  Micro-electro-mechanical system and nano-electro-mechanical system (MEMS and
NEMS) transistors are considered promising for size-reducing and
power-maximizing electronic devices. However, the tribopolymer which forms due
to the mechanical load to the surface contacts affects the conductivity between
the contacts dramatically. This is one of the challenging problems that prevent
widespread practical use of these otherwise promising devices. Here, we use
density functional theory (DFT) to investigate the mechanisms of tribopolymer
formation, including normal mechanical loading, the catalytic effect, as well
as the electrochemical effect of the metal contacts. We select benzene select
as the background gas, because it is one of the most common and severe
hydrocarbon contaminants. Two adsorption cases are considered: one is benzene
on the reactive metal surface, Pt(111), and the other is benzene on the noble
metal, Au(111). We demonstrate that the formation of tribopolymer is induced
both by the mechanical load and by the catalytic effect of the contact. First,
benzene molecules are adsorbed on the Pt surfaces. Then, due to the closure of
the Pt contacts, stress is applied to the adsorbates, making the C-H bonds more
fragile. As the stress increases further, H atoms are pressed close to the Pt
substrate and begin to bond with Pt atoms. Thus Pt acts as a catalyst,
accelerating the dehydrogenation process. When there is voltage applied across
the contacts, the catalytic effect is enhanced by electrochemistry. Finally,
due to the loss of H atoms, C atoms become more reactive and link together or
pile up to form tribopolymer. By understanding these mechanisms, we provide
guidance on design strategies for suppressing tribopolymer formation.
