| AVS 63rd International Symposium & Exhibition | |
| Thin Film | Monday Sessions |
| Session TF+PS+SE-MoA |
| Session: | Plasma-based Deposition Techniques and Film Characterization |
| Presenter: | Jeffry Kelber, University of North Texas |
| Authors: | B. Dong, University of North Texas A. Oyelade, University of North Texas E.M. Echeverria, University of Nebraska-Lincoln Y-S. Jun, University of California at Santa Barbara G.D. Stucky, University of California at Santa Barbara P.A. Dowben, University of Nebraska-Lincoln J.A. Kelber, University of North Texas |
| Correspondent: | Click to Email |
Many photovoltaic and photocatalytic systems employ band-bending at surfaces or interfaces to achieve electron-hole separation and functionality. Boron carbide-aromatic composites, formed by plasma-enhanced co-deposition of carboranes and aromatic precursors, present an alternative approach where such separation is achieved by aromatic coordination to the carborane icosahedra. Photoemission, density functional theory calculations, and variable angle spectroscopic ellipsometry demonstrate that for orthocarborane/pyridine and orthocarborane/aniline films, with controlled aromatic/orthocarborane ratios between 1:1 and 10: 1, 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. Recent photoabsorbance measurements show that in orthocarborane/pyridine films, the indirect band gap energy is significantly less than the exciton formation energy of 2.1 eV, allowing facile exciton elimination by phonon scattering of electrons into the conduction band at room temperature. The opposite is true for the PECVD orthocarborane film, where the exciton formation energy (2.4 eV) is less than the indirect band gap, inhibiting exciton elimination by electron-hole separation. The enhanced electron-hole separation, narrowed band gap, and significantly increased carrier lifetimes (350 µsec for PECVD orthocarborane/pyridine vs 35 µsec for PECVD orthocarborane), indicate the potential for greatly enhanced charge generation, as confirmed by zero-bias neutron voltaic studies. Those results--an 850% increase in charge generation per B atom for the PECVD pyridine/orthocarborane film relative to the PECVD orthocarborane film--indicate that the enhanced electron-hole separation and band gap narrowing observed for aromatic/orthocarborane films relative to PECVD orthocarborane, have 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). The authors would like to thank Shireen Adenwalla for technical assistance and discussion. James Hilfiker is also gratefully acknowledged for stimulating discussions.