Tailored Hydrogen-Free Carbon Films by Tuning the sp2/sp3 Configuration

Carbon configuration is a critical factor in determining the electrical, tribological, and mechanical properties of carbon films. In this work, we synthesized carbon films on a silicate glass by sputtering a graphite target in an Ar atmosphere while varying the DC sputtering power. Using Raman spectroscopy and photoemission analysis, we found that the carbon films produced at low sputtering power consisted mainly of sp(3) bonds, while sp(2) carbon ordering became dominant as the sputtering power increased. The sp(2)/sp(3) ratio of the carbon film controlled by the sputtering power is associated with two competing carbon deposition mechanisms: (1) collision between the incoming carbon ions and surface carbon atoms at low power (sub-plantation model) and (2) sp(3)-to-sp(2) rehybridization caused by excessive kinetic energy of the incoming carbon ions at high power (thermal relaxation). As the sputtering power increased, the friction, adhesion, and energy dissipation decreased, despite negligible topographical variations, while the conductivity rapidly increased. We fabricated a triboelectric nanogenerator with high durability by utilizing the low friction properties of the sputtered carbon films. Our results show a straightforward and effective way to control the properties of carbon films, which can be used as promising coating films for frictionless protection.