Lee Hee Soon, An Ki-Seok, Kim Yunsoo, Choi Cheol Ho
Department of Chemistry, College of Natural Sciences, Kyungpook National University, Taegu 702-701, South Korea.
J Phys Chem B. 2005 Jun 2;109(21):10909-14. doi: 10.1021/jp0505983.
The potential energy surfaces of one, two, and three water molecule sequential adsorptions on the symmetrically chlorinated Si(100)-2 x 1 surface were theoretically explored with SIMOMM:MP2/6-31G(d). The first water molecule adsorption to the surface dimer requires a higher reaction barrier than the subsequent second water molecule adsorption. The lone pair electrons of the incoming water molecule nucleophilically attack the surface Si atom to which the leaving Cl group is bonded, yielding an S(N)2 type transition state. At the same time, the Cl abstracts the H atom of the incoming water molecule, forming a unique four-membered ring conformation. The second water molecule adsorption to the same surface dimer requires a much lower reaction barrier, which is attributed to the surface cooperative effect by the surface hydroxyl group that can form a hydrogen bond with the incoming second water molecule. The third water molecule adsorption exhibits a higher reaction barrier than the first and the second water molecule adsorption channels but yields a thermodynamically more stable product. In general, it is expected that the surface Si-Cl bonds can be subjected to the substitution reactions by water molecules, yielding surface Si-OH bonds, which can be a good initial template for subsequent surface chemical modifications. However, oversaturations can be a competing side reaction under severe conditions, suggesting that the precise control of surface kinetic environments is necessary to tailor the final surface characteristics.
采用SIMOMM:MP2/6 - 31G(d)理论方法研究了一、二、三个水分子在对称氯化的Si(100)-2×1表面上依次吸附的势能面。第一个水分子吸附到表面二聚体上所需的反应势垒高于随后第二个水分子的吸附。进入的水分子的孤对电子亲核进攻与离去的Cl基团相连的表面Si原子，产生一个S(N)2型过渡态。同时，Cl夺取进入水分子的H原子，形成独特的四元环构象。第二个水分子吸附到同一表面二聚体上所需的反应势垒低得多，这归因于表面羟基与进入的第二个水分子形成氢键的表面协同效应。第三个水分子吸附表现出比第一个和第二个水分子吸附通道更高的反应势垒，但产生热力学上更稳定的产物。一般来说，预计表面Si-Cl键可被水分子进行取代反应，生成表面Si-OH键，这可以作为后续表面化学修饰的良好初始模板。然而，在苛刻条件下过饱和可能是一个竞争的副反应，这表明精确控制表面动力学环境对于定制最终表面特性是必要的。
