Seiji Heike

MAGNETIC NANOPARTICLE ARRAY WITH PERPENDICULAR CRYSTAL MAGNETIC ANISOTROPY

By using electron beam lithography, a continuous CoCrPt film with a perpendicular crystal magnetic anisotropy has been patterned into a magnetic nanoparticle array of 29 Gdot/in.2 with a 150 nm period, an 80 nm diameter, and a 44 nm height. Studies of magnetic properties using a magnetic force microscope and a vibrating sample magnetometer show that this patterning increases the remanent-to-saturation magnetization ratio from 0.2 of the continuous film to 1 of the particles, and that each particle has a single magnetic domain with perpendicular anisotropy. The application of this array to future high density magnetic recording media is discussed.

Interaction of Ga Adsorbates with Dangling Bonds on the Hydrogen Terminated Si(100) Surface

Adsorption of Ga on the hydrogen terminated Si(100)–2×1–H surface has been investigated by scanning tunneling microscopy (STM). We have found that the thermally deposited Ga atoms preferentially adsorb on the hydrogen-missing dangling bonds and on the surface impurities. We desorb hydrogen atoms by the STM current and fabricate atomic-scale dangling-bond wires, in the similar way as was reported by Lyding et al. [Appl. Phys. Lett. 64 (1994) 2010]. In order to fabricate more detailed dangling bond structures, several methods of manipulating (detaching, attaching and moving) the individual hydrogen atoms are tested. We are able to thermally deposit Ga atoms on a dangling-bond wire and fabricate an atomic-scale Ga wire on the Si surface.

A proposal of nanoscale devices based on atom/molecule switching

This paper proposes a very small switching device, called an atom relay, which would supersede present metal‐oxide‐semiconductor devices for the next decade. The basic configuration of an atom relay consists of an atom wire, a switching atom, and a switching gate, with total dimensions below 10 nm, and an operation speed at more than terahertz level. The operation principle of the atom relay is that a switching atom is displaced from the atom wire by the electric field supplied from the switching gate, and the atom relay exhibits an ‘‘off’’ state. The switching characteristics of the atom relay are demonstrated by simulation, and it is shown that the electron propagation is successfully cut if a gap of about 0.4 nm is formed in the atom wire by the displacement of the switching atom. A self‐relay structure, in which the switching atom is displaced by the electric field from the atom wire itself, enables a dynamic memory cell, and the functions are ascertained by simulation. Fundamental logic circuits as N...

Conductivity measurements of individual poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) nanowires on nanoelectrodes using manipulation with an atomic force microscope

We have prepared conducting polymer nanowires of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) with diameters under 10 nm by a molecular combing method, and have measured the conductivity of the individual PEDOT nanowires on platinum nanoelectrodes using manipulation with an atomic force microscope (AFM). The temperature dependence of the conductance was explained well by a quasi-one-dimensional variable range hopping model. The conductivity of two single nanowires was determined to be 0.6 and 0.09S∕cm, which is of the same order as that of PEDOT/PSS films. After all the nanowires crossed over the nanoelectrodes were cut off with AFM manipulation, the current was drastically decreased down to the background level. These results directly indicate that the conductivity was derived from the PEDOT nanowires on the nanoelectrodes.

SURFACE MODIFICATION MECHANISM OF MATERIALS WITH SCANNING TUNNELING MICROSCOPE

The surface modification mechanism with scanning tunneling microscope (STM) is investigated. Experiments in both ultrahigh vacuum and air are reported, using several kinds of materials to understand the mechanism systematically. Threshold voltages (Vt’s), which are defined as the voltages above which modification is possible under the STM tip, have linear dependence on the binding energies of the materials. Thus, the STM surface modification mechanism is attributed to the local sublimation induced by tunneling electrons. For the modification in air, it is also ascribed to the chemical reaction induced by tunneling electrons with adsorbed water, and the Vt’s also fit on this line by taking the reaction energy into consideration. Therefore, the process is a direct consequence of the high flux of low‐energy electrons incident on the surface from the STM tip.

Atomic resolution noncontact atomic force/scanning tunneling microscopy using a 1 MHz quartz resonator

A 1 MHz quartz length extension resonator is used as a force sensor for a noncontact atomic force/scanning tunneling microscope (AFM/STM). A tungsten probe tip glued onto the end of the quartz rod enables the detection of tunneling currents for STM observation. Au surface was observed in both AFM and STM modes. The resolution difference is discussed in terms of the insulating oxide layer on the tip. We also demonstrate the AFM/STM observation of the Si(111)-7×7 surface with atomic resolution in an ultrahigh vacuum.

Characteristics of scanning-probe lithography with a current-controlled exposure system

Characteristics of atomic force microscopy lithography using a current-controlled exposure feedback system are investigated by fabricating line-and-space patterns on the negative-type electron beam resist RD2100N. We find that the cross-sectional shape of the developed resist pattern depends on the amount of exposure. The resolution depends on the resist thickness,and a minimum line width of 27 nm is obtained for a 15-nm-thick resist. The proximity effect is evaluated by comparing a resist pattern with a model calculation. Electric-field mapping inside the resist is calculated,and an exposure mechanism is proposed to explain the characteristics.

Scanning Tunneling Spectroscopy of Dangling-Bond Wires Fabricated on the Si(100) -2 × 1 -H Surface

Tunneling spectroscopy of atomic-scale dangling-bond wires on a hydrogen-terminated Si(100)–2×1–H surface is studied using ultrahigh-vacuum scanning tunneling microscopy. Individual dangling bonds are fabricated by extracting hydrogen atoms one by one from the hydrogen terminated surface to form atomic-scale dangling-bond wires. These wires show a finite density of states at the Fermi level and do not show semiconductive band gaps. The results are compared with first-principles theoretical calculations.

SELF-ORGANIZED GROWTH OF FE NANOWIRE ARRAY ON H2O/SI(100)(2 N)

By evaporating Fe on to a water-terminated Si(100)(2×n) surface, we formed an Fe wire array reflecting the 2×n surface reconstruction. The average wire width was 2 nm and the period was 3 nm. The formation was caused by the deposited Fe atoms diffusing over the water-terminated flat area and being trapped at dimer vacancy lines. This array is applicable to magnetic devices.

Nanoneedle formation on silicon (111) surface using a scanning tunneling microscope tip

Nanoneedles are formed on the Si(111) surface when negative ramp voltages are applied to a scanning tunneling microscope (STM) tip. These nanoneedles allow the direct imaging of the STM tip, because of their extreme sharpness with an estimated diameter of ∼2 nm and a maximum height of 20 nm. In this article, voltage, time, and current dependences of nanoneedle growth are examined. Based on the experimental results, we propose that the nanoneedle formation mechanism is: Si atom extraction from the Si surface to the tip due to the applied high voltage, migration of the atoms to the tip apex, and redeposition from the tip apex to the sample surface. A nanoneedle can also be formed on the tip apex by applying a positive voltage and the growth process is observed by a nanoneedle on the sample surface.