Nanotribological properties and mechanisms of alkylthiol and biphenyl thiol self-assembled monolayers studied by AFM - art. no. 245412

Five kinds of alkylthiol and biphenyl thiol monolayers with different surfa ce terminals, spacer chains, and head groups were prepared using a self-ass embly method. The adhesion, friction, and wear properties were measured usi ng atomic force microscopy (AFM). It is found that hexadecane thiol (HDT) w ith a-CH3 terminal exhibits the smallest adhesive force and friction force because of the terminal group with its low work of adhesion and high-compli ance long carbon chain. Experimental results and a meniscus analysis indica te that the adhesive force varies linearly with work of adhesion of self-as sembled monolayers (SAMs). A molecular spring model is presented to clarify the lubrication mechanisms of SAMs. The molecular spring constant, as well as the inter molecular forces, dictates the magnitude of the coefficients of friction of SAMs. 4,4'-dihydroxybiphenyl (DHBp) on Si(111), due to its r igid biphenyl spacer chains, stronger interface bonds, and a hard substrate , has the best wear resistance. For all of the SAMs, the wear depth with no rmal load curves show critical normal loads. Below the critical normal load , SAMs undergo orientation, while at the critical normal load SAMs undergo severe wear at the interface due to the weak interfacial bond strengths. Th e influence of relative humidity on adhesive and frictional forces of SAMs can be mainly understood by comparing their terminal polarization propertie s and work of adhesion. At higher humidity, water capillary condensation ca n either increase friction through increased adhesion in the contact zone o r reduce friction through an enhanced water-lubricating effect.