Atomistic Molecular Dynamics Simulations of Chemical Force Microscopy David L. Patrick,* ,² John F. Flanagan, IV, ² Patrick Kohl, ² and Ruth M. Lynden-Bell ‡ Contribution from the Department of Chemistry, Western Washington UniVersity, 516 High Street, Bellingham, Washington 98225 and Atomistic Simulation Group, School of Mathematics and Physics, The Queen’s UniVersity of Belfast, Belfast BT7 1NN, Northern Ireland, U.K. Received February 6, 2003; E-mail: patrick@chem.wwu.edu Abstract: Chemical force microscopy and related force measurement techniques have emerged as powerful tools for studying fundamental interactions central to understanding adhesion and tribology at the molecular scale. However, detailed interpretation of these interactions requires knowledge of chemical and physical processes occurring in the region of the tip-sample junction that experiments cannot provide, such as atomic-scale motions and distribution of forces. In an effort to address some of these open issues, atomistic molecular dynamics simulations were performed modeling a chemical force microscope stylus covered with a planar C12 alkylthiolate self-assembled monolayer (SAM) interacting with a solid wall. A complete loading-unloading sequence was simulated under conditions of near-constant equilibrium, approximating the case of infinitely slow tip motion. In the absence of the solid wall, the stylus film existed in a fluid state with structural and dynamic properties similar to those of the analogous planar SAM at an elevated temperature. When the wall was brought into contact with the stylus and pressed against it, a series of reversible changes occurred culminating with solidification of the SAM film at the largest compressive force. During loading, the chemical composition of the contact changed, as much of the film’s interior was exposed to the wall. At all tip heights, the distribution of forces within the contact zone was uneven and subject to large local fluctuations. Analysis using the Johnson-Kendall-Roberts, Derjaguin-Muller-Toporov, and Hertz contacts mechanics models revealed significant deviations from the simulation results, with the JKR model providing best overall agreement. Some of the discrepancies found would be overlooked in an actual experiment, where, unlike the simulations, contact area is not separately known, possibly producing a misleading or incorrect interpretation of experimental results. These shortcomings may be improved upon by using a model that correctly accounts for the finite thickness of the compliant components and nonlinear elastic effects. Introduction In the science of adhesion and tribology, one of the most fundamental and important pairings is that between a small asperity and a flat surface. Macroscopic adhesive and tribo- logical phenomena ultimately originate with the microscopic properties of contacting surfaces, and the asperity-flat pairing is considered a model for the microscopic geometry occurring in most cases of technological relevance. 1 While the importance of single asperity contacts has been recognized for many decades, their direct experimental study accelerated dramatically with the invention of the atomic force microscope 2 (AFM), which enables controlled measurements involving asperity-flat pairings at nanometer-length scales and nanonewton forces. However, for quantitative measurements, one shortcoming of AFM is that the chemical and physical characteristics of the asperity are generally not well defined or controlled, especially when an experiment is performed under ambient conditions or in solution. 3 Aside from adjusting the bulk solution environment (e.g., pH), there is no practical way to control the chemistry of the probe or to prevent the formation of a contamination layer. Chemical force microscopy (CFM) is a variation of AFM which solves some of these problems by employing a stylus coated with a self-assembled monolayer (SAM) of chainlike molecules. 4 CFM provides a way to tailor the properties of the probe through chemical derivatization of the terminal group and chain, enabling measurement of chemically specific interactions between a small number of stylus and sample molecules with high spatial and force resolution. When compared to force measurements made using conventional unmodified tips, CFM therefore involves relatively well-controlled conditions. The technique has been used to map chemically distinct surface domains, 5 to quantitatively measure friction 6 and adhesion 7 forces for a variety of probe and surface chemical pairings, and to investigate the effect of different solvents 8 and solvent pH ² Western Washington University. ‡ The Queen’s University of Belfast. (1) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids; Clarendon Press: Oxford, 1985. (2) Binning, G.; Quate, C. F.; Gerber, Ch. Phys. ReV. Lett. 1986, 56, 930. (3) Iler, R. K. The Chemistry of Silica; Wiley-Interscience: New York, 1979. (4) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. Published on Web 05/10/2003 6762 9 J. AM. CHEM. SOC. 2003, 125, 6762-6773 10.1021/ja0345367 CCC: $25.00 © 2003 American Chemical Society