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With the continuous development of the Internet financial industry, its impact on people's daily life is growing. Under the influence of the information technology and the financial innovation, the volatility of the financial market has been significantly enhanced, and the financial risk has become increasingly serious. The COVID-19 in 2020 still has a profound impact on the global financial market. The reasonable measurement of the financial risks is the most important step in the financial risk management. Based on the sample analysis of the Standard & Poor's 500 Index from January 3, 2013 to June 30, 2020, this paper measures the risks by analyzing and calculating the value at risk (VaR) of the sample data by different methods. This paper mainly establishes the ARMA model, the GARCH model based on bayese statistics, and the POT model based on the extreme value theory, and compares and analyzes the VaR values obtained by the three models. Finally, the effectiveness of VaR is tested by VaR backtracking test, and the advantages and disadvantages of the methods are compared and evaluated.

The long–term stability of proton conductors is one of the most important factors in evaluating materials. Guest molecules can act as "bridges" for proton conduction channels and reside in the channels of covalent organic frameworks, but they are prone to leakage. Therefore, it is important to develop proton conductors with intrinsic proton conductivity. In this paper, we synthesized an intrinsically sulfonated covalent organic framework, TpPa–SO3H, which has a more stable proton conducting performance than that of TpPa@H2SO4 by loading guest molecules. Meanwhile, the proton conductivity of TpPa–SO3H was further improved by coating a superabsorbent polymer through an in situ reaction to obtain PANa@TpPa–SO3H (PANa: sodium polyacrylate). As a result, the modified composite exhibits an ultrahigh proton conductivity of 2.33 × 10−1 S cm−1 at 80 °C under 95% relative humidity (RH). The stability of PANa@TpPa–SO3H makes it an efficient proton transport platform with excellent proton conductivity and long–term durability. [ABSTRACT FROM AUTHOR]

Interfacial atomic configuration plays a vital role in the structural stability and functionality of nanocomposites composed of metal nanoparticles (NPs) and two−dimensional semiconductors. In situ transmission electron microscope (TEM) provides a real−time technique to observe the interface structure at atomic resolution. Herein, we loaded bimetallic NiPt truncated octahedral NPs (TONPs) on MoS2 nanosheets and constructed a NiPt TONPs/MoS2 heterostructure. The interfacial structure evolution of NiPt TONPs on MoS2 was in situ investigated using aberration−corrected TEM. It was observed that some NiPt TONPs exhibited lattice matching with MoS2 and displayed remarkable stability under electron beam irradiation. Intriguingly, the rotation of an individual NiPt TONP can be triggered by the electron beam to match the MoS2 lattice underneath. Furthermore, the coalescence kinetics of NiPt TONPs can be quantitatively described by the relationship between neck radius (r) and time (t), expressed as rn = Kt. Our work offers a detailed analysis of the lattice alignment relationship of NiPt TONPs on MoS2, which may enlighten the design and preparation of stable bimetallic metal NPs/MoS2 heterostructures. [ABSTRACT FROM AUTHOR]

Constructing a heterogeneous interface using different components is one of the effective measures to achieve the bifunctionality of nanocatalysts, while synergistic interactions between multiple interfaces can further optimize the performance of single-interface nanocatalysts. The non-precious metal nanocatalysts MoS2/NiSe2/reduced graphene oxide (rGO) bilayer sandwich-like nanostructure with multiple well-defined interfaces is prepared by a simple hydrothermal method. MoS2 and rGO are layered nanostructures with clear boundaries, and the NiSe2 nanoparticles with uniform size are sandwiched between both layered nanostructures. This multiple-interfaced sandwich-like nanostructure is prominent in catalytic water splitting with low overpotential for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) and almost no degradation in performance after a 20 h long-term reaction. In order to simulate the actual overall water splitting process, the prepared nanostructures are assembled into MoS2/NiSe2/rGO||MoS2/NiSe2/rGO modified two-electrode system, whose overpotential is only 1.52 mV, even exceeded that of noble metal nanocatalyst (Pt/C||RuO2~1.63 mV). This work provides a feasible idea for constructing multi-interface bifunctional electrocatalysts using nanoparticle-doped bilayer-like nanostructures. [ABSTRACT FROM AUTHOR]

As an important member of the IVA–VIA group compounds, 2D SnSe2 has emerged as a perfect platform for developing diverse applications, especially in high‐performance optoelectronic devices and data storage, etc. However, the bottom‐up synthesis of large‐area uniform, atomically thin SnSe2 crystals with controlled thicknesses has not yet been realized. Herein, we report the large‐area uniform growth of monolayer (1L), bilayer (2L), and few‐layer (FL) 1T‐SnSe2 single‐crystal flakes on mica substrates via a facile chemical vapor deposition (CVD) route. The feeding amount of Sn precursor and flow rate of hydrogen carrier are found to be the key parameters for the thickness‐controlled growth of uniform SnSe2 flakes. More intriguingly, obvious second harmonic generation (SHG) is revealed in the retained inversion symmetry structure of 1T‐SnSe2, with its intensity showing linear dependence with the thickness from monolayer to multilayers. The new findings reported herein should pave the ways for the thickness‐tunable growth of atomically thin SnSe2 crystals, and their unique optical property explorations and applications in nonlinear optics. [ABSTRACT FROM AUTHOR]