Chemical and Electronic Structure of Aromatic/Carborane Composite Films by PECVD for Neutron Detection
Description: Boron carbide-aromatic composites, formed by plasma-enhanced co-deposition of carboranes and aromatic precursors, present enhanced electron-hole separation as neutron detector. This is achieved by aromatic coordination to the carborane icosahedra and results in improved neutron detection efficiency. Photoemission (XPS) and FTIR suggest that chemical bonding between B atoms in icosahedra and aromatic contents with preservation of π system during plasma process. XPS, UPS, density functional theory (DFT) calculations, and variable angle spectroscopic ellipsometery (VASE) demonstrate that for orthocarborane/pyridine and orthocarborane/aniline films, states near the valence band maximum are aromatic in character, while states near the conduction band minimum include those of either carborane or aromatic character. Thus, excitation across the band gap results in electrons and holes on carboranes and aromatics, respectively. Further such aromatic-carborane interaction dramatically shrinks the indirect band gap from 3 eV (PECVD orthocarborane) to ~ 1.6 eV (PECVD orthocarborane/pyridine) to ~1.0 eV (PECVD orthocarborane/aniline), with little variation in such properties with aromatic/orthocarborane stoichiometry. The narrowed band gap indicate the potential for greatly enhanced charge generation relative to PECVD orthocarborane films, as confirmed by zero-bias neutron voltaic studies. The results indicate that the enhanced electron-hole separation and band gap narrowing observed for aromatic/orthocarborane films relative to PECVD orthocarborane, has significant potential for a range of applications, including neutron detection, photovoltaics, and photocatalysis. Acknowledgements: This work was supported by the Defense Threat Reduction Agency (Grant No.HDTRA1-14-1-0041). James Hilfiker is also gratefully acknowledged for stimulating discussions.
Date: December 2016
Creator: Dong, Bin