Effect of passivants in energy gap of Si47X24Y36 nano-clusters: A theoretical investigation

Passivation on the surface of the nano-cluster Si47 core cluster can lead to the formation of Si47X24Y36, where X and Y represent the inner and outer layer passivated positions, respectively. Accordingly, X and Y positions can accommodate one and three passivants, respectively. Herein, the density functional theory (DFT) and B3LYP method with the 6-31G(d) basis set were used to generate the electronic structures (HOMO, LUMO and energy gap between HOMO and LUMO) for two different fully passivated Si nano-clusters of Si47X60 and Si47X24Y36, both with a Td symmetry. The optimized structures were obtained with the local density approximation (LDA) implemented in SIESTA package. For fully passivated Si47X60 nano-clusters (X=–H, –CH3, –OH, –NH2, –F, –SiH3, –SH, –Cl, –C2H5 and –OCH3) alkyl passivants (–CH3 and –C2H5) affect insignificantly the calculated energy gaps while electron-withdrawing passivants give red-shifted electronic spectra. Same investigations were also conducted for the partially hydrogenated Si nano-clusters, inner layer passivated Si47X24H36 and outer layer passivated Si47H24Y36. The calculated energy gaps of Si nano-clusters with inner layer passivation (Si47X24H36) are close to that of Si47X60, both inner and outer layers passivated. Hence, the outer layer passivant effect is insignificant, which is also demonstrated by insignificant changes in energy gaps (≤0.5eV) for the Si nano-clusters with outer layer passivation since the difference of the Mülliken charge is ≤0.1eV.

Research Highlights►The electron-withdrawing passivants would cause an uneven electronic density around the Si nano-clusters and lower the calculated energy gap. For the inner layer passivants (SiX24H36), the passivation effect is clearly observed for the fully and with increasing passivant numbers, with a well defined tendency after passivant number equals 10.