Taiki Inoue

Decomposition of Ethanol and Dimethyl Ether during Chemical Vapor Deposition Synthesis of Single-Walled Carbon Nanotubes

In this study, we investigated carbon feedstock decomposition conditions on the synthesis of single-walled carbon nanotubes (SWNTs) by chemical vapor deposition. We simulated gas-phase thermal decomposition of ethanol and dimethyl ether (DME) at typical SWNT growth conditions using the chemical kinetic model, and confirmed the reaction trends and primary products using Fourier transform infrared (FT-IR) spectroscopy. Molar fractions were correlated against residence time in the reactor by adjusting the volumetric gas flow rate, and concentration profiles of reaction species were compared to the predicted decomposition mechanism. Signature peak intensities indicated concentrations of both ethanol and DME.

Chirality analysis of horizontally aligned single-walled carbon nanotubes: decoupling populations and lengths

Utilizing the aligned morphology of single-walled carbon nanotubes (SWCNTs) grown on crystal quartz substrates together with systematic Raman mapping measurements, the populations and lengths of SWCNTs with different chiralities (n,m) were independently evaluated. Chiralities of SWCNTs were assigned on a one-by-one basis by comparing radial breathing mode frequencies with the Kataura plot. The SWCNT lengths were determined by Raman mapping and/or scanning electron microscopy. Both the populations and lengths of the SWCNTs grown in this study with a diameter of 1.14 to 1.29 nm showed no clear dependence on their chiral angles.

Room temperature-processed inverted organic solar cells using high working-pressure-sputtered ZnO films

This study reports improved performance of inverted organic solar cells by using high working-pressure sputtered ZnO. Sputtering produces highly crystalline ZnO without the need for thermal annealing. However, photovoltaic devices fabricated by using sputtered ZnO have shown lower power conversion efficiencies than those made by using sol–gel ZnO. On the other hand, sol–gel ZnO limits the flexible application of inverted organic solar cells because of high-temperature annealing. Therefore, a new method of sputtering under high working pressure is developed. The power conversion efficiency of inverted organic solar cells fabricated using this high working-pressure-sputtered ZnO (η = 8.6%, VOC = 0.77 V, JSC = 15.6 mA cm−2, and FF = 0.72) is superior to that of conventional sol–gel ZnO-based devices (η = 7.8%, VOC = 0.73 V, JSC = 16.0 mA cm−2, and FF = 0.63). Furthermore, utilizing the low temperature process of sputtering, flexible application is successfully achieved using polyethylene terephthalate indium tin oxide films.

Water-assisted self-sustained burning of metallic single-walled carbon nanotubes for scalable transistor fabrication

Although aligned arrays of semiconducting single-walled carbon nanotubes (s-SWNTs) are promising for use in next-generation electronics owing to their ultrathin bodies and ideal electrical properties, even a small portion of metallic (m-) counterparts causes excessive leakage in field-effect transistors (FETs). To fully exploit the benefits of s-SWNTs for use in large-scale systems, it is necessary to completely eliminate m-SWNTs from as-grown SWNT arrays and thereby obtain purely semiconducting large-area arrays, wherein numerous FETs can be flexibly built. In this study, we performed electrical burning of m-SWNTs assisted by water vapor and polymer coating to eliminate m-SWNTs over a long length for the scalable fabrication of transistors from the remaining s-SWNT arrays. During the electrical-breakdown process, the combination of water vapor and the polymer coating significantly enhanced the burning of the SWNTs, resulting in a self-sustained reaction along the nanotube axis. We found that m-SWNT segments partially remaining on the anode side resulted from one-way burning from the initial breakdown position, where Joule-heating-induced oxidation first occurred. The s-SWNT-enriched arrays obtained were used to fabricate multiple FETs with a high on-off current ratio. The results indicate the advantages of this approach over conventional electrical breakdown for the large-scale purification of s-SWNTs.

Field emission and anode etching during formation of length-controlled nanogaps in electrical breakdown of horizontally aligned single-walled carbon nanotubes

We observe field emission between nanogaps and voltage-driven gap extension of single-walled carbon nanotubes (SWNTs) on substrates during the electrical breakdown process. Experimental results show that the gap size is dependent on the applied voltage and humidity, which indicates high controllability of the gap size by appropriate adjustment of these parameters in accordance with the application. We propose a mechanism for the gap formation during electrical breakdown as follows. After small gaps are formed by Joule heating-induced oxidation, SWNTs on the anode side are electrochemically etched due to physically-adsorbed water from the air and the enhanced electric field at the SWNT tips. Field emission is measured in a vacuum as a possible mechanism for charge transfer at SWNT gaps. The relationship between the field enhancement factor and geometric features of SWNTs explains both the voltage dependence of the extended gap size and the field emission properties of the SWNT gaps. In addition, the similar field-induced etching can cause damage to adjacent SWNTs, which possibly deteriorates the selectivity for cutting metallic pathways in the presence of water vapor.

Simple Fabrication Technique for Field-Effect Transistor Array Using As-Grown Single-Walled Carbon Nanotubes

A carbon nanotube field-effect transistor (CNT-FET) is a promising candidate for future electronic devices; however, its fabrication process is still challenging. We propose a simple fabrication technique for CNT-FET arrays using as-grown single-walled CNTs (SWNTs) as the gate channel. In this study, a hydrophobic self-assembled monolayer (SAM) was used to restrict the catalyst-supporting area after the fabrication of an electrode array. Since it is known that droplets are trapped at rough edges of a hydrophobic surface, the deposition of a liquid-based catalyst, followed by alcohol catalytic chemical vapor deposition (ACCVD) produced SWNTs that grew only at the corners of electrode edges. The current–voltage (I–V) characterization of FETs with a 40 µm channel width showed that 98% of the fabricated devices were electrically connected and more than 50% were functional FETs (ION/IOFF > 102).

On-Chip Sorting of Long Semiconducting Carbon Nanotubes for Multiple Transistors along an Identical Array

Ballistic transport and sub-10 nm channel lengths have been achieved in transistors containing one single-walled carbon nanotube (SWNT). To fill the gap between single-tube transistors and high-performance logic circuits for the replacement of silicon, large-area, high-density, and purely semiconducting (s-) SWNT arrays are highly desired. Here we demonstrate the fabrication of multiple transistors along a purely semiconducting SWNT array via an on-chip purification method. Water- and polymer-assisted burning from site-controlled nanogaps is developed for the reliable full-length removal of metallic SWNTs with the damage to s-SWNTs minimized even in high-density arrays. All the transistors with various channel lengths show large on-state current and excellent switching behavior in the off-state. Since our method potentially provides pure s-SWNT arrays over a large area with negligible damage, numerous transistors with arbitrary dimensions could be fabricated using a conventional semiconductor process, leading to SWNT-based logic, high-speed communication, and other next-generation electronic devices.

Carrier polarity engineering in carbon nanotube field-effect transistors by induced charges in polymer insulator

We present that the electrical conduction type in carbon nanotube field-effect transistors (CNT-FETs) can be converted by induced charges in a polyvinyl alcohol (PVA) insulator. When the CNT channels are covered with pure PVA, the FET characteristics clearly change from unipolar p-type to ambipolar. The addition of ammonium ions (NH4+) in the PVA leads to further conversion to unipolar n-type conduction. The capacitance − voltage characteristics indicate that a high density of positive charges is induced at the PVA/SiO2 interface and within the bulk PVA. Electrons are electrostatically accumulated in the CNT channels due to the presence of the positive charges, and thus, stable n-type conduction of PVA-coated CNT-FETs is observed, even under ambient conditions. The mechanism for conversion of the conduction type is considered to be electrostatic doping due to the large amount of positive charges in the PVA. A blue-shift of the Raman G-band peak was observed for CNTs coated with NH4+-doped PVA, which corre...

Quantitative study of bundle size effect on thermal conductivity of single-walled carbon nanotubes

Compared with isolated single-walled carbon nanotubes (SWNTs), thermal conductivity is greatly impeded in SWNT bundles; however, the measurement of the bundle size effect is difficult. In this study, the number of SWNTs in a bundle was determined based on the transferred horizontally aligned SWNTs on a suspended micro-thermometer to quantitatively study the effect of the bundle size on thermal conductivity. Increasing the bundle size significantly degraded the thermal conductivity. For isolated SWNTs, thermal conductivity was approximately 5000 ± 1000 W m–1 K–1 at room temperature, three times larger than that of the four-SWNT bundle. The logarithmical deterioration of thermal conductivity resulting from the increased bundle size can be attributed to the increased scattering rate with neighboring SWNTs based on the kinetic theory.

Digital Isotope Coding to Trace the Growth Process of Individual Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) are attracting increasing attention as an ideal material for high-performance electronics through the preparation of arrays of purely semiconducting SWCNTs. Despite significant progress in the controlled synthesis of SWCNTs, their growth mechanism remains unclear due to difficulties in analyzing the time-resolved growth of individual SWCNTs under practical growth conditions. Here we present a method for tracing the diverse growth profiles of individual SWCNTs by embedding digitally coded isotope labels. Raman mapping showed that, after various incubation times, SWCNTs elongated monotonically until their abrupt termination. Ex situ analysis offered an opportunity to capture rare chirality changes along the SWCNTs, which resulted in sudden acceleration/deceleration of the growth rate. Dependence on growth parameters, such as temperature and carbon concentration, was also traced along individual SWCNTs, which could provide clues to chirality control. Systematic growth studies with a variety of catalysts and conditions, which combine the presented method with other characterization techniques, will lead to further understanding and control of chirality, length, and density of SWCNTs.