Structure of H2Pc monolayers on InSb(111)A

The structure of free-base phthalocyanine (H2Pc) layers deposited onto the reconstructed InSb(111)A-(2x2) surface have been studied by low energy electron diffraction, scanning tunneling microscopy (STM), and van der Waals intermolecular interaction energy calculations. The InSb(111)A surface was prepared by gentle low energy ion bombardment and annealing with STM studies indicating two structural domains, both comprising (2x2) unit cells. One domain is consistent with the well-known In-vacancy buckling model, while the other is explained by an Sb-vacancy model. The diffraction pattern for H2Pc monolayers grown on InSb(111)A-(2x2) is a superposition of bright (root12xroot12)R30degrees spots and faint ring features. Two types of ordered structural domain were observed by STM. One domain comprises hexagonal unit cells which are rotated 30degrees with respect to the orientation of the substrate unit cell. This domain contributes to the (root12xroot12)R30degrees diffraction pattern. The second domain comprises quadratic unit cells which are randomly oriented and these contribute to the faint ring features in the diffraction pattern. The existence of the energetically unfavorable hexagonal structure implies a relatively strong substrate-molecule interaction. The lattice dimensions of the H2Pc unit cell deviate slightly from that expected on the basis of the calculations and possible reasons for this discrepancy are discussed.