Abstract
Several transition metals other than the largely used Cu and Ni can be, in principle, employed to catalyze carbon precursors for the chemical vapor deposition of graphene, because the thermodynamics of their alloying with carbon is well known. For example, the wealth of information in the Co-C phase diagram can be used to predict the properties of graphene grown in this way. It is, in fact, expected that growth occurs at a temperature higher than in Ni, with beneficial consequences to the mechanical and electronic properties of the final product. In this work, the growth of graphene onto Co film is presented together with an extensive Raman characterization of the structural properties of the material so far obtained. Previous results reporting the full coverage with negligible defective areas, in spite of discontinuities in the underlying metal, are confirmed, together with the occurrence of strain in the graphene sheet. Strain is deeply investigated in this work, in view of possible employment in engineering the material properties. The observed strain is ascribed to the high thermal mismatch with the substrate, even if an effect of the crystallographic transition of Co cannot be excluded.
Highlights
The thermodynamic approach has been largely employed in the study of the different techniques for the growth of innovative materials, since the birth of material science.Thermodynamic potentials, like the Gibbs’ free energy G = H − TS (H being the enthalpy, S the entropy, and T the absolute temperature), and its derivative with respect to the variation of the quantity of matter, the so-called chemical potential M = δG/δn, allow discriminating between ordered or disordered growth, or by crystalline or epitaxial processes
The mobility of electrical carriers in chemical vapor deposition (CVD) graphene is known to be limited by scattering with vacancies, grain boundaries, and distorted bonds, even at the nanoscale [16]: such departures from the perfect hexagonal lattice of graphene imply that some atoms vibrate at frequencies different from those of the perfect crystal
Overcomes 95%, whereas it is less than 90% in the case of deposition in the conventional quartz furnace. This suggests that the duration of the heating step plays a role [23], since in the first case temperature is raised at a rate of 5 ◦ C/s, due to instrumental limitations, and with a rate 10 times lower in the second. Another important difference is that we observe suspended graphene on the metal discontinuities in the rapid thermal processing (RTP) samples, and not in the ones processed in the quartz tube
Summary
Thermodynamic potentials, like the Gibbs’ free energy G = H − TS (H being the enthalpy, S the entropy, and T the absolute temperature), and its derivative with respect to the variation of the quantity of matter, the so-called chemical potential M = δG/δn, allow discriminating between ordered or disordered growth, or by crystalline or epitaxial processes. This approach, had no great success in the case of the growth of graphene onto catalyzing metals by chemical vapor deposition (CVD), probably because of the great complexity of this mechanism.
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