The role of vibration in surface-migration
1Lash Miller Chemical Laboratories, Department of Chemistry and Institute of Optical Sciences, University of Toronto, 80 St. George Street, Toronto, Ontario, M5S 3H6, Canada
2Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom
3Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-nano Devices, Renmin University of China, Beijing 100872, China
The motion of adsorbate molecules across surfaces is fundamental to self-assembly, material growth, and heterogeneous catalysis. Scanning Tunneling Microscopy (STM) has shown electron-induced long-range surface-migration of chemisorbed ethylene, benzene and related molecules, moving in directed fashion across Si(100) by distances averaging ~50 Å, and as great as 200 Å [K.R. Harikumar et al. Nature Chem. 3, 341 (2011); J. Phys. Chem. C 115, 22409 (2011)]. This motion was ascribed to the effect of a sudden repulsion operating between the adsorbate and the surface. Dynamical studies have not previously been made. They are presented here.
For ethylene at Si(100) we postulate that the energy of the electron (~2.9 eV) has been converted to repulsion. The repulsion is found to excite bending vibration of CH2 in the ethylene which, in subsequent V-T (vibration-to translation) energy-transfer, gives sufficient velocity of the ethylene centre-of-mass to break its two carbon-surface bonds. The resultant motion is that of ethylene arcing upwards in a ballistic trajectory to peak heights ~ 7 Å, thereby carrying the molecule tens of Angstroms perpendicular to the Si(100) dimer-rows --the direction of migration observed. Such 'cannon-ball' behaviour has been postulated previously. Here we compute its molecular dynamics for the first time under the influence of initial repulsion and subsequent van der Waals attraction.
These dynamics indicate that ballistic flight accounts for migration over substantial distances, but are insufficient to explain the observation of migration over the full range of distances. Long-range travel is due instead to a succession of such ballistic events, linked by elastic or super-elastic bounces at the silicon surface. The super-elasticity derives from further V-T vibrational energy-transfer in the course of the bounce [S.Y. Guo, PhD Thesis, U. of Toronto, 2015].
Additionally these calculations predict ballistic migration of neutral halogen atoms, coming from the electron-induced reaction of chemisorbed halides at Cu(110), a system under study here. In this case the downward acceleration returning the ballistic trajectory to the surface is due to long-range chemical attraction operating up to 8 Å above the surface.
Finally, in recent work some of us [A. Chatterjee et al., J. Phys. Chem. C 118, 25525 (2014); experiment and theory] have found surface migration occurring in the chemisorbed diradical carbene (CH2) on Cu(110) by a characteristic 'walking' along the copper rows, due to rocking vibration.
Surface migration would appear to be a field on the move.