제일원리계산을 통한 Si(001) 표면에 형성된 유기 분자선의 자기조립법에 관한 연구

Abstract

I have studied the self-assembly of one-dimensional (1D) organic molecular lines on the H-terminated Si(001)-2x1 surface using first-principles density-functional calculations. I present not only new growth mechanism of the allyl mercaptan (ALM) molecular line, which is an unique example of line across the dimer rows on the H-terminated Si(001)-2x1, but also novel fabrication methods for various self-assembled molecular lines. First, for the self-assembly of ALM molecular lines, I propose new chain reaction mechanism including formation of Si-S bond accompanying the associative desorption of H2. The proposed structural model of the ALM line shows that the molecules adsorb across two Si dimers in the adjacent dimer rows with the Si-C and Si-S bonds, thereby yielding a higher thermodynamic stability compared to other alkene lines (containing a single Si-C bond per molecule) grown along the dimer rows. This accounts for a successful growth of ALM lines which were observed to be stable even at a high temperature of 650 K. Second, I propose novel fabrication technique for the self-assembled heterogeneous molecular lines on the dangling-bond line generated on the H-terminated Si(001)-2x1 surface. Here, I choose pyridine and borine as Lewis base and Lewis acid, respectively, showing different behaviors in the chemical reactivity and selectivity on the dangling-bond line, leading to formation of the heterogeneous pyridine-borine line. In addition, I present the Peierls instability in the 1D borine line formed on the Si(001)-2x1 surface. I find that the borine line is stabilized by formation of a 1D charge density wave, accompanying a structural distortion with a double periodicity and a band-gap opening at the Fermi level. Third, I present new self-assembly method for the enhanced stability of 1D molecular lines on the H-terminated Si(001)-2x1 surface. Instead of a previously employed single dangling bond, I here employ a single H-free Si dimer as a reaction site, resulting in an enhanced stability of the radical intermediate for the o-phthalaldehyde (OP) molecule containing two carbonyl groups. This radical intermediate easily abstracts two H atoms from a neighboring Si dimer, thereby allowing the chain reaction for a 1D molecular line. Such a fabricated OP line will be stable at higher temperatures compared to previously reported alkene lines because of its enhanced stability. Finally, I present theoretical investigations of the self-assembled growth of 1D molecular lines directed across the dimer rows on the H-terminated Si(001)-2x1 surface. New growth mechanism of the 1D acetylacetone line is proposed, which involves the radical chain reaction initiated at two dangling-bond sites on one side of two adjacent Si dimers. It is also enabled that, if an H-free Si dimer were employed as the initial reaction site, a 1D acetylacetone line can grow along the dimer row. My findings represent the first insight into the growth of 1D molecular lines not only across but also along the dimer rows on the H-terminated Si(001)-2x1 surface.