the existing devices because of the limited understanding on fundamental physics, especially special photophysics in carrier dynamics, excitonic fine structures, excitonic quasiparticles, and spin‐related effect. Thus, there is still plenty of room to improve the performances of photoelectronic devices based on 2D RPPs by enhancing knowledge on fundamental photophysics. This review highlights the special photophysics of 2D RPPs that is fundamentally different from the conventional 3D congeners. It also provides the most recent progress, debates, challenges, prospects, and in‐depth understanding of photophysics in 2D perovskites, which is significant for not only boosting performance of solar cells, LEDs, photodetectors, but also future development of applications in lasers, spintronics, quantum information, and integrated photonic chips.
This review highlights the special photophysics of 2D perovskites that is fundamentally different from the conventional 3D congeners, which is significant for not only boosting performance of solar cells, light emitting diodes, photodetectors, but also future development of applications in lasers, spintronics, quantum information, and integrated photonic chips.
Phase transitions and phase engineering in two-dimensional MoS2 are important for applications in electronics and energy storage. By in situ transmission electron microscopy, we find that H-MoS2 ...transforms to T-LiMoS2 at the early stages of lithiation followed by the formation of Mo and Li2S phases. The transition from H-MoS2 to T-LiMoS2 is explained in terms of electron doping and electron–phonon coupling at the conduction band minima. Both are essential for the development of two-dimensional semiconductor-metal contacts based on MoS2 and the usage of MoS2 as anode material in Li ion batteries.
Organic–inorganic halide perovskites have attracted intensive attention due to their outstanding photophysical properties and high performance in optoelectronic devices. Moreover, the quality of ...perovskite films plays an important role in the performance of perovskite devices. The use of additives has been previously suggested as the simplest and effective method to improve the quality of perovskite films. However, only the cations or anions in most additives are capable of functioning as active species in improving the quality of perovskite films. Herein, we report that ammonium thiocyanate (NH 4 SCN) as an additive (contains NH 4 + and SCN − ions) can significantly improve the quality of perovskite films as compared to NH 4 I and Pb(SCN) 2 as additives containing the single active species of NH 4 + and SCN − , respectively. The synergistic effect between the ions in NH 4 SCN is favorable for controlling the nucleation and growth speed of the crystal grains, which evidently increases the grain size and markedly improves the film stability. Finally, the efficiency of the device based on NH 4 SCN (16.47%) is significantly enhanced without a hysteresis effect as compared to that of the devices with single ions as the active species, e.g. , NH 4 + in NH 4 I (13.97%) and SCN − in Pb(SCN) 2 (12.88%). Our findings suggest that the synergistic effect of anions and cations in additives is an effective strategy for realizing efficient and stable perovskites.
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