Two-dimensional materials are of current great interest for their promising applications to postsilicon microelectronics. Here we study, using first-principles calculations and a Monte Carlo simulation, the electronic structure and magnetism of CrI3 monolayer, whose bulk material is an interesting layered ferromagnetic (FM) semiconductor. Our results show that CrI3 monolayer remains FM with TC ∼ 75 K, and the FM order is due to a superexchange in the near-90 • Cr-I-Cr bonds. Moreover, we find that an itinerant magnetism could be introduced by carriers doping. Both electron doping and hole doping would render CrI3 monolayer half-metallic, and steadily enhance the FM stability. In particular, hole doping is three times as fast as electron doping in increasing TC, and a room temperature FM half-metallicity could be achieved in CrI3 monolayer via a half-hole doping. Therefore, CrI3 monolayer would be an appealing two-dimensional spintronic material.

Ternary Mg3(Bi,Sb)2 single crystals showing high thermoelectric performance are for the first time grown by the Mg flux method.

It has been demonstrated that topological nontrivial surface states can favor heterogeneous catalysis processes such as the hydrogen evolution reaction (HER), but a further decrease in mass loading and an increase in activity are still highly challenging. The observation of massless chiral fermions associated with large topological charge and long Fermi arc (FA) surface states inspires the investigation of their relationship with the charge transfer and adsorption process in the HER. In this study, it is found that the HER efficiency of Pt‐group metals can be boosted significantly by introducing topological order. A giant nontrivial topological energy window and a long topological surface FA are expected at the surface when forming chiral crystals in the space group of P213 (#198). This makes the nontrivial topological features resistant to a large change in the applied overpotential. As HER catalysts, PtAl and PtGa chiral crystals show turnover frequencies as high as 5.6 and 17.1 s−1 and an overpotential as low as 14 and 13.3 mV at a current density of 10 mA cm−2. These crystals outperform those of commercial Pt and nanostructured catalysts. This work opens a new avenue for the development of high‐efficiency catalysts with the strategy of topological engineering of excellent transitional catalytic materials.

Interlayer interactions in 2D materials, also known as van der Waals (vdWs) interactions, play a critical role in the physical properties of layered materials. It is fascinating to manipulate the vdWs interaction, and hence to “redefine” the material properties. Here, we demonstrate that in-plane biaxial strain can effectively tune the vdWs interaction of few-layer black phosphorus with thickness of 2-10 layers, using infrared spectroscopy. Surprisingly, our results reveal that in-plane tensile strain efficiently weakens the interlayer coupling, even though the sample shrinks in the vertical direction due to the Poisson effect, in sharp contrast to one’s intuition. Moreover, density functional theory (DFT) calculations further confirm our observations and indicate a dominant role of the puckered lattice structure. Our study highlights the important role played by vdWs interactions in 2D materials during external physical perturbations.

Because of the strong quantum confinement effect, few-layer γ-InSe exhibits a layer-dependent band gap, spanning the visible and near infrared regions, and thus recently has been drawing tremendous attention. As a two-dimensional material, the mechanical flexibility provides an additional tuning knob for the electronic structures. Here, for the first time, we engineer the band structures of few-layer and bulk-like InSe by uniaxial tensile strain and observe a salient shift of photoluminescence peaks. The shift rate of the optical gap is approximately 90-100 meV per 1% strain for four- to eight-layer samples, which is much larger than that for the widely studied MoS monolayer. Density functional theory calculations well reproduce the observed layer-dependent band gaps and the strain effect and reveal that the shift rate decreases with the increasing layer number for few-layer InSe. Our study demonstrates that InSe is a very versatile two-dimensional electronic and optoelectronic material, which is suitable for tunable light emitters, photodetectors, and other optoelectronic devices.

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The vibrational and electronic properties of 2-dimensinal (2D) materials can be efficiently tuned by external strain due to their good stretchability. Resonant Raman spectroscopy is a versatile tool to study the physics of phonons, electrons and their interactions simultaneously, which is particularly useful for the investigation of strain effect on 2D materials. Here, for the first time, we report the resonant Raman study of strained few-layer InSe (γ-phase). Under ~ 1% of uniaxial tensile strain, one order of magnitude enhancement of Raman intensity for longitudinal optical (LO) phonon is observed, while other modes exhibit only modest change. Further analysis demonstrates that it arises from the intraband electron-phonon scattering channel for LO phonon in resonance. The large enhancement of Raman intensity provides us a sensitive and novel method to characterize the strain effect and a mapping of the strain distribution in a wrinkled sample is demonstrated. In addition, we observed sizable redshifts of firstorder optical phonon modes. The shift rate exhibits phonon mode dependence, in excellent agreement with density functional perturbation theory (DFPT) calculations.Our study paves the way for sensitive strain quantification in few-layer InSe and its application in flexible electronic and optoelectronic devices. 3 / 41 I. INTRODUCTION Mechanical cleavage of graphene [1] by K. S. Novoselov et al. arouses tremendous research interest in 2D materials. A variety of 2D semimetals and semiconductors have been discovered ever since, such as transition metal dichalcogenides (TMDCs) [2], silicone [3], stanine [4] and black phosphorus [5,6]. Atomically thin indium selenide (γphase) joins the family lately with unique electronic properties [7-9]. Quantum Hall effect was observed in the high quality few-layer InSe electronic devices [9]. Strong quantum confinement in the out-of-plane direction gives rise to layer-dependent bandgap [7], covering a large range of visible and near infrared regions. Few-layer InSe has promised great application potentials in electronics and optoelectronics [10-12]. The mechanical stretchability of 2D materials opens the door for straining, to continuously and reversibly tune their lattice constants and electronic properties [13]. Raman spectroscopy is a crucial diagnostic tool to evaluate the strain effect. Phonon softening and splitting are commonly observed in 2D materials under uniaxial tensile strain, such as graphene [14,15], TMDCs [15,16] and black phosphorus [18,19], indicating the weakening of the bond strength and the symmetry-breaking. The band structure and electronic properties of 2D materials can be engineered efficiently via strain as well. For example, prominent strain-induced shift of the band gap and indirectto-direct bandgap transition were observed in multilayer TMDCs [20]. Owing to the small Young's modulus ( ~ 45 N/m) [21], the bandgap of few-layer InSe can be easily tuned by uniaxial tensile strain with shift rate up to 90-150meV/% [22,23]. Therefore, 5 / 41 II. EXPERI...

Large magnetic anisotropy energy (MAE) is desirable and critical for nanoscale magnetic devices. Here, using ligand-field level diagrams and density functional calculations, we well explain the very recent discovery [I. G. Rau et al., Science 344, 988 (2014)] that an individual Co adatom on a MgO (001) surface has a large MAE of more than 60 meV. More importantly, we predict that a giant MAE up to 110 meV could be realized for Ru adatoms on MgO (001), and even more for the Os adatoms (208 meV). This is a joint effect of the special ligand field, orbital multiplet, and significant spin-orbit interaction, in the intermediate-spin state of the Ru or Os adatoms on top of the surface oxygens. The giant MAE could provide a route to atomic scale memory.