Mechanochemistry at Nanoscale Metallic Contacts: How Stress and Voltage Drive Tribopolymerization.

Contact-induced reactions of interfacially confined molecules represent a widespread yet poorly understood class of mechanochemical phenomena, with broad implications for surface chemistry, tribology, and nanotechnology. Tribopolymerization─stress-induced polymerization of organic adsorbates into insulating nanolayers─causes conductance loss and limits the reliability of electrical contacts across length scales, particularly in nanoelectromechanical systems (NEMS). Using atomic force microscopy (AFM), we investigate how stress and voltage drive tribopolymer growth from ambient-adsorbed molecules in Pt/Pt nanocontacts. The measured kinetics follow a stress-assisted thermal activation model, confirming its mechanochemical origin. We develop a new contact-mechanics-corrected model that combines stress-dependent reaction kinetics with realistic contact mechanics. Using power-law tip geometries, this model accounts for inevitable wear-induced nonstandard tip shapes by integrating local reaction rates over the full, nonuniform stress distribution within the contact region. This enables accurate extraction of a unified activation volume (Δ = 5.6 ± 1.4 Å) across two decades of both contact area and stress, in sharp contrast to conventional analyses that neglect contact geometry and yield widely scattered activation volumes spanning 2 orders of magnitude. We further show that applied voltage accelerates tribopolymerization in a manner similar to stress, described through a newly introduced activation parameter and a field-induced bond-stretching model. Together, these results provide a general approach for quantifying coupled stress- and field-driven mechanochemical reactions at nanoscale interfaces, and offer mechanistic insights into tribopolymerization-induced electrical degradation of nanocontacts critical to device reliability.