Ryo Kitaura

• Ph.D.
• Principal Investigator at National Institute for Materials Science

Moiré superlattices formed in van der Waals (vdW) bilayers of 2D materials provide an ideal platform for studying previously undescribed physics, including correlated electronic states and moiré excitons, owing to the wide‐range tunability of their lattice constants. However, their crystal symmetry is fixed by the monolayer structure, and the lack...

This study investigates the optical and electronic properties of eight two-dimensional transition metal chalcogenides (TMDs)—MoS2, WS2, MoSe2, WSe2, MoTe2, WTe2, MoO2, and WO2—covalently functionalized with perylene, forming zero-dimensional/two-dimensional hybrid materials....

Aggregation-induced optical properties of molecules are at the forefront of materials sciences. Here, tetracyanoethylene (TCNE) is reacted and encapsulated in melamine. Crystallization in aqueous tetrahydrofuran solutions of melamine and TCNE at different concentrations yields colorful crystals with fluorescence emission at multiple wavelengths. Co...

The phenomena of pronounced electron-electron and electron-phonon interactions in one-dimensional (1D) systems are ubiquitous, which are well described by frameworks of Luttinger liquid, Peierls instability and concomitant charge density wave. However, the experimental observation of strong phonon-phonon coupling in 1D was not demonstrated. Herein...

We report on the photodetection properties of high-quality graphene encapsulated by hexagonal boron nitride under illumination with optical communication light. We demonstrate a gate-tunable photocurrent and zero-bias switching cycle operation at room temperature. Through gate- and temperature-dependent photocurrent measurements, we determine that...

In this study we develop a strategy to insulate 6,6 ‐Phenyl C61 butyric acid methyl ester (PCBM) on the basal plane of transition metal dichalcogenides (TMDs). Concretely single layers of MoS2, MoSe2, MoTe2, WS2, WSe2 and WTe2 and ultrathin MoO2 and WO2 were grown via chemical vapor deposition (CVD). Then, the thiol group of a PCBM modified with cy...

We have developed a microspectroscopy technique for measuring gate-modulated reflectance to probe excitonic states in two-dimensional transition metal dichalcogenides. Through the use of gate-modulated spectroscopy, we were able to detect excited states of excitons and trions, which would typically have weak optical signals in traditional reflectan...

In the present study, we investigated the intermolecular interactions between single-walled carbon nanotubes (SWCNTs) and encapsulated molecules by polarization resonance Raman microscopy. C70 encapsulated in SWCNTs is investigated under incident laser polarization parallel and perpendicular to the tube axis. We employed two excitation laser wavele...

We investigate the valley coherence in high and low-quality monolayer MoSe2 by polarization-resolved photoluminescence spectroscopy. The observed valley coherence is on the order of 10 % regardless of the sample quality, proving that the suppression of extrinsic effects does not improve the valley coherence. The valley decoherence time estimated ba...

We investigate the valley coherence in high and low-quality monolayer MoSe2 by polarization-resolved photoluminescence spectroscopy. The observed valley coherence is on the order of 10 % regardless of the sample quality, proving that the suppression of extrinsic effects does not improve the valley coherence. The valley decoherence time estimated ba...

We have developed a microspectroscopy technique for measuring gate-modulated reflectance to probe excitonic states in two-dimensional transition metal dichalcogenides. Successfully observing excited states of excitons from cryogenic to room temperature showed that this method is more sensitive to excitonic signals than traditional reflectance spect...

We observed a photovoltaic effect in MoS 2 nanoribbons prepared by a top-down approach. Two-terminal MoS 2 nanoribbons devices show a non-zero short-circuit photocurrent in response to light excitation; the photocurrent density reached ~ 5×10 ⁰ A/cm ² at a power of ~ 10 ⁴ W/cm ² . Photocurrent mapping has revealed that the photocurrent predominantl...

van der Waals (vdW) heterostructures, which can be assembled with various two-dimensional materials, provide a versatile platform for exploring emergent phenomena. Here, we report an observation of the photovoltaic effect in a WS2/MoS2 vdW heterostructure. Light excitation of WS2/MoS2 at a wavelength of 633 nm yields a photocurrent without applying...

We fabricated sensors by modifying the surface of MoS2 and WS2 with COVID-19 antibodies and investigated their characteristics, including stability, reusability, sensitivity, and selectivity. Thiols and disulfanes in antibodies strongly interact with vacant Mo or W sites of MoS2 or WS2, yielding durable devices that are stable for several days in t...

We have successfully confined trions into a one-dimensional restricted space of a MoSe 2 device with carbon nanotube (CNT) gate electrodes. The dry transfer process, including deterministic dry transfer of aligned CNTs, has led to an hBN-encapsulated MoSe 2 device with CNT back gate electrodes. In contrast to a location without CNT gate electrodes,...

Although the synthesis of monolayer transition metal dichalcogenides has been established in the last decade, synthesiz-ing nanoribbons remains challenging. In this study, we have developed a straightforward method to obtain nanoribbons with controllable widths (25-8000 nm) and lengths (1-50 μm) by O2 etching of the metallic phase in metal-lic/semi...

We have investigated the energetics and electronic structure of monolayer MoS 2 with periodic structural corrugations by density functional theory. The total energy of corrugated MoS 2 slightly increases with increasing corrugation height, which indicates that the MoS 2 sheet intrinsically and extrinsically possesses nanometer-scale structural corr...

Ultrathin lateral heterostructures of monolayer MoS2 and WS2 have successfully been realized with the metal-organic chemical vapor deposition method. Atomic-resolution HAADF-STEM observations have revealed that the junction widths of lateral heterostructures range from several nanometers to single-atom thickness, the thinnest heterojunction in theo...

We have successfully confined trions into a one-dimensional restricted space of a MoSe2 device with CNT gate electrodes. The dry transfer process, including deterministic dry transfer of aligned CNTs, has led to an hBN-encapsulated MoSe2 device with CNT back gate electrodes. In contrast to a location without CNT gate electrodes, applying voltage vi...

Low-dimensional (2D) materials, including carbon nanotubes, graphene, boron nitrides, and transition metal dichalcogenides (TMDs), have provided a platform to explore novel physics at the nano-scale. In addition to the fascinating properties of low-dimensional materials themselves, they allow exploring novel superstructures, such as heterojunctions...

Transition-metal dichalcogenides, such as MoS 2 , lack their inversion center in monolayers, exhibiting in-plane piezoelectricity at a nanoscale thickness. In conventional piezoceramics devices, the operating mechanism has been well established that piezocharges appear at crystal edges and how these charges act in capacitor structures. Although TMD...

Studying two-dimensional (2D) van der Waals (vdW) moiré superlattices and their interlayer interactions have received surging attention after recent discoveries of many new phases of matter that are highly tunable. Different atomistic registry between layers forming the inner and outer nanotubes can also form one-dimensional (1D) vdW moiré superlat...

The possibility of continuous Fermi level tuning of Nb-doped WSe 2 under an external electric field is investigated, using the density functional theory combined with the effective screening medium method. The Fermi level monotonically increases and decreases as the carrier concentration increases and decreases, respectively, by controlling the ext...

Different atomistic registry between the layers forming the inner and outer nanotubes can form one-dimensional (1D) van der Waals (vdW) moir\'e superlattices. Unlike the two-dimensional (2D) vdW moir\'e superlattices, effects of 1D vdW moir\'e superlattices on electronic and optical properties in 1D moir\'e superlattices are not well understood, an...

We have developed a simple and straightforward way to realize controlled post-doping towards 2D transition metal dichalcogenides (TMDs). The key idea is to use low-kinetic energy dopant beams and a high-flux chalcogen beam at the same time, leading to substitutional doping with controlled dopant densities. Atomic-resolution transmission electron mi...

Transition metal dichalcogenides are expected to be used in transparent, flexible, and highly efficient light-emitting devices. Exciton diffusion is a key factor in device applications. In this study, we measured the photoluminescence (PL) decay of excitons in a MoS2 monolayer synthesized by chemical vapor deposition on a sapphire substrate. The PL...

We describe the basal plane functionalization of chemically exfoliated molybdenum disulfide (ce-MoS2) nanosheets with a benzo-15-crown-5 ether (B15C5), promoted by the chemistry of diazonium salts en route to the fabrication and electrochemical assessment of an ion-responsive electrode. The success of the chemical modification of ce-MoS2 nanosheets...

The covalent functionalization of MoS2 with a perylenediimide (PDI) is reported and the study is accompanied by detailed characterization of the newly prepared MoS2‐PDI hybrid material. Covalently functionalized MoS2 interfacing organic photoactive species has shown electron and/or energy accepting, energy reflecting or bi‐directional electron acce...

The well‐known electron acceptor perylenediimide (PDI) is covalently attached to semiconducting MoS2, thus exposing the MoS2 material's electron donating potential in the novel MoS2–PDI hybrid. Abstract The covalent functionalization of MoS2 with a perylenediimide (PDI) is reported and the study is accompanied by detailed characterization of the n...

Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with the ultraflat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-...

Recent studies have revealed that van der Waals (vdW) heteroepitaxial growth of 2D materials on crystalline substrates, such as hexagonal boron nitride (hBN), leads to the formation of self-aligned grains, which results in defect-free stitching between the grains. However, how the weak vdW interaction causes a strong limitation on the crystal orien...

The luminescence yield of transition metal dichalcogenide monolayers frequently suffers from the formation of long-lived dark states, which include excitons with intervalley charge carriers, spin-forbidden transitions, and a large center-of-mass momentum located outside the light cone of dispersion relations. Efficient relaxation from bright excito...

The physical properties of molecular crystals are governed by the frontier orbitals of molecules. A molecular orbital, which is formed by superposing the atomic orbitals of constituent elements, has complicated degrees of freedom in the crystal because of the influence of electron correlation and crystal field. Therefore, in general, it is difficul...

The luminescence yield of transition metal dichalcogenide monolayers frequently suffers from the formation of long-lived dark states, which include excitons with intervalley charge carriers, spin-forbidden transitions, and a large center-of-mass momentum located outside the light cone of dispersion relations. Efficient relaxation from bright excito...

Excitons, quasiparticles composed of an electron and a hole, play an important role in optical responses in low-dimensional nanostructures. In this work, we have investigated exciton diffusion in monolayer MoSe2 encapsulated between flakes of hexagonal boron nitride (hBN/MoSe2/hBN). Through photoluminescence imaging and numerical solution of the tw...

In this study, we develop a new approach for stabilization of metallic phases of monolayer MoS2 through the formation of lateral heterostructures composed of semiconducting/metallic MoS2. The structure of metallic (a mixture of T and T′) and semiconducting (2H) phases was unambiguously characterized by Raman spectroscopy, x-ray photoelectron spectr...

Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with ultra-flat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-for...

Recent works have revealed that van der Waals (vdW) epitaxial growth of 2D materials on crystalline substrates, such as hexagonal boron nitride (hBN), leads to formation of self-aligned grains, which results in defect-free stitching between the grains. However, how the weak vdW interaction causes strong limitation on orientation of grains is still...

Excitons, quasi particles composed of an electron and a hole, play an important role in optical responses in low-dimensional nanostructures. In this work, we have investigated exciton diffusion in a monolayer MoSe2 encapsulated between flakes of hexagonal boron nitrides (hBN/MoSe2/hBN). Through PL imaging and numerical solving the 2D diffusion equa...

We report the first experimental observation of a strong-coupling effect in a one-dimensional moiré superlattice. We study one-dimensional double-wall carbon nanotubes (DWCNTs) in which van der Waals–coupled two single nanotubes form a one-dimensional moiré superlattice. We experimentally combine Rayleigh scattering spectroscopy and electron beam d...

Encapsulation by hexagonal boron nitride (hBN) has been widely used to address intrinsic properties of two-dimensional (2D) materials. The hBN encapsulation, however, can alter properties of 2D materials through interlayer orbital hybridization. In this paper, we present measurements of temperature dependence of photoluminescence intensity from mon...

Two-dimensional (2D) materials, including graphene, hexagonal boron nitrides, transition metal dichalcogenides (TMDs), and their heterostructures, have recently attracted a great deal of attention. Various 2D materials can respond to external stimuli differently, which leads to novel physical properties and possible high-performance device applicat...

Supercapacitors have attracted intensive research interest due to their advantages including longer cycling ability and higher power density. Porous heteroatom-doped carbons (PHCs) have been regarded as a class of promising electrode material for supercapacitor because of its unique porous, electronic and chemical properties. However, synthesis of...

We report the first experimental observation of strong coupling effect in one-dimensional moir\'e crystals. We study one-dimensional double-wall carbon nanotubes (DWCNTs) in which van der Waals-coupled two single nanotubes form one-dimensional moir\'e superlattice. We experimentally combine Rayleigh scattering spectroscopy and electron beam diffrac...

We have investigated atomic and electronic structure of grain boundaries in monolayer MoS2, where relative angles between two different grains are 0 and 60 degree. The grain boundaries with specific relative angle have been formed with chemical vapor deposition (CVD) growth on graphite and hexagonal boron nitride flakes; van der Waals interlayer in...

We present measurements of temperature dependence of photoluminescence intensity from monolayer MoS2 encapsulated by hexagonal boron nitride (hBN) flakes. The obtained temperature dependence shows an opposite trend to that of previously observed in a monolayer MoS2 on a SiO2 substrate. Ab-initio bandstructure calculations have revealed that monolay...

An investigation into the feasibility of using ultrashort laser pulses to induce deformations on a donor film coated in 2 dimensional materials as a printing technique for the contactless and impurity-free fabrication of devices based on these materials.

We have investigated atomic and electronic structure of grain boundaries in monolayer MoS2, where relative angles between two different grains are 0 and 60 degree. The grain boundaries with specific relative angle have been formed with chemical vapor deposition growth on graphite and hexagonal boron nitride flakes; van der Waals interlayer interact...

Monolayer molybdenum disulfide (MoS2) is an atomically thin semiconducting material with a direct bandgap. This physical property is attributable to atomically thin optical devices such as sensors, light-emitting devices, and photovoltaic cells. Recently, a near-unity photoluminescence (PL) quantum yield of a monolayer MoS2 was demonstrated via a t...

We show that blister-based-laser-induced forward transfer (BB-LIFT) can be used to cleanly desorb and transfer nano and micro-scale particles between substrates without exposing the particles to the laser radiation or to any chemical treatment that could damage the intrinsic electronic and optical properties of the materials. The technique uses las...

We systematically investigated changes of a photoluminescence (PL) spectral shape by picking a tungsten disulfide monolayer (1L-WS2) grown on a sapphire (SA) substrate and dropping it on various substrates such as SA, SiO2, and hexagonal boron nitride (hBN). An inhomogeneous contribution to the exciton PL linewidth was reduced by picking the CVD-gr...

Extending π-electron systems are among the most important topics in physics, chemistry and materials science because they can result in functional materials with applications in electronics and optics. Conventional processes for π-electron extension, however, can generate products exhibiting chemical instability, poor solubility or disordered struc...

A van der Waals (vdW) heterostructure composed of multi-valley systems can show excitonic optical responses from interlayer excitons that originate from several valleys in the electronic structure. In this work, we studied photoluminescence (PL) from a vdW heterostructure, WS2/MoS2, deposited on hexagonal boron nitride (hBN) flakes. PL spectra from...

Monolayer transition metal dichalcogenides (TMDCs) including WS2, MoS2, WSe2 and WS2, are twodimensional semiconductors with direct bandgap, providing an excellent field for exploration of manybody effects in 2-dimensions (2D) through optical measurements. To fully explore the physics of TMDCs, the prerequisite is preparation of high-quality sample...

Modulation of the local density of states of single-wall carbon nanotubes (SWCNTs) is induced by the encapsulation of europium nanowires (EuNWs). The observation of these modulated density of states using scanning tunneling microscopy/spectroscopy combined with density functional theory calculations is reported. The electronic modulation of SWCNTs...

Photochemical reactions of gaseous ethylene in femtosecond laser filaments are studied. It is shown that hydrogenated amorphous carbon (a-C:H) films and nanoparticles are produced by the strong interactions with the intense laser fields (ca. 10¹³ W cm¹²). The laser field intensity is found to be an important parameter to determine the sp²/sp³ carbo...

We investigates exciton-exciton annihilation (EEA) in tungsten disulfide (WS2) monolayers encapsulated by hexagonal boron nitride (hBN). It is revealed that decay signals observed by time-resolved photoluminescence (PL) are not strongly dependent on the exciton densities of hBN-encapsulated WS2 monolayers (WS2/hBN). In contrast, the sample without...

The crystal orientation of epitaxially grown hexagonal boron nitride (hBN) monolayers from graphene edges was investigated. Low-energy electron microscopy observations reveal that the orientation of individual hBN grains is dependent on the direction of the templated zigzag edges of graphene. Furthermore, the triangular atomic defects in hBN were u...

Two-dimensional (2D) semiconductors, including MoS 2 , WS 2 , MoSe 2 , etc., have provided a fascinating opportunity to explore optical properties in 2 dimensions. In particular, van der Waals (vdW) heterostructures composed of these 2D semiconductors, such as WS 2 /MoS 2 , offer a novel platform for optical physics arising from interlayer excitons...

Monolayer niobium diselenide (NbSe2) is prepared through molecular beam epitaxy with hexagonal boron nitride (hBN) as substrates. Atomic force microscopy and the Raman spectroscopy have shown that the monolayer NbSe2 grown on the hBN possesses triangular or truncated triangular shape whose lateral size amounts up to several hundreds of nanometers....

Wehave developed a facile and general method to passivate thin black phosphorus (BP) flakes with large-area high-quality monolayer hexagonal boron nitride (hBN) sheets grown by the chemical vapor deposition (CVD) method. In spite of the one-atom-thick structure, the high-quality CVD-grown monolayer hBN has proven to be useful to prevent the degrada...

Organic nanotubes (ONTs) are tubular nanostructures composed of small molecules or macromolecules that have found various applications including ion sensor/channels, gas absorption, and photovoltaics. While most ONTs are constructed by self-assembly processes based on weak noncovalent interactions, this unique property gives rise to the inherent in...

Large-area single-layer graphene can be grown on Cu foil by CVD, but for device applications, the layer must to be transferred onto an insulating substrate. As residual particles are often observed on transferred graphene, we investigated their origin using scanning electron microscopy and energy-dispersive X-ray spectrometry (EDX). The results sho...

Progress on researches of two-dimensional (2D) metals strongly relies on development of the growth technique. Studies on preparation of 2D metals have so far been limited, and this is in stark contrast to the situation of 2D semiconductors, where various layered semiconductors, including MoS2, WS2, MoSe2, WSe2, have been isolated in its monolayer f...

We report the single-crystal X-ray structure of La@C 70 (CF 3 ) 3 , which was obtained through in situ exohedral functionalization by means of trifluoromethylation [1]. The X-ray crystallographic study reveals that La@C 70 (CF 3 ) 3 is the first example of an endohedral rare-earth fullerene based on D 5 h -C 70 . The dramatically enhanced stability...

By utilizing graphene-sandwiched structures recently developed in this laboratory, we are able to visualize small droplets of liquids in nanometer scale. We have found that small water droplets as small as several tens of nanometers sandwiched by two single-layer graphene are frequently observed by TEM. Due to the electron beam irradiation during t...

Edge-dependent electronic properties of graphene nanoribbons (GNRs) have attracted intense interests. To fully understand the electronic properties of GNRs, the combination of precise structural characterization and electronic property measurement is essential. For this purpose, two experimental techniques using freestanding GNR devices have been d...

Research on atomic layers including graphene, hexagonal boron nitride (hBN), transition metal dichalcogenides (TMDCs) and their heterostructures has attracted a great deal of attention. Chemical vapor deposition (CVD) can provide large-area structure-defined high-quality atomic layer samples, which have considerably contributed to the recent advanc...

D5h -symmetric fullerene C70 (D5h -C70 ) is one of the most abundant members of the fullerene family. One longstanding mystery in the field of fullerene chemistry is whether D5h -C70 is capable of accommodating a rare-earth metal atom to form an endohedral metallofullerene M@D5h -C70 , which would be expected to show novel electronic properties. Th...

Single atom localization and identification is crucial in understanding effects which depend on the specific local environment of atoms. In advanced nanometer scale materials, the characteristics of individual atoms may play an important role. Here, we describe spectroscopic experiments (electron energy loss spectroscopy, EELS, and Energy Dispersed...

A simple method for the synthesis of linear-chain diamond-like nanomaterials, so-called diamantane polymers, is described. This synthetic approach is primarily based on a template reaction of dihalogen-substituted diamantane precursors in the hollow cavities of carbon nanotubes. Under high vacuum and in the presence of Fe nanocatalyst particles, th...

A simple method for the synthesis of linear-chain diamond-like nanomaterials, so-called diamantane polymers, is described. This synthetic approach is primarily based on a template reaction of dihalogen-substituted diamantane precursors in the hollow cavities of carbon nanotubes. Under high vacuum and in the presence of Fe nanocatalyst particles, th...

20-kV Diffractive Imaging of Graphene by using an SEM-based Dedicated Microscope - Volume 21 Issue S3 - Osamu Kamimura, Takashi Dobashi, Yosuke Maehara, Ryo Kitaura, Hisanori Shinohara, Kazutoshi Gohara

The valence of individual europium atoms confined in carbon nanotubes is successfully measured by using core-level electron energy loss spectroscopy. Changes in the oxidation state at the atomic scale have been observed in Eu atomic chains exposed to oxygen. A transitory behavior has been identified where multiple atoms show a signal consistent wit...

Nano-templated growth of graphene nanoribbons (GNRs) inside carbon nanotubes is a promising mean to fabricate ultrathin ribbons with desired side edge configuration. We report the optical properties of the GNRs formed in single-wall carbon nanotubes. By using coronene as the precursor, extended GNRs are grown via a high-temperature annealing at 700...

Carbon nanotubes (CNTs) are a novel synthetic material comprising only carbon atoms. Based on its rigidity, its electrical and heat conductivity and its applicability to surface manufacturing, this material is expected to have numerous applications in industry. However, due to the material's dimensional similarity to asbestos fibers, its carcinogen...

A fully flattened carbon nanotube (FNT), a graphene nanoribbon (GNR) analogue, provides a hollow space at edges for endohedral doping. Due to the unique shape of the hollow space of FNTs, novel types of low-dimensional arrangements of atoms and molecules can be obtained through endohedral doping into FNTs, which provides a new type of nanopeapods....

Although the inter-layer coupling in layered materials has attracted considerable interest due to its importance in determining physical properties of two-dimensional systems, studies on the inter-layer coupling in one-dimensional systems have so far been limited. Double-wall carbon nanotubes (DWCNTs) are one of the most fundamental and ideal model...

Atomically thin transition metal dichalcogenides (TMDCs) have attracted considerable interest due to the spin-valley coupled electronic structure and possibility in next-generation devices. Substrates are one of the most important factors to limit physical properties of atomic-layer materials, and, among various substrates so far investigated, hexa...

Thermally fragile tris( η 5 -cyclopentadienyl)erbium (ErCp3) molecules are encapsulated in single-wall carbon nanotubes (SWCNTs) with high yield. We realized the encapsulation of ErCp3 with high filling ratio by using high quality SWCNTs at an optimized temperature under higher vacuum. Structure determination based on high-resolution transmissio...

The unstable fullerene Ih-C80 can be stabilized greatly by encapsulation of two rare-earth metal atoms in its interior. Here we present a theoretical study on the electronic structures and stabilities of dimetallofullerene M2@Ih-C80 (M = Y, La, Gd, Lu). Density functional calculations demonstrate that La2@Ih-C80 has a closed-shell configuration, wh...

Graphene quantum dots (GQDs) have attracted a great deal of interest from the viewpoints of both fundamental studies and applications. Photoluminescence (PL) is one of the intriguing properties of GQDs, however, the mechanism of the PL in GQDs is not fully understood. In the present study, a drastic change in the PL from UV to red light region is r...