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

2D materials are the focus of an intense research effort because of their unique properties and their potential for revealing intriguing new phenomena. Phosphorene, a monolayer of black phosphorus, earned its place among the family of 2D semiconductor materials when recent results unveiled its high carrier mobility, high optical and UV absorption, and other attractive properties, which are of particular interest for optoelectronic applications. Unlike graphene, phosphorene has an anisotropic orthorhombic structure that is ductile along one of the in-plane crystal directions but stiff along the other. This results in unusual mechanical, electronic, optical and transport properties that reflect the anisotropy of the lattice. This Review summarizes the physical properties of phosphorene and highlights the recent progress made in the preparation, isolation and characterization of this material. The role of defects and doping is discussed, and phosphorene-based devices are surveyed; finally, the remaining challenges and potential applications of phosphorene are outlined. Phosphorene is a 2D material exhibiting remarkable mechanical, electronic and optical properties. In this Review, we survey fabrication techniques and discuss theoretical and experimental findings, exploring phosphorene from its fundamental properties to its implementation in devices.

Phosphorene: from theory to applications

We demonstrate first room temperature, and ultrabright single photon emission from a color center in two-dimensional multilayer hexagonal boron nitride. Density Functional Theory calculations indicate that vacancy-related centers are a likely source of the emission.

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Quantum emission from hexagonal boron nitride monolayers

Phosphorene, an emerging two-dimensional material, has received considerable attention due to its layer-controlled direct bandgap, high carrier mobility, negative Poisson's ratio and unique in-plane anisotropy. As cousins of phosphorene, 2D group-VA arsenene, antimonene and bismuthene have also garnered tremendous interest due to their intriguing structures and fascinating electronic properties. 2D group-VA family members are opening up brand-new opportunities for their multifunctional applications encompassing electronics, optoelectronics, topological spintronics, thermoelectrics, sensors, Li- or Na-batteries. In this review, we extensively explore the latest theoretical and experimental progress made in the fundamental properties, fabrications and applications of 2D group-VA materials, and offer perspectives and challenges for the future of this emerging field.

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Recent progress in 2D group-VA semiconductors: from theory to experiment

Phosphorene, the single- or few-layer form of black phosphorus, was recently rediscovered as a two-dimensional layered material holding great promise for applications in electronics and optoelectronics. Research into its fundamental properties and device applications has since seen exponential growth. In this Perspective, we review recent progress in phosphorene research, touching upon topics on fabrication, properties, and applications; we also discuss challenges and future research directions. We highlight the intrinsically anisotropic electronic, transport, optoelectronic, thermoelectric, and mechanical properties of phosphorene resulting from its puckered structure in contrast to those of graphene and transition-metal dichalcogenides. The facile fabrication and novel properties of phosphorene have inspired design and demonstration of new nanodevices; however, further progress hinges on resolutions to technical obstructions like surface degradation effects and nonscalable fabrication techniques. We also briefly describe the latest developments of more sophisticated design concepts and implementation schemes that address some of the challenges in phosphorene research. It is expected that this fascinating material will continue to offer tremendous opportunities for research and development for the foreseeable future.

Phosphorene: Fabrication, Properties, and Applications.

We have investigated the structure, adsorption energy, growth mode, diffusion barrier, magnetic property, dipole moment, and work function of Li, Na, Mg, Al, Cr, Fe, Co, Ni, Mo, Pd, Pt, and Au adsorption on phosphorene using first-principles density-functional theory. We have found that all the adatoms favor the hollow site of hexagonal. It seems that the Mg, Cr, Mo, and Au adatoms may have three-dimensional (3D) growth mode on phosphorene substrate, whereas all other adatoms prefer two-dimensional (2D) growth mode. The metallic state is observed in Li, Na, Al, and Cr doped systems, and the Cr doped phosphorene displays magnetic state. The other eight metal doped systems still preserve semiconducting band gap. Among them, we obtained spin polarized band structures in Fe, Co, and Au doped systems with a band gap. In particular, the Fe doped phosphorene can be a potential candidate for dilute magnetic semiconductor material because no clustering problem will take place. We found a variation in dipole moment...

First-Principles Study of Metal Adatom Adsorption on Black Phosphorene

Transition-metal dichalcogenides (TMDs) monolayers have been considered as important two-dimensional semiconductor materials for the study of fundamental physics in the field of spintronics. However, the out-of-plane mirror symmetry in TMDs may constrain electrons' degrees of freedom and it may limit spin-related applications. Recently, a newly synthesized Janus TMDs MoSSe was found to intrinsically possess both the in-plane inversion and the out-of-plane mirror-symmetry breaking. Here we performed first-principles calculations in order to systematically investigate the electronic band structures of a series of Janus monolayer TMDs with chemical formula $MXY\phantom{\rule{0.16em}{0ex}}(M=\mathrm{Mo},\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{and}\phantom{\rule{0.16em}{0ex}}X,Y=\mathrm{S},\phantom{\rule{0.16em}{0ex}}\mathrm{Se},\phantom{\rule{0.16em}{0ex}}\mathrm{Te})$. It is found that they possess robust electronic properties like their parent phases. We explored also the effect of perpendicular external electric field and in-plane biaxial strain on the Rashba spin splittings. The Zeeman-type spin splitting and valley polarization at $K({K}^{\ensuremath{'}})$ point are well preserved and we observed a Rashba-type spin splitting around the $\mathrm{\ensuremath{\Gamma}}$ point for all the $MXY$ systems. We have also found that these spin splittings can be enhanced by an external electric field collinear with the local electric field derived by the polar bonds and by the compressive strain. The Rashba parameters change linearly with the external electric field, but nonlinearly with the biaxial strain. The compressive strain is found to enhance significantly the anisotropic Rashba spin splitting.

Intrinsic and anisotropic Rashba spin splitting in Janus transition-metal dichalcogenide monolayers

With the aim of understanding recent experimental data concerning noncovalent n/p-doping effects in graphene samples, we have investigated the interactions between two prototypical donor and accept...

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Theoretical Study of the Interaction of Electron Donor and Acceptor Molecules with Graphene

Free-standing and flexible WS2 /WO2.9 /C hybrid membranes are synthesized and used as catalytic electrodes for electrochemical hydrogen evolution, exhibiting a high and stable catalytic activity. By virtue of the synergetic effect, a low onset overpotential of 20 mV and a Tafel slope of 36 mV dec-1 are achieved.

Noble-Metal-Free Hybrid Membranes for Highly Efficient Hydrogen Evolution.

A two-dimensional ferromagnetic semiconductor Cr2Ge2Te6 (CGT) was recently found to possess extraordinary characteristics and has great potential in the emerging field of spintronics. Using first-principles calculations, we examined the stabilities of this layered system by studying the cleavage energies and phonon dispersion. The ferromagnetic ground state has an in-plane spin polarization and bandgaps of about 0.26 eV by Perdew-Burke-Ernzerhof-van der Waals and 0.91 eV by the Heyd-Scuseria-Ernzerhof functional. Furthermore, we employed strain engineering and an external electric field to control the electronic and magnetic properties. In addition, we studied the magnetic anisotropy energy as well as its modulation under the electric field. We predict the CGT monolayer to be the easy plane ferromagnet, and the perpendicular electric field could affect the ferromagnetic stability along different directions. Our obtained results provide guidance for the potential applications of monolayer CGT for magnetic nanodevices, spintronic, and straintronic applications.

Tao Hu

Papers

First-Principles Study of Metal Adatom Adsorption on Black Phosphorene

Intrinsic and anisotropic Rashba spin splitting in Janus transition-metal dichalcogenide monolayers

Theoretical Study of the Interaction of Electron Donor and Acceptor Molecules with Graphene

Noble-Metal-Free Hybrid Membranes for Highly Efficient Hydrogen Evolution.

Magnetic and electronic properties of Cr2Ge2Te6 monolayer by strain and electric-field engineering