Takeuchi Noboru, Selloni Annabella
Department of Chemistry, Princeton University, Princeton, New Jersey 08540, USA.
J Phys Chem B. 2005 Jun 23;109(24):11967-72. doi: 10.1021/jp0507344.
Recent experimental work has shown that the addition of styrene molecules to hydrogen-terminated Si(001) surfaces leads to the formation of one-dimensional molecular structures through a radical-initiated surface chain reaction mechanism. These nanometric structures are observed to be directed parallel to the dimer rows on the H-Si(001)-(2 x 1) surface and perpendicular to the same rows on H-Si(001)-(3 x 1). Using periodic density functional theory (DFT) calculations, we have studied the initial steps of the radical chain mechanism on the H-Si(001)-(3 x 1) surface and compared them to analogous results for H-Si(001)-(2 x 1). On the H-Si(001)-(3 x 1) surface, one of the crucial steps of the surface chain reaction, namely, the abstraction of a H atom from a nearby surface hydride unit, is found to have a somewhat smaller activation energy in the direction perpendicular to the dimer rows (H abstraction from the nearest dihydride site) than along the rows (H abstraction from a neighboring dimer). Additionally, due to the steric repulsion between the styrene molecules and the SiH2 subunits, growth along the dimer rows is not thermodynamically favorable on the (3 x 1) surface. On the other hand, due to the absence of the SiH2 subunits, growth parallel to the Si dimer rows becomes favored on the H-Si(001)-(2 x 1) surface.
近期的实验工作表明，将苯乙烯分子添加到氢终止的Si(001)表面会通过自由基引发的表面链反应机制导致一维分子结构的形成。观察到这些纳米结构在H-Si(001)-(2×1)表面上与二聚体行平行，而在H-Si(001)-(3×1)表面上与二聚体行垂直。使用周期性密度泛函理论(DFT)计算，我们研究了H-Si(001)-(3×1)表面上自由基链机制的初始步骤，并将其与H-Si(001)-(2×1)的类似结果进行了比较。在H-Si(001)-(3×1)表面上，表面链反应的关键步骤之一，即从附近表面氢化物单元中提取一个H原子，发现在垂直于二聚体行的方向（从最近的二氢化物位点提取H）上的活化能比沿着行的方向（从相邻二聚体中提取H）略小。此外，由于苯乙烯分子与SiH2亚基之间的空间排斥，在(3×1)表面上沿着二聚体行生长在热力学上是不利的。另一方面，由于不存在SiH2亚基，在H-Si(001)-(2×1)表面上平行于Si二聚体行的生长变得有利。
