Monolayer-to-Mesoscale Modulation of the Optical Properties in 2D CrI_{3} Mapped by Hyperspectral Microscopy.

Magnetic 2D materials hold promise to change the miniaturization paradigm of unidirectional photonic components. However, the integration of these materials in devices hinges on the accurate determination of the optical properties down to the monolayer limit, which is still missing. By using hyperspectral wide-field imaging at room temperature, we reveal a nonmonotonic thickness dependence of the complex optical dielectric function in the archetypal magnetic 2D material CrI_{3} extending across different length scales: onsetting at the mesoscale, peaking at the nanoscale, and decreasing again down to the single layer. These results portray a modification of the electronic properties of the material and align with the layer-dependent magnetism in CrI_{3}, shedding light on the long-standing structural conundrum in this material. The unique modulation of the complex dielectric function from the monolayer up to more than 100 layers will be instrumental for understanding mesoscopic effects in layered materials and tuning light-matter interactions in magnetic 2D materials.

Strain Switching in van der Waals Heterostructures Triggered by a Spin-Crossover Metal-Organic Framework.

Van der Waals heterostructures (vdWHs) provide the possibility of engineering new materials with emergent functionalities that are not accessible in another way. These heterostructures are formed by assembling layers of different materials used as building blocks. Beyond inorganic 2D crystals, layered molecular materials remain still rather unexplored, with only few examples regarding their isolation as atomically thin layers. Here, the family of van der Waals heterostructures is enlarged by introducing a molecular building block able to produce strain: the so-called spin-crossover (SCO). In these metal-organic materials, a spin transition can be induced by applying external stimuli like light, temperature, pressure, or an electric field. In particular, smart vdWHs are prepared in which the electronic and optical properties of the 2D material (graphene and WSe2 ) are clearly switched by the strain concomitant to the spin transition. These molecular/inorganic vdWHs represent the deterministic incorporation of bistable molecular layers with other 2D crystals of interest in the emergent fields of straintronics and band engineering in low-dimensional materials.

Coexistence of structural and magnetic phases in van der Waals magnet CrI3.

CrI3 has raised as an important system to the emergent field of two-dimensional van der Waals magnetic materials. However, it is still unclear why CrI3 which has a ferromagnetic rhombohedral structure in bulk, changed to anti-ferromagnetic monoclinic at thin layers. Here we show that this behaviour is due to the coexistence of both monoclinic and rhombohedral crystal phases followed by three magnetic transitions at TC1 = 61 K, TC2 = 50 K and TC3 = 25 K. Each transition corresponds to a certain fraction of the magnetically ordered volume as well as monoclinic and rhombohedral proportion. The different phases are continuously accessed as a function of the temperature over a broad range of magnitudes. Our findings suggest that the challenge of understanding the magnetic properties of thin layers CrI3 is in general a coexisting structural-phase problem mediated by the volume-wise competition between magnetic phases already present in bulk.

Layer-Dependent Mechanical Properties and Enhanced Plasticity in the Van der Waals Chromium Trihalide Magnets.

The mechanical properties of magnetic materials are instrumental for the development of magnetoelastic theories and the optimization of strain-modulated magnetic devices. In particular, two-dimensional (2D) magnets hold promise to enlarge these concepts into the realm of low-dimensional physics and ultrathin devices. However, no experimental study on the intrinsic mechanical properties of the archetypal 2D magnet family of the chromium trihalides has thus far been performed. Here, we report the room temperature layer-dependent mechanical properties of atomically thin CrCl3 and CrI3, finding that the bilayers have Young's moduli of 62.1 and 43.4 GPa, highest sustained strains of 6.49% and 6.09% and breaking strengths of 3.6 and 2.2 GPa, respectively. This portrays the outstanding plasticity of these materials that is qualitatively demonstrated in the bulk crystals. The current study will contribute to the applications of the 2D magnets in magnetostrictive and flexible devices.

Quantum Rescaling, Domain Metastability, and Hybrid Domain-Walls in 2D CrI3 Magnets.

Higher-order exchange interactions and quantum effects are widely known to play an important role in describing the properties of low-dimensional magnetic compounds. Here, the recently discovered 2D van der Waals (vdW) CrI3 is identified as a quantum non-Heisenberg material with properties far beyond an Ising magnet as initially assumed. It is found that biquadratic exchange interactions are essential to quantitatively describe the magnetism of CrI3 but quantum rescaling corrections are required to reproduce its thermal properties. The quantum nature of the heat bath represented by discrete electron-spin and phonon-spin scattering processes induces the formation of spin fluctuations in the low-temperature regime. These fluctuations induce the formation of metastable magnetic domains evolving into a single macroscopic magnetization or even a monodomain over surface areas of a few micrometers. Such domains display hybrid characteristics of Néel and Bloch types with a narrow domain wall width in the range of 3-5 nm. Similar behavior is expected for the majority of 2D vdW magnets where higher-order exchange interactions are appreciable.

Deep-Learning-Enabled Fast Optical Identification and Characterization of 2D Materials.

Advanced microscopy and/or spectroscopy tools play indispensable roles in nanoscience and nanotechnology research, as they provide rich information about material processes and properties. However, the interpretation of imaging data heavily relies on the "intuition" of experienced researchers. As a result, many of the deep graphical features obtained through these tools are often unused because of difficulties in processing the data and finding the correlations. Such challenges can be well addressed by deep learning. In this work, the optical characterization of 2D materials is used as a case study, and a neural-network-based algorithm is demonstrated for the material and thickness identification of 2D materials with high prediction accuracy and real-time processing capability. Further analysis shows that the trained network can extract deep graphical features such as contrast, color, edges, shapes, flake sizes, and their distributions, based on which an ensemble approach is developed to predict the most relevant physical properties of 2D materials. Finally, a transfer learning technique is applied to adapt the pretrained network to other optical identification applications. This artificial-intelligence-based material characterization approach is a powerful tool that would speed up the preparation, initial characterization of 2D materials and other nanomaterials, and potentially accelerate new material discoveries.

Electrical control of 2D magnetism in bilayer CrI3.

Controlling magnetism via electric fields addresses fundamental questions of magnetic phenomena and phase transitions1-3, and enables the development of electrically coupled spintronic devices, such as voltage-controlled magnetic memories with low operation energy4-6. Previous studies on dilute magnetic semiconductors such as (Ga,Mn)As and (In,Mn)Sb have demonstrated large modulations of the Curie temperatures and coercive fields by altering the magnetic anisotropy and exchange interaction2,4,7-9. Owing to their unique magnetic properties10-14, the recently reported two-dimensional magnets provide a new system for studying these features15-19. For instance, a bilayer of chromium triiodide (CrI3) behaves as a layered antiferromagnet with a magnetic field-driven metamagnetic transition15,16. Here, we demonstrate electrostatic gate control of magnetism in CrI3 bilayers, probed by magneto-optical Kerr effect (MOKE) microscopy. At fixed magnetic fields near the metamagnetic transition, we realize voltage-controlled switching between antiferromagnetic and ferromagnetic states. At zero magnetic field, we demonstrate a time-reversal pair of layered antiferromagnetic states that exhibit spin-layer locking, leading to a linear dependence of their MOKE signals on gate voltage with opposite slopes. Our results allow for the exploration of new magnetoelectric phenomena and van der Waals spintronics based on 2D materials.

Recent progress in the assembly of nanodevices and van der Waals heterostructures by deterministic placement of 2D materials.

Designer heterostructures can now be assembled layer-by-layer with unmatched precision thanks to the recently developed deterministic placement methods to transfer two-dimensional (2D) materials. This possibility constitutes the birth of a very active research field on the so-called van der Waals heterostructures. Moreover, these deterministic placement methods also open the door to fabricate complex devices, which would be otherwise very difficult to achieve by conventional bottom-up nanofabrication approaches, and to fabricate fully-encapsulated devices with exquisite electronic properties. The integration of 2D materials with existing technologies such as photonic and superconducting waveguides and fiber optics is another exciting possibility. Here, we review the state-of-the-art of the deterministic placement methods, describing and comparing the different alternative methods available in the literature, and we illustrate their potential to fabricate van der Waals heterostructures, to integrate 2D materials into complex devices and to fabricate artificial bilayer structures where the layers present a user-defined rotational twisting angle.

A MoTe2-based light-emitting diode and photodetector for silicon photonic integrated circuits.

One of the current challenges in photonics is developing high-speed, power-efficient, chip-integrated optical communications devices to address the interconnects bottleneck in high-speed computing systems. Silicon photonics has emerged as a leading architecture, in part because of the promise that many components, such as waveguides, couplers, interferometers and modulators, could be directly integrated on silicon-based processors. However, light sources and photodetectors present ongoing challenges. Common approaches for light sources include one or few off-chip or wafer-bonded lasers based on III-V materials, but recent system architecture studies show advantages for the use of many directly modulated light sources positioned at the transmitter location. The most advanced photodetectors in the silicon photonic process are based on germanium, but this requires additional germanium growth, which increases the system cost. The emerging two-dimensional transition-metal dichalcogenides (TMDs) offer a path for optical interconnect components that can be integrated with silicon photonics and complementary metal-oxide-semiconductors (CMOS) processing by back-end-of-the-line steps. Here, we demonstrate a silicon waveguide-integrated light source and photodetector based on a p-n junction of bilayer MoTe2, a TMD semiconductor with an infrared bandgap. This state-of-the-art fabrication technology provides new opportunities for integrated optoelectronic systems.

Monolayer Tungsten Disulfide (WS2 ) via Chlorine-Driven Chemical Vapor Transport.

Large-scale production of high-quality tungsten disulfide (WS2 ) monolayers is a prerequisite for potential device applications using this promising transition metal dichalcogenide semiconductor. The most researched technique is chemical vapor deposition, typically involving the reaction of sulfur vapors with tungsten oxide. Other techniques such as physical vapor deposition have been explored with some success, but low vapor pressures make growth difficult. This study demonstrates a growth process that relies on halide-driven vapor transport commonly utilized in bulk crystal growth. Using a small amount of sodium chloride salt as a source of chlorine, nonvolatile WS2 can react to form gaseous tungsten chloride and sulfur. With an open tube system, a controlled reaction generates mono and few-layer WS2 crystals. Optical and physical characterization of the monolayer material shows good uniformity and triangular domains over 50 µm in length. Photoluminescence transient measurements show similar nonlinear exciton dynamics as exfoliated flakes, attributed to multiparticle physics. Requiring only the powder of the desired crystal and appropriate halide salt as precursors, the technique has the potential to realize other layered materials that are challenging to grow with current processes.

Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit.

Since the discovery of graphene, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin-valley coupling, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-density waves, and topological semimetals with edge transport. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered; such a crystal would be useful in many technologies from sensing to data storage. Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin-Wagner theorem. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect, whereas in trilayer CrI3 the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics, and van der Waals engineering to produce interface phenomena.

Enhanced superconductivity in atomically thin TaS2.

The ability to exfoliate layered materials down to the single layer limit has presented the opportunity to understand how a gradual reduction in dimensionality affects the properties of bulk materials. Here we use this top-down approach to address the problem of superconductivity in the two-dimensional limit. The transport properties of electronic devices based on 2H tantalum disulfide flakes of different thicknesses are presented. We observe that superconductivity persists down to the thinnest layer investigated (3.5 nm), and interestingly, we find a pronounced enhancement in the critical temperature from 0.5 to 2.2 K as the layers are thinned down. In addition, we propose a tight-binding model, which allows us to attribute this phenomenon to an enhancement of the effective electron-phonon coupling constant. This work provides evidence that reducing the dimensionality can strengthen superconductivity as opposed to the weakening effect that has been reported in other 2D materials so far.

Hybrid magnetic superconductors formed by TaS2 layers and spin crossover complexes.

The restacking of charged TaS2 nanosheets with molecular counterparts has so far allowed for the combination of superconductivity with a manifold of other molecule-intrinsic properties. Yet, a hybrid compound that blends superconductivity with spin crossover switching has still not been reported. Here we continue to exploit the solid-state/molecule-based hybrid approach for the synthesis of a layered TaS2-based material that hosts Fe(2+) complexes with a spin switching behavior. The chemical design and synthetic aspects of the exfoliation/restacking approach are discussed, highlighting how the material can be conveniently obtained in the form of highly oriented easy-to-handle flakes. Finally, proof of the presence of both phenomena is provided by the use of a variety of physical characterization techniques. The likely sensitivity of the intercalated Fe(2+) complexes to external stimuli such as light opens the door for the study of synergistic effects between the superconductivity and the spin crossover switching at low temperatures.

Coexistence of superconductivity and magnetism by chemical design.

Although the coexistence of superconductivity and ferromagnetism in one compound is rare, some examples of such materials are known to exist. Methods to physically prepare hybrid structures with both competing phases are also known, which rely on the nanofabrication of alternating conducting layers. Chemical methods of building up hybrid materials with organic molecules (superconducting layers) and metal complexes (magnetic layers) have provided examples of superconductivity with some magnetic properties, but not fully ordered. Now, we report a chemical design strategy that uses the self assembly in solution of macromolecular nanosheet building blocks to engineer the coexistence of superconductivity and magnetism in [Ni(0.66)Al(0.33)(OH)(2)][TaS(2)] at ∼4 K. The method is further demonstrated in the isostructural [Ni(0.66)Fe(0.33)(OH)(2)][TaS(2)], in which the magnetic ordering is shifted from 4 K to 16 K.

Synthesis and characterisation of 3,4-dialkoxy-substituted benzo-1,3,2-dithiazolyl radicals.

The conversion of 3,4-dialkoxy-benzenes into dialkoxy-benzo-1,3,2-dithiazolyls is described and representative examples (1 and 2) derived from benzodioxole and veratrol, respectively, are reported. Whilst 1 is a dimeric pi*-pi* dimer, the dimethoxy derivative 2 is monomeric in the solid state and exhibits antiferromagnetic interactions.

Confined growth of cyanide-based magnets in two dimensions.

Herein we report the first hybrid magnetic material resulting from the intercalation of a cyanide-based molecular magnet into a solid-state layered host. More specifically, the use of a diamagnetic cationic Zn(II)-Al(lII) layered double hydroxide host allows for the formation of an anionic two-dimensional ferromagnetic Ni(II)-Cr(III) Prussian Blue analogue, from the templated assembly of its ionic molecular components in the confined interlamellar space offered by the inorganic host.