Yuhei Miyauchi

Low-temperature synthesis of high-purity single-walled carbon nanotubes from alcohol

Abstract By using alcohol as the carbon source, a new simple catalytic chemical vapor deposition technique to synthesize high-purity single-walled carbon nanotubes at low temperature is demonstrated. Because of the etching effect of decomposed OH radical attacking carbon atoms with a dangling bond, impurities such as amorphous carbon, multi-walled carbon nanotubes, metal particles and carbon nanoparticles are completely suppressed even at relatively low reaction temperature such as 700–800 °C. By using methanol, generation of SWNTs even at 550 °C is demonstrated. The high-purity synthesis at low temperature promises large scale production at low cost and the direct growth of SWNTs on conventional semiconductor devices already patterned with aluminum.

Tunable Photoluminescence of Monolayer MoS2 via Chemical Doping

We demonstrate the tunability of the photoluminescence (PL) properties of monolayer (1L)-MoS2 via chemical doping. The PL intensity of 1L-MoS2 was drastically enhanced by the adsorption of p-type dopants with high electron affinity but reduced by the adsorption of n-type dopants. This PL modulation results from switching between exciton PL and trion PL depending on carrier density in 1L-MoS2. Achievement of the extraction and injection of carriers in 1L-MoS2 by this solution-based chemical doping method enables convenient control of optical and electrical properties of atomically thin MoS2.

Direct synthesis of high-quality single-walled carbon nanotubes on silicon and quartz substrates

Abstract A new technique of synthesizing high-quality single-walled carbon nanotubes (SWNTs) directly on the surface of silicon and quartz substrates has been developed by means of the low-temperature catalytic CVD method using ethanol. The proposed method does not employ conventional deposition/sputtering for the mounting of catalytic metals on the substrates, but it adopts an easy and costless liquid-based dip-coat approach without need of support/underlayer materials that were often used in previous studies. The substrate surface is blackened with a uniform layer of SWNTs after the CVD at an optimum condition. The optical absorption of ‘as-grown’ SWNTs has first been measured using thereby prepared SWNT-synthesized quartz substrate.

Characterization of single-walled carbon nanotubes catalytically synthesized from alcohol

High purity single-walled carbon nanotubes (SWNTs) were synthesized from ethanol by catalytic CVD method. The yield of SWNTs was determined based on TGA complemented by Raman and TEM analyses. The effects of CVD reaction time and pre-reduction of catalytic metal on the yield and quality of synthesized SWNTs were investigated. The SWNT yield of more than 40% was achieved over the weight of zeolite support powder with Fe/Co catalyst, which corresponded to more than 800% yield over the weight of the catalytic metal, within the CVD reaction time of 120 min assuring as-grown high quality.

Considerably improved photovoltaic performance of carbon nanotube-based solar cells using metal oxide layers

Carbon nanotube-based solar cells have been extensively studied from the perspective of potential application. Here we demonstrated a significant improvement of the carbon nanotube solar cells by the use of metal oxide layers for efficient carrier transport. The metal oxides also serve as an antireflection layer and an efficient carrier dopant, leading to a reduction in the loss of the incident solar light and an increase in the photocurrent, respectively. As a consequence, the photovoltaic performance of both p-single-walled carbon nanotube (SWNT)/n-Si and n-SWNT/p-Si heterojunction solar cells using MoOx and ZnO layers is improved, resulting in very high photovoltaic conversion efficiencies of 17.0 and 4.0%, respectively. These findings regarding the use of metal oxides as multifunctional layers suggest that metal oxide layers could improve the performance of various electronic devices based on carbon nanotubes.

Dependence of exciton transition energy of single-walled carbon nanotubes on surrounding dielectric materials

Abstract We theoretically investigate the environmental effect for optical transition energies of single-walled carbon nanotubes (SWNTs), by calculating the exciton transition energies of SWNTs. The static dielectric constants used in the exciton calculation can be expressed as a function of the dielectric constants of the surrounding material and that of the SWNT, in which the static and dynamic dielectric constants of the SWNT represent the screening effect of core electrons and the valence π electrons, respectively. The calculated results reproduce the environmental effect of the experimental transition energies for various surrounding materials and for various diameters of SWNTs.

Magnetic Brightening of Carbon Nanotube Photoluminescence through Symmetry Breaking

We report that symmetry breaking by a magnetic field can drastically increase the photoluminescence quantum yield of single-walled carbon nanotubes, by as much as a factor of 6, at low temperatures. To explain this we have developed a theoretical model based on field-dependent exciton band structure and the interplay of Coulomb interactions and the Aharonov-Bohm effect. This conclusively explains our data as the first experimental observation of dark excitons 5-10 meV below the bright excitons.

Nonlinear photoluminescence in atomically thin layered WSe2 arising from diffusion-assisted exciton-exciton annihilation

We studied multiexciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density and long-distance exciton diffusion, reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation (EEA) model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequent efficient EEA process determined the unusual multiexciton dynamics in atomically thin layered transition metal dichalcogenides.

Selective Optical Property Modification of Double-Walled Carbon Nanotubes by Fluorination

We found that by fluorination of double-walled carbon nanotubes (DWNTs), it is possible to suppress only the Raman radial breathing mode and absorption peaks from the outer (large diameter) tubes of DWNTs. In contrast, Raman signals from the inner shells showed no difference from the pristine DWNTs. The stability of the inner shells of fluorinated DWNTs was also confirmed from the photoluminescence (PL) map and the optical absorption spectra, which only showed the signals from the inner shells of DWNTs, with no distinct change in the optical properties of the inner tubes after fluorination. Our results indicate that once fluorinated, there exists only a weak, if not none, interaction between the inner tube and the outer fluorinated tube, proving that fluorination can be used to suppress the optical properties of carbon nanotubes without interfering the properties of inner tubes. The present finding can be important in electronic and sensor applications, keeping the inner tube from having unwanted contact with other substances that may distract from the inner tubes own characteristics.

Exploring the Origin of Blue and Ultraviolet Fluorescence in Graphene Oxide

We studied the fluorescence (FL) properties of highly exfoliated graphene oxide (GO) in aqueous solution using continuous-wave and time-resolved FL spectroscopy. The FL spectra of highly exfoliated GO showed two distinct peaks at ∼440 (blue) and ∼300 nm [ultraviolet (UV)]. The FL of GO in the UV region at ∼300 nm was observed for the first time. The average FL lifetimes of the emission peaks at ∼440 and ∼300 nm are 8-13 and 6-8 ns, respectively. The experimentally observed peak wavelengths of pH-dependent FL, FL excitation spectra, and the FL lifetimes are nearly coincident with those of aromatic compounds bound with oxygen functional groups, which suggests that the FL comes from sp(2) fragments consisting of small numbers of aromatic rings with oxygen functional groups acting as FL centers in the GO.