Electronic structure and evolution of dehydrogenation of orthocarborane, metacarborane, paracarborane, ortho-phosphacarborane and meta-phosphacarborane

This work compares the molecular films of three different isomers of closo-dicarbadodecaborane (orthocarborane (1,2-C2B 10H12), metacarborane (1,7-C2B10H 12), paracarborane (1,12-C2B10H12)) and two related icosahedral cage molecules, 1-phospha-2-carbadodecaborane (1,2-PCB10H11) and 1-phospha-7-carbadodecaborane (1,7-PCB 10H11) adsorbed on a variety of substrates. While the experimental electronic structure from combined photoemission and inverse photoemission studies of the molecular films are in good agreement with semi-empirical calculations for the isolated molecule, there is a shift in the chemical potential for each molecule. The experimental position of the molecular chemical potential implicates an influence of both interface and adsorbate dipole. The decomposition or dehydrogenation of the molecules was investigated as well. Decomposition results in a closing of the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gap. This reflects a transition from the molecular film to a semiconductor. These molecules are precursors to different types of semiconductors (n- or p-type). We find that some characteristics of the molecular films give us an insight into why they produce different types of semiconductors. Although the molecules are simply isomers of one another, a number of complexities are implicated in how the films are formed. Multiple polytypes of semiconducting boron carbides are indicated by this work, although the full range of polytypes has not been addressed