Keigo Otsuka

• PhD
• Professor (Assistant) at The University of Tokyo

The development of van der Waals heterostructures has introduced unconventional phenomena that emerge at atomically precise interfaces. For example, interlayer excitons in two-dimensional transition metal dichalcogenides show intriguing optical properties at low temperatures. Here we report on room-temperature observation of interface excitons in m...

When continued device scaling reaches the ultimate limit imposed by atoms, technology based on atomically precise structures is expected to emerge. Device fabrication will then require building blocks with identified atomic arrangements and assembly of the components without contamination. Here we report on a versatile dry transfer technique for de...

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-reso...

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...

The selective etching of carbon nanotubes has been widely explored as a post-synthetic route for enriching semiconducting species. As nanoelectronic applications increasingly demand pure semiconducting nanotubes for use in field-effect transistors and other optoelectronic devices, understanding the mechanistic basis of selective removal becomes cri...

Carbyne, a one-dimensional (1D) carbon allotrope with alternating triple and single bonds, has the highest known mechanical strength but is unstable to bending, limiting its synthesis to short linear chains. Encapsulation within carbon nanotubes (CNTs) stabilizes carbyne, forming confined carbyne (CC), thus enabling further research concerning attr...

Construction of transferable machine-learning interatomic potentials with a minimal number of parameters is important for their general applicability. Here, we present a machine-learning interatomic potential with the functional form of the bond-order potential for comprehensive exploration over the configuration space of carbon. The physics-based...

Carbyne, a one-dimensional (1D) carbon allotrope with alternating triple and single bonds, has the highest known mechanical strength but is unstable to bending, limiting synthesis to short linear chains. Encapsulation within carbon nanotubes (CNTs) stabilizes carbyne, forming confined carbyne (CC), thus enabling further research concerning attracti...

The controlled synthesis of 1D van der Waals (1D vdW) heterostructures, specifically single‐walled carbon nanotubes encapsulated within boron nitride nanotubes (SWCNT@BNNT), presents a challenge due to an incomplete understanding of the factors influencing BNNT growth. This study investigates the growth yield of SWCNT@BNNT heterostructures produced...

Recent advances in low-dimensional materials have enabled the synthesis of single-walled carbon nanotubes encapsulated in hexagonal boron nitride (BN) nanotubes (SWCNT@BNNT), creating one-dimensional van der Waals (vdW) heterostructures. However, controlling the quality and crystallinity of BNNT on the surface of SWCNTs using chemical vapor deposit...

Single-walled carbon nanotube (SWCNT) films exhibit exceptional optical and electrical properties, making them highly promising for scalable integrated devices. Previously, we employed SWCNT films as templates for the chemical vapor deposition (CVD) synthesis of one-dimensional heterostructure films where boron nitride nanotubes (BNNTs) and molybde...

Two-dimensional van der Waals heterostructures have introduced unconventional phenomena that emerge at atomically precise interfaces, and further development is expected in mixed-dimensional heterostructures. Here we discuss exciton physics in 1D-2D heterostructures consisting of one-dimensional carbon nanotubes and two-dimensional tungsten diselen...

Nanomaterials exhibit unique optical phenomena, in particular excitonic quantum processes occurring at room temperature. The low dimensionality, however, imposes strict requirements for conventional optical excitation, and an approach for bypassing such restrictions is desirable. Here we report on exciton transfer in carbon-nanotube/tungsten-disele...

A semiconductor/dielectric interface is one of the dominant factors in device characteristics, and a variety of oxides with high dielectric constants and low interface trap densities have been used in carbon nanotube transistors. Given the crystal structure of nanotubes with no dangling bonds, there remains room to investigate unconventional dielec...

The experimental synthesis of one-dimensional (1D) van der Waals heterostructures, first introduced in 2020 [1], has attracted much research attention. Discussions upon this coaxial stack of different atomic layers are not only limited to synthesis and characteristics, but also extended to applications. Many unique properties of the 1D van der Waal...

Organic color centers in single-walled carbon nanotubes have demonstrated exceptional ability to generate single photons at room temperature in the telecom range. Combining the color centers with pristine air-suspended nanotubes would be desirable for improved performance, but all current synthetic methods occur in solution which makes them incompa...

The development of van der Waals heterostructures has introduced unconventional phenomena that emerge at atomically precise interfaces. For example, interlayer excitons in two-dimensional transition metal dichalcogenides show intriguing optical properties at low temperatures. Here we report on room-temperature observation of interface excitons in m...

Nanomaterials exhibit unique optical phenomena, in particular excitonic quantum processes occurring at room temperature. The low dimensionality, however, imposes strict requirements for conventional optical excitation, and an approach for bypassing such restrictions is desirable. Here we report on exciton transfer in carbon-nanotube/tungsten-disele...

The nanotube/dielectric interface plays an essential role in achieving superb switching characteristics of carbon nanotube-based transistors for energy-efficient computation. Formation of van der Waals heterostructures with hexagonal boron nitride nanotubes could be an effective means to reduce interface state density, but the need for isolating na...

Atomically thin two-dimensional transition-metal dichalcogenide materials with van der Waals integration provide various interesting optoelectronic characteristics that can be used to realize highly efficient flexible solar cells and photosensors. We previously reported in-plane lateral one-dimensional Schottky junctions (SJs) on few-atom-layer 2H-...

The unique optical properties of 2D layered materials are attractive for achieving increased functionality in integrated photonics. Owing to the van der Waals nature, these materials are ideal for integrating with nanoscale photonic structures. Here a carefully designed air‐mode silicon photonic crystal nanobeam cavity for efficient control through...

When continued device scaling reaches the ultimate limit imposed by atoms, technology based on atomically precise structures is expected to emerge. Device fabrication will then require building blocks with identified atomic arrangements and assembly of the components without contamination. Here we report on a versatile dry transfer technique for de...

Single-walled carbon nanotubes have been a candidate for outperforming silicon in ultrascaled transistors, but the realization of nanotube-based integrated circuits requires dense arrays of purely semiconducting species. In order to directly growth such nanotube arrays on wafers, control over kinetics and thermodynamics in tube-catalyst systems pla...

Photoluminescent carbon nanotubes are expected to become versatile room-temperature single-photon sources that have applications in quantum information processing. Quantum emission from carbon nanotubes is often induced by localization of excitons or exciton-exciton annihilation. Here, we modify the local energy landscape of excitons by decorating...

Growing long and high-density arrays of semiconducting single-walled carbon nanotubes is the key to building high-performance electronics. From the growth process perspective, the density and length of carbon nanotubes are determined by their nucleation probability from individual catalysts, subsequent growth rates, and growth lifetime. Here, we st...

Organic color centers in single-walled carbon nanotubes have demonstrated exceptional ability to generate single photons at room temperature in the telecom range. Combining the color centers with pristine air-suspended nanotubes would be desirable for improved performance, but all current synthetic methods occur in solution which makes them incompa...

Ultrastrong coupling of light and matter creates new opportunities to modify chemical reactions or develop novel nanoscale devices. One-dimensional Luttinger-liquid plasmons in metallic carbon nanotubes are long-lived excitations with extreme electromagnetic field confinement. They are promising candidates to realize strong or even ultrastrong coup...

The unique optical properties of two-dimensional layered materials are attractive for achieving increased functionality in integrated photonics. Owing to the van der Waals nature, these materials are ideal for integrating with nanoscale photonic structures. Here we report on carefully designed air-mode silicon photonic crystal nanobeam cavities for...

We demonstrate narrow-linewidth and low-background light emission from an individual single-walled carbon nanotube integrated onto a photonic crystal nanobeam cavity and a waveguide operating in the telecom wavelength regime.

Single-walled carbon nanotubes have been a candidate for outperforming silicon in ultrascaled transistors, but the realization of nanotube-based integrated circuits requires dense arrays of purely semiconducting species. Control over kinetics and thermodynamics in tube-catalyst systems plays a key role for direct growth of such nanotube arrays, and...

Significance We recently synthesized coaxially nested one-dimensional van der Waals heterostructures in which boron nitride nanotubes or molybdenum disulfide nanotubes grew seamlessly on a single-walled carbon nanotube template. In this work, edge structures, nucleation sites, and crystal epitaxial relationships in heteronanotubes are unambiguously...

Nanotube-Based 1D Heterostructures In article number 2102585, Sofie Cambré, Shigeo Maruyama, Rong Xiang and co-workers review the current state-of-the-art in synthesizing and understanding one-dimensional heterostructures coupled by van der Waals forces. A particular focus is placed on the vast parameter space that can be used to design heterostruc...

1D van der Waals heterostructures based on carbon nanotube templates are raising a lot of excitement due to the possibility of creating new optical and electronic properties, by either confining molecules inside their hollow core or by adding layers on the outside of the nanotube. In contrast to their 2D analogs, where the number of layers, atomic...

In this work, we perform a follow-up and comprehensive study on the structural details and formation mechanism of chemical vapor deposition (CVD) synthesized one-dimensional (1D) van der Waals heterostructures. Edge structures, nucleation sites, crystal epitaxial relationships are clearly revealed using transmission electron microscopy (TEM). This...

One-dimensional (1D) van der Waals heterostructures based on carbon nanotube templates are raising a lot of excitement due to the possibility of creating new optical and electronic properties, by either confining molecules inside their hollow core or by adding layers on the outside of the nanotube. In contrast to their 2D analogues, where the numbe...

Photoluminescent carbon nanotubes are expected to become versatile single-photon sources that have applications in quantum information processing. Quantum emission from carbon nanotubes is often induced by localization of excitons or exciton–exciton annihilation. Here, we modify the local energy landscape of excitons by decorating nanoscale pentace...

Photoluminescent carbon nanotubes are expected to become versatile room-temperature single-photon sources that have applications in quantum information processing. Quantum emission from carbon nanotubes is often induced by localization of excitons or exciton-exciton annihilation. Here, we modify the local energy landscape of excitons by decorating...

We demonstrate an individual single-walled carbon nanotube light emitter integrated onto a microcavity and a waveguide operating in the telecom wavelength regime. Light emission from the carbon nanotube is enhanced at the cavity resonance and is efficiently extracted from the waveguide facet. We have transferred carbon nanotubes to a nanobeam cavit...

Organic color centers in single-walled carbon nanotubes have demonstrated exceptional ability to generate single photons at room temperature in the telecom range. Combining the color centers with pristine air-suspended tubes would be desirable for improved performance, but all current synthetic methods occur in solution which makes them incompatibl...

We demonstrate an individual single-walled carbon nanotube light emitter integrated onto a microcavity and a waveguide operating in the telecom wavelength regime. Light emission from the carbon nanotube is enhanced at the cavity resonance and is efficiently extracted from the waveguide facet. We have transferred carbon nanotubes to a nanobeam cavit...

When continued device scaling reaches the ultimate limit imposed by atoms, technology based on atomically precise structures is expected to emerge. Device fabrication will then require building blocks with identified atomic arrangements and assembly of the components without contamination. Here we report on a versatile dry transfer technique for de...

Hexagonal boron nitride is widely used as a substrate for two-dimensional materials in both electronic and photonic devices. Here, we demonstrate that two-dimensional hexagonal boron nitride is also an ideal substrate for one-dimensional single-walled carbon nanotubes. Nanotubes directly attached to hexagonal boron nitride show bright photoluminesc...

Hexagonal boron nitride is widely used as a substrate for two-dimensional materials in both electronic and photonic devices. Here, we demonstrate that two-dimensional hexagonal boron nitride is also an ideal substrate for one-dimensional single-walled carbon nanotubes. Nanotubes directly attached to hexagonal boron nitride show bright photoluminesc...

The charge interaction and corresponding doping effect between single‐walled carbon nanotubes (SWNTs) and various fullerene derivatives, namely, C60, phenyl‐C61‐butyric acid methyl ester (PC61BM), methano‐indenefullerene (MIF), 1′,1″,4′,4″‐tetrahydrodi[1,4]methanonaphthaleno[5,6]fullerene (ICBA), 1,4‐bis(dimethylphenylsilylmethyl)[60]fullerene (SIM...

To realize the efficient growth of single-walled carbon nanotubes (SWCNTs), the growth mechanism behind catalyst activity and growth rates needs to be elucidated at the single nanotube level. In this study, we synthesized SWCNTs with growth interruption, where only Ar, Ar/H2, or Ar/H2/H2O was introduced during a pause in the supply of a carbon sour...

Highly efficient exciton-exciton annihilation process unique to one-dimensional systems is utilized for super-resolution imaging of air-suspended carbon nanotubes. Through the comparison of fluorescence signals in linear and sublinear regimes at different excitation powers, we extract the efficiency of the annihilation processes using conventional...

Recently, W-based catalysts have provided a promising route to synthesize single-walled carbon nanotubes (SWCNTs) with specific chirality, but the mechanism of the growth selectivity is vaguely understood. We propose a strategy to identify the atomic structure as well as the structure evolution of the Co-W-C ternary SWCNT catalyst. The key is to us...

Environmental screening effects are large in carbon nanotubes due to their atomically thin nature, and therefore it is possible to control the optical properties with molecular adsorption [1,2]. Here we investigate adsorption effects of copper phthalocyanine molecules on excitons and trions in air-suspended carbon nanotubes [3]. Using photoluminesc...

Highly efficient exciton-exciton annihilation process unique to one-dimensional systems is utilized for super-resolution imaging of air-suspended carbon nanotubes. Through the comparison of fluorescence signals in linear and sublinear regimes at different excitation powers, we extract the efficiency of the annihilation processes using conventional...

We investigate adsorption effects of copper phthalocyanine molecules on excitons and trions in air-suspended carbon nanotubes. Using photoluminescence excitation spectroscopy, we observe that exciton energy redshifts gradually with the molecular deposition thickness. The trion emission is also observed at large deposition amounts, which indicates c...

We investigate adsorption effects of copper phthalocyanine molecules on excitons and trions in air-suspended carbon nanotubes. Using photoluminescence excitation spectroscopy, we observe that exciton energy redshifts gradually with the molecular deposition thickness. The trion emission is also observed at large deposition amounts, which indicates c...

Single-walled carbon nanotubes (SWNTs) are promising building blocks for next-generation high-performance electronics, e.g. field-effect transistors (FET). For the application, many properties, such as chirality, orientation, density and length, have to be well controlled. Despite significant progresses in controlling the SWNT chirality and density...

We present a systematic study on fabrication, characterization and high temperature surface enhanced Raman spectroscopic (SERS) performance of SiO2 coated silver nanoparticles (Ag@SiO2) on a flat substrate, aiming to obtain a thermal robust SERS substrate for the monitor of high temperature reactions. We confirm that a 10-15 nm SiO2 coating provide...

One of the most challenging tasks in nanotube research is to identify the different electronic types of nanotubes for device fabrication. The implementation of standard spectroscopy techniques at the single-tube level has remained a great task due to small nanotube signal and low spatial resolution. Scattering-type scanning near-field optical micro...

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-SW...

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 t...

Selective synthesis of single-walled carbon nanotubes (SWNTs) with controlled properties is an important research topic for SWNT studies. Here we report a thiophene-assisted chemical vapor deposition (CVD) method to directly grow highly conductive SWNT thin films on substrates, including transparent ones. By adding low concentration thiophene into...

Horizontally aligned single-walled carbon nanotubes (SWNTs) are promising for electronic applications such as field-effect transistors (FETs). In addition to the density-controlled CVD growth [1,2] and chirality characterization of aligned SWNTs on quartz substrates, we will discuss two manipulation techniques of SWNT arrays. First is the full-leng...

An organic film-assisted electrical breakdown technique is proposed to selectively remove metallic (m-) single-walled carbon nanotubes (SWNTs) in full length towards creation of pure semiconducting SWNT arrays which are available for the large-scale fabrication of field effect transistors (FETs). The electrical breakdown of horizontally aligned SWN...