The electronic structure of 1,2-PCB10H11 molecular films:: a precursor to a novel semiconductor

The band gaps and electronic structure of undoped films of molecular icosahedra of closo-1-phospha-2-carbadodecaborane (1,2-PCB10H11) are reported. 1,2-PCB10H11 adsorbs on Au and Ag substrates to generate molecular thin films with the Fermi level (chemical potential) placed closer to the lowest unoccupied molecular orbital than has been observed with closo-1,2-dicarbadodecaborane (1,2-C2B10H12, orthocarborane) adsorbed on Co, Cu or Ag. Both 1,2-PCB10H11 and 1,2-C2B10H12 molecular films exhibit an unoccupied molecular defect state above the Fermi level. The vibrational modes, observed in infra-red absorption, are close to the values expected for the isolated 1,2-PCB10H11 molecule. Consistent with the placement of the Fermi level in the molecular films, fabrication of heterojunction diodes from partially dehydrogenated 1,2-PCB10H11 indicates that the resultant PCB10Hx semiconductor film is more n-type than the corresponding boron carbide semiconductor formed from 1,2-C2B10H12, orthocarborane.