Masakazu Aono

Linear chain polymerization initiated by a scanning tunneling microscope tip at designated positions

A conjugated polymer nanowire can be created at any designated position in a monomolecular layer by initiating chain polymerization using a scanning tunneling microscope tip with a spatial precision of the order of 1 nm. The demonstration has been presented for a self-ordered monomolecular layer of 10,12-pentacosadiynoic acid, which is a diacetylene compound, adsorbed on a graphite surface. The polymer nanowires created have a length ranging from 5 to 300 nm, the length being controlled by domain boundaries or artificial defects in the molecular layer. The frequency of occurrence of chain polymerization is measured against the pulsed bias voltage, which suggests that the excitation of the molecule is caused by the inelastically tunneling electrons.

Surface structures and work functions of the LaB6 (100), (110) and (111) clean surfaces

The structures of the LaB6 (110) and (111) clean surfaces have been studied by angleresolved XPS, ISS and LEED. The LaB6 (110) clean surface has the relaxed and reconstructed c(2 × 2) structure where the surface lanthanum atoms are displaced toward the surface and a c(2 × 2) overlayer structure is caused by a displacement of the surface lanthanum atoms. The LaB6 (111) clean surface has the relaxed (1 × 1) structure where the surface lanthanum atoms are displaced toward the surface. The work functions of the (100), (110) and (111) surfaces have been measured from the width of UPS spectra and are ~2.3, ~2.5 and ~3.3 eV, respectively. The anisotropy of the work functions has been correlated with the surface structures. The origin of the unusually low work functions of the LaB6 (100) and (110) clean surfaces in refractory materials is interpreted by electric dipole moments produced by positive charges of surface lanthanum ions.

Development of a scanning tunneling microscope for in situ experiments with a synchrotron radiation hard-X-ray microbeam.

A scanning tunneling microscope dedicated to in situ experiments under the irradiation of highly brilliant hard-X-rays of synchrotron radiation has been developed. In situ scanning tunneling microscopy (STM) observation was enabled by developing an accurate alignment system in ultrahigh vacuum. Despite the noisy conditions of the synchrotron radiation facility and the radiation load around the probe tip, STM images were successfully obtained at atomic resolution. Tip-current spectra were obtained for Ge nano-islands on a clean Si(111) surface by changing the incident photon energy across the Ge absorption edge. A current modification was detected at the absorption edge with a spatial resolution of the order of 10 nm.

Construction of Independently Driven Double-Tip Scanning Tunneling Microscope

We construct an independently driven double-tip scanning tunneling microscope (STM) for evaluating electrical conduction within the micrometer scale under an ambient condition. Each independently driven STM unit has on atomic resolution and the tip approaches an intended position within 10 mm2 on the surface with three course driving stages and a piezoelectric device which has a maximum scan area of 10 µm. The current flow between the two tips through the material can be detected in the range from 0.1 pA to 100 nA. The measurement of the resistivity of regiorandom poly(3-octylthiophene) thin films was demonstrated using this system.

Surface states on the LaB6(100), (110) and (111) clean surfaces studied by angle-resolved ups

The surface states on the LaB6(100), (110) and (111) clean surfaces have been studied by means of angle-resolved UV photoelectron spectroscopy with unpolarized light. Surface states on LaB6(110) and (111) have been observed for the first time, and the energy-band structures have been determined. The surface states on LaB6(110) are located at ∼1.8 and ∼3.0 eV below EF. They are associated with the occurrence of the c(2 × 2) surface structure on LaB6(110). The surface states on LaB6(111) are located at ∼1.5 and ∼2 eV below EF. The dispersion of the lower band is relatively large (∼0.9 eV) compared with those on LaB6(100) and (110). The surface state at ∼2 eV below EF on LaB6(111) has a maximum energy at Γ point (k∥ = 0), while that on LaB6(100) has a minimum energy at Γpoint. The surface state at ∼2 eV below EFon LaB6(100) possesses Δ1 symmetry at k∥ = 0, and has even parity with respect to the (010) and (011) planes. The surface state at ∼1.8 eV below EF on LaB6(110) has even parity with respect to the (110) plane, and odd parity with respect to the (001) plane. The surface state at ∼3.0 eV below EF has even parity with respect to both the (110) and (001) planes. The low-lying surface state on LaB6(111) has Δ1 symmetry at k∥ = 0. All these surface states on the LaB6 clean surfaces are interpreted in terms of dangling bonds of the surface boron frameworks which are mainly B 2p in character.

Nanoscale wiring by controlled chain polymerization

Chain polymerization of amphiphilic diacetylene compounds in a monomolecular layer on a graphite substrate can be observed and controlled at a nanometer scale using a scanning tunneling microscope (STM). The wavelength dependence of photoreactivity suggests that the graphite substrate sensitizes the photoreaction. The chain polymerization can be initiated not only by irradiating light, but also by applying stimulation using an STM tip. We can also control the termination of chain polymerization by an artificial defect, so that we can create a polydiacetylene nanowire of designated length at any designated position.

Atomic scale modifications of hydrogen-terminated silicon 2×1 and 3×1 (001) surfaces by scanning tunneling microscope

Abstract Atomic scale desorption and deposition of hydrogen (H) atoms on Si(001)-(2×1)-H and Si(001)-(3×1)-H surfaces have been studied using clean and H covered tips from a scanning tunneling microscope. We report desorption of H atoms from these surfaces at positive and negative sample bias voltages with a resolution of one to two atomic rows and atomic scale phase transitions from 3×1 structures to 2×1 structures and vice versa. At positive sample bias, phase transitions from the 3×1 to 2×1 structures are accompanied with a large number of dangling bonds on the newly crated 2×1 structures, because the desorption of H from the 2×1 structure occurs at a lower tunnel current than the formation of the phase transition. At negative sample bias

Force Microscopy Study of SrTiO3(001) Surfaces with Single Atomic-Layer Steps

An atomically flat SrTiO3(001) surface was successfully obtained by combining ultrasonic agitation and subsequent annealing at 1000°C in air. The topographic images taken using an atomic force microscope revealed that the surface was composed of clear atomic-steps and atomically smooth terraces. The steps were 0.2 nm in height corresponding to half of the unit cell height of SrTiO3, or the single atomic-layer height. The terraces were categorized into two types; wide and narrow terraces appear alternately. Surface composition analyzed by coaxial impact collision ion scattering spectroscopy indicated that the topmost surface was covered by domains of the SrO and TiO2 layers, where the latter domain was dominant. The result suggests that the topmost surface of the narrow terrace was terminated by SrO and the wide terrace by TiO2. This was further examined by the use of a friction force microscopy technique with which we were able to distinguish the different surface terminations.

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB6(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB6(100), (110) and (111) clean surfaces. The surface states on LaB6(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB6(110) is increased to ∼3.8 eV by an oxygen exposure of ∼2 L. The surface states on LaB6(111) disappear at an oxygen exposure of ∼2 L where the work function has a maximum value of ∼4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB6(111) until an exposure of ∼2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ∼4.4 to ∼3.8 eV until an oxygen exposure of ∼100L. The initial sticking coefficient on LaB6(110) has the highest value of ∼1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB6 surfaces.

Atomic scale extraction of hydrogen atoms adsorbed on Si(001) with the scanning tunneling microscope

Abstract Atomic scale extraction of hydrogen atoms on Si(001)-(2 × 1)-H and -(3 × 1)-H surfaces between 300 and 610 K have been studied using a scanning tunneling microscope. Hydrogen atoms were extracted at positive and negative sample bias voltage. Site-selective desorption and reconstruction from the dihydride phase to the monohydride phase are found in the desorption of hydrogen atoms from Si(001)-(3 × 1)-H surfaces. STM-induced desorption of H atoms on a Si(001)-(3 × 1)-H surface occurs preferentially at dihydride units rather than monohydride units. The results support an electric field induced desorption.