Reactive Molecular Dynamics Investigation of the Thermal Stability of Organic Monolayers Grafted on Si(111)

F. A .SORIA; P. PAREDES-OLIVERA; E. M. PATRITO

Boston

Congreso; 2015 MRS Fall Meeting; 2015

MRS

Abstract Body: Organic monolayers covalently anchored to silicon surfaces represent a topic of fundamental and applied interest because such layers may be used as a thin dielectric, as a protection barrier or as a primer layers for use in microelectronics. The functionalization of silicon surface is also of interest in the development of chemical or biochemical sensors.A key issue for such applications is the chemical and thermal stability of the monolayers. In previous works we investigated the chemical stability of hydrogenated and alkylated Si(111) surfaces (1, 2) towards O2 and H2O oxidizing species using Density Functional Theory.In this work have we have employed reactive molecular dynamics simulations using the ReaxFF force field in order to understand the mechanisms of thermal decomposition of CH3, CH2CH3, CH2CH2CH3, CH2(CH2)2CH3,CH2(CH2)3CH3 y CH2(CH2)8CH3  grafted to the Si(111) surface. A maximum theoretical coverage of around 69% is predicted (3) for alkane monolayers on Si(111), implying that hydrogenated silicon atoms still remain on the surface.The only exception is the −CH monolayer which has every atop Si atom is bound to a methyl group. Surface SiH groups play a key role in the stability of the monoalyers. The thermal decomposition of the monolayers was investigated in the temperature range from 500 to 1500 K. The firststep in the decomposition mechanism involves the breakage of surface SiH bonds and the diffusion of hydrogen atoms into the bulk, leaving surface silyl radicals on the surface. These radicals first abstract hydrogen atoms from theCH group of beta carbon atoms which produces the desorption of alkene molecules. As the reaction proceeds, the surface coverage of organic molecules decreases and this allows the molecules to lie down on the surface as a consequence of the interaction with surface sylil radicals. This initiates the dehydrogenation of all the methylene groups. At the highest  temperatures a full dehydrogenation occurs giving rise to the formation of silicon carbide on the surface.The full coverage −CH monolayer as an exceptional thermal stability due to the absence of SiH groups. For this monolayer, the dehydrogenation of the methyl group occurs by a H transfer to the Si atom bound to the C atom whichhas a much higher energy barrier than the transfer of an H atom from a methylene group to an adjacent sylil radical as is the case for the long chain monolayers. For the most relevant elementary reaction steps identified in the MD simulations, Nudged Elastic Band calculations based on DFT energies were performed in order to obtain the energy profile along the reaction path from which activation energy barriers are calculated.(1) F. A. Soria, E. M. Patrito, P. Paredes-Olivera. J. Phys. Chem. C, 2012, 116, 24607.(2) F. A. Soria, P. Paredes-Olivera, E. M. Patrito J. Phys. Chem. C. 2015, 119, 284.(3) L. Scheres, B. Rijksen, M. Giesbers, H. Zuilhof Langmuir, 2011, 27, 972.