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The quinoid structure, a resonance structure of benzenoid, gives rise to peculiar chemical reactivity and physical properties. A complete characterization of its geometric and electronic properties...

Defects usually have an important role in tailoring various properties of two-dimensional (2D) materials. However, optical detection of defects, especially single-atom point defects, is very challenging in 2D layers. Here, we report our systematic studies on the Raman-activated defect vibrational modes in 2D semimetallic material by combining Raman spectroscopy, density functional theory (DFT) calculation and scanning tunneling microscopy (STM). We observed three common Raman-active vibrational modes located at 95 ($$A_{1g}^2$$ A 1 g 2 ), 228 ($$A_{1g}^1$$ A 1 g 1 ), and 304 cm−1 ($$B_{1g}^1$$ B 1 g 1 ) in ZrSiTe few-layers, consistent with our theoretical calculations. Moreover, a pronounced mode sitting at 131.7 cm−1 was found in the ZrSiTe monolayer. This mode fades out quickly in the bilayer (2L) and eventually disappears in 4L. The high-resolution STM images and DFT calculations suggest this mode to be an intralayer shear mode at the Brillouin zone boundary which is activated by atomic point defects, and STM-based inelastic tunneling spectrum further confirms the existence of such a defect mode. The appearance of such ‘forbidden’ modes in Raman spectra may pave an avenue for the optical characterization of single-atom point defects in metallic 2D layers.

Two-dimensional magnets have received increasing attention since Cr2Ge2Te6 and CrI3 were experimentally exfoliated and measured in 2017. Although layered ferromagnetic metals were demonstrated at room temperature, a layered ferromagnetic semiconductor with high Curie temperature (Tc) is yet to be unveiled. Here, we theoretically predicted a family of high Tc ferromagnetic monolayers, namely MnNX and CrCX (X = Cl, Br and I; C = S, Se and Te). Their Tc values were predicted from over 100 K to near 500 K with Monte Carlo simulations using an anisotropic Heisenberg model. Eight members among them show semiconducting bandgaps varying from roughly 0.23 to 1.85 eV. These semiconducting monolayers also show extremely large anisotropy, i.e. ∼101 for effective masses and ∼102 for carrier mobilities, along the two in-plane lattice directions of these layers. Additional orbital anisotropy leads to a spin-locked linear dichroism, in different from previously known circular and linear dichroisms in layered materials. Together with the mobility anisotropy, it offers a spin-, dichroism- and mobility-anisotropy locking. These results manifest the potential of this 2D family for both fundamental research and high performance spin-dependent electronic and optoelectronic devices.

Layered magnets have recently received tremendous attention, however spin-exchange coupling mechanism across their interlayer regions is yet to be revealed. Here, we report a Bethe-Slater-curve (BSC)-like behavior in nine transition metal dichalcogenide bilayers ($M{X}_{2}$, $M=\mathrm{V}$, Cr, Mn; $X=\mathrm{S}$, Se, Te) and established interlayer spin-exchange coupling mechanisms at their van der Waals gaps using first-principle calculations. The BSC-like behavior offers a distance-dependent interlayer antiferromagnetic (AFM) to ferromagnetic (FM) transition. This phenomenon is explained with the spin-exchange coupling mechanisms established using bilayer $\mathrm{CrS}{\mathrm{e}}_{2}$ as a prototype in this work. The Se ${p}_{\mathrm{z}}$ wave functions from two adjacent interfacial Se sublayers overlap at the interlayer region. The spin alignment of the region determines interlayer magnetic coupling. At a shorter interlayer distance, Pauli repulsion at the overlapped region dominates and thus favors antiparallel oriented spins leading to interlayer AFM. For a longer distance, kinetic-energy gain of polarized electrons across the bilayer balances the Pauli repulsion and the bilayer thus prefers an interlayer FM state. In light of this, the AFM-FM transition is a result of competition between Pauli and Coulomb repulsions and kinetic-energy gain. All these results open a route to tune interlayer magnetism and the revealed spin-exchange coupling mechanisms are paramount additions to those previously established ones.

Few-layer tellurium is an emerging quasi-one-dimensional layered material. The striking feature of Te is its presence as various few-layer allotropes (α-δ). Although these allotropes offer substantially different physical properties, only the α phase has been synthesized in neutral few-layers as it is so far the most stable few-layer form. Herein, we show that hole or electron doping could maintain a certain Te phase. The β, α, γ and δ phases appear as the most stable forms of Te bilayer, in sequence, with bandgap variations over 1 eV. In Te trilayer, a novel metallic chiral α + δ phase emerges, leading to the appearance of chirality. Transitions among these phases, understood at the wavefunction level, are accompanied by the emergence or elimination of inversion centers (α-β, α-γ, α-α + δ), structural anisotropy (α-γ, γ-δ) and chirality (α-α + δ), which could result in substantial changes in optical and other properties. In light of these findings, our work opens a new avenue for stabilizing different allotropes of layered materials; this is crucial for using their outstanding properties. This study also suggests the possibility of building mono-elemental electronic and optoelectronic heterostructures or devices, which are attractive for future applications in electronics.

Interlayer coupling is of vital importance for manipulating physical properties, e.g., electronic band gap, in two-dimensional materials. However, tuning magnetic properties in these materials is yet to be addressed. Here, we found the in-plane magnetic orders of $\mathrm{Cr}{\mathrm{S}}_{2}$ mono and few layers are tunable between striped antiferromagnetic (sAFM) and ferromagnetic (FM) orders by manipulating charge transfer between Cr ${t}_{2g}$ and ${e}_{g}$ orbitals. Such charge transfer is realizable through interlayer coupling, direct charge doping, or substituting S with Cl atoms. In particular, the transferred charge effectively reduces a portion of ${\mathrm{Cr}}^{4+}$ to ${\mathrm{Cr}}^{3+}$, which, together with delocalized S $p$ orbitals and their resulting direct S-S interlayer hopping, enhances the double-exchange mechanism favoring the FM rather than sAFM order. An exceptional interlayer spin-exchange parameter was revealed over $\ensuremath{-}10\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, an order of magnitude stronger than available results of interlayer magnetic coupling. It addition, the charge doping could tune $\mathrm{Cr}{\mathrm{S}}_{2}$ between $p$- and $n$-doped magnetic semiconductors. Given these results, several prototype devices were proposed for manipulating magnetic orders using external electric fields or mechanical motion. These results manifest the role of interlayer coupling in modifying magnetic properties of layered materials and shed considerable light on manipulating magnetism in these materials.

Inorganic halide perovskites CsPbX 3 (X = I, Br) have attracted tremendous attention in solar cell applications. However, the bulk form of the cubic phase CsPbX 3, which offers moderate direct bandgaps, is metastable at room temperature and tends to transform into a tetragonal or orthorhombic phase. Here, our density functional theory calculation results found that the surface energies of the cubic phase are smaller than those of the orthorhombic phase, although the bulk counterpart of the cubic phase is less stable than that of the orthorhombic phase. These results suggest a surface stabilization strategy to maintain the stability of the cubic phase at room temperature that an enlarged portion of surfaces shall change the relative stability of the two phases in nanostructured CsPbX 3. This strategy, which may potentially solve the long-standing stability issue of cubic CsPbX 3, was demonstrated to be feasible by our calculations in zero-, one-, and two-dimensional nanostructures. In particular, confined sizes from few to tens of nanometers could keep the cubic phase as the most thermally favored form at room temperature. Our predicted values in particular cases, such as the zero-dimensional form of CsPbI3, are highly consistent with experimental values, suggesting that our model is reasonable and our results are reliable. These predicted critical sizes give the upper and lower limits of the confined sizes, which may guide experimentalists to synthesize these nanostructures and promote likely practical applications such as solar cells and flexible displays using CsPbX 3 nanostructures.