Session T12: Focus Session: Graphene: Growth, Mechanical Exfoliation, and Properties - CVD on Metals

Characterization of Few Layer Graphene films Grown on Cu, Cu-Ni and SiC Substrates

The electronic structure of graphene depends on the number of graphene layers and the stacking sequence between the layers. Therefore, it is important to have a non-destructive technique for analyzing the overlayer coverage of graphene directly on the growth substrate. We have developed a technique using angle-resolved XPS to determine the average graphene thickness directly on metal foil substrates and SiC substrates. Since monolayer graphene films can be grown on Cu substrates, these samples are used as a standard reference for a monolayer of graphene. HOPG is used as a standard reference for bulk graphite. The electron mean free path of the C-1s photoelectron can be determined by analyzing the areas under the C-1s peaks of monolayer graphene/Cu and bulk graphite. With the electron mean free path, the graphene coverage of a film of arbitrary thickness can be determined by analyzing the area under the C-1s of that sample. Analysis of graphene coverages for graphene films grown on Cu-Ni substrates and of the thickness of both the graphene overlayer and intermediate buffer layer on SiC will be presented.   

Investigation of large-area graphene synthesized on palladium surface

We present a detailed study of large-area growth of graphene on palladium substrates. By studying the growth at different stages through appropriate variations of the temperature and time intervals of growth, and by investigating the growth at different regions of the metal substrate we have been able to observe a number of important properties of graphene growth on Pd surfaces. We have explored the nature of the as-synthesized graphene through a combination of electron microscopy and Raman spectroscopic analysis. Raman analysis of the graphene enables us to identify different kinds of as-synthesized graphene, including monolayer, turbostratic multi-layer, and mixed Bernal-turbostratic graphene layers. We further demonstrate a systematic study of the evolution of these different types of graphene as a function of temperature and growth-time.   

Crystallographic effects of copper substrate on graphene growth and fluorination

Graphene grown by chemical vapor deposition (CVD) on Cu is appealing due to supposed large-area monolayer growth and the low cost of the Cu foil substrate. However, this Cu substrate is inherently polycrystalline, with low and high index facets, annealing twins, and rough sites. We characterize CVD graphene growth on the Cu surfaces by combining Raman spectroscopy, electron-backscatter diffraction (EBSD), and scanning electron microscopy (SEM). We find that graphene growth on Cu(100) is multilayered and of low-quality, while growth on Cu(111) is monolayer and of high-quality. High index Cu facets containing a high percentage of (111) terraces are more monolayer-like than Cu(100). Graphene has a higher growth rate on (111) surfaces, growing the fastest on Cu(111). At temperatures below 900 \r{ }C, compact islands of graphene form from lowered growth rate. To open a bandgap in graphene, quantum confinement or covalent chemistry must be used. We do the latter by exposing our graphene films to XeF$_{2}$ gas, terminating in an insulating C$_{4}$F stoichiometry and covalent C-F bonds. Rougher facets fluorinate first, which allows possible bandgap engineering by the Cu crystallography. Additionally, film defects assist in fluorination effectiveness. We also show preliminary results on Cu crystallography effects for CVD of hexagonal boron nitride (h-BN).   

Barrier-Guided Growth of Micro- and Nano-Structured Graphene

The patterning of graphene is necessary for tuning its physical and electronic structure and for device-integration. Traditionally, patterning has been achieved via \textit{top-down} chemical or physical etching, which induces defects and disorder that degrades performance. In this work, we overcome these challenges through a fundamentally new \textit{bottom-up} growth method for the rational synthesis of patterned graphene called Barrier-Guided Chemical Vapor Deposition (BG-CVD). We deposit patterned barriers on the Cu surface, using scalable lithography methods, which guide the growth of graphene around them into any desired shape. The barriers locally passivate the surface, (i) preventing the decomposition of the methane, and (ii) blocking the growth of graphene on the barrier. By designing appropriate barrier layers, we have grown arbitrary patterns, nanoribbons, and nanoperforated graphene with features down to 25 nm with high mobility (215 cm$^{2}$/Vs). These materials are highly crystalline with domain size $>$4 $\mu $m and have 2-10x less edge defects than comparable top-down etched materials. The pattern reproducibility is $<$1 nm and thus, ultimately, should enable the bottom-up synthesis of sub-5 nm features. These results indicate BG-CVD as a superior production route to patterned graphene.   

Synthesis of Large-grain, Single-crystalline Graphene by a Novel Chemical Vapor Deposition Method and Electrical properties

Graphene, a two dimensional, honey comb arrangement of carbon atoms has drawn significant attention with its interesting physical and electronic properties. Tremendous efforts have been made to synthesize large-scale, high quality, single-layer graphene (SLG). Based on previous studies, CVD graphene with large grain size (less grain boundaries) and low defect density would show an enhancement of device mobility. Here we report a novel CVD method to synthesize graphene with grain size up to several hundreds of micrometers on copper foil. Raman surface map of individual graphene grain indicated that the large-grain graphene was single-layer and with very low defect density. Selected Area Electron Diffraction (SAED) also confirmed that the each individual graphene island was a single grain. Morphology study was also performed to investigate the relation between the shape of graphene and growth parameters. Furthermore, the large-grain graphene was transferred to SiO$_{2}$/Si for the field effect study, and the device mobility derived from the large-grain graphene was $\sim $ 5,200 cm$^{2}$/V/s. The achieved high device mobility indicates that the large-grain single-crystalline graphene is of great potential for graphene-based nanoelectronics.   

Mesoscale STM Study of Thermally Annealed Copper Foils

The growth of high quality graphene has become a topic of significance. There have been utilized several methods of material growth including the epitaxial growth on SiC, method of exfoliation of graphite, and chemical vapor deposition (CVD). The CVD approach typically utilizes foils of copper or nickel that are exposed to organic compounds at a high temperature. The purpose of the study is to investigate the role of the metal surface morphology during the growth process, relative grain size before and after thermal treatment, and relative flatness of the substrate after annealing. We investigated the effects of thermal annealing of polycrystalline Cu foil at the mesoscale using an ultrahigh vacuum (UHV) scanning tunneling microscope (STM). Prolonged low-temperature and rapid high-temperature annealing of the samples is being carried out and the resulting surface morphology will be reported. The STM observations reveal that the film quality is limited by grain boundaries.   

Graphene Crystal Growth and Device Integration

Graphene has unique electronic, chemical, thermal and physical properties and this is opening many opportunities for its use. However, to date the majority of the experiments have been performed on exfoliated graphene. There is a need to develop high quality, large area single crystal graphene for electronic applications. The discovery of graphene growth copper by chemical vapor deposition (CVD) has led to the growth of polycrystalline large area (square meters) films. The domain size for the baseline process is a few tens of microns in diameter but large ``crystals,'' 0.25 square mm, have been grown. However, even though the films are not yet fully single crystals the transport properties are equivalent to those of exfoliated graphene. The ultimate usefulness of any material for electronics is the ability to integrate it with dielectrics and metals. Graphene is chemically inert and will require special processes to integrate it with dielectrics and metals without interrupting its band structure. The objective of this presentation is to review and present new data on large area graphene crystal growth and integration of dielectrics and metals. Surface analysis of graphene with dielectrics and metals under various processing conditions will also be presented.   

In Situ Characterization of Alloy Catalysts for Low-Temperature Graphene Growth

Chemical vapor deposition (CVD) on transition metal catalysts offers a low-cost method of producing large-area graphene, but due to limited understanding of the underlying mechanism(s), growth control remains rudimentary. We use in situ X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) to monitor catalytic CVD on Ni based catalysts under typical reactor conditions. We thus develop a coherent model for graphene formation and show that graphene growth occurs during isothermal hydrocarbon exposure and is not limited to precipitation upon cooling. We introduce alloy catalysts to improve graphene growth by tuning reactivity and selectivity. We show that alloying polycrystalline Ni with Au allows low temperature ($<$450\r{ }C) CVD of predominantly monolayer ($>$74{\%}) graphene films with an average D/G peak ratio of $\sim $0.24 and domain sizes in excess of 220$\mu $m$^{2}$ [1]. We suggest that Au decorates a majority of high reactivity Ni surface sites, such as step edges, and lowers the stability of surface C. Au alloying thereby drastically lowers the graphene nucleation density, allowing more uniform and controlled growth at CMOS compatible temperatures. [1] Weatherup et al. Nano Lett. 11, 4154 (2011)   

ACCVD growth of mono- and bi-layer graphene and mechanism study

CVD on metal substrates has been proved to be effective in the synthesis of graphene. But before utilization in graphene electronics, the quality of the consequential graphene needs to be improved, and a reliable method to grow large-scale, high-quality bi-layer graphene is required. Besides, the mechanism of growth is not fully understood. Here we report utilizing Alcohol catalytic CVD (ACCVD) to produce mono- and bi-layer graphene. By applying alcohol as carbon source, we carefully studied how each parameter affects the growth process, and the defects and grain size of graphene. By changing growth conditions we could efficiently control the layer number (1 or 2). Afterwards, we conducted and compared the transfer procedures utilizing three commonly used mediators---PVA, PDMS and PMMA. We also employed $^{12}$C, $^{13}$C, and 2-$^{13}$C ethanol to synthesize graphene, and with the spectroscopy characterization results we are able to explain the growth mechanism on Cu and Ni substrates to a certain extent.   

Synthesis and sensing application of large scale bilayer graphene

We have synthesized large scale bilayer graphene by using Chemical Vapor Deposition (CVD) in atmospheric pressure. Bilayer graphene was grown by using CH4, H2 and Ar gases. The growth temperature was 1050${^\circ}$. Conventional FET measurement shows ambipolar transfer characteristics. Results of Raman spectroscopy, Atomic Force microscope (AFM) and Transmission Electron Microscope (TEM) indicate the film is bilayer graphene. Especially, adlayer structure which interrupt uniformity was reduced in low methane flow condition. Furthermore, large size CVD bilayer graphene film can be investigated to apply sensor devices. By using conventional photolithography process, we have fabricated device array structure and studied sensing behavior.   

CVD growth of graphene at low temperature

Graphene has attracted a lot of research interest owing to its exotic properties and a wide spectrum of potential applications. Chemical vapor deposition (CVD) from gaseous hydrocarbon sources has shown great promises for large-scale graphene growth. However, high growth temperature, typically 1000$^{\circ}$C, is required for such growth. In this talk, I will show a revised CVD route to grow graphene on Cu foils at low temperature, adopting solid and liquid hydrocarbon feedstocks. For solid PMMA and polystyrene precursors, centimeter-scale monolayer graphene films are synthesized at a growth temperature down to 400$^{\circ}$C. When benzene is used as the hydrocarbon source, monolayer graphene flakes with excellent quality are achieved at a growth temperature as low as 300$^{\circ}$C. I will also talk about our recent progress on low-temperature graphene growth using paraterphenyl as precursor. The successful low-temperature growth can be qualitatively understood from the first principles calculations. Our work might pave a way to economical and convenient growth route of graphene, as well as better control of the growth pattern of graphene at low temperature.