Department of Electrical Engineering, ‡Department of Chemistry, and §Department of Chemical Engineering and Materials Science, University of Southern California , Los Angeles, California 90089, United States.
Since its debut in 2004, graphene has attracted enormous interest because of its unique properties. Chemical vapor deposition (CVD) has emerged as an important method for the preparation and production of graphene for various applications since the method was first reported in 2008/2009. In this Account, we review graphene CVD on various metal substrates with an emphasis on Ni and Cu. In addition, we discuss important and representative applications of graphene formed by CVD, including as flexible transparent conductors for organic photovoltaic cells and in field effect transistors. Growth on polycrystalline Ni films leads to both monolayer and few-layer graphene with multiple layers because of the grain boundaries on Ni films. We can greatly increase the percentage of monolayer graphene by using single-crystalline Ni(111) substrates, which have smooth surface and no grain boundaries. Due to the extremely low solubility of carbon in Cu, Cu has emerged as an even better catalyst for the growth of monolayer graphene with a high percentage of single layers. The growth of graphene on Cu is a surface reaction. As a result, only one layer of graphene can form on a Cu surface, in contrast with Ni, where more than one layer can form through carbon segregation and precipitation. We also describe a method for transferring graphene sheets from the metal using polymethyl methacrylate (PMMA). CVD graphene has electronic properties that are potentially valuable in a number of applications. For example, few-layer graphene grown on Ni can function as flexible transparent conductive electrodes for organic photovoltaic cells. In addition, because we can synthesize large-grain graphene on Cu foil, such large-grain graphene has electronic properties suitable for use in field effect transistors.
"Some of these originated during growth itself and others were generated during the transfer process. The folded graphene, often referred to as " wrinkle " , is due to the difference of thermal expansion coefficient between graphene and the copper substrate . The tears can be generated by air pockets during the transfer process and removal of PMMA, as well as by the weak adhesion between graphene and substrate especially near the grain boundaries . "
[Show abstract][Hide abstract] ABSTRACT: Graphene was grown on copper and repeatedly transferred onto a GaAs semi-insulating substrate to form multilayers (1 to 10). These manually stacked graphene layers resulted in appreciable local variations of optical properties due to the local differences of stacking orders. In addition, most of the observed 2D/G intensity and area ratios of an n-multilayer CVD graphene is consistent with the characteristics of a single layer repeated n-times. However, multilayer graphene has many kinds of advantages for applications to optoelectronic devices. First, the G band shift is not related to the stacking order, proving that multilayer graphene reduces doping and strain effect from the substrate, which is confirmed by Raman results after metal electrode deposition. Second, the sheet resistance decreases with increasing number of layers and after thermal annealing. Another benefit of multilayer graphene is that each layer can be annealed after transfer, which greatly improves the sheet resistance and its lateral uniformity without intentional doping. We therefore conclude that multilayer CVD graphene is a good candidate for various GaAs-based electrical applications and its good electrical uniformity allows fabrication of devices on large scales.
"Although the growth of monolayer thick graphite on metal surface is known from 60 th May (1969) the full picture of growth mechanism is still developing Zhang et al. (2013). The problem of graphene growth by CVD is complex including the processes of absorption, dehydration, nucleation, etc. From the technological point of view the growth of graphene at atmospheric pressure is attractive. "
[Show abstract][Hide abstract] ABSTRACT: In this article we present the results of micro-Raman studies of graphene grown on copper foil surface by atmospheric pressure CVD using decane as precursor, nitrogen as carrier gas with zero flow of hydrogen. Analysis of Raman spectroscopy data showed that film contains spots with single layer thick graphene. We observed significant blue shift of 2D and G bands positions for mono-atomically thick graphene on copper foil. Following literature we relate this shift to the strain induced by the presence of copper substrate. Moreover, we observed changes in the defectiveness of graphene layers after the transfer, which was related to the appearance of chemically-induced defects and defects induced by changes in the mechanical strain.
Conference of Physics of Nonequilibrium Atomic Systems and Composites, PNASC 2015, 18-20 February 2015 and the Conference of Heterostructures for microwave, power and optoelectronics: physics, technology and devices (Heterostructures), 19 February 2015; 02/2015
"Epitaxial growth, chemical vapour deposition (CVD) and mechanical cleavage of graphite for instance lead to the formation of very high grade of graphene materials with very low sp3 carbon contents  . The dimension, ranging from nano-scale to micron sized and the number of layers superimposed across the graphene sheets may be finely tuned to produce materials with very versatile and specific bulk and surface properties . The development of 3D dense or porous architectures is therefore critical to fully benefit from the natural properties of graphene  . "
[Show abstract][Hide abstract] ABSTRACT: A direct approach to functionalize and reduce pre-shaped graphene oxide 3D architectures is demonstrated by gamma ray irradiation in gaseous phase under analytical grade air, N2 or H2. The formation of radicals upon gamma ray irradiation is shown to lead to surface functionalization of the graphene oxide sheets. The reduction degree of graphene oxide, which can be controlled through varying the γ-ray total dose irradiation, leads to the synthesis of highly crystalline and near defect-free graphene based materials. The crystalline structure of the graphene oxide and γ-ray reduced graphene oxide was investigated by x-ray diffraction and Raman spectroscopy. The results reveal no noticeable changes in the size of sp2 graphitic structures for the range of tested gases and total exposure doses suggesting that the irradiation in gaseous phase does not damage the graphene crystalline domains. As confirmed by X-ray photoemission spectroscopy, the C/O ratio of γ-ray reduced graphene oxide is increasing from 2.37 for graphene oxide to 6.25 upon irradiation in hydrogen gas. The removal of oxygen atoms with this reduction process in hydrogen results in a sharp 400 times increase of the electrical conductivity of γ-ray reduced graphene oxide from 0.05 S cm−1 to as high as 23 S cm−1. A significant increase of the contact angle of the γ-ray reduced graphene oxide bucky-papers and weakened oxygen rich groups characteristic peaks across the Fourier transform infrared spectra further illustrate the efficacy of the γ-ray reduction process. A mechanism correlating the interaction between hydrogen radicals formed upon γ-ray irradiation of hydrogen gas and the oxygen rich groups on the surface of the graphene oxide bucky-papers is proposed, in order to contribute to the synthesis of reduced graphene materials through solution-free chemistry routes.
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