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Register a new userWafer-size VO2 film prepared by water-vapor oxidant
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The growth of wafer-scale and uniform monoclinic VO2 film was a challenge if considering the multivalent vanadium atom and the various phase structures of VO2 compound. Directly oxidizing metallic vanadium film in oxygen gas seemed to be an easy way, while the oxidation parameters were extremely sensitive due to the critical preparation window. Here we proposed a facile thermal oxidation by water-vapor to produce wafer-scale VO2 films with high quality. Results indicated that by using the water-vapor oxidant, the temperature window for VO2 growth was greatly broadened. In addition, the obtained wafer-size VO2 film showed very uniform surface and sharp resistance change. The chemical reaction routes with water-vapor were calculated, which favored the VO2 film growth. Our results not only demonstrated that the water-vapor could be used as a modest oxidizing agent, but also showed the unique advantage for large size VO2 film preparation.
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Strain engineering vanadium dioxide thin films is one way to alter this materials characteristic first order transition from semiconductor to metal. In this study we extend the exploitable strain regime by utilizing the very large lattice mismatch of 8.78 % occurring in the VO$_2$/RuO$_2$ system along the c axis of the rutile structure. We have grown VO$_2$ thin films on single domain RuO$_2$ islands of two distinct surface orientations by atomic oxygen-supported reactive MBE. These films were examined by spatially resolved photoelectron and x-ray absorption spectroscopy, confirming the correct stoichiometry. Low energy electron diffraction then reveals the VO$_2$ films to grow indeed fully strained on RuO$_2$(110), exhibiting a previously unreported ($2times2$) reconstruction. On TiO$_2$(110) substrates, we reproduce this reconstruction and attribute it to an oxygen-rich termination caused by the high oxygen chemical potential. On RuO$_2$(100) on the other hand, the films grow fully relaxed. Hence, the presented growth method allows for simultaneous access to a remarkable strain window ranging from bulk-like structures to massively strained regions.
Uniform single layer graphene was grown on single-crystal Ir films a few nanometers thick which were prepared by pulsed laser deposition on sapphire wafers. These graphene layers have a single crystallographic orientation and a very low density of defects, as shown by diffraction, scanning tunnelling microscopy, and Raman spectroscopy. Their structural quality is as high as that of graphene produced on Ir bulk single crystals, i.e. much higher than on metal thin films used so far.
Polyynes are linear sp-carbon chains of finite length consisting in a sequence of alternated single and triple bonds and displaying appealing optical and electronic properties. A simple, low cost and scalable production technique for polyynes is the submerged arc discharge (SAD) in liquid, which so far, has been mainly exploited in organic solvents. In this work, we investigated in detail SAD in water as a cheap and non-toxic solvent for the production of polyynes. The role of process parameters such as current (10-25 A) and voltage (20-25 V) in the production yield have been investigated, as well as polyynes stability. Polyynes terminated by hydrogen (CnH2: n=6-16) were identified by High-Performance Liquid Chromatography (HPLC) coupled with UV-Visible absorption spectroscopy and with the support of density functional theory (DFT) calculations. Size-selected polyynes separated by HPLC were analyzed by surface enhanced Raman spectroscopy (SERS). The formation process was monitored by in situ SERS using an immersed fiber-optic Raman probe and employing Ag nanoparticles directly produced in the solution by SAD.
Possible existence of topologically protected surface in samarium hexaboride has created a strong need for investigations allowing to distinguish between properties coming from the surface states and those originating in the (remaining) bulk. Studies of SmB6 thin films represent a favorable approach allowing well defined variations of the bulk volume that is not affected by surface states. Moreover, thin films are highly desirable for potential technology applications. However, the growth of SmB6 thin films is accompanied by technology problems, which are typically associated with maintaining the correct stoichiometry of samarium and boron. Here we present feasibility study of SmB6 thin film synthesis by pulsed laser deposition (PLD) from a single stoichiometric SmB6 target. As proved by Rutherford Backscattering Spectrometry (RBS), we succeeded to obtain the same ratio of samarium and boron in the films as that in the target. Thin films revealing characteristic electrical properties of (crystalline) SmB6 were successfully deposited on MgO, sapphire, and glass-ceramics substrates, when the substrates were kept at temperature of 600$^circ$ C during the deposition. Performed electrical resistance studies have revealed that bulk properties of the films are only slightly affected by the substrate. Our results indicate that PLD is a suitable method for complex and intensive research of SmB6 and similar systems.
Vapor transportation is the core process in growing transition-metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD). One inevitable problem is the spatial inhomogeneity of the vapors. The non-stoichiometric supply of transition-metal precursors and chalcogen leads to poor control in products location, morphology, crystallinity, uniformity and batch to batch reproducibility. While vapor-liquid-solid (VLS) growth involves molten precursors at the growth temperatures higher than their melting points. The liquid sodium molybdate can precipitate solid MoS2 monolayers when saturated with sulfur vapor. Taking advantage of the VLS growth, we achieved three kinds of important achievements: (a) 4-inch-wafer-scale uniform growth of MoS2 flakes on SiO2/Si substrates, (b) 2-inch-wafer-scale growth of continuous MoS2 film with a grain size exceeding 100 um on sapphire substrates, and (c) pattern (site-controlled) growth of MoS2 flakes and film. We clarified that the VLS growth thus pave the new way for the high-efficient, scalable synthesis of two-dimensional TMDC monolayers.
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