Chemical forces on surfaces have a central role in numerous scientific and technological fields, including catalysis, thin film growth and tribology. Many applications require knowledge of the strength of these forces as a function of position in three dimensions, but until now such information has only been available from theory. Here, we demonstrate an approach based on atomic force microscopy that can obtain this data, and we use this approach to image the three-dimensional surface force field of graphite. We show force maps with picometre and piconewton resolution that allow a detailed characterization of the interaction between the surface and the tip of the microscope in three dimensions. In these maps, the positions of all atoms are identified, and differences between atoms at inequivalent sites are quantified. The results suggest that the excellent lubrication properties of graphite may be due to a significant localization of the lateral forces.

a b s t r a c tThe interface and electronic structure of thin ($20-74 nm) Co 3 O 4 (1 1 0) epitaxial films grown by oxygenassisted molecular beam epitaxy on MgAl 2 O 4 (1 1 0) single crystal substrates have been investigated by means of real and reciprocal space techniques. As-grown film surfaces are found to be relatively disordered and exhibit an oblique low energy electron diffraction (LEED) pattern associated with the O-rich CoO 2 bulk termination of the (1 1 0) surface. Interface and bulk film structure are found to improve significantly with post-growth annealing at 820 K in air and display sharp rectangular LEED patterns, suggesting a surface stoichiometry of the alternative Co 2 O 2 bulk termination of the (1 1 0) surface. Noncontact atomic force microscopy demonstrates the presence of wide terraces separated by atomic steps in the annealed films that are not present in the as-grown structures; the step height of %2.7 Å corresponds to two atomic layers and confirms a single termination for the annealed films, consistent with the LEED results. A model of the (1 Â 1) surfaces that allows for compensation of the polar surfaces is presented.

We present the design and first results of a low-temperature, ultrahigh vacuum scanning probe microscope enabling atomic resolution imaging in both scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM) modes. A tuning-fork-based sensor provides flexibility in selecting probe tip materials, which can be either metallic or nonmetallic. When choosing a conducting tip and sample, simultaneous STMMC-AFM data acquisition is possible. Noticeable characteristics that distinguish this setup from similar systems providing simultaneous STM/NC-AFM capabilities are its combination of relative compactness (on-top bath cryostat needs no pit), in situ exchange of tip and sample at low temperatures, short turnaround times, modest helium consumption, and unrestricted access from dedicated flanges. The latter permits not only the optical surveillance of the tip during approach but also the direct deposition of molecules or atoms on either tip or sample while they remain cold. Atomic corrugations as low as 1 pm could successfully be resolved. In addition, lateral drifts rates of below 15 p d h allow long-term data acquisition series and the recording of site-specific spectroscopy maps. Results obtained on Cu(1 1 1) and graphite illustrate the microscope's performance.