Controlling the number of layers in graphene using the growth pressure.

Monolayer graphene is commonly grown on Cu substrates due to the self-limiting nature of graphene synthesis by chemical vapor deposition (CVD). Consequently, the growth of multilayer graphene by CVD has proven to be relatively difficult. This study demonstrates that the number of layers in graphene synthesized on a copper substrate can be precisely set by controlling the partial pressure of hydrogen gas used in the CVD process. This study also shows that a pressure threshold exists for a distinct transition from monolayer to multilayer graphene growth. This threshold is shown to be the boundary where the graphene growth process on Cu by CVD is no longer a self-limiting process. In addition, the multilayer graphene synthesized through the pressure control method forms in the Volmer-Weber mode with an AB stacking structure.

Adhesion and Self-Healing between Monolayer Molybdenum Disulfide and Silicon Oxide.

The adhesion interactions of two-dimensional (2D) materials are of importance in developing flexible electronic devices due to relatively large surface forces. Here, we investigated the adhesion properties of large-area monolayer MoS2 grown on silicon oxide by using chemical vapor deposition. Fracture mechanics concepts using double cantilever beam configuration were used to characterize the adhesion interaction between MoS2 and silicon oxide. While the interface between MoS2 and silicon oxide was fractured under displacement control, force-displacement response was recorded. The separation energy, adhesion strength and range of the interactions between MoS2 and silicon oxide were characterized by analytical and numerical analyses. In addition to the fundamental adhesion properties of MoS2, we found that MoS2 monolayers on silicon oxide had self-healing properties, meaning that when the separated MoS2 and silicon oxide were brought into contact, the interface healed. The self-healing property of MoS2 is potentially applicable to the development of new composites or devices using 2D materials.

Growth of Monolayer Graphene on Nanoscale Copper-Nickel Alloy Thin Films.

Growth of high quality and monolayer graphene on copper thin films on silicon wafers is a promising approach to massive and direct graphene device fabrication in spite of the presence of potential dewetting issues in the copper film during graphene growth. Current work demonstrates roles of a nickel adhesion coupled with the copper film resulting in mitigation of dewetting problem as well as uniform monolayer graphene growth over 97 % coverage on films. The feasibility of monolayer graphene growth on Cu-Ni alloy films as thin as 150 nm in total is also demonstrated. During the graphene growth on Cu-Ni films, the nickel adhesion layer uniformly diffuses into the copper thin film resulting in a Cu-Ni alloy, helping to promote graphene nucleation and large area surface coverage. Furthermore, it was found that the use of extremely thin metal catalyst films also constraint the total amount of carbon that can be absorbed into the film during growth, which helps to eliminate adlayer formation and promote monolayer growth regardless of alloying content, thus improving the monolayer fraction of graphene coverage on the thinner films. These results suggest a path forward for the large scale integration of high quality, monolayer graphene into nanoelectronic and nanomechanical devices.

Large-Area Dry Transfer of Single-Crystalline Epitaxial Bismuth Thin Films.

We report the first direct dry transfer of a single-crystalline thin film grown by molecular beam epitaxy. A double cantilever beam fracture technique was used to transfer epitaxial bismuth thin films grown on silicon (111) to silicon strips coated with epoxy. The transferred bismuth films retained electrical, optical, and structural properties comparable to the as-grown epitaxial films. Additionally, we isolated the bismuth thin films on freestanding flexible cured-epoxy post-transfer. The adhesion energy at the bismuth/silicon interface was measured to be ∼1 J/m2, comparable to that of exfoliated and wet transferred graphene. This low adhesion energy and ease of transfer is unexpected for an epitaxially grown film and may enable the study of bismuth's unique electronic and spintronic properties on arbitrary substrates. Moreover, this method suggests a route to integrate other group-V epitaxial films (i.e., phosphorus) with arbitrary substrates, as well as potentially to isolate bismuthene, the atomic thin-film limit of bismuth.

Cracking of Polycrystalline Graphene on Copper under Tension.

Roll-to-roll manufacturing of graphene is attractive because of its compatibility with flexible substrates and its promise of high-speed production. Several prototype roll-to-roll systems have been demonstrated, which produce large-scale graphene on polymer films for transparent conducting film applications.1-4 In spite of such progress, the quality of graphene may be influenced by the tensile forces that are applied during roll-to-roll transfer. To address this issue, we conducted in situ tensile experiments on copper foil coated with graphene grown by chemical vapor deposition, which were carried out in a scanning electron microscope. Channel cracks, which were perpendicular to the loading direction, initiated over the entire graphene monolayer at applied tensile strain levels that were about twice the yield strain of the (annealed) copper. The spacing between the channel cracks decreased with increasing applied strain, and new graphene wrinkles that were parallel to the loading direction appeared. These morphological features were confirmed in more detail by atomic force microscopy. Raman spectroscopy was used to determine the strain in the graphene, which was related to the degradation of the graphene/copper interface. The experimental data allowed the fracture toughness of graphene and interfacial properties of the graphene/copper interface to be extracted based on classical channel crack and shear-lag models. This study not only deepens our understanding of the mechanical and interfacial behavior of graphene on copper but also provides guidelines for the design of roll-to-roll processes for the dry transfer of graphene.

Selective mechanical transfer of graphene from seed copper foil using rate effects.

A very fast, dry transfer process based on mechanical delamination successfully effected the transfer of large-area, CVD grown graphene on copper foil to silicon. This has been achieved by bonding silicon backing layers to both sides of the graphene-coated copper foil with epoxy and applying a suitably high separation rate to the backing layers. At the highest separation rate considered (254.0 µm/s), monolayer graphene was completely transferred from the copper foil to the target silicon substrate. On the other hand, the lowest rate (25.4 µm/s) caused the epoxy to be completely separated from the graphene. Fracture mechanics analyses were used to determine the adhesion energy between graphene and its seed copper foil (6.0 J/m(2)) and between graphene and the epoxy (3.4 J/m(2)) at the respective loading rates. Control experiments for the epoxy/silicon interface established a rate dependent adhesion, which supports the hypothesis that the adhesion of the graphene/epoxy interface was higher than that of the graphene/copper interface at the higher separation rate, thereby providing a controllable mechanism for selective transfer of graphene in future nanofabrication systems such as roll-to-roll transfer.

Ultra long-range interactions between large area graphene and silicon.

The wet-transfer of graphene grown by chemical vapor deposition (CVD) has been the standard procedure for transferring graphene to any substrate. However, the nature of the interactions between large area graphene and target substrates is unknown. Here, we report on measurements of the traction-separation relations, which represent the strength and range of adhesive interactions, and the adhesion energy between wet-transferred, CVD grown graphene and the native oxide surface of silicon substrates. These were determined by coupling interferometry measurements of the separation between the graphene and silicon with fracture mechanics concepts and analyses. The measured adhesion energy was 357 ± 16 mJ/m(2), which is commensurate with van der Waals interactions. However, the deduced traction-separation relation for graphene-silicon interactions exhibited a much longer range interaction than those normally associated with van der Waals forces, suggesting that other mechanisms are present.