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We investigated the growth of twisted graphene on graphene/silicon carbide (SiC-G) templates by metal-free chemical vapor deposition (CVD) through a sequential thermal (ST) process, which exploits the ultraclean surface of SiC-G without exposing the surface to air before CVD. By conducting control experiments with SiC-G templates exposed to air (AirE process), structural analysis by atomic force microscopy revealed that the nucleation density of CVD graphene (CVD-G) was significantly suppressed in the ST process under the same growth condition. The nucleation behavior on SiC-G surfaces is observed to be very sensitive to carbon source concentration and process temperature. The nucleation on the ultraclean surface of SiC-G prepared by the ST process requires higher partial pressure of carbon source compared with that on the surface by the AirE process. Moreover, analysis of CVD-G growth over a wide temperature range indicates that nucleation phenomena change dramatically with a threshold temperature of 1300{\deg}C, possibly due to arising of etching effects. The successful synthesis of twisted few-layer graphene (tFLG) was affirmed by Raman spectroscopy, in which analysis of the G' band proves a high ratio of twisted structure in CVD-G. These results demonstrate that metal-free CVD utilizing ultraclean templates is an effective approach for the scalable production of large-domain tFLG that is valuable for electronic applications., Comment: Authors' original version submitted to Crystal Growth & Design. Main manuscript: 23 pages, 6 figures. Supporting information: 1 pages

The creation of multilayer graphene (Gr), while preserving the brilliant properties of monolayer Gr derived from its unique band structure, can expand the application field of Gr to the macroscale. However, the energy-favorable AB stacking structure in the multilayer Gr induces a strong interlayer interaction and alters the band structure. Consequently, the intrinsic properties of each monolayer are degraded. In this work, we insert carbon nanotubes (CNTs) as nanospacers to modulate the microstructure of multilayer stacking Gr. Nanospacers can increase the interlayer distance and reduce the interlayer interaction. The Gr/CNT stacking structure is experimentally fabricated using a dry transfer method in a layer-by-layer manner. Raman spectroscopy verifies the reduction in the interlayer interaction within the stacking structure. Atomic force microscopy shows an increase in the interlayer distance, which can explain the weakening of the interlayer interactions. The microstructure of the stacked Gr and CNTs is studied by molecular dynamics simulation to systematically investigate the effect of CNT insertion. We found that the distribution distance, size, and arrangement of the CNT can modulate the interlayer distance. These results will help us to understand and improve the properties of the composite systems consisting of Gr and CNTs., Comment: This is the submitted version before review and revision

Kite growth is a process that utilizes laminar gas flow in chemical vapor deposition to grow long, well-aligned carbon nanotubes (CNTs) for electronic application. This process uses metal nanoparticles (NPs) as catalytic seeds for CNT growth. However, these NPs remain as impurities in the grown CNT. In this study, nanodiamonds (NDs) with negligible catalytic activity were utilized as nonmetallic seeds instead of metal catalysts because they are stable at high temperatures and facilitate the growth of low-defect CNTs without residual metal impurities. Results demonstrate the successful growth of over 100-$\mu$m-long CNTs by carefully controlling the growth conditions. Importantly, we developed an analysis method that utilizes secondary electron (SE) yield to distinguish whether or not CNTs grown from metal impurities. The absence of metallic NPs at the CNT tips was revealed by the SE yield mapping, whereas the presence of some kind of NPs at the same locations was confirmed by atomic force microscopy (AFM). These results suggest that most of the aligned CNTs were grown from nonmetallic seeds, most likely ND-derived NPs, via the tip-growth mode. Structural characterizations revealed the high crystallinity of CNTs, with relatively small diameters. This study presents the first successful use of nonmetallic seeds for kite growth and provides a convincing alternative for starting materials to prepare long, aligned CNTs without metal impurities. The findings of this study pave the way for more convenient fabrication of aligned CNT-based devices, potentially simplifying the production process by avoiding the need for the removal of metal impurities., Comment: Accepted version. Main manuscript: 26 pages, 6 figures. Supporting information: 8 pages, 9 figures

Since the strong interlayer interaction of AB-stacked graphene in bulk form degrades the superior property of single-layer graphene, formation of randomly stacked graphene is required to apply the high performances of graphene to macroscopic devices. However, conventional methods to obtain bulk-scale graphene suffer from a low crystallinity and/or the formation of a thermodynamically stable AB-stacked structure. This study develops a novel approach to produce bulk-scale graphene with a high crystallinity and high fractions of random stacking by utilizing the porous morphology of a graphene oxide sponge and an ultrahigh temperature treatment of 1500-1800 {\deg}C with ethanol vapor. Raman spectroscopy indicates that the obtained bulk-scale graphene sponge possesses a high crystallinity and a high fraction of random stacking of 80%. The large difference in the random-stacking ratio between the sponge and the aggregate samples confirms the importance of accessibility of ethanol-derived species into the internal area. By investigating the effect of treatment temperature, a higher random-stacking ratio is obtained at 1500 {\deg}C. Moreover, the AB-stacking fraction was reduced to less than 10% by introducing cellulose nanofiber as a spacer to prevent direct stacking of graphene. The proposed method is effective for large-scale production of high-performance bulk-scale graphene., Comment: 36 pages, 9 figures, Accepted manuscript (Submitted 13 September 2021, Accepted 14 September 2021)

In the synthesis of highly crystalline single-walled carbon nanotubes (SWCNTs), high growth temperatures are preferred, while the formation of impurity amorphous carbon (a-C) causes the termination of SWCNT growth and degrades its properties. To remove this by-product, H2O and CO2 have been employed in metal-catalyzed SWCNT growth systems because of their oxidizing ability. Recently, nonmetallic nanoparticles have become one of the growth seed candidates because of their high melting points and fewer metal impurities. In this study, by using nanodiamond-derived carbon nanoparticles as the solid growth seeds, we investigated the effect of CO2 and H2O on high-temperature SWCNT growth with two types of carbon-source supplies: C2H2 and C2H4. In this growth system, H2O showed oxidizing ability to etch a-C with either carbon sources. CO2 exhibited a similar a-C formation-preventing role in C2H4-supplied growth and achieved higher-purity SWCNTs with higher concentration of C2H4 than the case of H2O. However, in contrast to the other combinations, CO2 injection in the C2H2-supplied growth significantly enhanced the formation of a-C rather than the removal of it while the yield of SWCNTs was also increased, indicating the occurrence of the dehydration reaction between CO2 and C2H2. The present findings will lead to efficient growth of high-quality SWCNTs from nonmetallic growth seeds with the use of growth enhancers., Comment: 41 pages, 14 figures, Accepted manuscript (Submitted 7 October 2022, Accepted 3 November 2022)

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 via CNT gate electrodes significantly alters PL spectra at a location with CNT gate electrodes. PL imaging has revealed that image contrast from trions is linear along the CNT electrode underneath, consistent with 1D confinement of trions in response to the CNT local gating. The confinement width obtained from the PL image is 5.5 x 10^2 nm, consistent with nanoscale 1D confined trions with the diffraction limit broadening. This work has demonstrated electrical control of excitonic states at the nanoscale, leading to novel optoelectronic properties and exciton devices in the future.

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 is achieved by the direct synthesis of heteronanotubes on a CVD-compatible Si/SiO2 TEM grid, which enabled a transfer-free and non-destructive access to many intrinsic structural details. In particular, we have distinguished different shaped boron nitride nanotube (BNNT) edges, which are confirmed, by electron diffraction at the same location, to be strictly associated with its own chiral angle and polarity. We also demonstrate the importance of surface cleanness and isolation for the formation of perfect 1D heterostructures. Furthermore, we elucidate the handedness correlation between SWCNT template and BNNT crystals. This work not only provides an in-depth understanding of this new 1D heterostructure material group, but also, in a more general perspective, serves as an interesting investigation on crystal growth on highly curved (radius of a couple of nm) atomic substrates.

Single- and multi-walled molybdenum disulfide (MoS$_2$) nanotubes have been coaxially grown on small diameter boron nitride nanotubes (BNNTs) which were synthesized from heteronanotubes by removing single-walled carbon nanotubes (SWCNTs), and systematically investigated by optical spectroscopy. The strong photoluminescence (PL) from single-walled MoS$_2$ nanotubes supported by core BNNTs is observed in this work, which evidences a direct band gap structure for single-walled MoS$_2$ nanotubes with around 6 - 7 nm in diameter. The observation is consistent with our DFT results that the single-walled MoS$_2$ nanotube changes from an indirect-gap to a direct-gap semiconductor when the diameter of a nanotube is more than around 5 nm. On the other hand, when there are SWCNTs inside the heteronanotubes of BNNTs and MoS$_2$ nanotubes, the PL signal is considerably quenched. The charge transfer and energy transfer between SWCNTs and single-walled MoS$_2$ nanotubes were examined through characterizations by PL, XPS, and Raman spectroscopy. Unlike the single-walled MoS$_2$ nanotubes, multi-walled MoS$_2$ nanotubes do not emit light. Single- and multi-walled MoS$_2$ nanotubes exhibit different Raman features in both resonant and non-resonant Raman spectra. The method of assembling heteronanotubes using BNNTs as templates provides an efficient approach for exploring the electronic and optical properties of other transition metal dichalcogenide nanotubes.

The synthesis of one-dimensional van der Waals heterostructures was realized recently, which opens up new possibilities for prospective applications in electronics and optoelectronics. The even reduced dimension will enable novel properties and further miniaturization beyond the capabilities of its two-dimensional counterparts have revealed. The natural doping results in p-type electrical characteristics for semiconducting single-walled carbon nanotubes, while n-type for molybdenum disulfide with conventional noble metal contacts. Therefore, we demonstrate here a one-dimensional heterostructure nanotube of 11-nm-wide, with the coaxial assembly of semiconducting single-walled carbon nanotube, insulating boron nitride nanotube, and semiconducting molybdenum disulfide nanotube which induces a radial semiconductor-insulator-semiconductor heterojunction. When opposite potential polarity was applied on semiconducting single-walled carbon nanotube and molybdenum disulfide nanotube, respectively, the rectifying effect was materialized., Comment: 29 pages, 6 figures

The quantum-spin-Hall (QSH) phase of 2D topological insulators has attracted increased attention since the onset of 2D materials research. While large bulk gaps with vanishing edge gaps in atomically thin layers have been reported, verifications of the QSH phase by resistance measurements are comparatively few. This is partly due to the poor uniformity of the bulk gap induced by the substrate over a large sample area and/or defects induced by oxidation. Here, we report the observation of the QSH phase at room-temperature in the 1T'-phase of few-layer MoS2 patterned onto the 2H semiconducting phase using low-power and short-time laser beam irradiation. Two different resistance measurements reveal hallmark transport conductance values, ~e2/2h and e2/4h, as predicted by the theory. Magnetic-field dependence, scanning tunneling spectra, and calculations support the emergence of the room-temperature QSH phase. Although further experimental verification is still desirable, our results provide feasible application to room-temperature topological devices.

Chest radiography is a general method for diagnosing a patient's condition and identifying important information; therefore, radiography is used extensively in routine medical practice in various situations, such as emergency medical care and medical checkup. However, a high level of expertise is required to interpret chest radiographs. Thus, medical specialists spend considerable time in diagnosing such huge numbers of radiographs. In order to solve these problems, methods for generating findings have been proposed. However, the study of generating chest radiograph findings has primarily focused on the English language, and to the best of our knowledge, no studies have studied Japanese data on this subject. There are two challenges involved in generating findings in the Japanese language. The first challenge is that word splitting is difficult because the boundaries of Japanese word are not clear. The second challenge is that there are numerous orthographic variants. For deal with these two challenges, we proposed an end-to-end model that generates Japanese findings at the character-level from chest radiographs. In addition, we introduced the attention mechanism to improve not only the accuracy, but also the interpretation ability of the results. We evaluated the proposed method using a public dataset with Japanese findings. The effectiveness of the proposed method was confirmed using the Bilingual Evaluation Understudy score. And, we were confirmed from the generated findings that the proposed method was able to consider the orthographic variants. Furthermore, we confirmed via visual inspection that the attention mechanism captures the features and positional information of radiographs., Comment: 8 pages, 3 figures, 2 tables

Property by design is one appealing idea in material synthesis but hard to achieve in practice. A recent successful example is the demonstration of van der Waals (vdW) heterostructures,1-3 in which atomic layers are stacked on each other and different ingredients can be combined beyond symmetry and lattice matching. This concept, usually described as a nanoscale Lego blocks, allows to build sophisticated structures layer by layer. However, this concept has been so far limited in two dimensional (2D) materials. Here we show a class of new material where different layers are coaxially (instead of planarly) stacked. As the structure is in one dimensional (1D) form, we name it "1D vdW heterostructures". We demonstrate a 5 nm diameter nanotube consisting of three different materials: an inner conductive carbon nanotube (CNT), a middle insulating hexagonal boron nitride nanotube (BNNT) and an outside semiconducting MoS2 nanotube. As the technique is highly applicable to other materials in the current 2D libraries,4-6 we anticipate our strategy to be a starting point for discovering a class of new semiconducting nanotube materials. A plethora of function-designable 1D heterostructures will appear after the combination of CNTs, BNNTs and semiconducting nanotubes.