Effect of preparation on the commensurabilities and thermal expansion of graphene on Ir(111) between 10 and 1300 K
Résumé
The effects of the temperature on the structure of a single layer of graphene on Ir(111) have been investigated in situ in the growth chamber by grazing incidence x-ray diffraction between 10 and 1300 K. In addition, the effect of two growth temperatures has been studied. The graphene lattice parameter of the main, nonrotated phase displays a characteristic hysteresis whose lower and upper branches correspond to phases in which integer numbers (m,n) Ir of the Ir lattice parameters match integer numbers (p,q) Gr of the graphene lattice parameters, i.e., commensurate phases, (19,0) Ir × (21,0) Gr and (9,0) Ir × (10,0) Gr , respectively. With a higher growth temperature, graphene presents, in addition to a main nonrotated, incommensurate phase, domains with a small rotation (2.37 •) relative to the substrate, corresponding to a (7,2) Ir × (8,2) Gr commensurate phase. Despite the weak interaction between graphene and iridium, the thermal expansion coefficient of graphene is positive at all temperatures, even at liquid helium temperature contrary to free-standing graphene. It is also close to the iridium thermal expansion because of the tendency to form commensurate phases, which creates a strong link between the graphene and substrate lattices and decreases the elastic energy.
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