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In-plane heterostructure of hexagonal boron nitride and graphene (h-BN-G) has become a focus of graphene research owing to its tunable bandgap and intriguing properties. We report herein the synthesis of a quasi-freestanding h-BN-G monolayer heterostructure on a weakly coupled Ir(111) substrate, where graphene and h-BN possess distinctly different heights and surface corrugations. An atomically sharp zigzag type boundary has been found to dominate the patching interface between graphene and h-BN, as evidenced by high-resolution Scanning tunneling microscopy investigation as well as density functional theory calculation. Scanning tunneling spectroscopy studies indicate that the graphene and h-BN tend to exhibit their own intrinsic electronic features near the patching boundary. The present work offers a deep insight into the h-BN-graphene boundary structures both geometrically and electronically together with the effect of adlayer-substrate coupling.

Intercalation of metal atoms into the interface of graphene and its supporting substrate has become an intriguing topic for the sake of weakening the interface coupling and constructing metal atomic layers under inert covers. However, this novel behavior has rarely been reported on the analogous hexagonal boron nitride (h-BN) synthesized on metal substrates. Here, we describe a comparative study of Mn intercalation into the interfaces of graphene/Rh(111) and h-BN/Rh(111), by using atomically-resolved scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The intercalation was performed by annealing as-deposited Mn clusters, and the starting temperature of Mn intercalation into h-BN/Rh(111) was found to be ∼80 °C higher than that for graphene/Rh(111). Moreover, the intercalated islands of h-BN/Mn/Rh(111) usually possess more irregular shapes than those of graphene/Mn/Rh(111), as illustrated by temperature-dependent STM observations. All these experimental facts suggest a stronger interaction of Mn with h-BN/Rh(111) than that with graphene/Rh(111). Open image in new window

The atomic layer of hybridized hexagonal boron nitride (h-BN) and graphene has attracted a great deal of attention after the pioneering work of P. M. Ajayan et al. on Cu foils because of their unusual electronic properties (Ci, L. J.; et al. Nat. Mater. 2010, 9, 430-435). However, many fundamental issues are still not clear, including the in-plane atomic continuity as well as the edge type at the boundary of hybridized h-BN and graphene domains. To clarify these issues, we have successfully grown a perfect single-layer h-BN-graphene (BNC) patchwork on a selected Rh(111) substrate, via a two-step patching growth approach. With the ideal sample, we convinced that at the in-plane linking interface, graphene and h-BN can be linked perfectly at an atomic scale. More importantly, we found that zigzag linking edges were preferably formed, as demonstrated by atomic-scale scanning tunneling microscopy images, which was also theoretically verified using density functional theory calculations. We believe the experimental and theoretical works are of particular importance to obtain a fundamental understanding of the BNC hybrid and to establish a deliberate structural control targeting high-performance electronic and spintronic devices.

We report the synthesis of large-scale uniform graphene films on high carbon solubility substrates of Rh foils for the first time using an ambient-pressure chemical vapor deposition method. We find that, by increasing the cooling rate in the growth process, the thickness of graphene can be tuned from multilayer to monolayer, resulting from the different segregation amount of carbon atoms from bulk to surface. The growth feature was characterized with scanning electron microscopy, Raman spectra, transmission electron microscopy, and scanning tunneling microscopy. We also find that bilayer or few-layer graphene prefers to stack deviating from the Bernal stacking geometry, with the formation of versatile moiré patterns. On the basis of these results, we put forward a segregation growth mechanism for graphene growth on Rh foils. Of particular importance, we propose that this randomly stacked few-layer graphene can be a model system for exploring some fantastic physical properties such as van Hove singularities.

Due to strong interactions between epitaxial graphene and SiC(0001) substrates, the overlayer charge density induced by the interface charging effect is much more attenuated than that of exfoliated graphene on SiO2. We report herein a quantitive detection of the charge properties of few-layer graphene by surface potential measurements using electrostatic force microscopy (EFM). A minor difference in surface potential is observed to mediate a sequential assembly of metal-free phthalocyanine (H2Pc) on monolayer, bilayer and trilayer graphenes, as demonstrated by scanning tunneling microscopy (STM). In order to understand this, we further executed density functional theory (DFT) calculations which showed higher adsorption energies for Pc on thinner graphenes. In this case, we attribute the unique growth behavior of Pc to its variable adsorption energies on few-layer graphene, and in turn the layer charge variations from the viewpoint of energy minimizations. This work is expected to provide fundamental data useful for related nanodevice fabrications.

Graphene growth on the same metal substrate with different crystal morphologies, such as single crystalline and polycrystalline, may involve different mechanisms. We deal with this issue by preparing graphene on single crystal Ni(111) and on ∼300 nm thick Ni films on SiO2 using an ambient pressure chemical vapor deposition (APCVD) method, and analyze the different growth behaviors for different growth parameters by atomically-resolved scanning tunneling microscopy (STM) and complementary macroscopic analysis methods. Interestingly, we obtained monolayer graphene on Ni(111), and multilayer graphene on Ni films under the same growth conditions. Based on the experimental results, it is proposed that the graphene growth on Ni(111) is strongly templated by the Ni(111) lattice due to the strong Ni-C interactions, leading to monolayer graphene growth. Multilayer graphene flakes formed on polycrystalline Ni films are usually stacked with deviations from the Bernal stacking type and show small rotations among the carbon layers. Considering the different substrate features, the inevitable grain boundaries on polycrystalline Ni films are considered to serve as the growth fronts for bilayer and even multilayer graphene.

Understanding of the continuity and the microscopic structure of as-grown graphene on Cu foils through the chemical vapor deposition (CVD) method is of fundamental significance for optimizing the growth parameters toward high-quality graphene. Because of the corrugated nature of the Cu foil surface, few experimental efforts on this issue have been made so far. We present here a high-resolution scanning tunneling microscopy (STM) study of CVD graphene directly on Cu foils. Our work indicates that graphene can be grown with a perfect continuity extending over both crystalline and noncrystalline regions, highly suggestive of weak graphene-substrate interactions. Due to thermal expansion mismatch, defect-like wrinkles and ripples tend to evolve either along the boundaries of crystalline terraces or on noncrystalline areas for strain relief. Furthermore, the strain effect arising from the conforming of perfect two-dimensional graphene to the highly corrugated surface of Cu foils is found to induce local bonding configuration change of carbon from sp(2) to sp(3), evidenced by the formation of "three-for-six" lattices.

Microscopic features of graphene segregated on Ni films prior to chemical transfer—including atomic structures of monolayers and bilayers, Moire patterns due to non-AB stacking, as well as wrinkles and ripples caused by strain effects-have been characterized in detail by high-resolution scanning tunneling microscopy (STM). We found that the stacking geometry of the bilayer graphene usually deviates from the traditional Bernal stacking (or so-called AB stacking), resulting in the formation of a variety of Moire patterns. The relative rotations inside the bilayer were then qualitatively deduced from the relationship between Moire patterns and carbon lattices. Moreover, we found that typical defects such as wrinkles and ripples tend to evolve around multi-step boundaries of Ni, thus reflecting strong perturbations from substrate corrugations. These investigations of the morphology and the mechanism of formation of wrinkles and ripples are fundamental topics in graphene research. This work is expected to contribute to the exploration of electronic and transport properties of wrinkles and ripples. Open image in new window

Grain boundaries (GBs) of hexagonal boron nitride (h-BN) grown on Cu(111) were investigated by scanning tunneling microscopy/spectroscopy (STM/STS). The first experimental evidence of the GBs composed of square-octagon pairs (4|8 GBs) was given, together with those containing pentagon-heptagon pairs (5|7 GBs). Two types of GBs were found to exhibit significantly different electronic properties, where the band gap of the 5|7 GB was dramatically decreased as compared with that of the 4|8 GB, consistent with our obtained result from density functional theory (DFT) calculations. Moreover, the present work may provide a possibility of tuning the inert electronic property of h-BN via grain boundary engineering.

G raphene grown on the same substrate but under different growth conditions may evolve diverse characteristics and disparate growth mechanisms. To explore this issue, graphene is prepared on Rh(111) by both ultrahigh vacuum and ambient-pressure chemical vapor deposition methods and the different growth behaviors, the atomicscale structures, and the stacking geometry are analysed, mainly by virtue of scanning tunneling microscope. Interestingly, with ultrahigh vacuum chemical vapor deposition growth at 600 ° C, a template growth of graphene by the Rh(111) lattice is obtained, refl ected with the formation of a uniform graphene moire. In comparison, with the ambient-pressure chemical vapor deposition at 1000 ° C by different quenching processes, monolayer and randomly stacked few-layer polycrystalline graphene is achieved, probably directed by combined surface catalysis and segregation mechanisms. In this case, strong and weak interactions between graphene and Rh substrates are suggested, with the samples prepared under vacuum and ambientpressure conditions, respectively. This work is expected to contribute greatly to the exploration of interactions between graphene and a substrate, as well as the segregation mechanism of graphene growth on polycrystalline transitional metal substrates.

We report the fabrication of a novel epitaxial graphene(EG)/Mn/SiC(0001) sandwiched structure through the intercalation of as-deposited Mn atoms on graphene surfaces, with the aid of scanning tunneling microscope, low energy electron diffraction, and X-ray photoelectron spectroscopy. We found that Mn can intercalate below both sp(3)-hybridized carbon-rich interface layer and monolayer graphene, along with the formation of various embedded Mn islands showing different surface morphologies. The unique trait of the sandwiched system is that the strong interaction between the carbon-rich interface layer and SiC(0001) can be decoupled to some degrees, and contemporaneous, an n-doping effect is observed by mapping the energy band of the system using angle-resolved photoemission spectroscopy. Moreover, what deserves our special attention is that the intercalated islands can only evolve below monolayer graphene when a bilayer coexists, accounting for an intriguing graphene thickness-dependent intercalation effect. In the long run, we believe that the construction of graphene/Mn/SiC(0001) systems offers ideal candidates for exploring some intriguing physical properties such as the magnetic property of two-dimensional transition metal systems.

Taking advantage of ultrahigh electric field generated in electric-double-layer transistors (EDLTs), we investigated spin-orbit interaction (SOI) and its modulation in epitaxial trilayer graphene. It was found in magnetotransport that the dephasing length L(φ) and spin relaxation length L(so) of carriers can be effectively modulated with gate bias. As a direct result, SOI-induced weak antilocalization (WAL), together with a crossover from WAL to weak localization (WL), was observed at near-zero magnetic field. Interestingly, among existing localization models, only the Iordanskii-Lyanda-Geller-Pikus theory can successfully reproduce the obtained magnetoconductance well, serving as evidence for gate tuning of the weak but distinct SOI in graphene. Realization of SOI and its large tunability in the trilayer graphene EDLTs provides us with a possibility to electrically manipulate spin precession in graphene systems without ferromagnetics.

The synthesis of centimeter-scale uniform graphene on Pt foils was accomplished via a traditional ambient pressure chemical vapor deposition (CVD) method. Using scanning electron microscopy (SEM) and Raman spectroscopy, we reveal the macroscopic continuity, the thickness, as well as the defect state of as-grown graphene. Of particular importance is that the Pt foils after CVD growth have multifaceted texture, which allows us to explore the substrate crystallography effect on the growth rate and the continuity of graphene. By virtue of atomically resolved scanning tunneling microscopy (STM), we conclude that graphene grows mainly in registry with the symmetries of Pt(111), Pt(110), and Pt(100) facets, leading to hexagonal lattices and striped superstructures. Nevertheless, the carbon lattices on interweaving facets with different identities are connected seamlessly, which ensure the graphene growth from nanometer to micrometer levels. With these results, another prototype for clarifying the preliminary growth mechanism of the CVD process is demonstrated as an analogue of graphene on Cu foils.