In situ x-ray diffraction study of graphitic carbon formed during heating and cooling of amorphous-C/Ni bilayers

We examine graphitization of amorphous carbon (a-C) in a-C/Ni bilayer samples having the structure Si/SiO(2)/a-C(3-30 nm)/Ni(100 nm). In situ x-ray diffraction (XRD) measurements during heating in He at 3 degrees C/s to 1000 degrees C showed graphitic C formation beginning at temperatures T of 640-730 degrees C, suggesting graphitization by direct metal-induced crystallization, rather than by a dissolution/precipitation mechanism in which C is dissolved during heating and expelled from solution upon cooling. We also find that graphitic C, once formed, can be reversibly dissolved by heating to T>950 degrees C, and that nongraphitic C can be volatilized by annealing in H(2)-containing ambients.