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

Abstract

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".

Opening the way to molecular cycloaddition of large molecules on supported silicene

“…Moreover, previous studies on SiC and SiB with two boron defects have shown that the nitrogen atoms bond to Si adatoms, that is, lying above the surface. 26,27,30 Now, the distance between two nitrogen atoms of the H 2 Pc molecule is 6.78 Å. In agreement with this distance, there are only three possible adsorption sites for the H 2 Pc molecule on silicene layer.…”

“…• -boron surfaces (noted SiC and SiB, respectively). 26,27,30 In order to compare the adsorption of H 2 Pc on silicene/Ag with that on the surfaces of SiC, passivated SiB, and SiB with two defects (a boron defect is the substitution of one boron atom by a silicon one), the same parameters have been used in the present study. In particular, the integration of the Brillouin zone was reduced to the Γ k-point.…”

“…The number of layers in the present slab is similar to that used in a previous work. 30 The H 2 Pc molecule and the four top-most surface layers were allowed to relax during structural optimization, which is obtained when the residual forces are equal to 0.02 eV/Å. The rest of the atoms in the slab were fixed at their bulk-optimized lattice positions.…”

“…15,[18][19][20][21][22][23][24][25][69][70][71][72] In the case of the H 2 Pc molecule adsorption on the SiC and SiB surfaces, the structural study has shown that the H 2 Pc molecule stays nearly planar. 26,27,30 Clearly, it is not the case for the adsorption of H 2 Pc on silicene/Ag. Nevertheless, the N1-N2 and Si-N distances for these last systems (7.16 Å and 1.88 Å for H 2 Pc/SiC and 6.50 Å and TABLE I.…”

“…• -boron in presence of one boron defect. 30 In all papers, the calculations have shown that the H 2 Pc molecule remains planar after chemisorption. 26,27,30 The cycloaddition reaction seems to be a general mechanism of molecular adsorption on surfaces presenting top silicon atoms, as the Si(001) (2 × 1), Si(111) (7 × 7), Si(111)…”