Hexagonal Single Crystal Domains of Few-Layer Graphene on Copper Foils

Hexagonal-shaped single crystal domains of few layer graphene (FLG) are synthesized on copper foils using atmospheric pressure chemical vapor deposition with a high methane flow. Scanning electron microscopy reveals that the graphene domains have a hexagonal shape and are randomly orientated on the copper foil. However, the sites of graphene nucleation exhibit some correlation by forming linear rows. Transmission electron microscopy is used to examine the folded edges of individual domains and reveals they are few-layer graphene consisting of approximately 5−10 layers in the central region and thinning out toward the edges of the domain. Selected area electron diffraction of individual isolated domains reveals they are single crystals with AB Bernal stacking and free from the intrinsic rotational stacking faults that are associated with turbostratic graphite. We study the time-dependent growth dynamics of the domains and show that the final continuous FLG film is polycrystalline, consisting of randomly connected single crystal domains

Motion of Light Adatoms and Molecules on the Surface of Few-Layer Graphene

Low-voltage aberration-corrected transmission electron microscopy (TEM) is applied to investigate the feasibility of continuous electron beam cleaning of graphene and monitor the removal of residual species as present on few-layer graphene (FLG) surfaces. This combined approach allows us to detect light adatoms and evaluate their discontinuous sporadic motional behavior. Furthermore, the formation and dynamic behavior of isolated molecules on the FLG surface can be captured. The preferential source of adatoms and adsorbed molecules appeared to be carbonaceous clusters accumulated from residual solvents on the graphene surface. TEM image simulations provide potential detail on the observed molecular structures. Molecular dynamics simulations confirm the experimentally observed dynamics occurring on the energy scale imposed by the presence of the 80 kV electron beam and help elucidate the underlying mechanisms

Motion of Light Adatoms and Molecules on the Surface of Few-Layer Graphene

Low-voltage aberration-corrected transmission electron microscopy (TEM) is applied to investigate the feasibility of continuous electron beam cleaning of graphene and monitor the removal of residual species as present on few-layer graphene (FLG) surfaces. This combined approach allows us to detect light adatoms and evaluate their discontinuous sporadic motional behavior. Furthermore, the formation and dynamic behavior of isolated molecules on the FLG surface can be captured. The preferential source of adatoms and adsorbed molecules appeared to be carbonaceous clusters accumulated from residual solvents on the graphene surface. TEM image simulations provide potential detail on the observed molecular structures. Molecular dynamics simulations confirm the experimentally observed dynamics occurring on the energy scale imposed by the presence of the 80 kV electron beam and help elucidate the underlying mechanisms

All Chemical Vapor Deposition Growth of MoS₂:h-BN Vertical van der Waals Heterostructures

Vertical van der Waals heterostructures are formed when different 2D crystals are stacked on top of each other. Improved optical properties arise in semiconducting transition metal dichalcogenide (TMD) 2D materials, such as MoS₂, when they are stacked onto the insulating 2D hexagonal boron nitride (h-BN). Most work to date has required mechanical exfoliation of at least one of the TMDs or h-BN materials to form these semiconductor:insulator structures. Here, we report a direct all-CVD process for the fabrication of high-quality monolayer MoS₂:h-BN vertical heterostructured films with isolated MoS₂ domains distributed across 1 cm. This is enabled by the use of few-layer h-BN films that are more robust against decomposition than monolayer h-BN during the MoS₂ growth process. The MoS₂ domains exhibit different growth dynamics on the h-BN surfaces compared to bare SiO₂, confirming that there is strong interaction between the MoS₂ and underlying h-BN. Raman and photoluminescence spectroscopies of CVD-grown MoS₂ are compared to transferred MoS₂ on both types of substrates, and our results show directly grown MoS₂ on h-BN films have smaller lattice strain, lower doping level, cleaner and sharper interfaces, and high-quality interlayer contact

Examining the Edges of Multi-Layer Graphene Sheets

Multi-layer graphene (MLG) sheets were produced from highly ordered pyrolytic graphite by mechanical exfoliation. We use aberration-corrected, low-voltage, high-resolution transmission electron microscopy to study the atomic structure at the edges of the sheets. In general, we find two different types of edges of MLG sheets are produced, either parallel to a zigzag direction or an arm-chair direction. A MLG sheet with edges parallel to the zigzag direction was observed to have small random corrugations and was relatively straight. We observed edges parallel to the arm-chair direction with highly corrugated atomic structure and terracing. We show that the corrugations are due to sawtooth oscillations in the atomic structure with zigzag edge terminations along the general arm-chair direction

Atomic Structure of ABC Rhombohedral Stacked Trilayer Graphene

We distinguish between Bernal and rhombohedral stacked trilayer graphene using aberration-corrected high-resolution transmission electron microscopy. By using a monochromator to reduce chromatic aberration effects, angstrom resolution can be achieved at an accelerating voltage of 80 kV, which enables the atomic structure of ABC rhombohedral trilayer graphene to be unambiguously resolved. Our images of ABC rhombohedral trilayer graphene provide a clear signature for its identification. Few-layer graphene interfaces with ABC:BC:BCAB structure have also been studied, and we have determined the stacking sequence of each graphene layer and consequently the 3D structure. These results confirm that CVD-grown few-layer graphene can adopt an ABC rhombohedral stacking

Ultrathin All-2D Lateral Diodes Using Top and Bottom Contacted Laterally Spaced Graphene Electrodes to WS₂ Semiconductor Monolayers

The ultrathin nature of two-dimensional (2D) materials opens up opportunities for creating devices that are substantially thinner than using traditional bulk materials. In this article, monolayer 2D materials grown by the chemical vapor deposition method are used to fabricate ultrathin all-2D lateral diodes. We show that placing graphene electrodes below and above the WS2 monolayer, instead of the same side, results in a lateral device with two different Schottky barrier heights. Due to the natural dielectric environment, the bottom graphene layer is wedged between the WS2 and the SiO2 substrate, which has a different doping level than the top graphene layer that is in contact with WS2 and air. The lateral separation of these two graphene electrodes results in a lateral metal–semiconductor–metal junction with two asymmetric barriers but yet retains its ultrathin form of two-layer thickness. The rectification and diode behavior can be exploited in transistors, photodiodes, and light-emitting devices. We show that the device exhibits a rectification ratio up to 90 under a laser power of 1.37 μW at a bias voltage of ±3 V. We demonstrate that both the back-gate voltage and laser illumination can tune the rectification behavior of the device. Furthermore, the device can generate strong red electroluminescence in the WS2 area across the two graphene electrodes under an average flowing current of 2.16 × 10–5 A. This work contributes to the current understanding of the 2D metal–semiconductor heterojunction and offers an idea to obtain all-2D Schottky diodes by retaining the ultrathin device concept

Moiré Superlattice Structure of Pleated Trilayer Graphene Imaged by 4D Scanning Transmission Electron Microscopy

Moiré superlattices in graphene arise from rotational twists in stacked 2D layers, leading to specific band structures, charge density and interlayer electron and excitonic interactions. The periodicities in bilayer graphene moiré lattices are given by a simple moiré basis vector that describes periodic oscillations in atomic density. The addition of a third layer to form trilayer graphene generates a moiré lattice comprised of multiple harmonics that do not occur in bilayer systems, leading to nontrivial crystal symmetries. Here, we use atomic resolution 4D-scanning transmission electron microscopy to study atomic structure in bilayer and trilayer graphene moiré superlattices and use 4D-STEM to map the electric fields to show subtle variations in the long-range moiré patterns. We show that monolayer graphene folded into an S-bend graphene pleat produces trilayer moiré superlattices with both small (<2°) and larger twist angles (7–30°). Annular in-plane electric field concentrations are detected in high angle bilayers due to overlapping rotated graphene hexagons in each layer. The presence of a third low angle twisted layer in S-bend trilayer graphene, introduces a long-range modulation of the atomic structure so that no real space unit cell is detected. By directly imaging trilayer moiré harmonics that span from picoscale to nanoscale using 4D-STEM, we gain insights into the complex spatial distributions of atomic density and electric fields in trilayer twisted layered materials