Novel Cross-Linked Ortho-Carborane and Ortho-Carborane:Y (Y=1,4-Diaminobenzene, Pyridine, Benzene) Polymer Films: A New Class of Carborane-Based Materials with Tunable Electronic Structure

Abstract

A new class of semi-conducting boron carbide polymers has been formed from cross-linking of ortho-carborane (o-B10C2H12) or o-B10C2H12 in the presence of aromatic compounds (Y =1,4-diaminobenzene, pyridine, and benzene) to form B10C2HX and B10C2HX:Y, respectively. Core and valence band photoemission and molecular orbital calculations indicate that cross-linking of o-B10C2H12 results in site-specific bonding between boron sites opposite carbon sites on the carborane icosahedra. This site specificity is retained when cross-linking o-B10C2H12 in the presence of aromatic linking units; the linking units insert themselves between carborane icosahedra. This insertion results in bonding between boron sites opposite carbon sites in the carborane icosahedra to carbon sites on the aromatic moieties. B10C2HX:Y films exhibit shifts in the valence band maximum from -4.3 eV (i.e. 4.3 eV below the Fermi level) for B10C2Hx, to -2.6 eV, -2.2 eV and -1.7 eV for Y = benzene, pyridine, or 1,4-diaminobenzene, respectively. States near the top of valence band correspond to states localized on the linking unit. Films formed by plasma-enhanced chemical vapor deposition of o-B10C2H12 are p-doped, and such films show excellent rectifying characteristics when deposited on n-type Si--extremely promising for neutron detection and other device applications.