Vienna University of Technology in Austria has been the first developed the diode made by tungsten diselenide (WSe2). According to the experiment, this material can be used for ultra-thin flexible solar cells. It was expected in the market that it will take the place of graphene to become the next new-material concept, scientific experts and related enterprises indicates that the application of prospective studies are currently unclear.

 

Graphene is considered as one of the most promising electronic material, but it’s not suitable for build solar cells, which is why the Vienna University of Technology research team began to look for other similar materials like graphene, they want to find the one material that can be arranged by ultra-thin layers and has better electronic properties. Therefore, the researchers found tungsten diselenide, and its main structure is that the upper and lower layer of selenium connected to an intermediate layer of tungsten atoms, as graphene can absorb light, about 95% of the light can pass through it, but one-tenth of the remaining 5% of the light will be absorbed by the material, and converted into electricity.

Stress can make lattice shrink, increase energy band overlap, destroy this balance between carriers, which may induce a new electronic phase transition. Based on this consideration, cooperation team at Nanjing University of Artificial Microstructure Collaborative Innovation Center under the framework of the research carried out quickly both experimental and theoretical high pressure.

 

It was found that: at a pressure of 2.7GPa, when the temperature dropped to 3.1K or less, along with the gradual disappearance of the giant magnetoresistance effect, there was a sharp decline of the resistance. With further increase of the pressure, the emergence of zero resistance appear gradually, indicating the resistance steep drop corresponds to a stress-induced superconducting transition. At about 17GPa, the superconducting transition temperature TC reaches the maximum of 7K, and the accompanying pressures continue to increase, TC gradually decreased, showing a phase diagrama of superconducting pressure as a "dome" type. The study of different external magnetic field at high pressure resistance and magnetic susceptibility has been the further evidence of the existence of superconductivity. On the other hand, Theoretical calculations of high-pressure related show that TC beginning to increase with increasing pressure is the results of increasing density of states near the Fermi surface, and decreases of TC under higher pressure is attributed to the lattice structure instability.

Recently, Chinese Academy of Sciences Hefei Physical Institute of Solid State Physics, and the collaborative research team of Nanjing University and a strong magnetic field center, made new progress on the study of the WTe2 material physical properties under high pressure high magnetic field extreme conditions. The team used high-pressure diamond anvil cell technology, by studying the electrical transport and magnetic susceptibility, for the first time that observed the superconductivity of high-pressure-induced and magnetic resistance calculated by the theory of evolution and the superconductivity with pressure has been systematically studied, related to the results of "superconductivity and evolution of the electronic structure of the type" and titled by "tungsten telluride pressure-driven", published in "Nature - communication" on July 23, the solid researcher YangZhaorong and professor at Nanjing University Song Fengqi, Wan Xiangang are co-corresponding authors of this article.

 

High pressure is a clean, pure and powerful tool in researching new material properties, especially in the exploration of new superconducting materials. WTe2 is a kind of layered semi-metallic material, when the temperature is 0.53K, under strong magnetic field at 60T, the magnetoresistance can be up to 13,000,000%, and it still did not reach the saturation. This strange phenomenon has aroused wide attention of researchers at home and abroad. Recent theoretical studies suggest that it may be a kind of Weyl semi-metallic, magnetic resistance is considered to be related to the perfect match of the hole and electron carrier concentration of the Fermi surface nearby in the system, of which this perfect equilibrium is extremely sensitive for external perturbation, such as doping and stress.

Monolayer films of tungsten disulfide, just three atoms thick, have unique electronic valleys which can be manipulated with laser light. This finding, by MIT physics graduate student Edbert Jarvis Sie, Associate Professor Nuh Gedik, and colleagues, was significant enough to warrant placement on the cover of Nature Materials earlier this year.

 

The cover illustrates a tornado-like whorl of light, lifting an electronic band in the material to a higher energy state, which widens the band gap in the material. This widening is known as the optical Stark effect. The researchers, under senior author Nuh Gedik, the Lawrence C. (1944) and Sarah W. Biedenharn Career Development Associate Professor of Physics at MIT, found that applying circularly polarized laser light lifted the energy in one valley while leaving the energy in the other valley unaffected. "There are two valleys. If we switch the laser polarization, the effect switches to the other valley," Sie says. Gedik spoke about his group's research on topological insulators at the Materials Day Symposium, on Oct. 14, in Kresge Auditorium.

 

"In materials, electrons travelling in different directions experience different scattering potential with the atoms. This establishes an energy landscape as a function of electron's momentum, which can form a local minimum that we call a valley," Sie explains. "The phenomenon of the valleys only occurs in these extremely thin, monolayer forms of the tungsten sulfur compound, not in its bulk form. These valleys normally have the same energy. But as we apply this circularly polarized light, we can lift the energy of one valley relative to the other."

A magnetoresistive material is added with a magnetic field, whose resistance will change, which is a technically useful phenomenon, such as the utilization in sensors to read data of hard disk. Mazhar Ali and his colleagues now identify a kind of material (tungsten telluride), in which the magnetoresistance effect is abnormally large: the resistance can be changed by more than 13 million percent. Its remarkable reluctance will be displayed in very high magnetic fields and very low temperatures, so the actual application is currently impossible. But the discovery proposed new direction for study on the magnetoresistance, which may eventually lead to new applications of this effect.

 

Tungsten(IV) telluride (WTe2) is an inorganic semimetallic chemical compound. In October 2014, tungsten ditelluride was discovered to exhibit an extremely large magnetoresistance: 13 million percent with no known saturation point. The resistance is proportional to the square of the magnetic field. This may be due to the materiel being the first example of a compensated semimetal, in which the number of mobile holes is the same as the number of electrons. The structure of tungsten ditelluride is layered, and the substance can be exfoliated into thin sheets down to monolayers. However electrons can easily move between the layers, unlike in other two dimensional semiconductors. The fraction of charge carriers is 0.005 per formula unit (WTe2).

 

When subjected to pressure, the magnetoresistance effect in WTe2 is reduced. At a pressure of 10.5 GPa magnetoresistance disappears. Above this same pressure of 10.5 GPa tungsten ditelluride can become a superconducter. At 13.0 GPa the transition to superconductivity happens below 6.5K. WTe2 was also recently predicted to be a Weyl semimetal and, in particular, to be the first example of a "Type II" Weyl semimetal, where the Weyl nodes exist at the intersection of the electron and hole pockets.

Production Technology

There are two kinds of commercial process of isopropyl alcohol (IPA) preparation, which both use propylene as raw materials. The older method using refinery grade propylene and sulfuric acid catalyst indirect dehydration to produce isopropyl sulfate, then use steam hydrolyzed into sulfuric acid and IPA, crude IPA was purified by distillation. Newer chemical route is using chemical grade propylene (90% to 99%) to direct hydration, avoiding the use of sulfuric acid. Propylene and water through heating, the gas-liquid mixture into the trickle-bed reactor containing sulfonated polystyrene cation exchange resin under pressure.Also phosphoric acid catalyst by manipulation in a fixed bed reaction is carried out in the gas phase.

 

There is also a liquid line, using a soluble tungsten catalyst. IPA is obtained from the aqueous solution by distillation. Also a small amount of IPA from acetone hydrogenation in the liquid phase, but the process is only applicable to the excess acetone available place.

Demand and capacity

According to statistics, BP, Sasol and Shell's total production capacity of the European IPA was 424,010 tons in 2001, Western Europe sales was nearly 366,000 tons, and export 46,000 tons in 2001. Annual growth rate of Europe needs was 1% to 2%, it is expected that the growth will be higher in Central and Eastern Europe and Asia. IPA have a great potential for the production of derivatives, the growth of being used as a solvent is little or no.

 

The world's total production capacity is more than 2.42 million tons per year. Domo and Mitsui Chemicals built solvent (including IPA) combination unit of 80,000 tons per year in Germany Luna, and put into production in 2004.

On this basis, they proposed the catalytic surface atmospheric pressure chemical vapor deposition method with gold as growth substrate,which achieved the high quality, single uniform and large-area films preparation of millimeter-sized WS2 single crystals. The study found that the catalytic activity of gold and low solubility of tungsten in gold ensure the growth of uniform monolayer high quality WS2 crystals. In the meanwhile, the electrochemical bubbling method may be applied in realization of WS2 high quality transfer without gold substrate damaging. The single WS2 obtained by this method has high crystalline quality, showing optical and electrical properties compared with the material prepared by mechanical exfoliation of the proposed.

 

In addition, the research proposed non-destructive method of roll-to-roll and electrochemical bubbling combination for transferring, which achieve a low-cost continuous transfer of large-area single WS2 film to flexible transparent substrate, which also realize the preparation of large-area flexible transparent WS2 monolayer’s thin films transistors array and the electrical properties is still not attenuated after being bent hundreds of times .

The journal Nature Communications recently published online the results of the RenWencai study group of Institute of Metal Research, the research realize the low-cost and large-area preparation of high-quality uniform strict monolayer tungsten disulfide (WS2) single crystal and thin films, which laid the material foundation on the applications of its flexible electronic / optoelectronic devices and valley electronics and spin electronics field.

 

The researchers found that gold is the only metal that does not react with sulphur and generate sulfide at a high temperature and have a catalytic activity, which can effectively reduce the barrier of tungsten trioxide vulcanization process, and tungsten atoms in gold have a extremely low solubility at high temperature.