| AVS 60th International Symposium and Exhibition | |
| Applied Surface Science | Thursday Sessions |
| Session AS-ThP |
| Session: | Applied Surface Science Poster Session |
| Presenter: | F. Mangolini, University of Pennsylvania |
| Authors: | R.W. Carpick, University of Pennsylvania F. Mangolini, University of Pennsylvania J. Hilbert, University of Pennsylvania J.R. Lukes, University of Pennsylvania |
| Correspondent: | Click to Email |
Hydrogenated amorphous carbon (a-C:H) thin films are amongst the strongest, smoothest, and most lubricious coatings in existence. The impressive properties of these materials have resulted in their use in a wide range of applications. In particular, thin a-C:H films are employed as lynchpin materials to protect computer hard disks from corrosion and wear. Even though amorphous carbon-based materials have been studied for more than two decades, there is significant ambiguity regarding the mechanisms by which they transform in response to temperature or other energetic inputs. Quantifying the energetics and specifying the physical pathways of thermally-induced structural transformations have proven difficult. Progress has been limited by the challenges associated with the experimental investigation of the structure and bonding configuration of these materials in their thin film configurations, calling for the development of advanced analytical methods.
In this work, new insights into the thermally-induced structural evolution of a-C:H were gained by coupling experiments and molecular dynamics (MD) simulations. A new experimental methodology for quantitatively determining the bonding configuration of carbon in the near-surface region of a-C:H thin films was developed on the basis of X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES) and allowed the in situ XPS and XAES investigation of the thermally-induced structural evolution of a-C:H. Upon high vacuum annealing, three thermally-activated processes with an assumed Gaussian distribution of activation energies with mean value E and standard deviation σ occur in a-C:H: a) ordering and clustering of sp2-hybridized C (E=0.18 eV; σ=0.05 eV); b) scission of sp3 C-H bonds with formation of sp2-hybridized C (E = 1.7 eV; σ = 0.5 eV); and c) direct transformation of sp3- to sp2-hybridized C (E = 3.5 eV; σ = 0.5 eV). This first XPS-based study both demonstrates the low absolute energy barrier for clustering of the sp2 phases, and indicates that hydrogen enables conversion to sp2 hybridization in these films.
The experimental results were compared with the outcomes of MD simulations performed using the adaptive intermolecular reactive bond order potential. The atomic composition was chosen to match experiments, and the resulting structure was relaxed to the measured density. This enabled the direct visualization of the structure of a-C:H and its evolution as a function of temperature and time. We will discuss the comparison between the simulation and experimental results, emphasizing the insights gained from the fully atomistic picture provided by the atomistic simulations.