Formation Mechanism Of Graphene Research Articles

Highly Conductive Porous Graphene/Ceramic Composites for Heat Transfer and Thermal Energy Storage

AbstractA novel architecture of 3D graphene growth on porous Al2O3 ceramics is proposed for thermal management using ambient pressure chemical vapor deposition. The formation mechanism of graphene is attributed to the carbothermic reduction occurring at the Al2O3 surface to initialize the nucleation and growth of graphene. The graphene films are coated on insulating anodic aluminum oxide (AAO) templates and porous Al2O3 ceramic substrates. The graphene coated AAO possesses one‐dimensional isolated graphene tubes, which can act as the media for directional thermal transport. The graphene/Al2O3 composite (G‐Al2O3) contains an interconnected macroporous graphene framework with an extremely low sheet electrical resistance down to 0.11 Ω sq−1 and thermal conductivity with 8.28 W m−1 K−1. The G‐Al2O3 provides enormous conductive pathways for electronic and heat transfer, suitable for application as heat sinks. Such a porous composite is also attractive as a highly thermally conductive reservoir to hold phase change materials (stearic acid) for thermal energy storage. This work displays the great potential of CVD direct growth of graphene on dielectric porous substrates for thermal conduction and electronic applications.

A study of the key parameters, including the crucial role of H2 for uniform graphene growth on Ni foil

In this work we present a low-cost, scalable technique to fabricate few-layer graphene films via ambient pressure chemical vapor deposition (CVD) on a low-cost commercial polycrystalline Ni foil. Critical parameters including Ni thickness, cooling rate, and polycrystalline crystallographic orientation have been explored to understand the graphene formation mechanism and to obtain controlled carbon growth. We have studied the effect of operating conditions such as the synthesis time and feed composition, as well as the key role played by H2. The effect of the foil position in the reactor with respect to the feed flow was reported: the back Ni surface, not exposed to the feed flow, shows an increased carbon deposition with respect to the front one. An increase of the segregation and precipitation phenomena, as a function of the cooling rate and under Ni thickness increase was observed. By decreasing the synthesis time, it is possible to obtain 1L on areas showing an increased Ni(111) diffraction peak intensity. By varying the hydrogen concentration it is possible to control the final graphene formation on the nickel surface, obtaining quite uniform 2–3-layer graphene.

Atomic and Polymeric Carbon on Co(0001): Surface Reconstruction, Graphene Formation, and Catalyst Poisoning

Atomic carbon on Co(0001), deposited by ethylene decomposition, forms islands with a (√3 × √3)R30° structure at low C coverage (∼0.2 ML), whereas a high C coverage (0.5 ML, saturation) induces a reconstruction of the cobalt surface. Atomic carbon weakens the adsorption of CO and H2, but even a saturated atomic carbon layer does not block the surface for adsorption. Carbon–carbon coupling, i.e., polymeric carbon formation, was not observed for temperatures ≤630 K on the close-packed cobalt surface. Polymeric carbon, in the form of small graphene islands, forms on the close-packed terraces after heating of an acetylene-saturated surface. Graphene also forms upon heating of an atomic carbon covered surface on which ethylene was dosed at low temperature. In this case, step edges act as a nucleation point of the graphene islands, while their growth proceeds via the addition of C2Hx species. In both cases, hydrogenated forms of carbon rather than atomic carbon are key precursors for graphene growth. Graphene covers the cobalt surface, thereby inhibiting adsorption of CO and hydrogen completely. The described graphene formation mechanism is seen as a relevant, low temperature route to detrimental carbon that would deactivate a cobalt FT catalyst. Atomic carbon is more reactive than graphene, as it is oxidized at lower temperatures than graphene. The graphene islands formed at relatively low temperatures are of poor structural quality and contain (islands of) encapsulated cobalt atoms.

Step-templated CVD growth of aligned graphene nanoribbons supported by a single-layer graphene film

We present chemical vapor deposition (CVD) growth of a hybrid structure of aligned graphene nanoribbons (GNRs) supported by a single-layer graphene sheet. The step structure created on the epitaxial Co film is used to segregate arrays of aligned GNRs. Reflecting the highly ordered step structure of the Co catalyst, straight nanoribbons with high aspect ratio (>100) are formed. Analysis suggests that a large-area, single-layer graphene film also grows over the aligned GNRs, making a GNR-graphene hybrid structure. We also demonstrate the isolation of aligned GNRs by oxygen plasma treatment or partial transfer of the hybrid film. These findings on the formation of highly aligned GNRs give new insights into the formation mechanism of graphene and can be applied for more advanced graphene structure for future electronics.

Ellipsometry as a Real-Time Optical Tool for Monitoring and Understanding Graphene Growth on Metals

Despite the recent advances in the chemical vapor deposition (CVD) of graphene, it is still a great challenge to control the thickness of graphene especially in real-time during the growth. So far, there are no reports on the real-time monitoring of graphene growth. Here, we show for the first time real-time in situ kinetic monitoring of graphene deposition by CVD on nickel. We demonstrate an optical nondestructive method of dynamic spectroscopic ellipsometry for controlling and optimizing the catalyst cleaning and annealing and, consequently, the graphene deposition and properties. The kinetic ellipsometry monitoring also highlights the mechanism of graphene formation. Discussion shows the applicability and industrial scalability of this ellipsometric method to the control of large-area graphene formation on any substrate. This approach opens a way to in-line real-time graphene metrology and is helpful in guiding the graphene growth process as we try to achieve reproducible and controllable research as w...

Low-temperature rapid synthesis of high-quality pristine or boron-doped graphene via Wurtz-type reductive coupling reaction

High-quality graphene nanosheets are prepared via a rapid Wurtz-type reductive coupling (WRC) reaction without the assistance of any transition metal catalysts. This method involves a nearly stoichiometric reaction of tetrachloromethane (CCl4) and potassium (K) at 150–210 °C for as short as 10 min, which possesses great advantages compared with the solvothermal method. The layer number of the as-prepared graphene is mainly less than five. The formation mechanism of graphene is proposed, which comprises of three steps, stripping off the chlorines from CCl4 by the highly reductive K, the coupling and assembly of –CC– and the layer growth of hexagonal carbon clusters to form graphene. Many important factors determining the quality of graphene, such as the residual chlorines, the reaction temperature and the reductant, are discussed in detail. The low residual chlorine in the reaction favors the improvement of the graphene quality. Furthermore, controllable boron doping can be easily realized by adding an appropriate amount of BBr3. The developed method provides a cost-effective route to prepare high-quality pristine or doped graphene for mass production.

Preparation of Single- and Few-Layer Graphene Sheets Using Co Deposition on SiC Substrate

Single- and few-layer graphene sheets were fabricated by selective chemical reactions between Co film and SiC substrate. A rapid cooling process was employed. The number of layers and crystallinity of graphene sheets were controlled by process parameters. The formation mechanism of graphene was highly sensitive to carbon diffusion. Free carbon precipitated and then moved across the product layer that was composed mainly of cobalt-silicides. The graphene layer formed homogeneously on the surface and then transferred to the other substrate. This could provide a method for high-quality fabrication of wafer-sized graphene sheets.

Patterned Growth of Graphene over Epitaxial Catalyst

Rectangle- and triangle-shaped microscale graphene films are grown on epitaxial Co films deposited on single-crystal MgO substrates with (001) and (111) planes, respectively. A thin film of Co or Ni metal is epitaxially deposited on a MgO substrate by sputtering while heating the substrate. Thermal decomposition of polystyrene over this epitaxial metal film in vacuum gives rectangular or triangular pit structures whose orientation and shape are strongly dependent on the crystallographic orientation of the MgO substrate. Raman mapping measurements indicate preferential formation of few-layer graphene films inside these pits. The rectangular graphene films are transferred onto a SiO(2)/Si substrate while maintaining the original shape and field-effect transistors are fabricated using the transferred films. These findings on the formation of rectangular/triangular graphene give new insights on the formation mechanism of graphene and can be applied for more advanced/controlled graphene growth.

Analysis of the Formation Conditions for Large Area Epitaxial Graphene on SiC Substrates

We are aiming at understanding the graphene formation mechanism on different SiC polytypes (6H, 4H and 3C) and orientations with the ultimate goal to fabricate large area graphene (up to 2 inch) with controlled number of monolayers and spatial uniformity. To reach the objectives we are using high-temperature atmospheric pressure sublimation process in an inductively heated furnace. The epitaxial graphene is characterized by ARPES, LEEM and Raman spectroscopy. Theoretical studies are employed to get better insight of graphene patterns and stability. Reproducible results of single layer graphene on the Si-face of 6H and 4H-SiC polytypes have been attained. It is demonstrated that thickness uniformity of graphene is very sensitive to the substrate miscut.