Surface Stability and Electronic Structure of Hydrogen and Fluorine Terminated Diamond Surfaces: A First Principles Inve
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Surface Stability and Electronic Structure of Hydrogen and Fluorine Terminated Diamond Surfaces: A First Principles Investigation Fatih G. Sen1, Yue Qi2 and Ahmet T. Alpas1 1 NSERC/GM of Canada Industrial Research Chair, Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Ave, Windsor, ON, N9B 3P4 Canada. 2 Materials and Process Lab, GM Research and Development Center MC 480-106-224, Mound Road, Warren, MI, U.S.A. ABSTRACT The stability and electronic structure of fully H or F terminated and mixed H and F terminated diamond (111) surfaces were studied using first principles calculations. It was found that F atoms on the surface, like H, formed sp3 type bonding with C atoms, which resulted in a more stable 1x1 configuration rather than the π-bonded 2x1 construction of clean diamond. A phase diagram showing the stable surface composition regions was constructed as a function of chemical potentials of H and F. The diagram shows that the surface with 75% F (25% H) termination was unstable. The F terminated surface was more stable than H termination due to the formation of strong ionic C-F bonding and the close packing of the large F atoms. Due to the attractive forces developed between F atoms, a close packed surface was formed. Additionally, the exposure of C to the environment became restricted because of the large size of F atoms. Hence, F terminated diamond surface was more chemically inert compared to H terminated surface. To bring two F terminated surfaces together, a large repulsive force was required due to the negative charge on F atoms, and this led to low adhesion between two F terminated diamond surfaces compared to two H terminated surfaces. INTRODUCTION Diamond-like carbon (DLC) coatings improve the hardness, durability and chemical resistance of the tool materials that are used in machining [1]. The properties of DLC are dependent on sp3/sp2 bonding ratio of carbon atoms and their hydrogen content [2]. The termination of dangling bonds by hydrogen or hydroxide group, hinders the interaction of surface carbon atoms with the environment resulting a more stable surface and low coefficient of friction ( −2.91 eV/atom. It should be noted that the chemical potential needed for F termination is much lower than for H termination.
Figure 2. Surface phase diagram of stable structures of hydrogen and fluorine terminated diamond (111)1x1surfaces. The graph shows the predominant areas of stable surface terminations for a particular gas composition. The upper limit of chemical potentials of H and F are defined as the energies of atomic H ( µ H 0 − µ H = 2.281 eV/atom) and atomic F ( µ F 0 − µ F = 1.316 eV/atom).
The stability of surface structures was presented in the form of a surface phase diagram shown in Figure 2. Figure 2 also shows that the F terminated surfaces remain stable over the largest chemical potential space. When pure H2 and F2 gas sources are used in the surface preparation, a fully F terminated surface becomes stable. No 1H3F surface termin
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