Lee Jun-Ho, Lee Ji Young, Cho Jun-Hyung
BK21 Program Division of Advanced Research and Education in Physics and Research Institute of National Sciences, Hanyang University, 17 Haengdang-Dong, Seongdong-Ku, Seoul 133-791, Republic of Korea.
J Chem Phys. 2008 Nov 21;129(19):194110. doi: 10.1063/1.3021075.
Using first-principles density-functional calculations, we investigate the reaction of acetone on the Si(001) surface, which exhibits the conversion from a kinetically controlled reaction to thermodynamically controlled one by means of thermal anneal or the highly confined electron beam of the scanning tunneling microscopy (STM) tip. We identified the four different reaction pathways forming not only two kinds of di-sigma structures on top of a single Si dimer (termed as the [2+2] cycloaddition structure) and across the ends of two adjacent Si dimers but also two bridge-bonded dissociative structures (termed the "end-bridge" and "dimer-bridge" structures) involving two adjacent Si dimers. Our calculated energy profiles for the reaction pathways show not only that formation of the [2+2] cycloaddition structure is kinetically favored because of its low-energy barrier, but also that, as temperature increases, the kinetically favored [2+2] cycloaddition structure is converted to the more thermodynamically stable end-bridge and dimer-bridge structures via an intermediate state where the O atom forms a dative bond to the down Si atom of the buckled dimer. In addition, we find that the Si-C bonding (antibonding) states of the [2+2] cycloaddition structure appear at about 1-2 (2-3) eV below (above) the Fermi level, in which injected holes (electrons) through the STM tip can be created (trapped) to give rise to a Si-C bond breakage. These results manifest that the kinetically controlled reaction of acetone on Si(001) is associated with the [2+2] cycloaddition structure, rather than the alpha-H cleavage structure proposed by a recent STM experiment.
利用第一性原理密度泛函计算，我们研究了丙酮在Si(001)表面的反应，该反应通过热退火或扫描隧道显微镜(STM)针尖的高约束电子束实现了从动力学控制反应到热力学控制反应的转变。我们确定了四种不同的反应途径，不仅在单个硅二聚体顶部形成了两种二西格玛结构（称为[2+2]环加成结构），跨越两个相邻硅二聚体的端部，还形成了涉及两个相邻硅二聚体的两种桥键解离结构（称为“端桥”和“二聚体桥”结构）。我们计算的反应途径能量分布不仅表明[2+2]环加成结构因其低能垒在动力学上更有利，而且随着温度升高，动力学上有利的[2+2]环加成结构通过一个中间态转变为热力学上更稳定的端桥和二聚体桥结构，在该中间态中，O原子与弯曲二聚体的下方Si原子形成配位键。此外，我们发现[2+2]环加成结构的Si-C键（反键）态出现在费米能级以下约1-2（2-3）eV（以上），通过STM针尖注入的空穴（电子）可以在其中产生（捕获），从而导致Si-C键断裂。这些结果表明，丙酮在Si(001)上的动力学控制反应与[2+2]环加成结构有关，而不是最近STM实验提出的α-H裂解结构。
