Selective detection of short-range interaction forces was carried out with the second flexural mode of a commercially available dynamic mode cantilever. A higher mode has a higher spring constant and a lower mechanical quality factor, which are suitable for the small amplitude operation in dynamic force microscopy. With 0.70Å amplitude of the second flexural mode, atomically resolved constant frequency shift images of the Si(111)−7×7 reconstructed surface were obtained. The ultrasmall amplitude operation enabled imaging with high stability, due to the detection of the interaction force gradients at relatively long distances from the sample surface, and is an effective way to observe reactive surfaces while avoiding modifications and damaging of the tip and the sample.

Soft graphite surface was atomically resolved by ultrasmall amplitude dynamic force microscopy operating at 5 MHz. The giant corrugation amplitude of up to 85 pm appeared due to local vertical deformations of the graphite surface. In simultaneous scanning tunneling microscopy and dynamic force microscopy, all of the symmetric C atoms were resolved with the conservative interaction in the repulsive regime. Additionally, the dissipative interaction showed a large difference of asymmetric ␣ and ␤ site C atoms, arising from the different mechanical properties. The low stiffness of the graphite surface played a crucial role in the observations at room temperature.The graphite surface has been one of the most extensively studied surfaces with scanning probe microscopy. The surface with the hexagonal lattice has two different kind of sites, namely, the ␣ and the ␤ site C atoms. The ␣ site atoms are directly above in the adjacent planes, whereas the ␤ site atoms are located above the hollow sites. In general, the ␤ site atoms are only visible with scanning tunneling microscopy ͑STM͒. 1,2 Observation of all atoms has been challenging due to the asymmetry of the electric and mechanical properties. Observed corrugation amplitudes are usually varied and were up to several angstrom, 3,4 commonly known as the "giant corrugations." It is inconsistent to values of around 20 pm obtained by the first-principles calculation and the He scattering measurement. 5,6 It has been assumed that deformations of the graphite surface layer by interaction forces between the STM tip and the sample increase the observed corrugation amplitude. 7 Graphite has a laminated structure, and the surface layer is connected to the second layer with weak interactions, arising from the overlap of partially occupied pz orbitals perpendicular to the three hybridized orbitals ͓van der Waals ͑vdW͒ force type͔. On the other hand, in the plane, C atoms are strongly connected to each other by sp2 covalent bonds. This strong difference results in the high compliance in the laminated direction. Although this assumption is widely accepted, the giant corrugation has been observed only with STM, which traces the surface with a given charge density.Dynamic force microscopy ͑DFM͒ is another well-known technique to resolve surfaces with atomic resolution. 8 Since the tip-sample distance is usually regulated at a negative slope of the time-averaged interaction force gradient via a negative frequency shift of a cantilever, 9 all kind of surfaces, such as insulators, semiconductors, and metals, can be observed. 10 In the case of semiconductor surfaces, the strong interaction of the covalent bonding enables discriminations of surface atoms even in the same IV group. 11 On the other hand, vdW surfaces, such as graphite, carbon nanotubes, and C60, are one of the most challenging surfaces even for atomically resolved imaging due to their extremely low reactivity and small C-C distance of 142 pm. These features act to reduce the corrugation amplitude in DFM. Moreover since the st...

Lateral force gradient of down to 0.01 N/m on Si͑111͒-7 ϫ 7 was directly detected by dynamic lateral-force microscopy with an amplitude of 81 pm. Positive and negative torsional resonance frequency shifts of a silicon cantilever caused by the attractive interaction inward and outward tip ditherings were detected on adatom and nonadatom sites, respectively. The lateral force of down to subpiconewton was measurable with direct lateralforce spectroscopy. The converted lateral force predicts a possibility of the stick-slip motion in the noncontact region. The theoretical calculations were in good qualitative agreement with the experiments.

We present frequency modulation dynamic lateral force microscopy with true atomic resolution. Torsional resonance mode of a commercially available rectangular cantilever was used to detect interaction lateral force gradients caused between the tip and the sample surface. A slight negative frequency shift of the torsional resonance frequency was observed before contact to the silicon surface. Individual adatoms in a unit cell of the Si(111)-7×7 reconstructed surface were imaged with the constant frequency shift mode. Two sets of the neighboring corner adatoms and one set of the center adatoms on the dithering direction of the tip were connected on the image. This method has a great potential to observe friction between single atoms.

An atomic force microscope for nanocantilevers measuring from a few 100 nm to a few μm in length was implemented. The natural frequencies of the nanocantilevers lie in the range of 1 MHz to 1 GHz, and optical detection schemes adapted to their size and frequency range was selected. A helium neon laser with a beat frequency of 890 MHz was used as the laser source. The beat was shifted to 1090 MHz by an acousto-optical-modulator, and used as the carrier for heterodyne laser Doppler measurement. This enabled velocity measurement up to around 100 MHz. The probe beam of the Doppler interferometer was guided to the nanocantilever by a single mode polarization-maintaining optical fiber terminated by a collimating lens, a quarter wave plate, and a focusing lens. Reflected light was collected by the same optics and mixed with the reference beam. Self-excitation of the nanocantilever at its lowest natural frequency was implemented for an amplitude of 1 nmp-p at 36 MHz. The Q factor of the cantilever was 8000. Noise effective amplitude of the Doppler interferometer was smaller than 10 pmp-p above 10 MHz. Frequency detection was possible for a nanowire measuring 100 nm in width.

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The authors present an optically based method combining photothermal excitation and laser Doppler velocimetry of higher cantilever vibration modes for dynamic atomic force microscopy in liquid. The frequency spectrum of a silicon cantilever measured in water over frequencies ranging up to 10 MHz shows that the method allows us to excite and detect higher modes, from fundamental to fifth flexural, without enhancing spurious resonances. By reducing the tip oscillation amplitude using higher modes, the average tip-sample force gradient due to chemical bonds is effectively increased to achieve high-spatial-resolution imaging in liquid. The method's performance is demonstrated by atomic resolution imaging of a mica surface in water obtained using the second flexural mode with a small tip amplitude of 99 pm; individual atoms on the surface with small height differences of up to 60 pm are clearly resolved.

The authors analyze photothermal excitation of a single-crystalline silicon cantilever for higher vibration modes in liquid. The cantilever is bent by thermal stress generated by thermal diffusion in the direction perpendicular to the cantilever surface. Because the cantilever is made of a homogeneous material, thermal diffusion in the longitudinal direction does not generate thermal stress. Therefore, the higher vibration modes having small spatially periodic mode shapes are easily and effectively excited. The authors compared the excitation efficiency of two optical wavelengths, 405 and 780 nm. The 405 nm laser-diode beam was found to be 2.3-4.2 times more effective in exciting the second flexural mode compared with the 780 nm beam. These differences in excitation efficiency are attributed to the absorbance characteristics of silicon and were confirmed by measuring the transmitted light power ͑lost power͒ against the incident light power. Vibration amplitude of each mode was sufficient to operate dynamic-mode atomic force microscopy in liquid.