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High pressure or strain is an effective strategy for generating phase transformations in van der Waals (vdW) layered materials without introducing defects, but this approach remains difficult to perform consistently. We present a scalable and facile method for achieving phase transformation in vdW materials, wherein solid vdW materials are subject to internal thermal stress within a molten metal mantle as it undergoes cooling. This internal thermal stress is principally the product of differential thermal expansion between mantle and core and can be tuned by the mantle material and temperature conditions. We validated this approach by achieving phase transformation of red phosphorus to black phosphorus, and metallic 1T'- to semiconducting 2H-MoTe

Vertical van der Waals heterostructures (vdWhs), which are made by layer-by-layer stacking of two-dimensional (2D) materials, offer great opportunities for the development of extraordinary physics and devices such as topological superconductivity, robust quantum Hall phenomenon, electron-hole pair condensation, Coulomb drag, and tunneling devices. However, the size of vdWhs is still limited to the order of a few micrometers, which restricts the large-scale roll-to-roll processes for industrial applications. Herein, we report the sequential growth of a 14 in. vertical vdWhs on a rollable Al foil via chemical vapor deposition. By supplying chalcogen precursors to liquid transition-metal precursor-coated Al foils, we grew a wide range of individual 2D transition-metal dichalcogenide (TMD) films, including MoS

We report the synthesis of centimeter-scale monolayer WS2 on gold foil by chemical vapor deposition. The limited tungsten and sulfur solubility in gold foil allows monolayer WS2 film growth on gold surface. To ensure the coverage uniformity of monolayer WS2 film, the tungsten source-coated substrate was placed in parallel with Au foil under hydrogen sulfide atmosphere. The high growth temperature near 935 °C helps to increase a domain size up to 420 μm. Gold foil is reused for the repeatable growth after bubbling transfer. The WS2-based field effect transistor reveals an electron mobility of 20 cm(2) V(-1) s(-1) with high on-off ratio of ∼10(8) at room temperature, which is the highest reported value from previous reports of CVD-grown WS2 samples. The on-off ratio of integrated multiple FETs on the large area WS2 film on SiO2 (300 nm)/Si substrate shows within the same order, implying reasonable uniformity of WS2 FET device characteristics over a large area of 3 × 1.5 cm(2).

Hexagonal boron nitride (h-BN) has recently been in the spotlight due to its numerous applications including its being an ideal substrate for two-dimensional electronics, a tunneling material for vertical tunneling devices, and a growth template for heterostructures. However, to obtain a large area of h-BN film while maintaining uniform thickness is still challenging and has not been realized. Here, we report the systematical study of h-BN growth on Pt foil by using low pressure chemical vapor deposition with a borazine source. The monolayer h-BN film was obtained over the whole Pt foil (2 × 5 cm(2)) under100 mTorr, where the size is limited only by the Pt foil size. A borazine source was catalytically decomposed on the Pt surface, leading to the self-limiting growth of the monolayer without the associating precipitation, which is very similar to the growth of graphene on Cu. The orientation of the h-BN domains was largely confined by the Pt domain, which is confirmed by polarizing optical microscopy (POM) assisted by the nematic liquid crystal (LC) film. The total pressure and orientation of the Pt lattice plane are crucial parameters for thickness control. At high pressure (∼0.5 Torr), thick film was grown on Pt (111), and in contrast, thin film was grown on Pt (001). Our advances in monolayer h-BN growth will play an important role to further develop a high quality h-BN film that can be used for vertical tunneling, optoelectronic devices and growth templates for a variety of heterostructures.

This paper reports the findings of our efforts toward gaining a more complete understanding and utilization of galvanic replacement reactions involving manganese oxide with noble metals. It was revealed that the site of metal deposition is significantly affected by the variable oxidation state of manganese oxide. The use of carbon-encapsulated MnO nanoparticles as a reaction template led to metal growth specifically on the outermost surfaces of the carbon shells rather than on the MnO cores, which allowed for the selective decoration of the external surfaces of hollow carbon nanospheres with catalytic nanocrystals of various noble metals, including Pt, Pd, Rh, and Ir. By rearranging the sequence between carbon-shell coating and galvanic replacement processes, the deposited metal nanocrystals could be placed on the interior surfaces of hollow carbon nanospheres and, moreover, separately on the internal and the external surfaces, which may enable the respective control of the catalytic functionalities of each specific surface.

A core-shell structured Si nanoparticles@TiO2-x/C mesoporous microfiber composite has been synthesized by an electrospinning method. The core-shell composite exhibits high reversible capacity, excellent rate capability, and improved cycle performance as an anode material for Li-ion batteries. Furthermore, it shows remarkable suppression of exothermic behavior, which can prevent possible thermal runaway and safety problems of the cells. The improved electrochemical and thermal properties are ascribed to the mechanically, electrically, and thermally robust shell structure of the TiO2-x/C nanocomposite encapsulating the Si nanoparticles, which is suggested as a promising material architecture for a safe and reliable Si-based Li-ion battery of high energy density.

A novel electroless Pt deposition method was exploited by employing the galvanic replacement process occurring between the Mn(3)O(4) surface and PtCl(4)(2-) complexes. The newly discovered process provides a simple protocol to produce the catalytic nanocomposite, in which a high density of ultrafine Pt nanocrystals is stably immobilized in a homogeneously dispersive state on the surface of Mn(3)O(4) nanoparticles. When the eletrocatalytic activity was tested for the oxygen reduction reaction, which limits the rate of the overall process in proton-exchange membrane fuel cells, the resulting Pt/Mn(3)O(4) nanocomposite showed highly enhanced specific activity and durability, compared with those of the commercial Pt/C catalyst.

Use of random network carbon nanotube (CNT) transistors and their applications to complementary logic gates have been limited by several factors such as control of CNT density, existence of metallic CNTs producing a poor yield of devices, absence of stable n-dopant and control of precise position of the dopant, and absence of a scalable and cost-effective fabrication process. Here, we report a scalable and cost-effective fabrication of complementary logic gates by precisely positioning an air-stable n-type dopant, viologen, by inkjet printing on a separated semiconducting CNTs network. The obtained CNT transistors showed a high yield of nearly 100% with an on/off ratio of greater than 10(3) in an optimized channel length (∼9 μm). The n-doped semiconducting carbon nanotube transistors showed a nearly symmetric behavior in the on/off current and threshold voltage with p-type transistors. CMOS inverter, NAND, and NOR logic gates were integrated on a HfO2/Si substrate using the n/p transistor arrays. The gain of inverter is extraordinarily high, which is around 45, and NAND and NOR logic gates revealed excellent output on and off voltages. These series of whole processes were conducted under ambient conditions, which can be used for large-area and flexible thin film technology.

We report the synthesis of centimeter-scale, uniform 1T′- and 2H-MoTe2 thin films via the tellurization of Mo thin films. 1T′-MoTe2 was initially grown and converted gradually to 2H-MoTe2 over a prolonged growth time under a Te atmosphere. Maintaining excessive Te was essential for obtaining the stable stoichiometric 2H-MoTe2 phase. Further annealing under a lower partial pressure of Te at the same temperature, followed by a rapid quenching, led to the reverse phase transition from 2H-MoTe2 to 1T′-MoTe2. The orientation of the 2H-MoTe2 film was determined by the tellurization rate. Slow tellurization was the key for obtaining a highly oriented 2H-MoTe2 film over the entire area, while fast tellurization led to a 2H-MoTe2 film with a randomly oriented c-axis.

Hexagonal boron nitride (h-BN) is a promising material as a dielectric layer or substrate for two-dimensional electronic devices. In this work, we report the synthesis of large-area h-BN film using atmospheric pressure chemical vapor deposition on a copper foil, followed by Cu etching and transfer to a target substrate. The growth rate of h-BN film at a constant temperature is strongly affected by the concentration of borazine as a precursor and the ambient gas condition such as the ratio of hydrogen and nitrogen. h-BN films with different thicknesses can be achieved by controlling the growth time or tuning the growth conditions. Transmission electron microscope characterization reveals that these h-BN films are polycrystalline, and the c-axis of the crystallites points to different directions. The stoichiometry ratio of boron and nitrogen is close to 1:1, obtained by electron energy loss spectroscopy. The dielectric constant of h-BN film obtained by parallel capacitance measurements (25 μm(2) large areas) is 2-4. These CVD-grown h-BN films were integrated as a dielectric layer in top-gated CVD graphene devices, and the mobility of the CVD graphene device (in the few thousands cm(2)/(V·s) range) remains the same before and after device integration.

The doping/dedoping mechanism of carbon nanotubes (CNTs) with AuCl(3) has been investigated with regard to the roles of cations and anions. Contrary to the general belief that CNTs are p-doped through the reduction of cationic Au(3+) to Au(0), we observed that chlorine anions play a more important role than Au cations in doping. To estimate the effects of Cl and Au on CNTs, the CNT film was dedoped as a function of the annealing temperature (100-700 °C) under an Ar ambient and was confirmed by the sheet resistance change and the presence of a G-band in the Raman spectra. The X-ray photoelectron spectroscopy (XPS) analysis revealed that the doping level of the CNT film was strongly related to the amount of adsorbed chlorine atoms. Annealing at temperatures up to 200 °C did not change the amount of adsorbed Cl atoms on the CNTs, and the CNT film was stable under ambient conditions. Alternatively, Cl atoms started to dissociate from CNTs at 300 °C, and the stability of the film was degraded. Furthermore, the change in the amount of Cl atoms in CNTs was inversely proportional to the change in the sheet resistance. Our observations of the Cl adsorption, either directly or mediated by an Au precursor on the CNT surface, are congruent with the previous theoretical prediction.

We propose bis(trifluoromethanesulfonyl)imide [(CF(3)SO(2))(2)N](-) (TFSI) as a transparent strong electron-withdrawing p-type dopant in carbon nanotubes (CNTs). The conventional p-dopant, AuCl(3), has several drawbacks, such as hygroscopic effect, formation of Au clusters, decrease in transmittance, and high cost in spite of the significant increase in conductivity. TFSI is converted from bis(trifluoromethanesulfonyl)amine (TFSA) by accepting electrons from CNTs, subsequently losing a proton as a characteristic of a Brønsted acid, and has an inductive effect from atoms with high electronegativity, such as halogen, oxygen, and nitrogen. TFSI produced a similar improvement in conductivity to AuCl(3), while maintaining high thermal stability, and no appreciable change in transmittance with no cluster formation. The effectiveness of TFSI was compared with that of other derivatives.