Thermally Induced Structural Evolution of Silicon- and Oxygen-Containing Hydrogenated Amorphous Carbon: A Combined Spectroscopic and Molecular Dynamics Simulation Investigation

Abstract

Silicon- and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) coatings are amorphous thin-film materials composed of hydrogenated amorphous carbon (a-C:H), doped with silicon and oxygen. Compared to a-C:H, a-C:H:Si:O exhibits much lower susceptibility to oxidative degradation and higher thermal stability, making a-C:H:Si:O attractive for many applications. However, the physical mechanisms for this improved behavior are not understood. Here, the thermally induced structural evolution of a-C:H:Si:O was investigated in situ by X-ray photoelectron and absorption spectroscopy, as well as molecular dynamics (MD) simulations. The spectroscopy results indicate that upon high vacuum annealing, two thermally activated processes with a Gaussian distribution of activation energies with mean value E and standard deviation σ take place in a-C:H:Si:O: (a) ordering and clustering of sp carbon ( E ± σ = 0.22 ± 0.08 eV) and (b) conversion of sp- to sp-bonded carbon ( E ± σ = 3.0 ± 1.1 eV). The experimental results are in qualitative agreement with the outcomes of MD simulations performed using a ReaxFF potential. The MD simulations also indicate that the higher thermal stability of a-C:H:Si:O compared to a-C:H (with similar fraction of sp-bonded carbon and hydrogen content) derives from the significantly lower fraction of strained carbon-carbon sp bonds in a-C:H:Si:O compared to a-C:H, which are more likely to break at elevated temperatures.

Silicon Oxide‐Rich Diamond‐Like Carbon: A Conformal, Ultrasmooth Thin Film Material with High Thermo‐Oxidative Stability

“…This influences the resulting thermo‐oxidative stability of the material: while for PIIID films, the sp 3 fraction only changed from 78 ± 3% to 61 ± 3%; for PECVD a‐C:H:Si:O films, the sp 3 fraction drops substantially to 36 ± 3% upon vacuum annealing. It should be noted that the PECVD a‐C:H:Si:O already possesses a greater degree of pure thermal stability under vacuum annealing than an undoped a‐C:H by the metric of activation energy for sp 3 →sp 2 C rehybridization …”

“…Metal‐doping (Cr, Pt, Au) leads to a unique optical absorbance spectrum, which makes such materials attractive in solar energy applications, for example . a‐C:H:Si:O has been shown to exhibit higher thermal and oxidative stability . The authors of this work recently demonstrated that the a‐C:H:Si:O films grown by PECVD with silicon and oxygen levels of, respectively, 6 ± 1 at.% and 3 ± 1 at.% exhibit a slightly‐enhanced thermal stability under high vacuum (HV) conditions compared to a‐C:H (with similar fraction of sp 2 ‐bonded carbon and hydrogen content).…”

“…The authors of this work recently demonstrated that the a‐C:H:Si:O films grown by PECVD with silicon and oxygen levels of, respectively, 6 ± 1 at.% and 3 ± 1 at.% exhibit a slightly‐enhanced thermal stability under high vacuum (HV) conditions compared to a‐C:H (with similar fraction of sp 2 ‐bonded carbon and hydrogen content). Molecular dynamics simulations suggested that this slight enhancement in thermal stability is induced by the reduction of the number of strained carbon–carbon sp 3 bonds in a‐C:H:Si:O compared to a‐C:H, which are more likely to break at elevated temperatures . The introduction of silicon and oxygen in a‐C:H:Si:O was, however, found to drastically increase the thermo‐oxidative resistance through the formation of a thin silica surface layer, which prevents the underlying carbonaceous phase to be eroded.…”

“…A detailed description of the experimental methods is reported in the Supporting Information. Characterization of the commercial PECVD a‐C:H:Si:O has been previously reported …”

“…Upon vacuum annealing, a modest decrease in sp 3 fraction (from 78 ± 3% to 61 ± 3%) was detected. This can be due to the thermally‐induced scission of CH, CSi, and CC bonds . Changes during air annealing are more pronounced: a significant decrease in the intensity of the post‐edge region ( Figure a) was observed, which indicated a decrease in the number density of absorbing carbon atoms, in agreement with XPS results.…”