Controllable Growth,Characterization And Application In Photodetection Of Monolayer WS₂

Monolayer transition metal chalcogenides?TMDCs?,such as WS₂ and MoS₂,bring new opportunities to the next generation of optoelectronics due to their unique properties such as optical transparency,high carrier mobility and widely adjustable bandgap.Monolayer WS₂ has a band gap of².0eV and high luminous efficiency,which is mostly used for optical,electrical and optoelectronic properties.The growth of high quality monolayer WS₂ is a prerequisite for all researches or applications.Growth of monolayer WS₂ single crystals is easier,and it is facile to fabricate devices to study some physical properties.The preparation of large-area monolayer WS₂ film is a precondition for the application of monolayer WS₂.To this end,we have carried out the following experiments and research works:1.In this paper,the micron-grade monolayer WS₂ was generated on SiO₂/Si substrate through a self-built three zone tube furnaces chemical vapor deposition?CVD?system.Under the bi-temperature experiment condition,the SiO₂/Si substrates and the tungsten source were placed in the same corundum boat to increase the tungsten source concentration near the substrates,and adjusting the growth time is beneficial to grow monolayer WS₂ in the order of centimeters.The growth process of the film follows the thermodynamic growth mechanism:after the decomposition of the precursor on the substrate,WS₂ start nucleation and grow up,and then,the grains merged together to form the centimeter scale film.By characterization,we found that there are bi-and tri-layer unconventional grain boundaries in this centimeter scale film which is called Grain Wall?GW?.Meanwhile,two types of formation mechanisms were proposed.2.The electron mobility of monolayer WS₂ single crystal is 7.5 cm²/Vs,and the current on/off ratio is 10⁵,the responsivity?R?of device is 15 mA/W under the illumination of 457 nm.The fluorescence?FL?and photoluminescence?PL?for the monolayer WS₂ polycrystalline film could be adjusted by changing the gate voltage.The electron mobility of the monolayer WS₂ polycrystalline film was found to be 1.2 cm²/Vs.It was found that the photocurrent on the GW was stronger than that on the surrounding monolayer of WS₂ through measuring the photocurrent of the polycrystalline thin film.In addition,changing the temperature of monolayer WS₂ can affect the Raman and PL spectra.Particularly,the peaks of exciton,trion,biexciton and local states were observed at low temperatures,which is helpful to understand the process of optical excitation in the WS₂ film under low temperature.These electrical,optical,and optoelectronic studies are conductive to understand the physical properties of monolayer WS₂.3.The application of a thin sensitizing layer atop the TMDCs was proven to be a viable route to improve the photodetection performance.The method of coating narrow bandgap semiconductor sensitizers on the device was carried out to improve the photodetection ability of the monolayer WS₂.In actual experiments,tin selenide?SnSe?,black phosphorus?BP?and tin sulfide?SnS?nanosheets?NSs?were used as sensitizers for the preparation of hybrid photodetectors.The R of WS₂/SnSe,WS₂/BP and WS₂/SnS hybrid photodetectors can reach to 99,120 and 2000 mA/W,respectively,and the range of response spectrum was expanded to the near infrared range.In the experiment of combining SnS NSs with WS₂,it was found that adjusting the coverage ratio of SnS NSs on monolayer WS₂ can change the R of the hybrid photodetector,and the response time of the hybrid device are still maintained at a fast tens of microseconds level.This method of modifying the sensitizer has a great significance for improving the performance of two-dimensional materials and promoting the application of two-dimensional materials.4.The synthesis of wafer scale monolayer WS₂ film is the basis for the application of WS₂.The sulfur powder was replaced with H₂S gas as sulfur source.Controlling the gas flow is easy and accurate to adjust the sulfur vapor pressure,which improve the repeatability of sample growth.We also separate from the sulfur source and the tungsten source to the two gas paths,avoiding the“poisoning”of WO₃ and contributing to the continuous growth of WS₂.Because the growth of WS₂ follows a self-limiting growth mechanism,the WS₂ film was preferentially formed on the SiO₂/Si substrate and the wafer size WS₂ film is monolayer.It is found that the quality of monolayer films is excellent and uniform.Meanwhile,large numbers of devices measurement results showed that the electron mobility is around 0.3cm²/Vs.It is credible that the growth system and method can be applied to the growth of various wafer size sulfides,which greatly promoted the application of two-dimensional materials.