Observation of critical magnetic behavior in 2D carbon based composites

Two dimensional (2D) carbonaceous materials such as graphene and its derivatives, e.g., graphdiyne, have enormous potential possibilities in major fields of scientific research. Theoretically, it has been proposed that the perfect atomic lattice arrangement of these materials is responsible for their outstanding physical and chemical properties, and also for their poor magnetic properties. Experimentally, it is difficult to obtain a perfect atomic lattice of carbon atoms due to the appearance of structural disorder. This structural disorder is generated during the growth or synthesis of carbon-related materials. Investigations of structural disorder reveal that it can offer both advantages and disadvantages depending on the application. For instance, disorder reduces the thermal and mechanical stability, and deteriorates the performance of 2D carbon-based electronic devices. The most interesting effect of structural disorder can be seen in the field of magnetism. Disorder not only creates magnetic ordering within 2D carbon materials but also influences the local electronic structure, which opens the door for future spintronic devices. Although various studies on the disorder induced magnetism of 2D carbon materials are available in the literature, some parts of the above field have still not been fully exploited. This review presents existing work for the future development of 2D carbon-based devices.

High Photoresponsivity in Ultrathin 2D Lateral Graphene:WS2:Graphene Photodetectors Using Direct CVD Growth

We show that reducing the degree of van der Waals overlapping in all 2D ultrathin lateral devices composed of graphene:WS₂:graphene leads to significant increase in photodetector responsivity. This is achieved by directly growing WS₂ using chemical vapor deposition (CVD) in prepatterned graphene gaps to create epitaxial interfaces. Direct-CVD-grown graphene:WS₂:graphene lateral photodetecting transistors exhibit high photoresponsivities reaching 121 A/W under 2.7 × 10⁵ mW/cm² 532 nm illumination, which is around 2 orders of magnitude higher than similar devices made by the layer-by-layer transfer method. The photoresponsivity of our direct-CVD-grown device shows negative correlation with illumination power under different gate voltages, which is different from similar devices made by the transfer method. We show that the high photoresponsivity is due to the lowering of effective Schottky barrier height by improving the contact between graphene and WS₂. Furthermore, the direct CVD growth reduces overlapping sections of WS₂:Gr and leads to more uniform lateral systems. This approach provides insights into scalable manufacturing of high-quality 2D lateral electronic and optoelectronic devices.

High-Temperature Magnetism in Graphene Induced by Proximity to EuO

Addition of magnetism to spectacular properties of graphene may lead to novel topological states and design of spin logic devices enjoying low power consumption. A significant progress is made in defect-induced magnetism in graphene-selective elimination of p _z orbitals (by vacancies or adatoms) at triangular sublattices tailors graphene magnetism. Proximity to a magnetic insulator is a less invasive way, which is being actively explored now. Integration of graphene with the ferromagnetic semiconductor EuO has much to offer, especially in terms of proximity-induced spin-orbit interactions. Here, we synthesize films of EuO on graphene using reactive molecular beam epitaxy. Their quality is attested by electron and X-ray diffraction, cross-sectional electron microscopy, and Raman and magnetization measurements. Studies of electron transport reveal a magnetic transition at T_C^* ≈ 220 K, well above the Curie temperature 69 K of EuO. Up to T_C^*, the dependence R _xy( B) is strongly nonlinear, suggesting the presence of the anomalous Hall effect. The role of synthesis conditions is highlighted by studies of an overdoped structure. The results justify the use of the EuO/graphene system in spintronics.

Hydrogen-Assisted Growth of Large-Area Continuous Films of MoS2 on Monolayer Graphene

We show how control over the chemical vapor deposition (CVD) reaction chemistry of molybdenum disulfide (MoS₂) by hydrogen addition can enable the direct growth of centimeter-scale continuous films of vertically stacked MoS₂ monolayer on graphene under atmospheric pressure conditions. Hydrogen addition enables longer CVD growth times at high temperature by reducing oxidation effects that would otherwise degrade the monolayer graphene. By careful control of nucleation density and growth time, high-quality monolayer MoS₂ films could be formed on graphene, realizing all CVD-grown vertically stacked monolayer semimetal/semiconducting interfaces. Photoluminescence spectroscopy shows quenching of MoS₂ by the underlying graphene, indicating a good interfacial charge transfer. We utilize the MoS₂/graphene vertical stacks as photodetectors, with photoresponsivities reaching 2.4 A/W under 135 μW 532 nm illumination. This approach provides insights into the scalable manufacturing of high-quality two-dimensional electronic and optoelectronic devices.

Effect of defects controlled by preparation condition and heat treatment on the ferromagnetic properties of few-layer graphene

Magnetism in graphene has stimulated extensive studies to search for novel metal-free magnetic device. In this paper, we use a synthesis method far from equilibrium state named self-propagating high temperature synthesis (SHS) to produce few-layer graphene with different defect contents and then use a heat treatment process (vacuum-annealing and air-cooling) to further control the defects in graphene. We find that the type and content of defects in graphene can be controlled by adjusting the mole ratio of reactants (Mg: CaCO₃) for SHS reaction and the temperature of the subsequent heat treatment. The deviation of the ratio of reactants from stoichiometric ratio benefits the production of graphene with higher concentration of defects. It is indicated that the temperature of the heat treatment has remarkable influences on the structure of graphene, Raman-sensitive defects can be recovered partly by heat treatment while IR-sensitive defects are closely related with the oxidation and decomposition of the oxygen-containing groups at elevated temperature. This work indicates that SHS is a promising method to produce graphene with special magnetism, and the heat treatment is an effective way to further adjust the magnetism of graphene. This work sheds light on the study to develop carbon materials with controlled ferromagnetism.