Inelastic electron tunnelling spectroscopic study of lubrication

Abstract

Inelastic electron tunnelling spectroscopy reveals the vibrational spectrum of organic molecules on the oxide of a metal–oxide–metal junction1–3 . A vibrational mode of frequency ν appears in the tunnelling spectrum as a peak in the second derivative of voltage with respect to current, d2V/dI2, at a voltage V = hν/e (where h is the Planck's constant, and e is the electron charge). Both IR and Raman active modes can be observed. Tunnelling spectroscopy4,5 has been used as an effective surface probe in the studies of adhesion6–8, biochemistry9,10, water pollution11, damage due to electron beam irradiation12, or UV irradiation13, surface chemistry and catalysis14–18. We report here a new application of tunnelling spectroscopy—to lubrication—and present spectra of 1,2 hexadecanediol, hexadecanol and hexadecanoic acid and relate the results to their properties as boundary lubricants for aluminium. Because diols give the best results in both friction measurements and tunnelling spectra, we studied the adsorbtion of the smallest diol, ethylene glycol, and found that both OH groups react with the alumina surface.