Daisuke Nishide

Large-scale single-chirality separation of single-wall carbon nanotubes by simple gel chromatography

Monostructured single-wall carbon nanotubes (SWCNTs) are important in both scientific research and electronic and biomedical applications; however, the bulk separation of SWCNTs into populations of single-chirality nanotubes remains challenging. Here we report a simple and effective method for the large-scale chirality separation of SWCNTs using a single-surfactant multicolumn gel chromatography method utilizing one surfactant and a series of vertically connected gel columns. This method is based on the structure-dependent interaction strength of SWCNTs with an allyl dextran-based gel. Overloading an SWCNT dispersion on the top column results in the adsorption sites of the column becoming fully occupied by the nanotubes that exhibit the strongest interaction with the gel. The unbound nanotubes flow through to the next column, and the nanotubes with the second strongest interaction with the gel are adsorbed in this stage. In this manner, 13 different (n, m) species were separated. Metallic SWCNTs were finally collected as unbound nanotubes because they exhibited the lowest interaction with the gel.

Continuous Separation of Metallic and Semiconducting Carbon Nanotubes Using Agarose Gel

We have developed a novel method to separate metallic and semiconducting single-wall carbon nanotubes (SWCNTs) with high purities using agarose gel. When an SWCNTs/sodium dodecyl sulfate (SDS) dispersion was applied to a column containing agarose gel beads, semiconducting SWCNTs were trapped by the beads, while metallic SWCNTs passed through the column. After the semiconducting SWCNTs adsorbed to the beads were eluted with sodium deoxycholate solution, the column could be used for repeated separation. Because this continuous, repeatable separation method is applicable to a low-cost, large-scale process, it should enable the industrial production of metallic and semiconducting SWCNTs.

Discovery of surfactants for metal/semiconductor separation of single-wall carbon nanotubes via high-throughput screening.

We report novel surfactants that can be used for the separation of metallic (M) and semiconducting (S) single-wall carbon nanotubes (SWCNTs). Among the M/S separation methods using surfactants in an aqueous solution, sodium dodecyl sulfate plays a key role in density gradient ultracentrifugation (DGU) and agarose gel separations. In this study, we screened 100 surfactants for M/S separation using a high-throughput screening system. We identified five surfactants, which could be used for both DGU and agarose gel separations, suggesting that the basic principle of these separations is common. These surfactants have relatively low dispersibilities, which is likely due to their common structural features, i.e., straight alkyl tails and charged head groups, and appeared to enable M- and S-SWCNTs to be distinguished and separated. These surfactants should stimulate research in this field and extend the application of electrically homogeneous SWCNTs not only for electronics but also for biology and medicine.

Photoinduced Dispersibility Tuning of Carbon Nanotubes by a Water‐Soluble Stilbene as a Dispersant

Although single-walled carbon nanotubes (SWNTs) exhibit superior mechanical and electrical properties, [ 1 ] the industrial applications of SWNTs have been hindered by their insolubility in many conventional solvents. Bundled aggregates due to strong intertubular van der Waals interactions cause the poor solubility, which acts as an obstacle to both the purifi cation and handling of the SWNTs. In order to improve the solubility and to expand the potential application areas of the SWNTs, various approaches including covalent and noncovalent modifi cations have been reported. [ 2 ] In particular, noncovalent methods using solubilizing agents are of particular of interest because well-dispersed SWNTs can be easily prepared without a signifi cant decrease in the intrinsic electronic properties. Recently, a number of such dispersants with stimuli responsibility have been reported because tuning of the dispersibility of the SWNTs is desirable in many applications including switching devices, sensors, and drug delivery systems. For example, such dispersion or release control of the SWNTs has been examined using a metal complex, [ 3 ] a natural product, [ 4 ] and stimuli-responsive polymeric dispersants. [ 5 , 6 ]

Effective Separation of Carbon Nanotubes and Metal Particles from Pristine Raw Soot by Ultracentrifugation

A quick and effective method for the separation of single-wall carbon nanotubes (SWCNTs) from impurity particles has been demonstrated. High-purity SWCNTs and impurities, such as metal particles and amorphous carbons, were separated by ultracentrifugation from raw soot containing SWCNTs. The purity of each component was evaluated by scanning electron microscopy, thermogravimetric analysis, and Raman spectroscopy. The advantage of this method is that the impurity particles can be collected as residual soot in the ultracentrifugation process without any loss, while the impurity particles are more or less chemically modified or disappeared in previous purification protocols. The present technique can provide suitable samples for the research of both SWCNTs and impurity particles, particularly for nano-risk assessment.

Imaging and nanoprobing of graphene layers for interconnects by conductive atomic force microscopy

Graphene is a promising material to replace Cu-interconnect metallization under a width of 10 nm. We report a method for evaluating the graphene interconnect wiring structure by conductive atomic force microscopy (C-AFM), which enables the direct measurement of the two-dimensional (2D) resistance distribution and the coverage evaluation of multilayer graphene (MLG) grown on Ni interconnects using a 300 mm damascene process. The resistivity of exfoliated two-layer graphene was measured and a reasonable value of 30 µΩcm was obtained. We also measured the resistance of the MLG/Ni stack of 350 nm L/S patterns and confirmed the conduction paths of the MLG/Ni stack. It is demonstrated that the coverage of MLG on Ni interconnects can be estimated more precisely by C-AFM than by backscattered electron scanning electron microscopy (BSE-SEM) observation. C-AFM is demonstrated to be a potential technique for the local conductance evaluation of next-generation interconnects.

CMP process for selectively-grown carbon nanotubes in via structure

We have developed a CMP process providing a short polishing time and sufficient uniformity that is applicable to 300 mm scale carbon nanotube (CNT) via integration. To achieve our target, we have developed a new CMP process to make selectively grown CNT in a via hole. Spin on carbon (SOC)film was coated to protect nickel (Ni) catalyst and titanium nitride (TiN) film inside the via hole during SOC-CMP. The SOC-CMP process was used to perform two-step polishing to remove Ni catalyst and TiN film from the field area. CNT-CMP polishing time was reduced from 1690 sec to 200 sec and the yield ratio improved to 100%. CNT-CMP performance improved to a level usable for practical use by selectively growing CNT only from the via. We also evaluated the behavior of CNT-CMP during polishing using a damascene pattern wafer. Step height (Step height means a step between the wiring area and field area) occurred when CNT-SOG (spin-on glass) composite film was appeared upper the via hole but was reduced by concentrating the polishing pressure to project the CNT-SOG composite film. This result indicates that CNT-SOG composite film was polished by mechanical force.