Masaru Kishida

Metal-Coated Carbon Nanotube Tip for Scanning Tunneling Microscope

A metal-coated carbon nanotube (CNT) tip for scanning tunneling microscope (STM) has been developed. By pulsed laser deposition (PLD), a CNT attached to a tungsten tip was uniformly coated with 3–6 nm of tungsten thin layer. The observation of a Si(111)-7×7 surface using the tungsten-coated CNT tip demonstrated stable atomic imaging and potentiality of scanning tunneling spectroscopy (STS) measurement, which can be achieved from the pristine stage. It was demonstrated that the mechanical robustness and flexibility of the tip were maintained by virtue of the thin coated layer.

High-yield synthesis of conductive carbon nanotube tips for multiprobe scanning tunneling microscope

We have established a fabrication process for conductive carbon nanotube (CNT) tips for multiprobe scanning tunneling microscope (STM) with high yield. This was achieved, first, by attaching a CNT at the apex of a supporting W tip by a dielectrophoresis method, second, by reinforcing the adhesion between the CNT and the W tip by electron beam deposition of hydrocarbon and subsequent heating, and finally by wholly coating it with a thin metal layer by pulsed laser deposition. More than 90% of the CNT tips survived after long-distance transportation in air, indicating the practical durability of the CNT tips. The shape of the CNT tip did not change even after making contact with another metal tip more than 100 times repeatedly, which evidenced its mechanical robustness. We exploited the CNT tips for the electronic transport measurement by a four-terminal method in a multiprobe STM, in which the PtIr-coated CNT portion of the tip exhibited diffusive transport with a low resistivity of 1.8 kOmega/microm. The contact resistance at the junction between the CNT and the supporting W tip was estimated to be less than 0.7 kOmega. We confirmed that the PtIr thin layer remained at the CNT-W junction portion after excess current passed through, although the PtIr layer was peeled off on the CNT to aggregate into particles, which was likely due to electromigration or a thermally activated diffusion process. These results indicate that the CNT tips fabricated by our recipe possess high reliability and reproducibility sufficient for multiprobe STM measurements.

Electrical Characterization of Metal-Coated Carbon Nanotube Tips

Electrical characteristics of bare and metal-coated carbon nanotube (CNT) tips were investigated with an independently driven four-tip scanning tunneling microscope (STM). The CNT was glued on a W tip apex and wholly coated ex situ by metal thin layers. The resistance between the CNT-tip end and the W supporting tip scattered very widely from ca. 50 kΩ to infinity for the bare tips, while coating the tip with a 6-nm-thick PtIr film stably reduced the resistance to less than approximately 10 kΩ. The W coating was also effective for stabilizing the resistance, although they showed slightly larger resistance (ca. 50 kΩ). The metal-coated tips kept their low resistance and flexibility even after 100 repeated contacts to an object for conductivity measurements. They are expected to be useful for nanometer-scale transport measurements with multiprobe STM as well as for conventional single-tip STM.

Exploiting Metal Coating of Carbon Nanotubes for Scanning Tunneling Microscopy Probes

By exploiting the metal coating of carbon nanotube (CNT) tips for a scanning tunneling microscope (STM), we demonstrated atomic imaging/spectroscopy and showed their potential for electrical nanoprobes. A CNT glued to a W tip was uniformly coated with a thin W layer 3–6 nm thick. Using this tip, stable atomic imaging and spectroscopy were carried out on clean Si(111)-7×7 and Si(100)-2×1 surfaces. The mechanical flexibility of the coated CNT was maintained by virtue of the thin-layer coating, enabling repeated direct contact to the sample surface. Two W-coated CNT tips were brought together within a distance of approximately 50 nm. These results indicate that the tips are useful for electronic transport measurements on a nanometer scale after installation into a multiprobe STM.

Scanning Tunneling Spectroscopy Study of the ZnO(0001)?Zn Surface

The electronic structure of the ZnO(0001)–Zn surface was studied using scanning tunneling spectroscopy (STS) and first principles molecular dynamics. The STS spectrum indicated that the clean surface is n-type semiconducting with a band gap of about 3.3 eV. The local density of states (LDOS) calculated using ZnO slab model was in qualitative agreement with the STS spectrum, and revealed that occupied and unoccupied peaks originate from O and Zn atoms at the top bilayer of the surface, respectively. From the contour plots of LDOS, it was found that Zn atoms dominantly contribute to both occupied and unoccupied LDOS distributions and their broadening on the surface, which prevents atom-resolved scanning tunneling microscopy imaging of ZnO(0001) surface.

Synthesis of Metal-Alloy-Coated Nanowires toward Functional Scanning Probe Microscope

Using carbon nanotubes (CNTs) as templates, we have fabricated metal-alloy-coated nanowires by pulsed laser deposition. Superconducting and ferromagnetic materials were uniformly deposited around an isolated multiwalled CNT (MWNT), and reflected the shape of the MWNT. It was found that Nb3Sn and CoFe layers were deposited at a rate of about 0.13 nm/min, which indicates that their film thicknesses can be accurately controlled with nanometer accuracy. We also fabricated a superconductor-coated W tip, and obtained an scanning tunneling microscopy (STM) image of the Au(111) reconstructed surface at 2 K. These results indicate that nanowires synthesized using CNT templates can be used as materials for the tips of a functional scanning probe microscopy (SPM) which provide the nanoscale proximity of superconducting or magnetic nanowires.

Scanning Tunneling Microscopy Profiling of Steep Ridges Using Metal-Coated Carbon Nanotube Tip

We demonstrated the accurate imaging of steep ridges by scanning tunneling microscopy (STM) with a carbon nanotube (CNT) tip coated with a PtIr thin layer. Compared with the conventional tungsten tip, the PtIr-coated CNT tip could trace the shape of steep ridges (140 nm in width, 50 nm in height) more precisely with reduced artifacts originating from the finite shape of the tips. We also estimated the tip radius from the line profiles in the STM image, and proved that the tunneling current exactly flowed through the apex of the PtIr-coated CNT without bending or tilting of the tip during STM.

Synthesis of an Insulator-Coated Metal Tip with a 50-nm-Diameter Conductive Region at the Apex

We present a method for the fabrication of a passivated probe with a nanoscale conductive region. For an electrochemically sharpened W tip wholly coated with inner PtIr and outer SiO2 thin layers by pulsed laser deposition, the local removal of the SiO2 layer from the tip apex was achieved by electron-beam irradiation in a transmission electron microscope. The bared area of the tip apex was less than 50 nm in diameter. Scanning tunneling microscope imaging using the tip revealed that the active area at the apex was conductive. Moreover, the passivated tip showed stability to a biological culture solution.

Synthesis of inorganic thin-layer-coated carbon nanotubes toward passivated nanoprobes

We present a method for synthesizing inorganic-thin-layer-coated carbon nanotubes by pulsed laser deposition. Isolated multi-walled carbon nanotubes (MWNTs) were coated with inorganic thin layer in a multishell form. The product showed that various inorganic thin layers were uniformly wrapped around a MWNT, and reflected the shape of the MWNT. The thickness of the coated thin layer was precisely controlled with nanometer accuracy. Moreover, a metal-coated carbon nanotube (CNT) tip for scanning tunneling microscope (STM) has been developed. The observation of a Si(111)-7x7 surface using the metal-coated CNT tip demonstrated stable atomic imaging and potentiality of scanning tunneling spectroscopy (STS) measurement, which can be achieved from the pristine stage. It was demonstrated that the mechanical robustness and flexibility of the tip were maintained by virtue of the thin coated layer.