Chemical and electronic structure of composite films deposited by plasma-enhanced chemical vapor deposition from orthocarborane and pyridine source compounds

• Published in: Journal of electron spectroscopy and related phenomena Vol. 223; pp. 21 - 28
• Main Authors: Dong, Bin, Echeverria, E., Oyelade, A., Converse, D., Silva, J., Rimsza, J.M., Du, J., Driver, M.S., Hayworth, Benjamin, Shao, Nan, Gao, Yi, Mei, Wai-Ning, Dowben, P.A., Kelber, J.A.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.02.2018
• Subjects: Aromatic linking groups; Boron carbide; Density of states; PECVD; Density of states; PECVD; Aromatic linking groups; Boron carbide
• ISSN: 0368-2048; 1873-2526
• DOI: 10.1016/j.elspec.2017.12.003

•Semiconducting films were formed by plasma-enhanced vapor co-deposition of pyridine and orthocarborane.•Pyridine moieties are bonded to orthocarborane moieties at carborane boron sites.•States near the top of the valence band are associated with pyridine.•States near the bottom of the conduction band have spectral weight contributions from both orthocarborane and pyridine moieties.•The films exhibit an average coordination number of 3–4 pyridine moieties for each carborane. Semiconducting boron carbide films, with aromatic moieties, were formed by plasma-enhanced chemical vapor deposition (PECVD) using pyridine (C5H5N) and closo-1,2 dicarbadodecaborane (1,2-B10C2H12; ortho-carborane) as source compounds. X-ray photoelectron spectroscopy (XPS), in conjunction with pyridine/orthcarborane cluster density functional theory (DFT) calculations, indicates that such films consist of partially dehydrogenated orthocarboranes coordinated to pyridine moieties at icosahedral B sites. The XPS data also suggest a constant average coordination number of 3–4 pyridines per icosahedral carborane, indicating that higher pyridine content in the film results in greater areas of plasma-polymerized pyridine. Ultraviolet photoelectron spectroscopy (UPS) results and DFT cluster calculations indicate that electronic states near the valence band maximum are associated with pyridine moieties, while states near the conduction band minimum are associated with either carborane or pyridine moieties. Variable angle spectroscopic ellipsometry (VASE), the UPS data and theoretical results also indicate only gradual changes in indirect band gap energies with changing pyridine/orthocarborane stoichiometry, in agreement with previously reported results. The results presented here are consistent with and provide additional insight regarding recent neutron voltaic and photoluminescence results for these materials.