Materials research has driven the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices1,2 because identifying new magnetic materials is key to better device performance and design. Van der Waals crystals retain their chemical stability and structural integrity down to the monolayer and, being atomically thin, are readily tuned by various kinds of gate modulation3,4. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr2Ge2Te6 (ref. 5) and CrI3 (ref. 6) at low temperatures. Here we develop a device fabrication technique and isolate monolayers from the layered metallic magnet Fe3GeTe2 to study magnetotransport. We find that the itinerant ferromagnetism persists in Fe3GeTe2 down to the monolayer with an out-of-plane magnetocrystalline anisotropy. The ferromagnetic transition temperature, Tc, is suppressed relative to the bulk Tc of 205 kelvin in pristine Fe3GeTe2 thin flakes. An ionic gate, however, raises Tc to room temperature, much higher than the bulk Tc. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2 opens up opportunities for potential voltage-controlled magnetoelectronics7-11 based on atomically thin van der Waals crystals.

Gate-tunable Room-temperature Ferromagnetism in Two-dimensional Fe 3 GeTe 2

A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

Emerging in-plane anisotropic two-dimensional materials

Recent Progress on 2D Noble-Transition-Metal Dichalcogenides

High quality thin films of the ferromagnetic semiconductor EuO have been prepared and were studied using a new form of spin-resolved spectroscopy. We observed large changes in the electronic structure across the Curie and metal-insulator transition temperature. We found that these are caused by the exchange splitting of the conduction band in the ferromagnetic state, which is as large as 0.6 eV. We also present strong evidence that the bottom of the conduction band consists mainly of majority spins. This implies that doped charge carriers in EuO are practically fully spin polarized.

Exchange splitting and charge carrier spin-polarization in EuO

Due to the intriguing anisotropic optical and electrical properties, low-symmetry 2D materials are attracting a lot of interest both for fundamental studies and fabricating novel electronic and optoelectronic devices. Identifying new promising low-symmetry 2D materials will be rewarding toward the evolution of nanoelectronics and nano-optoelectronics. In this work, germanium diarsenide (GeAs2 ), a group IV-V semiconductor with novel low-symmetry puckered structure, is introduced as a favorable highly anisotropic 2D material into the rapidly growing 2D family. The structural, vibrational, electrical, and optical in-plane anisotropy of GeAs2 is systematically investigated both theoretically and experimentally, combined with thickness-dependent studies. Polarization-sensitive photodetectors based on few-layer GeAs2 exhibit highly anisotropic photodetection behavior with lineally dichroic ratio up to ≈2. This work on GeAs2 will excite interests in the less exploited regime of group IV-V compounds.

Highly In-Plane Anisotropic 2D GeAs2 for Polarization-Sensitive Photodetection.

Infrared (IR) photodetectors are finding diverse applications in imaging, information communication, military, etc. 2D metal chalcogenides (2DMCs) have attracted increasing interest in view of their unique structures and extraordinary physical properties. They have demonstrated outstanding IR detection performance including high responsivity and detectivity, high on/off ratio, fast response rate, stable room temperature operability, and good mechanical flexibility, which has opened up a new prospect in next-generation IR photodetectors. This Review presents a comprehensive summary of recent progress in advanced IR photodetectors based on 2DMCs. The rationale of the photodetectors containing photocurrent generation mechanisms and performance parameters are briefly introduced. The device performances of 2DMCs-based IR photodetectors are also systematically summarized, and some representative achievements are highlighted as well. Finally, conclusions and outlooks are delivered as a guideline for this thriving field.

2D Metal Chalcogenides for IR Photodetection.

Sodium‐Mediated Epitaxial Growth of 2D Ultrathin Sb2Se3 Flakes for Broadband Photodetection

Two-dimensional molecular crystals, consisting of zero-dimensional molecules, are very appealing due to their novel physical properties. However, they are mostly limited to organic molecules. The synthesis of inorganic version of two-dimensional molecular crystals is still a challenge due to the difficulties in controlling the crystal phase and growth plane. Here, we design a passivator-assisted vapor deposition method for the growth of two-dimensional Sb2O3 inorganic molecular crystals as thin as monolayer. The passivator can prevent the heterophase nucleation and suppress the growth of low-energy planes, and enable the molecule-by-molecule lateral growth along high-energy planes. Using Raman spectroscopy and in situ transmission electron microscopy, we show that the insulating α-phase of Sb2O3 flakes can be transformed into semiconducting β-phase under heat and electron-beam irradiation. Our findings can be extended to the controlled growth of other two-dimensional inorganic molecular crystals and open up opportunities for potential molecular electronic devices.

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Wei Han

Papers

Emerging in-plane anisotropic two-dimensional materials

Highly In-Plane Anisotropic 2D GeAs2 for Polarization-Sensitive Photodetection.

2D Metal Chalcogenides for IR Photodetection.

Sodium‐Mediated Epitaxial Growth of 2D Ultrathin Sb2Se3 Flakes for Broadband Photodetection

Two-dimensional inorganic molecular crystals.