Guo Hong, Ji Wei, Polanyi John C, Yang Jody S Y
Centre for the Physics of Materials and Department of Physics, McGill University, Montréal, Québec, Canada.
ACS Nano. 2008 Apr;2(4):699-706. doi: 10.1021/nn800017d.
An earlier experimental study by scanning tunneling microscopy (STM) from this laboratory described the use of "localized reaction" as a means to the electron- or photon-imprinting of self-assembled patterns of CH(3)Br(ad) as covalently bound Br-Si(s) at Si(111)-7x7. Here we show that the thermal surface bromination reaction by CH(3)Br(ad) is also highly localized, and present a detailed ab initio dynamical model for the reaction, using DFT. Localization is seen to be due to the coexistence in the reactive transition-state of the neighboring bonds being broken (C-Br) and formed (Br-Si). Both experiment and theory are consistent with a low energy-barrier, E(a) approximately 0.2 eV, for the thermal bromination of Si(111) by CH(3)Br(ad), and also for the desorption of intact CH(3)Br(g) (E(des) approximately 0.2 eV). Two physisorbed states of CH(3)Br(ad)/Si(111) (I and II) are distinguishable by STM at 50 K by their differing displacement from the underlying Si adatom. These states can be identified with similarly displaced states in the STM images simulated by DFT. At the elevated temperature of 80 K, a markedly displaced physisorbed state (III) appears in the STM image, indicated by DFT to have a configuration encountered along the reaction path immediately prior to the transition state. The electron-induced bromination of Si(111) by CH(3)Br(ad), and also electron-induced molecular desorption, are examined as a function of the energy of the incident electron, giving for both processes a threshold energy of E(e) approximately 1.8 eV in accord with ab initio theory, and a substantial yield of 10(-6) to 10(-5) Br-Si(s)/electron.
本实验室早期通过扫描隧道显微镜（STM）进行的一项实验研究描述了利用“局部反应”作为一种手段，将CH(3)Br(ad)的自组装图案以共价键结合的Br-Si(s)形式电子或光子印记在Si(111)-7x7上。在此我们表明，CH(3)Br(ad)的热表面溴化反应也是高度局部化的，并使用密度泛函理论（DFT）给出了该反应详细的从头算动力学模型。可以看出局部化是由于在反应过渡态中相邻键的断裂（C-Br）和形成（Br-Si）同时存在。实验和理论均表明，CH(3)Br(ad)对Si(111)进行热溴化反应以及完整的CH(3)Br(g)脱附（E(des)约为0.2 eV）的能垒较低，E(a)约为0.2 eV。在50 K时，STM可通过CH(3)Br(ad)/Si(111)的两种物理吸附态（I和II）与底层Si原子的不同位移来区分它们。这些态可与DFT模拟的STM图像中具有类似位移的态相对应。在80 K的升高温度下，STM图像中出现了一个明显位移的物理吸附态（III），DFT表明其构型是在反应路径上紧接过渡态之前遇到的。研究了CH(3)Br(ad)对Si(111)的电子诱导溴化反应以及电子诱导的分子脱附与入射电子能量的关系，结果表明这两个过程的阈值能量E(e)约为1.8 eV，这与从头算理论一致，并且每电子的产率高达10^(-6)至10^(-5) Br-Si(s)。
