Nami Choi

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.

Atomic Force Microscopy of Single-Walled Carbon Nanotubes Using Carbon Nanotube Tip

We succeeded in observing individual single-walled carbon nanotubes (SWNTs) using an atomic force microscope (AFM) in the tapping mode by paying attention to the preparation of both samples and AFM tips. To disentangle the bundles of SWNTs, we added a small amount of amine into N,N-dimethylformamide. To achieve a high resolution in tapping-mode AFM imaging, we used carbon nanotube (CNT) tips whose radii could be reduced. We were able to image individual SWNTs using CNT tips with widths that were half of those imaged using conventional silicon tips. With this improved resolution, we could clearly resolve the two SWNTs lying parallel on a mica substrate.

Effect of Amines on Single-Walled Carbon Nanotubes in Organic Solvents: Control of Bundle Structures

We have investigated the control of bundle structures of single-walled carbon nanotubes (SWNTs) in organic solvents using atomic force microscopy (AFM) and transmission electron microscopy (TEM). We have reported herein that organic amines are effective in untangling the bundled SWNTs in N,N-dimethylformamide (DMF). When Bucky paper was sonicated in DMF with 1% organic amine for 1 h, AFM images clearly showed mainly individual SWNTs. The time-dependent TEM observation revealed that the role of amine is to prevent rebundling of SWNTs in organic solvents.

Insertion process and electrical conduction of conjugated molecules in n-alkanethiol self-assembled monolayers on Au(111)

We have investigated an insertion process of conjugated molecules, 4-biphenylmethanethiol (BP) and [1,1′:4′,1″-terphenyl]-4-methanethiol (TP) into preassembled n-alkanethiol self-assembled monolayers on Au(111) using scanning tunneling microscopy (STM). Ex situ STM observation revealed that the insertion process is dependent on an organic solvent in which the conjugated molecules dissolve. The insertion of conjugated molecules with three phenyl rings (TP) was faster than that with two phenyl rings (BP). By analyzing the STM images, the lower limits of resistances for single BP and TP molecules were obtained as 22±5 and 36±5 GΩ, respectively.

Study of Field Emission Characteristics of Carbon with the Scanning Atom Probe

After obtaining the Fowler–Nordheim plot (F–N plot) of various carbon specimens including artificial diamonds and carbon nanotubes (CNTs), the surface composition of the specimens is analyzed by the scanning atom probe (SAP). Since the emitters vary from a sharp and slender CNT to a dull corner of a small diamond grain, the slope of the F–N plot is hardly related to the work function. Accordingly, the slope and intercept of the F–N plot are plotted on a Sasaki–Kaneko (S–K) chart. Even when the slopes of the F–N plots vary in a wide range, the points corresponding to the slopes and intercepts of the F–N plots are aligned on the S–K chart as suggested by the analytical calculation of the field strength directly above the emitter surface. Our results indicate that the work function of CNTs is smaller than that of the other carbon specimens and increases with the removal of the absorbed hydrogen.

LOW DIMENSIONAL STRUCTURE FORMATION IN SELF-ASSEMBLED MONOLAYERS ON AU(111)

Abstract We have studied the phase-separation of self-assembled monolayers (SAMs) of dialkyl disulfides on Au surface. Initially, the heat-induced phase-separation of SAMs consisting of unsymmetric disulfide molecules (CF 3 (CF 2 ) 5 (CH 2 ) 2 SS(CH 2 ) 7 CH 3 ) of hydrocarbon and fluorocarbon chains was investigated. We proved that the dimer structure cleaved on the Au surface. Subsequently, we formed one-dimensional nanostructures utilizing an anisotropic phase-separation of SAMs consisting of a mixture of the above unsymmetric disulfide and a symmetric disulfide of hydrocarbon (CH 3 (CH 2 ) 7 SS(CH 2 ) 7 CH 3 ). The wire-like structure with a width of 5 nm and a length ≈100 nm was observed by scanning tunneling microscopy.

Formation and evaluation of self-assembled monolayers derived from conjugated silylthiophene derivatives

We synthesized soluble rod-like silylthiophene, silylbithiophene, and silylquaterthiophene derivatives with various functional groups. Their self-assembled monolayer films on silicon oxide were evaluated by atomic force microscopy, X-ray photoelectron spectroscopy, water contact angle measurements, and fourier transform infrared-reflection absorption spectroscopy. We found that the molecular density (molecules/area) of the resultant films increased with thiophene moiety. This is probably due to the effective π-stacking of aromatic rings between molecules.

Differentiation of field-induced and native oxide using selective formation of self-assembled monolayer

The formation process of an alkylsiloxane self-assembled monolayer (SAM) on silicon oxide produced under various conditions was investigated. The growth rate of octadecyltrichlorosilane (OTS) SAM was dependent on the moisture level in the oxidation system. Area-selective oxidation of a silicon substrate with a native oxide layer was carried out under controlled humidity conditions by the field induced oxidation (FIO) technique using an atomic force microscope with a conductive cantilever. The silicon substrate partly FIO oxidized under dry nitrogen was immersed into the OTS solution, and the contact angle and topographic image showed that the OTS layer was formed only on the native oxide. In contrast, when the FIO was performed under a humidity of 88%, OTS SAM was formed both on FIO and native oxides. These results indicate that SAM formation on silicon oxides can be locally suppressed by FIO in a dry environment. Using this technique, we could fabricate an OTS SAM line structure as narrow as 22 nm.

Chemical Modification of SWNTs

One step synthesis of highly pure soluble single‐walled carbon nanotubes (s‐SWNTs) under mild condition is reported. SWNTs were functionalized long‐hydrophobic groups by oxidation and amidation. Raman and near infrared (NIR) absorption spectral data conformed that s‐SWNTs maintain the inherent properties of SWNTs.