Abstract
The dissociation of an ammonia molecule on a cluster of Si atoms simulating the 100 silicon crystal structure with two Si dimers has been investigated by means of the DFT and an approximate instanton methods. The model corresponds to the low coverage limit of the surface. Absolute rate constants of two different dissociation paths are evaluated together with deuterium isotope effects. It is demonstrated that, even at room temperatures, the process is dominated by tunneling and that dissociation to a silicon atom of the adjacent dimer, rather than a silicon within the same dimer, is the prevailing mechanism. This leads to creation of a metastable structure which will slowly decay through a two-step hydrogen atom migration towards the absolute minimum on the potential energy surface corresponding to the NH2 group and the hydrogen atom residing in the same dimer.
Highlights
Silicon (100)-(2 x 1) surface is known for its high chemical reactivity resulting from the presence of dangling bonds that leads to formation of Si dimers on the surface [1]
We have reported the kinetics of dissociation of a single ammonia molecule on a model cluster representing the Si(100) surface
The conventional path, tacitly assumed so far as the only one possible, is the one where the hydrogen atom is transferred to a Si atom in the same dimer in which the original NH3 molecule was adsorbed; as a result the NH2 group and the H atom reside on the same dimer
Summary
Silicon (100)-(2 x 1) surface is known for its high chemical reactivity resulting from the presence of dangling bonds that leads to formation of Si dimers on the surface [1]. Avouris and coworkers studied extensively the (100) surface [3] using scanning tunneling microscopy (STM), X-ray (XPS) and ultraviolet photoemission (UPS) spectroscopies. They found that at temperatures as low as 90 K, dissociation of ammonia occurs and that it leaves the Si dimers essentially intact. Dissociation of ammonia in the NH3 → NH2 + H reaction is the primary process to be studied It involves transfer of a hydrogen atom from the chemisorbed ammonia to a silicon atom located presumably within the same Si dimer that adsorbed the ammonia molecule. Some vibrational frequencies of modes involving adsorbed NH3, NH2 and H fragments on the (100) surface were obtained experimentally by means of high resolution vibrational electron energy loss spectroscopy (HREELS) [7,11,12]
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