Surfactant-free nanoflakes of n-type Bi2 Te3 and Bi2 Se3 are synthesized in high yields. Their suspensions are mixed to create nanocomposites with heterostructured nanograins. A maximum ZT (0.7 at 400 K) is achieved with a broad content of 10-15% Bi2 Se3 in the nanocomposites.
Ultrathin transition metal dichalcogenides (TMDs) have exotic electronic properties. With success in easy synthesis of high quality TMD thin films, the potential applications will become more viable in electronics, optics, energy storage, and catalysis. Synthesis of TMD thin films has been mostly performed in vacuum or by thermolysis. So far, there is no solution phase synthesis to produce large-area thin films directly on target substrates. Here, this paper reports a one-step quick synthesis (within 45-90 s) of TMD thin films (MoS , WS , MoSe , WSe , etc.) on solid substrates by using microwave irradiation on a precursor-containing electrolyte solution. The numbers of the quintuple layers of the TMD thin films are precisely controllable by varying the precursor's concentration in the electrolyte solution. A photodetector made of MoS thin film comprising of small size grains shows near-IR absorption, supported by the first principle calculation, exhibits a high photoresponsivity (>300 mA W ) and a fast response (124 µs). This study paves a robust way for the synthesis of various TMD thin films in solution phases.
2D metal chalcogenide thin films have recently attracted considerable attention owing to their unique physicochemical properties and great potential in a variety of applications. Synthesis of large-area 2D metal chalcogenide thin films in controllable ways remains a key challenge in this research field. Recently, the solution-based synthesis of 2D metal chalcogenide thin films has emerged as an alternative approach to vacuum-based synthesis because it is relatively simple and easy to scale up for high-throughput production. In addition, solution-based thin films open new opportunities that cannot be achieved from vacuum-based thin films. Here, a comprehensive summary regarding the basic structures and properties of different types of 2D metal chalcogenides, the mechanistic details of the chemical reactions in the synthesis of the metal chalcogenide thin films, recent successes in the synthesis by different reaction approaches, and the applications and potential uses is provided. In the last perspective section, the technical challenges to be overcome and the future research directions in the solution-based synthesis of 2D metal chalcogenides are discussed.
The controlled synthesis of large-area, atomically thin molybdenum and tungsten ditelluride (MoTe 2 and WTe 2 ) crystals is crucial for their emerging applications based on the attractive electronic properties. However, the solution phase synthesis of high-quality and large-area MoTe 2 or WTe 2 ultrathin films have not been achieved yet. In this study, we synthesized for the first time, large-area atomically thin MoTe 2 and WTe 2 films in solution phase, through rapid crystal formation directly on a conducting substrate. For the synthesis, we developed a new Te precursor. The crystal growth involves an in situ chemical transformation from Te nanoparticles into MoTe 2 or WTe 2 thin films. The synthesis enables precise control of the number of atomic layers over a large area, from a monolayer to multilayers. Micropatterned MoTe 2 thin films are also readily synthesized in situ using the same process. The photodetector made of 3-layer semiconducting MoTe 2 thin films exhibits high photoresponsivity (R λ ) over a broad spectral range (300−1100 nm) with a maximum in the near-IR region, including a R λ = 30 mA W −1 even at λ = 1.10 μm and a fast photoresponse (87 μs). Our synthesis method presents a crucial step in the solution phase synthesis of metal telluride ultrathin films and paves the way for their large-scale emerging applications.
Since 2003 Korea has experienced the highest suicide rate among OECD countries. One of the societal risk factors that triggers suicide is the contagious nature of suicide. This paper empirically examines the effect of celebrity suicide reports on subsequent copycat suicides, using daily suicide data and information of highly publicized suicide stories in Korea from 2005 to 2018. The findings from the Poisson regression model suggest that the number of public suicides soars after media reports on celebrity suicides. On average, the number of suicides in the population increased by 16.4% within just one day after the reports. Further analysis reveals that female and younger subgroups are more likely to be affected by celebrity suicides. Moreover, the public reacts more strongly to suicide incidents of celebrities of the same gender and even imitates the methods of suicide used by celebrities. This paper highlights the significance of careful and responsible media coverage of suicide stories to prevent copycat suicide. For policymakers, it is crucial to implement regulations not only for traditional media but also for new media where younger people can freely access unfiltered information.
Using two complementary approaches, this study examines the deterioration of the Korean labor market during the first 10 months of the global COVID-19 pandemic. Applying the synthetic control method, we first find that the COVID-19 outbreak has eliminated 1.1 million jobs (4.2% of nonfarm employment) nationwide in April 2020. However, a difference-in-differences approach shows that local variation in COVID-19 intensity, which captures the “regional” effect of the pandemic, explains only 9% of the national shock. The portion of the regional effect remains low until December. This is mainly because the nationwide fear and policies such as social distancing measures also have a “common” effect on local economies. Our findings suggest that the COVID-19 shock may last long in the labor market due to this common effect unless the risk of infection is completely eliminated.
Many
studies have reported the synthesis of two-dimensional (2D)
nanoplates and nanosheets of the layer-structured metal chalcogenides
but also have reported various structures far from the 2D shape. In
multicomponent compounds, the elemental concentrations keep changing
during the reaction; hence, the reactions are considered to be nonstoichiometric
from the nucleation and may include continuous changes in the structure
and composition. The different shapes of the metal chalcogenides with
the layered structure are attributed to the nonstoichiometric reactions.
However, it has not been studied if such nonstoichiometric reactions
eventually produce 2D nanoplates, and the mechanism involved in the
transformation has not been understood. This study investigates a
two-step chemical reaction as an extreme nonstoichiometric nucleation
and growth. It reveals that the layer-structured metal chalcogenides
(M2X3, M = Sb and Bi, X = Se and Te) with the R3̅m space group can be obtained
by transforming premade pure chalcogen (Se, Te) thick nanorods or
metal microparticles (Bi). By coupling our first-principles calculations
with a shape-prediction thermodynamic model, we have also examined
and rationalized the shape/aspect ratio of the M2X3 nanocrystals under different experimental growth conditions.
The tendency of anisotropic growth to form the 2D shapes during the
transformations are discussed on the basis of experimental results
and theoretical calculations.
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