The discovery of ferromagnetic two-dimensional van der Waals materials has opened up opportunities to explore intriguing physics and to develop innovative spintronic devices. However, controllable synthesis of these 2D ferromagnets and enhancing their stability under ambient conditions remain challenging. Here, we report chemical vapor deposition growth of air-stable 2D metallic 1T-CrTe2 ultrathin crystals with controlled thickness. Their long-range ferromagnetic ordering is confirmed by a robust anomalous Hall effect, which has seldom been observed in other layered 2D materials grown by chemical vapor deposition. With reducing the thickness of 1T-CrTe2 from tens of nanometers to several nanometers, the easy axis changes from in-plane to out-of-plane. Monotonic increase of Curie temperature with the thickness decreasing from ~130.0 to ~7.6 nm is observed. Theoretical calculations indicate that the weakening of the Coulomb screening in the two-dimensional limit plays a crucial role in the change of magnetic properties.

batteries is greatly limited by the highly insulating nature of S 8 /Li 2 S 2-x (x ≤ 1) and the dissolution of intermediate lithium polysulfides (Li 2 S n , 4 ≤ n ≤ 8) during charge/discharge process. [7][8][9] Over the past decades, massive efforts like encapsulating S 8 in conductive matrix, [10][11][12][13] protective coating layers, [14][15][16] and inducing interlayer between cathode and separator, [17][18][19] have been made to manipulate this deficiency, aiming to lighten shuttling and migration of Li 2 S n during long-term cycling and to improve the electrode kinetics. 2D materials with large specific area, such as graphene oxides (GOs), [20][21][22][23][24] MnO 2 , [25] Co 4 N, [26] MXene, [27] provide numerous anchoring sites and have been successfully employed as cathode hosts to suppress shuttling and migration of Li 2 S n in the Li-S batteries. Usually, heteroatoms doping is a general modification technique to further increase the polarity of 2D materials to adsorb Li 2 S n , giving birth to nitrogen-doped graphene, [28] nitrogen-doped MXene, [29] cobalt-doped porous carbon, [30] molybdenum-doped MoO 3 , [31] etc. However, the doping amount is very limited, which seriously restricts the improvement of their electrochemical performance. Comparing with the traditional doping strategy, intercalation can induce more heteroatoms in their van der Waals gap with good uniformity and change the properties of 2D materials more significantly. [32] For example, in our previous study, we proved that the n-type semiconducting SnS 2 can turn to a p-type semiconductor or metal after intercalation of different transition metal atoms. [33] Besides the electrical properties, the electrochemical properties of 2D materials might also be tuned effectively by this intercalation strategy.Here, ultrathin 2D layered α-MoO 3 nanoribbons with thickness of ≈10 nm have been synthesized and selected as the host. The strong polarity of MoO 3 together with its high specific surface area provides numerous active sites to bind sulfur species effectively, thus suppressing the "shuttle effect" obviously. Intercalation of metal tin (Sn) into van der Waals gap was further used to enhance the intrinsic conductivity of MoO 3 and improve the binding energy with sulfur species. Transmission electron microscopy (TEM) proved that Sn was inserted into the van der Waals gap of MoO 3 uniformly. First-principles calculations further certify that binding energy as large as 3.01 eV Heteroatom doping strategies have been widely developed to engineer the conductivity and polarity of 2D materials to improve their performance as the host for sulfur cathode in lithium-sulfur batteries. However, further improvement is limited by the inhomogeneity and the small amount of the doping atoms. An intercalation method to improve the conductivity and polarity of 2D-layered α-MoO 3 nanoribbons is developed here, thus, resulting in much improved electrochemical performance as sulfur host with better rate and cycle performance. The first principle calculations show t...

No abstract

Two-dimensional (2D) materials especially transition metal dichalcogenides (TMDs) have drawn intensive interest owing to their plentiful properties. Some TMDs with magnetic elements (Fe, Co, Ni, etc.) are reported to be magnetic theoretically and experimentally, which undoubtedly provide a promising platform to design functional devices and study physical mechanisms. Nevertheless, plenty of theoretical TMDs remain unrealized experimentally. In addition, the governable synthesis of these kinds of TMDs with desired thickness and high crystallinity poses a tricky challenge. Here, we report a controlled preparation of CoX2 (X = S, Se, and Te) nanosheets through chemical vapor deposition. The thickness, lateral scale, and shape of the crystals show great dependence on temperature, and the thickness can be controlled from a monolayer to tens of nanometers. Magneto-transport characterization and density function theory simulation indicate that CoSe2 and CoTe2 are metallic. In addition, unsaturated and linear magnetoresistance have been observed even up to 9 T. The conductivity of CoSe2 and CoTe2 can reach 5 × 106 and 1.8 × 106 S/m, respectively, which is pretty high and even comparable with silver. These cobalt-based TMDs show great potential to work as 2D conductors and also provide a promising platform for investigating their magnetic properties.

Since their discovery, two-dimensional (2D) materials have attracted significant research attention owing to their excellent and controllable physical and chemical properties. These materials have emerged rapidly as important material system owing to their unique properties such as electricity, optics, quantum properties, and catalytic properties. 2D materials are mostly bonded by strong ionic or covalent bonds within the layers, and the layers are stacked together by van der Waals forces, thereby making it possible to peel off 2D materials with few or single layers. The weak interaction between the layers of 2D materials also enables the use of van der Waals gaps for regulating the electronic structure of the system and further optimizing the material properties. The introduction of guest atoms can significantly change the interlayer spacing of the original material and coupling strength between the layers. Also, interaction between the guest and host atom also has the potential to change the electronic structure of the original material, thereby affecting the material properties. For example, the electron structure of a host can be modified by interlayer guest atoms, and characteristics such as carrier concentration, optical transmittance, conductivity, and band gap can be tuned. Organic cations intercalated between the layers of 2D materials can produce stable superlattices, which have great potential for developing new electronic and optoelectronic devices. This method enables the modulation of the electrical, magnetic, and optical properties of the original materials, thereby establishing a family of 2D materials with widely adjustable electrical and optical properties. It is also possible to introduce some new properties to the 2D materials, such as magnetic properties and catalytic properties, by the intercalation of guest atoms. Interlayer storage, represented by lithium-ion batteries, is also an important application of 2D van der Waals gap utilization in energy storage, which has also attracted significant research attention. Herein, we review the studies conducted in recent years from the following aspects: (1) changing the layer spacing to change the interlayer coupling; (2) introducing the interaction between guest and host atoms to change the physico-chemical properties of raw materials; (3) introducing the guest substances to obtain new properties; and (4) interlayer energy storage. We systematically describe various interlayer optimization methods of 2D van der Waals gaps and their effects on the physical and chemical properties of synthetic materials, and suggest the direction of further development and utilization of 2D van der Waals gaps.