Fe3GaTe2 (FGaT), a two-dimensional (2D) layered ferromagnetic metal, exhibits a high Curie temperature (TC) ~ 360 K along with strong perpendicular magnetic anisotropy (PMA), making it a promising material candidate for next-generation energy-efficient magnetic devices. However, the vast majority of studies on FGaT to date have been limited to millimeter-sized bulk crystals and exfoliated flakes, which are unsuitable for practical applications and integration into device processing. Also, its combination with other 2D materials to form van der Waals heterostructures has only been achieved by flake stacking. Consequently, the controlled large-scale growth of FGaT and related heterostructures remains largely unexplored. In this work, we demonstrate a breakthrough in the high-quality, large-scale growth of epitaxial FGaT thin films on single-crystalline graphene/SiC templates using molecular beam epitaxy. Structural characterization confirms the high crystalline quality of the continuous FGaT/graphene van der Waals heterostructures. Temperature-dependent magnetization and anomalous Hall measurements reveal robust PMA with an enhanced TC well above room temperature, reaching up to 400 K. Furthermore, X-ray absorption and X-ray magnetic circular dichroism spectra provide insight into the spin and orbital magnetic moment contributions, further validating the high TC and robust PMA. These findings are highly significant for the future development of high-performance spintronic devices based on 2D heterostructures, with potential applications in next-generation data storage, logic processing and quantum technologies.

The recent successes of superconducting qubits and the demonstration of quantum supremacy over classical bits herald a new era for information processing. Yet, the field is still in its infancy and there exist viable alternative candidates that can also store quantum information. In this review, we will highlight ideas, attempts, and the experimental progress to address nuclear spins in graphene, a readily available Dirac semimetal that consists of a single layer of carbon atoms. Carbon isotopes with a nuclear spin are rare in natural graphene. However, it is possible to enrich the spin-bearing 13C isotopes to produce large-scale graphene sheets, which constitute the testbed to store, transport, and retrieve spin information, or to engineer nanostructures. Here, the hyperfine interaction between the electron spins and the nuclear spins serves as an experimental control knob and mediator to address nuclear polarization and nuclear spin coherence times through electrical measurements. The exploitation of nuclear spins in graphene is thus an alluring perspective. We will discuss methods to synthesize 13C graphene and show experimental approaches and challenges to exploit the relatively weak hyperfine interaction in two-dimensional 13C graphene devices. The ultimate purpose, i.e., the exploitation of nuclear spins in graphene for information processing, is not within reach, but its potential for future applications merits a revisit of the current state-of-the-art.

Two-dimensional (2D) magnetic materials are promising building blocks for the realization of novel and ultra-compact devices for spintronics. Moreover, combining them with other 2D crystals to create van der Waals (vdW) heterostructures is a very attractive route to realize hybrid 2D systems exhibiting integrated functionalities. In this contribution, we report recent achievements on scalable epitaxial growth of Fe 5-x GeTe 2 on single crystalline graphene (on SiC) and WSe 2 (on Al 2 O 3 ) templates via molecular beam epitaxy (MBE). Structural studies show that epitaxial Fe 5-x GeTe 2 films with good crystalline quality and sharp vdW interfaces to graphene and WSe 2 could be realized. Importantly, magneto-transport measurements and SQUID magnetometry reveal ferromagnetic order persisting above 350 K with a predominant out-of-plane anisotropy (for the case of Fe 5-x GeTe 2 /graphene). These results represent an important advance beyond non-scalable flake exfoliation from bulk crystals, thus marking a crucial step towards the implementation of above room temperature ferromagnetic 2D materials in practical applications.

Van der Waals (vdW) heterostructures combining layered ferromagnets and other 2D crystals are promising building blocks for the realization of ultracompact devices with integrated magnetic, electronic, and optical functionalities. Their implementation in various technologies depends strongly on the development of a bottom-up scalable synthesis approach allowing for realizing highly uniform heterostructures with well-defined interfaces between different 2D-layered materials. It is also required that each material component of the heterostructure remains functional, which ideally includes ferromagnetic order above room temperature for 2D ferromagnets. Here, it is demonstrated that the large-area growth of Fe5-x GeTe2 /graphene heterostructures is achieved by vdW epitaxy of Fe5-x GeTe2 on epitaxial graphene. Structural characterization confirms the realization of a continuous vdW heterostructure film with a sharp interface between Fe5-x GeTe2 and graphene. Magnetic and transport studies reveal that the ferromagnetic order persists well above 300 K with a perpendicular magnetic anisotropy. In addition, epitaxial graphene on SiC(0001) continues to exhibit a high electronic quality. These results represent an important advance beyond nonscalable flake exfoliation and stacking methods, thus marking a crucial step toward the implementation of ferromagnetic 2D materials in practical applications.

Two-dimensional (2D) material research is rapidly evolving to broaden the spectrum of emergent 2D systems. Here, we review recent advances in the theory, synthesis, characterization, device, and quantum physics of 2D materials and their heterostructures. First, we shed insight into modeling of defects and intercalants, focusing on their formation pathways and strategic functionalities. We also review machine learning for synthesis and sensing applications of 2D materials. In addition, we highlight important development in the synthesis, processing, and characterization of various 2D materials (e.g., MXnenes, magnetic compounds, epitaxial layers, low-symmetry crystals, etc.) and discuss oxidation and strain gradient engineering in 2D materials. Next, we discuss the optical and phonon properties of 2D materials controlled by material inhomogeneity and give examples of multidimensional imaging and biosensing equipped with machine learning analysis based on 2D platforms. We then provide updates on mix-dimensional heterostructures using 2D building blocks for next-generation logic/memory devices and the quantum anomalous Hall devices of high-quality magnetic topological insulators, followed by advances in small twist-angle homojunctions and their exciting quantum transport. Finally, we provide the perspectives and future work on several topics mentioned in this review.

Heterostructures based on the stacking of two-dimensional materials with different electronic properties have been the subject of several studies addressing the development of novel devices with multiple functionalities. Graphene (G)/hexagonal boron nitride (h-BN) van der Waals (vdW) systems have been extensively investigated, including studies on the large-scale synthesis of h-BN on graphene/SiC(0001) substrate. This work is a theo-retical study using first-principles calculations of bilayer (G/h-BN) and trilayer (G/h-BN/G) heterostructures on SiC(0001) covered by a carbon buffer layer (SiC). The results show an energetic preference for h-BN encapsulation below a single layer of graphene on SiC, viz.: G/h-BN/SiC in the case of bilayer systems and G/h-BN/G/SiC in the case of trilayer systems. Indeed, the electronic structure calculations reveal the preservation of graphene linear energy band dispersion in the bilayer systems. The trilayer systems' electronic structure depends on the stacking order, with the emergence of parabolic bands in energetically less stable systems. Structural characterizations of these bilayers and trilayers on SiC were carried out based on simulations of C-1s core-level-shift (CLS) and carbon K-edge X-ray absorption near edge spectroscopy (XANES) with the goal of assisting future experimental spectroscopy in these graphene/h-BN vdW systems.

Binary alpha-Ga2O3 and ternary alpha-(AlxGa1-x)(2)O-3 thin films with x = 0.29 and 0.54 were deposited on a-plane Al2O3 substrates via plasma-assisted molecular beam epitaxy. The strain relaxation dynamics along three orthogonal crystal directions was investigated ex situ and in situ by synchrotron-based high-resolution x-ray diffraction. A pronounced in-plane anisotropy was observed as strain is preferably built up across [ 1 over bar 1.0] whereas along [00.1] it is effectively relieved. The particular behavior depends on the Al content with almost immediate plastic relaxation for x = 0 and 0.29. For x = 0.54, strain relaxation is elastic within the first monolayers. A dislocation glide on the r-plane slip system has been identified as the dominating plastic relaxation mechanism as layers grow thicker.

Using first‐principles and analytical potential atomistic simulations, production of defects in epitaxial graphene (EG) on SiC upon ion irradiation for ion types and energies accessible in helium‐ion microscope is studied. Graphene‐SiC systems consisting of the buffer (zero) graphene layer and SiC substrate, as well as one (monolayer) and two (bilayer) additional graphene layers, are focused on. The probabilities for single, double, and more complex vacancies to appear upon impacts of energetic ions in each graphene layer as functions of He‐ and Ne‐ion energies are calculated and the data are compared with those obtained for free‐standing graphene. The results indicate that the role of the substrate is minimal for He‐ion irradiation with energies above 5 keV, which can be associated with a low sputtering yield from this system upon ion irradiation, as compared with the common Si/SiO2 substrate. In contrast, SiC substrate has a significant effect on defect production upon Ne‐ion irradiation. The results can serve as a guide to the experiments on ion irradiation of EG to choose the optimum ion beam parameters for defect‐mediated engineering of such systems, for example, for creating nucleation centers to grow other 2D materials, such as h‐BN, on top of the irradiated EG.

Stacked hetero-structures of two-dimensional materials allow for a design of interactions with corresponding electronic and mechanical properties. We report structure, work function, and frictional properties of 1 to 4 layers of MoS2 grown by chemical vapor deposition on epitaxial graphene on SiC(0001). Experiments were performed by atomic force microscopy in ultra-high vacuum. Friction is dominated by adhesion which is mediated by a deformation of the layers to adapt the shape of the tip apex. Friction decreases with increasing number of MoS2 layers as the bending rigidity leads to less deformation. The dependence of friction on applied load and bias voltage can be attributed to variations in the atomic potential corrugation of the interface, which is enhanced by both load and applied bias. Minimal friction is obtained when work function differences are compensated.

Scalable fabrication of magnetic 2D materials and heterostructures constitutes a crucial step for scaling down current spintronic devices and the development of novel spintronic applications. With this prospect, we have recently succeeded in growing high quality and large area Fe3GeTe2 (F3GT) and Fe5-xGeTe2 (F5GT) epitaxial films via molecular beam epitaxy (MBE) on graphene/SiC(0001) templates. This 2D magnet is extremely promising for such applications, given its highly tunable properties and large potential for room temperature ferromagnetism (FM). Morphological and structural characterization of the films confirmed the realization of large-area, continuous layers with stable interfaces with graphene, and good crystalline quality. Magneto-transport and x-ray magnetic circular dichroism investigations confirmed a robust out-of-plane FM in the layers of F3GT with a Curie temperature of around 220 K. More interestingly, our most recent XMCD findings suggest that F5GT layers exhibit FM up to around 320 K. These results are comparable to state-of-the-art exfoliated flakes from bulk crystals and are highly relevant for further research on wafer-scale growth of van der Waals heterostructures combining FGT with other layered crystals such as transition metal dichalcogenides for the realization of multifunctional, atomically thin devices.

The hyperfine interaction between the spins of electrons and nuclei is both a blessing and a curse. It can provide a wealth of information when used as an experimental probing technique but it can also be destructive when it acts as a dephasive perturbation on the electronic system. In this paper, we fabricated large-scale single and multilayer isotopically purified 13C graphene Hall bars to search for interaction effects between the nuclear magnetic moments and the electronic system. We find signatures of nuclei with a spin in the analysis of the weak localization phenomenon that shows a significant dichotomy in the scattering times of monolayer 12C and 13C graphene close to the Dirac point. Microwave-induced electron spin flips were exploited to transfer momentum to the nuclei and build-up a nuclear field. The presence of a very weak nuclear field is encoded in a modulation of the electron Zeeman energy which shifts the energy for resonant absorption and reduces the g factor.

The introduction of atoms of different chemical species between epitaxial graphene and the SiC substrate by means of an intercalation process has been a reliable route to modify the interaction of this 2D material and its underlying substrate. Distinct atomic species have been intercalated so far and studies focusing on bond states of the electronically active layers were successful, retrieving the local chemical environment. However, the structure of interfacial layers is strongly affected whenever a more reactive atom is used in the intercalation process. In this work, we present experimental evidence based on X-ray crystal truncation rod scattering and photoelectron diffraction showing the coexistence of crystalline and amorphous regions in the oxidized interface. This interface between a bilayer graphene and the SiC substrate is generated by O-intercalation. Such fluctuations of the local structure are crucial to understand the abundant existence of silicon oxycarbides structures (SiOXCY) within the interfacial layer, which has been associated with the limited electronic properties of O-intercalated graphene bilayers. Consequently, this may be considered as a deterministic factor that affects device potentialities in these systems.

Van der Waals heterostructures have captured scientific attention due to their unique properties obtained through the stacking of 2D crystals. Large-area synthesis of such structures is crucial to implement them in future applications. In this work, we investigated the effect of the substrate on the structural properties of MoS2 grown by chemical vapor deposition on epitaxial graphene (EG)/SiC(0 0 0 1) and SiO2/Si(1 0 0). We also investigated the effect of each substrate on the thermal stability of the resulting MoS2 in face of hexagonal boron nitride (h-BN) growth by MBE on its surface. Structural characterization of samples was performed by atomic force microscopy and Raman spectroscopy. MoS2 islands grown on EG offered improved morphology in comparison to those formed on amorphous SiO2. Synchrotron-based grazing-incidence X-ray diffraction has confirmed epitaxial growth of MoS2 on EG at a relatively low temperature (of 600 degrees C). X-ray photoelectron spectroscopy data have revealed that MoS2/EG remains stable during h-BN growth, while the use of SiO2 as substrate led to its complete degradation. These results elucidate the effect of different substrates on the growth behavior and thermal stability of MoS2 and illustrate the great potential of EG as a template for scalable fabrication of high-quality van der Waals heterostructures.

We present magneto-photoluminescence measurements in a hybrid two-dimensional semiconductor/ferromagnetic structure consisting of MoSe2/hBN/Ni. When the nickel layer is magnetized, we observe circularly polarized photoluminescence of the trion peak in the MoSe2 monolayer under linearly polarized excitation. This build-up of circular polarization can reach a measured value of about 4% when the magnetization of Ni is saturated perpendicularly to the sample plane and changes its sign when the magnetization is reversed. The circular polarization decreases when the hBN barrier thickness increases. These results are interpreted in terms of a spin-dependent charge transfer between the MoSe2 monolayer and the nickel film. The build-up of circular polarization is observed up to 120 K, mainly limited by the trion emission that vanishes with temperature.

As different low-dimensional materials are sought to be incorporated into microelectronic devices, graphene integration is dependent on the development of band gap opening strategies. Amidst the different methods currently investigated, application of strain and use of electronic quantum confinement have shown promising results. In the present work, epitaxial graphene nanoribbons (GNR), formed by surface graphitization of SiC (0001) on crystalline step edges, were submitted to photochemical chlorination. The incorporation of Cl into the buffer layer underlying graphene increased the compressive uniaxial strain in the ribbons. Such method is a promising tool for tuning the band gap of GNRs.

Scalable fabrication of magnetic 2D materials and heterostructures constitutes a crucial step for scaling down current spintronic devices and the development of novel spintronic applications. Here, we report on van der Waals (vdW) epitaxy of the layered magnetic metal Fe3GeTe2 (FGT)-a 2D crystal with highly tunable properties and a high prospect for room temperature ferromagnetism (FM)-directly on graphene by employing molecular beam epitaxy. Morphological and structural characterization confirmed the realization of large-area, continuous FGT/graphene heterostructure films with stable interfaces and good crystalline quality. Furthermore, magneto-transport and x-ray magnetic circular dichroism investigations confirmed a robust out-of-plane FM in the layers, comparable to state-of-the-art exfoliated flakes from bulk crystals. These results are highly relevant for further research on wafer-scale growth of vdW heterostructures combining FGT with other layered crystals such as transition metal dichalcogenides for the realization of multifunctional, atomically thin devices.

Graphene nanoribbons (GNRs) are unique structures with interesting optical and electronic properties which have great potential of application in fields such as optoelectronics and nanoelectronics. Synthesis and characterization techniques of GNRs are quite important for the development of such technologies. In the case of the structural characterization, it is essential to develop quick and non-destructive approaches in order to determine properties such as thickness and uniformity of GNRs. In the present work we report the structural characterization of GNRs grown on stepped SiC (0001) surfaces by using a differential reflectance contrast (DRC) technique based on a near-field scanning optical microscope (NSOM). With this approach, it is possible to assess GNRs widths with dimension as small as 60 nm with a thickness of one or two graphene monolayers. Our results show that the DRC technique is powerful to analyze the morphology of GNRs grown on SiC (0001) substrates which is a promising wafer-scale plattform for the development of graphene-based nanoelectronics.

All-spin logic devices are based on the lateral spin valve (LSV) structure, which consists of two (or more) laterally separated ferromagnetic electrodes bridged by a nonmagnetic channel. The basic operation is switching of the bistable nanomagnets between their stable states representing binary data if enough torque is exerted on them [1]. The main building blocks of LSV devices are: spin injectors/detectors (ferromagnetic electrodes) and a spin transport channel. The suitable spin transport channel should allow for long spin lifetime and long distance spin propagation. The experimental studies of spin transport measurements identified graphene as the most favourable material for spin transport channel in spin-logic devices [2]. The key feature of the spin injectors/detectors is their electrons spins polarization at the Fermi energy. Hence, the ideal candidates for spin injection/detection are half-metallic ferromagnets which exhibit 100% spin polarization of conduction electrons. Examples of these are some Heusler alloys: NiMnSb, Co2FeSi, Co2MnSi, among others [3]. However, the growth of the Heusler alloys on the graphene was not studied before. Graphene properties depend strongly on the underlying material, number of layers, etc. Furthermore, the surface state affects adsorption, surface migration and aggregation of deposited atoms. Herein, we present the influence of the surface potential of the graphene substrate on growth properties of Co2FeSi Heusler alloys thin films deposited by molecular beam epitaxy. References: [1] B. Behin-Aein, D. Datta, S. Salahuddin and S, Datta, Proposal for an all-spin logic device with built-in memory, Nat. Nanotechnol. 5 266 (2010) [2] W. Han, R.K. Kawakami, M. Gmitra and J. Fabian, Graphene spintronics, Nat. Nanotechnol. 9 794 (2014) [3] C. Felser, G.H. Fecher, Spintronics: From Materials to Devices, Springer (2013)

The combination of two-dimensional (2D) materials into heterostructures enables the formation of atomically thin devices with designed properties. To achieve a high-density, bottom-up integration, the growth of these 2D heterostructures via van der Waals epitaxy (vdWE) is an attractive alternative to the currently mostly employed mechanical transfer, which is problematic in terms of scaling and reproducibility. Controlling the location of the nuclei formation remains a key challenge in vdWE. Here, a focused He ion beam is used to deterministically place defects in graphene substrates, which serve as preferential nucleation sites for the growth of insulating, 2D hexagonal boron nitride (h-BN). Therewith a mask-free, selective-area vdWE (SAvdWE) is demonstrated, in which nucleation yield and crystal quality of h-BN are controlled by the ion beam parameters used for defect formation. Moreover, h-BN grown via SAvdWE is shown to exhibit electron tunneling characteristics comparable to those of mechanically transferred layers, thereby lying the foundation for a reliable, high-density array fabrication of 2D heterostructures for device integration via defect engineering in 2D substrates.