Atsuto Okamoto

Optically tunable amino-functionalized graphene quantum dots.

Amino-functionalized graphene quantum dots (af-GQDs) with discrete molecular weights and specific edges were self-limitedly extracted from oxidized graphene sheet. Their optical properties can be precisely controlled only by the selective and quantitative functionalization at the edge sites. The af-GQDS exhibit bright colorful fluorescence under a single-wavelength excitation.

Ultrahigh-quality silicon carbide single crystals

Silicon carbide (SiC) has a range of useful physical, mechanical and electronic properties that make it a promising material for next-generation electronic devices. Careful consideration of the thermal conditions in which SiC {0001} is grown has resulted in improvements in crystal diameter and quality: the quantity of macroscopic defects such as hollow core dislocations (micropipes), inclusions, small-angle boundaries and long-range lattice warp has been reduced. But some macroscopic defects (about 1–10 cm-2) and a large density of elementary dislocations (∼ 104 cm-2), such as edge, basal plane and screw dislocations, remain within the crystal, and have so far prevented the realization of high-efficiency, reliable electronic devices in SiC (refs 12–16). Here we report a method, inspired by the dislocation structure of SiC grown perpendicular to the c-axis (a-face growth), to reduce the number of dislocations in SiC single crystals by two to three orders of magnitude, rendering them virtually dislocation-free. These substrates will promote the development of high-power SiC devices and reduce energy losses of the resulting electrical systems.

Growth of Carbon Nanowalls on a SiO2 Substrate by Microwave Plasma-Enhanced Chemical Vapor Deposition

We investigated the growth process of carbon nanowalls (CNWs) on a SiO2 substrate by microwave plasma-enhanced chemical vapor deposition (MPECVD). It is revealed that the CNWs are grown at the fine-textured structure on the SiO2 and the growth process does not require the catalyst. The CNW initially has a semicircular shape. The height, thickness, and mesh size increase with growth time. It is found that the height of CNWs as a function of time obeys the square root law. Extremely high growth rate, approximately 10 µm/h, is achieved, in contrast to previous studies.

Step structures and dislocations of SiC single crystals grown by modified Lely method

Step structures on the {0 0 0 1} facet of SiC single crystals grown by the modified Lely method were examined through an optical microscopy and an atomic force microscopy to discuss the dependence on the polytype and the polarity. The results were compared with those of CVD method. Step structures around micropipes were quite different for the different polarity of a growing surface. No micropipes were detected at the center of spirals with a step height of 1.5 nm for 6H-SiC. Quantitative morphological features could be corresponded to the variety of etch pits by chemical etching on the growing surface. The dislocations and their motions during the growth were also characterized through etching and polishing the grown crystal from the surface to the inside successively. The screw dislocations shifted outwards from the center of a giant spiral as the growth proceeded. In contrast, the edge dislocations glided in any directions. Discussion was carried out on these results.

Synthesis and characterization of multi- and single-wall carbon nanotubes by the catalytic vapor deposition method

Carbon nanotubes (CNTs) have been synthesized by catalytic decomposition of acetylene (catalytic chemical vapor deposition, the so-called CCVD method) over well dispersed metal particles (cobalt and iron) embedded in supports such as commercially available zeolites and folded sheets mesoporous material (FSM) at temperature above 600°C. A few individual single-wall carbon nanotubes (SWNTs) and bundles of SWNTs are observed in pristine soot synthesized with FSM supports at 900°C, whereas no SWNTs but only multi-wall CNTs (MWNTs) are observed in pristine soot synthesized at less than 800°C. The observed temperature dependence is similar to the synthetic result obtained with zeolites supports. The yield of CNTs (both SWNTs and MWNTs) is higher with Zeolite supports than with FSM. Transmission electron microscopy and Raman spectroscopy also revealed that surface pores of support materials could act as an excellent growth sites for SWNTs.

The photoelastic constant and internal stress around micropipe defects of 6H-SiC single crystal

Abstract Hexagonal silicon carbide (6H-SiC) single crystals made by a modified Lely method contain a ‘micropipe’ which is one kind of serious defects degrading device performance. The micropipe accompanies strong internal stress around itself. We determined the photoelastic constant in the plane of (00·1) 6H-SiC and then estimated the magnitude of the internal stress around the micropipes. The photoelastic constant was 2.73 brewster at λ =546 nm. The internal stress around the micropipe was estimated to be 113∼166 MPa.

Improved Diameter Control of Carbon Nanotubes Using Fe–V–O Nanoparticles as the Catalyst

Fe–O and Fe–V–O nanoparticles (NPs) containing 4.0, 8.1, 12.3, 16.4, and 20.6 at. % V were synthesized by a liquid phase process. Carbon nanotubes (CNTs) were grown from each NP using acetylene as the carbon source, in order to investigate the dependence of CNT diameter on V concentration. The results show that the diameters of the CNTs are larger than those of the NPs, but tend to approach that of the NP as V concentration increases. This suggests that the diameter control of CNTs can be improved by adding of V to Fe when Fe–O NPs are used as a catalyst of CNT growth.

Growth of Vertically Aligned Carbon Nanotubes from Highly Active Fe–Ti–O Nanoparticles Prepared by Liquid-Phase Synthesis

Monodispersed Fe–Ti–O nanoparticles and Fe–O nanoparticles were prepared by liquid-phase synthesis to be used as catalysts for carbon nanotube (CNT) growth. Vertically aligned, dense CNTs were obtained in the case of Fe–Ti–O nanoparticles, whereas few CNTs were grown when Fe–O particles were used. CNTs with a diameter of 4.3±0.7 nm were grown from Fe–10 at. % Ti nanoparticles with a diameter of 3.8±0.6 nm. These CNTs mainly consist of double- and triple-walled CNTs. The catalytic activity of nanoparticles peaks at around 20–25 at. % Ti. The diameter controllability of Fe–20 at. % Ti nanoparticles was lower than that of Fe–10 at. % Ti nanoparticles, although a 200-µm-thick CNT forest was grown within 10 min. The energy-dispersive X-ray spectroscopy (EDS) analysis of the Fe–Ti–O nanoparticles before and after CNT growth revealed the depletion of Ti in the nanoparticles during the CNT growth, indicating the extraction of Ti from the nanoparticles.

In situ Optical Imaging of Carbon Nanotube Growth

In situ optical imaging of vertically aligned carbon nanotube (CNT) growth has been performed. The thickness and growth rate of CNTs were accurately measured in real time using an image processor. A thickness variation ranging from approximately 7 to 250 µm was observed. As growth proceeded, growth rate degradation occurred with relaxing its gradient. However, our in situ observations revealed that growth rate recovered upon varying the growth parameters, such as temperature, pressure, and C2H2/H2 flow rate ratio, suggesting that in situ growth rate control is applicable to CNT growth.

Thermal Expansion Coefficient of a New Heavy Fermion Substance, CeInCu2

Thermal expansion coefficients α(K -1 ) of single crystalline CeInCu 2 have been measured from 4.2 K to 300 K by means of the strain gauge method. The α along the [110] direction is very large at low temperatures below ∼100 K compared with that of LaInCu 2 which has no 4f electron. This fact implies that there is a large contribution from the 4f electron in CeInCu 2 to the magnitude of α. The results are discussed with regard to the crystalline electric field splitting of the Ce 3+ ion.