Multilayered heterostructures of two-dimensional materials have recently attracted increased interest because of their unique electronic and optical properties. Here, we present chemical vapor deposition (CVD) growth of triangular crystals of monolayer MoS2 on single-crystalline hexagonal graphene domains which are also grown by CVD. We found that MoS2 grows selectively on the graphene domains rather than on the bare supporting SiO2 surface. Reflecting the heteroepitaxy of the growth process, the MoS2 domains grown on graphene present two preferred equivalent orientations. The interaction between the MoS2 and the graphene induced an upshift of the Raman G and 2D bands of the graphene, while significant photoluminescence quenching was observed for the monolayer MoS2. Furthermore, photoinduced current modulation along with an optical memory effect was demonstrated for the MoS2-graphene heterostructure. Our work highlights that heterostructures synthesized by CVD offer an effective interlayer van der Waals interaction which can be developed for large-area multilayer electronic and photonic devices.

The growth of single-layer graphene on Cu metal by chemical vapor deposition (CVD) is a versatile method to synthesize high-quality, large-area graphene. It is known that high CVD temperatures, close to the Cu melting temperature (1083 ºC), are effective for the growth of large graphene domains, but the growth dynamics of graphene over the high-temperature Cu surface is not clearly understood. Here, we investigated the surface dynamics of the single-layer graphene growth by using heteroepitaxial Cu(111) and Cu(100) films. At relatively lower temperatures, 900~1030 ºC, the as-grown graphene showed the identical orientation with the underlying Cu(111) lattice. However, when the graphene was grown above 1040 ºC a new stable configuration of graphene with 3.4º-rotation became dominant. This slight rotation is interpreted by the enhanced graphene-Cu interaction due to the formation of long-range ordered structure. Further increase of the CVD temperature gave the graphene which is rotated with a wide angle distributions, suggesting the enhanced thermal fluctuation of the Cu lattice. The band structures of CVD graphene grown at different temperatures are well correlated with the observed structural change of the graphene. The strong impact of high CVD temperature on a Cu catalyst was further confirmed by the structural conversion of a Cu(100) film to Cu(111) which occurred during the high temperature CVD process. Our work presents important insight on the growth dynamics of CVD graphene, which can be developed to high quality graphene for future high-performance electronic and photonic devices.

No abstract

In the hydrogenation-decomposition-desorption-recombination (HDDR) processed Nd-Fe-CoB -AlGa -Zr magnet powders, influence of the desorption-recombination (DR) temperatures on the microstructures was investigated by high-resolution scanning electron microscopy and scanning transmission electron microscopy. By applying the DR treatment under high vacuum (DR-Step 2) at relative low temperatures about 973 K, we can obtain the excellent texture in which the average size of Nd 2 Fe 14 B grains is refined to about 240 nm and the grain boundary (GB) phase is uniformly dispersed around the Nd 2 Fe 14 B grains. In the GB phase, furthermore, the Nd composition increases and the (Fe, Co) composition decreases drastically with decreasing the DR temperatures, and then Nd-rich GB phase with more than 65 at.％ Nd can be formed at 973 K. At 973 K, however, the region with unformed GB phase is remained about 10％ of the specimen, which leads to deterioration of the coercivity. It was found that the gently DR treatment is ver y effective not only for increasing the coercivity but also for improving the squareness in demagnetization curve.

The effects of replacing Nd by rare-earth mixture Nd 0.76 Pr 0.24 , which is equivalent to didymium, and hydrogen desorption rate at the early stage of desorption-recombination process on magnetic properties of HDDR Nd-Fe-B powders were studied. It was found that the Pr-substituted powder produced by a very slow desorption speed exhibits the same coercivity H cJ and approximately 8% higher remanence B r compared with the powder of pure Nd system prepared by the conventional desorption speed. Although corrosion resistance of the Pr-substituted powder is slightly inferior to that of the pure Nd system powder, it was confirmed that the Pr substitution does not strongly affect irreversible flux losses of the injection-molded bonded magnet fabricated from the powder after exposure to air at 100°C. These results indicate that didymium can be used as an alternative of the pure Nd to produce the HDDR powders. It is also expected that the usage of didymium can reduce the material cost eliminating the process for the separation of the rare-earth elements.