Laser fluence dependence of the elastic properties of diamond-like carbon films prepared by pulsed-laser deposition

We studied the temperature dependence of internal friction of variety of amorphous diamond-like carbon films prepared by pulsed-laser deposition. Like the most of amorphous solids, the internal friction below 10 K exhibits a temperature independent plateau, which is caused by the atomic tunnelling states-a measure of structure disorder. In this work, we have varied the concentration of sp3 versus sp2 carbon atoms by increasing laser fluence from 1.5 to 30 J/cm2. Our results show that the internal friction has a nonmonotonic dependence on sp3/sp2 ratio with the values of the internal friction plateaus varying between 6×10−5 and 1.1×10−4. We explain our findings as a result of a possible competition between the increase of atomic bonding and the increase of internal strain in the films, both of which are important in determining the tunneling states in amorphous solids. The importance of the internal strain in diamond-like carbon films is consistent with our previous study on laser fluence, doping, and annealing, which we will review as well. In contrast, no significant dependence of laser fluence is found in shear moduli of the films, which vary between 220 and 250 GPa.