Single-crystalline Monolayer Graphene Research Articles

Molecular dynamics simulation of graphene sinking during chemical vapor deposition growth on semi-molten Cu substrate

Copper foil is the most promising catalyst for the synthesis of large-area, high-quality monolayer graphene. Experimentally, it has been found that the Cu substrate is semi-molten at graphene growth temperatures. In this study, based on a self-developed C–Cu empirical potential and density functional theory (DFT) methods, we performed systematic molecular dynamics simulations to explore the stability of graphene nanostructures, i.e., carbon nanoclusters and graphene nanoribbons, on semi-molten Cu substrates. Many atomic details observed in the classical MD simulations agree well with those seen in DFT-MD simulations, confirming the high accuracy of the C–Cu potential. Depending on the size of the graphene island, two different sunken-modes are observed: (i) graphene island sinks into the first layer of the metal substrate and (ii) many metal atoms surround the graphene island. Further study reveals that the sinking graphene leads to the unidirectional alignment and seamless stitching of the graphene islands, which explains the growth of large single-crystal graphene on Cu foil. This study deepens our physical insights into the CVD growth of graphene on semi-molten Cu substrate with multiple experimental mysteries well explained and provides theoretic references for the controlled synthesis of large-area single-crystalline monolayer graphene.

Elastic straining of free-standing monolayer graphene

The sp2 nature of graphene endows the hexagonal lattice with very high theoretical stiffness, strength and resilience, all well-documented. However, the ultimate stretchability of graphene has not yet been demonstrated due to the difficulties in experimental design. Here, directly performing in situ tensile tests in a scanning electron microscope after developing a protocol for sample transfer, shaping and straining, we report the elastic properties and stretchability of free-standing single-crystalline monolayer graphene grown by chemical vapor deposition. The measured Young’s modulus is close to 1 TPa, aligning well with the theoretical value, while the representative engineering tensile strength reaches ~50-60 GPa with sample-wide elastic strain up to ~6%. Our findings demonstrate that single-crystalline monolayer graphene can indeed display near ideal mechanical performance, even in a large area with edge defects, as well as resilience and mechanical robustness that allows for flexible electronics and mechatronics applications.

Growth and carrier transport performance of single-crystalline monolayer graphene over electrodeposited copper film on quartz glass

The synthesis of single crystal graphene domains on cold rolling Cu foil has recently been reported. However, the cold rolling Cu foils have many rolling lines that increase the roughness of the foil. In this work, we developed an electrodeposited method to achieve high-quality Cu film on which the nucleation density of graphene decreased greatly. The Cu film was electrodeposited over chemical plated silver film on quartz glass. The roughness and thickness of the Cu film were positively correlated with the deposition temperature and current density, respectively. High-quality single crystal graphene with low nucleation density was achieved on electrodeposited Cu film. With increasing deposition temperature of Cu film from 20 to 65 °C, the nucleation density of graphene increased from 124 to 448 mm−2. The nucleation density of graphene increased from 124 to 192 mm−2 with increasing current density from 0.03 to 0.07 A/cm−2. Finally, a back-gated graphene field effect transistor (FET) was fabricated with a carrier transport performance of μh = ~4041 cm2V−1s−1 and μe = ~2580 cm2V−1s−1 at room temperature.

Single-Crystalline Monolayer Graphene Wafer on Dielectric Substrate of SiON without Metal Catalysts.

Many scientific and engineering efforts have been made to realize graphene electronics by fully utilizing intrinsic properties of ideal graphene for last decades. The most technical huddles come from the absence of wafer-scale graphene with a single-crystallinity on dielectric substrates. Here, we report an epitaxial growth of single-crystalline monolayer graphene directly on a single-crystalline dielectric SiON-SiC(0001) with a full coverage via epitaxial chemical vapor deposition (CVD) without metal catalyst. The dielectric surface of SiON provides atomically flat and chemically inert interface by passivation of dangling bonds, which keeps intrinsic properties of graphene. Atomic structures with a clean interface, full coverage of single-crystalline monolayer, and the epitaxy of graphene on SiON were confirmed macroscopically by mapping low energy electron diffraction (LEED) and Raman spectroscopy, and atomically by scanning tunneling microscopy (STM). Both of measured and calculated local density of states (LDOS) exhibit a symmetric and sharp Dirac cone with a Dirac point located at a Fermi level. Our method provides a route to utilize a single-crystalline dielectric substrate for ideal graphene growth for future applications.

Bioactive Functionalized Monolayer Graphene for High-Resolution Cryo-Electron Microscopy.

Single-particle cryo-electron microscopy (cryo-EM) has become one of the most essential tools to understand biological mechanisms at molecular level. A major bottleneck in cryo-EM technique is the preparation of good specimens that embed biological macromolecules in a thin layer of vitreous ice. In the canonical cryo-EM specimen preparation method, biological macromolecules tend to be adsorbed to the air-water interface, causing partial denaturation and/or preferential orientations. In this work, we have designed and produced a new type of cryo-EM grids using bioactive-ligand functionalized single-crystalline monolayer graphene membranes as supporting films. The functionalized graphene membrane (FGM) grids exhibit specific binding affinity to histidine (His)-tagged proteins and complexes. In cryo-EM, the FGM grids generate relatively low background for imaging and selectively anchor 20S proteasomes to the supporting film surface, enabling near-atomic-resolution 3D reconstruction of the complex. We envision that the FGM grids could benefit single particle cryo-EM specimen preparation with high reproducibility and robustness, therefore enhancing the efficiency and throughput of high-resolution cryo-EM structural determination.

Comparative study on the antioxidation behaviors of polycrystalline multilayer and single-crystalline monolayer graphene

Graphene is known effective as a short-term solution to protecting metals from oxidation. Past studies have shown that more layers and fewer defects (e.g. grain boundaries) enhance the antioxidation ability of graphene. However, it has remained unanswered which of these two parameters, namely, number of graphene layers and crystallinity of graphene, is more important when it comes to protecting metals. Herein, we aim to study this subject by comparing the oxidation behaviors of Cu covered by two different types of graphene. One is multilayered but contains a high density of grain boundaries, while the other is free of grain boundaries but comes with only one layer. Through oxidizing experiments in air and water-rich vapor conditions, failure mechanisms of graphene and change of underlying Cu (and Cu oxides) are thoroughly investigated using a wide range of analytical tools, including scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS). It is found that the single-crystalline, monolayer graphene is more effective than the counterpart in polycrystalline, multilayer format for protecting Cu against oxidation in both air and water-rich vapor conditions. Especially in air oxidizing case, the former graphene raises the temperature barrier by at least 100 °C as compared to the latter graphene. Our results therefore suggest that the crystallinity of graphene deserves more considerations when using graphene as the antioxidizing barrier.

Double quantum criticality in superconducting tin arrays-graphene hybrid

Two magnetic-field-induced quantum critical behaviors were recently discovered in two dimensional electron gas (2DEG) at LaTiO3/SrTiO3 interface and interpreted by disordered superconducting puddles coupled through 2DEG. In this scenario, the 2DEG is proposed to undergo a spontaneous phase separation and breaks up into locally superconducting puddles in a metallic matrix. However, as the inhomogeneous superconducting 2DEG is only illative, this proposal still lacks the direct experimental demonstration. Here, we artificially construct superconducting puddles-2DEG hybrid system by depositing tin nanoislands array on single crystalline monolayer graphene, where the two quantum critical behaviors are reproduced. Through the finite-size scaling analysis on magnetoresistivity, we show that the two quantum critical behaviors result from the intra-island and inter-island phase coherence, respectively, which are further illustrated by the phase diagram. This work provides a platform to study superconducting quantum phase transitions in a 2D system and helps to integrate superconducting devices into semiconductor technology.

Surface Engineering of Copper Foils for Growing Centimeter-Sized Single-Crystalline Graphene.

The controlled growth of high-quality graphene on a large scale is of central importance for applications in electronics and optoelectronics. To minimize the adverse impacts of grain boundaries in large-area polycrystalline graphene, the synthesis of large single crystals of monolayer graphene is one of the key challenges for graphene production. Here, we develop a facile surface-engineering method to grow large single-crystalline monolayer graphene by the passivation of the active sites and the control of graphene nucleation on copper surface using the melamine pretreatment. Centimeter-sized hexagonal single-crystal graphene domains were successfully grown, which exhibit ultrahigh carrier mobilities exceeding 25,000 cm(2) V(-1) s(-1) and quantum Hall effects on SiO2 substrates. The underlying mechanism of melamine pretreatments were systematically investigated through elemental analyses of copper surface in the growth process of large single-crystals. This present work provides a surface design of a catalytic substrate for the controlled growth of large-area graphene single crystals.

High quality sub-monolayer, monolayer, and bilayer graphene on Ru(0001)

High quality sub-monolayer, monolayer, and bilayer graphene were grown on Ru(0001). For the sub-monolayer graphene, the size of graphene islands with zigzag edges can be controlled by the dose of ethylene exposure. By increasing the dose of ethylene to 100 Langmuir at a high substrate temperature (800 °C), high quality single-crystalline monolayer graphene was synthesized on Ru(0001). High quality bilayer graphene was formed by further increasing the dose of ethylene while reducing the cooling rate to 5 °C/min. Raman spectroscopy revealed the vibrational states of graphene, G and 2D peaks appeared only in the bilayer graphene, which demonstrates that it behaves as the intrinsic graphene. Our present work affords methods to produce high quality sub-monolayer, monolayer, and bilayer graphene, both for basic research and applications.