Hideki Kawakatsu

An ultrasmall amplitude operation of dynamic force microscopy with second flexural mode

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 0.70A 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.

Photothermal excitation and laser Doppler velocimetry of higher cantilever vibration modes for dynamic atomic force microscopy in liquid

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 methods 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.

Dynamic lateral force microscopy with true atomic resolution

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.

Photothermal excitation of a single-crystalline silicon cantilever for higher vibration modes in liquid

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 power) against the incident light power. Vibration amplitude of each mode was sufficient to operate dynami...

An ultrahigh vacuum dynamic force microscope for high resonance frequency cantilevers

We present the design of an ultrahigh vacuum dynamic force microscope incorporating a heterodyne Doppler interferometer and a superheterodyne circuit with an intermediate frequency of 10.7MHz. The method allowed the use of a low-noise narrow-band analog phase-locked loop with a voltage controlled crystal oscillator for demodulating the frequency shifts caused by the interaction force gradients between the tip and the sample at the intermediate frequency. The system could be used for a conventional cantilever operating in its fundamental and higher modes, as well as for small or stiff cantilevers with high resonance frequency up to 100MHz. A preliminary measurement was demonstrated by the observation of the Si(111)−7×7 reconstructed surface with the second resonance of 1.6MHz with subangstrom amplitudes.

Atomically resolved observation of the quenched Si(111) surface with small amplitude dynamic force microscopy

Metastable reconstructed phases and highly disordered regions of the quenched Si(111) “1×1” phase with many silicon clusters were atomically resolved with a constant frequency mode of small amplitude dynamic force microscopy with the second flexural mode of a commercially available dynamic mode cantilever. Improved sensitivity due to the small amplitude dynamic force microscopy could operate at a relatively far distance from the sample surface with a given resolution and enable highly stable imaging with small interaction forces even on the Si(111) 1×1 metastable phases with silicon clusters. All of the individual atoms in the silicon cluster were atomically observed while avoiding deformations of the sample surface and the tip apex. In the case that the interaction forces of the imaging parameters were intently set to be ten times larger than those for stable imaging, arrangements of adatoms could easily be modified by mechanical interaction forces between the tip and the sample surface. The Si(111)-c(2×...

Mechanical atom manipulation with small amplitude dynamic force microscopy

We demonstrate atom manipulations of the Si(111)-(7×7) surface with small amplitude dynamic force microscopy at room temperature. Adatoms could not only be extracted and attached but also laterally manipulated in and over the half unit cell with a repulsive interaction force caused between the tip and the sample. In the case of a tip condition that gave a strong image contrast, an adatom beside a vacancy could be pulled to the neighboring stable site with a strong attractive force. Enhanced detection sensitivity due to the small amplitude of 4A could avoid accidental modifications of the surface during imaging.

Small single-crystal silicon cantilevers formed by crystal facets for atomic force microscopy

We have developed a batch fabrication method of small cantilevers formed by crystal facets of single-crystal silicon for improving the sensitivity of atomic force microscopy. In order to realize a small cantilever with a very sharp tip, we have employed KOH anisotropic etching and local oxidation of silicon. We have made two types of small cantilevers, the V-shaped triangular type and the bulk triangular type. The length of each cantilever is 20 microm. The tip of the V-shaped type is bridged by two wires with thickness of 0.6 mum. The bulk triangular type has a thickness of 1.5 microm. The frequency characteristics of the cantilevers vibrated using photothermal excitation were measured by laser Doppler velocimetry. The resonance frequency of the V-shaped type and the bulk triangular type were 687 kHz and 8.42 MHz, and their spring constants are estimated to be 0.7 N/m and 370 N/m, respectively.

Application of capillary forces and stiction for lateral displacement, alignment, suspension and locking of self-assembled microcantilevers

We report the surface-tension-powered self-assembly (displacement, alignment, pulling down and locking) of microcantilevers. Capillary forces-assisted displacement is realized by compression of four arrays of springs linked to the opposite lateral sides of the cantilevers. After in-plane translation along the initial cantilever orientation, the microstructure is pulled down and locked on the substrate by stiction. Spring and capillary forces are described with a simple analytical model. Complete self-assembly occurs when the layout of the whole system (the cantilever with its traveling spring structure, the surrounding area and the local distribution of the liquid) is well designed. We show that the presence of a water meniscus trapped at a step edge in the vicinity of the tip end of a microcantilever could lead to stiction failure before traveling of the structure. Small beams (6 µm long) protruding over step edges were fabricated by adding a mechanically assisted displacement step (with a microneedle) to the self-assembly experiment.

Atomically resolved amplitude modulation dynamic force microscopy with a high-frequency and high-quality factor cantilever

We demonstrate atomically resolved amplitude modulation ultrahigh vacuum dynamic force microscopy at room temperature. A feasible time response was obtained with 1.8MHz second resonance frequency of a commercially available silicon cantilever while keeping a high mechanical quality factor. Enhanced detection sensitivity due to small amplitude, high mechanical quality factor, and high resonance frequency enabled imaging of the Si(111)-7×7 surface at the attractive region. Three kinds of atomic contrast were obtained with same imaging parameters. Towards fast imaging, a constant amplitude shift image of 128×128pixels with atomic resolution was obtained within 4.75s.