The forces between the tip of a scanning tunneling microscope and an adsorbed molecule may be exploited to induce a range of translational and conformational modes of manipulation.[1–12] As shown by Bartels et. al.,[2] different modes of translational manipulation may be distinguished through the acquisition of the trajectory executed by the scanning tunneling microscopy (STM) instrument tip as the adsorbate moves across the surface. In most cases the tip height changes abruptly as the molecule is translated between discrete adsorption sites on the surface, enabling a distinction between attractive and repulsive modes of manipulation. The abrupt change occurs when the molecule hops, since there is an effective change in the tip–surface separation, which results, when operating under constant-current mode, in an adjustment of tip height. An interesting variant, which occurs for weakly bound adsorbates under low-temperature conditions, is a sliding mode, in which discontinuous changes in tip height are absent and the adsorbate moves across the surface in a quasi-continuous manner rather than hopping between discrete adsorption sites.[2] In this Communication, we describe a mode of manipulation in which a balance between attractive and repulsive tip–molecule forces results in the stabilization of a molecule at a series of intermediate positions between the adsorption sites adopted by a free molecule on the surface. We find that the tip induces a type of repulsive manipulation, in which abrupt molecular hops are suppressed through a residual attractive interaction with the tip. There are some similarities between the acquired STM line scans and those previously categorized as sliding in low-temperature experiments.[2] However, for the large, strongly bound, molecule considered here, the motion differs significantly from a sliding trajectory.

Our investigations are performed using C60 adsorbed on Si (100)-2 Â 1 (see Experimental Section), which provides a model system for combined experimental and theoretical studies of the manipulation of covalently bound molecules.[13–17] An STM image, and schematic, of a single C60 molecule on the Si (100)-2 Â 1 surface is shown in the inset of Figure 1. The molecule is adsorbed in a ‘trough’site midway between the dimer rows, which form on this surface.[18–20] Manipulation was performed both parallel and perpendicular to the dimer rows, while recording the tip trajectory as described in the Experimental Section. Typical tip trajectories are shown in Figure 1. All traces show a complex waveform with a lateral periodicity of na0, where a0 (3.84 A) is the lattice constant of the Si (100) surface and n ¼ 2, 3, or 4. We have shown [16, 17] that for manipulation along the trough (parallel to the dimer row) this long-range periodicity is due to molecular rolling. The large observed periodicity arises from the sequential bonding of the fullerene in different orientations during the rolling process. Fullerene rolling on this surface was proposed in the literature [13] and has also been discussed in the context of other surfaces.[9, 21, 22] In Figure 1a–d we show line scans for molecules manipulated parallel (a and b) and perpendicular (c and d) to the dimer rows. While these periodic traces are a signature of repulsive molecular rolling along the troughs or across the rows, the curves also illustrate a feature of great significance, which we attribute to a combined attractive and repulsive tip–molecule interaction. In particular, we observe a gradient of the tip trajectory after molecular manipulation commences, which is much smaller than that expected if the molecule simply hops directly to a neighboring adsorption site. An …