DOI: 10.1002/cphc.201200877 Nanoadhesion on Rigid Methyl-Terminated Biphenyl Thiol Monolayers: A High-Rate Dynamic Force Spectroscopy Study Hubert Gojzewski, [a, b] Michael Kappl, [c] Gunnar Kircher, [c] Wojciech Koczorowski, [a] Hans- Jürgen Butt, [c] and Arkadiusz Ptak* [a] 1. Introduction The development of micro- and nanoelectromechanical devi- ces is strongly limited by the phenomenon of adhesion and friction. At the nanoscale, the surface-to-volume ratio is ex- tremely large, and therefore, surface interactions—particularly van der Waals and meniscus forces—dominate over volume ef- fects such as mass inertia or thermal capacity. For this reason, scientists search for suitable lubricants for nanodevices. [1] Such lubricants should meet various requirements. For instance, they must not be fluid to prevent capillary forces, which may cause large adhesion. For the same reason, they cannot be hy- drophilic because a hydrophilic surface would lead to capillary condensation of water and, again, to a strong capillary attrac- tion. They should be resistant to high temperatures which can locally appear in micro- or nanodevices. Promising candidates seem to be molecularly thick organic films. The most interest- ing among them are self-assembled monolayers (SAMs), which are ordered molecular assemblies formed spontaneously by chemical adsorption of molecules on a solid surface. [2] They have stable, homogeneous, and well-ordered molecular struc- tures with a typical thickness of two nanometers. The micro/ nanoadhesive and tribological properties of SAMs, particularly alkane thiols and alkyl silanes, have been studied using an atomic force microscope (AFM) by several groups. [3–7] However, aromatic thiols offer a better organized film structure com- pared to alkane thiols due to their greater rigidity and larger intermolecular interactions. [8–11] Nevertheless, research on the nanoadhesion of aromatic thiol SAMs at the nanoscale has not been reported. The main goal of our paper is to compare the adhesion for biphenyl and alkyl thiols, and answer the question on the influ- ence of rigidity on the adhesion of methyl-terminated SAMs. We have also studied the influence of humidity on adhesion, which is important for micro- and nanodevices working in air. We have used an AFM [12] in the force spectroscopy mode [13, 14] to probe adhesive properties of 4-methyl-4’-mercaptobiphenyl SAMs. Evans and Ritchie showed—using Bell’s formula [15] —that measuring the adhesion force over a range of loading rates, that is, performing dynamic force spectroscopy (DFS) measure- ments, can provide a way to determine such parameters as the kinetic off-rate and the distance between the bound state and the transition state. [16] According to their model, a mean adhe- sion force (F ad ) depends logarithmically on the loading rate (r F ) [Eq. (1)]: F ad ¼ F b ln 1 F b r F k 0 off   ð1Þ Here, k 0 off is the intrinsic (force-free) rate constant and F b = k B T/x b is the so-called thermal fluctuation force (where k B is the Boltzmann constant, T is the absolute temperature, and x b is the distance between the bound state and the activation barri- er along the direction of the external pulling force). Equa- tion (1) provides a simple way to extract the value of x b and k 0 off from the slope and intercept of the F ad -versus-ln(r F ) curve fitted to the experimental data. That is why the Bell–Evans model has been widely applied by several research groups to study the interactions between or within biomolecules [17–21] as well as the nanoadhesion between an AFM tip and mica, [22] Nanoadhesion on a self-assembled monolayer of 4-methyl-4’- mercaptobiphenyl is measured using a modified atomic force microscope. The dependence of the adhesion force on the loading rate is analyzed with the Dudko–Hummer–Szabo model, and the kinetic and interaction potential parameters for a single terminal group are extracted. The energy and location of the activation barrier suggest that the adhesion is dominat- ed by van der Waals dispersion forces. The humidity effect on the nanoadhesion is also studied. The results are compared with previously measured values for methyl-terminated alkane thiols and the influence of the thiol rigidity on the adhesion force is discussed. [a] Dr. H. Gojzewski, Dr. W. Koczorowski, Dr. A. Ptak Institute of Physics, Poznan University of Technology Nieszawska 13A, PL-60965 Poznan (Poland) E-mail : arkadiusz.ptak@put.poznan.pl [b] Dr. H. Gojzewski Max Planck Institute of Colloids and Interfaces Am Mühlenberg 1 Golm, D-14476 Potsdam (Germany) [c] Dr. M. Kappl, G. Kircher, Prof. Dr. H.-J. Butt Max Planck Institute for Polymer Research Ackermannweg 10, D-55128 Mainz (Germany)  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2013, 14, 543 – 549 543 CHEMPHYSCHEM ARTICLES