石墨表面钠原子和团簇的密度泛函研究
Sodium atoms and clusters on graphite: a density functional study
摘要：
采用密度泛函理论、赝势和周期性边界条件研究了石墨（0001）表面钠原子和原子团簇（N<5）的动力学性质。观察到单个Na原子结合在表面上方2.45 A的中空位点处，吸附能为0.51 eV。0.06 eV的小扩散势垒表明平坦的势能面。增加Na覆盖导致弱的吸附物-基质相互作用，这在Na_3、Na_4、Na_5和（2x2）Na覆盖层的情况下与表面的较大分离中是明显的。Na_2的结合较弱，它具有一个完整的价电子层.底物的存在显著改变了Na_3、Na_4和Na_5的结构，Na_4和Na_5的平面性都发生了畸变。计算的形成能表明，原子团簇在能量上是有利的，开壳层团簇（如Na_3和Na_5）在石墨上比在气相中更丰富。对Na和Na_3的横向电荷密度分布的分析表明，在两种情况下，约0.5个电子的电荷转移。
英文摘要：
Sodium atoms and clusters (N<5) on graphite (0001) are studied using density functional theory, pseudopotentials and periodic boundary conditions. A single Na atom is observed to bind at a hollow site 2.45 A above the surface with an adsorption energy of 0.51 eV. The small diffusion barrier of 0.06 eV indicates a flat potential energy surface. Increased Na coverage results in a weak adsorbate-substrate interaction, which is evident in the larger separation from the surface in the cases of Na_3, Na_4, Na_5, and the (2x2) Na overlayer. The binding is weak for Na_2, which has a full valence electron shell. The presence of substrate modifies the structures of Na_3, Na_4, and Na_5 significantly, and both Na_4 and Na_5 are distorted from planarity. The calculated formation energies suggest that clustering of atoms is energetically favorable, and that the open shell clusters (e.g. Na_3 and Na_5) can be more abundant on graphite than in the gas phase. Analysis of the lateral charge density distributions of Na and Na_3 shows a charge transfer of about 0.5 electrons in both cases.
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