Car-Parrinello molecular dynamics simulations demonstrate that pulling a single thiolate molecule anchored on a stepped gold surface does not preferentially break the sulfur-gold chemical bond. Instead, it is found that this process leads to the formation of a monoatomic gold nanowire, followed by breaking a gold-gold bond with a rupture force of about 1.2 nN. The simulations also indicate that previous single-molecule thiolate-gold and gold-gold rupture experiments both probe the same phenomenon, namely, the breaking of a gold-gold bond within a gold nanowire.

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Gold-sulfur bonding is investigated theoretically using a variety of electronic structure methods, including the Becke-Perdew semilocal density functional, the B3LYP hybrid approach, the Hartree-Fock method, and the post Hartree-Fock approaches MP2 and QCISD͑T͒. Particular emphasis is given to adsorption structure and energetics in the case of weak and strong interactions of this general type, using up to five gold atoms and up to three carbon atoms in the aliphatic chain. It is found that all methods which take into account electron correlation, including the density functional methods, lead to quite similar structures. Concerning the energetics, the Becke-Perdew functional is found to overbind typically by about 5-15%. Quasiglobal structural relaxation based on ab initio simulated annealing clearly shows that the adsorption of thiolates onto gold clusters results in a dramatic distortion of the cluster framework. From a structural point of view the thiolate sulfur-gold bond has a distinctive directional ͑covalent͒ character which results in a clear preference for Au-S-C bond angles in the range of 103.5°and 108.7°. In general, dissociation into open-shell species is preferred against the formation of the ionic closed-shell counterparts if the sulfur-gold bond is forced to break. However, neutral closed-shell products can be favored if fragmentation of the gold cluster is allowed for as a dissociation channel. Finally, it is demonstrated that using ethyl or n-propyl chains instead of the methyl group leads to only small changes of the binding energies.

The performance of a scanning force microscope (SFM) operated in the dynamic mode at high oscillation amplitudes is determined by the response of the system to a given set of interaction forces between the probing tip and the sample surface. To clarify the details of the cantilever/tip dynamics two different aspects were investigated in experiment and computer simulation. First, the interaction forces dominating the oscillatory motion of the probe were varied by applying an additional electrostatic force field. It is shown that such variations in the attractive part of the interaction potential can cause a switching between two different oscillation states and thereby significantly contribute to the contrast obtained from phase imaging. Secondly, the interaction forces were kept constant but the system response itself was varied by modifying the effective quality factor of the oscillating cantilever with an active feedback circuit. This provides a means to influence the transition from the attractive to the partly repulsive interaction regime, i.e. the onset of the intermittent contact or tapping mode.Operating an SFM in the dynamic mode at high amplitudes (> 10 nm) offers the possibility of minimizing the contact time of the probing tip with the sample surface and thereby reduce lateral or friction forces involved in the scanning process. It also allows the collection of additional data related to different sample properties by recording the phase shift between the force driving the cantilever and its oscillation. In the last few years these features of operating the SFM in the dynamic mode [1, 2] were shown to be very useful to characterize several different kinds of sample surfaces, e.g. thin organic films, polymers, biological samples or even liquid droplets [3]. Although this has led to a steady increase in the number of possible applications, there are still several details of the interaction process between the tip and the sample that need further clarification. The overall goal must be to relate the experimentally accessible data, such as the amplitude and * Corresponding author phase signal, more or less directly to specific sample properties, such as topography, elasticity and viscoelasticity.Because highly nonlinear interaction forces are involved when the oscillating tip is in close proximity to a solid surface, the analysis of the dynamic system becomes quite complex. Therefore supplementary computer simulations based on proper mathematical models are useful to investigate the details of the interaction process. Basically, the equation of motion describing the dynamic properties of the probing tip has to be solved in such a way that the influence of different parameters characterizing the probe as well as the sample surface can be examined. There have been several reports recently on different approaches to this problem, providing analytical [4] as well as numerical [5][6][7][8][9][10][11] solutions. Most of them are based on the point-mass model, but there are also approaches which describe the comp...

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