By studying styrene, a simple hydrocarbon with a weak dipole moment we have investigated ferroelectric ordering and ferroelectric transitions at the single-molecule level. Low-temperature scanning tunneling microscopy imaging of individual styrene molecules reveals their internal structure, and hence the orientation of their dipole moment parallel to the Au{111} surface. At near monolayer coverages, both local ferroelectric ordering of the molecules and long-range antiferroelectric ordering of the domains are observed, and the dipole-dipole interaction energies quantified. A piezoelectric transition from ferroelectric to paraelectric ordering is observed upon further increase of the coverage. The effect of the STM tip in randomizing ordered domains is also discussed. This work demonstrates that important ferroelectric properties such as spontaneous polarization, long-range ordering and piezoelectricity can be achieved in nanoscale domains of a weakly polar molecule on a metal surface.

The invention of AFM by Binnig was triggered by his observations of force effects that occur when performing scanning tunneling microscopy experiments. The publication of first experiments with an “atomic force microscope” and the possibility of once obtaining atomic resolution by AFM1 lead to a surge in global research activities. However, it turned out that the road to an atom resolving force microscope was a long and winding one, a road that initially led away from STM. Measuring spring deflections through tunneling, the pivotal trick that promised great force resolution in AFM, proved to be impractical and it turned out that long-range interactions that are unimportant in STM are important in AFM. Almost a decade had passed before atomic resolution on silicon – STM’s early feat that captured the imagination of surface scientists – could be repeated by AFM in 1994. In these AFM experiments, cantilevers made from silicon with spring constants of about 20 N/m were used in the frequency-modulation mode (FM-AFM).2 In FM-AFM the cantilever is driven at a constant amplitude and forces acting between tip and sample are reflected in frequency changes. Intuitively, one would think that small oscillation amplitudes would be optimal in FM-AFM, because that would maximize the effects of the tip-sample forces on the cantilevers motion. However, amplitudes of about 10 nm were required when using silicon cantilevers with a stiffness of 20 N/m. Large amplitude operation has a disadvantage because it is strongly susceptible to long-range forces. To minimize the magnitude of long-range forces, sharp tips that were machined on the ends of silicon cantilevers had to be used. The introduction of force sensors that are about two orders of magnitude stiffer than usual silicon cantilevers has allowed stable small-amplitude operation and brought the AFM closer to a tool that focuses on chemical short-range forces.3 The qPlus sensor, an implementation of a self-sensing cantilever based on a quartz tuning fork, that is particularly simple to implement in STMs is now used in several laboratories4,5,6 at room temperature and low temperatures. In particular at low temperatures, measurements of chemical bonding forces at high-precision are now possible.7 Perspectives about the merits of adding AFM capabilities to previously pure STM experiments will be discussed. 1.G. Binnig, C. F. Quate, Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986). 2.T. Albrecht, P. Grutter, H. K. Horne, and D. Rugar, J. Appl. Phys. 69, 668 (1991).3.Franz J. Giessibl, “AFM’s path to atomic resolution”, Materials Today 8, 32 (2005).4.M. Maier, Pico Nr. I 2006 , page 4 (Omicron newsletter, http://www.omicron.de).5.M. Ternes, C. P. Lutz, A. Heinrich et al., unpublished.6.M. Heyde et al., Appl. Phys. Lett. 89, 263107 (2006).7.Sugimoto, Y. et al. Nature 446, 64–67 (2007).

Special and state selective chemistry on individual molecules is nowadays being driven by inelastically tunneled electrons from an STM tip. Molecules adsorbed on substrates such as metals or semiconductors have been some good examples. Such molecular manipulation is often coupled with the excitation of vibrational modes. Vibrational spectrum of a single molecule provides useful information not only for the chemical identification of the molecule [1] but also for investigating how molecular vibration can couple with the relevant