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

摘要

A new class of semi-conducting boron carbide polymers has been formed from cross-linking of ortho-carborane (o-B_(10_C_2H_(12)) or o-B_(10)C_2H_(12) in the presence of aromatic compounds (Y =1,4-diaminobenzene, pyridine, and benzene) to form B_(10)C_2H_X and B_(10)C_2H_X:Y, respectively. Core and valence band photoemission and molecular orbital calculations indicate that cross-linking of o-B_(10)C_2H_(12) results in site-specific bonding between boron sites opposite carbon sites on the carborane icosahedra. This site specificity is retained when cross-linking o-B_(10)C_2H_(12) 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. B_(10)C_2H_X:Y films exhibit shifts in the valence band maximum from -4.3 eV (i.e. 4.3 eV below the Fermi level) for B_(10)C_2Hx, 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-B_(10)C_2H_(12) 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.