2021年7月28日星期三

Chemical Deposition Method Preparing Monolayer Tungsten Disulfide

Monolayer tungsten disulfide is a kind of nano material with layered structure, it has better temperature stability than monolayer disulfide.

It has attracted much attention in recent years because of its better optical and electrical properties, in the semiconductor field such as transistors, tungsten disulfide is an indirect bandgap semiconductor, forbidden band width is  2.0eV, it has more space than the application of graphene in the field of electronic tube. At present, the most important methods to prepare monolayer tungsten disulfide in China are three methods: RF sputtering, reactive magnetron sputtering and chemical deposition.

In recent years, the research on the preparation of two-dimensional TMDCs by CVD(chemical vapor deposition) method is mainly focused on molybdenum disulfide, because MoS2 requires lower temperature and it is easy to control the growth process, tungsten disulfide seems much more complex.

monolayer tungsten disulfide image

There are two main ways to prepare tungsten disulfide film by CVD method in laboratory: one step reaction method and two step method. One step reaction is the method that directly make the reaction of S source with W source to generate WS2 and deposit on the target substrate in CVD furnace. The two step method is to pre deposit a layer of metal W film on the target substrate, the WS2 film is then synthesized by sulfidation in CVD furnace. The above two CVD methods do not perfectly solve the large-area controllable monolayer and minority layer WS2 films.

Overall, growth conditions on the preparation of WS2 is critical. In the process of two-dimensional WS2 prepared by CVD method, it has more stringent requirements on the instrument and growth process, and the cost of the experiment is high. We believe that in the near future, WS2 film development bottleneck will soon be widened to promote its device and industrialization process.

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Application of Monolayer Tungsten Disulfide Film in Photoelectric Field

Two dimensional transition metal chalcogenide (TMDCs) is considered as a potential star material for future applications of electronic devices because of its graphene like structure.

The current research more semiconductor materials are tungsten disulfide WS2, molybdenum disulfide MoS2, MoSe2 molybdenum selenide, WSe2 tungsten selenide and so on. Among them, WS2 has the most promising application in electronic devices because of its bipolar electron transport properties.

Single layer two W is a kind of layered material similar to graphite with a layer spacing of 0.7 nm. WS2 is an important photoelectric material. When WS2 is changed from bulk material to single layer, the material changes from indirect bandgap to direct band gap semiconductor, and the band gap will change from 1.3eV to 2.0eV, which makes the monolayer tungsten disulfide thin films have important potential applications in photoelectric detectors and solar cells.

Monolayers of tungsten disulfide films show excellent performance in photodetection device research. Some scholars have found that based on 10-layer thick WS2 thin-film optical transistor devices, has the ability to detect monochromatic light of different wavelengths. When the wavelength is 514 nm, the photoresponse time of the device is about 5.3 MS, and the photoresponse rate is about 10 times of that of the MoS2 film. 

tungsten disulfide film image

In the field of solar cells, tungsten disulfide has shown great potential due to its excellent light response and absorption characteristics, some scholars have constructed ITO /WS2 /Au Schottky solar cells. The test results show that visible photocurrent can be produced under visible light irradiation. They also studied the effects of single layer, double layer and multilayer graphene electrodes on Al/WS2/rGO, Schottky cells. The results show that the Schottky contact between graphene and WS2 film is easy to form, so that the built-in electric field is formed at the interface between graphene and WS2 film, the photoinduced electron flows from the WS2 film to the Al electrode, and the hole flows from the WS2 film to the graphene, the maximum photoelectric conversion efficiency of the solar cell is up to 3.3%.

At present, the mass production of single-layer tungsten disulfide film is still difficult, the finished product quality is unstable and the cost is also high, this may be that tungsten disulfide is not as famous as graphene main surface. In the future, it is still a difficult problem for scientists to find a cheap and large area continuous preparation method of WS2 film and guarantee the quality and quantity.

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How to Make Rare Earth Terbium Zinc Tungstate More Dazzling

Rare earth ions are rich in energy levels, their luminescence wavelengths can change from ultraviolet to infrared. They are often used as luminescent centers in luminescent materials.

Metal tungstate is a kind of important inorganic functional materials, including tungsten ore stone structure of zinc tungstate (ZnWO4) is a self luminous material type. WO42- itself has self excitation fluorescence, emits blue green light under ultraviolet irradiation, and can effectively transfer energy to rare earth ions. Therefore, it is widely concerned that in the chemical industry, the self luminescent tungstate doped rare earth element is the most effective LED light source acquisition scheme to change its fluorescence color.

rare earth terbium zinc tungstate fluorescence image

At present, the synthesis methods of rare earth doped ZnWO4 are sol gel method, combustion method, hydrothermal method and microemulsion method. Hydrothermal synthesis is a commonly used method for the synthesis of green phosphors ZnWO4:Tb3+. In the process of hydrothermal synthesis of  ZnWO4:Tb3+ phosphor, adding proper surfactant can greatly affect the structure and luminescent properties of  ZnWO4:Tb3+ phosphor.

In the rare earth doped ZnWO4 system, due to the unequal price of trivalent rare earth ions Tb3+ and Zn2+, Na+ is usually introduced into the synthesized ZnWO4:Tb3+as a charge compensation to enhance the luminescence intensity of the rare earth. In addition, some scholars have studied the effect of surfactants on the luminescence of rare earth zinc tungstate, polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB) and polyethyleneglycol 2000 (PEG-2000) were tested for surface active additives, concluded as follow:

Zinc rare earth terbium tungstate added with different surfactants had almost no effect on the position of the excitation spectrum, but had an influence on the emission intensity. The use of PEG-2000 as a surfactant is most effective for enhancing the luminescence intensity of phosphors. The lifetime of  ZnWO4:Tb3+ phosphors synthesized by using PVP as surfactant is longer than that of PEG-2000 and CTAB as surfactants. Combined with luminescence intensity and fluorescence lifetime, PVP is the most suitable surface additive for rare earth terbium zinc tungstate.

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Zinc Tungstate Whisker

Whisker is a kind of fiber which grows in single crystal form under artificial control.

Whisker's diameter is very small (micron order of magnitude), no defects in the usual materials (grain boundaries, dislocations, voids, etc.). Its atomic arrangement is highly ordered, so its intensity is close to the theoretical value of the complete crystal. Because the whisker reinforced composites have the potential of high strength, the research and development of whiskers have been paid much attention to.

zinc tungstate whisker image

Zinc tungstate whisker is mainly used in metal matrix composites, and it is a key high-tech material. It has been widely used in ceramic cutting tools, space shuttles, automobile parts, chemical industry, machinery and energy production. 

Zinc tungstate has a better scintillation performance than the currently widely used scintillation crystal, but also has the characteristics of not deliquescence and low price, and is a very promising scintillation crystal. The size of zinc tungstate whiskers is generally about 200~500nm, and it is unlikely to have obvious quantum size effect, therefore, scholars believe that the blue shift of the luminescence peak should be due to the crystal structure of the whisker is more complete, less defects, that is, the whisker growth of zinc tungstate is not difficult. 

Zinc tungstate whisker is prepared by solid phase sintering, that is, by mixing tungstic acid and tungsten oxide powder, ball milling, drying, grinding, sieving, tableting, sintering and other technological processes, finally, zinc tungstate whiskers were successfully prepared. Analysis of whisker structure with the XRD, found the structure of wolframite structure, smooth surface, thickness of single whisker is consistent, its cross section is round, the top of the whisker is sharp, and the growth of the whisker is best when the ratio of tungstic acid to tungsten oxide is 4:1, the sintering temperature is 1150 degrees, and the holding time is 240 min.

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Decoloration Mechanism of Double Doped Zinc Tungstate

The modification of zinc tungstate by doping rare earth elements is an effective way to improve its luminescent properties. The single cryst...