A new assisted molecular cycloaddition on boron doped silicon surfaces: a predictive DFT-D study

In the framework of the Density Functional Theory (DFT-D), we investigate the phthalocyanine (H2Pc) molecule adsorption on SiC(0001)3 × 3 and Si(111)√3 × √3R30°-B (SiB) surfaces, and particularly compare the involved molecular adsorptions. In the H2Pc–SiC(0001)3 × 3 system, the molecular adsorption can be ascribed to a [10+2] cycloaddition. The H2Pc–SiB system is considered in three cases: defectless SiB surface (denoted SiB-0D) and SiB surfaces presenting one or two boron defects (denoted SiB-1D and SiB-2D respectively). The SiB-0D surface is passivated by a charge transfer from the Si adatoms to the boron atoms and therefore no chemical bond between the molecule and the substrate is observed. A similar molecular adsorption as already evidenced in the H2Pc–SiC(0001)3 × 3 system is involved in the SiB-2D case. In the case of the SiB-1D surface, two Si–N bonds (Si1–N1 and Si2–N2) are observed. One of them, Si1–N1, is nearly similar to that found in the H2Pc–SiB-2D system, but the Si2–N2 bond is unexpected. The Bader charge analysis suggests that, in the presence of the H2Pc molecule, the boron atoms behave like an electron reservoir whose availability varies following the involved molecular adsorption process. In the SiB-1D case, charges are transferred from the substrate to the molecule, allowing the Si2–N2 bond formation. Such a kind of molecular adsorption, not yet observed, could be designed by “assisted pseudo cycloaddition”.