Nanoscrolls of Janus Monolayer Transition Metal Dichalcogenides.

Tubular structures of transition metal dichalcogenides (TMDCs) have attracted attention in recent years due to their emergent physical properties, such as the giant bulk photovoltaic effect and chirality-dependent superconductivity. To understand and control these properties, it is highly desirable to develop a sophisticated method to fabricate TMDC tubular structures with smaller diameters and a more uniform crystalline orientation. For this purpose, the rolling up of TMDC monolayers into nanoscrolls is an attractive approach to fabricating such a tubular structure. However, the symmetric atomic arrangement of a monolayer TMDC generally makes its tubular structure energetically unstable due to considerable lattice strain in curved monolayers. Here, we report the fabrication of narrow nanoscrolls by using Janus TMDC monolayers, which have an out-of-plane asymmetric structure. Janus WSSe and MoSSe monolayers were prepared by the plasma-assisted surface atom substitution of WSe2 and MoSe2 monolayers, respectively, and then were rolled by solution treatment. The multilayer tubular structures of Janus nanoscrolls were revealed by scanning transmission electron microscopy observations. Atomic resolution elemental analysis confirmed that the Janus monolayers were rolled up with the Se-side surface on the outside. We found that the present nanoscrolls have the smallest diameter of about 5 nm, which is almost the same as the value predicted by the DFT calculation. The difference in work functions between the S- and Se-side surfaces was measured by Kelvin probe force microscopy, which is in good agreement with the theoretical prediction. Strong interlayer interactions and anisotropic optical responses of the Janus nanoscrolls were also revealed by Raman and photoluminescence spectroscopy.

High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenides.

van der Waals (vdW) layered materials have attracted much attention because their physical properties can be controlled by varying the twist angle and layer composition. However, such twisted vdW assemblies are often prepared using mechanically exfoliated monolayer flakes with unintended shapes through a time-consuming search for such materials. Here, we report the rapid and dry fabrication of twisted multilayers using chemical vapor deposition (CVD) grown transition metal chalcogenide (TMDC) monolayers. By improving the adhesion of an acrylic resin stamp to the monolayers, the single crystals of various TMDC monolayers with desired grain size and density on a SiO2/Si substrate can be efficiently picked up. The present dry transfer process demonstrates the one-step fabrication of more than 100 twisted bilayers and the sequential stacking of a twisted 10-layer MoS2 single crystal. Furthermore, we also fabricated hBN-encapsulated TMDC monolayers and various twisted bilayers including MoSe2/MoS2, MoSe2/WSe2, and MoSe2/WS2. The interlayer interaction and quality of dry-transferred, CVD-grown TMDCs were characterized by using photoluminescence (PL), cathodoluminescence (CL) spectroscopy, and cross-sectional electron microscopy. The prominent PL peaks of interlayer excitons can be observed for MoSe2/MoS2 and MoSe2/WSe2 with small twist angles at room temperature. We also found that the optical spectra were locally modulated due to nanosized bubbles, which are formed by the presence of interface carbon impurities. The present findings indicate the widely applicable potential of the present method and enable an efficient search of the emergent optical and electrical properties of TMDC-based vdW heterostructures.

Air-stable and efficient electron doping of monolayer MoS2 by salt-crown ether treatment.

To maximize the potential of transition-metal dichalcogenides (TMDCs) in device applications, the development of a sophisticated technique for stable and highly efficient carrier doping is critical. Here, we report the efficient n-type doping of monolayer MoS2 using KOH/benzo-18-crown-6, resulting in a doped TMDC that is air-stable. MoS2 field-effect transistors show an increase in on-current of three orders of magnitude and degenerate the n-type behaviour with high air-stability for ∼1 month as the dopant concentration increases. Transport measurements indicate a high electron density of 3.4 × 1013 cm-2 and metallic-type temperature dependence for highly doped MoS2. First-principles calculations support electron doping via surface charge transfer from the K/benzo-18-crown-6 complex to monolayer MoS2. Patterned doping is demonstrated to improve the contact resistance in MoS2-based devices.

Nationwide Survey of Infectious Bronchitis Virus on Layer Farms in Japan During 2015.

After accumulating data through a nationwide survey, we characterized the recent prevalences and geographic distributions of various genotypes of infectious bronchitis virus (IBV) on layer farms in Japan. Reverse transcription PCR analysis of fecal samples revealed the presence of the IBV nucleoprotein (N) gene on approximately 30% of the farms surveyed. N-gene detection rates were higher in the Chugoku and Kyushu regions than in the remaining surveyed regions. Phylogenetic analysis of S1 gene sequences revealed that JP-I, JP-II, JP-III, and Massachusetts genotypes were particularly prevalent, with JP-I isolated throughout the country. Additionally, JP-II was the genotype detected most frequently in Chugoku, and JP-III was the most frequent in Kyushu. Unlike the previous results obtained in 1998 through 2003, the European-prevalent 4/91 genotype was no longer circulating in Japan. Moreover, the number of prefectures where multiple genotypes were detected simultaneously increased during that time.

Nota de Investigación- Muestreo nacional en Japón de los virus de la bronquitis infecciosa en granjas de postura durante el año 2015. Después de acumular datos a través de un muestreo a nivel nacional, se caracterizaron las prevalencias recientes y las distribuciones geográficas de varios genotipos del virus de la bronquitis infecciosa (IBV) en granjas de gallinas de postura en Japón. El análisis mediante transcripción reversa y PCR de muestras fecales reveló la presencia del gene de la nucleoproteína (N) del virus de la bronquitis infecciosa en aproximadamente el 30% de las granjas muestreadas. Las tasas de detección del gene N fueron más altas en las regiones de Chugoku y Kyushu en comparación con las regiones encuestadas restantes. El análisis filogenético de las secuencias del gene S1 reveló que los genotipos JP-I, JP-II, JP-III y Massachusetts eran particularmente prevalentes, siendo JP-I el genotipo aislado en todo el país. Además, JP-II fue el genotipo detectado con mayor frecuencia en Chugoku, y el genotipo JP-III fue el más frecuente en Kyushu. A diferencia de los resultados anteriores obtenidos desde el año 1998 hasta el 2003, el genotipo 4/91 prevalente en Europa ya no circulaba en Japón. Además, el número de prefecturas donde se detectaron de manera simultánea múltiples genotipos aumentó durante ese tiempo.