Kousuke Yokoyama

Carbon-Nanotube Tip for Highly-Reproducible Imaging of Deoxyribonucleic Acid Helical Turns by Noncontact Atomic Force Microscopy

A carbon nanotube (CNT) was used as a tip for a noncontact-mode atomic force microscope (NC-AFM). A CNT tip was attached to an Au/Si tip by a well-controlled procedure in a scanning-electron-microscope (SEM) chamber. The NC-AFM with the CNT tip produced highly reproducible images of right-handed helical turns of linear deoxyribonucleic acid (DNA) with a spacing of 3.5 ±1.0 nm. The full-width at half-maximum (FWHM) of the cross section of DNA measured was 3.1 ±0.6 nm.

High-resolution imaging of organic monolayers using noncontact AFM

Abstract Noncontact atomic force microscopy (AFM) provides useful technique for imaging organic molecules in high resolution. Here we present our recent advances in the noncontact AFM imaging of organic materials. (I) Molecular packing structures, defects and domain boundaries were clearly observed on adenine and thymine films. The noncontact AFM images revealed detailed features of the individual nucleic acid base molecules, thus allowing us to distinguish between adenine and thymine. (II) Both (√3×√3) R 30° structures and c (4×2) superlattice structures were resolved on alkanethiolate self-assembled monolayer (SAM) [CH 3 (CH 2 ) 8 SH] (nonanethiol) on Au(111). We found that the c (4×2) superlattice structures changed into (√3×√3) R 30° structures when the tip–surface distance decreased.

Atomic Resolution Imaging on Si(100)2 × 1 and Si(100)2 × 1 : H Surfaces with Noncontact Atomic Force Microscopy

We investigate the difference in atomic resolution images between the Si(100)2×1 reconstructed surface with a dangling bond and the Si(100)2×1:H monohydride surface without a dangling bond using noncontact atomic force microscopy. On the Si(100)2×1 surface, the distance between bright spots is 3.2 ±0.1 A, which is larger than that between silicon atoms. On the Si(100)2×1:H surface, the distance between bright spots is 3.5 ±0.1 A, which is in good agreement with that between hydrogen atoms. For the first time, individual hydrogen atoms are resolved. This means that the distance between measured bright spots forming dimers is increased by the hydrogen termination.

Correlation of frequency shift discontinuity to atomic positions on a Si(111)7 × 7 surface by noncontact atomic force microscopy

We succeeded in obtaining site-dependent frequency-shift curves on an atomic scale as a function of the tip-sample surface distance between a clean Si(111)7 x 7 surface and a clean active Si tip with a dangling bond using noncontact atomic force microscopy (NC-AFM). As a result, we found a discontinuous jump in the frequency-shift curve measured above active Si adatoms with a dangling bond, in contrast to a continuous frequency-shift curve measured above gaps between adjacent Si adatoms. These results suggest the possibility that the NC-AFM can be developed into a kind of spectroscopic tool, i.e. atomic force spectroscopy, which can measure the three-dimensional force-related map with true atomic resolution. Furthermore, we succeeded in suppressing the discontinuous jump in the frequency-shift curve by replacing the clean active Si tip apex with an oxidized inactive Si tip apex. This result suggests the possibility that we can control the interaction force between the tip and sample atoms on an atomic scale by placing a suitable atom on the tip apex.

Optical beam deflection noncontact atomic force microscope optimized with three-dimensional beam adjustment mechanism

We present a design and performance of an optical beam deflection noncontact atomic force microscope (nc–AFM). The optical deflection detection system can be optimized by the three-dimensional beam position adjustment mechanism (the slider which mounts laser diode module, the spherical rotors with mirror and the cylinder which mounts quadrant photodiode) using inertial stepping motors in an ultrahigh vacuum (UHV). The samples and cantilevers are easily exchanged in UHV. The performance of the instrument is demonstrated with the atomically resolved nc-AFM images for various surfaces such as Si(111)7×7, Cu(111), TiO2(110), and thymine/highly oriented pyrolytic graphite.

Atomic-scale structures on a non-stoichiometric TiO2(110) surface studied by noncontact AFM

Abstract We present atomic-scale structures of reconstructed phases of oxygen-deficient TiO2(110) observed by non-contact atomic force microscopy. This technique provided atomic structure of the surface terminated with added Ti2O3 rows including single oxygen atom vacancies. We successfully observed not only half steps along [112] caused by crystallographic shear planes belonging to {132} family but also novel half steps along [001]. On the basis of the idea that the half step formation has close relations with both the CS formation and diffusion of segregated impurities, we propose a structural model for half steps along [001].

Load dependence of sticking-domain distribution in two-dimensional atomic scale friction of NaF(100) surface

Remarkable transitions were found in atomic-scale friction-images of the NaF(100) surface and the corresponding sticking-domain distribution by detailed investigation of the load dependence using the two-dimensional frictional force microscope. The tip-position map reveals a new type of sticking-domain distribution pattern, which is different from the simple lattice periodicity.

Atomic Scale Origins of Force Interaction

The noncontact atomic force microscope (NC-AFM) using the frequency modulation (FM) detection method works as a three-dimensional forcemapping tool to investigate atomic scale origins of force interaction. For examples, we will show two different methods to map atomic force three-dimensionally; one by measuring site-dependent frequency-shift curves on an atomic scale as a function of the tip-sample distance between a clean Si(lll)7×7 surface and a clean Si tip, and the other by measuring the tip-sample distance dependence of atomically resolved NC-AFM image on Si(lll) √3x√3-Ag sample surface. Further, by placing a suitable atom on the tip apex, we investigated the possibility to control the interaction force between the tip and sample atoms on an atomic scale.

Frictional-Force Imaging and Friction Mechanisms with a Lattice Periodicity

The atomic force / lateral force microscope (AFM/LFM) using the optical lever method works as a two-dimensional frictional-force microscope (2D-FFM) in case of the atomically flat sample surface. On an atomic scale, the sliding body cannot be approximated to a uniform one but becomes a periodic one. As a result, the friction motion shows a lattice periodicity and becomes two-dimensional, because of the symmetry breaking of the stick-point distribution along the sliding (scanning) direction. Using a 2D-FFM, we can investigate the elementary process of the friction under the elastic deformation region, i.e., the two-dimensional nature of the atomic-scale friction with a lattice periodicity. Here, we show how to analyze the experimentally obtained 2D-FFM image patterns and how to deduce the load dependence of the effective adhesive region and also the stick-point distributions.

Structures of an Oxygen-Deficient TiO2(110) Surface Studied by Noncontact Atomic Force Microscopy

We investigated an oxygen-deficient TiO2(110) surface using noncontact atomic force microscopy, revealing the defect structures of topmost oxygen atoms. We successfully observed not only half-height steps along the [112] direction caused by crystallographic shear planes belonging to the {132} family but also novel half-height steps along [001]. The terrace between the half-height steps along the [112] direction was terminated with added Ti2O3 rows including some defect structures. We proposed that the half-height steps along the [001] direction were formed together with the half-height steps along the [112] direction.