Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. Yet, only a few dozen 2D materials have been successfully synthesized or exfoliated. Here, we search for 2D materials that can be easily exfoliated from their parent compounds. Starting from 108,423 unique, experimentally known 3D compounds, we identify a subset of 5,619 compounds that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van der Waals density functional theory, validated against experimental structural data and calculated random phase approximation binding energies, further allowed the identification of 1,825 compounds that are either easily or potentially exfoliable. In particular, the subset of 1,036 easily exfoliable cases provides novel structural prototypes and simple ternary compounds as well as a large portfolio of materials to search from for optimal properties. For a subset of 258 compounds, we explore vibrational, electronic, magnetic and topological properties, identifying 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors.

The family of 2D materials grows day by day, drastically expanding the scope of possible phenomena to be explored in two dimensions, as well as the possible van der Waals heterostructures that one can create. Such 2D materials currently cover a vast range of properties. Until recently, this family has been missing one crucial member -2D magnets. The situation has changed over the last two years with the introduction of a variety of atomically-thin magnetic crystals. Here we will discuss the difference between magnetic states in 2D materials and in bulk crystals and present an overview of the 2D magnets that have been explored recently. We will focus, in particular, on the case of the two most studied systems -semiconducting CrI3 and metallic Fe3GeTe2 -and illustrate the physical phenomena that have been observed. Special attention will be given to the range of novel van der Waals heterostructures that became possible with the appearance of 2D magnets, offering new perspectives in this rapidly expanding field.

Two-dimensional semiconductors such as MoS2 are an emerging material family with wide-ranging potential applications in electronics, optoelectronics, and energy harvesting. Large-area growth methods are needed to open the way to applications. Control over lattice orientation during growth remains a challenge. This is needed to minimize or even avoid the formation of grain boundaries, detrimental to electrical, optical, and mechanical properties of MoS2 and other 2D semiconductors. Here, we report on the growth of high-quality monolayer MoS2 with control over lattice orientation. We show that the monolayer film is composed of coalescing single islands with limited numbers of lattice orientation due to an epitaxial growth mechanism. Optical absorbance spectra acquired over large areas show significant absorbance in the high-energy part of the spectrum, indicating that MoS2 could also be interesting for harvesting this region of the solar spectrum and fabrication of UV-sensitive photodetectors. Even though the interaction between the growth substrate and MoS2 is strong enough to induce lattice alignment via van der Waals interaction, we can easily transfer the grown material and fabricate devices. Local potential mapping along channels in field-effect transistors shows that the single-crystal MoS2 grains in our film are well connected, with interfaces that do not degrade the electrical conductivity. This is also confirmed by the relatively large and length-independent mobility in devices with a channel length reaching 80 μm.

Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronicstructure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectivelylocalised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spinor-valued Wannier functions, more accurate methods to interpolate quantities in the Brillouin zone); improved usability (improved plotting routines, integration with arXiv:1907.09788v1 [cond-mat.mtrl-sci]

Magnetic layered van der Waals crystals are an emerging class of materials giving access to new physical phenomena, as illustrated by the recent observation of 2D ferromagnetism in Cr2Ge2Te6 and CrI3. Of particular interest in semiconductors is the interplay between magnetism and transport, which has remained unexplored. Here we report magneto-transport measurements on exfoliated CrI3 crystals. We find that tunneling conduction in the direction perpendicular to the crystalline planes exhibits a magnetoresistance as large as 10,000%. The evolution of the magnetoresistance with magnetic field and temperature reveals that the phenomenon originates from multiple transitions to different magnetic states, whose possible microscopic nature is discussed on the basis of all existing experimental observations. This observed dependence of the conductance of a tunnel barrier on its magnetic state is a phenomenon that demonstrates the presence of a strong coupling between transport and magnetism in magnetic van der Waals semiconductors.

The recent discovery of ferromagnetism in 2D van der Waals (vdW) crystals has generated widespread interest, owing to their potential for fundamental and applied research. Advancing the understanding and applications of vdW magnets requires methods to quantitatively probe their magnetic properties on the nanoscale. Here, we report the study of atomically thin crystals of the vdW magnet CrI 3 down to individual monolayers using scanning single-spin magnetometry, and demonstrate quantitative, nanoscale imaging of magnetisation, localised defects and magnetic domains. We determine the magnetisation of CrI 3 monolayers to be ≈ 16 µ B /nm 2 and find comparable values in samples with odd numbers of layers, whereas the magnetisation vanishes when the number of layers is even. We also establish that this inscrutable even-odd effect is intimately connected to the material structure, and that structural modifications can induce switching between ferro-and anti-ferromagnetic interlayer ordering. Besides revealing new aspects of magnetism in atomically thin CrI 3 crystals, these results demonstrate the power of single-spin scanning magnetometry for the study of magnetism in 2D vdW magnets.Magnetism in individual monolayers of vdW crystals has recently been observed in a range of materials, including semiconducting [3,4] and metallic [5][6][7] compounds. The discovery of such two dimensional magnetic order is per se non-trivial [8] and has triggered significant attention owing to emerging exotic phenomena including Kitaev spin liquids [9,10], or novel magneto-electric effects [11][12][13][14]. Remarkable efforts have led to the use of two-dimensional magnets as functional elements in spintronics, such as spin-filters [15, 16], spin-transistors [17], tunnelling magnetoresistance devices [18,19] or magnetoelectric switches [12][13][14]. Further advances hinge on methods for the quantitative study of the magnetic response of these atomically thin crystals at the nanoscale, but despite their central importance, the required experimental methods are still lacking. Indeed, transport ex-A z NV e NV z θ NV 3 µm 3 l a y e r s 2 l a y e r s B C, D 0.35 -0.35 0 B NV -B NV (mT) C 2 µm Magne�c stray-field map bias D 20 -20 0 σ (µ B /nm 2 ) 2 µm Magne�sa�on map FIG. 1.Nanoscale imaging of magnetism in twodimensional van der Waals magnets. A Schematic of the scanning single spin magnetometry technique employed in this work. A single Nitrogen-Vacancy (NV) electronic spin is scanned across few layer flakes of encapsulated CrI3 (encapsulation not shown). Stray magnetic fields from the sample are sensed by optically detected Zeeman shifts of the NV spin states, and imaged with nanoscale resolution (set by the sensor-sample separation zNV) by lateral scanning of the NV. The method detects magnetic fields along the NV spin quantisation axis eNV, at an angle θNV ∼ 54 • from the sample normal. B Optical micrograph of the CrI3 bi-and tri-layer flake of sample D1. C Magnetic field map of BNV across sample D1 recorded in a bias field B bias NV = 172.5...

We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a ‘hands-on’ approach, providing practical details and procedures as derived from literature as well as from the authors’ experience, in order to enable the reader to reproduce the results. Section is devoted to ‘bottom up’ approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section covers ‘top down’ techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers’ and modified Hummers’ methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by ...

At low energy, electrons in doped graphene sheets behave like massless Dirac fermions with a Fermi velocity, which does not depend on carrier density. Here we show that modulating a two-dimensional electron gas with a long-wavelength periodic potential with honeycomb symmetry can lead to the creation of isolated massless Dirac points with tunable Fermi velocity. We provide detailed theoretical estimates to realize such artificial graphenelike system and discuss an experimental realization in a modulation-doped GaAs quantum well. Graphene is a one-atom-thick two-dimensional ͑2D͒ electron system composed of carbon atoms on a honeycomb lattice.1 The lattice has two inequivalent sites in the unit cell that are analogous to the two spin orientations of a spin-1/2 particle. This observation opens the way to an elegant description of electrons in graphene as particles endowed with a pseudospin degree-of-freedom.1 At low energy, electrons in graphene are described by a 2D massless Dirac fermion ͑MDF͒ Hamiltonian, H D = v F · p, where v F is the bare Fermi velocity, which does not depend on carrier density, p is the 2D momentum measured from the corners of the Brillouin zone, and is the pseudospin operator constructed with two Pauli matrices ͕ i , i = x , y͖, which act on the sublattice pseudospin degree-of-freedom. It follows that the energy eigenstates are chiral, i.e., for a given p have pseudospins oriented either parallel ͑conduction band͒ or antiparallel ͑valence band͒ to p. The Dirac-like wave equation and the chirality of its eigenstates have a number of very intriguing implications.1 It would be highly desirable to have other materials with Dirac-like spectrum and a pseudospin degree-offreedom. One candidate is represented by HgTe/Hg͑Cd͒Te quantum wells ͑QWs͒ where MDFs are predicted to arise at a critical QW thickness.2 More recently, Park and Louie 3 proposed that MDFs can arise in any 2D electron gas ͑2DEG͒ if appropriately nanopatterned.Here we present an independent approach to the realization of "artificial graphene" in a nanopatterned 2DEG. We provide theoretical evidence for the occurrence of linearly dispersing energy bands in an artificially engineered honeycomb lattice, and we demonstrate a remarkable dependence of the Fermi velocity on the strength of the external potential in this system. We also define the conditions that the external periodic potential and the electron density must satisfy in order to achieve artificial MDFs. Finally we present the photoluminescence ͑PL͒ of the 2DEG confined in a highmobility modulation-doped GaAs/AlGaAs QW where a nanopatterning with honeycomb symmetry is achieved by dry etching. We believe that the development of patterned 2DEGs with tunable parameters will offer unprecedented opportunities to study fundamental interactions of MDFs in high-mobility semiconductor structures.We start our analysis by considering a 2DEG consisting of electrons with band mass m b = 0.067m ͑m is the bare electron mass in vacuum͒ confined in a GaAs/AlGaAs QW. The 2DEG is subjected to a...