First Principles Study Of Tribopolymerization On Conductive Material Surfaces

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Doctor of Philosophy (PhD)
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Computational Studies
Hybrid functionals
Physical Chemistry
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While tribopolymer, usually formed due to organic-molecule polymerization under contact surfaces frictions, is used to lubricate and to protect the surfaces of mechanical gears, it is considered as a major source of contamination to Micro-electro-mechanical system and Nano-electro-mechanical system (MEMS and NEMS) transistors. MEMS and NEMS, analogous to atomic force microscopy (AFM) devices, design with mechanical switching motions to reduce the power consumption caused by maintaining the open-circuit voltage in traditional field-effect transistors, can be considered as promising potential candidates to replace current generation transistors. However, the devices failure occurs usually due to adsorption of ambient molecules onto device contact surfaces following stress confined surface catalytic reactions to form tribopolymers, which are usually insulating and sticky. Experimental results only justify those tribopolymers to be hydrocarbon chains, but detailed composition and formation mechanism remain unclear. Here, we perform theoretical and computational studies using density functional theory methods to systematically and comprehensively investigate both the initial conditions of MEMS and NEMS devices contact surfaces with and without exposing the contamination gas molecules particularly benzene and model the mechanical switching cycles among many conducting materials such as metal oxides (RuO2) and metal sillisides (PtxSi). We found that the tribopolymerization mechanism lies on to surface catalytic effect enhanced by local confined stress, and a general trend follows for those materials as we investigated is: (i) chemisorption; (ii) dehydrogenation; (iii) polymerization. In particular, the applied normal stress lowers both the adsorption barrier and the reaction barriers lead to tribopolymerization. In addition, the weaker the initial binding strength of adsorbates the less the polymerization. With the knowledge acquired in our study, we are able to provide criteria for screening and designing tribopolymer-suppressing materials and stimulate the development of MEMS and NEMS to eventually reach practical manufacturing and make them widespread.

Andrew M. Rappe
Joseph E. Subotnik
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